Effect of Carbon Black Electrocatalyst Thickness and Composition on Electrosynthesis of Hydrogen Peroxide on Gas Diffusion Electrodes

The in-situ electrosynthesis of hydrogen peroxide (H2O2) from reduction of oxygen is a promising method to produce a strong oxidant and disinfectant for application in the water and wastewater treatment industry. Capable of producing low concentrations of H2O2 with no aeration requirements and harmless by-products of water and oxygen, the electrosynthesis of H2O2 using gas diffusion electrodes is advantageous. This research examines the impact that electrocatalyst thickness and composition have on the production efficiency of H2O2 at the gas diffusion layer of a gas diffusion electrode. From these results, the optimum electrocatalyst loading and composition with its respective energy requirements is assessed. An increase in electrocatalyst loading on the gas diffusion layer saw a greater thickness of the electrocatalyst layer, increasing the perpendicular diffusion pathway required for H2O2 to reach the electrolyte solution. At a current density of 1 mA/cm2, the lowest electrocatalyst loading of 0.5 mg/cm2 produced the highest H2O2 concentration of 807.54 mg/L and maximum current coulombic efficiency of 53%. The H2O2 produced from the additional three electrocatalyst loadings of 1.5, 3.0, and 5.0 mg/cm2 decreased linearly with increasing loading. An increase in the length of the diffusion pathway allows more time for H2O2 to accumulate and degrade to H2O or O2. As current density was increased, higher yields of H2O2 were achieved for all loadings, suggesting less chemical degradation of H2O2 to O2 as the higher electrocatalyst loadings significantly improved current efficiencies to match the lower electrocatalyst loadings. An energy input analysis calculating the mass produced/energy input in kg H2O2/kWh demonstrated a benefit in selecting higher electrocatalyst loadings at higher current densities. Although the highest electrocatalyst loading of 5.0 mg/cm2 produced the lowest concentrations of H2O2 at a current density of 5 mA/cm2, it accomplishes the greatest mass produced/energy input of 3.84 kg H2O2/kWh. The effect of electrocatalyst composition was examined as electrocatalyst loadings of 1.5 and 5.0 mg/cm2 were tested with and without proton exchange polymer, Nafion, in the electrocatalyst carbon ink. The electrocatalyst loadings comprised of a carbon ink without Nafion resulted in greater H2O2 concentrations and current efficiencies, with smaller differences between loadings compared to the results for loadings containing Nafion. At the cathode surface, pH increases rapidly, where H2O2 exists as its anion, hydroperoxide (HO2-). Interaction with the negatively charged sulfonic groups (SO3-) in the proton exchange polymer, Nafion, causes resistance to the mass transport of HO2- through the electrocatalyst layer. The HO2- can then accumulate, be degraded, and result in lower measured concentrations of H2O2 in the electrolyte