Dual Gas Diffusion H2/O2 Biofuel Cell in Direct Electron Transfer-Type System

Abstract

Biofuel cells are energy conversion systems in which enzymes are utilized as biocatalysts. Thanks to the characteristics of the enzyme reaction, the power device is able to operate under very mild and safe conditions (e.g. neutral pH, room temperature, and atmospheric pressure). In addition, the active sites in enzymes consist of nonprecious metals or organic compounds. Although conventional polymer electrolyte H2/O2 fuel cells are known as clean and efficient energy conversion devices, they usually require Pt catalysts, which are expensive. By utilizing hydrogenases and multicopper oxidases (MCOs) as catalysts for H2 oxidation and O2 reduction, respectively, a sustainable and environmentally friendly H2/O2 biofuel cell can be constructed. Bioelectrocatalysis, in which an enzyme reaction and an electrode reaction is coupled, is a key process for biofuel cells. The reaction is classified into two types: mediated electron transfer (MET) and direct electron transfer (DET). In DET-type systems, an enzyme can directly communicates with an electrode. This character makes it possible to construct a compact and simple biofuel cell and to minimize the thermodynamic loss in the electron transfer between an enzyme and an electrode. Combining a [NiFe]-hydrogenase-adsorbed bioanode and a MCO-adsorbed biocathode, we can construct a DET-type H2/O2 biofuel cell. The low solubility of the gaseous substrates (H2 and O2) frequently causes a power decline. To solve this critical problem, a gas diffusion electrode that can supply a gaseous substrate to an enzyme from the gas phase has been proposed. Moreover, the gas diffusion electrode can maintain high substrate concentrations near the active enzyme on the electrode. This system is also useful for overcoming the specific inactivation of [NiFe]-hydrogenases called “oxidative inactivation”, since the inactivation is thought to compete with H2 binding in the catalytic cycle. Furthermore, the gas diffusion electrode allows independent supply of gaseous substrates to the bioanode and the biocathode, thus avoiding the risk of explosion on mixing of H2 with and O2. In this research, we utilized O2-sensitive [NiFe]-hydrogenase from D. vulgaris Miyazaki F (DvMF) and bilirubin oxidase, one of the MCOs, from M. verrucaria (MvBOD) as biocatalysts for H2 oxidation and O2 reduction, respectively. We also constructed a novel gas diffusion bioelectrode with a sheet of waterproof carbon cloth as the electrode base and optimized the hydrophilicity/hydrophobicity of the electrode for both high gas permeability and high DET-type bioelectrocatalytic activity. The electrode exhibited a large catalytic current density of about 10 mA cm−2 in the steady state for both H2 oxidation and O2 reduction. DvMF-adsorbed bioanode and MvBOD-adsorbed biocathode were coupled to construct a dual gas diffusion H2/O2 biofuel cell in DET-type system. The dual gas diffusion system allowed the separate supply of gaseous substrates (H2 and O2) to the bioanode and biocathode, with consequent suppression of the oxidative inhibition of the hydrogenases. The open-circuit voltage of our H2/O2 biofuel cell is 1.14 V and is close to the standard motive force of the ideal H2/O2 cell (1.23 V). The cell exhibited a maximum power density of 8.4 mW cm−2 at a cell voltage of 0.7 V under quiescent conditions and H2 and O2 atmospheric conditions for the bioanode or the biocathode, respectively. The performance of our biofuel cell is 5.6 times better than that of the best H2/O2 biofuel cell and 3.2 times better than that of DET-type biofuel cells with other fuels in the literatures. Figure 1