Zirconia-based hollow anodes for improved molten regolith electrolysis

Abstract

This disclosure provides an electrochemical system and method for in situ resource utilization for the production of molecular oxygen and cathodic products from molten regolith. The regolith may be derived from resources on the moon, Mars, asteroids, and other destinations in outer space. The electrochemical system and method utilizes a novel hollow anode design. The disclosure provides an electrochemical system and method comprising a hollow anode comprising: an active anode material; and a solid state electrolyte in ionic communication with the active anode material; wherein the solid state electrolyte is conductive to oxygen anions; a cathode in ionic communication with the hollow anode; a liquid electrolyte comprising molten regolith; wherein the molten regolith establishes ionic communication between the cathode and the hollow anode; and an electronic unit configured to apply an electrical signal across the hollow anode and the cathode to generate molecular oxygen.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 551,627, filed Feb. 9, 2024, and U.S. Provisional Patent Application No. 63 / 635,123, filed Apr. 17, 2024, each of which is hereby incorporated by reference in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Award Number 80NSSC22K1184 awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention.BACKGROUND OF INVENTION

[0003] NASA's renewed interest in returning to the Moon and establishing a sustainable presence on the lunar surface has accelerated the timeline for improving in-situ resource utilization (ISRU) processes. ISRU has the potential to drastically decrease the cost of a lunar settlement by providing resources and materials for propellant, construction materials, life support, etc. Molten regolith electrolysis (MRE) is one ISRU process that seeks to directly electrolyze molten lunar regolith to produce O2, for propellant and life support, and ferrosilicon alloys, for construction materials. Lunar regolith is comprised of approximately 40% oxygen by weight, making it an ideal feedstock for O2 production. MRE has significant advantages over other ISRU processes targeting O2 production, but also requires operating temperatures at or above 1600° C. FIG. 1 shows a schematic of the electrochemistry involved at the traditional MRE cathode and anode. As a result of the high operating temperatures, it is challenging to find anode and cathode materials for extended operation in this corrosive environment of molten oxides. Additionally, the O2 generated is bubbled at the anode surface, which may present challenges with O2 collection and increased ohmic resistance at the anode / regolith interface.

[0004] Previous MRE experiments have successfully demonstrated O2 production using an iridium anode and molybdenum cathode1. The current anode design is iridium or iridium-coated graphite. Kim et al. measured the corrosion rate of iridium anodes in high-silica (acidic) and high-calcia / alumina (basic) melts and found that iridium has a significantly higher (˜20×) corrosion rate in basic oxide melts compared to acidic melts2. The lunar regolith melt will initially start as an acidic oxide melt due to the high concentration of silica. However, as electrolysis occurs, iron oxide and silica are the first species to be reduced. Schreiner et al. has modeled a MRE reactor and shown that for an operating temperature of 1900 K (1626° C.), all the iron oxide and ˜60% of the silica will be reduced to maximize the oxygen production efficiency of MRE3. This pushes the remaining regolith melt toward more basic conditions, likely increasing the corrosion rate of the iridium anode over time. Thus, it is unclear if iridium-based anodes can be used long term for MRE. Additionally, iridium is one of the least abundant elements in the Earth's crust, which is reflected in the cost of the metal. It is also extremely dense, hard, brittle, and strong, which makes machining iridium challenging, though this may be less of a concern with the method of coating iridium onto graphite developed by Shehetkovskiy et al.4

[0005] Bubbles at the surface of the anode can lead to increased ohmic resistance as they impede the movement of charged species to the anode. This reduces the efficiency of the electrolysis as a larger overpotential needs to be applied to drive electrolysis forward. This effect is exacerbated by the reduced gravity at the lunar surface, since bubble separation is largely dependent on the strength of the buoyancy force, which scales directly with surface gravity. Lomax et al. demonstrated the effects of reduced gravity on a water electrolysis cell by using parabolic flights to change the apparent gravity5. The results indicate that reduced gravity leads to the formation of additional smaller bubbles and an increased overpotential to drive the electrolysis at a set current density. It is likely that these effects will be more pronounced for lunar regolith melts since the surface tension of the melt is much higher than water's surface tension.SUMMARY OF THE INVENTION

[0006] Provided herein are electrochemical systems and methods for production of molecular oxygen and cathodic products from molten regolith that address the above noted and other challenges in the art. For example, disclosed herein is an electrochemical cell comprising a novel hollow anode. The hollow anode addresses the above noted and other challenges in the art by removing the direct interface between an active anode material and the molten regolith electrolyte. The hollow anode comprises a solid state electrolyte and an active anode material, wherein the solid state electrolyte shields the active anode material from the molten regolith electrolyte. By removing the active anode and molten regolith electrolyte interface, molten regolith electrolysis is carried out at the active anode surface, bypassing bubble formation, which improves the efficiency of MRE and reducing cost by reducing the degradation and waste of expensive active anode materials.

[0007] Aspects of the invention include an electrochemical system for producing molecular oxygen from regolith comprising: a hollow anode comprising: an active anode material; and a solid state electrolyte in ionic communication with the active anode material; wherein the solid state electrolyte is conductive to oxygen anions; a cathode in ionic communication with the hollow anode; a liquid electrolyte comprising molten regolith; wherein the molten regolith establishes ionic communication between the cathode and the hollow anode; and an electronic unit configured to apply an electrical signal across the hollow anode and the cathode to generate molecular oxygen.

[0008] Aspects of the invention include a process for producing molecular oxygen from regolith, the process comprising: providing an electrochemical system comprising: a hollow anode comprising: an active anode material; and a solid state electrolyte in ionic communication with the active anode material; wherein the solid state electrolyte is conductive to oxygen anions; a cathode in ionic communication with the hollow anode; a liquid electrolyte comprising molten regolith; wherein the liquid electrolyte establishes ionic communication between the hollow anode and the cathode; and applying an electrical signal across the hollow anode and the cathode to produce molecular oxygen at the hollow anode.

[0009] Without wishing to be bound by any particular theory, there may be discussion herein of beliefs or understandings of underlying principles relating to the devices and methods disclosed herein. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1: Schematic of MRE process showing electrochemistry involved at the traditional MRE cathode and anode. Schematic shows oxygen bubbles at the anode surface. Adapted from Schreiner. [1]

[0011] FIG. 2: Schematic of hollow anode functionality showing the hollow anode design comprising solid electrolyte shell and a metallic core. The metallic anode is shielded from the regolith melt by oxygen ion conducting material. Oxygen is produced on the interior of the solid electrolyte.

[0012] FIG. 3: Magnesia-stabilized zirconia in contact with molten lunar highlands regolith simulant (ExoLith LHS-1) for 3 hrs at 1600° C. in argon.

[0013] FIG. 4: Magnesia-stabilized zirconia in contact with molten lunar mare regolith simulant (NASA JSC-1A) for 3 hrs at 1600° C. in argon.

[0014] FIG. 5A: Schematic of the hollow anode design (intended for operational environment) and (FIG. 5B) the inverted hollow anode (used for validating technology).

[0015] FIG. 6: Image of supporting equipment and assembled cell for inverted hollow anode testing. Supporting equipment includes a potentiostat (to control the electric potential or current in the cell), a residual gas analyzer (to measure oxygen content in the outlet gas stream), and a high-temperature furnace (to heat the cell to 1,600° C.).

[0016] FIG. 7: Schematic of molten regolith electrolysis testbed utilizing the inverted hollow anode design.

[0017] FIG. 8: O2 content (left y-axis) and applied potential (right y-axis) for electrolysis of LHS-1 (highlands) lunar regolith simulant. Elapsed time is on x-axis and spans ˜65 min.

[0018] FIG. 9: O2 content (left y-axis) and applied potential (right y-axis) for electrolysis of JSC-1A (maria) lunar regolith simulant. Elapsed time is on x-axis and spans ˜65 min.

[0019] FIG. 10: Optical microscopy cross-sectional image of inverted hollow anode cell containing LHS-1 (highlands) regolith simulant after electrolysis.

[0020] FIG. 11: Optical microscopy cross-sectional image of inverted hollow anode cell containing JSC-1A (maria) regolith simulant after electrolysis.

[0021] FIG. 12: Optical and electron microscope images and Fe, Mo EDS maps for metallic alloy produced at cathode when electrolyzing LHS-1.

[0022] FIG. 13: Optical and electron microscope images and Fe, Mo EDS maps for metallic alloy produced at cathode when electrolyzing JSC-1A.

[0023] FIG. 14: O2 content (left y-axis) and applied potential (right y-axis) for 3-hr electrolysis of LHS-1 (highlands) lunar regolith simulant. Elapsed time is on x-axis and spans ˜185 min.

[0024] FIG. 15: O2 content (left y-axis) and applied potential (right y-axis) for 12-hr electrolysis of LHS-1 (highlands) lunar regolith simulant. Elapsed time is on x-axis and spans ˜725 min.

[0025] FIG. 16: Optical microscopy cross-sectional image of inverted hollow anode cell for Run #3 (LHS-1, 3-hour electrolysis).

[0026] FIG. 17: Optical microscopy cross-sectional image of inverted hollow anode cell for Run #4 (LHS-1, 12-hour electrolysis).

[0027] FIG. 18: Optical and electron microscope images and Fe, Si EDS maps of metallic alloys produced at cathode during Run #3 (LHS-1, 3-hour electrolysis).

[0028] FIG. 19: Optical and electron microscope images of metallic alloy produced at cathode during Run #4 (LHS-1, 12-hour electrolysis). Site 1 and 2 locations are marked with red boxes, with further analysis shown in FIGS. 20 and 21, respectively.

[0029] FIG. 20: Mo, Si, Fe, Ti, O EDS maps for Site 1 in FIG. 19. Ti is present in metallic cathodic products.

[0030] FIG. 21: Mo, Si, Fe, Ti, Mn EDS maps for Site 2 in FIG. 19. Ti and Mn is present in metallic cathodic products.

[0031] FIG. 22: Image of the lunar surface. Dark regions correspond to lunar maria and bright regions correspond to lunar highlands. [2]

[0032] FIG. 23: Starship HLS-Artemis 3. Starship (Human Landing System variant) can deliver 100 mt of payload to lunar surface. If lunar-derived O2 can be used to refuel on the lunar surface: payload can increase to ˜550 mt, cost ($ / mt) can be cut by ˜80%. [2]

[0033] FIG. 24: Lunar regolith is ˜40 wt % oxygen which makes it an ideal feedstock for oxygen production. Shows a comparison between chemical composition of lunar maria and lunar highlands.

[0034] FIG. 25: Molten regolith electrolysis (MRE). Directly electrolyzes molten regolith to generate O2 and Fe—Si alloys; high oxygen extraction efficiency (up to 100%); requires high operating temperatures (1600° C.); materials challenges: containment, anode.

[0035] FIG. 26: Crucible experiment. 8 mol % YSZ crucibles used for containment / reactivity testing; LHS-1 (highlands) tested; crucible size: 40 mm height, 40 mm ID; crucible loading: ˜30 g of LHS-1.

[0036] FIG. 27: Crucible experiment-LHS-1. Optical image of the YSZ / regolith interface shows YSZ dendrites suggesting dissolution-reprecipitation occurring near the YSZ / LHS-1 interface after LHS-1 was heated to 1600° C. for 3 hours in the YSZ crucible. Optical images of the bulk solidified LHS-1 indicate degradation of YSZ is diffusion limited.

[0037] FIG. 28: Microstructural characterization. EDS detected Y-depletion in reprecipitation region; Y-depletion causes reprecipitated YSZ to become monoclinic on cooling; confirmed with EBSD; YSZ cracks on cooling.

[0038] FIG. 29: Hollow anode for MRE. Use dense YSZ as a solid electrolyte and protective shell; designed to enable facile collection of O2; Effect on efficiency: decrease in additional mass transport resistance from solid YSZ; increase in O2 removed from system without bubbling through molten regolith.

[0039] FIG. 30: Inverted hollow anode for small-scale testing. Designed and built small-scale MRE reactor utilizing inverted hollow anode design; Test conditions: 1600° C. under flowing argon; constant current of 0.5 A; outlet gas composition measured using residual gas analysis.

[0040] FIG. 31: Shows oxygen content (vol %) vs. normalized time and electric potential at a constant current of 0.5 A applied to MRE reactor utilizing the inverted hollow anode embodiment.

[0041] FIG. 32: Post-electrolysis microscopy. YSZ cracks on cooling due to monoclinic phase transformation (similar to crucible); YSZ is limited to single-use applications for MRE.

[0042] FIG. 33: Cathodic products. Backscattered electron image; Fe EDS map; Mo EDS map; Fe detected in cathodic product (alloyed with Mo); Fe production confirmed.

[0043] FIG. 34: Powder compact experiments evaluated the equilibrium conditions of YSZ in contact with each regolith simulant (LHS-1 for highlands, JSC-1A for maria) at 1600° C. A 50:50 molar ratio was used for YSZ to regolith loading. Intimate contact between the YSZ and molten regolith was provided to avoid kinetic limitations of degradation and determine the equilibrium state of the system. From these experiments, it was determined that YSZ undergoes dissolution without the formation of other phases. In addition, the samples were quenched from 1600° C., allowing for measurements of the Zr and Y solubilities in both regolith stimulants.

[0044] FIG. 35: Space Launch System—Artemis 3. Refueling 27 mt (3%) with lunar-derived oxygen drops $ / kg of payload by half. [2]

[0045] FIG. 36: Grain sizes were measured in the total analysis region using electron backscatter diffraction and grain segmentation. The analysis region was broken up into three regions. The reprecipitation region has grains that are too small for accurate quantification of size. The surface region contained grains that are larger than the bulk region. A histogram of grain sizes was produced for each region to statistically shown this effect. Grain coarsening is occurring more rapidly in the surface region, which is in contact with molten regolith. Thus, this grain coarsening effect is accelerated by the presence of molten regolith. Larger grains have been shown to resist regolith penetration and dissolution, so this coarsening behavior may ultimately slow the rate of YSZ dissolution.

[0046] FIG. 37: Grain size analysis, as described for FIG. 36, was also performed on the exterior of the crucible (away from molten regolith). These data confirm that the thermal treatment alone does not account for the grain coarsening observed in the surface region. The bulk region and exterior regions have similar grain sizes, indicating that the coarser grains observed in the surface region are likely due to interactions with the molten regolith simulant.

[0047] FIG. 38: Oxygen production from lunar regolith. [2]

[0048] FIG. 39: MRE challenges—anode degradation. Iridium anode corrosion rates vary depending on slag composition; basic slags can cause significant degradation.

[0049] FIG. 40: MRE challenges—anode degradation. Hollow anode eliminates direct contact between metallic electrode and molten regolith.

[0050] FIG. 41: MRE challenges—Fe2+ / Fe3+ cycling. Due to shared electrolyte, Fe2+ / Fe3+ can be cycled during electrolysis; reduces current efficiency for oxygen production. ˜30-60% Faradaic efficiency for MRE process when reducing FeO reported.

[0051] FIG. 42: MRE challenges—Fe2+ / Fe3+ cycling. Fe2+ electrochemical oxidation is prevented by ZrO2 solid electrolyte; no contact with metallic electrode results in no electron transfer.

[0052] FIG. 43: MRE challenges—bubble detachment. O2 bubbles form at anode surface; electrolysis can stall if bubbles cover surface of electrode. Bubble separation is dependent on buoyance, dependent on surface gravity; lunar gravity is ˜⅙ of Earth gravity. 3× increase in bubble detachment time on lunar surface compared to Earth surface.

[0053] FIG. 44: Schematic showing hollow anode concept. Electrochemical cell comprises a hollow anode comprising a solid electrolyte shell and porous metallic electrode core. The porous metallic electrode core is shielded from the molten regolith electrolyte by the solid electrolyte shell.

[0054] FIG. 45: Crucible experiment. 8 mol % Y2O3—ZrO2 crucibles used for containment / reactivity testing; LHS-1 (Highlands) and JSC-1A (Mare) simulants tested. Slow heating and cooling rate (2° C. / min) in tube furnace.

[0055] FIG. 46: Crucible experiment—LHS-1; optical microscopy of cross-sectioned crucible containing LHS-1 after testing. Experimental conditions are as described in FIG. 45. Optical microscopy revealed the presence of YSZ dendrites forming near the YSZ crucible wall.

[0056] FIG. 47: Crucible experiment—LHS-1; Scanning electron microscopy images of YSZ / regolith interface from the crucible used in FIG. 45 and FIG. 46. Energy dispersive X-ray spectroscopy mapping was performed to confirm the composition of the dendrites is YSZ.

[0057] FIG. 48: Crucible experiment—JSC-1A; optical microscopy of cross-sectioned crucible containing JSC-1A after testing. FIG. 45 describes experimental test. Optical microscopy revealed the presence of YSZ dendrites forming near the YSZ crucible wall and in the bulk of the solidified JSC-1A.

[0058] FIG. 49: Crucible experiment—JSC-1A; scanning electron microscopy images of YSZ / regolith interface from the crucible used in FIGS. 45 and 48. Energy dispersive X-ray spectroscopy mapping was performed to confirm the composition of the dendrites is YSZ.

[0059] FIG. 50: Crucible experiments: dissolution and grain boundary penetration are primary mechanisms for degradation; JSC-1A causes more degradation on YSZ; more YSZ dissolved in JSC-1A. Questions raised: Crucibles were slow-cooled; phases and solubilities detected may be forming on cooldown; ZrO2 dendrites likely forming as solidification product. Equilibrium solubility of Zr at 1600° C. in LHS-1 and JSC-1A? Equilibrium solubility of Y at 1600° C. in LHS-1 and JSC-1A? Crystalline reaction phases at 1600° C.?

[0060] FIG. 51: Powder compact experiment. 6 mol % Y2O3—ZrO2 powders mixed with regolith simulants and cold pressed into pellets (Pt crucibles for containment); 2 molar ratios tested: 25:75 and 50:50 (simulant: YSZ). Air quenched after heat treatment (˜150° C. / min).

[0061] FIG. 52: Powder XRD—LHS-1: YSZ. Anorthite is dominant crystalline phase in LHS-1; anorthite peaks observed after mixing and disappear after heat treatment; regolith is amorphous based on composition and cooling rate. No additional crystalline phases observed.

[0062] FIG. 53: Powder XRD—JSC-1A: YSZ. Anorthite is dominant crystalline phase in JSC-1A; anorthite peaks observed after mixing and disappear after heat treatment; regolith is amorphous based on composition and cooling rate. No additional crystalline phases observed.

[0063] FIG. 54: Powder compact SEM / EDS—LHS-1: YSZ. Composition measured over 30 sites for regolith and 5 sites for YSZ using EDS; Zr solubility in molten regolith at 1600° C. calculated using FactSage 8.2.

[0064] FIG. 55: Powder compact SEM / EDS-JSC-1A: YSZ. Composition measured over 30 sites for regolith and 5 sites for YSZ using EDS; Zr solubility in molten regolith at 1600° C. calculated using FactSage 8.2.

[0065] FIGS. 56A-56B: Reactivity summary. FIG. 56A: Crucible experiments; dissolution and grain boundary penetration are primary mechanisms for degradation; JSC-1A causes more degradation on YSZ; more YSZ dissolved in JSC-1A. FIG. 56B: Powder compact experiments: JSC-1A has higher solubility of Zr and Y; Zr-JSC-1A: 2.0%, LHS-1:1.6%; Y-JSC-1A: 1.7%, LHS-1:1.6%; no Zr / Y crystalline reaction products.

[0066] FIG. 57: Schematic of inverted hollow anode design.

[0067] FIG. 58: Crucible experiment—LHS-1. Y in YSZ returns to nominal value after initial degraded layer.

[0068] FIG. 59: Crucible experiment—JSC-1A. ˜100 um Y depletion layer (compared to nominal YSZ) after initial degraded layer.

[0069] FIGS. 60A-60B: Schematic of (FIG. 60A) envisioned integration of hollow anode into existing MRE reactors and (FIG. 60B) inverted hollow anode used to perform proof-of-principle experiments.

[0070] FIG. 61A: O2 content of outlet gas stream and (FIG. 61B) potential across electrolysis cell vs. elapsed time since electrolysis start for all three electrolysis experiments.

[0071] FIG. 62: Optical microscopy of the 1-hour electrolysis cell and EDS elemental maps of the cathodic products generated. EDS analysis area is denoted with the orange box in the optical microscope image.

[0072] FIG. 63: Optical microscopy of the 3-hour electrolysis cell and EDS elemental maps of the cathodic products generated. EDS analysis area is denoted with the orange box in the optical microscope image.

[0073] FIG. 64: Optical microscopy of the 12-hour electrolysis cell and EDS elemental maps of the cathodic products generated. EDS analysis area is denoted with the orange box in the optical microscope image.

[0074] FIGS. 65A-65B: FactSage predictions of (FIG. 65A) the percent atomic elemental composition of the remaining regolith and (FIG. 65B) the cathodic product composition vs. O2 extraction efficiency. Dashed vertical lines represent the O2 extraction efficiency achieved in the 1-hour (0.4%), 3-hour (1.2%), and 12-hour (3.7%) electrolysis cells.

[0075] FIG. 66: Area-weighted histogram of YSZ grain diameters (5 μm bins) at beginning-of-life (BOL), 1-hour electrolysis, 3-hour electrolysis, and 12-hour electrolysis.

[0076] FIG. 67: Raman spectra of YSZ from beginning-of-life (BOL), 1-hour, 3-hour, and 12-hour electrolysis cells.

[0077] FIG. 68: Quantitative EDS summary of Mo oxides observed on interior of YSZ after electrolysis experiments. Data taken from 3-hour electrolysis cell. All values are in at %.

[0078] FIG. 69: Quantitative EDS summary (at %) of cathodic product region in 1-hour electrolysis cell (FIG. 3). All values are in at %.

[0079] FIG. 70: Quantitative EDS summary (at %) of cathodic product region in 3-hour electrolysis cell (FIG. 4). All values are in at %.

[0080] FIG. 71: Quantitative EDS summary (at %) of cathodic product region in 12-hour electrolysis cell (FIG. 5). All values are in at %.

[0081] FIG. 72: FactSage predictions of the remaining regolith composition (full and zoomed) vs. O2 extraction efficiency. The O2 extraction efficiency achieved in the 1-hour (0.4%), 3-hour (1.2%), and 12-hour (3.7%) electrolysis cells.

[0082] FIG. 73: FactSage predictions of the cathodic product composition (full and zoomed) vs. O2 extraction efficiency. The O2 extraction efficiency achieved in the 1-hour (0.4%), 3-hour (1.2%), and 12-hour (3.7%) electrolysis cells.REFERENCES CORRESPONDING TO BRIEF DESCRIPTION OF DRAWINGS

[0083] [1] Samuel S. Schreiner et al., 2016, Adv. Space Res. 57 1585.

[0084] [2] Adapted from NASA STMD's ISRU workshop. URL: https: / / www.hou.usra.edu / meetings / lunarisru2019 / presentations / Sanders.pdf.STATEMENTS REGARDING CHEMICAL COMPOUNDS AND NOMENCLATURE

[0085] In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.

[0086] The term “porous” refers to a membrane, electrode, film, layer, coating, or any other material or item that has porosity. The term “porosity” refers to the amount of a material or item, such of a membrane, corresponding to an absence of said material or item, such as absence corresponding to pores, such as apertures, channels, voids, etc. Generally, porosity refers to absence of said material or item within the physical bounds of said material or item, such as due to the material or item having a porous internal structure. Porosity may be expressed as the percentage of the volume of a material or item, which corresponds to pores, such as apertures, channels, voids, etc., relative to the total volume occupied by the material or item.

[0087] The term “permeable” or “permeable to molecular oxygen” refers to a membrane, electrode, film, layer, active anode material, coating, or any other material or item that is permeable to molecular oxygen (O2) or through which molecular oxygen may flow through or penetrate. For example, a porous material, such as a porous active anode material, may be permeable to molecular oxygen. Optionally, a liquid active anode material, not characterized as having pores, may be permeable to molecular oxygen. The term “permeable” is not to be limited to “permeable to molecular oxygen,” for example, a material may be permeable to other liquids or gases such that the material allows liquids or gases to pass through it.

[0088] The term “electrochemical system” refers to devices and / or device components, such as one or more electrochemical cells, that perform electrochemistry. As used herein, the terms “electrochemical system,”“electrochemical cell”“electrolytic system,” and “electrolytic cell” can be used interchangeably and are generally intended to refer to a device or device components that perform electrochemistry. Electrochemistry refers to conversion of chemical energy into electrical energy or electrical energy into chemical energy. Chemical energy can correspond to a chemical change or chemical reaction. Electrochemistry can thus refer to a chemical change (e.g., a chemical reaction of one or more chemical species into one or more other species) generating electrical energy and / or electrical energy being converted into or used to induce a chemical change. An example of a chemical change or chemical reaction includes oxidation of oxygen anions (O2−). Electrical energy refers to electric potential energy, corresponding to a combination of electric current and electric potential in an electrical circuit. An exemplary electrochemical system or electrochemical cell is an electrolytic cell. An electrolytic cell uses electrical energy to drive a non-spontaneous redox reaction. Electrochemical cells have two or more electrodes (e.g., negative and positive electrodes; e.g., cathode and anode, respectively) and one or more electrolytes.

[0089] The term “electrolysis” means any process that causes a chemical reaction by passing an electrical current through a material or applying an electric potential or voltage to a material.

[0090] As used herein, the term “cathode” refers to an electrode in an electrochemical system, electrolytic system, or an electrolytic cell and refers to an electrode through which an electric current flows from in an electrochemical system. For example, during electrolysis, or application of a sufficient current or voltage across the cathode and the anode, current will flow from the cathode in order to reduce metal, non-metal, or metalloid species in the molten regolith. For example, Si cations or Fe cations may be reduced into elemental Si or Fe at the cathode. In an embodiment, the cathode comprises a refractory metal, meaning that the cathode comprises a metal or combination of metals that remain solid at the operating temperature of embodiments of the electrochemical system disclosed herein. Exemplary cathode materials include Mo, W, Ta, Nb, Re, a combination of these, or an alloy thereof. Exemplary cathode materials for use in embodiments disclosed herein will not undergo electrochemical reduction themselves, and instead will facilitate electrochemical reduction of components of the molten regolith.

[0091] The term “hollow anode” refers to an exemplary anode design of the present invention for use in a molten regolith electrolysis reactor, for example. The design comprises an oxygen anion conducting solid state electrolyte and an active anode material. It will be understood that any geometry of the solid state electrolyte and the active anode material that allows for the hollow anode to be in contact with the molten regolith is feasible. More specifically, the solid state electrolyte preferably shields the active anode material from coming into direct contact with the molten regolith. For molten regolith electrolysis, the solid state electrolyte, may be, for example, conductive to oxygen anions present in the molten regolith. Oxygen anion conductivity allows for oxygen anions to diffuse to the active anode material where the oxygen anions may be oxidized to molecular oxygen. The active anode material may be permeable, defined elsewhere herein, to molecular oxygen, allowing for molecular oxygen produced at the anode to be collected after it is generated at the active anode material through an electrochemical reaction.

[0092] The term “active anode material,” used interchangeably with “metallic current collector” refers to the portion of the anode, or hollow anode, at which oxidation occurs, or in other words, a chemical change being oxidation occurs at the active anode material. In the present disclosure, the active anode material may be shielded by the solid state electrolyte, such that the active anode material does not come into direct physical contact with other components of the electrochemical cell. For example, the active anode material may be shielded by the solid state electrolyte from coming into direct physical contact with the liquid electrolyte, for example, in an exemplary embodiment, the solid state electrolyte shields the active anode material from coming into contact with the molten regolith electrolyte during molten regolith electrolysis. Exemplary active anode materials include, for example, a metal or metal alloy that is a solid at operating temperatures, for example, Ir, Pt, Cr2O3, an alloy thereof, or a combination thereof. The active anode material may be porous to allow molecular oxygen generated at the active anode material to permeate the active anode material and be collected by an oxygen collection device. The active anode material may comprise a metal or metal alloy that is liquid at operating temperature. For example, the active anode material may comprise Pt, Pd, Au, Ag, Rh, Os, Ru, a combination thereof, or an alloy thereof. One of skill in the art will understand that liquid metals are permeable to molecular oxygen, and therefore, a liquid metal active anode material may be characterized as nonporous.

[0093] The term “electrolyte” refers to a medium that provides an ion transport mechanism between the cathode and anode of an electrochemical cell. In an embodiment, the electrolyte may comprise a liquid electrolyte, for example, the molten regolith may serve as the electrolyte in an exemplary embodiment.

[0094] As used herein, the terms “solid electrolyte,”“solid-state electrolyte,” and “solid state electrolyte” are interchangeable and intended to have an equivalent meaning, referring to an ionic conductor. Preferably, the solid state electrolyte is entirely a solid material. Preferably a solid electrolyte is not an electrolyte conventionally characterized or art-known as a liquid electrolyte, a gel electrolyte, or a polymer electrolyte. Optionally, a solid electrolyte is a rigid material. Optionally, a solid state electrolyte of the present invention selectively conducts oxygen anions (O2-). Optionally, for example, the composition of the solid state electrolyte comprises oxygen vacancies, which preferably facilitate oxygen anion conductivity in the solid electrolyte. Optionally, the oxygen anion conductivity of the solid state electrolyte increases with increasing operating temperature. Preferably, but not necessarily, the solid electrolyte has divalent ion conductivity due to hopping, optionally, vacancy-mediated hopping, of the divalent ion within the solid electrolyte. For example, the solid state ion conductivity of the solid electrolyte can occur via movement of ions via defects in a crystal structure of the solid electrolyte. Preferably, a solid electrolyte is characterized by a crystal structure. Optionally, the solid state electrolyte comprises rare earth or alkaline earth-doped zirconia (ZrO2), hafnia (HfO2), ceria (CeO2), thoria (ThO2), urania (UO2). Optionally, the solid state electrolyte comprises a ceramic, for example, a ceramic suitable for high temperature electrochemical cells. Preferably, for example, the solid state electrolyte may comprise Yttria-stabilized zirconia (YSZ) comprising zirconium dioxide (ZrO2) doped with yttrium oxide (Y2O3) to generate oxygen vacancies and enhance oxygen anion conductivity. Optionally, the solid state electrolyte may comprise hafnia, ceria, thoria, or urania doped with yttrium oxide. Optionally, zirconia, hafnia, ceria, thoria, urania, or any combination thereof may be doped with other stabilizers, such as, for example, magnesium oxide (MgO), cerium oxide (CeO2), calcium oxide (CaO), lanthanum oxide (La2O3), scandium oxide (Sc2O3), gadolinium oxide (Gd2O3), or any combination of these. Any other oxides which demonstrate oxygen anion conductivity, are preferably chemically and mechanically stable, at least for a time, at operating temperatures, may be used for the solid state electrolyte. The solid state electrolyte can be any shape, for example, a tubular shape for holding the molten regolith. The solid state electrolyte may also be a tubular shape and hold the active anode material. The solid state electrolyte may be any shape which allows for the solid state electrolyte to contact the liquid molten regolith electrolyte on one side and the active anode material on another side such that the active anode material does not contact the molten regolith electrolyte.

[0095] Generally, the term “doped” has an art-recognized meaning and refers to a host material, such as zirconia, comprising other elements (dopants), such as yttria, generally as a result of said other elements being intentionally introduced to the host material, during formation of the host material and / or after the host material is formed. As used herein, the term doped or doping is not exclusive of the art-known terms alloyed or alloying, respectively. Generally, description of a host material doped with a dopant material or element may be understood to refer to the host material comprising the dopant material or element.

[0096] “Ionic communication” refers to the arrangement of two or more materials or items such that ions can be transported to, passed through, and / or from one material or item to another. Generally, ions can pass through ionically conducting materials such as ionically conducting liquids, such as water, or through solid ionic conductors, for example a solid state electrolyte. Preferably, but not necessarily exclusively, as used herein, transport or conduction of ions refers to transport or conduction of ions in an electrolyte, preferably a solid state electrolyte. For example, in some embodiments two materials or items are in ionic communication with one another if a path of ion flow is provided directly between the two materials or items. For example, the electrolyte comprising regolith and the active anode material separated by the solid state electrolyte. In some embodiments, two materials or items are in ionic communication with one another if an ion flow path is provided indirectly between the two materials or items, such as by including one or more other materials or items or ion flow paths between the two materials or items. In one embodiment, two materials or items are not necessarily in ionic communication with one another unless ions from the first material or item are drawn to, past and / or through the second material or item, such as along an ion flow path.

[0097] The term “regolith” as used herein refers to a layer of heterogeneous and loose deposits overlaying solid rock. Optionally, regolith may include soil, rock fragments, dust and other related materials and is present on, for example, Earth, the Moon, Mars, some asteroids and other extraterrestrial planets and moons. The term “Lunar regolith” refers to a deposition of unconsolidated, loose, heterogeneous superficial deposits covering solid rock, such as the Moon. The regolith often comprises dust, broken rocks, and other related materials and is present on the Moon. The term lunar soil is often used interchangeably with “lunar regolith” but typically refers to the finer fraction of regolith, that which is composed of grains one centimeter in diameter or less. Lunar regolith is generally 4-5 m thick in mare areas and 10-15 m in the older highland regions. Lunar regolith is the result of the continuous impact of meteoroids large and small and the steady bombardment of charged particles from the sun and stars. Lunar regolith is made up of, for example, rock chips, mineral fragments, impact and volcanic glasses and a component only found on the Moon called “agglutinates.” The ratio of these various components varies widely among lunar regolith found in different locations on the Moon. “Agglutinates” are defined as aggregates of mineral fragments held together with glass, and are a major component of, for example, lunar soils, composing up to 60 or 70% of some soils. They are formed when a micrometeorite impact melts a small amount of soil. The mean grain size of typical lunar regolith ranges from 40-800 μm with most falling, for example, between 45-100 μm. Regolith, for example lunar regolith, may be characterized as comprising minerals. Examples of minerals include, but are not limited to, plagioclase, olivine, augite, orthopyroxene, pegeonite, ilmenite, chromite, quartz, cristobalite, and whitlockite. Regolith, for example, lunar regolith may be characterized as comprising oxides. For example metal oxides, metalloid oxides, and non-metal oxides. Examples of metal oxides include, but are not limited to, FeO, MgO, CaO, Al2O3, TiO2, Na2O, K2O, and MnO. Examples of metalloid oxides include, but are not limited to, SiO2, GeO2, As2O3, Sb2O3, or TeO2. Examples of non-metal oxides include, but are not limited to P2O5, and P2O3. Components of regolith may be characterized by an acidity or basicity, such that during electrolysis, for example, the overall acidic character of the molten regolith may change.

[0098] The term “molten regolith electrolysis” or “MRE” refers to an electrochemical technique for extracting oxygen and metals from regolith, for example, from lunar regolith or Martian regolith. In general, to perform MRE, regolith may be fed to an electrochemical system and heated until it melts. The electrochemical system may comprise an anode and a cathode immersed in the molten regolith and a voltage or a current may be applied across the anode and the cathode to drive electrolysis. In one embodiment, melting the regolith or maintaining the melt may be carried out by, for example, Joule heating, which involves applying a current or voltage across the anode the cathode, and requires, for example, higher applied voltages and / or currents than is needed for electrolysis of the molten regolith. In this embodiment, for example, an AC voltage may be applied for heating, with a superimposed DC voltage for electrolysis. Upon application of a sufficient current or voltage, electrochemical reactions occur at the anode and the cathode immersed in the molten regolith. For example, metals ions, metalloid ions, or non-metal ions, and any combination thereof, present in the molten regolith may be reduced at the cathode and oxygen anions may be oxidized at the anode to form molecular oxygen.

[0099] The term “Joule heating” broadly includes the process of passing an electric current through a conductor to release heat. Without being bound by theory of operation, joule heating can be caused by interactions between the moving particles that form the current (usually, but not always, electrons) and the atomic ions that make up the body of the conductor. Charged particles in an electric circuit can be accelerated by an electric field but give up some of their kinetic energy each time they collide with an ion. The increase in the kinetic or vibrational energy of the ions manifests itself as heat and a rise in the temperature of the conductor. Hence energy can be transferred from the electrical power supply to the conductor and any materials with which it is in thermal contact. The term “thermal contact” refers to the relationship between two elements, for example the hollow anode or the cathode and the regolith before it is heated to molten regolith or the hollow anode or the cathode and the molten regolith that are in sufficient proximity to transfer thermal energy between them directly or indirectly.

[0100] The term “electronic unit” as used herein is a broad term and will have its ordinary and customary meaning to a person skilled in the art and will not be limited to a special or customized meaning. The term may specifically refer to a unit, for example, a unit that can be treated as a single component, configured to perform at least one electronic function, but is not limited thereto. Specifically, the electronic unit may have at least one interface for connection to an electrochemical system, and the electronic unit may provide at least one electronic function for interacting with the electrochemical system. For example, the electronic unit may interface with an electrochemical system to control the electric potential or current in the electrochemical system. An example of an electronic unit may include, for example, a potentiostat.

[0101] The term “electrical signal” as used herein, may be used to refer to the voltage, current, or both the voltage and current being applied to the electrochemical system to drive electrolysis of the molten regolith.

[0102] The term “faradaic efficiency” is defined as the ratio of product produced in an electrolysis system and the theoretical maximum of that product based on the total current passed. The Faradaic efficiency is a measure of how much of the applied current is used to electrochemically produce the desired product, for example O2. Based on the amount and duration of the applied current, a theoretical maximum of product produced can be calculated assuming every electron transferred is used to produce the product. For example, in an embodiment, the total amount of O2 produced can be measured. The ratio of the measured O2 and the theoretical maximum O2 gives the Faradaic efficiency.

[0103] The term “metal alloy” refers to an alloy of two or more metals or metalloids. For example, a metal alloy may be characterized as a solid solution of two or more metal or metalloid elements (e.g., the metal elements being in the form of atoms or ions in the solid solution), a mixture of metallic phases, or an intermetallic compound. A metal alloy can be characterized as comprising metallic bonding. In certain embodiments, a metal, rather than a metal alloy, refers to a metallic material whose chemical formula has one metal element (i.e., its composition has substantially or essentially one metal element).

[0104] The term “refractory material” as used herein has its ordinary meaning in the art. A refractory material is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures, for example, at temperatures above 1,500° C. A refractory material, may be, for example, an inorganic, non-metallic compound that may be porous or non-porous. A refractory material may be characterized by a crystallinity. For example, a refractory material may be crystalline, polycrystalline, amorphous, or a composite. A refractory material may comprise, for example, oxides, carbides or nitrides of the following elements: silicon, aluminum, magnesium, calcium, boron, chromium and zirconium. In embodiments, a refractory material may be zirconia or doped zirconia, hafnia or doped hafnia, ceria or doped ceria, thoria or doped thoria, urania or doped uriana, or any combination thereof.

[0105] The term “refractory metal” has its art recognized meaning and refers to a metal material (present as an element or alloy, if appropriate also with non-metallic additives, for example, oxides, carbides, etc.) that has a melting point of ≥2000° C. Examples of refractory metals include Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, or an alloy thereof. In certain embodiments, refractory metals may be used for the cathode and / or the active anode material. For example, in embodiments, the cathode material may comprise Mo. In embodiments, for example, the active anode material may comprise Ir. The active anode material may comprise, for example, porous Ir to facilitate oxygen permeability.

[0106] The term “batch operation” is used to describe an exemplary embodiment as disclosed herein. In an embodiment, for example, the electrochemical system may be provided and further comprise a container to receive and hold the molten regolith during electrolysis. Batch operation refers to this configuration. Batch operation may refer to a method of carrying out an electrolysis process wherein the process undergoes thermal cycles, for example, is heated to a temperature to generate molten regolith for electrolysis, then subsequently cooled to replace a cathode material, or, for example, an anode material, or for example, to provide a fresh batch of regolith to the container. Therefore, the process for MRE may be carried out in “batches.”

[0107] The term “mobile operation” refers to an exemplary embodiment as disclosed herein, wherein no container is provided to receive and hold the regolith. Instead, the electrochemical system is provided directly to an ambient environment comprising regolith and the method for electrochemical processing of regolith is performed, for example, directly on the lunar surface. Mobile operation may also refer to providing the electrochemical system for processing of molten regolith further configured to be operated in a mobile fashion, wherein the electrochemical system is provided on a mobile system, such as a rover. The electrochemical system may be moved across the lunar surface via the mobile system, for example via the rover, in order to access fresh lunar regolith for MRE directly on the lunar surface.

[0108] The term “maintain,” as used herein may relate to maintaining over a certain period of time a condition of the electrochemical system. For example, molten regolith electrolysis requires the regolith component to be liquid, for example, molten. Regolith, for example lunar regolith, may have a melting point of between about 1,110° C. to about 1,400° C. depending on the composition of the regolith. Thus, the temperature of lunar regolith, for example, must be maintained at greater than or equal to 1,500° C. in order to carry out molten regolith electrolysis in an electrochemical system. More preferably, molten regolith electrolysis is carried out temperatures greater than or equal 1,600° C. in order to maintain the regolith melt.

[0109] In an embodiment, a composition or compound of the invention, such as an alloy or precursor to an alloy, is isolated or substantially purified. In an embodiment, an isolated or purified compound is at least partially isolated or substantially purified as would be understood in the art. In an embodiment, a substantially purified composition, compound or formulation of the invention has a chemical purity of 95%, optionally for some applications 99%, optionally for some applications 99.9%, optionally for some applications 99.99%, and optionally for some applications 99.999% pure.DETAILED DESCRIPTION OF THE INVENTION

[0110] In the following description, numerous specific details of the devices, device components and methods of the present invention are set forth in order to provide a thorough explanation of the precise nature of the invention. It will be apparent, however, to those of skill in the art that the invention can be practiced without these specific details.

[0111] The invention can be further understood by the following non-limiting examples.Example 1: Zirconia-Based Hollow Anodes for Improved Molten Regolith Electrolysis

[0112] Disclosed herein is a novel hollow anode design for use in a MRE reactor that can mitigate the aforementioned challenges. The hollow anode design contains a solid electrolyte shell and a metallic core. The metallic anode is shielded from the regolith melt by an oxygen ion conducting ceramic, allowing electron transfer to occur in the interior of the shell. O2 is produced on the interior of the shell, bypassing the regolith melt. This enables facile collection of O2, while avoiding any subsequent dissolution / reaction that may occur as O2 is bubbled through molten regolith in the standard MRE operation. FIG. 2 shows a schematic of the optional materials useful in this design.

[0113] In the design provided in FIG. 2, stabilized zirconia (hereafter referred to as zirconia) is the solid electrolyte shell, meaning that the metallic anode is no longer in contact with the regolith melt. Additionally, zirconia is significantly less expensive than iridium, so the amount of material lost per dollar through corrosion is favorable with zirconia vs. iridium. Reactivity testing of magnesia-stabilized zirconia indicate that the material can dissolve into the melt but does not appear to react with the oxide species of the melt. Zirconia recession rates can optionally be improved using different stabilizers, combinations of stabilizers, and dopant concentrations. Examples of stabilizers include but are not limited to CeO2, CaO, La2O3, Y2O3, SC2O3, and Gd2O3.

[0114] The zirconia solid electrolyte necessitates the migration of oxygen anions to the metallic anode during electrolysis, avoiding bubble formation at the electrolyte / regolith interface since the electron transfer and formation of molecular oxygen occurs on the interior of the shell. This addresses bubble formation concerns, despite the additional mass transport step added in the electrolysis process. The generation of O2 in the interior of the zirconia shell also simplifies the collection process since O2 can be pumped directly from the interior of the tube, avoiding the regolith melt where it can undergo dissociation and additional reactions.

[0115] One design of the metallic anode core, or active anode material, is porous iridium. Iridium resists oxidation at the extreme temperatures of the MRE process, as evidenced by Iridium's usage as the state-of-the-art anode. A porous anode is necessary since iridium is solid at operating temperatures and the O2 gas must be allowed to escape from the zirconia / metal interface. However, since the anode is no longer in contact with the molten regolith, other less expensive metals may be used in this application instead, since the main reason iridium is used as the anode is that it corrodes slowly in the molten regolith. As an example, alternatively, porous platinum may be used as the active anode material. A porous, solid anode may limit the efficiency of electrolysis since it introduces a mass transfer resistance to the process, where oxygen anions must migrate to a zirconia / metal interface to form molecular oxygen. In another embodiment, optionally, a metallic anode fills the interior with a metal that is molten and has no stable oxide at operational temperatures. The liquid metal has a higher contact area with the zirconia shell as compared to a solid porous active anode material, reducing the mass transfer limitation while still allowing electron transfer. Additionally, the liquid can allow gaseous O2 to escape, unlike a dense solid. The choice of metal requires that no reactions between O2 and the liquid metal occur, e.g., oxide formation at operational temperatures and that the metal may be liquid at operational temperatures. Depending on the operating temperature, palladium, platinum, gold, silver, ruthenium, rhodium, and osmium, as well as alloys of these metals, are suitable for use in this optional design with a liquid metal active anode material.

[0116] Experiments are conducted to elucidate how zirconia behaves at 1600° C. in contact with molten regolith simulants. The composition of lunar regolith varies depending on location on the lunar surface. The lunar surface can be split into two major regions—lunar mare and lunar highlands. The average regolith composition measured during the Apollo era is provided in Table 1.6 Regolith simulants for the mare (NASA JSC-1A) and highlands (ExoLith LHS-1) are tested against magnesia-stabilized zirconia (provided by Zircoa). In 3-hour exposures, the zirconia samples lost ˜3% volume in the highlands simulant melt and ˜17% in the mare simulant melt. The highlands composition appears to be more suitable for maintaining a low degradation rate of the zirconia. The duration of these exposure experiments was also extended to 6-12 hrs as disclosed elsewhere herein. These times are consistent with literature-reported operation of a MRE reactor on Earth.TABLE 1Average lunar regolith compositions.6Mare [wt %]Highlands [wt %]Al2O312.925.1CaO11.014.9Cr2O30.40.1FeO17.06.3K2O0.10.1MgO9.67.6MnO0.20.1Na2O0.40.4P2O50.20.1SiO243.745.0TiO25.20.5

[0117] FIGS. 3 and 4 show cross-sectioned exposure samples in contact with highlands and mare regolith simulants, respectively. Though there is some observable damage to the zirconia, zirconia has degraded less than the alumina containment vessel in both images. Alumina is considered a refractory material, suitable for use in high temperature environments. It is significant that the zirconia degrades less than alumina in contact with the molten oxides from the lunar regolith simulants. The high temperature stability of any anode material is the largest challenge to address for long-term MRE operation.

[0118] Based on the exposure tests, a zirconia / porous iridium hollow anode is fabricated and is likely to survive short-term electrochemical testing at 1600° C. As an alternative, a zirconia / porous platinum hollow anode is fabricated. Zirconia's ability to conduct oxygen anions is well studied and zirconia is commonly used as a solid electrolyte in oxygen sensors and high temperature fuel cells. Once the hollow anodes are fabricated, the electrochemical response is measured in the molten regolith melt to determine how the addition of zirconia to the system affects reaction kinetics and overpotential required for electrolysis. Most applications utilizing zirconia's oxygen conductivity operate below 1000° C., while this application targets temperatures greater than or equal to 1500° C., and more preferably temperatures greater than or equal to 1600° C. The higher temperature results in higher oxygen conductivity, minimizing the effect of the additional mass transfer step compared to lower temperature applications. In order to further reduce mass transfer resistance, the alternative liquid metal anode design, described above, is used to further minimize efficiency losses that stem from limited contact area between a porous, solid metal anode and the zirconia shell.

[0119] One method of electrochemical characterization is cyclic voltammetry which is performed on both the hollow anode and bare Ir anode (as a control). As an alternative, electrochemical characterization is performed on both the hollow anode and bare platinum anode. These experiments elucidate the potential at which electrolysis occurs, and thereby inform the operational voltage range for oxygen production. Additionally, the peak current is used to estimate the effects that zirconia layer has on the mass transport of O2-anions to the working electrode surface. If O2-diffusion through zirconia is not rate limiting, then the peak current for the bare Ir anode and the hollow anode are the same after normalizing by the anode surface area. Conversely, if O2-diffusion is the limiting step, then the peak current for the hollow anode is lower than the bare Ir anode, with the magnitude of this difference qualitatively indicating diffusion rates.

[0120] Electrochemical impedance spectroscopy (EIS) is also used to quantify the effect of zirconia on the overall kinetics of this system. EIS is a powerful characterization technique that can separate the effects of different microscopic processes occurring at different rates. EIS is used to determine the effect of an additional solid electrolyte in series with the liquid electrolyte. The Randles circuit is a simplified equivalent circuit model that is used to represent the bare Ir anode case. The addition of zirconia in the hollow anode design manifests as an additional solid zirconia electrolyte resistance term in series with the molten regolith electrolyte resistance. Equivalent circuit models for this system are refined with the resulting data. Through EIS, the diffusion of O2− through zirconia is quantified and is used in future models to estimate the rate of oxygen production. Though a reduction in O2 production rate is expected with the addition of zirconia, the resulting hollow anode remains more cost effective per hour of operation that bare Ir and enables facile collection of O2. These benefits outweigh the slower overall production rate provided the decrease is small.REFERENCES CORRESPONDING TO EXAMPLE 1

[0121] 1 Aislinn H. Sirk et al., 2010, ECS Trans. 28 367

[0122] 2 Hojong Kim et al., 2011, J. Electrochem. Soc. 158 E101

[0123] 3 Samuel S. Schreiner et al., 2016, Adv. Space Res. 57 1585

[0124] 4 Anatoliy Shchetkovskiy et al., 2010, Earth and Space 2010

[0125] 5 Bethany A. Lomax et al., 2022, Nature Commun. 13 583

[0126] 6 Douglas B. Stoeser et al., 2010, NASA Technical Reports Server, NASA / TM-2010-216438Example 2. Zirconia-Based Hollow Anodes for Improved Molten Regolith Electrolysis

[0127] Description: An operational molten electrolysis cell was designed and fabricated to provide validation for the hollow anode embodiment. In designing the cell, an optional embodiment, wherein the hollow anode is inverted to allow for ease of testing geometry and reducing the complexity of the assembly process. This embodiment is meant for testing and validation purposes, while the embodiment shown in FIG. 5A is how the hollow anode is intended to be used in its operational environment. FIGS. 5A-5B summarize the change from the hollow anode to an inverted hollow anode for laboratory testing. There is no directionality for ionic conduction of O2− through the yttria-stabilized zirconia (YSZ) shell, allowing the inverted hollow anode to function as a small-scale testbed for the hollow anode design.

[0128] FIG. 6 provides an image of the supporting equipment needed to perform molten regolith electrolysis using an inverted hollow anode design. These include a potentiostat (to control the electric potential or current in the cell), residual gas analyzer (to measure O2 content in the outlet gas stream), and a high-temperature furnace (to heat the electrochemical system to 1600° C.). FIG. 6 also provides an image of the feedthroughs on the assembled cell. Two of these feedthroughs act as the gas inlet and outlet, while the other two feedthroughs are used to apply electrical potential to the molybdenum cathode and the platinum anode. FIG. 7 is a schematic of the interior of the assembled inverted hollow anode cell used to perform the proof-of-concept experiments.

[0129] Experimental results: Two experiments were performed to validate the inverted hollow anode cell. The first experiment used LHS-1 lunar regolith simulant (procured from Space Resource Technologies) to represent the lunar highlands regolith. The second experiment used JSC-1A lunar regolith simulant (procured from NASA Marshall Space Flight Center) to represent the lunar maria regolith. In these experiments, the inverted hollow anode cell was heated to 1600° C. and held for 3 hours, with a mass flow controller providing 100 sccm of argon gas. While at temperature, the potentiostat was programmed to be in galvanostatic mode and provide 0.5 A of current to the cell. The outlet gas was monitored by a residual gas analyzer for the duration of this electrolysis. After the electrolysis was completed, the cell was cooled to ambient temperature, mounted in epoxy, and cross-sectioned for microscopy and compositional analysis.

[0130] FIGS. 8 (LHS-1) and 9 (JSC-1A) show the O2 content of the outlet gas stream and potential provided by the potentiostat to maintain a constant current of 0.5 A through the cell. These data confirm that O2 is being produced by the inverted hollow anode cell when current is flowing through the cell. There is a clear increase in the O2 content of the outlet gas stream when current is applied and a decrease in O2 content when the current is stopped. O2 production is associated with the electrochemical decomposition of FeO / Fe2O3 in the lunar regolith simulants. This provides proof that O2− anions from the lunar regolith simulant are being transported through the YSZ shell in order to produce O2 at the platinum anode.

[0131] The O2 composition data can also be used to calculate a Faradaic efficiency for oxygen production. The Faradaic efficiency is a measure of how much of the applied current is used to electrochemically produce the desired product (O2). Based on the amount and duration of the applied current, a theoretical maximum of O2 produced can be calculated assuming every electron transferred is used to produce O2. Since the flow rate through the system is known (100 sccm of argon), the O2 composition data can be integrated to provide the total amount of O2 measured during electrolysis. The ratio of the measured O2 and the theoretical maximum O2 gives the Faradaic efficiency. Table 2 below summarizes the measured O2 and the calculated Faradaic efficiency for both LHS-1 and JSC-1A electrolysis experiments.TABLE 2Measured O2 and calculated Faradaic efficienciesfor LHS-1 and JSC-1A electrolysis.Total chargeTheoretical max.FaradaicRegolithtransferredof O2Measured O2efficiencysimulant(C)(mL)(mL)(%)LHS-11799.63104.3548.9947.0%JSC-1A1799.52104.3443.6241.8%

[0132] In experiments electrolyzing JSC-1A, Sirk et al. observed Faradaic efficiencies of 30-60% using bare iridium anodes (the current baseline design for molten regolith electrolysis anode).1 The calculated Faradaic efficiencies for both regolith simulants (Table 2) are consistent with the literature-reported values for molten regolith electrolysis utilizing an iridium anode. Thus, it does not appear that the inclusion of YSZ in the hollow anode design negatively impacts the Faradaic efficiency of FeO / Fe2O3 reduction and the resulting O2 production.

[0133] FIGS. 10 (LHS-1) and 11 (JSC-1A) are optical microscope images of the cross-sectioned and polished inverted hollow anode cell after electrolysis. In these images, there is a clear bulb of metallic electrolysis product at the tip of the Mo cathode. FIGS. 12 (LHS-1) and 13 (JSC-1A) provide a higher magnification of this region, along with scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDS) maps of the elemental composition in this region. The EDS maps reveal that Fe has been electrochemically produced and alloys with the Mo cathode. The presence of metallic Fe confirms that FeO / Fe2O3 reduction is the complementary half-reaction to O2 production at the Pt anode. This is proof that metals can be produced at the cathode when utilizing a hollow anode design for molten regolith electrolysis.

[0134] Summary: Molten regolith electrolysis experiments were performed with an inverted hollow anode design. O2 production was confirmed using a residual gas analyzer and only occurred when current was passed through the cell. Metallic Fe production at the cathode was confirmed using EDS elemental analysis. An estimation of the Faradaic efficiency is provided and is consistent with molten regolith electrolysis experiments utilizing an iridium anode, which is the current baseline anode design. Comparable Faradaic efficiencies between the YSZ / Pt hollow anode and iridium anode suggest that the inclusion of YSZ does not negatively impact the efficiency of this process.REFERENCES CORRESPONDING TO EXAMPLE 21 Aislinn H. Sirk et al., 2010, ECS Trans. 28 367Example 3. Zirconia-Based Hollow Anodes for Improved Molten Regolith Electrolysis

[0136] Extended duration electrolysis of LHS-1 (highlands simulant)

[0137] Description: The fabrication of a molten regolith electrolysis cell utilizing the inverted hollow anode design was disclosed in the Example 2. Two additional experiments were performed in the same cell design to extend the electrolysis duration, specifically with LHS-1 (lunar highlands regolith simulant).

[0138] Experimental results: Two additional experiments were performed to validate the inverted hollow anode cell for longer electrolysis durations. Both experiments used LHS-1 lunar regolith simulant (procured from Space Resource Technologies) to represent the lunar highlands regolith. The first experiment (Run #3) had an electrolysis duration of 3 hours. The second experiment (Run #4) had an electrolysis duration of 12 hours. In these experiments, the inverted hollow anode cell was heated to 1600° C. with a mass flow controller providing 100 sccm of argon gas. While at temperature, the potentiostat was programmed to be in galvanostatic mode and provide 0.5 A of current to the cell. The outlet gas was monitored by a residual gas analyzer for the duration of this electrolysis. After the electrolysis was completed, the cell was cooled to ambient temperature, mounted in epoxy, and cross-sectioned for microscopy and compositional analysis.

[0139] FIGS. 14 (LHS-1, 3 hours) and 15 (LHS-1, 12 hours) show the O2 content of the outlet gas stream and potential provided by the potentiostat to maintain a constant current of 0.5 A through the cell. These data confirm that O2 is being produced by the inverted hollow anode cell when current is flowing through the cell. There is a clear increase in the O2 content of the outlet gas stream when current is applied and a decrease in O2 content when the current is stopped. Due to the longer electrolysis duration, there are new cathodic reactions that balance oxygen production at the anode. In the previous experiments, the electrochemical reduction of FeO / Fe2O3 in the lunar regolith simulants was the primary cathodic reaction. In these extended duration experiments, in addition to the FeO / Fe2O3 reduction observed in previous runs (see Example 2, Update 1), SiO2 and P2O5 reduction was also observed in both experiments, while TiO2 and MnO reduction was observed in the Run #4 (12-hour electrolysis).

[0140] FIGS. 16 (Run #3, 3-hour electrolysis) and 17 (Run #4, 12-hour electrolysis) are optical microscope images of the cross-sectioned and polished inverted hollow anode cell after electrolysis. In these images, metallic regions near the cathode are the cathodic products formed during electrolysis.

[0141] In Run #4, there is a decrease in the O2 content of the outlet stream observed around 5 hours as based on FIG. 15. This behavior is attributed to two potential effects—the onset of electronic conductivity in YSZ and cathodic products shorting the cell. It has been shown that YSZ will have electronic conductivity with the application of a DC bias. This electronic conductivity can effectively short the cell in that some of the current applied to the cell will flow as electrons through the cell without driving the desired Faradaic reaction (O2 production). This effect is exacerbated by the presence of a liquid cathodic product. In normal operation, the molten regolith is not expected to have significant electronic conductivity, typically only conducting electrons through polaron hopping on the Fe2+ / Fe3+ cations. In this case, current can only flow if the Faradaic reactions are occurring at the cathode and anode, though it is possible for O2 to be generated on the interior of the YSZ instead of on the exterior.

[0142] However, the molten cathodic products are expected to be electronically conductive since they are mostly metallic. Based on FIG. 17, it is clear that the cathodic products are not forming a liquid layer that coats the cathode. Instead, the cathodic products appear to be diffusing away from the cathode. If any of the liquid metal contacts the YSZ, there will be a direct electronic short through the cell, leading to reduced Faradaic efficiencies and reduced oxygen production. There is some evidence of this at the cathode. The cathode is heavily degraded at the region nearest to the Pt anode. It is believed that the accelerated degradation is a result of elevated temperatures in that region due to Joule heating from electronic conduction. This effect is due to the confined geometry of the inverted hollow anode and is not likely be a problem in larger MRE reactors.

[0143] FIGS. 18 (Run #3, 3-hour electrolysis) and 19 (Run #4, 12-hour electrolysis) provide a higher magnification of the cathodic products, along with scanning electron microscope (SEM) images. FIG. 18 also includes energy-dispersive X-ray spectroscopy (EDS) maps of the elemental composition in this region. FIG. 19 highlights two regions of interest, with Site 1 at the cathodic products near the Mo cathode and Site 2 at a droplet of cathodic products found in the solidified LHS-1. FIGS. 20 (Site 1) and 21 (Site 2) provide EDS maps of more elements to show the presence of Ti and Mn in the metallic cathodic products.

[0144] For Run #3, the EDS maps (FIG. 18) reveal that Fe and Si have been electrochemically produced and alloy with the Mo cathode. P is also detected in smaller quantities (not shown here). The presence of these species reveals that FeO / Fe2O3 and SiO2 reduction are the complementary half-reaction to O2 production at the Pt anode. This is significant since SiO2 is the most abundant oxide species in lunar regolith. Demonstrating O2 production from SiO2 confirms that high oxygen extraction efficiencies (>20%) are possible through MRE with a hollow anode.

[0145] For Run #4, the EDS maps (FIGS. 20 and 21) reveal that Fe, Si, Ti, and Mn have been electrochemically produced and alloy with the Mo cathode. Similar to Run #3, P (not shown) is also detected in smaller quantities. The presence of Ti and Mn suggest that TiO2 and MnO reduction are occurring simultaneously with SiO2 reduction. These results confirm that the hollow anode can be used for longer durations in an MRE reactor and is able to produce oxygen at the anode, regardless of the multiple cathodic reactions occurring.

[0146] The O2 composition data can also be used to calculate a Faradaic efficiency for oxygen production. The Faradaic efficiency is a measure of how much of the applied current is used to electrochemically produce the desired product (O2) and is determined by the ratio of the measured O2 and the theoretical maximum O2. Based on the amount and duration of the applied current, a theoretical maximum of O2 produced can be calculated assuming every electron transferred is used to produce O2. Since the flow rate through the system is known (100 sccm of argon), the O2 composition data can be integrated to provide the total amount of O2 measured during electrolysis. Table 3 below summarizes these calculations as well as the parameters for all MRE tests performed to date. A column was also added for oxygen extraction efficiency. This is calculated from the mass of O2 produced, divided by the mass of the regolith simulant used. Many MRE experiments report results on this basis, so it has been included to more easily compare results between different MRE systems.TABLE 3Summary of test results for LHS-1 and JSC-1A electrolysis.AnodicVol. O2RunElectrolysisCathodicFaradaicproducedExtraction#duration [h]Simulantproductsefficiency[mL]efficiency11LHS-1Fe47%49mL0.4%21JSC-1AFe, P42%44mL0.3%33LHS-1Fe, P, Si53%166mL1.2%412LHS-1Fe, P, Si,42%524mL3.7%Ti, Mn

[0147] In experiments electrolyzing JSC-1A, Sirk et al. observed Faradaic efficiencies of 30-60% using bare iridium anodes (the current baseline design for molten regolith electrolysis anode).1 The calculated Faradaic efficiencies for both regolith simulants (Table 3) are consistent with the literature-reported values for molten regolith electrolysis utilizing an iridium anode. Thus, it does not appear that the inclusion of YSZ in the hollow anode design negatively impacts the Faradaic efficiency of FeO / Fe2O3 reduction and the resulting O2 production. Sirk et al. did not investigate SiO2, P2O5, TiO2, and MnO reduction, so there are no comparisons for these species undergoing reduction. However, the overall Faradaic efficiency remains ˜40%, suggesting that these species do not significantly impact the electrolysis process.

[0148] Summary: Two additional molten regolith electrolysis experiments targeting longer electrolysis durations were performed with an inverted hollow anode design. In Run #3, the electrolysis period was 3 hours, while in Run #4, the electrolysis period was 12 hours. O2 production was confirmed using a residual gas analyzer and only occurred when current was passed through the cell. For Run #3, Fe, Si, and P production at the cathode was observed using EDS elemental analysis. For Run #4, Fe, Si, Ti, Mn, and P production was observed using EDS elemental analysis. A summary of cathodic products and Anodic Faradaic and Extraction efficiencies for all MRE experiments with a hollow anode is presented. Faradaic efficiency across all runs is consistent with MRE experiments utilizing an iridium anode, which is the current baseline anode design. Comparable Faradaic efficiencies between the YSZ / Pt hollow anode and iridium anode suggest that the inclusion of YSZ does not negatively impact the efficiency of this process.REFERENCES CORRESPONDING TO EXAMPLE 31 Aislinn H. Sirk et al., 2010, ECS Trans. 28 367Example 4. Improving Molten Regolith Electrolysis with Zirconia-Based Hollow Anode Technology

[0150] Abstract: Molten regolith electrolysis is a promising in-situ resource utilization technology that targets O2 production through the direct electrolysis of molten lunar regolith. However, there are still challenges associated with molten regolith electrolysis, such as bubble detachment and O2 separation and collection at the anode. A hollow anode, comprised of an oxygen-conducting yttria-stabilized zirconia shell and a platinum current collector, is disclosed herein to address these challenges. Experimental results from an inverted hollow anode reactor successfully demonstrates that molten regolith electrolysis can be performed through a solid electrolyte. This is surprising, as YSZ and other ceramic solid electrolytes may undergo parasitic reactions with species present in molten regolith. One anticipated reaction is the generation of ZrSiO4 at the YSZ surface which would disrupt oxygen anion conduction and hinder or stop the migration of oxygen anions to the active anode material. Therefore, the stability and performance observed for the hollow anode disclosed herein is unexpected and supports a range of applications including MRE. The elemental composition of both the cathodic products (primarily Fe and Si) and solidified lunar regolith simulant are reported with increasing electrolysis duration. These observations are supported by a thermochemical model built using FactSage to provide compositions of the cathodic products and solidified regolith simulant with increasing O2 removal. Finally, the behavior of yttria-stabilized zirconia in the hollow anode application is characterized, and provides guidance for the design and operation of a yttria-stabilized zirconia hollow anode to enable integration into full-scale molten regolith electrolysis reactors.1. Introduction

[0151] NASA's Artemis program seeks to return humans to the lunar surface for the first time since the Apollo program, with a long-term goal of establishing a sustainable human presence on the lunar surface [1]. In support of these goals, there has been a renewed focus on developing lunar in-situ resource utilization (ISRU) technologies that can extract and process lunar resources into valuable commodities. O2 production on the lunar surface is considered a critical ISRU technology to enable a long-term lunar presence by reducing the cost of resupply launches to the lunar surface, while also curtailing the reliance on terrestrially-produced O2 [2]. The O2 is used as the oxidizer for launch vehicles such as NASA's Space Launch System and SpaceX's Starship and is approximately 70% of the initial mass of these launch vehicles [3,4]. Any lunar-produced O2 that can be refueled either on the lunar surface or in lunar orbit can provide significant cost savings in the long run [5].

[0152] There are several O2 production processes that have been investigated, either targeting a feedstock of lunar regolith or lunar ice [6,7]. The present study focuses on a specific ISRU technology known as molten regolith electrolysis (MRE) that seeks to directly electrolyze molten lunar regolith, producing Fe—Si alloys at the cathode and O2 at the anode [6,7]. MRE is a promising technology because it can achieve a high oxygen extraction efficiency and produces two useful commodities in O2 and metals [8]. Guerrero-Gonzalez et al. performed a system analysis of different ISRU technologies targeting O2 and metals production and found that MRE was the most effective ISRU process in terms of mass payback ratio [9].

[0153] However, there are still challenges associated with MRE, arising from its harsh operational environment. To fully melt lunar regolith, temperatures of approximately 1600° C. are necessary [6]. At this temperature, contact with the corrosive molten regolith can cause significant degradation to refractory materials

[10] . This is further complicated at the anode, where O2 is produced, causing an oxidizing environment to exist at the anode surface, in addition to contact with molten regolith at 1600° C. Reviews indicate that current MRE reactors utilize an Ir or Ir-coated anode to survive these conditions [11,12]. Wang et al. investigated Ir corrosion rates when used as the anode in MRE and measured a corrosion rate of ˜8 mm / year, which is lost through dissolution to the molten regolith electrolyte

[13] . Given the high cost and scarcity of Ir metal, it is unclear if Ir anodes are a cost-effective solution for long-term MRE reactors.

[0154] In addition to the materials challenges at the anode, there are other inefficiencies when performing MRE on the lunar surface. The O2 produced at the anode surface is formed at a liquid / solid interface and produces bubbles. The molten regolith has a high viscosity and surface tension while operation on the lunar surface reduces acceleration due to gravity

[14] . The combination of these factors indicates that it is more difficult for bubbles to separate from the anode surface, decreasing the effective surface area of the anode, and potentially causing electrolysis to stall

[15] . Burke et al. investigated bubble separation in MRE reactors and found that bubbles generally become larger and bubble detachment time increases with the reduced lunar gravity

[16] . Beyond bubble separation, there is also a reduction in O2 capture efficiency associated with bubbling O2 through molten regolith into a shared reactor headspace. This allows O2 to reoxidize partially reduced Fe2+ species in the molten regolith electrolyte or gaseous reduced metal species, such as Na, K, or Mg in the reactor headspace

[14] .

[0155] Disclosed herein is a new type of anode, referred to as the hollow anode, that can mitigate some of the aforementioned risks associated with MRE. The hollow anode is designed to allow for the facile collection of O2 after it has been produced through MRE. FIG. 1(a) provides a schematic of the envisioned integration of a hollow anode in a MRE reactor, based on the work of Grossman et al.

[17] . The hollow anode contains a solid electrolyte shell and a metallic current collector. Within this design, the goal is to force O2− from the molten regolith to migrate through an oxygen-conducting solid electrolyte to the metallic current collector. Once O2− reaches the current collector, electron transfer can occur, allowing for oxidation of O2− into O2. The generated O2 is fully contained within the solid electrolyte shell, enabling the collection of O2 without contact between the molten regolith and O2. In addition, O2 is now generated at a solid / solid interface, eliminating the bubble formation challenges discussed earlier.2. Hollow Anode Design

[0156] In an embodiment, the materials used for the hollow anode are yttria-stabilized zirconia (YSZ) for the solid electrolyte shell and Pt for the metallic current collector. YSZ is a well-known oxygen-ion conductor, as well as a refractory oxide ceramic. Previous work by the authors has shown that YSZ demonstrates controlled degradation in contact with molten regolith simulants at 1600° C.

[18] (see, e.g., K. Yu, J. Stokes, B. Harder, L. Reidy, K. T. Faber, “Thermochemical interactions between yttria-stabilized zirconia and molten lunar regolith simulants”, Journal of the American Ceramic Society 107 (2024) 7119-7130. https: / / doi.org / 10.1111 / jace.19821, which is incorporated by reference herein for all purposes and particularly with reference to the behavior of YSZ in various molten regolith compositions). Pt is used as the current collector since it is not sensitive to oxidizing environments at high temperatures

[19] . Unlike the Ir anodes discussed earlier, the Pt used in the hollow anode does not directly contact molten regolith, avoiding the dissolution of Pt into the molten regolith electrolyte. The combined YSZ / Pt anode shown in FIG. 60A is referred to as the hollow anode.

[0157] Anodes with a similar design to the hollow anode have been used previously in lower temperature applications. Guo et al. studied the use of YSZ / Ag anodes in oxy-fluoride fluxes at 1200° C. and found that the optical basicity of the oxy-fluoride flux has a significant effect on YSZ degradation

[20] . Gao et al. investigated the use of magnesia-stabilized zirconia (MSZ) / Pt anodes in FeO-containing slags at 1450° C. and found that the inclusion of MSZ as a solid electrolyte does not negatively impact the performance of their electrolysis cell [21,22]. Both experiments were performed at lower temperatures than the MRE operational temperature of 1600° C. Moreover, the flux (Ca2+, Mg2+, Si4+, Y3+) or slag (Ca2+, Mg2+, Si4+, Al3+, Fe2+ / Fe3+) used contained relatively few cationic species compared to the 10-component molten regolith simulant used in these experiments.

[0158] This work focuses on the design, fabrication and testing of a laboratory-scale MRE cell, utilizing an inverted hollow anode, which successfully demonstrates that MRE can be performed with a hollow anode for, for example, up to 12 hours. Residual gas analysis confirms the production of O2 when potential is applied to the cell. The elemental composition of both the cathodic products and solidified regolith simulant are presented, indicating that FeO and SiO2 are the primary oxide species being electrolyzed. A FactSage thermochemical model is also built to predict the cathodic product and remaining regolith simulant compositions, which is compared to the experimentally measured compositions from the MRE cells. The results of laboratory-scale testing reveal interactions between YSZ and molten regolith simulant that will be discussed and extended to provide design considerations for a hollow anode in full-scale MRE reactors.2. Materials and Methodology2.1 Molten Regolith Electrolysis Experiments

[0159] All MRE runs were performed in the inverted hollow anode configuration, shown in FIG. 60B. In this embodiment, the YSZ tube functions as both the primary containment vessel and as the solid electrolyte for O2−. An outer alumina tube was used to isolate the gaseous environment around the cell and to act as a secondary containment vessel. A mass flow controller (FMA series, OMEGA) was used to provide 20) a consistent Ar flow to serve as the carrier gas. The gas outlet was connected to a residual gas analyzer (HPR-20, Hiden Analytical) for compositional analysis to track the production of O2. The sealed electrolysis reactor was partially placed into a high-temperature box furnace (Rapid Temp Furnace, CM Furnaces) for heating to the operational temperature. The electrolysis cell was designed such that the lower ¼ portion of the cell, shown in FIG. 60B, containing the lunar regolith simulants is exposed to operational temperature, while the upper portion with electrical leads, gas fittings, and tube gaskets (not shown) remained near ambient temperature. A potentiostat (VersaStat 4, Princeton Applied Research) was attached to the cathode and anode leads to perform electrolysis.2.1.1 Electrolysis Cell Fabrication

[0160] A custom stainless steel endplate was designed and machined to provide a mounting point for the interior 10.5 wt % YSZ tube and a sealing surface for the exterior alumina tube (McDanel Advanced Material Technologies). The endplate also contains feedthroughs for the cathode, anode, gas inlet tube, and gas outlet tube. To fabricate the electrolysis cell, a YSZ tube was loaded with approximately 20 grams of lunar highlands regolith simulant and mounted onto the endplate. The cathode is an alumina-sheathed Mo rod that is inserted into the center of the endplate and pushed down the YSZ tube through the loose regolith simulant. The anode consists of a Pt wire mesh that is adhered to the exterior of the YSZ tube with Pt ink (Fuel Cell Store). The gas inlet and outlet tubes were then attached to the endplate. Finally, the entire assembly was lowered into the exterior alumina tube and sealed off by a Viton gasket at the endplate.2.1.2 Electrolysis Conditions

[0161] MRE was performed for three different durations-1 hour, 3 hours, and 12 hours. The operational temperature of the electrolysis cell was 1600° C. for all three durations. An additional hour was added before and after electrolysis to allow the cell to stabilize before starting electrolysis and to perform electrochemical characterization such as cell resistance determination. All experiments were run under 100 sccm of flowing Ar. The experiments were performed with a constant current set at 0.5 A on the potentiostat. The cathode surface area is ˜10 cm2 and the anode pad surface area is ˜1 cm2, giving an anodic current density of ˜0.5 A / cm2.2.2 Lunar Regolith Simulant

[0162] The lunar south pole is the targeted landing site for NASA's Artemis missions and could potentially be suitable for a permanent lunar settlement

[23] . As a result, MRE experiments were performed on lunar highlands regolith simulant, which is more representative of the regolith at the lunar south pole

[24] . LHS-1 (Space Resource Technologies) was selected as the highlands regolith simulant. The composition of LHS-1

[25] , and a comparison to lunar highlands regolith returned from Apollo 16

[26] , are presented in Table 4. Though there are some compositional variations in specific oxides (CaO, MgO, Na2O), the terrestrially-produced LHS-1 is comparable in the majority oxides, SiO2 and Al2O3, to the lunar regolith returned by Apollo 16. It is noted that though Table 4 presents the composition using a binary oxide basis, both the lunar regolith and regolith simulant are comprised of mixtures of oxide glasses and minerals. However, MRE seeks to fully melt the regolith to form an ionic liquid electrolyte and does not require the presence of specific minerals, such as ilmenite (FeTiO3) for hydrogen reduction

[27] .TABLE 4Major abundances of LHS-1 simulant and highlands lunar regolith (wt %)SpeciesSiO2FeOMgOCaOAl2O3TiO2Na2OK2OMnOP2O5LHS-151.22.71.612.826.60.62.90.50.10.1(Simulant)Apollo 1645.34.24.917.227.70.40.40.1<0.1—(#64501)2.3 Characterization

[0163] The geometry of the MRE cells was designed to allow for characterization of all heated cell components, including the YSZ solid electrolyte, cathodic products, and solidified regolith simulant, after each experiment. After cooling, each cell was mounted in epoxy and cross-sectioned down the middle of the cell. One half of each cell was polished to 0.05 μm to prepare for subsequent characterization.2.3.1 Optical Microscopy

[0164] All cells were optically imaged at 30× (VHX-7000, Keyence) and stitched together to capture macroscopic changes resulting from high-temperature operation and electrolysis. Further investigation of regions of interest were performed at 50×-200× (VHX-2000, Keyence). These regions of interest provided guidance for higher-magnification electron microscopy. ImageJ was used to measure YSZ wall recession from these images

[28] .2.3.2 Electron Microscopy

[0165] Scanning electron microscopy (SEM; 1550VP FESEM, ZEISS) was used with the backscattered electron (BSE) detector to image the cathodic products and solidified regolith simulant within each cell. BSE image brightness is determined by the average atomic number of the elements present, with lighter elements appearing darker. Since O has a low atomic number, BSE images can provide an initial analysis of which regions are metallic (the cathode and cathodic products) vs. oxide (the remaining LHS-1). Energy dispersive spectroscopy (EDS; X-Max EDS, Oxford Instruments) was used to assess the elemental composition of the cathodic products and solidified regolith simulant using EDS mapping and point analysis.

[0166] Beyond SEM and EDS, electron backscatter diffraction (EBSD; HKL EBSD, Oxford Instruments) was used to determine the grain structure and phase of YSZ. Specifically, regions of YSZ in contact with molten regolith were analyzed to elucidate the compositional and microstructural changes of YSZ for each electrolysis cell. Each EBSD analysis region was 300 μm wide (along a YSZ / LHS-1 interface) and 700 μm deep (from YSZ / LHS-1 interface into YSZ interior) with a 1.5 μm step size. MTEX, an EBSD analysis software package, was used for grain segmentation and grain size analysis on the collected EBSD maps to provide statistical data on grain sizes

[29] .2.3.3 Raman Spectroscopy

[0167] A confocal Raman microscope (inVia Raman, Renishaw) was used on the center of the YSZ walls to qualitatively determine the effect of extended electrolysis on YSZ. All electrolysis cells and a polished fragment of the beginning-of-life (BOL) YSZ were characterized by Raman spectroscopy. For this analysis, a 20× lens was used with 10% laser power, a 30-second acquisition time, with 3 total accumulations. Background subtraction was performed using the WIRE software package (Renishaw). The resulting data were normalized for intensity by MATLAB.2.3.4 FactSage Analysis

[0168] FactSage 8.3 was used to construct a thermochemical model for the removal of O2 from molten lunar regolith at 1600° C.

[30] . The LHS-1 composition from Table 4 was input as the initial composition. The FToxid and FTlite databases were used to determine the equilibrium slag (regolith simulant) and metal (cathodic product) compositions, respectively. The basis for O2 removal is extraction efficiency and is defined as the mass of O2 produced divided by the mass of regolith. The model was used to generate the equilibrium slag and metal compositions from an extraction efficiency of 0% (representing BOL) to 5%, in 0.1% increments. It is noted that these results do not account for the kinetics of electrolysis and are only able to determine the thermodynamic equilibrium composition assuming that O2 is removed from the system. As a result, Mo (cathode) and YSZ / Pt (anode) are not included in this analysis since their dissolution into the molten LHS-1 is a kinetically-limited process.3. Results3.1 Electrolysis Data

[0169] The volumetric percentage of O2 in the outlet gas stream and the potential required to maintain 0.5 A of current through the cell are presented in FIGS. 61A-61B. Elapsed time in these plots is counted from the application of constant current to the cell. In all cells, the O2 content increased from ˜0.005 vol % to ˜1 vol % within a few minutes of starting electrolysis. The current is applied for 1 hour, 3 hours, and 12 hours. The time at which the potential curve terminates denotes the end of electrolysis. Within a few minutes of ending electrolysis, the O2 content decreases to ˜0.005 vol %. These results indicate that O2 is being produced when current is applied to the cell, providing a proof-of-principle validation that MRE can be performed through the YSZ solid electrolyte and demonstrating that the hollow anode design is feasible.

[0170] The initial potential behavior for the three electrolysis cells is comparable, suggesting that the electrolysis cell fabrication procedure is robust. All three cells exhibited an increase in potential over the first ˜80 minutes of operation. In this initial period, the expected cathodic reaction is the reduction of Fe2+ / Fe3+, coming from FeO / Fe2O3 in the regolith. The increase in potential is attributed to two factors—the mass transport limitation of FeO / Fe2O3 and the increase in regolith viscosity. When electrolysis first occurs, the molten regolith is well-mixed, with FeO / Fe2O3 available at the cathode surface for reduction. As electrolysis continues, the FeO / Fe2O3 concentration near the cathode surface is depleted as it is reduced to Fe0, requiring a larger potential to cause FeO / Fe2O3 to migrate to the cathode. In addition to this effect, FeO / Fe2O3 is also a glass network modifier which increases regolith viscosity with decreasing concentration

[31] . Thus, as FeO / Fe2O3 is depleted from the molten regolith, the viscosity of the remaining molten regolith increases, further causing the potential to increase to maintain a consistent rate of FeO / Fe2O3 diffusing to the cathode surface.

[0171] In the 3-hour and 12-hour electrolysis cells, there is an inflection point in the potential curve at approximately 90 minutes. Based on FactSage predictions (Section 3.4), this inflection point is an indication that nearly all the FeO / Fe2O3 in LHS-1 has been electrolyzed. After this point, Si4+ reduction, from SiO2, becomes the dominant cathodic reaction, which is consistent with previously reported models

[14] . Due to the significantly higher concentration of SiO2 compared to FeO in LHS-1, there is less of a mass transport limitation, causing the potential to increase at a slower rate after the inflection point. In contrast to FeO / Fe2O3, SiO2 is a glass network former. The continued removal of SiO2 through electrolysis causes the regolith viscosity to decrease, thereby increasing the diffusivity of cations in the remaining regolith, allowing more SiO2 to diffuse to the cathode surface. These two effects result in a slower increase in the potential applied after the inflection point in both the 3-hour and 12-hour electrolysis cells.

[0172] The 12-hour electrolysis cell potential alone indicates that its slower rate of increase continues until ˜420 minutes (7 hours) of electrolysis. At this point, the potential starts to decrease and contains more noise. The decrease in potential is associated with a lower cell resistance, which is attributed to the onset of electronic conductivity in YSZ and will be discussed in more detail in Section 3.5.2. The noise in the data is attributed to the expanding cathodic products. As more cathodic product is generated, its volume increases, until an electronic short between the cathode and the YSZ wall is formed. Since the cathodic products are liquid at operational temperature, the noise in the potential originates from shorts forming and disappearing as the liquid cathodic product is repositioned by natural convection within the cell.

[0173] In all three electrolysis experiments, the highest O2 content measured occurs at the start of electrolysis and decreases over time. This behavior is also attributed to the onset of electronic conductivity in YSZ and is discussed in more detail in Section 3.5.2 [32-34]. As YSZ becomes more electronically conductive, O2 is not solely generated on the exterior of the YSZ but also can be generated on the interior of the inverted hollow anode. The gas outlet tube samples from the exterior of the YSZ. Any O2 that is produced on the interior of the cell will likely oxidize the exposed portion of the alumina-sheathed Mo cathode. Evidence of this behavior has been observed in the interior of the YSZ (FIG. 68). The oxidation of Mo produces volatile MoOx, which then condenses in cold regions of the cell. The O2 measured by the residual gas analyzer is the lower bound of O2 produced since it cannot detect the Mo oxidation products that have condensed on cold spots in the electrolysis cell.

[0174] The volume of O2 produced can be calculated by integrating the O2 content data with a known gas flow rate from the mass flow controller. The electrolysis duration can be used in conjunction with the known current applied to the cell to calculate the total charge transferred during electrolysis. The anodic half-reaction is assumed to be:2O2−→O2(g)+4e−

[0175] The total charge transferred can be used to determine the maximum O2 that could be produced by assuming that every e-participates in the Faradaic reaction. The anodic Faradaic efficiency then can be calculated by dividing the measured O2 produced by the theoretical maximum O2 produced. Extraction efficiency is defined as the mass of O2 produced divided by the mass of regolith processed, with the maximum possible extraction efficiency for LHS-1 being approximately 46%. A summary of the aforementioned calculations and experimental measurements for each electrolysis experiment is presented in Table 5.TABLE 5Summary of molten regolith electrolysis experimentsCathodicAnodicVolumeElectrolysisproductsFaradaicof O2Extractiondurationobservedefficiencyproducedefficiency1 hrFe47% 49 mL0.4%3 hrFe, P, Si53%166 mL1.2%12 hr Fe, P, Si, Ti, Mn42%524 mL3.7%

[0176] The calculated anodic Faradaic efficiencies are in line with those reported in studies utilizing a bare Ir anode instead of a hollow anode. Sirk et al. performed MRE with a Mo cathode and Ir anode, reporting Faradaic efficiencies of approximately 30-60% when electrolyzing an FeO-containing molten regolith

[35] . This suggests that the inclusion of YSZ does not significantly reduce the expected Faradaic efficiency of MRE.3.2 Cathodic Products

[0177] Optical microscope images of each electrolysis cell and EDS elemental maps of the cathodic products generated in each electrolysis cell are presented in FIGS. 62 (1-hour electrolysis), 63 (3-hour electrolysis), and 64 (12-hour electrolysis). A more quantitative EDS point analysis of each region is provided in FIGS. 69, 70, and 71. The observed cathodic products are also summarized in Table 5. The EDS maps presented consist of the observed cathodic products, except for O and Al. Al2O3 is not able to be electrolyzed at these durations, so the O and Al maps are provided as an indication of the position of the remaining regolith in each EDS region.

[0178] The EDS analysis for the 1-hour electrolysis cell indicates that Fe is the primary cathodic product. FIG. 62 shows that there is a significant amount of Fe detected in the cathodic product, and it has alloyed with Mo from the cathode. The alloying suggests that the Fe detected is Fe0, since Fe2+ / Fe3+ are unable to diffuse into Mo. The Fe and Mo maps also indicate that there are two different Fe—Mo alloys within the cathodic product. However, the bulb shape of the cathodic products indicates that it is liquid at operational temperature, and likely phase separates on cooling, which is consistent with the Fe—Mo phase diagram

[36] . No Si is detected within the cathodic product, which demonstrates that SiO2 electrolysis has not been achieved in the 1-hour electrolysis cell.

[0179] In contrast to the 1-hour electrolysis cell, EDS maps of the 3-hour electrolysis cell (FIG. 63) show the presence of both Fe and Si in the cathodic products. In addition, there is a detectable amount of P observed in the cathodic products. The Fe, Si, and P are also alloyed with Mo from the cathode. However, unlike the 1-hour cell (FIG. 62), there does not appear to be significant phase separation in the cathodic products, indicating that the overall composition of the cathodic product is stable down to ambient temperature. The observation of Si in the cathodic products is significant since it indicates that SiO2 electrolysis is being performed. To the authors' awareness, this is the first reported experimental confirmation of Si production from the direct electrolysis of a molten lunar regolith simulant. Given that SiO2 is the most abundant oxide species in LHS-1, this result provides a path for MRE with a hollow anode to achieve O2 extraction efficiencies up to 25%.

[0180] The cathodic products within the 12-hour electrolysis cell no longer form a dense film along the cathode surface and have begun migrating away from the cathode. Due to the cell design, there is likely a thermal gradient within the molten regolith. The top of the molten regolith is expected to be cooler since it can radiate to the cold side of the electrolysis cell. The separation of the cathodic products from the cathode is attributed to the growing size of the liquid cathodic product layer. This results in a layer thickness that exceeds the flow boundary layer induced by convection from a thermal gradient, causing the cathodic products to be mixed with the remaining molten regolith.

[0181] Within the cathodic products, Ti and Mn are observed in addition to the previously observed Fe, Si, and P from the shorter duration electrolysis experiments. There is also now significant Si produced, such that Si in the cathodic products has a higher concentration (brighter in Si EDS map) than Si remaining in the molten regolith. A lower concentration of P is detected in the 12-hour electrolysis cell compared to the 3-hour electrolysis cell. Based on FactSage calculations (see Section 3.4), it is expected that P2O5 electrolysis occurs simultaneously with FeO reduction. Thus, by the end of the 3-hour electrolysis experiment, most of the P2O5 is likely already electrolyzed. The lower concentration of P in the 12-hour cell is attributed to dilution of the cathodic products with the continued production of Si.

[0182] The production of fully reduced cationic components from the molten regolith is expected for MRE. The presence of these reduced species in all three electrolysis cells provides further validation beyond O2 production that MRE can be performed through a YSZ solid electrolyte. Though the production of these cathodic products is not the focus of the present study, it is important to confirm that the presence of YSZ at the anode does not significantly alter the expected cathodic half-reactions that balance the anodic half-reaction of O2 production.3.3 Solidified Regolith Analysis

[0183] Extended electrolysis of molten regolith alters the regolith composition since specific oxides in the regolith are preferentially reduced. The changing regolith (electrolyte) composition can cause both the thermophysical properties (i.e. viscosity, density, etc.) and the electrical conductivity to vary from the BOL LHS-1 properties. The average solidified LHS-1 composition for each of the three electrolysis cells were measured using EDS. These were compared to previous studies by the authors of the BOL composition of the molten LHS-1 when exposed to 1600° C. in a YSZ crucible

[18] . A comparison between the BOL composition and the post-electrolysis composition for each electrolysis experiment is presented in Table 6.TABLE 6Average regolith composition after melting and electrolysis (at %)SpeciesSiFeMgCaAlTiNaKYZrOBOL18.171.011.325.3611.680.231.870.310.240.6059.201 hr18.210.471.305.2311.700.221.900.320.170.4760.013 hr18.060.041.455.8712.410.221.870.310.240.6358.9112 hr 15.230.001.787.2112.950.172.320.340.370.6658.97

[0184] The composition changes detected in the LHS-1 are consistent with the observed cathodic products. A clear decrease in Fe concentration is detected from BOL to 12 hours of electrolysis, which is expected since Fe is electrolyzed across all three electrolysis experiments. A minor reduction in the Si concentration is detected in the 3-hour electrolysis cell and becomes a significant reduction in the 12-hour electrolysis cell, consistent with the continued electrolysis of SiO2. The Ti concentration is consistent with BOL across the 1-hour and 3-hour electrolysis results but begins to decrease in the 12-hour electrolysis result, also consistent with the observation of Ti in the cathodic products of the 12-hour electrolysis result. Across all electrolysis experiments and the BOL specimen, Y and Zr are detected due to the dissolution of YSZ by molten regolith. This is discussed in further detail in Section 3.5.1.3.4 FactSage Thermochemical Results

[0185] The FactSage thermochemical predictions of O2 removal from LHS-1 are presented in FIGS. 65A-65B. The full dataset of FactSage predictions is provided in FIGS. 72 and 73. At 0% extraction efficiency (no O2 removed from regolith), the only thermodynamically stable phase is the molten regolith, as expected at BOL. FIG. 65A presents only the species that are major components of the regolith (Ca, Al) or are involved in electrolysis (Fe, Si, Ti). O is omitted from FIG. 65A for the sake of clarity but is approximately 60 at % throughout the electrolysis. As the extraction efficiency is increased, cathodic products begin to form in low absolute mass quantities. In FIG. 65B, the normalized composition of cathodic products is presented. There is significant variation in the elemental composition of the cathodic products at low O2 extraction efficiencies that is attributed to the relatively small mass of cathodic product that initially forms compared with the regolith.

[0186] These FactSage predictions generally agree with experimental observations, even though YSZ was not included in the FactSage model. Based on the results from FIG. 65A, it appears that the switchover between predominantly FeO electrolysis to predominantly SiO2 electrolysis occurs near an O2 extraction efficiency of 1%. This is consistent with the observation that Si is not present in the cathodic products of the 1-hour electrolysis cell but is present in the cathodic products of the 3-hour electrolysis cell where the electrolysis cell achieved an O2 extraction efficiency of 1.2%. Similarly, TiO2 electrolysis is not predicted until an O2 extraction efficiency of approximately 2%, which agrees with the experimental observation that Ti is not detected in cathodic products until the 12-hour electrolysis cell, which achieved an O2 extraction efficiency of 3.7%. These results also provide further evidence that the inclusion of YSZ as a solid electrolyte does not significantly impact which oxides are electrolyzed during MRE.3.5 YSZ Interactions3.5.1 Thermochemically-Induced Interactions

[0187] Previous work by the authors investigated the thermochemical interactions of YSZ in contact with molten regolith simulants and determined that there are two mechanisms at work—specifically, the dissolution of YSZ and the depletion of Y from YSZ

[18] . It is expected that both mechanisms are occurring in the electrolysis cells and will eventually lead to YSZ failure in long-term electrolysis experiments. Thus, it is paramount to investigate the rates of dissolution to provide guidance for design lifetimes of YSZ hollow anodes in a MRE environment.

[0188] Evidence of YSZ dissolution is observed in the three electrolysis cells. Table 6 shows that Y and Zr are both detected in the solidified LHS-1, indicating that YSZ is dissolving into the molten LHS-1 at operational temperatures. YSZ dissolution by molten silicates is well-studied and has been observed in other systems

[37] . Previous work by the authors has shown that the equilibrium solubility of Y and Zr in LHS-1 at 1600° C. is approximately 1.4 at % and 1.2 at %, respectively

[18] . The measured concentrations of Y and Zr in these electrolysis cells is lower than the solubility limit, which is expected. These experiments were cooled slowly compared to the quenching experiments used to determine the equilibrium solubility of Y and Zr. As a result, the YSZ dissolved into molten LHS-1 is able to nucleate and grow within the molten regolith as the temperature is decreased. Such YSZ particles are observed at the bottom of the 1-hour and 3-hour cells in FIGS. 62 and 63. The tan-colored opacity observed in the bottom half of the 12-hour electrolysis cell (FIG. 64) is also due to YSZ particles that have nucleated from the molten LHS-1 on cooling.

[0189] Furthermore, YSZ wall recession is observed in these electrolysis cells due to dissolution. The recession only occurs where YSZ is in contact with molten LHS-1. Using the portion of YSZ that remains above the molten regolith liquid line as a baseline for the original wall thickness, wall recession is calculated for the YSZ in contact with molten LHS-1. The average wall recession is determined for each electrolysis experiment and summarized in Table 7. The electrolysis durations and the time spent at 1600° C. are both provided in Table 7, since it is thermal exposure to molten LHS-1, and not electrolysis itself, that causes YSZ dissolution.TABLE 7YSZ wall recession due to dissolution by molten LHS-1ElectrolysisTime atYSZ recessionduration1600° C.(μm)1 hr3 hr22 ± 103 hr5 hr54 ± 1212 hr 14 hr 85 ± 19

[0190] Based on Table 7, it appears that the rate of YSZ wall recession is slowing with increasing time in contact with molten LHS-1 at 1600° C. This result suggests that there are additional effects slowing the rate of YSZ dissolution. Kowalski et al. evaluated varying YSZ grain sizes when exposed to molten silicates similar in composition to the molten LHS-1 in this study

[38] . They showed that the grain size of YSZ can influence the rate of penetration and dissolution, with larger grains being preferred for minimizing silicate penetration

[38] . To better understand YSZ in the current study, EBSD was performed at the YSZ / LHS-1 in each cell to generate a grain microstructure map. Additionally, the same EBSD analysis was applied to a piece of YSZ that was not exposed molten LHS-1 to serve as the BOL reference. The EBSD results are summarized in a grain size histogram, provided in FIG. 66.

[0191] From FIG. 66, it is clear that grain coarsening is occurring. The molten LHS-1 is penetrating along YSZ grain boundaries and increasing the diffusivity of Y2O3 and ZrO2 along grain boundaries, allowing for coarsening to occur. Increasing durations in contact with molten LHS-1 at 1600° C. pushes the average grain diameter to larger values. The slowing of YSZ dissolution is attributed to the grain coarsening behavior discussed here.

[0192] It is noted that all three electrolysis cells exhibit YSZ cracking on cooling (seen in FIGS. 62, 63, and 64). The cracking was observed on the exterior of the cell prior to cross-sectioning. After cross-sectioning, optical and electron microscopy of each cell shows no evidence of regolith penetration along the cracks in the YSZ. These results suggest that the YSZ is cracking on cooling, specifically below the temperature at which LHS-1 solidifies, since cracking while LHS-1 is still liquid would cause the regolith to seep through the YSZ along cracks. This behavior is consistent with what was observed by the authors in previous work using YSZ crucibles to contain LHS-1

[18] .

[0193] The cause of YSZ cracking is due to Y depletion from the YSZ. Pure ZrO2 undergoes a monoclinic-to-tetragonal phase transition between 800° C.-1100° C.

[39] . This phase transition is accompanied by a change in the unit cell volume which gives rise to cracking. Y2O3 is a common stabilizer used to suppress this phase transition and allows YSZ to survive thermal cycling. However, exposure to molten regolith causes Y2O3 to diffuse out of YSZ and into the molten regolith, forming a YSZ region that is depleted of the Y2O3 stabilizer

[18] . Due to the low stabilizer content, the YSZ in this region will undergo the tetragonal-to-monoclinic phase transition on cooling. It is this phase transition that causes the YSZ to crack on cooling.

[0194] As further evidence, Table 6 shows that average measured concentrations of Y and Zr are 0.26 at % and 0.59 at %, resulting in a ratio of Y:Zr of approximately 0.44. In contrast, the ratio of Y:Zr in YSZ at BOL is approximately 0.1, significantly lower than what is observed in the molten regolith. To achieve the 0.44 ratio measured, Y2O3 must be preferentially diffusing out of the YSZ. The EBSD analysis used to determine grain sizes can also provide the crystallographic phase of YSZ and reveals that there is an approximately 100 μm thick layer of YSZ that is monoclinic at the YSZ / LHS-1 interface. The presence of monoclinic YSZ at the interface provides further evidence of Y depletion and reveals the cause of YSZ cracking.

[0195] 3.5.2 Potential-Induced Interactions

[0196] All the electrolysis cells exhibited a decrease in cell resistance after their respective electrolysis periods, with longer durations causing a larger decrease in resistance. A summary of cell resistances is presented in Table 8. There are three contributing factors to the changes in cell resistance—an increase in cathodic surface area, changes in regolith viscosity / conductivity, and the onset of electronic conductivity in YSZ. The cathode surface is already significantly larger than the anode (˜10 cm2 vs ˜1 cm2) at BOL, so an increase in the cathode surface area, resulting from the evolution of liquid cathodic products, is unlikely to cause the magnitude of cell resistance changes observed. The regolith viscosity is inversely related to the regolith electrical conductivity. In the 1-hour electrolysis cell, the removal of FeO / Fe2O3 is expected to increase the regolith viscosity, decrease the regolith conductivity, and cause the cell resistance to increase if the regolith viscosity is the dominant factor affecting the cell resistance. However, Table 8 reveals that a resistance decrease is observed in the 1-hour electrolysis cell, indicating that regolith conductivity is not the dominant factor in cell resistance. Thus, the resistance decrease is attributed to the onset of electronic conductivity in the YSZ.TABLE 8Cell resistances at 1600° C. before and after electrolysisElectrolysisInitialFinal%durationresistanceresistanceChange1 hr1.54 Ω1.44 Ω−6.5%3 hr1.59 Ω1.12 Ω−29.6%12 hr 1.46 Ω0.78 Ω−46.6%

[0197] It has been reported in literature that YSZ can have a significant electronic conductivity in low pO2 environments and / or under an applied DC bias [32-34]. The MRE cell environment exposes YSZ to both conditions, in addition to a higher temperature than typical YSZ applications require. The combination of these factors leads to a high concentration of O2− vacancies forming within the YSZ crystal structures, which can interact with the electrons in the crystal structure to form F centers, creating a reduced YSZ structure. These F centers give rise to electron states within the band gap of YSZ, allowing high electronic conductivity to exist in a material that is typically an electron insulator

[40] . The decrease in cell resistance is a result of this behavior. As a DC bias is applied at 1600° C., more O2− vacancies are formed, allowing for more electron states to open within the band gap of the YSZ, increasing the electronic conductivity of YSZ. This behavior is exacerbated with increasing duration of applied DC potential, thereby resulting in a lower cell resistance (Table 8) with increasing electrolysis duration.

[0198] The formation of F centers in YSZ not only increases the electronic conductivity of YSZ but also manifests as a darkening in the color of YSZ [40,41]. This effect was also observed in the cells. In FIGS. 62, 63, and 64, the optical image of the electrolysis cells shows a clear darkening of YSZ from white to gray as the electrolysis duration is increased. Raman spectra of the YSZ in all three electrolysis cells, as well as a BOL sample, are presented in FIG. 67. The A1g (260 cm−1) and Eg (640 cm−1) Raman modes are labeled in FIG. 67. The Raman spectra are normalized to the intensity of the Eg peak, which is denoted with the horizontal dashed line for each of the spectra. The associated labels summarize the intensity of the A1g peak relative to the largest intensity Eg peak.

[0199] Hastak et al. demonstrated that Raman spectroscopy can be used to qualitatively determine the difference in O2− vacancy concentration of YSZ at varying degrees of reduction

[42] . Specifically, the A1g Raman mode becomes broader and lower in intensity as the O2− vacancy concentration increases

[42] . This effect was also observed in the YSZ of the electrolysis cells. The highest intensity YSZ A1g peak occurs in the BOL sample, which has not been reduced or exposed to the MRE conditions. Each electrolysis run shows a decreased A1g intensity compared to the BOL sample, suggesting that more O2− vacancies are present with increasing electrolysis duration. This is consistent with the observed cell resistance behavior (Table 8) and the YSZ color in the electrolysis cells (FIGS. 62, 63, 64). Based on these results, it is expected that YSZ will become further reduced and electronically conductive with increasing electrolysis duration.4. Implications for Hollow Anode Design

[0200] The results from the inverted hollow anode experiments conducted in this study provide insight into how a hollow anode can incorporated into a MRE reactor. The experimental results presented indicate that MRE can be successfully performed using YSZ / Pt at the anode, without altering the metallic products that form at the cathode. For a hollow anode utilizing YSZ as the solid electrolyte, there are three primary mechanisms affecting electrolysis performance—the onset of electronic conductivity in YSZ, the thermochemical dissolution of YSZ into molten regolith, and the cracking of YSZ, induced by Y depletion and thermal cycling. Each mechanism has been previously discussed in the context of the electrolysis experiments performed. In this section, the implications of each mechanism on YSZ hollow anodes in a prototypical MRE reactor are assessed with potential solutions.4.1 YSZ Regeneration

[0201] The increasing electronic conductivity of YSZ with increasing electrolysis duration is undesirable in a hollow anode application. If YSZ is electronically conductive, O2 generation is not isolated to the YSZ / Pt interface and can be produced at the YSZ / regolith interface. Within the hollow anode design, this may cause O2 to be generated as if the anode were Ir or an ideal inert anode, allowing the produced O2 to be bubbled through the molten regolith. This would reintroduce bubble separation and reoxidation challenges, resulting in a lower O2 production efficiency. A method of regenerating YSZ performance is important to enable sustainable high O2 production efficiencies using a hollow anode.

[0202] As discussed previously, the electronic conductivity observed in YSZ is due to the formation of reduced YSZ, with a high concentration of O2− vacancies. Thus, the high concentration of O2− must be decreased to decrease the electronic conductivity of YSZ. The proposed method of regeneration is to pause MRE after a set amount of time and allow O2 to dissociate and reabsorb into the YSZ crystal structure (reoxidation), filling the excess O2− vacancies. The reversible uptake of O2 into YSZ and subsequent metal-to-insulator transition is well-studied

[43] . However, further work is necessary to determine the correct conditions (pO2 and duration) to regenerate the performance of YSZ in a hollow anode application. Guo et al. investigated the thermochemical darkening of YSZ for oxygen sensor applications in low pO2 environments and provided a similar suggestion that YSZ should be periodically reoxidized to prolong the performance of YSZ

[41] . The same type of thermochemical reoxidation is proposed here to reduce the electronic conductivity of YSZ, increasing the O2 production efficiency of the hollow anode. Based on FIGS. 61A-61B, it appears that regeneration should be performed approximately every 6 hours if the same current density (0.5 A / cm2) is used.4.2 Design Life

[0203] YSZ undergoes dissolution in contact with molten regolith. For highlands regolith, this process appears to be relatively slow when compared to the timescale of increasing YSZ electronic conductivity. From Table 7, an average of 85 μm of wall recession was observed for 12 hours of electrolysis, providing a wall recession rate of approximately 7 μm / hr. Linearly extrapolating these values suggests that a week of operation would result in approximately 1.2 mm of recession, roughly half of the original YSZ wall thickness. Based on these values, it appears that a YSZ hollow anode would need to be changed out every one to two weeks if used continuously in contact with highlands regolith. However, these results do not account for the decrease in the rate of YSZ wall recession that stems from grain coarsening at the interface. Thus, the conservative design life of one to two weeks is likely a lower bound on the usable life of YSZ in contact with molten highlands regolith. It is possible that grain coarsening can significantly decrease the rate of YSZ dissolution and extend the design life of YSZ hollow anodes, but further experiments are needed to quantify the expected design life of YSZ hollow anodes.4.3 Thermal Cycling Behavior

[0204] The Y depletion caused by interactions between YSZ and molten regolith will cause a YSZ to crack on cooling. Since no regolith penetration is detected along YSZ cracks in the inverted hollow anode, the cracks are assumed to form below the solidification temperature of LHS-1. This behavior indicates that YSZ hollow anodes are expected to survive through only a single thermal cycle. However, given the relatively long usable life of YSZ, the expectation is that MRE reactors incorporating a hollow anode would not be thermally cycled until the design life of the YSZ is reached (one to two weeks). Since the cracking would occur after the usable life of a YSZ hollow anode is reached, it would be replaced regardless. Large-scale O2 production rates (>10 MT / year) are necessary to meet NASA's production goal for a technology demonstration

[44] , which will likely require an MRE reactor to operate continuously and with a high duty cycle. The continuous operation of a MRE reactor will decrease the number of thermal cycles, ideally extending high temperature operation to the full design life of a YSZ hollow anode.5. Conclusions

[0205] This study presents the hollow anode as an alternative anode design to the state-of-the-art Ir anodes currently used in MRE reactors. The hollow anode allows for the facile collection of O2 by utilizing a YSZ solid electrolyte in combination with a Pt current collector, allowing produced O2 to be removed without contacting the molten regolith. This design addresses both the bubble formation and reoxidation challenges associated with MRE. Additionally, the hollow anode can be a more cost-effective solution compared with bare Ir anodes.

[0206] Experimental results from an inverted hollow anode reactor demonstrate that MRE can be successfully performed through a YSZ solid electrolyte for up to 12 hours. Residual gas analysis indicates that O2 is produced when a constant current is applied to the cell, confirming that anodic O2 production is not affected by the YSZ solid electrolyte. Subsequent SEM / EDS analysis of the cross-sectioned cells reveal the compositions of both the cathodic products and solidified regolith simulant. The primary cathodic products are Fe and Si, which is consistent with previous MRE models. Furthermore, the FeO and SiO2 content of the solidified regolith simulant decreases with increasing electrolysis duration, indicating that FeO and SiO2 electrolysis is achieved in the inverted hollow anode reactor. To the authors' knowledge, this is the first reported experimental confirmation of SiO2 electrolysis in a MRE reactor. This is significant since SiO2 is the most abundant oxide species in lunar regolith. Demonstrating O2 production from SiO2 provides confirmation that high O2 electrolysis efficiencies (>25%) are possible through MRE utilizing a hollow anode.

[0207] Post-electrolysis analysis of the YSZ reveals that there are three primary mechanisms affecting electrolysis performance—the onset of electronic conductivity in YSZ, the thermochemical dissolution of YSZ into molten regolith, and the cracking of YSZ, induced by Y depletion and thermal cycling. The rates associated with each mechanism are estimated from the inverted hollow anode cells. From these rates, preliminary design guidelines for YSZ hollow anodes in a prototypical MRE reactor are identified. Given the extreme operating environment of MRE, the observed interactions between YSZ and LHS-1 are expected. However, the overall dissolution rate of YSZ indicates that electrolysis durations longer than the 12 hours achieved in this study are possible, potentially allowing for a YSZ hollow anode to operate for two weeks.REFERENCES CORRESPONDING TO EXAMPLE 4

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[44] NASA, Lunar Infrastructure Foundational Technologies-1: Request for Information, (2023). https: / / nspires.nasaprs.com / external / viewrepositorydocument / cmdocumentid=967421 / s olicitationld=% 7B23D0FE0E-099D-77A8-3E06-2C516D900762% 7D / viewSolicitationDocument=1 / LIFT-1% 20RFI%20STMD%20Amednment%201.pdf (accessed Dec. 23, 2024).Certain Exemplary Aspects and Embodiments

[0252] Various aspects are contemplated and disclosed herein, several of which are set forth in the paragraphs below. It is explicitly contemplated and disclosed that any aspect or portion thereof can be combined to form an aspect. Moreover, the terms “any preceding aspect” and “any one of the preceding aspects” means any aspect that appears prior to the aspect that contains such phrase (for example, the sentence “Aspect 15: The electrochemical system or process of any of the preceding aspects, wherein the active anode material comprises a porous solid metal . . . ” means that any Aspect prior to Aspect 15 is referenced, including letter versions). For example, it is contemplated and disclosed that, optionally, any method or system of any of the below aspects may be useful with or combined with any other aspect provided below. Further, for example, it is contemplated and disclosed that any embodiment or aspect described above may, optionally, be combined with any of the below listed aspects or any portion(s) thereof.

[0253] Aspect 1: an electrochemical system for producing molecular oxygen from regolith comprising: a hollowing anode comprising: an active anode material; and a solid state electrolyte in ionic communication with the active anode material; wherein the solid state electrolyte is conductive to oxygen anions; a cathode in ionic communication with the hollow anode; a liquid electrolyte comprising molten regolith; wherein the molten regolith establishes ionic communication between the cathode and the hollow anode; and an electronic unit configured to apply an electrical signal across the hollow anode and the cathode to generate molecular oxygen.

[0254] Aspect 2: a process for producing molecular oxygen from molten regolith, the process comprising: providing an electrochemical system comprising: a hollow anode comprising: an active anode material; and a solid state electrolyte in ionic communication with the active anode material; wherein the solid state electrolyte is conductive to oxygen anions; a cathode in ionic communication with the hollow anode; a liquid electrolyte comprising molten regolith; wherein the liquid electrolyte establishes ionic communication between the hollow anode and the cathode; and applying an electrical signal across the hollow anode and the cathode to produce molecular oxygen at the hollow anode.

[0255] Aspect 3: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may comprise a shell that shields the active anode material from the liquid electrolyte. Preferably, the solid state electrolyte may comprise a shell that shields the active anode material from the liquid electrolyte comprising molten regolith.

[0256] Aspect 4: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may conduct oxygen anions from the liquid electrolyte to the active anode material. Preferably, the solid state electrolyte may conduct oxygen anions from the molten regolith to the active anode material.

[0257] Aspect 5: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte has a thickness between 1 mm to 10 mm. Optionally the solid state electrolyte has a thickness between 1.2 mm to 9.8 mm. Optionally, the solid state electrolyte has a thickness between 1.4 mm to 9.6 mm. Optionally, the solid state electrolyte has a thickness between 1.6 mm to 9.4 mm. Optionally, the solid state electrolyte has a thickness between 1.8 mm to 9.2 mm. Optionally, the solid state electrolyte has a thickness between 2.0 mm to 9.0 mm. Optionally, the solid state electrolyte has a thickness between 2.2 mm to 8.8 mm. Optionally, the solid state electrolyte has a thickness between 2.4 mm to 8.6 mm. Optionally, the solid state electrolyte has a thickness between 2.6 mm to 8.4 mm. Optionally, the solid state electrolyte has a thickness between 2.8 mm to 8.2 mm. Optionally, the solid state electrolyte has a thickness between 3.0 mm to 8.0 mm. Optionally, the solid state electrolyte has a thickness between 3.2 mm to 7.8 mm. Optionally, the solid state electrolyte has a thickness between 3.4 mm to 7.6 mm. Optionally, the solid state electrolyte has a thickness between 3.6 mm to 7.4 mm. Optionally, the solid state electrolyte has a thickness between 3.8 mm to 7.2 mm. Optionally, the solid state electrolyte has a thickness between 4.0 mm to 7.0 mm. Optionally, the solid state electrolyte has a thickness between 4.2 mm to 6.8 mm. Optionally, the solid state electrolyte has a thickness between 4.4 mm to 6.6 mm. Optionally, the solid state electrolyte has a thickness between 4.6 mm to 6.4 mm. Optionally, the solid state electrolyte has a thickness between 4.8 mm to 6.2 mm. Optionally, the solid state electrolyte has a thickness between 5.0 mm to 6.0 mm. Optionally, the solid state electrolyte has a thickness between 5.2 mm to 5.8 mm. Optionally, the solid state electrolyte has a thickness between 5.4 mm to 5.6 mm. Optionally, the thickness of the solid state electrolyte changes over the course of electrolysis in the electrochemical system or process due to degradation of the solid state electrolyte in the liquid electrolyte, for example, degradation in the molten regolith.

[0258] Aspect 6: The electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may at least partially be in contact with the active anode material. For example, contact may refer to direct or indirect physical contact between the solid state electrolyte and the active anode material.

[0259] Aspect 7: The electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte has an oxygen anion conductivity of at least 0.1 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 0.2 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 0.4 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 0.6 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 0.8 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 1.0 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 1.2 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 1.4 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 1.6 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 1.8 S / cm at 1600° C. Optionally, the solid state electrolyte has an oxygen anion conductivity of at least 2.0 S / cm, at 1600° C. Optionally, the solid state electrolyte is characterized by an oxygen anion conductivity that allows oxygen anions to diffuse from the liquid electrolyte, preferably the molten regolith, to the active anode material.

[0260] Aspect 8: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may be selectively conductive to oxygen anions.

[0261] Aspect 9: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may not react with oxide species. For example, the solid state electrolyte may not react with O2− species. For example, the solid state electrolyte may be chemically inert to oxide species, for example O2− species.

[0262] Aspect 10: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may comprise a refractory ceramic.

[0263] Aspect 11: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may be selected from the group consisting of doped zirconia, doped hafnia, doped ceria, doped thoria, doped urania, or any combination thereof.

[0264] Aspect 12: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may comprise yttria stabilized zirconia.

[0265] Aspect 13: the electrochemical system or process of any of the preceding aspects, wherein the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 6 mol % to 14 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 7 mol % to 13 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 8 mol % to 13 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 9 mol % to 12 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 10 mol % to 11 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 14 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 13 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 12 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 11 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 10 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 9 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 8 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 7 mol %. Optionally the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 5 mol % to 6 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer with a concentration of 6 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 7 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 8 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with concentration of 9 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 10 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 11 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 12 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant, or a stabilizer, with a concentration of 13 mol % to 15 mol %. Optionally, the solid state electrolyte may comprise a dopant or a stabilizer with a concentration of 14 mol % to 15 mol %. Optionally, the dopant, or the stabilizer, introduces oxygen vacancies into the solid state electrolyte for facilitating oxygen anion conductivity.

[0266] Aspect 14: the electrochemical system or process of any of the preceding aspects, wherein the dopant comprises MgO, CeO2, CaO, La2O3, Y2O3, Sc2O3, Gd2O3, or any combination thereof.

[0267] Aspect 15: the electrochemical system or process of any of the preceding aspects, wherein the active anode material comprises a porous solid metal. Optionally, the active anode material comprises a porous refractory solid metal, a porous refractory solid metal oxide, a porous solid metal alloy, or any combination thereof. The solid active anode material is porous such that molecular oxygen generated at the hollow anode is able to diffuse through the active anode material and preferably be collected.

[0268] Aspect 16: the electrochemical system or process of any of the preceding aspects, wherein the porous solid metal comprises Ir, Pt, or Cr2O3, an alloy thereof, or any combination thereof. Preferably the solid metal or solid metal oxide is stable in the oxidizing environment at the hollow anode.

[0269] Aspect 17: the electrochemical system or process of any of the preceding aspects, wherein the active anode material comprises a liquid metal.

[0270] Aspect 18: the electrochemical system or process of any of the preceding aspects, wherein the liquid metal comprises Pd. Optionally, the liquid metal comprises Pt. Optionally, the liquid metal comprises Au. Optionally, the liquid metal comprise Ag. Optionally, the liquid metal comprises Ir. Optionally, the liquid metal comprises Ru. Optionally, the liquid metal comprises Rh. Optionally, the liquid metal comprises Os. Optionally, the liquid metal comprises an alloy of Pd, Pt, Au, Ag, Ir, Ru, Rh, or Os. Optionally, the liquid metal comprises any combination of Pd, Pt, Au, Ag, Ir, Ru, Rh, or Os.

[0271] Aspect 19: the electrochemical system or process of any of the preceding aspects, wherein the active anode material comprises a metallic core at least partially enclosed by the solid state electrolyte.

[0272] Aspect 20: the electrochemical system or process of any of the preceding aspects, wherein the active anode material is permeable to molecular oxygen. Optionally, the active anode material is permeable to oxygen anions. Optionally the active anode material is permeable to molecular oxygen, oxygen anions, or any combination thereof.

[0273] Aspect 21: the electrochemical system or process of any of the preceding aspects, wherein the active anode material comprises carbon. Optionally, the active anode material comprises carbon coated with a solid metal. Optionally, the active anode material comprises carbon coated with Ir.

[0274] Aspect 22: the electrochemical system or process of any of the preceding aspects, wherein the cathode comprises Mo. Optionally, the cathode comprises Nb. Optionally, the cathode comprises Ta. Optionally, the cathode comprises W. Optionally, the cathode comprises Re. Optionally, the cathode comprises any alloy of Mo, Nb, Ta, W, or Re. Optionally, the cathode comprises any combination of Mo, Nb, Ta, W, or Re.

[0275] Aspect 23: the electrochemical system or process of any of the preceding aspects, wherein the cathode is any electronically conductive material that is solid at temperatures between 1,500° C. and 2,000° C. Optionally, the cathode is any electronically conductive material that is solid at 1,500° C. Optionally, the cathode is any electronically conductive material that is solid at a temperature of 1,600° C. Optionally, the cathode is any electronically conductive material that is solid at a temperature of 1,700° C. Optionally, the cathode is any electronically conductive material is solid at a temperature of 1,800° C. Optionally, the cathode is any electronically conductive material that is solid at 1,900° C. Optionally, the cathode is any electronically conductive material that is solid at 2,000° C.

[0276] Aspect 24: the electrochemical system or process of any of the preceding aspects, wherein the electrical signal comprises a current. Optionally, the electrical signal comprises a voltage. Optionally, the electrical signal comprises a current, a voltage, or any combination thereof.

[0277] Aspect 25: the electrochemical system or process of any of the preceding aspects, wherein the voltage is at least 1.0V. Optionally, the voltage is applied to drive electrolysis of molten regolith to produce molecular oxygen at the hollow anode. Optionally, a large overpotential, or overvoltage is applied to necessitate joule heating. Optionally, the applied voltage comprises a DC voltage. Optionally, the applied voltage comprises an AC voltage.

[0278] Aspect 26: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith comprises lunar regolith. Optionally, the molten regolith comprises Martian regolith. Optionally, the molten regolith comprises terrestrial regolith. Optionally, the molten regolith comprises extra-terrestrial regolith. Optionally, the molten regolith comprises lunar regolith, Martian regolith, terrestrial regolith, extra-terrestrial regolith, or any combination thereof.

[0279] Aspect 27: the electrochemical system or process of any of the preceding aspects, wherein the lunar regolith comprises mare regolith. Optionally, the lunar regolith comprises highlands regolith. Optionally, the lunar regolith comprises any combination of mare regolith and highlands regolith.

[0280] Aspect 28: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith comprises Al2O3. Optionally, the molten regolith comprises CaO. Optionally, the molten regolith comprises Cr2O3. Optionally, the lunar regolith comprises FeO. Optionally, the lunar regolith comprises Fe2O3. Optionally, the lunar regolith comprises K2O. Optionally, the lunar regolith comprises K2O. Optionally, the lunar regolith comprises MgO. Optionally, the lunar regolith comprises MnO. Optionally, the lunar regolith comprises Na2O. Optionally, the lunar regolith comprises P2O5. Optionally, the lunar regolith comprises SiO2. Optionally, the lunar regolith comprises TiO2. Optionally, the lunar regolith comprises any combination of Al2O3, CaO, Cr2O3, FeO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SiO2, or TiO2.

[0281] Aspect 29: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith comprises at least FeO. Optionally, the molten regolith comprises at least Fe2O3. Optionally, the molten regolith comprises at least SiO2. Optionally, the molten regolith comprises any combination of at least FeO, Fe2O3, SiO2. Optionally, the molten regolith comprises a combination of FeO and SiO2.

[0282] Aspect 30: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith comprises at least 10% oxygen by weight. Optionally, the molten regolith comprises at least 15% oxygen by weight. Optionally, the molten regolith comprises at least 20% oxygen by weight. Optionally, the molten regolith comprises at least 25% oxygen by weight. Optionally, the molten regolith comprises at least 30% oxygen by weight. Preferably, the molten regolith comprises at least 35% oxygen by weight. Even more preferably, the molten regolith comprises at least 40% oxygen by weight. Even more preferably, the molten regolith comprises at least 45% oxygen by weight. Optionally, the molten regolith comprises at least 50% oxygen by weight.

[0283] Aspect 31: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith comprises at least 10% SiO2 by weight. Optionally, the molten regolith comprises at least 15% SiO2 by weight. Optionally, the molten regolith comprises at least 20% SiO2 by weight. Optionally, the molten regolith comprises at least 25% SiO2 by weight. Optionally, the molten regolith comprises at least 30% SiO2 by weight. Optionally, the molten regolith comprises at least 35% SiO2 by weight. Preferably, the molten regolith comprises at least 40% SiO2 by weight.

[0284] Aspect 32: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith comprises at least 2% FeO by weight. Optionally, the molten regolith comprises at least 3% FeO by weight. Optionally, the molten regolith comprises at least 4% FeO by weight. Optionally, the molten regolith comprises at least 5% FeO by weight. Optionally, the molten regolith comprises at least 6% FeO by weight. Optionally, the molten regolith comprises at least 7% FeO by weight. Optionally, the molten regolith comprises at least 8% FeO by weight. Optionally, the molten regolith comprises at least 9% FeO by weight. Optionally, the molten regolith comprises at least 10% FeO by weight. Optionally, the molten regolith comprises at least 11% FeO by weight. Optionally, the molten regolith comprises at least 12% FeO by weight. Optionally, the molten regolith comprises at least 13% FeO by weight. Optionally, the molten regolith comprises at least 14% FeO by weight. Optionally, the molten regolith comprises at least 15% FeO by weight. Optionally, the molten regolith comprises at least 16% FeO by weight. Optionally, the molten regolith comprises at least 17% FeO by weight. Optionally, the molten regolith comprises at least 18% FeO by weight. Optionally, the molten regolith comprises at least 19% FeO by weight. Optionally, the molten regolith comprises at least 20% FeO by weight. Optionally, the molten regolith comprises at least 21% FeO by weight. Optionally, the molten regolith comprises at least 22% FeO by weight. Optionally, the molten regolith comprises at least 23% FeO by weight. Optionally, the molten regolith comprises at 24% FeO by weight. Optionally, the molten regolith comprises at least 25% FeO by weight.

[0285] Aspect 33: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith is characterized by a viscosity of 0.1 Pa·s to 50 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 1 Pa·s to 49 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 2 Pa·s to 48 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 3 Pa·s to 47 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 4 Pa·s to 46 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 5 Pa·s to 45 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 6 Pa·s to 44 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 7 Pa·s to 43 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 8 Pa·s to 42 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 9 Pa·s to 41 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 10 Pa·s to 40 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 11 Pa·s to 39 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 12 Pa·s to 38 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 13 Pa·s to 37 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 14 Pa·s to 36 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 15 Pa·s to 35 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 15 Pa·s to 34 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 16 Pa·s to 33 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 17 Pa·s to 32 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 18 Pa·s to 31 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 19 Pa·s to 30 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 20 Pa·s to 29 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 21 Pa·s to 28 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 22 Pa·s to 27 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 23 Pa·s to 26 Pa·s at 1,600° C. Optionally, the molten regolith is characterized by a viscosity of 24 Pa·s to 25 Pa·s at 1,600° C.

[0286] Aspect 34: the electrochemical system or process of any of the preceding aspects, wherein the molten regolith is characterized by a surface tension of 100 N / m to 1,000 N / M at 1,600° C. Optionally, the molten regolith is characterized by a surface tension of 200 N / m to 900 N / m at 1,600° C. Optionally, the molten regolith is characterized by a surface tension of 300 N / m to 800 N / m at 1,600° C. Optionally, the molten regolith is characterized by a surface tension of 400 N / m to 700 N / m at 1,600° C. Optionally, the molten regolith is characterized by a surface tension of 500 N / m to 600 N / m at 1,600° C.

[0287] Aspect 35: the electrochemical system or process of any of the preceding claims wherein the faradaic efficiency for molecular oxygen is greater than or equal to 10%. Optionally, the faradaic efficiency for molecular oxygen is greater than or equal to 15%. Optionally, the faradaic efficiency for molecular oxygen is greater than or equal to 20%. Optionally, the faradaic efficiency for molecular oxygen is greater than or equal to 25%. Optionally, the faradaic efficiency for molecular oxygen is greater than or equal to 30%. Preferably, the faradaic efficiency for molecular oxygen is greater than or equal to 35%. Preferably, the faradaic efficiency for molecular oxygen is greater than or equal to 40%. Preferably, the faradaic efficiency for molecular oxygen is greater than or equal to 45%.

[0288] Aspect 36: the electrochemical system or process of any of the preceding aspects, wherein molecular oxygen is produced at the hollow anode via the reaction given by equation FX1:O2−2e−+12O2(g)  (FX1).Aspect 37: the electrochemical system or process of any of the preceding aspects, wherein one or more metal cation is reduced at the cathode. Optionally, one or more non-metal cations is reduced at the cathode. Optionally, one or more metalloid cations is reduced at the cathode. Optionally, Al3+ is reduced at the cathode. Optionally, Ca2+ is reduced at the cathode. Optionally, Cr3+ is reduced at the cathode. Optionally, Fe2+ is reduced at the cathode. Optionally, Fe3+ is reduced at the cathode. Optionally, K+ is reduced at the cathode. Optionally, Mg2+ is reduced at the cathode. Optionally, Mn2+ is reduced at the cathode. Optionally, Na+ is reduced at the cathode. Optionally, P5+ is reduced at the cathode. Optionally, Si4+ is reduced at the cathode. Optionally, Ti4+ is reduced at the cathode. Preferably, any one of Fe2+, Fe3+, Si4+, or any combination thereof, is reduced at the cathode.

[0290] Aspect 38: the electrochemical system or process of any of the preceding aspects, wherein Fe is generated at the cathode. Optionally, Si is generated at the cathode. Preferably, Fe and Si are generated at the cathode. Optionally, an alloy of Fe and Si are generated at the cathode.

[0291] Aspect 39: the electrochemical system or process of any of the preceding aspects, further comprising a module for collecting the molecular oxygen generated at the hollow anode.

[0292] Aspect 40: the electrochemical system or process of any of the preceding aspects, wherein the electrochemical system is configured to heat and maintain the molten regolith at a temperature greater than or equal to 1,500° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature greater than or equal to 1,550° C. Preferably, the electrochemical system is configured to heat and maintain the molten regolith at a temperature greater than or equal to 1,600° C. Optionally, the electrochemical is configured to heat and maintain the molten regolith at a temperature greater than or equal to 1,650° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature greater than or equal to 1,700° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature of greater than or equal to 1,750° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature of greater than or equal to 1,800° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature of greater than or equal to 1,850° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature of greater than or equal to 1,900° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature of greater than or equal to 1,950° C. Optionally, the electrochemical system is configured to heat and maintain the molten regolith at a temperature of greater than or equal to 2,000° C.

[0293] Aspect 41: the electrochemical system or process of any of the preceding aspects, wherein the process is carried out via batch operation.

[0294] Aspect 42: the electrochemical system or process of any of the preceding aspects, wherein the process is carried out via mobile operation.

[0295] Aspect 43: the electrochemical system or process of any of the preceding aspects, wherein the electrochemical system is operated at a temperature greater than or equal to 1,500° C. and less than or equal to 2,000° C.

[0296] Aspect 44: the electrochemical system or process of any of the preceding aspects, wherein a hollow anode product comprises molecular oxygen and a cathode product comprises Si. Optionally, the cathode product comprises Fe. Optionally, the cathode product comprises an alloy of Si and Fe. Optionally, the cathode product comprises any combination of Si and Fe.STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

[0297] All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

[0298] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.

[0299] As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” includes a plurality of such cells and equivalents thereof known to those skilled in the art. As well, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, and “having” can be used interchangeably. The expression “of any of claims XX-YY” (wherein XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form, and in some embodiments is interchangeable with the expression “as in any one of claims XX-YY.”

[0300] When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.

[0301] Every device, system, formulation, combination of components, or method described or exemplified herein can be used to practice the invention, unless otherwise stated.

[0302] Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.

[0303] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art as of their publication or filing date and it is intended that this information can be employed herein, if needed, to exclude specific embodiments that are in the prior art.

[0304] For example, when composition of matter are claimed, it should be understood that compounds known and available in the art prior to Applicant's invention, including compounds for which an enabling disclosure is provided in the references cited herein, are not intended to be included in the composition of matter claims herein.

[0305] As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” may be replaced with either of the other two terms. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

[0306] One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

Claims

1. An electrochemical system for producing molecular oxygen from regolith comprising:a hollow anode comprising:an active anode material; anda solid state electrolyte in ionic communication with the active anode material;wherein the solid state electrolyte is conductive to oxygen anions;a cathode in ionic communication with the hollow anode;a liquid electrolyte comprising molten regolith; wherein the molten regolith establishes ionic communication between the cathode and the hollow anode; andan electronic unit configured to apply an electrical signal across the hollow anode and the cathode to generate molecular oxygen.

2. The electrochemical system of claim 1, wherein the solid state electrolyte comprises a shell that shields the active anode material from the liquid electrolyte, and wherein the solid state electrolyte conducts oxygen anions from the liquid electrolyte to the active anode material.

3. (canceled)4. The electrochemical system of claim 1, wherein the solid state electrolyte has a thickness of between 1 mm to 10 mm, and is at least partially in contact with the active anode material.

5. (canceled)6. (canceled)7. The electrochemical system of claim 1, wherein the solid state electrolyte is selectively conductive to oxygen anions, has an oxygen anion conductivity of at least 0.1 S / cm at 1600° C., and does not react with oxide species in the molten regolith.

8. (canceled)9. The electrochemical system of claim 1, wherein the solid state electrolyte comprises a refractory ceramic, and is selected from the group consisting of doped zirconia, doped hafnia, doped ceria, doped thoria, doped urania, or any combination thereof.

10. (canceled)11. The electrochemical system of claim 1, wherein the solid state electrolyte comprises yttria stabilized zirconia.

12. The electrochemical system of claim 1, wherein the solid state electrolyte comprises a dopant with a concentration of 5 mol % to 15 mol %, and the dopant comprises MgO, CeO2, CaO, La2O3, Y2O3, SC2O3, Gd2O3, or any combination thereof.

13. (canceled)14. The electrochemical system of claim 1, wherein the active anode material comprises a porous solid metal, a metal oxide, or any combination thereof, and the porous solid metal or metal oxide comprises Ir, Pt, Cr2O3, an alloy thereof, or any combination thereof.

15. (canceled)16. The electrochemical system of claim 1, wherein the active anode material comprises a liquid metal, and the liquid metal comprises Pd, Pt, Au, Ag, Ir, Ru, Rh, Os, an alloy thereof, or any combination thereof.

17. (canceled)18. The electrochemical system of claim 1, wherein the active anode material comprises a metallic core at least partially enclosed by the solid state electrolyte, and the active anode material is permeable to oxygen anions, molecular oxygen, or any combination thereof.

19. (canceled)20. The electrochemical system of any of claim 1, wherein the active anode material comprises carbon, carbon coated with a solid metal, or any combination thereof.

21. The electrochemical system of claim 1, wherein the cathode comprises a refractory metal selected from the group consisting of Mo, Nb, Ta, W, Re, an alloy thereof, or any combination thereof, and the cathode is solid at temperatures between 1,500° C. and 2,000° C.

22. (canceled)23. The electrochemical system of claim 1, wherein the electrical signal comprises a current, a voltage, or any combination thereof, and the voltage is at least 1.0 V.

24. (canceled)25. The electrochemical system of claim 1, wherein the molten regolith comprises lunar regolith, Martian regolith, terrestrial regolith, or any combination thereof, and wherein the lunar regolith comprises mare regolith, highlands regolith, or any combination thereof.

26. (canceled)27. The electrochemical system of claim 1, wherein the molten regolith comprises Al2O3, CaO, Cr2O3, FeO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SiO2, TiO2, or any combination thereof.

28. The electrochemical system of claim 1, wherein the molten regolith comprises FeO, Fe2O3, SiO2, or any combination thereof.

29. The electrochemical system of claim 1, wherein the molten regolith comprises at least 35% oxygen by weight, at least 40% SiO2 by weight, from 2% to 25% FeO by weight, or any combination thereof.

30. (canceled)31. (canceled)32. The electrochemical system of claim 1, wherein the molten regolith is characterized by a viscosity of 0.1 Pa·s to 50 Pa·s at 1,600° C., and a surface tension of 10 N / m to 1,000 N / m at 1,600° C.

33. (canceled)34. The electrochemical system of claim 1, wherein the faradaic efficiency for molecular oxygen is greater than or equal to 30%.

35. (canceled)36. The electrochemical system of claim 1, wherein one or more metal, non-metal, or metalloid cations selected from the group consisting of Al3+, Ca2+, Cr3+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+, P5+, Si4+, or Ti+4 is reduced at the cathode and wherein Fe, Si, an alloy thereof, or any combination thereof is generated at the cathode.

37. (canceled)38. The electrochemical system of claim 1 further comprising a module for collecting the molecular oxygen, and a container for holding the liquid electrolyte.

39. The electrochemical system of claim 1, wherein the electrochemical system is configured to heat and maintain the molten regolith at a temperature greater than or equal to 1,500° C. and less than or equal to 2,000° C.

40. (canceled)41. A process for producing molecular oxygen from regolith, the process comprising:providing an electrochemical system comprising:a hollow anode comprising:an active anode material; anda solid state electrolyte in ionic communication with the active anode material; wherein the solid state electrolyte is conductive to oxygen anions;a cathode in ionic communication with the hollow anode;a liquid electrolyte comprising molten regolith; wherein the liquid electrolyte establishes ionic communication between the hollow anode and the cathode; andapplying an electrical signal across the hollow anode and the cathode to produce molecular oxygen at the hollow anode.

42. The process of claim 41, wherein the process is carried out via mobile operation.

43. (canceled)44. The process of claim 41, further comprising operating the electrochemical system at a temperature greater than or equal to 1,500° C. and less than or equal to 2000° C. and wherein a hollow anode product comprises molecular oxygen and a cathode product comprises Si, Fe, an alloy thereof, or any combination thereof.

45. (canceled)

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