Biofuel Cells. Группа авторов

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СКАЧАТЬ evaluation in half-cell tests demonstrated the superior activity for alcohol oxidation than PC and MWCNTs alone. Nonetheless, full-cell tests were not presented.

      1.4.5 Flexible Bioelectrodes for Flexible BFCs

The last section of supports for biofuel cells is devoted to the recent advances in flexible electrodes for the development of flexible BFCs. The most recent works have been focused on enzymatic biofuel cells rather than in microbial biofuel cells, where most of these works operate with glucose and oxygen as fuel and oxidant, respectively. Hui et al. [126] used nickel foam coated with gold as electrodes to decrease ohmic resistances, while the flexibility is achieved using agarose as gel electrolyte, a cellulose acetate membrane, and silicone rubber as cases. This flexible BFC constructed using glucose oxidase and laccase achieved a maximum power density of 2.32 mW cm⁻² with an OCV close to 0.6 V. Niiyama et al. [127] constructed a flexible BFC using a carbon cloth modified with MgO. The reported BFC employed a flavine adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) as bioanode, and bilirubin oxidase (BOD) as biocathode. The OCV displayed by this flexible device was 0.75 V, while the maximum power density was 2.0 mW cm⁻². Another strategy to gain flexibility is through the development of graphene paper as reported by Shen et al. [128]. They used pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH) as bioanode and bilirubin oxidase as biocathode. With this configuration, an OCV of 0.66 V and a maximum current density of 4.03 μW cm⁻² were obtained.

      It is worth mentioning that in all of these works, bending tests were not presented, nor in other revised works [129]. Thus, the development of fully functional bendable and flexible biofuel cells is still a hot topic area.

1.5 Electron Transfer Phenomena

      1.5.1 Enzyme-Electrode Electron Transfer

      For the enzymatic redox reactions to be useful in systems containing bioelectrodes (such as fuel cells), the electrode must replace one of the half reactions of the enzyme in its natural environment. This is, the electrode must function as the final electron acceptor or donor, depending on whether it is working as a bioanode or biocathode, respectively. For this to occur, the electrons must travel between the enzyme active sites and the electrode surface in a process termed “electron transfer”. In the context of enzymatic electrodes, this term is employed more loosely than in pure electrochemical sciences, in that the process can involve non-electrochemical charge transport steps, for instance, the diffusion of a mediator (see below) to/from the electrode surface.

In some enzymes, like glucose oxidase, the presence of the enzyme and its substrate at the electrode is, in general, not enough to produce an electrochemical response [3, 55]. As pointed out in Section 1.2.1, the active site of glucose oxidase is buried deep inside the protein. The distance between the redox cofactor at the active site and the electrode surface is too large for efficient electron tunneling. In its initial experiment, Davies noted that, when methylene blue was added to the GOx/glucose solution, an electrochemical response was observed [3]. This charge transport mechanism, termed mediation, opened the door to a whole new class of electrodes.

      Mediated electrodes incorporate a molecule that acts as an electron carrier between the enzyme’s active site and the electrode surface. After reacting with their substrate, enzymes change their oxidation state. In their native environment, the enzyme then reacts with another molecule (e.g. O₂ for GOx) and returns to their original oxidation state. The mediators must be able to substitute these natural electron donors/acceptors and readily exchange electrons with the enzyme cofactor. To this end, they must possess the adequate size and charge to be able to access the active site, which can be insulated by oligopeptide and saccharide shells. Furthermore, mediators must also be capable of undergoing a reversible reaction on the electrode surface.

      A variety of transition metal (e.g. Os, Fe, Ru, Co) coordination compounds have been employed to this end [16, 56]. Among the ligants used are derivatives of cyclopentadiene [7, 44, 51, 63–65], bipyridine (bpy) [50, 56, 66] and phenanthroline [130]. Careful modification of these ligands with electron donor or withdrawing groups allow for a fine tuning of the redox potential of the mediator. In general, electron donating substituents will increase the electron density of the mediator, therefore making it easier to oxidize. Experimentally, this can be observed as a shift in its formal redox potential

in the negative direction. The opposite effect is observed for electron withdrawing functional groups.

      Organic molecules have also been used as mediators. As pointed out earlier, methylene blue (a phenothiazine) was one of the first mediators employed, although it is no longer common in enzymatic biofuel cells. Other phenothiazines have been used as well, including methylene green [42], toluidine blue [131] and thionine [6]). Pyrroloquinoline quinone (PQQ), a cofactor of several enzymes, including glucose dehydrogenase has also been used as a mediator for enzymes that do not naturally employ it. Glucose oxidase and lactate dehydrogenase, for example, have been shown to couple with PQQ-containing self-assembled monolayers to shuttle electrons to Au surfaces [132]. It is believed that the fact that the mediator reaction is a 2-electron one, similarly to the enzymatic reaction, can help simplify the electron transfer process [133].

Ramanavicius’ group has studied the use of phenanthroline derivatives (not coordinated to a metal) as mediators for glucose oxidase. They reported that amine electron-donating substituents performed more favorably than nitro electron-withdrawing groups [55]. They note this is in contrast with the above-described results for ligands in metal complexes and cannot be rationalized purely in terms of the effects of electron density on the redox potential. As well, they successfully used a dione phenanthroline derivative to mediate the anode reaction in a GOx-based biofuel cell [13].

In order for a molecule to work as a mediator, its formal redox potential

must have the appropriate value relative to the one for the enzyme cofactor . At the anode, must be higher than so that the mediator is capable of reoxidizing the reduced enzyme back to its active state. Conversely, must be lower than at the cathode (Figure 1.10). It is important to realize that these requirements necessary introduce thermodynamic potential losses, therefore decreasing the open circuit voltage of the cell. Therefore, a balance must be sought to have a large enough separation between and to drive the mediation reaction thermodynamically but to minimize the open circuit potential losses. In any case, the potential losses are compensated for with the increased kinetics caused by the presence of the mediator.

      Depending on the enzyme employed, the reaction between the enzyme and the mediator might be in competition with the one between the enzyme and its natural acceptor. Such is the case of GOx, in which oxygen competes for the electrons with the mediator. In these cases, the concentration of the mediator must be high enough to favor a predominant enzymemediator reaction. On the other hand, when using glucose dehydrogenase for example, the enzyme activity is not dependent on oxygen and therefore no competition exists [5].

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