**2.4 Analytical method**

Two protocols, polarity and cyclic voltammetry techniques, were adopted to analyze experimental data in terms of voltage and current density.

### **2.4.1 Polarity curve**

Polarization curves were obtained using an adjustable external resistance. Power and current were calculated based on following equations:

$$P = \mathbf{I} \times \mathbf{E} \tag{1}$$

when the MFC was not in use. Neutral red and potassium permanganate were also supplied by Merck Company (Darmstadt, Germany) as mediators and oxidizer agent in continues mode, respectively. The schematic diagram, photographic images and auxiliary equipments of the fabricated MFC cell in batch and continuous systems are shown in Fig. 2. In continuous operation, the MFC was continuously fed with the prepared media in an up-flow mode using

(a)

(b)

Two protocols, polarity and cyclic voltammetry techniques, were adopted to analyze

Polarization curves were obtained using an adjustable external resistance. Power and

*P=I×E* (1)

experimental data in terms of voltage and current density.

current were calculated based on following equations:

Fig. 2. Schematic diagram of cubic two chamber MFC in batch (a) and continues (b) mode

an adjustable peristaltic pump (THOMAS, Germany).

**2.4 Analytical method** 

**2.4.1 Polarity curve** 

$$\mathbf{I} \equiv (\mathbb{E}/\mathbb{R}\_{\text{ext}}) \tag{2}$$

where P is generated power and E measured cell voltage; Rext denotes external resistance and I indicates produced current. The online recorded produced current and power were normalized by the surface area of the used membrane. Analog digital data acquisition was fabricated to record data point in every 4 min. Measurements were carried out at variable resistances which were imposed to the MFC. The current in the MFC was automatically calculated and recorded dividing the obtained voltage by the specified resistance. Then, the system provides power calculation by multiplication of voltage and current. The provisions were provided for online observation of polarization curve showing the variation of power density and MFC voltage with respect to current. The online system had the ability to operate automatically or manually. While it operates in auto-mode, the assembled relays were able to regulate automatically the resistances. Voltage of MFC was amplified and then data were transmitted to a microcontroller by an accurate analog to digital converter. The microcontroller was also able to send the primary data to a computer by serial connection. In addition, a special function of MATLAB software (7.4, 2007a) was used to store and display synchronically the obtained data. The power, current and voltage were automatically recorded by the computer connected to the system.

Columbic efficiency can be calculated by division of total coulombs obtained from the cell and theoretical amount of coulombs that can be produced from glucose (Equation 3):

$$\text{CE} = (\text{C}\_{\text{fl}} \text{/C}\_{\text{T}}) \times 100 \tag{3}$$

Total coulombs are obtained by integrating the current variation over time (*Cp*), where CT is the theoretical amount of coulombs that can be produced from carbon source, calculated as follows:

$$C\_T \equiv \left( FbSV. M^{\Box} \right) \tag{4}$$

For continuous flow through the system, CE can be calculated on the basis of generated current at steady state conditions as follows (Logan et al., 2006):

$$\text{CE} = \text{MI}/\text{Fbg} \Delta \text{S} \tag{5}$$

In equation (4), *F* is Faraday's constant , b the number of moles of electrons produced per mole of substrate (24 mol of electrons were produced in glucose oxidation in anaerobic anode chamber), *S* the substrate concentration, *q* flow rate of substrate and *M* the molecular weight of used substrate (*M*= 180.155 g.mol-1) (Allen and Bennetto, 1993; Oh and Logan, 2006).

In batch mode, polarization curves were obtained at steady state condition by setting an adjustable resistance in data logger. When the MFC was operated in continuous mode, the concentration of glucose in the feed tank solution was kept constant at 30 g.l-1. Several hydraulic retention times (HRT) were examined in continuous operation. The HRT was measured from the volume of medium and the inward flow rate to the anode compartment of MFC.

### **2.4.2 Cyclic Voltammetry (CV)**

Beside the polarity curve, cyclic Voltammeter (IVUM soft, Ivium Technology, Netherland) was also used to analyze for testing oxidation and reduction of organic materials. The potential range of -400 mV to 1000 mV was applied. The working electrode and sense

Effect of Mass Transfer on Performance of Microbial Fuel Cell 239

After incubation 10 hours after incubation At SS condition

**Power (mW.m-2)**

0.0

**Voltage (mV)**

0

10 hours after incubation and at steady state condition

50

100

150

200

250

300

0.2

0.4

0.6

0.8

1.0

**Current (mA.m-2)** 0 2 4 6 8 10 12 14 16

> After incubation 10 hours after incubation At SS conditio

(a)

**Current (mA.m-2)**

(b)

Fig. 3. Generated power density (a) and voltage (b) as function of current density at start up,

0 2 4 6 8 10 12 14 16

electrode were joined together to measure oxidation and reduction peaks. Carbon paper (NARA, Guro-GU, Seoul, Korea) was used as the working electrode and Platinum (Platinum, gauze, 100 mesh, 99.9% meta basis, Sigma Aldrich) as the counter electrode. Also, Ag/AgCl (Ag/AgCl, sat KCl, Sensortechnik Meinsberg, Germany) electrode was utilized as reference electrode. Voltage rate of 50 mV.S-1 was chosen as scan rate in CV analysis.
