Since enzymes are highly selective, there are limited problems associated with fuel crossover from the anode to the cathode in a biofuel cell. If an anode and cathode are both selective, then a polymer electrolyte membrane is no longer required to separate the anode and cathode solutions. Topcagic et al. have devel­oped the first ethanol membraneless biofuel cell. At the cathode, bilirubin oxidase has been chosen to replace platinum as the reducing catalyst to increase specificity of the cathode. A schematic showing the simplicity of a membraneless biofuel cell can be seen in Figure 12.7.

Alternatives for platinum found in the literature typically use laccase enzyme as studied by Heller’s Group [27]. Laccase lowers the power of a biofuel cell due to the maximum turnover rate of laccase occuring at pH 5.0 and deactivation in the presence of chlorine ions. Bilirubin oxidase has been chosen as a catalyst for future studies, because it has optimum performance in a physiological envi­ronment (near-neutral pH and presence of various ions). The second problem associated with many biocathodes in the literature is that electrodes are osmium — based creating a toxicity hazard to the surrounding environment. Topcagic et al. have replaced the osmium-based mediator with a ruthenium-based complex of

similar structure that is less toxic and has a higher self-exchange rate. A third problem associated with anodes and cathodes in the literature is a technique of immobilizing the enzyme at the electrode surface. Literature enzyme immobili­zation employs covalent bonding of the enzyme to the surface of the electrode or to the mediator. This method does not protect the enzyme from its surroundings and its optimal activity is lowered due to the conformational change that resulted from physically attaching the enzyme to the surface to the electrode [23]. How­ever, instead of physically attaching the enzyme, Topcagic et al. have immobilized it in modified Nafion®; therefore optimum enzyme activity is retained and the enzyme is protected from the surrounding environment.

The membraneless biofuel cell operates at room temperature, which varies from 20-25°C in a phosphate buffer pH 7.15 containing 1.0 mM ethanol. Since the polymer electrolyte membrane has been eliminated, the electrodes have to be specific enough to work in same compartment. Both electrodes, bioanode and biocathode, consisted of immobilized enzyme casting solution at the surface of the 1-cm2 carbon fiber paper. The performance of NAD+-dependent ADH and PQQ-dependent ADH bioanodes coupled to biocathodes was also studied and is

Electrical Load

summarized in Table 12.3. The bioanode used for most studies had PQQ-depen — dent alcohol dehydrogenase enzymes, while the biocathode had bilirubin oxidase, bilirubin with Ru(bpy)3+2 immobilized at the surface of the electrode. Maximum open circuit potential is 1.04 V with maximum current density of 8.47 mA/cm2 [28]. For the membraneless system comprised of a PQQ-dependent ADH anode and bilirubin oxidase biocathode, the fuel cell has an increased lifetime of greater than 353%, increased open circuit potential of 15%, and increased power density of 97% compared to the NAD+-dependent ADH bioanode [28].

CONCLUSIONS

Research has succeeded in increasing the stability of enzymes at the electrode surface, which in turn increases the open circuit potential, current and power of a biofuel cell. Also, by eliminating the need for an electrocatalyst layer and a polyelectrolyte membrane, it reduces the cost of production of the current biofuel cell and simplifies fabrication. Replacing NAD+-dependent ADH with PQQ — dependent ADH is a step toward reaching the goal of increasing the overall lifetime of the biofuel cell for future use in multiple power applications. The most important phenomenon to examine is the increase in lifetime of the mem­braneless system with PQQ-dependent bioanodes. The PQQ-dependent bioanodes are more stable than NAD+-dependent bioanodes. In addition to better data results, PQQ-dependent ADH serves as an invaluable replacement for NAD+-dependent ADH due to the simplicity it offers in bioanode fabrication.