Effects of water constituents on the stability of gas diffusion electrode during electrochemical hydrogen peroxide production for water and wastewater treatment

Electrochemically producing hydrogen peroxide (H2O2) from oxygen reduction reaction (ORR) with natural air diffusion electrode (NADE) is an attractive way to supply H2O2 for decentralized water treatment. In this study, the stability of NADE during H2O2 electroproduction in varying water matrices were evaluated, including synthetic electrolyte solutions (0.05 M Na2SO4) with or without calcium ions (Ca2+, 200 mg/L) and/or humic acid (HA, 40 mg/L), as well as a selected municipal wastewater (92.7 mg/L Ca2+, 3.6 mg/L Mg2+, and 23.9 mg/L total organic carbon). The results show that NADEs maintained a good stability during H2O2 electroproduction in Na2SO4 solutions regardless of the presence of HA. However, Ca2+ (and Mg2+) could form significant amounts of mineral precipitates on the surface and in the internal pores of NADEs during H2O2 electroproduction. These mineral precipitates can negatively influence H2O2 production by impeding the oxygen, electron, and proton transfer processes involved in ORR to H2O2. Moreover, the mineral precipitates shifted the NADEs from hydrophobic to hydrophilic, which may promote H2O2 reduction to H2O at the NADEs. Consequently, the apparent current efficiencies of H2O2 production decreased substantially from initially ∼90% to 50%–70% as the NADEs were continuously used for 60 h in the Ca-containing solutions and selected wastewater. These results indicate that water constituents that are commonly present in real water matrices, especially Ca2+, can cause serious deterioration of NADE stability during H2O2 electroproduction. Therefore, proper strategies are needed to mitigate electrode fouling during H2O2 electroproduction with NADEs in practical water and wastewater treatment.