WO2019058275A1

WO2019058275A1 - Durable oxygen evolution electrocatalysts

Info

Publication number: WO2019058275A1
Application number: PCT/IB2018/057212
Authority: WO
Inventors: Kazuhiro Takanabe; Keisuke OBATA
Original assignee: King Abdullah University Of Science And Technology
Priority date: 2017-09-19
Filing date: 2018-09-19
Publication date: 2019-03-28
Also published as: US20200308720A1

Abstract

Oxygen evolution electrocatalysts, electrodes using oxygen evolution electrocatalysts, and methods of making oxygen evolution electrocatalysts are provided. The oxygen evolution electrocatalyst includes an oxide electrocatalyst and a permselective amorphous layer deposited on the oxide electrocatalyst. In this regard, the permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions and evolved O2 through the permselective amorphous layer.

Description

DURABLE OXYGEN EVOLUTION ELECTROCATALYSTS

CROSS-REFERENCED TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application No. 62/560,438, filed September 19, 2017, 62/565,732, filed September 29, 2017, and 62/626,963 filed February 6, 2018, which are hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The presently-disclosed invention relates generally to providing stable and selective electrocatalysts for use in the oxygen evolution reaction, and more particularly to oxygen evolution electrocatalysts, electrodes using oxygen evolution electrocatalysts, and methods of making oxygen evolution electrocatalysts.

BACKGROUND

Hydrogen is one of the energy carriers that can effectively store intermittent energy from renewables, such as solar and wind power. A lot of techniques are studied to generate hydrogen and oxygen from water via, for example, water electrolysis and photocatalytic water splitting. Since the oxygen evolution reaction (OER) is a kinetically sluggish reaction compared with the hydrogen evolution reaction (HER) in water splitting, the development of highly active, durable and cost-effective electrocatalysts for OER is desired. NiFeO_x is one of the most active electrocatalysts towards OER in alkaline conditions and it is known that Fe has a critical role to improve activity while the reason why doping of Fe boosts the kinetics of OER is still under discussion. Although a lot of research has been devoted to decrease overpotential via development of layered double hydroxide (LDH) structure, compositional control, or deposition on conductive support with high surface area, the stability of NiFeOx under harsh oxidative conditions has not been considered. Indeed, the efforts relating to decreasing overpotential to date have resulted in either increased overpotential or decreased iron content in the electrocatalyst during a stability test. Accordingly, there still exists a need for an electrocatalyst that provides improved stability and selectivity in the oxygen evolution reaction (OER). BRIEF SUMMARY OF THE INVENTION

One or more embodiments of the invention may address one or more of the aforementioned problems. Certain embodiments provide oxygen evolution electrocatalysts, electrodes using oxygen evolution

electrocatalysts, and methods of making oxygen evolution electrocatalysts. In one aspect, an oxygen evolution electrocatalyst is provided. The oxygen evolution electrocatalyst may include an oxide electrocatalyst, and a permselective amorphous layer deposited on the oxide electrocatalyst. The permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst.

In another aspect, an electrode using an oxygen evolution electrocatalyst is provided. The electrode may include a substrate and an oxygen evolution electrocatalyst. The oxygen evolution electrocatalyst may include an oxide electrocatalyst, and a permselective amorphous layer deposited on the oxide electrocatalyst. The permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst.

In yet another aspect, a method of making an oxygen evolution electrocatalyst is provided. The method may include providing an oxide electrocatalyst, and depositing a permselective amorphous layer on the oxide electrocatalyst via anodic deposition. The permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst. BRIEF DESCRIPTION OF THE DRAWING(S)

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. la illustrates results from electrocatalytic stability tests on electrocatalysts in accordance with certain embodiments of the invention.

FIGs. lb and lc are cyclic voltammograms of electrodes in accordance with certain embodiments of the invention.

FIG. Id is a Tafel plot taken from the cyclic voltammograms of FIGs. lb and lc.

FIGs. 2a and 2b are scanning electron microscope (SEM) images of electrocatalysts in accordance with certain embodiments of the invention.

FIG. 2c is an SEM image of an electrode in accordance with certain embodiments of the invention.

FIG. 2d is a Raman spectra of electrodes in accordance with certain embodiments of the invention.

FIG. 3a illustrates Nyquist plots of electrocatalysts in accordance with certain embodiments of the invention.

FIG. 3b illustrates Bode plots of electrocatalysts in accordance with certain embodiments of the invention.

FIG. 4a illustrates cyclic voltammograms for electrodes in accordance with certain embodiments of the invention.

FIG. 4b illustrates Faradaic efficiency of oxygen during controlled current electrolysis for electrocatalysts in accordance with certain embodiments of the invention.

FIGs. 5a and 5b are cyclic voltammograms for substrates and electrodes in accordance with certain embodiments of the invention.

FIG. 5c is a chronoamperogram of an electrode in accordance with certain embodiments of the invention. FIG. 6 illustrates peak position of Ni reduction peaks taken from FIGs. lb and lc.

FIG. 7 illustrates XRD diffractograms of various substrates in accordance with certain embodiments of the invention.

FIGs. 8a and 8b are XPS spectra of the CeOx layer in accordance with certain embodiments of the invention.

FIGs. 9a and 9b illustrate equivalent circuits for (a) Randies circuit and (b) Voigt circuit in accordance with certain embodiments of the invention.

FIG. 10 is a cyclic voltammogram of electrodes in accordance with certain embodiments of the invention.

FIG. 11 is a chronoamperogram of electrocatalysts in accordance with certain embodiments of the invention.

FIG. 12a is a chronoamperogram of electrocatalysts in accordance with certain embodiments of the invention.

FIG. 12b illustrates electrocatalytic stability test results for electrodes in accordance with certain embodiments of the invention.

FIGs. 13a-13e illustrate Faradaic efficiency of oxygen during controlled current electrolysis for electrodes in accordance with certain embodiments of the invention.

FIG. 14 illustrates electrocatalytic stability test results for electrodes in accordance with certain embodiments of the invention.

FIG. 15A and 15B illustrates cyclic voltammograms of electrodes in accordance with certain embodiments of the invention.

FIGs. 16a and 16b are SEM images of electrodes in accordance with certain embodiments of the invention.

FIGs. 17a and 17b are cyclic voltammograms of electrodes in accordance with certain embodiments of the invention. FIG. 18 is a schematic block diagram illustrating a method of making an oxygen evolution electrocatalyst in accordance with certain embodiments of the invention.

FIG. 19 illustrates a cross-section of an electrode in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, this inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. As used in the specification, and in the appended claims, the singular forms "a", "an", "the", include plural referents unless the context clearly dictates otherwise.

Through combined effort and ingenuity, the inventors have developed a highly durable OER electrocatalyst by deposition of a permselective amorphous layer on an oxide electrocatalyst. The permselective amorphous layer includes a mixed oxide and hydroxide layer which prevents diffusion of redox ions while allowing OH^" and evolved O2 to permeate through. Improved stability of the oxide electrocatalyst can be attributed to the permselectivity of the amorphous layer, which can regulate diffusion of redox anions, such as iodide, ferrocyanide and ferrate, between the electrolyte and oxide electrocatalyst. In this regard, the oxygen evolution electrocatalyst is a highly active and durable OER catalyst that undergoes harsh oxidative condition.

I. Oxygen Evolution Electrocatalyst

In accordance with certain embodiments, oxygen evolution electrocatalysts are provided. The oxygen evolution electrocatalyst includes an oxide electrocatalyst and a permselective amorphous layer deposited on the oxide electrocatalyst. In this regard, the permselective amorphous layer may prevent diffusion of redox ions (e.g., iodide, ferrocyanide, ferrate, etc.) but permit diffusion of hydroxide ions and evolved O2 through the permselective amorphous layer. Without intending to be bound by theory, the diffusion of hydroxide ions to the oxide electrocatalyst under the permselective amorphous layer allows the evolution of oxygen in the OER while preventing diffusion and further dissolution of easily-dissolved iron species from the underlying oxide to the electrolyte. Accordingly, the resulting oxygen evolution electrocatalyst described herein may achieve significantly improved stability over bare electrocatalysts (i.e.

electrocatalysts without the permselective amorphous layer).

In accordance with certain embodiments, for example, the oxide electrocatalyst may comprise an oxide of a transition metal. In some embodiments, for instance, the oxide electrocatalyst may comprise at least one of NiOx, CoO_x, FeO_x, MnO_x, IrO_x, RuO_x, or any combination thereof to form a mixed oxide, where "x" is the number of oxygen atoms in the oxide electrocatalyst and may depend on the number of metal cations present, the number of oxidation states, and/or the like as understood by one of ordinary skill in the art. For example, in some embodiments, "x" may be from 2-8, and in further embodiments, "x" may be from 2-4. In certain embodiments, the oxide electrocatalyst may comprise NiFeO_x. In other embodiments, for instance, the oxide electrocatalyst may comprise CoFeO_x. In some embodiments, for example, the oxide electrocatalyst may comprise a thickness from about 10 nm to about 300 nm. By way of example only, the thickness of the CoO_x electrocatalyst of FIG. 16A is about 10 nm, while the thickness of the NiFeO_x electrocatalyst of FIG. 2C is about 300 nm.

According to certain embodiments, for instance, the oxide electrocatalyst may comprise an LDH structure or an amorphous structure. In further embodiments, for example, the oxide electrocatalyst may comprise a spinel composition. Spinels are a mineral oxide having the general formula of AB2O4 and may be supported on a plurality of support oxides. As such, the A component is tetrahedrally coordinated with the oxygens and the B component is octahedrally coordinated with the oxygens. Spinels may include a transition metal (e.g., iron (Fe), manganese (Mn), nickel (Ni), cobalt (Co), copper (Cu), vanadium (V), silver (Ag), titanium (Ti), etc.) and an "other metal" (i.e., aluminum (Al), magnesium (Mg), gallium (Ga), tin (Sn), thallium (Tl), lead (Pb), bismuth (Bi), or indium (In)).

In some embodiments, for example, a spinel composition may include copper, nickel, cobalt, iron, manganese, or chromium at any concentration, including quaternary, ternary, and binary combinations thereof. Ternary combinations may include Cu— Mn— Fe, Cu— Mn— Co, Cu— Fe-Ni, Cu-Co-Fe, Cu-Mn-Ni, Cu-Mn-Co, and Cu-Co-Ni. Binary combinations may include Cu— Mn, Cu— Fe, Cu— Co, Cu— Ni, Cu— Ni, Mn— Fe, Mn— Co, Mn— Ni, Co— Ni, and Co— Fe. In further embodiments, for instance, the oxide electrocatalyst may comprise one or more of CoFe20₄ and NiFe20₄.

In accordance with certain embodiments, for example, the permselective amorphous layer may comprise a catalytically inactive material. In some embodiments, for instance, the permselective amorphous layer may comprise at least one of CeO_x, TiO_x, A10_x, ZnO_x, ZrO_x, SiO_x, CaO_x, MgO_x, or any combination thereof, where "x" is the number of oxygen atoms in the oxide electrocatalyst and may depend on the number of metal cations present, the number of oxidation states, and/or the like as understood by one of ordinary skill in the art. For example, in some embodiments, "x" may be from 2-8, and in further embodiments, "x" may be from 2-4. In further embodiments, for example, the permselective amorphous layer may comprise a thickness from about 100 nm to about 600 nm. In some embodiments, for instance, the permselective amorphous layer may comprise a thickness from about 100 nm to about 200 nm. If the thickness of the permselective amorphous layer is less than 100 nm, for example, the permselective amorphous layer begins to lose its selectivity. In certain embodiments, for instance, the permselective amorphous layer may be deposited approximately uniformly (i.e. homogenously) on the oxide electrocatalyst.

II. Electrode

In another aspect, electrodes using oxygen evolution electrocatalysts are provided. As shown in FIG. 19, the electrode 200 includes a substrate 202 and an oxygen evolution electrocatalyst 204. The oxygen evolution electrocatalyst 204 includes an oxide electrocatalyst 206 and a permselective amorphous layer 208 deposited on the oxide electrocatalyst 206. In this regard, the permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst, as previously discussed herein.

In accordance with certain embodiments, for example, the substrate may comprise any conductive substrate suitable for use with the oxygen evolution electrocatalyst as understood by one of ordinary skill in the art. In some embodiments, for instance, the substrate may comprise at least one of cobalt, gold, nickel, a fluorine-doped tin oxide (FTO) substrate, or any combination thereof. In further embodiments, for example, the substrate may comprise a gold-coated FTO substrate. By way of example only, approximately 200 nm of gold may be deposited on approximately 500 nm of FTO substrate; however, the substrate may comprise any thickness as long as it remains conductive.

III. Method of Making an Oxygen Evolution Electrocatalyst

In yet another aspect, methods of making oxygen evolution electrocatalysts are provided. As shown in FIG. 18, the method 100 includes providing an oxide electrocatalyst at block 101, and depositing a permselective amorphous layer on the oxide electrocatalyst via anodic deposition at block 102. In this regard, the permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst, as previously discussed herein.

In accordance with certain embodiments, for example, depositing the permselective amorphous layer may comprise depositing the permselective amorphous layer approximately uniformly on the oxide electrocatalyst. As mentioned herein, anodic deposition may be used to deposit the

permselective amorphous layer on the oxide electrocatalyst. By way of example only, the anodic deposition may utilize an electrolyte (i.e.

deposition solution) including from about 0.1 M to about 0.5 M

Ce(N03)3- 6H2O at room temperature. A constant anodic potential may be applied to the deposition solution, and the deposition solution may be bubbled with argon before adding metal salts, and the argon atmosphere may be maintained during anodic deposition. EXAMPLES

The following examples are provided for illustrating one or more embodiments of the present invention and should not be construed as limiting the invention.

Material Preparation

Gold-coated fluorine-doped tin oxide (FTO) substrate was prepared by sputter-coating 20 nm of Ti followed by 190 nm of gold.

Electrodeposition of NiFeO_x on the gold-coated FTO substrate was performed by cathodic electrochemical deposition. -20 mA cm^"2 was applied for 2 min in an electrolyte solution containing 50 mM NH4OH, 25 mM H2SO4, 9 mM NiSO 7H₂0 and 9 mM FeSO 7H₂0 with a carbon paper as a counter electrode. The pH of the deposition solution was adjusted to 2.5. When NiFeCeOx was prepared, 3, 6 or 9 mM of Ce(N03)3- 6H20 was also added to the deposition solution. A CeO_x layer was formed on the NiFeOx electrode by anodic deposition in the electrolyte containing 0.4 M Ce(N0₃)3- 6H2O and 0.4 M CH3COONH4. A constant anodic potential of 1.1 V vs Ag/AgCl was applied for 6 h with resting period of 1 min in every 30 min. Deposition solution was stirred by a magnetic stir during deposition. pH of 0.4 M CH3COONH4 was adjusted to 8 by 4 M NaOH solution and it turned to pH 7 after adding Ce(NO)3- 6H20. Deposition solutions were bubbled with Ar (99.9999%) for at least 30 min before adding metal salts and Ar atmosphere was maintained during deposition.

Physical Characterization

X-ray diffraction (XRD) was collected with a Bruker D8 Advanced A25 diffractometer in the Bragg-Brentano geometry (with Cu Ka radiation at 40 kV and 40 mA). Data sets were acquired in continuous scanning mode over a 2Θ range of 0-80°. Raman spectra was obtained by an Olympus BXFMILHS microscope with a He/Ne laser, which has excitation at 633 nm. X-ray photoelectron spectroscopy (XPS) spectra were obtained with an AMICUL KRATOS using Al anode at 10 kV and 15 mA. A peak maximum of C IS at 284.8 eV was used as an internal standard to correct the binding energies. To obtain loading of catalysts on the substrates, catalysts were dissolved in 1 mL of aqua regia for 12 h and then diluted in 9 mL of Milli-Q water. Induced coupled plasma (ICP) measurements were performed for the solutions using an ICP-OES Varian 72 ES. Scanning electron microscopy (SEM) images were obtained with Nova Nano 630 scanning electron microscope from FEI Company. Cross-section views were taken using an FEI Helios NanoLab 400S FIB/SEM dual-beam system equipped with a Ga⁺ ion source. The surfaces of the electrode were covered by C and Pt layers by electron and ion beam to protect the sample from the milling.

Electrochemical measurement

1 M KOH solution (pH = 14) and 0.5 M K0.eH2.4BO3 solution (pH = 9.4) were prepared from KOH, H3BO4, and Milli-Q water (18 ΜΩ cm). Purification of 1 M KOH solution was conducted. Electrochemical measurements were performed using a BioLogic VMP3 potentiostat. Pt wire and Ni foam were used as counter electrodes in purified KOH and other solutions, respectively. Hg/HgO (1 M NaOH) (ALS CO., Ltd) and Ag/AgCl (Saturated KC1) (ALS CO., Ltd) were used as reference electrodes in KOH solution and KBi solution, respectively. All potentials are reported with respect to the reversible hydrogen electrode (RHE). Potentials were reported with R-correction unless otherwise specified. Solution resistance R_s was measured by impedance spectroscopy (100 mHz - 10 kHz, 10 mV

amplitude). Before and during all the measurements, Ar (99.9999%) or O2 (99.999%) gas was continuously supplied to the electrochemical cell.

Product gas from gas tight cell was quantified with a gas chromatography (GC-8A; Shimadzu Co. Ltd.) equipped with a TCD detector and a Molecular sieve 5A column using Ar (99.999 %) as a carrier gas. Ar was flowed at 22 SCCM in the electrochemical cell and outlet gas is connected to a sampling loop in the GC. Example 1

NiFeOx was prepared on a Au/Ti/FTO substrate by cathodic deposition and used as a base electrode. The amorphous CeO_x layer was formed on the NiFeOx by applying a constant anodic potential of 1.7 V vs. RHE in the deposition solution containing 0.4 M cerium nitrate and 0.4 M ammonium acetate (pH = 7) for 6 h at room temperature. Without intending to be bound by theory, in the deposition solution, Ce³⁺ is oxidized and precipitates on the anode because of the lower solubility of Ce⁴⁺ compared to Ce³⁺. As shown in FIG. 5A, which is a cyclic voltammogram for Au/Ti/FTO in 0.4 M Ce(NO)3 + 0.4 M CH3COONH4 (pH = 7) under Ar atomosphere (condition: 20 mV s^_1 and 298 K), oxidation of Ce³⁺ appeared around 1.0 V vs. RHE in the 1^st cycle and disappeared from the 2^nd cycle. Depending on the potential of the anode, Ce³⁺ can be also oxidized by O2 evolved from the anode and deposited on it.

Because Ni(OH)2 is not conductive but becomes conductive once it is oxidized to NiOOH, in order to deposit CeO_* on top of the NiFeO_x, more anodic potential was applied than the potential for oxidization of Ce³⁺ where oxygen can evolve, as shown in FIG. 5B, which is a cyclic voltammogram for an

NiFeOx electrode in 0.4 M Ce(NO)₃ + 0.4 M CH3COONH4 (pH = 7) under Ar atomosphere (condition: 20 mV s^"1 and 298 K).

The bulk composition of electrodes were quantified by inductively coupled plasma (ICP), shown in Tables 1 and 2 below. Table 1 shows the composition of bare NiFeOx and CeOx/ NiFeOx electrodes. Table 2 shows the composition of NiFeCeOx catalysts prepared by cathodic deposition in deposition solution containing 25 mM NH4SO4, 9 mM N1SO4, 9 mM FeS04 and x mM Ce(N03)3.

Table 1

Bare NiFeOx 1.3 0.64 0.33

CeOx/NiFeOx 1.3 0.66 1.6 0.34

Table 2

3 1.3 0.22 0.025 Nio.84Feo.14Ceo.02Ox

6 0.47 0.10 0.038 Nio.77Feo.i6Ceo.07Ox

9 0.62 0.098 0.070 Nio.79Feo.12Ceo.09Ox

The composition of Ni and Fe was maintained during deposition of CeOx. The total amount of Ce deposited was 1.6 μπιοΐ cm^"2 quantified by ICP while total charge passed during deposition was 14 C cm^"2 which

corresponds to 150 μπιοΐ cm^"2 of electrons, as shown in FIG. 5C, which is a chronoamperogram of a NiFeO_x electrode at 1.7 V vs. RHE during the deposition process of CeO_x in 0.4 M Ce(N0₃)3 + 0.4 M CH3COONH4 (pH = 7) under Ar atomosphere. Therefore the efficiency of deposition was not high in this anodic deposition process.

FIG. 1A shows an electrocatalytic stability test by controlled current electrolysis of uncoated and CeOx-coated NiFeOx at 20 mA cm^"2 in 1 M KOH. Uncoated NiFeOx reached 20 mA cm^"2 at 1.53 V vs. RHE in the beginning of the stability test, but the overpotential increased more than 60 mV in 96 h. On the other hand CeOx coated NiFeOx maintained its overpotential for 96 h while the potential to achieve 20 mA cm^"2 was similar to the uncoated NiFeOx at the initial stage. In the cyclic voltammetry (CV) measured immediately after stability test (96 h), slight recovery of current towards OER was observed at the 2^nd cycle, without going back completely to the original current density measured before stability test on the bare NiFeOx electrode, as shown in the cyclic voltammogram of FIG. IB.

Cathodic sweep or resting potential may help to recover the current of deactivated NiFeOx to some extent, but the reason still remains unclear. In contrast, although a slight increase of overpotential (-10 mV) was observed during the stability test for the CeOx coated NiFeOx, by potential sweeping, current for the OER recovered completely to the original one, which was measured before the stability test, as shown in the cyclic voltammogram of FIG. 1C. This full recovery was in marked contrast relative to that for the bare NiFeOx electrode. Importantly, as shown in FIG. ID, the Tafel slope remained 40 mV dec^"1 after CeOx deposition, which suggests that the CeOx layer did not affect the fundamental reaction mechanism of NiFeOx towards OER. When the redox peaks of Ni^2+/3+ in the CVs measured before and right after stability test are compared, as shown in FIG. 6, the redox peaks showed a similar negative shift after anodic polarization on bare and CeOx coated NiFeOx, although they had different stability. This indicates that the peak position of Ni^2+/3+ does not reflect the activity for OER. Surface SEM images show almost all the NiFeO_x was uniformly covered by CeOx layer, as shown in comparing FIG. 2A, which shows a bare NiFeOx electrode, with FIG. 2B, which shows a CeO_x coated NiFeOx electrode. Although cathodically deposited NiFeOx had a coarse structure on the substrate, the CeOx layer was successfully formed with thickness ranged 100-200 nm on top of the NiFeOx, as shown in the cross-section SEM image of a CeOx coated NiFeOx electrode milled by focused ion beam (FIB) of FIG. 2C. As shown in FIG. 7, the XRD diffraction pattern did not show any peaks from crystalline Ce02 and NiFeOx, while it shows only a broad peak around 25° indicating that the electrochemically deposited oxide or hydroxide consists of an amorphous phase. Raman spectra show O-Ce-0 vibration around 450 cm^"1 with broad peak of Ni-0 vibration from Ni(OH)2 around 560 cm^"1, as shown in FIG. 2D. When compared with commercial Ce02, the vibration of O-Ce-0 was shifted negatively and became broader. Without intending to be bound by theory, this change of peak can be caused by particle size or coexistence of Ce³⁺. XPS spectra of Ce 3d showed multiplet splitting composed of Ce³⁺ and Ce⁴⁺,^{26 28} which indicates that Ce³⁺ was precipitated together with Ce⁴⁺, as shown in FIG. 8 A. As shown in FIG. 8B, O ls spectra showed two main peaks at 529 eV and 532 eV, which are characteristic of oxide and hydroxide, respectively. Since XPS is a surface sensitive technique, peaks from Ni and Fe underneath the CeOx layer were not detected.

Impedance spectra shown in FIG. 3A of bare and CeOx coated NiFeOx electrodes show a single semicircle which arises from the parallel connection of charge transfer resistance (RCT) towards OER and double layer capacitance (Cdi), as shown in the Randies circuit shown in FIG. 9A. The corresponding peak appeared around 100 Hz in the phase of the Bode plot shown in FIG. 3B. A thick oxide film is often reported to have an additional semicircle in the Nyquist plot and corresponding peak around 1 kHz in the Bode plot. This additional semicircle is assigned to the parallel connection of resistance (Rf) and capacitance (Cf) from oxide film, as shown in the Voigt circuit in FIG. 9B, and induces potential drop through the oxide film. CeOx coated NiFeOx did not show any other semicircle, which also implies CeOx layer itself did not cause potential loss through the layer in alkaline condition.

Although the CeOx layer improved the stability of NiFeOx, CeOx itself is not active for OER because deposition of CeOx on Au substrate did not show any current around 1.5 V vs. RHE, as shown in the cyclic voltammogram of FIG. 10. As shown in the chronoamperogram of FIG. 11 , when Ce³⁺ precursor was not present in the deposition process, stability of the NiFeOx was not improved, which confirms that deposition of CeOx layer is essential to obtain stable OER performance. In other words, acetate ion or ammonium ion in the deposition solution did not contribute to the improved stability. To confirm the importance of CeOx layer, different compositions of NiFeCeOx electrodes were also prepared by adding Ce(NO)3 in the solution for cathodic deposition of oxide catalysts. The composition of deposition solutions and electrocatalysts deposited are shown in Table 2 above. Doping of Ce to the Ni-based catalyst improved the kinetics of OER, and 3-5 nm of segregated Ce02 was observed after electrochemical measurements were taken, although the contribution of Ce02 to the improved kinetics is still under investigation. As shown in FIGS. 12A and 12B, Nio.84Feo.14Ceo.02Ox showed improved current compared with NiFeOx without Ce. However, none of the NiFeCeOx electrodes improved the stability as much as CeOx coated NiFeOx. Ce-doped NiFeOx can be also stabilized by deposition of CeOx layer showing a slightly smaller overpotential at 20 mA cm^"2 compared with CeOx/NiFeOx. This universal improvement of OER stability with the CeOx layer is directed to the permselectivity of the layer, which prevents dissolution of iron species from the catalysts. Example 2

Permselectivity of the amorphous CeO_x layer was investigated by electrochemical measurements in the presence of various kinds of reducing agents which can be oxidized on NiFeO_x. FIG. 4A shows representative cyclic voltammograms of bare and CeO_x coated NiFeO_x electrodes in 1 M KOH with and without 0.2 M K4Fe(CN)6. In the alkaline solution with the ferrocyanide redox ion, bare NiFeO_x displays oxidation and reduction peaks of ferrocyanide and ferricyanide, which has redox formal potential at 1.33 V vs. RHE in pH 14, followed by redox peaks of Ni^2+/3+. On the other hand, CeOx coated NiFeO_x did not show any redox peaks of ferrocyanide ion and only redox peaks from Ni^2+/3+ were observed, which suggests the CeO_* layer blocked the permeation and charge transfer of ferrocyanide. A slight difference was observed between the solution with and without ferrocyanide in the potential range from 1.4 to 1.5 V vs. RHE, which suggests that there was still leak current for redox reactions.

This permselectivity was also confirmed by gas quantification of O2 during controlled current electrolysis at 10 mA cm^"2, as shown in FIG. 13 A. In the case of bare NiFeO_x, oxygen did not evolve in the initial 3 h, which means redox oxidation dominated on the bare NiFeOx surface rather than

OER. After 3 h of electrolysis, oxygen started to evolve, which comes from decreasing concentration of redox species during the electrolysis. Because this choronopotentiometry is done in a batch electrochemical cell, redox concentration kept decreasing, and oxygen evolution could start when redox diffusion limiting current density became lower than applied current density. On the other hand, 90 % of the faradaic efficiency was maintained on CeOx/NiFeOx after 7 h.

Example 3

To investigate the permselectivity of the amorphous CeO_* layer further, Faradaic efficiencies of O2 in the presence of different kinds of reducing agents, such as iodide, methanol, ethanol and iso-propanol, were further evaluated, as shown in FIGS. 13B-13E, and summarized in FIG. 4B. In alkaline solution, iodide can be oxidized to iodate (eq. 1, shown below). Methanol and ethanol are oxidized to corresponding carboxylic acids (eq. 2 and 3, shown below), and iso-propanol is oxidized to acetone (eq. 4, shown below).

Γ + 6ΟΗ^"→ +3H₂0 + 6e^" ^) CH₃OH + 50H^"→ HCOCT + ₄H₂0 + 4e^"

C₂H_sOH + 50ΙΓ→ CH_jCOCT + 4H₂0 + 4e^~

CH₃CH(OH)CH₃ + 20ΙΓ→ CH₃COCH₃ + 2H₂0 + 2e^" ^ Clear improvement of selectivity toward O2 was observed in the solution with redox anions rather than neutral alcohols. This trend suggests that diffusion of reducing agent is impacted by their charges. Since the isoelectronic point of CeCh is around 7, CeO_x layer should be negatively charged and repulse anions which resulted in suppression of diffusion of anion through the layer in alkaline condition. Although the OH^" ion is also negatively charged, the CeO_* layer electrodeposited by anodic polarization is reported to have hydrous disordered structure, which could contribute the diffusion of OH^" to the NiFeO_x catalyst underneath the layer. Example 4

Coating of the CeOx layer can be applied to seawater splitting to improve the stability of NiFeO_x catalysts. The overpotential of NiFeO_x increased more than 60 mV in 6 h in the solution with 1 M KCl, while it increased 30 mV in the solution without KCl, as shown in FIG. 14. There is no oxidation of CI^" (eq. 5, shown below) in alkaline conditions because the redox potential is reported to be 1.72 V vs. RHE, which is higher than the potential observed.

Cr + 20H^"→C10^" + H₂0 + 2e^" ^

CI^" seems to facilitate the deactivation of NiFeOx, although the reason of promoted degradation is not well understood. On the other hand, the overpotential of CeOx coated NiFeOx increased 15 mV, while the potential was comparable to the bare NiFeOx at the beginning of stability test. This result suggests that CeOx/NiFeQx can be an attractive candidate for seawater splitting which does not require purification process.

Example 5

Suppressed diffusion of anion was also observed in the reaction condition of OER. In near neutral 0.5 M K0.6H2.4BO4 solution (pH = 9.4), the overpotential towards OER drastically increased by deposition of CeO_x, while those in 1 M KOH were quite similar between bare and CeO_x coated NiFeOx, as shown in FIGS. 15A and 15B. In neutral pH (< 11), buffer anion is required to remove protons produced from water during OER because insufficiency of OH^" induces reactant switching from OH^" (eq. 6, shown below) to water (eq. 7, shown below). Increased overpotential by the CeOx layer also indicates that the layer prevented diffusion of the buffer anion.

40H^"→0₂ + 2H₂0 + 4e^~ ^ 2H₂0 + 4B(OH)^"→ 0₂ + 4H₃B0₃ + 4e^~ ^

From these results, it can be seen that the CeOx layer has

permselectivity which prevents redox anions from diffusing through while it allows OH^" ion to evolve O2 from OER catalysts underneath the layer. This permselectivity may contribute to the improved stability of NiFeOx in the long term current electrolysis in alkaline solution. The permselective layer may have also suppressed Fe0₄ ^2~ anion to diffuse to the electrolyte, which resulted in maintaining the active sites in NiFeOx during anodic polarization. Example 6

A thin film of cobalt phthalocyanine was deposited by thermal evaporation for 10 min at room temperature. The deposited cobalt phthalocyanine was transformed to CoOx by annealing in air at 400 °C for 30 min. CeOx deposition was conducted on the CoOx electrode following the deposition procedure mentioned above. A constant anodic current (10 μΑ cm^"2) was applied for 1 h under an Ar atmosphere. Surface SEM images show that almost all the CoO_x was uniformly covered by the CeOx layer, as shown in comparing FIG. 16A, which shows a bare CoO_x electrode, with FIG. 16B, which shows a CeO_x coated CoO_x electrode. Although deposited CoO_x had a nanoparticle structure on the substrate, the CeOx layer was successfully formed on top of the CoO_x.

As shown in the cyclic voltammogram of a bare CoOx electrode in FIG. 17 A, bare CoOx showed a drastic decrease of the current and a positive shift of the onset potential with potential cycles that were attributed to the gradual loss of the Co species from the electrode surface. In contrast, the CeOx coating did not alter the initial onset, i.e., the electrocatalytic activity of CoOx. These observations are attributed to the confinement introduced by the layer where the dissolved Co species is maintained within the layer, assisting self-healing, as shown in the cyclic voltammogram of FIG. 17B. Thus, the essential role of the additional CeOx layer for the stability was confirmed to be universal regardless of the OER catalyst.

Non-Limiting Exemplary Embodiments

Having described various aspects and embodiments of the invention herein, further specific embodiments of the invention include those set forth in the following paragraphs.

Certain embodiments provide oxygen evolution electrocatalysts, electrodes using oxygen evolution electrocatalysts, and methods of making oxygen evolution electrocatalysts. In one aspect, an oxygen evolution electrocatalyst is provided. The oxygen evolution electrocatalyst may include an oxide electrocatalyst, and a permselective amorphous layer deposited on the oxide electrocatalyst. The permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst.

In accordance with certain embodiments, for example, the oxide electrocatalyst may comprise an oxide of a transition metal. In some embodiments, for instance, the oxide electrocatalyst may comprise at least one of NiOx, CoO_x, FeO_x, MnO_x, IrO_x, RuO_x, or any combination thereof to form a mixed oxide. In further embodiments, for example, the oxide electrocatalyst may comprise NiFeO_x or CoFeO_x. In certain embodiments, for instance, the oxide electrocatalyst may comprise NiFeO_x.

In accordance with certain embodiments, for example, the permselective amorphous layer may comprise a catalytically inactive material. In some embodiments, for instance, the permselective amorphous layer may comprise at least one of CeO_x, TiO_x, A10_x, ZnO_x, ZrO_x, SiO_x, CaO_x, MgO_x, or any combination thereof. In further embodiments, for example, the permselective amorphous layer may comprise CeO_x.

According to certain embodiments, for instance, the permselective amorphous layer may comprise a thickness from about 100 nm to about 600 nm. In some embodiments, for example, the permselective amorphous layer may be deposited approximately uniformly on the oxide electrocatalyst.

In accordance with certain embodiments, for example, the substrate may comprise a conductive substrate. In some embodiments, for instance, the substrate may comprise at least one of cobalt, gold, nickel, a fluorine- doped tin oxide (FTO) substrate, or any combination thereof. In further embodiments, for example, the substrate may comprise a gold-coated FTO substrate.

In yet another aspect, a method of making an oxygen evolution electrocatalyst is provided. The method may include providing an oxide electrocatalyst, and depositing a permselective amorphous layer on the oxide electrocatalyst via anodic deposition. The permselective amorphous layer may prevent diffusion of redox ions but permit diffusion of hydroxide ions to the oxide electrocatalyst.

In accordance with certain embodiments, for example, depositing the permselective amorphous layer comprises depositing the permselective amorphous layer approximately uniformly on the oxide electrocatalyst. In accordance with certain embodiments, for example, the oxide electrocatalyst may comprise an oxide of a transition metal. In some embodiments, for instance, the oxide electrocatalyst may comprise at least one of NiOx, CoO_x, FeO_x, MnO_x, IrO_x, RuO_x, or any combination thereof to form a mixed oxide. In further embodiments, for example, the oxide electrocatalyst may comprise NiFeO_x or CoFeO_x. In certain embodiments, for instance, the oxide electrocatalyst may comprise NiFeO_x.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which the inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

We Claim:

1. An oxygen evolution electrocatalyst comprising:

an oxide electrocatalyst; and

a permselective amorphous layer deposited on the oxide

electrocatalyst,

wherein the permselective amorphous layer prevents diffusion of redox

ions but permits diffusion of hydroxide ions to the oxide electrocatalyst.

2. The oxygen evolution electrocatalyst according to claim 1, wherein the oxide electrocatalyst comprises an oxide of a transition metal.

3. The oxygen evolution electrocatalyst according to claim 1 or 2, wherein the oxide electrocatalyst comprises at least one of NiO_x, CoO_x, FeOx, MnOx, IrO_x, RuO_x, or any combination thereof to form a mixed oxide.

4. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the oxide electrocatalyst comprises NiFeO_x or CoFeO_x.

5. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the oxide electrocatalyst comprises NiFeO_x.

6. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the permselective amorphous layer comprises a catalytically inactive material.

7. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the permselective amorphous layer comprises at least one of CeO_x, TiO_x, A10_x, ZnO_x, ZrO_x, SiO_x, CaO_x, MgO_x, or any combination thereof.

8. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the permselective amorphous layer comprises CeO_x.

9. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the permselective amorphous layer comprises a thickness from about 100 nm to about 600 nm.

10. The oxygen evolution electrocatalyst according to any one of the preceding claims, wherein the permselective amorphous layer is deposited approximately uniformly on the oxide electrocatalyst.

11. An electrode comprising:

a substrate; and

an oxygen evolution electrocatalyst, the oxygen evolution electrocatalyst

comprising:

an oxide electrocatalyst; and

a permselective amorphous layer disposed on the oxide electrocatalyst,

wherein the permselective amorphous layer prevents diffusion of redox

ions but permits diffusion of hydroxide ions to the oxide

electrocatalyst.

12. The electrode according to claim 11, wherein the substrate comprises a conductive substrate.

13. The electrode according to claim 11 or 12, wherein the substrate comprises at least one of cobalt, gold, nickel, a fluorine-doped tin oxide (FTO) substrate, or any combination thereof.

14. The electrode according to any one of claims 11 to 13, wherein the substrate comprises a gold-coated FTO substrate.

15. The electrode according to any one of claims 11 to 14, wherein the oxide electrocatalyst comprises an oxide of a transition metal.

16. The electrode according to any one of claims 11 to 15, wherein the oxide electrocatalyst comprises at least one of NiO_x, CoO_x, FeO_x, MnO_x, IrO_x, RuO_x, or any combination thereof to form a mixed oxide.

17. The electrode according to any one of claims 11 to 16, wherein the oxide electrocatalyst comprises NiFeO_x or CoFeO_x.

18. The electrode according to any one of claims 11 to 17, wherein the oxide electrocatalyst comprises NiFeO_x.

19. The electrode according to any one of claims 11 to 18, wherein the permselective amorphous layer comprises a catalytically inactive material.

20. The electrode according to any one of claims 11 to 19, wherein the permselective amorphous layer comprises at least one of CeO_x, TiO_x, A10_x, ZnO_x, ZrO_x, SiO_x, CaO_x, MgO_x, or any combination thereof.

21. The electrode according to any one of claims 11 to 20, wherein the permselective amorphous layer comprises CeO_x.

22. The electrode according to any one of claims 11 to 21, wherein the permselective amorphous layer comprises a thickness from about 100 nm to about 600 nm.

23. The electrode according to any one of claims 11 to 22, wherein the permselective amorphous layer is deposited approximately uniformly on the oxide electrocatalyst.

24. A method of making an oxygen evolution electrocatalyst, the method comprising:

providing an oxide electrocatalyst; and

depositing a permselective amorphous layer on the oxide electrocatalyst via

anodic deposition,

wherein the permselective amorphous layer prevents diffusion of redox

ions but permits diffusion of hydroxide ions to the oxide electrocatalyst.

25. The method according to claim 24, wherein depositing the permselective amorphous layer comprises depositing the permselective amorphous layer approximately uniformly on the oxide electrocatalyst.

26. The method according to claim 24 or 25, wherein the oxide electrocatalyst comprises an oxide of a transition metal.

27. The method according to any one of claims 24 to 26, wherein the oxide electrocatalyst comprises at least one of NiO_x, CoO_x, FeO_x, MnO_x, IrO_x, RuO_x, or any combination thereof to form a mixed oxide.

28. The method according to any one of claims 24 to 27, wherein the oxide electrocatalyst comprises NiFeO_x or CoFeO_x.

29. The method according to any one of claims 24 to 28, wherein the oxide electrocatalyst comprises NiFeO_x.

30. The method according to any one of claims 24 to 29, wherein the permselective amorphous layer comprises at least one of CeO_x, TiO_x, A10_x, ZnO_x, ZrO_x, SiO_x, CaO_x, MgO_x, or any combination thereof.

31. The method according to any one of claims 24 to 30, wherein the permselective amorphous layer comprises CeO_x.