The ladder towards understanding the oxygen evolution reaction

Abstract

Understanding the atomic-scale mechanistic details of the oxygen evolution reaction (OER) remains an unresolved challenge in electrochemistry owing to the complexity of the OER. In this short review we discuss how, with the advent of new experimental and computational methodologies, the OER can be treated with increasingly sophisticated models to aid in our complete understanding. For the case of steady state catalyst surfaces, we define a six-rung ladder of complexity to frame how far this understanding reaches and in which aspects our understanding could still improve.

Introduction

Solid liquid interfaces have attracted considerable attention in recent years, due to the promise of electrochemistry to provide a direct pathway from abundant molecules to valuable chemical feedstocks or high-energy molecules by the use of green electricity [1,2]. One prominent example is the green production of hydrogen in polymer electrolyte membrane (PEM) electrolyzers at large current densities and output pressures [3, 4, 5]. The lifetime of such devices is often limited by degrading anodes, caused by the harsh conditions of the oxygen evolution reaction (OER) in acidic electrolytes [6,7]. To find strategies mitigating such limitations and reduce costs, scientists try to understand how the OER occurs on the surface of the electrocatalyst.

As the exemplary reaction pathway in Figure 1 illustrates, the processes on the surface are complex. The OER is a multistep, heterogeneously catalyzed reaction, in which several intermediates and transition states are expected. Their energies are closely connected to the state of the surface, as they are chemically bound to it [8,9]. Adding to the complexity, the educt, the product, and the released protons are dynamically solvated in the electrochemical double layer, in which the distribution of the ions and the associated potential drop is not well known on an atomic length scale.

Reaching an understanding at the highest level of complexity cannot be achieved in one leap. The analysis needs to be simplified and made more complex with the advancing state of research and its technological capabilities. Since it might not be obvious to a newcomer at which rung on this ladder of complexity we currently stand, we want to provide a short overview of how far the understanding of the OER reaches and in which aspects our understanding could still improve. For a more extensive review of the literature, however, we refer to other reviews on the topic [3,5,10, 11, 12]. This review will be structured by six rungs on a ladder towards the understanding of the OER.