Modeling Carbon Nanotube Connectivity and Surface Activity in a Contact Lens Biofuel Cell

Biofuel cells are often limited by the current density produced by the cathode; this is especially true when such fuel cells are scaled down to fit a desired application. Herein, we created a computational model to examine the effects of carbon nanotube (CNT) connectivity and surface activity on the current density of a biofuel cell cathode. The model was motivated by the creation of a novel contact lens biofuel cell that, although more stable and biocompatible than previously reported designs, was cathode limited. The device produced a maximum current density of 22 ± 4 μA cm−2, and a maximum power density of 2.4 ± 0.9 μW cm−2 (at 0.163 V) with an open-circuit voltage of 0.44 ± 0.08 V. Computational results showed that in a Nafion film containing 1.6% CNTs by volume, less than 20% of the CNT fibers were connected to the electrode, assuming a planar electrode. The simulations predicted that a three-fold increase in CNT loading would lead to a roughly two-fold increase in total CNT connectivity. The simulations further estimated that for the CNTs connected to the electrode, only 21% of their sidewalls were contributing to cathodic current, meaning that the remaining surfaces were not electrochemically active. Given the low bilirubin oxidase (BOD) enzyme surface concentration, which was experimentally found to be 1.24 × 10−13 mol cm−2, it is likely that large portions of the CNT surfaces are not connected to enzymes. This result validates the push by the research community to increase BOD and laccase adsorption/orientation to CNT surfaces.

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