Anion exchange membrane electrolyzer with a self-supporting oxygen evolution electrode without platinum group metals

Abstract

A fluorine-containing nickel iron oxyhydroxide electrocatalyst is used as an anode in an anion exchange membrane electrolyzer for hydrogen production.

Description

[0001] Statement Regarding Federally Sponsored Research or Development

[0002] This invention was made with government support under Awards DE-AR0000771 and DE-AR0001149, awarded by the U.S. Department of Energy, Advanced Research Projects Agency-Energy (ARPA-E). The government has certain rights in this invention.

[0003] Cross - Reference to Related Applications

[0004] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 019,968, filed May 4, 2020, the entire disclosure of which is incorporated herein by reference. TECHNICAL FIELD

[0005] Fluorinated nickel iron oxyhydroxide electrocatalysts are disclosed. These electrocatalysts can be used in electrochemical devices such as anion exchange membrane electrolyzers (AEMELs) and in methods for producing hydrogen gas (H 2 2). BACKGROUND ART

[0006] Producing green hydrogen through low-temperature water electrolysis is considered a promising large-scale and long-term technology for storing and transporting intermittent renewable wind and solar energy across continents and industrial sectors [1] . In particular, green hydrogen has the unique ability to eliminate carbon emissions in industries that are otherwise difficult to decarbonize, such as ammonia synthesis, steelmaking, and transportation, especially heavy-duty vehicles.

[0007] Since 1927 [2,3] Traditional alkaline electrolyzers (AELs) operating with 25-40 wt% potassium hydroxide (KOH) or sodium hydroxide (NaOH) as the electrolyte have been commercial technologies. AELs have a service life of up to 30 to 40 years, and their inexpensive platinum group metal (PGM)-free catalysts and stack components can reduce capital costs [3] . However, due to the formation of bubbles adsorbed on the electrode surface in the liquid electrolyte and thick diaphragms (especially at high current densities), resulting in high internal resistance, their voltage efficiency is low [4] . The concentrated liquid electrolyte also generates shunt currents, leading to efficiency losses and hardware corrosion problems. Due to the slow ion transport in the liquid electrolyte, AELs also experience a slow transient response, making it difficult to utilize intermittent renewable energy [4] .

[0008] Hydroxide exchange membrane electrolyzers (HEMELs) offer an alternative solution that retains the low-cost advantage of AELs while using an improved design of proton exchange membrane electrolyzers (PEMELs), which benefits from a solid electrolyte membrane and a zero-gap configuration to reduce internal resistance. Through such a configuration that replaces the strong acid proton-conducting membrane of PEMELs with a hydroxide-conducting polymer membrane, HEMELs can eliminate the need for expensive PGM electrocatalysts and noble metal-coated titanium-based stack materials. The zero-gap solid electrolyte assembly also features high voltage efficiency, high current density, fast dynamic response, and the ability to operate under pressure differences. [5] .

[0009] One of the biggest improvements of HEMELs compared to AELs is that they can be operated by feeding water instead of corrosive alkaline electrolytes. However, to achieve high-performance water-fed HEMELs, advanced hydroxide exchange membranes (HEMs) and hydroxide exchange ionomers (HEIs) are required. These two components are responsible for the transport path of hydroxide ions through the electrolyzer. Therefore, HEMs and HEIs exhibit high hydroxide conductivity and excellent chemical and mechanical stability to avoid a reduction in electrolyzer performance and durability.

[0010] Wang et al. [6] reported the performance of a water-fed HEMEL single cell using PGM catalysts (platinum black for the cathode and IrO 2 ) and unstable commercial HEMs and HEIs. They achieved a current density of 399 mA cm -2 at 1.8 V, but had poor durability in pure water. Another HEMEL study using PGM-free catalysts (Ni-Mo for the cathode and Ni-Fe for the anode) and self-made HEMs and HEIs showed a current density close to 300 mA cm -2 at 1.8 V and a short-term durability of 8 hours. [7] . In a recent study, Kim et al. [8] reported a high-performance PGM-free HEMEL that uses a model quaternized polyphenylene HEM and a quaternary ammonium-type polystyrene HEI with a high ion exchange capacity (IEC) (3.3 mequiv.g -1 ). The single cell test produced a current density of 906 mA cm -2 at 1.8 V, but even this showed a short-term performance decline (<10 h) and long-term instability. One of the main reasons for the performance degradation is that the catalyst is easily eroded during operation because using a high IEC HEI weakens the binding strength with the catalyst, making it difficult for the catalyst to remain while withstanding water flow scouring and gas evolution.

[0011] Several commercial HEMs and HEIs have recently been developed, including Orion TMI TM , which is a quaternary ammonium-functionalized aromatic polymer produced by Orion Polymer [9] . Ecolectro developed Aemion, which is a phosphorus-functionalized polyethylene conductive polymer

[10] , and Ionomr Innovations Inc. synthesized polybenzimidazolium HEIs and HEMs

[11] . All materials have reached a point where further increases in conductivity and IEC are hindered by dissolution in water.

[0012] Another key limiting factor for HEMEL performance is the electrochemistry reaction resistance, which depends on the catalytic activity of the electrodes used, especially for the slow oxygen evolution kinetics in the anode

[12] . Transition metal oxyhydroxides (MOOH, where M = Fe, Co, and Ni) are considered to be one of the most promising OER candidates among PGM-free catalysts in alkaline environments [13-15] . They are also considered to be the actual active species of oxides, dichalcogenides, nitrides, and phosphides generated by irreversible surface reconstruction during the catalytic process [16-22] . However, a relatively large overpotential (>400 mV) is still required to meet the industrial application level (>500 mA cm -2 ).

[0013] Therefore, there is a need for an oxygen evolution electrocatalyst used as the anode in AEMEL and HEMEL, which is not easily eroded during the operation of the electrolyzer, thereby improving performance and long-term stability. SUMMARY OF THE INVENTION

[0014] The present disclosure relates to fuel cell systems, electrochemical pumps, and methods for using them to reduce the concentration of carbon dioxide in the air and generate electricity.

[0015] For example, the present disclosure relates to a fluorine-containing nickel iron oxyhydroxide electrocatalyst.

[0016] In addition, the present disclosure relates to a self-supporting oxygen evolution electrode without platinum group metals (PGM), which comprises an electrocatalyst in the pores of a gas diffusion layer containing nickel foam.

[0017] Further, the present disclosure relates to an anion exchange membrane electrolyzer (AEMEL) for generating hydrogen from water. The AEMEL includes an anode containing an anode electrocatalyst, the anode electrocatalyst including a fluorine-containing nickel iron oxyhydroxide electrocatalyst for forming oxygen and water from hydroxide ions; a cathode including a cathode electrocatalyst for forming hydrogen and hydroxide ions from water; and an anion exchange membrane adjacent to and spaced apart from the anode and the cathode for transporting hydroxide ions from the cathode to the anode.

[0018] The present disclosure also relates to a method for preparing a fluorine-containing nickel iron oxyhydroxide electrocatalyst. The method includes

[0019] immersing compressed nickel foam in an aqueous solution containing iron(III) nitrate hexahydrate and sodium fluoride and having oxygen flowing above the surface of the solution for at least 8 hours to form the fluorine-containing nickel iron oxyhydroxide electrocatalyst; and washing the fluorine-containing nickel iron oxyhydroxide electrocatalyst with water. 2

[0020] Other objects and features will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In Figure 1 Figure 1 Figure 1

[0022] x Ni y OOH and Fe x Ni y OOH-20F of (b) XRD pattern and (c) high-resolution F 1s XPS spectrum. Figure 1 x Ni y OOH-20F of (d) SEM, (e) TEM, and (f) HRTEM images.

[0022] Figure 2 is a schematic diagram of a single-cell AEMEL.

[0023] In Figure 3

[0024] Figure 4

[0025] Figure 5 Figure 5 Figure 5is the polarization curve of an AEL using a Pt / C / HEI cathode, Fe x Ni y OOH-20F anode, a Zirfon membrane (500 μm), and a 1.0 M KOH aqueous electrolyte at 80 °C.

[0026] In Figure 6 Figure (a) is a schematic diagram of the configuration of a water-fed HEMEL using a Pt / C cathode and a self-supported Fe x Ni y OOH-20F anode. In Figure 6 Figure (b) is the polarization curve of a water-fed HEMEL using Fe x Ni y OOH-20F and Ir / C anode catalysts at cell temperatures of 80 °C and 90 °C. In Figure 6 Figure (c) is a comparison chart of the cell performance of the water-fed HEMEL of the present invention and the literature ("this work") (j 1.8 ).

[0027] In Figure 7 Figure (a) and (b) are the (a) polarization curve and (b) EIS curve of water-fed HEMELs as a function of HEI loading at 80 °C. The EIS data was measured at a current density of 100 mA cm -2 . In Figure 7 Figure (c) illustrates the equivalent circuit used to simulate the EIS data. The Nyquist plot was fitted to an equivalent circuit consisting of a resistor in series with three other resistors, each resistor in parallel with a constant phase element (CPE).

[58] R 1 represents the ohmic resistance of the current collector, catalyst layer, membrane, and all contact resistances. R 2 corresponds to the charge transfer resistance of the electron / ion conducting element.

[58] R 3 is related to the kinetic resistance of the oxygen evolution and hydrogen evolution reactions. The oxygen evolution reaction catalyzed by PGM-free Fe x Ni y OOH-20F is much slower than the hydrogen evolution reaction catalyzed by the PGM Pt / C catalyst. Therefore, the kinetic resistance at the cathode is considered negligible compared to the anode. R 4 is related to the mass transfer effect. In Figure 7 Figure (d) is a chart of the simulated R 1 , R 2 , R 3 and R 4 values at different HEI loadings.

[0028] Figure 8 At 80 °C, the short-term durability performance of water-fed HEMEL at current densities from 100 mA cm -2 to 500 mA cm -2 is shown in the graph.

[0029] In Figure 9 , panels (a), (c), and (d) are graphs of the long-term stability performance of water-fed HEMEL at 200 mA cm -2 and 80 °C, (c) an XRD pattern, and (d) a high-resolution F1s XPS spectrum of the Fe -2 Ni x OOH-20F / HEI anode obtained after 160 hours of continuous operation at 200 mA cm y and 80 °C. In Figure 9 , panel (b) is an SEM image of the Fe x Ni y OOH-20F / HEI anode obtained after 160 hours of continuous operation at 200 mA cm -2 and 80 °C.

[0030] Figure 10 is a graph of the long-term stability performance of water-fed HEMEL at 500 mA cm -2 and 80 °C.

[0031] Figure 11 is a bar graph of the molar ratio of Fe to Ni (Fe / Ni) in Fe x Ni y OOH and Fe x Ni y OOH-nF (n = 10, 20, and 30) measured using a microwave plasma atomic emission spectrometer (MP-AES).

[0032] In Figure 12 , panels (a) to (d) are SEM images of (a) Fe x Ni y OOH, (b) Fe x Ni y OOH-10F, (c) Fe x Ni y OOH-20F, and (d) Fe x Ni y OOH-30F.

[0033] In Figure 13 , panels (a) to (d) are (a) a CV curve, (b) a polarization curve, (c) a Tafel slope, and (d) η 100With j ECSA Nickel iron oxyhydroxide (Fe x Ni y OOH), fluorine-doped nickel iron oxyhydroxide (Fe x Ni y OOH-nF, where n is the concentration of F at 10, 20, or 30 mM in the reactants - ), and PGM Ir / C (20 wt%) catalyst, which were measured in an O 2 -saturated 1.0 M KOH solution.

[0034] In Figure 14 , panels (a) and (b) are (a) SEM image and (b) corresponding EDX analysis of the (Fe, Co, Ni)OOH layer prepared by immersing nickel foam into an O 2 -saturated Fe(NO 3 ) 3 and Co(NO 3 ) 2 solution.

[0035] Figure 15 is the chart of the electrochemical impedance spectroscopy (EIS) of Fe x Ni y OOH and Fe x Ni y OOH-20F electrodes measured at 1.60 V vs. RHE, which has an AC oscillation with an amplitude of 10 mV at frequencies from 100 kHz to 100 mHz. The EIS spectrum was fitted using an equivalent circuit consisting of two parallel units of the ohmic resistance (R s ), the charge transfer resistance (R ct ) at the interface between the catalyst and the electrolyte, the mass transfer resistance (R mass ), and constant phase elements (CPE ct and CPE mass )(inserted) in series. [10,11]

[0036] In Figure 16 , panels (a) and (b) are the CV curves of Fe x Ni y OOH and (b) Fe x Ni y OOH-20F measured in the non-Faradaic potential region, and in Figure 16 , panel (c) is the chart of the corresponding double-layer capacitance (C dl ).

[0037] In Figure 17 , panels (a) and (b) are (a) Fe x Niy OOH and (b) Fe x Ni y The first to twentieth CV cycles of the OOH-20F catalyst measured in saturated 1.0 M KOH solution. Compared with Fe 2 Ni x Ni y OOH, the OER current of Fe x Ni y OOH-F-2 increases from the first to the twentieth CV cycle.

[0038] In Figure 18 Platelets (a) to (c) are the high-resolution (a) Ni 2p, (b) Fe 2p, and (c) O 1s XPS spectra of Fe x Ni y OOH and Fe x Ni y OOH-20F. The peaks at 856.1 eV and 873.8 eV in the high-resolution Ni 2p XPS spectrum are attributed to the 2p3 / 2 and 2p1 / 2 peaks of Ni(II)-OH respectively,

[50] and the peaks at binding energies of 861.7 eV and 879.8 eV belong to satellite peaks. In the high-resolution Fe 2p XPS spectrum, the peaks at 711.2 eV and 724.4 eV are attributed to the 2p3 / 2 and 2p1 / 2 peaks of FeO(OH) respectively, [5,6] the peaks at 714.2 eV and 727.4 eV are characteristic of Fe 3+ .

[53] The corresponding shake-up satellite peaks are located at 719.0 eV and 732.6 eV. The peaks at binding energies of 530.0 eV, 531.5 eV, and 533.0 eV in the high-resolution O 1s XPS correspond to Fe / Ni-O, OH, and adsorbed H 2 O respectively. [8,9]

[0039] In Figure 19 Platelets (a) to (d) are the high-resolution (a) F 1s, (b) Ni, 2p, (c) Fe, 2p, and (d) O 1s XPS spectra of Fe 2 Ni x OOH-20F recorded after 20 consecutive CV cycles in saturated 1.0 M KOH solution. The high-resolution Ni 2p, Fe 2p, and O 1s spectra of Fe y Ni x OOH-20F after 20 repeated CV cycles are compared with the original Fe y Ni x Ni ySimilar to OOH-20F, the F 1s peak corresponding to the (Fe, Ni)-F bond disappeared, indicating that F - ions were leached out during the CV cycle.

[0040] Figure 20 Figure shows a comparison of the cell performance of HEMEL of the present invention with that of the literature when reacting with 1.0 M potassium hydroxide solution.

[0041] In Figure 21 , panels (a) to (c) are high-resolution (a) Ni 2p, (b) Fe 2p, and (c) O 1s XPS spectra of the Fe -2 Ni x OOH-20F / HEI anode obtained after a 160-hour stability test under the condition of 200 mA cm y .

[0042] Figure 22 is the SEM image of the Fe x Ni y OOH-20F / HEI anode obtained before the stability test.

[0043] In the entire figure, corresponding reference numerals indicate corresponding parts. Detailed Description of the Invention

[0044] An in-situ dissolved oxygen and galvanic corrosion method is used to synthesize a fluorinated nickel-iron oxyhydroxide electrocatalyst. Preferably, a vertically aligned fluorinated nickel-iron oxyhydroxide nanosheet array is formed on nickel foam and used as a highly active platinum-group metal (PGM)-free self-supporting oxygen evolution electrode. This electrode can be integrated with a highly conductive anion exchange membrane and an ionomer into an anion exchange membrane electrolyzer (AEMEL). For example, when integrated with a highly conductive poly(arylpiperidinium) (PAP) hydroxide exchange membrane and an ionomer into a pure water-fed hydroxide exchange membrane electrolyzer (HEMEL), the vertically aligned fluorinated nickel-iron oxyhydroxide nanosheet array formed on nickel foam can be used as the anode. Such HEMEL achieved a performance of 1020 mA cm -2 under the conditions of 1.8 V and 90 °C, and can continuously and stably operate at 200 mA cm -2 for 160 hours without eroding the electrocatalyst. Such AEMEL and HEMEL can be used for large-scale production of low-cost hydrogen using intermittent renewable energy.

[0045] The present disclosure relates to a fluorinated nickel-iron oxyhydroxide electrocatalyst. The electrocatalyst is designated as Fe x Ni y OOH-nF, where n is the F in the reactants used in the electrocatalyst synthesis reaction -Molar concentration, where x and y are Fe x Ni y The molar ratio of Fe and Ni in the OOH - nF catalyst, which is measured by microwave plasma atomic emission spectrometry (MP - AES). The electrocatalyst can be used as an anode in AEMEL (such as HEMEL).

[0046] As shown by high - resolution fluoride (F) 1s X - ray photoelectron spectroscopy, the electrocatalyst can have a single F1s peak. Preferably, the single F1s peak is located at a binding energy of 684.0 eV.

[0047] The electrocatalyst may comprise a three - dimensional sponge - like network structure as determined by scanning electron microscopy (SEM) imaging.

[0048] The Fe / Ni molar ratio of the electrocatalyst determined by microwave plasma atomic emission spectrometry (MP - AES) is less than 4.0, preferably from about 2.0 to about 3.2.

[0049] The formula of the electrocatalyst may be Fe x Ni y OOH, where x ranges from about 0.75 to about 0.83, and y ranges from about 0.26 to about 0.38.

[0050] In addition to Fe and Ni, the electrocatalyst may further comprise at least one metal, and the at least one metal includes Ce, Cr, Cu, Co, Mo, Ru, Pd, Pt, Ir, Rh, Os, Ag, Au, Re, Ta, Ti, V, W, Mn, Zn, Sn, Sb, In, Ga, Bi, Pb or Zr. For example, Co is present in the electrocatalyst of Example 4.

[0051] As determined by high - angle annular dark - field scanning transmission electron microscopy (HAADF - STEM), the electrocatalyst can be in the form of a vertically oriented and interpenetrating nanosheet array. Each nanosheet has a thickness of about 2 nm to 3 nm as determined by high - magnification transmission electron microscopy (TEM) imaging.

[0052] The fluorinated nickel - iron oxyhydroxide electrocatalyst can be in the form of a nanosheet array on compressed nickel foam. Such nanosheet arrays can be grown in situ on nickel foam to form a catalyst - coated substrate.

[0053] Due to the fluoride - leaching - induced surface reconstruction shown in Table 1, the fluorinated nickel - iron oxyhydroxide electrocatalyst exhibits significantly higher catalytic activity in alkaline electrolytes (such as KOH) than other Ni - Fe catalysts. More precisely, when fluoride ions leach out from the electrocatalyst, it induces surface reconstruction to expose more NiOOH active sites, thereby enhancing the catalytic activity.

[0054] Table 1. Comparison of the oxygen evolution reaction (OER) performance of the fluorine-doped iron nickel oxyhydroxide catalyst with previously reported Ni-Fe catalysts.

[0055]

[0056] Another aspect of the present disclosure relates to a method for preparing a fluorine-containing nickel iron oxyhydroxide electrocatalyst. The method includes immersing compressed nickel foam in an aqueous solution containing iron(III) nitrate hexahydrate and sodium fluoride above the surface of the solution under an oxygen flow, soaking for at least 8 hours with oxygen flowing above the surface of the solution to form the fluorine-containing nickel iron oxyhydroxide electrocatalyst; washing the fluorine-containing nickel iron oxyhydroxide electrocatalyst with water. 2 The method may further include compressing the nickel foam with a force of at least 4448 N to form the compressed nickel foam. For example, the nickel foam can be compressed with a force of about 4448 N to about 13344 N or about 4448 N (1000 lbf).

[0057] The method may further include immersing the compressed nickel foam in an acidic aqueous solution to remove residual oxides from the compressed nickel foam, and then washing the compressed nickel foam with water to remove the acidic solution.

[0058] The iron(III) nitrate hexahydrate and the sodium fluoride are present in the oxygen-rich aqueous solution in a molar ratio of about 2:1 to about 1:1.5.

[0059] The oxygen-rich aqueous solution is formed by bubbling oxygen through an aqueous solution containing iron(III) nitrate hexahydrate and sodium fluoride. 2 The oxygen-rich aqueous solution is formed by bubbling oxygen through an aqueous solution containing iron(III) nitrate hexahydrate and sodium fluoride.

[0060] The oxygen-rich aqueous solution is formed by bubbling oxygen through an aqueous solution containing iron(III) nitrate hexahydrate and sodium fluoride. 2 The oxygen-rich aqueous solution is formed by bubbling oxygen through an aqueous solution containing iron(III) nitrate hexahydrate and sodium fluoride.

[0061] The flow rate of the oxygen flow above the surface can be about 40 sccm to about 100 sccm.

[0062] The method may further include removing the fluorine-containing nickel iron oxyhydroxide electrocatalyst from the compressed nickel foam. For example, the electrocatalyst can be removed from the nickel foam by sonication.

[0063] An in-situ growth of the fluorine-containing nickel iron oxyhydroxide catalyst on the compressed nickel foam can be achieved using a galvanic corrosion process. When the compressed nickel foam is immersed in an oxygen-rich Fe(NO 2 ) 3 ) 3 and NaF solution, the oxidants (Fe 3+ and O 2 ) will promote the oxidation of surface Ni species to Ni 2+ ( Figure 1a). Then the foam coordinates with anionic OH - and F - where the F - concentration varies. The complete characterization data of Fe x Ni y OOH-nF are included in Example 2.

[0064] The in-situ growth mechanism for forming the Fe x Ni y OOH-nF anode provides several advantages over other electrodes fabricated using a catalyst-coated substrate (CCS) configuration. The electrocatalyst grows directly on a compressed nickel foam substrate via a simple galvanic / dissolved oxygen corrosion mechanism, where the nickel foam substrate serves both as a catalyst support and as a gas diffusion layer (GDL), replacing the expensive titanium microporous layer (MPL) found in PEMELs.

[0065] The conductive nickel foam provides an electron pathway for the catalytically active sites. These active sites are present throughout the pores of the GDL rather than being individually sprayed on the surface of the GDL, thereby increasing the utilization of the electrocatalyst.

[0066] The growth mechanism promotes stable contact between the electrocatalyst and the GDL because the electrocatalyst grows directly on the GDL, which is one of the reactants in the synthesis process. Such stable contact eliminates the problem of catalyst loss at high current densities and during long-term operation, thus demonstrating for the first time a 160-hour stability using a high IEC HEI.

[0067] The simple one-step impregnation process for fabricating the electrocatalyst also eliminates the need for a cumbersome hand-spraying fabrication method.

[0068] Another aspect of the present disclosure relates to an AEMEL for generating hydrogen. Figure 2 A schematic diagram showing an example of an AEMEL is presented. Figure 2Figure 10 shows a single cell AEMEL configuration 10 having an anode 12 including an anode electrocatalyst that includes a fluorinated nickel iron oxyhydroxide electrocatalyst for forming oxygen and water from hydroxide ions. Anode 12 may also include a substrate such as nickel foam, such that the anode has the form of a cathode-coated substrate. The substrate also serves as a gas diffusion layer on the anode side of the AEMEL. Cathode 14 includes a cathode electrocatalyst for forming hydrogen and hydroxide ions from water. Anion exchange membrane 16 is adjacent to and spaced from anode 12 and cathode 14 and transports hydroxide ions from cathode 14 to anode 12. A gas diffusion layer 18 may be present between cathode 14 and cathode end plate 20. A DC power supply 22 conducts electrons from the anode to the cathode. Anode end plate 24 is adjacent to the anode. Feed ports 26 and 30 supply water or an aqueous alkaline electrolyte, such as KOH or NaOH, to the AEMEL. Water and oxygen are discharged from outlets 28 and 30 on the anode side. Hydrogen is discharged from outlet 32 on the cathode side. The anodic reaction is the oxygen evolution reaction (OER):

[0069]

[0070] The cathodic reaction is the hydrogen evolution reaction (HER):

[0071]

[0072] The water fed to cathode 14 may contain a hydroxide-conducting electrolyte for forming oxygen and water from hydroxide ions. The hydroxide-conducting electrolyte may include KOH or NaOH, preferably KOH.

[0073] Preferably, the feed stream entering feed inlet 26 is pure water that does not include any alkaline electrolyte to minimize corrosion.

[0074] The fluorinated nickel iron oxyhydroxide electrocatalyst may be in the pores of a gas diffusion layer that includes nickel foam.

[0075] Anion exchange membrane 16 may include an anion exchange polymer and an electronically conductive material or an electronically conductive anion exchange polymer. For example, the anion exchange polymer may include quaternary ammonium or imidazolium groups and a polymer backbone that does not include ether groups.

[0076] The anion exchange polymer includes poly(arylpiperidinium), alkylammonium-functionalized poly(arylenealkylene), substituted imidazolium-functionalized poly(arylenealkylene), alkylammonium-functionalized poly(styrene), substituted imidazolium-functionalized poly(styrene), alkylammonium-functionalized poly(styrene-co-divinylbenzene), substituted imidazolium-functionalized poly(styrene-co-divinylbenzene), alkylammonium-functionalized poly(styrene-block-ethylene-co-butadiene-block-styrene), substituted imidazolium-functionalized poly(styrene-block-ethylene-co-butadiene-block-styrene), alkylammonium-functionalized poly(ethylene), substituted imidazolium-functionalized poly(ethylene), alkylammonium-functionalized poly(tetrafluoroethylene), substituted imidazolium-functionalized poly(tetrafluoroethylene), alkylammonium-functionalized poly(ethylene-co-tetrafluoroethylene), substituted imidazolium-functionalized poly(ethylene-co-tetrafluoroethylene), polyethyleneimine, poly(diallylammonium), or a combination thereof. Poly(arylpiperidinium) is preferred.

[0077] The electronically conductive material may include carbon, nickel, stainless steel, silver, a conductive polymer, or a combination thereof. For example, the electronically conductive material may include nanowires or nanotubes.

[0078] The cathode electrocatalyst includes silver, a silver alloy, silver supported on carbon, a silver alloy supported on carbon, platinum, a platinum alloy, platinum supported on carbon, a platinum alloy supported on carbon, palladium, a palladium alloy, palladium supported on carbon, a palladium alloy supported on carbon, manganese oxide, manganese oxide supported on carbon, cobalt oxide, cobalt oxide supported on carbon, heteroatom-doped carbon (X-C, where X includes one or more of N, C, B, P, S, Se, or O), metal-heteroatom-carbon (M-X-C, where X includes one or more of N, C, B, P, S, Se, or O and M includes one or more of Fe, Ce, Cr, Cu, Co, Mo, Ni, Ru, Pd, Pt, Ir, Rh, Os, Ag, Au, Re, Ta, Ti, V, W, Mn, Zn, Sn, Sb, In, Ga, Bi, Pb, or Zr), perovskite (ABX 3 , where A includes one or more of Ca, Sr, Ba, Sc, Y, La, Ce, Zr, Cu, Zn, Sb, Bi, B includes one or more of Al, Ti, Mn, Fe, Co, Ni, W, Pd, and X includes one or more of O, Se, S), perovskite supported on carbon (ABX 3 , where A includes one or more of Ca, Sr, Ba, Sc, Y, La, Ce, Zr, Cu, Zn, Sb, Bi, B includes one or more of Al, Ti, Mn, Fe, Co, Ni, W, Pd, and X includes one or more of O, Se, S), or a combination thereof. Platinum supported on carbon is preferred.

[0079] Before applying the cathode catalyst, the ionomer interlayer can be directly applied to the cathode side of the anion exchange membrane. Such interlayers provide a hydroxide conduction network. All experiments were conducted using PAP membranes and ionomers. The PAP membranes and ionomers are described in U.S. Patent No. 10,290,890, with U.S. application serial number 16 / 651,622, PCT publication number WO 2019 / 068051, which is incorporated herein by reference in its entirety. The preferred cathode ionomer is PAP-TP-85.

[0080] The gas diffusion layer 18 on the cathode side of the AEMEL can comprise any suitable material known in the art, such as carbon paper. For example, the GDL can comprise Toray carbon paper 060 with 5% and 10% wet proofing, and / or SGL carbon paper 29BC.

[0081] Before applying the anode catalyst, the ionomer interlayer can be directly applied to the anode side of the anion exchange membrane. Such interlayers provide a hydroxide conduction network. All experiments were conducted using PAP membranes and ionomers. The PAP membranes and ionomers are described in U.S. Patent No. 10,290,890, with U.S. application serial number 16 / 651,622, PCT publication number WO2019 / 068051, which is incorporated herein by reference in its entirety. The preferred anode ionomer is PAP-TP-85-MQN.

[0082] Current is supplied to the AEMEL by a power source.

[0083] An example of the HEMEL described herein is a single cell assembled by using Pt / C catalyst (TKK) as the cathode catalyst, Fe x Ni y OOH-20F as the anode catalyst, and an alkali-stable and highly OH-conductive PAP-TP-85 HEM and HEIs with an IEC of 2.4 mmol g -1 . [32,33] The Pt / C catalyst and PAP-TP-85 HEIs were sprayed on the HEM to form a porous cathode with a Pt loading of 0.94 mg Pt cm -2 and an HEI loading of 30 wt%, as Figure 3 shown, wherein the catalyst particles form an electron conduction network and the HEIs adsorbed on the catalyst surface form an OH - conduction network. The anode is a self-supporting Fe x Ni y OOH-20F electrode with a catalyst loading of 4.8 mg cm -2 and coated with an IEC of 3.2 mmol g -1of IEC's PAP-TP-85-MQN HEI (as described in Example 13 of PCT Publication No. WO 2019 / 068051). Figure 4 shows the polarization curves of HEMEL in contact with a KOH aqueous electrolyte at 80 °C. By increasing the KOH concentration from 10 mM to 1000 mM, the performance is significantly improved because the externally supplied OH - ions increase the ionic conductivity of HEM and HEI and reduce the ohmic resistance (from 0.32 ohm cm for 10 mM KOH 2 to 0.06 ohm cm for 1000 mM KOH 2 ), and increase the reaction rate towards OER. At a voltage of 1.74 V, the performance of PAP HEM with 1000 mM KOH aqueous electrolyte is as high as 1500 mA cm -2 , which is much higher than that of Zirfon TM membrane-based AEL under similar experimental conditions ( Figure 5 ), further demonstrating the high ionic conductivity of PAP HEM. In addition, the performance of HEMEL is much better than that of previously reported solid-state alkaline water electrolyzers using 1.0 M KOH electrolyte ( Figure 5 ), [34–37] and is close to that of PGM catalyst-based PEMEL shown in Table 2:

[0084] Table 2. MEA specifications and performance of HEMEL in contact with 1.0 M KOH electrolyte compared to previously reported PEMELs.

[0085]

[0086] When HEMEL is operated with water instead of an alkaline electrolyte, corrosion problems can be avoided. Figure 6 a schematically shows the configuration of a typical water-fed HEMEL, where PAP-TP-85 HEI and Pt / C catalyst are sprayed onto HEM to form the cathode, while PAP-TP-85-MQN HEI is loaded onto a self-supported Fe x Ni y OOH-20F electrode by dip coating to form the anode. Figure 7 a shows the polarization curves of water-fed HEMELs with different HEI loadings at the anode. Notably, the current density at a cell potential of 1.8 V (j 1.8 ) is maximized at an optimal HEI loading of 0.8 mg cm -2 because as the HEI loading increases, ion transfer and OER kinetics are improved, as Figure 7as shown by the reduced ohmic resistance and OER kinetic resistance in b and 7d. However, when the HEI loading was increased to 0.9 mg cm -2 as can be seen from the increase in mass transfer resistance ([[]] Figure 7 d), the too thick HEI layer at the anode restricted the oxygen release, which led to a slight deterioration of the HEMEL performance.

[0087] The performance of the water-fed HEMEL was optimized to 1020 mA cm -2 for j 1.8 ( Figure 6 b). In contrast, when the PGM Ir / C catalyst was used to replace Fe x Ni y OOH-20F at the anode, the performance of the HEMEL decreased significantly, and under similar experimental conditions, j 1.8 dropped to 240 mA cm -2 at 80 °C and j 1.8 dropped to 290 mA cm -2 at 90 °C. This indicates that the excellent performance of the HEMEL described in this paper, compared with many advanced HEMELs ( Figure 6 c) [6,7,38–43] is even superior to the previously reported HEMELs that work with potassium carbonate aqueous electrolyte. [44,45] This excellent performance can be attributed to several factors described below.

[0088] The water-fed HEMEL with self-supported Fe x Ni y OOH-20F at the anode had a low ohmic resistance of 0.19 Ω cm 2 , lower than 0.23 Ω cm 2 of the previously reported water-fed HEMELs using PGM catalysts, [6] and 0.30 Ω cm 2 of the Zirfon membrane-based AEL that works with KOH aqueous electrolyte.

[36] It is also comparable to PEMEL (i.e., 0.10 - 0.13 Ω cm 2 ).

[46]

[0089] The self-supported Fe x Ni y OOH-nF electrode as the anode catalyst exhibited excellent OER activity via F - leaching-induced self-reconstruction (Table 1), [23,24] and promoted the electron transfer from the catalyst layer to the current collector, with 0.33 Ω cm 2 and 0.58 Ω cm2 In comparison, this results in a lower ohmic resistance (0.19 Ω cm 2 ) and OER kinetic resistance (0.32 Ω cm 2 ).

[0090] Weak metal-fluorine bonds in the electrocatalyst have been shown to gradually evolve into highly active metal-(oxy)hydroxide bonds during CV cycling, as indicated by the disappearance of the (Fe,Ni)-F bond after multiple consecutive cycles. Additionally, the Ni(II) / Ni(III) oxidation peak depends on the number of exposed NiOOH active sites and is considered an indicator of OER activity, which is evident in the electrocatalyst, especially after multiple repeated cycles.

[0091] PAP-TP-85 and PAP-TP-85-MQN HEMs and HEIs exhibit higher OH - conductivity than previously reported, including A201, AS-4, FFA-3, and aQAPS, as shown in Table 3:

[0092] Table 3. Ion exchange capacity (IEC) and OH - conductivity (σ OH - ) comparison of previously reported HEMs and HEIs.

[0093] Materials <![CDATA[IEC / mmol g -1 > <![CDATA[σ OH - / mS cm -2 > References PAP - TP - 85 2.4 <![CDATA[78 a ,175 b > HEM / HEI of the Present Invention PAP - TP - 85 - MQN 3.2 <![CDATA[150 a ,]]> HEM / HEI of the Present Invention Tokuyama A201 1.8 <![CDATA[42 a > Tokuyama Corporation Tokuyama A901 1.8 <![CDATA[38 a > Tokuyama Corporation AS4 1.4 <![CDATA[14 a > Tokuyama Corporation FFA - 3 2.0 <![CDATA[30 a >

[59] aQAPS 1.0 <![CDATA[100 b >

[60] LDPE 2.6 <![CDATA[145 c >

[61] QPE - X16Y11 1.9 <![CDATA[144 b >

[62] PVB - MPY 1.7 <![CDATA[159 c >

[63] NC5Q - PPQ - 60 2.6 <![CDATA[96 b >

[64] S70P30 4.0 <![CDATA[115 b >

[65] PFB 3.6 <![CDATA[124 b >

[66] 50PPOFC6NC6 1.9 <![CDATA[42 a ,140 b >

[67] BPN1 - 100 2.7 <![CDATA[122 b >

[68] QPAEN - 0.4 1.8 <![CDATA[116 b >

[69] FPAE - 3B - 3.0 - PD 1.2 <![CDATA[98 b >

[70] QAPPT 2.5 <![CDATA[137 b >

[71] TPQPOH 1.1 <![CDATA[27 a >

[72] PPO_Pip1.7 1.7 <![CDATA[18 a ,101 b >

[73]

[0094] a Data were collected from liquid water under room temperature conditions. b Data were collected from liquid water at T = 80 °C. c Data were collected at T = 80 °C and 95% relative humidity.

[0095] Durability is an important consideration for commercial applications. Most previously reported water-fed HEMELs have a short lifespan (<100 h), and during durability testing, their performance deteriorates rapidly, mainly due to the irreversible chemical degradation of HEI and HEM, especially for HEI in close contact with the catalyst. [8,42,47] First, the short-term durability of water-fed HEMEL was investigated at different current densities. It was observed that after continuous operation for 4 h at current densities from 80 °C, 100 mA cm -2 to 500 mA cm -2 , the cell potential hardly decayed ( Figure 8 ). Figure 9 a shows the long-term durability performance measured at a current density of 200 mA cm -2 and conditions of 80 °C. Due to the catalyst activation of HEM and HEI and complete HCO3 - / OH - Upon exchange, the battery potential decreased from 1.71 V to 1.63 V within the first 3 h of operation and then increased slowly at a rate of 0.56 mV h -1 −1 during the subsequent 160 h. Even under the condition of 500 mA cm -2 −2, after continuous operation at 80 °C for 70 h, the battery potential was still below 1.9 V, with a degradation rate of 1.81 mV h -1 ( Figure 10 ). As shown in Table 4, the long-term durability was significantly improved compared with the previously reported water-fed HEMEL:

[0096] Table 4. Comparison of the durability of water-fed HEMEL with previously reported HEMEL operating under similar conditions.

[0097]

[0098] The improved long-term durability is attributed to the following features. PAP HEM and HEI showed good alkaline stability and no obvious degradation occurred after 2000 h in 1.0 M KOH solution at 100 °C. [32,33] In addition, the self-supporting Fe x Ni y OOH-20F electrode showed excellent structural and chemical stability during the catalytic process. It was found that after continuous operation at 200 mA cm -2 −2 and 80 °C for 160 h, the vertically oriented nanosheet array structure ( Figure 9 b) and the crystal phase and chemical configuration of Fe x Ni y OOH-20F were well preserved ( Figure 9 c and 8). A peak at 688.0 eV corresponding to the C-F bond rather than the (Fe,Ni)-F bond appeared in the high-resolution F 1s XPS spectrum ( Figure 9 d), indicating that the HEI molecules were still attached to the catalyst surface after long-term operation to facilitate the transport of OH - , while the anionic F x Ni y in the Fe - OOH-20F catalyst was leached out due to the weak metal-fluorine bond during the OER process.

[24] However, the outermost HEI layer on the anode surface was mostly degraded and / or washed away by the water flow and oxygen ( Figure 10 and 9 b), which led to a slow increase in the battery potential with the extension of the measurement time.

[0099] By using a combination of HEM, HEI, and OER anode catalysts, the single-cell HEMEL described herein can achieve excellent performance and long-term durability. The HEMEL described herein is an effective water electrolysis technology for narrowing the gap between laboratory and commercial-scale production of low-cost hydrogen using intermittent renewable energy.

[0100] Hydrogen has been used in industry to refine petroleum to reduce its sulfur content, treat metals, produce fertilizers, purify glass, protect electronic products, and process food. Hydrogen can also be used as a hydrogen fuel, for example, in a hydrogen fuel cell, to generate electricity to power an electrical system.

[0101] Hydrogen produced via AEMEL using intermittent renewable energy (wind and solar), seawater, and wastewater can improve the utilization efficiency of renewable energy and reduce the cost of hydrogen production.

[0102] Due to its low cost, high voltage efficiency, high hydrogen purity, and high outlet pressure, AEMEL is a promising distributed electrolytic hydrogen production model.

[0103] The anode is not only used for water electrolysis to produce hydrogen but also in a flow cell to facilitate the electrochemical reduction of carbon dioxide and nitrogen.

[0104] The present invention has been described in detail. It is obvious that modifications and variations can be made without departing from the scope of the present invention as defined by the appended claims.

[0105] Examples

[0106] The following non-limiting examples are provided to further illustrate the present invention.

[0107] Example 1: Synthesis of Nickel Iron Oxyhydroxide and Fluorine-Doped Nickel Iron Oxyhydroxide Nanosheet Arrays Grown Directly on Compressed Nickel Foam

[0108] After compression at a pressure of 1000 lbs., nickel foam (2.5 cm × 2.5 cm) with a thickness of 280 μm was immersed in 1.0 M H 2 SO 4 aqueous solution for 1 hour to remove residual oxides, and then washed with deionized water until the acid was completely removed. A fluorine-doped nickel iron oxyhydroxide catalyst grown directly on compressed nickel foam was prepared via a one-step method. Iron(III) nitrate hexahydrate (Fe(NO 3 ) 3 ·6H 2 O, 20 mM) and sodium fluoride (NaF, 10 - 30 mM) were dissolved in 20 mL of deionized water. Then, O 2 gas was bubbled through the solution for 10 minutes. Subsequently, the compressed nickel foam was immersed in the above solution at room temperature for 12 hours, with continuous O2 flow. After washing with deionized water, the product was labeled as Fe x Ni y OOH-nF, where n represents the NaF concentration (10, 20, and 30 mM) in the reactants.

[0109] For comparison, nickel iron oxyhydroxide (Fe x Ni y OOH) catalyst was synthesized following the same procedure without adding NaF during the preparation.

[0110] Fe x Ni y OOH and Fe x Ni y OOH-nF had a mass loading of approximately 4.8 mg cm -2 .

[0111] Example 2: Electro-catalyst characterization.

[0112] Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) mapping analyses were performed on an Auriga 60 Crossbeam at an accelerating voltage of 3 kV. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) measurements were carried out on a Talos TM F200C at an accelerating voltage of 200 kV. X-ray diffraction (XRD) was performed using a Bruker D8 XRD with Cu kα irradiation, with a step size of 0.05° and a scanning rate of 0.025° s -1 . X-ray photoelectron spectroscopy (XPS) was measured using a Thermo Scientific TM K-Alpha TM XPS system with a resolution of 0.3 - 0.5 eV from a monochromatic aluminum anode X-ray source with Kα radiation (1486.6 eV). The Fe x Ni y OOH and Fe x Ni y OOH-nF catalysts were separated from the compressed nickel foam and then dissolved in HNO 3 aqueous solution (2 wt%) and the Fe / Ni molar ratio was determined via microwave plasma atomic emission spectrometry (MP-AES, Agilent 4100).

[0113] Figure 1 a schematically shows the formation mechanism of fluorine-doped nickel iron oxyhydroxide grown in situ on compressed nickel foam.

[0114] Figure 1 The XRD pattern in b shows the diffraction peaks of nickel foam (2θ = 44.5° and 51.8°) and three other diffraction peaks at 2θ = 11.9°, 16.9° and 35.3°. These diffraction peaks are those of Fe x Ni y OOH and Fe x Ni y OOH-20F (characteristic peaks of FeOOH in JCPDS 01-075-1594), and they are consistent with the appearance of Fe(III)-(OH)O and Ni(II)-OH species in the high-resolution Fe 2p and Ni 2p XPS spectra ( Figure 18 ).

[0115] In the high-resolution F1s XPS spectrum, the F1s peak at a binding energy of 684.0 eV reveals the presence of (Fe,Ni)-F bonds in Fe x Ni y OOH-20F ( Figure 1 c).

[48]

[0116] The Fe x Ni y OOH has an Fe / Ni molar ratio of 4.6 as determined by a microwave plasma atomic emission spectrometer (MP-AES). When the F - concentration in the reactants increases to 30 mM, this molar ratio drops to 2.0 ( Figure 11 ). This is because at 25 °C, the strong coordination interaction (stability constant (K - ) between F 3+ anions and Fe f cations ( 15 ) results in a decrease in the free Fe 3+ concentration in the reactants.

[0117] Figure 1 The scanning electron microscope (SEM) images in d and 12a show the three-dimensional sponge-like network structures of Fe x Ni y OOH and Fe x Ni y OOH-20F, which are composed of vertically oriented and interpenetrating arrays of nanosheets. In addition, as the F - concentration increases, the thickness and size of the nanosheets gradually decrease ( Figure 12 ), which may be due to the lattice strain caused by F - incorporation. Figure 1 Fe x Ni yThe high-magnification TEM images of OOH-20F confirmed the ultrathin nanosheet structure with a thickness of 2 nm to 3 nm. According to the XRD results, the lattice fringe with d = 0.52 nm corresponds to the lattice distance of the (200) plane of FeOOH( Figure 1 f).

[0118] Example 3: Electrochemical Electrocatalyst Characterization

[0119] The OER catalytic activity of the electrocatalyst in Example 1 was measured on a VMP-300 multi-channel electrochemical workstation in O 2 saturated 1.0 M KOH solution. At 100 mA cm -2 (η 100 ) the overpotential was calculated as follows:

[0120] η = E 100 -1.23 (1)

[0121] where E 100 is the OER polarization potential relative to RHE at 100 mA cm -2 after correction by iR-compensation, and the O 2 / H 2 O equilibrium potential is 1.23 V.

[0122] The Ir / vulcan XC-72 catalyst (20 wt%), Nafion TM solution (40 μL), and isopropanol (960 μL) were sonicated in an ice-water bath for 1 h and then sprayed onto both sides of compressed nickel foam at a total mass loading of approximately 4.8 mg cm -2 as a comparative benchmark PGM OER catalyst (denoted as "Ir / C" in the figure).

[0123] The internal resistance (R) was obtained from the electrochemical impedance spectroscopy (EIS) measured at 10 mV in the frequency range of 100 kHz to 0.01 Hz at open circuit voltage. The electrochemically active surface area (ECSA) was calculated based on the Fe x Ni y OOH and Fe x Ni y OOH-nF electrodes in N 2 saturated 1.0 M KOH solution for the electrochemical double-layer capacitance (C dl ). The current measured in the non-Faradaic potential region (i c , mA cm -2 ) should originate from double-layer charging, so C dl was calculated from the double-layer charging current (i c , mA cm-2 ) and scan rate (ν, mV s -1 ) obtained as follows:

[0124] C dl = i c / v (2).

[0125] Estimate the ECSA and roughness factor (RF) from C according to Equations 3 and 4: dl ECSA = C

[0126] / C dl (3) s RF = ECSA / A (4)

[0127] where C

[0128] is the specific capacitance of the material with an atomically smooth planar surface and should be 0.040 mF cm s in 1.0 M KOH. A is the geometric area of the electrode (2.0 cm -2

[49] ). 2 )

[0129] The OER activity of Fe 2 Ni x OOH and Fe y Ni x OOH-nF catalysts was measured using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques in O y -saturated 1.0 M KOH aqueous electrolyte. As can be seen from the CV curves in the first 20 cycles shown Figure 17 , the OER current of Fe x Ni y OOH-20F increased significantly. This was accompanied by a positive shift in the oxidation peak potential of Ni(II) / Ni(III), while under similar measurements, Fe x Ni y OOH showed no obvious change. The weak metal-fluorine bond in Fe x Ni y OOH-20F is thought to gradually evolve into a highly active metal-(oxy)hydroxide bond during the CV cycles, [23,24] as indicated by the disappearance of the (Fe,Ni)-F bond after 20 consecutive cycles ( Figure 19 ). In addition, the Ni(II) / Ni(III) oxidation peak depends on the number of exposed NiOOH active sites and is considered an indicator of OER activity, [25–28] which is obvious in Fe x Ni y OOH-20F, especially after 20 repeated cycles. In Fe x Niy Hardly visible in OOH( Figure 13 a), further demonstrating Fe x Ni y F in OOH-20F - ion leaching induces surface reconstruction, thereby exposing more NiOOH active sites and enhancing catalytic activity.

[0130] The polarization curves measured at 5 mV s -1 via iR compensation are further used to compare the OER activities. When Fe x Ni y OOH and Fe x Ni y OOH-nF species grow on compressed nickel foam, among all Fe x Ni y OOH and Fe x Ni y OOH-nF catalysts and uncoated nickel foam, Fe x Ni y OOH-20F shows the highest OER activity( Figure 13 b). More precisely, Fe x Ni y OOH-20F has an overpotential 63 mV lower than that of Fe -2 几何面积 (η 100 ) and even 90 mV lower than that of the PGM Ir / C catalyst at 100 mA cm x Ni y OOH. The exceptional OER activity is mainly attributed to two factors. First, F - leaching induces the formation of a catalytically active layer on the surface to improve electron conductivity, electron transport, and mass transfer

[23] . This is also illustrated by the decrease in the ohmic resistance, charge transfer resistance, and mass transfer resistance from Fe x Ni y OOH to Fe x Ni y OOH-20F catalyst( Figure 15 ). Second, the self-reconstruction caused by F - leaching increases the number of exposed active sites and the electrochemically active surface area (ECSA), manifested as an increase in the double-layer capacitance (C dl ) of the non-Faradaic region from 13.3 mF cm x Ni y OOH to 16.1 mF cm -2 of Fe x Ni y OOH-20F -2( Figure 16 )。Fe x Ni y The smaller Tafel slope (66.1 mV dec -1 ) of Fe x Ni y OOH compared with 124.5 mV dec -1 of Fe -1 Ni - OOH and 82.2 mV dec Figure 13 ) of the Ir / C catalyst further demonstrates that the incorporation and leaching of F Figure 13 d summarizes the relationship between η 100 and the specific current density at 1.55 V and the RHE normalized with respect to the ECSA (j ECSA @1.55 V). The j x Ni y @1.55 V values of Fe ECSA Ni x OOH-nF are all higher than those of Fe y Ni x OOH, especially for the value of Fe y Ni - OOH-20F, further confirming that the leaching-induced reconstruction significantly enhances the intrinsic OER activity by exposing effective active species and improving electron transport. In addition, the optimized Fe x Ni y OOH-20F catalyst has overpotentials of 280 mV and 348 mV at geometric surface area current densities of 100 mA cm -2 and 500 mA cm -2 respectively, which meet the requirements of industrial applications (less than 400 mV at 500 mA cm -2 ), and are comparable to the previously reported Ni-Fe-based catalysts grown on uncompressed foam metals by more complex methods (Table 1). [14,15,23,29-31]

[0131] Example 4: Synthesis of nickel-iron-cobalt oxyhydroxide and fluorine-doped nickel-iron oxyhydroxide nanosheet arrays directly grown on compressed nickel foam

[0132] The facile electrocatalyst synthesis method in Example 1 has been used to prepare another multi-metal oxyhydroxide nanosheet array (Fe, Ni, Co)OOH( Figure 14 ). After compression at a pressure of 1000 lbs., a 280-μm-thick nickel foam (2.5 cm × 2.5 cm) was immersed in 1.0 M H 2 SO 4The solution was kept in an aqueous solution for 1 hour to remove residual oxides, and then washed with deionized water until the acid was completely removed. A nickel-iron-cobalt oxyhydroxide catalyst grown directly on compressed nickel foam was prepared via a one-step method. Iron(III) nitrate hexahydrate (Fe(NO 3 )) 3 ·6H 2 O, 20 mM) and cobalt(II) nitrate hexahydrate (Co(NO 3 )) 3 ·6H 2 O, 20 mM) were dissolved in 20 mL of deionized water. Then, O 2 gas was bubbled through the solution for 10 minutes. Subsequently, the compressed nickel foam was immersed in the above solution at room temperature for 12 hours with a continuous O 2 flow above the liquid surface. After washing with deionized water, the product (Fe, Co, Ni)OOH on the nickel foam was obtained.

[0133] Using this method, sodium fluoride (NaF, 10 - 30 mM) was added to the solution containing iron(III) and cobalt(II) nitrates to produce the (Fe, Co, Ni)OOH-nF electrocatalyst.

[0134] Example 5: Fabrication of HEMEL

[0135] As Figure 6 shown in a, HEMEL includes a flow channel plate, a cathode gas diffusion layer (GDL), a cathode, HEM, and an anode. TGP-H-60 Toray carbon paper (5% moisture-proof) was used as the GDL of the cathode.

[0136] According to our previous method, a poly(arylpiperidinium) hydroxide exchange membrane (PAP HEM) with a thickness of 20 μm in the form of carbonate was prepared from N-methyl-4-piperidone, 2,2,2-trifluoroacetophenone, and p-terphenyl,

[32] where the molar ratio between N-methyl-4-piperidone and the aryl monomer was 85%. Poly(arylpiperidinium) hydroxide exchange ionomers (PAPHEIs) were synthesized via the method of PAP HEM,

[32] and dissolved in anhydrous ethanol with a concentration of 5 wt% in the form of carbonate. At room temperature, PAP HEI was PAP-TP-85 in the cathode with an ion exchange capacity (IEC) of 2.4 mmol g -1 , OH - conductivity of 78 mS cm -1 , and when it was PAP-TP-85-MQN in the cathode, the IEC was 3.2 mmol g -1 , OH - conductivity of 150 mS cm -1 .

[0137] To prepare the cathode, first, the Pt / C catalyst (47 wt%, TKK), deionized water, isopropyl alcohol, and PAP-TP-85HEI solution were ultrasonically treated in an ice-water bath for 1 hour to obtain a well-dispersed catalyst ink. Then, with the help of a spray gun (Iwata, Japan), the catalyst ink was sprayed onto the PAP HEM using a manual spraying method to produce a cathode (hydrogen evolution electrode) with a Pt loading of 0.94 mg Pt cm -2 , and an HEI loading of 30 wt%. The electrode area was 5 cm 2 .

[0138] To prepare an anode without platinum group metals (PGM), PAP-TP-85-MQNHEIs were loaded onto the Fe x Ni y OOH-20F electrode to form an anode (oxygen evolution electrode) using the dip coating method. The loading of HEI in the anode was calculated based on the weight change of ten samples before and after the dip coating process.

[0139] For comparison, a PGM anode was prepared by spraying a catalyst ink composed of an Ir / C catalyst (20 wt%), deionized water, isopropyl alcohol, and PAP-TP-85HEI solution on both sides of compressed nickel foam. The total mass loading of the Ir / C catalyst was 4.8 mg cm -2 , and the PAP-TP-85HEI loading was 30 wt%.

[0140] Example 6: HEMEL Cell Performance Evaluation

[0141] The cell performance and durability of HEMEL were characterized using the following water electrolysis device, which consisted of a membrane electrode assembly (MEA), a graphite end plate with three serpentine channels on the cathode side, and a titanium end plate with three serpentine grooves on the anode side. A KOH aqueous solution or pure water with different concentrations was fed into the anode at a flow rate of 3 mL min -1 . An Arbin battery test device was used to provide the voltage and current required for the water decomposition reaction. By stepping the current density from 10 mA cm -2 in increments of 10 mA cm -2 to 1000 mA cm -2 , the polarization curves (current density vs. cell voltage) of HEMEL were recorded at 80 °C and 90 °C, respectively, and each current density was maintained for one minute. At 200 mA cm -2 and 500 mA cm -2Durability was tested at a current density of Figure 15 in which Fe x Ni y OOH and Fe x Ni y OOH-20F electrodes were measured at 1.60 V vs. RHE with an AC oscillation of 10 mV amplitude at frequencies from 100 kHz to 100 mHz. The EIS spectra were fitted using an equivalent circuit consisting of two parallel units of the ohmic resistance (R s ) and the charge transfer resistance (R ct ) at the interface between the catalyst and the electrolyte, the mass transfer resistance (R mass ) and constant phase elements (CPE ct and CPE mass )(inserted) in series. [10,11]

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[0225] When introducing elements of the present invention or preferred embodiments thereof, the articles "a", "an", "the", and "said" are intended to mean that there is one or more of the elements. The terms "comprising", "including", and "having" are intended to be inclusive and mean that there may be additional elements other than the recited elements.

[0226] In view of the foregoing, it will be seen that several objects of the present invention have been achieved and other advantageous results have been obtained.

[0227] Since various changes can be made to the above-described apparatus and method without departing from the scope of the present invention, all matter contained in the above description and shown in the accompanying drawings should be interpreted as illustrative and not restrictive.

Claims

1. A fluorine-containing nickel iron oxyhydroxide electrocatalyst is prepared by immersing a compressed gas diffusion layer containing nickel into an aqueous solution rich in O 2 containing iron nitrate and a fluoride salt.

2. The electrocatalyst according to claim 1, which has a single F 1s peak shown by high-resolution fluoride (F) 1s X-ray photoelectron spectroscopy.

3. The electrocatalyst according to claim 2, wherein the single F 1s peak is located at a binding energy of 684.0 eV.

4. The electrocatalyst according to claim 1, which comprises a three-dimensional sponge-like network structure determined by scanning electron microscopy (SEM) imaging.

5. The electrocatalyst according to claim 1, which comprises a vertically oriented and interpenetrating nanosheet array determined by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).

6. The electrocatalyst according to claim 5, wherein each nanosheet has a thickness of 2 nm to 3 nm determined by high-magnification transmission electron microscopy (TEM) imaging.

7. The electrocatalyst according to claim 1, wherein the Fe / Ni molar ratio of the electrocatalyst determined by microwave plasma atomic emission spectrometry (MP-AES) is less than 4.

0.

8. The electrocatalyst according to claim 7, wherein the Fe / Ni molar ratio of the electrocatalyst determined by MP-AES is 2.0 to 3.

2.

9. The electrocatalyst according to claim 1, wherein the formula of the electrocatalyst is Fe x Ni y OOH, where the range of x is from 0.75 to 0.83, and the range of y is from 0.26 to 0.

38.

10. The electrocatalyst according to claim 1, which further comprises at least one metal other than Fe and Ni, and the at least one metal comprises Ce, Cr, Cu, Co, Mo, Ru, Pd, Pt, Ir, Rh, Os, Ag, Au, Re, Ta, Ti, V, W, Mn, Zn, Sn, Sb, In, Ga, Bi, Pb or Zr.

11. A method for preparing a fluorinated nickel iron oxyhydroxide electrocatalyst, the method comprising: Immerse a compressed nickel-containing gas diffusion layer in an aqueous solution containing iron(III) nitrate hexahydrate and a fluoride salt, and soak for at least 8 hours with oxygen flowing above the surface of the solution to form the fluorinated nickel iron oxyhydroxide electrocatalyst; and 2 ​ washing the fluorinated nickel iron oxyhydroxide electrocatalyst with water.

12. The method according to claim 11, further comprising compressing the nickel-containing gas diffusion layer with a force of at least 4448 N to form the compressed nickel-containing gas diffusion layer.

13. The method according to claim 11, further comprising immersing the compressed nickel-containing gas diffusion layer in an acidic aqueous solution to remove residual oxides from the compressed nickel-containing gas diffusion layer, and then washing the compressed nickel-containing gas diffusion layer with water to remove the acidic solution.

14. The method according to claim 11, wherein the iron(III) nitrate hexahydrate and the fluoride salt are present in the aqueous solution rich in O in a molar ratio ranging from 2:1 to 1:1.

5. 2 ​ 15. The method according to claim 11, wherein the aqueous solution rich in O 2 is formed by bubbling oxygen through an aqueous solution containing iron(III) nitrate hexahydrate and a fluoride salt.

16. The method according to claim 11, further comprising removing the fluorinated nickel iron oxyhydroxide electrocatalyst from the compressed nickel-containing gas diffusion layer.

17. The method according to claim 16, wherein the fluorinated nickel iron oxyhydroxide electrocatalyst is removed via ultrasonic treatment.

18. The electrocatalyst according to any one of claims 1 to 9, which is prepared by the method according to any one of claims 11 to 17.

19. A self-supporting oxygen evolution electrode without platinum group metals (PGM), which comprises the electrocatalyst according to any one of claims 1 to 10 in the pores of a gas diffusion layer comprising a nickel substrate.

20. An anion exchange membrane electrolyzer (AEMEL) for generating hydrogen from water, the AEMEL comprising: An anode, which includes an anode electrocatalyst, the anode electrocatalyst including a fluorine-containing nickel iron oxyhydroxide electrocatalyst according to any one of claims 1 to 10, for forming oxygen and water from hydroxide ions; A cathode, which includes a cathode electrocatalyst for forming hydrogen and hydroxide ions from water; and An anion exchange membrane, which is adjacent to and separated from the anode and the cathode, and is used for transporting hydroxide ions from the cathode to the anode.

21. The AEMEL according to claim 20, wherein the water fed to the cathode or the anode contains a hydroxide-conducting electrolyte for forming oxygen and water from hydroxide ions.

22. The AEMEL according to claim 21, wherein the hydroxide-conducting electrolyte contains potassium hydroxide.

23. The AEMEL according to claim 20, wherein the water fed to the cathode or the anode does not contain an alkaline electrolyte.

24. The AEMEL according to claim 20, wherein the fluorine-containing nickel iron oxyhydroxide electrocatalyst is in the pores of a gas diffusion layer comprising a nickel substrate.

25. The AEMEL according to claim 20, wherein the membrane comprises an anion exchange polymer.

26. The AEMEL according to claim 25, wherein the anion exchange polymer comprises a quaternary ammonium or imidazolium group and a polymer backbone without an ether group.

27. The AEMEL according to claim 25, wherein the anion exchange polymer comprises poly(arylpiperidinium), alkylammonium-functionalized poly(arylenealkylene), substituted imidazolium-functionalized poly(arylenealkylene), alkylammonium-functionalized poly(styrene), substituted imidazolium-functionalized poly(styrene), alkylammonium-functionalized poly(styrene-co-divinylbenzene), substituted imidazolium-functionalized poly(styrene-co-divinylbenzene), alkylammonium-functionalized poly(styrene-block-ethylene-co-butadiene-block-styrene), substituted imidazolium-functionalized poly(styrene-block-ethylene-co-butadiene-block-styrene), alkylammonium-functionalized poly(ethylene), substituted imidazolium-functionalized poly(ethylene), alkylammonium-functionalized poly(tetrafluoroethylene), substituted imidazolium-functionalized poly(tetrafluoroethylene), alkylammonium-functionalized poly(ethylene-co-tetrafluoroethylene), substituted imidazolium-functionalized poly(ethylene-co-tetrafluoroethylene), polyethyleneimine, poly(diallylammonium), or a combination thereof.

28. The AEMEL according to claim 27, wherein the anion exchange polymer comprises poly(arylpiperidinium).

29. The AEMEL according to claim 20, wherein the cathode electrocatalyst comprises silver, a silver alloy, silver supported on carbon, a silver alloy supported on carbon, platinum, a platinum alloy, platinum supported on carbon, a platinum alloy supported on carbon, palladium, a palladium alloy, palladium supported on carbon, a palladium alloy supported on carbon, manganese oxide, manganese oxide supported on carbon, cobalt oxide, cobalt oxide supported on carbon, heteroatom-doped carbon, metal-heteroatom-carbon, perovskite, perovskite supported on carbon, or a combination thereof.

30. The AEMEL according to claim 29, wherein the heteroatom-doped carbon is represented by X-C, where X comprises one or more of N, C, B, P, S, Se, or O.

31. The AEMEL according to claim 29, wherein the metal-heteroatom-carbon is represented by M-X-C, where X comprises one or more of N, C, B, P, S, Se, or O, and M comprises one or more of Fe, Ce, Cr, Cu, Co, Mo, Ni, Ru, Pd, Pt, Ir, Rh, Os, Ag, Au, Re, Ta, Ti, V, W, Mn, Zn, Sn, Sb, In, Ga, Bi, Pb, or Zr.

32. The AEMEL according to claim 29, wherein the perovskite is represented by ABX 3 , where A comprises one or more of Ca, Sr, Ba, Sc, Y, La, Ce, Zr, Cu, Zn, Sb, Bi, B comprises one or more of Al, Ti, Mn, Fe, Co, Ni, W, Pd, and X comprises one or more of O, Se, S.

33. The AEMEL according to claim 29, wherein the carbon-supported perovskite is represented by ABX 3 , wherein A comprises one or more of Ca, Sr, Ba, Sc, Y, La, Ce, Zr, Cu, Zn, Sb, Bi, B comprises one or more of Al, Ti, Mn, Fe, Co, Ni, W, Pd, and X comprises one or more of O, Se, S.

34. The AEMEL according to claim 20, wherein the cathode electrocatalyst comprises carbon-supported platinum.

35. The AEMEL according to claim 20, further comprising a gas diffusion layer adjacent to the cathode.

36. The AEMEL according to claim 20, further comprising an ionomer layer on the cathode and / or an ionomer layer on the anode.

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