Patent Publication Number: US-2023132969-A1

Title: Membrane electrode assembly catalyst material

Description:
TECHNICAL FIELD 
     The present disclosure relates to ternary and quaternary iridium oxide catalyst materials for membrane electrode assemblies (MEA) for hydrogen-generating devices, a method of identifying the same, and a method of producing the same. 
     BACKGROUND 
     Hydrogen-producing devices such as fuel cells and electrolyzers are becoming increasingly popular due to their ability to produce clean energy. But cost of their individual components has remained to be a hurdle to large scale production. Due to the harsh environment of the fuel cells and electrolyzers, only a limited number of materials has been identified as suitable for production of their components such as electrodes and reaction catalysts. Most of the traditional materials include rare elements which are cost prohibitive. 
     SUMMARY 
     In an embodiment, a catalyst for a membrane electron assembly (MEA) is disclosed. The catalyst includes a ternary oxide material having at least one composition of formula (I): 
       Ir x M 1-x O 2   (I),
 
     where
 
x is any number between about 0.25 and 0.75, and
 
     M is Ag, Au, Ba, Bi, Ca, Ce, Eu, Ge, Hf, La, Nd, Os, Pd, Pr, Re, Rh, Se, Sm, Tl, or W, 
     the material being configured to catalyze an oxygen evolution reaction (OER) and to increase stability, activity, or both of the catalyst. The MEA may be a polymer-electron membrane (PEM) MEA. The MEA may be a fuel cell MEA. The catalyst may include a first composition and a second composition of the formula (I), the first and second compositions having different M, x values, or both. M may be Bi. x may be about 0.25 to 0.5. The catalyst may further include at most about 50 wt. % of Ir, Ru, IrO 2 , RuO 2 , or a combination thereof, based on the total weight of the catalyst. The ternary oxide material may form a nanoparticle layer on an anode of the MEA. 
     In another embodiment, a catalyst of a membrane electron assembly (MEA) is disclosed. The catalyst may include a quaternary oxide material having at least one composition of formula (II): 
       Ir x Bi y M z O 2   (II),
 
     where
 
x, y, z is each individually and independently any number between about 0.25 and 0.75, x+y+z=1, and
 
     M is Ag, Au, Ba, Ca, Ce, Eu, Ge, Hf, La, Mo, Nb, Nd, Os, Pd, Pt, Pr, Re, Rh, Ru, Sb, Se, Sm, Sn, Ta, Tl, Ti, W, Y, or Zr, 
     the material being configured to catalyze an oxygen evolution reaction (OER) and increase stability, activity, or both of the catalyst. The MEA may be a polymer-electron membrane (PEM) MEA. The MEA may be a MEA in a fuel cell stack. M may be Ce, Sb, Se, or Sn. The quaternary oxide material may include at least two different compositions of the formula (II). Each of the at least two compositions may have different constituents, but the same values of numeric subscripts. The catalyst may further include Ir, Ru, IrO 2 , RuO 2 , or a combination thereof. 
     In a yet another embodiment, a membrane electron assembly (MEA) is disclosed. The MEA may include an OER catalyst material having a first material including 
     (a) a ternary oxide material having at least one composition of formula (I): 
       Ir x M 1-x O 2   (I),
 
     where
 
x is any number between about 0.25 and 0.75; and
 
     M is Ag, Au, Ba, Bi, Ca, Ce, Eu, Ge, Hf, La, Nd, Os, Pd, Pr, Re, Rh, Se, Sm, Tl, or W, and 
     (b) a quaternary oxide material having at least one composition of formula (II): 
       Ir x Bi y M z O 2   (II)
 
     where
 
x, y, z is each individually and independently any number between about 0.25 and 0.75, x+y+z=1, and
 
     M is Ag, Au, Ba, Ca, Ce, Eu, Ge, Hf, La, Mo, Nb, Nd, Os, Pd, Pt, Pr, Re, Rh, Ru, Sb, Se, Sm, Sn, Ta, Tl, Ti, W, Y, or Zr, 
     the material of the formulas (I) and (II) being configured to catalyze oxygen evolution reaction (OER) and increase stability, activity, or both of the catalyst. The MEA may be a polymer-electron membrane (PEM) MEA. The MEA may be a fuel cell MEA. M in the formula (II) may be Se, Sn, Sb, or Ce. M in the formula (I) may be Bi. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic depiction of a non-limiting example of proton-exchange membrane fuel cell (PEMFC) including a MEA; 
         FIG.  2    shows schematically principles of electrolysis in a MEA; 
         FIG.  3    shows a schematic of a MEA stack having individual cells tailored with the herein-disclosed material to achieve higher activity, stability, or both; 
         FIG.  4    shows a phase diagram between H 3 O and IrO 2 ; 
         FIG.  5    shows a plot categorizing each studied Ir 0.75 M 0.25 O 2  species (vs. pure IrO 2 ) based on chemical reactions against H, H 3 O, OH, OOH, O, and CO and thermodynamic decomposition; 
         FIG.  6    shows a plot categorizing each studied Ir 0.5 M 0.5 O 2  species (vs. pure IrO 2 ) based on chemical reactions against H, H 3 O, OH, OOH, O, and CO and thermodynamic decomposition; and 
         FIG.  7    shows a plot categorizing each studied Ir 0.25 M 0.75 O 2  species (vs. pure IrO 2 ) based on chemical reactions against H, H 3 O, OH, OOH, O, and CO and thermodynamic decomposition. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed. 
     The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
     It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components. 
     As used herein, the term “substantially,” “generally,” or “about” means that the amount or value in question may be the specific value designated or some other value in its neighborhood. Generally, the term “about” denoting a certain value is intended to denote a range within +/−5% of the value. As one example, the phrase “about 100” denotes a range of 100+/−5, i.e. the range from 95 to 105. Generally, when the term “about” is used, it can be expected that similar results or effects according to the invention can be obtained within a range of +/−5% of the indicated value. The term “substantially” may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such instances, “substantially” may signify that the value or relative characteristic it modifies is within ±0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or 10% of the value or relative characteristic. 
     It should also be appreciated that integer ranges explicitly include all intervening integers. For example, the integer range 1-10 explicitly includes 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Similarly, the range 1 to 100 includes 1, 2, 3, 4, . . . 97, 98, 99, 100. Similarly, when any range is called for, intervening numbers that are increments of the difference between the upper limit and the lower limit divided by 10 can be taken as alternative upper or lower limits. For example, if the range is 1.1. to 2.1 the following numbers 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0 can be selected as lower or upper limits. 
     In the examples set forth herein, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. 
     For all compounds expressed as an empirical chemical formula with a plurality of letters and numeric subscripts (e.g., CH 2 O), values of the subscripts can be plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures. For example, if CH 2 O is indicated, a compound of formula C (0.8-1.2 )H (1.6-2.4) O (0.8-1.2) . In a refinement, values of the subscripts can be plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures. In still another refinement, values of the subscripts can be plus or minus 20 percent of the values indicated rounded to or truncated to two significant figures. 
     As used herein, the term “and/or” means that either all or only one of the elements of said group may be present. For example, “A and/or B” means “only A, or only B, or both A and B”. In the case of “only A”, the term also covers the possibility that B is absent, i.e. “only A, but not B”. 
     It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way. 
     The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps. 
     The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. 
     The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter. 
     With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms. 
     The term “one or more” means “at least one” and the term “at least one” means “one or more.” The terms “one or more” and “at least one” include “plurality” as a subset. 
     The description of a group or class of materials as suitable for a given purpose in connection with one or more embodiments implies that mixtures of any two or more of the members of the group or class are suitable. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. First definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
     Fuel cells, or electrochemical cells, that convert chemical energy of a fuel (e.g. H 2 ) and an oxidizing agent into electricity through a pair of electrochemical half (redox) reactions, have become an increasingly popular hydrogen-fuel-generating technology. Fuel cells are now a promising alternative transportation technology capable of operating without emissions of either toxins or green-house gases. One of the current limitations of wide-spread adoption of this clean and sustainable technology is related to clean production of H 2  fuel. 
     A proton-exchange membrane fuel cell (PEMFC) represents an environment friendly alternative to internal combustion engines for a variety of vehicles such as cars and buses. A PEMFC typically features a relatively high efficiency and power density. A very attractive feature of the PEMFC engine are no carbon emissions, provided that the hydrogen fuel has been gained in an environmentally friendly manner. Besides being a green engine, the PEMFC may be used in other applications such as stationary and portable power sources. 
     The PEMFC technology; however, presents a number of challenges connected to its maintenance, sustainable performance over time, longevity, and production cost. For example, the PEMFC has a highly corrosive environment requiring materials capable of withstanding the challenging conditions. While focus is on the overall performance of the fuel cells, incremental improvements of individual components of the PEMFC are needed. 
     A non-limiting example of a PEMFC is depicted in  FIG.  1   . A core component of the PEMFC  10  that helps produce the electrochemical reaction needed to separate electrons is the Membrane Electrode Assembly (MEA)  12 . The MEA  12  includes subcomponents such as electrodes (cathode, anode), catalysts, and polymer electrolyte membranes. Besides MEA  12 , the PEMFC  10  typically includes other components such as current collectors  14 , gas diffusion layer(s)  16 , gaskets  18 , and bipolar plate(s)  20 . 
     Different types of MEA may be incorporated, for example a proton-exchange membrane (PEM) electrolyzer stack. A PEM electrolyzer is an electrochemical device designed to convert electricity and water into hydrogen and oxygen, which may be in turn used to store energy. The PEM electrolyzer utilizes electrolysis for hydrogen production. Besides fuel cells, the PEM electrolyzer may be utilized in other applications including industrial, residential, and military applications and technologies focused on energy storage such as electrical grid stabilization from dynamic electrical sources including wind turbines, solar cells, or localized hydrogen production. 
     A depiction of the electrolysis principal, utilized by a PEM electrolyzer, with relevant reactions is depicted in  FIG.  2   . The electrolyzer  30  includes the PEM  32 , anode  34 , and cathode  36 . During electrolysis, water is broken down into oxygen and hydrogen in anodic and cathodic electrically driven evolution reactions. The reactant liquid water (H 2 O) permeates through the anode  34  porous transportation layer (PTL) to the anode catalyst layer, where the oxygen evolution reaction (OER) occurs. The protons (H + ) travel via the PEM  32 , and electrons (e−) conduct through an external circuit during the hydrogen evolution reaction (HER) at the cathode  36  catalyst layer. The anodic OER requires a much higher overpotential than the cathodic HER. It is the anodic OER which determines efficiency of the water splitting due to the sluggish nature of its four-electron transfer. 
     Different materials are used to produce the PEM electrolyzer  30 . An example of the anode PTL layer material may be titanium (Ti) and the cathode PTL layer may be carbon-based materials such as carbon paper, carbon fleece, etc. The PEM  32 , anode  34 , and cathode  36  may be surrounded by bipolar or separator plates which may be made, for example, from Ti, or gold- or platinum-coated Ti metals. 
     Catalysts are typically used on the anode  34  and the cathode  36  to assist with the half-reaction processes. The typical catalyst material on the cathode  36  is platinum (Pt) while the typical catalyst used on the anode  34  is ruthenium (Ru), iridium (Ir), Ir—Ru, ruthenium oxide (RuO 2 ), iridium oxide (IrO 2 ), or iridium-ruthenium oxide (Ir—Ru—O) due to a combination of a relatively high activity and durability. But large-scale use and production of PEM electrolyzes, and fuel cells utilizing PEM electrolyzes, requires substantial amount of the catalyst materials, which poses a problem for the industry. Out of all PEM electrolyzer components, the anode catalyst is the most expensive constituent due to use of the rare metals Ir and/or Ru, and lack of opportunity to reduce its cost through economies-of-scale effects. 
     At the anode  34 , Ir typically catalyzes the EOR (H 2 O→2H + +½O2+2e − ); and, at the cathode  36 , Pt typically catalyzes the HER (2H + +2e − →H 2 ). The cell temperature typically ranges from 50 to 80° C. The cell voltage in the electrolyzer  30  is rather high compared to a fuel cell (greater than 1.23 V), typically ranging from 1.8 to 2.2 V vs. SHE at full load. Due to high operating voltage, the electrolyzer  30  materials may undergo further catalyst degradation (e.g., metal dissolution that can lead to the loss in electrochemically active surface area), which may affect the entire electrolyzer  30  stack system throughout its lifetime. 
     There are typically two important design factors for selecting the PEM electrolyzer anode  34  catalyst: 1) catalytic activity and 2) catalyst stability or durability during high voltage operation. While noble metals such as Ir, Ru, or Pt are known to be “immune” against corrosion, high voltage operation that oxidizes the surface of the metal may still trigger dissolution. For example, IrO 2  is actively used for PEM electrolyzer applications which can add value in terms of catalyst stability. Adding Ru (or another transition metal like Nb) to IrO 2  may increase the catalytic activity for the OER, when compared to pure IrO 2  catalyst. But Ru and the transition metal may leach out in the acidic environment with elevated voltage operation. This may lead to reduced electrochemical surface area (ECSA) loss and PEM electrolyzer degradation. Due to the dissolution of these expensive catalyst materials and high cost associated with their acquirement, a large-scale production is unsustainable, costly, and impracticable. 
     Additionally, the same electrolysis principles described above with respect to the PME electrolyzer  30  apply to the PEMFC anode. When the fuel cell is operated under harsh operating conditions such as rapid load change or subzero start-up, fuel starvation may occur. Upon the fuel starvation at the anode, hydrogen is no longer sufficient to provide the needed protons and electrons so water electrolysis reaction and carbon corrosion may occur. The corrosion may deteriorate and compromise the anode materials. To prevent the degradation, an OER catalyst may be added to the anode to promote water electrolysis reaction over carbon corrosion. 
     Thus, there is a need to identify alternative materials with high activity, good stability, and strong acid tolerance at high oxidation potentials which may fully or at least partially replace Ir and Ru as catalysts at the electrolyzer anode  34  and/or at the PEMFC anode. 
     In one or more embodiments, a material is disclosed. The material may be a binary oxide. The material may be an OER catalyst material. The material may be a first, second, and/or third material. The material may include, comprise, consist essentially of, or consist of one or more compositions of formula (I): 
       Ir x M 1-x O 2   (I),
 
     where
 
x is any number between about 0.1 and 0.99, and
 
M is an element from the Period 4, 5, or 6 of the Periodic Table of Elements.
 
     In formula (I), x may be any number between about 0.1 and 0.99. x may be about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, or a range including any two of the disclosed numerals. A non-limiting example of the range may be about 0.25-0.50, 0.50-0.75, or 0.25-0.75. Another non-limiting example of the range may be about 0.10-0.90, 0.20-0.80, or 0.30-0.70. 
     In formula (I), at least the following condition may apply: x+(1−x)=1. 
     In formula (I), M may be an element from Period 4 of the Periodic Table of Elements and may include Ca, Ti, Ge; Period 5 of the Periodic Table of Elements and may include Y, Zr, Nb, Mo, Rh, Pd, Ag, Sn, Sb; or Period 6 of the Periodic Table of Elements and may include Ba, La, Ce, Pr, Nd, Sm, Eu, Hf, Ta, W, Re, Os, Pt, Au, Tl, or Bi. In formula (I), M may be from Group IIA, IIIA, IVA, VA, VIA, IB, IIB, IIIB, IVB, VB, VIB, VIIB, or VIIIB. In formula (I), M may be an alkaline earth metal, coinage metal, volatile metal, icoasagen, tetrel, pentel, chalcogen, transition metal, port-transition metal, metalloid, nonmetal, or lanthanoid. 
     In formula (I), M may be an element selected from the group consisting of Ca, Ti, Ge, Y, Zr, Nb, Mo, Rh, Pd, Ag, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Hf, Ta, W, Re, Os, Pt, Au, Tl, and Bi. In formula (I), M may be an element selected from the group consisting of Ag, Au, Ba, Bi, Ca, Ce, Eu, Ge, Hf, La, Nd, Os, Pd, Pr, Re, Rh, Sb, Se, Sm, Sn, Tl, and W. In formula (I), M may be an element selected from the group consisting of Bi, Ce, Sb, Se, and Sn. 
     Non-limiting examples of binary oxides of formula (I) may include Ir x Ca x-1 O 2 , Ir x Ti x-1 O 2 , Ir x Ge x-1 O 2 , Ir x Y x-1 O 2 , Ir x Zr x-1 O 2 , Ir x Nb x-1 O 2 , Ir x Mo x-1 O 2 , Ir x Rh x-1 O 2 , Ir x Pd x-1 O 2 , Ir x Ag x-1 O 2 , Ir x Sn x-1 O 2 , Ir x Sb x-1 O 2 , Ir x Ba x-1 O 2 , Ir x La x-1 O 2 , Ir x Ce x-1 O 2 , Ir x Pr x-1 O 2 , Ir x Nd x-1 O 2 , Ir x Sm x-1 O 2 , Ir x Eu x-1 O 2 , Ir x Hf x-1 O 2 , Ir x Ta x-1 O 2 , Ir x W x-1 O 2 , Ir x Re x-1 O 2 , Ir x Os x-1 O 2 , Ir x Pt x-1 O 2 , Ir x Au x-1 O 2 , Ir x Tl x-1 O 2 , or Ir x Bi x-1 O 2 , 
     Further non-limiting examples of binary oxides of formula (I) may include Ir 0.25 Ag 0.75 O 2 , Ir 0.5 Ag 0.5 O 2 , Ir 0.75 Ag 0.25 O 2 , Ir 0.25 Au 0.75 O 2 , Ir 0.5 Au 0.5 O 2 , Ir 0.75 Au 0.25 O 2 , Ir 0.25 Ba 0.75 O 2 , Ir 0.5 Ba 0.5 O 2 , Ir 0.75 Ba 0.25 O 2 , Ir 0.25 Bi 0.75 O 2 , Ir 0.5 Bi 0.5 O 2 , Ir 0.75 Bi 0.25 O 2 , Ir 0.25 Ca 0.75 O 2 , Ir 0.5 Ca 0.5 O 2 , Ir 0.75 Ca 0.25 O 2 , Ir 0.25 Ce 0.75 O 2 , Ir 0.5 Ce 0.5 O 2 , Ir 0.75 Ce 0.25 O 2 , Ir 0.25 Eu 0.75 O 2 , Ir 0.5 Eu 0.5 O 2 , Ir 0.75 Eu 0.25 O 2 , Ir 0.25 Ge 0.75 O 2 , Ir 0.5 Ge 0.5 O 2 , Ir 0.75 Ge 0.25 O 2 , Ir 0.25 Hf 0.75 O 2 , Ir 0.5 Hf 0.5 O 2 , Ir 0.75 Hf 0.25 O 2 , Ir 0.25 La 0.75 O 2 , Ir 0.5 La 0.5 O 2 , Ir 0.75 La 0.25 O 2 , Ir 0.25 Nd 0.75 O 2 , Ir 0.5 Nd 0.5 O 2 , Ir 0.75 Nd 0.25 O 2 , Ir 0.25 Os 0.75 O 2 , Ir 0.5 OS 0.5 O 2 , Ir 0.75 Os 0.25 O 2 , Ir 0.25 Pd 0.75 O 2 , Ir 0.5 Pd 0.5 O 2 , Ir 0.75 Pd 0.25 O 2 , Ir 0.25 Pr 0.75 O 2 , Ir 0.5 Pr 0.5 O 2 , Ir 0.75 Pr 0.25 O 2 , Ir 0.25 Re 0.75 O 2 , Ir 0.5 Re 0.5 O 2 , Ir 0.75 Re 0.25 O 2 , Ir 0.25 Rh 0.75 O 2 , Ir 0.5 Rh 0.5 O 2 , Ir 0.75 Rh 0.25 O 2 , Ir 0.25 Sb 0.75 O 2 , Ir 0.5 Sb 0.52 , Ir 0.75 Sb 0.25 O 2 , Ir 0.25 Se 0.75 O 2 , Ir 0.5 Se 0.5 O 2 , Ir 0.75 Se 0.25 O 2 , Ir 0.25 Sm 0.75 O 2 , Ir 0.5 Sm 0.5 O 2 , Ir 0.75 Sm 0.25 O 2 , Ir 0.25 Sn 0.75 O 2 , Ir 0.5 Sn 0.5 O 2 , Ir 0.75 Sn 0.25 O 2 , Ir 0.25 Tl 0.75 O 2 , Ir 0.5 Tl 0.5 O 2 , Ir 0.75 Tl 0.25 O 2 , Ir 0.25 W 0.75 O 2 , Ir 0.5 W 0.5 O 2 , or Ir 0.75 W 0.25 O 2 . 
     In one or more embodiments, another or second material may be disclosed. The material may be a ternary oxide. The material may be an OER catalyst material. The material may be a first, second, and/or third material. The material may include, comprise, consist essentially of, or consist of one or more compositions of formula (II): 
       Ir x Bi y M z O 2   (II),
 
     where
 
x, y, z is each individually and independently any number between about 0.1 and 0.98, x+y+z=1, and
 
M is an element from the Period 4, 5, or 6 of the Periodic Table of Elements.
 
     In formula (II), x, y, and z may be each individually and independently about 0.1 and 0.99. x, y, and/or z may be about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, or a range including any two of the disclosed numerals. A non-limiting example of the range for x, y, and/or z may be about 0.10-0.90, 0.20-0.80, or 0.30-0.70. Another non-limiting example of the range for x, y, and/or z may be about 0.25-0.5, 0.5-0.75, or 0.25-0.75. 
     In formula (II), at least the following condition may apply: x+y+z=1. 
     In formula (II), M may be an element from Period 4 of the Periodic Table of Elements and may include Ca, Ti, Ge; Period 5 of the Periodic Table of Elements and may include Y Zr, Nb, Mo, Rh, Pd, Ag, Sn, Sb; or Period 6 of the Periodic Table of Elements and may include Ba, La, Ce, Pr, Nd, Sm, Eu, Hf, Ta, W, Re, Os, Pt, Au, Tl, or Bi. In formula (I), M may be from Group IIA, IIIA, IVA, VA, VIA, IB, IIB, IIIB, IVB, VB, VIB, VIIB, or VIIIB. In formula (I), M may be an alkaline earth metal, coinage metal, volatile metal, icoasagen, tetrel, pentel, chalcogen, transition metal, port-transition metal, metalloid, nonmetal, or lanthanoid. 
     In formula (II), M may be an element from the Period 4 of the Periodic Table of Elements and may include Se, Period 4 of the Periodic Table of Elements and may include Sb, or Period 6 of the Periodic Table of Elements and may include Ce. In formula (II), M may be an element from Group VA, VIA, or IIIB. In formula (II), M may be a chalcanoid, metalloid, metal, lanthanoid, or nonmetal. 
     In formula (I), M may be an element selected from the group consisting of Ca, Ti, Ge, Y, Zr, Nb, Mo, Rh, Pd, Ag, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Hf, Ta, W, Re, Os, Pt, Au, T, and Bi. In formula (II), M may be Se, Sb, or Ce. In formula (II), M may be selected from the group consisting of Se, Sb, and Ce. 
     Non-limiting example ternary oxides of formula (II) may include Ir 0.33 Bi 0.33 Se 0.33 O 2 , Ir 0.33 Bi 0.33 Sn 0.33 O 2 , Ir 0.33 Bi 0.33 Sb 0.33 O 2 , Ir 0.33 Bi 0.33 Ce 0.33 O 2 , Ir 0.25 Bi 0.25 Se 0.5 O 2 , Ir 0.25 Bi 0.5 Se 0.25 O 2 , Ir 0.5 Bi 0.25 Se 0.25 O 2 , Ir 0.25 Bi 0.25 Sn 0.5 O 2 , Ir 0.25 Bi 0.5 Sn 0.25 O 2 , Ir 0.5 Bi 0.25 Sn 0.25 O 2 , Ir 0.25 Bi 0.25 Sb 0.5 O 2 , Ir 0.25 Bi 0.5 Sb 0.25 O 2 , Ir 0.5 Bi 0.25 Sb 0.25 O 2 , Ir 0.25 Bi 0.25 Ce 0.5 O 2 , Ir 0.25 Bi 0.5 Ce 0.25 O 2 , or Ir 0.5 Bi 0.25 Ce 0.25 O 2 . 
     In one or more embodiments, the material of formula (I) may be combined with the material of formula (II). In one or more embodiments, a MEA may include one composition, at least one composition, or more than one composition of the material of formula (I) and one composition, at least one composition, or more than one composition of the material of formula (II). 
     One or more oxides of the formulas (I), (II), or both may form a protective, stabilizing, and/or active layer. The material of the formulas (I), (II), or both may form an internal layer, external layer, or both with respect to adjoining, adjacent, or integral bulk region. The bulk region may be an electrode. The electrode may be an anode, cathode, or both of a MEA, PEM electrolyzer, or PEMFC. The material and/or the layer including the material may form a catalyst or be part of a catalyst. The catalyst may be a part of a MEA, PEM electrolyzer, or PEMFC electrode. The material of the formula (I), (II), or both may be used as an OER catalyst in a MEA (e.g. MEA of a PEMFC or an electrolyzer MEA), an anode OER catalyst in a PEM electrolyzer, or as an additive or OER catalyst in a PEMFC anode. Alternatively, the material of formula (I), (II), or both may be used on a PEMFC cathode. 
     The material may be in a form of nanoparticles. The nanoparticles may have the same or different size, diameter, dimensions, orientation, structure, facets content, composition in each layer. The loading of the oxides of the formulas (I), (II), or both may be different or the same within the layer(s). It is contemplated that more than one layer including the oxides of the formulas (I), (II), or both may be formed. The layers may have the same or different architecture, loading of individual oxides, types of oxides, size of the oxide nanoparticles, the like, or a combination thereof. 
     Furthermore, variation of catalyst loading levels (e.g., gradient) may be used to lead to different OER activities and current density within the catalyst material/catalyst layer/electrode/cell/stack/MEA/electrolyzer/PEMFC. In other words, homogenization of the current density may be realized by tailoring the catalyst material/catalyst layer/electrode/cell/stack/MEA/electrolyzer/PEMFC by redistributing the catalyst loading. 
     The material of formula (I), (II), or both may be used in addition to traditional electrolyzer catalyst material(s) such as Ir, Ru, Ir—Ru, IrO 2 , RuO 2 , Ir—Ru—O. The material of the formula (I), (II), or both may replace a portion of the traditional electrolyzer catalyst material, especially toward the bulk region of the nanoparticles. For example, about 5 to 99, 10 to 80, or 20 to 70 wt. % of the traditional electrolyzer catalyst material may be replaced with the material of the formula (I), (II), or both. For example, about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 99, 100 wt. % of the traditional electrolyzer catalyst material may be replaced with the material of the formula (I), (II), or both. In a non-limiting specific example, an OER catalyst includes about 20 to 40 wt. % of Ir, Ru, Ir—Ru, IrO 2 , RuO 2 , Ir—Ru—O, or a combination thereof, and the remainder such as about 60 to 80 wt. % of the material of the formula (I), (II), or both. 
     The material of choice for the OER catalyst may be tailored to a specific application. For example, a more stable oxide of the formula (I), (II), or both may be placed within the MEA stack, electrolyzer stack, or PEMFC stack in the location requiring higher stability. Alternatively, or in addition, a more active oxide of the formula (I), (II), or both may be placed within the MEA stack, electrolyzer stack, or PEMFC stack in the location requiring higher activity. The MEA, electrolyzer, or PEMFC stack may thus be designed to maximize activity and stability by using different oxides of the formula (I), (II), or both in different locations. 
     For example, it was discovered that Bi-containing oxide of the formula (I), (II), or both is more stable than IrO 2 . It was also discovered that Se-, Sb-, and Ce-containing oxides of the formula (I), (II), or both have increased activity in comparison to IrO 2 . Thus, an electrolyzer or PEMFC cell and/or stack may include a first material including Bi-containing oxide of the formula (I), (II), or both to increase stability and/or a second material including Se-, Sb-, and Ce-containing oxides of the formula (I), (II), or both to increase activity. The first and second material may be used to partially or entirely replace a traditional MEA material/electrolyzer material/PEMFC electrode material, the third material, or be included together with the third material. 
     In the MEA, electrolyzer, and/or PEMFC region(s) that experience the least degradation, the performance and cost may be optimized by selecting the material of formula (I), (II), or both, structured to deliver the highest catalytic activity. In the non-limiting example, the region(s), cell(s), layer(s), catalyst(s), or a combination thereof may incorporate the material of formula (I), (II), or both including Se, Sn, Sb, Ce, Ti, Zr, Ta, W, Nb, Mo, Re, Ru, Os, or a combination thereof. 
     Similarly, the material of formula (I), (II), or both that are more stable may be utilized in the MEA, electrolyzer, and/or PEMFC region(s) that lead to a fast degradation. In a non-limiting example, the region(s), cell(s), layer(s), catalyst(s), may incorporate the material of formula (I), (II), or both including Bi, Y, La, Pr, Nd, Sm, Eu, Ag, Hf; Ba, Rh, Pd, Pt, Au, Tl, or a combination thereof. 
     Table 1 shows oxides of formulas (I) and (II) having higher stability, higher activity, and equal activity and stability with respect to IrO 2 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Reaction tendency of species in comparison to IrO 2   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Higher stability than IrO 2   
                 Bi, Sm, La, Nd 
               
               
                 Higher activity than IrO 2   
                 Ce, Se, Zr, Sb, Ti, Ta, W, Nb, Mo 
               
               
                 Equal stability and activity as IrO 2   
                 Sn, Pr 
               
               
                   
               
            
           
         
       
     
     The material may be further arranged such that different MEA within a single stack include different disclosed species at various locations, depending on susceptibility to corrosion and desired performance (activity, stability). For example, a MEA stack may include a first material with one or more species of the material of formula (I), (II), or both in a number of first cell(s). A number of second cell(s), adjacent to the first cell(s), may include the disclosed material of formula (I), (II), or both with at least partially or completely different species/elements/M. A number of third cell(s) adjacent to the second cell(s) on the opposite side than the first cell(s) may include the material of formula (I), (II), or both with yet different species than the first and second cell(s). Alternatively, the second cell(s) may be adjacent to the first cell(s) on both sides. It is contemplated that various arrangements may be made within the MEA, PEM electrolyzer, PEMFC stack(s). 
     In a non-limiting example, shown in  FIG.  3   , a MEA stack  50  features first cells  52 , second cells  54 , and third cells  56 . The first cells  52 , the second cells  54 , and the third cells  56  each have a different composition of the catalyst material of the formulas (I), (II), or both. For example, the second cell(s)  54  may include the material of formulas (I), (II), or both focused on increasing activity of the catalyst material/catalyst layer/electrode/cell/stack/MEA/electrolyzer/PEMFC, the third cell(s)  56  may include the material of formulas (I), (II), or both focused on increasing stability of the catalyst material/catalyst layer/electrode/cell/stack/MEA, and the first cell(s)  52  may include the material of formula (I), (II), or both focused on sustainability, practicality, and lower production price of the catalyst material/catalyst layer/electrode/cell/stack/MEA, thus replacing a higher volume or traditional OER catalyst materials than the second and third cell(s)  54 ,  56 . 
     The material of the formulas (I), (II), or both may be synthesized in the following manner. Metal containing precursors of the disclosed species may be annealed with desired stoichiometric amount in oxidizing (air or O 2 ) or reducing heat treatment condition using N 2 , Ar, or H 2  mixture gas. The heat treatment temperature may range from about 150 to 1500° C. to yield a desired ternary oxides or doped composition. The heat treatment time may vary from about 30 seconds to 48 hrs. The metal precursors may be prepared by solid-state synthesis route (e.g., ball milling process), co-precipitation process (e.g., solution-based process), sol-gel process, hydrothermal process, or the like. The oxide specie(s) may be deposited on to a designated support materials (carbon, metal, ceramic, etc.) during the synthesis process or as a post-treatment step. Deposition techniques may include, but are not limited to, physical vapor deposition, chemical vapor deposition, atomic layer deposition, or solution-based approach, etc. 
     The electrode fabrication may include the following process. The oxide or the oxide on a support (see above) may be deposited on a membrane, a decal material, or a PTL with an ink containing additional ionomer and solvent(s) using typical deposition technique, followed by drying and/or annealing steps. 
     To reveal the structural and morphological details of the herein-disclosed oxide materials, X-ray diffraction (XRD) technique may be used to identify crystal structure. Different crystal structures may be found: e.g., cubic, tetragonal, trigonal, orthorhombic, monoclinic, etc. It may be possible to find other XRD peaks due to impurity and/or phase decomposition. For more accurate size distribution, high-resolution transmission electron microscope (HR-TEM) imaging technique may be used. 
     The above-mentioned material of the formulas (I) and (II) was identified using database-driven materials screening. While typically, a surface-based slab DFT model may be used to understand thermodynamic stability, metal mixing, element segregation toward surface or bulk, OER activity, and durability, both human and CPU times are quite expensive to build DFT slab models, carry out atomistic simulation, and analyze the results. Additionally, while the DFT slab models are ideal for a simple metal or a binary oxide system such as pure Ir, Ru, IrO 2 , and RuO 2 , even modeling binary metallic catalyst such as Ir x R 1-x  becomes very complicated due to the increased degree of freedoms in structural generation. Instead, a different approach was adopted to identify suitable material to replace the traditional electrolyzer and PEMFC electrode materials. The approach is described below in the Experimental section. 
     Experimental Section 
     In the first step, RuO 2 , IrO 2 , and PtO 2  were examined against corrosive species H, H 3 O, OH, OOH, O, and CO. Analysis of various reaction enthalpy E rxn (eV/atom) values of the studied species in reducing and oxidizing reactions revealed tendencies of Ru and Ir compositions to lean more towards either higher activity or higher stability. For example, RuO 2  typically shows enhanced OER performance—i.e., more activity than IrO 2 —but leads to poor stability due to corrosion from the strong acidity at the perfluorosulfonic membrane and high anodic potential at OER. On the other hand, IrO 2  is a more resistive material to OER in the acidic environment, but IrO 2  exhibits lower performance than RuO 2 . Other Ir and Ru compositions were studied. Specific reaction parameters which reveal tendencies of materials to be more active (like RuO 2 ) or more stable (like IrO 2 ) were identified. The relevant reaction parameters were then studied with respect to 56 elements of the Periodic Table: Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Co, Cu, Zn, Ga, Ge, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Te, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Pt, Au, Tl, and Bi. 
     The “interface reactions” module kit, publicly available from materialsproject.org was used. The decomposition products of Ir 0.75 M 0.25 O 2 , where M represented each element named above, was conducted. The loading of Ir was chosen to be higher than loading of M. Because PEM electrolyzer operates in acidic conditions, the decomposition products of the studied material should be “acid stable.” Decomposition products of each studied element were identified, and stable compositions determined. The Ir 0.75 M 0.25 O 2  compositions with stable decomposition products included M=Ca, Ti, Ge, Se, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Hf, Ta, W, Re, Os, Pt, Au, Tl, and Bi. 
     Next, the thermodynamic decomposition of Ir 0.75 Mo 25 O 2  at its given chemical space was studied. For example, Ir 0.75 Ru 0.25 O 0.2  tends to thermodynamically decompose to 0.75 IrO 2  and 0.25 RuO 2 , where both oxides belong to a tetragonal crystal system (P4 2 /mnm). But Ir 0.75 Pt 0.25 O 2  thermodynamically decomposes to 0.75 IrO 2  and 0.25 PtO 2 . PtO 2  belongs to orthorhombic crystal system (Pnnm). Each phase mixture was examined to evaluate whether the decomposition products are tetragonal or non-tetragonal structures. Percentage of non-tetragonal phase in all phase mixtures was determined, and penalty points (PP) based on this value were assigned to non-tetragonal structures. Table 2 summarizes the thermodynamic decomposition reactions for Ir 0.75 M 0.25 O 0.2  and their assigned penalty points (PP dcmp ). 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Thermodynamic decomposition of Ir 0.75 M 0.25 O 2  products with penalty 
               
               
                 points assigned to non-tetragonal structures and acid stability 
               
            
           
           
               
               
               
               
            
               
                 M 
                 Decomposition Reaction 
                 PP dcmp   
                 Acid Stability 
               
               
                   
               
               
                 Ca 
                 Ca 0.25 Ir 0.75 O 2  → 0.25 IrO 3  + 0.25 CaIrO 3  + 0.25 IrO 2   
                 0.667 
                 Maybe (Ca 2+  unstable) 
               
               
                 Ti 
                 Ti 0.25 Ir 0.75 O 2  → 0.25 TiO 2  + 0.75 IrO 2   
                 0.000 
                 Passivates (TiO 2 ) 
               
               
                 Ge 
                 Ge 0.25 Ir 0.75 O 2  → 0.25 GeO 2  + 0.75 IrO 2   
                 0.250 
                 Stable 
               
               
                 Se 
                 Ir 0.75 Se 0.25 O 2  → 0.75 IrO 2  + 0.25 SeO 2   
                 0.000 
                 Passivates (SeO 2 ) 
               
               
                 Y 
                 Y 0.25 Ir 0.75 O 2  → 0.125 IrO 3  + 0.125 Y 2 Ir 2 O 7  + 0.375 IrO 2   
                 0.400 
                 Maybe (Y 3+  unstable) 
               
               
                 Zr 
                 Zr 0.25 Ir 0.75 O 2  → 0.25 ZrO 2  + 0.75 IrO 2   
                 0.250 
                 Passivates (ZrO 2 ) 
               
               
                 Nb 
                 Nb 0.25 Ir 0.75 O 2  → 0.125 Nb 2 O 5  + 0.062 Ir + 0.688 IrO 2   
                 0.214 
                 Passivates (NbO x ) 
               
               
                 Mo 
                 Mo 0.25 Ir 0.75 O 2  → 0.25 MoO 2  + 0.75 IrO 2   
                 0.000 
                 Passivates (MoO x ) 
               
               
                 Ru 
                 Ir 0.75 Ru 0.25 O 2  → 0.75 IrO 2  + 0.25 RuO 2   
                 0.000 
                 Noble Metal (immune) 
               
               
                 Rh 
                 Ir 0.75 Rh 0.25 O 2  → 0.75 IrO 2  + 0.25 RhO 2   
                 0.000 
                 Noble Metal (immune) 
               
               
                 Pd 
                 Ir 0.75 Pd 0.25 O 2  → 0.25 PdO 2  + 0.75 IrO 2   
                 0.000 
                 Noble Metal (immune) 
               
               
                 Ag 
                 Ag 0.25 Ir 0.75 O 2  → 0.083 Ag 3 O 4  + 0.167 IrO 3  + 0.583 IrO 2   
                 0.300 
                 Noble Metal (immune) 
               
               
                 Sn 
                 Sn 0.25 Ir 0.75 O 2  → 0.25 SnO 2  + 0.75 IrO 2   
                 0.000 
                 Passivates (SnO 2 ) 
               
               
                 Sb 
                 Sb 0.25 Ir 0.75 O 2  → 0.25 SbO 2  + 0.75 IrO 2   
                 0.250 
                 Passivates (SbO x ) 
               
               
                 Ba 
                 Ba 0.25 Ir 0.75 O 2  → 0.25 Ba(IrO 3 ) 2  + 0.25 IrO 2   
                 0.500 
                 Maybe (Ba 2+  unstable) 
               
               
                 La 
                 La 0.25 Ir 0.75 O 2  → 0.083 La 3 Ir 3 O 11  + 0.083 IrO 3  + 0.417 IrO 2   
                 0.285 
                 Maybe (La 3+  unstable) 
               
               
                 Ce 
                 Ce 0.25 Ir 0.75 O 2  → 0.25 CeO 2  + 0.75 IrO 2   
                 0.250 
                 Passivates (CeO 2 ) 
               
               
                 Pr 
                 Pr 0.25 Ir 0.75 O 2  → 0.083 Pr 3 IrO 7  + 0.083 IrO 3  + 0.583 IrO 2   
                 0.222 
                 Maybe (Pr 3+  unstable) 
               
               
                 Nd 
                 Nd 0.25 Ir 0.75 O 2  → 0.083 Nd 3 IrO 7  + 0.083 IrO 3  + 0.583 IrO 2   
                 0.222 
                 Maybe (Nd 3+  unstable) 
               
               
                 Sm 
                 Sm 0.25 Ir 0.75 O 2  → 0.083 IrO 3  + 0.083 Sm 3 IrO 7  + 0.583 IrO 2   
                 0.222 
                 Maybe (Sm 3+  unstable 
               
               
                 Eu 
                 Eu 0.25 Ir 0.75 O 2  → 0.125 IrO 3  + 0.125 Eu 2 Ir 2 O 7  + 0.375 IrO 2   
                 0.400 
                 Maybe Eu 3+  unstable) 
               
               
                 Hf 
                 Hf 0.25 Ir 0.75 O 2  → 0.25 HfO 2  + 0.75 IrO 2   
                 0.250 
                 Passivates (HfO 2 ) 
               
               
                 Ta 
                 Ta 0.25 Ir 0.75 O 2  → 0.125 Ta 2 O 5  + 0.062 Ir + 0.688 IrO 2   
                 0.143 
                 Passivates (Ta 2 O 5 ) 
               
               
                 W 
                 Ir 0.75 W 0.25 O 2  → 0.25 WO 3  + 0.625 IrO 2  + 0.125 Ir 
                 0.000 
                 Passivates (WO 3 ) 
               
               
                 Re 
                 Re 0.25 Ir 0.75 O 2  → 0.25 ReO 3  + 0.125 Ir + 0.625 IrO 2   
                 0.250 
                 Passivates (ReO 2 ) 
               
               
                 Os 
                 Ir 0.75 Os 0.25 O 2  → 0.75 IrO 2  + 0.25 OsO 2   
                 0.250 
                 Passivates (OsO x ) 
               
               
                 Pt 
                 Ir 0.75 Pt 0.25 O 2  → 0.25 PtO 2  + 0.75 IrO 2   
                 0.250 
                 Noble Metal (immune) 
               
               
                 Au 
                 Ir 0.75 Au 0.25 O 2  → 0.125 IrO 3  + 0.625 IrO 2  + 0.125 Au 2 O 3   
                 0.375 
                 Noble Metal (immune) 
               
               
                 Tl 
                 Tl 0.25 Ir 0.75 O 2  → 0.125 IrO 3  + 0.125 Tl 2 O 3  + 0.625 IrO 2   
                 0.286 
                 Passivates (Tl 2 O 3 ) 
               
               
                 Bi 
                 Bi 0.25 Ir 0.75 O 2  → 0.083 Bi 3 Ir 3 O 11  + 0.083 IrO 3  + 0.417 IrO 2   
                 0.285 
                 Passivates (BiO x ) 
               
               
                   
               
            
           
         
       
     
     Generally, noble metals are immune in the acidic region, and there are metals that passivate (e.g., TiO 2 ) which are also stable in the acidic regions. Some metals that are known to be not stable in the acid (e.g., Ca) when decomposition product is not a pure metal or a binary oxide but forms a ternary oxide (e.g., CaIrO 3 ) were included. 
     Further analysis included testing of chemical reactivity of each oxide system in oxidizing conditions (against OH, OOH, 0), reducing conditions (against H and H 3 O), and CO poisoning or carbon corrosion at high potential: CO+H 2 O→H 2 +CO 2 . 
     For studying these reactions, each ternary oxide was tested during the most thermodynamically stable reaction pathway (i.e., at its minimum reaction enthalpy in 2D phase space between ternary oxide catalyst phase and H, H 3 O, OH, OOH, O, and CO). IrO 2  catalyst was chosen as a reference material to evaluate each Ir 0.75 M 0.25 O 2  phase. A phase diagram between H 3 O (representative oxidizing agent: H 2 O+H) and IrO 2  PEM electrolyzer catalyst was generated. The phase diagram is shown in  FIG.  4   , where the molar fraction (x) indicates the amount of H 3 O and IrO 2 . For example, x=0 represents pure IrO 2 , and x=1 represents 100% H 3 O. The most stable reaction between two species takes place at its minimum reaction enthalpy Ern. As can be seen in  FIG.  4   , the strongest decomposition reaction occurs at molar fraction x=0.8, where 0.8H 3 O and 0.2IrO 2  react to form 0.2 Ir and 1.2 H 2 O as decomposition products. The reaction enthalpy (E Rxn ) between H 3 O and IrO 2  is −0.238 eV/atom. 
     The most thermodynamically stable reaction (at minimum E rxn ) for each studied Ir 0.75 M 0.25 O 2  ternary oxide catalyst phase was determined and compared to IrO 2 . Evaluating such reactions against H, H 3 O, OH, OOH, O, and CO (also called the PEM electrolyzer species) accounts for situations, where both PEM electrolyzer species and potential catalyst materials are abundantly present, where decomposition reactions may proceed at the minimum reaction enthalpy (i.e., the most favorable condition). By evaluating these reactions, the following information was obtained: (1) the amount of species (H, H 3 O, OH, OOH, O, and CO) each ternary oxide catalyst is capable of consuming at its thermodynamic equilibrium and (2) how favorable is the most stable decomposition reaction (i.e., what is the magnitude of E rxn,min ). 
     Tables 3 and 4 summarize the H and H 3 O reactions respectively for Ir 0.75 M 0.25 O 2 . For Tables 3 and 4, when molar ratio is different between H/oxide, normalization to H/oxide of IrO 2 , which is 2, was made. For example, Ba 0.25 Ir 0.75 O 2  in Table 3 shows lower H/oxide value (1.75) when compared to IrO 2 . Normalization of H/oxide=2 for Ba 0.25 Ir 0.75 O 2  further increases the E rxn  to a higher value. A higher H or H 3 O/oxide ratio indicates that an OER catalyst can take more PEM electrolyzer species per mol. It was discovered that in the reducing conditions, a lower H or H 3 O/oxide ratio and increased E rxn,H  values indicate more active OER catalyst (RuO 2 -like) and a higher H or H 3 O/oxide ratio and lower E rxn,H  values indicate more stable OER catalyst (IrO 2 -like). 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Chemical reactivity of Ir 0.75 M 0.25 O 2  against H 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ref. 
                 H 2  Reaction for IrO 2   
                 H/oxide 
                 E rxn, H   
               
               
                   
               
               
                 IrO 2   
                 0.4 H 2  + 0.2 IrO 2  → 0.2 Ir + 0.4 H 2 O 
                 2.00 
                 −0.646 
               
               
                   
               
               
                 M 
                 H 2  Reaction for Ir 0.75 M 0.25 O 2   
                 H/oxide 
                 E rxn, H   
               
               
                 Ca 
                 0.389 H 2  + 0.222 Ca 0.25 Ir 0.75 O 2  → 0.056 Ca(HO) 2  + 0.333 H 2 O + 0.167 Ir 
                 1.75 
                 −0.685 
               
               
                 Ti 
                 0.375 H 2  + 0.25 Ti 0.25 Ir 0.75 O 2  → 0.375 H 2 O + 0.063 TiO 2  + 0.188 Ir 
                 1.50 
                 −0.565 
               
               
                 Ge 
                 0.375 H 2  + 0.25 Ge 0.25 Ir 0.75 O 2  → 0.062 GeO 2  + 0.375 H 2 O + 0.187 Ir 
                 1.50 
                 −0.565 
               
               
                 Se 
                 0.4 H 2  + 0.2 Ir 0.75 Se 0.25 O 2  → 0.025 IrSe 2  + 0.4 H 2 O + 0.125 Ir 
                 2.00 
                 −0.675 
               
               
                 Y 
                 0.3825 H 2  + 0.235 Y 0.25 Ir 0.75 O 2  → 0.059 YHO 2  + 0.353 H 2 O + 0.176 Ir 
                 1.63 
                 −0.621 
               
               
                 Zr 
                 0.375 H 2  + 0.25 Zr 0.25 Ir 0.75 O 2  → 0.375 H 2 O + 0.063 ZrO 2  + 0.188 Ir 
                 1.50 
                 −0.565 
               
               
                 Nb 
                 0.368 H 2  + 0.264 Nb 0.25 Ir 0.75 O 2  → 0.368 H 2 O + 0.005 Nb 12 O 29  + 0.198 Ir 
                 1.39 
                 −0.541 
               
               
                 Mo 
                 0.4 H 2  + 0.2 Mo 0.25 Ir 0.75 O 2  → 0.4 H 2 O + 0.05 MoIr 3   
                 2.00 
                 −0.602 
               
               
                 Ru 
                 0.4 H 2  + 0.2 Ir 0.75 Ru 0.25 O 2  → 0.05 Ir 3 Ru + 0.4 H 2 O 
                 2.00 
                 −0.631 
               
               
                 Rh 
                 0.4 H 2  + 0.2 Ir 0.75 Rh 0.25 O 2  → 0.05 Ir 3 Rh + 0.4 H 2 O 
                 2.00 
                 −0.653 
               
               
                 Pd 
                 0.4 H 2  + 0.2 Ir 0.75 Pd 0.25 O 2  → 0.4 H 2 O + 0.05 Pd + 0.15 Ir 
                 2.00 
                 −0.703 
               
               
                 Ag 
                 0.4 H 2  + 0.2 Ag 0.25 Ir 0.75 O 2  → 0.4 H 2 O + 0.05 Ag + 0.15 Ir 
                 2.00 
                 −0.743 
               
               
                 Sn 
                 0.4 H 2  + 0.2 Sn 0.25 Ir 0.75 O 2  → 0.4 H 2 O + 0.05 SnIr + 0.1 Ir 
                 2.00 
                 −0.572 
               
               
                 Sb 
                 0.2 Sb 0.25 Ir 0.75 O 2  + 0.4 H 2  → 0.025 Sb 2 Ir + 0.4 H 2 O + 0.125 Ir 
                 2.00 
                 −0.604 
               
               
                 Ba 
                 0.389 H 2  + 0.222 Ba 0.25 Ir 0.75 O 2  → 0.056 BaH 8 O 5  + 0.167 H 2 O + 0.167 Ir 
                 1.75 
                 −0.634 
               
               
                 La 
                 0.3825 H 2  + 0.235 La 0.25 Ir 0.75 O 2  → 0.059 La(HO) 3  + 0.294 H 2 O + 0.176 Ir 
                 1.63 
                 −0.615 
               
               
                 Ce 
                 0.375 H 2  + 0.25 Ce 0.25 Ir 0.75 O 2  → 0.062 CeO 2  + 0.375 H 2 O + 0.187 Ir 
                 1.50 
                 −0.565 
               
               
                 Pr 
                 0.235 Pr 0.25 Ir 0.75 O 2  + 0.3825 H 2  → 0.059 Pr(HO) 3  + 0.294 H 2 O + 0.176 Ir 
                 1.63 
                 −0.625 
               
               
                 Nd 
                 0.3825 H 2  + 0.235 Nd 0.25 Ir 0.75 O 2  → 0.059 Nd(HO) 3  + 0.294 H 2 O + 0.176 Ir 
                 1.63 
                 −0.624 
               
               
                 Sm 
                 0.3825 H 2  + 0.235 Sm 0.25 Ir 0.75 O 2  → 0.059 Sm(HO) 3  + 0.294 H 2 O + 0.176 Ir 
                 1.63 
                 −0.621 
               
               
                 Eu 
                 0.3845 H 2  + 0.231 Eu 0.25 Ir 0.75 O 2  → 0.019 Eu 3 O 4  + 0.385 H 2 O + 0.173 Ir 
                 1.66 
                 −0.607 
               
               
                 Hf 
                 0.375 H 2  + 0.25 Hf 0.25 Ir 0.75 O 2  → 0.375 H 2 O + 0.063 HfO 2  + 0.188 Ir 
                 1.50 
                 −0.565 
               
               
                 Ta 
                 0.3665 H 2  + 0.267 Ta 0.25 Ir 0.75 O 2  → 0.367 H 2 O + 0.033 Ta 2 O 5  + 0.2 Ir 
                 1.37 
                 −0.540 
               
               
                 W 
                 0.4 H 2  + 0.2 Ir 0.75 W 0.25 O 2  → 0.05 Ir 3 W + 0.4 H 2 O 
                 2.00 
                 −0.591 
               
               
                 Re 
                 0.2 Re 0.25 Ir 0.75 O 2  + 0.4 H 2  → 0.05 ReIr 3  + 0.4 H 2 O 
                 2.00 
                 −0.565 
               
               
                 Os 
                 0.4 H 2  + 0.2 Ir 0.75 Os 0.25 O 2  → 0.4 H 2 O + 0.05 Os + 0.15 Ir 
                 2.00 
                 −0.637 
               
               
                 Pt 
                 0.4 H 2  + 0.2 Ir 0.75 Pt 0.25 O 2  → 0.4 H 2 O + 0.05 Pt + 0.15 Ir 
                 2.00 
                 −0.681 
               
               
                 Au 
                 0.4 H 2  + 0.2 Ir 0.75 Au 0.25 O 2  → 0.4 H 2 O + 0.05 Au + 0.15 Ir 
                 2.00 
                 −0.737 
               
               
                 Tl 
                 0.4 H 2  + 0.2 Tl 0.25 Ir 0.75 O 2  → 0.4 H 2 O + 0.05 Tl + 0.15 Ir 
                 2.00 
                 −0.679 
               
               
                 Bi 
                 0.2 Bi 0.25 Ir 0.75 O 2  + 0.4 H 2  → 0.4 H 2 O + 0.025 Bi 2 Ir + 0.125 Ir 
                 2.00 
                 −0.623 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Chemical reactivity of Ir 0.75 M 0.25 O 2  against H 3 O 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ref. 
                 H 3 O Reaction for IrO 2   
                 Ratio 
                 E rxn, H3O   
               
               
                   
               
               
                 IrO 2   
                 0.8 H 3 O + 0.2 IrO 2  → 0.2 Ir + 1.2 H 2 O 
                 4.00 
                 −0.238 
               
               
                   
               
               
                 M 
                 H 3 O Reaction for Ir 0.75 M 0.25 O 2   
                 Ratio 
                 E rxn, H3O   
               
               
                   
               
               
                 Ca 
                 0.778 H 3 O + 0.222 Ca 0.25 Ir 0.75 O 2  → 0.056 Ca(HO) 2  + 1.111 H 2 O + 0.167 Ir 
                 3.50 
                 −0.262 
               
               
                 Ti 
                 0.75 H 3 O + 0.25 Ti 0.25 Ir 0.75 O 2  → 1.125 H 2 O + 0.063 TiO 2  + 0.187 Ir 
                 3.00 
                 −0.226 
               
               
                 Ge 
                 0.75 H 3 O + 0.25 Ge 0.25 Ir 0.75 O 2  → 0.063 GeO 2  + 1.125 H 2 O + 0.188 Ir 
                 3.00 
                 −0.226 
               
               
                 Se 
                 0.8 H 3 O + 0.2 Ir 0.75 Se 0.25 O 2  → 0.025 IrSe 2  + 1.2 H 2 O + 0.125 Ir 
                 4.00 
                 −0.249 
               
               
                 Y 
                 0.765 H 3 O + 0.235 Y 0.25 Ir 0.75 O 2  → 0.059 YHO 2  + 1.118 H 2 O + 0.176 Ir 
                 3.26 
                 −0.243 
               
               
                 Zr 
                 0.75 H 3 O + 0.25 Zr 0.25 Ir0.75O 2  → 1.125 H 2 O + 0.063 ZrO 2  + 0.187 Ir 
                 3.00 
                 −0.226 
               
               
                 Nb 
                 0.733 H 3 O + 0.267 Nb 0.25 Ir 0.75 O 2  → 0.033 Nb 2 O 5  + 1.1 H 2 O + 0.2 Ir 
                 2.75 
                 −0.222 
               
               
                 Mo 
                 0.75 H 3 O + 0.25 Mo 0.25 Ir 0.75 O 2  → 1.125 H 2 O + 0.063 MoO 2  + 0.187 Ir 
                 3.00 
                 −0.226 
               
               
                 Ru 
                 0.8 H 3 O + 0.2 Ir 0.75 Ru 0.25 O 2  → 0.05 Ir 3 Ru + 1.2 H 2 O 
                 4.00 
                 −0.233 
               
               
                 Rh 
                 0.8 H 3 O + 0.2 Ir 0.75 Rh 0.25 O 2  → 0.05 Ir 3 Rh + 1.2 H 2 O 
                 4.00 
                 −0.241 
               
               
                 Pd 
                 0.8 H 3 O + 0.2 Ir 0.75 Pd 0.25 O 2  → 1.2 H 2 O + 0.05 Pd + 0.15 Ir 
                 4.00 
                 −0.259 
               
               
                 Ag 
                 0.8 H 3 O + 0.2 Ag 0.25 Ir 0.75 O 2  → 1.2 H 2 O + 0.05 Ag + 0.15 Ir 
                 4.00 
                 −0.274 
               
               
                 Sn 
                 0.75 H 3 O + 0.25 Sn 0.25 Ir 0.75 O 2  → 0.063 SnO 2  + 1.125 H 2 O + 0.188 Ir 
                 3.00 
                 −0.226 
               
               
                 Sb 
                 0.235 Sb 0.25 Ir 0.75 O 2  + 0.765 H 3 O → 0.029 Sb 2 O 3  + 1.147 H 2 O + 0.176 Ir 
                 3.26 
                 −0.227 
               
               
                 Ba 
                 0.778 H 3 O + 0.222 Ba 0.25 Ir 0.75 O 2  → 0.056 BaH 8 O 5  + 0.944 H 2 O + 0.167 Ir 
                 3.50 
                 −0.243 
               
               
                 La 
                 0.765 H 3 O + 0.235 La 0.25 Ir 0.75 O 2  → 0.059 La(HO) 3  + 1.059 H 2 O + 0.176 Ir 
                 3.26 
                 −0.240 
               
               
                 Ce 
                 0.75 H 3 O + 0.25 Ce 0.25 Ir 0.75 O 2  → 0.063 CeO 2  + 1.125 H 2 O + 0.188 Ir 
                 3.00 
                 −0.226 
               
               
                 Pr 
                 0.235 Pr 0.25 Ir 0.75 O 2  + 0.765 H 3 O → 0.059 Pr(HO) 3  + 1.059 H 2 O + 0.176 Ir 
                 3.26 
                 −0.244 
               
               
                 Nd 
                 0.765 H 3 O + 0.235 Nd 0.25 Ir 0.75 O 2  → 0.059 Nd(HO) 3  + 1.059 H 2 O + 0.176 Ir 
                 3.26 
                 −0.244 
               
               
                 Sm 
                 0.765 H 3 O + 0.235 Sm 0.25 Ir 0.75 O 2  → 0.059 Sm(HO) 3  + 1.059 H 2 O + 0.176 Ir 
                 3.26 
                 −0.243 
               
               
                 Eu 
                 0.765 H 3 O + 0.235 Eu 0.25 Ir 0.75 O 2  → 0.029 Eu 2 O 3  + 1.147 H 2 O + 0.176 Ir 
                 3.26 
                 −0.236 
               
               
                 Hf 
                 0.75 H 3 O + 0.25 Hf 0.25 Ir 0.75 O 2  → 1.125 H 2 O + 0.063 HfO 2  + 0.187 Ir 
                 3.00 
                 −0.226 
               
               
                 Ta 
                 0.733 H 3 O + 0.267 Ta 0.25 Ir 0.75 O 2  → 1.1 H 2 O + 0.033 Ta 2 O 5  + 0.2 Ir 
                 2.75 
                 −0.222 
               
               
                 W 
                 0.8 H 3 O + 0.2 Ir 0.75 W 0.25 O 2  → 0.05 Ir 3 W + 1.2 H 2 O 
                 4.00 
                 −0.218 
               
               
                 Re 
                 0.286 Re 0.25 Ir 0.75 O 2  + 0.714 H 3 O → 0.071 ReO 3  + 1.071 H 2 O + 0.214 Ir 
                 2.50 
                 −0.217 
               
               
                 Os 
                 0.8 H 3 O + 0.2 Ir 0.75 Os 0.25 O 2  → 1.2 H 2 O + 0.05 Os + 0.15 Ir 
                 4.00 
                 −0.235 
               
               
                 Pt 
                 0.8 H 3 O + 0.2 Ir 0.75 Pt 0.25 O 2  → 1.2 H 2 O + 0.05 Pt + 0.15 Ir 
                 4.00 
                 −0.251 
               
               
                 Au 
                 0.8 H 3 O + 0.2 Ir 0.75 Au 0.25 O 2  → 1.2 H 2 O + 0.05 Au + 0.15 Ir 
                 4.00 
                 −0.272 
               
               
                 Tl 
                 0.789 H 3 O + 0.211 Tl 0.25 Ir 0.75 O 2  → 0.026 Tl 2 O + 1.184 H 2 O + 0.158 Ir 
                 3.74 
                 −0.253 
               
               
                 Bi 
                 0.235 Bi 0.25 Ir 0.75 O 2  + 0.765 H 3 O → 0.029 Bi 2 O 3  + 1.147 H 2 O + 0.176 Ir 
                 3.26 
                 −0.239 
               
               
                   
               
            
           
         
       
     
     Tables 5, 6, and 7 summarize Ir-M-O chemical reactivity with OH, OOH, and O at its most stable thermodynamic reaction between the OER catalyst and the PEM electrolyzer species. In Tables 5, 6, and 7, when molar ratio is different between the oxidizing agent and the catalyst, normalization to 2 was made. For example, the ratio between OH and IrO 2  in Table 5 is 2—i.e., 0.667 OH (or, 0.333 H 2 O 2 ) per 0.333 IrO 2 . Ir 0.75 NB 0.25 O 2  in Table 5 shows lower OH/oxide value (1.75) when compared to IrO 2 . Normalization of OH/oxide to 2 for Ir 0.75 Nb 0.25 O 2  increases the E rxn  to become a higher value. A higher OH, OOH, or O/oxide ratio indicates that an OER catalyst can take more PEM electrolyzer species per mol. In the oxidizing conditions, the goal was to identify a higher amount of OH, OOH, or O per oxide, meaning, the OER catalyst is capable of absorbing more PEM electrolyzer species per mol. It was discovered that in the oxidizing conditions, a higher OH, OOH, or O/oxide ratio and lower E rxn,H  values indicate more active OER catalyst (RuO 2 -like) and a lower OH, OOH, or O/oxide ratio and higher E rxn,H  values indicate more stable OER catalyst (IrO 2 -like). 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Chemical reactivity of Ir 0.75 M 0.25 O 2  against OH 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ref. 
                 OH Reaction for IrO 2   
                 Ratio 
                 E rxn, OH   
               
               
                   
               
               
                 IrO 2   
                 0.333 H 2 O 2  + 0.333 IrO 2  → 0.333 IrO 3  + 0.333 H 2 O 
                 2.00 
                 −0.070 
               
               
                   
               
               
                 M 
                 OH Reaction for Ir 0.75 M 0.25 O 2   
                 Ratio 
                 E rxn, OH   
               
               
                   
               
               
                 Ca 
                 0.25 H 2 O 2  + 0.5 Ca 0.25 Ir 0.75 O 2  → 0.375 IrO 3  + 0.125 Ca(HO) 2  + 0.125 H 2 O 
                 1.00 
                 −0.062 
               
               
                 Ti 
                 0.3 H 2 O 2  + 0.4 Ti 0.25 Ir 0.75 O 2  → 0.3 IrO 3  + 0.3 H 2 O + 0.1 TiO 2   
                 1.50 
                 −0.061 
               
               
                 Ge 
                 0.3 H 2 O 2  + 0.4 Ge 0.25 Ir 0.75 O 2  → 0.3 IrO 3  + 0.1 GeO 2  + 0.3 H 2 O 
                 1.50 
                 −0.061 
               
               
                 Se 
                 0.357 H 2 O 2  + 0.286 Ir 0.75 Se 0.25 O 2  → 0.071 H 10 SeO 8  + 0.214 IrO 3  + 0.036 O 2   
                 2.50 
                 −0.086 
               
               
                 Y 
                 0.278 H 2 O 2  + 0.444 Y 0.25 Ir 0.75 O 2  → 0.333 IrO 3  + 0.111 YHO 2  + 0.222 H 2 O 
                 1.25 
                 −0.055 
               
               
                 Zr 
                 0.429 H 2 O 2  + 0.571 Zr 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.429 H 2 O + 0.143 ZrO 2   
                 1.50 
                 −0.061 
               
               
                 Nb 
                 0.318 H 2 O 2  + 0.364 Nb 0.25 Ir 0.75 O 2  → 0.273 IrO 3  + 0.045 Nb 2 O 5  + 0.318 H 2 O 
                 1.75 
                 −0.100 
               
               
                 Mo 
                 0.1665 H 2 O 2  + 0.667 Mo 0.25 Ir 0.75 O 2  → 0.167 MoO 3  + 0.167 H 2 O + 0.5 IrO 2   
                 0.50 
                 −0.124 
               
               
                 Ru 
                 0.556 H 2 O 2  + 0.444 Ir 0.75 Ru 0.25 O 2  → 0.333 IrO 3  + 0.111 RuO 4  + 0.556 H 2 O 
                  2.505 
                 −0.115 
               
               
                 Rh 
                 0.3 H 2 O 2  + 0.4 Ir 0.75 Rh 0.25 O 2  → 0.3 IrO 3  + 0.3 H 2 O + 0.1 RhO 2   
                 1.50 
                 −0.061 
               
               
                 Pd 
                 0.3 H 2 O 2  + 0.4 Ir 0.75 Pd 0.25 O 2  → 0.3 IrO 3  + 0.1 PdO 2  + 0.3 H 2 O 
                 1.50 
                 −0.061 
               
               
                 Ag 
                 0.385 H 2 O 2  + 0.615 Ag 0.25 Ir 0.75 O 2  → 0.154 AgHO 2  + 0.462 IrO 3  + 0.308 H 2 O 
                 1.25 
                 −0.057 
               
               
                 Sn 
                 0.3 H 2 O 2  + 0.4 Sn 0.25 Ir 0.75 O 2  → 0.3 IrO 3  + 0.1 SnO 2  + 0.3 H 2 O 
                 1.50 
                 −0.061 
               
               
                 Sb 
                 0.364 Sb 0.25 Ir 0.75 O 2  + 0.318 H 2 O 2  → 0.273 IrO 3  + 0.045 Sb 2 O 5  + 0.318 H 2 O 
                 1.75 
                 −0.086 
               
               
                 Ba 
                 0.1665 H 2 O 2  + 0.667 Ba 0.25 Ir 0.75 O 2  → 0.167 Ba(IrO 3 ) 2  + 0.167 IrO 3  + 0.167 H 2 O 
                 0.50 
                 −0.031 
               
               
                 La 
                 0.269 H 2 O 2  + 0.462 La 0.25 Ir 0.75 O 2  → 0.308 IrO 3  + 0.038 La 3 IrO 7  + 0.269 H 2 O 
                 1.16 
                 −0.050 
               
               
                 Ce 
                 0.429 H 2 O 2  + 0.571 Ce 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 CeO 2  + 0.429 H 2 O 
                 1.50 
                 −0.061 
               
               
                 Pr 
                 0.444 Pr 0.25 Ir 0.75 O 2  + 0.278 H 2 O 2  → 0.333 IrO 3  + 0.111 Pr(HO) 3  + 0.111 H 2 O 
                 1.25 
                 −0.059 
               
               
                 Nd 
                 0.278 H 2 O 2  + 0.444 Nd 0.25 Ir 0.75 O 2  → 0.333 IrO 3  + 0.111 Nd(HO) 3  + 0.111 H 2 O 
                 1.25 
                 −0.057 
               
               
                 Sm 
                 0.269 H 2 O 2  + 0.462 Sm 0.25 Ir 0.75 O 2  → 0.308 IrO 3  + 0.038 Sm 3 IrO 7  + 0.269 H 2 O 
                 1.16 
                 −0.054 
               
               
                 Eu 
                 0.273 H 2 O 2  + 0.727 Eu 0.25 Ir 0.75 O 2  → 0.364 IrO 3  + 0.091 Eu 2 Ir 2 O 7  + 0.273 H 2 O 
                 0.75 
                 −0.041 
               
               
                 Hf 
                 0.3 H 2 O 2  + 0.4 Hf 0.25 Ir 0.75 O 2  → 0.3 IrO 3  + 0.3 H 2 O + 0.1 HfO 2   
                 1.50 
                 −0.061 
               
               
                 Ta 
                 0.318 H 2 O 2  + 0.364 Ta 0.25 Ir 0.75 O 2  → 0.273 IrO 3  + 0.318 H 2 O + 0.045 Ta 2 O 5   
                 1.75 
                 −0.100 
               
               
                 W 
                 0.1665 H 2 O 2  + 0.667 Ir 0.75 W 0.25 O 2  → 0.167 WO 3  + 0.5 IrO 2  + 0.167 H 2 O 
                 0.50 
                 −0.140 
               
               
                 Re 
                 0.571 Re 0.25 Ir 0.75 O 2  + 0.2145 H 2 O 2  → 0.143 ReH 3 O 5  + 0.429 IrO 2   
                 0.75 
                 −0.170 
               
               
                 Os 
                 0.25 H 2 O 2  + 0.5 Ir 0.75 Os 0.25 O 2  → 0.125 OsO 4  + 0.375 IrO 2  + 0.25 H 2 O 
                 1.00 
                 −0.215 
               
               
                 Pt 
                 0.3 H 2 O 2  + 0.4 Ir 0.75 Pt 0.25 O 2  → 0.3 IrO 3  + 0.1 PtO 2  + 0.3 H 2 O 
                 1.50 
                 −0.061 
               
               
                 Au 
                 0.278 H 2 O 2  + 0.444 Ir 0.75 Au 0.25 O 2  → 0.333 IrO 3  + 0.278 H 2 O + 0.056 Au 2 O 3   
                 1.25 
                 −0.056 
               
               
                 Tl 
                 0.278 H 2 O 2  + 0.444 Tl 0.25 Ir 0.75 O 2  → 0.333 IrO 3  + 0.056 Tl 2 O 3  + 0.278 H 2 O 
                 1.25 
                 −0.056 
               
               
                 Bi 
                 0.4 Bi 0.25 Ir 0.75 O 2  + 0.3 H 2 O 2  → 0.1 BiO 2  + 0.3 IrO 3  + 0.3 H 2 O 
                 1.50 
                 −0.059 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Chemical reactivity of Ir 0.75 M 0.25 O 2  against OOH 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ref. 
                 OOH Reaction for IrO 2   
                 Ratio 
                 E rxn, OOH   
               
               
                   
               
               
                 IrO 2   
                 0.4 HO 2  + 0.6 IrO 2  → 0.6 IrO 3  + 0.2 H 2 O 
                 0.67 
                 −0.032 
               
               
                 M 
                 OOH Reaction for Ir 0.75 M 0.25 O 2   
                 Ratio 
                 E rxn, OOH   
               
               
                 Ca 
                 0.333 HO 2  + 0.667 Ca 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 Ca(HO) 2  + 0.083 O 2   
                 0.50 
                 −0.032 
               
               
                 Ti 
                 0.333 HO 2  + 0.667 Ti 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 H 2 O + 0.167 TiO 2   
                 0.50 
                 −0.027 
               
               
                 Ge 
                 0.333 HO 2  + 0.667 Ge 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 GeO 2  + 0.167 H 2 O 
                 0.50 
                 −0.027 
               
               
                 Se 
                 0.4 HO 2  + 0.6 Ir 0.75 Se 0.25 O 2  → 0.05 H 4 SeO 5  + 0.45 IrO 3  + 0.1 H 2 SeO 4   
                 0.67 
                 −0.048 
               
               
                 Y 
                 0.294 HO 2  + 0.706 Y 0.25 Ir 0.75 O 2  → 0.529 IrO 3  + 0.176 YHO 2  + 0.059 H 2 O 
                 0.42 
                 −0.022 
               
               
                 Zr 
                 0.333 HO 2  + 0.667 Zr 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 H 2 O + 0.167 ZrO 2   
                 0.50 
                 −0.027 
               
               
                 Nb 
                 0.368 HO 2  + 0.632 Nb 0.25 Ir 0.75 O 2  → 0.474 IrO 3  + 0.079 Nb 2 O 5  + 0.184 H 2 O 
                 0.58 
                 −0.076 
               
               
                 Mo 
                 0.143 HO 2  + 0.857 Mo 0.25 Ir 0.75 O 2  → 0.214 MoO 3  + 0.071 H 2 O + 0.643 IrO 2   
                 0.17 
                 −0.119 
               
               
                 Ru 
                 0.25 HO 2  + 0.75 Ir 0.75 Ru 0.25 O 2  → 0.187 RuO 4  + 0.563 IrO 2  + 0.125 H 2 O 
                 0.33 
                 −0.098 
               
               
                 Rh 
                 0.333 HO 2  + 0.667 Ir 0.75 Rh 0.25 O 2  → 0.5 IrO 3  + 0.167 H 2 O + 0.167 RhO 2   
                 0.50 
                 −0.027 
               
               
                 Pd 
                 0.333 HO 2  + 0.667 Ir 0.75 Pd 0.25 O 2  → 0.5 IrO 3  + 0.167 PdO 2  + 0.167 H 2 O 
                 0.50 
                 −0.027 
               
               
                 Ag 
                 0.294 HO 2  + 0.706 Ag 0.25 Ir 0.75 O 2  → 0.176 AgHO 2  + 0.529 IrO 3  + 0.059 H 2 O 
                 0.42 
                 −0.026 
               
               
                 Sn 
                 0.333 HO 2  + 0.667 Sn 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 SnO 2  + 0.167 H 2 O 
                 0.50 
                 −0.027 
               
               
                 Sb 
                 0.632 Sb 0.25 Ir 0.75 O 2  + 0.368 HO 2  → 0.474 IrO 3  + 0.079 Sb 2 O 5  + 0.184 H 2 O 
                 0.58 
                 −0.058 
               
               
                 Ba 
                 0.143 HO 2  + 0.857 Ba 0.25 Ir 0.75 O 2  → 0.214 Ba(IrO 3 ) 2  + 0.214 IrO 3  + 0.071 H 2 O 
                 0.17 
                 −0.012 
               
               
                 La 
                 0.28 HO 2  + 0.72 La 0.25 Ir 0.75 O 2  → 0.48 IrO 3  + 0.06 La 3 IrO 7  + 0.14 H 2 O 
                 0.39 
                 −0.018 
               
               
                 Ce 
                 0.333 HO 2  + 0.667 Ce 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 CeO 2  + 0.167 H 2 O 
                 0.50 
                 −0.027 
               
               
                 Pr 
                 0.712 Pr 0.25 Ir 0.75 O 2  + 0.288 HO 2  → 0.027 Pr 3 IrO 7  + 0.507 IrO 3  + 0.096 Pr(HO) 3   
                 0.40 
                 −0.026 
               
               
                 Nd 
                 0.288 HO 2  + 0.712 Nd 0.25 Ir 0.75 O 2  → 0.027 Nd 3 IrO 7  + 0.507 IrO 3  + 0.096 Nd(HO) 3   
                 0.40 
                 −0.024 
               
               
                 Sm 
                 0.28 HO 2  + 0.72 Sm 0.25 Ir 0.75 O 2  → 0.48 IrO 3  + 0.06 Sm 3 IrO 7  + 0.14 H 2 O 
                 0.39 
                 −0.023 
               
               
                 Eu 
                 0.2 HO 2  + 0.8 Eu 0.25 Ir 0.75 O 2  → 0.4 IrO 3  + 0.1 Eu 2 Ir 2 O 7  + 0.1 H 2 O 
                 0.25 
                 −0.016 
               
               
                 Hf 
                 0.333 HO 2  + 0.667 Hf 0.25 Ir 0.75 O 2  → 0.5 IrO 3  + 0.167 H 2 O + 0.167 HfO 2   
                 0.50 
                 −0.027 
               
               
                 Ta 
                 0.368 HO 2  + 0.632 Ta 0.25 Ir 0.75 O 2  → 0.474 IrO 3  + 0.184 H 2 O + 0.079 Ta 2 O 5   
                 0.58 
                 −0.076 
               
               
                 W 
                 0.143 HO 2  + 0.857 Ir 0.75 W 0.25 O 2  → 0.214 WO 3  + 0.643 IrO 2  + 0.071 H 2 O 
                 0.17 
                 −0.136 
               
               
                 Re 
                 0.8 Re 0.25 Ir 0.75 O 2  + 0.2 HO 2  → 0.067 ReH 3 O 5  + 0.067 Re 2 O 7  + 0.6 IrO 2   
                 0.25 
                 −0.168 
               
               
                 Os 
                 0.25 HO 2  + 0.75 Ir 0.75 Os 0.25 O 2  → 0.187 OsO 4  + 0.563 IrO 2  + 0.125 H 2 O 
                 0.33 
                 −0.228 
               
               
                 Pt 
                 0.333 HO 2  + 0.667 Ir 0.75 Pt 0.25 O 2  → 0.5 IrO 3  + 0.167 PtO 2  + 0.167 H 2 O 
                 0.50 
                 −0.027 
               
               
                 Au 
                 0.294 HO 2  + 0.706 Ir 0.75 Au 0.25 O 2  → 0.529 IrO 3  + 0.147 H 2 O + 0.088 Au 2 O 3   
                 0.42 
                 −0.024 
               
               
                 Tl 
                 0.294 HO 2  + 0.706 Tl 0.25 Ir 0.75 O 2  → 0.529 IrO 3  + 0.088 Tl 2 O 3  + 0.147 H 2 O 
                 0.42 
                 −0.024 
               
               
                 Bi 
                 0.667 Bi 0.25 Ir 0.75 O 2  + 0.333 HO 2  → 0.167 BiO 2  + 0.5 IrO 3  + 0.167 H 2 O 
                 0.50 
                 −0.023 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Chemical reactivity of Ir 0.75 M 0.25 O 2  against O 2   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ref. 
                 O 2  Reaction for IrO 2   
                 Ratio 
                 E rxn, O   
               
               
                   
               
               
                 IrO 2   
                 0.25 O 2  + 0.5 IrO 2  → 0.5 IrO 3   
                 0.50 
                 −0.040 
               
               
                   
               
               
                 M 
                 O→ Reaction for Ir 0.75 M 0.25 O→ 
                 Ratio 
                 E rxn, O   
               
               
                   
               
               
                 Ca 
                 0.2 O 2  + 0.8 Ca 0.25 Ir 0.75 O 2  → 0.6 IrO 3  + 0.2 CaO 
                 0.25 
                 −0.015 
               
               
                 Ti 
                 0.2145 O 2  + 0.571 Ti 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 TiO 2   
                 0.38 
                 −0.032 
               
               
                 Ge 
                 0.2145 O 2  + 0.571 Ge 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 GeO 2   
                 0.38 
                 −0.032 
               
               
                 Se 
                 0.2335 O 2  + 0.533 Ir 0.75 Se 0.25 O 2  → 0.067 Se 2 O 5  + 0.4 IrO 3   
                 0.44 
                 −0.039 
               
               
                 Y 
                 0.238 O 2  + 0.762 Y 0.25 Ir 0.75 O 2  → 0.571 IrO 3  + 0.095 Y 2 O 3   
                 0.31 
                 −0.022 
               
               
                 Zr 
                 0.2145 O 2  + 0.571 Zr 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 ZrO 2   
                 0.38 
                 −0.032 
               
               
                 Nb 
                 0.2335 O 2  + 0.533 Nb 0.25 Ir 0.75 O 2  → 0.4 IrO 3  + 0.067 Nb 2 O 5   
                 0.44 
                 −0.093 
               
               
                 Mo 
                 0.25 O 2  + 0.5 Mo 0.25 Ir 0.75 O 2  → 0.375 IrO 3  + 0.125 MoO 3   
                 0.50 
                 −0.134 
               
               
                 Ru 
                 0.278 O 2  + 0.444 Ir 0.75 Ru 0.25 O 2  → 0.333 IrO 3  + 0.111 RuO 4   
                 0.63 
                 −0.121 
               
               
                 Rh 
                 0.2145 O 2  + 0.571 Ir 0.75 Rh 0.25 O 2  → 0.429 IrO 3  + 0.143 RhO 2   
                 0.38 
                 −0.032 
               
               
                 Pd 
                 0.2145 O 2  + 0.571 Ir 0.75 Pd 0.25 O 2  → 0.429 IrO 3  + 0.143 PdO 2   
                 0.38 
                 −0.032 
               
               
                 Ag 
                 0.184 O 2  + 0.632 Ag 0.25 Ir 0.75 O 2  → 0.053 Ag 3 O 4  + 0.474 IrO 3   
                 0.29 
                 −0.026 
               
               
                 Sn 
                 0.2145 O 2  + 0.571 Sn 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 SnO 2   
                 0.38 
                 −0.032 
               
               
                 Sb 
                 0.2335 O 2  + 0.533 Sb 0.25 Ir 0.75 O 2  → 0.4 IrO 3  + 0.067 Sb 2 O 5   
                 0.44 
                 −0.071 
               
               
                 Ba 
                 0.1 O 2  + 0.8 Ba 0.25 Ir 0.75 O 2  → 0.2 IrO 3  + 0.2 Ba(IrO 3 ) 2   
                 0.13 
                 −0.012 
               
               
                 La 
                 0.184 O 2  + 0.632 La 0.25 Ir 0.75 O 2  → 0.421 IrO 3  + 0.053 La 3 IrO 7   
                 0.29 
                 −0.020 
               
               
                 Ce 
                 0.2145 O 2  + 0.571 Ce 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 CeO 2   
                 0.38 
                 −0.032 
               
               
                 Pr 
                 0.184 O 2  + 0.632 Pr 0.25 Ir 0.75 O 2  → 0.421 IrO 3  + 0.053 Pr 3 IrO 7   
                 0.29 
                 −0.026 
               
               
                 Nd 
                 0.184 O 2  + 0.632 Nd 0.25 Ir 0.75 O 2  → 0.421 IrO 3  + 0.053 Nd 3 IrO 7   
                 0.29 
                 −0.026 
               
               
                 Sm 
                 0.184 O 2  + 0.632 Sm 0.25 Ir 0.75 O 2  → 0.421 IrO 3  + 0.053 Sm 3 IrO 7   
                 0.29 
                 −0.026 
               
               
                 Eu 
                 0.1365 O 2  + 0.727 Eu 0.25 Ir 0.75 O 2  → 0.364 IrO 3  + 0.091 Eu 2 Ir 2 O 7   
                 0.19 
                 −0.018 
               
               
                 Hf 
                 0.2145 O 2  + 0.571 Hf 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 HfO 2   
                 0.38 
                 −0.032 
               
               
                 Ta 
                 0.2335 O 2  + 0.533 Ta 0.25 Ir 0.75 O 2  → 0.4 IrO 3  + 0.067 Ta 2 O 5   
                 0.44 
                 −0.093 
               
               
                 W 
                 0.25 O 2  + 0.5 Ir 0.75 W 0.25 O 2  → 0.375 IrO 3  + 0.125 WO 3   
                 0.50 
                 −0.149 
               
               
                 Re 
                 0.1365 O 2  + 0.727 Re 0.25 Ir 0.75 O 2  → 0.091 Re 2 O 7  + 0.545 IrO 2   
                 0.19 
                 −0.185 
               
               
                 Os 
                 0.1665 O 2  + 0.667 Ir 0.75 Os 0.25 O 2  → 0.167 OsO 4  + 0.5 IrO 2   
                 0.25 
                 −0.260 
               
               
                 Pt 
                 0.2145 O 2  + 0.571 Ir 0.75 Pt 0.25 O 2  → 0.429 IrO 3  + 0.143 PtO 2   
                 0.38 
                 −0.032 
               
               
                 Au 
                 0.1925 O 2  + 0.615 Ir 0.75 Au 0.25 O 2  → 0.462 IrO 3  + 0.077 Au 2 O 3   
                 0.31 
                 −0.028 
               
               
                 Tl 
                 0.1925 O 2  + 0.615 Tl 0.25 Ir 0.75 O 2  → 0.077 Tl 2 O 3  + 0.462 IrO 3   
                 0.31 
                 −0.028 
               
               
                 Bi 
                 0.2145 O 2  + 0.571 Bi 0.25 Ir 0.75 O 2  → 0.429 IrO 3  + 0.143 BiO 2   
                 0.38 
                 −0.028 
               
               
                   
               
            
           
         
       
     
     To study the CO poisoning or corrosion, chemical reactivity was studied against CO. CO reactions follow the same trend as reducing conditions i.e., H and H 3 O reactions above. 
     
       
         
           
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Chemical reactivity of Ir 0.75 M 0.25 O 2  against CO 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ref. 
                 CO Reaction 
                 Ratio 
                 E rxn, CO   
               
               
                   
               
               
                 IrO 2   
                 0.333 IrO 2  + 0.667 CO → 0.667 CO 2  + 0.333 Ir 
                 2.00 
                 −0.532 
               
               
                   
               
               
                 M 
                 CO Reaction 
                 Ratio 
                 E rxn, CO   
               
               
                   
               
               
                 Ca 
                 0.364 Ca 0.25 Ir 0.75 O 2  + 0.636 CO → 0.545 CO 2  + 0.091 CaCO 3  + 0.273 Ir 
                 1.75 
                 −0.622 
               
               
                 Ti 
                 0.4 Ti 0.25 Ir 0.75 O 2  + 0.6 CO → 0.6 CO 2  + 0.1 TiO 2  + 0.3 Ir 
                 1.50 
                 −0.465 
               
               
                 Ge 
                 0.4 Ge 0.25 Ir 0.75 O 2  + 0.6 CO → 0.6 CO 2  + 0.1 GeO 2  + 0.3 Ir 
                 1.50 
                 −0.465 
               
               
                 Se 
                 0.667 CO + 0.333 Ir 0.75 Se 0.25 O 2  → 0.667 CO 2  + 0.208 Ir + 0.042 IrSe 2   
                 2.00 
                 −0.561 
               
               
                 Y 
                 0.381 Y 0.25 Ir 0.75 O 2  + 0.619 CO → 0.619 CO 2  + 0.048 Y 2 O 3  + 0.286 Ir 
                 1.62 
                 −0.515 
               
               
                 Zr 
                 0.4 Zr 0.25 Ir 0.75 O 2  + 0.6 CO → 0.6 CO 2  + 0.1 ZrO 2  + 0.3 Ir 
                 1.50 
                 −0.465 
               
               
                 Nb 
                 0.421 Nb 0.25 Ir 0.75 O 2  + 0.579 CO → 0.053 Nb 2 O 5  + 0.579 CO 2  + 0.316 Ir 
                 1.38 
                 −0.445 
               
               
                 Mo 
                 0.667 CO + 0.333 Mo 0.25 Ir 0.75 O 2  → 0.667 CO 2  + 0.083 MoIr 3   
                 2.00 
                 −0.488 
               
               
                 Ru 
                 0.333 Ir 0.75 Ru 0.25 O 2  + 0.667 CO → 0.083 Ir 3 Ru + 0.667 CO 2   
                 2.00 
                 −0.517 
               
               
                 Rh 
                 0.333 Ir 0.75 Rh 0.25 O 2  + 0.667 CO → 0.083 Ir 3 Rh + 0.667 CO 2   
                 2.00 
                 −0.539 
               
               
                 Pd 
                 0.333 Ir 0.75 Pd 0.25 O 2  + 0.667 CO → 0.667 CO 2  + 0.25 Ir + 0.083 Pd 
                 2.00 
                 −0.589 
               
               
                 Ag 
                 0.333 Ag 0.25 Ir 0.75 O 2  + 0.667 CO → 0.667 CO 2  + 0.083 Ag + 0.25 Ir 
                 2.00 
                 −0.629 
               
               
                 Sn 
                 0.6 CO + 0.4 Sn 0.25 Ir 0.75 O 2  → 0.6 CO 2  + 0.1 SnO 2  + 0.3 Ir 
                 1.50 
                 −0.465 
               
               
                 Sb 
                 0.333 Sb 0.25 Ir 0.75 O 2  + 0.667 CO → 0.042 Sb 2 Ir + 0.667 CO 2  + 0.208 Ir 
                 2.00 
                 −0.490 
               
               
                 Ba 
                 0.364 Ba 0.25 Ir0.75O 2  + 0.636 CO → 0.545 CO 2  + 0.091 BaCO 3  + 0.273 Ir 
                 1.75 
                 −0.585 
               
               
                 La 
                 0.381 La 0.25 Ir 0.75 O 2  + 0.619 CO → 0.048 La 2 CO 5  + 0.571 CO 2  + 0.286 Ir 
                 1.62 
                 −0.534 
               
               
                 Ce 
                 0.6 CO + 0.4 Ce 0.25 Ir 0.75 O 2  → 0.1 CeO 2  + 0.6 CO 2  + 0.3 Ir 
                 1.50 
                 −0.465 
               
               
                 Pr 
                 0.381 Pr 0.25 Ir 0.75 O 2  + 0.619 CO → 0.619 CoO + 0.048 Pr 2 O 3  + 0.286 Ir 
                 1.62 
                 −0.355 
               
               
                 Nd 
                 0.381 Nd 0.25 Ir 0.75 O 2  + 0.619 CO → 0.619 CO 2  + 0.048 Nd 2 O 3  + 0.286 Ir 
                 1.62 
                 −0.510 
               
               
                 Sm 
                 0.381 Sm 0.25 Ir 0.75 O 2  + 0.619 CO → 0.619 CO 2  + 0.048 Sm 2 O 3  + 0.286 Ir 
                 1.62 
                 −0.511 
               
               
                 Eu 
                 0.364 Eu 0.25 Ir 0.75 O 2  + 0.636 CO → 0.545 CO 2  + 0.091 EuCO 3  + 0.273 Ir 
                 1.75 
                 −0.548 
               
               
                 Hf 
                 0.4 Hf 0.25 Ir 0.75 O 2  + 0.6 CO → 0.6 CO 2  + 0.1 HfO 2  + 0.3 Ir 
                 1.50 
                 −0.465 
               
               
                 Ta 
                 0.421 Ta 0.25 Ir 0.75 O 2  + 0.579 CO → 0.579 CO 2  + 0.053 Ta 2 O 5  + 0.316 Ir 
                 1.38 
                 −0.445 
               
               
                 W 
                 0.667 CO + 0.333 Ir 0.75 W 0.25 O 2  → 0.667 CO 2  + 0.083 Ir 3 W 
                 0.50 
                 −0.477 
               
               
                 Re 
                 0.333 Re 0.25 Ir 0.75 O 2  + 0.667 CO → 0.667 CO 2  + 0.083 ReIr 3   
                 2.00 
                 −0.451 
               
               
                 Os 
                 0.333 Ir 0.75 Os 0.25 O 2  + 0.667 CO → 0.667 CO 2  + 0.25 Ir + 0.083 Os 
                 2.00 
                 −0.523 
               
               
                 Pt 
                 0.333 Ir 0.75 Pt 0.25 O 2  + 0.667 CO → 0.667 CO 2  + 0.25 Ir + 0.083 Pt 
                 2.00 
                 −0.567 
               
               
                 Au 
                 0.333 Ir 0.75 Au 0.25 O 2  + 0.667 CO → 0.667 CO 2  + 0.25 Ir + 0.083 Au 
                 2.00 
                 −0.623 
               
               
                 Tl 
                 0.348 Tl 0.25 Ir 0.75 O 2  + 0.652 CO → 0.043 Tl 2 CO 3  + 0.609 CO 2  + 0.261 Ir 
                 1.87 
                 −0.585 
               
               
                 Bi 
                 0.381 Bi 0.25 Ir 0.75 O 2  + 0.619 CO → 0.571 CO 2  + 0.048 Bi 2 CO 5  + 0.286 Ir 
                 1.62 
                 −0.523 
               
               
                   
               
            
           
         
       
     
     The results of the (a) thermodynamic decomposition analysis (i.e., tetragonal vs. non-tetragonal phase decomposition) and (b) data from the Tables 3-8 (chemical reactions against H, H 3 O, OH, OOH, O, and CO) are shown in  FIG.  5    depicting three categories by functionality—species that enhance 1) stability (“Stable Catalyst”), 2) activity (“Active Catalyst”), or are expected to have 3) similar behavior as pure IrO 2  (“Similar to IrO 2 ”). As can be observed from  FIG.  5   , the activity and/or stability of the OER catalyst and/or PEMFC electrode may be tuned by adding and/or replacing Ir- or Ru-based traditional materials with more economical, suitable, and attainable species. The discovery thus has a potential of saving cost, improving performance, durability, sustainability, and increasing production quantities feasibility as well as enabling large scale manufacture of the MEA, OER catalyst, and/or PEMFC electrode having at least comparable characteristics as a traditional IrO 2  OER catalyst. Tables 9-11 below further summarize the stability-enhancing and activity-enhancing ternary oxide species disclosed herein, focusing on known or unknown acid stability and practicality due to availability and lower cost of the herein-disclosed oxide species. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Stability enhancing ternary oxide species 
               
            
           
           
               
               
               
            
               
                   
                 Tiers for stability enhancing 
                   
               
               
                   
                 ternary oxide species 
                 M in Ir 0.75 M 0.25 O 2   
               
               
                   
                   
               
               
                   
                 S-tier 1: improves stability 
                 Bi 
               
               
                   
                 and contains practical element 
               
               
                   
                 S-tier 2: improves stability, 
                 Y, La, Pr, Nd, Sm, Eu 
               
               
                   
                 but unknown acid stability 
               
               
                   
                 S-tier 3: improves stability, 
                 Ag, Hf; (Ba) 
               
               
                   
                 but slightly expensive element 
               
               
                   
                 (and, unknown acid stability) 
               
               
                   
                 S-tier 4: improves stability, 
                 Rh, Pd, Pt, Au, Tl 
               
               
                   
                 but more expensive than Ru 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Activity enhancing ternary oxide species 
               
            
           
           
               
               
               
            
               
                   
                 Tiers for activity enhancing 
                   
               
               
                   
                 ternary oxide species 
                 M in Ir 0.75 M 0.25 O 2   
               
               
                   
                   
               
               
                   
                 A-tier 1: improves activity 
                 Nb, Mo, Ta, W 
               
               
                   
                 and contains practical element 
               
               
                   
                 A-tier 2: improves activity, 
                 Re, Ru, Os 
               
               
                   
                 but expensive 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Ternary oxide species with similar performance as IrO 2   
               
            
           
           
               
               
               
            
               
                   
                 Tiers for cost saving 
                   
               
               
                   
                 ternary oxide species 
                 M in Ir 0.75 M 0.25 O 2   
               
               
                   
                   
               
               
                   
                 C-tier 1: no disadvantage 
                 Ti, Se, Zr, Sn, Sb, Ce 
               
               
                   
                 (similar to IrO 2  in FIG. 2) 
               
               
                   
                 C-tier 2: unknown acid 
                 Ca, Ge 
               
               
                   
                 stability, or slightly expensive 
               
               
                   
                   
               
            
           
         
       
     
     Similar screening process and analysis were repeated for an increased concentration of M and reduced amount of Ir for the S-tier 1, S-tier 2, A-tier 1, and C-tier 1 species from Tables 9-11: Ir 0.5 M 0.5 O 2  and Ir 0.25 M 0.75 O 2 , respectively. 
     From thermodynamic decomposition analysis, it was found that Ca and Y formed CaO and Y 2 O 3  unstable in the acidic condition. In addition, Eu was eliminated from further screening due to O 2  gas release during thermodynamic decomposition.  FIG.  6    and Table 12 show results of the analysis for 15 elements for Ir 0.5 M 0.5 O 2 , where M=Ti, Se, Zr, Nb, Mo, Sn, Sb, La, Ce, Pr, Nd, Sm, Ta, W, and Bi.  FIG.  6    depicts three categories by functionality—species that enhance 1) stability, 2) activity, or are expected to 3) have similar behavior as pure IrO 2 . 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Assignment of different tiers for OER catalysts 
               
               
                 leading to stability/activity enhancement, or cost 
               
               
                 saving with metal substitution in Iro.5Mo.5O2 
               
            
           
           
               
               
            
               
                   
                 M in Ir 0.5 M 0.5 O 2   
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Tiers for stability enhancing OER catalyst 
                   
               
               
                   
                 S-tier1: improves stability and includes 
                 Bi 
               
               
                   
                 practical element 
               
               
                   
                 S-tier2: improves stability, but 
                 La, Nd, Sm 
               
               
                   
                 acid stability unknown 
               
               
                   
                 Tiers for activity enhancing OER catalyst 
               
               
                   
                 A-tier1: improves activity and 
                 Ti, Se, Zr, Sb, 
               
               
                   
                 includes practical element 
                 Ce, Ta, W, Nb 
               
               
                   
                 A-tier2: improves activity, but 
                 Mo 
               
               
                   
                 might be too active (less stable) 
               
               
                   
                 Tiers for cost saving OER catalyst 
               
               
                   
                 C-tier1: no disadvantage (similar to IrO 2 ) 
                 Sn 
               
               
                   
                 C-tier2: unknown acid stability 
                 Pr 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  7    and Table 13 show results of the analysis for 10 elements (1 st -tier Ir 0.5 M 0.5 O 2  oxide species) for Ir 0.25 M 0.75 O 2 , where M=Bi, Ti, Se, Zr, Sb, Ce, Ta, W, Nb, and Sn.  FIG.  7    depicts three categories by functionality—species that enhance 1) stability, 2) activity, or 3) are expected to have similar behavior as pure IrO 2 . As can be seen in  FIG.  7    and Table 13, in reduced Ir concentration, Bi substitution may lead to stability while adding other elements shifts the species to become more active. Yet, when activity is too high, it is likely that the material will become less stable. 
     The described research revealed the overall capabilities of the following studied species, captured in Table 13. 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Ir 0.75 M 0.25 O 2  species summary 
               
            
           
           
               
               
            
               
                   
                 M in Ir 0.75 M 0.25 O 2   
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Tiers for stability enhancing OER catalyst 
                   
               
               
                   
                 S-tier1: improves stability and includes 
                 Bi 
               
               
                   
                 practical element 
               
               
                   
                 Tiers for activity enhancing OER catalyst 
               
               
                   
                 A-tier1: improves activity and includes 
                 Se, Sn, Sb, Ce 
               
               
                   
                 practical element 
               
               
                   
                 A-tier2: improves activity, but might be 
                 Ti, Zr, Ta, W, Nb 
               
               
                   
                 too active (less stable) 
               
               
                   
                   
               
            
           
         
       
     
     Based on the findings summarized in Table 13, various compositions of Ir—Bi-M-O material, where M=Se, Sn, Sb, and Ce were analyzed. The quaternary system may supply the stability-increasing Bi in combination with an activity-enhancing element and cost savings due to the use of less expensive elements than Ir. Comparison of Ir 0.33 Bi 0.33 M 0.33 O 2  composition with Ir 0.25 Bi 0.25 M 0.5 O 2  shows that as concentration of Se, Sn, Sb, and Ce increases, the material becomes more active. When the amount of Bi increases in Ir 0.33 Bi 0.33 M 0.33 O 2  to Ir 0.25 Bi 0.5 M 0.25 O 2 , the corresponding material is predicted to become more stable. The results are summarized in Table 14 below. 
     In the plot of  FIG.  7   , a stable catalyst is one having relative shift &gt;=110% and an active catalyst as having relative shift &lt;=90%. There is a strong correlation between the x and y axes since the axes are not independent. The x axis is a summation of the penalty points (PP) by weight and the y axis is a % difference between the sum of PP for IrO 2  vs. Ir 0.75 M 0.25 O 2 . The sum of PP includes PP for chemical reactions against H, H 3 O, OH, OOH, O, and CO, and thermodynamic decomposition. 
     
       
         
           
               
             
               
                 TABLE 14 
               
             
            
               
                   
               
               
                 Quaternary Ir—Bi—M—O compositions tested in comparison with 
               
               
                 pure IrO 2  as an OER catalyst in a PEM electrolyzer application 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Relative shift 
                 Advantages 
               
               
                   
                 Composition tested 
                 vs. IrO 2  (%) 
                 vs. pure IrO 2   
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Ir 0.33 Bi 0.33 Se 0.33 O 2   
                 127.9 
                 Stability, Cost 
               
               
                   
                 Ir 0.33 Bi 0.33 Sn 0.33 O 2   
                 108.6 
                 Cost 
               
               
                   
                 Ir 0.33 Bi 0.33 Sb 0.33 O 2   
                 121.2 
                 Stability, Cost 
               
               
                   
                 Ir 0.33 Bi 0.33 Ce 0.33 O 2   
                 102.5 
                 Cost 
               
               
                   
                 Ir 0.25 Bi 0.25 Se 0.5 O 2   
                 107.5 
                 Cost 
               
               
                   
                 Ir 0.25 Bi 0.25 Sn 0.5 O 2   
                 89.1 
                 Activity, Cost 
               
               
                   
                 Ir 0.25 Bi 0.25 Sb 0.5 O 2   
                 91.0 
                 Cost 
               
               
                   
                 Ir 0.25 Bi 0.25 Ce 0.5 O 2   
                 82.2 
                 Activity, Cost 
               
               
                   
                 Ir 0.25 Bi 0.5 Se 0.25 O 2   
                 129.3 
                 Stability, Cost 
               
               
                   
                 Ir 0.25 Bi 0.5 Sn 0.25 O 2   
                 110.5 
                 Stability, Cost 
               
               
                   
                 Ir 0.25 Bi 0.5 Sb 0.25 O 2   
                 124.7 
                 Stability, Cost 
               
               
                   
                 Ir 0.25 Bi 0.5 Ce 0.25 O 2   
                 106.6 
                 Cost 
               
               
                   
                   
               
            
           
         
       
     
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications.