The stability number as a metric for electrocatalyst stability benchmarking

Abstract

Reducing noble metal loading and increasing specific activity of oxygen evolution catalysts are omnipresent challenges in proton exchange membrane (PEM) water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements (e.g. perovskites, IrNiO x , etc.). However, proper verification of the stability of these materials is still pending. In this work dissolution processes of various iridium-based oxides are explored by introducing a new metric, defined as the ratio between amount of evolved oxygen and dissolved iridium. The so called Stability-number is independent of loading, surface area or involved active sites and thus, provides a reasonable comparison of diverse materials with respect to stability. Furthermore it can support the clarification of dissolution mechanisms and the estimation of a catalyst's lifetime. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to formation of highly active amorphous iridium oxide, the instability of which is explained by participation of activated oxygen atoms, generating short-lived vacancies that favour dissolution. These insights are considered to guide further research which should be devoted to increasing utilization of pure crystalline iridium oxide, as it is the only known structure that guarantees a high durability in acidic conditions. In case amorphous iridium oxides are used, solutions for stabilization are needed.

A Dissolution/Precipitation Equilibrium on the Surface of Iridium‐Based Perovskites Controls Their Activity as Oxygen Evolution Reaction Catalysts in Acidic Media

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Recently,I r V -based perovskite-like materials were proposed as oxygen evolution reaction (OER) catalysts in acidic media with promising performance.H owever,i ridium dissolution and surface reconstruction were observed, questioning the real active sites on the surface of these catalysts.In this work, Sr 2 MIr (V) O 6 (M = Fe,Co) and Sr 2 Fe 0.5 Ir 0.5 (V) O 4 were explored as OER catalysts in acidic media. [7][8][9][10][11] Thef ew materials established as suitable OER catalysts in acidic media are mostly Ir-based metal oxides,such as rutile IrO 2 , [12,13] 14] or more recently Ni-substituted IrO x . 4H + + O 2 + 4e À ).

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“…[1,2] While alarge variety of transition metal oxides have been studied as promising OER catalysts in alkaline media, [3][4][5][6] thedesign of active and stable catalysts in acidic media has proven challenging. [11,20,21] However,dissolution of iridium, alkali and/or rare earth elements in acidic electrolytes has been observed after close examinations of some Ir V -based perovskites,such as La 2 LiIr (V) O 6[20] and Ba 2 PrIr (V) O 6 , [7,19] indicating their drastic structural instabilities in harsh acidic conditions. [15][16][17] Recently,Ir-based perovskites have been reported as promising candidates in acidic media.…”

“…[18][19][20] Thehigh OER activity of these Ir-based perovskites was ascribed to the formation of electrophilic O (IÀ) surface species favoring the nucleophilic attack of water. [7,19] Coupling electrochemical measurements with iridium dissolution studies,w ep ropose au biquitous dissolution-electrodeposition/precipitation mechanism for the origin of OER activities measured for these perovskites.Structural and electronic properties of Sr 2 FeIr (V) O 6 , Sr 2 Fe 0.5 Ir 0.5 (V) O 4 ,a nd Sr 2 CoIr (V) O 6 are shown in the Supporting Information (Figures S1-S4) and their catalytic behaviors in Figures 1a-c. Sr 2 CoIr (V) O 6 exhibits an enhanced OER activity compared to commercial micron-size IrO 2 (ca. [12] Furthermore,the presence of IrO 2 nanoparticles was revealed on the surface of La 2 LiIr (V) O 6 after cycling.…”

“…[20] and Ba 2 PrIr (V) O 6 , [7,19] indicating their drastic structural instabilities in harsh acidic conditions. [12] Furthermore,the presence of IrO 2 nanoparticles was revealed on the surface of La 2 LiIr (V) O 6 after cycling.…”

Screening Bifunctional Pt Based NSTF Catalysts for Durability with the Rotating Disk Electrode: The Effect of Ir and Ru

“…This concept has been experimentally explored and proposed as a durability performance metric. 36,37 Considering the nature of the AST- Figure 3 shows that when a constant current is applied to the electrode under study, there is a period before a steady potential is reached during which a portion of the current goes to charging the electrode's pseudocapacitance. Therefore another proportionality constant (μ) can be introduced:…”