Glucose Oxidation in Viologen-Mediated Fuel Cells Using a Non-Precious-Metal Anode

Abstract

Glucose has a great potential as a renewable source of energy for fuel-cell applications. However, rapid and complete oxidation of glucose in fuel cells has been a challenge. To address these issues, this study examines the use of dimethyl viologen (MV) as an electron mediator. MV has the potential of oxidizing glucose to a large extent at a relatively fast rate. In a MV-mediated glucose fuel cell, MV homogeneously oxidizes glucose. Subsequently, MV oxidized at the anode of an electrochemical cell, generating current. Since the electrochemical reaction of MV does not require a catalyst, MV-mediated fuel cells also offer a potential cost advantage over precious-metal-based fuel cells. This study investigates the efficiency of MV-mediated glucose oxidation. The study also evaluates the rate capability of an MV-mediated glucose fuel cell in terms of current density and power density. Glucose oxidation efficiency was experimentally investigated in an electrochemical cell. To do so, the impact of various factors including the initial molar ratio of MV to glucose and the rate of the electrochemical reaction were examined. In addition, 13C-NMR was used to identify reaction products in order to obtain insight into the oxidation mechanism. Our experimental results show that the oxidation efficiency depends on the rate of electrochemical reaction and, over a certain range, can be increased by increasing that rate. Moreover, the oxidation efficiency can also be increased by increasing the initial molar ratio of MV to glucose. The maximum oxidation efficiency observed was 22%, which is almost three times larger than that observed in precious-metal-based fuel cells. 13C-NMR results confirmed that the main oxidation product was glycolic acid, consistent with the measured oxidation efficiency. To investigate the rate capability of a MV-mediated glucose fuel cell, a mathematical model was developed. A fuel cell device was also made and rate experiments were performed. In such a cell, the homogeneous reaction of glucose with the mediator and the electrochemical reaction of the mediator at the anode are coupled, so that both types of reactions contribute to and have the potential to limit the overall rate. The model helped us to better understand the influence of each type of reaction on the overall rate. Methods for enhancing the overall rate were also explored. Based on the model, system optimization has the potential to produce power densities about one order of magnitude larger than values reported to date in the literature.