ELECTRODE-KINETICS AND SPECIES TRANSPORT IN DIRECT OXIDATION METHANOLFUEL-CELLS - PREFACE

Polarisation losses due to limited reaction rates at both electrodes a s well as fuel crossover to the cathode are the principal loss mechani sms in a state-of-the-art direct oxidation methanol fuel cell (DMFC) c onsisting of a Pt-Ru alloy anode, Pt cathode, proton conducting membra ne electrolyte and liquid fuel supply. The objective of this study has been accurate quantification and discussion of the relative importanc e of these loss mechanisms. The electrode reactions and species transp ort phenomena have been studied both experimentally and by numerical c alculations. In the experimental part of the work, the kinetics of the electrode reactions has been studied on PTFE bonded electrodes in a h alf cell set-up in sulphuric acid. Galvanostatic polarisation data on carbon supported Pt and Pt-Ru catalyst materials has been analysed usi ng the theory of adsorption on heterogeneous surfaces. In contrast to earlier kinetic models for methanol oxidation, the steady-state model presented is valid over a wide range of surface coverages by the react ion intermediates. The model has been extended to study simultaneous m ethanol oxidation and oxygen reduction processes in conditions which m ay be present in the DMFC cathode. The extended model shows that the t wo processes are running in parallel on Pt surface which is causing ch emical oxidation of methanol. In the theoretical part of the work, a F ORTRAN 77 simulation programme has been written to study the loss mech anisms in a DMFC single cell. The simulation results show that the con version losses due to the fuel crossover are as important a loss mecha nism as the polarisation losses. As the conversion losses decrease and polarisation losses increase as functions of the current density, the overall efficiency comprising the two effects is maximised at interme diate current densities. The crossover accounts typically for an equiv alent current density of 50 - 200 mA/cm(2) and the overall efficiency is 15 - 25 %. Further improvements in methanol oxidation catalysis and methanol tolerant oxygen reduction catalysts or methanol impermeable electrolyte membranes are still needed before efficient high power den sity direct oxidation methanol fuel cells are to be realised.