Electrochemical production of hydrogen peroxide via two-electron water oxidation using boron-doped diamond

The two-electron water oxidation reaction (2e- WOR) is an unorthodox yet promising pathway for the electrochemical production, during water electrolysis, of hydrogen peroxide (H2O2), a strong, green, and versatile oxidant, with applications in paper, pulp and textile bleaching, chemical synthesis of organic and inorganic peroxides, and wastewater treatment, to name a few. Mass-produced by the centralised and complex anthraquinone auto-oxidation (AO) process, researchers have, over the last century, explored alternative, and competitive, methods for generating H2O2 in a localised, sustainable, and risk-free manner. Electrochemistry may offer such a route, and the two-electron oxygen reduction reaction (2e- ORR) has received widespread attention within the research community for its high faradaic efficiencies (FEH2O2 > 90%) and impressive H2O2 production rates, using carbon-based electrocatalysts in alkaline media. Nonetheless, it is generally accepted that, to compete with the industrial behemoth that is the AO process, exploitation of the anode during the 2e- ORR (or even electrochemical water splitting) to generate aqueous H2O2 solutions, instead of oxygen gas (O2), is imperative. Consequently, over the past decade, a plethora of studies have investigated the capability of metal oxides to catalyse the unconventional 2e- WOR, with notable progress made concerning the selectivity of the proposed 2e- WOR metal oxide electrocatalysts, albeit at low electric currents. At elevated current densities (j > 100 mA cm-2), these state-of-the-art materials exhibit a diminished electrochemical performance, particularly concerning H2O2 production and accumulation, as well as chemical and mechanical stability. The ideal 2e- WOR catalyst-candidate will withstand hundreds of hours of electrolysis at current densities in the order of 102 - 103 mA cm-2, while synchronously delivering a large H2O2 output (molar concentration order of 0.1 M) with (virtually) total selectivity. Accordingly, the research detailed within this PhD dissertation is centred on the development, and testing, of such a highly active, selective, and robust anode, alongside the engineering of an equally-essential and compatible aqueous supporting electrolyte, for the assembly of an enhanced, overall, 2e- WOR system. Boron-doped diamond (BDD) is selected as a prospective 2e- WOR electrocatalyst for its well-documented ability to oxidise organic pollutants (due to the formation of the hydroxyl radical) and its utilisation to electro-synthesise persulfates, industrially. Thus, a custom-made BDD-titanium (BDD/Ti) anode is initially introduced as a potential 2e- WOR catalyst-candidate and it is found that in an H-cell containing 25 mL of potassium hydrogen carbonate (2 M KHCO3) as an anolyte, H2O2 concentrations of up to 29 mM can be attained at current densities of 300 mA cm-2 with H2O2 production rates of 19.7 μmol cm-2 min-1, albeit at the cost of a moderate faradaic efficiency (maximum 28%). By modifying the key features of the BDD coating (boron doping level, sp3/sp2 ratio, crystallite size, coating thickness and roughness), and replacing the Ti substrate with niobium (Nb), six new bespoke BDD/Nb anodes are manufactured and assessed for their capability to further optimise the 2e- WOR, in 25 mL of a mixed potassium carbonate and bicarbonate (K2CO3/KHCO3) electrolyte, specifically engineered to utilise the redox catalytic effect of the carbonate ion (CO32-) on H2O2 production via water oxidation. It is discovered that the likely synergistic relationship between this novel electrocatalyst-electrolyte combination results in the electrosynthesis of H2O2 concentrations in the order of 0.11 M, at applied current densities of 200 - 300 mA cm-2, with H2O2 production rates reaching 76.4 μmol cm-2 min-1, faradaic efficiencies up to 87% and partial current densities of 247 mA cm-2, for ten hours of continuous electrolysis. Lastly, the impact of aqueous K2CO3 solutions and their primary characteristics (concentration, conductivity, and pH) on the 2e- WOR using BDD/Nb is scrutinised in parallel with some electrochemical kinetics experiments and an appraisal of the most accurate techniques for the chemical quantification of electrochemically produced H2O2. Highly concentrated (4 - 5 M) K2CO3 mixtures (25 mL) allow for the acquisition of current density values of around 0.5 A cm-2 and faradaic efficiencies above 90% at overpotentials of approximately 1,000 mV for the 2e- WOR, and it is noted that potassium permanganate (KMnO4) titration is a dependable method for determining the amount of H2O2 electro-synthesised in carbonate. These results firmly establish BDD and K2CO3 as a suitable and cost-effective electrocatalyst-electrolyte combination for the optimised and stable electrochemical production of H2O2 via the 2e- WOR at industrially relevant current densities. Combined with an equally-desirable and compatible cathodic process like the 2e- ORR, or even the reduction of carbon dioxide (CO2) to coveted organic commodities like ethylene (C2H4), this work can lay the foundation for the construction of parallel paired electrosynthesis reactors that can generate two valuable chemical molecules using one single pass of electric charge, in a cost-effective, localised and waste-free approach that will rival mainstream industrial standards of chemical synthesis.