The Subtle Helpers: How Spectator Ions Govern Efficiency in Electrolytic Water Splitting

Abstract

Recently a growing magnitude of research reveals that the electrolyte species usually regarded as inert 'spectators' exert a non-negligible influence on electrode catalytic performance, which sparked considerable interest in the scientific community, suggesting the potential emergence of a new frontier in electrocatalysis. Despite growing recognition of their significance, the mechanistic participation of spectator ions in electrochemical processes remains incompletely understood, with fundamental aspects of their interfacial behavior and catalytic modulation still awaiting systematic elucidation. To elucidate how the electrochemical performance can be modulated through electrolyte effects caused by spectator ions within the double layer microenvironment, we systematically investigated the impact of: 1) different alkali metal cations on platinum for hydrogen evolution (HER) and oxygen evolution (OER) reactions in alkaline electrolytes, and 2) acidic anions on platinum under acidic conditions electrolytes. Our experimental results show distinct activity trends: for HER, cation efficacy: Li⁺ > Na⁺ > K⁺ while anion efficacy: H2PO4- > HSO4- > ClO4-; for OER, cation efficacy: Li⁺ > Na⁺ > K⁺ while anion efficacy: ClO4- > HSO4- > H2PO4-.Combining DFT result one can conclude the adsorption of spectator ions polarizes the Pt surface, forcing electrons near or far away from the surface and shifting the d-band center (ε d ) of the Pt. This tuning for chemical activity of Pt would change adsorption of reaction intermediates and effect the performance. Furthermore under the alkaline condition, order degree of hydrogen bond network with the cation doping present obvious distinction that would effect the water dissociation. These findings provide fundamental mechanistic insights for rational catalyst design through microenvironment engineering approaches.