Parameters in Evaluating BES Performance

19.3.1 Electrode Potential and Power Output

All BES processes share a same principle—EAB in the anode chamber oxidize biodegradable substrates and generate electron flow (i. e. current) to reduce the electron acceptors in the cathode chamber. Therefore, the most direct way to evaluate the performance of a BES is the measurement of circuit voltage (V). The current (I) passing through an external resistance (Rext) can be measured using a current meter, or simply calculated based on the Ohm’s law:

1 = IT (19Л)


The V represents the direct output of a reactor, and it can also be describes as:

V = OCV — IRint (19.2)

where OCV is the open circuit voltage of the reactor, and Rint is the overall internal resistance. Generally, Rint represents the total current-related resistance loss, which is the sum of system ohmic resistance caused by the resistance of the electrodes and electrolyte, the activation loss caused by biochemical reactions on the elec­trode surface, and the charge transfer resistance caused by the limitation of mass transfer and concentration polarization (Logan et al. 2006). Rjnt is considered to connect with Rext in series. The power density (P) from an MFC is inversely proportional to the total system resistance squared according to:

The traditional approach of reporting power density from a BES or MFC reactor was to operate the reactor with a static Rext or applied potential then transiently obtaining polarization data by changing the Rext at a 5-30 min interval or conduct a voltammetry sweep (Logan et al. 2006; Lyon et al. 2010; Ren et al. 2011b). Figure 19.5 shows typical polarization and power density curves obtained from a lab scale MFC reactor, and it shows that the MFC voltage is practically inversely proportional to the output current, and there exists a pair of voltage and

Fig. 19.5 Polarization curve and power density curve obtained from a lab scale microbial fuel cell reactor

current that delivers the maximum power when Rext is equal to Rint (Pinto et al. 2011; Ren et al. 2011b).

Compared to other alternative energy systems, BES/MFC is a small power system due to its thermodynamic limitation. The MFC anode potential is generally around -0.3 V (versus Normal Hydrogen Electrode, NHE), which is set by the respiratory enzymes of bacteria that metabolize electron donors. Take acetate (5 mM, pH = 7) as an example:

Anode: 2 HCO — + 9H++ 8 e — ! CH3COO — + 4 H2O. .

OCP anode = -0.296 V 1 — j

The cathode potential is around +0.8 V when oxygen is used as the terminal electron acceptor (Logan et al. 2006; Meehan et al. 2011).

Cathode: O2 + 4H++ 4e-! 2 H2O OCPCathode = +0.80V (19.5)

Such characteristics determine that the voltage of an air-cathode MFC is generally less than 0.8 V and the current output is usually in the range of a few mA due to overpotential and other losses. Other chemical oxidants, such as ferricyanide or permanganate could provide a higher cathode potential, but air cathodes have been widely adopted in MFCs because it is free and sustainable (Logan 2008; Ren et al. 2007b). Higher power generation using single or multiple MFCs can be achieved by applying electronic harvesting systems and developing larger stack systems. (Aelterman et al. 2006; Dewan et al. 2008; Park and Ren 2012; Wang et al. 2012).

Power density has been a key parameter to demonstrate the performance of a reactor. Traditionally, the power output is normalized to projected electrode area, defined as surface power density, so it is possible to compare the performance of different systems. However, such parameter sometimes may not represent a fair comparison when different systems use different electrode configurations and materials. For example, when replacing carbon cloth (* 100 m2/m3) anode with high surface area brush anode (*9,600 m2/m3), the normalized cathode power density increased from 0.6 to 2.4 W/m2, but the normalized anode power density did not change that much (Logan et al. 2007). Therefore, more and more studies
begin to use volumetric power density, which is the power output normalized by the reactor volume, because it represents the reactor overall power output and aligns with reactor size and water treatment capability. In general, the higher the electrode density, the better the reported volumetric power density, but very high electrode surface areas may pose another challenge of clogging when actual wastewater is used as the substrate.

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