Development and Comparison of Membrane-/Mediator-Free Enzymatic Biofuel Cells Using Carbon Nanotube-Based Supports of Varying Morphology
Authors: Campbell Alan S. , Islam Mohammad F. , Russell Alan J. 
Published: February 2020

Enzymatic biofuel cells (EBFCs) convert the chemical energy present in renewable biofuels such as sugars and alcohols into electrical energy using enzymes at one or both electrodes. These devices have shown much promise in the continuous powering of implantable devices due to their physiologically available fuels and lack of harmful products. A fully enzymatic biofuel cell consists of two separate electrode systems: an anodic enzyme system that serves to oxidize the fuel and transfer electrons to an external circuit, and a cathodic enzyme system that accepts and transfers electrons from the external circuit to a final electron acceptor (generally molecular oxygen). Crucial to this function is the transfer of electrons between enzyme and electrode either directly or through a redox mediator. For efficient and reliable electron transfer to proceed, the working enzymes must be immobilized at or near the electrode surface, particularly in direct electron transfer-based cells. However, enzyme immobilization onto solid supports has also been reported to decrease enzymatic activity. Further, this activity reduction has been shown to be dependent on the surface properties of the chosen support. We report on the development of three separate EBFC systems with widely varying morphologies in terms of specific surface area (SSA), pore size and surface curvature in order to show the impact of each of these characteristics on overall performance. In all studies, glucose oxidase (GOX) and bilirubin oxidase were physically adsorbed onto the selected electrode material to form enzymatically active anodes and cathodes, respectively. One of the supports was gold/multi-wall carbon nanotube (MWCNT) fiber paddles that were formed by depositing gold nanoparticles and MWCNTs (diameter = 11.5 nm) onto electrospun polyacrylonitrile fibers with a diameter of ~1 µm and resulted in a SSA of 3.6 m2 g-1 and micrometer sized pores. Gold/MWCNT fiber paddle EBFCs yielded power densities of 0.4 µW cm-2 with an open circuit voltage (OCV) of 0.22 V and GOX loadings of 2.0 x 10-10 mol cm-2. The other two supports were formed through the gelation of individually dispersed single-wall carbon nanotubes (SWCNTs) with 1 nm diameters. Graphene-coated SWCNT gels possessed a specific surface area of 686 m2 g-1 with a majority of pores less than 10 nm in diameter. Graphene-coated SWCNT gel EBFCs produced power densities of 3.6 µW cm-2 with an OCV of 0.22 V and GOX loadings of 1.0 x 10-11 mol cm-2. In comparison, Graphene/SWCNT cogels had a specific surface area of 846 m2 g-1 with a majority of pores greater than 10 nm in diameter. Graphene/SWCNT cogel EBFCs resulted in power densities of 190 µW cm-2 with an OCV of 0.61 V and GOX loadings of 8.65 x 10-9 mol cm-2. All reported biofuel cell systems operated without the need for external redox mediators or electrode separating membranes. Improvement in anodic enzyme loadings and resulting performances were attributed to differences in electrode support morphology. We postulate that larger pore sizes provided increased GOX loadings and GOX supported on SWCNTs of higher curvature compared to MWCNTs retained a greater fraction of their functionality. Additionally, the effective entrapment of GOX within graphene/SWCNT cogels allowed for direct electron transfer to proceed between the buried GOX active site and the electrode surface. Graphene/SWCNT cogel power densities were further rejuvenated by a simple wash and reloading procedure. We believe the studied systems provide an important comparison and design principle for improving enzymatic power generation.