(52d) An Experimental Model of Formic Acid Adsorption on Copper Electrocatalysts

Formic acid fuel cells have garnered considerable interest due to their safety, minimal fuel crossover and superior energy density compared to other proton-exchange fuel cells. Successful design of better electrocatalysts for formic acid oxidation should consider the reaction mechanism, which includes a first adsorption step of formic acid on a charged surface. Thus, understanding the adsorption behavior of this species under different reaction conditions is critical to optimize its conversion to CO₂ and improve cell performance. In this work, we investigated the adsorption of formic acid on copper at different temperatures and potentials by using surface enhanced infrared spectroscopy.

Polarization of copper electrodes in the presence of formic acid led to the formation of a band centered at 1710 cm^-1, which was attributed to the C=O stretching mode of adsorbed formate. We showed that, at –0.2 V, the coverage of this anion follows a Langmuir-type isotherm for all temperatures between 25 and 75 °C (Fig. A). Higher temperature led to increased formate coverage for a given bulk concentration, which allowed us to estimate a positive DDH = 6.33 kJ/mol associated with the surface replacement of adsorbed electrolyte species by formate (Fig B). Further kinetic studies and isotherms collected at different potentials suggest that there is slightly more formate adsorbed at more positive potentials for a given temperature. Our results suggest that quantitatively understanding the binding strength of formic acid to charged electrodes is key to better design electrocatalysts under operating conditions that favor formic acid oxidation.