Cell Engineering Can Mitigate Cathode Scaling during Water Electrolysis in the Presence of Mg²⁺

Direct seawater electrolysis might play an important role in distributed hydrogen production but is constrained by the natural ionic composition of seawater. A specific challenge at the cathode is scaling with low-solubility, electrically-insulating magnesium and calcium hydroxides resulting from a reaction of naturally present Mg2+ and Ca2+ with OH– generated by the hydrogen evolution reaction. In theory, this should be resolved by the transport of protons generated by the anodic oxygen evolution reaction, but regular devices do not provide this sufficiently. Herein, we demonstrate that the pH imbalance during the electrolysis of Mg2+-containing unbuffered water can be mitigated by a suitable cell design. We present a real-time visualization of the pH gradients evolving during unbuffered water electrolysis (0.6 M Na2SO4 + 0.053 M MgSO4, pH ≈ 7), and show how these induce Mg(OH)2 precipitation depending on forced convection and distance between the anode and cathode. Minimizing the latter significantly suppresses the Mg(OH)2 scaling when the contact of the cathode with the electrolyte solution is restricted to one side facing the anode and a H2 gas escape pathway through a water-impermeable membrane is provided on the other side. This cell design enables stable water electrolysis at 0.1 A cm–2 on a day-long time scale at both ambient (23 ± 2 °C) and industrially relevant temperatures (80 ± 1 °C). Beyond seawater electrolysis, this simple strategy might be also applicable to magnesium–seawater batteries.