(Invited) How to Expand the Scope of Cyclic Voltammetry with the Thin-Film Rotating (Ring) Disk Electrode to Investigate Oxygen Reduction Reaction Electrocatalysts (2021)

The performance of electrochemical energy conversion and storage (EECS) devices is modulated by a cascade of phenomena that include: (i) the redox processes taking place at the electrodes; (ii) the transport of charge, with a particular reference to the migration of ions through the electrolyte; and (iii) the transport of mass involved in the provision of reactants/removal of products to/from the electrodes. These phenomena are intimately correlated with one another and with all the different features of the functional materials adopted in the fabrication of the specific EECS device. Thus, it is often very complex to unravel the different contributions to overall system performance. In addition, it is not practical to pursue a purely “trial-and-error” approach where all the possible combinations of functional materials are used to fabricate and test full-fledged EECS prototypes, as this would entail too massive an investment in terms of both resources and time. For this reason, there is a great need to implement experimental approaches able to elucidate the details of specific electrochemical features ( e.g. , reaction kinetics) of the various functional materials used in EECS devices, at the same time minimizing the cloaking effects due to: (i) the formation of complex interfaces with other components of the device; and (ii) spatial and temporal inhomogeneity in the motion of both charged and neutral species through the system. The resulting information is critical in the development of advanced functional materials bestowing to the EECS device a performance and durability beyond the state of the art. Perhaps the most successful and widely adopted of such approaches is cyclic voltammetry conducted with a thin-film rotating disk electrode or ring-disk electrode (CV-TF-R(R)DE) [1]. CV-TF-R(R)DE is typically used to screen quickly the performance and the details of the reaction mechanism of electrocatalysts (ECs) for the oxygen reduction reaction (ORR). The latter process is widely exploited in a broad variety of EECS devices including fuel cells (FCs) and metal-air batteries. The ORR is particularly critical since it often bottlenecks the operation of the entire system. Even though the CV-TF-R(R)DE method is very popular and broadly used in the scientific community, most researchers do not use all its remarkable capabilities. Here it is shown how to implement both innovative and more traditional (but no longer widespread) approaches to exploit CV-TF-R(R)DE to its full potential, with the purpose to study the most relevant electrochemical features of ORR ECs and quickly identify the most promising candidates for application in FCs or other electrochemical devices [2]. Particular emphasis is devoted to analysis techniques meant to study in detail the ORR kinetic features of vastly different ECs, allowing for quantitative performance comparisons at the same high level of accuracy without risking serious distortions due to an excessive impact of corrections during data analysis. It is also shown how to adopt the CV-TF-R(R)DE approach to understand quantitatively whether an ORR EC exhibits improved morphology/mass transport features in comparison with a Pt/C benchmark. In summary, it is elucidated how to expand significantly the scope of CV-TF-R(R)DE studies with respect to the level typically achieved in the state of the art, taking into consideration both kinetic and morphology/mass transport features by means of a simple morphokinetic (MK) correlation map. Acknowledgements The research leading to the results reported in this work has received funding from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells e HELPER” funded by the University of Padova. References [1] Y. Garsany, O.A. Baturina, K.E. Swider-Lyons, S.S. Kocha, Anal. Chem. 82(15) (2010) 6321-6328. [2] V. Di Noto, E. Negro, A. Nale, G. Pagot, K. Vezzù, P. Atanassov, Curr. Opinion Electrochem. 25 (2021) 100626.