Mechanistic Insights into Electrochemical Oxidation of Small Alcohol Molecules

Abstract

Small alcohol molecules such as methanol and ethanol represent promising fuels to power fuel cells for mobile electronic devices, transportation and stationary applications. Ethanol is of particular interest due to its renewability by generation from biomass feedstocks, large energy density and compatibility with existing infrastructures for fuel storage and delivery. Although substantial progress has been made in the recent years for the development of direct ethanol fuel cell (DEFC) technology, obstacles are still present for the practical implication, largely in the lack of efficient and selective electrocatalysts for complete oxidation of ethanol to CO2. Platinum (Pt) is commonly used as electrocatalyst in fuel cells, including DEFCs. However, ethanol oxidation on Pt is found to be incomplete and produces undesired partial oxidation products such as acetaldehyde and acetic acid. It is thereby considered that Pt itself cannot cleave the C-C bond in ethanol molecules and a second transition metal such as Rh and Ir is needed to facilitate this rate- and selectivity-limiting step. However, evidences are also present in the literature for the formation of strongly binding adsorbates, such as -CO and -CH x , at low potentials (<0.5 V vs. RHE), from both in situ molecular spectroscopy (e.g., Infrared (IR) and surface enhanced Raman spectroscopy (SERS)) and differential electrochemical mass spectroscopy (DEMS) studies. These results suggest that oxidative removal of these C1 species, instead of C-C bond cleavage, is the sole rate-limiting factor on Pt catalysts. In attempt to resolve this controversy, we have performed systematic studies of methanol, ethanol and ethylene glycol oxidation on polycrystalline Pt electrodes. In particular, surface-specific sum frequency generation (SFG) spectroscopy is combined with electrochemistry to determine the reaction intermediates and evaluate their dependence on electrode potentials. It is found that C1 adsorbates form at low potentials, and the coverages increase as the electrode potential is raised from 0.1 V to ~0.4 V. While the -CO feature drops above 0.4 V for methanol and ethylene glycol oxidation, it persists up to 0.5 V and then drops for ethanol oxidation. Our results confirm the presence of active Pt sites for C-C bond cleavage and underline the role of -CH x species generated from the β-carbon in governing the kinetics of alcohol oxidation reactions. It is proposed that the slow transition from -CH x to -CO gives rise to the higher overpotentials for ethanol oxidation compared to methanol and ethylene glycol oxidation.