(23c) Electrocatalytic Generation of H2O2: Carbon Based Material Synthesis and Device Design for Portable Low Cost Water Purification

Hydrogen peroxide, produced at a rate of 3 million tons/year, is one of the most important industrial chemicals widely used as a bleaching and oxidation agent.¹ Compared to chlorine which is the most commonly used water purification agent at the moment, hydrogen peroxide does not generate harmful byproducts after decomposition, and the higher oxidation potential makes it a more favourable reagent for treatment of water pollutants.² Hydrogen peroxide is mainly produced in large-scale facilities using a batch anthraquinone oxidation process, facing the challenge of high transportation and capital cost as well as explosive hazard during storage. For further applications in developing world a selective electrochemical catalyst towards H₂O₂ production is important and a prototype fuel cell/electrolyzer-solar cell device needs to be designed to enable continuous, small-scale and decentralized production of H₂O₂.

Through DFT calculation, we have predicted that different types of nitrogen-doped (N-doped) carbon or carbon with defects are able to selectively catalyze a 2-electron transfer oxygen reduction. We synthesized different types of carbon based materials and tested them along with some commercially available mesoporous carbons. Our results suggest that carbon defect sites without heteroatom doping could serve as the active center. The best catalyst by far is able to catalyse the reaction with no over-potential under alkaline condition with over 90% selectivity.

In this study, prototype devices, both fuel-cell and electrolyzer setups have also been designed and tested. Early results in a fuel-cell setup shows that H₂O₂ has higher affinity towards the aqueous phase as compared to the gas phase. Also H₂O₂ can be easily decomposed in contact with transition metals cations and reductive organic compounds. Considering the fact that electrocatalytic H₂O₂production will have the largest impact in the developing world, itâ??s desirable to have a simple material manufacturing process and low material cost. All these considerations lead to the current electrolyzer design using plastic with no Nafion membrane and precious metal catalysts involved.

In our electrolyzer design, the produced H2O2 is up-concentrated in the electrolyte. At the current stage, the electrolyzer cell is able to operate with 50 mA overall current at pH 13 with an applied cell potential 1.7 V and faradaic efficiency close to 100%. The device can also be connected to a PV system as the energy supply. A ten-minute operation will able to accumulate 10ml solution with the concentration around 500mg/L.

1. Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G., Hydrogen peroxide synthesis: An outlook beyond the anthraquinone process. Angew Chem Int Edit 2006, 45(42), 6962-6984.

2. Chu, S.; Majumdar, A., Opportunities and challenges for a sustainable energy future. Nature 2012, 488 (7411), 294-303.