Mathematical and Experimental Analysis of the Rate Performance of Viologen-Mediated Glucose Fuel Cells

Abstract

Glucose is a desirable source of energy for fuel cell applications. However, its slow oxidation rate on nonprecious metal electrodes has been a challenge. Viologens can potentially mitigate this challenge as they homogeneously oxidize glucose and then transfer electrons to inert electrodes with fast kinetics. This study aims to better understand the factors that determine the effectiveness of viologen as a mediator for glucose oxidation. The relative significance of the key physical processes including homogeneous reaction, mass transfer, and electrochemical reaction was evaluated by dimensional analysis and detailed simulations. While all processes were important under certain conditions, mass transfer was the principal limiting step. Mass transfer was initially improved by flow; however, this impact was counterbalanced by the decreased concentration of the reduced mediator at high flow rates. The maximum obtainable current density was close to 200 mA cm−2, which corresponded to a predicted anode polarization of 300 mV. This current density is noticeably higher than rates available from biological cells and comparable to values for precious-metal-based cells. Thus, viologen-mediated fuel cells offer high rates without the additional cost associated with precious metal electrodes. Finally, the approach presented can be used for process development and optimization of any mediated system.