Innovations in Fuel Cell Technologies
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Innovations in Fuel Cell Technologies
350Hardcover
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Innovations in Fuel Cell Technologies
By Robert Steinberger-Wilckens, Werner Lehnert
The Royal Society of Chemistry
Copyright © 2010 Royal Society of ChemistryAll rights reserved.
ISBN: 978-1-84973-033-4
CHAPTER 1
Printed Enzymatic Current Sources
MATTI VALKIAINEN, SAARA TUURALA, MARIA SMOLANDER AND OTTO-VILLE KAUKONIEMI
VTT Technical Research Centre of Finland, Biologinkuja 5, Espoo, P.O. Box 1000, FI-02044 VTT, Finland
1.1 Introduction
Biofuel cells are devices capable of transforming chemical energy directly to electrical energy via electrochemical reactions involving enzymatic catalysis replacing precious metal catalysts. Operational principles are the same in bio- fuel cells and in conventional fuel cells, but the operating conditions, catalysts and materials, as well as fuels utilised differ considerably from conventional fuel cells.
In a microbial fuel cell the chemical energy is converted to electrical energy by the catalytic reaction of microorganisms, which produce their own enzyme catalysts. These microbial fuel cells have application areas such as wastewater treatment. Another group of biofuel cells comprises of enzymatic fuel cells that are equipped with puri?ed enzymes which are further on dealt with in this chapter.
In an enzymatic biofuel cell various oxidising and reducing enzymes, i.e. oxido-reductases are applied as biocatalysts for the anodic or cathodic half-cell reactions. The electron transfer process in glucose oxidase (GOx) half-cell reaction using glucose as the fuel was first shown to take place by Yahiro et al. Recently, the state-of-the-art of enzyme catalysed fuel cells were reviewed by Minteer et al., Davis and Higson and Cooney et al. The introduction of enzymes enables the operation of the cell under mild conditions and the utilisation of various, renewable chemicals as fuels. Biofuel cells can be utilised in various applications, including miniaturised electronic devices, self-powered sensors and portable electronics. It is also anticipated that implanted biofuel cells could utilise body fluids, particularly blood, as the fuel source for the generation of electrical power, which may then be used to activate pacemakers, insulin pumps, prosthetic elements, or biosensing systems.
A power source integrated with printed electronics could have a remarkable market potential in several mass-marketed consumer products, e.g. as package integrated functionalities (sensors, displays, or entertaining features, etc.) or as part of diagnostic devices. One of the main requirements is that the power source should be biodegradable or possible to incinerate with normal household waste. This demand is not easily met by traditional battery technology. The material costs of the power source should be reasonable, should not significantly increase the price of the product, and the cells should also to be made from roll to roll in a cost effective way. As an alternative power source the miniaturised biological, enzyme catalysed fuel cell, has the potential to be developed to meet these demands.
Biofuel cell research started in the early 1960s and activity has increased greatly since year 2000. The cumulative number of publications up to early 2010 is about 1800 including 1500 SciTech publications and 300 patents. The research has been very lively at St Louis University, where Professor Shelley Minteer and her group have authored altogether more than 100 papers and patents. The patenting has been led by companies such as Sony, Toyota and Canon.
In this chapter the possibility to utilise biological catalysts, enzymes as the active components of a printed power sources, i.e. biofuel cells, will be discussed. As a background for the realisation of this type of innovative concept we will first describe, in detail, those biological fuel cells that are potentially applicable for series production, with special focus on their performance figures. Potential printing methods and existing applications of power sources will also be discussed generally, thereafter mass-producible applications involving the use of enzymes are discussed first generally and then focussing on the production of enzymatically active layers by printing. Finally, the concept of a printed biofuel cell is presented.
1.2 Enzyme Catalysts in Fuel Cells
An active and stable biocatalyst is necessary to realise an enzyme-based power source. Enzymes are biocatalysts consisting of amino acids with the exception that the active centre of the enzyme may contain metal ions or other nonmetallic compound co-factors. The three-dimensional structure of the enzyme molecule determines its substrate specificity. Substrate refers here to the compound, which is modified during catalysis. The most suitable redox enzyme types for bioelectrodes are those having relatively tightly bound co-factors (such as metal ions, pyrroloquinoline quinone (PQQ) and flavin adenine dinucleotide (FAD) or haem), which are needed to carry out the electron transfer within the enzyme.
The majority of enzymatic biofuel cells utilise mediated electron transfer (MET) type bioelectrocatalysis, which is applicable for many redox enzymes. Mediators are redox species with reversible electrochemistry that can transfer electrons between the co-enzyme/co-factor of an oxido-reductase enzyme and the electrode. These mediators can be either in the electrolyte solution or immobilised in the electrodes. Figure 1.1 illustrates the principle of mediated electron transfer on a bioelectrode. The MET type bioelectrocatalysis often offers current density advantage over the direct electron transfer (DET) type as long as the mediator concentration is sufficiently high; however, MET has also some disadvantages like thermodynamic loss and potential mediator leakage. Recently, the studies on DET have become an object of growing interest as reviewed by Cooney et al.
The indicator of the catalytic performance, enzyme activity, is related to the reaction rate. The SI unit used to express this is katal (kat) which expresses the amount of enzyme that is needed to transform 1 mol of substrate in 1 s (distinct from the activity unit, U, which expresses the amount of enzyme needed to transform 1 µmol in 1 min. 1 kat = 60 000 000 U). The unit is defined in a way that represents the maximum catalytic power of the enzyme in specified environmental conditions. When the amount of enzyme activity on the electrode is known the approximate theoretical limit for electron transfer and cell current caused by the characteristic enzyme activity can be calculated. For instance, in the case of two electrons liberated per reaction at the molecular level and enzyme activity of 1 nkat cm-2 the theoretical limiting current of 190 µA cm-2 is obtained.
1.3 Enzyme-based Microsystems for Power Production
In this chapter the state of the art of biofuel cells is examined from the point of view of the developments in the cell structure. Biofuel cell constructions are presented in three different categories: biofuel cells constructed in a liquid chamber, biofuel cells based on carbon fibre design and biofuel cell constructions suitable for large-scale production. Different biofuel cell structures and their potential construction or manufacturing methods are discussed and the performance of the different biofuel cell constructions is reviewed in the following chapters and in Tables 1.1 and 1.2.
1.3.1 Biofuel Cells Constructed in a Liquid Chamber
A liquid chamber or a bulk electrochemical cell is a basic tool for studying the performance of enzymatic electro-active layers. Disk electrodes are covered with a paste containing electro-catalytic agents. The size of the disk electrodes in the characterisation studies is usually a few square centimetres and a common practice is to add the component acting as a mediator into the buffer solution. In the studies considered below, the electrode surface material was typically gold or glassy carbon, but porous carbon has also been used.
In 1998 Willner et al. presented a liquid chamber biofuel cell construction based on golden disk electrodes (ca. 0.2 cm2). The open circuit voltage (OCV) was 0.31 V and the short circuit current density 114 µA cm-2. The power output of the cell was 32 µW at ambient temperature, which corresponds to a power density of ca. 160 µW cm-2. Glucose oxidase from Aspergillus niger was used as the anode enzyme on a roughened golden disk electrode modified with a cystamine monolayer and electron acceptors PQQ and FAD. Microperoxidase-11 (MP-11) was harnessed as the cathodic enzyme and covalently linked to a cystamine monolayer on a roughened golden electrode surface.
In Katz et al. Willner and collaborators utilised the similar electrode structure in a membraneless liquid chamber. In this design the anode worked in an aqueous electrolyte and the cathode in a non-aqueous electrolyte (Dichloromethane), where cumene peroxide was used as the oxidant. The open circuit voltage for this design was 0.99 V. The area of the disk electrodes was 0.4 cm2 and the maximum power output of the device was 540 µW, which corresponds to the maximum power density of 4300 µW cm-2. The short circuit current density was 830 µA cm-2.
When the suitability of a fuel cell concept for mass-production is considered, the affordable price of the electrode materials, biocatalysts and mediators are essential requirements. The study reported by Liu et al. confirmed that glassy carbon and noble metal electrodes could be substituted with a porous carbon support. Here the enzymes used were glucose oxidase from Aspergillus niger and laccase from Coriolus versicolor and the cost-efficient mediators were ferrocene monocarboxylic acid (FeMCA) and 2,2'-azino-bis-(3-ethylbenzthiazo-line-6-sulfonic acid) diammonium salt (ABTS). Carbon nanotubes and chitosan were mixed with both enzymes before they were spread onto porous carbon, and the area of the electrode was 7 mm2. The important dependence of the cell performance on pH was studied. The highest power density of 99.8 µW cm-2 was measured at pH 4.0, whereas it decreased to 14.75 µW cm-2 at pH 5.0 and was only 2.0 µW cm-2 at pH 7.0. In addition, the same behaviour appears in the open circuit voltage, which was 0.66 V at pH 4.0, 0.55 V at pH 5.0, and 0.16 V at pH 7.0. The short circuit current density was 950 µA cm-2 at pH 4.0, 738 µA cm-2 at pH 5.0, and 144 µA cm-2 at pH 7.0.
Zhang et al. studied a novel type of direct methanol biocatalytic fuel cell (DMBFC) based on enzymatic conversion of methanol by methanol dehydrogenase (MDH) from Methylobacterium extorquens at the anode. The terminal electron acceptor at the cathode was potassium permanganate. Performance characteristics achieved were: open circuit voltage 1.4 V, power density 0.25 mW cm-2 and current density 0.38 mA cm-2 at the operating voltage of 0.67 V, and a continuous operation time of 2 weeks.
1.3.2 Miniature Membraneless Biofuel Cells
Miniature biofuel cells described in this chapter are comprised of two bioelectrocatalyst-coated carbon fibre electrodes inside a small test cell without a separator. This miniature cell structure differs from the liquid chamber construction where covered disk or wire (i.e. bigger) electrodes are mainly used and usually a separator is needed. The most remarkable features of these miniature biofuel cells are their small size and structural simplicity due to which they are potential power sources for medical applications, e.g. hypodermic implants. For example, a sensor–transmitter could be powered by a miniature biofuel cell that continuously produces a few microwatts, of which less than 1 µW will be consumed by the sensor; the transmitter will require most of the power. The transmitted information will be easily acquired outside the body with a small ultra-capacitor, that stores about 10 µJ, which is enough for a 1 ms burst of 10 mW (1–10 GHz) every 10 s.
In 2001 Chen et al. studied a cell where carbon fibres were placed in two 1 mmx1 mm grooves bored in a 3 cm long polycarbonate support. The gap between the grooves was 400 µm and the active area of the fibres was 0.44 mm2. Glucose oxidase from Aspergillus niger and laccase from Coriolus hirsutus were used as the anode and cathode enzymes, respectively, and osmium complexes were used as mediators. A cross-linking agent poly(ethylene glycol) diglycidyl ether (PEGDGE) was used to immobilise the enzymes. The power density for this design was 64 µW cm-2 at 23 °C and 137 µW cm-2 at 37 °C. The true power output of the device was 280 nW and 600 nW, respectively.
In 2002 Tsujimura et al. 12 demonstrated that a membraneless biofuel cell is capable of operating in a physiological buffer solution, which enables biofuel cells to be used e.g. in living plants. In 2003, Heller and Mano et al. demonstrated a miniature glucose–O2 biofuel cell implanted in a grape (Figure 1.2). The electrodes were 7 µm in diameter and 2 cm in length. On the anode, glucose oxidase from Aspergillus niger was immobilised and electrically connected onto the carbon fibre. Respectively, on the cathode, bilirubin oxidase from Trachyderma tsunodae was immobilised and electrically connected onto the carbon fibre.
1.3.3 Biofuel Cell Constructions Suitable for Large-scale Production
The Sony Corporation has developed a biofuel cell that uses carbohydrates (sugars) as its fuel and enzymes as its catalyst. The first published passive-type test cells achieved a power output of 50 mW. This output was high enough to power an MP-3 player with two speakers (Figure 1.3). The second passive-type test cells were capable of generating a power of over 100 mW, high enough to operate a radio-controlled car.
Sony's biofuel cell is based on glucose dehydrogenase (GDH) from a Bacillus sp. and diaphorase (DI) from Bacillus stearothermophilus as the anode enzymes and methyl-1,4-naphthoquinone (vitamin K3, VK3) or 2-amino-1,4-naphtho-quinone (ANQ) as the anode mediator. β-Nicotinamide adenine dinucleotide disodium salt (NADH) is added as the soluble co-factor of the dehydrogenase. The cathodic enzyme and mediator are bilirubin oxidase (BOD) from Myrothecium verucaria and K3[Fe(CN)6], respectively (Figure 1.4). The performance of Sony's biofuel cells was achieved by applying different approaches described below.
In order to increase the maximum current density, enzymes and mediators were effectively entrapped on ozone treated carbon fibre (CF) electrodes retaining the enzymatic activity. CF sheets were used because CF has a higher surface area and a higher porosity, allowing an undisturbed transport of fuels.
Sakai et al. reported that the catalytic current density of the manufactured CF bioanode was lower than assumed at first, despite the fact that the evaluated surface area of the CF electrode was higher than previously. It was presumed that the low catalytic current density was due to insufficient proton diffusion in the CF bioanode. In order to solve this problem, the effects of buffer concentration in the glucose containing electrolyte solution (phosphate buffered saline, PBS) were examined. The current density was considerably improved in 1.0 M PBS compared to the previously used electrolyte of 0.1 M PBS. It was also noticed that the current density decreased when the buffer concentration was higher than 1.0 M, presumably due to enzymatic activities of GDH and DI. On the other hand it was observed that the higher ionic strength of the buffer could beneficially suppress the pH drop during the operation of the CF bioanode. As a conclusion, Sakai et al. proposed two rate determining steps for the reaction: the insufficient proton diffusion at low buffer concentrations (<1.0 M) and inactivation of the enzymatic activity at higher buffer concentrations (>1.0 M). The concentration of the buffer was therefore optimised for the immobilised enzymes.
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Excerpted from Innovations in Fuel Cell Technologies by Robert Steinberger-Wilckens, Werner Lehnert. Copyright © 2010 Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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