Non-Noble Bifunctional Amorphous Metal Boride Electrocatalysts for Selective Seawater Electrolysis

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 (Co-B) showed the best overpotential values of 182 mV for HER and 305 mV for OER, to achieve 10 mA/cm2, 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/cm2 was 1.85 V for the Co-B catalyst when used in a 2-electrode assembly. The Co-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 mono-metallic 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. Green hydrogen production from seawater is highly feasible since approximately 96.5 % of the Earth's surface is covered by water resource. It is necessary to develop an effective electrocatalyst that can withstand the harsh seawater condition. Meanwhile, monometallic amorphous transition metal borides synthesized by one step facile chemical reduction method shows a superior activity, stability and similar to 99 % selectivity for seawater electrolysis. Amongst three TMBs, amorphous Co-B electrocatalyst exhibited an overpotential of 182 mV and 305 mV, respectively at 10 mA/cm2 for HER and OER in alkaline simulated seawater. The optimized Co-B catalyst were extensively studied in three different alkaline media where it showed negligible decrease in the activity. The overall cell voltage of the Co-B outperforms the benchmark electrocatalyst Pt/RuO2 at higher current density. Also, Co-B sustains 50 long hours stability for both HER and OER without degradation which makes them suitable for commercial applications. image