Electrocatalysis of Direct Methanol and Ethanol Oxidation in Polymer Electrolyte Fuel Cells

In recent decades, significant efforts have been focused on the direct electrochemical oxidation of alcohol and hydrocarbon fuels. Organic liquid fuels are characterized by high energy density and the electromotive force associated with their electrochemical combustion to CO₂ is comparable to that of hydrogen combustion to water. Among the liquid organic fuels, methanol and ethanol have promising characteristics in terms of reactivity at low temperatures, storage and handling. Compared with ethanol, methanol has the significant advantage of faster reaction kinetics and higher selectivity to CO₂ formation for the electrochemical oxidation process.

Highly dispersed carbon-supported bimetallic PtRu and PtSn alloy catalysts are widely recognized among the most performing anode formulations for these processes. Alloying Pt with Ru and Sn promotes oxidation of methanol and ethanol by the adsorption of OH species at considerably lower overpotentials and, thus, favoring  the occurrence of a bifunctional mechanism. Furthermore, the electronic effect caused by a second metal on the neighboring Pt atoms affects the strength of CO adsorption on the catalyst surface. This causes a decrease of the coverage of poisoning CO intermediate species. Catalysts characterized by a high degree of alloying and metallic behavior on the surface appear to be very active towards methanol oxidation.

However, beside the alloyed catalysts, noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx, TiOx and VOx) can be suitable promoters for methanol and ethanol oxidation in acidic environment.  An effective use of such oxide promoters in combination with the alloy catalysts can provide a multifunctional catalytic system.  Recent studies  carried out in our laboratory have shown that IrOx can give rise to a significant promoting effect, even larger than that of RuOx, both in the case of methanol and ethanol oxidation. Whereas, the electrocatalytic enhancement produced by the valve metal oxides is generally  lower than IrOx and RuOx but well evident. These results are interpreted in terms of the different water displacement mechanisms for the various oxides and the related effects on adsorbed CO removal.

The effect of temperature is also discussed with reference to the coverage of adsorbed methanolic residues or change in selectivity in the case of ethanol electro-oxidation.

For the ethanol oxidation process, anode catalyst selectivity towards CO₂, acetic acid and acetaldehyde reaction products is discussed in relation to the alloy and oxide content in the catalyst. In particular, SnO_x species on the surface of Sn-rich Pt-Sn-based electrocatalyst appear to assist the further oxidation of ethanolic adsorbates, leading to larger yields of acetic acid and CO₂.

In addition to the enhancement of reaction rates, there is an effect of the promoter on the stability of the bimetallic alloy as evidenced by accelerated stress tests.

All these evidences seem to indicate that a multifunctional catalyst may represent a valid route to enhance performance and reliability of methanol and ethanol electro-oxidation processes at low temperature.