Operando Evidence for a Universal Oxygen Evolution Mechanism on Thermal and Electrochemical Iridium Oxides

Abstract

Progress in the development of proton exchange membrane (PEM) water electrolysis technology requires decreasing the anode overpotential, where the sluggish multistep oxygen evolution reaction (OER) occurs. This calls for an understanding of the nature of the active OER sites and reaction intermediates, which are still being debated. In this work, we apply synchrotron radiation-based near-ambient pressure X-ray photoelectron and absorption spectroscopies under operando conditions in order to unveil the nature of the reaction intermediates and shed light on the OER mechanism on electrocatalysts most widely used in PEM electrolyzers-electrochemical and thermal iridium oxides. Analysis of the O K-edge and Ir 4f spectra backed by density functional calculations reveals a universal oxygen anion red-ox mechanism regardless of the nature (electrochemical or thermal) of the iridium oxide. The formation of molecular oxygen is considered to occur through a chemical step from the electrophilic O species, which itself is formed in an electrochemical step.

Metallic Iridium Thin-Films as Model Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)—Morphology and Activity

“…The O 1s spectra in Figure 5b of the Ac catalysts prior to and after OER testing revealed three different oxygen species, which can be assigned to adsorbed water (~533.1 eV), lattice oxygen (~530.1 eV), and hydroxyl groups (~531.5 eV). All samples are distinguished by a high surface hydroxyl group and very small lattice oxygen contents, typical for electrochemically grown iridium oxide [20]. The O 1s XP spectra indicate that the oxide layers in the electrochemically treated samples are largely amorphous, which explains the absence of any crystalline IrO x phase in the SAED of these samples, as discussed earlier.…”

Pt Dopant: Controlling the Ir Oxidation States toward Efficient and Durable Oxygen Evolution Reaction in Acidic Media

“…Since the amorphous Ir III species contain electronic defects in its cationic (Ir III ) and anionic frameworks (O I− ), it can promote the formation of OO bond with lower kinetic barriers via acid–base mechanism than rutile IrO 2 (Ir IV ), thereby improving the OER activity. [ 8,41,42,45–48 ] The relative percentages of Ir III for a ‐PN‐IPN frame/C and a ‐IPN cage/C were 3.3 and 2.7 times than that of the a ‐PN‐IN frame/C, respectively. However, it should be noted that the absolute number of surface Ir III sites of the latter is bigger due to the much larger surface IrO x area as demonstrated by CO stripping, resulting in better OER activity (Figure S15, Supporting Information).…”

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. [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] electrodeposited IrO x [14] or more recently Ni-substituted IrO x . [15][16][17] Recently,Ir-based perovskites have been reported as promising candidates in acidic media.…”