Sub-volt conversion of activated biochar and water for H2 2 production near equilibrium via biochar-assisted water electrolysis

Sluggish water oxidation reactions limit water electrolysis for H2 2 production, which can be alleviated by the use of carbon-based materials like agricultural wastes as reducing agents. Biochar from such biomass can reduce equilibrium cell potentials at standard conditions from 1.23 V to 0.21 V by avoiding direct water splitting at the anode. However, some challenges hinder biochar oxidation, including poor biochar binding, electrode caking, and surface passivation. We find that enhanced C/O ratio, crystallinity, and negative zeta potential improve biochar oxidation kinetics at moderate temperatures. Smaller particle sizes and better mixing prevent electrode caking, enhancing biochar stability. Here, we report sub-volt biochar-coupled H2 2 production, often referred to as a biochar-assisted water electrolysis (BAWE), yielding '250 mA/gcat cat H2 2 current at 100% Faradaic efficiency. Over 1 mA current was observed at a near-equilibrium cell potential of 0.2 V cell potential. Using a single-junction solar cell-powered BAWE, '15 mA H2 2 is generated at 1 Sun, resulting in '4.8% solar-to-hydrogen efficiency, equivalent to '35% when the energy of H2 2 relative to H2O 2 O (without biochar) is assumed.