Activated chemical bonds in nanoporous and amorphous iridium oxides favor low overpotential for oxygen evolution reaction

Abstract

To date, the search for active, selective, and stable electrocatalysts for the oxygen evolution reaction (OER) has not ceased and a detailed atomic-level design of the OER catalyst remains an outstanding (if not, compelling) problem. Considerable studies on different surfaces and polymorphs of iridium oxides (with varying stoichiometries and dopants) have emerged over the years, showing much higher OER activity than the conventionally reported rutile-type IrO2. Here, we have considered different metastable nanoporous and amorphous iridium oxides of different chemical stoichiometries. Using first-principles electronic structure calculations, we investigate the (electro)chemical stability, intercalation properties, and electronic structure of these iridium oxides. Using an empirical regression model between the Ir-O bond characteristics and the measured OER overpotentials, we demonstrate how activated Ir-O bonds (and the presence of more electrophilic oxygens) in these less understood polymorphs of iridium oxides can explain their superior OER performance observed in experiments.

Green synthesis of water splitting electrocatalysts: IrO2 nanocages via Pearson's chemistry

“…Even if their exact role is still poorly understood, the presence of a high number of surface metal atoms and the structure of nanoporous and amorphous/small-crystallites containing iridium oxides greatly improve the flexibility of the Ir charge states, promote the presence of electrophilic oxygens when compared to their crystalline counterparts and lead to high electrochemical activity towards the OER. 27,46…”

“…However, for such purpose, the calcination temperature to convert the Ir precursor into IrO 2 is of high importance since it is well known that amorphous iridium oxide materials or materials with very small crystallite size show higher electrochemical activity than their wellcrystallized counterparts. 27,[44][45][46] The optimal calcination temperature usually ranges between 400 C and 500 C, where the crystallisation of rutile IrO 2 occurs. Lower calcination temperature leads to unstable materials and higher one to larger crystals and lower activity.…”

Iridium-based Electrocatalysts for Acidic Oxygen Evolution Reaction: Engineering Strategies to Enhance the Activity and Stability

“…Reproduced with permission. 87 Copyright 2022, Springer Nature. Operando X-ray absorption spectroscopy.…”

“…20 Copyright 2017, American Chemical Society. (b) Averaged projected crystal orbital Hamilton population (-pCOHP) between the Ir and O atoms in R-IrO2 and Ho-IrO2.Reproduced with permission 87. Copyright 2022, Springer Nature.…”

Cation Transport Effect on Nickel Iron Oxyhydroxide Electrodes in the Oxygen Evolution Reaction

“…Current studies on the electrolysis of water are focused on optimizing the reaction kinetics of the anode by developing multiple types of OER catalysts. , Despite much effort, however, the OER still suffers from large overpotential due to the insufficient electron and mass transfer on the surface of the electrocatalyst . The rational optimization of the anodic electrochemical process is a high-priority goal in the current technique, whereas it is still challenging. , …”