Gas-Diffusion and Direct Electron Transfer-Type Bioanode for Hydrogen Oxidation with Oxygen-Tolerant [Nife]-Hydrogenase As an Electrocatalyst

              In order to realize the promising hydrogen energy conversion system, a superior catalyst to oxidize hydrogen is needed.Nowadays, the power devices such as an H₂/O₂ fuel cell rely on platinum as a catalyst, which is a rare metal and thus too expensive. Many researchers have explored alternative catalysts. From the aspect of bioelectrochemistry, hydrogenase that catalyzes the redox reaction between hydrogen and proton have received a lot of attention, and the bioelectrocatalytic properties has extensively been characterized all over the world.

              The discovery of O₂- and CO-tolerant [NiFe]-hydrogenase is one of the breakthroughs essential for the construction of the aforementioned biofuel cell, and the unique properties have been investigated from several viewpoints. One critical problem in utilization of [NiFe]-hydrogenase is reversible inactivation caused by the anaerobic oxidation at high electrode potential or high solution potential. Several spectroscopic studies have revealed that the catalytic cycle proceeds in three states: Ni-SI (active silent form), Ni-R (H₂-reduced form), Ni-C (one-electron oxidized form of Ni-R), and that the inactivation generates a Ni(III) state form known as Ni-B by one-electron oxidation of Ni-SI, in which a hydroxide ligand is coordinated to the Ni atom in a bridging position with respect to the Fe(II).

              A membrane-bound [NiFe]-hydrogenase (MBH) from Hydrogenoviblio marinus allows a direct electron transfer (DET)-type bioelectrocatalysis for the H₂ oxidation and is an O₂-tolerant promising enzyme for the construction of enzymatic H₂/O₂ biofuel cells.

              From the practical viewpoint of the electricity production, the H₂ depletion near the electrode surface and the oxidative and reversible inactivation (as a competitive inhibition) of [NiFe]-hydrogenases limit the H₂ oxidation, and consequently causes the power decline. In this research, the Michaelis constant has been evaluated as 0.57 mM under steady-state conditions for the DET-type H₂ oxidation by MBH chemically immobilized on Ketjien black mesoporous carbon electrode. In order to spontaneously supply H₂ from the gas phase and to avoid the inactivation, an MBH-adsorbed gas-diffusion-type electrode has been constructed. The maximum current density of H₂ oxidation has reached about 6 mA cm^-2 at 0 V (vs. Ag|AgCl|sat. KCl electrode) under quiescent (passive) and H₂ atmospheric conditions.