Decreased gas-diffusion electrode porosity due to increased electrocatalyst loading leads to diffusional limitations in cathodic H2O2 electrosynthesis

Abstract

The effect of carbon black electrocatalyst loading on the efficiency of cathodic hydrogen peroxide (H2O2) electrosynthesis in neutral buffered catholytes using gas-diffusion electrodes (GDEs) was evaluated. Increased carbon black loadings on carbon cloth-based GDEs (from 0.5 to 1.5 mg/cm2 to 3.3 mg/cm2) reduced the cathodic coulombic efficiency significantly although no significant advantage in terms of electrochemical performance was noted for the highest loading studied in linear sweep voltammograms. Decreasing cathodic coulombic efficiency during a given batch H2O2 electrosynthesis experiment was a result of increasing catholyte pH, which occurred irrespective of the carbon black loading, and could be alleviated using a higher buffer concentration. Scanning electron microscopy (SEM) and X-ray computed tomography (CT) reconstructions of the GDEs with the different carbon black loadings showed that increased loading leads to lower GDE porosity, which likely causes mass transport limitations, that result in consumption of H2O2in-situ in the electrocatalyst layer, either due to electrochemical reduction or decomposition in high pH conditions. Future studies should focus on engineering GDE structures that allow enhanced mass transport rates for H2O2 out of the GDE, thus also achieving high coulombic efficiencies.