Enhancing biochar oxidation reaction with the mediator of Fe2+/Fe3+ or NO2-/NO3- for efficient hydrogen production through biochar-assisted water electrolysis

The biochar-assisted water electrolysis (BAWE) is a promising method for clean biomass utilization and hydrogen production. But it is limited by the slow anode biochar oxidation reaction (BOR). To enhance the BOR, the mediator of Fe2+/Fe3+ or NO2-/NO3- was added to the pyrolytic biochar (PB) and tested using electrochemical methods. Both Fe2+ and NO2- showed stronger oxidation activity than Fe3+ and NO3-, and thereby presented more significant enhanced role on BOR, including the reduction in onset potential and the enlargement of current density. Fe2+/Fe3+ existence resulted in the five-step mechanism such as adsorption, redox reaction in solution, oxidation at electrode surface, and passive films formation on the biochar surface. 0.5 mol/L or less of Fe2+/Fe3+ was suggestable, while the lower acid concentration was necessary to avoid side reaction with NO2- existence. The Fe2+/Fe3+ acted as catalysts due to the almost unchanged total iron ions during continuous BAWE process, whereas a high consumption of NO2- occurred. The cathode hydrogen production was less influenced by mediator and showed comparable with the theoretical value of Faraday's law. In addition, the activation energy of BOR was significantly reduced with Fe2+ and NO2- mediation. The present work provides a favorable strategy for improving BOR and energy-saving production of hydrogen via BAWE process.