Kinetics, mass transport characteristics and model of the direct methanol fuel cell anode

The steady-state performance of a small scale (cross sectional electrode area 9cm liquid feed direct methanol fuel cell (DMFC) has been investigated in a series of galvanostatic experiments. The effect of methanol concentration (0.125 - 2.0mol dm-), pH of the anode feed (1-7), cell temperature (T= 70 C - 90 C), and cathode air pressure (0.15 - 0.30 MPha) was investigated. The DMFC consisted of a perfluorinated polymer electrolyte membrane (Nafion TM 117), a carbon-supported platinum-ruthenium anode catalyst and a carbon supported platinum cathode catalyst. Increasing the methanol concentration leads to a significant dorp in the cell voltage in general and especially the open circuit cell voltage, which is decreased to about 0.6V - 0.7V at 90 C (cf. the thermodynamically predicted cell voltage is 1146V at the same temperature). As has been proved by model simulation, this was partly caused by methanol crossover to the cathode [1,2]. At low current densities the apparent methanol reaction order became negative at the higher methanol concentrations. A echanistic analysis revealed that this could be explained by strong adsorption of reaction intermediates, such as CO, on the anode catalyst surface. The investigation of the pH of methanol feed solution showed that there was some benefit in power output with operation at low pH. As expected, an increase in temperature lead to an improvement in overall cell performance over the complete range of operating states, i.e. voltage, due to the increase in oxygen partial pressure at the cathode and a reduction in the extent of methanol crossover. The experimental data have been used to validate a model of the DMFC which includes the anode and cathode kinetics, the methanol crossover phenomenon and mass transfer to the anode catalyst layer. Model simulations with the validated model revealed that the DMFC operates with internal mass transfer and chemical microkinetics dominating the process of methanol oxidation. The performance limitations arising from methanol oxidation kinetics and methanol crossover can be reduced by dynamic variation of the methanol feed concentration