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 electrocatalysts for water splitting. Moreover, being associated with free energy changes of the overall process (according Arrhenius formula) the TOF serves as a significant metrics that deals with 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 making it hard to distinguish whether the catalytic activity is due to the quality of active sites or due greater number of reactive centres. TOF, on the other hand, defines per site activity shedding light on the real efficiency of the individual active sites. While catalyst with larger ECAS may exhibit higher current densities, their TOF can be significantly lower due to less efficient active sites. This showcases the importance of optimizing not just the quantity, but the quality and electronic environment of active sites to achieve efficient electrocatalysis. Further detail kinetic analysis, considering multi-step electrocatalytic process reveals that the rate constant or TOF is mainly govern by the rate-determining step (RDS) of the catalytic cycle and the nature of the active site involved. Conventional electrochemical and non-electrochemical ways of determining electrochemical active site (ECAS) for an electrocatalyst are facing a serious limitation as the calculated TOF value from this does not reflect its intrinsic nature. ECAS determination via various electrochemical methods suffers from a strong dependence 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 by assuming 100% atom utilization. Moreover, none of the reported procedure consider the importance of RDS in the catalytic cycle. Using theoretical analysis, in-situ spectroscopic technique and various electrochemical analyses have proven effective in identifying the nature of RDS and active sites involved. Therefore, integrating such advance measurement along with standard electro/non-electrochemical technique can provide a more accurate picture of TOF which certainly would help to developed effective electrocatalyst for sustainable hydrogen production in future.