Electron-mediated carbohydrate fuel cells: Characterizing the homogeneous viologen-mediated electron transfer rate of carbohydrate oxidation

One area of exploration for renewable energy is the development of fuel cells, including carbohydrate fuel cells that can extract energy from carbohydrates. Viologen electron mediators have been shown to enhance energy extraction and improve carbohydrate conversion efficiencies, although the limiting step appears to be the homogeneous rate at which electrons are first transferred from the carbohydrate to the viologen. In this work, electron transfer rates for various monosaccharides in the presence of methyl viologen were studied in the absence of a fuel cell to isolate the homogeneous rate. Using glucose as the model carbohydrate, a rigorous mechanistic model of the homogeneous electron transfer rate was developed and showed a first-order dependence on OH− concentration, a first-order dependence on carbohydrate concentration, a zero-order dependence when the methyl viologen concentration was ≫ 0.4 mM, and an increasing rate with incubation time when glucose was incubated in a buffer solution prior to exposure to methyl viologen. The incubation effect had a strong dependence on pH and was consistent with interconversion between glucose and fructose. The mechanistic model, which agreed well with experimental data, can be useful for identifying process improvements to carbohydrate fuel cells, especially when the homogenous rate is a limiting step.