Enzymatic Fuel Cell Research Articles

NADH Bioelectrocatalysis at a Naphthoquinone Redox Polymer

Nicotinamide adenine dinucleotide-dependent (NAD) enzymes are used by a wide-spread of dehydrogenases which have been used in a large portion of enzymatic fuel cells (EFCs) to date. The oxidation of an EFC’s fuel (such as glucose) results in the generation of enzymatically-generated diffusive reduced cofactor (NADH). Various electrocatalysts and mediators have been employed to electro-oxidize NADH at low potential, such as phenothiazines and rhodium catalysts.1, 2 Naphthoquinones and anthraquinones have also been employed for NADH oxidation in high power EFCs, however their use typically requires an additional enzyme to mediate electron transfer between the quinone and NADH (diaphorase).3 We report the use of a naphthoquinone redox polymer to oxidize NADH, enzymatically generated from glucose oxidation by NAD-dependent glucose dehydrogenase. The naphthoquinone redox polymer serves as a support for enzyme immobilization. Additionally, NADH is oxidized without the requirement of diaphorase.4

(Invited) Overcoming Biocathode Limitations in Bfcs: Enhancing O2 Concentration with Micelles

Enzymatic fuel cells (EFCs) are capable of utilizing a wide range of sugars and short-chain alcohols as fuel sources. Using an enzymatic catalyst at either the anode or cathode, EFCs are able to operate under physiological pH and temperature. Recent advancements in bioanodic materials have allowed for generation of current densities in excess of 2 mA cm-2; however, the majority of EFCs are limited by current density generated at the biocathode. Typical enzymatic biocathodes utilize a copper oxidase, such as laccase or bilirubin oxidase, to bioelectrocatalytically reduce molecular oxygen to H2O. However, biocathodes are often limited by slow diffusion of O2 to the electrode interface, which is ultimately the result of relatively low solubility of O2 in aqueous solutions (mole fraction solubility, xO2 = 2.3×10-5). By contrast, the solubility of O2 in organic solvents such as nonane or perfluorooctane (xO2 = 2.1×10-3 and 5.3×10-3, respectively) are much higher than in aqueous conditions. We have developed a strategy to combine the high O2 solubility of organic solvents with the biocompatibility afforded by aqueous solutions, in which phospholipid micelles are used to trap O2 and transport it to the cathode surface. By modifying the phospholipid composition, various properties of the corresponding micelle can be tuned to cause dispersion upon contact with the electrode surface thereby increasing the localized O2 concentration. The net result is a dramatic increase in current density generated at the biocathode.

(Invited) Electrode Nanostructuration for Electricity Production By H2/O2 Enzymatic Fuel Cells

Enzymatic H2/O2 fuel cells (EFC) recently emerged as attractive devices for small power applications [1]. In this “green” fuel cell, hydrogenase and multicopper oxidase enzymes would replace scarce and expensive chemical catalysts at the anodic and cathodic sides respectively. Advantages of biocatalysts include bioavailability, high specificity that allows to avoid the costly membrane separator between the two cell compartments, high efficiency, very low over-voltages for the transformation of the substrates. Screening the biodiversity allows furthermore to identify enzymes showing outstanding properties, such as thermostability and insensitivity to CO which is one of the major drawback of the platinum catalyst used in fuel cells. [2]. One of the main challenges to EFC development is to achieve high power performances and stability in order EFCs become a self-sufficiency source of energy. The first issue is to optimize the electron transfer rate between the enzyme and the electrochemical interface. This will allow to improve the turn over frequency of the immobilized enzyme, thus the catalytic current under a low overvoltage. The second issue is to enhance the loading of electrically connected enzymes to reach catalytic currents compatible with a practical application. These two issues suppose i) the knowledge of the electron pathways from the enzyme active site to the electrode, ii) the knowledge of the key amino acid residues involved in the interfacial recognition, and iii) the development of biocompatible 3D networks. Previous works in our laboratory have shown how the combination of electrochemistry, PMIRRAS and modeling helps in the understanding of the modular basis for such an efficient enzyme immobilization on planar electrochemical interfaces. We were then able to design the first H2/O2 biofuel cell delivering a power density of 300 µW.cm-2 [3]. In this work we discuss the rational design of nanostructured interfaces able to enhance the performances of the H2/O2 biofuel cells [4]. We show that coupling electrochemistry to SPR provides essential kinetic data to get a full understanding of the electron process as a function of various parameters (chemistry of the electrochemical interface, pH, time, applied potential,…). We highlight that gold nanoparticles and various carbon nanomaterials, from nanotubes to nanofibers, can serve as efficient platforms for enzyme immobilization with enhanced long-term stability. We especially focus on the hierarchical porosity and the chemical functionalization of the nanomaterials. Their respective role on mass transport limitation during catalysis and stable loading of the biocatalyst is discussed as a function of the nature of the enzyme. We then develop new strategies based on modification of carbon felts by these nanomaterials. The performances of the new H2/O2 biofuel cells based on these optimized biohybrids are presented. We demonstrate that these EFCs deliver a cell power required to feed a wireless electronic device [5-6]. References (1) A. de Poulpiquet et al., ChemElectroChem, (2014) (2) M. Guiral et al., Adv. Microbial. Physiol., (2012) (3) A. Ciaccafava et al., Angew. Chem. (2012) ; Electrochem. Comm. (2012) (4) K. Monsalve et al., Bioelectrochem. (2015) ; A. de Poulpiquet et al., PCCP (2014) (5)A. de Poulpiquet et al., Electrochem. Com., (2014) ; N. Lalaoui et al., Chem. Comm. (2015) ; K. Monsalve et al., Electrochem. Comm. (2015)

Direct Electron Transfer Mechanisms Applied Toward the Development of an Enhanced Photoelectrochemical Biofuel Cell

Photoelectrochemical biofuel cells (PEBFCs) are hybrid devices which combine the utility of enzymatic redox catalysts with the photon absorbing characteristics of a dye-sensitized solar cell (DSSC). The pairing of these two technologies has been demonstrated to result in a system that improves on the performance of either a traditional enzymatic fuel cell (EFC) or DSSC operating independently1. Furthermore, due to the increased potential provided by a PEBFC relative to an EFC the range of oxidizers used in the cathode can be broadened to produce a variety of useful reduced fuels. Traditionally, PEBFCs have relied on NAD+/NADH intermediates for charge transfer between the anodic redox enzyme and the porphyrin sensitizer. However, recent work by our group has shown that quinoprotein glucose dehydrogenases are capable of efficient, mediator free direct electron transfer (DET) to immobilized heme porphyrins2. DET allows for a further reduction in the electrode overpotential compared to many mediated systems and therefore provides more usable potential for energy conversion. In the current work a PEBFC utilizing DET mechanisms was developed with the goal of providing efficient electron transfer between the enzymatic catalyst and the porphyrin sensitizer on the anode. The electron transfer was enhanced by photo-excitation of the sensitizer and induced charge separation to a covalently bound n-type semi-conductor (Figure 1). The resulting oxidized sensitizer then readily accepts electrons from the quinoproteins adsorbed to the electrode surface. The open circuit potential (Voc) of the illuminated photo anode was more negative than the same anode in a dark condition by approximately 80 mV (top of Figure 2). The generated current of the illuminated photoanode was also considerably higher at all potentials tested (bottom of Figure 2). To complete the PEBFC, the photoanode is coupled with a cathode utilizing an oxidase enzymatic catalyst and overall performance in terms of surface characteristics, Voc, and total power are characterized. Further applications with regards to possible uses in energy conversion and reforming of simple, abundant substrates such as glucose to more readily useable fuels such as hydrogen will be discussed. 1. M. Hambourger, G. Kodis, M. D. Vaughn, G. F. Moore, D. Gust, A. L. Moore and T. A. Moore, Dalton Transactions, 9979 (2009). 2. R. J. Lopez, S. Babanova, K. Artyushkova and P. Atanassov, Bioelectrochemistry, 105, 78 (2015). Figure 1

Novel Graphene-Modified Poly(styrene-b-isoprene-b-styrene) Enzymatic Fuel Cell with Operation in Plant Leaves

ABSTRACTA carboxylated poly(styrene-b-isoprene-b-styrene) triblock copolymer was synthesized for the construction of an enzymatic fuel cell. Glucose oxidase and bilirubin oxidase were chemically immobilized via the carboxylated functional groups of the polymer. The enzymatic fuel cell working electrodes were modified with graphene to accelerate the electron transfer rate of the system. Essential design and operational parameters were carefully optimized for improving the power of the enzymatic fuel cell. A power density of 20 µW cm−2 with only 4 µg of immobilized bilirubin oxidase was generated from 30 mM glucose at 0.72 V. The improved enzymatic fuel cell was tested in a plant leaf. A power density of 14 nW cm−2 was generated with glucose produced by photosynthesis reactions conducted during 30 min in the leaf.

Utilization of hydrolysate from lignocellulosic biomass pretreatment to generate electricity by enzymatic fuel cell system

The waste hydrolysate after dilute acid pretreatment (DAP) of lignocellulosic biomass was utilized to generate electricity using an enzymatic fuel cell (EFC) system. During DAP, the components of biomass containing hemicellulose and other compounds are hydrolyzed, and glucose is solubilized into the dilute acid solution, called as the hydrolysate liquid. Glucose oxidase (GOD) and laccase (Lac) were assembled on the electrode of the anode and cathode, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured, and the maximum power density was found to be 1.254×103μW/cm2. The results indicate that the hydrolysate from DAP is a reliable electrolyte containing the fuel of EFC. Moreover, the impurities in the hydrolysate such as phenols and furans slightly affected the charge transfer on the surface of the electrode, but did not affect the power generation of the EFC system in principal.

Tuning Structural Changes in Glucose Oxidase for Enzyme Fuel Cell Applications.

Stabilization and electrical contacting of redox enzymes with electrodes are fundamental requirements for bioelectronics devices, including biosensors and enzyme fuel cells (EFCs). In this study, we show increased glucose oxidase (GOx) stability by immobilization with Nafion. The immobilization process affected GOx conformation but was not detrimental to its activity, which was maintained for more than 120 days. The GOx/Nafion system was interfaced to a carbon cloth electrode and assembled in a prototypal EFC fed with glucose. Polarization and power density curves demonstrated that GOx/Nafion system was able to generate power, exploiting a Nafion-assisted electron transfer process to the electrode. Our findings are consistent with the onset of pH-dependent conformational equilibrium for the enzyme secondary structure and its active site. Significantly, the protective effect exerted by Nafion on the enzyme structure may be tuned by varying parameters such as the pH to fabricate durable EFCs with good electrocatalytic performance.

Generating power from transdermal extracts using a multi-electrode miniature enzymatic fuel cell

The development of self-powered wearable biodevices is highly attractive for a number of applications, such as health monitoring and drug delivery. Enzymatic fuel cells (EFCs) hold great potential as power sources for such devices, since they can generate power from physiological fluids and operate at body temperature.In this study, we present a cascade of three EFCs embedded in a compact and handy single channel device and we demonstrate for the first time power generation from iontophoresis extracts obtained from pig skin. The EFCs implement non-toxic highly-porous gold electrodes; an easy-to-reproduce procedure is adopted for the immobilization of glucose oxidase and laccase at the anode and cathode respectively; no external mediators are used; and the system design can easily be further miniaturized.When electrically connected in parallel, the EFCs generated a power output close to the sum of the power generated by each unit, with peak values of 0.7µW (flow-through mode) and 0.4µW (batch mode), at a glucose concentration of 27mM. When the device was fed with transdermal extracts, containing only 30μM of glucose, the average peak power was proportionally lower (0.004µW).

Use of nonimmobilized enzymes and mediators achieved high power densities in closed biobatteries

AbstractThe immobilization of redox enzymes on the surface of electrodes is a typical practice for enzymatic fuel cells (EFCs) but enzyme immobilization usually results in low retaining enzymatic activities and low power densities. Here, we investigated an alternative solution – the use of nonimmobilized thermostable enzymes (e.g., NAD‐based glucose 6‐phosphate dehydrogenase and flavin‐containing diaphorase) and a nonimmobilized mediator (i.e., benzyl viologen or 9,10‐anthraquinone‐2,7‐disulphonic acid, AQDS) for achieving high power densities from glucose 6‐phosphate in closed EFCs (called biobatteries). At the same enzyme loading, power densities of biobatteries increased in an order from the case of immobilized enzymes and immobilized vitamin K3 to the case of nonimmobilized enzymes and immobilized vitamin K3 to the case of nonimmobilized enzymes and nonimmobilized AQDS. The maximum power density of the biobattery based on nonimmobilized enzymes and AQDS was 1.1 mW cm−2 of anode at room temperature, 34‐times of the biobattery based on immobilized enzymes and immobilized vitamin K3. When enzyme loading was increased by another 10‐fold, the highest power density of biobattery was increased to 2.4 mW cm−2 at 37°C. The biobattery based on nonimmobilized enzymes and AQDS retained 60% of its initial current density after running for 2 h. When repeatedly being used for 6 rounds, the biobattery had 35% of its initial current density after 12 days. Our results suggest that closed biobatteries equipped with nonimmobilized thermostable enzymes and mediators feature simple system configuration and have high power densities.

Rates and Routes of Electron Transfer of [NiFe]-Hydrogenase in an Enzymatic Fuel Cell.

Hydrogenase enzymes are being used in enzymatic fuel cells immobilized on a graphite or carbon electrode surface, for example. The enzyme is used for the anodic oxidation of molecular hydrogen (H2) to produce protons and electrons. The association and orientation of the enzyme at the anode electrode for a direct electron transfer is not completely resolved. The distal FeS-cluster in [NiFe]-hydrogenases contains a histidine residue which is known to play a critical role in the intermolecular electron transfer between the enzyme and the electrode surface. The [NiFe]-hydrogenase graphite electrode association was investigated using Brownian Dynamics simulations. Residues that were shown to be in proximity to the electrode surface were identified (His184, Ser196, Glu461, Glu464), and electron transfer routes connecting the distal FeS-cluster with the surface residues were investigated. Several possible pathways for electron transfer between the distal FeS-cluster and the terminal amino acid residues were probed in terms of their rates of electron transfer using DFT methods. The reorganization energies λ of the distal iron-sulfur cluster and coronene as a molecular model for graphite were calculated. The reorganization energy of the distal (His)(Cys)3 cluster was found to be not very different from that of a standard cubane clusters with a (Cys)4 coordination. Electronic coupling matrix elements and rates of electron transfer for the different pathways were calculated according to the Marcus equation. The rates for glutamate-mediated electrode binding were found to be incompatible with experimental data. A direct electron transfer from the histidine ligand of the distal FeS-cluster to the electrode yielded rates of electron transfer in excellent agreement with experiment. A second pathway, however, from the distal FeS-cluster to the Ser196 residue was found to be equally efficient and feasible.

Design of a mediated enzymatic fuel cell to generate power from renewable fuel sources

The present work reported a compartment-less enzymatic fuel cell (EFC) based on newly synthesized Poly(pyrrole-2-carboxylic acid-co-3-thiophene acetic acid) film containing glucose oxidase and laccase effectively wired by p-benzoquinone incorporated into the copolymer structure. The resulting system generated a power density of 18.8 µW/cm2 with 30 mM of glucose addition at +0.94 V at room temperature. Improvements to maximize the power output were ensured with step-by-step optimization of electrode fabrication design and operational parameters for operating the system with renewable fuel sources. We demonstrated that the improved fuel cell could easily harvest glucose produced during photosynthesis to produce electrical energy in a simple, renewable and sustainable way by generating a power density of 10 nW/cm2 in the plant leaf within 2 min. An EFC for the first time was successfully operated in municipal wastewater which contained glycolytic substances to generate electrical energy with a power output of 3.3 µW/cm2.

Naphthoquinone Derivatives As Low-Potential Electron Mediators of Fad-Dependent Glucose Dehydrogenase

Flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) is rapidly emerging as an alternative glucose-oxidizing enzyme in enzymatic fuel cells (EFCs). FAD-GDH has the benefit over its commonly used counterpart, glucose oxidase (GOx), of not undergoing the parasitic reduction of O2 to H2O2, which can be troublesome in EFC applications. Additionally, the FAD cofactor has a relatively low redox potential in comparison to heme-based and PQQ-dependent enzymes, which can allow for a much higher theoretical cell potential. However, FAD-GDH is unable to facilitate rapid rates of direct electron transfer, and therefore requires a low-overpotential redox mediator to take full advantage of the low oxidation potential of its FAD cofactor. We have prepared a range of naphthoquinone analogues that are capable of mediating electron transfer between FAD-GDH and carbon electrodes, with onset potentials ranging between -0.425 to -0.150 V (vs. SCE), at pH 7.4. Several naphthoquinone derivatives were immobilized either onto a poly(ethylenimine) backbone or directly onto the outer shell of FAD-GDH to yield a naphthoquinone redox hydrogel and a naphthoquinone-labeled FAD-GDH. Both methods demonstrated large catalytic current densities at low onset potentials. When coupled with an O2-reducing biocathode (bilirubin oxidase, direct electron transfer-type) the glucose/O2 EFC possessed a large open-circuit potential (OCP) of 0.864 V and was able to deliver a maximum current density of 5.4 mA cm-2. The EFC reached its maximum power density (2.3 mW cm-2) at 0.55 V.

Quinone Electrochemistry: From the Electron Transport Chain to Electron Mediators and Orientational Moieites

Quinones play a vital role in electron transport chains (ETC) in mitochondrial (eukaryotic) and bacterial (prokaryotic) cells. Within the mitochondrial ETC, ubiquinone (coenzyme Q) is responsible for transporting electrons and protons from complexes I and II to complex III, where the ETC ultimately produces adenosine triphosphate (ATP) from ATP synthase. Within bacterial cells, ubiquinone and menaquinone also act as electron and proton carriers between large membrane-bound complexes. Quinone functionality is also required for many oxidoreductase enzymes, which utilize a pyrroloquinoline quinone (PQQ) cofactor. Naphthoquinone derivatives have been investigated for their suitability within enzymatic fuel cells (EFCs). At the bioanode, the electrochemical properties of naphthoquinone derivatives can be exploited to yield a bioanodic architecture that can mediate electron transfer between anodic enzymes and the electrode. In contrast, the chemical properties (not electrochemical redox properties) of the same naphthoquinone electrode architecture can be utilized at the biocathode of an EFC to efficiently orientate oxygen-reducing enzymes to undergo the direct bioelectrocatalytic reduction of oxygen. The use of the same electrode architecture at the bioanode and biocathode of EFCs results in a simplified EFC design, which ultimately results in lower manufacturing costs of devices.

Employing FAD-dependent glucose dehydrogenase within a glucose/oxygen enzymatic fuel cell operating in human serum

Flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) is emerging as an oxygen-insensitive alternative to glucose oxidase (GOx) as the biocatalyst for bioelectrodes and bioanodes in glucose sensing and glucose enzymatic fuel cells (EFCs). Glucose EFCs, which utilize oxygen as the oxidant and final electron acceptor, have the added benefit of being able to be implanted within living hosts. These can then produce electrical energy from physiological glucose concentrations and power internal or external devices. EFCs were prepared with FAD-GDH and bilirubin oxidase (BOx) to evaluate the suitability of FAD-GDH within an implantable setting. Maximum current and power densities of 186.6±7.1μAcm−2 and 39.5±1.3μWcm−2 were observed when operating in human serum at 21°C, which increased to 285.7±31.3μAcm−2 and 57.5±5.4μWcm−2 at 37°C. Although good stability was observed with continual near-optimal operation of the EFCs in human serum at 21°C for 24h, device failure was observed between 13–14h when continually operated at 37°C.

Rational design of quinones for high power density biofuel cells.

Enzymatic fuel cells (EFCs) are devices that can produce electrical energy by enzymatic oxidation of energy-dense fuels (such as glucose). When considering bioanode construction for EFCs, it is desirable to use a system with a low onset potential and high catalytic current density. While these two properties are typically mutually exclusive, merging these two properties will significantly enhance EFC performance. We present the rational design and preparation of an alternative naphthoquinone-based redox polymer hydrogel that is able to facilitate enzymatic glucose oxidation at low oxidation potentials while simultaneously producing high catalytic current densities. When coupled with an enzymatic biocathode, the resulting glucose/O2 EFC possessed an open-circuit potential of 0.864 ± 0.006 V, with an associated maximum current density of 5.4 ± 0.5 mA cm-2. Moreover, the EFC delivered its maximum power density (2.3 ± 0.2 mW cm-2) at a high operational potential of 0.55 V.

Development of Electron Transfer Mediator Using Modified Graphite Oxide/Cobalt for Enzymatic Fuel Cell

In an enzymatic fuel cell system, mediation of electron transfer is one of most important components supporting enzymes on electrode surfaces and transferring electrons from oxidation reaction. Various materials were tested as mediators for efficient electron transfer. The electrode preparation process was optimized using the selected material. Cobalt (Co) showed good performance as an electron transfer mediator. CoCl2 of 0.02 mol was the most effective concentration for a high-quality graphite oxide (GO)/Co composite. A reaction time of 18 h was optimal for significantly improved power density. Self-assembled glucose oxidase (GOx) and laccase were used as the anode biocatalyst and cathode biocatalyst, respectively, on Au electrodes. The enzymatic fuel cell produced a cell voltage of −0.578 V vs Ag/AgCl. The highest power density (1,058 μW/cm2) was generated at the optimal condition. Molecular properties of the developed electron transfer mediator using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were identified by observing the changes of the composite at the micro-surface level.

Biological Fuel Cells: Cardinal Advances and Critical Challenges

The always-young field of bioelectrochemistry has seen an unusual development: a move away from its comfortable analytical roots toward energy harvesting and power generation, as manifested by an explosion of projects, symposia and published papers. Wiley(-VCH) publications have provided a platform for this emerging trend by not only being the home for many of those individual breakthrough and developmental papers, but also by dedicating the pages of its well-established Electroanalysis for the first special issue on biological fuel cells (or biofuel cells) [1] in 2010, a special issue on microbial fuel cells in ChemSusChem in 2012, [2] and in 2014 publishing a book that is a compendium of the enzymatic fuel cell effort to date and another dedicated to implanting these biofuel cells in living organisms. [3, 4] Now, this growing, exciting and captivating field is presenting itself on the pages of the special issue of one of the newest members of the Wiley family, ChemElectroChem. The guest-editors are among the most engaged contributors to this field, and you will find an authorship list that will both convince you of the depth of the questions asked and the breadth of the solutions suggested. We are very happy to present these 30 + original contributions and focused topical reviews that provide a snapshot of a burgeoning field so dynamic and explosive in ideas and, sometimes, controversy, that the only problem the guest-editors had, was how not to lose the momentum. We thank all the contributors for trusting this issue with their best work of 2014. It was an amazing experience to not hear a single “no” in the process of recruiting this “by invitation only” volume, but rather to only receive suggestions on who else should be invited as well. This speaks volumes of the sense of camaraderie, mutual respect and promotion of the best science in the field of electrochemistry of biological systems for energy conversion, harvesting and power generation. Biological fuel cells (BFCs) are usually understood as devices that utilize biological systems (enzymes, organelles, microorganisms or tissues) and their intrinsic catalytic activity for energy transformation reactions. As those processes are usually associated with ubiquitous fuels, available in the environment, both natural and anthropogenic, the practical application of the BFCs is often associated with energy harvesting. The two main classes of these devices are the enzymatic fuel cells (EFCs) and microbial fuel cells (MFCs) divided by the primary use of either isolated enzymes or living microorganisms as catalysts. A broader definition also includes an abiotic fuel cell (or specific electrodes) that are designed to function by converting biologically available fuel (biomass, components in cellular or intercellular fluids of a living organism), regardless of the active catalytic components. Such abiotic electrodes are usually combined in a hybrid device with a bio-electrode (enzymatic or microbial), or an abiotic cell is used for energy harvesting while implanted in living tissue. The full diversity of these approaches is captured in this special issue. The EFCs are predominantly associated with small/ miniature devices with relatively high power density output that can be designed as a bio-battery, either containing all their fuel in a compartment of the cell or as a flow-through, open system that utilizes an external fuel stream. Often EFCs share design concepts with electrochemical biosensors, with the principal difference being the spontaneity of the electrochemical reactions happening at the electrodes. EFCs, however, are used as self-powered sensors and/or logical devices that can be integrated as electrical actuators in complex systems. The critical drawback of the EFCs has always been the timespan of their operation, which is critically limited by the environmental stability of immobilized enzymes while operating in abiotic environments. This special issue addresses the issues of integration of enzymes and nanomaterials, both carbonaceous (carbon nanotubes, their composites or synthetic hierarchically structured carbons) and metallic nanoparticles. Special attention is paid to the state and modification of the electrode interface to facilitate the charge transfer between the enzyme and the electrode for co-factors, among them PQQ, FAD and NADH. The works presented here address the issues of fuel flexibility and design for utilization of complex fuels, such as monosaccharides, sugars, starches and others. The design solutions vary from laboratory test units and well-defined electrodes to practical prototypes suitable for in vivo implantation or environmental testing.

Recent advances in carbon nanotube-based enzymatic fuel cells.

This review summarizes recent trends in the field of enzymatic fuel cells. Thanks to the high specificity of enzymes, biofuel cells can generate electrical energy by oxidation of a targeted fuel (sugars, alcohols, or hydrogen) at the anode and reduction of oxidants (O2, H2O2) at the cathode in complex media. The combination of carbon nanotubes (CNT), enzymes and redox mediators was widely exploited to develop biofuel cells since the electrons involved in the bio-electrocatalytic processes can be efficiently transferred from or to an external circuit. Original approaches to construct electron transfer based CNT-bioelectrodes and impressive biofuel cell performances are reported as well as biomedical applications.

Enzymatic Fuel Cells Solely Supplied with Unpurified Cellobiose Dehydrogenase and Laccase in Microorganism’s Culture Supernatants

AbstractEnzymatic electrodes have great potential for catalysing the direct conversion of chemical compounds into electricity with the use of redox enzymes. However, expensive and time‐consuming enzyme purification and the frequent need to add mediators are considered as drawbacks of enzymatic electrodes. We report a biofuel cell at pH 5, supplied with the unpurified enzymes laccase and cellobiose dehydrogenase (CDH) in crude culture supernatant, without the further addition of mediators. A maximum power output of 6.2±1.2 μW cm−2 was achieved by using supernatants containing laccase from Trametes versicolor (2.84 UmL−1) and CDH (0.90 UmL−1) from a recombinant yeast Yarrowia lipolytica YPC4. In comparison, the supply of purified enzymes (laccase: 2.40 UmL−1, CDH: 0.15 UmL−1) in a buffer solution yielded only about a twofold higher power density. Our results demonstrate the feasibility of using unpurified laccase and CDH in an enzymatic biofuel cell, which can simplify its construction and operation.

Miniature Direct Electron Transfer Based Enzymatic Fuel Cell Operating in Human Sweat and Saliva

AbstractWe present data on operation of a miniature membrane‐less, direct electron transfer based enzymatic fuel cell in human sweat and saliva. The enzymatic fuel cell was fabricated following our previous reports on miniature biofuel cells, utilizing gold nanoparticle modified gold microwires with immobilized cellobiose dehydrogenase and bilirubin oxidase. The following average characteristics of miniature glucose/oxygen biodevices operating in human sweat and saliva, respectively, were registered: 580 and 560 mV open‐circuit voltage, 0.26 and 0.1 μW cm–2 power density at a cell voltage of 0.5 V, with up to ten times higher power output at 0.2 V. When saliva collected after meal ingestion was used, roughly a two‐fold increase in power output was obtained, with a further two‐fold increase by addition of 500 μM glucose. Likewise, the power generated in sweat at 0.5 V increased two‐fold by addition of 500 μM glucose.