Mediated biocatalytic cathodes for fuel cells

Mediated biocatalytic cathodes based on redox polymers and the oxygen-reducing enzyme laccase were fabricated, characterized, and incorporated into fuel cell systems. A membrane-electrode assembly incorporating the bio-cathode with a platinum-based anode was demonstrated in operation with hydrogen and methanol fuels. The enzyme and redox polymer were immobilized on carbon-fiber paper and supplied with air-saturated citrate buffer. The hydrogen fuel cell exhibited an open circuit potential of 1.1 V and a maximum current density of 6 mA/cm2. With 10 M methanol fuel, the open-circuit potential was 0.8 V and maximum current density was 4 mA/cm2. Tolerance of the laccase cathode to the presence of methanol was demonstrated by polarization in the presence of high methanol feed concentration.; Performance of the bio-cathode on a rotating-disk electrode was examined as a function of the ionic strength. When buffer concentration was lowered from 100 mM to 50 muM, the oxygen reduction current density decreased by more than two orders of magnitude. Electron transport in the film via the redox polymer was measured by two independent methods, cyclic voltammetry and impedance spectroscopy, and was not significantly dependent on ionic strength. Measurements of enzyme kinetics confirmed that catalytic activity, and not mediator electron transport, was the limiting factor in catalytic current density at micromolar-scale ionic strength.; The mediated bio-cathode was tested in the membrane-electrode assembly with gas-phase oxygen and air. The electrode reached current densities of 1 mA/cm2 but was unstable because of water loss from the cathode. Flow rate, humidification, and oxygen concentration of the cathode gas stream had a strong effect on stability but not current density. Cyclic voltammetry and electrochemical impedance experiments revealed that ionic conductivity in the cathode decreased as water was lost.; Finally, mediated laccase cathodes were fabricated via co-electrodeposition of redox polymer and enzyme onto planar and porous carbon electrodes. After one hour of electrodeposition time, the electrodeposited bio-cathode films exhibited catalytic current density on par with that of chemically crosslinked films. The specific activity in terms of current per mole of redox polymer or enzyme was significantly higher for electrodeposited films.