Promoting and controlling electron transfer of furfural oxidation efficiently harvest electricity, furoic acid, hydrogen gas and hydrogen peroxide

Abstract

Conventional chemical oxidation of aldehydes such as furfural to corresponding acids by molecular oxygen usually needs high pressure to increase the solubility of oxygen in aqueous phase, while electrochemical 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 generation 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 (Pmax) 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, Pmax 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 proposed, referring to one-electron and two-electron reaction pathways depending on the fate of adsorbed hydrogen atom transferred from furfural aldehyde group.