(Invited) Overcoming Biocathode Limitations in Bfcs: Enhancing O2 Concentration with Micelles

Abstract

Enzymatic fuel cells (EFCs) are capable of utilizing a wide range of sugars and short-chain alcohols as fuel sources. Using an enzymatic catalyst at either the anode or cathode, EFCs are able to operate under physiological pH and temperature. Recent advancements in bioanodic materials have allowed for generation of current densities in excess of 2 mA cm-2; however, the majority of EFCs are limited by current density generated at the biocathode. Typical enzymatic biocathodes utilize a copper oxidase, such as laccase or bilirubin oxidase, to bioelectrocatalytically reduce molecular oxygen to H2O. However, biocathodes are often limited by slow diffusion of O2 to the electrode interface, which is ultimately the result of relatively low solubility of O2 in aqueous solutions (mole fraction solubility, xO2 = 2.3×10-5). By contrast, the solubility of O2 in organic solvents such as nonane or perfluorooctane (xO2 = 2.1×10-3 and 5.3×10-3, respectively) are much higher than in aqueous conditions. We have developed a strategy to combine the high O2 solubility of organic solvents with the biocompatibility afforded by aqueous solutions, in which phospholipid micelles are used to trap O2 and transport it to the cathode surface. By modifying the phospholipid composition, various properties of the corresponding micelle can be tuned to cause dispersion upon contact with the electrode surface thereby increasing the localized O2 concentration. The net result is a dramatic increase in current density generated at the biocathode.