Electrochemical exsolution of metal / alloy nanoparticles

Abstract

Provided herein are systems and methods for producing nano-particle-decorated catalytic surfaces by applying a current to conductive metal oxides. The nano-particle-decorated catalytic surfaces provide enhanced catalytic activity and are disclosed as anodes for oxidation reactions and cathodes for reduction reactions. Methods of electrocatalysis using the nano-particle decorated catalytic surfaces are also disclosed.

Description

Electrochemical Exsolution of Metal / Alloy NanoparticlesCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority to, and the benefit of, U.S. provisional application no. 63 / 470,581, filed on June 2, 2023, which is hereby incorporated by reference herein in its entirety.STATEMENT OF GOVERNMENT SUPPORT

[0002] This invention was made with government support under grant / contract no. 1932638 awarded by the National Science Foundation and grant / contract no. DE-FG02-07ER15896 awarded by the Department of Energy. The government has certain rights in the invention.BACKGROUND

[0003] Perovskite oxides are widely used for energy and chemical conversion processes as mixed ionic and electronic conductors (MIECs) in solid oxide electrocatalytic cells. These materials have garnered significant attention in the scientific community due to their exceptional redox properties, which are linked to their tunable oxygen ion mobility, high stability under both oxidizing and reducing environments, and resistance to coke formation. To improve the electronic and ionic conductivity, stability, or activity of these materials, many studies have focused on doping the A or B sites of perovskite oxides.

[0004] It has been proven that a controlled reducing environment at elevated temperatures may be used to induce the exsolution of B-site atoms onto the surface. During thermochemical exsolution, B-site transition metal atoms undergo reduction and migrate towards the surface, where they segregate into nanoparticles. By contrast to the physical deposition techniques, exsolution results in the formation of well-distributed and uniform sized nanoparticles that adhere firmly to the parent perovskite surface with superior thermochemical stability.

[0005] It is noteworthy that exsolution of metal nanoparticles can be triggered by applying cathodic polarization to the perovskite oxide electrode. It has been also reported that the electrochemical exsolution could be induced by anodic polarization due to incomplete oxidation and local charge transfer. The electrochemical exsolution is time efficient given that it can be achieved within several minutes by applying polarization. This is why the electrochemical exsolution is referred to as “electrochemical switching”. In contrast, the thermal exsolution under reducing atmosphere normally requires several hours of treatment due to the slowdiffusion and low driving force (oc(knT) / 2 In Pm). However, studies on the electrochemical exsolution are limited so far because the process requires high temperature, electrical lead connections, controlled gas environment, which make in situ and ex situ observation difficult.

[0006] Recent years have seen significant research efforts, seeking out catalysts that can maximize the yield of C2 hydrocarbons, such as ethane and ethylene. Based on the evidence thus far, however, it has been suggested that the low yields of C2 hydrocarbons is due to the rate of hydrogen abstraction from C2 products being greater than the rate of hydrogen abstraction from methane on most heterogeneous catalysts. A kinetic study has predicted that catalytic processes for oxidative coupling of methane (OCM) can yield C2 hydrocarbons up to an upper limit of 28-30%. In light of this, other approaches need to be considered for OCM. The yields to C2 hydrocarbons follow a half-order dependence on O2 partial pressure while deep oxidation of CH4 to CO and CO2 shows first-order dependence. As such, one strategy to improve the C2 selectivity is to use a membrane reactor able to operate OCM at low partial pressure of oxygen. Particularly, solid oxide cells (SOCs) consisting of solid oxide electrolyte membrane and electrodes that are able to regulate oxygen-ion flux based on fine control of appropriate current. SOCs are a doubly attractive option because of the additional valuable products that can be generated on the cathode depending on the reaction of choice, such as the electrolysis of H2O or CO2, while performing OCM at the anode. A further advantage of SOCs is that electricity can be spontaneously generated during OCM. Even with these advantages, OCM using SOC is still challenging since most high temperature catalysts have mixed ionic and electronic conductivity that chemically drives CH4 to CO and CO2.

[0007] Light olefins, such as ethylene and propylene, are key chemical feedstocks in a wide variety of industries producing goods that the modem world relies on, including but not limited to plastics, polymers, detergents, surfactants, antifreeze, and synthetic fuel. The conventional route for light olefin production is catalytic cracking of crude oil derivatives, which is an energy-intensive process with a large carbon footprint. Oil refining is a primary source of olefin feedstock and is affected by unexpected fluctuations in oil / oil derivative prices and global climate change. An alternative source of olefin feedstock is of high importance for stability of resources and to reduce the impacts of climate change caused by oil production and refining. Converting shale gas which is a mixture of methane and light alkanes in a single unit capable of performing oxidative dehydrogenation (ODH) of light alkanes and OCM together at locations where the olefins are needed or where the feedstock is available has a significant potential to provide a distributed on-board olefin production strategy.SUMMARY

[0008] Provided herein are systems and methods for producing nano-particle-decorated catalytic surfaces using applied electric current. The nano-particle-decorated catalytic surfaces provide enhanced catalytic activity and are disclosed as anodes for oxidation reactions and cathodes for reduction reactions. Methods of electrocatalysis using the nano-particle decorated catalytic surfaces are also disclosed.

[0009] The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

[0010] In some aspects, a method of forming a nano-particle-decorated catalytic surface is disclosed. The method including: providing a conductive metal oxide substrate to a system, and applying a cathodic voltage to a working electrode.

[0011] In some aspects, the conductive metal oxide substrate includes a mixed metal oxide.

[0012] In some aspects, the mixed metal oxide includes a perovskite, complex perovskite (e.g., double perovskite), or Ruddlesden-Popper phase (layered perovskite).

[0013] In some aspects, the mixed metal oxide includes the perovskite having a formula of (AZ)a(XY)O3, wherein a is about 1 or less than 1; A and Z are the same or different elements; and X and Y are the same or different elements.

[0014] In some aspects, A and Z are chosen from La, Sr, Pr, Ba, or Ca, and X and Y are chosen from Co, Fe, Ni, Mn, Ti, Re, Cu, Cr, or Mo .

[0015] In some aspects, the method includes applying the cathodic voltage until a plurality of nanoparticles are exsolved onto a surface of the conductive metal oxide substrate.

[0016] In some aspects, the applied voltage is varied until a desired density of nanoparticles is achieved.

[0017] In some aspects, the plurality of metal nanoparticles includes metal or metal alloy nanoparticles, and wherein the applied voltage is varied until a desired metal or metal alloy is formed (e.g. varying the ratio of metals in the alloy).

[0018] In some aspects, the nanoparticles nucleate at oxygen vacancies on the surface of the conductive metal oxide substrate.

[0019] In some aspects, the system is at or below a relevant phase transition temperature of the conductive metal oxide substrate.

[0020] In some aspects, a nano-particle-decorated catalytic surface formed by the disclosed method is described. The nano-particle-decorated catalytic surface including a reduced mixed metal oxide substrate and a plurality of metal or metal alloy nanoparticles on a surface of the reduced mixed metal oxide substrate, wherein at least one metal is common in both the reduced mixed metal oxide substrate and the plurality of metal of metal alloy nanoparticles.

[0021] In some aspects, the nano-particle-decorated catalytic surface is used as an anode catalyst for an oxidation reaction or as a cathode for a reduction reaction.

[0022] In some aspects, the disclosure related to an electrode composition including a reduced perovskite substrate having a formula of (AZ)(XY)Os and a plurality of metal or metal alloy nanoparticles on a surface of the reduced perovskite substrate, wherein at least one metal is common in both the reduced perovskite substrate and the plurality of metal or metal alloy nanoparticles.

[0023] In some aspects, A and Z are chosen from La, Sr, Pr, Ba, or Ca. For example, X and Y are chosen from Co, Fe, Mn, Ti, Re, Cu, Cr, or Mo.

[0024] In some aspects, the reduced perovskite substrate includes reduced (LaSr)(CoFe)O3 and wherein the plurality of metal nanoparticles includes Co metal or CoFe metal alloy.

[0025] In some aspects, the electrode composition is used as an anode catalyst for an oxidation reaction or as a cathode for a reduction reaction.

[0026] In some aspects, a method of electrocatalysis is described. The method including providing the disclosed electrode composition to an electrochemical system; providing a current to the electrochemical system; and streaming a feed gas into the electrochemical system.

[0027] In some aspects, providing a current includes providing an anodic current, and wherein streaming feed gas includes methane, C2-C6 alkanes, C2-C6 alkenes, or combinations thereof.

[0028] In some aspects, providing a current includes providing an cathodic current, and wherein streaming feed gas includes NOx, CO2, H2O, O2 or combinations thereof.

[0029] In some aspects, the electrochemical system is a solid oxide electrochemical cell.DESCRIPTION OF DRAWINGS

[0030] Fig. 1 shows a schematic illustration of the electrocatalytic reaction system.

[0031] Figs. 2A-2B show SEM images of LSCF cathode top surface (secondary electron) (Fig. 2A) and cross-section (backscattered electron) (Fig. 2B).

[0032] Figs. 3 A-3D show SEM secondary electron (top) and backscattered electron (bottom) images of LSCF cathode after applying -1.3 V of cathodic potential under helium at 700 °C for 0.5 minutes (Fig. 3A), one minute (Fig. 3B), and two minutes (Fig. 3C); and STEM image (Fig. 3D) of the electrochemically exsolved nanoparticle with elemental mapping of (d2) La, (d3) Sr, (d4) Fe, (d5) Co, and (d6) O after two minutes of -1.3 V cathodic polarization at 700 °C.

[0033] Figs. 4A-4C show an illustration of the operando XANES analysis on LSCF electrode at 45° angle to the incident X-ray beam (Fig. 4A); the operando XANES results on Fe K edge (Fig. 4B) and Co K edge (Fig. 4C) at room temperature, 650 °C under open-circuit voltage, and under -5 mA of cathodic current.

[0034] Fig. 5 shows current density as a function of cell voltage at different scan rates at 700 °C under 60 ccm of 5% H2O-5% CO2.

[0035] Fig. 6 shows current density and the resulting area specific resistance as a function of cell voltage at a scan rate of 1 mV s ' .

[0036] Figs. 7A-7C show SEM secondary electron (top) and backscattered electron (bottom) images of LSCF electrode after cathodic polarization at cell voltages of -0.25 V (Fig. 7A), - 0.5V (Fig. 7B), -1.5V (Fig. 7C) under the co-electrolysis condition for ten minutes.

[0037] Fig. 8 shows a schematic illustration of the electrochemical behavior of LSCF electrode at different cell voltages.

[0038] Figs. 9A-9C show current density as a function of cell voltage for co-electrolysis of H2O and CO2 at a scan rate of 1 mV s'1at different temperatures (Fig. 9A), the corresponding ASR (Fig. 9B), and Arrhenius plots of ASR at different cell voltages as a function of temperature (Fig. 9C).

[0039] Figs. 10A-10B show electrochemical impedance spectra (Fig. 10A) and the resultant polarization resistance of LSCF and eRed-LSCF at 850 °C for H2O, CO2, and H2O / CO2 electrolysis (Fig. 10B). Impedance spectra was fitted using the equivalent circuit in (a). eRed- LSCF was prepared by applying -2.0 V of cathodic polarization for two minutes at 850 °C.

[0040] Fig. 11 shows electrochemical impedance spectra of LSCF (top) and eRed-LSCF (bottom) for co-electrolysis of H2O and CO2 at different temperatures and Arrhenius plots of Rp and Rs.

[0041] Figs. 12A-12C production rates of H2 and CO (Fig. 12A) and the ratio of H2 to CO (Fig. 12B), the cell overpotential for co-electrolysis of H2O and CO2 on eRed-LSCF (Fig. 12C).

[0042] Figs. 13A-13B shows SEM secondary electron (left) and backscattered electron (right) images of Lao.7Sro.2Coo.2Feo.803-8 electrode following the application of -1 V of cathodic current under helium at 700 °C for one minute (Fig. 13 A), two minutes (Fig. 13B).

[0043] Fig. 14 shows selectivity evolution for electrocatalytic OCM on the LSCF anode cell at 850 °C at 100 mA / cm2.

[0044] Fig. 15 shows evolution of hetero-phases on LSCF under reducing environment: (a) XRD patterns of LSCF, H2-Red-LSCF (LSCF reduced under 5% H2 / N2) , and CH4-Red-LSCF (LSCF reduced under CEL),

[0045] Figs. 16A-16C shows STEM images of LSCF, H2-Red-LSCF, and CH4-Red-LSCF with average elemental counts in the specified area shown by a square (Fig. 16 A), LSCF to reduced phase transition (Fig. 16B), illustration of physically deposited CoFe nanoparticle and exsolved CoFe nanoparticles on LSCF (Fig. 16C).

[0046] Fig. 17 shows Rietveld refinement analysis on EE-Red-LSCF (top) and CH4-Red-LSCF (bottom).

[0047] Figs. 18A-18B show TPR profiles of LSCF under 5% H2 / N2 and TPO profiles of Red- LSCF under 2% Ch / He (Fig. 18 A), XRD patterns of LSCF, EE-Red-LSCF, and re-oxidized Red- LSCF treated with 5% Ch / He for one hour at 850 °C (Reoxi-Red-LSCF) (Fig. 18B).

[0048] Figs. 19A-19D show electrocatalytic OCM results on the LSCF cell at 850 °C at various current densities including production rate of C2+ hydrocarbon and the ratio of alkene to alkane (Fig. 19A), carbon selectivities and methane conversion (Fig. 19B), time-on stream of the cell potential (Fig. 19C), Nyquist plot and polarization resistance (Fig. 19D).

[0049] Figs. 20A-20B show long-term test of Red-LSCF for the electrocatalytic OCM under 100 mA cm2at 850 °C including the Red-LSCF cell potential and Faradaic efficiency (Fig. 20A), C2+ hydrocarbons selectivity and production rate (Fig. 20B).

[0050] Figs. 21A-21B show (a) Surface models of CoFe (100) and LSCF (110) (Fig. 21 A), and DFT evaluated free energy diagrams of surface reactions on CoFe and LSCF (110) surface at 0 V RHE (Fig. 2 IB).

[0051] Fig. 22 shows X-ray photoelectron spectra of LSCF and Red-LSCF.

[0052] Figs. 23 A-23B show XANES spectra on Fe K edge (Fig. 23 A) and Co K edge (Fig. 23B) LSCF and Red-LSCF.

[0053] Figs. 24A-24B show in situ DRIFTS spectra on LSCF and Red-LSCF under 30 ccm of 10% CF / He at 450 °C (Fig. 24A) and peak intensity ratios compared to the highest peak of CFL at 3015 cm-1(Fig. 24B).

[0054] Fig. 25 shows TPSR profiles on LSCF and Red-LSCF under 30 ccm of 10% CFh / He.

[0055] Fig. 26 shows In-situ TPD-DRIFTS of CO2 on LSCF and Red-LSCF at various temperatures.

[0056] Figs. 27A-27C show in situ measurement of electrical conductivity under different atmospheres including LSCF under air as a function of temperature (Fig. 27A), LSCF under different atmospheres at 850 °C (Fig. 27B), Red-LSCF under 5% H2 / N2 as a function of temperature (Fig. 27C). The inset figures show Arrhenius plots with activation energy.

[0057] Fig. 28 shows electrocatalytic OCM results on the LSCF anode at 850 °C.DETAILED DESCRIPTION

[0058] To facilitate understanding of the disclosure set forth herein, a number of terms are defined below. Unless defined otherwise, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference.General Definitions

[0059] As used in this specification and the following claims, the terms “comprise” (as well as forms, derivatives, or variations thereof, such as “comprising” and “comprises”) and “include” (as well as forms, derivatives, or variations thereof, such as “including” and “includes”) are inclusive (i.e., open-ended) and do not exclude additional elements or steps. For example, the terms "comprise" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Other than where noted, all numbers expressing quantities of ingredients, reaction conditions, geometries, dimensions, and so forth used in the specification and claims are to be understood at the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope ofthe claims, to be construed in light of the number of significant digits and ordinary rounding approaches.

[0060] Accordingly, these terms are intended to not only cover the recited element(s) or step(s) but may also include other elements or steps not expressly recited. Furthermore, as used herein, the use of the terms “a”, “an”, and “the” when used in conjunction with an element may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” Therefore, an element preceded by “a” or “an” does not, without more constraints, preclude the existence of additional identical elements.

[0061] It is understood that when combinations, subsets, groups, etc. of elements are disclosed (e.g., combinations of components in a composition, or combinations of steps in a method), that while specific reference of each of the various individual and collective combinations and permutations of these elements may not be explicitly disclosed, each is specifically contemplated and described herein.

[0062] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. By “about” is meant within 5% of the value, e.g., within 4, 3, 2, or 1% of the value. When such a range is expressed, another aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. A range may be construed to include the start and the end of the range. For example, a range of 10% to 20% (i.e., range of 10% - 20%) can includes 10% and also includes 20%, and includes percentages in between 10% and 20%, unless explicitly stated otherwise herein.

[0063] As used herein, the terms "may," "optionally," and "may optionally" are used interchangeably and are meant to include cases in which the condition occurs as well as cases in which the condition does not occur. Thus, for example, the statement that a formulation "may include an excipient" is meant to include cases in which the formulation includes an excipient as well as cases in which the formulation does not include an excipient.

[0064] A “control” is an alternative subject or sample used in an experiment for comparison purposes. A control can be "positive" or "negative."

[0065] Terms used herein will have their customary meaning in the art unless specified otherwise. The organic moieties mentioned when defining variable positions within the general formulae described herein (e.g., the term “halogen”) are collective terms for the individual substituents encompassed by the organic moiety.

[0066] As used herein, "alkyl" means a straight or branched chain saturated hydrocarbon moieties such as those containing from 1 to 10 carbon atoms. A “higher alkyl” refers to saturated hydrocarbon having 11 or more carbon atoms. A “Ce-Cie” refers to an alkyl containing 6 to 16 carbon atoms. Likewise, a “C6-C22” refers to an alkyl containing 6 to 22 carbon atoms. Representative saturated straight chain alkyls include methyl, ethyl, n-propyl, n- butyl, n-pentyl, n-hexyl, n-septyl, n-octyl, n-nonyl, and the like; while saturated branched alkyls include isopropyl, sec-butyl, isobutyl, tert-butyl, isopentyl, and the like.

[0067] As used herein, the term “olefin” and “alkenyl” refers to unsaturated, straight or branched hydrocarbon moieties containing a double bond. Unless otherwise specified, C2-C24 (e.g., C2-C22, C2-C20, C2-C18, C2-C16, C2-C14, C2-C12, C2-C10, C2-C8, C2-C6, or C2-C4) alkenyl groups are intended. Alkenyl groups may contain more than one unsaturated bond. Examples include ethenyl, 1 -propenyl, 2-propenyl, 1 -methyl ethenyl, 1-butenyl, 2-butenyl, 3-butenyl, 1- m ethyl- 1 -propenyl, 2-methyl-l -propenyl, l-methyl-2-propenyl, 2-methyl-2-propenyl, 1- pentenyl, 2-pentenyl, 3 -pentenyl, 4-pentenyl, 1 -methyl- 1-butenyl, 2-methyl- 1-butenyl, 3-methyl-1-butenyl, l-methyl-2-butenyl, 2-methyl-2-butenyl, 3-methyl-2-butenyl, l-methyl-3-butenyl, 2- methyl-3-butenyl, 3-methyl-3-butenyl, l,l-dimethyl-2-propenyl, 1,2-dimethyl-l -propenyl, 1,2- dimethyl-2-propenyl, 1 -ethyl- 1 -propenyl, l-ethyl-2-propenyl, 1 -hexenyl, 2-hexenyl, 3 -hexenyl, 4-hexenyl, 5-hexenyl, 1 -methyl- 1 -pentenyl, 2-methyl-l-pentenyl, 3 -methyl- 1 -pentenyl, 4- m ethyl- 1 -pentenyl, l-methyl-2-pentenyl, 2-methyl-2-pentenyl, 3-methyl-2-pentenyl, 4-methyl-2-pentenyl, l-methyl-3-pentenyl, 2-methyl-3 -pentenyl, 3 -methyl-3 -pentenyl, 4-methyl-3- pentenyl, l-methyl-4-pentenyl, 2-methyl-4-pentenyl, 3-methyl-4-pentenyl, 4-methyl-4-pentenyl, l,l-dimethyl-2-butenyl, l,l-dimethyl-3-butenyl, 1,2-dimethyl- 1-butenyl, l,2-dimethyl-2- butenyl, l,2-dimethyl-3-butenyl, 1,3-dimethyl-l-butenyl, l,3-dimethyl-2-butenyl, 1,3-dimethyl-3-butenyl, 2,2-dimethyl-3-butenyl, 2,3 -dimethyl- 1-butenyl, 2,3-dimethyl-2-butenyl, 2,3- dimethyl-3-butenyl, 3,3-dimethyl-l-butenyl, 3,3-dimethyl-2-butenyl, 1 -ethyl- 1-butenyl, 1-ethyl- 2-butenyl, l-ethyl-3-butenyl, 2-ethyl- 1-butenyl, 2-ethyl-2-butenyl, 2-ethyl-3-butenyl, 1,1,2- trimethyl-2-propenyl, 1 -ethyl- l-methyl-2-propenyl, l-ethyl-2-m ethyl- 1 -propenyl, and 1 -ethyl -2-methyl-2-propenyl. The term “vinyl” refers to a group having the structure -CH=CH2; 1- propenyl refers to a group with the structure-CH=CH-CH3; and 2- propenyl refers to a group with the structure -CH2-CH=CH2. Asymmetric structures such as (Z1Z2)C=C(Z3Z4) are intended to include both the E and Z isomers. This can be presumed in structural formulae herein wherein an asymmetric alkene is present, or it can be explicitly indicated by the bond symbol C=C.

[0068] "Alkoxy" refers to an alkyl group as defined above with the indicated number of carbon atoms attached through an oxygen bridge. Examples of alkoxy include, but are not limited to, methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, s-butoxy, t-butoxy, n- pentoxy, and s- pentoxy. Preferred alkoxy groups are methoxy, ethoxy, n-propoxy, i- propoxy, n-butoxy, s- butoxy, t-butoxy.

[0069] The terms "halogen" and "halo" refer to fluorine, chlorine, bromine, and iodine.

[0070] The term "substituted" refers to a molecule wherein at least one hydrogen atom is replaced with a substituent. When substituted, one or more of the groups are "substituents." The molecule can be multiply substituted. In the case of an oxo substituent ("=O"), two hydrogen atoms are replaced. Example substituents within this context can include halogen, hydroxy, alkyl, alkoxy, nitro, cyano, oxo, carbocyclyl, carbocycloalkyl, heterocarbocyclyl, heterocarbocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, -NRaRb, -NRaC(=O)Rb, - NRaC(=O)NRaNRb, -NRaC(=O)ORb, - NRaSChRb, -C(=O)Ra, -C(=O)ORa, -C(=O)NRaRb, - OC(=O)NRaRb, -ORa, -SRa, -SORa, - S(=O)2Ra, -OS(=O)2Ra and -S(=O)2ORa. Ra and Rb in this context can be the same or different and independently hydrogen, halogen hydroxyl, alkyl, alkoxy, alkyl, amino, alkylamino, dialkylamino, carbocyclyl, carbocycloalkyl, heterocarbocyclyl, heterocarbocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl.

[0071] The term "optionally substituted," as used herein, means that substitution with an additional group is optional and therefore it is possible for the designated atom to be unsubstituted. Thus, by use of the term “optionally substituted” the disclosure includes examples where the group is substituted and examples where it is not.

[0072] Reference will now be made in detail to specific aspects of the disclosed materials, compounds, compositions, articles, and methods, examples of which are illustrated in the accompanying Examples and Figures.

[0073] In some embodiment, a method is described for forming alloy nanoparticle-decorated catalytic surface. The method comprises providing a conductive metal oxide substrate to a system; and applying a cathodic voltage to the working electrode.

[0074] The described method results in the reduction of the conductive metal oxide substrate and exsolution of metal or metal alloy nanoparticles onto the surface of the reduced substrate, which forms the metal or metal alloy nanoparticle-decorated catalytic surface.

[0075] In some examples, the cathodic voltage is applied until a plurality of nanoparticles are exsolved onto a surface of the conductive metal oxide substrate. The duration and magnitude of the applied voltage is varied to achieve a desired nanoparticle-decorated catalytic surface.

[0076] For example, the applied voltage is varied until a desired density of nanoparticles is achieved, where increasing the duration of the applied voltage results in an increased density and / or size of the plurality of nanoparticles. In the example the voltage may be applied for about 1 to 2 minutes, about 1 to 3 minutes, about 1 to 4 minutes, about 1 to 5 minutes, about 1 or more minutes, about 2 to 3 minutes, about 2 to 4 minutes, about 2 to 5 minutes, about 2 or more minutes, about 3 to 4 minutes, about 3 to 5 minutes, or about 3 or more minutes.

[0077] In other examples, the plurality of metal nanoparticles includes metal or metal alloy nanoparticles. The applied voltage is varied until a desired metal or metal alloy is formed (e.g. varying the ratio of metals in the alloy).

[0078] In the described method, the applied cathodic voltage may be about -1 volts. In an example, the applied voltage induces reduction of the conductive metal oxide substrate, for example, the cathodic voltage may be about -0.1 V, about -0.2 V, about -0.3 V, about -0.4 V, about -0.5 V, about -0.6 V, about -0.7 V, about -0.8 V, about -0.9 V, about -1.0 V, about -1.1 V, about -1.2 V, or about -1.3 V. The cathodic voltage may be chosen and applied until a plurality of nanoparticles are exsolved onto the surface of the perovskite-type substrate.

[0079] It will be shown in the examples that the nanoparticles nucleate at oxygen vacancies on the surface of the conductive metal oxide substrate.

[0080] In some aspects, the conductive metal oxide substrate includes a mixed metal oxide. In some aspects, the mixed metal oxide is a perovskite, complex perovskite (e.g., double perovskite), or Ruddlesden-Popper phase (layered perovskite).

[0081] In some aspects, the mixed metal oxide includes the perovskite having a formula of (AZ)a(XY)O3, wherein a is 1 or less than 1; A and Z are the same or different elements; and X and Y are the same or different elements. In some aspects, the plurality of nanoparticles includes X, Y, or alloys thereof. It is contemplated that the perovskite may be doped with catalytically active elements in the X / Y positions (e.g. B-site of perovskite), which may be part of the metal alloy nanoparticles.

[0082] For example, A and Z are chosen from La, Sr, Pr, Ba, or Ca. For example, X and Y are chosen from Co, Fe, Ni, Mn, Ti, Re, Cu, Cr, or Mo . For example, a is about 1, about 0.9, about 0.8, about 0.7, or about 0.6.

[0083] In some examples, the perovskite has a formula of (LaSr)(CoFe)O3. In some examples, the perovskite has a formula of Lao.7Sro.2Coo.2Feo.8O3, and the plurality of metal or metal alloy nanoparticles include Co metal and / or CoFe metal alloy.

[0084] In some examples, the perovskite has a formula of (LaSr)(NiFe)O3. In some examples, the perovskite has a formula of Lao.7Sro.2Nio.2Feo.8O3, and the plurality of metal or metal alloy nanoparticles include Ni and / or NiFe alloy.

[0085] In some aspects, a nano-particle decorated catalytic surface is made using the disclosed method. The electrode composition includes a reduced mixed metal oxide substrate and a plurality of metal or metal alloy nanoparticles on a surface of the reduced mixed metal oxide substrate, wherein at least one metal is common in both the reduced mixed metal oxide substrate and the plurality of metal of metal alloy nanoparticles.

[0086] In some aspects, the reduced mixed metal substrate is a perovskite, complex perovskite (e.g., double perovskite), or Ruddlesden-Popper phase (layered perovskite).

[0087] In some aspects, the perovskite has a formula of (AZ)I-&(XY)O3, where b is less than 0.1; A and Z are the same or different elements; and X and Y are the same or different elements. In some aspects, the plurality of nanoparticles includes X, Y, or alloys thereof. It is contemplated that the perovskite may be doped with catalytically active elements in the X / Y positions (e.g. B- site of perovskite), which may be part of the metal alloy nanoparticles.

[0088] For example, A and Z are chosen from La, Sr, Pr, Ba, or Ca. For example, X and Y are chosen from Co, Fe, Ni, Mn, Ti, Re, Cu, Cr, or Mo. For example, a is about 1, about 0.9, about 0.8, about 0.7, or about 0.6. For example, b is about 0.1, about 0.09, about 0.08, about 0.07, about 0.06, about 0.05, about 0.04, about 0.03, about 0.02, about 0.01, or less.

[0089] In some examples, A is La, Z is Sr, X is Co, Y is Fe. In some examples, the perovskite has a formula of Lao.7Sro.2Coo.2Feo.8O3, and the plurality of metal or metal alloy nanoparticles include Co and / or CoFe alloy.

[0090] In some examples, A is La, Z is Sr, X is Ni, Y is Fe. In some examples, the perovskite has a formula of Lao.7Sro.2Nio.2Feo.8O3, and the plurality of metal or metal alloy nanoparticles include Ni and / or NiFe alloy.

[0091] In some aspects, the nano-particle decorated catalytic surface is an electrocatalyst. For example, the nano-particle decorated catalytic surface is an anode catalyst for an oxidation reaction or as a cathode for a reduction reaction.

[0092] In yet another embodiment, an electrode composition comprising a reduced perovskite substrate and a plurality of metal or metal alloy nanoparticles on a surface of the reduced perovskite substrate, wherein at least one metal is common in both the reduced perovskite substrate and the plurality of metal or metal alloy nanoparticles.

[0093] In some aspects, the reduced perovskite substrate has a formula of (AZ)I-&(XY)O3, where b is less than 0.1; A and Z are the same or different elements; and X and Y are the same or different elements. In some aspects, the plurality of metal or metal alloy nanoparticles includes X, Y, or alloys thereof. It is contemplated that the perovskite may be doped with catalytically active elements in the X / Y positions (e.g. B-site of perovskite), which may be part of the metal alloy nanoparticles.

[0094] For example, A and Z are chosen from La, Sr, Pr, Ba, or Ca. For example, X and Y are chosen from Co, Fe, Mn, Ti, Re, Cu, Cr, or Mo. For example, a is about 1, about 0.9, about 0.8, about 0.7, or about 0.6. For example, b is about 0.1, about 0.09, about 0.08, about 0.07, about 0.06, about 0.05, about 0.04, about 0.03, about 0.02, about 0.01, or less.

[0095] In some examples, A is La, Z is Sr, X is Co, Y is Fe. In some examples, the perovskite has a formula of Lao.7Sro.2Coo.2Feo.8O3, and the plurality of metal or metal alloy nanoparticles include Co and / or CoFe alloy.

[0096] The described electrode composition may be used as an electrocatalyst. For example, the electrode composition is used as an anode catalyst for an oxidation reaction or as a cathode for a reduction reaction.

[0097] In some aspects, the electrode composition is part of a solid oxide electrochemical cell.

[0098] In other embodiments, a method of electrocatalysis is disclosed. The method includes (a) providing the reduced perovskite substrate having a general formula of (AZ)(XY)O3 and plurality of metal or metal alloy nanoparticles on a surface of the reduced perovskite substrate to an electrochemical system; (b) providing a current to the electrochemical system; and (c) streaming a feed gas into the electrochemical system.

[0099] In some aspects, providing a current includes providing an anodic current, and wherein streaming feed gas includes methane, C2-C6 alkanes, C2-C6 alkenes, or combinations thereof.

[0100] In other aspects, providing a current includes providing a cathodic current, and wherein streaming feed gas includes N0x, CO2, H2O, O2 or combinations thereof.

[0101] In the described method, the applied cathodic voltage may be about -1 volts. In an example, the cathodic voltage may be about -0.1 V, about -0.2 V, about -0.3 V, about -0.4 V, about -0.5 V, about -0.6 V, about -0.7 V, about -0.8 V, about -0.9 V, about -1.0 V, about -1.1 V, about -1.2 V, or about -1.3 V.

[0102] In some aspects, the reduced perovskite substrate has a formula of (AZ)I-&(XY)O3, where b is less than 0.1; A and Z are the same or different elements; and X and Y are the same or different elements. In some aspects, the plurality of metal or metal alloy nanoparticles includes X, Y, or alloys thereof. It is contemplated that the perovskite may be doped with catalytically active elements in the X / Y positions (e.g. B-site of perovskite), which may be part of the metal alloy nanoparticles.

[0103] For example, A and Z are chosen from La, Sr, Pr, Ba, or Ca. For example, X and Y are chosen from Co, Fe, Mn, Ti, Re, Cu, Cr, or Mo. For example, a is about 1, about 0.9, about 0.8, about 0.7, or about 0.6. For example, b is about 0.1, about 0.09, about 0.08, about 0.07, about 0.06, about 0.05, about 0.04, about 0.03, about 0.02, about 0.01, or less.

[0104] In some examples, A is La, Z is Sr, X is Co, Y is Fe. In some examples, the perovskite has a formula of Lao.7Sro.2Coo.2Feo.8O3, and the plurality of metal or metal alloy nanoparticles include Co and / or CoFe alloy.

[0105] In some examples, a method of electrocatalytic oxidative dehydrogenation of light alkanes and / or oxidative coupling of methane is described. The method may comprise providing a reduced (LaSr)(CoFe)O3 perovskite substrate and Co metal and / or CoFe metal alloy nanoparticles on the surface of the reduced (LaSr)(CoFe)O3 perovskite substrate to an electrochemical system, providing an anodic current to the electrochemical system, and streaming feed gas into the electrochemical system. In some examples, the method may comprise providing a reduced Lao.7Sro.2Coo.2Feo.8O3 perovskite substrate and Co metal and / or CoFe metal alloy nanoparticles on the surface of the reduced Lao.7Sro.2Coo.2Feo.8O3 perovskite substrate to the electrochemical system.

[0106] It is contemplated that for different perovskite-type substrates, the temperature of the system can be adjusted accordingly according to the described method.EXAMPLES

[0107] While testing the activity / selectivity of the lanthanum ferrite-type perovskite oxide anodes in electrocatalytic OCM process, we observed that the selectivity for coupling products (hydrocarbons with 2 or more carbons) increased from 20% to 53% over the first three hours of the reaction with a significant decrease in COx selectivity (Fig. 14). The characterization of the post-reaction button cell showed that B-site metal nanoparticles migrated from the perovskite structure to the anode surface and created an electrode surface decorated with metal -alloy nanoparticles (Figs. 16A-16C) under a reducing atmosphere and at high temperatures.

[0108] The migration of B-site metal / alloy nanoparticles from a parent perovskite structure is called “exsolution” also known as “solid-state recrystallization” or “intelligent selfregeneration”. The exsolution process leads to a controlled formation of metal / alloy nanoparticles with a uniform distribution. The exsolved particles are anchored to the parent perovskite surface, providing excellent mechanical and thermal stability by preventing agglomeration and sintering. Exsolution can occur under a reducing atmosphere and at high temperatures.

[0109] The significant selectivity increase we observed was due to the exsolution of metal / alloy nano particles from the bulk perovskite structure. Although exsolution of nano particles can happen in reducing environments and high temperatures (thermochemical exsolution), this process is slow and takes a long time.

[0110] In order to reduce the time needed to create a nanoparticle-decorated electrode surface, we adopted an alternative route which is “exsolution via cathodic voltage” (electrochemical exsolution) under an inert, reducing, or oxidizing atmosphere. In this process, a cathodic voltage / current is applied to the anode or cathode catalyst to force the formation of oxygen vacancies in the crystalline structure of the perovskite oxides which are considered as the nucleation sites for B-site metal / alloy nanoparticle exsolution.[OHl] The preliminary results are very encouraging and prove that the electrochemical exsolution concept is achievable. The scanning electron microscopy (SEM) images of the posttreatment Lao.7Sro.2Coo.2Feo.8O3 (LSCF) electrodes showed that the electrochemical exsolution of Co metal CoFe alloy nanoparticles happens in 1-2 minutes when a cathodic voltage of -IV is applied under helium at 700 °C (Figs. 13A-13B).

[0112] This invention holds a significant potential to make the electrocatalytic production routes a promising alternative to the conventional processes for the production of value-addedchemicals by enhancing the selectivity of the process towards the desired products using green electricity derived from renewable energy sources such as wind and solar.Example 1: Electrochemical exsolution of metal nanoparticles from perovskite oxide upon electrolysis.

[0113] This study discloses the electrochemical reduction of LSCF perovskite during electrolysis, aiming to understand the exsolution of metal nanoparticles. The exsolution of metal nanoparticles from perovskite electrodes can significantly enhance their electrochemical performance in electrolysis. By applying cathodic polarization to the perovskite oxide electrode, the exsolution process was shown to be electrochemically induced within a few minutes. Additionally, a user-designed X-ray absorption spectroscopy operando cell was employed to analyze the edge energy change of the B-site atoms during electrolysis. The electrochemical reduction of perovskite and the subsequent exsolution of the B-site metal nanoparticles were investigated by scanning the cell voltage, providing an understanding of the electrochemical behavior during electrolysis. The electrochemical switching point, characterized by a decrease in the incremental area-specific resistance, was identified. This study offers insights into the electrochemical exsolution process of metal nanoparticles from perovskite oxide electrodes.

[0114] Introduction. ABO3 perovskite oxides are widely used for energy and chemical conversion processes as mixed ionic and electronic conductors (MIECs) in solid oxide electrocatalytic cells. These materials have garnered significant attention in the scientific community due to their exceptional redox properties, which are linked to their tunable oxygen ion mobility, high stability under both oxidizing and reducing environments, and resistance to coke formation [1-4], To improve the electronic and ionic conductivity, stability, or activity of these materials, many studies have focused on doping the A or B sites of perovskite oxides.

[0115] It has been proven that a controlled reducing environment at elevated temperatures may be used to induce the exsolution of B-site atoms onto the surface [5-13], During thermochemical exsolution, B-site transition metal atoms undergo reduction and migrate towards the surface, where they segregate into nanoparticles. By contrast to the physical deposition techniques, exsolution results in the formation of well-distributed and uniform sized nanoparticles that adhere firmly to the parent perovskite surface with superior thermochemical stability [14-17],

[0116] It is noteworthy that exsolution of metal nanoparticles can be triggered by applying cathodic polarization to the perovskite oxide electrode. It has been also reported that the electrochemical exsolution could be induced by anodic polarization due to incomplete oxidationand local charge transfer [18-20], The electrochemical exsolution is time efficient given that it can be achieved within several minutes by applying polarization. This is why the electrochemical exsolution is referred to as “electrochemical switching”

[0019] , In contrast, the thermal exsolution under reducing atmosphere normally requires several hours of treatment due to the slow diffusion and low driving force (oc(knT) / 2 In Pm)

[0021] , However, studies on the electrochemical exsolution are limited so far because the process requires high temperature, electrical lead connections, controlled gas environment, which make in situ and ex situ observation difficult.

[0117] The goal of the present work is to provide insights into the electrochemical reduction of the working electrode and the resultant exsolution during the electrolysis process. The electrochemical switching point was presented with area-specific resistance by scanning the cell voltage. Xray absorption spectroscopy data was obtained to evaluate the elemental edge energy during electrolysis using a user-designed operando cell.

[0118] Experimental

[0119] Preparation of catalysts. Lao.7Sro.2Coo.2Feo.8O3 (LSCF) perovskite, as reported in our earlier publications, was synthesized by a citric acid ethylenediaminetetraacetic acid complexation method

[0022] , A mixture of metal-nitrate salts dissolved in a stoichiometric amount in 100 ml of deionized water. Ethylenediaminetetraacetic acid was added to the solution with a 1 : 1 molar ratio to the total metal ions. Ammonium hydroxide was also added to the solution to stabilize pH at 6 at 60 °C. After adding ethylene glycol and citric acid, the solution was heated to 90 °C while maintaining pH of 6. After evaporation, the gel formed was dried at 150 °C overnight. The dried black powder was calcined at 1000 °C for 5 hours to produce crystalline LSCF.

[0120] Catalyst characterization. X-ray diffraction patterns were acquired using a Bruker D8 Lead Xray powder diffractometer equipped with a Cu Ka X-ray source. The voltage and current in the generator were 40 kV and 40 mA, respectively. Scans were conducted with a 29 range from 20° to 60°, and the step size was set at 0.019° per 0.5 seconds. Crystallographic open database (COD) was used to identify phases

[0023] , Miller indices were calculated from the XRD patterns by Rietveld refinement method using General Structure Analysis System-2 (GSAS-2) software

[0024] , In-situ XRD was performed with an Anton Paar HTK1200 oven at temperature ranges of 30 °C to 800 °C.

[0121] Operando X-ray Absorption Near Edge Spectroscopy (XANES) was carried out in fluorescence mode at Sector 10-ID of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source (APS, Argonne National Laboratory). Detailed information about the user-made operando cell design can be found in a previous study

[0025] , The button cell was located at the center of the F-factor at 45° angle to the incident X-ray beam. The inlet stream was 50 ccm of 3% H2O / N2 at 650 °C. The current was applied using Keithley 6220. The collected XANES data was processed using Athena software

[0026] ,

[0122] The visual examination of the nanoparticles was conducted using a scanning transmission electron microscope (STEM), a TECNAI F20 equipped with an energy dispersive X-ray spectrometer (EDS). Scanning electron microscope images (SEM) were taken using FEI Apreo instrument with an EDS. SEM images of the LSCF cathode after cathodic polarization were captured to examine the surface of LSCF. The reactor was filled with helium during the cooling process.

[0123] Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data was acquired using Thermoelectron Nicolet 6700 FTIR. In a 1 :20 ratio, powder samples were diluted with potassium bromide (KBr). The samples were pretreated with He at 450 °C prior to temperatureprogrammed desorption (TPD) -DRIFTS in order to desorb any additional adsorbate. DRIFTS spectra were collected under He at various temperatures after CO2 or H2O exposure at 50 °C.

[0124] The electrical conductivity of the catalyst samples was evaluated using pellets that were prepared by compressing the powder using a hydraulic press, followed by sintering at 1300 °C for five hours. Four gold wires were affixed to the pellet using gold paste at four designated points. A Keithley 6220 current source was connected to two of the wires to provide current, while a Keithley 6182 sensitive nanovoltmeter was connected to the other two wires to measure the resulting voltages. In situ measurement was performed under 50 ccm of 5% H2 / He atmosphere.

[0125] Evaluation of electrocatalytic performance. The electrochemical button cells were made by screen-printing on commercial yttria-stabilized zirconia (YSZ) electrolyte (25 mm diameter, 125 pm thickness, Nextech Materials). The cathode side was coated with gadolinium-doped ceria (GDC) interlayer that was sintered under air at 1400 °C for two hours. LSCF powder was printed with a 0.28 cm2surface area on the GDC interlayer and sintered at 1200 °C under air for two hours. A mixture of (La0.80Sr0.20)0.95Mn03- (Y203)0.08(Zr02)0.92 (LSM-YSZ;Nextech Materials) in a weight ratio of 1 : 1 was screen-printed on the anode side and sintered at1200 °C under air for two hours. A gold wire and platinum paste were used to attach the gold mesh to the electrodes. A total flow rate of 60 ccm was used for the electrocatalytic performance test with a feed gas concentration of 5% H2O, 5% CO2, and balanced He at the cathode. Electrochemical impedance spectra (EIS) were collected within the frequency range of 1 MHz to 10 mHz. The current was scanned by increasing the cell voltage from 0 to -2 V at different scan rates ranging 0.1-100 mV s ' . The equivalent circuit model of RS(RPIQI) (RP2Q2) was used, which is composed of an ohmic resistance RS and two polarization resistances (RP) and constant phase elements (Q) in parallel. Gas products were quantified with an on-line gas chromatograph (Shimadzu 2014) equipped with a pulse discharge helium ionization detector (PDHID).

[0126] Results and Discussion

[0127] Electrochemical reduction ofLSCF. Electrochemical reduction of the LSCF perovskite and co-electrolysis of H2O and CO2 were conducted in the experimental setup depicted in Fig. 1. The top surface and cross-section images of the LSCF button cell are shown in Figs. 2A-2B. The LSCF cathode was well densified to provide ionic and electrical conductivity while also having enough porosity to facilitate the transport of reactants. The gadolinium-doped ceria (GDC) layer, which has a thickness of 4.6 pm, was placed between the LSCF cathode and YSZ electrolyte to prevent any reaction between the two materials.

[0128] In order to isolate the effect of cathodic polarization from gas phase oxidation / reduction, the electrochemical reduction of the LSCF electrode was conducted under helium at 700 °C. As no gas phase reactant was reduced by the applied current, the electrons were assumed to participate in reducing the lattice oxygen within the LSCF perovskite electrode. Scanning electron microscope (SEM) images in Figs. 3A-3C illustrate the top surface of the LSCF electrode after applying a cathodic potential of -1.3 V for 0.5-2 min. Nanoparticles were found in secondary electron SEM image even after 0.5 min of polarization (Fig. 3A (top)). The backscattered electron SEM image (Fig. 3 A (bottom)) revealed compositional differences between the exsolved nanoparticles and the parent perovskite. Over time, the nanoparticles grew in size and exhibited uniformity, covering the entire perovskite surface after two minutes of potential application. In Fig. 3D, the Scanning Transmission Electron Microscope (STEM) image and corresponding elemental mapping for different elements are presented. Fig. 3D (dl) shows the STEM image of an exsolved metal nanoparticle, protruding to the right from the bulk, a result of subjecting the sample to a cathodic polarization of -1.3 V at 700 °C for 2 min. In Fig. 3D (d2, d3, d4, d5, d6), the elemental mapping of lanthanum (La), strontium (Sr), iron (Fe),cobalt (Co), and oxygen (O), respectively, are presented. The figures clearly illustrate that cobalt is the sole element detected on the exsolved nanoparticle, signifying the presence of pure cobalt metal. This result unequivocally confirms the selective exsolution of cobalt nanoparticles on the LSCF surface.

[0129] The exsolved nanoparticles emerged following a cathodic polarization of -1.3 V at 700 °C for two minutes. Notably, the elemental mapping reveals decreased counts of O, Fe, and La on the nanoparticles, while Co remains present on the particle surface. This observation strongly supports the phenomenon of selective exsolution of cobalt nanoparticles on the LSCF perovskite surface. The exsolution of CoFe bimetallic nanoparticles investigated in our previous study required a high temperature (850 °C), reducing environment (5% H2 / N2), and long duration time (three hours) to acquire the similar size of nanoparticles with this study

[0027] , Notably, the fast process of electrochemical reduction relies on charge transfer throughout the perovskite structure, contrasting with the slow diffusion of metal atoms and relatively low driving force (oc(knT) / 2 In Pm) for thermochemical exsolution

[0021] , This process can be understood through the following expressions based on Kroger-Vink notation:

[0130] Thermochemical exsolution

[0131] Electrochemical exsolution

[0132] where Bl indicates a transition metal occupying the B-site of the perovskite lattice, O denotes a lattice oxygen in the perovskite structure, V™ denotes a B-site vacancy, Fodenotes an oxygen vacancy, and B surface denotes exsolved transition metal.

[0133] Applied electrons directly result in the lattice oxygen being ionized and oxygen vacancy being formed and eventually, the B-site transition metal being reduced. The electrochemical exsolution of metal nanoparticles proved to be more efficient than thermochemical reduction. Additionally, this study reveals that the exsolved metal nanoparticles obtained here are larger than those reported in the literature. For instance, Ni nanoparticles exsolved from Lao.43Cao.37Nio.o6Tio.9403-Y measured 15 nm after applying 2 V of cathodic polarization for 150 seconds at 900 °C, while exsolved Ni nanoparticles from Lao.4Cao.4Tio.88Feo.o6Nio.o603-8 ranged from 15 to 42 nm after applying 5 V of cathodic or anodic bipolar shock at 800 °C [18,19], The considerable size of the exsolved particles (30-200 nm) observed in this study can be attributedto the 10% A-site deficient stoichiometry of the Lao.7Sro.2Coo.2Feo.8O3 perovskite and the B-site being doped 20% cobalt.

[0134] Operando XANES analysis. An operando characterization of the LSCF electrode under the electrochemical reaction conditions was conducted to investigate the oxidation state of Co and Fe atoms. The experimental setup utilized a user-designed operando cell, as previously reported, connected to a button cell with a ring-shaped current collector

[0025] , This arrangement allows for the central area of the electrode to be exposed to the incident X-ray beam at a 45° angle, as depicted in Fig. 4 A. The operando X-ray absorption near-edge structure (XANES) measurements were carried out in 3% H2O / N2. Notably, the Co K-edge energy on the LSCF electrode exhibited a shift to lower values as the temperature increased from room temperature to 650 °C, while the Fe K-edge energy remained unchanged. The absence of pre-edge features associated with metallic Fe and Co atoms in the XANES indicated that these atoms maintained their oxidized forms under open-circuit voltage conditions at elevated temperatures. The Fe XANES showed no significant changes following the application of a 5 mA cathodic current. In contrast, the Co XANES displayed a shift to lower energy and revealed the appearance of preedge features associated with metallic cobalt. This result is consistent with the observations from STEM image with elemental mapping presented in Fig. 3D. The accurate edge energy calculation for the XANES was challenging due to noise caused by the applied current.Considering that the penetration depth of the incident X-ray beam is approximately 5 pm for Fe and Co atoms with edge energies of 7112 eV and 7717 eV, respectively, these findings suggest that the electrochemical reduction of Co atoms on the LSCF cathode is not limited to the surface but also occurs within the bulk, aligning with previous literature results

[0018] , It is noteworthy that although the current collector was attached only to the perimeter of the LSCF electrode in a ring shape, electrochemical reduction was observed even in regions where direct contact with the current collector was absent. This observation implies that electrochemical reduction of LSCF may occur throughout the entire electrode.

[0135] Electrochemical behavior during electrolysis. The electrochemical reduction of the LSCF electrode and the resulting exsolution can be influenced by the presence of oxidant gases such as H2O or CO2 during electrolysis. This is due to the potential re-oxidation of the reduced LSCF and the exsolved metal nanoparticles. To investigate the electrochemical behavior of the LSCF cell under electrolysis conditions (5% H2O, 5% CO2, and balanced He), an analysis of the relationship between cell voltage and current density was conducted.

[0136] The measurement of current density was performed by increasing the cell voltage at various scan rates ranging from 0.1 to 200 mV s ' . Fig. 5 shows that higher current densities are observed at faster scan rates, as expected. Another important observation from the polarization curves is the fact that at each scan rate, there is an inflection point, i.e., a point where the slope changes. This point may represent electrochemical reduction of the LSCF and the progression of the exsolution. This finding suggests that the exsolution of metal particles likely takes place at specific voltage values, thereby influencing the electrochemical behavior of the cell. Such electrochemical behavior has been reported as an activation process of the electrode in the literature

[0028] ,

[0137] To provide a detailed explanation of the electrochemical behavior, the area-specific resistance (ASR) was derived from the I-V curve at 700 °C and a scan rate of 1 mV s ', as shown in Fig. 6. The ASR showed a decline, an increase, and a subsequent decline, which can be explained by dividing the process into three intervals accordingly.

[0138] At lower cell voltage (left side of Fig. 6), the LSCF electrode is likely in a fully oxidized form. At this point, the current density is low and ASR is high. The slight decrease in the ASR with current density is because of the decrease in polarization resistance due to increased voltage. The reduction of H2O and / or CO2 occurs at a faster rate compared to the supply of electrons. This suggests that the oxidation of the LSCF electrode by oxidants is faster than its reduction by the supplied electrons. SEM images of the LSCF electrode reveal a clean surface without any exsolved nanoparticles after applying a polarization of -0.25 V for 10 minutes under electrolysis conditions (Fig. 7A). Thus, the LSCF electrode maintains a fully oxidized state as depicted in Fig. 8.

[0139] In the region shown in yellow, the LSCF electrode undergoes reduction due to the applied electrons. It is noteworthy that despite an increase in cell voltage, the current density does not increase during this stage, while the ASR substantially increases up to 77.6 cm2. This increase in ASR can be attributed to the decreased electrical conductivity of the LSCF electrode. LSCF perovskite is a -type electric conductor with charge carriers consisting of Fe2+ / 3+ / 4+and Co3+ / 4+ion pairs

[0027] , When the supply of electrons exceeds the reduction of H2O or CO2, the surplus electrons begin to reduce the lattice oxygen and cobalt in the LSCF perovskite electrode. Then, the concentration of charge carriers decreases and consequently electrical conductivity of LSCF perovskite reduces, resulting in an increased ASR. The electrical conductivity of the electrochemically reduced LSCF is challenging to measure; however, the conductivity of the thermochemically reduced LSCF was measured and found to decrease from 161.1 S cm1underoxidizing atmosphere to 4.5 S cm1under 5% H2 / N2 atmosphere at 850 °C. SEM images reveal the presence of exsolved metal nanoparticles on the LSCF electrode after cathodic polarization of -0.5 V for 10 minutes through secondary and backscattered electron images in Fig. 7B. These nanoparticles are predominantly small, and the distinction in brightness between the nanoparticles and the parent LSCF perovskite is not distinctive at certain points, particularly in larger particles, as indicated by the red arrows, even in the presence of helium as a protective gas during the cooling process. This observation suggests that the exsolved nanoparticles may undergo oxidation by H2O and / or CO2 on the electrode surface as depicted in Fig. 8, which explains the increase in the ASR despite the presence of exsolved nanoparticles during this stage.

[0140] The electrochemical switching point is identified as the point (-0.98 V) where the trend of increasing ASR reverses and starts to a decrease, as shown in Fig. 6. At this point, the supply of electrons surpasses the oxidation of LSCF and exsolved cobalt nanoparticles by H2O and / or CO2. Consequently, exsolved nanoparticles begin to maintain their metallic form against oxidation by H2O or CO2 as shown in Fig. 8.

[0141] In the third region in Fig. 6 shown on the far right panel, the LSCF electrode is in a fully reduced state, when the cell voltage is above -0.98 V, with numerous exsolved metal nanoparticles, which improve the electrical conductivity of the electrode. At this stage, the electron supply is faster than the reduction of H2O and CO2. As a result, oxygen from the reduction of H2O and CO2 instantaneously converts into oxygen ions rather than oxidizes the exsolved cobalt nanoparticles or the LSCF perovskite, as illustrated in Fig. 8. SEM secondary and backscattered electron images in Fig. 7C display large metal nanoparticles (-200 nm) with distinct brightness. The LSCF perovskite electrode becomes decorated with abundant metal nanoparticles that provide metallic conductivity. The cobalt nanoparticles, exhibiting conductivity two orders of magnitude greater than LSCF, exsolved on the LSCF surface may augment the electrode’s conductivity, thereby leading to a reduction in the ASR

[0029] , The substantial size and the sustained metallic state of the exsolved Co particles are the key factors in enhancing the ASR during the reduced stage, a stark contrast to the reducing stage. This outcome contrasts with the observed decrease in the electrical conductivity of the thermochemically reduced LSCF perovskite. To examine the difference between electrochemical and thermochemical reduction, a SEM image of LSCF perovskite cathode, thermochemically reduced under a 5% H2 / N2 atmosphere at 850 °C for a duration time of two hours, was taken. The exsolved particles depicted in the SEM image, with an approximate sizeof 60 nm and a sparse number density, may not have a substantial contribution to the conductivity of the electrode.

[0142] The electrochemical switching explored in this study diverges from the activation process typically seen in SOFCs / SOECs that contain pre-deposited or pre-exsolved transition metal (such as nickel) nanoparticles, for various reasons. Firstly, electrochemical exsolution can be accomplished within mere minutes, a stark contrast to the thermochemical exsolution process which requires several hours, followed by an activation process. Secondly, as the cell voltage increases to the electrochemical switching point, the LSCF cathode undergoes deactivation due to the reduction of the LSCF cathode (and the decrease in the density of the charge carriers) and the consequent decrease in its electrical conductivity caused by cathodic polarization. Thirdly, the change in instantaneous resistance at the electrochemical switching point is much more pronounced compared to the activation process. The instantaneous resistance goes up to 1477 cm2at -0.85 V, then diminishes to 1.7 cm2at -2 V. This substantial fluctuation in instantaneous resistance is a unique characteristic that sets the electrochemical exsolution and switching apart from the activation process described in the literature

[0030] ,

[0143] The effect of temperature on the electrochemical behavior of the LSCF cell was examined over a temperature range of 700 °C to 850 °C during electrolysis. Fig. 9A depicts the relationship between current density and cell voltage at different temperatures, using a scan rate of 1 mV s '. Notably, higher temperatures corresponded to elevated current densities due to the enhanced electrolysis of H2O and CO2. To focus on a specific voltage range following a flat region, a zoomed graph in the inset of Fig. 9A is presented, capturing the initial take-off point. Interestingly, the take-off cell voltage demonstrated a decrease with increasing temperature. This trend is also evident in the ASR graph presented in Fig. 9B. As temperature increased, ASRs exhibited a decline, while the electrochemical switching points occurred at lower cell voltages. This behavior can be attributed to the formation of exsolved metal nanoparticles. As the temperature rises, the rate of thermochemical oxidation of metal nanoparticles and LSCF perovskite by gas phase oxidants, H2O and CO2, accelerates. Simultaneously, the electrochemical reduction of LSCF and the exsolution of metal nanoparticles by supplied electrons also occur at a faster rate due to the reduced resistance. Consequently, the shift to lower voltage in the electrochemical switching point with increasing temperature suggests that the impact of temperature on enhancing the electrochemical reduction outweighs its effect on promoting thermochemical oxidation.

[0144] Fig. 9C presents Arrhenius plots illustrating the activation energy for SOECs at various cell voltages. The calculated activation energies for ASRs at -0.5 V and -0.75 V, which lie before the electrochemical switching point, were found to be 0.35 eV and 0.34 eV, respectively. The similar activation energy values across the increasing cell voltage range suggest that gas diffusion and the adsorption of gas-phase oxidants are the primary factors influencing the ASRs at these voltage levels, where electrochemical switching has not yet occurred. However, at -1.25 V, the activation energy for ASRs notably increased to 1.13 eV. Subsequently, the activation energy values gradually decreased with further increases in cell voltage, measuring 0.82 eV at - 1.50 V, 0.66 eV at -1.75 V, and 0.57 eV at -2.00 V. It should be noted that charge transfer resistance is a thermal activation process, whereas gas diffusion and adsorption are predominantly pressure-dependent, rather than being influenced by temperature [31,32], This implies that at lower voltages with low activation energy, gas diffusion and adsorption may represent the primary barriers to the electrolysis of H2O and CO2. The activation energy of 1.13 eV observed at a cell voltage of -1.25 V, the highest among the ASRs, can be attributed to the presence of electrochemically reduced LSCF surfaces containing metal nanoparticles, which improve the adsorption of gas-phase oxidants and enhance the electrical conductivity of the electrode. Given the high activation energy (1.13 eV) observed at the -1.25 V cell voltage, it can be inferred that the primary barrier to electrolysis is the charge transfer. However, further reduction of the LSCF electrode at higher cell voltages and the resulting exsolution of metal nanoparticles provide metallic conductivity at higher cell voltages, thereby causing a decrease in activation energy.

[0145] Electrolysis of H2O and CO2. The electrocatalytic performance of LSCF and electrochemically reduced LSCF (eRed-LSCF) for the electrolysis of H2O, CO2, and H2O / CO2 was investigated by comparing Nyquist plots under open circuit voltage. The resulting polarization resistances (Rp) were also determined. eRed-LSCF was prepared by applying -2 V of cathodic polarization for two minutes at 850 °C. It should be noted that 850 °C represents the maximum electrolysis temperature utilized in this study and extending the duration beyond two minute did not yield any further improvements in resistance. The exsolved particles on eRed- LSCF exhibit approximately 300 nm of size and remarkably high number density. Fig. 10A presents Nyquist plots obtained under open circuit voltage for the electrolysis of H2O, CO2, and H2O / CO2 on both LSCF and eRed-LSCF electrodes. The impedance spectra were fitted with three different resistances, including the ohmic resistance (Rs) and the polarization resistance (Rp). The Rs value, which corresponds to the high-frequency intercepts, did not show significantdifference for all three reactions regardless of the LSCF cathode reduction. This value can be attributed to factors such as the electrolyte thickness, contact resistances, and the resistances of current collectors and contacts

[0033] , The Rp, calculated by fitting the impedance spectra using the equivalent circuit in Fig. 10B, displayed a 76-78% decrease on the eRed-LSCF cell compared to the LSCF cell, indicating a lower electric energy requirement for H2O, H2O / CO2, and CO2 electrolysis.

[0146] Fig. 11 illustrates the impedance spectra on the LSCF and eRed-LSCF cells for coelectrolysis of H2O and CO2 at different temperatures, showing a decrease in both Rs and Rp with increasing temperature. The values of Rs were similar for both cells, resulting in an activation energy of 0.86 eV for LSCF and 0.88 eV for eRed-LSCF, as depicted in the inset figures. This activation energy can be attributed to O2ion conduction through the yttria- stabilized zirconia (YSZ) electrolyte and typically falls within the range of 0.7-1.4 eV

[0034] , Furthermore, the eRed-LSCF cell demonstrated significantly improved performance compared to the LSCF cell. It exhibited a lower Rp with an estimated activation energy of 0.88 eV, which corresponds to a 22% reduction compared to the Rp of 1.13 eV observed in the LSCF cell. This enhanced electrocatalytic performance in the eRed-LSCF cell can be attributed to improved adsorption properties for FLO and CO2, as well as reduced oxidation states of B-site transition metals facilitated by exsolved nanoparticles and abundant oxygen vacancies. The reduction of LSCF electrode, through either electrochemical or thermochemical means, induces the formation of oxygen vacancies as shown in Eqs. 1 and 2. These vacancies serve as active adsorption sites for both H2O and CO2.

[0147] The normalized production rate of H2 and CO during the co-electrolysis of H2O and CO2 on the eRed-LSCF cell at different temperatures and current densities is presented in Fig. 12A. The Faradaic efficiency consistently was above 93% for all temperatures and current densities, indicating the exceptional efficiency of the eRed-LSCF cell. Such high Faradaic efficiency, which is a distinguishing characteristic of high-temperature solid oxide electrolysis cells (SOECs), verifies the minimized electron consumption for carbon formation on the cathode surface. The formation of carbon during CO2 electrolysis can significantly impede cathode performance by obstructing active sites

[0035] , Interestingly, despite the equimolar supply of H2O and CO2, H2O electrolysis predominated over CO2 electrolysis due to the slower kinetics associated with CO2 electrolysis

[0036] , It has been reported that reverse water-gas shift reaction (r-WGS) could be responsible for the production of CO during co-electrolysis of H2O / CO2 [36,37], It has been reported that the extent of r-WGS reaction during co-electrolysis ofH2O / CO2 depends on operating temperature, gas composition, applied current, and voltage [38,39], Fig. 12B illustrates the significant influence of temperature and current density on the H2 / CO ratio during the co-electrolysis of H2O and CO2. The slower kinetics of CO2 electrolysis contribute to a higher H2 / CO ratio at lower temperatures, while r-WGS reactions tend to occur at higher temperatures, resulting in increased CO production. Additionally, a higher current density intensifies H2 production in the reaction environment, thereby reducing the H2 / CO ratio. Lastly, Fig. 12C demonstrates the stability of the eRed-LSCF cell over a three hour period during coelectrolysis, showing no noticeable increase in voltage. This indicates the excellent stability of the eRed-LSCF cell under prolonged operating conditions, even without a safety gas. Although the absence of a safety gas enhances the efficiency of electrolysis, the necessity of a safety gas to maintain a reduced state of transition metals in the state-art electrodes, such as Ni / YSZ, remains a subject of debate [28, 40-42], However, the findings in this study suggest that the use of a safety gas is unnecessary. Other studies have reported the stability of perovskite electrodes, both with and without nanoparticles, during electrolysis without the co-feeding of a safety gas [35,43,44],

[0148] Conclusions. This study provides a comprehensive understanding of the electrochemical behavior during electrolysis by investigating the electrochemical reduction of LSCF perovskite and the subsequent exsolution of metal nanoparticles. By applying cathodic polarization for a short duration, successful electrochemical reduction of LSCF perovskite and exsolution of metal nanoparticles were achieved, as confirmed by microscopic images displaying the exsolution of metal nanoparticles. Operando X-ray absorption near-edge structure analysis conducted during electrolysis demonstrated a significant reduction in the oxidation state of B-site atoms, particularly cobalt, within the LSCF electrode. The electrochemical behavior of the LSCF electrode during electrolysis exhibited dynamic changes in current density and area-specific resistance in response to variations in cell voltage, attributable to instantaneous alterations in electrical conductivity and surface adsorption properties. The electrochemical switching point, marked by the transition from an increasing trend to a decreasing trend in area-specific resistance, was identified. Remarkably, the eRed-LSCF electrode displayed excellent stability and significantly lowered polarization resistance during the electrolysis of H2O and / or CO2, without the requirement of a safety gas.

[0149] Additional examples may be found in J. Kim, et al “Electrochemical exsolution of metal nanoparticles from perovskite oxide upon electrolysis,” App. Catalysis B: Environmental, Vol. 344 (2024) 123603), which is incorporated herein in its entirety.Example 2: Bimetallic CoFe nanoparticles on (La,Sr)FeOs perovskite: Its effect on electrocatalytic oxidative coupling of methane.

[0150] This study presents in situ reduction of lanthanum strontium cobalt ferrite (LSCF) perovskite as an effective method for modifying its surface properties and enhancing its electrocatalytic reactivity for oxidative coupling of methane (OCM). The evolution of heterophases during the reduction of LSCF resulted in CoFe nanoparticles being formed at the surface. The in situ reduced LSCF cell for OCM could be operated in either an ion pump or a fuel cell mode. High selectivity of 63% and 10.2% were reported for C2+ hydrocarbons and C3H6, respectively. Density Functional Theory (DFT) calculations on LSCF and CoFe revealed that the high selectivity of C2+ hydrocarbons on the LSCF primarily stems from the presence of CoFe nanoparticles. In situ DRIFTS conducted under CH4 proved that complete oxidation of CH4 can be effectively inhibited by reducing LSCF, and control of oxygen supply is an important parameter for selective conversion of CH4 to higher order hydrocarbon.

[0151] Introduction. In recent decades, technology has greatly advanced the refinement of shale gas, which produces abundant methane [1], Methane is a small and stable molecule characterized by strong C-H bonds (439.3 kJ moE1), negligible electron affinity, large ionization energy, and low polarizability [2], In order to convert methane into chemicals, such as alcohols and olefins, a steam reforming reaction is typically involved, which is an endothermic and inefficient process. As an alternative, it may be more efficient to eliminate the reforming step from the process and convert methane directly into valuable compounds. In a heterogeneous catalysis process, it is possible to perform the direct conversion of methane either by non- oxidative or oxidative coupling [2], During oxidative coupling of methane (OCM), CH4 undergoes partial oxidation, leading to the formation of an unpaired methyl radical (CH3*), which couples with another methyl radical to form C2H6 (Eq. 3). The reaction may continue by a subsequent oxidative dehydrogenation of C2H6 (Eq. 4), thus releasing C2H4[3],2CH4 + ’ / 2O2 ^CH3CH3 + H2O (3)AH298K = -87.8 kJ moE12CH4+ O2 ^CH2CH2+ 2H2O (4)AH298K = -140.4 kJ moE1CH4+ O2 ^CO2+ 2H2O (5)AH298K = -801.3 kJ mol1

[0152] Recent years have seen significant research efforts, seeking out catalysts that can maximize the yield of C2 hydrocarbons, such as ethane and ethylene. Based on the evidence thus far, however, it has been suggested that the low yields of C2 hydrocarbons is due to the rate of hydrogen abstraction from C2 products being greater than the rate of hydrogen abstraction from methane on most heterogeneous catalysts [4], A kinetic study has predicted that catalytic processes for OCM can yield C2 hydrocarbons up to an upper limit of 28-30% [5], In light of this, other approaches need to be considered for OCM. The yields to C2 hydrocarbons follow a half-order dependence on O2 partial pressure while deep oxidation of CH4 to CO and CO2 shows first-order dependence (Eq. 5) [6], As such, one strategy to improve the C2 selectivity is to use a membrane reactor able to operate OCM at low partial pressure of oxygen. Particularly, solid oxide cells (SOCs) consisting of solid oxide electrolyte membrane and electrodes that are able to regulate oxygen-ion flux based on fine control of appropriate current. SOCs are a doubly attractive option because of the additional valuable products that can be generated on the cathode depending on the reaction of choice, such as the electrolysis of H2O or CO2, while performing OCM at the anode [7], A further advantage of SOCs is that electricity can be spontaneously generated during OCM. Even with these advantages, OCM using SOC is still challenging since most high temperature catalysts have mixed ionic and electronic conductivity that chemically drives CEE to CO and CO2 [7-9],

[0153] In previous work, the exsolved B-site metal nanoparticles on the perovskite surface were induced by exposing it to H2 atmosphere was recognized as an important factor for the improvement of electrocatalytic activity for CO2 electrolysis

[0010] , It is reasonable to expect that the exsolution of B-site atoms could be triggered by the exposure of the perovskite to other reducing gases such as CO or hydrocarbons. It is for this reason that an in situ reduction of an anode would be feasible, resulting in the modification of its surface characteristics which lead to enhanced electrocatalytic activity, during the oxidation reaction of hydrocarbons. In this study, it was hypothesized that the catalytic properties of the perovskite oxide surface can be modified by exposing it to CEE gas. A Lao.7Sro.2Coo.2Feo.8O3 (LSCF) catalyst was explored as an anode in a solid oxide cell working either in fuel cell or ion pump mode for OCM to produce ethylene and propylene. The in situ exsolution of bimetallic CoFe nanoparticles and the resulting improvement in electrocatalytic performance of LSCF anode for OCM were verified by surface and bulk analysis. DFT calculations were also conducted to confirm the selective OCM reaction pathways on the exsolved CoFe nanoparticles and the surface of LSCF by examining the various active sites.

[0154] Experimental

[0155] Preparation of catalysts. A citric acid-ethylenediaminetetraacetic acid (EDTA) complexation method was used to synthesize A-site deficient Sr and Co doped lanthanum ferrite perovskite, Lao.7Sro.2Coo.2Feo.8O3 (LSCF), as reported in an earlier publication

[0011] , Reduced LSCF samples (Red-LSCF) were prepared by reducing the calcined LSCF powders under 5% H2 / N2 at 850 °C.

[0156] Evaluation of electrocatalytic performance. A screen-printing technique was used to make electrocatalytic button cells on commercial Yttria-stabilized zirconia (YSZ) electrolyte (25 mm diameter, 125 pm thickness, Nextech Materials). On the anode side, a gadolinium doped ceria (GDC) interlayer was printed and sintered at 1400 °C for two hours under N2 flow. LSCF mixed with GDC at 40 wt% was printed with 0.72 cm2on the GDC layer and sintered at 1200 °C under air. A cathode side LSM-YSZ layer was printed and sintered at 1200 °C under air. The gold wire was applied to the electrodes with the use of gold paste. In the electrocatalytic performance tests, pure CEL (99.99%), flowed to the anode side at a rate of 10 seem and experiments were performed at 800 °C, 825 °C, and 850 °C. The electrochemical impedance spectra (EIS) were collected in the frequency range from 1 MHz to 10 mHz. The equivalent circuit model of RS(RPIQI)(RP2Q2) was used consisting of an ohmic resistance (Rs) and two contributions to polarization impedance (Rp) and constant phase elements (Q) in parallel. Quantification of the gas products was performed on an on-line gas chromatograph (Shimadzu 2014) equipped with a pulse discharge ionization detector (PDHID) and a flame ionization detector (FID).

[0157] Characterizations. The analysis of X-ray diffraction patterns of the catalysts was performed on a Bruker D8 Lead X-ray powder diffractometer equipped with a Cu Ka X-ray source. In the generator, the voltage was 40 kV and the current was 40 mA. Scans were performed in a 29 range of 20-60°, and the step size was set at 0.014° per 0.5 seconds. Phase identification was done using the crystallographic open database (COD). Rietveld refinement was used to calculate Miller indices from the XRD patterns using General Structure Analysis Sy stem -2 (GSAS-2) software

[0012] , The extent of exsolution,was calculated as the ratio of moles of CoFe to moles of the parent perovskite based on the refined mass fraction

[0013] , Crystal structures were visualized by VESTA

[0014] , In situ XRD required the use of an Anton Paar HTK1200 oven at various temperatures from 30 °C to 800 °C.

[0158] The Raman spectra were obtained with a Horiba LabRAM HR-800 Raman Spectrometer equipped with an asymmetric Czerny Turner spectrometer (1200 g mm1grating) and a CCD detector (1024 x 256 pixels, each pixel measuring 26 pm x 26 pm). Five scans were taken over an exposure time of 60 seconds to collect the spectrum.

[0159] Analysis of the surface elemental oxidation state and composition was carried out by X- ray photoelectron Spectroscopy (XPS) using a Kratos Axis Ultra XPS instrument equipped with a monochromated Al Ka X-ray source (1254 eV, 12 kV, 10 mA) and a charge neutralizer at 2.05 A of element current, 1.3 V of filament bias, and 3.6 V of charge balance. C 1 s standard peak at 284.5 eV was used for calibration for each scan. Eight sweeps of high-resolution scans for La 3d, Sr 3d, Co 2p, and Fe 2p were performed with a 450 ms dwell time in the narrowed binding energy ranges.

[0160] X-ray Absorption Near Edge Spectroscopy (XANES) was conducted at Sector 10-BM of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source (APS, Argonne National Laboratory). The instrument was operated in transmission mode for all samples. The catalyst powders were diluted with boron nitride in a ratio of 1 :5. The Fe K-edge and Co K-edge were scanned at 7112 eV and 7717 eV, respectively, for both catalyst samples. The corresponding metal foils were used as references to calibrate the K-edge spectra. Athena software was utilized to process the collected data

[0015] ,

[0161] The scanning transmission electron microscope (STEM) was used for an investigation of LSCF and Red-LSCF using a TECNAI F20 TEM operating at 200 kV equipped with an energy dispersive X-ray spectroscopy (EDS).

[0162] Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data were collected using Thermoelectron Nicolet 6700 FTIR equipped with an MCT detector. Powder samples were diluted with potassium bromide (KBr) in a 1:20 ratio of catalyst to KBr. For in situ DIRFTS under CH4, the DRIFTS spectra of the pretreated samples were acquired under 30 seem of 10% CHr / He at 450 °C. For temperature-programmed desorption (TPD) -DRIFTS, the samples were pretreated with helium at 450 °C to desorb any additional adsorbed species. The samples were exposed to CO2 at 50 °C and DRIFTS spectra was collected at various temperatures under helium.

[0163] For temperature-programmed reduction / oxidation (TPR / TPO), 30 seem of gas mixture was flowed through 50 mg of catalyst powders and the effluence was analyzed by an MKS Cirrus mass spectrometer. 5% H2 / N2 was used for TPR and 2% Ch / He or 5% CCh / He was forTPO. The reactor was heated from room temperature to 1000 °C with a ramp rate of 10 °C min-1.

[0164] For measurement of electrical conductivity, the sample pellets were prepared by compressing the powder in a hydraulic press, followed by sintering at 1300 °C for five hours. A gold paste was used to connect four gold wires to four points on the pellet. The two leads with current application were connected to Keithley 6220 current source. The two leads with voltage measurement were connected to Keithley 6182 sensitive nanovoltmeter.

[0165] Computational details. All plane wave DFT calculations were performed using the projector augmented wave pseudopotentials provided in the Vienna ab initio simulation package (VASP) [16,17], The spin polarized Perdew-Burke-Ernzerhof (PBE) exchange-correlation with Hubbard U corrections S were used with a plane wave expansion cutoff of 400 eV. U corrections were only applied for LSCF structure with the 4 eV on Co and F

[0018] , La and Fe of LaFeCh (Pm3m phase) were replaced with Sr and Co, respectively to model LSCF system

[0019] , In terms of CoFe, the same phase (Pm3m) had been used to model the generated CoFe on LSCF by the exsolution. The PBE bulk lattice constants of CoFe (a = b = c = 2.84 A) and LSCF (a = b = c = 3.89 A) were used to fix the lateral dimensions of CoFe and LSCF slabs, respectively. Recent computational studies had proposed that (100) with Fe termination for CoFe was most energetically favorable

[0020] , For LSCF slab, many of perovskite structures dominantly have (110) facets

[0021] , Based on these results, computational studies focused on the CoFe (100) and LSCF (110) slabs. The simulated slabs consisted of four layers with fixed two bottom layers, but other layers are allowed to relax until the forces are less than 0.03 eV / A.

[0166] All computational slab models included a vacuum spacing of ~ 20 A which is sufficient to reduce the periodic interaction in the surface normal direction. In terms of system size, 2 ^ 2 unit cells with corresponding 2 x 2 x 1 Monkhorst-Pack k-point meshes were employed.

[0167] In the present study, the free energy, G, of adsorbed species was obtained with Eq. 3 where A* denotes an adsorbed molecule A on a surface. DFT was used to evaluate the energy (EDFT) and vibrational frequencies of the adsorbed molecules. The frequencies are used along with statistical partition functions to calculate the zero point energy (ZPE), and vibrational contributions to entropy (Svib) and internal energy (Uvib).G(A*) = EDFT + ZPE + Uvib - TSvib (6)

[0168] A similar expression but one that incorporates RT, rotational and translational degrees of freedom can be used to obtain the free energy of molecular species under ideal gas conditions(G(A(g)). We evaluated the potential dependent reaction free energy (AG) for a coupled protonelectron elementary reaction by using the computational hydrogen electrode (CHE) model

[0022] , The AG of A* +H++e“ -^AH* can be described by

[0169] where * indicates an adsorbed molecule on the surface. In the CHE method we use the equivalence of free energy of the proton-electron pair and a hydrogen molecule in the standard state at 0 V of the reference hydrogen electrode (RHE) potential.

[0170] Results and Discussion

[0171] Evolution of hetero-phases . In situ X-ray diffraction (XRD) was used to examine the crystal structure of the synthesized Lao.7Sro.2Coo.2Feo.8O3 (LSCF) powders under air and helium at various temperatures. LSCF has a distorted orthorhombic crystal structure at room temperature with a 21 nm of crystallite size. XRD analysis under air and helium gave evidence that LSCF is thermochemically stable up to temperatures of 800 °C. The oxygen vacancy per formula unit of LSCF was calculated, and the stoichiometric formula of LSCF was determined to be Lao.7Sro.2Coo.2Feo.8O2.9i8 under helium at 1000 °C.

[0172] LSCF underwent structural transformation under 5% H2 / N2 at 850 °C as demonstrated by the XRD spectra in Fig. 15. The main XRD peaks of LSCF remained after three hours of reduction (H2-Red-LSCF), while the additional peaks of bimetallic CoFe, La2FeO4, and La2Os were observed. It is important to note that LSCF could be reduced during CH4 activation since the lattice oxygen from LSCF was consumed to produce H2O and CO2. To verify the structural transformation of LSCF during CH4 activation, LSCF treated with 10% CH4 / He for three hours at 850 °C (CH4-Red-LSCF) was examined by XRD. XRD pattern of the CH4-Red-LSCF experienced the same structural transformation with H2-Red-LSCF, displaying peaks present in the bimetallic CoFe alloy and La2FeO4. Following are the steps involved during the structural transformation: nABO^(n- )AB(h + AO + BO2 (8)BO2 + H2 ^B + H2O ABO3 + AO -^A2BO (9)

[0173] Consequently, the reduction of LSCF could cause the formation of a B-site zerovalent metal, an A2BO4 oxide phase, an A-site metal oxide, and oxygen vacancies as demonstrated in Fig. 16B, modifying the structural and catalytic properties of the parent perovskite.

[0174] Scanning transmission electron microscopy (STEM) equipped with energy dispersive X- ray spectroscopy (EDS) was used to investigate surface morphology of LSCF, EE-Red-LSCF, and CEU-Red-LSCF. In Fig. 16A, LSCF showed uniform morphology without any noticeable heterogeneous phase. On the other hand, EE-Red-LSCF clearly exhibited a heterogeneous phase on the surface, indicative of exsolved metal particles. Based on EDS micrographs, atomic counts on the area indicated by a square shape on the STEM images showed that the chemical compositions in the “B” areas are close to LSCF perovskite, while the atomic counts of Co and Fe were found to be substantially higher with approximately equal ratios of Co to Fe in the “A” areas on both EE-Red-LSCF and CEL-Red-LSCF. It can be suggested that the nanoparticles are composed of a metallic CoFe alloy, which agrees with the XRD results. The chemical composition of CoFe alloy nanoparticles could vary locally

[0023] , Nanoparticles exsolved from CEL-Red-LSCF appear to have a core-shell appearance, as if they were coated with a thin layer. The shell layer could not be analyzed by EDS, but it can be assumed to be an amorphous carbon phase deposited on the metal surface since graphitic carbon was not detected by XRD. The results of temperature-programmed oxidation by CO2 (CO2-TPO) and the sequential temperature-programmed oxidation by O2 (O2-TPO) showed that deposited carbon on CEL-Red- LSCF was mostly amorphous carbon, which can be readily removed

[0024] , It was also found that about half of the diameter of each nanoparticle is embedded onto the surface of LSCF so that each nanoparticle is firmly anchored on the LSCF surface, thereby preventing agglomeration while facilitating ionic transfer, as demonstrated in Fig. 16C. Similar observations have been reported for Ni doped perovskites [25,26],

[0175] Based upon the XRD patterns of Red-LSCF, Rietveld refinement method was applied for a quantitative analysis in Fig. 17. The perovskite phase was still the dominant structure in both H2-Red-LSCF (44.1%) and CEL-Red-LSCF (51.1%) after the reduction. Each sample exhibited substantial amounts of Ruddlesden-Popper phase, La2FeO4, that provides a superior level of electrical conductivity over the parent perovskite because of its oxygen hyper stoichiometry

[0027] , The CoFe alloy was estimated to be present at weight fractions of 11.3% and 8.3% on H2- Red-LSCF and CEL-Red-LSCF, respectively. The extent of exsolution, the ratio of moles ofCoFe to the moles of the parent perovskite, was calculated to be 0.520 for Eb-Red-LSCF and 0.224 for CEL-Red-LSCF

[0013] , The extent of exsolution can be controlled by various factors such as reduction temperature, duration time, partial pressure of oxygen, and stoichiometry of perovskite. The A-site deficiency of the perovskite in this study could contribute to thesignificant extent of exsolution since it has been demonstrated that A-site deficient materials display increased migration of B-site cations compared to their stoichiometric equivalents

[0028] ,

[0176] The transformed structure of LSCF and the exsolved nanoparticles can be reversed to the original LSCF structure under oxidizing conditions as verified by XRD and temperatureprogrammed experiments in Figs. 18A-18B. The reduction features obtained by following the H2O (m / z = 18) signal in the temperature-programmed reduction (TPR) profile in Fig. 18A were observed at two different temperatures; one between 400 °C and 600 °C and the other above 700 °C. The first peak was due to the formation of oxygen vacancies, which was the first step of the reduction process and, therefore, can be associated with the partial reduction of Fe3+and Co3+ions to lower valent cations. At temperatures higher than 700 °C, the reduction of the B-site cations continues, leading to a broad reduction peak. This peak resulted from metal ions being reduced to the metallic phase, causing nanoparticles to be exsolved on the surface. Temperatureprogrammed oxidation (TPO) was conducted on FL-Red-LSCF to estimate the temperature at which reoxidation occurs. The oxygen consumption peak (m / z = 32) was observed at 610 °C in the TPO profile. In addition, the XRD pattern of FL-Red-LSCF in Fig. 18B was completely reversible to the original perovskite structure with no impurity after one hour of oxidation under 5% 02 / He atmosphere at 850 °C. Accordingly, the exsolved CoFe nanoparticles on the surface could be readily redistributed when needed.

[0177] Electrocatalytic oxidative coupling of methane . The electrocatalytic performance of LSCF was investigated for oxidative methane coupling (OCM) with 10 seem of CPU flow. The product selectivity of OCM was observed for a period of three hours under 100 mA cm2at 850 °C. A 32.5% increase in the selectivity of C2+ hydrocarbons, including C2H4, C2H6, and C3H6, was observed from 19.9% to 52.4% over the first three hours, whereas the selectivity for carbon oxides decreased from 80.1% to 47.6%. In situ reduction of LSCF by CPU is likely to be responsible for changes in the selectivity, since LSCF was demonstrated to be reduced at 850 °C under CPU.

[0178] The electrocatalytic OCM reaction on the in situ reduced LSCF anode produced more C2+ hydrocarbons as the current density increased (Fig. 19A and Fig. 28). It was estimated that the carbon balance of the product stream was close to 100% with no more than 2% error. In comparison with open circuit voltage (OCV), the production rate of C2+ hydrocarbons increased fourteen times at 150 mA cm2, verifying that the catalytic formation of C2+ hydrocarbons was an electrocatalytically activated process and that oxidative coupling of CPU was predominant over non-oxidative coupling. The increase in the ratio of alkene to alkane implies that theoxidative dehydrogenation of C2H6 was also electrocatalytically facilitated, resulting in C2H4 being produced about four times more than C2H6. It is noteworthy that selectivity to C3H6, which was the most valuable product from OCM, was at 10.23% at 75 mA cm2. Previous studies in the literature have reported propylene selectivities to be less than 2% or negligible [29,30], This was a significant improvement over existing technology as the large difference in kinetic diameters between C3H6 (0.47 nm) and CP (0.38 nm) allow for an easy gas separation, whereas C2H6 (0.39 nm) and C2H4 (0.39 nm) require a more energy-intensive separation process.

[0179] Furthermore, the question of whether the formation of C2H4 was a result of dehydrogenation of C2H6 or whether coupling of CH2 was also considered to help in the understanding of the reaction pathway. It was found that the ratio of C2H2 to C2H4 increased as the current density increased, while the ratio of C2H6 to C2H4 decreased. These results suggest that the dehydrogenation of C2H6 was the primary step for C2H4 production.

[0180] The in situ reduced LSCF cell voltages for OCM remained stable for 180 minutes (Fig. 19C). The stability of the cell at varying current densities can be attributed to the thermochemical stability of the anode after in situ reduction of LSCF under the operating conditions for OCM. Moreover, the cell voltage at 25 mA cm2was 0.41 V indicating that the cell was in a fuel cell regime, capable of producing electricity during OCM catalysis. The in situ LSCF cell also proved to be efficient for the electrocatalytic OCM, showing over 90% of Faradaic efficiency in Fig. 28. Raman spectrum on the post-OCM in situ reduced LSCF anode shows small peaks representative of amorphous carbon with no observable peak corresponding to stable graphitic carbon. These results demonstrate that the efficiency loss associated with the formation of carbon on the surface of the in situ reduced LSCF anode was minimal. The ohmic resistance, Rs, of the cell remained unchanged at around 1.25 cm2at various current densities (Fig. 19D). The polarization resistance, Rp, considerably decreased with the increase in the current densities. The polarization resistance at high frequency, RHF, mainly attributed to charge transfer, was much higher than the one at low frequency, RLF, corresponding to mass transfer.

[0181] An investigation of the effect of temperature on electrocatalytic OCM has been conducted in the range of 800-850 °C. The C2+ hydrocarbon production rate, the C2+ hydrocarbon selectivity, the ratio of alkene to alkane, and C3H6 selectivity were all observed the highest at 850 °C at all applied current densities. It is likely because the cleavage of the strong C-H bond in CH4 molecule requires higher temperature than the subsequent reactions such as methyl coupling to C2H6, further dehydrogenation of C2H6 to C2H4, or oxidation to CO and CO2.

[0182] Figs. 20A-20B show the results of a long-term electrocatalytic OCM performed on Red- LSCF anode at a constant current density of 100 mA cm2and an operating temperature of 850 °C. As shown in Fig. 20A, the Red- LSCF anode showed stable performance over a 100-hour period, with a passivation rate less than 0.5 mV h1and a Faradaic efficiency greater than 90%, monitored every five hours. The selectivity and production rate of C2+ hydrocarbons were also maintained without significant change for the period of time in Fig. 20B. This indicates that the Red-LSCF anode is thermochemically stable under the operating conditions of the electrocatalytic OCM.

[0183] Computational results. The potential-dependent reaction free energy (AG) of methane C-C coupling reactions on CoFe and LSCF at 0 V RHE has been studied using DFT. Multiple configurations were explored for each of the intermediates on the surfaces, and a most favored configurations was found.

[0184] Both surfaces of CoFe (100) and LSCF (110) have coordinatively unsaturated (cus) surface metal sites and cus oxygen atoms. The cus sites of transition metal oxides have been proposed to be generally active due to the instability of its electronic structures

[0031] , Fig. 21 A represents CoFe (100) surface with Fecus surface atoms and LSCF with Lacus and Ocus. Co metal atoms of CoFe (100) and LSCF (110) surface are not exposed to the surface. Fig. 21B displays the free energy diagrams of surface reaction on CoFe (100) and LSCF (110) surface calculated by DFT. The y-axis scale is greater on LSCF than CoFe. The initial activation of C-H bond cleavage requires 0.25 eV less energy on the CoFe surface than on LSCF. To form value- added products that were experimentally observed, the generated CH3* on CoFe could undergo two different pathways of C-C coupling: C2H6* formation or dehydrogenation to CH2* . The proposed C2H6* formation is a non-electrocatalytic step due to the absence of proton-electron transfer. It could continue to occur in gas-phase [7], C2H6* weakly binds on the surface with the low adsorption energy of 0.54 eV; C2H6* stability is competitive with the free energy requirement of subsequent dehydrogenation of C2H5 formation (0.56 eV). Considering the activation energy of dehydrogenation and the large entropic contributions of desorption, the C2H6 desorption is expected to occur more frequently than the subsequent dehydrogenation step. The proposed free energy analysis is well in line with the experimental results showing high selectivity of C2He(g)

[0032] ,

[0185] C2H4* formations can be achieved by the dehydrogenation of C2H5* or the non- electrocatalytic C-C coupling of CH2* on CoFe. Both steps are thermodynamically favorable, suggesting that C2H4* would be easily formed on CoFe surface. C2H4 desorption isthermodynamically less favorable (1.19 eV) than the dehydrogenation (0.21 eV). However, the large entropic contributions of desorption (large desorption prefactor) as well as the kinetic effects of dehydrogenation would allow C2H4 desorption to compete with further dehydrogenation (but still less facile than dehydrogenation). The generated C2H2* from the thermodynamically favorable step of C2H3* dehydrogenation is stably adsorbed with the large desorption energy of 3.41 eV and likely to undergo oxidation or further dehydrogenation steps. The above computational results of C2H4* and C2H2* correspond well to the experimentally observed higher selectivity of C2H4 and low selectivity of C2H2.

[0186] Other pathways for the formation of carbon and longer chain hydrocarbons (CsHx) were also explored. The simulations predicted that the carbon formation by subsequent dehydrogenation from CH3 are facile: the CH3 —> CH2 (0.15 eV), CH2 —> CH (exothermicity), and CH —> C (0.07 eV). It suggests that carbon coking would competitively occur together with other surface reactions, and it was experimentally observed on CoFe particles as an amorphous layer in Fig. 16A. The C-C coupling forming C3H6 is thermodynamically favorable, and the generated C3H6 can either desorb (1.3 eV) or undergo subsequent dehydrogenations (0.21 eV). C3H6 desorption is expected to compete with the further reaction when considering the large entropic contribution of desorption. Overall, the computational results related to the formations of C3H6, C2H6, and C2H4 showed the low free energy requirements on CoFe surface corresponding to the low applied potentials activating the reactions.

[0187] The oxidation pathway starting with CH3* (CH3* —> CH3O* —> CH2O*) assumes that the oxygen source is LSCF, based on the experimental results. The CH3O* formation (0.61 eV) and the subsequent dehydrogenation (0.03 eV) were predicted to be exothermic reactions resulting in a slower oxidation rate compared to the C-C coupling reactions discussed above. Even though the other oxidation steps could require less energy, the oxidation rate would still be slow due to the lack of surface oxygens on CoFe. The results of DFT calculations suggest that the C-C coupling reaction dominates the reaction pathway, thereby producing high selectivity of C2+ hydrocarbons.

[0188] On LSCF surface, C-C coupling and the oxidation steps from CH3* are all thermodynamically favorable, except for CH3* dehydrogenation. The subsequent dehydrogenation step of C2H6* —> C2H5* formation has a large thermodynamic uphill (1.45 eV). In addition, the small desorption energy of C2H6 (0.22 eV) causes C2H6 desorption to be favored rather than dehydrogenation. These thermodynamic hindrances of C2H5* and CH2* formations and the favorable C2H6 desorption consequently hamper the formation of C2H4. Interestingly,C3H6 formation from C-C coupling of CH3* was predicted to be readily achieved due to the thermodynamic favorability on LSCF. The desorption of C3H6 is strongly favored over further dehydrogenation because of the low adsorption stability (0.11 eV (desorption) vs. 1.16 eV (dehydrogenation). Overall, the LSCF is expected to favorably produce C2He(g) and C3He(g); however, these pathways are competitive with the oxidation reactions which corresponds to the low selectivity toward C2+ hydrocarbons. By comparing the computational results of CoFe and LSCF, it was found that CoFe and LSCF are both attributed to generation of C2He(g) and C3He(g); however, the generation of C2H4(g) primarily occurs on CoFe while oxidation is dominant on LSCF.

[0189] Analysis of electronic structure. The electronic structure of LSCF and Red-LSCF was investigated to reveal the mechanism for improved performance of Red-LSCF for OCM. X-ray photoelectron spectroscopy (Fig. 22) and X-ray absorption spectroscopy (Figs. 23 A-23B) techniques were used to characterize these samples.

[0190] There were two main peaks in the XPS spectrum of La 3d on LSCF: La 3d3 / 2 and La 3d5 / 2, which were located at 833.23 eV and 850.03 eV, respectively, due to a 16.8 eV spinorbital splitting, which suggests that La3+dominates the spectrum

[0033] , Furthermore, satellite peaks at 837.3 eV and 854.1 eV are also evidence for the trivalent oxidation state of La ions, which are 3.7 eV higher than the peaks along the 3 ds / 2 and 3d3 / 2 core lines

[0034] , Observations of all the above spectral characteristics indicate that lanthanum in LSCF is in an oxidation state of 3+ coordinated by an oxide ion environment. After the reduction of LSCF, the La 3d3 / 2 peak was shifted by 1.13-834.36 eV. This can be explained by the fact that both lanthanum oxide and carbonate contribute to the La 3d spectrum of Red-LSCF. Lanthanum atoms form lanthanum oxide, La2O3, during the reduction process of LSCF as explained by the results of XRD in Fig. 15. La2O3 having a strong basicity is typically converted to lanthanum carbonate, La2O2CO3, by CO2 adsorption on its surface under air

[0035] , Since La2O3-based catalysts are known for high C2 hydrocarbon selectivity for OCM, the evolution of La2O3 after the reduction of LSCF would contribute to the improved selectivity toward C2+ hydrocarbons for OCM

[0035] ,

[0191] Co 2p XPS spectrum on LSCF was deconvoluted into two different oxidation states. Those peaks in the region of Co 2p3 / 2 were observed at 779.8 eV for Co3+and 781.5 eV for Co2+, respectively

[0036] , The XPS spectrum of Co 2p on Red-LSCF, each peak in the region of Co 2p3 / 2 was located at 780.0 eV for Co3+and 782.8 eV for Co2+, respectively. These shifts in binding energy between LSCF and Red-LSCF were due to the oxygen vacancy formed during the reduction under H2. Moreover, a peak corresponding to zero-valent Co species was observedat 777.8 eV in the regions of Co 2p3 / 2. In accordance with the results of TEM and XRD, this peak is evidence of bimetallic CoFe nanoparticles that have been exsolved on the surface during the reduction step.

[0192] Analysis of the binding energies between the Fe 2p3 / 2 peak and the Fe2+and Fe3+satellite peaks has recently been reported as a method for determining the presence of Fe2+and Fe3+

[0037] ,

[0193] Fe3+compounds normally have about 8 eV of AFe2p3, while Fe2+compounds have 4.3-5.65 eV. In the XPS spectra, AFe2p3is 8.53 eV for Fe3+2p3 / 2 satellite, indicating Fe ions existed as Fe3+mixture. The peaks at the binding energy of 709.4 eV and 711.1 eV were assigned to Fe3+2p3 / 2 and Fe4+2p3 / 2, respectively, for LSCF

[0038] , The Fe ions have different oxidation states which provide redox couples, enabling the LSCF to function as a p-type charge carrier

[0039] , After the reduction of LSCF, the XPS spectra in the region of Fe 2p3 / 2 has three deconvoluted peaks. The peaks for Fe3+2p3 / 2 and Fe4+2p3 / 2 were shifted to the binding energy of 710.1 eV, and 711.8 eV, respectively. The composition of Fe ions with higher oxidation state decreased from 64.7% to 53.2% for Fe4+. Moreover, it displayed the peak at 706.3 eV associated with the zerovalent iron from the exsolved bimetallic CoFe nanoparticles. Such zerovalent bimetallic CoFe nanoparticles would provide highly selective conversion of CPU toward C2+ hydrocarbons.

[0194] The XPS spectra of Sr 3d in LSCF and Red-LSCF was composed of a pair of species with an oxidation state of 2+ . Sr existed in three divalent oxide forms with different 3ds / 2 binding energies at 131.8, 132.8, and 133.6 eV. The first value is close to that reported for Lao.6Sro.4Coo.8Feo.203-8, and the other two peaks may reflect strontium oxide (SrO) and strontium carbonate (SrCCh) [10,40], The reduction of LSCF resulted in an increased concentration of oxygen vacancies on the surface facilitating an adsorption of CO2, and strontium atoms easily react with CO2 to form stable SrCCh. SrO with strong basicity formed on LSCF could have contributed to the improvement in selectivity of C2+ hydrocarbons as SrO are likely to segregate on the surface

[0035] ,

[0195] Figs. 23A and 23B illustrate XANES spectral analysis of the Co K-edge and Fe Kedge for LSCF and Red-LSCF with reference metal foils and oxides. Observations of Fe K-edge XANES spectra in Fig. 23A show a shift downward of - 1.3 eV from 7127.6 eV to 7126.3 eV after the reduction. As the absorption edge energy of the reference compound Fe2O3 is 7126.14eV, it is obvious that Red-LSCF and LSCF exhibit a predominantly 3+ oxidation state. Additionally, two peak features are observed on both samples, namely pre-edge peak Ai and post-edge peak A2

[0041] , In response to the reduction of LSCF, the Ai peak shifted slightly from 7114.3 eV to 7112.9 eV and its intensity increased. The decrease in the edge A2 was observed indicative of the reduced oxidation state of Fe ions in Red-LSCF.

[0196] The Co K-edge energy decreased to 7717.7 eV from 7723.6 eV attributed to the reduction treatment in Fig. 23B. The inset in Fig. 23B shows the split of the pre-edge peak of Co K-edge on LSCF, but not on Red-LSCF, meaning that cobalt predominantly exists in the zerovalent form on Red-LSCF. Co pre-edge peak on Red-LSCF exhibits a high intensity near the Co foil, and the one on LSCF shows a low intensity comparable to CO2O3. In addition, it was found that there was a significant reduction in the white line intensity in the Co K-edge XANES spectrum following the reduction of LSCF. The reason for this is that Red-LSCF containing the metallic species has less well-defined energy levels than LSCF, resulting in lower intensity of the white line closer to Co foil. The ligand-to-metal charge transfer (LMCT) also undergoes a mixed configuration in the ground and excited states following the reduction of LSCF, which leads to a dispersion of the white line

[0042] ,

[0197] The average oxidation state of cobalt and iron ions in the samples were estimated based on the linearly fitted oxidation state as a function of energy. The reduction of the samples resulted in the oxidation state of iron decreasing from 3.03 to 2.81 and cobalt from 2.27 to 1.28.

[0198] In-situ DRIFTS and TPSR under CID. In order to gain insight into the surface dynamics during CH4 activation, in-situ CH4-DRIFTS experiments were performed on LSCF and Red- LSCF catalysts under CFL atmosphere at 450 °C. As shown in Figs. 24A-24B, in situ CFL- DRIFTS spectra on LSCF showed two pairs of adsorption peaks in addition to the peaks of gaseous CH4. The peaks characterized by the IR bands in the region of 2360 cm-1 and 2325 cm1correspond to molecularly adsorbed CO2

[0043] , The peaks located at 2175 cm1and 2110 cm1correspond to molecularly adsorbed CO

[0044] , Those peaks originated from the oxidation of CH4 on the surface of LSCF. The intensity of those peaks decreased progressively in 20 minutes. The peak intensity ratio compared to the highest peak of CH4 at 3015 cm1clearly illustrates the peak evolution of the carbon oxides in Fig. 24B. I2360 / 13015 (CO2 / CH4) was estimated above 0.9 for the first ten minutes of the reaction and gradually decreased to 0.5, and I2175 / 13015 (CO / CH4) behaved similarly. This decrease in peak intensity ratio is likely due to the limited amount of surface oxygen on the LSCF. The in situ DRIFTS with CH4 on Red-LSCF also displayed the IR bands corresponding to molecularly adsorbed CO2 and CO. The DRIFTS spectra also showedtwo peaks located at 1470 cm1and 850 cm1which are ascribed to the adsorbed C2H6

[0045] , The intensity ratio on Red-LSCF in Fig. 24B showed that I1470 / I3015 (C2H6 / CH4) increased from 0 to 0.23, while I2360 / 13015 (CO2 / CH4) and I2175 / 13015 (CO / CH4) decreased to below 0.1 within ten minutes. This result implies that the surface oxygen available from Red-LSCF was rapidly consumed, and CFL coupling took place predominantly when oxygen was limited. Therefore, it could be suggested that CH4 coupling reactions to C2+ hydrocarbons are competing with the oxidation of CH4 and controlling the oxygen supply is a key parameter for a selective conversion of CH4on Red-LSCF.

[0199] In addition, with the introduction of CFL, a broad negative band was found at 3480 cm1in the DRIFTS spectra of Red-LSCF. Since the peak in that region is corresponding to the stretch mode of OH group, the negative intensity of the peak implies that the hydroxyl groups on the surface of Red-LSCF were consumed during CH4 adsorption and activation

[0046] , The dissociative adsorption of CH4 might predominantly occur on the surface of the exsolved bimetallic CoFe nanoparticles at such low operating temperature of 450 °C. Then, in view of the well-known phenomenon of hydrogen spillover, the adsorbed H atoms dissociated from CH4 on CoFe nanoparticles may react with OH groups on the surface of LSCF, releasing H2O

[0047] , An additional negative peak near 1610 cm1can be attributed to bidentate carbonate

[0048] , As demonstrated by the XPS results, carbonate species readily form on the reduced surface of LSCF. With the introduction of CH4, the negative peak was immediately observed, indicating that the two bidentate carbonates, consisting of La2O2CO3 and SrCOs, are consumed rapidly to react with CH4.

[0200] An experiment concerning the activation of CH4 by LSCF and Red-LSCF was conducted by monitoring temperature-programmed surface reaction with CH4 (CH4-TPSR). Fig. 25 shows the product molecules arising from the reaction as CH4, H2, and CO from each catalyst. The formation of CO and H2 on LSCF took off sharply at a temperature of 760 °C. As of 867 °C, the CO production was completed, but the H2 production was still in progress up to 930 °C. It was also observed that small amounts of H2O and CO2 evolved during the reaction, which could be seen in the 50 times enlarged plots. It is interesting that CO2 signal abruptly decreased at 770 °C, suggesting a contribution from the reverse-water gas shift (r-WGS) reaction. It is contemplated that in situ reduction under CH4 flow caused the exsolution of CoFe nanoparticles on the surface of LSCF during the heating process, resulting in the reaction mechanism being changed. For Red-LSCF, CH4 activation toward H2 and CO took place readily when the temperature was above 672 °C. In light of the much lower temperature for activating CH4 on Red-LSCF than onLSCF, it is apparent that Red-LSCF is a more effective catalyst for CPU activation than LSCF. In addition, TPSR profiles taken on Red-LSCF showed to have a peak intensity of H2 greater than CO, supporting the validity of the finding that the deep oxidation of CH4 can be suppressed by the reduction of LSCF. In the enlarged plot, CO2 signal was much lower than CO signal in the entire temperature range, indicating that CPU activation on CoFe primarily produces CO rather than CO2. It supports again that the abrupt decrease in CO2 signal intensity in TPRS on LSCF is due to the in situ exsolved CoFe nanoparticles.

[0201] The majority of carbon products from TPSR was CO rather than CO2 and C2+ hydrocarbons on both LSCF and Red-LSCF. Given that CO was produced without CO2 from OCM under OCV in Fig. 19B, the surface oxygen on LSCF perovskite can be assumed to be responsible for the production of CO2.

[0202] Analysis of surface basicity. It is important to note that heterogeneous phases such as La2Os, La2FeO4, and exsolved CoFe nanoparticles formed during the reduction of LSCF could affect its adsorption and desorption properties. According to the statistical analysis of published data from the last three decades on OCM, Zavyalova et al. have discovered several key factors for the OCM catalysts and demonstrated strong basicity as one of the most crucial factors for the selective formation of C2+ products

[0049] , It is not surprising that an extremely strong base is required to activate CPU, given that CPU is an extremely weak acid. TPD-DRIFTS using CO2 as a probe molecule was conducted on LSCF and Red-LSCF to illustrate the difference in the acidic / basic characteristics of both samples. As shown in Fig. 26, CO2 was molecularly adsorbed onto both samples as evidenced with the peaks at 2380 cm1and 2330 cm1

[0043] , When the temperature was raised to 350 °C, it is apparent from both samples that the peak intensities of physiosorbed CO2 decreased to zero. On the other hand, chemisorbed forms of carbonate were only observed on the Red-LSCF with the adsorption peaks located at 1530 cm1and 1380 cm corresponding to bidentate and monodentate carbonate, respectively

[0044] , It was observed that adsorbed carbonates could withstand temperatures up to 350 °C. The peaks of carbonated species could be originated from the formation of lanthanum carbonate (T^CLCCh) and strontium carbonate (SrCCh) in agreement with XPS analysis shown in Fig. 22. According to this, the presence of oxygen vacancies and exsolution of CoFe alloy nanoparticles induced by the reduction of LSCF promotes the basicity of LSCF and facilitates CO2 adsorption. It is a well- established fact that thermodynamically stable carbonate is a substantial constituent in initiating the oxidation of CPU on its surface

[0050] , Therefore, it can be stated that the surface of Red-LSCFis more basic than that of LSCF and the reduction of LSCF is an effective way of promoting the catalytic characteristics of LSCF for OCM.

[0203] Electrical conductivity. The electrical conductivity of LSCF and Red-LSCF was measured in air and 5% H2 / N2 environment in Figs. 27A-27C to evaluate the conductivity of the LSCF electrodes under the environment similar with the electrocatalytic OCM. A high electrical conductivity was observed with LSCF at elevated temperature under air in Fig. 27A. According to the linearly fit Arrhenius plot, LSCFs follow a small polaron conduction mechanism with Fe3+ / Fe4+redox couples acting as / >-type carriers

[0011] , The LSCF pellet was highly conductive under air at elevated temperatures with a maximum conductivity of 173 S cm proving to be a reliable electrode.

[0204] A substantial decrease in electrical conductivity was observed for LSCF in Fig. 27B when exposed to 5% H2 / N2 at 850 °C, from 173 S cm1to 6.1 Swithin 15 minutes and then gradually decreased more to 4.5 SSuch decrease in electrical conductivity has been observed on (Lao.75Sro.25)o.95Cro.5Mno.503 under reducing environment, while strontium titanate- based perovskite showed an increased in conductivity after reduction [51,52], The abrupt decrease in the electrical conductivity could not be explained by the structural transformation of LSCF which occurs much slower. Then, it could be attributed to the formation of oxygen vacancies. Electrical conductivity of lanthanum ferrite-based perovskites have shown the decrease in conductivity at high temperatures since outgoing oxide ions leave oxygen vacancies resulting in the reduced concentration of Fe ions redox pairs

[0053] , In addition to oxidation state of B-site ions, electric conductivity may vary depending on the Fermi level. According to an in- situ AP-XPS study, electron holes tend to be located on O 2p states since oxygen vacancies are much larger than electronic defects at oxygen partial pressures of less than 0.1 mbar

[0054] , A further observation was made in an in situ XAS study that the anodic polarization resulted in depopulation of electronic states of the Fe3+ / Fe4+redox couples near the Fermi level

[0055] ,

[0205] After exposure to the reducing atmosphere, 3% H2O with 5% H2 / N2 was introduced while measuring the conductivity since H2O is a product for OCM reaction affecting the conductivity of the LSCF anode. The electrical conductivity increased up to 18.2 S cm1under the humid conditions, which is above the generally acceptable level of conductivity, 1 S cm1for porous electrode structures and 10 S cm1for densified electrode materials for SOEC / SOFC

[0056] , Therefore, the reduced LSCF may work properly as an electrocatalysts for the electrocatalytic OCM under a humid reducing environment with sufficient electrical conductivity.

[0206] The redox reversibility was also observed in the electrical conductivity in Fig. 27B. When 10% O2 / H2 was introduced, the conductivity increased back to the original level in a few minutes. A noteworthy observation is that the electrical conductivity of Red-LSCF had a negative dependency on temperature in Fig. 27C. However, it seems to still follow the small polaron conduction mechanism considering the linear fit Arrhenius plot. Such negative relation of electrical conductivity to temperature has been studied on Sr2FeMoo.65Nio.35O6

[0057] , The decrease in the electrical conductivity with the increase in the temperature is likely due to the abundant oxygen vacancies resulted from the reducing atmosphere and high temperatures.

[0207] Conclusions. The exsolved B-site metal nanoparticles on the perovskite surface has been recognized as an important factor in the improvement of electrocatalytic activity. By reducing LSCF perovskite in-situ, its surface properties were modified and its electrocatalytic activity for OCM was improved by 32.5%. XRD and STEM equipped with EDS experiments proved that the evolution of hetero-phases during LSCF reduction under H2 or CH4 resulted in bimetallic CoFe nanoparticles being formed at the surface of LSCF at 850 °C. The structural evolution of LSCF appeared to be completely reversible as reflected in XRD and TPR / TPO patterns, indicating that sintering of nanoparticles on the surface could be addressed by reoxidation and subsequent reduction to re-distribute the exsolved CoFe nanoparticles. As a result of using the LSCF anode either in ion pump mode or in fuel cell mode, the highest C2+ hydrocarbons selectivity of 63% and C3H6 selectivity of 10.2% were achieved. DFT calculations on LSCF and CoFe revealed that the high selectivity of the reduced in situ LSCF for OCM arises primarily from CoFe nanoparticles. The elemental analysis on the surface and in the bulk of the LSCF and the reduced LSCF by XPS and XANES verified the reduced oxidation state of Co and Fe ions, including zerovalent species. The results of TPSR and in situ DRIFTS conducted under CH4 flow revealed that the reduced LSCF can successfully suppress deep oxidation of CH4, and that oxygen supply is an important parameter for selective conversion of CH4. The more basic surface of Red-LSCF than that of LSCF examined by in situ TPD-DRIFTS using CO2 was also one of the key factors for the improved selectivity of C2+ hydrocarbons from the electrocatalytic OCM.

[0208] Additional examples may be found in Jaesung Kim, et al “In-situ exsolution of bimetallic CoFe nanoparticles on (La,Sr)FeO3 perovskite: Its effect on electrocatalytic oxidative coupling of methane,” App. Catalysis B: Environmental, Vol. 321 (2023) 122026), which is incorporated herein in its entirety.Discussion

[0209] The most important concern in OCM and ODH reactions is controlling selectivity and keeping the desired product (olefins) from further oxidizing to COx. Since olefins are more reactive than their alkane counterparts, in the presence of gas phase oxygen, preventing further oxidation to carbon oxides becomes a real challenge. Solid oxide electrochemical cell (SOEC)- type reactors consisting of solid oxide electrolyte membranes and electrodes that are capable of regulating oxide ion flux based on fine control of an electrical bias can offer an efficient way to selectively produce light olefins from shale gas.

[0210] In a typical electrocatalytic OCM / ODH performed in an SOEC-type reactor, oxide ions generated on the cathode (counter electrode) are transported to the anode surface through a dense solid oxide electrolyte to react with methane and / or light alkanes present in the shale gas mixture. SOEC reactor set-up is similar to solid oxide fuel cell (SOFC), but instead of being used for generating power, an external current is applied to control the oxide ion transport through the electrolyte. The advantages of the electrocatalytic route over the thermo-catalytic OCM / ODH are (i) the presence of oxide ions on the anode lifts the thermodynamic limitation of the direct dehydrogenation and coupling, (ii) reactants / products and gas phase oxygen never come in contact with each other, hence eliminating the possibility of further oxidation of olefins to CO2 and H2O, and (iii) oxide ions help gasify / oxidize any carbonaceous species that may deposit on the anode catalyst surface and cause catalyst deactivation. The anode catalyst also plays an important role in the electrocatalytic ODH / OCM process.

[0211] The systems and methods of the appended claims are not limited in scope by the specific systems and methods described herein, which are intended as illustrations of a few aspects of the claims and any systems and methods that are functionally equivalent are intended to fall within the scope of the claims. Various modifications of the systems and methods in addition to those shown and described herein are intended to fall within the scope of the appended claims. Further, while only certain representative systems components and method steps disclosed herein are specifically described, other combinations of the components and method steps also are intended to fall within the scope of the appended claims, even if not specifically recited. Thus, a combination of steps, elements, components, or constituents may be explicitly mentioned herein; however, other combinations of steps, elements, components, and constituents are included, even though not explicitly stated.REFERNCES:Example 1 :[1] R.R. Hudgins, P.L. Silveston, C.-Y. Li, A.A. Adesina, Partial Oxidation and Dehydrogenation of Hydrocarbons. Periodic Operation of Chemical Reactors, Elsevier,, 2013, pp. 79-122.[2] R. Watanabe, Y. Hondo, K. Mukawa, C. Fukuhara, E. Kikuchi, Y. Sekine, Stable and selective perovskite catalyst for dehydrogenation of propane working with redox mechanism, J. Mol. Catal. A Chem. 377 (2013) 74-84, https: / / doi.Org / 10.1016 / j. molcata.2013, 04,033.[3] S. Royer, D. Duprez, F. Can, X. Courtois, C. Batiot-Dupeyrat, S. Laassiri, H. Alamdari, Perovskites as substitutes of noble metals for heterogeneous catalysis: dream or reality, Chem. Rev. 114 (2014) 10292-10368, https: / / doi.org / 10.1021 / cr500032a.[4] K. Chen, E. Iglesia, A.T. Bell, Isotopic tracer studies of reaction pathways for propane oxidative dehydrogenation on molybdenum oxide catalysts, J. Phys. Chem. B 105 (2001) 646- 653, https: / / doi.org / 10.1021 / ip00210Qx.[5] K. Huang, An emerging platform for electrocatalysis: perovskite exsolution, Sci. Bull. 61 (2016) 1783-1784, https: / / doi.org / 10, 1007 / sl 1434-016-1194-8.[6] D. Neagu, V. Kyriakou, I.L. Roiban, M. Aouine, C. Tang, A. Caravaca, K. Kousi, I. Schreur- Piet, I S. Metcalfe, P. Vernoux, M.C.M. van de Sanden, M.N. Tsampas, In situ observation of nanoparticle exsolution from perovskite oxides: from atomic scale mechanistic insight to nanostructure tailoring, ACS Nano 13 (2019) 12996-13005, https: / / doi.org / 10.1021 / acsnano.9b05652.[7] D. Neagu, G. Tsekouras, D.N. Miller, H. Menard, J.T. Irvine, In situ growth of nanoparticles through control of non-stoichiometry, Nat. Chem. 5 (2013) 916-923, https: / / doi.org / 10.lQ38 / nchem.1773.[8] Y. Sun, J. Li, Y. Zeng, B.S. Amirkhiz, M. Wang, Y. Behnamian, J. Luo, A-site deficient perovskite: the parent for in situ exsolution of highly active, regenerable nano-particles as SOFC anodes, J. Mater. Chem. A 3 (2015) 11048-11056, https: / / doi.org / 10.1039 / C5TA01733E.[9] S.-K. Otto, K. Kousi, D. Neagu, L. Bekris, J. Janek, I.S. Metcalfe, Exsolved nickel nanoparticles acting as oxygen storage reservoirs and active sites for redox CH4 conversion, ACS Appl. Ener. Mater. 2 (2019) 7288-7298, https: / / doi.org / 10.1021 / acsaem.9b01267.

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Claims

WHAT IS CLAIMED IS:

1. A method of forming a nano-particle-decorated catalytic surface, the method comprising: providing a conductive metal oxide substrate to a system, and applying a cathodic voltage to a working electrode.

2. The method of claim 1, wherein the conductive metal oxide substrate comprises a mixed metal oxide.

3. The method of claim 2, wherein the mixed metal oxide comprises a perovskite, complex perovskite (e.g., double perovskite), or Ruddlesden-Popper phase (layered perovskite).

4. The method of claim 3, wherein the mixed metal oxide comprises the perovskite having a formula of (AZ)a(XY)03, wherein a is about 1 or less than 1; A and Z are the same or different elements; and X and Y are the same or different elements.

5. The method of claim 4, wherein A and Z are chosen from La, Sr, Pr, Ba, or Ca, and X and Y are chosen from Co, Fe, Ni, Mn, Ti, Re, Cu, Cr, or Mo .

6. The method of any one of claims 1-5, comprising applying the cathodic voltage until a plurality of nanoparticles are exsolved onto a surface of the conductive metal oxide substrate.

7. The method of claim 6, wherein the applied voltage is varied until a desired density of nanoparticles is achieved.

8. The method of claim 6, wherein the plurality of metal nanoparticles comprises metal or metal alloy nanoparticles, and wherein the applied voltage is varied until a desired metal or metal alloy is formed (e.g. varying the ratio of metals in the alloy).

9. The method of any one of claims 6-8, wherein the nanoparticles nucleate at oxygen vacancies on the surface of the conductive metal oxide substrate.

10. The method of any one of claims 1-9, wherein the system is at or below a relevant phase transition temperature of the conductive metal oxide substrate.

11. A nano-particle-decorated catalytic surface formed by the method of any one of claims 1-10, the nano-particle-decorated catalytic surface comprising a reduced mixed metal oxide substrate and a plurality of metal or metal alloy nanoparticles on a surface of the reduced mixed metal oxide substrate, wherein at least one metal is common in both the reduced mixed metal oxide substrate and the plurality of metal of metal alloy nanoparticles.

12. The nano-particle-decorated catalytic surface of claim 11, wherein the nano-particle- decorated catalytic surface is used as an anode catalyst for an oxidation reaction or as a cathode for a reduction reaction.

13. An electrode composition comprising a reduced perovskite substrate having a formula of (AZ)(XY)O3 and a plurality of metal or metal alloy nanoparticles on a surface of the reduced perovskite substrate, wherein at least one metal is common in both the reduced perovskite substrate and the plurality of metal or metal alloy nanoparticles.

14. The electrode composition of claim 13, wherein A and Z are chosen from La, Sr, Pr, Ba, or Ca, and wherein X and Y are chosen from Co, Fe, Mn, Ti, Re, Cu, Cr, or Mo.

15. The electrode composition of claims 13 or 14, wherein the reduced perovskite substrate comprises reduced (LaSr)(CoFe)O3 and wherein the plurality of metal nanoparticles comprises Co metal or CoFe metal alloy.

16. The electrode composition of any one of claims 13-15, wherein the electrode composition is used as an anode catalyst for an oxidation reaction or as a cathode for a reduction reaction.

17. A method of electrocatalysis, the method comprising: a. providing the electrode composition of any one of claims 13-16 to an electrochemical system; b. providing a current to the electrochemical system; and c. streaming a feed gas into the electrochemical system.

18. The method of claim 17, wherein providing a current comprises providing an anodic current, and wherein streaming feed gas comprises methane, C2-C6 alkanes, C2-C6 alkenes, or combinations thereof.

19. The method of claim 17, wherein providing a current comprises providing an cathodic current, and wherein streaming feed gas comprises N2, NOx, CO2, H2O, O2 or combinations thereof.

20. The method of any one of claims 17-19, wherein the electrochemical system is a solid oxide electrochemical cell.

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