Apparent versus Cross-Validated Selectivity in Seawater Electrolysis: Linking Product Analysis to Catalyst–Electrolyte Interface Dynamics

Selectivity in seawater electrolysis is commonly evaluated by comparing the oxygen evolution reaction (OER) and chlorine evolution reaction (CER). However, the products measured after electrolysis do not always directly represent those initially formed at the catalyst-electrolyte interface. Chlorine generated at the anode can partition between the gas and liquid phases and interconvert among Cl2, HOCl, and OCl- depending on pH, reaction time, and mass transport, while further oxidation can produce more highly oxidized oxychlorine species such as chlorate and perchlorate. At the same time, catalyst reconstruction, corrosion, and dissolution can alter the active surface and contribute to currents that are not associated with the measured products. In this Review, we examine the major analytical methods used to evaluate OER/CER selectivity, including gas chromatography, ion chromatography, iodometric and N,N-diethyl-p-phenylenediamine (DPD) assays, rotating ring-disk electrodes, and complementary electrochemical and operando techniques. We discuss what each method actually measures, its major limitations, and the experimental conditions required for reliable Faradaic-efficiency determination. We then distinguish apparent selectivity, obtained from limited or individual product measurements, from cross-validated selectivity, established by complementary gas- and liquid-phase product analyses together with pH, mass-balance, and catalyst-stability measurements. Based on these considerations, we propose a tiered evidence framework for evaluating and reporting OER/CER selectivity. Integrating product analysis with time-resolved characterization of the catalyst-electrolyte interface will provide a more reliable basis for understanding structure-selectivity relationships and for evaluating seawater electrolysis catalysts under realistic operating conditions.