Engineering electrodes and electrolytes for two-electron water oxidation to H2O2

The efficient conversion of renewable energy to useful chemicals and fuels is a promising means by which to decrease global reliance on unsustainable chemicals and processes which have been the root of our economy for generations. The primary devices capable of achieving these transformations are electrolyzers, which combine renewable energy with another plentiful resource, water, to produce valuable chemicals. Holistically, electrolyzers are limited by the kinetically sluggish reaction of water oxidation, which typically produces oxygen gas (O2). Though O2 is of use in submarine and aerospace applications, it often is vented to the environment when produced on land. Decades of research have focused on catalysis for improving the efficiency of water oxidation to O2, but economically viable systems remain elusive in part due to the cost of fabricating electrodes with precious metal catalysts. Recently the selective oxidation of water to produce a more versatile chemical, hydrogen peroxide (H2O2), has gained attention as an alternative means of valorizing and possibly improving the efficiency of the electrochemical oxidation of water. Despite this value proposition, the conditions, materials, and systems necessary to efficiently oxidize water through this two-electron process to H2O2 demand further research. This thesis presents research in developing water oxidation systems toward improved value and thermodynamic performance. Through the development of a novel electroless deposition procedure, we establish a method for depositing nickel as a non-precious water oxidation catalyst on silicon anodes used for the production of O2. This synthesis method is shown to be suitable for several micro and nanostructured silicon substrates and it improves the cost of fabrication and manufacturability of anodes for water oxidation by replacing state-of-the-art precious metal catalysts like iridium and ruthenium with nickel. Still, the value of this water oxidation system is limited as the evolved product is O2 gas. Accordingly, the remainder of this thesis focuses on enhancing the production of H2O2 from water oxidation. To do so, standardized procedures for two-electron water oxidation experiments are developed. Particularly important are experiments for accurately measuring H2O2 accumulated in electrochemical environments, for which an adapted cobalt-carbonate ultraviolet-visible spectroscopic procedure is found to be robust across a wide array of water oxidation environments. After standardized experiments for evaluating H2O2 production performance are established, these methods are utilized to measure the performance of a known, promising metal oxide catalyst in bismuth vanadate (BiVO4). Through a simple wet chemical procedure, the BiVO4 host is doped with gadolinium, showing improved activity (110 mV decrease in overpotential), selectivity (~99% faradaic efficiency toward H2O2) and catalytic lifetime for H2O2 production. In addition to improving the anode, we investigate the role that the electrolyte, especially bicarbonate (HCO3-), plays in selectively generating H2O2. Through an array of electrochemical and spectroscopic experiments, we find that HCO3- is directly oxidized at modest potentials (ca. 2.5 V vs. RHE) on BiVO4 anodes, which leads to subsequent, homogeneous water oxidation to H2O2. At higher potentials, CO32- plays a promotional role in H2O2 production. These results lead to engineering the composition of the electrolyte toward greater H2O2 selectivity. Ultimately, we find that an optimized electrolyte composition (0.5M KHCO3 / 3.5M K2CO3) produces H2O2 at a rate more than tenfold greater than the commonly used 2M KHCO3 electrolyte without sacrificing stability or increasing material cost. With the demonstrated improvement in H2O2 production rate and electrochemical stability, these electrolyte and anode engineering procedures can help enable the valorization of water oxidation as a means of renewable energy storage and distributed H2O2 production

Keywords: Mechanical Engineering.
Advisor: Zheng Xiaolin
Cappelli Mark A. (Mark Antony)
Wang Hai