Biofuel Cells. Группа авторов

and has been reported to result in an increase of almost an order of magnitude in current compared to cells without air-breathing cathodes [170].

      1.6.2 Microbiological Catalysis Control

      The use of bioelectrodes helps diminishing the overpotential in both, anode and cathode. The reduction of CO₂ to CH₄ and acetic acid follows metabolic pathways that depend on the cathode potential. Jiang et al. [171] reported that exclusive formation of methane and hydrogen was obtained in the range from −850 to −950 mV, whereas the simultaneous formation of CH₄, H₂ and acetate occurred in potentials more negatives than −950 mV.

Cathode potentials have been compared by Blanchet et al. [79]. The authors tested −0.36 and −0.66 V/SHE, finding that the former potential was appropriate for CO₂ reduction whereas the second potential resulted in hydrogen production in addition to CH₄. Thus, acetate was produced in an amount of 244 mg L⁻¹. In the same way, Siegert et al. [172] observed that methane production increased with more negative cathode potential in the order −650 mV > −600 mV > −550 mV. The products recovered were CH₄, acetate and some cases formate.

      Co-products are expected in MECs, which represent another advantage if they are high value-added metabolites. For instance, acetate has been produced in a membrane-less system at potentials lower than −1.0 V/SHE, but by varying the potential to −0.4 V, the production increased to 600 mg L⁻¹ in 9 days [173]. Similarly, Nie et al. [174] obtained 540 mg L⁻¹ after 8 days, and Marshall et al. [175], using graphite granules at −0.59 V/SHE, produced up to 10,500 mg L⁻¹ over 20 days.

1.7 Recent Applications of Bioelectrocatalysis

      1.7.1 Biosensors

      One of the great limitations of enzymatic biofuel cells is the low energy output compared to the well-established inorganic counterparts. However, together with their mild operation conditions, this has made them a suitable candidate as an implantable power source [21].

Besides the obvious applications in energy conversion, enzymatic biofuel cells have been proposed as so called “self-powered” biosensors. Katz’s original idea was to use the open circuit potential as an indicator of the concentration of the fuel (glucose or lactate) [132]. However, subsequent developments used an amperometric approach, in which a constant resistance would be connected to the fuel cell and the measured current across it taken as analytical signal [146]. It must be noted, however, that the term “self-powered biosensor” is somewhat misleading. While it is true that it is not necessary to apply a potential difference to the electrochemical cell (i.e. they are galvanic cells), measurement of the electrochemical response does require external power. Recently, Pellitero and coworkers developed a true self-powered biosensor based on an enzymatic biofuel cell. They ingeniously coupled a mediated GOx anode with a transparent indium tin oxide cathode in which Prussian blue is reduced to its colorless form (sometimes referred to as Prussian white). The geometrical arrangement of their electrodes and electrolyte (loaded in a lateral flow membrane) allows to use their cathode as an electrochromic display, in which the discolored distance is proportional to the glucose concentration of the sample [176].

      Finally, it must be noted that, in parallel with the advances in enzymatic electrodes for fuel cells, significant research has been conducted in the use of enzymatic bioelectrodes as components of electrolytic biosensors [177]. The main difference is that, in this case, electrodes are used as part of an electrolytic cell, rather than a galvanic one. Therefore, a potential is typically applied and chronoamperometric measurements used to obtain the analytical signal. Although seemingly distinct, both fields share many common interests and challenges, including increasing the current output and achieving better stability.

      1.7.2 Microbial Catalyzed CO₂ Reduction

      Microbial bioelectrocatalysis has become important for storage and energy conversion, synthesis of valuable products such as hydrogen and methane, and waste treatment among others.

The production of methane from CO₂ reduction in microbial biocathodes has been proposed as a frontier technology. The first study on CO₂ conversion to CH₄ was reported by Cheng et al. in 2008 [178]. The interest of using CO₂ as gaseous substrate lies on their availability as atmospheric gas and waste gas; it is also land-independent and ease to handle (Figure 1.13).

      The MECs conjugate characteristics of an electrochemical process and an enzymatic-type process for CO₂ reduction [179]. The difference is that bacteria are utilized as catalyst either at the cathode or both at the anode and cathode. The diverse possible pathways for CO₂ reduction are still uncertain. However, at least two mechanisms are envisaged for this reaction:

Figure 1.13 Integration of bioprocesses with microbial electrolysis cells (MEC).

indirect production when methane is formed in the bulk and direct when methane is formed near the biocathode (Figure 1.14). The mechanisms for biocatalyzed methane formation are diverse since the sources of reactants are multiple; moreover, the metabolic pathways of electroactive microorganisms receiving electrons are still poorly understood.

      Rosenbaum et al. stated various hypothesis on the molecular phenomena [180]:

      1 • Direct electron transfer involves c-type cytochrome and electron transfer chains

      2 • Direct electron transfer includes cytochrome linked to hydrogenase partnerships

      3 • A mediated electron transfer to a periplasmic hydrogenase takes place.

      At a bioelectrochemical process level, one mechanism for CO₂ reduction is explained by hydrogen production at the cathode, which is then utilized by bacteria to reduce CO₂. The biochemical pathway covers homoacetogenic fermentation by chemolithotrophic species. Examples of chemolithotrophic acetogens are Clostridium aceticum and Acetobacterium woodii [173].

Jiang et al. reported that formation of CH₄ from CO₂ can follow two pathways, through the direct use of electrical current for hydrogen formation or via biohydrogen production and then CO₂ bioelectrochemical reduction. Hydrogen produced through water electrolysis can provide the substrate for methanogens to produce methane. However, abiotic hydrogen production requires the use of catalyst whereas biohydrogen does not [171].

Figure 1.14 Overview of hypothetic mechanism for CH₄ production from CO₂. (a) Indirect mechanism, (b) Direct mechanism, (c) Alternative direct mechanism.

      Blanchet et al. agree that hydrogen is produced on the cathode as a reactant for the microbial reduction of CO₂. They propose that two consecutive steps occurs, hydrogen production by water electrolysis and then reduction of CO₂ by microbial species that utilize that hydrogen [79].

      Two pathways are described by Villano et al., hydrogenotrophic methanogenesis and direct extracellular electron transfer. The contribution of each pathway depends on the set cathode potential. The author points that the reactants for CH₄ formation, electrons and CO₂ are produced by