Abstract: Conventional chemical oxidation of aldehydes such as furfural to corresponding acids by molecular oxy-gen usually needs high pressure to increase the solubility of oxygen in aqueous phase, while electro-chemical oxidation needs input of external electric energy. Herein, we developed a liquid flow fuel cell (LFFC) system to achieve oxidation of furfural in anode for furoic acid production with co-production of hydrogen gas. By controlling the electron transfer in cathode for reduction of oxygen, efficient gener-ation of electricity or production of H₂O₂ were achieved. Metal oxides especially Ag₂O have been screened as the efficient catalyst to promote the oxidation of aldehydes, while liquid redox couples were used for promoting the kinetics of oxygen reduction. A novel alkaline-acidic asymmetric design was also used for anolyte and catholyte, respectively, to promote the efficiency of electron transfer. Such an LFFC system achieves efficient conversion of chemical energy of aldehyde oxidation to electric energy and makes full use the transferred electrons for high-value added products without input of external energy. With (VO₂)₂SO₄ as the electron carrier in catholyte for four-electron reduction of oxygen, the peak output power density (P_max) at room temperature reached 261 mW/cm² with furoic acid and H₂ yields of 90% and 0.10 mol/mol furfural, respectively. With anthraquinone-2-sulfonate (AQS) as the cathodic electron carrier, P_max of 60 mW/cm² and furoic acid, H₂ and H₂O₂ yields of 0.88, 0.15 and 0.41 mol/mol furfural were achieved, respectively. A new reaction mechanism on furfural oxidation on Ag₂O anode was pro-posed, referring to one-electron and two-electron reaction pathways depending on the fate of adsorbed hydrogen atom transferred from furfural aldehyde group.

Key words: Oxidation of furfural, Liquid flow fuel cell, Electricity generation, Hydrogen production, Electron transfer