Oxygen evolution reaction electrodes

Abstract

OER electrodes are comprised of low cost fibrous PTLs conformally coated with one to three layers of metal oxides, metal oxyhydroxides, metal hydroxides or a combination thereof. The OER electrodes comprise a unique structure and morphology which achieve a low overpotential and excellent durability in use. The structure, morphology and loading of the electrocatalyst is easily tunable and appropriately scalable to the required application in an AEM and AEL electrolyzer. AEM electrolyzers comprising the OER electrodes achieve high performance and promising cell durability.

Description

OXYGEN EVOLUTION REACTION ELECTRODESFIELD OF THE INVENTION

[0001] Catalysts and electrodes for oxygen evolution reaction are provided for use in water electrolysis. These catalysts and electrodes can be used in anion exchange membrane electrolyzers, alkaline electrolyzers, and other devices in which oxygen evolution reaction is required.BACKGROUND OF THE INVENTION

[0002] Green hydrogen generation by water electrolysis at low temperatures has been considered an efficient approach to realize low-cost hydrogen at large scale. Bryan Pivovaret al,. Electrochem. Soc. Interface, 201827, 47. Abbasi, R. et al,. Adv. Mater. 2019, 31,1805876. Alkaline electrolyzers (AELs) are relatively mature and were commercialized since 1920s. LeRoy, R. L. Int. J. Hydrogen Energy ,1983, 8, 401-417. These electrolyzers use inexpensive platinum-group-metal (PGM) free catalyst in the stacks and have demonstrated a service lifetime of 30-40 years, which translates to low capital cost for this technology. However, AELs suffer from low voltage efficiency due to high internal resistance caused by gas bubbles in the liquid electrolyte and on the electrode surface. The high concentration of liquid electrolyte in the electrolyzers (40 wt% NaOH or KOH) not only contributes to the loss of efficiency but also cause hardware corrosion issues.

[0003] Anion Exchange membrane electrolyzer (AEMEL) technology is emerging as it combines the advantages of two other water electrolysis technologies, i.e., AELs and proton exchange membrane electrolyzers (PEMELs). By using a solid electrolyte membrane, i.e., an anion exchange membrane, the AEMELs have a zerogap design of PEMELs and operate in significantly less harsh conditions of dilute KOH or NaOH, or even DI water. These unique properties of AEMELs allows them to shift away from expensive hardware and PGM catalysts used in PEMELs, such as titanium-based plates and PGM catalysts, and yet to yield high-voltage efficiency and fast dynamic response that AELs are not capable of. Zhang, Y et al., Electrochem. Commun. 2007, 9, 667.

[0004] One of the biggest challenges for AEMELs at present is the sluggish oxygen evolution reaction (OER) in the anode. Zeng, K, et al., Prog. Energ Combust.Sei. 2010, 36, 307-326. Transition metal oxides are promising candidates as OER catalysts due to their low-cost and decent activity. Xiao, J. W., et al. ACS catal. 2021 , 11, 264. Hu, C, et al. ACS appl. Mater. Interfaces 2018, 10, 33124. However, these transition metal containing OER catalyst often suffer from high overpotential (>400 mV) to meet the industrial applications as well as <100 h performance stability. Park, J.E., et al. App. Catal., B 2018, 237, 140. Jang, M. J., et al., J. Mater. Chem. A 2020, 8, 4290.

[0005] To maintain the low cost of the OER catalyst / electrode while improving its performance and stability in AEMELs, herein, a series of non-PGM OER catalysts / electrodes are provided.SUMMARY OF THE INVENTION

[0006] An OER electrode is provided which comprises one layer of a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof on the fibers of a porous transport layer (PTL) wherein, the fibers of the PTL comprise stainless steel, nickel (Ni), aluminum (Al), copper (Cu) or a combination thereof, and the OER catalyst layer on the fibers of the PTL comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal is Ni, Co, Fe, manganese or a combination thereof.

[0007] An OER electrode is provided which comprises two layers of a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof on the fibers of a PTL, wherein: the fibrous PTL comprises stainless steel, Ni, Al or Cu or a combination thereof, the first layer on the fibers of PTL serves as a corrosion protection layer and comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal comprises Ni, cobalt (Co), titanium (Ti), niobium (Nb) or a combination thereof, and the second layer which is on the first layer on the fibers of the PTL is an OER catalyst and comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal comprises Ni, Co, Fe, Cu, manganese ora combination thereof.

[0008] An OER electrode is also provided which comprises three layers of a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof on the fibers of a PTL, wherein: the fibrous PTL comprises stainless steel, Ni, Al, Cu or a combination thereof, the first layer on the fibers of PTL serves as a corrosion protection layer and comprises a metal oxide, a metal oxyhydroxide, a metalhydroxide or a combination thereof wherein the metal comprises Ni, cobalt (Co), titanium (Ti), niobium (Nb) or a combination thereof, and the second layer which is on the first layer serves as a radical scavenger layer and comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal comprises Cu, Zr, Ce, Zn, Sr, Ba or Mn or a combination thereof, and the third layer which is on the second layer is an OER catalyst and comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal comprises Ni, Co, Fe, Mn or a combination thereof.

[0009] An AEM electrolyzer is provided which comprises an AEM membrane and any of the aforementioned OER electrodes and has performance of 0.2 - 20 A / cm2at 1.8 V.

[0010] An AEM electrolyzer is provided which comprises an AEM membrane and any of the aforementioned OER electrodes and has performance loss rate of 0.1 uV / h to 400 uv / h at 1 A / cm2.

[0011] An AEM electrolyzer comprising the OER electrode can provide an OER overpotential of 50-400 mV at 20 mA / cm2.

[0012] An AEM electrolyzer comprising the OER electrode can provide a Tafel slope of 20 - 60 mV / decade.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 illustrates an exemplary anion exchange membrane electrolyzer.

[0014] Figure 2 illustrates an exemplary cross-section of a fibrous PTL and polarization and durability curves.

[0015] Figure 3 depicts polarization and durability curves for the exemplary OER electrode with an electrocatalyst layer on fibers of a PTL.

[0016] Figure 4 illustrates an exemplary OER electrode-2 with one layer of a metal oxide, a metal oxyhydroxide or a metal hydroxide on fibers of a PTL.

[0017] Figure 5 illustrates an SEM image of OER electrode-2 and polarization and durability curves.

[0018] Figure 6 depicts polarization and durability curves for OER electrode-2.

[0019] Figure 7 illustrates an exemplary OER electrode-3 with one layer of a metal oxide, a metal oxyhydroxide or a metal hydroxide on fibers of a PTL.

[0020] Figure 8 depicts polarization and durability curves for OER electrode-3.

[0021] Figure 9 illustrates an exemplary OER electrode-4 with two layers of a metal oxide, a metal oxyhydroxide or a metal hydroxide on fibers of a PTL.

[0022] Figure 10 depicts SEM images of the bare PTL fibers, nickel electrodeposited on such fibers, cobalt electrodeposited on the nickel coated PTL fibers, and the OER electrode-4.

[0023] Figure 11 depicts an SEM image of a single fiber of OEM electrode-4, a line scan image of such fiber and EDX and XRD for OER electrode-4.

[0024] Figure 12 depicts a schematic of MPL on PTL and the SEM crosssection of OER electrode-4.

[0025] Figure 13 shows polarization, galvanostatic electrochemical impedance spectroscopy and durability curves for OER electrode-4.

[0026] Figure 14 depicts OER electrode-4 polarization and durability curves.

[0027] Figure 15 is a plot of cobalt loading in the electrode vs. current density, a polarization curve for OER electrode-4 and a plot regarding degradation rate of electrolyzer cells.

[0028] Figure 16 shows an exemplary OER electrode-5 with three layers of a metal oxide, a metal oxyhydroxide or a metal hydroxide on fibers of a PTL.

[0029] Figure 17 shows polarization and durability curves for OER electrode-5.DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] An OER electrode is provided which comprises an OER catalyst comprising a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof on fibers of a PTL.

[0031] The fibers of the PTL comprises stainless steel, nickel (Ni), aluminum (Al), copper (Cu) or a combination thereof.

[0032] Preferably, the PTL of the OER electrode comprises Ni or stainless steel or a combination thereof, and preferably, the PTL comprises Ni.

[0033] The catalyst layer on the fibers of the PTL is the OER catalyst which comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof comprising Ni, Co, Fe or Manganese or a combination thereof.

[0034] The OER catalyst has a total metal loading of 0.05 - 200 mg / cm2.

[0035] The loading of Co in the catalyst layer can be 0.05 - 50 mg / cm2.

[0036] The loading of Ni in the catalyst layer can be 0 - 50 mg / cm2.

[0037] The loading of Fe in the catalyst layer can be 0 - 50 mg / cm2.

[0038] The loading of Mn in the catalyst layer can be 0 - 50 mg / cm2.

[0039] The particle size of the catalyst metal oxides, oxyhydroxides or hydroxides on fibers of the PTL is from 0.001 urn - 10 urn.

[0040] The fibers on the top 5 to 200 pm of the PTL is coated with a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof to form a microporous layer (MPL).

[0041] Another OER electrode is provided which comprises two layers of a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof on a fibrous PTL.

[0042] The fibers of the PTL comprises stainless steel, Ni, Al or Cu or a combination thereof.

[0043] The first layer on the fibers of the PTL serves as the corrosion protection layer and it comprises a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal is Ni, Cobalt (Co), titanium (Ti) or niobium (Nb) or a combination thereof.

[0044] The second layer on the fiber of a PTL is the OER catalyst which comprises a metal oxide, a metal oxyhydroxide or a metal hydroxide, wherein the metal is Ni, Co, Fe, Mn, Cu, or a combination thereof.

[0045] The OER catalyst has a total metal loading from 0.05 - 200 mg / cm2.

[0046] The loading of Co in the catalyst is from 0.05 to 50 mg / cm2.

[0047] The loading of Ni, Fe or Mn in the catalyst layer is from 0 to 50 mg / cm2.

[0048] The particle size of the catalyst on the fibers of the PTL is 0.001 urn -10 urn.

[0049] The thickness of the catalyst layer on fibers of the PTL is 0.1 - 10 urn.

[0050] The fibers on the top 5 to 200 pm of the PTL is coated with the OER catalyst to form a microporous layer (MPL).

[0051] Preferably, the fibrous PTL of the OER electrode comprises stainless steel.

[0052] Preferably, the metal oxide, metal oxyhydroxide, or metal hydroxide of the first layer over the fibers of the PTL comprises Ni, Cu or a combination thereof,and preferably, the first layer comprises a nickel oxide, a nickel oxyhydroxide, or a combination thereof.

[0053] Preferably, the metal oxide, metal oxyhydroxide or metal hydroxide of the catalyst layer over the fibers of the PTL comprises Co, Cu or a combination thereof, and preferably, the second layer comprises cobalt oxide, cobalt oxyhydroxide, cobalt hydroxide or a combination thereof.

[0054] Another OER electrode is provided which comprises three layers of a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof on a fibrous PTL.

[0055] The fibers of the PTL comprises stainless steel, Ni, Al or Cu or a combination thereof.

[0056] The first layer on the fibers of the PTL, which serves as the corrosion protection layer, comprises an oxide, an oxyhydroxide, or a hydroxide of a metal, wherein the metal is Ni, Cobalt (Co), Ti, Niobium (Nb) or a combination thereof.

[0057] The second layer of metal oxide, metal oxyhydroxide or metal hydroxide on the fibers serves as a radical scavenger layer and it comprises an oxide, an oxyhydroxide, or a hydroxide of a metal, wherein the metal is Cu, Zr, Ce, Zn, Sr, Ba, Mn or a combination thereof.

[0058] The third layer of the metal oxide, metal oxyhydroxide, or metal hydroxide on the fibers of PTL is the OER catalyst comprises Ni, Co, Fe, Mn or a combination thereof.

[0059] The thickness of each of the three layers on the fibers of the PTL is 0.01 - 30 urn.

[0060] The total loading of metals in the protection layer is 0.05 - 200 mg / cm2.

[0061] The total loading of metals in the radical scavenger layer is 0.05 - 200 mg / cm2.

[0062] The loading of Cu in the radical scavenger layer is 0.05 - 50 mg / cm2.

[0063] The loading of Co in the radical scavenger layer is 0 - 50 mg / cm2.

[0064] The loading of Zr in the radical scavenger layer is 0 - 50 mg / cm2.

[0065] The loading of Ce in the radical scavenger layer is 0 - 50 mg / cm2.

[0066] The OER catalyst layer has a total metal loading from 0.05 - 200 mg / cm2.

[0067] The loading of Co in the catalyst is from 0.05 to 50 mg / cm2.

[0068] The loading of Ni, Fe or Mn in the catalyst layer is from 0 to 50 mg / cm2.

[0069] The particle size of the catalyst on the fibers of the PTL is 0.001 um - 10 um.

[0070] The thickness of the catalyst layer on fibers of the PTL is 0.1 - 10 um.

[0071] The fibers on the top 5 to 200 pm of the PTL is coated with the OER catalyst to form a microporous layer (MPL).

[0072] The fiber of the PTLs used in all OER electrodes ranges from 0.1 to 50 microns in diameter.

[0073] The thickness of the PTL used in all OER electrodes ranges from 10 to 1000 microns in total thickness.

[0074] The OER electrodes can optionally comprise anion exchange ionomers with a loading of 0.1 - 10 mg / cm2

[0075] An AEM electrolyzer is provided which comprises an AEM membrane and any of the forementioned OER electrodes and has performance of 0.2 - 20 A / cm2at 1.8 V.

[0076] An AEM electrolyzer is provided which comprises an AEM membrane and any of the forementioned OER electrodes and has performance loss rate of 0.1 uV / h to 400 uV / h at 0.5 A / cm2.

[0077] The AEM electrolyzers can further comprise a hydrogen evolution catalyst electrode comprising platinum (Pt), platinum ruthenium (PtRu), nickel molybdenum (NiMo) or nickel or a combination thereof.

[0078] The AEM electrolyzer can provide an OER overpotential of 50-400 mV at 20 mA / cm2

[0079] An AEM electrolyzer comprising the OER electrode can provide a Tafel slope of 20-60 mV / decade.

[0080] The AEM electrolyzer has performance of 0.2- 10 A / cm2at 1.8 V at 80 °C with 10 Mm KOH electrolyte.

[0081] The AEM electrolyzer has performance of 1- 20 A / cm2at 1.8 V at 80 °C with 1M KOH electrolyte.

[0082] The AEM electrolyzers can have an active area between 5 cm2to 1000 cm2.

[0083] An AEM electrolyzer stack can be built by integrating the aforementioned single AEM electrolyzer cell.

[0084] Preferably, the electrolyzer comprises Piperion® membrane, which is commercially available from Versogen, Inc., Newark, DE, and is disclosed in U.S.Patent Nos. 10,290,890 and 11,512,156, which are incorporated herein by reference in their entirety.

[0085] Preferably, the ionomer in any of the electrodes comprises Piperion® ionomer, which is commercially available from Versogen, Inc., Newark, DE, and is disclosed in U.S. Patent Nos. 10,290,890 and 11 ,512,156.

[0086] The OER electrodes are made by electrodeposition processes which are well known in the art and are described in the working examples below. For example, the PTL substrate can undergo pre-treatment to remove oxide layers by acetone rinse followed by cathodic alkaline electro-cleaning using a base such as NaOH for a few minutes. A metal oxide-, metal oxyhydroxide-, or metal hydroxide-catalyst or a combination thereof, can be coated on the electro-cleaned PTL substrate by electrodeposition at current density as described in the examples in a metal salt solution (such as a metal nitrate or metal sulfate solution) for a few minutes to obtain a desired metal loading.EXAMPLES EXAMPLE 1

[0087] An OER electrode listed in this example comprises one layer of catalyst comprising Co layered hydroxide, Co oxyhydroxide, Co hydroxide or a combination thereof coated on Ni porous transport layer fibers / substrate. This electrode is named OER electrode-1 which depicts eCo / Ni PTL. Electrodeposition was used for fabricating this OER electrode-1. Figure 1 panel (a) shows the OER electrode-1 preparation steps and panel (b) shows the structure of OER electrode-1.

[0088] Firstly, the Ni PTL substrate undergoes pre-treatment steps to remove oxide layers by acetone rinse followed by cathodic alkaline electro-cleaning using 1.0 M NaOH at 6 V for few minutes. A cobalt-based metal oxide-, metal oxyhydroxide-, or metal hydroxide-catalyst or a combination thereof, was coated on the electrocleaned Ni substrate by electrodeposition at a current density of 25 mA / cm2in 0.2 M cobalt nitrate solution for 3 minutes to obtain a cobalt loading of 0.3 mg / cm2. Figure 2 panel (a) shows the scanning electron microscopy (SEM) cross-section image of the eCo / Ni OER electrode-1 done in 1M KOH electrolyte. The OER electrode-1 shows the growth of ultra-thin nanoflake structures on the surface of Ni substrate. The eCo / Ni OER electrode-1 was found to have an overpotential of 294 mV at 20mA / cm2with a tafel slope of 52.7 mV / decade as shown in the OER polarization curves in Figure 2 panel (b) done in 1M KOH electrolyte.

[0089] The AEM water electrolyzer tests were conducted in the 5 cm2electrolyzer hardware at 80 ± 1 °C with various concentrations of KOH while having same cathode (PGM catalyst on carbon paper) and AEM membrane (PiperlON - 40 pm thickness). Figure 2 panel (c) shows the polarization curves of the 5 cm2AEM electrolyzer cell with eCo / Ni OER electrode-1. A current density of 1.5 A / cm2 and 1.2 A / cm2was achieved at 1.8 V in 10 mM KOH and 0.5 mM KOH electrolytes. Figure 2 panel (d) shows the durability curve of the 5 cm2electrolyzer cell at 80 °C in 0.5 mM KOH at constant current density of 1 A / cm2. A degradation rate of 5.6 pV / h(0 to 100h) was achieved.

[0090] Figure 3 panel (a) shows the three-electrode OER polarization curves for OER electrode-1 and OER electrode-1 a done in 1M KOH electrolyte. Figure 3 panel (b) depicts 5 cm2AEM cell polarization curves with OER electrode-1 and OER electrode-1 a done at 80 °C in 10 mM KOH electrolyte. Figure 3 panel (c) shows durability of the 5 cm2electrolyzer cell with OER electrode-1 and OER electrode-1 with higher Co loading (named as OER electrode-1 a) done at 80 °C in 0.5 mM KOH at 1 A / cm2hold. By increasing Co loading from 0.3 to 0.7 mg / cm2, the OER electrode-1 a showed a lower OER overpotential of 279 mV at 20 mA / cm2. Similarly, the OER electrode-1 a achieved a higher current density of 1.9 A / cm2at 1.8 V in 10 mM KOH electrolyte. The 5 cm2cell with OER electrode-1 a had a degradation rate of 223 pV / h(oto 1 ooh).EXAMPLE 2

[0091] An OER electrode listed in this example comprises of one layer of catalyst based on Co oxide, Co oxyhydroxide, Co hydroxide or a combination thereof coated on stainless steel (SS) porous transport layer fibers / substrate. This electrode is named OER electrode-2 which depicts eCo / SS PTL. Electrodeposition was used for fabricating the OER electrode-2. Figure 4 panel (a) shows the OER electrode-2 preparation steps and panel (b) shows the structure of OER electrode-2.

[0092] The SS PTL substrate undergoes pre-treatment steps to remove oxide layers by acetone rinse followed by anodic alkaline electro-cleaning using 1.0 M NaOH at 6 V for few minutes and an optional 1 M sulfuric acid dip to neutralize the surface. Cobalt based oxide-, oxyhydroxide- or hydroxide- catalyst was coated onthe electro-cleaned SS substrate by electrodeposition at 30 mA / cm2current density in 0.2 M cobalt nitrate solution for 6 minutes to obtain a cobalt loading of 0.7 mg / cm2. Figure 5 panel (a) shows the scanning electron microscopy (SEM) cross-section image of the eCo / SS OER electrode-2. The OER electrode-2 shows uniform growth of ultra-thin nanoflake structures grown perpendicular to the surface of every single SS fiber of the PTL substrate. The eCo / SS OER electrode-1 was found to have an overpotential of 267 mV at 20 mA / cm2with a tafel slope of 30.2 mV / decade as shown in the OER polarization curves in Figure 5 panel (b) done in 1M KOH electrolyte. The OER overpotential of OER electrode-2 was lower than the OER electrode-1 by at least by 12 mV indicating the essentialness to increase the OER active site density by increasing the surface area of the substrate. It is to be noted that the SS PTL fiber diameter ranges from 2 to 10 pm, while the Ni PTL fiber diameter ranges from 10 to 50 pm is essential in providing a high surface area for growth. The AEM water electrolyzer tests were conducted in the 5 cm2electrolyzer hardware at 80 ± 1 °C with various concentrations of KOH while having same cathode (PGM catalyst on carbon paper) and AEM membrane (PiperlON - 40 pm thickness). Figure 5 panel (c) shows the polarization curves of the 5 cm2AEM electrolyzer cell with eCo / SS OER electrode-2. A current density of 1.3 A / cm2 and 0.51 A / cm2was achieved at 1.8 V in 10 mM KOH and 0.5 mM KOH electrolytes. Figure 5 panel (d) shows the durability curve of the 5 cm2electrolyzer cell at 80 °C in 0.5 mM KOH at constant current density of 1A / cm2. A degradation rate of 219.5 pV / h(0 to 80h) was achieved.

[0093] Figure 6 panel (a) shows the three-electrode OER polarization curves for OER electrode-2 and OER electrode-2a done in 1 M KOH electrolyte. Figure 6 panel (b) shows the 5 cm2AEM cell polarization curves with OER electrode-2 and OER electrode-2a done at 80 °C in 10 mM KOH electrolyte. Figure 6 panel (c) shows durability of the 5 cm2electrolyzer cell with OER electrode-2 and OER-electrode-2 with higher Co loading (named as OER electrode-2a) done at 80 °C in 0.5 mM KOH at 1 A / cm2 hold. The electrodeposition time was increased to 12 minutes to increase Co loading from 0.7 to 1.0 mg / cm2. The OER electrode-2a showed a lower OER overpotential of 258 mV at 20 mA / cm2in 1.0 M KOH, while there was not great improvement in performance of 5 cm2electrolyzer cell performance in 10 mM KOH electrolyte. However, the 5 cm2cells with OERelectrode-2a achieved a current density of 1.2 A / cm2in 0.5 mM KOH electrolyte, which was higher than the OER electrode-2 performance in 0.5 mM KOH electrolyte.

[0094] The 5 cm2cell with OER electrode-2a had a degradation rate of 195.2 pV / h(oto oh) as shown in the durability curve in Figure 6 panel (c). Using SS PTL substrates results in leaching of anions such as FeO42’, CrO42’, etc. corresponding to a concentration of 1 to 250 ppb into the electrolyzer feed water as evidenced by inductive coupled plasma mass spectroscopy (ICP-MS). This may have detrimental effects on the electrolyzer voltage and component degradation. Using SS PTL requires additional corrosion protection layers to implement in AEM electrolyzers.EXAMPLE 3

[0095] An OER electrode listed in this example comprises of one layer of Co oxide, Co oxyhydroxide, Co hydroxide or a combination thereof coated on titanium (Ti) porous transport layer fibers / substrate. This electrode is named OER electrode-3 which depicts eCo / Ti PTL. Electrodeposition was used for fabricating the OER electrode-3. Figure 7 panel (a) shows the OER electrode-3 preparation steps and panel (b) shows the structure of OER electrode-3.

[0096] The Ti PTL substrate undergoes pre-treatment steps to remove surface contaminants by acetone rinse. Co oxide, Co oxyhydroxide, Co hydroxide, or a combination thereof was coated on the Ti substrate by electrodeposition at 30 mA / cm2current density in 0.2 M cobalt nitrate solution for 6 minutes to obtain a cobalt loading of 1.0 mg / cm2. Figure 8 panel (a) shows the three-electrode OER polarization curves of OER electrode-3 done in 1M KOH electrolyte and was found to have an overpotential of 434 mV at 20 mA / cm2with a tafel slope of 122.9 mV / decade. The AEM water electrolyzer tests were conducted in the 5 cm2electrolyzer hardware at 80 ± 1 °C with various concentrations of KOH while having same cathode (PGM catalyst on carbon paper) and AEM membrane (PiperlON - 40 pm thickness). Figure 8 panel (b) shows the polarization curves of the 5 cm2AEM electrolyzer cell with eCo / Ti OER electrode-3 done at 80 °C at different KOH concentration electrolytes. A current density of 1.5 A / cm2and 1.2 A / cm2was achieved at 1.8 V in 10 mM KOH and 0.5 mM KOH electrolytes. Figure 7 panel (c) shows the durability curve of the 5 cm2electrolyzer cell at 80 °C in 0.5 mM KOH at constant current density of 1A / cm2. A degradation rate of 722.1 pV / h(0 to 80h) was achieved.EXAMPLE 4

[0097] An OER electrode listed in this example comprises two layers of compounds comprising metal oxide, metal oxyhydroxide, metal hydroxide, ora combination thereof deposited on individual fibers of SS PTL substrate. The first layer comprises of Ni-based oxide, oxyhydroxide, hydroxide or a combination thereof and functions as a corrosion protection layer for the SS substrate to reduce the metal anion leaching into electrolyzer feed water and a nanoparticle seed layer enabling to increase the OER active site density. The second layer comprises a Co-based oxide, oxyhydroxide, hydroxide, or a combination thereof which serves as the OER electrocatalyst. The two layers may contribute to OER electrocatalytic activity. This electrode is named OER electrode-4 which depicts eCoNi / SS PTL. Electrodeposition was used for fabricating the OER electrode-4. Figure 9 panel (a) shows the electrode preparation steps and panel (b) shows the structure of OER electrode-4. The SS PTL substrate undergoes several pre-treatment steps to remove surface oxide layers by acetone rinse followed by anodic alkaline electro-cleaning using 1.0 M NaOH at 6 V for few minutes and an optional 1M sulfuric acid dip to neutralize the surface. Ni-based oxide, oxyhydroxide, hydroxide or a combination thereof was coated on the electro-cleaned SS substrate by electrodeposition at a current density of 30 mA / cm2in 0.2 M nickel nitrate solution for 6 minutes to obtain a Ni loading of 0.2 mg / cm2on both sides of the PTL.

[0098] Figure 10 panel (a) shows the SEM cross-section image of bare SS PTL before electrodepositing any catalyst layers. Figure 10 panel (b) shows the SEM image of the electrode after electrodepositing the first layer of Ni on the electrocleaned SS substrate. The Ni layer was found to have a nanoparticle morphology with nanoparticles size ranging from 1 nm to 100 nm. This Ni nanoparticle layer provides corrosion protection for the SS PTL. These Ni nanoparticle layers also function to provide more surface area for depositing the second layer which is the Co electrocatalyst layer. In the next step, a layer of Co-based oxide, oxyhydroxide, hydroxide or a combination thereof was electrodeposited on the Ni nanoparticle layer by electrodeposition at 25 mA / cm2current density in 0.2 M cobalt nitrate solution for 12 minutes to obtain a Co loading of 1.0 to 1.2 mg / cm2. Figure 10 panel (c) shows the SEM image of the OER electrode-4 after deposition of Co-based oxide, oxyhydroxide, hydroxide, or a combination thereof on Ni coated SS PTL - OERelectrode-4. The Co-based oxide, oxyhydroxide, hydroxide, ora combination thereof was found to have ultra-thin nano flakes perpendicularly grown on the Ni nanoparticles to form nanoflower morphology as shown in the high magnification image of OER electrode-4 in Figure 10 panel (d). The structures are oriented perpendicular to the surface of the PTL exposing OER active sites. The OER electrode-4 has uniform growth of ultra-thin nanoflake structures on the surface of every single SS fiber of the PTL substrate.

[0099] Figure 11 panel (a) shows the SEM cross-section image of a single fiber of OER electrode-4. The corresponding EDX line scan image is shown in Figure 11 panel (b) which provides the thickness of electrodeposited Ni and Co layer as 1 pm. Figure 11 panels (c) to (f) shows the EDX mapping of the OER electrode-4, which complements the line scan results indicating the Co and Ni layer on single SS fiber. Fe is mapped to represent the SS substrate. The XRD of the OER electrode-4 is shown in the Figure 11 panel (g). It exhibits diffraction peaks centered at 9.7° and 33.5° which can be indexed as the (001) plane of a-Ni(OH)2 and (100) plane of [3-Ni(OH)2. The diffraction peaks centered at 10.6° and 22.2° can be indexed as (003) plane of a-Co(OH)2 and (110) plane of CoOOH. Based on recent literature, the diffraction peaks present in the 2-theta region of 9° to 11° may also be attributed to the CoNi- based oxide, oxyhydroxide, hydroxide, or a combination thereof (Li, Ruchun, et al. Scientific Reports 6.1 (2016): 18737). The sharp crystalline peak at 43.6° can be indexed as Ni (111 ) plane from the stainless steel PTL substrate. The XRD analysis depicts that the OER electrode-4 has a mixture of metal hydroxides and metal oxyhydroxides.

[0100] Figure 12 panels (a) and (b) show a schematic drawing of the MPL on PTL and the SEM cross-section of the OER electrode-4, respectively. The crosssection image reveals the top 10 to 100 pm thickness of the PTL has formed microporous layer (MPL) structure resulting from oxide, oxyhydroxide, or hydroxide layers comprising Ni and Co accommodated in the pores of the PTL. The anode PTL facing the counter electrode during the electrodeposition results higher loading of Co and Ni enabling this MPL structure formation. This MPL structure in the OER electrode is essential to keep the ionomer layer on the top of the electrode, without seeping through the entire thickness of the PTL, facing the AEM membrane in the electrolyzer to provide higher effective ionic conductivity and efficient catalyst utilization to enable high performance AEM electrolyzer.

[0101] The eCoNi / SS OER electrode-4 was found to have a lower overpotential of 224 mV at 20 mA / cm2with a tafel slope of 32.9 mV / decade as shown in the OER polarization curves done in 1M KOH electrolyte in Figure 13 panel (a). The OER overpotential of OER electrode-4 is lowest among the OER electrodes reported in this patent application indicating the excellent intrinsic catalytic activity of this electrode structure towards OER. The low overpotential and low tafel slope are the result of the presence of more exposed OER active sites due to the addition of a nanoparticle Ni seed layer. The AEM water electrolyzer tests were conducted in the 5cm2electrolyzer hardware at 80 ± 1 °C with various concentrations of KOH while having same cathode (PGM catalyst on carbon paper) and AEM membrane (PiperlON - 40 pm thickness). Figure 13 panel (b) shows the polarization curves of the 5 cm2AEM electrolyzer cell with eCoNi / SS OER electrode-4 done at 80 °C at different KOH concentration electrolytes. A current density of 3.85 A / cm2, 2.9 A / cm2and 1.35 A / cm2was achieved at 1.8 V in 10 mM KOH, 0.5 mM KOH and pure DIW electrolytes, respectively. Figure 13 panel (c) shows the galvanostatic electrochemical impedance spectroscopy done at 0.90 A / cm2at 80 °C at different KOH concentration electrolytes for the 5 cm2electrolyzer cell with OER electrode-4. The OER electrode-4 was found to have a high frequency resistance (HFR) of about 60 mOhm-cm2with 10 mM KOH electrolyte, lowest among the OER electrodes reported in this patent application. The HFR increases by decreasing the concentration of the electrolyte owing to the decreasing ionic conductivity. The MPL structure of the OER electrode-4 enabled the smoother interface between the anode and membrane. The cobalt oxide, cobalt oxyhydroxide, or cobalt hydroxide structures have higher electronic conductivity which results in having lower interfacial contact resistance (ICR) at the plate to anode interface. All these aspects of OER electrode-4 structure add up to provide a lower HFR to enable high performance AEM electrolyzer. Figure 13 panel (d) shows the initial durability studies of the 5 cm2electrolyzer cells with OER electrode-4 done at different durability conditions. At 80 °C in 0.5 mM KOH at constant current density of 1A / cm2, the OER electrode-4, resulted in a degradation rate of 286.1 V / h(oto88h). While in an accelerated durability condition of 90 °C at current density of 1.5 A / cm2in 10 mM KOH resulted in a degradation rate of 567.6 V / h(oto88h). At 10 barg hydrogen backpressure, the 5 cm2electrolyzer cell at 90 °C in 0.5 mM KOH electrolyte at 1.5 A / cm2resulted in a degradation rate of 103.1 V / h(oto88h).

[0102] The OER electrode-4 was tested in 1.0 M KOH electrolyte at 80 °C and 60 °C to compare and provide a comprehensive concentration study as shown in the polarization curves of Figure 14 panel (a). The AEM electrolyzer with OER electrode-4 achieved a current density of 4.5 A / cm2at 1.8 V at 80 °C and 3 A / cm2at 1.8 V at 60 °C in 1.0 M KOH electrolyte. While at a cell voltage of 2.0 V the electrolyzer with OER electrode-4 achieved a current density of 8.0 A / cm2at 80 °C and 5.2 A / cm2at 60 °C in 1.0 M KOH electrolyte, this cell resulted in a degradation rate of 167.1 pV / h(oto ooh) in 1.0 M KOH at 60 °C over a period of 1000 h as shown in Figure 14 panel (b).

[0103] The effect of Co loading in the AEM electrolyzer performance was studied by varying the time for electrodepositing Co on the Ni coated SS PTL substrate. Figure 15 panel (a) shows the plot of Co loading in the electrode vs. current density at 1.8 V. With increasing Co loading from 0.6 to 1.5 mg / cm2, the current density was increased from 2.0 A / cm2to 3.0 A / cm2at 1.8 V in 10 mM KOH electrolyte in 5cm2electrolyzer cell, indicating the scope to increase the OER active site density with increasing the loading. However, increasing the Co loading greater than 1.5 mg / cm2starts to decrease in the current density indicating filling the pores of the PTL resulting in increased mass transport resistance due to inefficient transport of water feed and oxygen gas. Figure 15 panel (b) shows the corresponding polarization curve of OER electrode-4 with a loading 1.5 mg / cm2and 3.0 mg / cm2in 10 mM KOH. Due to low porosity resulting from high Co loading, the polarization curves start to curve up in the high current density regions where the mass transport of liquid and gas gets difficult when there are not much pores in the PTL. This signifies the importance of optimizing the catalyst loading on non-PGM electrocatalysts on free-standing OER electrodes. This method of making the electrocatalyst leverages the ability to tune the loading to achieve an in-situ MPL formation resulting in high AEM electrolyzer performance. The corresponding effect of degradation rate of the electrolyzer cells are shown in Figure 15 panel (c). With increasing the Co loading on the catalyst layer, the voltage degradation increases, which may indicate that we might start to lose active sites due to catalyst dissolution at higher Co loadings.EXAMPLE 5

[0104] OER electrode listed in this example comprises three layers of metal oxide, metal oxyhydroxide, metal hydroxide, or a combination thereof deposited on individual fibers of SS PTL substrate. Figure 16 panel (a) shows the OER electrode-5 preparation steps and panel (b) shows the structure of OER electrode-5. The first layer comprises Ni oxide, Ni oxyhydroxide, Ni hydroxide or a combination thereof and functions as a corrosion protection layer for the SS substrate to reduce the metal anion leaching into electrolyzer feed water and a nanoparticle seed layer enabling to increase the OER active site density. The second layer comprises a Cu, Cu oxide, Cu oxyhydroxide, Cu hydroxide or a combination thereof which serves as the radical scavenger layer. The initial degradation observed in the 5cm2electrolyzer cells of OER electrode-4 may be attributed to the degradation of the membrane or ionomer on the anode from the reactive oxygen species (ROS) or radicals released from OER reactions at the surfaces of the Cu-based compounds. Performing gel-permeation chromatography (GPC) of the membrane after an initial 100 h of durability at 80 °C, 1A / cm2hold in 0.5 mM KOH or 10 mM KOH electrolytes has resulted in loss in the average molecular weight of the membrane by 10 kDa. To understand this better, OER electrode in this example has a Cu layer as a radical scavenger. This layer may also be attributed to OER electrocatalysis. Following this thin layer of Cu, the next Co electrocatalyst layer is electrodeposited. This electrode is named OER electrode-5 which depicts eCoCuNi / SS PTL.

[0105] Similar to the OER electrode-4, the SS PTL substrate undergoes several pre-treatment steps to remove surface oxide layers by acetone rinse followed by anodic alkaline electro-cleaning using 1.0 M NaOH at 6 V for few minutes and an optional 1 M sulfuric acid dip to neutralize the surface. Ni oxide, Ni oxyhydroxide or Ni hydroxide was coated on the electro-cleaned SS substrate by electrodeposition at a current density of 30 mA / cm2in 0.2 M nickel nitrate solution for 6 minutes to obtain a Ni loading of 0.2 mg / cm2on both sides of the PTL. In the next step, a Cu oxide layer was electrodeposited on the Ni nanoparticle layer by at 25 mA / cm2current density in 0.2 M copper sulfate solution for 1.5 minutes to obtain a Cu loading of 1.0 mg / cm2. Followed by this, a layer of Co-based oxide, oxyhydroxide or hydroxide was electrodeposited on the CuNi nanoparticle layer at 25 mA / cm2current density in 0.2 M cobalt nitrate solution for 12 minutes to obtain a Co loading of 1.0 to 1.2 mg / cm2

[0106] The AEM water electrolyzer tests were conducted in the 5 cm2electrolyzer hardware at 80 ± 1 °C with various concentrations of KOH while having same cathode (PGM catalyst on carbon paper) and AEM membrane (PiperlON - 40 pm thickness). Figure 17 panel (a) shows the polarization curves of the 5 cm2AEM electrolyzer cell with eCoCuNi / SS OER electrode-5 done at 80 °C. A current density of 2.8 A / cm2, and 1.4 A / cm2was achieved at 1.8 V in 10 mM KOH and 0.5 mM KOH electrolytes, respectively. Figure 17 panel (b) shows the durability studies of the 5 cm2electrolyzer cells with OER electrode-5 done at 80 °C in 0.5 mM KOH at constant current density of 1 A / cm2, that resulted in a degradation rate of 229.3 pV / h(o to 420h).DEFINITIONS

[0107] When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are 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 listed elements.

[0108] The term “Anion exchange ionomers” refers to polymers that facilitate the transport of anions.

[0109] The “Piperion®” refers to the brand of anion exchange ionomers / polymer / membrane developed and sold by Versogen, Inc. and as described in U.S. Patent Nos. 10,290,890 and 11,512,156, which are incorporated herein by reference in their entirety.

Claims

WHAT IS CLAIMED IS:

1. An oxygen evolution reaction (OER) electrode comprising:an OER catalyst comprising a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal is nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn) or a combination thereof ; anda porous transport layer (PTL), wherein:the OER catalyst being on at least a portion of fibers of the PTL;the fibers of the PTL comprising stainless steel, nickel (Ni), aluminum (Al), copper (Cu) or a combination thereof.

2. An OER electrode comprising:an OER catalyst comprising a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal is nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), manganese (Mn) or a combination thereof;a corrosion protection layer comprising a metal oxide, a metal oxyhydroxide, metal hydroxide or a combination thereof wherein the metal is nickel (Ni), cobalt (Co), titanium (Ti), niobium (Nb) or a combination thereof; anda porous transport layer (PTL), wherein:fibers of the PTL comprise stainless steel, Ni, Al, Cu, or a combination thereof;the corrosion protection layer being on at least a portion of the fibers of the PTL, andthe OER catalyst being on at least a portion of the corrosion protection layer.

3. An OER electrode comprising:an OER catalyst comprising a metal oxide, a metal oxyhydroxide, a metal hydroxide, or a combination thereof wherein the metal is nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), manganese (Mn) or a combination thereof;a radical scavenging layer comprising a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal is copper (Cu), zirconium (Zr), cerium (Ce), zinc (Zn), strontium (Sr), barium (Ba), manganese (Mn) or a combination thereof;a corrosion protection layer comprising a metal oxide, a metal oxyhydroxide, a metal hydroxide or a combination thereof wherein the metal is nickel (Ni), cobalt (Co), titanium (Ti), niobium (Nb) or a combination thereof; anda porous transport layer (PTL), wherein:fibers of the PTL comprise stainless steel, Ni, Al, Cu, or a combination thereof;the corrosion protection layer being on at least a portion of the fibers of the PTL;the radical scavenging layer being on at least a portion of the corrosion protection layer; andthe OER catalyst being on at least a portion of the radical scavenging layer.

4. The OER electrode of claim 3, wherein the radical scavenging layer is about 0.01 to about 20 urn thick.

5. The OER electrode of claim 3 or 4, wherein total loading of metals in the radical scavenging layer is about 0.05 to about 200 mg / cm2.

6. The OER electrode of any one of claims 3-5, wherein loading of Cu in the radical scavenging layer is about 0.05 to about 50 mg / cm2.

7. The OER electrode of any one of claims 3-6, wherein loading of Co in the radical scavenging layer is up to about 50 mg / cm2.

8. The OER electrode of any one of claims 3-7, wherein the loading of Zr in the radical scavenging layer is up to about 50 mg / cm29. The OER electrode of any one of claims 3-8, wherein loading of Ce in the radical scavenging layer is up to about 50 mg / cm2.

10. The OER electrode of any one of claims 3-9, wherein particle size of the metal oxide, the metal oxyhydroxide or the metal hydroxide in the radical scavenger layer is about 0.001 to about 10 pm.

11. The OER electrode of any one of claims 2-10, wherein the corrosion protection layer is about 0.01 to about 30 urn thick.

12. The OER electrode of any one of claims 2-11 , wherein total loading of metals in the corrosion protection layer is about 0.05 to about 200 mg / cm2.

13. The OER electrode of any one of claims 2-12, wherein loading of Ni in the corrosion protection layer is about 0.05 to about 50 mg / cm2.

14. The OER electrode of any one of claims 2-13, loading of Ti in the corrosion protection layer is about 0.05 to about 50 mg / cm2.

15. The OER electrode of any one of claims 2-14, loading of Nb in the corrosion protection layer is up to about 50 mg / cm2.

16. The OER electrode of any one of claims 2-15, loading of Co in the corrosion protection layer is up to about 50 mg / cm2.

17. The OER electrode of any one of claims 1-16, wherein the fibers of the PTL have a diameter of about 0.1 to about 50 pm.

18. The OER electrode of any one of claims 1 -17, wherein the PTL has a thickness of about 10 to about 1000 pm.

19. The OER electrode of any one of claims 1-18, wherein the OER catalyst layer has a thickness of about 0.01 to about 10 pm.

20. The OER electrode of any one of claims 1 -19, wherein the OER catalyst has a particle size of about 0.001 to about 10 pm.

21. The OER electrode of any one of claims 1 -20, wherein the OER catalyst has a total metal loading of about 0.05 to about 200 mg / cm2.

22. The OER electrode of any one of claims 1 -21 , wherein the OER catalyst has a loading of Co of about 0.05 to about 50 mg / cm223. The OER electrode of any one of claims 1 -22, wherein the OER catalyst has a loading of Ni of up to about 50 mg / cm2.

24. The OER electrode of any one of claims 1 -23, wherein the OER catalyst has a loading of Fe of up to about 50 mg / cm2.

25. The OER electrode of any one of claims 1 -24, wherein the OER catalyst has a loading of Mn of up to about 50 mg / cm2.

26. The OER electrode of any one of claims 1 -25, wherein the OER catalyst forms a microporous layer (MPL) on the fibers on the upper 5 to 200 pm of the PTL.

27. The OER electrode of any one of claims 1-26, further comprising an anion exchange ionomer with a loading of about 0.1 to about 10 mg / cm2.

28. The OER electrode of claim 27, wherein the anion exchange ionomer comprises Piperion® ionomer.

29. An anion exchange membrane (AEM) electrolyzer comprising an AEM membrane and the OER electrode of any one of claims 1 -28.

30. The AEM electrolyzer of claim 29, further comprising a hydrogen evolution catalyst electrode comprising platinum (Pt), platinum ruthenium (PtRu), nickel molybdenum (NiMo), nickel, or a combination thereof.

31. The AEM electrolyzer of claim 29 or 30, having performance of about 0.2 to about 20 A / cm2at 1.8 V.

32. The AEM electrolyzer of any one of claims 29-31 , having a performance loss rate of about 0.1 uV / h to about 400 uV / h at 0.5 A / cm2.

33. The AEM electrolyzer of any one of claims 29-32, providing an OER overpotential of about 50 to about 400 mV at 20 mA / cm234. The AEM electrolyzer of any one of claims 29-33, providing a Tafel slope of about 25 to about 40 mV / decade.

35. The AEM electrolyzer of any one of claims 29-34, having a performance of about 0.2 to about 10 A / cm2at 1.8 V and 80 °C with 10 Mm KOH electrolyte.

36. The AEM electrolyzer of any one of claims 29-35, having performance of about 1 to about 20 A / cm2at 1.8 V and 80 °C with 1 M KOH electrolyte.

37. The AEM electrolyzer of any one of claims 29-36, having an active area between 5 cm2to 1000 cm2.

38. The AEM electrolyzer of any one of claims 29-37, wherein the AEM membrane comprises Piperion® membrane.

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