Effect of redox polymer synthesis on the performance of a mediated laccase oxygen cathode

Water-soluble, crosslinkable redox polymers are synthesized from poly(N-vinylimidazole) and an osmium complex, with variation in the concentration and reactant mole ratio of these precursor materials. Higher reactant concentration results in increased osmium loading on the redox polymer backbone, with products produced having a range of 19–23 imidazole repeat units per pendant osmium redox site. Performance of oxygen-reducing enzyme electrodes prepared from the redox polymers and laccase from Trametes versicolor on vitreous carbon rotating disks displays a high dependence on osmium mass fraction, with observed current densities varying by nearly a factor of two over the range osmium fractions studied. The highest-performing electrode produces a current density of 2.1 ± 0.2 mA/cm2 at 0.65 V and 900 rpm in 0.1 M pH 4 citrate buffer at 40 °C, which to our knowledge is greater than any previous analogous electrode. The gain in catalytic performance with osmium mass fraction is attributed to both increased charge transport and mediation kinetics. When scaled to high surface area composite electrodes 13 ± 0.8 mA/cm2 is produced at a rotation rate of 900 rpm, and current densities exceed 20 mA/cm2 when mass transport limitation is further eliminated.