Ryuhei Nakamura

Nitrogen-doped carbon nanomaterials as non-metal electrocatalysts for water oxidation

Efficient and low-cost electrocatalysts for the oxygen evolution reaction are essential components of renewable energy technologies, such as solar fuel synthesis and providing a hydrogen source for powering fuel cells. Here we report that the nitrogen-doped carbon materials function as the efficient oxygen evolution electrocatalysts. In alkaline media, the material generated a current density of 10 mA cm(-2) at the overpotential of 0.38 V, values that are comparable to those of iridium and cobalt oxide catalysts and are the best among the non-metal oxygen evolution electrocatalyst. The electrochemical and physical studies indicate that the high oxygen evolution activity of the nitrogen/carbon materials is from the pyridinic-nitrogen- or/and quaternary-nitrogen-related active sites. Our findings suggest that the non-metal catalysts will be a potential alternative to the use of transition metal-based oxygen evolution catalysts.

Microbial fuel cell

Without the use of mediators which can increase the current density to provide a microbial fuel cell. The microbial fuel cell 1 has an aggregate (4) comprise a three-dimensional structure formed by the conductive fine particles 2 and microorganisms (3). Agglomerate 4, the conductive fine particles (2) rest with Nella 3 together as soon dispersed amongst each other, the conductive fine particles (2) kkirido connected to one another rest with Nella maintain 3, and the three-dimensional structure as a whole It is formed. As a result, a break and Nella 3, conductive fine particles even rest on the surface and Nella (3a) and, Shh and Nella (3b) in a position away in the vertical direction from the surface of the electrode 103 of the electrode 103 it can maintain by the following formula (2) it can be transmitted to the electronic more Shh and Nella 3.

Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions

The development of Mn-oxide electrocatalysts for the oxidation of H(2)O to O(2) has been the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O(2)-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidation reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochemical techniques, we demonstrate that the stabilization of surface-associated intermediate Mn(3+) species relative to charge disproportionation is an effective strategy to lower the overpotential for water oxidation by MnO(2). The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO(2) electrodes effectively stabilized the Mn(3+) species, resulting in an ~500-mV negative shift of the onset potential for the O(2) evolution reaction at neutral pH.

Rate enhancement of bacterial extracellular electron transport involves bound flavin semiquinones

Extracellular redox-active compounds, flavins and other quinones, have been hypothesized to play a major role in the delivery of electrons from cellular metabolic systems to extracellular insoluble substrates by a diffusion-based shuttling two-electron-transfer mechanism. Here we show that flavin molecules secreted by Shewanella oneidensis MR-1 enhance the ability of its outer-membrane c-type cytochromes (OM c-Cyts) to transport electrons as redox cofactors, but not free-form flavins. Whole-cell differential pulse voltammetry revealed that the redox potential of flavin was reversibly shifted more than 100 mV in a positive direction, in good agreement with increasing microbial current generation. Importantly, this flavin/OM c-Cyts interaction was found to facilitate a one-electron redox reaction via a semiquinone, resulting in a 103- to 105-fold faster reaction rate than that of free flavin. These results are not consistent with previously proposed redox-shuttling mechanisms but suggest that the flavin/OM c-Cyts interaction regulates the extent of extracellular electron transport coupled with intracellular metabolic activity.

Self-Constructed Electrically Conductive Bacterial Networks†

Microbial attachment to mineral surfaces is a fundamental process for initiating a broad range of biochemical and geological events in a natural environment. The genus Shewanella, which consists of dissimilatory metal-reducing bacteria often found in subsurface sediments, has the ability to recognize the surface of iron(III) oxides and initiate extracellular electron transfer (ET) to the attached iron oxides as a terminal process in its metabolism. This is an important process for its influence on the biogeochemical cycling of iron, and it has also gained attention not only for a new aspect of the metabolic strategy of microorganisms, but also for its applicability in microbial fuel cells. The outer-membrane (OM) redox proteins, c-type decaheme cytochromes (c-Cyt), play a crucial role in mediating ET from the cell to iron(III) oxides. 11] A great deal of research has been focused on the electrochemical and spectroscopic investigation of the purified OM proteins. However, few studies have been performed by directly monitoring the ET process of intact cells, and therefore the mechanism of this process has largely remained unsolved. Herein, we report the ability of S. loihica PV-4 to selfassemble into an electrically conductive network in the presence of iron(III) oxides, and demonstrate the role of semiconductive nanominerals in promoting a long-distance extracellular ET process in the bacterial network. To probe the extracellular ET of intact cells of S. loihica PV-4, we used a single-chamber, three-electrode system, with lactate as a carbon source and an electron donor. An optically transparent conductive-glass, tin-doped In2O3 (ITO) electrode, with a surface area of 1.8 cm, was used as the working electrode, and placed on the bottom surface of the reactor. A current was generated immediately after adding the cells into the reactor (Figure 1a), and reached a constant value 0.4– 0.6 mA. Current generation is a consequence of electrical

Respiratory interactions of soil bacteria with (semi)conductive iron-oxide minerals.

Pure-culture studies have shown that dissimilatory metal-reducing bacteria are able to utilize iron-oxide nanoparticles as electron conduits for reducing distant terminal acceptors; however, the ecological relevance of such energy metabolism is poorly understood. Here, soil microbial communities were grown in electrochemical cells with acetate as the electron donor and electrodes (poised at 0.2 V versus Ag/AgCl) as the electron acceptors in the presence and absence of iron-oxide nanoparticles, and respiratory current generation and community structures were analysed. Irrespective of the iron-oxide species (hematite, magnetite or ferrihydrite), the supplementation with iron-oxide minerals resulted in large increases (over 30-fold) in current, while only a moderate increase (∼10-fold) was observed in the presence of soluble ferric/ferrous irons. During the current generation, insulative ferrihydrite was transformed into semiconductive goethite. Clone-library analyses of 16S rRNA gene fragments PCR-amplified from the soil microbial communities revealed that iron-oxide supplementation facilitated the occurrence of Geobacter species affiliated with subsurface clades 1 and 2. We suggest that subsurface-clade Geobacter species preferentially thrive in soil by utilizing (semi)conductive iron oxides for their respiration.

Analyses of Current-Generating Mechanisms of Shewanella loihica PV-4 and Shewanella oneidensis MR-1 in Microbial Fuel Cells

ABSTRACT Although members of the genus Shewanella have common features (e.g., the presence of decaheme c-type cytochromes [c-cyts]), they are widely variable in genetic and physiological features. The present study compared the current-generating ability of S. loihica PV-4 in microbial fuel cells (MFCs) with that of well-characterized S. oneidensis MR-1 and examined the roles of c-cyts in extracellular electron transfer. We found that strains PV-4 and MR-1 exhibited notable differences in current-generating mechanisms. While the MR-1 MFCs maintained a constant current density over time, the PV-4 MFCs continued to increase in current density and finally surpassed the MR-1 MFCs. Coulombic efficiencies reached 26% in the PV-4 MFC but 16% in the MR-1 MFCs. Although both organisms produced quinone-like compounds, anode exchange experiments showed that anode-attached cells of PV-4 produced sevenfold more current than planktonic cells in the same chamber, while planktonic cells of MR-1 produced twice the current of the anode-attached cells. Examination of the genome sequence indicated that PV-4 has more c-cyt genes in the metal reductase-containing locus than MR-1. Mutational analysis revealed that PV-4 relied predominantly on a homologue of the decaheme c-cyt MtrC in MR-1 for current generation, even though it also possesses two homologues of the decaheme c-cyt OmcA in MR-1. These results suggest that current generation in a PV-4 MFC is in large part accomplished by anode-attached cells, in which the MtrC homologue constitutes the main path of electrons toward the anode.

Electronic Absorption Spectra and Redox Properties of C Type Cytochromes in Living Microbes

Shewanella is an electrogenic microbe that has significant content of c type cytochromes (ca. 0.5 mM). This feature allows the optical absorption spectra of the cell-membrane-associated proteins to be monitored in vivo in the course of extracellular respiratory electron-transfer reactions. The results show significant differences to those obtained in vitro with purified proteins.

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.

Thermoelectricity Generation and Electron–Magnon Scattering in a Natural Chalcopyrite Mineral from a Deep‐Sea Hydrothermal Vent

Current high-performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+x Fe1-x S2 , obtained from a deep-sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron-magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earths crust.