Highly Permeable Gas Diffusion Electrodes with Hollow Carbon Nanotubes for Bilirubin Oxidase-Catalyzed Dioxygen Reduction

A gas diffusion electrode system that can supply gaseous substrates to enzymes from the gas phase in bioelectrocatalysis is essential to increase the mass-transfer of gaseous substances; such a system can also address the problem of mass-transfer-limited reactions for gaseous substrates with low solubility in the dissolved system. We have constructed gas diffusion dioxygen (O2)-reducing biocathodes with bilirubin oxidase (BOD) as an electrocatalyst. Novel carbon nanotubes (CNTs) with hollow structures were used as a carbon material to increase the gas permeability. We achieved a steady-state current density of more than 30 mA cm−2 at pH 5 in direct electron transfer- (DET-) type bioelectrocatalytic O2 reduction under quiescent conditions. The significance of the structural characteristics of the hollow CNTs was discussed based on microscopic observations. The hollow CNTs also provided platforms for mediated electron transfer-(MET-) type bioelectrocatalysis of BOD (not only in monolayer but in multilayers). MET-type reaction of BOD proceeded effectively at pH 7.0, at which the DET-type activity decreased down to one third of that at pH 5.0, and can cover a weak point of BOD as a decrease in the enzyme activity at neutral pH.