More recent research in developing long-term stability in biofuel cell systems has focused on studying a new enzymatic system. Initial studies have been successful in utilizing PQQ-dependent ADH as a catalyst at the anode of a biofuel cell. Pyrroloquinoline quinone (PQQ)-dependent alcohol dehydrogenase (ADH) has been chosen to replace NAD+-dependent ADH in order to extend the lifetime and simplicity of the fuel cell. PQQ is the coenzyme of PQQ-dependent ADH, and it remains electrostatically attached to PQQ-dependent ADH; therefore, the enzyme and coenzyme will remain in the membrane leading to an increased lifetime and activity for the biofuel cell. Also, PQQ-dependent ADH possesses desirable electrochemistry (it has the ability to transition between its oxidized and reduced state). The coenzyme PQQ has quasireversible electrochemistry and a low overpotential at an unmodified carbon electrode. This eliminates the need for an electrocatalyst layer, thereby simplifying the process of forming high — surface-area bioanodes.

PQQ-dependent ADH bioanodes fabricated and tested by Treu et al. using the same procedures and methods as for the NAD+-dependent ADH system had desirable results. The PQQ-dependent ADH enzymatic system has shown life­times of greater than 1 year of continuous use, open circuit potentials of 1.0 V and power densities of up to 3.61 mW/cm2 for a 1.0 mM ethanol solution at room temperature in a static system [25].

The PQQ-dependent ADH-based biofuel cell outperformed the NAD+-depen — dent ADH-based fuel cell in all aspects of traditional systems. For the traditional system of PQQ-dependent ADH anode coupled with a traditional platinum cath­ode, the PQQ-based enzymatic system increased the overall lifetime by greater than 711%, increased open circuit potential by 67%, and increased power density by 251% [25]. There was no correlation between power and different pHs for the PQQ-dependent ADH biofuel cell [26].

PQQ-dependent ADH has optimum selectivity for ethanol, but will oxidize other alcohols. Similar fuel cells have been developed for methanol, butanol, and propanol using PQQ-dependent ADH bioanodes. The performance of those fuel cells is shown in Table 12.3. PQQ-dependent ADH is a desirable substitute for a biocatalyst at the anode of a biofuel cell. Eliminating the poly(methylene green) layer simplifies the fabrication of a bioanode and lowers the IR drop leading to an increase in performance. Research has shown that PQQ-dependent ADH enables simple and timely electrode fabrication, along with impressive open circuit potentials, current densities, power densities, and lifetime for a complete ethanol/oxygen biofuel cell.