Discovery of crystal structure–stability correlation in iridates for oxygen evolution electrocatalysis in acid

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Abstract

We provide general descriptions regarding the structure–stability correlations of iridium-based complex-oxide catalysts for oxygen evolution reaction in acid media.

Abstract

One of the critical bottlenecks in many energy-conversion devices associated with the oxygen evolution reaction (OER) is to find adequate catalysts that both show high activity and endure extreme pH conditions of an electrolyte under anodic potentials. As Ir-based compositions are regarded to show better resistance to acidic environments than any other transition metals, a surge of research on many complex iridates recently has been carried out. However, the intrinsic structure–property relationship in iridates remains elusive, although several examples showing exceptional OER activity were reported. Here we discover that eleven different A _xIr _yO _z-type oxides (A = Ca, Sr, Ba, Y, Pr, Nd) with high OER activity can be categorized into three distinct groups in terms of the [IrO ₆] connection geometry. Furthermore, a notable common correlation among the crystal structure, activity variation, and stability during the OER is identified in each of the groups. If the iridates consist of a strong edge- or face-sharing [IrO ₆] configuration, their stability and activity are preserved during a remarkably large number of anodic cycles. In addition to comparing the OER performance between many high-activity iridates, our findings emphasize that the [IrO ₆] connectivity is a crucial structural factor governing the overall longevity of Ir-based catalysts.

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Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction.

Charles C L McCrory, Suho Jung, Jonas C Peters … (2013)

Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm(-2) per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can determine electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alkaline solutions: CoO(x), CoPi, CoFeO(x), NiO(x), NiCeO(x), NiCoO(x), NiCuO(x), NiFeO(x), and NiLaO(x). The oxygen-evolving activity of an electrodeposited IrO(x) catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alkaline solution, every non-noble metal system achieved 10 mA cm(-2) current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrO(x) was unstable under oxidative conditions in acidic solutions.