Controlling product distributions from ethanol electrolysis

Electrolysis of bioethanol in proton exchange fuel cells has the potential to become an important technology for sustainable production of green hydrogen and platform chemicals. However, the economic competitiveness of the process relative to water electrolysis is uncertain and will depend strongly on the demand for acetic acid, acetaldehyde, and other by-products. Controlling the product distribution is consequently a crucial element for development of ethanol electrolysis technology. This review focuses on the factors that determine product distributions and mechanistic models of the reaction pathways. Most research on catalysts for electrochemical oxidation of ethanol has focused on increasing current densities and selectivity for the complete oxidation to carbon dioxide. However, a viable technology will require valorization of other products to offset the cost of ethanol. It is therefore important to evaluate and compare catalysts based on the net cost of hydrogen production. Fundamental studies of ethanol oxidation in aqueous electrolytes combined with density functional theory models have provided a comprehensive view of pathways and mechanisms, and of the factors that control product distributions. This provides a background for understanding product distributions obtained from electrolysis cells, and ethanol fuel cells. Generally, it has been found that lower ethanol concentrations and higher temperatures favour the complete oxidation of ethanol to carbon dioxide. Production of acetic acid peaks at intermediate concentrations, and exclusive production of acetaldehyde can be obtained at high concentrations. Pure Pt and PtRh catalysts provide the highest selectivity for complete oxidation. Current densities at low potentials can be increased by combining Pt with oxyphilic metals such as Ru and Sn, although this leads to low selectivity for complete oxidation.