Elsevier

Journal of Power Sources

Volume 196, Issue 22, 15 November 2011, Pages 9260-9269
Journal of Power Sources

Direct hybrid glucose–oxygen enzymatic fuel cell based on tetrathiafulvalene–tetracyanoquinodimethane charge transfer complex as anodic mediator

https://doi.org/10.1016/j.jpowsour.2011.07.060Get rights and content

Abstract

TTF–TCNQ has been used for the first time as a mediator in a direct glucose fuel cell operating on gas-phase oxygen. It has been shown that TTF–TCNQ forms highly irregular porous structure, which emphasizes the importance of optimization of mass transport and kinetic resistance in the catalyst layer. Kinetics resistance can be optimized by variation of the mediator and/or enzyme loading, while mass transport resistance mainly by the variation of other structural parameters such as electrode thickness. The optimized anode reached limiting current densities of nearly 400 μA cm−2 in presence of 5 mM glucose under rotation. The enzymatic fuel cell exhibited unexpectedly high OCV values (up to 0.99 V), which were tentatively ascribed to different pH conditions at the anode and the cathode. OCV was influenced by glucose crossover and was decreasing with an increase of glucose concentration or flow rate. Although the performance of the fuel cell is limited by the enzymatic anode, the long-term stability of the fuel cell is mainly influenced by the Pt cathode, while the enzymatic anode has higher stability. The fuel cell delivered power densities up to 120 μW cm−2 in presence of 5 mM glucose, depending on the glucose flow rate.

Highlights

► Modular hybrid fuel cell with enzymatic anode and Pt cathode, separated by Nafion. ► 3D-enzymatic anode based on TTF–TCNQ and glucose oxidase. ► High power density at 5 mM glucose, operation with oxygen. ► Limited long-term stability due to change of pH at cathode side.

Introduction

Enzymatic fuel cells belong to the group of biofuel cells. The characteristic feature of this fuel cell type is the application of enzymes (biocatalysts) instead of noble metal catalysts. Enzymatic fuel cells are a promising type of fuel cells for niche applications, which benefit from the utilization of enzymes as highly efficient natural catalysts. Advantages of enzymes include activity at mild conditions, lower price and substrate selectivity, which can theoretically enable a membraneless design [1]. The utilization of biocatalysts offers larger number of possible fuels and oxidants as natural substrates of the respective enzymes. The typical fuel in enzymatic biofuel cells is glucose [1] but other sugars, such as fructose [2] and lactose [3], as well as lower aliphatic alcohols, such as ethanol [4] and glycerol [5] have been also used. Contrary to the variety of fuel types, oxygen is almost exclusively used as an oxidant in biofuel cells [1]. Glucose–oxygen enzymatic biofuel cells are promising as power sources for implantable devices [6]. Major drawbacks of these systems are the still low power output and the limited stability.

The enzymes that are employed in this type of fuel cells are called oxidoreductases and catalyze oxidation and reduction processes involving transport of electrons. Oxidoreductases can be coupled with the electrode surface, thus forming enzymatic electrodes, and the electron transfer process can be followed by different electrochemical methods. To construct a biofuel cell system based on redox enzymes, the following points have to be considered: (1) enzyme immobilization; (2) communication between the enzyme and the electrode surface (type of electron transfer); (3) enzyme kinetics; (4) enzymatic electrode architecture; (5) coupling of the electrodes and design of the overall system. The first three points have been extensively studied in the past, mainly regarding the application of these systems as amperometric biosensors. As a result of these activities many preparation methods for enzymatic electrodes can be found in literature. However, only few of them can be employed in systems, where energy production is the main application.

Some examples of mediators in enzymatic electrodes, which have found an application in glucose biofuel cells are ferrocene [7], tetrathiafulvalene (TTF) [8], 8-hydroxyquinoline-5-sulfonic acid (HQS) [9] and Os redox hydrogels [10]. So far, the best performance exhibit Os hydrogels with redox centers attached to a polymer backbone [1]. However, the procedure of synthesizing Os redox hydrogels is usually complicated and involves several steps [11]. In addition, in respect to the application of enzymatic biofuel cells as implantable power sources, some issues, associated with the toxicity of Os-containing compounds may arise [12]. Another type of mediator, which in our opinion has a great promise for biofuel cell application is the charge transfer complex (CTC) known as organic conductive salt, based on tetrathiafulvalene and tetracyanoquinodimethane (TTF–TCNQ). This complex has been so far mainly used as a mediator in enzymatic electrodes for biosensor applications as outlined in a recent review [13]. Glucose biosensors based on TTF–TCNQ exhibit high current densities, high oxygen tolerance and remarkable stability under continuous operation [14], [15]. Khan et al. have shown that glucose biosensors based on TTF–TCNQ can retain up to 40% of their initial response after 100 days of continuous operation and exhibit low sensitivity to the oxygen in normal buffer solutions [14]. However, despite these promising features, TTF–TCNQ anode, to our best knowledge, has not been employed in an enzymatic fuel cell so far. In addition, CTC has several other advantages. Enzymatic electrodes based on TTF–TCNQ do not require complicated modification procedures, in fact they can be prepared as simply as carbon paste electrodes by mixing of the respective components [16]. The TTF–TCNQ salt is commercially available and has high electronic conductivity, which is beneficial for lowering the ohmic resistance within the electrode layer. The morphology of the CTC-crystals can be tuned by variation of the experimental conditions [17]. They can be also prepared in form of nanoparticles [18]. These strategies can be applied to tune the catalytic properties of the CTC and/or to increase the catalytically active surface area. Both TTF and TCNQ have low toxicity, which is attributed to their low solubility in water and physiological fluids [19]. In addition, the catalytic properties of the CTC and the overpotential for glucose oxidation can be further improved by lowering of the redox potentials of its components, e.g. of TTF [20].

To construct an enzymatic electrode for a biofuel cell application the electrode architecture has to be carefully designed and optimized. In the case of enzymes the number of active sites per volume is generally lower than in the case of noble metal electrodes, which issue is crucial in the case of energy-producing systems. This motivates the use of three-dimensional electrode structures instead of monolayer or thin layer configurations [1]. Depending on the type of electron transfer (mediated or direct), a complex network between the enzyme, the mediator and the electron conductive surface has to be established. To tackle experimentally this issue, the influence of the loadings of the respective components in the catalyst layer (composition) and the overall electrode architecture has to be studied, which has not been extensively investigated so far.

The next important aspect is the design of the whole fuel cell system. Little emphasis has been put on this issue in the past and most of the studies have been focused on single electrodes studies. In this work a hybrid biofuel cell device, based on the combination of enzymatic anode and a Pt cathode has been developed. The structure of the cathode comprises the membrane electrode assembly (MEA) design, which has been adopted from the conventional fuel cell technology. The operation with gas-phase oxygen compensates the low solubility of oxygen in aqueous solutions. The combination with a noble metal catalyst electrode allows for testing of the enzymatic electrode performance under fuel cell conditions. Similar strategy has been used both for anodes [21], [22], [23] and for cathodes [4], [5], [24]. Such systems have been usually referred to as biofuel cells, despite of the presence of a non-bio component. The fuel cell device in this study can be used as a platform for investigation of different enzymatic anodes and give additional information about their behavior in a whole fuel cell system when combined with a cathode with “known” catalytic properties.

To summarize, in this paper some aspects regarding the development and design of a hybrid enzymatic fuel cell, such as: (a) the utilization of TTF–TCNQ mediator and the optimization of the three-dimensional electrode structure, (b) anode and cathode coupling and fuel cell design and (c) fuel cell system characterization have been covered.

Section snippets

Chemicals and materials

Glucose oxidase (EC 1.1.3.4, GOx) from Aspergillus niger was supplied by Fluka. All other chemicals including glucose, TTF, TCNQ and polyvinyl sulfate potassium salt (PVS) as well as tetrahydrofuran (THF) and acetonitrile (ACN) were of analytical reagent grade and purchased from Sigma–Aldrich. Ultrapure water from Millipore was used in all experiments.

Stainless steel was used as a mechanical and electrical support for the preparation of enzymatic electrodes. Discs with a diameter of 11 mm and 1 

Enzymatic electrode characterization with SEM

A SEM image of the enzymatic electrode assembly cross section, peeled off from the stainless steel support, is shown in Fig. 3. Three layers can be clearly distinguished and the thickness of every layer has been estimated from the micrograph as follows: 2 μm polypyrrole layer, 60 μm catalyst layer, composed of CTC crystals with adsorbed GOx, and 5 μm gelatin layer. As can be seen in Fig. 3 the catalyst layer of the enzymatic electrode is characterized by randomly distributed CTC bundles and

Conclusions

In the present study an enzymatic anode based on a charge-transfer complex (TTF–TCNQ) and glucose oxidase has been applied for the first time in a hybrid glucose–oxygen fuel cell. The CTC forms highly irregular porous structure, which serves a matrix for enzyme immobilization. This points out the importance of the enzyme distribution optimization within the catalyst layer. The optimized electrode reached high current densities and the fuel cell reached high power densities in presence of low

Acknowledgments

The authors would like to thank Leonardo Sarmento and Torsten Schröder, who contributed to the design of the hybrid enzymatic fuel cell device and the fuel cell testing facility.

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