Date of Award
August 2020
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Environmental Engineering and Earth Sciences
Committee Member
Sudeep C Popat
Committee Member
David L Freedman
Committee Member
David A Ladner
Abstract
The electrochemical synthesis of H2O2 by the cathodic reduction of O2 is an on-site alternative to the current industrial scale method of production. Carbon-based gas diffusion electrodes (GDEs) are selective to H2O2 synthesis and inexpensive compared to their precious metal counterparts. The research presented here examines the role electrolyte pH in the cathode has on the efficiency of H2O2 electrosynthesis, particularly as it pertains to the rate of H2O2 production on the electrode and subsequent degradation in the cathode chamber. From these results, the optimization of the cathode surface and environmental conditions is then considered.
The overall performance was dependent on the recirculation rate of the electrolyte in 4-hour batch experiments. Increasingly turbulent conditions at the surface of the cathode decreased the diffusion layer thickness and accelerated the mass transport of co-synthesized H2O2 and OH- with peak performance occurring at a catholyte recirculation rate of 60 mL/min with a maximum cathodic coulombic efficiency (CCE) of 68%. Minimal residence time on the surface of the cathode reduces the chance for the deleterious electrochemical reduction of H2O2 to H2O. High recirculation rates were favored at initial reaction times (t < 2 hour) but the rising bulk electrolyte pH caused by the diffusion of OH- resulted in a larger drops in CCE over time. Alkaline environments yielded the highest H2O2 concentrations with a maximum concentration of 1.78 g/L in pH 13.5 after a 4-hour reaction time. The highest concentrations of H2O2 synthesized at pH 13.5 were in spite of the rapid degradation that occurred in alkaline conditions. Bulk, pH-driven degradation rates peaked at pH 12 while concurrent bulk and electrochemical reduction was rampant and nearly uniform across all pH regimes. A Tafel analysis demonstrated a mechanistic shift in the catalytic reaction taking place at pH ≥ 11.5 and is hypothesized to be in favor of the 2-electron reduction pathway, thus demonstrating that pH also influences the selectivity of the reaction. High overpotential Tafel slopes transitioned from ~240 mV/decade to 120 mV/decade at acidic and neutral to alkaline conditions. Stable production efficiencies were achieved in concentrated buffer solutions that effectively neutralized the bulk degradation pathway that appears with increasing pH.
Recommended Citation
Lindsay, Spencer, "Effect of Electrolyte pH on the Electrosynthesis of Hydrogen Peroxide on Carbon Black-based Gas Diffusion Electrodes" (2020). All Theses. 3384.
https://open.clemson.edu/all_theses/3384