Falk Harnisch

On the use of cyclic voltammetry for the study of anodic electron transfer in microbial fuel cells

In this communication we discuss, by means of the metal reducing bacterium Geobacter sulfurreducens, a strategy to use cyclic voltammetry for the study of anodic bioelectrocatalytic electron transfer in microbial fuel cells.

Improvement of the anodic bioelectrocatalytic activity of mixed culture biofilms by a simple consecutive electrochemical selection procedure

In this paper we demonstrate that the anodic, bioelectrocatalytic performance of wastewater inoculum based, mixed culture microbial biofilms can be considerably improved by using a consecutive, purely electrochemical selection and biofilm acclimatization procedure. The procedure may represent an alternative to a repetitive mechanical biofilm removal, re-suspension and electrochemically facilitated biofilm formation. By using the proposed technique, the bioelectrocatalytic current density was increased from the primary to the secondary biofilm from 250 microAcm(-2) to about 500 microAcm(-2); and the power density of respective microbial fuel cells could be increased from 686 mWm(-2) to 1487 mWm(-2). The electrochemical characterization of the biofilms reveals a strong similarity to Geobacter sulfurreducens biofilms, which may indicate a dominating role of this bacterium in the biofilms.

The study of electrochemically active microbial biofilms on different carbon-based anode materials in microbial fuel cells

In this communication we show that the achievable maximum current density for mature wastewater-based microbial biofilms is strongly dependent on the electrode material and the operation temperature. On graphite and polycrystalline carbon rods, the catalytic current of about 500 microA cm(-2) (projected surface area) at 30 degrees C was achieved. Carbon fiber veil or carbon-paper based materials, having a large microbially-accessible surface gave a projected current density approximately 40% higher than on graphite rod. In contrast, the biofilm cannot form well on graphite foil. Elevating the temperature from 30 to 40 degrees C increased current density by 80% on graphite rod anodes. Interestingly, the formal potential of the active site (-0.12 V (vs. standard hydrogen electrode (SHE))) is similar to all electrocatalytically active microbial biofilms and to that found for Geobacter sulfurreducens in previous studies. In addition, the real surface area values measured by BET surface area technique cannot provide a reasonable explanation for suitability of an electrode material for the formation of electrochemically active biofilm.

Modeling the ion transfer and polarization of ion exchange membranes in bioelectrochemical systems

An explicit numerical model for the charge balancing ion transfer across monopolar ion exchange membranes under conditions of bioelectrochemical systems is presented. Diffusion and migration equations have been solved according to the Nernst-Planck Equation and the resulting ion concentrations, pH values and the resistance values of the membrane for different conditions were computed. The modeling results underline the principle limitations of the application of ion exchange membranes in biological fuel cells and electrolyzers, caused by the inherent occurrence of a pH-gradient between anode and cathode compartment, and an increased ohmic membrane resistance at decreasing electrolyte concentrations. Finally, the physical and numerical limitations of the model are discussed.