Electrochemical Oxidation of Primary Alcohols Using a Co2NiO4 Catalyst: Effects of Alcohol Identity and Electrochemical Bias on Product Distribution

The electrochemical reduction of CO2 and H2O to solar fuels remains a promising strategy for storing intermittent energy sources in the form of chemical bonds. These electrochemical reductions occurring at the cathode typically are coupled to the oxygen evolution reaction at the anode. The electrochemical oxidation of organic alcohols in the alcohol oxidation reaction is a promising alternative anode reaction that occurs at decreased operating potentials compared to the oxygen evolution reaction and that produces more valuable products than O2. Co2NiO4 is a particularly promising catalyst for the oxidation of alcohols, able to promote alcohol oxidation at current densities of 10 mA cm–2 at potentials of only 1.42 V vs reversible hydrogen electrode (RHE) in alkaline aqueous conditions, significantly less positive than typical potentials required for the oxygen evolution reaction. In this work, we study the alcohol oxidation reaction by Co2NiO4 for a series of straight-chain primary alcohols of increasing chain length from ethanol to n-pentanol. We show that the product distribution for alcohol oxidation depends on the alcohol chain length, changing from primarily aldehyde products for shorter-chain alcohols to primarily carboxylic acid products for longer-chain alcohols. These results suggest that alcohols are oxidized sequentially to first aldehydes and then carboxylic acids at Co2NiO4. During the oxidation of longer-chain alcohols, the aldehyde intermediates are retained at the catalyst surface for longer times, facilitating further oxidation to terminal carboxylic acid products. We also explored the potential-dependent activities and product distributions for n-butanol oxidation at Co2NiO4, and showed that alcohol oxidation is able to outcompete chloride oxidation in aqueous solutions containing Cl– at seawater concentrations. These studies provide further insight into the alcohol oxidation reaction at Co2NiO4 and highlight its promise as an alternative anode reaction for the production solar fuels.