Iron phosphate modified calcium iron oxide as an efficient and robust catalyst in electrocatalyzing oxygen evolution from seawater.

Solar fuel generation using seawater as the proton source is fascinating but challenging due to the detrimental chlorochemistry, the lack of active and stable oxygen evolution catalysts operating at seawater pH (∼8) and high turnover conditions. In the present study, iron phosphate modified calcium iron oxide (CaFeOx|FePO4) modified FTO electrodes were prepared, and their electrocatalytic properties towards the oxygen evolution reaction in both synthetic and natural seawater solutions were investigated. CaFeOx|FePO4 was prepared by electrodepositing FePO4 onto CaFeOx, prepared by spin-coating and a follow-up annealing process, under a constant applied current for different durations. Mg2+-induced fouling significantly reduces the activity of CaFeOx and slows down the activation process of CaFeOx, but can be mitigated by surface-modification of CaFeOx with FePO4. In addition, the presence of additional electrodeposited iron phosphate on the surface of CaFeOx attenuates the production of corrosive hypochlorite from chloride oxidation. With these unique properties of FePO4, the activated CaFeOx|FePO4 electrode shows high activity and stability under high turnover conditions, reaching 10 mA cm-2 at an overpotential of ∼710 mV, with a moderate increase in η (∼70 mV), mainly due to the change in solution pH, over 10 h of electrolysis in phosphate-buffered (0.5 M, pH 7) seawater solution.