A Formalism Adopting Thin-Film Rotating Ring-Disk Electrode Studies to Compare Electrocatalysts for the Oxygen Reduction Reaction (ORR)

Several families of electrochemical energy conversion and storage devices (e.g., fuel cells and metal-air batteries) exploit the oxygen reduction reaction (ORR) in their operation. Very often, the ORR is the slowest electrochemical process in these systems and plays the most relevant role to degrade their energy conversion efficiency. Thus, to devise high-performing ORR electrocatalysts (ECs) is of crucial importance for practical applications. Unfortunately, to understand in detail the operating mechanism of an ORR EC is typically complex and time-consuming, requiring the implementation of a broad spectrum of advanced techniques and extensive data analysis. Consequently, there is a strong need to design simple methods capable to yield information on the most critical features of an ORR EC for screening purposes. In this regard, cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) is a very popular approach. The latter allows for the facile study of the kinetics of an electrochemical process as promoted by an EC and minimizes the influence of complex spurious phenomena (e.g., charge and mass transport). In this contribution a general method is discussed allowing for the correlation of the outcome of conventional electrochemical experiments with the critical features determining the performance of an ORR EC. Such features include prominently: (i) the kinetic activation barrier of the ORR; and (ii) the accessibility of O2 to the active sites. The proposed method: (i) adopts the CV-TF-RRDE setup; (ii) does not lean on the simplifications associated with the conventional Butler-Volmer kinetic description of electrochemical processes; and (iii) does not make assumptions on the specific features of the EC. As a result, the proposed method allows to compare accurately the kinetic performance of ORR ECs exhibiting a completely different chemistry. Finally, it is shown that the figure of merit considered in this method, E(j Pt(5%)), is much more accurate than other popular figures of merit to gauge the ORR such as the half-wave potential E1/2. Acknowledgements This research has received funding from (a) the European Union’s Horizon 2020 research and innovation program under grant agreement 881603 (b) the project ‘Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells’ funded by EIT Raw Materials, (c) Alkaline membranes and (platinum group metals)-free catalysts enabling innovative, open electrochemical devices for energy storage and conversion d AMPERE, FISR 2019 project funded by the Italian Ministry of University and Research, and (d) the project ‘Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells d HELPER’ funded by the University of Padova. PJK and IAR (University of Warsaw) were supported in part by the National Science Center (NCN, Poland) under Opus Project 2018/29/B/ ST5/02627.