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Recent research in our group has shown that mixture-casting Nafion with quaternary ammonium bromides can increase the electrochemical flux of redox couples through the membrane and allow for larger redox species to diffuse to the electrode surface. The research has also suggested that when these salts are cast with Nafion micellar pore size is changing. Therefore, it was proposed that the quaternary ammonium salts could be employed to tailor the structure of the Nafion membrane for immobilizing enzymes in the polymer. For cations with a high affinity for the sulfonic acid groups of Nafion, the modified structure of Nafion can also help to stabilize the enzyme and increase activity by providing a protective outer shell and an ideal chemical environment that resists a decrease in pH within the pore structure. This research examines the ability to immobilize dehydrogenase enzymes in Nafion that has been modified with quaternary ammonium bromides. Fluorescence assays, fluorescence microscopy, and cyclic voltammetric studies were employed to analyze the ability to immobilize an enzyme within the membrane, to determine the activity of the immobilized enzyme and to examine the transport of coenzyme within the membrane. Dehydrogenase enzymes immobilized in tetrabutylammonium bromide/Nafion membranes have shown high catalytic activity and enzyme active lifetimes of greater than 45 days. A variety of dehydrogenase enzymes have been successfully immobilized in the membrane, including: alcohol dehydrogenase, aldehyde dehydrogenase, glucose dehydrogenase, and lactic dehydrogenase.

The key operating characteristics of biofuel cells, high energy density and low power delivery, make them ideal power choices for military, industrial, and niche wireless sensor network applications. There is virtually universal demand for higher energy power supplies for wireless sensors to enable longer operation, more frequent data capture, and higher bandwidth sensing techniques such as video. Advanced biofuel cell technology has been demonstrated for such applications that is capable of 2 -4Â increase in energy density compared to incumbent batteries of a similar size.

A biofuel cell is an electrochemical device in which the energy stored in a fuel, such as ethanol, is converted to electrical energy by the means of the catalytic activity of enzymes. Biofuel cells have traditionally suffered from low power densities and short lifetimes due to the fragility of the enzyme catalyst. Utilizing a novel quaternary ammonium salt treated Nafion membrane for enzyme immobilization in a biofuel cell results in increases in power densities and enzyme lifetimes to commercially viable levels. Additionally, this method provides sufficient protection to develop a membrane electrode assembly style (MEA) biofuel cell, an important step for commercialization. Previously, it has not been possible to create a MEA-style biofuel cell due to the denaturing of the enzyme that would occur at the high temperatures experienced during the heat pressing step of fabrication. Quaternary ammonium salt treated Nafion membranes provide sufficient protection for the enzyme to retain activity after exposure to temperatures of 140°C. Thus, a MEA-style biofuel cell can be created. Preliminary results yield biofuel cell MEAs with power densities ranging from 0.15 to 1.49 mW/cm2 and open circuit potentials of 0.360 to 0.599 V.

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