Eggshell membrane-derived metal sulfide catalysts for seawater splitting†

Abstract

To replace depleting freshwater resources, seawater, with its abundance and economy, has become a more favourable option for water electrolysis. However, seawater electrolysis necessitates electrocatalysts with excellent activity as well as resistance to Cl⁻ corrosion. Herein, we utilized a biowaste, eggshell membranes, as a versatile platform to fabricate sulfide electrocatalysts for the oxygen revolution reaction (OER). Structural analyses including X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy tests indicated that the introduction of iron into the cobalt sulfide lattice greatly modified the structures of the sulfide. Electrochemical tests and operando electrochemical Raman spectroscopy showed that the introduction of Fe adjusted the electronic structure of Co₉S₈, facilitating the formation of Co⁴⁺ species, which serve as the major active sites for OER, thereby effectively improving the catalyst performance. The optimal Co₈FeS₈/ESM-900 sample can achieve a current density of 10 mA cm⁻² in alkaline freshwater, simulated seawater, and natural seawater at overpotentials of 270, 271, and 324 mV, respectively, which are lower than the overpotentials of 273, 272, and 337 mV obtained from IrO₂. The sulphate passivation layer formed during the OER process can effectively repel Cl⁻, leading to outstanding corrosion resistance. The Co₈FeS₈/ESM-900 catalyst can be continuously operated in seawater electrolysis for 200 000 s. A (−)Pt/C||Co₈FeS₈/ESM-900(+) electrolyzer required only 1.629, 1.623, and 1.648 V to yield a current density of 10 mA cm⁻² for the electrolysis of alkaline freshwater, simulated seawater, and natural seawater, respectively, which are superior to the performance of the (−)Pt/C||IrO₂(+) electrolyzer. In virtue of its low cost, high efficiency and outstanding stability, the catalyst reported in this study is promising in practical seawater electrolysis.