The discovery of adsorbed iodine from trace impurity as a key trigger for promoted oxygen reduction reaction at Pt electrodes

Organic cations, such as tetraalkylammonium salts, have been reported to enhance oxygen reduction reaction (ORR) kinetics. However, the promotion effect is not well reproducible and precise mechanisms remain unclear. In order to unveil possible impact from trace impurities, we reevaluate the ORR activity of Pt(1 1 1) electrodes in both acidic and alkaline solutions containing tetraalkylammonium cations. A significant enhancement in ORR activity (with a positive shift of ∼40–45 mV in the half-wave potential for ORR is only observed with tetrahexylammonium cation from one company. By conducting a systematic inquiry, we find that cation hydrophobicity is unlikely to be the dominant origin of the observed effect under the low-concentration regime and experimental conditions studied here. After considering the impact of most species present under our experimental conditions and excluding the most plausible alternative explanations, we identify that an iodine adlayer (I*) is likely a major contributor to the enhanced ORR kinetics. This adlayer forms on the Pt(1 1 1) surface probably through adsorption of trace iodine-derived impurities present in the organic cationic reagent. Our result overturns the previous consensus that I* always poisons the electrocatalytic reaction. Our systematic studies unveil that whether I* acts as a poison or a promoter, its coverage is crucial. This finding offers a new strategy for optimizing catalyst design through deliberate control of additive levels, furthermore, it also highlights the critical importance of rigorous impurity monitoring in electrochemical experiments. The impact of organic cations in electrocatalytic activity as well as necesscity of exploring the broader implications of trace impurities in electrocatalysis are discussed.