Water-repellent-treated enzymatic electrode for passive air-breathing biocathodic reduction of oxygen

•Oxygen reduction current reached 20 mA cm− 2 at 0.25 V vs. Ag|AgCl sat. at pH 7.•Oxygen is passively supplied to the biocathode through the PTFE film from air.•Water-repellents were directly applied to the biocathode.

A passive-type air-breathing biocathode using bilirubin oxidase as enzyme, ferricyanide as redox mediator, and poly-l-lysine as linker was investigated. Water-repellents from Scotchgard™ with dissolved in methyl isobutyl ketone were directly applied to the enzyme- and mediator-modified biocathode. The partially water-repelled biocathode was placed on a gas-permeable polytetrafluoroethylene (PTFE) membrane and immersed in a buffer solution, and O2 was passively supplied to the biocathode through the PTFE membrane from air. Cathodic currents were dependent on the buffer concentration and amount of applied water repellent. Steady-state catalytic current densities reached 24 and 32 mA cm− 2 at 0 V vs. Ag|AgCl under air and oxygen atmospheric conditions, respectively, using a biocathode modified with 0.32 mg cm− 2 of the water repellent and 2.0 M imidazole buffer at pH 7.