Fuel cells convert chemical energy directly into electricity through an oxidation reaction at the anode and a reduction reaction at the cathode. In enzymatic biofuel cells, traditional fuel cell catalysts (e.g. Pt, Pd, Ru) are replaced by fuel oxidizing enzymes (e.g. Glucose Oxidase, Alcohol Dehydrogenase, Fructose Dehydrogenase) at the anode and oxygen reducing enzymes (e.g. Laccase, Bilirubin Oxidase) at the cathode. Currently the main limitations of enzymatic biofuel cells are low power output and limited lifetimes. Principal Investigators David Schmidtke and Daniel Glatzhofer at the University of Oklahoma, Norman Campus have hypothesized on the limiting factors. In enzymatic biofuel cells, the redox center of most enzymes is buried in the protein shell and electrically inaccessible. Thus most redox enzymes do not normally exchange electrons with an electrode. They propose to increase the power output of enzymatic biofuel cells by synthesizing redox polymers that increase the rate of electron transfer (i.e. current flow)between the redox enzymes and electrodes. These novel PEI-based redox polymers produce high currents by efficiently collecting and shuttling electrons between the enzyme¡¦s redox center and the electrode surface. These polymers contain metal species, ferrocene compounds, linked to the polymer. Tuning the system will require synthesis of polymers with different lengths of spacer arms. The PIs are targeting a 20-200 fold improvement in power output.
Enzymatic biofuel cells will not solve the nation¡¦s energy needs. However, as the demand for portable energy increases, there develops a need for alternative renewable energy sources. Enzymatic biofuel cells are an attractive energy conversion technology because they operate at mild temperatures of20-40?aC and under neutral pH and consume substrates such as sugars that are readily available in biological systems. Because of their inherent selectivities, enzyme based anodes and cathodes can operate in the same compartment without separating membranes thereby reducing size and weight. They have potential applications for both in vivo (e.g. pacemakers, implantable sensors) and ex vivo (e.g. remote site sensing, mobile electronics) systems.
The investigators intend to use their research as a vehicle to increase the attractiveness of the field to potential students. Their belief, based on studies, is that a student¡¦s attitude toward science and his achievement in science improves with hands-on experiences. To help increase the likelihood that students will continue their studies in science and engineering, proposed educational activities with research opportunities include Research Opportunities for Undergraduate students and Summer Research Internships for Underrepresented Groups, matched with Research Opportunities for Middle School Teachers.
The results from this project establish a new class of redox polymers based on the attachment of ferrocene redox couples (Fc) to linear poly(ethyleneimine) (LPEI). Redox polymers are a special class of polymers that have metal species, such as iron or osmium, attached to a polymer backbone. These metal species allow the polymers to collect or donate electrons from other molecules such as enzymes. When redox enzymes such as glucose oxidase, laccase, or fructose dehydrogenase are immobilized in these novel ferrocene redox polymers (Fc-LPEI) electrons can be efficiently shuttled between the enzyme’s redox center and an electrode surface thereby producing an electrical current. We demonstrate that these redox polymers allow for the construction of miniature enzymatic biofuel cells that utilize simple sugars, such as glucose and fructose, as their fuel sources. Over the course of the research, we showed that the ability of the redox polymers to electrically communicate with the enzymes depends on the length the spacer (1 – 6 carbons) between the ferrocene and the LPEI backbone. Redox polymers with a 1 carbon (Fc-C1-LPEI) or 3-carbon (Fc-C3-LPEI) spacer produced current densities > 1 mA/cm2. We also demonstrated that the voltage (i.e. redox potential) at which the ferrocene centers could accept or donate electrons could be lowered by 100 – 200 millivolts by modifying the ferrocene centers with methyl (-CH3) groups. Alternatively we raised the redox potential of the polymers by 100 millivolts by modifying the ferrocene centers with a chloro (-Cl) group to produce a novel redox polymer FcCl-C3-LPEI. The ability to develop low potential redox polymers for the biofuel cell anode and high redox potential polymers for the biofuel cell cathode was significant because it allowed for an increased cell potential and power output. In addition to developing different forms of the Fc-LPEI redox polymers, we also showed that these polymers could be combined with nanostructured materials such as single-walled carbon nanotubes (SWNTs), or carbon felt electrodes to produce current densities as high as 11 mA/cm2. Finally we have demonstrated that these redox polymers can be incorporated into multi-enzyme cascades which allow for multiple pairs of electrons to be extracted from one sugar molecule and thus producing a larger current and biofuel cell power output. The research results obtained during the course of this project have been disseminated to the research community and general public via peer-reviewed journal publications and presentations at local and national meetings. To date there have been 11 peer reviewed journal publications in such prestigious journals such as ACS Catalysis, Langmuir, Journal of the Electrochemical Society, and the Journal of Physical Chemistry. Three additional papers are currently under review or have been accepted for publication. There was a considerable effort to couple these research activities to education and outreach activities. In specific through this NSF-funded project we provided research opportunities to 6 graduate students and 10+ undergraduate students. A number of these students were women or from under-represented groups. In addition to these research activities we also gave presentations on the research to high school-aged boys and girls to provide them an opportunity to learn about cutting-edge research, and to positively impact their attitudes towards science and engineering. Finally we developed an integrative laboratory module, based on our research, for use in an advanced undergraduate laboratory class. The laboratory module starts with an organic synthesis and characterization of our basic ferrocene-modified polymer (Fc-LPEI), electrochemical evaluation of the polymer, fabrication of bioanodes using the polymer, electrochemical evaluation of the bioelectrode, and finally, incorporation of the bioanode into a biofuel cell and its electrochemical evaluation. The laboratory module was incorporated into our CHEM 4444 (Advanced Synthesis and Characterization) course in Fall of 2013 with an initial group of 6 students. The students were able to complete the laboratory module in a period of 2.5 weeks, and the laboratory experiment was very well received by the students. Based on the feedback from this experience, we are modifying the module to improve the procedures and will incorporate it into the curriculum again in Fall 2014. We will then produce a final laboratory module and a laboratory manuscript to be submitted to the Journal of Chemical Education.
