Organic redox catalysts for oxygen electroreduction to hydrogen peroxide : an application to drinking water treatment

Conventional H₂O₂ production entails an energy and capital intensive Riedl-Pfleiderer process, which is advantageous based on the economy of scale, yet it generates large amounts of toxic waste. The electrochemical synthesis of H₂O₂ can potentially emerge in small and remote applications, where the transportation and handling of concentrated H₂O₂ can be avoided. The commercial Dow-Huron electrolysis cell has shown some success in the pulp and paper industry. However, its highly caustic product (pH > 13) may limit its wide-spread application. Electrocatalytically, the two electron reduction of O₂ in near neutral or acidic media has proven challenging. In addition to cobalt macrocycle-based catalysts, quinone-based redox catalysts have also been successfully demonstrated as viable electrocatalysts. The present work reports the synthesis of a novel riboflavinyl-anthraquinone (RF-AQ) compound which showed redox catalytic activity for O₂ reduction to H₂O₂. Cyclic voltammetry with a rotating ring-disk electrode assembly was employed to characterize the catalyst. Chromoamperommetry experiments in a batch electrolysis cell were performed, using 0.5 M H₂SO₄ saturated with O₂, up to 24 hours at 21°C and 1barabs to demonstrate the longer term H₂O₂ synthesis. Modifications of the Vulcan XC72 by RF-AQ adsorption increased the onset potential of the O₂ reduction reaction by up to 50 mV compared to Vulcan XC72 alone. A H₂O₂ selectivity of up to 85 ± 5% was observed for the RF-AQ catalyst. Chronoamperommetry, via constant potential control at 0.1V vs. RHE, with the 10 wt% RF-AQ catalyst (composite loading of 2.5 mg cm⁻²) generated H₂O₂ with an initial rate (in two hours) of 21 µmol hr⁻¹ cm⁻² (normalized by the electrode geometric area) and accumulated up to 425 µmol cm⁻² (normalized by the electrode area) in 24 hours with a current density of about 1.3 mA cm⁻² at 70 ± 5% current efficiency. While the unmodified Vulcan XC72, with a similar catalyst weight loading and the same cathode potential, generated H₂O₂ with an initial rate of 6 µmol hr⁻¹ cm⁻² (normalized by electrode area) and accumulated only up to 140 µmol cm⁻² (normalized by electrode area) in 24 hours with a current density of about 0.55 mA cm⁻² at 55 ± 5% current efficiency.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat