Bioelectrochemical Energy Conversion
Enzymatic fuel cells (EFC) are biomimetic devices which convert chemical energy of fuel directly into electrical energy by employing enzymes as biocatalysts. The basic working principle of EFC mimics effectively the cellular respiration of living cells. Similar to cellular respiration, glucose (or another energy-rich metabolite) is supplied to the system as an electron donor, while oxygen plays the role of the final electron acceptor. Usually the fuel is only partially oxidized. E.g., if glucose oxidase (GOx) is used as an anode catalyst, D-glucono-1,5-lactone, which hydrolyzes further to gluconic acid, will be the main oxidation product . Thus, EFC can be considered as co-generation system supplying electrical energy as well as valuable chemicals.
We developed recently a hybrid enzymatic glucose oxygen fuel cell utilizing the organic salt tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) as anodic mediator (Figure 1a) . The EFC exhibits high OCV values (up to 0.99 V) and it delivered power densities up to 120 μW cm−2 at limiting current density of nearly 400 μA cm-2 in 5 mM glucose solution (Figure 1b) . These performance data are quite encouraging. Nevertheless, for real world applications, the performance and stability of EFC must be improved further. For this purpose a detailed understanding of the underlying enzymatic reaction mechanism, which is coupled to the charge and mass transport within the catalyst layer, is indispensable. For improved understanding of enzymatic electrode kinetics and limitations advanced electroanalytical techniques, such as Nonlinear Frequency Response Analysis (NFRA) might be very helpful. NFRA is similar to Electrochemical Impedance Spectroscopy (EIS), but while in classical EIS input signals with small amplitudes are used, NFRA applies harmonic perturbations of larger amplitudes. Thereby, one obtains higher-order system responses which contain valuable information about the system’s nonlinearities [3, 4]. To establish the modeling and experimental routine of NFRA we used an example of a simple electrochemical reaction (Figure 1c) [3, 4]. Furthermore, FRA was employed for the identification of the mechanism and the kinetics of the enzymatically catalyzed reaction .
Figure 1: a) Schematic presentation of the hybrid enzymatic fuel cell; b) polarization (black squares) and power curves (red circles) of the hybrid fuel cell  c) Schematic illustration of the working principle of Nonlinear Frequency Response Analysis (NFRA) [3,4][less]
Figure 1: a) Schematic presentation of the hybrid enzymatic fuel cell; b) polarization (black squares) and power curves (red circles) of the hybrid fuel cell  c) Schematic illustration of the working principle of Nonlinear Frequency Response Analysis (NFRA) [3,4]
 Ivanov, I., T. Vidaković-Koch, K. Sundmacher: Recent Advances in Enzymatic Fuel Cells: Experiments and Modeling, Energies 3, 803-846 (2010).
 Ivanov, I., T. Vidaković-Koch and K. Sundmacher: Development of a hybrid glucose-oxygen enzymatic fuel cell based on tetrathiafulvalene-tetracyanoquinodimethane charge transfer complex as anodic mediator, J. Power Sources, 196, 9260-9269 (2011).
 Vidaković-Koch, T., V. Panić, M. Andrić, M. Petkovska and K. Sundmacher: NFRA of the Ferrocyanide Oxidation Kinetics. Part I: A Theoretical Analysis J. Phys. Chem. C 115, 17341-17351 (2011).
 Panić, V., T. Vidaković-Koch, M. Andrić, M. Petkovska and K. Sundmacher: NFRA of the Ferrocyanide Oxidation Kinetics. Part II: Measurement Routine and Experimental Validation J. Phys. Chem. C 115, 17351-17358 (2011).
 T. Vidaković-Koch, V.K. Mittal, T.Q.N. Do, M. Varničić, K. Sundmacher, “Application of electrochemical impedance spectroscopy for studying of enzyme kinetics”, Electrochim. Acta, in press, available online 13 March 2013.