Research on the Microbial Fuel Cell Stimulated by DC Electric Field

Chen Tao, Liu Qing-yu※, Li Jin-yang, Hu Yan-qing College of Engineering, Shenyang Agricultural University

Shenyang, China 13940089060@126.com

Abstract—To study the properties of MFC stimulated by DC electric field. Seven identical two-chamber MFCs were constructed in this study. The MFCs were stimulated by DC electric field of ±5V, ±3V, ±1V and 0V respectively after they were started at 12h, 48h and 84h. And every stimulation under 30 minutes. The results showed that the start-up time was shortened by lower DC electric field(±1V) and the electrochemical properties were improved. The start-up time was delayed by higher positive DC electric field(+3V, +5V) and the electrochemical properties were damaged. The start-up time was shortened by higher negative DC electric field(-3V, -5V), but the electrochemical properties were damaged. It is proved that lower external DC electric field can be used to short the start-up time and improve the properties of MFC.

Keywords-DC electric field; stimulate; start-up time; electrochemical properties; MFC

1. Introduction Microbial fuel cell (MFC) is a device which transform

chemical energy into electrical energy via microbial organisms. Recent studies showed that a mixed bacterial inoculum used in anodic chamber can get better power. When mixed bacteria was used as inoculum, it is helpful to enrich electricigens and hand up the biofilm when a lower positive potential was poised on anode. A lower negative potential was poised on anode can accelerate the growth of electricigens and enhance their activity during start-up time[1]. The electricigens possess electronics when they are growth at anodic electrode. So, if we add a certain direction of DC electric field on MFC’s two electrodes, the electricigens’ physiological properties will be affected[2]. Thus affect the properties of MFC.

In this study, mixed bacteria were used as inoculum in anodic chamber. Seven identical two-chamber MFCs were constructed which were stimulated by different DC electric field. The properties of microbial fuel cell affected by DC electric field were studied.

2. Materials and methods

2.1 device and start-up Each cell was built by bolting two identical square glass

compartments together. Both anodic chamber and cathodic chamber’s dimensions are 6 cm×6 cm×6 cm. Between the two chambers, the cation exchange membrane(CEM) was used as separator. Its effective area is 12.56 cm2. In order to remove its

surface impurities, CEM must be boiled 30min at 80 by 30% H2O2 and 0.5 mol/L H2SO4 respectively before used. Both anodic and cathodic electrode were made of graphite plate, which dimension is 20 mm×60 mm×5 mm and total area is 32 cm2. It must be soaked in liquid of 1 mol•L-1 HCl and 1 mol•L-1 NaOH respectively more than 10 hours before used. Titanium wire was used to tie to electrode. Reference electrode (vs AgCl/Ag)was straight into the anodic chamber. Reactor schematic diagram is shown in Figure 1.

H +

e - e -Rex

Rec or de r Comput e r

1. anodic electrode 2. cathodic electrode

3. reference electrode

4. rubber plug 5. Anode feed and discharge tube

6. Cathode feed and discharge tube 7. CEM 8. Bolt Figure 1. Two-chamber MFC reactor schematic diagram

Cell voltages across the external resistance and anodic potential were recorded by a paperless recorder (12 channels, Hangzhou Meikong company) with a 60 seconds interval. AgCl/Ag electrode(0.195V vs. Standard hydrogen electrode, SHE) was used as reference electrode when measure anode potential.

Anode inoculum was taken from sludge of Shenyang Mantang River sewage treatment plant and cultivated one month. Simulate wastewater was used as anolyte, which components include NaHCO3 3.13 g/L, NH4Cl 0.31 g/L, NaH2PO4 4.97 g/L, Na2HPO4 2.75 g/L, (NH4)SO4 0.56 g/L, KCl 0.13 g/L, glucose 2 g/L. Besides, a variety of trace elements are necessary for micro-organisms[3]. 100 mg/L of KMnO4 a.q was used as cathodic solution. Both anode and cathode were operated in fed-batch mode.

※Corresponding author: Liu Qing-yu, Professor, PhD supervisor, Mainly research on renewable energy. Email: 13940089060@126.com

978-1-4577-0547-2/12/$31.00 ©2012 IEEE

2.2 Test methods Seven identical two-chamber MFCs were constructed in

this study. The external DC electric field was generated by a steady voltage and current power(30V 5A, Zhaoxin company). It is named positive electric field when the DC power’s positive electrode is connected to the anode and negative electrode is connected to the cathode. Otherwise, it is named negative electric field. External electric field were seted at ±1V, ±3V, ±5V respectively and 0V as control. Stimulation test was started after MFCs were started at 12h, 48h and 84h and every stimulation under 30 minutes. The MFCs were allowed to remain at open-circuit when stimulated[4].

2.3 projects and methods of measurement All MFCs was operated under a fixed load(R) of 1000Ω

during all the experimental period(except stimulation time). Anodic potential(Ua) and cell voltages across the external resistance(U) were recorded and stored by paperless recorder automaticly. Current density(jv) was calculated according to jV(mA/m3)=I/V=U/RV; and power density(Pv) was calculated according to PV(mW/m3)=jV=U2/RV; where, V(m3) represented the total volume of anode and cathode chamber; R(Ω) represented the external load.

Internal resistance(Rint) was measured according to polariyzation curves. Polariyzation curves were obtained varying external resistances from 100000 to 50Ω, with every resistor tested for 15 minutes. We can draw the curve of U which was changed with I. The number of the curve’s slope was the MFC’s internal resistance(Rint) [5].

3. Results and analysis

3.1 Effect on the MFC’s start-up time by different electric field 1) The impact on the current density

Current density(jV) can reflect the power output of MFC during start-up and operating time. Figure 2 showed MFC’s current density curve stimulated by different DC electric field.

0

50

100

150

200

250

300

350

0 50 100 150 200Time/h

Curre

nt density/mA

·m-3

0V +1V +3V +5V -1V -3V -5V

Figure 2. Current density curve stimulated by different DC electric field

Fig. 2 shows the current density’s trend in MFC’s start-up time are similar with the growth of micro-organisms. Which is inseparable with the growth of electricigens on anodic electrode. Electricigens growing up on anodic electrode as start-up time goes on. The biofilm are stability after MFCs were started 80 hours, and the current density tends to be stable. In addition, stimulated by DC electric field of ±1V, -3V can not only short the start-up time(all were 10 hours faster

than control) but also increase current density when MFCs were stable(+1V increased 15～17mW/m3, -1V increased 16～18 mW/m3, -3V only increased 4～8 mW/m3). Stimulated by -5V DC electric field can short start-up time, was 20 hours faster than control, but its current density was 18～24 mW/m3 lower than control after stable. Stimulated by +3V and +5V DC electric field have a same start-up time with control but their current density were lower than control apparently, decreased 46～52 mW/m3 and 127～131 mW/m3 respectively.

2) The impact on the anodic potential The viariation of anodic potential reflect biofilm that

electricigens growth on anodic electrode. The growth of anaerobic microorganisms require a lower potential. External DC electric field can change the environmental potential that electricigens need. What is more, the electrochemical properties of MFC were changed[6]. Figure 3 showed MFC’s anodic potential curve of MFCs stimulated by different DC electric field.

-500

-400

-300

-200

-100

0

100

200

10 20 30 40 50 60 70 80 90 100 110 120Time/h

Anodic potential/mV

0V +1V +3V +5V -1V -3V -5V

Figure 3. Anode potential curve stimulated by different DC electric field

As shown in Fig. 3, the viariation of anodic potential has a same trend with current density. Anodic potential decrease as start-up time goes on and keep steady after 80 hours. Anode potential of MFCs which were stimulated by DC electric field of ±1V decrease faster and get stable earlier than control. Anodic potential of MFCs stimulated by -3V and -5V DC electric field decrease much more faster during start-up time but higher than control when they are stable. Anodic potential of MFCs stimulated by +3V and +5V decrease slower during start-up time and higher than control apparently when MFCs were stable.

In summary, lower DC electric field stimulate on MFCs can increase current density and short start-up time. Higher negative DC electric field can short start-up time, but cann’t increase current density. Higher positive DC electric field not only delay start-up time but also decrease current density.

3.2 Effect on the electrochemical properties by different electric field

1) The impact on the internal resistance and power output Internal resistance and power density are two important

indicators to evaluate properties of MFC. In MFC system, internal resistance is related to electricigens that grow up on anodic electrode. The more electricigens grow up on anodic electrode, the less internal resistance will be. Power density reflects the ability of power that MFC can provide. To a certain MFC which has a fixed volume, power density is only related

with internal resistance and open circuit voltage. The less of internal resistance and the higher of open circuit voltage, the higher power density will be. The maximum power density will be achieved when the external resistance equal to the internal resistance[7]. Fig. 4 and Fig. 5 are polarization curve and power density curve respectively after stable operation of MFCs.

0

100

200

300

400

500

600700

800

900

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Current density/mA·m-3

Cell vol

tages/mV

0V +1V +3V +5V -1V -3V -5V

Figure 4. Variation of the cell voltage with current density after stable

operation

0

50

100

150

200

250

300

350

0 200 400 600 800 1000 1200 1400 1600Current density/mA·m-3

Power

densit

y/mW·

m-3

0V +1V +3V +5V -1V -3V -5V

Figure 5. Variation of the output power density with current density after

stable operation

Linear fit the linear region of polarization curve. The slope represent MFC’s internal resistance Rin. Quadratic fit power density curve. The peak of quadratic curve is the maximum power density Pmax. All MFCs’ Rin and Pmax were shown in table 1.

TABLE1 MFCS’ RESISTANCE RIN AND THE MAXIMUM OUTPUT POWER DENSITY PMAX AFTER STABLE OPERATION

Electric field strength

+5V +3V +1V 0V -1V -3V -5V

inR /Ω 4932 4039 1058 1652 1018 2094 3343

maxP /mW·m-3

8.0 20.1 255.2 105.8 281.6 110.9 31.8

Table 1 shows that there was an important impact on internal resistance and power density after stable of MFCs stimulated by external DC electric field.The internal resistance of MFCs which were stimulated by +1V and -1V DC electric field were decreased 594Ω and 634Ω respectively. The internal resistance of MFCs which were stimulated by -3V, -5V, +3V and +5V DC electric field were increased 442Ω, 1691Ω, 2387Ω and 3280Ω respectively. The maximum power density of MFCs which were stimulated by -3V, +1V and -1V DC electric field were increased 5.9mW/m3, 149.4

mW/m3 and 175.8 mW/m3 respectively. The maximum power density of MFCs which were stimulated by -5V, +3V and +5V DC electric field were decreased 74mW/m3, 85.7 mW/m3 and 97.8 mW/m3 respectively.

2) The impact on the open circuit voltage Open circuit voltage is MFC’s voltage when the external

load tend to infinite, which represent the maximum voltage of MFC’s output, is one of the most important property parameters of MFC. To further verify the effect of MFCs which were stimulated by DC electric field, open circuit voltages were still measured after MFCs operated stably. Fig 6 is variation curve of open circuit voltage. It shows that open circuit voltage of MFCs which stimulated by +1V, -1V, -3V and 0V have been increased as operate time goes on and reached a similar level finally. Despite the open circuit voltage of MFC stimulated by -5V has a substantial increase at later period, but still lower than the first four groups. However, the open circuit voltage of MFCs stimulated by +3V and +5V always keep at a low level.

200

300

400

500

600

700

800

900

1000

100 150 200 250 300 350 400 450 500

Time/h

Open c

irc

uit

vol

tage

/mV 0V +1V +3V +5V -1V -3V -5V

Figure 6. MFCs’ open circuit voltage curve

In summary, MFCs were stimulated by lower DC electric field, especially lower negative DC electric field can decrease MFC’s internal resistance and increase MFC’s power density effectively. MFCs were stimulated by higher DC electric field, especially higher positive DC electric field can increase MFC’s internal resistance and decrease MFC’s power density effectively. This may be because lower electric stimulation activate electricigens in the anodic chamber at start-up time. And negative DC electric field make the anodic potential remained at a low level which is suitable for electricigens’ growth. Otherwise, higher electric stimulation is disadvantage to the growth of electricigens on anodic electrode, what is more, it may cause a permanent damage to the anodic biofilm.

4. Conclusion 1) Both lower positive and negative DC electric field(±1V)

stimulate can short start-up time and increase current output. Higher positive DC electric field(+3V, +5V) stimulate decrease current output and can not short start-up time. Higher negative DC electric field(-3V, -5V) stimulate can short start-up time but decrease current output.

2) Both lower positive and negative DC electric field(±1V) stimulate can decrease MFC’s internal resistace and increase power density and open circuit voltage. And negative DC electric field(-1V) stimulate is more conductive to improve MFC’s properties. Both higher positive and negative DC

electric field stimulation are harmful to MFC’s properties, especially by higher positive DC electric field.

REFERENCES [1] Aelterman, P. , Freguia, S. , Keller, J. , Verstraete, W. , Rabaey, K. The

anode potential regulates bacterial activity in microbial fuel cells. Appl Microbiol Biotechnol, 2008, 78: 409-418.

[2] Schkov ,V. , Velizarov, S. , Agathos, S.N. , Lukova, V. Bacterrial denitrification of waste water stimulated by constant electric field. Biochemical Engineering Journal, 2004, 17: 141-145.

[3] D.R. Lovley, E.J.P. Phillips. Novel Mode of Microbial Energy Metabolism:Organic Carbon Oxidation Coupled to Dissmilatory Reduction of Iron or Manganese. Applied and Environmental Microbiology, 1988, 54(6): 1472-1480.

[4] Min Sun, Lei Zhang, Guo-Ping Sheng, Xian-Wei Liu, Chang-Rong Xia, Zhe-Xuan Mu, et al. The external electric field manipulates anode biofilm catalytic activity in microbial fuel cells. Journal of Biotechnology, 2008, 136S: S402-S459.

[5] Liang Peng, Fan Ming-zhi, Cao Xiao-xin, Huang Xia, Wang Cheng. Composition and Measurement of the Apparent Internal Resistance in Microbial Fuel Cell. Environmental Sience, 2007, 28(8): 1894-1898.

[6] Finkelstein D.A. , Tender L.M. , Zeikus J.G. Effect of electrode potential on electrode-reducing microbiota. Environmental Science ＆ Technology, 2006, 40: 6990-6995.

[7] Bruce E. Logan. Microbial Fuel Cells. New Jersey: John Wiley＆ Sons; 2008.