Biofuel Cells

Figure 1.7 (a) Schematic representation of a polarization curve for an ideal and a real BFC, and (b) representation of current density and potential losses during time.

In general, the classification of the reported nanomaterials for BFCs is presented in Figure 1.9; being carbon allotropes the most reported materials. Carbon materials that are of high interest in BFCs in recent years are buckypaper, carbon paper and nitrogen-doped graphene. Therefore, the discussion will be centered on them.

Figure 1.8 Structural and electronic modifications of supports to improve the electrocatalytic properties in biofuel cells.

Figure 1.9 Types of supports reported for biofuel cells.

Buckypapers are thin sheets composed of entangled carbon nanotubes, where the thickness can be modulated from tens of nanometers to hundreds of micrometers [99]. Walgama et al . prepared buckypaper with different thickness, finding that 87 μm was the minimal thickness required to develop a mechanically stable electrode to be in contact with aqueous solutions for BFCs [100]. The group of Serge Cosnier has published interesting works related to the use of buckypaper bioelectrodes functionalized with several mediators for glucose biofuel cells. In a recent work, their group used 1,10-phenanthroline-5,6-dione (PLQ) as mediator in a glucose biofuel cell achieving open circuit voltages (OCVs) between 0.67 and 0.74 V, and power densities up to 24 mW cm −3in a single compartment BFC [101]. Additionally, the same group reported the use of buckypaper functionalized with a pyrene–polynorbornene homopolymer for a flexible lactate BFC [102]. This BFC delivered an OCV of 0.74 V, and a maximum power density of 520 μW cm −2. Güven et al . used buckypaper as bioelectrodes for pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase/laccase glucose BFC [103]. The function of buckypaper was to diminish the electrochemical barriers for a direct communication of these enzymes. Thus, a maximum OCV of 0.44 V was achieved, while 49.16 μW cm −2was the highest achieved power density. Bollella et al . reported a miniaturized glucose BFC based on buckypaper electrodes using PQQ-dependent glucose dehydrogenase and bilirubin oxidase. This BFC achieved an OCV of 0.6 V and a maximum power density close to 10 μW. In addition, the authors implanted the BFC in a living slug obtaining an OCV of 0.31 V, and a power density of 2.4 μW (~4-fold lower to that obtained in ideal conditions) [104]. Hou and Liu reported the use of buckypaper for the incorporation of flavin adenine dinucleotide-glucose dehydrogenase and laccase in a glucose BFC coupled to a supercapacitor based on carbon nanotubes and polyaniline [105]. This combined device achieved 0.8 V and a maximum power density of 608 μW cm −2.

Torrinha et al . used a typical methodology reported for buckypaper to prepare paper-like electrodes using Vulcan carbon black, reduced graphene (rG) electrode and carbon nanotubes (buckypaper electrode) [106]. Glucose oxidase and bilirubin oxidase were deposited onto these electrodes, and a finger-powered glucose biofuel cell was constructed. This cell has the advantage of avoiding the use of external pumps to drive the anodic and cathodic streams, using the finger force for that purpose. The authors found that paper-like electrode of rG outperformed buckypaper and Vulcan carbon black paper-like electrodes. Escalona-Villalpando et al . evaluated the use of nanofoam-like carbon paper in hybrids bioanode/cathode & anode/biocathode glucose nanofluidic BFCs and in a full glucose nanofluidic BFC [107]. The authors found that the OCV in the glucose oxidase & Pt/C hybrid BFC was 0.55 V, and it can be increased to 0.91 V using a AuAg anode with a laccase biocathode. In addition, the full BFC achieved 0.44 V and a maximum power density of 3.2 mW cm −2. Escalona-Villalpando et al . also reported the effect of stacked microfluidic biofuel cells on the power density. For this purpose, they worked with glucose dehydrogenase as anodic biocatalyst and bilirubin oxidase as cathodic biocatalyst [108]. A single cell BFC enabled 0.78 V and 0.36 mW cm −2, while four cells stacked in parallel achieved 0.53 mW cm −2, and these cells stacked in series enabled an OCV of 1.27 V and a power density of 0.38 mW cm −2. However, the most effective way that they found to improve the cell performance was stacking four cells 1 and 2 in series and 3 and 4 in parallel, achieving an OCV of 1.23 and a power density of 0.42 mW cm −2.

Another kind of carbon paper electrodes consisted in carbon fiber arrays; Koushanpour et al . developed an all glucose biofuel cell using this electrode and the H 2O 2generated in the anode as oxidant [109]. They reported that the use of Meldola’s blue as catalyst for the electro-oxidation of NADH and hemin as catalyst for H 2O 2reduction resulted in OCVs close to 0.5 V.

The doping of carbon-based materials with heteroatoms (N, B, P, and S) is a route to activate the π electrons through creation of charge sites, being these responsible of an enhanced conductivity and activity toward the oxygen reduction reaction (ORR). Highly conductive supports like graphene have been modified with heteroatoms for their use as cathodes in hybrid biofuel cells. Du et al . grew N-doped carbon nanotubes on reduced graphene oxide (rGO) nanosheets to improve the performance of a microbial fuel cell (MFC) [110]. The maximum power density achieved by this biofuel cell was 1,329 mW cm −2, which was 1.37 times higher to that achieved by benchmarked Pt/C, and the improvement was associated to the strong covalent bonds formed between the carbon nanotubes and graphene facilitating the electron transfer between these interfaces. Zhong et al . followed a similar strategy developing a N-doped hierarchical carbon [111]. This material also contained Fe species in its structure, and was obtained through the carbonization of metal–organic frameworks (MOFs). This material was used as cathode in a microbial fuel cell using carbon felt and carbon cloth as anode and cathode, respectively, and the highest performance reported was 1,607.2 mW cm −3.

N-doped materials have been used in the anode compartment of microbial fuel cells. Guan et al . [112] synthesized N-doped carbon dots on carbon paper electrodes to improve microbial immobilization. One of the first findings was that the biofilm has 2 times higher thickness in this electrode in contrast with an unmodified carbon paper electrode. In addition, the cell performance was boosted because the extracellular electron transfer process from the microorganisms to the electrode was improved. Zhang et al . [113] used a N-doped graphene as support for a Mo 2C nanocatalyst to improve the hydrogen evolution reaction in a microbial fuel cell stacked with an ammonia electrolytic cell. The authors reported a maximum power density of 536 mW cm −2, achieved using four air-cathode MFCs stacked in series. Guo et al . [114] also improved the anode of a MFC synthesizing a N-doped 3D expanded graphite foam, which displayed a maximum power density of 739 mW cm −2, 17.4 times higher than the performance obtained by a simple graphite foil. The activity improvement was attributed to a higher surface area which allowed a bigger growth of the biofilm.