Simulation of Multistep Enzyme-Catalyzed Methanol Oxidation in Biofuel Cells

Abstract

We report a simulation of multi-step enzyme catalysis and cofactor-mediated electron transfer in a bio-anode of a methanol biofuel cell. Three nicotinamide adenine dinucleotide (NAD₊) dependent dehydrogenase enzymes, namely alcohol dehydrogenase (ADH), aldehyde dehydrogenase (AlDH), and formate dehydrogenase (FDH) combine to achieve complete oxidation of methanol to carbon dioxide. Oxidation of methanol by the enzyme cascade produces the reduced co-factor, NADH, which is oxidized to regenerate NAD₊ at an electrode comprised of a poly(methylene green) catalyst electrodeposited on carbon paper. Kinetic parameters for the enzymatic oxidation reactions were experimentally determined between pH 6.5 and 9 and were employed in the simulation to evaluate the effect of pH. Model results compare closely to previously reported experimental data over a broad pH range. Model-based analysis indicates that cell performance is controlled by NAD₊ transport and NADH oxidation kinetics for electrode specific areas lower than 3 m₂ cm₋₃ and shifted to enzyme kinetic control at higher specific area. An optimal electrode design is predicted to achieve current density exceeding 10 mA cm₋₂.