- Email: [email protected]
Anode modification by electrochemical oxidation: A new practical method to improve the performance of microbial fuel cells
Biochemical Engineering Journal 60 (2012) 151–155
Contents lists available at SciVerse ScienceDirect
Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej
Short communication
Anode modification by electrochemical oxidation: A new practical method to improve the performance of microbial fuel cells Minghua Zhou ∗ , Meiling Chi, Hongyu Wang, Tao Jin Key Laboratory of Pollution Processes and Environmental Criteria of Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 30 January 2011 Received in revised form 10 October 2011 Accepted 31 October 2011 Available online 7 November 2011 Keywords: Microbial fuel cell Electrochemical oxidation Carbon mesh Power generation Cyclic voltammetry Electrode modification
a b s t r a c t Electrochemical oxidation as a convenient and effective method was established for anode modification to improve the performance of microbial fuel cells (MFCs). The anode modification was realized by one-step electrochemical treatment in one of the three electrolytes (nitric acid, ammonium nitrate, ammonium persulfate) at ambient temperature. The performances of MFCs before and after anode modification were compared, confirming that all these anode modifications posed positive effects. The maximum power density of the MFC with the anode modified by nitric acid was 792 mW/m2 , which was 43% larger than the unmodified control (552 mW/m2 ). Furthermore, the Coulombic efficiency (CE) significantly promoted about 71% from 14% (the unmodified MFC) to 24%. It revealed that the electrochemical oxidation resulted in the change of the anode properties, such as surface morphology, internal resistance and anode potential, and thus benefited to the microbial attachment and electron transfer on the anode surface, which might contribute to the performance improvement of the MFCs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) are novel processes that use bacteria as the catalysts to oxidize organic (inorganic) matter and directly convert chemical energy into electricity [1,2]. Particularly, it has been an ever-growing interest in using MFCs for wastewater treatment, and up to now, a variety of wastewaters have been attempted [1–3], such as domestic wastewater [4], starch processing wastewater [5] and chemical wastewater [6]. However, the bottlenecks restricting its application are still the poor power density and the low treatment efficiency. Anode material has been regarded as an important factor that influences energy conversion because it links microbiology and electrochemistry although the performance of MFCs depends on a complex system of parameters [3]. Presently, carbon-based materials are the most widely used anode materials due to their good electrical conductivity, strong biocompatibility and low cost [7,8]. In order to improve the performance of MFCs, various chemical and physical approaches for anode modification have been employed. Baked at 1100 ◦ C for 12 h in a kiln keeping anaerobic with N2 gas, Park and Zeikus developed the graphite anodes modified by metal
∗ Corresponding author. Tel.: +86 22 23507800; fax: +86 22 23507800. E-mail addresses: [email protected], [email protected] (M. Zhou). 1369-703X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2011.10.014
(Fe3+ or Mn4+ ), on which an enhanced electrical energy production was observed comparing with the conventional graphite anodes [9]. Cheng used ammonia gas to modify carbon cloth and received an excellent power output, but the electrode preparation required a thermogravimetric analyzer (TGA) and conducted in a furnace up to 700 ◦ C with a total treatment time of 180 min [10]. Tungsten carbide was explored as a promising anodic electrocatalyst for MFCs, however, the anode synthesis was performed in a carbon monoxide stream for 8–10 h at the temperature over 750 ◦ C [11]. In summary, although considerable enhancements for the performance of MFCs were observed, these modification techniques either needed complicated apparatus or multiple steps, and/or high temperature conditions and long treatment time, which would obviously increase the MFCs costs and hinder their application [3]. In the present work, a simple, quick and cost-effective electrochemical approach was attempted for anode modification, operating at ambient temperature. Electrochemical oxidation technology has been used as an effective method to improve the mechanical strength and adhesion of carbon fiber [12,13] as well as the electron transfer from bacteria to electrodes [14]. In this paper, carbon mesh anodes that were modified by three different electrolytes (nitric acid, ammonium nitrate, ammonium persulfate) were investigated, and the performance of MFCs was compared before and after anode modification. It proved that electrochemical modification could alter the anode surface characteristics and
Voltage(mV)
500
CM-C CM-N CM-A CM-P
400 300
CM-C-V CM-N-V CM-A-V CM-P-V
900
CM-C-P CM-N-P CM-A-P CM-P-P
2
900
600
Voltage (mV)
A
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155
Power Density (mW/m )
152
600
600
300
300
200 100 0 0
0
20
40
60
80
100
120
140
160
1
2
3
4
0
Current Density (mA/m2)
180
Time (h)
B
Fig. 2. Polarization curves and power density curves as a function of current density with four MFCs (error bars SD based on the voltages in the experiments run in triplicate).
30 Coulombic efficiency
2.2. MFC construction and operation
CE (%)
20
10
0
CM-C
CM-N
CM-A
CM-P
MFC Fig. 1. Cell voltage (A) and Coulombic efficiency (B) of four MFCs conditions: substrate: glucose 1 g/L, external load 1000 , initial pH 7.
thus increase the power generation and chemical oxygen demand (COD) removal efficiency, providing a convenient and promising method to promote the development of MFCs. 2. Experimental 2.1. Anode modification Carbon mesh (CM; Jilin Carbon Plant, China) was used as anode material, which was firstly cleaned by acetone overnight and then rinsed drastically in ultrapure water before use (this unmodified anode we called “CM-C”). Electrochemical oxidation was operated in an undivided electrolytic cell at ambient temperature, using the CM-C (effective area which was the projected area, 4 cm2 ) as the anode and graphite rod as the cathode with a 2.5 cm electrode gap. The electrolysis current density was kept at 1.25 mA/cm2 and electrolysis time was 30 min. Three types of modified anodes were obtained according to different electrolytes: nitric acid (65–68%, CM-N), ammonium nitrate (112 g/L, CM-A), and ammonium persulfate (160 g/L, CM-P). All modified anodes were washed three times with distilled water before being used in MFCs. All of the MFCs in this study had run for more than three months. Under the same operating conditions, the performance on voltage, pH variation, and COD removal showed good repeatability. And the experiment of polarization curves and Coulombic efficiency (CE) analysis were performed in triplicate.
Single chamber air-cathode MFCs with a volume of 14 mL and an electrode gap of 2 cm were constructed as described [15]. Four MFCs with different pretreatment anodes (CM-C, CM-N, CM-A, and CMP) and the same cathode of carbon cloth containing 0.35 mg/m2 Pt catalyst were started up. The MFCs were inoculated with anaerobic sludge (TEDA Sewage Treatment Plant, Tianjin) and a 50 mM phosphate buffered nutrient solution (PBS: NH4 Cl 0.31 g/L, KCl 0.13 g/L, NaH2 PO4 ·2H2 O 3.32 g/L, Na2 HPO4 ·12H2 O 10.36 g/L; trace minerals 12.5 mL/L; vitamins 5 mL/L) containing 1.0 g/L glucose as substrate. This solution was switched to a feed solution containing glucose (1.0 g/L) and PBS (50 mM) until the similar output voltage produced over two consecutive cycles. The feed solution was replaced when the voltage dropped below 50 mV. The external resistance was fixed at 1000 and the pH was adjusted to 6.9–7.0. All tests were operated in a 30 ± 0.5 ◦ C temperature-controlled biochemical incubator. 2.3. Analytical techniques and calculations Cell voltage was measured every minute with a data acquisition system (PISO-813, ICP DAS Co., Ltd.). Polarization curves were obtained by varying external resistances from 1000 to 50 when the cycles were stable. A reference electrode (KCl saturated calomel electrode, SCE) was used to measure the anode potential. Power density and current density were calculated based on the effective area of anode according to Ref. [7]. The Coulombic efficiency was obtained as follows: n
CE =
Ui ti
i=1
Rex FbCODV
×M
where Ui is the output voltage (V) at time ti , F is Faraday’s constant (96,485 C/mol), b is the number of electrons exchanged per mole of oxygen (=4), COD is the removal of COD (mg/L), M is the molecular weight of oxygen (=32 g/mol), and V is the wastewater volume (L). COD were measured with the closed reflux spectrophotometric method on a commercial COD detector (HACH, DRB 200, DR/890 Colorimeter, USA). Cyclic voltammogram (CV) was carried out using an electrochemical station (CHI600D, Shanghai, Chenghua) in a standard three-electrode system at a scan rate of 50 mV/s. The anode was the working electrode, the counter electrode was a platinum sheet and
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155
153
Fig. 3. SEM (A) and cyclic voltammograms (B) of the unmodified and modified anodes.
the reference electrode was a SCE. All experiments were performed in the 50 mM PBS at room temperature. The surface morphologies of the anodes before incubation were examined by a field emission scanning electron microscope (FESEM, Hitachi, S-4700). 3. Results and discussions 3.1. MFC performance The four MFCs were all reached stable maximum voltages after inoculation for about 120 h. Fig. 1 shows the voltage output (A) and Coulombic efficiency (B) of these four MFCs. Clearly, the anode modification affected the peak voltage and its sustainable period.
Among these MFCs, the CM-N achieved the highest voltage of 540 mV, which was much larger than that on CM-C (508 mV). More importantly, the sustainable periods near the maximum voltage for all modified MFCs were considerably improved. For example, the sustainable period of CM-N MFC increased from 9 h of the unmodified MFC to 16.5 h, which led to the Coulombic efficiency significantly increased by 71% (from 14% to 24%). Although the cell voltages of CM-P MFC (496 mV) and CM-A MFC (485 mV) were less than that of the control, the sustainable periods were both much longer (Fig. 1(A)), which resulted in a slight increase on their CE (Fig. 1(B)). Fig. 2 demonstrates the polarization curves and power density curves as a function of current density of four MFCs, which are
154
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155
obtained by the steady discharging method [16]. It was noteworthy that after anode modification the internal resistance of CM-N and CM-A MFC was reduced to 122 and 125 , which was 23% and 21% lower than the unmodified one (158 ), respectively. Besides, the internal resistance of CM-P MFC was 160 , which was similar with the unmodified one. It has been demonstrated that the decrease in internal resistance is an important factor for improving the performance of MFCs, which can also be confirmed by the power density in the present work. The maximum power densities after anode modification were all observed larger than that of the control one (CM-C; 552 mW/m2 ). The power density of MFC after nitric acid treatment increased by 43% to 792 mW/m2 , the one after ammonium nitrate treatment increased by 33% to 736 mW/m2 , and the one after ammonium persulfate treatment was 567 mW/m2 . Compared with other anode modification methods [9–11], this electrochemical approach is much simpler and quicker, operating at ambient temperature with the cheap apparatus of power supply (direct current). Moreover, this approach is also efficient for the promotion of MFCs performance with a power density increased by 43% to 792 mW/m2 , as disclosed above. Kim et al. [17] modified the anode with ferric oxide by a chemical vapor deposition (CVD) technique, using the professional and expensive CVD equipment, but only increased the power density to 30 mW/m2 . Scott et al. [18] investigated a range of carbon and carbon modified anode by deposition or polymerization or programmed heat, and the best anodes were made from carbon modified with quinone/quinoid groups, obtaining a maximum power density of 30 mW/cm2 . However, its preparation needed a complex technical procedure such as programmed heat up to 900 ◦ C. In contrast, our method only required 30 min treatment at room temperature and the power output was much larger. Therefore, simple operation, cheap modification equipments, short operation time and high modification performance are all beneficial to the application in engineering. Other characteristics of these four MFCs were also explored. Though the anode modification played an insignificant role on open-circuit voltage (OCV), it did affect the anode potentials, which all decreased from −157 mV of the unmodified one to around −220 mV. It supposed that the anode potential should be as low as possible to improve the performance of MFCs since it determined the interaction between bacteria and the electrode substrate [19]. And it was established that −200 mV might be the best anode potential to sustain enhanced current and maximum power generation [20]. The anode potential change in the present work suggested the anode modification should be beneficial to improve the performance of the MFCs, which was also verified by the power generation and the COD removal efficiency. The volumetric power density (Pv ) of CM-N MFC increased from the unmodified CM-C MFC of 28–39 W/m3 , and the Pv of CM-A MFC was also up to 36 W/m3 . The COD removal efficiencies after modification were all higher than the unmodified one (71.0%) and the CM-N MFC achieved the best result of 81.7%.
3.2. Anode characterization Fig. 3(A) illustrates the SEM of these carbon mesh anodes. Obviously, compared with the relatively smooth surface of the unmodified anode, much rougher surface and deeper cracks on the modified anodes were observed, especially on the CM-N and CM-A anode. These surface variations might have enhanced cell attachment. Fig. 3(B) shows the CV of four anodes without biofilm. Although no obvious oxidation/reduction peaks appeared, much higher redox currents on the modified anodes were observed. This fact indicated that the modified anodes displayed much better electrontransfer properties than the unmodified one. This substantial
alteration in electrochemical properties may contribute to the performance improvement of the MFCs. 4. Conclusions This study revealed that the electrochemical oxidation of anode was an effective and practical method to improve the performance of MFCs both on power generation and wastewater treatment efficiency, which indicated this approach was promising for application. The following conclusions could be drawn: (1) The modification in three electrolytes all posed positive performance. The anode modified by nitric acid achieved the best power production, which was increased by 43% from 552 to 792 mW/m2 . And the one modified by ammonium nitrate increased power by 33% to 736 mW/m2 . (2) The electrochemical oxidation of anode led to the change of the electrochemical properties of the anodes. The internal resistance of CM-N and CM-A MFC was found decreased to 122 and 125 , which was much lower than the unmodified control (158 ). And the anode potentials after modification were all decreased from the original −157 mV to about −220 mV. (3) Confirmed by SEM and CV, the increasing surface area and current response might be responsible for the improvement of the performance of MFCs. Acknowledgements This research was financially supported by NCET (08-0296), SRF for ROCS, SEM (2009-1001), China National Water Project (No. 2008ZX07314-001), and the Natural Science Foundation of Tianjin (No. 09JCYBJC08000). References ˝ [1] B.E. Logan, B. Hamelers, R. Rozendal, U. Schroder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5292. [2] Z. Du, H. Li, T. Gu, A state of the art review on microbial fuel cells: a promising technology for wastewater treatment and bioenergy, Biotechnol. Adv. 25 (2007) 464–482. [3] M.H. Zhou, M.L. Chi, J.M. Luo, H.H. He, T. Jin, An overview of electrode materials in microbial fuel cells, J. Power Sources 196 (2011) 4427–4435. [4] B. Min, B.E. Logan, Continuous electricity generation from domestic wastewater and organic substrates in a flat plate microbial fuel cell, Environ. Sci. Technol. 38 (2004) 5809–5814. [5] N. Lu, S.G. Zhou, L. Zhuang, J.T. Zhang, J.R. Ni, Electricity generation from starch processing wastewater using microbial fuel cell technology, Biochem. Eng. J. 43 (2009) 246–251. [6] S.V. Mohan, G. Mohanakrishna, B.P. Reddy, R. Saravanan, P.N. Sarma, Bioelectricity generation from chemical wastewater treatment in mediatorless (anode) microbial fuel cell (MFC) using selectively enriched hydrogen producing mixed culture under acidophilic microenvironment, Biochem. Eng. J. 39 (2008) 121–130. [7] B.E. Logan, Anode materials, in: Microbial Fuel Cells, John Wiley & Sons, New Jersey, 2008, pp. 62–68. [8] K. Watanabe, Recent developments in microbial fuel cell technologies for sustainable bioenergy, J. Biosci. Bioeng. 6 (2008) 528–536. [9] D.H. Park, J.G. Zeikus, Improved fuel cell and electrode designs for producing electricity from microbial degradation, Biotechnol. Bioeng. 81 (2003) 348–355. [10] S.A. Cheng, B.E. Logan, Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells, Electrochem. Commun. 9 (2007) 492–496. ˝ F. Scholz, Evaluation [11] M. Rosenbaum, F. Zhao, M. Quaas, H. Wulff, U. Schroder, of catalytic properties of tungsten carbide for the anode of microbial fuel cells, Appl. Catal. B: Environ. 74 (2007) 261–269. [12] T. Momma, X.J. Liu, T. Osaka, Y. Ushio, Y. Sawada, Electrochemical modification of active carbon fiber electrode and its application to double-layer capacitor, J. Power Sources 60 (1996) 249–253. [13] A. Fukunaga, S. Ueda, Anodic surface oxidation for pitch-based carbon fibers and the interfacial bond strengths in epoxy matrices, Compos. Sci. Technol. 60 (2000) 249–254. [14] X.H. Tang, K. Guo, H.R. Li, Z.W. Du, J.L. Tian, Electrochemical treatment of graphite to enhance electron transfer from bacteria to electrodes, Bioresour. Technol. 102 (2011) 3558–3560.
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155 [15] S.A. Cheng, H. Liu, B.E. Logan, Increased power generation in a continuous flow MFC with advective flow through the porous anode and reduced electrode spacing, Environ. Sci. Technol. 40 (2006) 2426–2432. [16] P. Liang, M.Z. Fan, X.X. Cao, X. Huang, C. Wang, Composition and measurement of the apparent internal resistance in microbial fuel cell, Environ. Sci. 28 (2007) 4981–8981 (In Chinese). [17] J.R. Kim, B. Min, B.E. Logan, Evaluation of procedures to acclimate a microbial fuel cell for electricity production, Appl. Microbiol. Biotechnol. 68 (2005) 23–30.
155
[18] K. Scott, G.A. Rimbu, K.P. Katuri, K.K. Prasad, I.M. Head, Application of modified carbon anodes in microbial fuel cells, Process Safe. Environ. 85 (2007) 481–488. [19] A. Rinaldi, B. Mecheri, V. Garavaglia, S. Licoccia, P.D. Nardo, E. Traversa, Engineering materials and biology to boost performance of microbial fuel cells: a critical review, Energy Environ. Sci. 1 (2008) 417–429. [20] P. Aelterman, S. Freguia, J. Keller, W. Verstraete, K. Rabaey, The anode potential regulates bacterial activity in microbial fuel cells, Appl. Microbiol. Biotechnol. 78 (2008) 409–418.
Contents lists available at SciVerse ScienceDirect
Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej
Short communication
Anode modification by electrochemical oxidation: A new practical method to improve the performance of microbial fuel cells Minghua Zhou ∗ , Meiling Chi, Hongyu Wang, Tao Jin Key Laboratory of Pollution Processes and Environmental Criteria of Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 30 January 2011 Received in revised form 10 October 2011 Accepted 31 October 2011 Available online 7 November 2011 Keywords: Microbial fuel cell Electrochemical oxidation Carbon mesh Power generation Cyclic voltammetry Electrode modification
a b s t r a c t Electrochemical oxidation as a convenient and effective method was established for anode modification to improve the performance of microbial fuel cells (MFCs). The anode modification was realized by one-step electrochemical treatment in one of the three electrolytes (nitric acid, ammonium nitrate, ammonium persulfate) at ambient temperature. The performances of MFCs before and after anode modification were compared, confirming that all these anode modifications posed positive effects. The maximum power density of the MFC with the anode modified by nitric acid was 792 mW/m2 , which was 43% larger than the unmodified control (552 mW/m2 ). Furthermore, the Coulombic efficiency (CE) significantly promoted about 71% from 14% (the unmodified MFC) to 24%. It revealed that the electrochemical oxidation resulted in the change of the anode properties, such as surface morphology, internal resistance and anode potential, and thus benefited to the microbial attachment and electron transfer on the anode surface, which might contribute to the performance improvement of the MFCs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) are novel processes that use bacteria as the catalysts to oxidize organic (inorganic) matter and directly convert chemical energy into electricity [1,2]. Particularly, it has been an ever-growing interest in using MFCs for wastewater treatment, and up to now, a variety of wastewaters have been attempted [1–3], such as domestic wastewater [4], starch processing wastewater [5] and chemical wastewater [6]. However, the bottlenecks restricting its application are still the poor power density and the low treatment efficiency. Anode material has been regarded as an important factor that influences energy conversion because it links microbiology and electrochemistry although the performance of MFCs depends on a complex system of parameters [3]. Presently, carbon-based materials are the most widely used anode materials due to their good electrical conductivity, strong biocompatibility and low cost [7,8]. In order to improve the performance of MFCs, various chemical and physical approaches for anode modification have been employed. Baked at 1100 ◦ C for 12 h in a kiln keeping anaerobic with N2 gas, Park and Zeikus developed the graphite anodes modified by metal
∗ Corresponding author. Tel.: +86 22 23507800; fax: +86 22 23507800. E-mail addresses: [email protected], [email protected] (M. Zhou). 1369-703X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2011.10.014
(Fe3+ or Mn4+ ), on which an enhanced electrical energy production was observed comparing with the conventional graphite anodes [9]. Cheng used ammonia gas to modify carbon cloth and received an excellent power output, but the electrode preparation required a thermogravimetric analyzer (TGA) and conducted in a furnace up to 700 ◦ C with a total treatment time of 180 min [10]. Tungsten carbide was explored as a promising anodic electrocatalyst for MFCs, however, the anode synthesis was performed in a carbon monoxide stream for 8–10 h at the temperature over 750 ◦ C [11]. In summary, although considerable enhancements for the performance of MFCs were observed, these modification techniques either needed complicated apparatus or multiple steps, and/or high temperature conditions and long treatment time, which would obviously increase the MFCs costs and hinder their application [3]. In the present work, a simple, quick and cost-effective electrochemical approach was attempted for anode modification, operating at ambient temperature. Electrochemical oxidation technology has been used as an effective method to improve the mechanical strength and adhesion of carbon fiber [12,13] as well as the electron transfer from bacteria to electrodes [14]. In this paper, carbon mesh anodes that were modified by three different electrolytes (nitric acid, ammonium nitrate, ammonium persulfate) were investigated, and the performance of MFCs was compared before and after anode modification. It proved that electrochemical modification could alter the anode surface characteristics and
Voltage(mV)
500
CM-C CM-N CM-A CM-P
400 300
CM-C-V CM-N-V CM-A-V CM-P-V
900
CM-C-P CM-N-P CM-A-P CM-P-P
2
900
600
Voltage (mV)
A
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155
Power Density (mW/m )
152
600
600
300
300
200 100 0 0
0
20
40
60
80
100
120
140
160
1
2
3
4
0
Current Density (mA/m2)
180
Time (h)
B
Fig. 2. Polarization curves and power density curves as a function of current density with four MFCs (error bars SD based on the voltages in the experiments run in triplicate).
30 Coulombic efficiency
2.2. MFC construction and operation
CE (%)
20
10
0
CM-C
CM-N
CM-A
CM-P
MFC Fig. 1. Cell voltage (A) and Coulombic efficiency (B) of four MFCs conditions: substrate: glucose 1 g/L, external load 1000 , initial pH 7.
thus increase the power generation and chemical oxygen demand (COD) removal efficiency, providing a convenient and promising method to promote the development of MFCs. 2. Experimental 2.1. Anode modification Carbon mesh (CM; Jilin Carbon Plant, China) was used as anode material, which was firstly cleaned by acetone overnight and then rinsed drastically in ultrapure water before use (this unmodified anode we called “CM-C”). Electrochemical oxidation was operated in an undivided electrolytic cell at ambient temperature, using the CM-C (effective area which was the projected area, 4 cm2 ) as the anode and graphite rod as the cathode with a 2.5 cm electrode gap. The electrolysis current density was kept at 1.25 mA/cm2 and electrolysis time was 30 min. Three types of modified anodes were obtained according to different electrolytes: nitric acid (65–68%, CM-N), ammonium nitrate (112 g/L, CM-A), and ammonium persulfate (160 g/L, CM-P). All modified anodes were washed three times with distilled water before being used in MFCs. All of the MFCs in this study had run for more than three months. Under the same operating conditions, the performance on voltage, pH variation, and COD removal showed good repeatability. And the experiment of polarization curves and Coulombic efficiency (CE) analysis were performed in triplicate.
Single chamber air-cathode MFCs with a volume of 14 mL and an electrode gap of 2 cm were constructed as described [15]. Four MFCs with different pretreatment anodes (CM-C, CM-N, CM-A, and CMP) and the same cathode of carbon cloth containing 0.35 mg/m2 Pt catalyst were started up. The MFCs were inoculated with anaerobic sludge (TEDA Sewage Treatment Plant, Tianjin) and a 50 mM phosphate buffered nutrient solution (PBS: NH4 Cl 0.31 g/L, KCl 0.13 g/L, NaH2 PO4 ·2H2 O 3.32 g/L, Na2 HPO4 ·12H2 O 10.36 g/L; trace minerals 12.5 mL/L; vitamins 5 mL/L) containing 1.0 g/L glucose as substrate. This solution was switched to a feed solution containing glucose (1.0 g/L) and PBS (50 mM) until the similar output voltage produced over two consecutive cycles. The feed solution was replaced when the voltage dropped below 50 mV. The external resistance was fixed at 1000 and the pH was adjusted to 6.9–7.0. All tests were operated in a 30 ± 0.5 ◦ C temperature-controlled biochemical incubator. 2.3. Analytical techniques and calculations Cell voltage was measured every minute with a data acquisition system (PISO-813, ICP DAS Co., Ltd.). Polarization curves were obtained by varying external resistances from 1000 to 50 when the cycles were stable. A reference electrode (KCl saturated calomel electrode, SCE) was used to measure the anode potential. Power density and current density were calculated based on the effective area of anode according to Ref. [7]. The Coulombic efficiency was obtained as follows: n
CE =
Ui ti
i=1
Rex FbCODV
×M
where Ui is the output voltage (V) at time ti , F is Faraday’s constant (96,485 C/mol), b is the number of electrons exchanged per mole of oxygen (=4), COD is the removal of COD (mg/L), M is the molecular weight of oxygen (=32 g/mol), and V is the wastewater volume (L). COD were measured with the closed reflux spectrophotometric method on a commercial COD detector (HACH, DRB 200, DR/890 Colorimeter, USA). Cyclic voltammogram (CV) was carried out using an electrochemical station (CHI600D, Shanghai, Chenghua) in a standard three-electrode system at a scan rate of 50 mV/s. The anode was the working electrode, the counter electrode was a platinum sheet and
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155
153
Fig. 3. SEM (A) and cyclic voltammograms (B) of the unmodified and modified anodes.
the reference electrode was a SCE. All experiments were performed in the 50 mM PBS at room temperature. The surface morphologies of the anodes before incubation were examined by a field emission scanning electron microscope (FESEM, Hitachi, S-4700). 3. Results and discussions 3.1. MFC performance The four MFCs were all reached stable maximum voltages after inoculation for about 120 h. Fig. 1 shows the voltage output (A) and Coulombic efficiency (B) of these four MFCs. Clearly, the anode modification affected the peak voltage and its sustainable period.
Among these MFCs, the CM-N achieved the highest voltage of 540 mV, which was much larger than that on CM-C (508 mV). More importantly, the sustainable periods near the maximum voltage for all modified MFCs were considerably improved. For example, the sustainable period of CM-N MFC increased from 9 h of the unmodified MFC to 16.5 h, which led to the Coulombic efficiency significantly increased by 71% (from 14% to 24%). Although the cell voltages of CM-P MFC (496 mV) and CM-A MFC (485 mV) were less than that of the control, the sustainable periods were both much longer (Fig. 1(A)), which resulted in a slight increase on their CE (Fig. 1(B)). Fig. 2 demonstrates the polarization curves and power density curves as a function of current density of four MFCs, which are
154
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155
obtained by the steady discharging method [16]. It was noteworthy that after anode modification the internal resistance of CM-N and CM-A MFC was reduced to 122 and 125 , which was 23% and 21% lower than the unmodified one (158 ), respectively. Besides, the internal resistance of CM-P MFC was 160 , which was similar with the unmodified one. It has been demonstrated that the decrease in internal resistance is an important factor for improving the performance of MFCs, which can also be confirmed by the power density in the present work. The maximum power densities after anode modification were all observed larger than that of the control one (CM-C; 552 mW/m2 ). The power density of MFC after nitric acid treatment increased by 43% to 792 mW/m2 , the one after ammonium nitrate treatment increased by 33% to 736 mW/m2 , and the one after ammonium persulfate treatment was 567 mW/m2 . Compared with other anode modification methods [9–11], this electrochemical approach is much simpler and quicker, operating at ambient temperature with the cheap apparatus of power supply (direct current). Moreover, this approach is also efficient for the promotion of MFCs performance with a power density increased by 43% to 792 mW/m2 , as disclosed above. Kim et al. [17] modified the anode with ferric oxide by a chemical vapor deposition (CVD) technique, using the professional and expensive CVD equipment, but only increased the power density to 30 mW/m2 . Scott et al. [18] investigated a range of carbon and carbon modified anode by deposition or polymerization or programmed heat, and the best anodes were made from carbon modified with quinone/quinoid groups, obtaining a maximum power density of 30 mW/cm2 . However, its preparation needed a complex technical procedure such as programmed heat up to 900 ◦ C. In contrast, our method only required 30 min treatment at room temperature and the power output was much larger. Therefore, simple operation, cheap modification equipments, short operation time and high modification performance are all beneficial to the application in engineering. Other characteristics of these four MFCs were also explored. Though the anode modification played an insignificant role on open-circuit voltage (OCV), it did affect the anode potentials, which all decreased from −157 mV of the unmodified one to around −220 mV. It supposed that the anode potential should be as low as possible to improve the performance of MFCs since it determined the interaction between bacteria and the electrode substrate [19]. And it was established that −200 mV might be the best anode potential to sustain enhanced current and maximum power generation [20]. The anode potential change in the present work suggested the anode modification should be beneficial to improve the performance of the MFCs, which was also verified by the power generation and the COD removal efficiency. The volumetric power density (Pv ) of CM-N MFC increased from the unmodified CM-C MFC of 28–39 W/m3 , and the Pv of CM-A MFC was also up to 36 W/m3 . The COD removal efficiencies after modification were all higher than the unmodified one (71.0%) and the CM-N MFC achieved the best result of 81.7%.
3.2. Anode characterization Fig. 3(A) illustrates the SEM of these carbon mesh anodes. Obviously, compared with the relatively smooth surface of the unmodified anode, much rougher surface and deeper cracks on the modified anodes were observed, especially on the CM-N and CM-A anode. These surface variations might have enhanced cell attachment. Fig. 3(B) shows the CV of four anodes without biofilm. Although no obvious oxidation/reduction peaks appeared, much higher redox currents on the modified anodes were observed. This fact indicated that the modified anodes displayed much better electrontransfer properties than the unmodified one. This substantial
alteration in electrochemical properties may contribute to the performance improvement of the MFCs. 4. Conclusions This study revealed that the electrochemical oxidation of anode was an effective and practical method to improve the performance of MFCs both on power generation and wastewater treatment efficiency, which indicated this approach was promising for application. The following conclusions could be drawn: (1) The modification in three electrolytes all posed positive performance. The anode modified by nitric acid achieved the best power production, which was increased by 43% from 552 to 792 mW/m2 . And the one modified by ammonium nitrate increased power by 33% to 736 mW/m2 . (2) The electrochemical oxidation of anode led to the change of the electrochemical properties of the anodes. The internal resistance of CM-N and CM-A MFC was found decreased to 122 and 125 , which was much lower than the unmodified control (158 ). And the anode potentials after modification were all decreased from the original −157 mV to about −220 mV. (3) Confirmed by SEM and CV, the increasing surface area and current response might be responsible for the improvement of the performance of MFCs. Acknowledgements This research was financially supported by NCET (08-0296), SRF for ROCS, SEM (2009-1001), China National Water Project (No. 2008ZX07314-001), and the Natural Science Foundation of Tianjin (No. 09JCYBJC08000). References ˝ [1] B.E. Logan, B. Hamelers, R. Rozendal, U. Schroder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5292. [2] Z. Du, H. Li, T. Gu, A state of the art review on microbial fuel cells: a promising technology for wastewater treatment and bioenergy, Biotechnol. Adv. 25 (2007) 464–482. [3] M.H. Zhou, M.L. Chi, J.M. Luo, H.H. He, T. Jin, An overview of electrode materials in microbial fuel cells, J. Power Sources 196 (2011) 4427–4435. [4] B. Min, B.E. Logan, Continuous electricity generation from domestic wastewater and organic substrates in a flat plate microbial fuel cell, Environ. Sci. Technol. 38 (2004) 5809–5814. [5] N. Lu, S.G. Zhou, L. Zhuang, J.T. Zhang, J.R. Ni, Electricity generation from starch processing wastewater using microbial fuel cell technology, Biochem. Eng. J. 43 (2009) 246–251. [6] S.V. Mohan, G. Mohanakrishna, B.P. Reddy, R. Saravanan, P.N. Sarma, Bioelectricity generation from chemical wastewater treatment in mediatorless (anode) microbial fuel cell (MFC) using selectively enriched hydrogen producing mixed culture under acidophilic microenvironment, Biochem. Eng. J. 39 (2008) 121–130. [7] B.E. Logan, Anode materials, in: Microbial Fuel Cells, John Wiley & Sons, New Jersey, 2008, pp. 62–68. [8] K. Watanabe, Recent developments in microbial fuel cell technologies for sustainable bioenergy, J. Biosci. Bioeng. 6 (2008) 528–536. [9] D.H. Park, J.G. Zeikus, Improved fuel cell and electrode designs for producing electricity from microbial degradation, Biotechnol. Bioeng. 81 (2003) 348–355. [10] S.A. Cheng, B.E. Logan, Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells, Electrochem. Commun. 9 (2007) 492–496. ˝ F. Scholz, Evaluation [11] M. Rosenbaum, F. Zhao, M. Quaas, H. Wulff, U. Schroder, of catalytic properties of tungsten carbide for the anode of microbial fuel cells, Appl. Catal. B: Environ. 74 (2007) 261–269. [12] T. Momma, X.J. Liu, T. Osaka, Y. Ushio, Y. Sawada, Electrochemical modification of active carbon fiber electrode and its application to double-layer capacitor, J. Power Sources 60 (1996) 249–253. [13] A. Fukunaga, S. Ueda, Anodic surface oxidation for pitch-based carbon fibers and the interfacial bond strengths in epoxy matrices, Compos. Sci. Technol. 60 (2000) 249–254. [14] X.H. Tang, K. Guo, H.R. Li, Z.W. Du, J.L. Tian, Electrochemical treatment of graphite to enhance electron transfer from bacteria to electrodes, Bioresour. Technol. 102 (2011) 3558–3560.
M. Zhou et al. / Biochemical Engineering Journal 60 (2012) 151–155 [15] S.A. Cheng, H. Liu, B.E. Logan, Increased power generation in a continuous flow MFC with advective flow through the porous anode and reduced electrode spacing, Environ. Sci. Technol. 40 (2006) 2426–2432. [16] P. Liang, M.Z. Fan, X.X. Cao, X. Huang, C. Wang, Composition and measurement of the apparent internal resistance in microbial fuel cell, Environ. Sci. 28 (2007) 4981–8981 (In Chinese). [17] J.R. Kim, B. Min, B.E. Logan, Evaluation of procedures to acclimate a microbial fuel cell for electricity production, Appl. Microbiol. Biotechnol. 68 (2005) 23–30.
155
[18] K. Scott, G.A. Rimbu, K.P. Katuri, K.K. Prasad, I.M. Head, Application of modified carbon anodes in microbial fuel cells, Process Safe. Environ. 85 (2007) 481–488. [19] A. Rinaldi, B. Mecheri, V. Garavaglia, S. Licoccia, P.D. Nardo, E. Traversa, Engineering materials and biology to boost performance of microbial fuel cells: a critical review, Energy Environ. Sci. 1 (2008) 417–429. [20] P. Aelterman, S. Freguia, J. Keller, W. Verstraete, K. Rabaey, The anode potential regulates bacterial activity in microbial fuel cells, Appl. Microbiol. Biotechnol. 78 (2008) 409–418.