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

Sluggish water oxidation reactions limit water electrolysis for H₂ 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 production, often referred to as a biochar-assisted water electrolysis (BAWE), yielding ∼250 mA/g_cat H₂ 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 is generated at 1 Sun, resulting in ∼4.8% solar-to-hydrogen efficiency, equivalent to ∼35% when the energy of H₂ relative to H₂O (without biochar) is assumed.