Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction

TLDR

Objective evaluation of electrocatalyst activity for water oxidation is essential for energy conversion technologies, yet current assessment methods lack standardization, hindering comparison of activity and stability. The authors present a standardized protocol to evaluate the activity, stability, and Faradaic efficiency of electrodeposited oxygen‑evolving electrocatalysts. The protocol determines electrochemically active surface area, measures overpotential at 10 mA cm⁻² as the primary figure of merit, calculates turnover frequencies, and is applied to a range of metal‑oxide systems in acidic and alkaline media, including IrOₓ for comparison. In alkaline solution, all non‑noble metal catalysts reach 10 mA cm⁻² at overpotentials between 0.35 and 0.43 V, whereas in acidic media every system except IrOₓ is unstable under oxidative conditions.

Abstract

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.