Developing a fundamental understanding of oxygen evolution reaction (OER) is essential to advance state-of-the-art energy conversion and storage technologies such as electrolysis. Different from acidic conditions, non-noble metal catalysts, such as Ni and Co based oxides, can be used for the OER in alkaline media to reduce cost significantly¹. As a heterogeneous reaction, OER occurs at the interface formed between the catalyst layer surface and adjacent liquid electrolyte. The interaction of adsorbed OH^- ions with the charged catalyst interface forms an electronic double layer (EDL) for charge storage and transfer. In principle, the EDL microstructure and chemical state governed by the interfacial electric potential has a significant influence on electrocatalytic performance of OER². EDL thickness is determined mostly by the interactive force between catalyst and ionic species and is generally a few hundred angstroms. Due to its reaction nature and extreme thickness, real time in-situ visualization of the EDL microstructure and electrochemical behaviors is not possible. Therefore, developing effective electrochemical diagnostic tools is imperative to analyze EDL performance.

This current work is focused on designing electrochemical experiments to characterize various interfacial behaviors during alkaline OER. It is found that the catalyst activity measured by traditional linear sweep voltammetry method depends strongly on the applied potential scan rate. While noble-metal oxide IrO₂ shows enhanced activity as scan rate increases, opposite behavior is observed for non-noble transition-metal oxide, such as NiCo₂O₄. Further characterizations by thin-film rotating ring disk electrode technique reveal that this reversed trend can be ascribed to their intrinsic structural and chemical features, as well as drastic reconstruction of EDL microstructure during OER. Compared with IrO₂, NiCo₂O₄ catalyst shows higher resistances of OH^-, O₂ and electron transport, capacitive and pseudocapacitive charging process, and electrocatalytic OER. These combined resistances of NiCo₂O₄ catalyst prolong the time required to reach steady state OER performance in response to potential perturbation. Furthermore, results from chronoamperometry demonstrate that the continuous EDL reconstruction of NiCo₂O₄ is favorable for OH^- adsorption and OER. In contrast, continuous EDL reconstruction of IrO₂ is harmful for OER due to Ir dissolution and Ir⁵⁺ formation. Based on the collective electrochemical observations, three new insights are gained: (1) how to reliably characterize OER catalyst performance, (2) how to evaluate EDL structure and its interfacial behaviours, and (3) how to demonstrate EDL modification during continuous electrochemical operation.

References

1. Varcoe JR, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 7, 3135-3191 (2014).
2. Li G, Anderson L, Chen Y, Pan M, Chuang P. A. New insights into evaluating catalyst activity and stability of oxygen evolution reactions in alkaline media. Sustainable Energy & Fuels, (2017), DOI:10.1039/C7SE00337D