Platinum is the most active single-component catalyst for methanol electrooxidation in DMFCs, however poisoning reactions on the surface in acidic electrolytes render the anode ineffective under target operating conditions. As an approach to designing better catalysts for this system, a number of in situ, ex situ and on-line techniques have been utilized to obtain information on the nature of the poisoning intermediate(s). While significant advances have been made, no current in situ technique can yield detailed quantitative information on practical (i.e. supported, dispersed) electrocatalysts. Nuclear magnetic resonance (NMR) spectroscopy is a quantitative, non-destructive method of probing the chemical environment of a specific nucleus. During the last two decades the technique has been used successfully in the field of gas-phase catalysis as a tool for identifying and characterizing chemisorbed species on practical catalysts. Our research in collaboration with Prof. Jeffrey A. Reimer has successfully extended the application of NMR spectroscopy to studies of surface poisoning of carbon-supported platinum and platinum-alloy DMFC anodes in operating electrochemical cells.
We have constructed a glass three-electrode electrochemical cell for use in a narrow-bore (5 cm) spectrometer operating at a proton frequency of 270 MHz. The working electrode material is commercially prepared 20% Pt/Vulcan XC-72 supported on thin carbon cloth. This cloth is rolled tightly to form a cylindrical porous plug, filling the volume of the NMR coil with an active catalyst surface area on the order of 3m2. The electromagnetic coupling of the conductive electrode material with the coil presents a special problem for these experiments. To minimize this effect, a porous separator is wound with the cloth to electronically insulate adjacent layers of the plug.
We have carried out studies of the model system of CO adsorbed on Pt. CO is an important electrocatalyst poison present in reformed methanol and hydrocarbon fuels used in fuel cells. We used a circulation system for adsorption of 13C-enriched CO from saturated aqueous H2SO4. As an indirect monitor we used voltammetry to observe the displacement of adsorbed H2 from the Pt surface by the irreversibly adsorbed CO. We have studied the 13C NMR signal arising from 13CO adsorbed on the electrodes described above open-circuit conditions, and at a variety of controlled potentials. We have been successful in identifying three different surface species resulting from the adsorption of CO on Pt/C, Pt-Ru/C, and Pt-Sn/C at 25C. We are extending these studies to higher temperatures and other electrocatalysts, using CO and CH3OH as the adsorbates.