Electrode Reactions in MFC

A typical MFC reactor contains an anodic chamber, a cathodic chamber and a PEM partitioning the two cham­bers. Figure 9.1 shows a dual-chamber MFC.

Anode Reaction

The microbes in the anodic chamber oxidize sub­strates such as glucose, acetate and some refractory organics. For example, glucose is oxidized as follows to generate electrons, protons and carbon dioxide (Pham et al., 2006):

C6H12O6 + 6H2O/6CO2 + 24H+ + 24e~ (9.1)

Because electrons cannot "swim" in an aqueous solu­tion, the oxidation reaction must occur in a biofilm that is capable of transferring electrons to the anode. In the absence of a suitable oxidant in the anodic chamber to absorb the electrons, electrons will be transferred to the anode by the biofilm (Zhao et al., 2009). The electrons reach the cathode via an external circuit linking the anode and the cathode, where they are used to reduce an oxidant such as oxygen (Figure 9.1). A load is placed on the external circuit to harvest the electricity. To main­tain electroneutrality, protons must carry an equal amount of positive charges from the anodic chamber to the cathodic chamber usually through a PEM. Inefficient proton migration will result in accumulation of protons that causes acidity in the anodic chamber (Xu et al., 2012).

In the anodic chamber, anaerobic conditions are very important to guarantee the substrate oxidation by the microbes through anaerobic respiration (Liu et al., 2005b; Logan et al., 2006). Oxygen leaked into the anodic chamber from outside air or through diffusion from the cathodic chamber (Figure 9.1) would reduce Coulombic efficiency of the MFC by directly oxidizing the organic matter in the anodic chamber. In this case, energy will be released as low-grade heat instead of electricity. A PEM plays an important role of preventing oxygen diffusion from the cathodic chamber to the anodic cham­ber (Li et al., 2011), while allowing positive charges to go through it via a proton exchange process. If nonoxygen oxidants such as sulfate and nitrate are present in suffi­cient quantities in the anodic chamber feed stream, the biofilm on the anode must not be able to catalyze their reduction because it would divert the electrons released from oxidation of organic matters for the local reduction of sulfate or nitrate. A buffer solution that usually con­tains NH4Q, NaH2PO4, Na2HPO4, KCl, and so on is often used to enhance the proton transfer in laboratory MFC investigations (Liu et al., 2011). The presence of a buffer solution increases the conductivity, thus reducing internal resistance of the MFC (Liu et al., 2005a).

Cathode Reaction

The cathode reaction has a major impact on MFC performance. The electrons coming from the anode via the external circuit, the protons coming from the anodic chamber via the PEM and the electron acceptors (e. g. O2) will react with the help of catalysts on the cathode (Pham et al., 2006):

24H+ + 24e~ + 6O2 / 12H2O (9.2)

Reactions (9.1) and (9.2) form a thermodynamically favorable redox reaction, that is, the aerobic oxidation of glucose. However, a thermodynamically favorable reaction may not proceed at an appreciable rate if the kinetics is too slow. In an MFC, anode and cathode reac­tions almost always require catalysis. For the anode reaction, a biofilm is required to catalyze organic carbon oxidation and electron transfer. For the cathode, oxygen reduction rate is very slow without catalysis. The cathodic reaction efficiency depends on the concentra­tion and type of electron acceptors, proton concentra­tion, electrode structure and its catalytic ability (Zhou et al., 2012).

In order to improve electricity generation, a good cat­alytic cathode is crucial since the catalysts can reduce the activation energy and thus greatly increase the reaction rate. Currently, for oxygen reduction, platinum (Pt) appears to be most effective. However, it is extremely expensive and, thus, unrealistic for most practical appli­cations even when only Pt coating is used. Some alterna­tive catalysts have been explored such as MnOx, CoTMPP, PbO2, iron(II) phthalocyanine (FePc) and recently the biocathode (Roche and Scott, 2008; Zhou et al., 2011).

Oxygen is the most popular acceptor because of its high standard potential (0.818 mV), low cost and envi­ronmental "friendliness". However, the rate of oxygen reduction is very low on the cathode surface, resulting in a high overpotential, which is one of the most impor­tant limiting factors in MFCs (Gil et al., 2003). Potassium ferricyanide (K3[Fe(CN)6]) can overcome this handicap (Logan et al., 2006; Nevin et al., 2008; Park and Zeikus,

2003) . However, the regeneration of K3[Fe(CN)6]is a problem because it usually is not sufficiently oxidized by oxygen. It needs to be replenished periodically (Franks and Nevin, 2010). In addition, K3[Fe(CN)g] can diffuse into the anodic chamber through the PEM, thereby influencing the desired anaerobic conditions of anodic chamber (Logan et al., 2006). Potassium perman­ganate is also used as an acceptor, and the power density was reported to be higher than that with K3[Fe(CN)g] and oxygen (You et al., 2006). In practice, wastewater streams are low-grade energy sources that are pale in comparison to pure fuels such as hydrogen or ethanol as a fuel. This inherently means that a large volume of water must be treated to harvest a sufficient amount of electricity. This makes all externally added soluble catalysts impractical, limiting them to academic investigations.

To overcome the requirement for catalysis by oxygen oxidation on the cathode, biocathodes have been explored (Biocathodes Section). Various biofilms have been tested on cathodes to biocatalyze oxygen or a nonoxygen oxidant such as nitrate and perchlorate (Shea et al., 2008; Srikanth et al., 2012; Zhang and Angelidaki, 2012).