Beyond traditional TOF: unveiling the pitfalls in electrocatalytic active site determination

Abstract

Turnover frequency (TOF) is a fundamental metric for evaluating the intrinsic activity of an electrocatalyst for water splitting. Being associated with free energy changes in the overall process (according to the Arrhenius formula), TOF serves as a significant metric that deals with the molecular origin of electrocatalytic activity compared to conventional current density or overpotential as the standard descriptors. For instance, current density signifies the overall rate of an electrochemical reaction; however, it is influenced by the number of electrochemical active sites (ECASs), which makes it difficult to distinguish whether the catalytic activity is due to the quality of the active sites or due to a greater number of reactive centres. TOF, on the other hand, defines the per-site activity, shedding light on the real efficiency of individual active sites. This perspective highlights that a higher ECAS does not always guarantee superior intrinsic activity or efficiency of an electrocatalyst. Although catalysts with a larger ECAS may exhibit higher current densities, their TOF can be significantly lower due to the less efficient active sites. This shows the importance of optimizing not only the quantity but also the quality and electronic environment of the active sites to achieve efficient electrocatalysis. Further detailed kinetic analysis, considering a multi-step electrocatalytic process, reveals that the rate constant or TOF is mainly governed by the rate-determining step (RDS) of the catalytic cycle and the nature of the active site involved. Conventional electrochemical and non-electrochemical methods for determining electrochemical active sites (ECASs) for an electrocatalyst face serious limitations because the calculated TOF value does not reflect its intrinsic nature. ECAS determination via various electrochemical methods is strongly dependent on the catalyst loading, scan rate, and substrate selected for electrochemical analysis. Direct measurement of ECAS via ICP-MS and structural characterization may lead to overestimation if 100% atom utilization is assumed. Moreover, none of the reported procedures considers the importance of RDS in the catalytic cycle. Recent advancements in using theoretical analysis, in situ spectroscopic techniques, and various electrochemical analyses have proven effective in identifying the nature of the RDS and the active sites involved. Therefore, integrating such advanced measurements with standard electro/non-electrochemical techniques can provide a more accurate picture of TOF, which would certainly help develop effective electrocatalysts for sustainable hydrogen production in the future.