Proton exchange membrane flow reactor with ozone-treated gas diffusion layers for production of pure H2O2 in aqueous and methanol solutions†

Electrocatalytic production of pure H₂O₂via the two-electron oxygen reduction reaction (2e⁻ ORR) in a proton exchange membrane (PEM) flow reactor is an ideal process for on-site and on-demand H₂O₂ usage. However, the low selectivity is challenging due to the successive reduction of generated H₂O₂ remaining on the electrocatalyst in a membrane electrode assembly with a gas diffusion layer (GDL). This study investigates the selective 2e⁻ ORR in an H₂–O₂ PEM flow reactor (PEMFR) using a cobalt electrocatalyst with various cathode GDLs to optimize the transport of solvent to recover the generated H₂O₂. The effects of wettability and hydrophobicity were tested using pristine, polytetrafluoroethylene (PTFE) treated, and ozone-treated carbon GDLs under simultaneous water and O₂ gas flow. The ozone-treated GDL, with slight hydrophilicity, continuously produced 46 mM H₂O₂ (1560 ppm) with a faradaic efficiency (FE) of ∼75% at a current density of 20–30 mA cm⁻². In contrast, pristine and PTFE-treated GDLs resulted in a low H₂O₂ FE below 15%. The ozone-treated GDL on the flow channel side was critical for transporting liquid water to recover the generated H₂O₂. The production of H₂O₂ in methanol instead of water also achieved ∼80% FE, without applying energy bias.