Investigation of methanol transport and reaction kinetics in direct methanol fuel cells

Direct Methanol Fuel Cells (DMFCs) have been demonstrated extensively as electrical power sources for portable applications. In DMFCs, the chemical energy stored in methanol is converted directly to electrical energy through a number of chemical, transport and kinetic processes. The overall efficiency of the DMFC system can be improved by optimizing these processes with precise control over operating conditions. The goal of this research is to evaluate optimal operating conditions and system design for improving the DMFC's electrical performance through a combination of experimental strategies and process models.;A DMFC system incorporating metal foams as the flow field was designed to increase system efficiency. The influence of metal foam parameters and operating conditions on fuel cell performance was investigated. Our results indicated that due to the opposing effects of methanol concentration on anode and cathode kinetics, there exists an optimal value of methanol concentration at each current density that will yield the highest electrical performance. A control algorithm employing feedback from the fuel cell voltage was implemented to dynamically adjust the methanol feed concentration for peak DMFC performance. Additionally, water and methanol crossover fluxes across the membrane were also measured to understand their transport rates under different conditions.;The physico-chemical processes in DMFCs were investigated by developing an accurate mathematical model coupling mass transport with reaction kinetics within the five-layer membrane electrode assembly of the DMFC. An experimental scheme was developed to measure the overpotential contributions of anode methanol oxidation, cathode oxygen reduction and cathode methanol oxidation. Subsequently, the kinetic constants for these three reactions are characterized for various catalyst loadings. The model predicted that methanol undergoes electrochemical adsorption on the Pt/C cathode catalyst layer, followed by both electrochemical and chemical oxidation. The overpotential loss due to methanol oxidation on the cathode with 2 mg/cm2 catalyst loading is as large as 80 mV at 20 mA/cm 2. Our model indicated that most of the methanol adsorbed on the cathode catalyst undergoes purely chemical oxidation with oxygen and causes mass transport limitations for oxygen electro-reduction. We also found that the transport of methanol to the anode catalyst layer was significantly enhanced by the convection of CO2 bubbles towards the flow field. This model should prove useful in optimizing the supply rates of methanol and oxygen in DMFCs.