Direct bio-electrocatalysis by multi-copper oxidases: Gas-diffusion laccase-catalyzed cathodes for biofuel cells

We have studied the bio-electroreduction of oxygen based on direct electron transfer (DET) between laccase and the electrode. Laccase enzymes from two different sources, namely, tree laccase from Rhus vernicifera, and fungal laccase from Trametes hirsuta were used in the study. The gas-diffusion cathode was made using a mixture of teflonized carbon and untreated carbon black, with a nickel mesh that served as a current collector, sandwiched between a hydrophobic gas diffusion layer, and a hydrophilic biocatalytic layer with physically adsorbed laccase enzyme. High current densities: up to 1 mA cm−2 under oxygen (for bio-electrocatalytic oxygen reduction) and increased stability (up to 30 days) has been achieved using teflonized carbon blacks at gas–electrode interface, high surface area carbon black for loading the enzyme.Gas diffusion laccase-catalyzed cathode demonstrates a number of advantageous properties including good adhesion, biocompatibility and high bio-electrocatalytic properties. An open circuit potential (OCP) of 600 mV at pH 7 for tree laccase (R. vernicifera) and 725 mV at pH 5 for fungal laccase (T. hirsuta) at zero current densities were obtained with respect to SHE reference electrode. Tafel plots obtained confirmed different DET characteristics for the two sources of laccase enzymes, which could suggest different mechanism of charge transfer: 4-electron electroreduction of oxygen using fungal laccase and 2-electron electroreduction using tree laccase. The performance of the cathode was studied in galvanostatic mode and polarization curves at various conditions are reported including those obtained under air and neat oxygen feed from the gas phase.