Modelling - Mediator Reduction

Mediator Reduction

In a Microbial Fuel Cell, microorganisms provide the electrons for the anode from the oxidation of the substrate(s) in the intracellular metabolic pathways. The transfer of the electrons to electrodes has been demonstrated for several species, though this process is inefficient as far as Coulombic yield and current generation are concerned [A]. Hence, the application of electrochemical mediators is essential for the construction of a Microbial Fuel Cell. Mediators are usually small water-soluble molecules, which are capable of undergoing the redox transformations. The mediator acts as an electron shuttle, enhancing the kinetics of the electron transfer. This approach has been proven to be generally quite successful and many substances were tested for their potential as electron shuttles.

There are two classes of mediators, endogenous and exogenous mediators. The endogenous mediators are generated by the bacteria and can be secreted to the medium and then be oxidized at the electrode. The exogenous mediators are redox molecules that are chemically synthesized and must be added into the anode chamber of the Microbial Fuel Cell in order to enable electron transfer from bacterial metabolic pathways to the anode.

Therefore, the anodic performance dependents not only on the nature and the rate of the metabolism, but on the nature and the rate of electron transfer from the mediator to the anode as well. Several exogenous mediators have been already studied in regard to their effect on electron transfer and so their impact on the electricity generation. Among those mediators are methylene blue (MB), neutral red (NR), thionin, ferricyanide, humic acid or methyl viologen. In our model we modelled the influence of the three of the mediators:

• N-methyl phenazine (NMP)methosulfate,
• 1-methoxy-5-methyl phenazine (MNMP) methosulfate and
• Meldola Blue (MB)

Reaction Kinetics

In contrast to the first bottle neck reaction of our model, which has been described as the Michaelis-Menten reaction, the redox reaction between the bacterial metabolites and the oxidized mediator can be considered as a first order reaction:

The kinetic properties of the electrocatalytic oxidation of NADH and reduction of the soluble mediator, regarding rate constants were studied by cyclic voltammetry and chronoamperometry (Griindig et al., 1995). The measured rates are shown in the table 1:

Table 1: Kinetic characteristics for k₂ studied with cyclic voltammetry(CV) and chronoamperometry(CA) for the mediators:NMP, M-NMP and MB. The calculated rates obtained from (Griindig et al., 1995

References