Biochar-Assisted Water Electrolysis

This study has experimentally proven an approach to integrate electric energy and chemical energy of biomass into chemical energy of hydrogen by biochar-assisted water electrolysis (BAWE). This type of electrolysis, in other words, electrochemical gasification, consists of hydrogen formation at the cathode and biochar oxidation at the anode, instead of O2 formation. Different from traditional gasification of biochar, BAWE is operated at a temperature below 100 °C and normal pressure. Linear sweep voltammetry showed that the electrolysis of acidified water, when suspended with biochar, occurred at an interelectrode potential as low as 0.5 V, which was much smaller than 1.23 V, the standard potential to split water into hydrogen and oxygen at 25 °C. The performance of biochar depended significantly upon the carbonization temperature for its preparation. It was found that 850 °C was the best carbonization temperature that provided an optimum combination of specific surface area and carbon-type distribution. It was revealed by continuous BAWE that the formation of O-containing functional groups on the biochar surface was predominant over CO2 formation at the anode, while H2 was formed obeying stoichiometry at the cathode. Accumulation of the O-containing groups on the biochar surface decreased its electrochemical reactivity, slowing the electrolysis. Thermal treatment at 850 °C removed the major portion of O-containing groups from the spent biochar, fully recuperating its electrochemical reactivity. CO2 gasification enhanced the biochar activity, and its effect went far beyond the heat treatment. On the basis of the above-mentioned characteristics of BAWE, its combination with CO2 gasification as the biochar recuperator as well as syngas producer is proposed.