Microbial bioelectrocatalysts

First we will discuss the similarities and differences between conventional chemical and microbial bioelectrocatalysts. Subsequently, the principal mechanisms of the extracellular electron transfer (EET) and the technology aiming at their exploitation will be introduced. Finally, the biological and electrochemical methods that allow the analysis of the microbial electron transfer and identification of the respective mediators, proteins, and microorganisms will be addressed. 

Microbial bioelectrocatalysts are living cells. As a consequence, they need a certain amount of energy for their maintenance and proliferation. Cells use energy gained to produce ATP and NAD(P)H, the two main energy carriers. In order to generate these, they need to build up internal potential gradients, which externally lead to a loss of useful potential. In the context of bioelectrocatalysis, this... 

When comparing the microbial bioelectrocatalyst with a conventional, noble metal-based electrocatalyst for low-temperature fuel cells, it appears that the aforementioned energetic disadvantage could be compensated by a number of advantages (Table 8.1). 

Table 8.1 Operating conditions of a microbial bioelectrocatalyst and an electrocatalyst in a low-temperature fuel cell.

The characterization of microbial bioelectrocatalysts and development of methods and techniques are therefore one of the most active and vital fields. One might distinguish two different types of methods (i) electrochemical methods (see Section 8.5.1) and (ii) biological methods (see Section 8.5.2). Whereas the former generally focus on the identification and characterization of the electron transfer, the latter are devoted to the identification, analysis, and spatial distribution of the bioelectrocatalytic cells and their environment. 

As already discussed, microorganisms extract a certain share of energy for their living from the maximum theoretically exploitable energetic difference. In case of anodic bioelectrocatalysis, the energy difference is situated between the microbial substrate, that is, the fuel, and the potential of the terminal electron transfer site. Furthermore, and like in conventional electrocatalytic systems [5], several energetic losses at the bioelectrocatalyst-electrode interface occur (Figure 8.2). 

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