Determination of the reaction orders for electrode reactions

Information of reaction orders is prerequisite in unveiling the mechanism(s) of complex electrocatalytic reactions, which is of great help in benchmarking the intrinsic electrocatalytic performance and in establishing the structure–activity relationship. However, electrochemical reaction orders for only few electrocatalytic reactions have hitherto been unambiguously quantified, due to the complexities of the reaction themselves and the complexities of interfacial environments. The apparent reaction orders may depend on the coverage of the adsorbed reactant, reactive intermediates at the electrode interface, their adsorption behavior, the occurrence of parallel pathways as well as existence pre or postchemical reactions. In this short review, theories and methods used for determination of the reaction orders for electrode reactions are summarized and exemplified by taking hydrogen evolution/oxidation reaction (HER/HOR) and oxygen reduction reaction (ORR) under rotating disk electrode configuration as model reactions. Frequently encountered challenges in accurate determination the reaction orders for complex electrocatalytic reactions are discussed.