Composition and distribution of internal resistance in an enzymatic fuel cell and its dependence on cell design and operating conditions

A variety of sugar-based enzymatic fuel cells (EFCs) are able to completely oxidize fuels catalyzed by enzyme cascades, achieving high energy densities. However, the poor power output of EFCs limits their potential applications. In the present study, the composition of internal resistance throughout the EFCs affected by various factors, including the separator, enzyme loading, electron acceptor, applied voltage and operation time, was characterized by electrochemical impedance spectroscopy (EIS). Total resistance is divided into solution-separator resistance, charge transfer resistance, and diffusion resistance, respectively. The Nafion 212 membrane was found to yield a small solution-separator resistance and a high power density. Increased enzyme loading led to reduced internal resistance and improved cell performance, generating a maximum power density of 0.17 mW cm^-2. Using potassium ferricyanide to replace oxygen as the electron acceptor could improve cathode performance significantly and resulted in a 4-fold increase in the power density. EIS was also performed for EFCs operated continuously for 16 h. Power output decreased distinctly over time, while the internal resistance, primarily the diffusion resistance, increased. Additionally, altering operation voltages had an impact on diffusion resistances. These results can be summarized that diffusion plays a rather important role in deciding the power and future efforts should be made towards increasing the mass transfer in EFCs.