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Effect of nitrogen addition on the performance of microbial fuel cell anodes
Bioresource Technology 102 (2011) 395–398
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of nitrogen addition on the performance of microbial fuel cell anodes Tomonori Saito a,b, Maha Mehanna a, Xin Wang c, Roland D. Cusick a, Yujie Feng c, Michael A. Hickner b, Bruce E. Logan a,* a
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, United States Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, United States c State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No 73 Huanghe Road, Nangang District, Harbin 150090, China b
a r t i c l e
i n f o
Article history: Received 31 March 2010 Received in revised form 10 May 2010 Accepted 18 May 2010 Available online 17 June 2010 Keywords: Microbial fuel cell Anode treatment Bacterial adhesion Diazonium functionalization
a b s t r a c t Carbon cloth anodes were modified with 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate to increase nitrogen-containing functional groups at the anode surface in order to test whether the performance of microbial fuel cells (MFCs) could be improved by controllably modifying the anode surface chemistry. Anodes with the lowest extent of functionalization, based on a nitrogen/carbon ratio of 0.7 as measured by XPS, achieved the highest power density of 938 mW/m2. This power density was 24% greater than an untreated anode, and similar to that obtained with an ammonia gas treatment previously shown to increase power. Increasing the nitrogen/carbon ratio to 3.8, however, decreased the power density to 707 mW/m2. These results demonstrate that a small amount of nitrogen functionalization on the carbon cloth material is sufficient to enhance MFC performance, likely as a result of promoting bacterial adhesion to the surface without adversely affecting microbial viability or electron transfer to the surface. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) use electricity-generating bacteria (exoelectrogens) to oxidize organic matter and produce current. Many strains of bacteria are known to be capable of exogenous electron transfer and are abundant in rivers, ocean sediments, domestic and agricultural wastewaters (Logan et al., 2006). The use of exoelectrogenic bacteria in MFCs shows great promise for using wastewater for renewable energy production. Recently, there have been several reports on pilot scale tests using MFCs and microbial electrolysis cells (MECs) (Logan, 2010). Improving the current and power densities in these systems, however, remains a major challenge. Several chemical and physical modifications of anode surfaces have successfully been used to increase MFC power densities. High temperature treatment of graphite fibers in air (450 °C for 30 min) can result in increased power, likely due to the increase in the surface N/C ratio (Feng et al., 2010; Wang et al., 2009). Heating carbon cloth surfaces at high temperatures with ammonia gas (700 °C for 1 h) reduced the start up time of an MFC and also increased power, likely as a result of increased amine groups on the surface of the carbon cloth, but detailed changes in the surface chemistry were not investigated (Cheng and Logan, 2007). Other reported anode
* Corresponding author. Address: Department of Civil and Environmental Engineering, The Pennsylvania State University, 231Q Sackett Building, University Park, PA 16802, United States. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: [email protected] (B.E. Logan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.05.063
modification methodologies include oxidation of the anode surface using acid (Lowy et al., 2006; Scott et al., 2007) or electrochemical methods (Lowy and Tender, 2008), incorporation of anthraquinone (Feng et al., 2010; Lowy and Tender, 2008; Lowy et al., 2006), addition of polyaniline or polypyrrole (Feng et al., 2010; Qiao et al., 2007; Schroder et al., 2003; Scott et al., 2007; Zou et al., 2008), or the use of carbon nanotubes (Qiao et al., 2007; Sharma et al., 2008; Sun et al., 2010; Tsai et al., 2009; Zou et al., 2008). Increased power and current in all of these cases was likely due to a combination of factors including: (1) enhanced electrical conductivity, (2) increased surface area, and (3) increased biocompatibility of the anode surface (Scott et al., 2007). Understanding the correlation of the anode performance and the anode surface chemistry will be important for continuing to improve in the performance of MFCs. Increasing the surface charge, for example by protonated amine groups (Cheng and Logan, 2007), can increase the adhesion of negatively charged bacteria (Terada et al., 2006) and produce an improved surface for electron transfer between the bacteria and the carbon. The success of heat and ammonia treatments that increased the nitrogen content and the positive surface charge (from 0.38 to 3.99 meq m2) (Cheng and Logan, 2007; Feng et al., 2010; Wang et al., 2009) suggests that amine functionalization of the carbon surface could be useful method to improve microbial adhesion and power generation. An investigation of anode performance with a controlled degree of predetermined amine functional groups has not been previously undertaken with the ammonia gas treatment due to the difficulty in identifying the structure of the amine
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groups added to the surface, and the lack of a method to control the degree of functionalization. In the present study, we modified carbon cloth anodes to different extents with a specific amine functional group (dimethylaniline), using 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate, and determined the effect of this modification on the power generation of MFCs. We compared the performance of anodes with different amounts of dimethylaniline functionalization to anodes that were heat or ammonia treated using single-chamber, air-cathode MFCs. 2. Methods 2.1. Anodes Carbon cloth (CC; type A, BASF Fuel Cell Inc., Somerset, NJ) was used as received. The cloth was treated using four different concentrations of 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate (diazonium salt, Wako Chemicals) (A–D), by heating in air (CH) (Wang et al., 2009), or by heating in an ammonia gas atmosphere (CA) (Cheng and Logan, 2007) (Table S1). Dimethylaniline functionalization was performed by placing 1 g of carbon cloth (0.083 mol carbon) into 70 mL DI water containing 0.01 (A), 0.1 (B), 0.2 (C and D) (mol/mol of carbon) of diazonium salt and 0.02 (A), 0.2 (B), 0.4 (C and D) equivalent (mol/mol of carbon) of hypophosphorous acid, (H3PO2, 50 wt.% in water, Aldrich) in a 100 mL round-bottomed flask (Fig. S1). To ensure the carbon cloth was completely wetted, it was first soaked in methanol, then rinsed with water several times before treatment. Reactions were performed at room temperature (25 °C, A–C) or 60 °C (D) for 18 h. Addition of H3PO2 was used to enhance diazonium salt reactivity (Toupin and Belanger, 2008) so that required amount of diazonium salt reagent could be reduced. The heat-treated CH anode was prepared by heating the carbon cloth in a muffle furnace at 450 °C for 30 min as previously described (Wang et al., 2009), while the ammonia treated CA anodes were prepared by treating the carbon cloth at 700 °C for 1 h in a 5% ammonia gas as previously described (Cheng and Logan, 2007). Surface area measurements of these prepared anode materials were not performed as it has previously been shown that there is a lack of correlation between measured surface area values and corresponding performance (Liu et al., 2010). The atomic compositions of the anodes were examined using Xray photoelectron spectroscopy (XPS, PHI Model 5600 MultiTechnique) with a monochromated Al Ka X-ray source (Wang et al., 2009). Samples were dried under reduced pressure at 80 °C prior to analysis. Spectra were obtained over a scan range of 1350– 0 eV, recorded and stored using the PHI ACCESSTM data system, and analyzed using CasaXPS software (Version 2.3.12Dev9). All peaks were identified except Auger peaks. 2.2. MFC configuration and operation Cube-shaped single-chambered MFCs were machined to contain a cylindrical chamber 7 cm2 in projected area, and 2 cm in length, with the electrodes placed on either side of the chamber (liquid volume 14 mL). Cathodes with catalyst loading of 0.5 mg/ cm2 of platinum were prepared by application of a Nafion binder solution (Aldrich, 5 wt.%) containing platinum on carbon black catalyst (10 wt.% platinum on Vulcan XC-72, E-TEK, Sommerset NJ) on wet-proofed carbon cloth (type B-1B, E-TEK) on the electrolyte side, and four PTFE diffusion layers on the air side. Reactors were inoculated with domestic wastewater (Pennsylvania State University Wastewater Treatment Plant) and fed sodium acetate (1 g/L) in 50 mM phosphate buffer (PBS) solution (4.576 g/L Na2HPO4, 2.452 g/L NaH2PO4H2O; pH 7.0; conductiv-
ity of 7 mS/cm) with 0.31 g/L NH4Cl, 0.13 g/L KCl, trace minerals (12.5 mL/L), and vitamins (5 mL/L). MFCs were operated in fedbatch mode at 30 °C. The voltage (V) was monitored across a fixed external resistor (1000 X) using a multimeter and data acquisition system (2700 Keithley, USA). Electrode potentials were measured using a digital multimeter (22-813, Radioshack, USA) with a Ag/AgCl reference electrode (RE-5B, Bioanalytical Systems Inc.; 0.20 V versus a standard hydrogen electrode, SHE). Polarization and power density curves were obtained by measuring the stable voltage at 20 min intervals generated under various external resistances ranging between 1000 and 50 X. Current (I = V/R) and power (P = IV) were calculated as previously described (Logan et al., 2006).
3. Results and discussion 3.1. Modification of carbon cloth anodes Increasing the concentration of diazonium salt reactant during the carbon cloth treatment correlated with an increase in the N1s intensity in the XPS spectra (Fig. S2) and calculated N/C ratios (Table 1). The N/C ratios of the carbon surface increased from 0.7 to 1.8 (A–C) upon addition of higher concentrations of diazonium salt reactant (Table 1). Higher reaction temperatures also contributed to an increase in N1s intensity, and thus the N/C ratio (1.8, C; 3.8, D). N/C ratios were 0.5 for the CH anode, and 1.2 for the CA anode, which are similar to those for the A (0.7) and B (1.0) anodes. The gain in nitrogen content upon heating or ammonia gas treatment is consistent with the XPS results in previous reports for heat-treated carbon fiber brush (Feng et al., 2010), heat-treated carbon mesh (Wang et al., 2009) and ammonia treated carbon mesh (Wang et al., 2009). A significant amount of charged nitrogen (protonated nitrogen), which should show up as peaks at 402 eV (Jansen and Vanbekkum, 1995), was not observed in XPS spectra for the dimethylaniline functionalized carbon cloth anodes (A–D), whereas the presence of peaks at 400 eV corresponds to deprotonated nitrogen and suggests the presence of deprotonated dimethylaniline groups (Fig. S2). The presence of the deprotonated dimethylaniline groups should be consistent at pH 7 due to the pKa 5. The presence of protonated nitrogen for the heat-treated anode (CH) was also insignificant in the XPS spectrum, although the increased protonated amine from XPS spectra for a heat-treated carbon fiber brush anode correlated to the increased anode performance in a previous report (Feng et al., 2010). These contrasting results are probably due to the different materials used in these studies. Thermogravimetric analysis for each anode sample was also performed, but the difference in weight loss upon heating was insignificant (0.1–0.2 wt.%; data not shown). XPS was therefore the more sensitive analytical method to quantify the degree of functionalization due to surface modification.
Table 1 N/C ratios of carbon cloth anodes as determined from XPS.
A B C D CC CH CA
C1s (%)
N1s (%)
O1s (%)
N/C (%)
97.8 95.9 94.8 93.6 95.2 97.7 96.6
0.7 1.0 1.7 3.6 0 0.5 1.1
1.6 3.2 3.5 2.8 3.4 1.8 2.3
0.7 1.0 1.8 3.8 0 0.5 1.2
T. Saito et al. / Bioresource Technology 102 (2011) 395–398
Fig. 1. (A) Cell voltage as a function of time for MFCs with different carbon cloth anodes. (B) Maximum cell voltage values for each cycle. A–D, dimethylaniline functionalized carbon cloth anodes; CH, heat-treated carbon cloth anode; CA, ammonia treated carbon cloth anode; CC, untreated carbon cloth anode.
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Fig. 2. (A) Power density and (B) corresponding anode potentials for MFCs with different anodes.
3.2. MFC performance All the MFCs exhibited similar acclimation times of 60 h, defined as the time to reach the first reproducible maximum power density (Fig. 1). While there was no significant differences between the time for acclimation with plain carbon cloth (CC) or the treated anodes, the acclimation time for the MFC with the ammonia treated anode (CA) was consistent with previous reports (Cheng and Logan, 2007). These results on acclimation times and power densities were found to be reproducible based on repeated experiments (data not shown). The performance of the treated anodes after 60 h was found to be the same as that of the plain carbon anodes (CC) under conditions of a single fixed resistance. However, it was previously shown that performance of the ammonia treated anodes slightly exceeded that of the plain material (Cheng and Logan, 2007). The lack of an increase in power in the present work could have been due to the use of different anode materials here (type A cloth, BASF Fuel Cell Inc.,) compared to those used in previous tests (non-wet proofed, type A, E-TEK). We were not able to obtain exactly the same electrode materials as E-TEK had discontinued sales of the previously used materials. The maximum power density of the anodes with the lowest loading of dimethylaniline (A) was slightly greater than that of the heat (CH) or ammonia (CA) treated anodes, but its performance was much larger than that of the untreated carbon cloth (CC) (Fig. 2). The trend in anode potential (Fig. 2) based on polarization data show relatively large changes, whereas there were relatively small changes in cathode potentials (Fig. S3). This indicated that the measured differences in anode performance were the origin of the changes observed in the MFC performance as the anode treatments were varied, and not changes in cathode performance. Incorporation of dimethylaniline functional groups as predetermined by the diazonium reagent used, which was observed as an
increased N/C ratio, successfully improved power production, consistent with previous studies. It should be noted that no heat was introduced upon functionalization for A, B, and C, which provides support that the presence of amines at the anode surface enhanced the anode performance independent of the effects of heat treatment. However, the maximum power density did not increase with higher N/C ratios achieved by additional deposition of dimethyaniline. Maximum power densities were reduced for anodes with higher loadings of dimethyaniline (B–D), compared to the A anode and the heat (CH) and ammonia treated (CA) anodes. The highest loading (D) produced a lower power density than the plain (CC) carbon electrode. These results demonstrated that excessive dimethylaniline loading impeded performance. While adhesion of bacteria should increase with amine functionalization, highly functionalized surfaces can reduce bacterial viability (Terada et al., 2006). N,N0 -dimethylaniline is also known to possess moderate aquatic toxicity (daphnid acute 48 h LC50 5 mg/L, algae acute 24 h LC50 340 mg/L, fish acute <96 h LC50 33 mg/L) (Sanderson and Thomsen, 2009). In addition, the electrical resistance of the surface measured by a multimeter was not changed as a result of different levels of amine loading. Our results therefore suggest that the lower amount of dimethylaniline functionalization created the optimum condition relative to bacterial adhesion for improving MFC anode performance. This optimal range of the N/C ratio is reinforced by similar N/C ratios for the heat and ammonia treated anodes, compared to other electrodes based on XPS data (Table 1). Although the types of amine functional groups on the CH and CA anodes likely were different than those of the dimethyaniline group, these electrodes performed similarly when they had similar N/C ratios. This was probably due to similar amounts of deprotonated or protonated nitrogen present for A, CH and CA anodes, which was confirmed by XPS
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analysis. Therefore, it can be concluded that the N/C ratio is important for controlling anode performance. 4. Conclusion We have demonstrated that dimethylaniline functionalization of carbon cloth anodes enhanced MFC performance. The lowest amount of dimethylaniline provided the highest maximum power density, suggesting that the absence of nitrogen at the anode surface or too high a nitrogen content impedes anode performance. Low N/C ratios were also observed with ammonia and air treated anodes, which suggested that controlling the nitrogen content in anodes was important to achieve enhanced MFC performance. These findings that methods to increase nitrogen content of the anode electrode, including ammonia, heat, or diazonium treatment, demonstrate a pathway to improved MFC performance through anode modifications. Acknowledgements This research was supported by the National Science Foundation (CBET-08,03,137), Award KUS-11-003-I3 from the King Abdullah University of Science and Technology, and the National Creative Research Groups of China (No. 50821002). We thank Dr. Timothy B. Tighe for helping XPS analysis. We also thank Mr. David W. Jones and Ms. Ellen M. Bingham for technical support for this research. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2010.05.063. References Cheng, S., Logan, B.E., 2007. Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells. Electrochem. Commun. 9, 492–496. Feng, C., Ma, L., Li, F., Mai, H., Lang, X., Fan, S., 2010. A polypyrrole/anthraquinone-2, 6-disulphonic disodium salt (PPy/AQDS)-modified anode to improve performance of microbial fuel cells. Biosens. Bioelectron. 25, 1516–1520.
Feng, Y., Yang, Q., Wang, X., Logan, B.E., 2010. Treatment of carbon fiber brush anodes for improving power generation in air-cathode microbial fuel cells. J. Power Sources 195, 1841–1844. Jansen, R.J.J., Vanbekkum, H., 1995. XPS of nitrogen-containing functional-groups on activated carbon. Carbon 33, 1021–1027. Liu, Y., Harnisch, F., Fricke, K., Schroeder, U., Climent, V., Feliu, J.M., 2010. The study of electrochemically active microbial biofilms on different carbon-based anode materials in microbial fuel cells. Biosens. Bioelectron. 25, 2167–2171. Logan, B.E., 2010. Scaling up microbial fuel cells and other bioelectrochemical systems. Appl. Microbiol. Biotechnol. 85, 1665–1671. Logan, B.E., Hamelers, B., Rozendal, R., Schrorder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K., 2006. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 40, 5181–5192. Lowy, D.A., Tender, L.M., 2008. Harvesting energy from the marine sediment-water interface III. Kinetic activity of quinone- and antimony-based anode materials. J. Power Sources 185, 70–75. Lowy, D.A., Tender, L.M., Zeikus, J.G., Park, D.H., Lovley, D.R., 2006. Harvesting energy from the marine sediment-water interface II – kinetic activity of anode materials. Biosens. Bioelectron. 21, 2058–2063. Qiao, Y., Li, C.M., Bao, S.J., Bao, Q.L., 2007. Carbon nanotube/polyaniline composite as anode material for microbial fuel cells. J. Power Sources 170, 79–84. Sanderson, H., Thomsen, M., 2009. Comparative analysis of pharmaceuticals versus industrial chemicals acute aquatic toxicity classification according to the United Nations classification system for chemicals. Assessment of the (Q)SAR predictability of pharmaceuticals acute aquatic toxicity and their predominant acute toxic mode-of-action. Toxicol. Lett. 187, 84–93. Schroder, U., Niessen, J., Scholz, F., 2003. A generation of microbial fuel cells with current outputs boosted by more than one order of magnitude. Angewandte Chemie-Int. Ed. 42, 2880–2883. Scott, K., Rimbu, G.A., Katuri, K.P., Prasad, K.K., Head, I.M., 2007. Application of modified carbon anodes in microbial fuel cells. Process Saf. Environ. Prot. 85, 481–488. Sharma, T., Reddy, A.L.M., Chandra, T.S., Ramaprabhu, S., 2008. Development of carbon nanotubes and nanofluids based microbial fuel cell. Int. J. Hydrogen Energy 33, 6749–6754. Sun, J.-J., Zhao, H.-Z., Yang, Q.-Z., Song, J., Xue, A., 2010. A novel layer-by-layer selfassembled carbon nanotube-based anode: preparation, characterization, and application in microbial fuel cell. Electrochim. Acta 55, 3041–3047. Terada, A., Yuasa, A., Kushimoto, T., Tsuneda, S., Katakai, A., Tamada, M., 2006. Bacterial adhesion to and viability on positively charged polymer surfaces. Microbiology-Sgm 152, 3575–3583. Toupin, M., Belanger, D., 2008. Spontaneous functionalization of carbon black by reaction with 4-nitrophenyldiazonium cations. Langmuir 24, 1910–1917. Tsai, H.Y., Wu, C.C., Lee, C.Y., Shih, E.P., 2009. Microbial fuel cell performance of multiwall carbon nanotubes on carbon cloth as electrodes. J. Power Sources 194, 199–205. Wang, X., Cheng, S., Feng, Y., Merrill, M.D., Saito, T., Logan, B.E., 2009. Use of carbon mesh anodes and the effect of different pretreatment methods on power production in microbial fuel cells. Environ. Sci. Technol. 43, 6870–6874. Zou, Y.J., Xiang, C.L., Yang, L.N., Sun, L.X., Xu, F., Cao, Z., 2008. A mediatorless microbial fuel cell using polypyrrole coated carbon nanotubes composite as anode material. Int. J. Hydrogen Energy 33, 4856–4862.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of nitrogen addition on the performance of microbial fuel cell anodes Tomonori Saito a,b, Maha Mehanna a, Xin Wang c, Roland D. Cusick a, Yujie Feng c, Michael A. Hickner b, Bruce E. Logan a,* a
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, United States Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, United States c State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No 73 Huanghe Road, Nangang District, Harbin 150090, China b
a r t i c l e
i n f o
Article history: Received 31 March 2010 Received in revised form 10 May 2010 Accepted 18 May 2010 Available online 17 June 2010 Keywords: Microbial fuel cell Anode treatment Bacterial adhesion Diazonium functionalization
a b s t r a c t Carbon cloth anodes were modified with 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate to increase nitrogen-containing functional groups at the anode surface in order to test whether the performance of microbial fuel cells (MFCs) could be improved by controllably modifying the anode surface chemistry. Anodes with the lowest extent of functionalization, based on a nitrogen/carbon ratio of 0.7 as measured by XPS, achieved the highest power density of 938 mW/m2. This power density was 24% greater than an untreated anode, and similar to that obtained with an ammonia gas treatment previously shown to increase power. Increasing the nitrogen/carbon ratio to 3.8, however, decreased the power density to 707 mW/m2. These results demonstrate that a small amount of nitrogen functionalization on the carbon cloth material is sufficient to enhance MFC performance, likely as a result of promoting bacterial adhesion to the surface without adversely affecting microbial viability or electron transfer to the surface. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) use electricity-generating bacteria (exoelectrogens) to oxidize organic matter and produce current. Many strains of bacteria are known to be capable of exogenous electron transfer and are abundant in rivers, ocean sediments, domestic and agricultural wastewaters (Logan et al., 2006). The use of exoelectrogenic bacteria in MFCs shows great promise for using wastewater for renewable energy production. Recently, there have been several reports on pilot scale tests using MFCs and microbial electrolysis cells (MECs) (Logan, 2010). Improving the current and power densities in these systems, however, remains a major challenge. Several chemical and physical modifications of anode surfaces have successfully been used to increase MFC power densities. High temperature treatment of graphite fibers in air (450 °C for 30 min) can result in increased power, likely due to the increase in the surface N/C ratio (Feng et al., 2010; Wang et al., 2009). Heating carbon cloth surfaces at high temperatures with ammonia gas (700 °C for 1 h) reduced the start up time of an MFC and also increased power, likely as a result of increased amine groups on the surface of the carbon cloth, but detailed changes in the surface chemistry were not investigated (Cheng and Logan, 2007). Other reported anode
* Corresponding author. Address: Department of Civil and Environmental Engineering, The Pennsylvania State University, 231Q Sackett Building, University Park, PA 16802, United States. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: [email protected] (B.E. Logan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.05.063
modification methodologies include oxidation of the anode surface using acid (Lowy et al., 2006; Scott et al., 2007) or electrochemical methods (Lowy and Tender, 2008), incorporation of anthraquinone (Feng et al., 2010; Lowy and Tender, 2008; Lowy et al., 2006), addition of polyaniline or polypyrrole (Feng et al., 2010; Qiao et al., 2007; Schroder et al., 2003; Scott et al., 2007; Zou et al., 2008), or the use of carbon nanotubes (Qiao et al., 2007; Sharma et al., 2008; Sun et al., 2010; Tsai et al., 2009; Zou et al., 2008). Increased power and current in all of these cases was likely due to a combination of factors including: (1) enhanced electrical conductivity, (2) increased surface area, and (3) increased biocompatibility of the anode surface (Scott et al., 2007). Understanding the correlation of the anode performance and the anode surface chemistry will be important for continuing to improve in the performance of MFCs. Increasing the surface charge, for example by protonated amine groups (Cheng and Logan, 2007), can increase the adhesion of negatively charged bacteria (Terada et al., 2006) and produce an improved surface for electron transfer between the bacteria and the carbon. The success of heat and ammonia treatments that increased the nitrogen content and the positive surface charge (from 0.38 to 3.99 meq m2) (Cheng and Logan, 2007; Feng et al., 2010; Wang et al., 2009) suggests that amine functionalization of the carbon surface could be useful method to improve microbial adhesion and power generation. An investigation of anode performance with a controlled degree of predetermined amine functional groups has not been previously undertaken with the ammonia gas treatment due to the difficulty in identifying the structure of the amine
396
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groups added to the surface, and the lack of a method to control the degree of functionalization. In the present study, we modified carbon cloth anodes to different extents with a specific amine functional group (dimethylaniline), using 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate, and determined the effect of this modification on the power generation of MFCs. We compared the performance of anodes with different amounts of dimethylaniline functionalization to anodes that were heat or ammonia treated using single-chamber, air-cathode MFCs. 2. Methods 2.1. Anodes Carbon cloth (CC; type A, BASF Fuel Cell Inc., Somerset, NJ) was used as received. The cloth was treated using four different concentrations of 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate (diazonium salt, Wako Chemicals) (A–D), by heating in air (CH) (Wang et al., 2009), or by heating in an ammonia gas atmosphere (CA) (Cheng and Logan, 2007) (Table S1). Dimethylaniline functionalization was performed by placing 1 g of carbon cloth (0.083 mol carbon) into 70 mL DI water containing 0.01 (A), 0.1 (B), 0.2 (C and D) (mol/mol of carbon) of diazonium salt and 0.02 (A), 0.2 (B), 0.4 (C and D) equivalent (mol/mol of carbon) of hypophosphorous acid, (H3PO2, 50 wt.% in water, Aldrich) in a 100 mL round-bottomed flask (Fig. S1). To ensure the carbon cloth was completely wetted, it was first soaked in methanol, then rinsed with water several times before treatment. Reactions were performed at room temperature (25 °C, A–C) or 60 °C (D) for 18 h. Addition of H3PO2 was used to enhance diazonium salt reactivity (Toupin and Belanger, 2008) so that required amount of diazonium salt reagent could be reduced. The heat-treated CH anode was prepared by heating the carbon cloth in a muffle furnace at 450 °C for 30 min as previously described (Wang et al., 2009), while the ammonia treated CA anodes were prepared by treating the carbon cloth at 700 °C for 1 h in a 5% ammonia gas as previously described (Cheng and Logan, 2007). Surface area measurements of these prepared anode materials were not performed as it has previously been shown that there is a lack of correlation between measured surface area values and corresponding performance (Liu et al., 2010). The atomic compositions of the anodes were examined using Xray photoelectron spectroscopy (XPS, PHI Model 5600 MultiTechnique) with a monochromated Al Ka X-ray source (Wang et al., 2009). Samples were dried under reduced pressure at 80 °C prior to analysis. Spectra were obtained over a scan range of 1350– 0 eV, recorded and stored using the PHI ACCESSTM data system, and analyzed using CasaXPS software (Version 2.3.12Dev9). All peaks were identified except Auger peaks. 2.2. MFC configuration and operation Cube-shaped single-chambered MFCs were machined to contain a cylindrical chamber 7 cm2 in projected area, and 2 cm in length, with the electrodes placed on either side of the chamber (liquid volume 14 mL). Cathodes with catalyst loading of 0.5 mg/ cm2 of platinum were prepared by application of a Nafion binder solution (Aldrich, 5 wt.%) containing platinum on carbon black catalyst (10 wt.% platinum on Vulcan XC-72, E-TEK, Sommerset NJ) on wet-proofed carbon cloth (type B-1B, E-TEK) on the electrolyte side, and four PTFE diffusion layers on the air side. Reactors were inoculated with domestic wastewater (Pennsylvania State University Wastewater Treatment Plant) and fed sodium acetate (1 g/L) in 50 mM phosphate buffer (PBS) solution (4.576 g/L Na2HPO4, 2.452 g/L NaH2PO4H2O; pH 7.0; conductiv-
ity of 7 mS/cm) with 0.31 g/L NH4Cl, 0.13 g/L KCl, trace minerals (12.5 mL/L), and vitamins (5 mL/L). MFCs were operated in fedbatch mode at 30 °C. The voltage (V) was monitored across a fixed external resistor (1000 X) using a multimeter and data acquisition system (2700 Keithley, USA). Electrode potentials were measured using a digital multimeter (22-813, Radioshack, USA) with a Ag/AgCl reference electrode (RE-5B, Bioanalytical Systems Inc.; 0.20 V versus a standard hydrogen electrode, SHE). Polarization and power density curves were obtained by measuring the stable voltage at 20 min intervals generated under various external resistances ranging between 1000 and 50 X. Current (I = V/R) and power (P = IV) were calculated as previously described (Logan et al., 2006).
3. Results and discussion 3.1. Modification of carbon cloth anodes Increasing the concentration of diazonium salt reactant during the carbon cloth treatment correlated with an increase in the N1s intensity in the XPS spectra (Fig. S2) and calculated N/C ratios (Table 1). The N/C ratios of the carbon surface increased from 0.7 to 1.8 (A–C) upon addition of higher concentrations of diazonium salt reactant (Table 1). Higher reaction temperatures also contributed to an increase in N1s intensity, and thus the N/C ratio (1.8, C; 3.8, D). N/C ratios were 0.5 for the CH anode, and 1.2 for the CA anode, which are similar to those for the A (0.7) and B (1.0) anodes. The gain in nitrogen content upon heating or ammonia gas treatment is consistent with the XPS results in previous reports for heat-treated carbon fiber brush (Feng et al., 2010), heat-treated carbon mesh (Wang et al., 2009) and ammonia treated carbon mesh (Wang et al., 2009). A significant amount of charged nitrogen (protonated nitrogen), which should show up as peaks at 402 eV (Jansen and Vanbekkum, 1995), was not observed in XPS spectra for the dimethylaniline functionalized carbon cloth anodes (A–D), whereas the presence of peaks at 400 eV corresponds to deprotonated nitrogen and suggests the presence of deprotonated dimethylaniline groups (Fig. S2). The presence of the deprotonated dimethylaniline groups should be consistent at pH 7 due to the pKa 5. The presence of protonated nitrogen for the heat-treated anode (CH) was also insignificant in the XPS spectrum, although the increased protonated amine from XPS spectra for a heat-treated carbon fiber brush anode correlated to the increased anode performance in a previous report (Feng et al., 2010). These contrasting results are probably due to the different materials used in these studies. Thermogravimetric analysis for each anode sample was also performed, but the difference in weight loss upon heating was insignificant (0.1–0.2 wt.%; data not shown). XPS was therefore the more sensitive analytical method to quantify the degree of functionalization due to surface modification.
Table 1 N/C ratios of carbon cloth anodes as determined from XPS.
A B C D CC CH CA
C1s (%)
N1s (%)
O1s (%)
N/C (%)
97.8 95.9 94.8 93.6 95.2 97.7 96.6
0.7 1.0 1.7 3.6 0 0.5 1.1
1.6 3.2 3.5 2.8 3.4 1.8 2.3
0.7 1.0 1.8 3.8 0 0.5 1.2
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Fig. 1. (A) Cell voltage as a function of time for MFCs with different carbon cloth anodes. (B) Maximum cell voltage values for each cycle. A–D, dimethylaniline functionalized carbon cloth anodes; CH, heat-treated carbon cloth anode; CA, ammonia treated carbon cloth anode; CC, untreated carbon cloth anode.
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Fig. 2. (A) Power density and (B) corresponding anode potentials for MFCs with different anodes.
3.2. MFC performance All the MFCs exhibited similar acclimation times of 60 h, defined as the time to reach the first reproducible maximum power density (Fig. 1). While there was no significant differences between the time for acclimation with plain carbon cloth (CC) or the treated anodes, the acclimation time for the MFC with the ammonia treated anode (CA) was consistent with previous reports (Cheng and Logan, 2007). These results on acclimation times and power densities were found to be reproducible based on repeated experiments (data not shown). The performance of the treated anodes after 60 h was found to be the same as that of the plain carbon anodes (CC) under conditions of a single fixed resistance. However, it was previously shown that performance of the ammonia treated anodes slightly exceeded that of the plain material (Cheng and Logan, 2007). The lack of an increase in power in the present work could have been due to the use of different anode materials here (type A cloth, BASF Fuel Cell Inc.,) compared to those used in previous tests (non-wet proofed, type A, E-TEK). We were not able to obtain exactly the same electrode materials as E-TEK had discontinued sales of the previously used materials. The maximum power density of the anodes with the lowest loading of dimethylaniline (A) was slightly greater than that of the heat (CH) or ammonia (CA) treated anodes, but its performance was much larger than that of the untreated carbon cloth (CC) (Fig. 2). The trend in anode potential (Fig. 2) based on polarization data show relatively large changes, whereas there were relatively small changes in cathode potentials (Fig. S3). This indicated that the measured differences in anode performance were the origin of the changes observed in the MFC performance as the anode treatments were varied, and not changes in cathode performance. Incorporation of dimethylaniline functional groups as predetermined by the diazonium reagent used, which was observed as an
increased N/C ratio, successfully improved power production, consistent with previous studies. It should be noted that no heat was introduced upon functionalization for A, B, and C, which provides support that the presence of amines at the anode surface enhanced the anode performance independent of the effects of heat treatment. However, the maximum power density did not increase with higher N/C ratios achieved by additional deposition of dimethyaniline. Maximum power densities were reduced for anodes with higher loadings of dimethyaniline (B–D), compared to the A anode and the heat (CH) and ammonia treated (CA) anodes. The highest loading (D) produced a lower power density than the plain (CC) carbon electrode. These results demonstrated that excessive dimethylaniline loading impeded performance. While adhesion of bacteria should increase with amine functionalization, highly functionalized surfaces can reduce bacterial viability (Terada et al., 2006). N,N0 -dimethylaniline is also known to possess moderate aquatic toxicity (daphnid acute 48 h LC50 5 mg/L, algae acute 24 h LC50 340 mg/L, fish acute <96 h LC50 33 mg/L) (Sanderson and Thomsen, 2009). In addition, the electrical resistance of the surface measured by a multimeter was not changed as a result of different levels of amine loading. Our results therefore suggest that the lower amount of dimethylaniline functionalization created the optimum condition relative to bacterial adhesion for improving MFC anode performance. This optimal range of the N/C ratio is reinforced by similar N/C ratios for the heat and ammonia treated anodes, compared to other electrodes based on XPS data (Table 1). Although the types of amine functional groups on the CH and CA anodes likely were different than those of the dimethyaniline group, these electrodes performed similarly when they had similar N/C ratios. This was probably due to similar amounts of deprotonated or protonated nitrogen present for A, CH and CA anodes, which was confirmed by XPS
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analysis. Therefore, it can be concluded that the N/C ratio is important for controlling anode performance. 4. Conclusion We have demonstrated that dimethylaniline functionalization of carbon cloth anodes enhanced MFC performance. The lowest amount of dimethylaniline provided the highest maximum power density, suggesting that the absence of nitrogen at the anode surface or too high a nitrogen content impedes anode performance. Low N/C ratios were also observed with ammonia and air treated anodes, which suggested that controlling the nitrogen content in anodes was important to achieve enhanced MFC performance. These findings that methods to increase nitrogen content of the anode electrode, including ammonia, heat, or diazonium treatment, demonstrate a pathway to improved MFC performance through anode modifications. Acknowledgements This research was supported by the National Science Foundation (CBET-08,03,137), Award KUS-11-003-I3 from the King Abdullah University of Science and Technology, and the National Creative Research Groups of China (No. 50821002). We thank Dr. Timothy B. Tighe for helping XPS analysis. We also thank Mr. David W. Jones and Ms. Ellen M. Bingham for technical support for this research. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2010.05.063. References Cheng, S., Logan, B.E., 2007. Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells. Electrochem. Commun. 9, 492–496. Feng, C., Ma, L., Li, F., Mai, H., Lang, X., Fan, S., 2010. A polypyrrole/anthraquinone-2, 6-disulphonic disodium salt (PPy/AQDS)-modified anode to improve performance of microbial fuel cells. Biosens. Bioelectron. 25, 1516–1520.
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