Mechanistic Insight from Acetate Oxidation in Three-Electrode Cells and Flow Cells

Abstract

Recent years has seen a tremendous growth in finding electrochemical pathways to produce chemicals and fuels because it is believed that pairing low-carbon electricity (solar, wind, nuclear) with renewable carbon sources and electrochemical processes will lead to a reduction in greenhouse gas emissions. One of the possible sources renewable carbon is biomass. During biomass upgrading, one of the byproducts is bio-oil, which contains a large amount of acetic acid. Acetic acid itself is a relatively low value product. Upgrading acetic acid (or its alkaline counterpart acetate) to a wider range of valuable products (e.g., methanol, ethylene) is extremely attractive. This can be done through thermochemical or electrochemical partial oxidation.Though acetic acid reactions have been studied for many years, details about its overall reaction mechanism still remain unknown. The most known and defended pathway in the literature is Kolbe electrolysis, where C-C bonds are formed from dimerization of methyl radicals as shown in Equation 1.2CH3COOH → 2CO2 + C2H6 + 2H+ + 2e- (Equation 1)However, aqueous environments have shown deviations from this dimerization. It is often overlooked that these reactions occur at high potentials and the state and role of the formed surface oxide are not well described in the literature. It is even not known what water-oxide mechanism dominates for the formation of alcohol-based products. Although prior work has shown that a three-electrode batch cell can produce the typical Kolbe products (ethane and carbon dioxide) at high faradaic efficiencies and steady state when the electrolyte bath is held at the pKa (4.7) and high voltage (>3.0V vs. RHE) – our previous studies have shown that the reaction environment (e.g. concentration, addition of supporting electrolyte, pH) can vary the effluent composition and reaction rate considerably. Also, our recent work has shown that transitioning the same catalyst to a lower-water environment in a flow cell can yield a completely different product profile than in aqueous media.Therefore, the purpose of this study is to demonstrate how the catalyst and reacting environment affect the formation of various C1 and C2 products from acetate electrochemical oxidation. To do this, custom-made two-electrode (similar to an alkaline exchange membrane electrolyzer) and three-electrode cells are used. Another variable that is manipulated is the voltage profile, where both constant voltage and pulsed experiments are performed. In the flow cell, several anode electrocatalysts (e.g., Pt, PtRu, NiO, IrOx) were dispersed in inks and sprayed onto porous transport layers. The anode feed was 0.5M potassium acetate. No gases were fed to the cathode, but hydrogen was collected from the exhaust. The anode effluent (gas and liquid mixture) was separated downstream in a simple vessel with UHP helium flowing through the incoming effluent to carry the gas for GC/MS analysis. Liquid samples were analyzed by ex situ and NMR with D2O solvent. Two-electrode and three-electrode results will be combined to provide new insights into acetate oxidation and pathways for future research and development will be discussed.