Construction of a Bioelectrochemical Dihydrogen/Formate Interconversion System and a Bio-Solar Cell

Modern societies have energy and environmental issues such as fossil fuel depletion and excessive carbon dioxide (CO2) emissions. Therefore, an effective use of renewable energy resources instead of fossil fuels is essential for the development of a sustainable energy economy. Particularly, dihydrogen (H2), formic acid (HCOOH) and sunlight are expected to be new energy sources. These advantages are as follows; H2 emits no harmful substances in use; HCOOH has a high energy density and is a liquid fuel that can be easily stored and transported; sunlight is universally available on the ground. However, the electrochemical conversion of the above energy has the following problems; a catalyst for H2 oxidation, platinum, is expensive; byproducts such as carbon monoxide are generated in CO2 reduction at usual electrodes; solar cell materials are sometimes expensive and environmentally harmful. In order to overcome these problems, we focused on biocatalysts, which are ecofriendly, and have high catalytic performance and selectivity under mild conditions. Furthermore, we constructed two electro-enzymatic energy conversion devices: an H2/HCOO- interconversion system and a bio-solar cell. In an H2/HCOO- interconversion system, we applied two enzymes which catalyze bidirectional redox reactions: [NiFe]-hydrogenase from Desulfovibrio vulgaris Miyazaki F (H2ase; EC 1.12.2.1) and tungsten-containing formate dehydrogenase from Methylobacterium extorquens AM1 (FoDH1; EC 1.2.1.2). H2ase and FoDH1 catalyze the H+/H2 and CO2/HCOO- interconversion, respectively, and we attempted to generate HCOO- without external power supply by the coupling of two bioelectrocatalytic reactions: direct electron transfer (DET)-type H2 oxidation catalyzed by H2ase and mediated electron transfer (MET)-type CO2 reduction catalyzed by FoDH1 using benzyl viologen (BV) as a mediator (H2 + CO2 → HCOO- + H+). H2ase was adsorbed on a bioanode functionalized with p-phenylenediamine, which leads to orientations suitable for DET-type bioelectrocatalysis of H2ase, while FoDH1 and BV were adsorbed on a biocathode. In addition, gas-diffusion-type electrodes were employed for high-speed supply of gaseous substrates. The two electrodes were short-circuited in a cell, and CO2 and H2 gasses were spontaneously supplied from the outside of the biocathode and the bioanode, respectively. The amount of formate in the electrolyte solution increased almost linearly with the reaction time. On the other hand, when the feed gasses were replaced with Ar gas to remove CO2 and H2 from the system, the amount of formate decreased with the time. This is due to the reversed reaction: (HCOO- + H+ → H2 + CO2). These results mean that the difference in equilibrium potentials of the H+/H2 and CO2/HCOO- couples determines the driving force of the reaction, which is reversed depending on conditions, and the interconversion between H2 and HCOO- was experimentally demonstrated. A bio-solar cell is an electric energy device mimicking natural photosynthesis and extracts excited electrons outside the photosynthetic system. In this study, we constructed a bio-solar cell using the thylakoid membrane from spinach as an anodic photo-bioelectrocatalyst which oxidizes water (H2O) to dioxygen (O2) and the recombinant bilirubin oxidase (BOD; EC 1.3.3.5) produced by expressing the gene of BOD from Myrothecium verucarria in Pichia pastoris as a cathodic bioelectrocatalyst which reduces O2 to H2O. In addition, we investigated the characteristics of a thylakoid membrane-functionalized bioanode with a mediator of hexaammineruthenium(III/II) ion ([Ru(NH3)6]3+/2+), which has the following characteristics: fast electrode kinetics, high stability and solubility, and somewhat low redox potential. The photo-driven bioanode prepared by applying the mixture of thylakoid membranes and water-dispersed multi-walled carbon nanotubes on a gold-spattered indium tin oxide (ITO) electrode showed the photocurrent caused by MET-type photo-bioelectrocatalytic H2O oxidation in a buffer (pH 7.0) containing 5 mM Ru(NH3)6Cl3 under the light condition. The performance of the bioanode (0.2 mA cm- 2 at 0 V vs. Ag|AgCl|sat. KCl) is better than that in the previous report using 1,2-naphthoquinone as a mediator. Then, a bio-solar cell was constructed by combining the bioanode with the BOD-modified gas-diffusion-type biocathode, and O2 was spontaneously supplied from the outside of the biocathode. The cell exhibited an open-circuit voltage of 0.61 V and a maximum power density of 50 μW cm- 2 at a cell voltage of 0.42 V under quiescent conditions. To the best of our knowledge, the power density of our cell unit is the highest among those reported for the bio-solar cells to date. These works lead to the artificial photosynthetic system, which generates H2 or HCOO- from sunlight and H2O (and CO2), and they might be a breakthrough for the sustainable energy economy.