Approaches for measuring the surface areas of metal oxide electrocatalysts for determining their intrinsic electrocatalytic activity

Great attention has been recently drawn to metal oxide electrocatalysts for electrocatalysis-based energy storage and conversion devices. To find the optimal electrocatalyst, a prerequisite is an activity metric that reasonably evaluates the intrinsic electrocatalytic activity of a particular catalyst. The intrinsic activity is commonly defined as the specific activity which is the current per unit catalyst surface area. Thus, the precise assessment of intrinsic activity highly depends on the reliable measurement of catalyst surface area, which calls for the knowledge of experimental approaches for determining the surface areas of metal oxide electrocatalysts. This tutorial review aims to summarize and analyze the approaches for measuring the surface areas of metal oxide electrocatalysts for evaluating and comparing their intrinsic electrocatalytic activities. We start by comparing the popular metrics for activity estimation and highlighting the importance of surface-area-normalized activity (i.e. specific activity) for intrinsic chemistry analysis. Second, we provide some general guidelines for experimentally measuring the electrochemically active surface area (ECSA). Third, we review the methods for the surface area measurement of metal oxide electrocatalysts. The detailed procedure for each method is explicitly described to provide a step-by-step manual that guides researchers to perform the measurement; the rationales and uncertainties for each method are discussed to help readers justify the reliable assessment of surface area. Next, we give our recommendations on selecting a rational experimental approach for the surface area measurement of a particular metal oxide electrocatalyst. Lastly, we discuss the future challenges of ECSA measurement and present an exemplary novel ECSA technique.