Experimental Challenges in the Electrocatalytic Benchmarking of Multicomponent Alloys

Abstract

Multicomponent alloys, especially complex solid solutions (CSS), and more specifically high entropy alloys (HEA) have emerged as promising new types of electrocatalysts [1] for numerous reactions of significance for energy conversion. Their exceptional properties and the large number of degrees of freedom available for tuning their properties make CSSs promising candidates for overcoming the limitations of traditional catalysts: activity limitations imposed by the scaling, as well as stability issues.Despite the large interest they have garnered, their potential still remains to be understood and harnessed, as their electrochemical behavior still needs to be understood. Significant experimental challenges need to be addressed and widely applied standards need to be implemented in order to understand and correctly asses the electrocatalytic behavior of such catalysts. The electrochemical surface area (ECSA) determination, identification of active sites, understanding their operando behavior, effects of crystallinity [2] as well as basic experimental challenges, such as careful Ohmic drop correction, are some of the most important experimental challenges in the electrocatalytic assessment of CSSs.Here, an overview of CSS catalysts for the oxygen reduction reaction to date will be given (Figure 1A) [1]. Traditional methods used to evaluate the ECSA of well-known systems (e.g. noble metal electrodes) are discussed in relation to their use in CSS evaluation. For instance, the application of double layer charging current techniques, surface limited reactions and surface science techniques all have their advantages and drawbacks for the determination of the electrochemical surface area in terms of ease of application, size of errors introduced and reliability. Additionally, crystallinity (more generally, the effects of long- and short-range order, Figure 1B), phase separations, oxide layer growth and other caveats during electrocatalytic benchmarking of CSSs will be discussed. Figure 1