OXYGEN EVOLUTION CATALYST COMPOSITIONS, FORMS, AND MANUFACTURING OF THE SAME

Description

INCORPORATION BY REFERENCE OF RELATED PATENT APPLICATIONS

This application is based upon and claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 63/704,259, filed Oct. 7, 2024, the entire contents of all of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present application relates to electrochemical components that facilitate the oxygen evolution reaction inside electrochemical devices that use anion exchange membranes. The invention of such components includes the catalyst elemental composition and methods of creating the same in a membrane electrode assembly.

BACKGROUND

Water electrolysis, also known as “water splitting”, is the decomposition of liquid water (H₂O) into oxygen gas (O₂) and hydrogen gas (H₂). Oxygen gas is produced at the anode (reaction 1.1) and hydrogen gas is produced at the cathode (reaction 1.2). Hydrogen gas plays a key role in industrialized society both as a direct (alternative) energy source and reagent in many important industrial processes, including the Haber process (for producing ammonia used to make most agricultural fertilizers). Oxygen gas may also be used as an oxidizing reagent or simply as a component of breathable air. For example, astronauts and cosmonauts residing at the International Space Station (ISS) rely on water electrolysis to maintain their life-supporting oxygen supply.

To make water electrolysis a commercially viable process, electrolyzers are needed. Commercial electrolyzers are devices that optimize mass transport (input and output feedstocks, cooling fluids, liquid electrolyte), ionic transport (reactants and products of the electrochemical half-reactions), electron transport, temperature, and the gradients thereof in order to efficiently produce chemical feedstocks in a commercially relevant manner. Electrolyzers contain a plurality of electrolyzer cells, each of which are constructed from many components, including a membrane electrode assembly (MEA). MEAs are the electrochemically active portion of electrolyzers, wherein an anode electrode and a cathode electrode are separated from each other by a membrane (also known as a separator). Electrodes carry out electrochemical half-reactions (i.e., reactions 1.2 and 1.3), producing and consuming ionic species (i.e., OH), which are conducted from one electrode to the other via the membrane. Different membrane chemistries conduct different types of ionic species, and thereby membrane chemistry strongly affects the design of electrodes, particularly the design of the catalyst layer contained in or on the electrode.

As a result, three primary electrolyzer approaches are currently used: alkaline water electrolyzer (AWEL), proton exchange membrane electrolyzer (PEMEL), and anion exchange membrane electrolyzer (AEMEL). In the case of AWE and AEMEL, a combination of the liquid electrolyte (i.e. pure water, aqueous KOH) and the membrane causes an alkaline chemistry near the electrode catalyst layers (pH>7). In alkaline chemistries, conventional wisdom states that Nickel (Ni) and Nickel-Iron (NiFe) catalysts are the most catalytically active of the non-precious metal catalysts. Some studies even place NiFe catalysts above precious metal catalysts in terms of thermodynamic efficiency. This means that they are the most thermodynamically efficient for the oxygen evolution reaction in relation to the current density (rate of reaction). The prevailing wisdom favoring NiFe catalysts is consistent throughout academic literature and commercially active electrode manufacturers alike (e.g., https://doi.org/10.1016/j.jpowsour.2019.227375) (last accessed on Sep. 18, 2024). Unfortunately, NiFe catalyst technology is still insufficient to enable large-scale, commercially viable water electrolysis, and catalysts with even higher efficiency are needed to make water electrolysis commercially viable. Thus, new electrodes and commercially viable methods of manufacturing the same are needed.

SUMMARY

The inventors of the present application have developed highly efficient anode catalysts, forms of incorporating them into anode electrodes, and methods of manufacturing the same. Such anodes have been specifically designed and optimized for AEMEL but may readily be used in AWEL and PEMEL. Such anodes have been specifically designed for an electrolyzer facilitating water electrolysis but may readily be used in any electrochemical device which facilitates OER (see Reaction 1.2 below).

Specifically, this application provides highly efficient anode catalysts, which are based on and include Ni, Fe, and Cr. The catalyst chemistries discussed herein are highly novel in the aspect that Cr inclusion in an OER catalyst is not practiced and most often discouraged (e.g., https://doi.org/10.1021/ja510442p) (last accessed on Sep. 18, 2024). In particular, the inventors of the present application have developed two separate Ni/Fe/Cr catalyst domains, described herein with reference to the relative weight ratios of Ni, Fe, and Cr, that provide unexpected and superior performance in comparison to traditional NiFe catalysts. The catalysts of the present application need not be purely composed of Ni, Fe, and Cr and may include metallic and non-metallic impurities and dopants. Indeed, dopants and impurities with and without an intended function are expected considering the need for commercially available and/or scalable materials. Unexpectedly, the inclusion of chromium in nickel-iron catalyst compositions provides significant performance improvements, achieving 0.5-1.6 kWh/kgH₂ efficiency gains compared to the industry-standard catalyst compositions, despite the prevailing technical teaching that chromium inclusion should be avoided in oxygen evolution reaction applications.

Recognizing the need for commercially viable anodes, the present application also provides various forms that the catalysts discussed herein may take in or on an anode electrode. Manufacturing methods are also specified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts tested NiFeCr catalytic compositions and two highly OER efficient regions in the NiFeCr elemental composition parameter space.

FIG. 2 depicts a schematic of a slot-die coating operation.

FIG. 3 depicts a schematic of a spray coating operation.

FIG. 4 depicts an embodiment of a chemical bath operation.

FIG. 5 depicts embodiments of an electrochemical bath operation.

FIG. 6 depicts embodiments of a thermochemical treatment operation.

FIG. 7 depicts results obtained in Example 2 herein.

DETAILED DESCRIPTION OF EMBODIMENTS

Definitions

As used herein, the following definitions will apply unless otherwise indicated.

In the context of the present application, the term “catalyst” means a material that causes or accelerates a change or reaction without itself being rapidly altered in the process, by means of increasing the rate of reaction (i.e. current density) relative to thermodynamic conditions (i.e. overpotential).

The term “electrode” means the combination of a substrate and its corresponding catalyst layer.

The term “ionomer” means a functional polymer that may or may not be cross-linked and contains a functional group that enables conduction of ions and water through itself.

The term “membrane” means a polymeric film which can conduct ions.

The term “thermodynamic efficiency” means the enthalpy of the reaction products (H₂, O₂) represented as a fraction of the amount of energy introduced into the electrolyzer in the form of electrical energy. Given the relative ease of measuring a cell voltage, rather than a single electrode's electrochemical voltage, thermodynamic efficiency is represented as water electrolysis cell efficiency (reaction 1.1) (cell voltage multiplied by current).

The term “current density” refers to the current per unit area (two-dimensional) passing through a cell.

The term “water electrolysis” in the present application refers to the following reaction:

Water electrolysis is described by two half-reactions, which happen simultaneously inside an electrolyzer. In alkaline conditions (i.e. in AWEL, AEMEL) the reactions are driven in the anion exchange form, balanced by the ion transport across the AEM:

These reactions may also be carried out in a proton exchange membrane electrolyzer (PEMEL), wherein the half-reactions are written with cations (i.e. H⁺) as the mobile species transported across a proton exchange membrane (PEM), such as Nafion. These reactions may also be carried in a bipolar membrane (BPMEL), wherein the bipolar membrane (BPM) is composed of an AEM on one side and a PEM on the other side, and the BPM is oriented relative to the anode and cathode to maximize thermodynamic efficiency.

OER Catalyst Compositions

Preferred embodiments of the present application include a catalyst having two or three of the elements Nickel (Ni), Iron (Fe), and Chromium (Cr). In such embodiments, the catalyst is a metal alloy, a mixed metal oxide, or another metal-containing substance such as a metal-organic complex. In some embodiments, the catalyst is purely composed of Ni, Fe, and/or Cr. In some embodiments, the catalyst is 51-100% composed of Ni, Fe, and/or Cr, and the balance thereof is composed of other metallic and non-metallic elements (e.g., Nb, Ta, Mo, W, Mn, Ru, Co, Rh, Ir, Pd, Pt, Cu, Al, Sn, N, and C) for functional and/or incidental reasons. In such embodiments, functional reasons can include but are not limited to enhancement in thermodynamic efficiency and inhibition of corrosion. In such embodiments, incidental reasons can include but are not limited to manufacturing impurities, and the composition of commercially available materials discussed herein. In all embodiments, catalyst composition is described as Ni, Fe, and Cr weight percentages, regardless of the weight content of other elements. For example, three catalysts, one containing Ni(25%)Fe(25%)Cr(50%), one containing Ni(24%)Fe(24%)Cr(48%)Mn(4%), and one containing Ni(20%)Fe(20%)Cr(40%)Co(20%) are all considered a Ni(25%)Fe(25%)Cr(50%) catalyst in the present application.

The inventors of the present application have developed catalyst compositions encompassed within two regions (102, 103) of the NiFeCr parameter space (100) depicted in FIG. 1, that demonstrate a remarkably high thermodynamic efficiency. In a first, Nickel-rich region (102), the catalyst may contain 50-90 wt % Ni, 10-50 wt % Cr, 0-40 wt % Fe. In particular, the first region (102) is bounded by a triangular perimeter in the NiFeCr parameter space having vertices at (90 wt % Ni, 0 wt % Fe, 10 wt % Cr), (50 wt % Ni, 0 wt % Fe, 50 wt % Cr), and (50 wt % Ni, 40 wt % Fe, 10 wt % Cr). In some embodiments, the first region (102) includes values falling on the triangular perimeter; in other embodiments, the first region (102) excludes these values.

In some embodiments, the Nickel-rich NiFeCr catalyst may specifically contain 50-70 wt % Ni, 20-40 wt % Fe, and 10-30 wt % Cr; or 50-70 wt % Ni, 0-20 wt % Fe, and 30-50 wt % Cr; or 50-70 wt % Ni, 0-20 wt % Fe, and 10-30 wt % Cr; or 70-90 wt % Ni, 0-20 wt % Fe, and 10-30 wt % Cr.

In some embodiments, the catalyst is contained in a subregion bounded by a triangular perimeter in the NiFeCr parameter space having the vertices set forth in Table 1 below:

TABLE 1
Region	Vertex 1 (wt % values)	Vertex 2 (wt % values)	Vertex 3 (wt % values)
1A	(90 Ni, 0 Fe, 10 Cr)	(80 Ni, 10 Fe, 10 Cr)	(80 Ni, 0 Fe, 20 Cr)
1B	(80 Ni, 10 Fe, 10 Cr)	(70 Ni, 20 Fe, 10 Cr)	(70 Ni, 10 Fe, 20 Cr)
1C	(80 Ni, 10 Fe, 10 Cr)	(70 Ni, 10 Fe, 20 Cr)	(80 Ni, 0 Fe, 20 Cr)
1D	(80 Ni, 0 Fe, 20 Cr)	(70 Ni, 10 Fe, 20 Cr)	(70 Ni, 0 Fe, 30 Cr)
1E	(70 Ni, 20 Fe, 10 Cr)	(60 Ni, 30 Fe, 10 Cr)	(60 Ni, 20 Fe, 20 Cr)
1F	(70 Ni, 20 Fe, 10 Cr)	(60 Ni, 20 Fe, 20 Cr)	(70 Ni, 10 Fe, 20 Cr)
1G	(70 Ni, 10 Fe, 20 Cr)	(60 Ni, 20 Fe, 20 Cr)	(60 Ni, 10 Fe, 30 Cr)
1H	(70 Ni, 10 Fe, 20 Cr)	(60 Ni, 10 Fe, 30 Cr)	(70 Ni, 0 Fe, 30 Cr)
1I	(70 Ni, 0 Fe, 30 Cr)	(60 Ni, 10 Fe, 30 Cr)	(60 Ni, 0 Fe, 40 Cr)
1J	(60 Ni, 30 Fe, 10 Cr)	(50 Ni, 40 Fe, 10 Cr)	(50 Ni, 30 Fe, 20 Cr)
1K	(60 Ni, 30 Fe, 10 Cr)	(50 Ni, 30 Fe, 20 Cr)	(60 Ni, 20 Fe, 20 Cr)
1L	(60 Ni, 20 Fe, 20 Cr)	(50 Ni, 30 Fe, 20 Cr)	(50 Ni, 20 Fe, 30 Cr)
1M	(60 Ni, 20 Fe, 20 Cr)	(50 Ni, 20 Fe, 30 Cr)	(60 Ni, 10 Fe, 30 Cr)
1N	(60 Ni, 10 Fe, 30 Cr)	(50 Ni, 20 Fe, 30 Cr)	(50 Ni, 10 Fe, 40 Cr)
1O	(60 Ni, 10 Fe, 30 Cr)	(50 Ni, 10 Fe, 40 Cr)	(60 Ni, 0 Fe, 40 Cr)
1P	(60 Ni, 0 Fe, 40 Cr)	(50 Ni, 10 Fe, 40 Cr)	(50 Ni, 0 Fe, 50 Cr)

In some embodiments, the catalyst comprises a composition set forth in Table 2 below:

	TABLE 2
	Composition	Ni (wt %)	Fe (wt %)	Cr (wt %)

	1001	85 ± 5	5 ± 5	15 ± 5
	1002	75 ± 5	5 ± 5	25 ± 5
	1003	75 ± 5	15 ± 5	15 ± 5
	1004	65 ± 5	5 ± 5	35 ± 5
	1005	65 ± 5	15 ± 5	25 ± 5
	1006	65 ± 5	25 ± 5	15 ± 5
	1007	55 ± 5	5 ± 5	45 ± 5
	1008	55 ± 5	15 ± 5	35 ± 5
	1009	55 ± 5	25 ± 5	25 ± 5
	1010	55 ± 5	35 ± 5	15 ± 5

The inventors of the present application have discovered the beneficial effects of Cr inclusion extend across both the nickel-rich and iron-rich compositional domains discussed herein, Without being bound by theory, it is believed Cr provides a fundamental improvement to oxygen evolution catalysis across the full range of compositions of the domains set forth herein.

FIG. 1 also depicts a second, Iron-rich region (103), in which the catalyst may contain 0-45 wt % Ni, 15-60 wt % Cr, and 40-85 wt % Fe. In particular, the second region (103) is bounded by a triangular perimeter in the NiFeCr parameter space having vertices at (45 wt % Ni, 40 wt % Fe, 15 wt % Cr), (0 wt % Ni, 40 wt % Fe, 60 wt % Cr), and (0 wt % Ni, 85 wt % Fe, 15 wt % Cr). In some embodiments, the second region (103) includes the values falling on the triangular perimeter; in other embodiments, the second region (103) excludes these values.

In some embodiments, the Iron-rich NiFeCr catalyst may specifically contain 20-45 wt % Ni, 40-65 wt % Fe, and 15-40 wt % Cr; or 0-25 wt % Ni, 60-85 wt % Fe, and 15-40 wt % Cr; or 5-20 wt % Ni, 45-60 wt % Fe, and 20-35 wt % Cr; or 0-25 wt % Ni, 40-65 wt % Fe, and 35-60 wt % Cr.

In some embodiments, the catalyst is contained in a subregion bounded by a triangular perimeter in the NiFeCr parameter space having the vertices set forth in Table 3 below:

TABLE 3
Region	Vertex 1 (wt % values)	Vertex 2 (wt % values)	Vertex 3 (wt % values)
2A	(45 Ni, 40 Fe, 15 Cr)	(35 Ni, 50 Fe, 15 Cr)	(35 Ni, 40 Fe, 25 Cr)
2B	(35 Ni, 50 Fe, 15 Cr)	(25 Ni, 60 Fe, 15 Cr)	(25 Ni, 50 Fe, 25 Cr)
2C	(35 Ni, 50 Fe, 15 Cr)	(25 Ni, 50 Fe, 25 Cr)	(35 Ni, 40 Fe, 25 Cr)
2D	(35 Ni, 40 Fe, 25 Cr)	(25 Ni, 50 Fe, 25 Cr)	(25 Ni, 40 Fe, 35 Cr)
2E	(25 Ni, 60 Fe, 15 Cr)	(15 Ni, 70 Fe, 15 Cr)	(15 Ni, 60 Fe, 25 Cr)
2F	(25 Ni, 60 Fe, 15 Cr)	(15 Ni, 60 Fe, 25 Cr)	(25 Ni, 50 Fe, 25 Cr)
2G	(25 Ni, 50 Fe, 25 Cr)	(15 Ni, 60 Fe, 25 Cr)	(15 Ni, 50 Fe, 35 Cr)
2H	(25 Ni, 50 Fe, 25 Cr)	(15 Ni, 50 Fe, 35 Cr)	(25 Ni, 40 Fe, 35 Cr)
2I	(25 Ni, 40 Fe, 35 Cr)	(15 Ni, 50 Fe, 35 Cr)	(15 Ni, 40 Fe, 45 Cr)
2J	(15 Ni, 70 Fe, 15 Cr)	(5 Ni, 80 Fe, 15 Cr)	(5 Ni, 70 Fe, 25 Cr)
2K	(15 Ni, 70 Fe, 15 Cr)	(5 Ni, 70 Fe, 25 Cr)	(15 Ni, 60 Fe, 25 Cr)
2L	(15 Ni, 60 Fe, 25 Cr)	(5 Ni, 70 Fe, 25 Cr)	(5 Ni, 60 Fe, 35 Cr)
2M	(15 Ni, 60 Fe, 25 Cr)	(5 Ni, 60 Fe, 35 Cr)	(15 Ni, 50 Fe, 35 Cr)
2N	(15 Ni, 50 Fe, 35 Cr)	(5 Ni, 60 Fe, 35 Cr)	(5 Ni, 50 Fe, 45 Cr)
2O	(15 Ni, 50 Fe, 35 Cr)	(5 Ni, 50 Fe, 45 Cr)	(15 Ni, 40 Fe, 45 Cr)
2P	(15 Ni, 40 Fe, 45 Cr)	(5 Ni, 50 Fe, 45 Cr)	(5 Ni, 40 Fe, 55 Cr)
2Q	(5 Ni, 80 Fe, 15 Cr)	(0 Ni, 85 Fe, 15 Cr)	(0 Ni, 80 Fe, 20 Cr)
2R	(5 Ni, 80 Fe, 15 Cr)	(0 Ni, 80 Fe, 20 Cr)	(5 Ni, 75 Fe, 20 Cr)
2S	(5 Ni, 75 Fe, 20 Cr)	(0 Ni, 80 Fe, 20 Cr)	(0 Ni, 75 Fe, 25 Cr)
2P	(5 Ni, 75 Fe, 20 Cr)	(0 Ni, 75 Fe, 25 Cr)	(5 Ni, 70 Fe, 25 Cr)
2R	(5 Ni, 70 Fe, 25 Cr)	(0 Ni, 75 Fe, 25 Cr)	(0 Ni, 70 Fe, 30 Cr)
2S	(5 Ni, 70 Fe, 25 Cr)	(0 Ni, 70 Fe, 30 Cr)	(5 Ni, 65 Fe, 30 Cr)
2T	(5 Ni, 65 Fe, 30 Cr)	(0 Ni, 70 Fe, 30 Cr)	(0 Ni, 65 Fe, 35 Cr)
2U	(5 Ni, 65 Fe, 30 Cr)	(0 Ni, 65 Fe, 35 Cr)	(5 Ni, 60 Fe, 35 Cr)
2V	(5 Ni, 60 Fe, 35 Cr)	(0 Ni, 65 Fe, 35 Cr)	(0 Ni, 60 Fe, 40 Cr)
2W	(5 Ni, 60 Fe, 35 Cr)	(0 Ni, 60 Fe, 40 Cr)	(5 Ni, 55 Fe, 40 Cr)
2X	(5 Ni, 55 Fe, 40 Cr)	(0 Ni, 60 Fe, 40 Cr)	(0 Ni, 55 Fe, 45 Cr)
2Y	(5 Ni, 55 Fe, 40 Cr)	(0 Ni, 55 Fe, 45 Cr)	(5 Ni, 50 Fe, 45 Cr)
2Z	(5 Ni, 50 Fe, 45 Cr)	(0 Ni, 55 Fe, 45 Cr)	(0 Ni, 50 Fe, 50 Cr)
2AA	(5 Ni, 50 Fe, 45 Cr)	(0 Ni, 50 Fe, 50 Cr)	(5 Ni, 45 Fe, 50 Cr)
2AB	(5 Ni, 45 Fe, 50 Cr)	(0 Ni, 50 Fe, 50 Cr)	(0 Ni, 45 Fe, 55 Cr)
2AC	(5 Ni, 45 Fe, 50 Cr)	(0 Ni, 45 Fe, 55 Cr)	(5 Ni, 40 Fe, 55 Cr)
2AD	(5 Ni, 40 Fe, 55 Cr)	(0 Ni, 45 Fe, 55 Cr)	(0 Ni, 40 Fe, 60 Cr)

In some embodiments, the catalyst comprises a composition set forth in Table 4 below:

	TABLE 4
	Composition	Ni (wt %)	Fe (wt %)	Cr (wt %)

	2001	40 ± 5	45 ± 5	20 ± 5
	2002	30 ± 5	45 ± 5	30 ± 5
	2003	30 ± 5	55 ± 5	20 ± 5
	2004	20 ± 5	45 ± 5	40 ± 5
	2005	20 ± 5	55 ± 5	30 ± 5
	2006	20 ± 5	65 ± 5	20 ± 5
	2007	10 ± 5	45 ± 5	50 ± 5
	2008	10 ± 5	55 ± 5	40 ± 5
	2009	10 ± 5	65 ± 5	30 ± 5
	2010	10 ± 5	75 ± 5	20 ± 5
	2011	5 ± 5	45 ± 5	55 ± 5
	2012	5 ± 5	55 ± 5	45 ± 5
	2012	5 ± 5	65 ± 5	35 ± 5
	2013	5 ± 5	75 ± 5	25 ± 5
	2014	5 ± 5	80 ± 5	20 ± 5

FIG. 1 also depicts embodiments of discrete catalyst compositions falling within the regions (102) and (103). Further details regarding the embodiments and associated experimental results are provided in Example 1.

The discrete catalyst compositions depicted in FIG. 1 were measured in an AEMEL at low current density, specifically 0-0.1 A/cm², and demonstrated a thermodynamic efficiency 0-2 kWh/kgH2 higher than a conventional NiFe catalyst (101) having a composition comprising Ni(80%)Fe(20%). Low current density is used as a benchmark to isolate the effects of the catalyst composition from the effects of the various other factors that influence electrolyzer efficiency.

In some embodiments, the highly efficient NiFeCr catalyst is Nickel-rich, and specifically contains 50-90% Ni, 0-40% Fe, and 10-50% Cr (102). In some embodiments, the highly efficient NiFeCr catalyst is Iron-rich, and specifically contains 0-45% Ni, 40-85% Fe, and 15-60% Cr (103).

In some embodiments, the catalyst comprises only two of the three elements nickel, iron, and chromium. For example, nickel-chromium catalysts without iron, or iron-chromium catalysts without nickel, may be prepared using the manufacturing methods described herein. In some embodiments, these compositions exhibit improved OER activity compared to conventional nickel-iron catalysts.

Forms of an OER Catalyst Layer and Manufacturing of the Same

In the preferred embodiment, an electrode is comprised of the catalyst layer supported by a substrate. In the preferred embodiment, the substrate is a thin layer (10s μm to 100s μm) which is electrically conductive and porous for mass transport (i.e. produced oxygen gas, liquid electrolyte). In some embodiments, the substrate thickness is 10-100 μm or 100-500 μm. In some embodiments, the substrate layer is a metallic felt, woven mesh, sintered plate, foam, or expanded metal, wherein the chosen metal is a corrosion-resistant metal or alloy, such as Ni or stainless steel (i.e. 304, 316/316L). In some embodiments, the corrosion resistance of the substrate is improved by electroplating it with titanium, gold, etc. In some embodiments, the substrate layer is a carbon-based fiber felt, composed of graphitic carbon, carbon black, coke, or other allotropes thereof.

Another aspect of the present application relates to the different forms that an OER catalyst may take as a catalyst layer on an electrode. There are various forms of that a catalyst layer can occupy on the substrate (i.e. a discrete layer adjacent to the substrate) or in the substrate (i.e. a layer contained within the substrate pores). The form of the catalyst layer strongly determines manufacturing method, discussed herein, and strongly influences thermodynamic efficiency. In a preferred embodiment, a catalyst layer is manufactured using one or multiple of the following techniques in a roll-to-roll manufacturing line, processing rolls of substrate at 0.1-25 m/min. The following forms of catalyst layers need not be mutually exclusive. In some embodiments, the substrate is passed through a cleaning step before the catalyst layer manufacturing step(s).

Form #1: Ink-Based Catalyst Layer

In some embodiments, the discussed catalyst layer is created via drying of a catalyst ink. In such embodiments, an ink is prepared with a solvent (i.e. water, alcohols), a catalyst powder (i.e. nanopowder), and, optionally, ionomers (i.e. anion exchange ionomers) and binders (i.e. functionalized hydrocarbons). The ink is dispensed onto the substrate and the solvents evaporate to leave behind an adhered catalyst layer.

In some embodiments, ink-based catalyst layers are created via continuous slot-die operation. A schematic of a slot-die coating operation is depicted in FIG. 2. Specifically, FIG. 2 depicts a substrate (200) passing around a roller (203) and under a slot-die (202). A liquid to be coated (201) is coated onto the substrate (200). As the roller (203) continues to turn, a coated substrate (204) is produced. A slot-die is an instrument that uniformly and continuously dispenses a catalyst ink onto a rectangular footprint via an engineered cavity (the “die”), which is downstream of a pump and other ink-processing equipment. The user controls the pump rate and the position of the die relative to the substrate in order to control various coating parameters (i.e. quantity of dispensed ink per surface area, uniformity of the dispensed ink). In this embodiment, a roll of substrate passes around a roller and passes under a slot-die, which distributes the coating fluid across a desired coating width before coating the substrate. The substrate moves relative to the slot-die as the coating is coated onto the substrate. In some embodiments, the slot-die delivers a catalyst ink. In some embodiments, the substrate is heated before, while, or after passing under the spray nozzle(s) to modulate drying of the catalyst layer.

In some embodiments, ink-based catalyst layers are created via continuous spray coating operation. A schematic of a spray coating operation is depicted in FIG. 3. In this embodiment, a substrate (300) passes around a roller (303) and passes under a spray nozzle (302), which atomizes the coating fluid (301) into droplets, evenly distributed across a fixed area. As the roller continues, a substrate coated with catalyst layer (304) is produced. In some embodiments, multiple spray nozzles are aligned in the cross-machine direction to span the desired coating width. In such embodiments, the substrate moves relative to the spray nozzle(s) as the coating is coated onto the substrate. In other embodiments, the spray nozzle(s) deliver a catalyst ink. In some embodiments, the substrate is heated before, while, or after passing under the spray nozzle(s) to modulate drying of the catalyst layer.

Form #2: Ink-Free Catalyst Layer

In some embodiments, the discussed catalyst layer is ink-free, wherein the catalyst is deposited onto or into a substrate and originates from an aqueous or non-aqueous metal ion solution. Removing the conventional catalyst ink from the manufacturing process may dramatically reduce manufacturing cost; for example, metal salts are most often less expensive than nanoparticles and subsequent ink preparation steps. In another preferred embodiment, ink-free catalyst layers are created via chemical bath impregnation, electroplating, or thermochemical growth.

In some embodiments, an ink-free catalyst layer is created via chemical impregnation in a continuous chemical bath. Chemical impregnation is the process of exposing metal ions (most often Fe, Cr) to a substrate that readily absorbs those metal ions (most often Ni), leaving behind a surface containing the catalyst compositions discussed herein. In some embodiments, the solution is heated and contains various organic and non-organic additives to facilitate the reaction. In some embodiments, the substrate is rolled through a subsequent rinsing step to remove residual chemical bath and controllably halt the reaction. A schematic of a chemical bath operation is depicted in FIG. 4. In this embodiment, a substrate (402) passes around a roller (404), which is submerged into a chemical bath (401) in a bath tank (400) that creates an ink-free catalyst layer over a defined residence time, which is 1-10 seconds, or 10-100 seconds, or 100-1000 seconds. As the roller continues, a substrate coated with catalyst layer (403) is produced. In such embodiments, all surfaces in contact with the bath are inert to the composition of the chemical bath (i.e. composed of or coated with PTFE). In some embodiments, the substrate is heated before, while, or after passing through the bath to facilitate the creation of the ink-free catalyst layer and prevent thermal gradients across the bath. In some embodiments, a continuous slot-die operation or a continuous spray coating operation are used to deliver controlled quantities of a chemical bath fluid.

In some embodiments, an ink-free catalyst layer is created via electroplating (also known as electrodeposition) in a continuous electrochemical bath. Specifically, in FIG. 5, a substrate (502) passes around a roller (504), which is submerged in an electroplating bath (501) contained in a bath tank (500). As the roller continues, a substrate electroplated with a catalyst layer (503) is produced. In such embodiments, a continuous electrochemical bath is achieved by introducing a stationary counter electrode in parallel to the substrate and applying a voltage to the substrate against the counter electrode, as depicted in FIG. 5. In some embodiments, the residence time of the substrate near the counter electrode may be 1-10 seconds, 10-100 seconds, or 100-1000 seconds. In some embodiments, the counter electrode is a sacrificial material to uptake reaction products or replenish reactants. In some embodiments, the counter electrode is inert to the bath. In some embodiments, the geometry of the counter electrode (505) is parallel to the substrate, such that the substrate experiences a consistent electrochemical potential throughout the entire path length, for example as shown by (A) in FIG. 5. In some embodiments, the geometry of the counter electrode (506) is localized near the roller, for example as shown by (B) in FIG. 5. In some embodiments, the tank (500) that contains the electrochemical bath, or a particular surface of the tank, is used as the counter electrode, for example as shown by (C) in FIG. 5. In some embodiments, multiple rollers (504) are included in the bath to modify the position and geometry of the substrate relative to the counter electrode. In some embodiments, the distance between the substrate and the counter electrode is controlled to remain at a controlled value, which may be 0.1 cm to 1 cm, or 1 cm to 5 cm, or 5 cm to 20 cm. In some embodiments, voltage is applied to the passing substrate (the working electrode) via electrical leads that are stationary (i.e., brushes), rotating (i.e., wheels), or incorporated into the substrate handling equipment (i.e., an idle roller, 504). In some embodiments, all other surfaces in contact with the bath are inert to the composition of the chemical bath (i.e. composed of or coated with PTFE).

In some embodiments, an ink-free catalyst layer is created via thermochemical treatment (here referred to as “thermochemical growth”). Thermochemical growth is the process of exposing a substrate to higher-than-room temperature in a reactive, gaseous atmosphere to modify its surface composition and/or oxidation state. For example, under certain conditions, stainless steel surfaces become iron-rich when heated in O₂. In some embodiments, the substrate is exposed to 25-212° C., 212-400° C., 400-1000° C., or 1000-2000° C. In some embodiments, the reactive atmosphere contains oxidizing agents (including but not limited to O₂, H₂O, Cl₂), reducing agents (including but not limited to H₂, CO, HCl), and inert agents (including but not limited to N₂, Ar). In some embodiments, oxidizing and reducing agents comprise 1-10 ppm, 10-100 ppm, 100-1000 ppm, 0.1-1%, 1-10%, or 10-100% of the reactive atmosphere, while inert agents comprise the balance. In such embodiments, continuous thermochemical growth manufacturing is achieved by introducing a heater into a roll-to-roll line.

Specifically, as depicted in FIG. 6, a substrate (600) passes around a roller (601) and under a heater (602), which maintains a high-temperature region (603) along the path of the substrate to thermochemically grow a catalyst layer on the substrate (604). In such embodiments, the residence time of the substrate is 1-10 seconds, 10-100 seconds, or 100-1000 seconds, and is modulated via heater footprint and substrate speed. In such embodiments, a reactive atmosphere is created via nozzles, equipment enclosures, and exhaust ventilation. In other embodiments, batch thermochemical growth manufacturing is achieved by introducing a large quantity of substrate into a furnace of reactor (options include but are not limited to convection furnaces and tank reactors).

In some embodiments, the ink-free catalyst layer is subsequently coated with ionomer and binder for tuning of the local chemistry, thus a further enhanced thermodynamic efficiency and/or chemical stability of the catalyst layer.

Form #3: Naturally Occurring Catalyst Layer

An aspect of the present application includes the design of a catalyst layer that is naturally occurring on the surface of the substrate (i.e. the surface of individual wires in a woven mesh), thereby removing the need for a discrete catalyst layer manufacturing step or enhancing the performance of the catalyst layer it supports as a substrate. This is achieved by selecting a substrate material, in the forms discussed herein, whose surface naturally has high thermodynamic efficiency. In such embodiments, the naturally occurring catalyst layer has an elemental composition identical to the alloy's surface elemental composition (i.e. naturally occurring Cr-rich surface in stainless steels), while the elemental composition and/or oxidation state may naturally change throughout the electrolyzer operation. Notably, the selected substrate material (i.e., a standard metal alloy) is not conventionally used in electrolyzer applications, such as AEMEL, especially not as a naturally occurring catalyst layer. In some embodiments, the standard alloy substrate layer must be passivated to enhance chemical stability, processed continuously via chemical bath (i.e. CitriSurf application) or electrochemical bath (i.e. electrocleaning).

In some embodiments, a highly efficient, Ni-rich NiFeCr catalyst (102) is obtained by using the naturally occurring catalyst layer in standard metal alloys, including but not limited to Inconel alloys, Hastelloy alloys, Nichrome alloys, and Incoloy alloys. With respect to FIG. 1, the inventors of the present application prepared inventive electrodes having naturally occurring catalyst layers from substrates composed of Inconel 601 (labelled “A” in FIG. 1), Inconel 625 (“B”), Hastelloy C-267 (“C”), Hastelloy C-2000 (“D”), Nichrome60 (off standard specification) (“E”), and Nichrome80 (“F”) in an AEMEL.

In some embodiments, a highly efficient, Fe-rich NiFeCr catalyst (103) is obtained by using the naturally occurring catalyst layer in standard metal alloys, including but not limited to stainless steels, Incoloy alloys, Nitronic alloys, and FeCr/FeCrAl/FeCrAlY alloys. With respect to FIG. 1, the inventors of the present application prepared inventive electrodes having naturally occurring catalyst layers from substrates composed of Stainless Steel 304/304L/316/316L (labelled “G” in FIG. 1), and Incoloy 25-6Mo (“H”) in an AEMEL. FIG. 1 also depicts an FeCr alloy (“I”) within the Fe-rich catalyst space (103).

In some embodiments, a highly efficient, Ni-rich or Fe-rich NiFeCr catalyst comprises an alloyed material, such as one of the foregoing materials, which may or may not be obtained using a naturally occurring catalyst layer. If not embodied as a naturally occurring catalyst layer, the catalyst may be deposited, coated, grown, or otherwise applied using standard techniques discussed herein.

In some embodiments, the naturally occurring catalyst layer is subsequently coated with ionomer and binder for tuning of the local chemistry, thus a further enhanced thermodynamic efficiency and/or chemical stability of the catalyst layer.

EXAMPLES

In Example 1, the inventors of the present application prepared electrode compositions having naturally occurring catalyst layers from substrates composed of Inconel 601, Inconel 625, Hastelloy C-267, Hastelloy C-2000, Nichrome80, Nichrome60, Stainless Steel 304/304L, Stainless Steel 316/316L, and Incoloy 25-6Mo. The electrodes were tested in an AEMEL at low current density, specifically 0-0.1 A/cm², at conditions of 65° C. and atmospheric pressure, in 0-100 mM potassium carbonate aqueous anolyte and an absence of catholyte. The electrodes were evaluated against a reference NiFe electrode contained in an adjacent cell inside the same AEMEL. The thermodynamic efficiency improvement, in reference to a Ni(80%)Fe(20%) electrode, for each electrode composition was recorded and is reproduced in Table 5 below. Efficiency improvement is reported as a range to reflect the noise and expected precision of the technique used to isolate catalytic effects.

TABLE 5
Cell Voltage Improvement	Equivalent Efficiency
Compared to NiFe	Improvement	Alloys Tested
0-20 mV	0-0.5 kWh/kgH2	Inconel 601, Hastelloy C-2000, Stainless
		Steel.
20-40 mV	0.5-1.1 kWh/kgH2	Hastelloy C-267, Nichrome 60, Nichrome
		80.
40-60 mV	1.1-1.6 kWh/kgH2	Inconel 625, Incoloy 25-6Mo.

In Example 2, the inventors the inventors of the present application prepared prototype electrodes, targeting catalyst layers across the nickel-rich region (102). The electrodes were tested in an AEMEL for 100s and 1000s of hours, at a commercially relevant current density (0.5-2 A/cm2), temperature (65° C.), and electrolyte (0-100 mM potassium carbonate aqueous anolyte). The electrodes were evaluated against a reference anode contained in an adjacent cell inside the same AEMEL. Results are depicted in FIG. 7, with gaps in the data are due to test bay downtime. The thermodynamic efficiency improvement is herein demonstrated to be highly durable, and, moreover, able to improve over time. This behavior is remarkably superior to commercial electrolyzers, which decay over time.

FURTHER EMBODIMENTS

A-1. A catalyst for an electrode of an electrolytic cell, comprising:

• • a metal containing substance having at least two of the three elements Nickel (Ni), Iron (Fe), and Chromium (Cr).

A-2. The catalyst of A-1,

• • wherein the metal containing substance comprises 50-90 wt % Ni, 0-40 wt % Fe, 10-50 wt % Cr,
  • wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

A-3. The catalyst of A-1,

• • wherein the metal containing substance comprises 0-45 wt % Ni, and 40-85 wt % Fe, 15-60 wt % Cr,
  • wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

A-4. The catalyst of A-2,

• • wherein the metal containing substance is contained within a region in the NiFeCr parameter space bounded by a triangular perimeter having vertices at (90 wt % Ni, 0 wt % Fe, 10 wt % Cr), (50 wt % Ni, 0 wt % Fe, 50 wt % Cr), and (50 wt % Ni, 40 wt % Fe, 10 wt % Cr),
    wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

A-5. The catalyst of A-3,

• • wherein the metal containing substance is contained within a region in the NiFeCr parameter space bounded by a triangular perimeter having vertices at (45 wt % Ni, 40 wt % Fe, 15 wt % Cr), (0 wt % Ni, 40 wt % Fe, 60 wt % Cr), and (0 wt % Ni, 85 wt % Fe, 15 wt % Cr).

A-6. The catalyst of any of A-1 to A-5,

• • wherein the catalyst comprises 51-100 wt % the metal containing substance.

B-1. An electrode for an electrolytic cell, comprising the catalyst of any of A-1 to A-6.

B-2. The electrode of B-1,

• • wherein the electrode comprises a substrate layer and a catalyst layer, wherein the catalyst layer comprises the catalyst of any of A-1 to A-6.

B-3. The electrode of B-2,

• • wherein the substrate layer comprises an electrically conductive and porous media.

B-4. The electrode of B-3,

• • wherein the substrate layer comprises a metallic felt, woven mesh, sintered plate, foam, or expanded metal.

B-5. The electrode of any of B-2 to B-4,

• • wherein the catalyst layer comprises an ink comprising a catalyst powder having the catalyst of any of A-1 to A-6 and optionally contains ionomers and binders.

B-6. The electrode of any of B-2 to B-4,

• • wherein the catalyst layer comprises an electroplated layer containing the catalyst of any of A-1 to A-6.

B-7. The electrode of any of B-2 to B-4,

• • wherein the catalyst layer comprises a layer of the substrate containing the catalyst of any of A-1 to A-6 which has been impregnated via a chemical bath.

B-8. The electrode of any of B-2 to B-4,

• • wherein the catalyst layer comprises a layer of the substrate containing the catalyst of any of A-1 to A-6 which has been grown on the substrate via thermochemical growth.

B-9. The electrode of any of B-2 to B-4,

• • wherein the catalyst layer is a naturally forming surface layer of the substrate layer.

B-10. The electrode of B-9,

• • wherein the substrate layer comprises a material having 50-90 wt % Ni, 0-40 wt % Fe, 10-50 wt % Cr,
  • wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

B-11. The electrode of B-9,

• • wherein the substrate layer comprises a material having 0-45 wt % Ni, and 40-85 wt % Fe, 15-60 wt % Cr,
  • wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

B-12. The electrode of B-10,

• • wherein the substrate layer comprises a material within a region in the NiFeCr parameter space bounded by a triangular perimeter having vertices at (90 wt % Ni, 0 wt % Fe, 10 wt % Cr), (50 wt % Ni, 0 wt % Fe, 50 wt % Cr), and (50 wt % Ni, 40 wt % Fe, 10 wt % Cr),
    wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

B-13. The electrode of B-11,

• • wherein the substrate layer comprises a material within a region in the NiFeCr parameter space bounded by a triangular perimeter having vertices at (45 wt % Ni, 40 wt % Fe, 15 wt % Cr), (0 wt % Ni, 40 wt % Fe, 60 wt % Cr), and (0 wt % Ni, 85 wt % Fe, 15 wt % Cr),
    wherein weight percentages (wt %) of Ni, Fe, and Cr in the metal containing substance are defined relative to one another and are independent of the presence or concentration of other elements or materials in the metal containing substance.

B-14. The electrode of any of B-10 to B-13,

• • wherein the substrate layer comprises a material selected from the group consisting of Inconel 601, Inconel 625, Hastelloy C-267, Hastelloy C-2000, Nichrome80, Nichrome60, a Fe (72%)Cr(28%) alloy, and Incoloy 25-6Mo.

C-1. An electrolytic cell comprising an electrode of any of B-1 to B-14.

C-2. The electrolytic cell of C-1, wherein the cell performs the oxygen evolution reaction at one of its electrodes.

C-3. The electrolytic cell of C-1, wherein the cell performs water electrolysis.

D-1. An electrolysis stack comprising an electrolytic cell of any of C-1 to C-3.

E-1. A method for preparing a catalyst composition for an electrode of an electrolytic cell, comprising:

• • forming a catalyst ink by preparing a solvent, adding a powder comprising the catalyst of any of A-1 to A-6, and optionally adding ionomers and/or binders,
  • applying the catalyst ink to an electrode.

E-2. The method of E-1,

• • wherein the catalyst ink is applied to the electrode via a slot-die coating operation.

E-3. The method of E-1,

• • wherein the catalyst ink is applied to the electrode via a spray coating operation.

F-1. A method for preparing a catalyst composition for an electrode of an electrolytic cell, comprising:

• • preparing an aqueous or non-aqueous metal ion solution containing the catalyst of any of A-1 to A-6,
  • applying the catalyst via chemical bath impregnation, electroplating, or thermochemical growth.

G-1. A method for preparing a catalyst composition for an electrode of an electrolytic cell, comprising:

• • providing an electrode substrate having a naturally occurring catalyst layer.

G-2. The method of G-1, wherein the electrode substrate comprises a material having the composition of any of B-10 to B-14.