Biofuel Cells. Группа авторов

Чтение книги онлайн.

Figure 1.12 Characterization of the performance of a GOx anode and a Lac cathode through cyclic voltammetry (a) and power curves (b) at two different pH values. Black lines and symbols correspond to pH = 5.5 while grey ones are obtained at pH = 7.4. Republished with permission of the Royal Society of Chemistry, from Chem. Commun., Saravanan Rengaraj et al., 47, 2011, 11861–11863; permission conveyed through Copyright Clearance Center, Inc.

      Recently, laminar flow has been explored as another alternative to separate anolyte and catholyte solutions without the need for a membrane. In electrode compartments of reduced dimensions, low Reynolds number regimes exist, preventing solutions from mixing through turbulence. Therefore, the membrane can be obviated. In practice, such systems have taken the form of microfluidic fuel cells in which glucose oxidase, lactate oxidase and laccase and have been employed [107, 148, 166].

      The performance of the cell is also determined by the availability of fuel and oxidant at the anode and cathode respectively. Glucose, like many of the fuels, can be employed in solution at high concentrations, therefore ensuring availability at the anode. Oxygen, on the other hand, has a much lower solubility in aqueous solutions, as well as a low diffusion coefficient. Therefore, cathode performance usually limits the overall cell output. A good strategy to increase the availability of oxygen is the incorporation of “air-breathing” cathodes. These electrodes are typically composed of a carbon conductor (in the form of paper) with a hydrophobic membrane. This membrane is generally fabricated with some fluoropolymer and allows the gas exchange between the catholyte and the atmosphere without allowing solution to leak. Such electrodes are therefore exposed to both oxygen dissolved in solution and in gas form from the atmosphere. This strategy has been used in inorganic [144, 167, 168] and biological [42, 148, 169] fuel cell cathodes 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].