Performance Comparison of Microbial Fuel Cells in Some Different Conditions

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International Journal of Energy and Power (IJEP) Volume 4, 2015 doi: 10.14355/ijep.2015.04.014

Performance Comparison of Microbial Fuel Cells in Some Different Conditions Li-ping Fan*1, Dan-dan Xu1, Yu-jiao Zheng1, Chong Li1,2 College of Environment and Safety Engineering, Shenyang University of Chemical Technology, Shenyang, China

1

University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria

2

*1

fanliping@syuct.edu.cn; 294218603@qq.com

Abstract Microbial fuel cell is a promising technology for bioenergy. Some problems of microbial fuel cell such as stability, reliability and efficiency must be resolved before putting it into large number of applications. This paper compared and analyzed the operation data of the microbial fuel cell under some different conditions through experiments. Several conditions such as different electron acceptor, different catholyte, and different aeration status are tested. Experimental results showed that when the mixture of NaCl and K3[Fe(CN)6] is used as catholyte and aeration is added, the overall performance of the microbial fuel cell is good.

Keywords Microbial Fuel Cell; Wastewater Treatment; Power Generation; Catholyte

Introduction The high energy requirement of conventional sewage treatment systems are demanding for the alternative treatment technology which will require less energy for its efficient operation and recover useful energy to make this operation sustainable [1]. Application of microbial fuel cell (MFC) for wastewater treatment could be an attractive alternative to reduce the treatment cost and generate electricity [2,3].

Microbial fuel cell is a kind of biological reaction device which converts chemical energy to electrical energy during substrate oxidation with the help of microorganisms. MFC produces electricity while purifying wastewater and produce no pollution to the environment during the power generation process [4-6]. MFC is regarded as a new wastewater treatment process which has the advantages of high-efficiency, low energy consumption, clean and environmental protection [7].

Microbial fuel cell belongs to a complex biological electrochemical reaction system, many factors such as substrate concentration, environmental temperature, biological environment, load disturbance and some others will have significant impacts on its performance. Therefore, some problems of microbial fuel cell such as stability, reliability and efficiency must be resolved before putting it into large number of applications [8,9].

This paper studies on the purifying effect and power performance of MFC treating restaurant wastewater. A double chamber microbial fuel cell is used to run in different electrolyte, different electrode, and different aeration mode, so as to find suitable operating condition for MFC. Materials and Methods System Configuration A dual chamber microbial fuel cell is used in the study. It is mainly composed of the cathode chamber and anode chamber, proton exchange membrane, cathode and anode. The cathode chamber and anode compartments are in the same size as 500mL, and they are separated by a proton exchange membrane. Both the anode and cathode are made up of carbon cloth, and their surface area is 6cm Ă— 7cm, and copper wire is used to link to the carbon cloth to transmit current. An external resistor is connected as a load to form a complete closed circuit, and a parallel data

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acquisition card is connected to record the instant voltage generated by microbial fuel cell. Then, the data acquired by the data acquisition card is delivered to connect computer for memory and process. The complete experimental apparatus is shown in Fig. 1. e e e data card load

carbon felt

catholyte catholyte

exchange membrane

effluent effluent

carbon felt

anolyte

wastewater

FIG. 1 EXPERIMENTAL SYSTEM OF DUAL-CHAMBER MICROBIAL FUEL CELL

Material Preparation The proton exchange membrane used in the experiment is Nafion117. The exchange membrane also needs to be pretreated before use. First boil it in H2O2 solution with the volume fraction of 5% for 1 hour for removal of organic impurities in the film, then rinse it repeatedly with deionized water, and then put it into H2SO4 at the concentration of 1.0 mol/L to boil for 1 hour to make it transform into H+ type, and then rinse it several times with deionized water, then put it into deionized water to boil for 1 hour with deionized water, and finally rinse it with deionized water for 5-6 times to remove the residual H2SO4 in the membrane. Then put the processed membrane in deionized water to spare.

Anode and cathode materials used in the microbial fuel cell are carbon felt. Before use, carbon felt needs to preprocess so as to remove the surface impurities and metal ions. The carbon felt is connected to copper wire to form electrode. The joint between copper wire and carbon felt is sealed with silica gel to prevent electrochemical corrosion in the process of experiment.

The activated sludge used in anode compartment is collected from the bottom of the campus lake. After domestication and cultivation, the activated sludge is used in the anode compartment to provide microbial population for the microbial fuel cell.

Restaurant wastewater is used as the substrate of the microbial fuel cell. Restaurant wastewater is very different from other urban sewage. The main ingredient of canteen waste water is starch, dietary fiber, animal fats and other organic matter. It presents high nutrients, high fat content, easy fermented smell and other characteristics, which can be highly biodegradable. It has a high B/C value, and is suitable for using as the substrate of a microbial fuel cell.

In the experiments, two kinds of canteen wastewater are used as the substrate of the microbial fuel cell respectively to test the effect of substrate concentration on the performance of the microbial fuel cell. One is using the collected cafeteria wastewater as the anode liquid directly without any treatment. Another is using the diluted cafeteria wastewater as anode liquid. Mixing dilution water with the canteen wastewater by 2:1, the diluted cafeteria wastewater is prepared. The dilution water is composed of lake water and phosphate buffer solution (PBS) with the volume ratio of 1:1 configured.

Three kinds of catholyte are tested in these experiments. One is using NaCl with 0.4mol/L concentration as the catholyte, the second is using a kind of 0.2mol/L K3[Fe (CN) 6] as catholyte, the third is using the mixed liquid of 0.4mol/L NaCl and 0.2mol/L K3[Fe (CN) 6] as catholyte.

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Data Processing Method The power performance of microbial fuel cells is usually evaluated by the current density value. It can be described as I A=

I U = A RA

(1)

in which, IA is the current density (A/m2), U is the output voltage of the microbial fuel cell, A is the anode surface area, R is the external resistance and fixed at 100Ω in the experiments.

In order to analyze the sewage treatment effect of the microbial fuel cell, COD and BOD removal rate were tested.

The influent wastewater and effluent BOD5 are determined by the BOD5 tester OxiTop. The calculation formula is as follows

= BOD5 [ FM (Vv + Vp ) / Vp ] - FMVv / Vp Bw = FMVv / Vp

(2) (3)

where F is a constant 20, M is the digital display value of the instrument, Vv is the inoculated dilution water volume, Vp is the wastewater volume, and Bw is the blank value which equals to 1.

Because the cafeteria wastewater has a large biodegradability, its BOD value can be estimated as 80% of the value of COD. Then COD can be calculated as COD =

BOD5 80%

(4)

Analysis and Discussion Many factors, such as pH, temperature, anode and cathode materials, catholyte, substrate type, concentration, dissolved oxygen and so on, affect the purifying performance and power generation performance of a microbial fuel cell. In the condition of using canteen wastewater as substrate, effects of different concentrations substrate, different aeration mode and different catholyte on microbial fuel cell are tested and compared.

Impact of Different Catholyte A major role of Catholyte is to provide electron receptor. Reduction potential of the cathode electron acceptor affects the output voltage of the microbial fuel cell to a great extent. The commonly used cathode electron acceptors mainly include oxygen (O2), potassium ferricyanide (K3[Fe(CN)6] ) [10-12].

From the point of pursuing good power generation performance, electron acceptor must meet the following conditions: high redox potential, low cost, and the product itself gives no pollution to the environment. This paper contrasts two electron acceptors: oxygen and potassium ferricyanide. The power generation performance and the purifying performance of the microbial fuel are tested under the condition of different substrate concentration.

When the oxygen is used as the electron acceptor, the reactions occurring in the microbial fuel cell can be described as follows: Anode： microbe C6H12O6+6H2O  → 6CO2+24 H++24 e-

(5)

Cathode： microbe 6O2+24 H++24 e-  → 12 H2O

(6)

When oxygen is used as an electron acceptor, the product is water that releases no pollution to the environment.

The low solubility of oxygen in water limits its application in microbial fuel cells as electron acceptor. Potassium ferricyanide is widely used in microbial fuel cells, but its deficiency is its lower standard electrode potential. When 88

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K3 [Fe(CN)6] is used as the electron acceptor, the following chemical reaction process will appear in the cathode [Fe(CN)6 ]3− +e- → [Fe(CN)6 ]4 −

(7)

The current density curves corresponding to the two different cathode electron acceptor are shown in Fig. 2, in which the tested substrate is the diluted restaurant wastewater. The current density curves corresponding to the two different cathode electron acceptor with original restaurant wastewater are shown in Fig. 3.

10.0

10.0

9.5

2

O K3[ Fe(CN)6]

8.5

O2

9.0

K3[ Fe(CN)6]

8.5

Current density IA/(mA/m2)

2 Current density IA/( mA/m )

9.0

9.5

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0

0

20

40

60

80

100

120

0

20

40

60

80

100

120

Time t/h

Time t/h

FIG. 3 CURRENT DENSITY OF DIFFERENT ELECTRON ACCEPTOR WITH RAW WASTEWATER

FIG. 2 CURRENT DENSITY OF DIFFERENT ELECTRON ACCEPTOR WITH DILUTED WASTEWATER

It can be seen from Fig.2 and Fig.3, the current density of the MFC with O2 as the cathode electron acceptor is higher than that of K3[Fe(CN)6] as electron acceptor, whether the diluted wastewater or the original wastewater is used as the substrate of the microbial fuel cell. So when oxygen is used as electron acceptor, the power generating effect of the microbial fuel cell is better than that of K3[Fe(CN)6] as electron acceptor. This shows that the oxidation of oxygen is stronger than that of K3[Fe(CN)6]. The power generation performance of the microbial fuel has great relationship with the electron acceptor. The stronger the oxidation of the electron acceptor, the better the power generation performance of a microbial fuel cell is.

It can also be found by comparing Fig. 2 with Fig. 3 that the power generation current density of the microbial fuel cell with raw wastewater as substrate is lower instead than that with diluted wastewater as substrate.

The COD removal rates of the microbial fuel cell with the two different electron acceptors are shown in Table 1. It can be seen from the table that the wastewater treatment effect of the microbial fuel cell is seriously affected by the electron acceptor. When oxygen is used as the electron acceptor, the COD removal rate of the microbial fuel cell is higher than that of K3[Fe(CN)6] being used.

In sum, the cathode electron acceptor not only affects the power generation performance of the microbial fuel cell, but also influences the wastewater treatment efficiency of the microbial fuel cell. If oxygen is used as electron acceptor, the overall performance of the microbial fuel cell is better than that of potassium ferricyanide as electron acceptor. Therefore, it is necessary to choose a suitable cathode electron acceptor for microbial fuel cells to achieve the best comprehensive performance. The traditional MFC using dissolved oxygen as electron acceptor consumes a lot of energy in aeration, and its efficiency limited by dissolved oxygen concentration. When using K3[Fe(CN)6] as electron acceptor, [Fe(CN)6]3- turns to [Fe(CN)6]4- by accepting electrons. [Fe(CN)6]3- continues to be consumed, therefore the catholyte needs to change to provide adequate electron acceptor after the system operating for a period of time, which causes the system cannot work continuously and its application is limited.

TABLE 1: COD REMOVALWITH DIFFERENT ELECTRON ACCEPTOR

Substrate

Electron Acceptor K3[Fe(CN)6]

O2

Diluted wastewater

24.8%

31.5%

Raw wastewater

29.1%

46.6%

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Impact of Aeration Mode In the following experiments, NaCl and the mixture of NaCl with K3[Fe(CN)6]are used as the catholyte to provide electron acceptor for the MFC. Two aeration modes are tested: one state is natural aeration and the other is continuous aeration. At first, the microbial fuel cell operates in the condition of natural aeration; from the time of 60h, the aeration pump begins to work to aerate continuously to the cathode chamber. The experimental results are shown in Fig. 4 and Fig. 5. Fig.4 corresponds to the current density of MFC with diluted wastewater as fuel, and Fig.5 corresponds to the current density of MFC with raw wastewater.

Some phenomena can be seen from these two figures. First consider the situation of using diluted wastewater as the substrate of the MFC, as shown in Fig. 4. From the time of adding aeration at 60h, the current density of the microbial fuel cell increased to some extent, especially for the MFC using the mixture of NaCl and K3[Fe(CN)6] as catholyte. For the situation of using raw wastewater as substrate, as shown in Fig. 5, effect of aeration on the current density is more obvious. When aeration is added to the system, the current density of MFC using the mixture of NaCl and K3[Fe(CN)6] increased significantly, but the current density of MFC using NaCl presented no obvious change. That is to say, when we use the mixture of NaCl and K3[Fe(CN)6] as catholyte, with or without aeration has a great effect on the power performance of the MFC. When aeration is imposed to the system and raw wastewater is used as the substrate of the MFC, using the mixture of NaCl and K3[Fe(CN)6]as catholyte will receive good power generation effect.

14

9.5

NaCl NaCl+K3[ Fe(CN)6]

Current density IA/(mA/m2)

9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5

Current density IA/( mA/m2)

10.0

NaCl NaCl+ K3[ Fe(CN)6]

12

10

8

6

5.0 4

4.5 4.0 0

20

40

60

80

100

120

Time t/h FIG. 4 CURRENT DENSITY OF DIFFERENT DISSOLVED OXYGEN WITH DILUTED WASTEWATER

0

20

40

60

80

100

120

140

160

Time t/h FIG. 5 CURRENT DENSITY OF DIFFERENT DISSOLVED OXYGEN WITH RAW WASTEWATER

Conclusions The cathode electron acceptor has an impact on the performance of microbial fuel cells, the stronger the electron acceptor oxidizing, the better the power generation performance and sewage purification effect of the microbial fuel cell to produce. Also the performance of a microbial fuel cell is influenced by the anode chamber substrate.

Suitable concentration substrate can make microbial fuel cell to achieve the best effect. If the COD concentration of the substrate is too high, the microbial strains cannot digest well and thus influence the activity of microorganism strains, which affects the performance of MFC on wastewater treatment and power generation.

Dissolved oxygen has a great influence on the performance of microbial fuel cell. Sufficient dissolved oxygen concentration is helpful to generate power and purify wastewater.

Studies have shown that many factors affect the performance of a microbial fuel cell. In order to make microbial fuel cells to achieve high efficiency, a number of factors must be taken into account for selecting the appropriate operating conditions. Further study will focus on finding suitable operation condition and further optimization of operation parameters for microbial fuel cells to improve efficiency to make the microbial fuel cell achieve high efficiency and realizing industrialization.

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ACKNOWLEDGMENT

This work was supported by the Science and Technology special fund of Shenyang City under Grant F14-207-6-00, and the Science and Technology Research Project of Liaoning Education Department under Grant L2012140, the National Natural Science Foundation of China under Grant 61143007.

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Li-ping Fan, University professor of Shenyang University of Chemical Technology, Ph.D. The author’s major is Control Engineering. More than 50 papers were received by EI or SCI and 3 patents had been published.

Dan-dan Xu, studying for master degree in Shenyang University of Chemical Technology. Her major is Environmental Science and Engineering.

Yu-jiao Zheng, studying for master degree in Shenyang University of Chemical Technology. Her major is Environmental Science and Engineering.

Chong Li, master degree in engineering, studying for PhDs in University of Chemical Technology and Metallurgy-Sofia. The author’s major is Control Engineering.

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