V with a maximum power density of 0.9 mW cm−2.
1.4.4 Metal–Organic Framework (MOF)-Based Carbonaceous Materials as Bioelectrodes for BFCs
MOFs have advantages over typical carbon supports such as tailorable properties from the preparation method, large specific surface area (SSA), high porosity and easy modification with metal atoms and heteroatoms like nitrogen [116]. Porous carbon supports obtained through calcination of MOFs have attracted attention in the energy conversion area because the high specific surface area and ordered porous structure enable a convenient path for electron transfer [117]. In MFCs, these materials are highly used to improve the oxygen reduction, and there are several works focused on this topic as was highlighted in a recent review [118]. Wang et al. [117] obtained a hollow material based on Cu/Co/N with a BET surface area of 286 m2 g−1 and a half-wave potential shifted to more positive values in contrast with a benchmarked Pt/C electrocatalyst. This improvement was attributed to the electron properties of this MOF-derived material, which displayed an OCV of 0.68 V, and a maximum power density of 1,008 mW cm−2. This performance is comparable to that obtained with N-doped materials [110–114]. Zhong et al. [119] obtained a MOF-derived electrocatalyst for oxygen reduction reaction in MFCs using Zr-based MOF UiO66-NH2 and incorporating Co–Nx active components. This electrocatalyst had a BET surface area of 279 m2 g−1 and, similarly to the Cu/Co/N, this material had a more positive half-wave potential (35 mV) than the benchmarked Pt/C. The cell performance evaluation indicated that this material achieved an OCV of 0.39 V and a maximum power density of 299.62 mW cm−2, which was slightly lower to that obtained by Pt/C (312.59 mW cm−2). Wang et al. [120] used the isoreticular metal organic framework-3 (IRMOF-3) modified with g-C3N4 nanosheets to obtain a N-doped carbon material with a BET surface area of 686.41 m2 g−1. This material displayed superior activity in half-cell and full-cell experiments, the half-wave potential and current density were superior to those obtained for benchmarked Pt/C (0.89 vs. 0.79 V and 6.35 vs. 5.51 mA cm−2, respectively). Additionally, the maximum power density was 1,402.8 mW cm−3, which was 110 mW cm−3 higher to that obtained by Pt/C. Xe et al. [121] reported an electrocatalyst for oxygen reduction reaction in MFCs based on zeolitic imidazolate framework (ZIF-8), this new material displayed a BET surface area of 1,416.19 m2 g−1, which is larger to that previously mentioned. Consequently, the maximum power density was 2,103.4 mW cm−2, which also at least 3 and 3 times higher to that reported in the previous works. The ZIF-8 was then modified with polypyrrole to fabricate a polyhedral porous carbon embedded N-doped carbon networks (PPC/NC) [122]. This material presented a surface area of 342.3 m2 g−1, but the improvement in the electron transfer allowed achieving power densities of 2,401 mW cm−2, which was 3.3 times higher than the control, and between 1.7 and 8 times higher to that obtained in the previous works herein discussed. Finally, Luo et al. [123] modified the ZIF-8 with FeS to dope the resulting carbon material with Fe, N, and S heteroatoms. This modified material had a BET surface area of 598 m2 g−1, and the presence of heteroatoms improved the power density of a MFC, displaying a maximum value of 1,196 mW cm−2 with an OCV of 0.71 V.
On the other hand, the use of MOF for the preparation of enzymatic electrodes is limited. Li et al. [124] developed an enzymatic BFC encapsulating laccase in the ZIF-8 MOF. This electrode array was combined with bacterial cellulose and carboxylated carbon nanotubes achieving OCVs close to 0.3 V, and a maximum current density of 3.68 W m−3. Zhang et al. [125] reported the use of the IRMOF-8 impregnated in carbon nanotubes to develop a porous carbon intercalated by multi-walled carbon nanotubes (PC/MWCNTs) as anode for the immobilization of alcohol dehydrogenase. This material had a BET surface area of 1,166 m2 g-1, while the electrochemical 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