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Decorating anode with bamboo-like nitrogen-doped carbon nanotubes for microbial fuel cells
Electrochemistry Communications 14 (2012) 71–74
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Decorating anode with bamboo-like nitrogen-doped carbon nanotubes for microbial fuel cells Suqin Ci a, c, Zhenhai Wen b, Junhong Chen b,⁎, Zhen He c,⁎⁎ a b c
Key Laboratory of Nondestructive Testing (Ministry of Education), School of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China Department of Mechanical, University of Wisconsin—Milwaukee, 3200 North Cramer Street, Milwaukee, WI, 53211, USA Department of Civil Engineering and Mechanics, University of Wisconsin—Milwaukee, 3200 North Cramer Street, Milwaukee, WI, 53211, USA
a r t i c l e
i n f o
Article history: Received 13 October 2011 Received in revised form 5 November 2011 Accepted 6 November 2011 Available online 20 November 2011 Keywords: Carbon nanotubes Nitrogen doping Anode Microbial fuel cell
a b s t r a c t Anode electrodes play a key role in generating electricity from microbial fuel cells (MFCs) because they directly affect microbial activities. This communication reports the preparation of nitrogen-doped carbon nanotubes with a bamboo-like nanostructure (Bamboo-NCNTs) by catalytic pyrolysis of ethylene diamine and application of the Bamboo-NCNTs as anode-modifying materials in MFCs. The Bamboo-NCNTs significantly improved performance of an MFC in current production and power output, and reduced internal resistance of the anode compared with conventional CNTs-modified and unmodified anodes. The improved performance could be attributed to the increased active sites induced by multiple joint structures and enhanced biocompatibility originated from nitrogen dopant. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) are bio-electrochemical systems that directly convert chemical energy in organic compounds to electricity via microbial metabolism. Over the past decade, the power density of MFCs has improved by several orders of magnitude [1]; however, tremendous effort is still required to promote the power output of MFCs before practical application can be realized. The anode materials, which act as the important medium of electron transport and can greatly affect the growth and activities of microbes, are one of the key components in determining the power generation [2,3]. The ideal anode material should promote bacterial attachment and facilitate electron transfer. Conventional carbon materials (e.g., carbon cloth, carbon paper) have been widely used as anode because of their good conductivity, stability, and low cost; however, their limited surface area and active reaction sites restrict biofilm formation and power production. To address these issues, one promising approach is modifying carbon electrodes with nanomaterials, which have high surface area [4,5]. Carbon nanotubes (CNTs) have been studied as anode-modifying materials in MFCs to increase power output [5,6]. Despite their high surface area, conventional CNTs should be further improved for a better biocompatibility for bacterial formation and adhesion
⁎ Corresponding author. Tel.: + 1 414 229 2615; fax: + 1 414 229 6958. E-mail addresses: [email protected] (J. Chen), [email protected] (Z. He). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.11.006
[7]. Recently, nitrogen-doped CNTs (NCNTs) were successfully applied in biosensors due to their unique electronic properties and good biocompatibility [8]. In addition, N-dopant can create defects that break the chemical inertness of pure CNTs and result in rich hydrophilic (N) defects in NCNTs, which was reported to be beneficial for facilitating electron transfer to and from proteins on cell membranes [9,10]. The use of NCNTs as the anode material in MFCs has not been reported. In this communication, we demonstrate a facile method to prepare the nitrogen-doped carbon nanotubes with a bamboo-like nanostructure (Bamboo-NCNTs), and use them as anode-modifying materials in MFCs with significantly improved performance compared with carbon cloth and non-doped CNTs. 2. Experimental 2.1. Synthesis and characterization of nitrogen-doped CNTs Carbon nanotubes with a diameter of 20 nm were synthesized through the catalytic decomposition of ethylene on Fe/Al2O3 catalyst in a nano-agglomerate fluidized-bed reactor [11]. Unlike aerosol chemical vapor deposition used previously [12], we herein prepared bamboo-NCNTs by catalytic pyrolysis of ethylene diamine at 750 °C, in which the iron nanoparticles, through decomposition of ferrocene, were used as the catalysts to grow carbon nanotubes. The morphology and nanostructures of the samples were characterized by using transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS).
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2.2. MFC setup and measurement The Bamboo-NCNTs were mixed with 5% Nafion solution in an ultrasonic bath, and then deposited on both sides of carbon cloth—CC (1.5 cm× 3 cm) with a loading of ~5 mg/cm2. The same procedure was used to prepare a CNT-based anode electrode. A batch-type MFC with two chambers was constructed by connecting two glass bottles (120 mL each) with cation exchange membrane (CEM, Membrane International Inc., Ringwood, NJ, USA) as a separator. For a fair comparison, three anodes, namely Bamboo-NCNTs, CNTs and bare CC, were installed in the same anode chamber without contact between each other; this arrangement could minimize the effect of the (different) cathode on experimental results. To ensure that the location of electrode placement did not affect current generation, a strong mixing via magnetic stirring was applied to the anode chamber to minimize the difference of substrate supply and ion transport among the anode electrodes. In addition, both CNT-modified electrodes were about 1 cm further than bare CC from the CEM; therefore the advantages of
the CNT-modified electrodes did not benefit from the electrode arrangement. The three anodes shared and were connected individually with copper wires to the same carbon brush cathode (Gordon Brush Mfg. Co. Inc., CA, USA) by using 1.0 M K3[Fe(CN)6] as electron acceptors in 0.1 M phosphate buffer solution (PBS, pH= 7.0). Therefore, three MFCs were formed as the Bamboo-NCNTs-MFC, CNTs-MFC, and CC-MFC, respectively. The anolyte was prepared according to a previous report, [13] with sodium acetate (1 g/L) as an electron donor. A mixture of aerobic and anaerobic sludge from a local wastewater treatment plant (South Shore, Milwaukee, WI, USA) was inoculated as the biocatalyst in the anode chamber. Polarization curves were constructed using a potentiostat (Gamry Instruments, Warminster, PA, USA) at a scan rate of 0.5 mV/s. The cell voltage was recorded by digital multimeter (Keithley Instruments, Inc., Cleveland, OH, USA). The densities of power and current were calculated based on the surface area of the anode. Coulombic efficiency (CE) was calculated according to a previous report [13]. Electrochemical impedance spectroscopy (EIS) tests were conducted
Fig. 1. SEM (a) and TEM (b) images of the CNTs, SEM (c–d), and TEM (e) images of Bamboo-NCNTs, (f) high-resolution XPS of N1s in Bamboo-NCNTs.
S. Ci et al. / Electrochemistry Communications 14 (2012) 71–74
using a potentiostat at the open circuit potential in a frequency range of 50 mHz–100 kHz. 3. Results and discussion The morphologies of CNTs and Bamboo-NCTNs were observed using SEM and TEM, which showed that the tube nanostructure of CNTs has a diameter of around 20 nm (Fig. 1a and b). The CNT fiber can be clearly observed in a general morphology of a filament bundle of bambooNCNTs at low resolution (Fig. 1c). The magnified SEM image manifests the structure of the nanotubes in the bamboo-NCNTs (Fig. 1d). We also observed that the samples have a diameter range of 20–100 nm, with a distinct bamboo joint structure in the TEM images of the Bamboo-NCNTs (Fig. 1e). The high-resolution XPS of N1s spectrum (Fig. 1f) can be fitted to three components of the binding energy: pyridine-type nitrogen atoms (398.2 eV), pyrrolic-type N doping (399.9 eV), and graphite-type nitrogen atoms (401.2 eV) [14], demonstrating that the nitrogen was successfully doped into the bambooNCNTs. In addition, the XPS measurement reveals the surface of the Bamboo-NCNTs has a C/N molar ratio of about 12.3. Electricity generation in the three MFCs was initially examined using the batch cycle operation at an external resistor of 10 Ω. A reproducible cycle of current density was obtained with the three MFCs after three months of microbial cultivation (Fig. 2a), indicating the biofilm formation and the bacterial adhesion reached a steady state on the anode electrodes. The current generation was clearly affected by the modifying materials of the anodes. The CC-MFC produced a peak current density of only 0.86 ± 0.06 A m− 2, while the CNTs-MFC produced a much higher current density of 2.31 ± 0.10 A m − 2. Notably, the BambooNCNTs-MFC delivered a peak current density of 3.63 ± 0.06 A m − 2,
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which is almost 1.6 times greater than that of the CNTs-MFC, and more than 4 times greater than that of the CC-MFC. The BambooNCNT-MFC produced a CE of 11.5 ± 0.3%, which is 1.5 and 5.2 times greater than that of the CNTs-CC-MFC (7.5± 0.1%) and CC-MFC (2.2 ± 0.1%). These results demonstrate that the Bamboo-NCNTs modification of the anode can improve the current generation and enhance the CE in MFCs. The polarization curves were used to confirm the above findings. Although the three MFCs showed a slight difference in open circuit voltage (Voc), the maximum power density of the Bamboo-NCNTsMFC improved remarkably compared with the CNTs-MFC and CC-MFC (Fig. 2b). Specifically, the Bamboo-NCNTs-MFC achieved a maximum power density of 1.04 W m− 2 at the current density of 3.16 A m− 2, while the maximum power density of the CNTs-MFC was 0.71 W m− 2 at the current density of 2.22 A m− 2. As expected, the CC-MFC generated a maximum power density of only 0.47 W m− 2 at the current density of 0.31 A m− 2. In addition, the CC-MFC exhibited an overshoot in power production [15], likely caused by inefficient electricity generation during the polarization test. To understand the effect of anode modification on individual potentials, we further investigated the variation of the anode and cathode potentials. The cathode potentials of three MFCs behaved similarly, but the anode potentials showed a significant difference (Fig. 2c and d), suggesting that the anode potential is highly affected by the modification and application of CNTs, especially BambooNCNTs, which slowly increased the anode potential. Therefore, at the same current densities, the potential of the Bamboo-NCNTs anode was more negative than those of CNTs and CC anodes, resulting in the highest current density among the three MFCs. A lower anode potential at the fixed current density indicates lower overpotential
Fig. 2. (a) Cell current density as a function of time with an external resistance of 10 Ω, (b) polarization curves and power density curves for three MFCs. Variation of cathode potentials (c) and anode potentials (d) of the three MFCs.
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measurement. Second, better biocompatibility, due to N-doping in Bamboo-NCNTs, might induce the absorption and growth of favored microbial on the electrode for lowing overpotential and increasing biocatalytic activity. 4. Conclusions In summary, the nitrogen-doped carbon nanotubes with a bamboo structure were successfully used to modify the anode in an MFC, and significantly improved the MFC performance compared with unmodified carbon cloth and CNTs anode. The super electrochemical performance can be ascribed to the synergetic effect of its unique structures, higher surface area, and N-doping in bamboo-NCNTs, which could provide more active sites for interface electrochemical reaction and better biocompatibility. Fig. 3. Nyquist curve of the EIS test for the MFCs equipped with the Bamboo-NCNTs anode, CNTs anode and CC anode, respectively.
Acknowledgements for acetate electrooxidation [16,17], while a larger current density at the fixed potential suggests higher activity for acetate electrooxidation. The lower anode potential with the Bamboo-NCNTs modification generated more electricity, in accordance with the above results of batch tests and polarization curves, confirming that the BambooNCNTs can substantially enhance anode performance. The charge-transfer properties of the anodes at their corresponding Voc were analyzed using EIS. As illustrated in the Nyquist curves (Fig. 3), all plots consist of semicircles at high-frequencies and straight-line features in a low-frequency region, corresponding with the charge transfer resistance (Rct) at the anode interface and the diffusion resistance, respectively. An equivalent circuit model (inset of Fig. 3) was used to fit the EIS data. The Bamboo-NCNTs anode had an Rct of 23 Ω, which is much lower than 68 Ω of the CNTs anode and 390 Ω of the CC anode, demonstrating that the Bamboo-NCNT anode has superior performance for acetate oxidation, which is likely due to the increased surface area of the electrode and improved biofilm formation. The overall internal resistance (Ri) for the BambooNCNTs anode was 57 Ω, while the CNTs anode and the CC anode had an Ri of 641 Ω and 3610 Ω, respectively. The Bamboo-NCNTs anode showed the largest double-layer capacitor (Cdl) with 0.31 F compared with those of the CNTs anode (0.031 F) and CC anode (0.0088 F), suggesting that the Bamboo-NCNTs anode had a larger surface area available for electron transfer. The Bamboo-NCNTs-MFC outperforms the CNTs-MFC and CC-MFC in terms of electricity generation, which is likely due to the following reasons. First, the bamboo nanotube structures of the NCNTs may provide rich defect sites along the inner wall of CNTs, and the BambooNCNTs showed higher specific surface areas (SSA) (205.3 m2/g) than that of CNTs (165.8 m 2/g) based on nitrogen adsorption–desorption
This work was financially supported by the National Science Foundation (CBET-1033505 and CMMI-0900509), the U.S. Department of Energy (DE-EE0003208), We Energies, and the National Natural Science Foundation of China (no. 20903055). We thank Michelle Schoenecker (UW—Milwaukee) for her assistance with manuscript proofreading. References [1] I.S. Kim, K.J. Chae, M.J. Choi, Environmental Engineering Research 13 (2008) 51. [2] B. Cercado-Quezada, M.L. Delia, A. Bergel, Electrochemistry Communications 13 (2011) 440. [3] V. Sharma, P.P. Kundu, Enzyme and Microbial Technology 47 (2010) 179. [4] Y. Fan, S. Xu, R. Schaller, J. Jiao, F. Chaplen, H. Liu, Biosensors and Bioelectronics 26 (2011) 1908. [5] Y. Qiao, C.M. Li, S.J. Bao, Q.L. Bao, Journal of Power Sources 170 (2007) 79. [6] Y. Zou, C. Xiang, L. Yang, L.X. Sun, F. Xu, Z. Cao, International Journal of Hydrogen Energy 33 (2008) 4856. [7] M.A. Hussain, M.A. Kabir, A.K. Sood, Current Science 96 (2009) 664. [8] S. Deng, G. Jian, J. Lei, Z. Hu, H. Ju, Biosensors and Bioelectronics 25 (2009) 373. [9] J.C. Carrero-Sanchez, L. Elías, R. Mancilla, G. Arrellín, H. Terrones, J.P. Laclette, M. Terrones, Nano Letters 6 (2006) 1609. [10] N.Q. Jia, L.J. Wang, L. Liu, Q. Liu, Z.Y. Jiang, Electrochemistry Communications 7 (2005) 349. [11] Z.H. Wen, Q. Wang, J.H. Li, Advanced Functional Materials 17 (2007) 2772. [12] A.A. Koos, M. Dowling, K. Jurkschat, A. Crossley, N. Grobert, Carbon 47 (2009) 30. [13] Z. He, N. Wagner, S.D. Minteer, L.T. Angenent, Environmental Science and Technology 40 (2006) 5212. [14] L.Y. Feng, Y.Y. Yan, Y.G. Chen, L.J. Wang, Energy Environmental Science 4 (2011) 1892. [15] V.J. Watson, B.E. Logan, Electrochemistry Communications 13 (2011) 54. [16] C. Feng, L. Ma, F. Li, H. Mai, X. Lang, S. Fan, Biosensors and Bioelectronics 25 (2010) 1516. [17] Y. Zhang, G. Mo, X. Li, W. Zhang, J. Zhang, J. Ye, X. Huang, C. Yu, Journal of Power Sources 196 (2011) 5402.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Decorating anode with bamboo-like nitrogen-doped carbon nanotubes for microbial fuel cells Suqin Ci a, c, Zhenhai Wen b, Junhong Chen b,⁎, Zhen He c,⁎⁎ a b c
Key Laboratory of Nondestructive Testing (Ministry of Education), School of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China Department of Mechanical, University of Wisconsin—Milwaukee, 3200 North Cramer Street, Milwaukee, WI, 53211, USA Department of Civil Engineering and Mechanics, University of Wisconsin—Milwaukee, 3200 North Cramer Street, Milwaukee, WI, 53211, USA
a r t i c l e
i n f o
Article history: Received 13 October 2011 Received in revised form 5 November 2011 Accepted 6 November 2011 Available online 20 November 2011 Keywords: Carbon nanotubes Nitrogen doping Anode Microbial fuel cell
a b s t r a c t Anode electrodes play a key role in generating electricity from microbial fuel cells (MFCs) because they directly affect microbial activities. This communication reports the preparation of nitrogen-doped carbon nanotubes with a bamboo-like nanostructure (Bamboo-NCNTs) by catalytic pyrolysis of ethylene diamine and application of the Bamboo-NCNTs as anode-modifying materials in MFCs. The Bamboo-NCNTs significantly improved performance of an MFC in current production and power output, and reduced internal resistance of the anode compared with conventional CNTs-modified and unmodified anodes. The improved performance could be attributed to the increased active sites induced by multiple joint structures and enhanced biocompatibility originated from nitrogen dopant. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) are bio-electrochemical systems that directly convert chemical energy in organic compounds to electricity via microbial metabolism. Over the past decade, the power density of MFCs has improved by several orders of magnitude [1]; however, tremendous effort is still required to promote the power output of MFCs before practical application can be realized. The anode materials, which act as the important medium of electron transport and can greatly affect the growth and activities of microbes, are one of the key components in determining the power generation [2,3]. The ideal anode material should promote bacterial attachment and facilitate electron transfer. Conventional carbon materials (e.g., carbon cloth, carbon paper) have been widely used as anode because of their good conductivity, stability, and low cost; however, their limited surface area and active reaction sites restrict biofilm formation and power production. To address these issues, one promising approach is modifying carbon electrodes with nanomaterials, which have high surface area [4,5]. Carbon nanotubes (CNTs) have been studied as anode-modifying materials in MFCs to increase power output [5,6]. Despite their high surface area, conventional CNTs should be further improved for a better biocompatibility for bacterial formation and adhesion
⁎ Corresponding author. Tel.: + 1 414 229 2615; fax: + 1 414 229 6958. E-mail addresses: [email protected] (J. Chen), [email protected] (Z. He). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.11.006
[7]. Recently, nitrogen-doped CNTs (NCNTs) were successfully applied in biosensors due to their unique electronic properties and good biocompatibility [8]. In addition, N-dopant can create defects that break the chemical inertness of pure CNTs and result in rich hydrophilic (N) defects in NCNTs, which was reported to be beneficial for facilitating electron transfer to and from proteins on cell membranes [9,10]. The use of NCNTs as the anode material in MFCs has not been reported. In this communication, we demonstrate a facile method to prepare the nitrogen-doped carbon nanotubes with a bamboo-like nanostructure (Bamboo-NCNTs), and use them as anode-modifying materials in MFCs with significantly improved performance compared with carbon cloth and non-doped CNTs. 2. Experimental 2.1. Synthesis and characterization of nitrogen-doped CNTs Carbon nanotubes with a diameter of 20 nm were synthesized through the catalytic decomposition of ethylene on Fe/Al2O3 catalyst in a nano-agglomerate fluidized-bed reactor [11]. Unlike aerosol chemical vapor deposition used previously [12], we herein prepared bamboo-NCNTs by catalytic pyrolysis of ethylene diamine at 750 °C, in which the iron nanoparticles, through decomposition of ferrocene, were used as the catalysts to grow carbon nanotubes. The morphology and nanostructures of the samples were characterized by using transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS).
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2.2. MFC setup and measurement The Bamboo-NCNTs were mixed with 5% Nafion solution in an ultrasonic bath, and then deposited on both sides of carbon cloth—CC (1.5 cm× 3 cm) with a loading of ~5 mg/cm2. The same procedure was used to prepare a CNT-based anode electrode. A batch-type MFC with two chambers was constructed by connecting two glass bottles (120 mL each) with cation exchange membrane (CEM, Membrane International Inc., Ringwood, NJ, USA) as a separator. For a fair comparison, three anodes, namely Bamboo-NCNTs, CNTs and bare CC, were installed in the same anode chamber without contact between each other; this arrangement could minimize the effect of the (different) cathode on experimental results. To ensure that the location of electrode placement did not affect current generation, a strong mixing via magnetic stirring was applied to the anode chamber to minimize the difference of substrate supply and ion transport among the anode electrodes. In addition, both CNT-modified electrodes were about 1 cm further than bare CC from the CEM; therefore the advantages of
the CNT-modified electrodes did not benefit from the electrode arrangement. The three anodes shared and were connected individually with copper wires to the same carbon brush cathode (Gordon Brush Mfg. Co. Inc., CA, USA) by using 1.0 M K3[Fe(CN)6] as electron acceptors in 0.1 M phosphate buffer solution (PBS, pH= 7.0). Therefore, three MFCs were formed as the Bamboo-NCNTs-MFC, CNTs-MFC, and CC-MFC, respectively. The anolyte was prepared according to a previous report, [13] with sodium acetate (1 g/L) as an electron donor. A mixture of aerobic and anaerobic sludge from a local wastewater treatment plant (South Shore, Milwaukee, WI, USA) was inoculated as the biocatalyst in the anode chamber. Polarization curves were constructed using a potentiostat (Gamry Instruments, Warminster, PA, USA) at a scan rate of 0.5 mV/s. The cell voltage was recorded by digital multimeter (Keithley Instruments, Inc., Cleveland, OH, USA). The densities of power and current were calculated based on the surface area of the anode. Coulombic efficiency (CE) was calculated according to a previous report [13]. Electrochemical impedance spectroscopy (EIS) tests were conducted
Fig. 1. SEM (a) and TEM (b) images of the CNTs, SEM (c–d), and TEM (e) images of Bamboo-NCNTs, (f) high-resolution XPS of N1s in Bamboo-NCNTs.
S. Ci et al. / Electrochemistry Communications 14 (2012) 71–74
using a potentiostat at the open circuit potential in a frequency range of 50 mHz–100 kHz. 3. Results and discussion The morphologies of CNTs and Bamboo-NCTNs were observed using SEM and TEM, which showed that the tube nanostructure of CNTs has a diameter of around 20 nm (Fig. 1a and b). The CNT fiber can be clearly observed in a general morphology of a filament bundle of bambooNCNTs at low resolution (Fig. 1c). The magnified SEM image manifests the structure of the nanotubes in the bamboo-NCNTs (Fig. 1d). We also observed that the samples have a diameter range of 20–100 nm, with a distinct bamboo joint structure in the TEM images of the Bamboo-NCNTs (Fig. 1e). The high-resolution XPS of N1s spectrum (Fig. 1f) can be fitted to three components of the binding energy: pyridine-type nitrogen atoms (398.2 eV), pyrrolic-type N doping (399.9 eV), and graphite-type nitrogen atoms (401.2 eV) [14], demonstrating that the nitrogen was successfully doped into the bambooNCNTs. In addition, the XPS measurement reveals the surface of the Bamboo-NCNTs has a C/N molar ratio of about 12.3. Electricity generation in the three MFCs was initially examined using the batch cycle operation at an external resistor of 10 Ω. A reproducible cycle of current density was obtained with the three MFCs after three months of microbial cultivation (Fig. 2a), indicating the biofilm formation and the bacterial adhesion reached a steady state on the anode electrodes. The current generation was clearly affected by the modifying materials of the anodes. The CC-MFC produced a peak current density of only 0.86 ± 0.06 A m− 2, while the CNTs-MFC produced a much higher current density of 2.31 ± 0.10 A m − 2. Notably, the BambooNCNTs-MFC delivered a peak current density of 3.63 ± 0.06 A m − 2,
73
which is almost 1.6 times greater than that of the CNTs-MFC, and more than 4 times greater than that of the CC-MFC. The BambooNCNT-MFC produced a CE of 11.5 ± 0.3%, which is 1.5 and 5.2 times greater than that of the CNTs-CC-MFC (7.5± 0.1%) and CC-MFC (2.2 ± 0.1%). These results demonstrate that the Bamboo-NCNTs modification of the anode can improve the current generation and enhance the CE in MFCs. The polarization curves were used to confirm the above findings. Although the three MFCs showed a slight difference in open circuit voltage (Voc), the maximum power density of the Bamboo-NCNTsMFC improved remarkably compared with the CNTs-MFC and CC-MFC (Fig. 2b). Specifically, the Bamboo-NCNTs-MFC achieved a maximum power density of 1.04 W m− 2 at the current density of 3.16 A m− 2, while the maximum power density of the CNTs-MFC was 0.71 W m− 2 at the current density of 2.22 A m− 2. As expected, the CC-MFC generated a maximum power density of only 0.47 W m− 2 at the current density of 0.31 A m− 2. In addition, the CC-MFC exhibited an overshoot in power production [15], likely caused by inefficient electricity generation during the polarization test. To understand the effect of anode modification on individual potentials, we further investigated the variation of the anode and cathode potentials. The cathode potentials of three MFCs behaved similarly, but the anode potentials showed a significant difference (Fig. 2c and d), suggesting that the anode potential is highly affected by the modification and application of CNTs, especially BambooNCNTs, which slowly increased the anode potential. Therefore, at the same current densities, the potential of the Bamboo-NCNTs anode was more negative than those of CNTs and CC anodes, resulting in the highest current density among the three MFCs. A lower anode potential at the fixed current density indicates lower overpotential
Fig. 2. (a) Cell current density as a function of time with an external resistance of 10 Ω, (b) polarization curves and power density curves for three MFCs. Variation of cathode potentials (c) and anode potentials (d) of the three MFCs.
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measurement. Second, better biocompatibility, due to N-doping in Bamboo-NCNTs, might induce the absorption and growth of favored microbial on the electrode for lowing overpotential and increasing biocatalytic activity. 4. Conclusions In summary, the nitrogen-doped carbon nanotubes with a bamboo structure were successfully used to modify the anode in an MFC, and significantly improved the MFC performance compared with unmodified carbon cloth and CNTs anode. The super electrochemical performance can be ascribed to the synergetic effect of its unique structures, higher surface area, and N-doping in bamboo-NCNTs, which could provide more active sites for interface electrochemical reaction and better biocompatibility. Fig. 3. Nyquist curve of the EIS test for the MFCs equipped with the Bamboo-NCNTs anode, CNTs anode and CC anode, respectively.
Acknowledgements for acetate electrooxidation [16,17], while a larger current density at the fixed potential suggests higher activity for acetate electrooxidation. The lower anode potential with the Bamboo-NCNTs modification generated more electricity, in accordance with the above results of batch tests and polarization curves, confirming that the BambooNCNTs can substantially enhance anode performance. The charge-transfer properties of the anodes at their corresponding Voc were analyzed using EIS. As illustrated in the Nyquist curves (Fig. 3), all plots consist of semicircles at high-frequencies and straight-line features in a low-frequency region, corresponding with the charge transfer resistance (Rct) at the anode interface and the diffusion resistance, respectively. An equivalent circuit model (inset of Fig. 3) was used to fit the EIS data. The Bamboo-NCNTs anode had an Rct of 23 Ω, which is much lower than 68 Ω of the CNTs anode and 390 Ω of the CC anode, demonstrating that the Bamboo-NCNT anode has superior performance for acetate oxidation, which is likely due to the increased surface area of the electrode and improved biofilm formation. The overall internal resistance (Ri) for the BambooNCNTs anode was 57 Ω, while the CNTs anode and the CC anode had an Ri of 641 Ω and 3610 Ω, respectively. The Bamboo-NCNTs anode showed the largest double-layer capacitor (Cdl) with 0.31 F compared with those of the CNTs anode (0.031 F) and CC anode (0.0088 F), suggesting that the Bamboo-NCNTs anode had a larger surface area available for electron transfer. The Bamboo-NCNTs-MFC outperforms the CNTs-MFC and CC-MFC in terms of electricity generation, which is likely due to the following reasons. First, the bamboo nanotube structures of the NCNTs may provide rich defect sites along the inner wall of CNTs, and the BambooNCNTs showed higher specific surface areas (SSA) (205.3 m2/g) than that of CNTs (165.8 m 2/g) based on nitrogen adsorption–desorption
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