Anodic Electron Transfer Mechanisms

Schroder U. Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency. Phys Chem ChemPhys 2007 9 2619-2629. 

This chapter introduces the principles and applications of MFCs, with emphases on the nature of electricity-producing bacteria, anodic electron transfer mechanisms, power generation of MFCs and efficiency of electrode materials. Different types of MFCs and other microbial-electrochemical conversion devices are also discussed. 

Schematic diagram of the energy flux in anMFC. (Reprinted with permission from U. Schrdder, Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency, Phys. Chem. Chem. Phys. 9, 2007, 2619-2629. Copyright 2007 The Royal Society of Chemistry.)...

The anodic electron transfer mechanism in MFC is a key issue in understanding how MFCs work. As discussed above, the redox active species at the end of the electron transfer chain links the solid electrode in MFCs anodes, completing the exo-cellular electron transfer (Figure 2.3). These linking species, for example, may be a soluble redox shuttle, an outer membrane redox protein or a pili (nanowire). For an efficient electron transfer, the linking species must fulfill the following requirements [6] ... 

A further increase in anodic polarization lowers still further the Fermi level ersc)(ti) which gradually approaches the valence band edge Cy at the electrode interface as shown in Fig. 8-21. As the anodic polarization increases, the concentration of interfacial holes in the valence band increases, thus causing the anodic electron transfer to change from the conduction band mechanism to the valence band mechanism. 

On the basis of oxidation potentials, current-potential relationships, and isotope effects, an electron-transfer mechanism is suggested for the anodic oxidation of methyl N,N-dialkyl substituted carbamates, which can reasonably explain the formation of all three types of products. Also, N-acylazacycloalkanes are converted anodically at a platinum electrode in R0H-Et4NBF4 into a-monoalkoxy or a,a -dialkoxy derivatives depending on the electrolysis conditions employed.198... 

The cyclization mechanism is unknown but a one-electron transfer mechanism has been suggested (1997JOC9177). Anodic oxidation of 2-hydroxyiminohydrazones 345 gives respectable yields of 1,2,3-triazole 1-oxides 346 (1982ZC25). 

Biofilms, Electroactive, Fig. 2 Depicting the different (so far described) bacterial electron transfer mechanisms in biofihns on the example of an anodic biofihn... 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

The rate constant of electron transfer (ks) and anodic and cathodic electron transfer coefficients (aa and ac) of the SODs at various pH values were estimated with Laviron s equation and summarized in Table 6.5. Interestingly, the fastest electron transfer of the SODs was essentially achieved in a neutral solution, probably in agreement with the biological conditions for the inherent catalytic mechanisms of the SODs for 02" dismutation, although the electrode processes of the SODs follow a different mechanism. 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

Heterogeneous Processes at a Platinum Electrode. The series of organometals I-IV are also readily oxidized electrochemical ly (9). Thus we can apply the same steric probe to the corresponding anodic process for which the analogous mechanism for heterogeneous electron transfer at an electrode [E] is represented by an electrochemical EC sequence shown in Scheme II. 

It seems that no general mechanistic description fits all these experiments. Some of the reactions proceed via an addition-elimination mechanism, while in others the primary step is electron transfer from the arene with formation of a radical cation. This second mechanism is then very similar to the electrochemical anodic substitution/addition sequence. 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

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