The products of methanol crossover through the acid‐doped polybenzimidazole polymer electrolyte membrane (PBI PEM) to the cathode of a prototype direct methanol fuel cell (DMFC) were analyzed using multipurpose electrochemical mass spectrometry (MPEMS) coupled to the cathode exhaust gas outlet. It was found that the methanol crossing over reacts almost quantitatively to CO2 at the cathode with the platinum of the cathode acting as a heterogeneous catalyst. The cathode open‐circuit potential is inversely proportional to the amount of CO2 formed. A poisoning effect on the oxygen reduction also was found. Methods for the estimation of the methanol crossover rate at operating fuel cells are suggested.

A real-time Fourier transform infrared spectroscopy (FTIRS) analysis of the products of methanol oxidation in a prototype direct-methanol fuel cell operating at high temperatures (150 to 185°C) is reported here. The methanol oxidation products on platinum black and platinum-ruthenium catalyst surfaces were determined as a function of the fuel cell operating temperature, current density, and methanol/water mole ratio. Neither formaldehyde nor formic acid was detected in anode exhaust gas at all cell operating conditions. The product distributions of methanol oxidation obtained by on-line FTIRS are consistent with our previous results obtained by on-line mass spectroscopy under similar conditions. With pure methanol in anode feed, methanaldimethylacetal was found to be the main product, methyl formate and CO2 were also found. However, when water was present in the anode feed, the main product was C02, and the formation of methanaldimethylacetal and methyl formate decreased significantly with increase of the water/methanol mole ratio. Increase of cell operating temperature enhanced the formation of CO2 and decreased the formation of methanaldimethylacetal and methyl formate. Pt/Ru catalyst is more active for methanol oxidation and has a higher selectivity toward CO2 formation than Ptblack. Nearly complete methanol oxidation, i.e., the product was almost exclusively C02, was achieved using a Pt/Ru catalyst and a water/methanol mole ratio of 2 or higher in the anode feed at a temperature of 185°C or above.

The electro-oxidation of formic acid was studied in a direct-oxidation polymer-electrolyte fuel cell at 170°C using real-time mass spectrometry. The results are compared with those obtained for methanol oxidation under the same conditions. Formic acid was electrochemically more active than methanol on both Pt-black and Pt/Ru catalysts. The polarization potential of formic acid oxidation was ca. 90 to 100 mV lower than that of methanol.The oxidation of formic acid was dependent on the water/formic acid mole ratio. The best anode performance was obtained using a water/formic acid mole ratio of -2. In addition, Pt/Ru catalyst was more active than Pt-black for formic acid oxidation. The mass spectrometric results showed that CO2 is the only reaction product of formic acid oxidation.The results are discussed in terms of possible formic acid oxidation mechanisms.

Acid-Doped Polybenzimidazoles: A New Polymer Electrolyte.-The proton conductivity and MeOH vapor permeability are measured for polybenzimidazole (PBI) films doped with H3PO4 and the applicability of the films as potential polymer electrolytes for hydrogen/air and direct MeOH fuel cells is studied. Long-term tests show that the PBI electrolyte is stable at elevated temp. even in the presence of H2, O2, and Pt. A H2/O2 cell with the PBI-based electrolyte and Pt/C electrodes was successfully operated at 200 mA/cm2 for over 200 h without decay of the performance. The max. power observed for an unoptimized cell was 0.25 W/cm2 at 700 mA/cm2. The MeOH/O2 fuel cell with PBI-H3PO4 membrane, Pt-Ru anode, and Pt black cathode produces over 0.1 W/cm2 at 250-500 mA/cm2. The low MeOH permeability of the electrolyte reduces the adverse effects of MeOH crossover. -(WAINRIGHT, J. S.; WANG, J.-T.; WENG, D.; SAVINELL, R. F.; LITT, M.; J. Electrochem. Soc. 142 (1995) 7, L121-L123; Dep. Chem. Eng., Case West. Reserve Univ., Cleveland, OH 44106-7217, USA; EN)