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The global scarcity of freshwater resources has recently driven the need to explore abundant seawater as an alternative feedstock for hydrogen production by water-splitting. This route comes with new challenges for the electrocatalyst, which has to withstand harsh saline water conditions with selectivity towards oxygen evolution over other competing reactions. Herein, a series of amorphous metal borides based on the iron triad metals (Co, Ni, and Fe), synthesized by a simple one-step chemical reduction method, displayed excellent bifunctional activity for overall seawater splitting. Amongst the chosen catalysts, amorphous cobalt boride (CoÀ B) showed the best overpotential values of 182 mV for HER and 305 mV for OER, to achieve 10 mA/cm 2 , in alkaline simulated seawater. This superior activity was owed to the enrichment of the metal site with excess electrons (HER) and the in-situ surface transformation (OER), as confirmed by various means. In alkaline simulated seawater, the overall cell voltage required to achieve 100 mA/ cm 2 was 1.85 V for the CoÀ B catalyst when used in a 2-electrode assembly. The CoÀ B catalyst showed negligible loss in activity even after 1000 cycles and 50 h potentiostatic tests, thus demonstrating its industrial viability. The selectivity of the catalyst was established with Faradaic efficiency of above 99 % for HER and 96 % for OER, with no detection of chloride products in the spent electrolyte. This study using the monometallic boride catalysts will turn to be a precursor to exploit other complex metal boride systems as potential candidates for seawater electrolysis for large-scale hydrogen production.

Amorphous materials are used in multitude of catalytic applications, including electrocatalytic water-splitting. Identification and investigation of active sites in amorphous catalysts are rarely reported, mainly owing to the complexity of the systems. Herein, we report an amorphous bifunctional Co–W–B electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The optimized Co–W–B catalyst showed promising overpotential values of 97 mV (HER) and 292 mV (OER), respectively, to achieve 10 mA cm–2 in 1 M KOH, with good stability. The promoting effect of W in Co–B was investigated experimentally, while computational tools were used to identify all the possible catalytic sites in an amorphous Co–W–B model and classify the most preferred sites for HER and OER. The presence of multi-catalytic sites with specific selectivity toward HER and OER was observed, which explained the bifunctional activity of Co–W–B. This study will foster better understanding of the origin of catalytic activity in similar amorphous systems.

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