Fourier transformed alternating current voltammetry in electromaterials research: Direct visualisation of important underlying electron transfer processes

Recent advances in materials science have significantly broadened the range of electrodes available for use in dynamic forms of electrochemistry. In the modern era of dc voltammetry when the kinetic model of electron transfer with coupled chemical reactions was introduced, initial emphasis in theory-experiment comparison was placed on use of the ideal homogenous liquid mercury electrode with significant attention also given to polycrystalline and faceted metal electrodes. Nowadays, there are a plethora of carbon-based electrodes such as glassy carbon, edge and basal plane graphite, boron doped diamond, graphene and carbon nanotubes that may be extremely heterogeneous. These are supplemented by chemical modifications designed for example to improve the efficiency of electrocatalysis. In this review, it is shown that analysis of the higher harmonics available in large amplitude Fourier transformed alternating current voltammetry (FTacV) allows key processes to be detected, that are masked under commonly used dc voltammetric conditions. In particular it is shown how underlying fast electron processes that facilitate carbon dioxide reduction at tin electrodes and oxygen evolution at cobalt based electrodes can be directly detected and analysed for the first time. FTacV also experimentally reveals that structural defects or adatoms can give rise to well-defined higher order ac harmonics suggesting that a fast electron transfer process is associated with the active sites during electrocatalytic oxidation or reduction. Importantly, electron transfer processes often can be evaluated by FTacV in the presence and absence of the electrocatalysis, unlike dc voltammetric methods. The ability to analyse third and higher order ac harmonics that are essentially devoid of background charging current and which allow the electron transfer and catalytic steps to be resolved, presents new opportunities for fundamental advances in understanding complex electrochemical reaction mechanisms taking place at heterogeneous electrodes. Related advantages in studying electron transfer of surface confined metallo-enzymes or proteins in the presence and absence of their catalytically oxidised or reduced biologically relevant substrate partners are also surveyed. Finally, prospects for providing quantitative accounts of complex reactions at highly heterogeneous electrodes by FTacV are considered.