Hydrated Electrocatalysis: To Boost the Selectivity for the Oxygen Evolution Reaction in Seawater Electrolysis

Abstract

The increase in the production of renewable electricity offers the opportunity to transition from the usage of fossil-based hydrogen to green hydrogen. While mass production of green hydrogen by water electrolysis demands substantial freshwater resources, the abundant availability of seawater provides a promising opportunity to directly use it as an electrolyte in a water electrolyzer. However, a major challenge in seawater electrolysis is the low selectivity for oxygen evolution compared to the chlorine evolution at the anode. To address this, we proposed a strategy to boost the selectivity for oxygen evolution by hydrated electrocatalysis, in which water is itself part of the catalyst. Water molecules that are coordinately bonded to the active catalytic sites play a significant role in enhancing oxygen evolution selectivity. This approach was demonstrated with Prussian blue analogue electrocatalysts in acidified simulated seawater electrolyte using rotating ring disk electrode voltammetry. Microkinetic modeling was employed to correlate the coverage by the reactants (H₂O and Cl^–) with selectivity. Notably, the crystal water coverage on hydrated electrocatalysts emerged as the partial descriptor for the selectivity of the oxygen evolution reaction. To gain insights for coverage by crystal water and Cl^–, the thermogravimetric analysis combined with Rietveld refinement and microkinetic Tafel analysis was performed. In a nutshell, we explored the question: if the reactant molecule (H₂O) is an integral part of the catalyst, can it promote the corresponding electrochemical oxidation reaction (O₂ evolution) over its competitor (Cl₂ evolution)?