The pH of the seawater catholyte decreased from an initial value of 8.5 to 1.6 after 20 h of HER. Additionally, we observed the deposition of material on the cathode after 20 h. EDX elemental analysis of the deposited material showed that it mainly consisted of magnesium and oxygen in addition to trace amounts of calcium, sodium, potassium, sulfur, and chlorine (Figure and Table ). These are all components of the model seawater electrolyte. We hypothesize that the constant formation of OH -during hydrogen evolution at the cathode forms an alkaline diffusion layer that causes a layer of Mg(OH)2 precipitate to form on the electrode. The deposited layer is stabilized by a diffusion layer formed at the electrode surface, which traps hydroxide from neutralizing the hydronium entering the catholyte as the cation from the BPM. The Mg(OH)2 layer dissolved from the electrode surface and the pH of the acidified seawater catholyte recovered to 8.5 when the electrode was left in it over a 48-hour period without electrolysis. This layer formation been previously observed and strategies for avoiding these depositions have been studied elsewhere. The anolyte pH was unaffected by the electrocatalysis and was measured to be 14.2 before and after all reactions regardless of duration of the experiment.