Biochar and Fe2+ mediation in hydrogen production by water electrolysis: Effects of physicochemical properties of biochars

Electrolysis of water to produce hydrogen can consume excessive renewable power and generate high-value hydrogen. Biochar-assisted water electrolysis for producing pure and green hydrogen, substituting the oxygen evolution reaction at the anode with biochar oxidation reaction (BOR), could significantly reduce the starting potential and increase the current density during water electrolysis, combining excessive renewable electricity and biomass waste utilization. However, slow BOR limits the electrolysis. In this study, the mediator of Fe2+ and biochars with different treatments were used to enhance the BOR in electrochemical experiments. Pickling pretreatment improves the performance of pyrolysis and hydrothermal biochars in BAWE containing Fe2+ media, especially the latter, which has the highest oxidation current density of 180 mA/cm2 at an anode voltage of 1.2 V vs. MSE. The effects of different oxygen-containing functional groups on the current density are determined by comparing different biochars before and after pickling. The abundance of –OH and CO groups favors the increase of oxidation current, while the C–O groups from anhydride and ether play a negative role. Pickling allows biochars to obtain a higher specific surface area and enrich its pores, thereby improving electrolytic performance. Further activation of KOH can increase the specific surface area and make the pore structure, especially the micropores, more abundant, which is conducive to the further increase of oxidation current. This work is expected to lead to a more efficient use of biochar in the BAWE process.