Selective electrolysis of water under artificial seawater conditions using transition metal borate anodes

Abstract

Direct seawater electrolysis overcomes one major hurdle in sustainable hydrogen production, viz. the requirement to use high-purity water, which is currently a scarce resource for ∼80% of the global population. On the other hand, the natural composition of seawater makes it unsuitable for direct use in conventional electrolyzer systems, primarily due to the occurrence of the chlorine evolution reaction (CER) at the anode, instead of the desired oxygen evolution reaction (OER), and the build-up of insulating Mg2+/Ca2+ hydroxides at the cathode. Herein, we present an electrolyzer fed with artificial seawater with no added electrolyte and enabling selective splitting of H2O, rather than H2 generation coupled to the CER. This selectivity was provided by the anodes derived from a cobalt iron boride material, which was predicted to favor the OER rather than the CER by the density functional theory calculations. The compact design of the electrode-separator assembly suppressed the cathodic Mg/Ca(OH)2 precipitation. Robust operation of the electrolyzer over one week was demonstrated at 100 mA cm−2 with a cell voltage of ca. –2.6 V at 80 ± 1°C. These operating conditions were selected based on a preliminary techno-economic analysis as a realistic benchmark for cost-competitive hydrogen production.