Method for producing hydrogen by decomposition of seawater, hydrogen generator, and method for producing hydrogen generator

Abstract

A method for producing hydrogen is provided. A method for producing hydrogen includes providing a solution and immersing a device in the solution. The device includes a substrate having a surface, an array of conductive protrusions supported by the substrate and extending outward from the surface of the substrate, and a plurality of catalyst nanoparticles disposed on the array of conductive protrusions. The solution includes dissolved sodium chloride (NaCl).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 433,746, filed December 19, 2022, entitled "Hydrogen Production by Seawater Splitting," the entire disclosure of which is expressly incorporated herein by reference. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with federal government support under Grant No. W911NF-21-1-0337 awarded by the USA Army Research Office. The government has certain rights in this invention. [Technical Field]

[0003] The present disclosure relates generally to photoelectrodes, photocatalytic devices, and other devices for hydrogen generation via water splitting and / or other reactions. [Background technology]

[0004] Hydrogen is a clean energy carrier that can replace fossil fuels due to its sufficiently high energy density and zero carbon emissions. Among several possible methods, water electrolysis has been proposed as one of the most promising routes for generating high-purity H2 at low temperatures. By using renewable energy to power water electrolysis with seawater, which is an abundant hydrogen source on Earth, a sustainable and environmentally friendly hydrogen economy can be realized. Acidic seawater prepared by mixing an acidic electrolyte with seawater is an attractive aqueous solution for highly efficient hydrogen evolution reaction (HER) because of the very high proton concentration near the cathode. However, in acididic seawater, competition with both chloride oxidation reaction (COR) and oxygen evolution reaction (OER) is inevitable at the anode. Since COR is kinetically favorable (although thermodynamically unfavorable) compared to OER, the slow 4-electron kinetics of OER results in low O2 selectivity. The difference in the standard electrode potential between OER and COR can be gradually increased to 0.48 V by raising the pH to 7.5. Therefore, COR can be suppressed under alkaline conditions.

[0005] Electrode catalysts have been developed for H2 production by alkaline seawater splitting. However, under alkaline conditions with a pH exceeding about 9.5, high concentrations of OH - ions can lead to the precipitation of insoluble salts (e.g., Mg(OH)2 and Ca(OH)2). A feasible approach to address the drawbacks of acidic and alkaline seawater is to use neutral or weakly alkaline (7.5 < pH < 9.5) seawater. In neutral electrolytes, the kinetic bottleneck of low HER activity is attributed to the Volmer step of water dissociation (H2O + e - → H + + OH - ). In this regard, various electrode catalysts have addressed the H-OH cleavage in water molecules to enhance HER under low overpotential. However, the HER activity in seawater with a neutral pH is still inferior to that in acidic and alkaline electrolytes.

[0006] Compared to electrochemical methods, photoelectrochemical (PEC) water splitting can achieve more efficient H2 generation by integrating semiconductor and cocatalyst materials. An electrical bias (e.g., from a solar cell) can shift the onset potential positively, promoting hydrogen generation with less power consumption. Over the past few decades, many semiconductor materials have been developed and applied as photocathodes for solar water splitting. However, conventional semiconductor materials, such as Si, GaAs, and InP, are easily corroded even in freshwater due to surface holes accumulated under light irradiation. The stability of such semiconductors is further reduced in seawater due to strong oxidizing agents (e.g., chlorine and hypochlorite). Although protective films on photoelectrodes have improved stability in freshwater, the number of photocathodes that operate efficiently in seawater has been limited. Summary of the Invention

[0007] According to one aspect of the present disclosure, a method for producing hydrogen includes providing a solution and immersing a device in the solution. The device includes a substrate having a surface, an array of conductive protrusions supported by the substrate and extending outward from the surface of the substrate, and a plurality of catalyst nanoparticles disposed on the array of conductive protrusions. The solution includes dissolved sodium chloride (NaCl).

[0008] According to another aspect of the present disclosure, a device includes a substrate having a front surface and a back surface opposite the front surface, an array of conductive protrusions supported by the substrate and extending outward from the front surface of the substrate, a plurality of catalytic nanoparticles disposed on the array of conductive protrusions, a first contact coupled to the back surface and to which charge carriers photogenerated in the substrate migrate in a photoelectrochemical (PEC) mode of operation of the device, and a second contact coupled to the array of conductive protrusions to provide charge carriers to the array of conductive protrusions in an electrochemical (EC) mode of operation of the device.

[0009] According to yet another aspect of the present disclosure, a device includes a substrate having a surface, an array of conductive protrusions supported by the substrate and extending outward from the surface of the substrate, wherein each conductive protrusion of the array of conductive protrusions comprises a metal nitride semiconductor material, and a plurality of metal catalysts disposed on the array of conductive protrusions such that a metal / metal nitride interface is established, the metal / metal nitride interface being configured to promote hydrogen production via water splitting.

[0010] According to yet another aspect of the present disclosure, a method for fabricating a hydrogen generation device includes providing a substrate having a surface and growing an array of nanowires on the surface of the substrate. Each nanowire in the array of nanowires extends outward from the surface of the substrate. Each conductive protrusion in the array of conductive protrusions includes a metal nitride semiconductor material. The method for fabricating a hydrogen generation device further includes depositing a plurality of metal catalyst nanoparticles over the array of nanowires such that a metal / metal nitride interface is established. The metal / metal nitride interface is configured to promote hydrogen production via water splitting.

[0011] In relation to any one of the aforementioned aspects, the devices and / or methods described herein may alternatively or additionally include or involve any combination of one or more of the following aspects or features: the solution includes seawater; the solution is a phosphate buffer solution; providing the solution includes lowering the pH of the solution; the method further includes irradiating the device such that charge carriers are photogenerated in the device to support hydrogen production; illuminating the device includes irradiating a backside of a substrate of the device with sunlight; the method further includes applying a bias voltage to the device. The device includes contacts coupled to the backside of the substrate. Applying the bias voltage includes applying the bias voltage to the contacts in a photoelectrochemical (PEC) operation mode of the device; the method further includes switching the bias voltage to apply the bias voltage to the array of conductive protrusions in an electrochemical operation mode of the device when the device is not illuminated; the device includes a eutectic alloy disposed on the surface and in electrical communication with the array of conductive protrusions, and switching the bias voltage includes providing the bias voltage to the array of conductive protrusions via the eutectic alloy. Immersing the device includes placing the device in a flow cell reactor through which the solution flows. Each conductive protrusion of the array of conductive protrusions and each catalytic nanoparticle of the plurality of catalytic nanoparticles establish a metal / metal nitride interface configured to promote hydrogen production via water splitting. The substrate includes silicon. Each conductive protrusion of the array of conductive protrusions includes a III-nitride semiconductor material. Each catalytic nanoparticle of the plurality of catalytic nanoparticles includes a platinum nanocluster. The first and second contacts include a eutectic alloy on the back surface of the substrate and on the front surface of the substrate, respectively. The device further includes a switch for applying a bias voltage to either the first contact or the second contact. The device further includes an oxide passivation layer covering the plurality of catalytic nanoparticles. The oxide passivation layer includes titanium oxide. Each conductive protrusion of the array of conductive protrusions has a semiconductor composition. The semiconductor composition of each conductive protrusion of the array of conductive protrusions is terminated with nitrogen along the surface of the conductive protrusion. Each conductive protrusion of the array of conductive protrusions includes a nanowire.Each catalyst nanoparticle of the plurality of catalyst nanoparticles comprises platinum. Each conductive protrusion of the array of conductive protrusions and each catalyst nanoparticle of the plurality of catalyst nanoparticles establish a metal / metal nitride interface configured to promote hydrogen production via water splitting. Each catalyst of the plurality of catalysts comprises platinum. Each catalyst of the plurality of catalysts comprises a rare earth element. The metal nitride semiconductor material is GaN. The method further includes forming first and second electrical connections to the array of nanowires and the substrate, respectively. Forming the first and second electrical connections to the array of nanowires and the substrate includes disposing a eutectic alloy on the front surface and the back surface of the substrate, respectively. The method further includes passivating the plurality of catalyst nanoparticles and the array of nanowires with an oxide layer. Passivating the plurality of catalyst nanoparticles and the array of nanowires includes depositing titanium oxide via atomic layer deposition. Depositing the plurality of catalyst nanoparticles includes performing a photo-deposition procedure using platinum. Growing the array of nanowires includes performing a molecular beam epitaxy (MBE) procedure under N-rich conditions. The eutectic alloy includes GaIn. The method further includes passivating the backside of the substrate and the electrical connections to the array of nanowires with epoxy.

[0012] For a more complete understanding of the present disclosure, reference should be made to the following detailed description and the accompanying drawings, in which like reference numerals identify like elements throughout. [Brief explanation of the drawings]

[0013] [Figure 1]An example shows Pt nanoclusters on GaN nanowires grown on n+-p Si wafers, including (a) a schematic diagram of the fabrication of a Pt / GaN / Si photocathode by epitaxial growth of GaN nanowires (NWs) on SiP-n wafers and photodeposition of Pt nanoclusters (NCs), (b) a 45° tilted view SEM image of the Pt / GaN / Si photocathode, (c) a HAADF-STEM image of the Pt / GaN NWs, and STEM-EDS elemental maps of (d) Ga, (e) N, and (f) Pt, and (g) Ga2p3 / 2, (h) N1s, and (i) Pt4f XPS spectra of the GaN / Si and Pt / GaN / Si electrodes. [Figure 2] 1 shows graphical plots of photoelectrochemical seawater hydrogen generation according to some examples, where (a) is the LSV curves of Si, Pt / Si, GaN / Si, and Pt / GaN / Si measured in a three-electrode configuration in 0.5M NaCl solution under AM 1.5G 1 sun illumination or in the dark; (b) is the LSV curves of Pt / GaN / Si in six aqueous solutions including acidic solutions (pH=0) of 0.5M H2SO4 and 0.5M NaCl+0.5M H2SO4, neutral solutions (pH=7.4) of 1M PBS and 0.5M NaCl+1M PBS, and weakly alkaline solutions of 0.5M NaCl (pH=9.1) and seawater (pH=8.2); (c) is the ABPE of Pt / GaN / Si in seawater, 0.5M NaCl, and 0.5M NaCl+1M PBS; and (d) is the ABPE of Pt / GaN / Si in 0.5M NaCl+1M PBS. LSV curves of Si, Pt / Si, GaN / Si, and Pt / GaN / Si measured in a two-electrode configuration in NaCl; the inset shows the starting potential for each electrode. (e) The amount of H produced and the faradaic efficiency of Pt / GaN / Si in 0.5 M NaCl at -3 V vs. IrOx are shown. The faradaic efficiency is nearly 100%. (f) The stability of Pt / GaN / Si in 0.5 M NaCl and seawater at -3 V is shown. The photocurrent density retains more than 85% of its initial value after 15 hours of reaction. The inset shows the LSV curves before and after the stability test. [Figure 3]Schematic representation of water dissociation with an example where the Pt / GaN interface promotes water splitting, including optimized structures and schematic diagrams of calculated energy changes for (a) water dissociation on Pt(111), (b) proton transfer from surface NH to Pt clusters, and (c) water dissociation at Pt-Ga sites at the Pt / GaN interface and subsequent H spillover to the Pt surface. Energy changes are shown in eV, and white, red, blue, green, and gray spheres represent H, O, N, Ga, and Pt atoms, respectively. [Figure 4] (a) An example switchable dual-contact electrode (e.g., a PEC / EC switchable electrode) is shown, in which the front contact is placed on the front side of a GaN nanowire (NW) and the back contact is placed on the back side of an n+-p Si wafer, with a GaIn eutectic sandwiched between the contact (e.g., a Cu contact) and the substrate for ohmic contact; (b) a schematic diagram of the front contact for electrochemical HER in the dark and (c) the back contact for photoelectrochemical HER in the light; (d) LSV curves of the electrodes measured in a two-electrode configuration in 0.5 M NaCl solution; (e) onset potentials and current densities at −2 V and −3 V for the front and back contacts, where the onset potential is defined as the potential at −10 mA / cm; and (f) a schematic diagram of the amount of H produced and the measured faradaic efficiency at −2 V and 3 V under dark (front contact) and light (back contact) conditions. [Figure 5] Figure 1 shows concentrated solar PEC water splitting according to some examples, including (a) a schematic and (b) a photograph of the liquid flow cell for the PEC water splitting examples where light illuminates the backside of an n+-p Si wafer and HER occurs at the frontside of Pt / GaN nanowires (NWs), as well as graphical plots of LSV curves measured for Pt / GaN / Si electrodes in (c) a three-electrode and (d) a two-electrode configuration in 0.5 M NaCl under different light intensities, and (e) a chronoamperometry curve and (f) further graphical plots of the amount of H produced and the faradaic efficiency measured under light intensities of 1, 3, 6, and 9 suns at -3 V. [Figure 6]1 is a schematic and block diagram of an electrochemical system having a photocathode with an array of nanostructures having a protection device for stable hydrogen generation by water splitting, according to an example. [Figure 7] 1 is a flow diagram of a method for manufacturing a photocathode having a protection device according to an example; [Figure 8] 1 is a flow diagram of a method for producing hydrogen from a solution containing sodium chloride (e.g., seawater), according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Embodiments of the disclosed devices, systems, and methods may assume a variety of forms. Specific embodiments are shown in the drawings and described below, with the understanding that the disclosure is intended to be illustrative. The disclosure is not intended to limit the invention to the specific embodiments described and illustrated herein.

[0015] Photoelectrochemical, photocatalytic, and other systems and devices (e.g., photocathodes or photoelectrodes) for hydrogen production and / or other chemical reactions are described. Hydrogen is produced from solutions containing dissolved sodium chloride (NaCl), such as seawater. Methods of hydrogen production, e.g., using the devices and systems, are also described.

[0016] The device can include a metal / metal nitride interface configured to promote water splitting. The interface can be established between a conductive protrusion (e.g., nanowire) composed of or otherwise including a metal nitride semiconductor material such as GaN and metal nanoparticles (e.g., nanoclusters) disposed on (e.g., distributed throughout) the conductive protrusion. Methods for fabricating the device (e.g., a photoelectrode or photocatalytic device) are also described.

[0017] During operation, the disclosed device is immersed in seawater or other solution containing dissolved NaCl. The pH of the solution may be lowered in preparation for hydrogen production. However, the solution may be acidic or alkaline. In some cases, the solution is a phosphate buffer solution.

[0018] The disclosed devices can be irradiated for charge carrier generation and photoelectrochemical operation. For example, the backside of the device's substrate can be irradiated to generate charge carriers. In some cases, solar radiation can be used to irradiate the device. Thus, the disclosed devices can be configured for solar-assisted or solar-driven seawater hydrogen generation and / or other chemical reactions. Water splitting can also be solar-driven, in the sense that a bias voltage applied to the contacts of the disclosed devices is supported by a photovoltaic device, such as a solar cell.

[0019] In some cases, the disclosed devices include multiple contacts to support both photoelectrochemical (PEC) and electrochemical (EC) operation. The PEC mode of operation may be used when the device is illuminated (e.g., during the day), and the EC mode of operation may be used when light is not available (e.g., at night). The device may include switches (e.g., switching electrodes) for applying bias voltages to the respective contacts of the device for the PEC and EC modes of operation. For example, a eutectic alloy may be disposed on the substrate surface and on the backside opposite the surface for electrical connection to the contacts.

[0020] Although described in the context of photoelectrochemical water splitting, the disclosed devices and systems may be used in other chemical reaction contexts and applications. For example, the disclosed devices and systems may be useful in conjunction with various types of photocatalytic and / or other systems and / or in conjunction with other reactions, including, for example, N reduction, CO reduction to various fuels and other chemicals, NO reduction, and urea synthesis.

[0021] In some cases, the disclosed devices may include an oxide passivation layer for protection of catalysts and / or other elements of the disclosed photoelectrodes (e.g., photocathode), photocatalytic devices, or other devices. The oxide passivation layer may be provided by a thin conformal layer (e.g., a conformal oxide layer). In some cases, the conformal layer is composed of titanium oxide, although additional or alternative oxides may be used. The conformal layer may have a thickness on the order of the size of each catalyst nanoparticle. The thin nature of the conformal layer is configured so that the layer does not inhibit, for example, charge carrier movement from the photoelectrode structure(s) to reaction sites along the photoelectrode. Nevertheless, the conformal layer is thick enough to cover and therefore protect the catalyst arrangement. Protection of the catalyst nanoparticles and / or other surface passivation is thus provided despite the thin nature of, for example, the oxide or other conformal layer.

[0022] Alternatively or additionally, surface protection is provided by the nitrogen-based surface of the disclosed photoelectrode. When including an array of nanostructures, each nanostructure may be composed of or otherwise include a compound semiconductor that establishes a nitrogen-based surface. For example, the nanostructures may be composed of or otherwise include GaN in a nitrogen-terminated configuration. Alternatively or additionally, the disclosed photoelectrode may have one or more surfaces on which a nitrogen or other nitrogen-based layer is disposed.

[0023] In some cases, the disclosed devices can include arrays of N-terminated nanowires (e.g., GaN nanowires) grown on Si. The nanowires can effectively protect the underlying Si photocathode from degradation, for example, for over 3000 hours in acidic electrolytes. Furthermore, as described herein, cocatalysts supported on GaN nanowires exhibit unique catalytic performance resulting from the cocatalyst-GaN interface while remaining stable for enhanced seawater splitting.

[0024] An example of a Si photocathode with Pt nanoclusters (NCs) on GaN nanowires is described below. The efficiency and stability of the Si photocathode applied to seawater PEC hydrogen generation are addressed. GaN nanowires are loaded with catalytic Pt nanoclusters and grown on the Si photocathode. The nanowire-nanocluster arrangement catalyzes water splitting while protecting the Si wafer from corrosion. Furthermore, DFT calculations show that the Pt-Ga sites at the Pt / GaN interface promote the adsorption and activation of water molecules. Dissociated H * The atoms spill over into neighboring Pt nanoclusters, promoting H evolution. As a result, the exemplary Pt / GaN / Si photocathode exhibits a current of −10 mA / cm at 0.15 and 0.39 V vs. RHE (VRHE). 2 The photocathode used in the example was a two-electrode configuration made of IrO x -10 mA / cm at a small potential of -1.45 V 2 The photocathode exhibited a current density of approximately -169 mA / cm under concentrated sunlight (9 suns) and operated stably for over 120 hours, demonstrating superior performance compared to photocathodes without GaN nanowires. 2 The high current density produced high purity hydrogen (H2) continuously.

[0025] Although described herein in connection with electrodes having GaN-based nanowire arrays for water splitting, the disclosed devices and systems are not limited to GaN-based nanowire arrays. A wide variety of other types of nanostructures and other conductive protrusions can be used. In some cases, the electrodes of the disclosed systems do not include arrays of nanowires, but instead include protrusions of other shapes. Accordingly, the nature, structure, composition, properties, shape, and other aspects of the electrodes may vary.

[0026] Although described herein in connection with electrodes having platinum nanocluster catalysts for water splitting, the disclosed devices and methods are not limited to platinum nanoclusters. Other materials and other nanoparticle structures may be used. Accordingly, the nature, structure, composition, characteristics, shape, and other aspects of the nanoparticle catalyst may vary.

[0027] FIG. 1 shows an example photocathode with a PtGaN / Si microstructure, where GaN nanowires are grown by plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich conditions. + The GaN nanowires were grown on a -p Si(100) substrate (Figure 1, part a). Pt nanoclusters were deposited to coat the surface of the GaN nanowires. In this case, the nanoclusters were applied by photochemical deposition, further details of which are provided below. Scanning electron microscope (SEM) images show that the GaN nanowires are planar n-type nanowires with lengths of approximately 400 nm and diameters of approximately 50 nm. + -p Si substrate (Figure 1, part b). The Pt nanoclusters are not clearly visible in the SEM image due to their small size.

[0028] The microstructures of the GaN nanowires and Pt nanoclusters were analyzed by scanning transmission electron microscopy (STEM). In the high-angle annular dark-field (HAADF)-STEM image (Figure 1, part c), Z-contrast from locally segregated Pt nanoclusters was observed on the GaN nanowires due to the large difference in atomic numbers between Pt(78) and Ga(31). The atomic distribution was elucidated by STEM-EDS elemental maps (Figure 1, parts d-f). Ga, N, and Pt elements were found to be uniformly distributed throughout the entire surface of the GaN nanowires. The X-ray diffraction (XRD) pattern of the Pt / GaN / Si microstructure showed GaN (002) and (004) peaks. However, due to the small size of the microcrystalline Pt nanoclusters, the XRD peaks of microcrystalline Pt nanoclusters were not detected.

[0029] X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface bonding states of the GaN / Si and Pt / GaN / Si microstructures. 3 / 2 The XPS spectrum was deconvoluted with the main peak of the Ga-N bond (1118.4 eV) and the minor peak of the Ga-O or Ga-OH bond (1119.6 eV) (Figure 1, part g). In the N 1s XPS spectrum, photoelectrons originating from N-Ga (398.2 eV) and NH (399.7 eV) were detected along with Ga LMM Auger electrons (Figure 1, part h). The intensities and energy positions of the bonding states were similar to those of the Ga 2p bond after the deposition of Pt nanoclusters. 3 / 2 and N 1s spectra remained nearly identical, indicating that the N-rich GaN nanowires were stable to photochemical reactions in aqueous solution during photodeposition. 5 / 2 and 4f 7 / 2 The Pt4fXPS spectrum with two doublets is shown as Pt 0 (71.3 and 74.5 eV) and Pt 2+ (72.3 and 75.6 eV) bonds were analyzed (Figure 1, part i). The Pt nanoclusters mainly contain metal-Pt bonds, with Pt 2+ A small portion of the bonding is likely due to surface-absorbed oxygen or Pt-N bonding at the interface between Pt and GaN. For comparison, a Pt / Si reference electrode was used, with a planar n + -pSi was prepared by photodeposition of Pt nanoclusters and showed Pt4fXPS spectra with similar bonding states.

[0030] The performance of the example device in photoelectrochemical seawater splitting was then evaluated. The PEC hydrogen evolution reaction was performed on the planar n +The simulation was performed using a simulated AM1.5G solar spectrum incident on the Pt-decorated GaN nanowire array at an angle perpendicular to the Si wafer. The conduction band edge of GaN is located above the redox potential for the hydrogen evolution reaction, indicating a suitable energy band structure for electron transfer. An electrical bias was applied to the backside of the Si wafer. Linear sweep voltammetry (LSV) curves of the Si, Pt / Si, GaN / Si, and Pt / GaN / Si microstructures were measured in 0.5 M NaCl (pH = 9.1) in a three-electrode configuration at 1 sun (100 mW / cm). 2 ) under irradiation (Figure 2, part a). 2 The potential at (η 10 ) were −1.42, −1.41, −0.83, and 0.16 VRHE for the Si, Pt / Si, GaN / Si, and Pt / GaN / Si microstructures, respectively. Both GaN nanowires and Pt nanoclusters improved the activity of the PEC hydrogen evolution reaction. The synergistic interaction between GaN nanowires and Pt nanoclusters played a useful role in achieving high catalytic activity for the Pt / GaN / Si microstructure. The saturation current density (approximately 35 mA / cm) of the GaN / Si and Pt / GaN / Si microstructures was −1.42, −1.41, −0.83, and 0.16 VRHE for the Si, Pt / Si, GaN / Si, and Pt / GaN / Si microstructures, respectively. 2 ) without GaN nanowires (approximately 30 mA / cm 2 ) because the GaN nanowires increase the surface area and reduce Fresnel reflection losses. The Pt / GaN / Si microstructure exhibited negligible current density in the dark due to the absence of photogenerated charge carriers.

[0031] An exemplary Pt / GaN / Si microstructure was evaluated in six different solutions (Figure 2, part b) to investigate the effects of pH and ions in the solution. The Pt / GaN / Si microstructure exhibited high PEC hydrogen evolution reaction activity (η ) in 0.5 M H2SO4 solution (pH = 0) and 1 M phosphate buffer solution (PBS) (pH = 7.4), even with dissolved NaCl. 10 >0.3V RHE ) was shown. H near the electrode surface +Although the hydrogen evolution reaction performance is slightly worse than that in acidic and neutral solutions due to the lack of ions, the hydrogen evolution reaction in 0.5 M NaCl (pH = 9.1) and seawater (pH = 8.2) is stable at applied potentials > 0 V. RHE The Pt / Si microstructure was still demonstrated to be catalytically inert. As a result, high ABPEs of 7.9% in 0.5M NaCl + 1M PBS, 1.6% in 0.5M NaCl, and 1.7% in seawater were achieved (Figure 2c). Meanwhile, the PEC hydrogen generation reaction performance of the Pt / Si microstructure significantly decreased in 0.5M NaCl and 0.5M NaCl + 1M PBS. These findings revealed that the Pt / Si microstructure exhibited poor catalytic activity due to the slow kinetics of water dissociation in neutral electrolytes. Furthermore, the LSV curves of the Pt / GaN / Si microstructure were identical in solutions with different NaCl concentrations, confirming the feasibility of hydrogen generation in seawater.

[0032] To test practical photoelectrolysis, iridium oxide (IrO) x ) LSV curves were measured in 0.5 M NaCl solution using a two-electrode configuration against a counter electrode (Figure 2, part d). 10 were −3.41, −3.47, −2.85, and −1.88 V for the Si, Pt / Si, GaN / Si, and Pt / GaN / Si microstructures, respectively. 10 improved to -1.33 and -1.45 V when measured in acidic (0.5 M NaCl + 0.5 M H2SO4) and phosphate buffered (0.5 M NaCl + 1 M PBS) seawater, respectively. To the best of applicant's knowledge, -1.45 V is the best η value among PEC and ECHER electrodes in neutral pH seawater. 10 is.

[0033] The faradaic efficiency of H2 and O2 generated from the Pt / GaN / Si microstructure with a two-electrode configuration was further evaluated in 0.5 M NaCl and seawater. x At -2V, the H2 production rate is 275 μmol / cm 2 / h, and the O2 production rate is 128 μmol / cm in 0.5 M NaCl. 2 / h. The H2 / O2 ratio (2.15) is close to the theoretical value for water splitting (2). x ) is applied to the photocathode, the rates of H2 and O2 production are 600 and 216 μmol / cm3 in 0.5M NaCl, respectively. 2 / h (Figure 2, part e), and 575 and 159 μmol / cm / h in seawater. Regardless of the electrolyte and applied potential, the faradaic efficiency of H was nearly 100%, confirming the high efficiency of electron utilization. On the other hand, the faradaic efficiency of O was significantly higher when the potential was changed to IrO. x By increasing the voltage from -2V to -3V, the ion concentration decreased from 93% to 61-75%. - It is likely due to an oxidation reaction.

[0034] The exemplified device exhibited stability with little degradation in the chronoamperometry (CA) and LSV curves after 15 hours of reaction in 0.5 M NaCl and seawater (Figure 2, part f). There were no significant changes in the morphology and microstructure of the Pt·GaN·Si microstructure. Clearly, the physical and chemical stability of the Si photoelectrode with GaN nanowires and Pt nanoclusters is greatly improved compared to the rapid degradation of the Pt / Si microstructure. Furthermore, the performance of the Pt / GaN / Si microstructure could be fully restored after redeposition of Pt nanoclusters, indicating that removal of Pt nanoclusters in seawater is the cause of degradation. Although a thin TiO layer (2 nm) on the Pt / GaN / Si microstructure slightly reduced catalytic activity, the Pt nanoclusters were stabilized on the GaN nanowires, resulting in long-term stability exceeding 120 hours in pH-neutral seawater.

[0035] Theoretical modeling was conducted to investigate the mechanistic role played by the Pt / GaN binary system via periodic density functional theory (DFT) calculations. While Pt and other catalysts are capable of proton reduction, Pt is not good at reducing water under neutral or alkaline conditions because water dissociation is a bottleneck for efficient hydrogen evolution under these conditions. Therefore, modeling focused on the water dissociation step on the Pt(111) and Pt / GaN interfaces to provide a mechanistic understanding of the favorable performance of the Pt / GaN binary system.

[0036] We first analyzed the dissociation of water on a Pt(111) surface. The H-OH bond cleavage is highly endothermic on Pt(111), with an energy change of 0.72 eV (Figure 3, part a). This is consistent with the experimental fact that Pt is not an effective catalyst for H2 generation in neutral or alkaline solutions. It is known that water dissociates on the m-plane of GaN due to the highly polar surface. When water molecules approach the GaN surface, they spontaneously form H2. + and OH - It dissociates into OH - coordinates to Ga, and H + is bonded to N. However, the proton transfer from the surface NH groups to the Pt clusters was found to be highly endothermic by 1.38 eV, likely due to the high pKa of the NH groups (Figure 3, part b). At the same time, the dissociation of water at the Pt-Ga sites at the Pt / GaN interface was found to be highly favorable with an energy change of -0.67 eV (Figure 3, part c).

[0037] Under photoelectrochemical water splitting conditions, a gallium oxynitride (GaON) layer can be formed on the surface of GaN. Further details regarding GaON layers are described in WO 2022 / 187133 ("Crystallographic- and Oxynitride-Based Surface Stabilization"), the entire disclosure of which is incorporated herein by reference.

[0038] To investigate the dependence of water dissociation energy on GaN and GaON surfaces, we investigated water dissociation at the Pt-Ga site at the Pt / GaON interface. The energy change of water dissociation is similar to that at the Pt / GaN interface. Compared to water dissociation on Pt(111), water dissociation at the Pt-Ga site benefits from an asymmetric atomic environment that promotes heterolytic cleavage of H-OH bonds to form Pt-H and Ga-OH. * H spontaneously spills onto neighboring Pt atoms with a ΔE of -0.15 eV. Therefore, the overall energy change due to water splitting at the Pt / GaN interface is -0.81 eV. The efficient water dissociation at the Pt-Ga site is likely responsible for the high activity of seawater splitting on Pt / GaN / Si in neutral pH or weakly alkaline solutions. Interestingly, the difference in water dissociation energy ΔE when comparing Pt(111) with the Pt-Ga site is 1.53 eV, which is the same as the difference in overpotential (Δη) between the Pt / Si and Pt / GaN / Si electrodes. 10 = 1.58 V), demonstrating the usefulness of Pt-Ga sites at the Pt / GaN interface for achieving efficient water splitting.

[0039] Here, we describe the operation of an exemplary device in concentrated solar PEC seawater splitting. Concentrated sunlight increases the number of photogenerated electrons in the semiconductor, increasing the photocurrent for the PEC hydrogen evolution reaction. However, the reaction rate under concentrated sunlight with vigorous H evolution can be limited by mass transport and the shielding of active sites by gas bubbles. A flow cell can simultaneously solve these two problems by fast delivery of reagents and physical detachment of gas bubbles from the electrode surface. Therefore, a liquid flow cell reactor can be used in concentrated solar PEC seawater splitting to demonstrate high photocurrent density (Figure 4, part a). Light is n + The Pt / GaN / Si microstructure was exposed to the backside of the Pt / GaN / Si wafer, while the PEC hydrogen evolution reaction occurred at the front side of the reactor (Figure 4, part b). In this example, a 0.5 M NaCl solution was continuously flowing at a rate of 20 ml / min during the measurements. RHE η 10 and approximately -19mA / cm under sunlight. 2A saturation current density of about −58, −118, and −163 mA / cm was achieved (Figure 4, part c). When the light was increased to 3, 6, and 9 suns, the saturation current densities were about −58, −118, and −163 mA / cm, respectively. 2 In the two-electrode configuration, the Pt / GaN / Si microstructure also showed a gradual increase in photocurrent density, reaching approximately −165 mA / cm at −3.2 V under solar illumination. 2 reached high values ​​(Figure 4, part d). The linear correlation between light intensity and saturation current density indicates that light intensity and the number of photogenerated electrons at the photoelectrode are limiting factors for the PEC hydrogen evolution reaction in the flow cell.

[0040] CA measurements were performed using IrO x The current densities for each reaction period (-20, -60, -121, and -169 mA / cm, respectively) were evaluated at -3 V for 1, 3, 6, and 9 suns (Figure 4, part e). 2 ) was stable in 0.5 M NaCl, a record high photocurrent density for the PEC hydrogen evolution reaction between photocathodes in aqueous electrolytes.

[0041] The amount and faradaic efficiency of H2 production were monitored under different light intensities at -3 V for 4 h (Figure 4, part f). The production rate increased from 359 to 3351 μmol / cm2 as the incident irradiance increased from 1 to 9 suns. 2 / h. The faradaic efficiency was nearly 100% regardless of light intensity, confirming that all of the photocurrent was involved in the hydrogen evolution reaction. Furthermore, two individual photocathodes were tested in 0.5 M NaCl at 9 suns and -3 V for 4 hours. Both samples showed 166 and 157 mA / cm without degradation. 2 The photocurrent density was 160 mA / cm, demonstrating remarkable stability even under intense gas bubbling. Furthermore, concentrated sunlight experiments were performed in phosphate buffer solution (0.5 M NaCl + 1 M PBS) and showed high photocurrent densities (160 mA / cm). 2 The Pt / GaN / Si microstructure of the exemplary device thus provides a highly efficient, selective, and stable photocathode for seawater splitting.

[0042] In some cases, the disclosed devices may be configured for both PEC (light) and electrochemical (dark) operation, and the devices may include switches or switchable electrodes to support both modes of operation, as described below.

[0043] GaN nanowires can be degenerately doped n-type and have high conductivity due to the dense packing of Pt nanoclusters. Therefore, electrical contact at the front side of the GaN nanowire / Si wafer can establish a cathode for the electrochemical hydrogen evolution reaction. Therefore, the Pt / GaN / Si microstructure can function under either light or dark conditions by switching the contact position (Figure 5, part a). During the electrochemical hydrogen evolution reaction in the dark, an electrical bias injects electrons into the GaN nanowires, which then drift to the Pt nanoclusters, resulting in H2 evolution (Figure 5, part b). In contrast, during the photoinduced PEC hydrogen evolution reaction, n-type GaN nanowires with a narrow bandgap (approximately 1.1 eV) are used. + The -pSi substrate generates electron-hole pairs upon solar irradiation (Figure 5, part c). The photogenerated electrons in the conduction band migrate toward the n-type GaN nanowires and Pt nanoclusters for the hydrogen evolution reaction, while the holes migrate to the back contact and counter electrode to participate in the anodic reaction.

[0044] The performance of the switchable electrodes was evaluated using an exemplary device immersed in a 0.5 M NaCl solution (Figure 5, part d). When the front contact was used, i.e., when operating in electrochemical mode, there was no photoresponse and the LSV curves under dark and light conditions overlapped. In contrast, when the back contact was used (PEC mode), there was no photocurrent in the dark and η 10 , and 0.11 V for the three-electrode configuration under one solar irradiation, respectively. RHE, -1.83 V in the two-electrode configuration. The built-in potential of the Si pn junction can shift the onset potential positively by about 0.4 V compared to the electrochemical reaction, allowing for a relatively high current density (-17.5 mA / cm) at a low operating voltage (-2 V). 2 ) can be achieved (Figure 5, part e). However, the photocurrent density is limited by minority carrier diffusion in reverse bias as well as the number of photogenerated charge carriers, approximately 35 mA / cm. 2 The electrochemical hydrogen evolution reaction was performed in the dark using the front contact, resulting in a higher yield of H at −3 V (−58 mA / cm). 2 ) was observed. The high quality of the degenerately doped n-type GaN nanowires, evidenced by the enhanced saturation photocurrent density of the Pt / GaN / Si assembly compared to the Pt / Si microstructure, allows for the use of majority carriers for the hydrogen evolution reaction due to factors such as the absence of Fermi level pinning and efficient electron transport to Pt at the nonpolar sidewalls. Furthermore, the productivity and faradaic efficiency of the Pt / GaN / Si microstructure were measured at -2 V and -3 V under dark and light conditions (Figure 5, part f). At -2 V, the generation rates were 100 and 276 μmol / cm in the dark and light, respectively. 2 / h, indicating that the PEC hydrogen evolution reaction is more favorable than the electrochemical hydrogen evolution reaction under light irradiation. On the other hand, at a more negative potential of -3 V, the electrochemical HER generation rate (1125 μmol / cm) 2 / h) is the PEC hydrogen evolution reaction (562 μmol / cm 2 / h), which was about two times higher than that of the conventional Pt / GaN / Si assembly. Remarkably, the faradaic efficiency was nearly 100% under all conditions. Therefore, the Pt / GaN / Si assembly can operate as a photocathode during the day and as an electrocatalyst for the continuous production of clean hydrogen at night.

[0045] The above examples demonstrate that Pt nanoclusters bound to GaN nanowires are highly active for the PEC hydrogen evolution reaction in acidic, neutral, and weakly alkaline solutions of seawater. DFT calculations further confirmed that the Pt-Ga sites at the Pt / GaN interface promote water dissociation and facilitate H2 evolution. Therefore, Pt / GaN on Si electrodes is a synergistic binary system for achieving highly efficient catalytic hydrogen evolution over a wide pH range. The Pt / GaN / Si microstructure exhibits excellent η with high ABPE values ​​of 1.7%, 1.6%, and 7.9% in seawater (pH = 8.2), 0.5 M NaCl (pH = 0.1), and phosphate-buffered seawater (pH = 7.4), respectively. 10 =0.15 and 0.39V RHE In the two-electrode configuration, the photocathode operated for 120 hours, achieving approximately -169 mA / cm under concentrated sunlight (9 suns). 2 The Pt / GaN / Si microstructure produced a record-high photocurrent density of 1000 kJ / cm². The measured efficiency and stability are among the highest reported to date for the PEC hydrogen evolution reaction in seawater. Importantly, the Pt / GaN / Si microstructure can operate uniquely under both light (PEC) and dark (electrochemical) conditions. The reported findings indicate that the Pt / GaN / Si assembly can be used as an energy-saving electrode to promote catalytic reactions and with superior performance compared to conventional photoelectrolysis systems.

[0046] FIG. 6 illustrates a system 100 for hydrogen generation by water splitting, according to one example. The system 100 may also be configured for other reactions. The system 100 may be configured as an electrochemical system. In this example, the electrochemical system 100 is a photoelectrochemical (PEC) system that uses sunlight and / or other radiation to promote hydrogen generation and water splitting. Water splitting may be assisted or unassisted. For example, unassisted water splitting (e.g., solar-only powered operation) can be performed in conjunction with InGaN nanowires having tunnel junctions. The manner in which the PEC system 100 is illuminated may vary. The wavelength and other characteristics of the radiation may vary accordingly.

[0047] The electrochemical system 100 includes one or more electrochemical cells 102. For ease of illustration and description, a single electrochemical cell 102 is shown. The electrochemical cell 102 and other components of the electrochemical system 100 are also shown schematically in FIG. 6 for ease of description. The cell 102 includes an electrolyte solution 104. The solution 104 includes dissolved NaCl and may be acidic, neutral, or alkaline, as described herein. In some cases, CO2 and / or other sources are applied. In some cases, the electrolyte solution is saturated with CO2. Additional or alternative electrolytes may be used, as described below. Further details regarding an example electrochemical system 100 are provided below.

[0048] In the example of FIG. 6, the electrochemical cell 102 has a three-electrode configuration. The electrochemical cell 102 includes a working electrode 108, a counter electrode 110, and a reference electrode 112, each immersed in the electrolyte 104. The counter electrode 110 may be or include a metal wire, such as a platinum wire. The reference electrode 112 may be configured as a reversible hydrogen electrode (RHE) (e.g., Ag / AgCl filled with 3M KCl). The positioning of the reference electrode 112 may differ from the example shown. For example, the reference electrode 112 may be adjacent to the counter electrode 110 in other cases. The configuration of the counter electrode 110 and the reference electrode 112 may vary. For example, the counter electrode 110 may be connected to a counter electrode 112 to detect the oxidation of water (4H2O⇔2O2 + 8e - +8H + ) occurs, or may otherwise include a photoanode. In some cases, the counter electrode 110 is made of IrO x It is configured as an electrode or IrO x The configuration of the electrochemical cell may vary. For example, in other cases, two-electrode or other configurations may be used.

[0049] Hydrogen evolution occurs at the working electrode 108 as follows.

[0050] Hydrogen generation: 2H2O + 2e -⇔H2+2OH - To this end, electrons can flow from the counter electrode 110 through a circuit path external to the electrochemical cell 102 to the working electrode 108. The working electrode 108 and counter electrode 110 can therefore be considered the cathode and anode, respectively. As mentioned above, water dissociation can also occur at the working electrode 108.

[0051] 6, the working and counter electrodes are separated from each other by a membrane 114, for example a proton exchange membrane. The structure, composition, construction, and other properties of the membrane 114 can vary.

[0052] In the example of FIG. 6, the circuit path includes a voltage source 116 of the electrochemical system 100. The voltage source 116 is configured to apply a bias voltage between the working electrode 108 and the counter electrode 110. The bias voltage can be used to establish the ratio of CO reduction to hydrogen (H) evolution at the working electrode and / or another reaction ratio. The circuit path can include additional or alternative components. For example, the circuit path may include a potentiometer in some cases. In other cases, no bias voltage is applied (e.g., in an unassisted system).

[0053] In this example, the working electrode 108 is configured as a photocathode. Light 118, such as solar radiation, may be incident on the working electrode 108 as shown. Thus, the electrochemical cell 102 may be considered a photoelectrochemical cell and configured as such. In such a case, illumination of the working electrode 108 may generate charge carriers within the working electrode 108. Electrons arriving at the surface of the working electrode 108 may then be used for hydrogen generation. The photogenerated electrons may augment electrons provided via the current path. Alternatively, or additionally, the electrons provided via the current path may recombine with photogenerated holes at a backside or other contact. Further details regarding example photocathodes are provided below.

[0054] The working electrode 108 includes a substrate 120. The substrate 120 of the working electrode 108 can form part of the architecture, scaffolding, or other support structure of the working electrode 108. The substrate 120 can be homogeneous or composite. For example, the substrate 120 can include any number of layers or other components. Thus, the substrate 120 may or may not be monolithic. The shape of the substrate 120 can also vary. For example, the substrate 120 may or may not be planar or flat.

[0055] 6, the substrate 120 is doped or otherwise configured to exhibit a junction. Thus, the substrate 120 of the working electrode 108 can be active (functional) with respect to photogeneration of charge carriers. Alternatively or additionally, the substrate 120 is passive (e.g., structural). The substrate 120 can be configured and act as a support structure for the catalyst arrangement of the working electrode 108, as described below. Alternatively or additionally, the substrate 120 can be composed of or otherwise include a material suitable for growth or other deposition of the catalyst arrangement of the working electrode 108.

[0056] In the active or functional case, substrate 120 may include a light-absorbing material. The light-absorbing material is configured to generate charge carriers upon sunlight or other illumination. The light-absorbing material has a band gap such that incident light generates charge carriers (electron-hole pairs) within the substrate. Part or all of substrate 120 may be configured for photogeneration of electron-hole pairs. To this end, substrate 120 may include a semiconductor material. In some cases, substrate 120 is composed of or includes silicon. For example, substrate 120 may be provided as a silicon wafer.

[0057] The silicon may be doped. In the example of FIG. 6, substrate 120 includes heavily n-doped layer 122, moderately or lightly p-doped layer 123, and heavily p-doped layer 124. The arrangement of layers 122-124 establishes a junction within substrate 120. The doping arrangement may vary. For example, one or more components of substrate 120 may be undoped (intrinsic) or effectively undoped. Substrate 120 may include alternative or additional layers, including, for example, support layers or other structural layers. In other cases, substrate 120 is not light-absorbing.

[0058] The substrate 120 of the working electrode 108 establishes a surface upon which the catalytic device is disposed. In some cases, a catalyst support structure or scaffold for the electrode 108 is provided as described below. As described below, the catalyst support structure may include an array of conductive protrusions extending outward from the surface of the substrate 120. In other cases, the catalyst arrangement does not include conductive protrusions. For example, the catalyst arrangement may include one or more planar structures, such as one or more layers, supported by the substrate 120.

[0059] 6, the working electrode 100 includes an array of nanostructures 126 (or other conductive protrusions) supported by a substrate 120. Each nanostructure 126 is configured to extract charge carriers (e.g., electrons) from the substrate 120. The extraction transports the electrons along the nanostructure 126 to an external site for use in hydrogen generation.

[0060] In some cases, each nanostructure 126 is configured as a nanowire. Each nanostructure 126 can have a semiconductor composition. In some cases, the semiconductor composition includes a semiconductor core. For example, the core may be composed of or include a III-V nitride semiconductor material, such as gallium nitride (GaN). Additional or alternative semiconductor materials may be used, including, for example, indium gallium nitride (InGaN) and / or other III-V nitride semiconductor materials.

[0061] The core of each nanowire or other nanostructure 126 may be or include a pillar-, post-, or other elongated structure that extends outward (e.g., upward) from the plane of the substrate 120. The semiconductor nanowires or other nanostructures 126 may be grown or formed as described in U.S. Pat. No. 8,563,395, the entire disclosure of which is incorporated herein by reference. The nanostructures 126 may be referred to herein as nanowires, with the understanding that the dimensions, size, shape, composition, and other properties of the nanostructures 126 or other conductive protrusions may vary.

[0062] The semiconductor composition of each nanostructure 126 may or may not be configured to facilitate the reaction(s) supported by electrochemical system 100. The semiconductor composition may be configured for photogeneration of charge carriers, as described below. Alternatively or additionally, the semiconductor composition may be configured to act as a catalyst for the reaction. The semiconductor composition may provide other functions, including, for example, protection of substrate 120, as described above in connection with the GaN example. Additional or alternative semiconductor materials may be used, including, for example, indium nitride, aluminum nitride, boron nitride, aluminum oxide, silicon, and / or alloys thereof.

[0063] The semiconductor composition of each nanostructure 126 can be configured to provide surface passivation and / or other protection for the photoelectrode 108. For example, in some cases, the semiconductor composition is terminated with nitrogen along the surface of the nanostructure 126. The nitrogen termination or other nitrogen-based aspects of the nanostructure 112 can protect the nanostructure 126 and / or other components of the electrode 108 (e.g., the substrate 120) during operation, for example, from corrosion. In other cases, alternative or additional nitrogen-based protection schemes can be used. For example, a nitrogen-containing layer can be deposited or otherwise disposed along the surface of each nanostructure 126 and / or other elements of the electrode 108.

[0064] Nanostructures 126 can facilitate hydrogen evolution and / or another chemical reaction in one or more ways. For example, each nanostructure 126 can be configured to extract charge carriers (e.g., electrons) generated at substrate 120. The extraction transports the electrons along nanostructure 126 to an external site for use in hydrogen evolution and / or another chemical reaction. The composition of nanostructures 122 can also form an interface well suited for hydrogen evolution and / or another chemical reaction, as described herein.

[0065] Each nanostructure 126 may be or include a pillar-, post-, or other elongated structure that extends outward (e.g., upward) from the plane of the substrate 120. The dimensions, size, shape, composition, and other properties of the nanostructures 126 may vary. For example, each nanostructure 126 may or may not be elongated, such as a nanowire. Thus, other types of nanostructures from the substrate 120 may be used, such as nanocrystals of various shapes.

[0066] In some cases, nanostructures 126 may be configured to generate electron-hole pairs upon irradiation. For example, nanostructures 122 may be configured to absorb light at a different frequency than other light-absorbing components of electrode 108. For example, one light-absorbing component, such as substrate 120, may be configured to absorb in the visible or infrared wavelength range, while another component may be configured to absorb light at ultraviolet wavelengths. In other cases, nanostructures 126 are the only light-absorbing component of electrode 108. In still other cases, substrate 120 is the only light-absorbing component of electrode 108.

[0067] In some cases, each nanowire 126 may include a layered or segmented arrangement of semiconductor materials. For example, in the example of III-nitrides, the layers or segments of the arrangement may have different III-group (e.g., indium and gallium) compositions. One or more layers or segments in the arrangement may be configured for absorption in a respective wavelength range. Each nanowire 126 may include one or more segments with a compound semiconductor composition (e.g., InGaN) configured for photogeneration of charge carriers. Other layers or segments may be directed toward establishing a tunnel junction. Each nanowire 126 may include a segment with a compound semiconductor composition (e.g., InGaN) configured to establish a tunnel junction. Each nanowire 126 may also include additional or alternative segments, including, for example, a segment between the tunnel junction and the substrate 120.

[0068] In other cases, layered arrangements of semiconductor materials are used to establish multi-band structures, such as quadruple-band structures. Each layer or segment of the structure may have a different semiconductor composition to establish a different bandgap. Different bandgaps can be useful in absorbing different wavelengths of light.

[0069] Other layer arrangements may also be used. For example, further details regarding the formation and construction of multi-band structures, including triple-band structures, are provided in U.S. Patent No. 9,112,085 ("High efficiency broadband semiconductor nanowire devices") and U.S. Patent No. 9,240,516 ("High efficiency broadband semiconductor nanowire devices"), the disclosures of which are incorporated by reference in their entireties.

[0070] The semiconductor composition of each nanowire 126 may be configured to improve the efficiency of water splitting in additional ways. For example, in some cases, the semiconductor composition of each nanowire 126 may include doping to promote charge carrier separation and extraction and to facilitate the establishment of a photochemical diode. For example, the dopant concentration of the semiconductor composition may vary laterally.

[0071] In examples involving Group III nitride compositions, the dopant may be or include magnesium. Further details regarding how magnesium doping promotes charge carrier separation and extraction are described in U.S. Patent No. 10,576,447 ("Methods and systems relating to photochemical water splitting"), the entire disclosure of which is incorporated by reference. Depending on the semiconductor light absorber selected, additional or alternative dopant materials may be used, including, for example, silicon, carbon, and beryllium.

[0072] The semiconductor device can further include catalytic nanoparticles 136 disposed on the array of nanowires 126. The nanoparticles 136 are distributed across or along the exterior surface (e.g., sidewall) of each nanowire 126. The nanoparticles 136 are configured to promote the proton reduction reaction. In some cases, each nanoparticle 136 includes a metal such as platinum. Other metals or materials, such as rare earth metals, may also be used. Still other metals, including alloys and / or other metals or combinations of metals, may be used. Further details regarding the formation, configuration, functionality, and other properties of the nanoparticles 136 in conjunction with the nanowire array are described in one or more of the above-referenced U.S. patents.

[0073] The distribution of the nanoparticles 136 may be uniform or non-uniform. Thus, the nanoparticles may be randomly distributed across each nanowire 126. The schematic configuration of Figure 6 is shown for ease of explanation.

[0074] The electrode 108 also includes an oxide layer 138 covering the catalytic nanoparticles 136 and each nanostructure 126 in the array of nanostructures 126. The oxide layer 138 acts as a passivation and / or other protective layer. For example, the oxide layer 138 may be configured to protect the nanoparticles 136 and / or passivate the surface of the nanostructures 126. In some cases, the oxide layer 138 is composed of or includes titanium oxide. Alternative or additional oxide materials may be used, including, for example, aluminum oxide, gallium oxide (and / or aluminum), and / or gallium oxynitride (and / or aluminum) (or oxynitrides).

[0075] The oxide layer 138 can have a thickness on the order of the size of each catalytic nanoparticle 136. For example, the thickness of the oxide layer 138 can be in the range of about 1 nm to about 2 nm, although other thicknesses may be used. In these and other cases, the oxide layer 138 conformally covers the array of catalytic nanoparticles 136 and nanostructures 126. For example, the oxide layer 138 can conformally cover the sidewalls and other surfaces of the nanostructures 126, as shown in FIG. 6. The conformal coverage of the nanostructure surfaces and nanoparticles is shown schematically for ease of illustration. The thin nature of the oxide layer 138 allows it to provide a protective function without adversely affecting charge carrier transport and other catalysis of hydrogen evolution and / or other reactions occurring at the electrode 108.

[0076] The nanowires 126 and nanoparticles 136 are not shown to scale in the schematic diagram of Figure 6. The shapes of the nanowires 126 and nanoparticles may vary from the example shown.

[0077] Nanoparticle-nanowire catalyst arrangements may be fabricated on a substrate (e.g., a silicon substrate) by nanostructural engineering. In one example, molecular beam epitaxial (MBE) growth of nanowires is followed by photodeposition of nanoparticles. The photodeposition of nanoparticles may be configured to selectively deposit nanoparticles on each side of the nanowire. Further details regarding an exemplary fabrication procedure are provided below, for example, in connection with FIG. 7.

[0078] The placement of platinum nanoclusters or other metal catalytic nanoparticles on an array of nanowires or conductive protrusions establishes a metal / metal nitride interface. As mentioned above, the metal / metal nitride interface is configured to promote hydrogen production via water splitting. For example, in the GaN-platinum example, the Ga-Pt interface promotes water splitting through water dissociation.

[0079] Nanowires 126 may facilitate water splitting in alternative or additional ways. For example, each nanowire 126 may be configured to extract charge carriers (e.g., electrons) generated within the substrate (e.g., as a result of light absorbed by substrate 120). Extraction transports the charge carriers along nanowire 126 to an external site for use in water splitting or other reactions. Thus, for example, nanowire 126 may form an interface well suited for hydrogen evolution, CO reduction, and / or other reactions.

[0080] The device 100 is configured to operate in a PEC (illuminated) mode and an electrochemical (non-illuminated) mode. The system 100 and / or the working electrode 108 may include a switch 140 for selectively applying a bias voltage. To that end, the working electrode 108 includes multiple contacts. In the example of FIG. 6 , the working electrode 108 includes a first contact 142 coupled to the backside of the substrate 120 and through which photogenerated charge carriers in the substrate 120 migrate in the photoelectrochemical (PEC) mode of operation. The working electrode 108 includes a second contact 144 coupled to the array of conductive protrusions 126 to provide charge carriers to the array of conductive protrusions 126 in the electrochemical (EC) mode of operation. Thus, the switch 140 can provide a bias voltage to either the first contact 142 or the second contact 144. As described herein, one or both of the contacts 142, 144 can include a eutectic alloy on the backside and front sides of the substrate, respectively. For example, a Ga-In eutectic can be coupled to a Cu back contact and an n-type eutectic for ohmic contact. + The -pSi wafer may be sandwiched between the silicon wafer and the silicon substrate. Additional or alternative electrical connections may be used to apply a bias voltage.

[0081] 7 illustrates a method 200 for fabricating a photoelectrode or other semiconductor device for photocatalytic water splitting, PEC water splitting, or other photocatalytic reactions, according to one example. Method 200 may be used to fabricate any of the photoelectrodes or other devices described herein, or another device. Method 200 may include additional, fewer, or alternative actions. For example, method 200 may or may not include one or more actions directed to forming a back contact for the device (act 230).

[0082] Method 200 may begin at operation 202, where a substrate is prepared or otherwise provided. The substrate may be or may be formed from a silicon wafer. In one example, a 2-inch Si wafer was used, although wafers of other (e.g., larger) sizes may be used. Other semiconductors and substrates may also be used.

[0083] The substrate may have a planar or non-planar surface. In some cases, operation 202 includes operation 204, in which one or more procedures are performed to clean the substrate. In some cases, a wet or other etching procedure is performed to define the surface. For example, the etching procedure may be or include a crystallographic etching procedure. In the example of a silicon substrate, the crystallographic etching procedure may be a KOH etching procedure. In such cases, if the substrate is <100> If the surface has an orientation, the wet etching procedure <111> It is established that the pyramidal textured surface includes a surface oriented along the surface.

[0084] Operation 202 may include fewer, additional, or alternative operations. For example, in the example of Figure 7, operation 202 includes operation 206 in which oxide is removed.

[0085] In one example, a prime-grade polished silicon wafer is etched in a KOH solution (e.g., 1.8 wt% KOH with 20 vol% isopropanol) at 80 °C for 30 minutes to form a microtextured surface with Si pyramids. After neutralization with concentrated hydrochloric acid, the substrate surface is washed with acetone and / or methanol, and the native oxide is removed with 10% hydrofluoric acid.

[0086] 7, operation 202 includes operation 208, which performs one or more doping procedures (e.g., thermal diffusion procedures) to form doped regions or layers, thereby establishing junctions, as described herein. Alternatively, the substrate is initially provided with a desired dopant concentration profile.

[0087] In one example, a (100) Si wafer was used to fabricate n +We fabricated p-Si junctions by spin-coating phosphorus as an n-type dopant on the front side of a double-side polished p-type Si(100) wafer and boron as a p-type dopant on the other side to obtain ohmic metal contacts. The spin-coated wafers were then annealed at 950 °C for 4 h under a nitrogen atmosphere.

[0088] Method 200 includes act 210, in which an electrode or other device structure is grown or otherwise formed on a substrate. In some cases, an array of nanowires or other nanostructures is grown or otherwise formed on the substrate. Each nanowire is formed on the surface of the substrate such that each nanostructure extends outward from the surface of the substrate. Each nanostructure may have a semiconductor composition, as described herein. In one example, GaN nanowires were grown by plasma-assisted molecular beam epitaxy (MBE) under N-rich conditions.

[0089] Growth of the nanostructures may be accomplished in act 212, in which a molecular beam epitaxy (MBE) procedure is performed. The substrate may be rotated during the MBE procedure so that each nanostructure is shaped as a cylindrical nanostructure. Thus, each nanostructure may have a circular cross-sectional shape, as opposed to a plate- or sheet-like nanostructure.

[0090] The MBE procedure can be carried out under nitrogen-rich conditions, which can result in nitrogen-terminated sidewalls and other surfaces, as described herein.

[0091] In some cases, the MBE procedure can be modified to fabricate an arrangement of layers or segments of each nanowire. Various parameters can be adjusted to achieve different composition levels, thereby forming multiple segments. For example, the substrate temperature can be adjusted in operation 214. The beam equivalent pressure may alternatively or additionally be adjusted. In some cases, the dopant cell temperature is adjusted to control the doping of the nanowires (e.g., Mg doping).

[0092] In one example, plasma-assisted molecular beam epitaxy was used to generate n under nitrogen-rich conditions with an N flow rate of 1.0 standard cubic centimeters per minute. + -pSi wafer n + GaN nanowires were grown on the side of the substrate. The substrate temperature was maintained at 790 °C, and the growth time was approximately 2 h. The forward plasma power was 350 W, and the Ga flux beam equivalent pressure (BEP) was 5 × 10 -8 It was Torr.

[0093] Method 200 further includes act 216, in which catalytic nanoparticles are deposited across the array of nanowires. Deposition of the nanoparticles may be achieved through performance of a photo-deposition procedure in act 218. The nanoparticles may be composed of or otherwise include a metal, such as platinum, or other metallic material (e.g., a rare earth metal). A drying operation 220 may then be performed. Further details regarding photo-deposition procedures are described in one or more of the above-referenced U.S. patents.

[0094] In one example, catalyst nanoparticles were deposited using a photodeposition procedure in which GaN / Si microstructures were placed on a Teflon holder in a glass chamber with quartz windows. 10 μl of 0.2 M chloroplatinic acid hydrate (99.9%, Sigma-Aldrich), 55 ml of deionized water, and 11 ml of methanol were poured into the glass chamber. The chamber was evacuated for 5 minutes using a vacuum pump. It was then irradiated for 30 minutes using a 300 W xenon lamp. During photoirradiation, platinum ions from the chloroplatinic acid hydrate were reduced and deposited on the GaN nanowires.

[0095] Method 200 may then include act 222, in which the nanostructures are covered with a passivation or protective layer. The protective layer may be or otherwise include a conformal oxide layer deposited in act 224. The oxide layer may be composed of or otherwise include titanium oxide. The deposition of act 224 may include performing an atomic layer deposition (ALD) procedure and / or may be otherwise configured to control the thickness of the oxide layer as described herein.

[0096] In one example, a 2 nm TiO2 passivation layer was deposited on a Pt / GaN / Si microstructure by thermal atomic layer deposition (ALD) at 250 °C using tetrakis(dimethylamino)titanium as a precursor.

[0097] Method 200 includes operation 226 in which electrical connections are made. In some cases, the electrical connections are to the backside of the substrate and to the nanowire array, as described herein. In the example of FIG. 7 , operation 226 includes, in operation 228, disposing a eutectic alloy (e.g., Ga-In) on the backside and front sides of the substrate to form the connections. Wires can then be attached to the contacts in operation 230. For example, connections to the backside contacts can be established by connecting Cu wires to ohmic contacts by applying Ga-In eutectic paste.

[0098] Method 200 may include one or more additional acts directed to forming contacts for the device. For example, in some cases, method 200 includes act 232 in which one or more contacts or sides of the electrode are passivated (e.g., via application of epoxy).

[0099] Method 200 may include additional or alternative actions. For example, method 200 may include the act of dicing the photocathode into smaller pieces, for example, using a diamond pen.

[0100] 8 illustrates a method 300 of hydrogen production, according to an example. Method 300 may be implemented using the devices and systems described herein and / or another device or system. Method 300 may be directed to hydrogen generation in seawater or other solutions containing dissolved NaCl.

[0101] Method 300 includes act 302, in which a solution is provided. Optionally, in operation 304, the pH of the solution is lowered. For example, the solution may be a phosphate buffer solution. The solution may or may not constitute or include seawater. For example, the solution may be seawater-based or otherwise derived or formed from seawater.

[0102] In step 306, a photocathode or other device is immersed in the solution. As described herein, the device includes a substrate having a surface, an array of conductive protrusions supported by the substrate and extending outward from the surface of the substrate, and a plurality of catalyst nanoparticles disposed on the array of conductive protrusions. As described above, in operation 308, the apparatus can be placed in a flow cell reactor.

[0103] In some cases, method 300 includes act 310, in which the device is irradiated such that charge carriers are photogenerated within the device to support hydrogen production. Operation 310 may include irradiating the backside of the substrate of the device with sunlight in operation 312. Alternative or additional portions of the photocathode or PEC system may be irradiated or otherwise irradiated.

[0104] In operation 314, a bias voltage may be applied to the device, as described above. The bias voltage may be applied in a PEC mode of operation (e.g., while the device is illuminated in operation 310). Optionally, in operation 316, a bias voltage is applied to the backside of the substrate of the photocathode via an electrical connection.

[0105] As described above, method 300 may also include act 318 in which the bias voltage is switched for operation in dark (non-illuminated) conditions. Switching the bias voltage supports operation in an electrochemical mode of operation. For example, electrode switching may provide a bias voltage via an electrical connection to the nanowire array in act 320.

[0106] Method 300 may include additional or alternative operations. For example, method 300 may include another operation in which a bias voltage is switched back for electrical connections to the backside of the substrate for PEC operation.

[0107] Described above are apparatus and methods for providing seawater electrolysis to produce clean hydrogen fuel. Examples of high-performance photocathodes for seawater hydrogen evolution reactions have been realized, such as photocathodes modified with Pt nanoclusters anchored on GaN nanowires, which have dramatically improved activity and stability for the hydrogen evolution reaction in seawater. + In these examples, the Pt-Ga sites at the Pt / GaN interface promote the dissociation of water molecules and generate H for efficient H generation. * to neighboring Pt atoms. An exemplary Pt / GaN / Si photocathode exhibits −10 mA / cm at 0.15 and 0.39 V vs. RHE. 2 and high applied bias photon-to-current efficiencies of 1.6% and 7.9% in seawater (pH = 9.1) and phosphate-buffered seawater (pH = 7.4), respectively. Under concentrated sunlight (9 suns), a current density of approximately 169 mA / cm was achieved. 2 A photocurrent density of 1000 kJ / cm2 was also demonstrated. Furthermore, disclosed above is an example of a Pt / GaN / Si electrode configured to continuously generate H2 even under dark conditions by simply switching electrical contacts. Thus, the disclosed device and method provide an efficient, stable, and energy-saving electrode for H2 production from seawater splitting.

[0108] The term "about" is used herein to include deviations from a specified value that would be understood by one of ordinary skill in the art to be effectively the same as the specified value, e.g., due to the absence of any appreciable, detectable, or otherwise significant difference in the operation, results, properties, or other aspects of the disclosed methods, devices, and systems.

[0109] Although the present disclosure has been described with reference to particular embodiments, these embodiments are merely illustrative and do not limit the present disclosure. Modifications, additions, and / or deletions can be made to the embodiments without departing from the spirit and scope of the present disclosure.

[0110] The foregoing description is given for clarity of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

1. 1. A method for producing hydrogen, comprising: Providing a solution; and immersing the device in the solution; The device comprises: a substrate having a surface; an array of conductive protrusions supported by the substrate and extending outwardly from a surface of the substrate; a plurality of catalytic nanoparticles disposed on the array of conductive protrusions; The method for producing hydrogen, wherein the solution comprises dissolved sodium chloride (NaCl).

2. The method of claim 1 , wherein the solution comprises seawater.

3. The method of claim 1 , wherein the solution is a phosphate buffer solution.

4. The method of claim 1 , wherein providing the solution comprises lowering the pH of the solution.

5. 10. The method of claim 1, further comprising illuminating the device such that charge carriers are photogenerated within the device to support the hydrogen production.

6. The method of claim 5 , wherein irradiating the device comprises irradiating a backside of the substrate of the device with sunlight.

7. The method of claim 1 further comprising applying a bias voltage to the device.

8. the device comprises a contact coupled to a backside of the substrate; 8. The method of claim 7, wherein applying the bias voltage comprises applying the bias voltage to the contacts in a photoelectrochemical (PEC) mode of operation of the device.

9. 8. The method of claim 7, further comprising switching the bias voltage to apply the bias voltage to the array of conductive protrusions in an electrochemical mode of operation of the device when the device is not illuminated.

10. the device includes a eutectic alloy disposed on a surface and in electrical communication with an array of conductive protrusions; The method of claim 1 , wherein switching the bias voltage comprises providing the bias voltage to the array of conductive protrusions through the eutectic alloy.

11. The method of claim 1 , wherein immersing the device comprises placing the device in a flow cell reactor through which the solution flows.

12. 10. The method of claim 1, wherein each conductive protrusion of the array of conductive protrusions and each catalytic nanoparticle of the plurality of catalytic nanoparticles establishes a metal / metal nitride interface configured to promote the hydrogen production via water splitting.

13. the substrate comprises silicon; each conductive protrusion of the array of conductive protrusions comprises a Group III nitride semiconductor material; The method of claim 1 , wherein each catalytic nanoparticle of the plurality of catalytic nanoparticles comprises a platinum nanocluster.

14. A device, a substrate having a front surface and a back surface opposite the front surface; an array of conductive protrusions supported by the substrate and extending outwardly from a surface of the substrate; a plurality of catalyst nanoparticles disposed on the array of conductive protrusions; a first contact coupled to the backside surface through which charge carriers photogenerated within the substrate migrate in a photoelectrochemical (PEC) mode of operation of the device; a second contact coupled to the array of conductive protrusions to provide charge carriers to the array of conductive protrusions in an electrochemical (EC) mode of operation of the device.

15. The device of claim 14 , wherein the first contact and the second contact comprise a eutectic alloy on the back surface of the substrate and the front surface of the substrate, respectively.

16. 15. The device of claim 14, further comprising a switch for applying a bias voltage to either the first contact or the second contact.

17. The device of claim 14 , further comprising an oxide passivation layer covering the plurality of catalytic nanoparticles.

18. 20. The device of claim 17, wherein the oxide passivation layer comprises titanium oxide.

19. each conductive protrusion of the array of conductive protrusions has a semiconductor composition; 15. The device of claim 14, wherein the semiconductor composition of each conductive protrusion of the array of conductive protrusions is terminated with nitrogen along a surface of the conductive protrusion.

20. The device of claim 14 , wherein each conductive protrusion in the array of conductive protrusions comprises a nanowire.

21. The device of claim 14 , wherein each catalytic nanoparticle of the plurality of catalytic nanoparticles comprises platinum.

22. 15. The device of claim 14, wherein each conductive protrusion of the array of conductive protrusions and each catalytic nanoparticle of the plurality of catalytic nanoparticles establishes a metal / metal nitride interface configured to promote hydrogen production via water splitting.

23. A device, a substrate having a surface; an array of conductive protrusions supported by the substrate and extending outwardly from a surface of the substrate, each conductive protrusion in the array of conductive protrusions comprising a metal nitride semiconductor material; a plurality of metal catalysts disposed on the array of conductive protrusions such that a metal / metal nitride interface is established; The device, wherein the metal / metal nitride interface is configured to promote hydrogen production via water splitting.

24. 24. The device of claim 23, wherein each catalyst of the plurality of catalysts comprises platinum.

25. 24. The device of claim 23, wherein each catalyst of the plurality of catalysts comprises a rare earth element.

26. 24. The device of claim 23, wherein the metal nitride semiconductor material is GaN.

27. 1. A method for manufacturing a hydrogen generation device, comprising: providing a substrate having a surface; growing an array of nanowires on a surface of a substrate such that each nanowire in the array extends outward from the surface of the substrate; each conductive protrusion of the array of conductive protrusions comprises a metal-nitride semiconductor material; The method further comprises depositing a plurality of metal catalyst nanoparticles across the array of nanowires such that a metal / metal-nitride interface is established; The method wherein the metal / metal-nitride interface is configured to promote hydrogen production via water splitting.

28. 30. The method of claim 27, further comprising forming first and second electrical connections to the array of nanowires and the substrate, respectively.

29. 30. The method of claim 28, wherein forming the first and second electrical connections to the array of nanowires and the substrate comprises disposing a eutectic alloy on a front surface of the substrate and a back surface of the substrate, respectively.

30. 28. The method of claim 27, further comprising passivating the plurality of catalytic nanoparticles and the array of nanowires with an oxide layer.

31. 31. The method of claim 30, wherein passivating the plurality of catalytic nanoparticles and the array of nanowires comprises depositing titanium oxide by atomic layer deposition.

32. 28. The method of claim 27, wherein depositing the plurality of catalytic nanoparticles comprises performing a photo-deposition procedure using platinum.

33. 30. The method of claim 27, wherein growing the array of nanowires comprises performing a molecular beam epitaxy (MBE) procedure in N-rich conditions.

34. 28. The method of claim 27, wherein the eutectic alloy comprises GaIn.

35. 28. The method of claim 27, further comprising passivating the backside of the substrate and the electrical connections to the array of nanowires with epoxy.

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