Carbohydrates have great potential as a renewable and abundant source of energy. Biofuel cells represent a possible way of exploiting this source of energy in the form of electricity. However, carbohydrate oxidation in biofuel cells faces challenges related to both the rate of conversion and the extent to which the fuel can be converted. This experimental study examines the use of viologen compounds as a homogeneous catalyst to address these challenges. Specifically, the study seeks to quantify the rate and extent of the homogeneous reaction between viologen and carbohydrates as the basis for development of a practical biofuel cell. The heterogeneous electrochemical reaction of viologen is also examined, and issues related to coupling of the homogeneous and heterogeneous reactions are investigated and optimized. Methods for increasing the rate and extent of reaction are also explored. To do this, experiments were performed at different pH values, temperatures, concentrations of catalyst and carbohydrates, and electrode potentials. The homogeneous reaction rate was measured by tracking O2
uptake over time or by spectroscopic measurement of the viologen concentration. When the homogeneous reaction was coupled with electrochemical regeneration of the catalyst, the current provided a direct measurement of the reaction rate. The overall conversion was estimated by integrating the rate data or by measuring the final concentration for experiments without catalyst regeneration. In addition, 13
C NMR was used to examine reaction products in order to provide insight into the oxidation mechanism and conversion efficiency. Tests were performed with several different carbohydrates including dihydroxyacetone, glyceraldehyde, xylose and glucose.
Experiments with three-carbon carbohydrates showed rapid reaction of the fuel. The extent of reaction for these fuels was approximately 30%; in other words, 30% of the possible electrons were extracted from the fuel relative to the electron removal required for full oxidation to carbon dioxide. NMR data showed Formate as a principal product, and additional tests indicated that the rate of Formate oxidation is sufficiently slow such that it should be considered a “dead-end” product. Coupling of a Formate fuel cell would enhance the process efficiency. The reaction rates observed for the longer carbon chains were considerably slower, and measurement of the extents of reaction are still in progress. These rates are also slow relative to that of the heterogeneous electrochemical reaction; therefore, the homogeneous rate would likely control the performance of the fuel cell for these fuels. An optimal biofuel cell design must account for both the absolute reaction rates and the relative rates of the homogeneous and heterogeneous (electrochemical) reactions. Finally, slow degradation of catalyst was observed under some conditions (e.g., high pH) and catalyst stability is a factor that should be considered in practical system design.