Biofuel Cells

Conventional MFCs utilize abiotic cathodes; however, limitations for the oxygen reduction reaction are present similarly to what is observed in electrochemical cells. The use of biocathodes as an alternative to metallic cathodes was proposed in 2005 [25].

The biocathodes are classified as a function of the terminal electron acceptor available. Aerobic biocathodes utilize oxygen as final acceptor; of electrons electron transfer occurs via the reduction of a mediator such as iron and manganese and then, the mediator is oxidized by the bacteria. Anaerobic biocathodes directly reduce acceptors such as nitrate and sulfate [26].

Suitable bacteria for indirect electron transfer are Leptothix discophora , which is able to reduce MnO 2to manganese ion [27], and Acidithiobacillus ferrooxidans an iron oxidizing bacterium [28], while consortia of electroactive microorganisms for direct transfer can be found in sediment and anaerobic sludge [29, 30]. An exhaustive evaluation of the electroactivity capabilities of diverse bacterial species was reported by Cournet et al . [31].

Hybrid microbial bioelectrochemical systems include MFC fueled by light energy; this is achieved by oxygenic photosynthetic organisms, such as microalgal and cyanobacteria species. Photosynthetic organisms transfer electrons to the anode, or to heterotrophic microorganisms which in turn transfer the charge to the anode.

Energy pathways in cyanobacteria occur in the thylakoid membranes containing respiratory electron transfer chain components and in the cytoplasmic membrane with a respiratory electron transfer short chain. Since the electron transfer is not adapted for extracellular electron transport, mutant strains for electron export ability are being obtained [32]. Synechocystis have three respiratory terminal oxidase complexes for the reduction of oxygen and mutants lacking respiratory terminal oxidases showed increased ferricyanide reduction rate [32]. However, the mechanism for electron excretion to the periplasmatic space and beyond remains unresolved; hypothesis on the presence of nanowires, an assimilatory metal reduction pathway, and endogenous mediators have been stated.

One additional advantage of MFCs over general bioreactors is the possibility to adapt the microbial metabolism of the inoculum as function of the set electrode potential. This procedure enables one to increase the selectivity of the reactions and the galvanic mode of operation can become electrolytic [33].

Table 1.1 Cell potential for typical reactions with microbial bioanode, and a microbial biocathode.

Microbial electrolysis cells (MECs) operate under a constant electrical supply, therefore this energy represents an operation cost and a decrease in the energy efficiency. To overcome these issues, the use of alternative energy technologies has been proposed [34]. Another alternative to reduce the cost of energy supply is an intermittent operation for conversion of CO 2into organic products using a biocathode [35].

The thermodynamic cell voltage for hydrogen production from acetate oxidation in a bioanode, and methane production in a biocathode from water oxidation is resumed in Table 1.1. At least theoretically, these reactions present advantages compared to totally abiotic processes for gaseous fuel production. For instance, water electrolyzers require 1.23 V for hydrogen formation while the bioelectrolysis of water requires 0.134 V.

Although the first proof-of-concept biofuel cells employed the enzymes freely in solution [3, 4] this approach is poorly applicable in practice. Enzymes are costly and losing them with the fuel and oxidant flow turns operation expensive. Therefore, most of the reported enzymatic biofuels include enzymes immobilized on the electrode surface.

A number of strategies have been developed to immobilize enzymes on solid supports and a significant number of reviews have been published explaining the advantages and disadvantages of each approach, often proposing a classification of the methods based on criteria that is not standardized [36–41]. As well, these reviews often focus on the immobilization of enzymes on supports that, while solid, are generally a mobile part of a bioreactor (the so-called carriers). This section, instead, focuses on the description of the different strategies in the context of the immobilization on an electrode surface, giving representative examples of their use in fuel cell research.

In general, enzyme immobilization is a balancing act between the external forces holding the enzyme on the support and the internal forces that maintain the enzyme conformation, and therefore, its function. The addition of interactions can stabilize the enzyme but, if they are too strong, they can modify the conformation of the active site or even denature the enzyme. As well, the addition of dense composite materials around the enzymes can create mass transport limitations that need to be kept in mind; else the catalytic performance can be severely affected.

Immobilized enzymes are usually evaluated measuring the current produced with different concentrations of substrate. In solution, the relationship between the enzymatic reaction rate (V) and the substrate concentration ([S]) is given by the Michaelis–Menten equation ( Equation (1.1)).

[1.1]

where V maxis the maximum rate at a given enzyme concentration and K Mis the Michaelis–Menten constant that represents mainly the enzyme’s affinity for the substrate. The enzymatic rate can be measured by a number of techniques, spectrophotometric ones being particularly popular. Once the oxidoreductase is immobilized on the electrode, part of the exchanged electrons in the enzymatic reaction end up / come from the electrode. Therefore, the measured current does depend as well from the substrate concentration. An analogous equation has been derived which employs the apparent Michaelis–Menten constant as shown in Equation (1.2).

[1.2]

In this case, the value depends not only on the enzyme–substrate affinity but also on substrate partition between the solution and the film, and mass-transfer limitations due to the film structure [39].

According to the forces involved, immobilization strategies can be classified as either physical or chemical. The first group includes adsorption, polymer entrapment and electrostatic binding. In adsorption, enzymes are weakly bound to the electrode surface via mainly Van der Waals forces, hydrophobic interactions and hydrogen bonds ( Figure 1.4a) [40]. The main advantage of this method is the simple procedure required. Typically, the electrode is incubated in a solution of the enzyme, after which it is rinsed to remove the unbound enzymes. Laccase [42] and fructose dehydrogenase [7] have been shown to present direct electron transfer (see Section 1.5.1 ) when adsorbed on carbon electrodes. The main drawback of adsorption immobilization is the lability of the enzymes. Since no strong interaction is present between the enzyme and the electrode, enzyme leaching is a common limitation of the electrodes prepared in this manner, which can be aggravated if the conditions of the environment change [40].