(690d) Homogeneous Reaction Kinetics of Carbohydrates with Viologen Catalysts in Biofuel Cell Applications

The production of renewable energy from biomass has been demonstrated using processes such as Fischer-Tropsch synthesis, direct fermentation of biomass sugars, biomass conversion to syngas followed by fermentation, and utilization of biomass carbohydrates in biofuel cells. Processes have been used to produce either electrical power, heat, liquid biofuels, or hydrogen. However, limitations and challenges for commercial viability still exist with all of these processes. Challenges inhibiting biofuel cell processes include low power output, low biological catalyst durability, and low carbohydrate conversion. Recently, a potentially transformative breakthrough has occurred with the discovery of a homogeneous non-biological viologen catalyst that facilitates carbohydrate conversion to electricity.

The viologen catalyst serves as a mediator or “electron shuttle” to extract electrons from the carbohydrate to form a reduced viologen. The reduced viologen is then oxidized at the anode of a biofuel cell to generate power. The combination of homogeneous viologen reduction by the fuel coupled with oxidation of the viologen at the electrode provides a continuous electron shuttle, enabling energy to be extracted from carbohydrates. For a biofuel cell process, the viologen catalyst provides greater durability relative to that achievable with biological catalysts.

Our initial studies have shown that viologens are effective in shuttling electrons from a wide variety of carbohydrates (C1 to C6 compounds). The homogeneous reaction rates and the efficiency of electron extraction were found to vary with reaction conditions such as pH, temperature, and reactant concentrations. Optimization of the reaction rates and efficiency are critical towards developing a viable biofuel cell using biomass carbohydrates as the feedstock.

Studies were conducted to determine the homogeneous rate law that describes the reaction between the carbohydrate and viologen. Specifically, studies were performed using NMR and spectroscopic techniques to develop a mechanistic description of the reaction rate applicable for a wide range of conditions. Studies of viologen stability were also performed to quantify potential viologen decomposition conditions that could compromise carbohydrate conversion efficiency. This work will present the mechanistic model of the carbohydrate-viologen reaction and provide insights towards optimizing reactor conditions that enhance reaction rates and carbohydrate conversion efficiency.