Biofuel Cells. Группа авторов

Biofuel Cells - Группа авторов


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→ O2 + 4H+ + 4e- CO2 + 8 H+ + 8 e → CH4 + 2H2O E = 1.064 V E = 0.820 V/NHE E = −0.244V/NHE

      Microbial electrolysis cells (MECs) operate under a constant electrical supply, therefore this energy represents an operation cost and a decrease in the energy efficiency. To overcome these issues, the use of alternative energy technologies has been proposed [34]. Another alternative to reduce the cost of energy supply is an intermittent operation for conversion of CO2 into organic products using a biocathode [35].

      1.3.1 Immobilization of Enzymes on Electrodes

      Although the first proof-of-concept biofuel cells employed the enzymes freely in solution [3, 4] this approach is poorly applicable in practice. Enzymes are costly and losing them with the fuel and oxidant flow turns operation expensive. Therefore, most of the reported enzymatic biofuels include enzymes immobilized on the electrode surface.

      In general, enzyme immobilization is a balancing act between the external forces holding the enzyme on the support and the internal forces that maintain the enzyme conformation, and therefore, its function. The addition of interactions can stabilize the enzyme but, if they are too strong, they can modify the conformation of the active site or even denature the enzyme. As well, the addition of dense composite materials around the enzymes can create mass transport limitations that need to be kept in mind; else the catalytic performance can be severely affected.

as shown in Equation (1.2).

      In this case, the

value depends not only on the enzyme–substrate affinity but also on substrate partition between the solution and the film, and mass-transfer limitations due to the film structure [39].

      In polymer entrapment (Figure 1.4b), enzymes are physically trapped between the network of the polymer chains. Although some interactions between the enzymes and the polymer matrix might exist, they are not the main cause for enzymes to be fixed in place. Electropolymerization [43] and photopolymerization [44] in the presence of enzyme have been used to trap glucose oxidase in the polymer layer. An interesting approach has been the use of tetrabutylammonium bromide (TBAB)-modified Nafion, to immobilize alcohol dehydrogenase, aldehyde dehydrogenase and even nanotube-bound laccase [7, 9]. The exchange of the protons in the Nafion for hydrophobic alkyl ammonium ions reduces the acidity of the polymer environment and widens the channels to allow the diffusion of relatively large enzymes substrates and cofactors [45].

      Chemical


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