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...