The Utilization of Chaotropic Salts to Reduce the Overpotential of Water Splitting in Water Electrolysis

Abstract

This study investigates the influence of sodium thiocyanate (NaSCN) as chaotropic electrolyte additive on the physicochemical properties and electrochemical performance of aqueous NaOH for alkaline water electrolysis. Density, viscosity, and electrical conductivity were measured experimentally and compared with empirical correlations and classical MD simulations. A biased-random walk model was used to estimate the contribution of Grotthuss-type transport to OH^– mobility. Electrochemical performance was evaluated on Pt-mesh electrodes using cyclic voltammetry. NaSCN addition increased electrolyte density and generally increased viscosity. Although it increased the number of charge carriers, its effect on conductivity depended strongly on composition. Positive conductivity increments were observed in some diluted mixtures, whereas concentrated mixtures exhibited reduced conductivity because of increased viscosity, ionic crowding, and correlated ion motion. Classical simulations reproduced NaSCN conductivity but underestimated NaOH conductivity because the nonreactive force field excluded structural OH^– transport. The conductivity-based analysis indicated that NaSCN decreased the corrected OH^– self-diffusion coefficient and increased its estimated hopping lifetime, suggesting suppression of Grotthuss transport. NaSCN also substantially inhibited the electrochemical reactions on Pt. The HER overpotential magnitude at –10 mA/cm2 increased approximately from 116 mV to 300–310 mV, while the apparent OER overpotential at 10 mA/cm2 increased approximately from 459 mV to as much as 1082 mV. These effects are attributed to a combination of reduced ionic mobility, increased transport resistance, modification of the electrical double layer, and specific adsorption of SCN^– on Pt. Sulfate detected after cyclic voltammetry further supported the occurrence of parasitic SCN^– oxidation, indicating that part of the anodic current could not be assigned exclusively to OER. Overall, NaSCN did not improve alkaline water electrolysis performance under the investigated conditions. The results demonstrate that chaotropicity alone is insufficient for selecting electrolyte additives; ionic transport, catalyst interactions, and electrochemical stability mas also be considered.