Biocatalytic fuel cells design

Contrary to traditional fuel cells, biocatalytic fuel cells are in principle very simple in design [1], Fuel cells are usually made of two half-cell electrodes, the anode and cathode, separated by an electrolyte and a membrane that should avoid mixing of the fuel and oxidant at both electrodes, while allowing the diffusion of ions to/from the electrodes. The electrodes and membrane assembly needs to be sealed and mounted in a case from which plumbing allows the fuel and oxidant delivery to the anode and cathode, respectively, and exhaustion of the reaction products. In contrast, the simplicity of the biocatalytic fuel cell design rests on the specificity of the catalyst brought upon by the use of enzymes. 

Many assembled glucose-02 biocatalytic fuel cells have been reported in recent years. This section aims at giving an overview of the methods that have been used to build these biocatalytic fuel cells and of the performances obtained by these devices depending on their different designs. 

A remaining crucial technological milestone to pass for an implanted device remains the stability of the biocatalytic fuel cell, which should be expressed in months or years rather than days or weeks. Recent reports on the use of BOD biocatalytic electrodes in serum have, for example, highlighted instabilities associated with the presence of 02, urate or metal ions [99, 100], and enzyme deactivation in its oxidized state [101]. Strategies to be considered include the use of new biocatalysts with improved thermal properties, or stability towards interferences and inhibitors, the use of nanostructured electrode surfaces and chemical coupling of films to such surfaces, to improve film stability, and the design of redox mediator libraries tailored towards both mediation and immobilization. 

The main potential of mediated ET lies in the increase of current densities, as the essential challenge of designing biofuel cells is to increase the biocatalytic power of these devices. Biofuel cells presently reach a power output in the range of about 10 to 10 Wcm . Practical conventional fuel cells operate in the range of about 1 to 10 Wcm [303]. Taking the calculations from Barton and coworkers into consideration [70], in which, as mentioned above, the theoretical current density of a monolayer was estimated to be about SOpAcm" one would require thousands of layers to obtain a current density above 10 mA cm f... 

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