Biochar-assisted water electrolysis for energy-saving hydrogen production: Evolution of corn straw-based biochar structure and its enhanced effect on Cr(VI) removal

Biochar-assisted water electrolysis (BAWE) offers a novel strategy for clean hydrogen production and biochar utilization. Here we propose the production of activated biochar via limited biochar electrooxidation over Ni foam and its application to remove Cr(VI). The corn straw-based biochar oxidation reaction delivers 100 mA cm-2@1.528 V vs. RHE, and reaches 98% Faradaic efficiency for H2 production. The electrical consumption of 4.16 kWh Nm-3 H2 for BAWE is lower than 5.07 kWh Nm-3 H2 for water electrolysis. After 10 h electrolysis at 5 mA cm-2, due to the direct oxidation of biochar at electrode and indirect oxidation by reactive oxygen species in solution, the activated biochar is formed with increased pore size and specific surface area, and full of -OH, C-O, C=O, and COOH groups. The removal rate of Cr(VI) by activated biochar reaches 72.79%, much higher than the 10.96% for original biochar, which is attributed to the well-developed mesoporous structure of activated biochar for Cr(VI) adsorption, and the enriched -OH and COOH for Cr(VI) reduction and Cr(III) complexation. These findings confirm the feasibility of electrochemical activation of biochar in BAWE process, which demonstrate a new avenue for biochar upgrading and energy-saving hydrogen production for a sustainable future.