The subtle helpers: how spectator ions govern the efficiency of electrolytic water splitting

Abstract

Recently, a growing number of studies reveal that the electrolyte species usually regarded as inert ‘spectators’ exert a non-negligible influence on the catalytic performance of electrodes, which has sparked considerable interest in the scientific community, suggesting the potential emergence of a new frontier in electrocatalysis. However, comprehensive investigations into the participation of the spectator ions in electrochemical processes remain at an early stage, leaving numerous questions and complexities unresolved. To elucidate how electrochemical performance can be modulated through the electrolyte effects caused by the spectator ions within a double-layer microenvironment, we systematically investigate the impact of (1) different alkali metal cations on platinum for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline electrolytes and (2) acidic anions on platinum in acidic electrolytes. Our experimental results show distinct activity trends: for the HER, cation efficacy: Li⁺ > Na⁺ > K⁺ and anion efficacy: H₂PO₄⁻ > HSO₄⁻ > ClO₄⁻; for the OER, cation efficacy: Li⁺ > Na⁺ > K⁺ and anion efficacy: ClO₄⁻ > HSO₄⁻ > H₂PO₄⁻. From density functional theory (DFT) results, one can conclude that the adsorption of the spectator ions polarizes the Pt surface, driving electrons toward or away from the surface and shifting the d-band center (ε_d) of the Pt. This tuning of the chemical activity of Pt changes the adsorption of reaction intermediates and affects the performance. Furthermore, under alkaline conditions, the order degree of the hydrogen bond network with cation doping presents an obvious distinction that will affect the water dissociation. These findings provide fundamental mechanistic insights for rational catalyst design through microenvironment engineering approaches.