The Influence of Alkaline Earth Metal Cations on the Performance of Platinum Electrodes during Brine Electrolysis

The performance of platinum electrodes towards the hydrogen evolution reaction (HER) during pH-neutral brine electrolysis was investigated in the presence of alkaline earth metal cations, with a focus on Mg²⁺ ions. This study aimed to understand the impact of these cations and/or their hydroxide deposits on electrode performance. Electrochemical measurements, including chronoamperometry and polarization curves, were conducted in pH-neutral electrolytes with varying Mg²⁺ concentrations (0 mM to 55 mM) in 1 M Na₂SO₄ solution. Contrary to concerns that Mg(OH)₂ deposits might impair performance, Pt electrodes exhibited improved HER activity across all Mg²⁺ concentrations compared to Mg-free electrolytes. To distinguish the effects of hydroxide deposits from solvated cation interactions with the electrocatalyst, other divalent cations (Ca²⁺, Sr²⁺, Mn²⁺) with varying solubility product constants were also tested. All cations enhanced HER performance, including those unlikely to form deposits under the tested conditions. Our results point to cation acidity and Gibbs free energy of hydration as effective descriptors for predicting the role of divalent cations in HER kinetics. In situ image analysis revealed that Mg(OH)₂ deposits are periodically removed by evolving hydrogen bubbles, preventing the blockage of active sites. In situ Raman spectroscopy further confirmed the formation of Mg(OH)₂ deposits during HER.