Biofuel cells are energy sources that rely on biocatalysts for the transformation of renewable feedstocks to electrical power via bioelectrochemical pathways. The overall objective of this project is to develop a biofuel cell that relies on glucose, a sugar derived from lignocellulosic biomass, as the sole carbon fuel source for electrical power generation. The device will generate electrons from glucose by a series of enzymatic reactions which utilize commercial enzymes as the catalytic elements. The biofuel cell will contain an assembly of biocatalysts on high surface area electrodes composed of gold microfibers or composite gold carbon nanotube fibers. The high surface area will enable the immobilization and stabilization of a high density of enzymes, and will allow for better diffusion of substrates to and from the electrode. The proposed research will provide insight into the fabrication of enzymatic electrode on gold microfibers as the scaffold for construction of a practical biofuel cell. Towards this end, there are three research tasks: (1) Microfiber fabrication; (2) enzyme immobilization and electrochemical redox properties; (3) biofuel cell design, fabrication and study of effects of flow on performance.
Broader Impacts
The proposed effort will integrate the proposed research into education activities with a multi-tiered approach to training and mentoring, with emphasis on the recruitment and training of women and under-represented minorities, and co-mentoring experiences for undergrads and graduate students. Outreach activities include school visits, workshops at the annual Carnegie Science Center SciTech Festival, middle school summer camp and training of high school students. A simplified version of the biofuel cell will be integrated with K-12 educational activities via the existing research experiences for teachers (RET) and mobile lab initiative.
The ultimate goal of this project is the development of stable power source for blood-powered implantable devices. The part of that long term goal that was undertaken here was the fabrication and testing of stable enzyme driven electrodes that result in high powered biofuel cells compatible with in vivo conditions. The most effective reported biofuel cells, with power densities of 1.5 mW/cm2, employ mediators to shuttle electrons from the enzyme to the electrode’s conducting surface. Since most of these mediators are toxic these electrodes cannot be used in vivo. We are therefore confined to the use of electrodes the function by direct electron transfer (DET) from the enzyme to the electrode. DET is less efficient than mediated transfer and DET electrodes must compensate by having a more enzyme in contact with the electrode surface. Our initial solution was to fabricate electrodes from electrospun fiber mats coated with carbon nanotubes to provide high surface area/volume substrata for enzyme. The electrodes are non-woven fiber meshes generated by electrospinning a solution of polyacrylonitrile containing gold nanoparticles. The fibers are then exposed to electro-less deposition of gold (nucleated by the nanoparticle inclusions) and electrophoretic deposition of multi-walled carbon nanotubes. The end result is a wire mesh electrode with an accessible surface area. A schematic of the process is shown in Figure 1. We demonstrated DET with the glucose oxidase (GOX) as the anodic enzyme. Billirubin oxidase (BOD), the cathode enzyme, is well known perform DET. Biofuel cells using GOX and BOD were shown to produce electricity by oxidizing glucose (GOX) and reducing oxygen (BOD) achieving power densities of 0.02 ± 0.01 µW/cm2, a long way from optimal. Calculations of GOX concentrations on the electrode surface showed very little attached enzyme leaving ample room for optimization. A number of optimization experiments increased GOX binding, however, failed to increase the power. The conclusion was that only a small percentage of the bound GOX was in an orientation that was optimal for DET. The solution to enzyme occupancy and orientation presented itself in the form of a new material from Dr. Mohammad Islam’s research at Carnegie Mellon University. Dr. Islam has developed methods to produce aerogels from pure single-walled carbon nanotubes (SWCNT) either as the sole constituent or supported with graphene. These materials are highly conductive and we began testing their suitability as bioelectrodes. Both the SWCNT and the graphene supported SWCNT gels showed huge increases in power density over the electrospun electrodes reaching levels of 15.0 ± 1.0 µW/cm2. Pore size distribution analysis combined with effective GOX concentration measurements of these gels indicate that most of the enzyme is on or only slightly below the surface of the 3 dimensional structure. Further, calculations indicated that the available binding sites on the surface were nearly saturated with the bound enzyme. In order to improve enzyme loading we developed a new material comprised of a co-gel of micro-flaked graphene and SWCNT (a schematic of the synthesis is shown in Figure 2). This resulted in gels with larger pores and correspondingly larger enzyme loading capacity. Biofuel cells with this material as electrodes (a schematic is shown in Figure 3) were assembled and tested. The power density from the first batch of was 140 ± 20 µW/cm2 nearly 4 orders of magnitude higher than the electrospun electrodes. As with all previous electrodes the stability of the response is not optimal. Future experiments will use the graphene-SWCNT co-gels as the basis for increasing enzyme loading and developing stability of the electrodes.
