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

Abstract

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 x 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.

O2Reduction in Enzymatic Biofuel Cells

“…Both SW-CNTs 142,296,297,[299][300][301][302][303][304] and MW-CNTs 218,244,295,298,303,[305][306][307][308] the CNT-connectivity drops off once the distance to the electrode surface becomes higher than the CNT size (Fig 22). 312 Reproduced with permission from [312]. Copyrigth 2016 Elsevier…”

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment

“…The aforementioned wearable sensors track human body's physical activities in dry conditions by detecting changes in electrical signals, which provide the variety of physiological information, but do not include the body's biomolecular state. Noninvasive wearable electrochemical sensors have been developed (Figure b) to detect target analytes in accessible biofluids such as saliva, tears, sweat, and skin ISF, while the use of blood is challenging to access in vitro wearable platform . Electrochemical sensors are placed close to the fluid origination, local stimulation, and on‐demand or ideal sites for continuous monitoring, tracking the concentration and its changes for various types of analytes such as ions, biomolecules, and proteins, and then converting to electrical signal changes through various techniques such as potentiometry, voltammetry, chronoamperometry, and electrochemical impedance spectroscopy .…”

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

“…To maximize the specific surface area for enzyme loading and the contact area of the electrodes, the common structure adopted to realize the wearable EFC includes multiple thin layers with specific functions, like a basic conductive layer for current collection (e.g., gold, platinum, conducting polymer, etc. ), a nanoengineered highly porous layer with a large surface area (e.g., carbon nanotubes, graphene nanosheets, carbon or metal nanoparticles, etc.,) for enzyme and cofactor immobilization, another layer for mediator/crosslinker reservoir, and eventually, a final ion-conducting layer (e.g., Nafion 117 solution) as protection from leakage and any damage from the external environment [206,207]. The combination of these layers makes the final architecture of the EFC; if connected in space by series or parallel, they can determine various compact designs and stacks.…”