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Aarne Halme, Xiachang Zhang

Biological fuel cell is a device that realizes the conversion of biochemical energy into electrical energy. The basic principle is that the process of substrate oxidation by microorganisms or enzymes in the fuel cells offers electrons for electricity production. Since the conversion is not restricted by the Carnot cycle, the theoretical efficiency can be as high as 90%. There are two types of microbial fuel cell. One involves the utilization of electroactive metabolites, e.g., hydrogen, converted by microbial metabolism or enzyme reaction from substrate, and another involves the utilization of mediators as electron transporters from a certain metabolic pathway of the microorganism or enzyme to electrodes. So far we have studied the structure (including the size and shape) of the fuel cell device, modelling and control of the process. Also, we have tested two kinds of microorganism (baker's yeast and bacteria from Finnish Gulf) in our fuel cell system. In the current study, we are going to study the small enzyme fuel cell and try to apply the enzymatic fuel cell for small electronic devices like mobile phone. The detailed information on the enzymatic fuel cell is available.

Microorganisms used for the biological fuel cell

After using a baker's yeast as microorganism in the biological fuel cell a few years ago, bacteria have been used for the study. The bacteria were obtained from the bottom of the Finnish Gulf. One of the main reasons to use bacteria is that the bacteria could survive in a higher pH solution. The higher pH solution offers a lower anodic potential, which makes a higher potential difference in the terminals of the fuel cell.

Structure of the fuel cell

Different shapes of the anode and the cathode are studied. It seems that the thickness of the anode compartment should be less than 2 cm and larger membrane area is needed for a high energy output. On the other hand, the size of the biological fuel cell is important factor for the efficient energy conversion from chemical to electrical energy. The smaller size of the fuel cell improves the efficient.

Modelling and control of the biological fuel cell processes

The model was set up on the basis of the experimental results and analysis of biochemical and electrochemical processes. Simulation of the process shows that the model describes the process reasonably well. The analysis of model simulation illustrates how the current output depends on the substrate concentration, mediator concentration and other main variables. The relationship between the current output and over-voltage is revealed from the modelling study. Based on the experimental and modelling of the process, proper control of bacteria metabolism can convert most of substrate to electroactive intermediates.

Reaction diagram of the biological fuel cell

A biological fuel cell device