Oxidation efficiency of glucose using viologen mediators for glucose fuel cell applications with non-precious anodes

Glucose is a potential source of energy for fuel cell applications. However, its complete oxidation has been a challenge. Dimethyl viologen, as an electron mediator, has been shown to promote high levels of glucose oxidation under aerobic conditions. Nevertheless, the efficiency of viologen-mediated glucose oxidation has been low in electrochemical experiments. In this study, viologen-mediated oxidation of glucose was investigated under anaerobic electrochemical conditions to understand the factors that impact the oxidation efficiency. Of particular interest was the improvement of electrochemical oxidation for glucose fuel cell applications. An experimental cell was developed to electrochemically reoxidize the mediator as it was homogeneously reduced by glucose under anaerobic conditions. In contrast, the mediator was reoxidized by direct reaction with oxygen under aerobic conditions. The aerobic oxidation efficiency was 75%, three times larger than the maximum efficiency in the electrochemical cell. C-13-NMR results show that the main product formed under aerobic conditions was formic acid, whereas glycolic acid was the principal product formed in the electrochemical cell. Carbonate was only formed under aerobic conditions. Therefore, the use of oxygen to reoxidize the mediator also directly influenced the glucose oxidation pathway. In the electrochemical cell, the oxidation efficiency depended on the electrochemical reaction rate of the mediator and was higher at faster rates. The efficiency also depended on the initial molar ratio of the mediator to glucose. The maximum oxidation efficiency of glucose in the electrochemical cell was approximately 22%, which is about three times larger than the maximum efficiency for precious-metal-based anodes.