• Energy Research

Lactococcus lactis is a gram-positive, normally homolactic fermenter that is known to produce several kinds of membrane associated quinones, which are able to mediate electron transfer to extracellular electron acceptors such as Fe(3+), Cu(2+) and hexacyanoferrate. Here we show that this bacterium is also capable of performing extracellular electron transfer to anodes by utilizing at least two soluble redox mediators, as suggested by the two-step catalytic current developed. One of these two mediators was herein suggested to be 2-amino-3-dicarboxy-1,4-naphthoquinone (ACNQ), via evaluation of standard redox potential, ability of the bacterium to exploit the quinone when exogenously provided, as well as by high performance liquid chromatography coupled with UV spectrum analysis. During electricity generation, L. lactis slightly deviated from its normal homolactic metabolism by excreting acetate and pyruvate in stoichiometric amounts with respect to the electrical current. In this metabolism, the anode takes on the role of electron sink for acetogenic fermentation. The finding that L. lactis self-catalyses anodic electron transfer by excretion of redox mediators is remarkable as the mechanisms of extracellular electron transfer by pure cultures of gram-positive bacteria had previously never been elucidated.

Biofuel cells are a next-generation energy device because they use renewable fuels with high energy density and safety. We have developed passive-type biofuel cell units, which generate a power over 100 mW (80 cm3, 39.7 g). Our biofuel cell, in which two-electron oxidation of glucose and four-electron reduction of O2 occurs at pH 7 in mediated bioelectrochemical processes under quiescent conditions, accomplished the maximum power density of 1.45 ± 0.24 mW cm−2 at 0.3 V. This performance was achieved by introducing three technologies: (1) Enzymes and mediator are densely entrapped on carbon-fiber electrodes with the enzymatic activity retained, (2) the concentration of buffer in electrolyte solution was optimized for the immobilized enzymes, and (3) the cathode structure was designed to supply O2 efficiently. The cell units with a multi-stacked structure successfully operate a radio-controlled car (16.5 g), which demonstrates the potential of biofuel cells in practical applications.

It is essential to construct a biofuel cell-based sensor and develop an effective strategy to detect glucose without any potentiostat circuitry in order to create a simple and miniaturized device. In this report, an enzymatic biofuel cell (EBFC) is fabricated by the facile design of an anode and cathode on a screen-printed carbon electrode (SPCE). To construct the anode, thionine and flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) are covalently immobilized via a crosslinker to make a cross-linked redox network. As a cathode, the Pt-free oxygen reduction carbon catalyst is employed alternative to the commonly used bilirubin oxidase. We proposed the importance of EBFC-based sensors through the connection of anode and cathode; they can identify a short-circuit current by means of applied zero external voltage, thereby capable of glucose detection without under the operation of the potentiostat. The result shows that the EBFC-based sensor could be able to detect based on a short-circuit current with a wide range of glucose concentrations from 0.28 to 30 mM. Further, an EBFC is employed as a one-compartment model energy harvester with a maximum power density of (36 ± 3) μW cm- 2 in sample volume 5 μL. In addition, the constructed EBFC-based sensor demonstrates that the physiological range of ascorbic acid and uric acid shows no significant effect on the short-circuit current generation. Moreover, this EBFC can be used as a sensor in artificial plasma without losing its performance and thereby used as a disposable test strip in real blood sample analysis.

Biofuel cells (BFCs) with enzymatic electrocatalysts have attracted significant attention, especially as power sources for wearable and implantable devices; however, the applications of BFCs are limited owing to the limited O2 supply. This can be addressed by using air-diffusion-type bilirubin oxidase (BOD) cathodes, and thus the further development of the hierarchical structure of porous electrodes with highly effective specific surface areas is critical. In this study, a porous layer of gold is deposited over magnesium-oxide-templated carbon (MgOC) to form BOD-based biocathodes for the oxygen reduction reaction (ORR). Porous gold structures are constructed via electrochemical deposition of gold via dynamic hydrogen bubble templating (DHBT). Hydrogen bubbles used as a template and controlled by the Coulomb number yield a porous gold structure during the electrochemical deposition process. The current density of the ORR catalyzed by BOD without a redox mediator on the gold-modified MgOC electrode was 1.3 times higher than that of the ORR on the MgOC electrode. Furthermore, the gold-deposited electrodes were modified with aromatic thiols containing negatively charged functional groups to improve the orientation of BOD on the electrode surface to facilitate efficient electron transfer at the heterogeneous surface, thereby achieving an ORR current of 12 mA cm−2 at pH 5 and 25 °C. These results suggest that DHBT is an efficient method for the fabrication of nanostructured electrodes that promote direct electron transfer with oxidoreductase enzymes.

To clarify the major factor caused by oxygen-enhancing charge production of Shewanella decolorationis NTOU1 towards a polarized anode, a series of experimental runs (i.e., with/without ambient air flushing and with/without ammonia addition as nitrogen source) were conducted in this study. Within 6-day of operation at +0.4 V vs. Ag|AgCl and starting with 35 mM of lactate, consistently the electrical charge production under the aerobic condition was higher than that under the anaerobic condition. In all the experimental runs, the values of nicotinamide adenine dinucleotide (NADH) production were found to be correlated positively and significantly with the charge production, but the highest Coulombic efficiency of 18% was observed under the anaerobic conditions without ammonia addition while the lowest charge production occurred. Those results indicate that NADH production enhanced by oxygen is the leading cause of the increase of the charge production, but the biomass production and the oxygen reduction would both consume NADH electrons and lead to lower electron recoveries. In addition, whether under constant aerobic or anaerobic, or alternating aerobic/anaerobic conditions, chronoamperometric results made it possible to rule out other factors, like lactate uptake rate or cell growth, which might increase the charge production under aerobic conditions. By using high performance liquid chromatography, some diffusive flavins (e.g., 0.5 microM of riboflavin) were found under the aerobic condition, but were not found under the anaerobic one. However, from results of cyclic voltammetry (CV), the signals of flavins were found to be approximately the same under both conditions. Although it is inferred that oxygen renders the flavins secreted extracellularly, that is not the major effect of oxygen for boosting the charge production. Furthermore, bound flavins under anaerobic condition were found to be effectively electrocatalytic according to sigmoidal CV result.

We demonstrate a flexible paper-based biofuel cell using porous carbon inks for high power output. The power density of the fabricated biofuel cell reached 0.12 mW cm(-2) (at 0.4 V), which is the highest output power reported to date, to the best of our knowledge.

One-compartment biofuel cells without separators have been constructed, in which d-fructose dehydrogenase (FDH) from Gluconobacter sp. and laccase from Trametes sp. (TsLAC) work as catalysts of direct electron transfer (DET)-type bioelectrocatalysis in the two-electron oxidation of d-fructose and four-electron reduction of dioxygen as fuels, respectively. FDH adsorbs strongly and stably on Ketjen black (KB) particles that have been modified on carbon papers (CP) and produces the catalytic current with the maximum density of about 4 mA cm(-2) without mediators at pH 5. The catalytic wave of the d-fructose oxidation is controlled by the enzyme kinetics. The location and the shape of the catalytic waves suggest strongly that the electron is directly transferred to the KB particles from the heme c site in FDH, of which the formal potential has been determined to be 39 mV vs. Ag|AgCl|sat. KCl. Electrochemistry of three kinds of multi-copper oxidases has also been investigated and TsLAC has been selected as the best one of the DET-type bioelectrocatalyst for the four-electron reduction of dioxygen in view of the thermodynamics and kinetics at pH 5. In the DET-type bioelectrocatalysis, the electron from electrodes seems to be transferred to the type I copper site of multi-copper oxidases. TsLAC adsorbed on carbon aerogel (CG) particles with an average pore size of 22 nm, that have been modified on CP electrodes, produces the catalytic reduction current of dioxygen with a density of about 4 mA cm(-2), which is governed by the mass transfer of the dissolved dioxygen. The FDH-adsorbed KB-modified CP electrodes and the TsLAC-adsorbed CG-modified CP electrodes have been combined to construct one-compartment biofuel cells without separators. The open-circuit voltage was 790 mV. The maximum current density of 2.8 mA cm(-2) and the maximum power density of 850 microW cm(-2) have been achieved at 410 mV of the cell voltage under stirring.

We report the first example of a liposome-based energy conversion system that is useful for entrapping enzymes and NAD coenzyme to accelerate multi-step enzymatic reactions. The liposome generates a much higher catalytic current compared with the non-liposome system, which is in good consistency with numerical simulations.