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Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells
Electrochemistry Communications 9 (2007) 492–496 www.elsevier.com/locate/elecom
Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells Shaoan Cheng, Bruce E. Logan
*
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA, 16802, USA Received 10 October 2006; accepted 11 October 2006 Available online 9 November 2006
Abstract Changes in microbial fuel cell (MFC) architecture, materials, and solution chemistry can be used to increase power generation by microbial fuel cells (MFCs). It is shown here that using a phosphate buffer to increase solution conductivity, and ammonia gas treatment of a carbon cloth anode substantially increased the surface charge of the electrode (from 0.38 to 3.99 meq m 2), and improved MFC performance. Power increased to 1640 mW m 2 (96 W m 3) using a phosphate buffer, and further to 1970 mW m 2 (115 W m 3) using an ammonia-treated electrode. The combined effects of these two treatments boosted power production by 48% compared to previous results using this air-cathode MFC. In addition, the start up time of an MFC was reduced by 50%. 2006 Elsevier B.V. All rights reserved. Keywords: Microbial fuel cell; Biofuel cell; Ammonium treatment; Positive charge; Power density
1. Introduction Microbial fuel cells (MFCs) use bacteria to directly catalyze the conversion of organic matter to electricity. Many different strains of bacteria are now known to be capable of exogenous electron transfer. These bacteria, known as exoelectrogens, include iron reducing bacteria such as Shewanella putrefaciens [1–3] and several Geobacteraceae strains [4–8], but also clostridia such as Clostridium butyricum [9] and pseudomonads [10,11]. Reactors inoculated domestic wastewater, and river and marine sediments [1,6] also produce power in MFCs with a variety of substrates including glucose, acetate, lactate, and proteins, as well as with domestic and animal wastewaters [12,25,14–19]. The ability to generate power using bacteria already in the wastewater make MFCs a promising technology for wastewater treatment [12,16,20]. However, in order to make the process economical, power densities need to be increased. Several factors have been studied to improve the performance of an MFC including acclimation of the inoculum *
Corresponding author. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: [email protected] (B.E. Logan).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.023
[10], presence or absence of a cation exchange material [12,21], increased solution conductivity by varying the solution ionic strength [25], reducing the electrode spacing, varying cell architectures to direct flow through the anode towards the cathode [15], and using different cathode materials [5,22–24]. Power was increased from 720 to 1330 mW m 2 through an increase in the solution ionic strength using NaCl from 100 to 400 mM [13]. With continuous, power was further be increased to 1540 mW m 2 by providing flow through a porous carbon cloth anode with additional decreases in electrode spacing [15]. Power production by MFCs has been increased by binding different materials to the anode. In many cases, materials have been chosen based on their ability to transfer electrons. For example, binding the electron mediator neutral red to the anode increased current by 10-fold using Shewanella putrefaciens [26]. Power was also increased using other known electron mediators such as anthraquinone-1,6-dissulfonic acid (AQDS) or 1,4-Naphthoquinone (NQ) [27]. However, in these cases the maximum power achieved was low (<120 mW/m2). Exoelectrogenic bacteria such as S. oneidensis [28] and G. sulfurreducens [29] produce nanowires that facilitate
S. Cheng, B.E. Logan / Electrochemistry Communications 9 (2007) 492–496
electron transfer to carbon electrodes. While the exact molecules that convey electrons to the surface of the electrode have yet to be identified, it has been shown in the literature that adding specific chemicals to highly conductive surfaces such as gold can greatly affect electron transfer. For example, Whitesides and co-workers [30,31] have shown how different self assembled monolayers (SAMs) of chemicals can affect electron transfer from cytochrome c to gold electrodes. It has been shown that precipitating iron oxides onto carbon electrodes reduced acclimation times needed to produce power in MFCs using mixed cultures [32]. Similarly, adding Mn4+ [14], Fe3O4, or Fe3O4 and Ni2+ to graphite anodes [27] also increased power production. It is also well known that the presence of positively charged compounds to naturally occurring surfaces such as quartz and sand increases the adhesion of negatively-charged bacteria due to electrostatic attraction of the cells to the surface [33–35]. We reasoned based on these studies that inefficient attachment of bacterial nanowires to carbon cloth electrodes could be limiting power production. In this paper, we show that increasing the positive charge on the carbon surface using an ammonia gas treatment reduces the enrichment time and increases power production in a mixed culture MFC. 2. Materials and methods 2.1. Electrodes The anode was either ammonia-treated or plain carbon cloth (non-wet proofed, type A, E-TEK). Ammonia gas treatment on carbon cloth was conducted using a thermogravimetric analyzer (TGA) [36]. The furnace temperature was ramped up to 700 C at 50 C/min using nitrogen gas (70 mL/min) before switching the gas feed to 5% NH3 in helium gas. The sample was then held at 700 C for 60 min, before being cooled back to room temperature under nitrogen gas (70 mL/min) over 120 min. The carbon cloth cathode contained a Pt catalyst (0.5 mg cm 2 Pt) and four diffusion layers was prepared as previously described [24]. Both electrodes had a projected surface area of 7 cm2. 2.2. MFC inoculation and operation Single chamber air-cathode MFCs were constructed as previous described with the electrode spacing set at 2 cm [15]. The MFCs were inoculated with domestic wastewater (50/50 v/v) collected from the primary clarifier of the Pennsylvania State University Wastewater Treatment Plant and a phosphate buffered nutrient solution (PBS, 50 mM; [15]) containing 1 g/L sodium acetate. This solution was replaced until the similar output voltage produced over two consecutive cycles, typically requiring five or more solution changes over 120 h (1 kX fixed external resistance). The solution was then switched to a feed solution containing sodium acetate (1 g L 1) and a higher concentration of PBS (200 mM). The 200 mM PBS solution contained: NH4Cl (0.31 g L 1);
493
NaH2PO4 Æ H2O (19.88 g L 1); Na2HPO4 Æ H2O (11 g L 1); KCl (0.13 g L 1), and a metal (12.5 mL) and vitamin (5 mL) solution [23]. The feed solution was replaced when the voltage dropped below 20 mV, forming one complete cycle of operation. Polarization curves were obtained by measuring the stable voltage generated at various external resistances, and then used to evaluate the maximum power density [37]. At each external resistance, the MFC ran for at least two complete operation cycles. All tests were conducted in a 30 C temperature-controlled room. 2.3. Analytical measurements and calculations Cell voltage across an external resistor was recorded using a multimeter with a data acquisition system (2700, Keithly). Current density was calculated as i = V/RA, where V (mV) is the voltage, R (X) the external resistance, and A (cm2) the geometric surface area of the anode electrode. Power density (mW m 2) was calculated as P = 10iV (10 is used for unit conversions), and Coulombic efficiency was calculated as Ec = Cp/Cth · 100%, where Cp (C) is the total Coulombs calculated by integrating the current over time, and Cth is the theoretical amount of Coulombs available from the oxidation of acetate. Surface charge was conducted on a Mettler Toledo DL53 titrator (Mettler Toledo Inc., Columbus, OH) according to the method presented by Chen et al. [38] using a 0.01 M NaCl electrolyte. Carbon cloth (about 0.15 g) was cut to small pieces (5 mm · 5 mm) before adding the electrolyte (200 mL). Titrations were conducted with a pseudo-equilibration time of 10 min, with each sample analyzed in duplicate. 3. Results 3.1. Acclimation time Following inoculation, the MFC containing the untreated carbon cloth anode required 150 h before reaching the first maximum power production (Fig. 1). The reactor was then refueled five times before the cell voltages became reproducible in terms of maximum voltages and duration of current generation. Using the ammoniatreated carbon cloth anode, the first maximum power cycle was reduced to 60 h, with a reproducible cycle of voltage production requiring a number of refueling cycles similar to that obtained with the untreated anode. The reduction of time needed to produce the initial maximum voltage using the ammonium-treated anode was found to be reproducible in additional tests (data not shown). These results suggest that bacterial attachment to the anode electrode was greatly improved using the ammonium treatment process. 3.2. Reactor performance The maximum power density and coulombic efficiency were both increased as a result of increased phosphate
494
S. Cheng, B.E. Logan / Electrochemistry Communications 9 (2007) 492–496
70
0.6 Treated Untreated
50
0.4 0.3
40 30 20
0.2
10
0.1
0
0 0
100
200
Fig. 1. Enrichment with a mixed solution of phosphate buffered nutrient medium (50 mM) containing 1 g/L sodium acetate with domestic wastewater (50/50 v/v) for MFCs with different anodes. Each spike in power generation was followed by re-fueling of the reactor with new substrate, resulting in the next cycle of power generation.
concentration and ammonium gas treatment of the anode. The maximum power density of the MFC with a 200 mM phosphate buffer (untreated anode) was 1640 mW m 2 (Fig. 2) versus 1330 m 2 previously found by increasing solution conductivity using NaCl [25]. Using an ammonium-treated anode, the maximum power density increased to 1970 mW m 2 and volumetric power density increased to 115 W/m3 (Fig. 2). This is the highest power density obtained to date using an air-cathode MFC, and represents an increase of 48% based on surface area or volume compared to previous results using this reactor operated in a fed batch mode (1330 mW m 2, 77 W m 2; [25]). The Coulombic efficiency (CE) with the ammonia-treated anode ranged from 30% to 60% depending on the current density, with values approximately 20% higher than those obtained with untreated anode and the phosphate buffer (Fig. 3). These CEs with phosphate buffer are similar to those previously obtained using NaCl to increase system performance (25–61%; [25]). The increased performance of the anode can be attributed to the increased surface charge of the carbon cloth. Ammonia treatment of the carbon cloth increased the surface charge from 0.38 meq m 2 to 3.99 meq m 2 at pH 7. 2000 -2
Power density (mW m )
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
1500 1000 V-Treated V-Untreated P-Treated P-Untreated
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0.2
0.4
500
0.6
0.8
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Current density (mA cm ) Fig. 2. Power density for MFCs with different anodes, 200 mM phosphate buffered nutrient medium containing 1 g/L sodium acetate.
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-2
Current density (mA cm )
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Time (h)
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Fig. 3. Coulombic efficiency for MFCs with different anodes, 200 mM phosphate buffered nutrient medium containing 1 g/L sodium acetate.
The increase in positive charge was due to the formation of nitrogen-containing surface functional groups on the carbon cloth surface during the ammonium treatment, which has been previously shown by elemental analysis of the surface [38]. 4. Discussion Ammonia treatment of the carbon anode, in concert with several other recently developed techniques to increase power in MFCs, resulted here in a maximum power density of 1970 mW m 2 based on anode surface area, or 115 W/m3 based on reactor volume. This power density larger by a factor of 7.5 times as much power as first achieved with this basic reactor system (292 mW m 2, [12]). Increases in power density have resulted from removal of a proton exchange membrane bonded to the cathode (494 mW m 2 [12]), reduced electrode spacing (1210 mW m 2 [13]), and increases in solution conductivity (1330 mW m 2 [13]). The higher initial power production of the untreated anode measured here (1640 mW m 2) than that previously obtained in the same system (1330 mW m 2 [13]) was due to the use of a different electrolyte to increase solution conductivity. In the previous study, NaCl was used to increase solution ionic strength. Increasing the Cl concentration can affect the oxygen reduction activity of the cathode [39,40]. Here we used a phosphate buffer which not only increased solution conductivity but also eliminated this adverse effect of Cl on the cathode. Furthermore, a high concentration of PBS provided additional buffering capacity at the electrodes to reduce the effect of pH increases at the cathode due to the rapid consumption of protons at higher power densities [41]. The improvement in power generation with the ammonium-treated anode could result from two different factors: increased adhesion of bacteria during reactor start up, and an increase in the efficiency of electron transfer from the bacteria to the surface. The substantial reduction in acclimation time demonstrates that bacterial adhesion to the surface was important. The higher power density of the system suggests that electron transfer was also improved due to the ammonium treatment. At higher power densities
S. Cheng, B.E. Logan / Electrochemistry Communications 9 (2007) 492–496
(>0.5 mA cm 2; Fig. 2), we observed from the polarization curve that the cell voltage remained high, suggesting that the presence of functional groups at the surface improved electron flow to the electrode surface. Polarization curves of the ammonia-treated and the untreated anodes were identical at lower current densities (<0.3 mA cm 2), indicating that the activation energy needed for electron transfer to the surface was unaffected. This increase in power is not due to ammonia as a fuel, as it has been shown that electricity is not generated from ammonia in an MFC [42]. This increase in MFC performance was due to nearly an order-of-magnitude increase in the surface charge (3.99 meq m 2 at pH 7) of the electrode. In preliminary experiments, the carbon cloth anode was treated with H2 and then CH4 gas under the same temperature conditions. However, this treatment reduced the charge of carbon cloth surface, and MFC tests with this material showed a decrease in the maximum power density (data not shown). Thus, these results indicate that the improved performance obtained here with ammonia gas treatment was due to increased surface charge, and not simply from high temperature treatment of the carbon cloth electrode. Ammonium treatment of carbon electrodes should be a widely applicable method for increasing power generation with different anode materials. For example, other anode materials used in previous MFC studies such as carbon paper [13], graphite granules [43], graphite felt [5], and reticulated vitreous carbon (RVC, [44]) could all be treated using a similar ammonium gas treatment. Treatment of these materials can be expected to facilitate bacterial adhesion and increased electron transfer to the anode surface. 5. Conclusions Ammonium treatment of the anode increased the surface charge of carbon cloth electrodes and improved MFC performance. The increase in power density to 1970 mW m 2 (115 W m 3) resulted both improved bacterial adhesion and the improved performance of the system at higher current densities (0.5 to 0.9 mA cm 2). The results demonstrate a new approach for improving power generation in MFCs. Acknowledgments The authors thank Dr. Weifang Chen and Adam Redding for help with surface charge measurements and ammonium gas treatment. This research was supported by National Science Foundation Grant BES-0401885. References [1] H.J. Kim, H.S. Park, M.S. Hyun, I.S. Chang, M. Kim, B.H. Kim, Enzyme Microbiol. Technol. 30 (2002) 145. [2] B.H. Kim, H.J. Kim, M.S. Hyun, D.H. Park, J. Microbiol. Biotechnol. 9 (1999) 127. [3] H.J. Kim, M.S. Hyun, I.S. Chang, B.H. Kim, J. Microbiol. Biotechnol. 9 (1999) 365.
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[4] D.R. Bond, D.R. Lovley, Appl. Environ. Microbiol. 69 (2003) 1548. [5] J.K. Jang, T.H. Pham, I.S. Chang, K.H. Kang, H. Moon, K.S. Cho, B.H. Kim, Process Biochem. 39 (2004) 1007. [6] D.R. Bond, D.E. Holmes, L.M. Tender, D.R. Lovley, Science 295 (2002) 483. [7] C.E. Reimers, L.M. Tender, S. Ferig, W. Wang, Environ. Sci. Technol. 35 (2001) 192. [8] L.M. Tender, C.E. Reimers, H.A. Stecher, D.E. Holmes, D.R. Bond, D.A. Lowy, K. Pilobello, S.J. Fertig, D.R. Lovley, Nat. Biotechnol. 20 (2002) 821. [9] H.S. Park, B.H. Kim, H.S. Kim, H.J. Kim, G.T. Kim, M. Kim, I.S. Chang, Y.K. Park, H.I. Chang, Anaerobe 7 (2001) 297. [10] K. Rabaey, N. Boon, M. Ho¨fte, W. Verstraete, Appl. Environ. Microbiol. 70 (2004) 5373. [11] K. Rabaey, N. Boon, S.D. Siciliano, M. Verhaege, W. Verstraete, Environ. Sci. Technol. 39 (2005) 3401. [12] H. Liu, B.E. Logan, Environ. Sci. Technol. 38 (2004) 4040. [13] H. Liu, S.A. Cheng, B.E. Logan, Environ. Sci. Technol. 39 (2005) 5488. [14] D.H. Park, J.G. Zeikus, Biotechnol. Bioeng. 81 (2003) 348. [15] S.A. Cheng, H. Liu, B.E. Logan, Environ. Sci. Technol. 40 (2006) 2426. [16] G.C. Gil, I.S. Chang, B.H. Kim, M. Kim, J.K. Jang, H.S. Park, H.J. Kim, Biosens. Bioelectron. 18 (2003) 327. [17] J. Heilmann, B.E. Logan, Water Environ. Res. 78 (2006) 531. [18] Y. Zuo, Y.P.C. Maness, B.E. Logan, Engery & Fuels 20 (2006) 1716. [19] B. Min, J.R. Kim, S.E. Oh, J.M. Regan, B.E. Logan, Water Res. 39 (2005) 4961. [20] H. Liu, R. Ramnarayanan, B.E. Logan, Environ. Sci. Technol. 38 (2004) 2281. [21] S. Oh, B.E. Logan, Appl. Microbiol. Biotechnol. 70 (2006) 162. [22] F. Zhao, F. Harnisch, U. Schro¨der, F. Scholz, P. Bogdanoff, I. Herrmann, Electrochem. Commun. 7 (2005) 1405. [23] S.A. Cheng, H. Liu, B.E. Logan, Environ. Sci. Technol. 40 (2006) 364. [24] S.A. Cheng, H. Liu, B.E. Logan, Electrochem. Commun. 8 (2006) 489. [25] H. Liu, S.A. Cheng, B.E. Logan, Environ. Sci. Technol. 39 (2005) 658. [26] D.H. Park, J.G. Zeikus, Appl. Microbiol. Biotechnol. 59 (2002) 58. [27] D.A. Lowy, L.M. Tender, J.G. Zeikus, D.H. Park, D.R. Lovley, Biosens. Bioelectron. 21 (2006) 2058. [28] Y.A. Gorby, S. Yanina, J.S. McLean, K.M. Rosso, D. Moyles, A. Dohnalkova, T.J. Beveridge, I.S. Chang, B.H. Kim, K.S. Kim, D.E. Culley, S.B. Reed, M.F. Romine, D.A. Saffarini, E.A. Hill, L. Shi, D.A. Elias, D.W. Kennedy, G. Pinchuk, K. Watanabe, S. Ishii, B.E. Logan, K.H. Nealson, J.K. Fredrickson, PNAS 103 (2006) 11358. [29] G. Reguera, K.D. McCarthy, T. Mehta, J.S. Nicoll, M.T. Tuominen, D.R. Lovley, Nature 435 (2005) 1098. [30] E. Ostuni, R.G. Chaoman, M.N. Liang, G. Meluleni, G. Pier, D.E. Ingber, G.M. Whitesides, Langmuir 17 (2001) 6336. [31] X. Chen, R. Ferrigno, J. Yang, G.M. Whitesides, Langmuir 18 (2002) 7009. [32] J.R. Kim, B. Min, B.E. Logan, Appl. Microbiol. Biotechnol. 68 (2005) 23. [33] W.P. Johnson, B.E. Logan, Water Res. 30 (1996) 923. [34] M. Elimelech, M. Nagai, C.H. Ko, J.N. Ryan, Environ. Sci. Technol. 34 (2000) 2143. [35] B.K. Li, B.E. Logan, Colloids Surf. B: Biointer 36 (2004) 81. [36] W.F. Chen, F.S. Cannon, J.R. Rangel-Mendez, Carbon 43 (2005) 581. [37] B.E. Logan, P. Aelterman, B. Hamelers, R. Rozendal, U. Schro¨eder, J. Keller, S. Freguiac, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006) 5181. [38] W.F. Chen, F.S. Cannon, J.R. Rangel-Mendez, Carbon 43 (2005) 573.
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[39] V. Stamenkovic, N.M. Markovic, P.N. Ross Jr., J. Electroanal. Chem. 500 (2001) 44. [40] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, R.J. Behm, J. Electroanal. Chem. 508 (2001) 41. [41] R.A. Rozendal, H.V.M. Hamelers, C.J.N. Buisman, Environ. Sci. Technol. 40 (2006) 5206.
[42] J.-R. Kim. PhD dissertation, Dept. Civil & Environ. Eng., Penn State University, University Park, PA (2006). [43] K. Rabaey, P. Clauwaert, P. Aelterman, W. Verstraete, Environ. Sci. Technol. 39 (2005) 8077. [44] Z. He, S.D. Minteer, L.T. Angenent, Environ. Sci. Technol. 39 (2005) 5262.
Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells Shaoan Cheng, Bruce E. Logan
*
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA, 16802, USA Received 10 October 2006; accepted 11 October 2006 Available online 9 November 2006
Abstract Changes in microbial fuel cell (MFC) architecture, materials, and solution chemistry can be used to increase power generation by microbial fuel cells (MFCs). It is shown here that using a phosphate buffer to increase solution conductivity, and ammonia gas treatment of a carbon cloth anode substantially increased the surface charge of the electrode (from 0.38 to 3.99 meq m 2), and improved MFC performance. Power increased to 1640 mW m 2 (96 W m 3) using a phosphate buffer, and further to 1970 mW m 2 (115 W m 3) using an ammonia-treated electrode. The combined effects of these two treatments boosted power production by 48% compared to previous results using this air-cathode MFC. In addition, the start up time of an MFC was reduced by 50%. 2006 Elsevier B.V. All rights reserved. Keywords: Microbial fuel cell; Biofuel cell; Ammonium treatment; Positive charge; Power density
1. Introduction Microbial fuel cells (MFCs) use bacteria to directly catalyze the conversion of organic matter to electricity. Many different strains of bacteria are now known to be capable of exogenous electron transfer. These bacteria, known as exoelectrogens, include iron reducing bacteria such as Shewanella putrefaciens [1–3] and several Geobacteraceae strains [4–8], but also clostridia such as Clostridium butyricum [9] and pseudomonads [10,11]. Reactors inoculated domestic wastewater, and river and marine sediments [1,6] also produce power in MFCs with a variety of substrates including glucose, acetate, lactate, and proteins, as well as with domestic and animal wastewaters [12,25,14–19]. The ability to generate power using bacteria already in the wastewater make MFCs a promising technology for wastewater treatment [12,16,20]. However, in order to make the process economical, power densities need to be increased. Several factors have been studied to improve the performance of an MFC including acclimation of the inoculum *
Corresponding author. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: [email protected] (B.E. Logan).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.023
[10], presence or absence of a cation exchange material [12,21], increased solution conductivity by varying the solution ionic strength [25], reducing the electrode spacing, varying cell architectures to direct flow through the anode towards the cathode [15], and using different cathode materials [5,22–24]. Power was increased from 720 to 1330 mW m 2 through an increase in the solution ionic strength using NaCl from 100 to 400 mM [13]. With continuous, power was further be increased to 1540 mW m 2 by providing flow through a porous carbon cloth anode with additional decreases in electrode spacing [15]. Power production by MFCs has been increased by binding different materials to the anode. In many cases, materials have been chosen based on their ability to transfer electrons. For example, binding the electron mediator neutral red to the anode increased current by 10-fold using Shewanella putrefaciens [26]. Power was also increased using other known electron mediators such as anthraquinone-1,6-dissulfonic acid (AQDS) or 1,4-Naphthoquinone (NQ) [27]. However, in these cases the maximum power achieved was low (<120 mW/m2). Exoelectrogenic bacteria such as S. oneidensis [28] and G. sulfurreducens [29] produce nanowires that facilitate
S. Cheng, B.E. Logan / Electrochemistry Communications 9 (2007) 492–496
electron transfer to carbon electrodes. While the exact molecules that convey electrons to the surface of the electrode have yet to be identified, it has been shown in the literature that adding specific chemicals to highly conductive surfaces such as gold can greatly affect electron transfer. For example, Whitesides and co-workers [30,31] have shown how different self assembled monolayers (SAMs) of chemicals can affect electron transfer from cytochrome c to gold electrodes. It has been shown that precipitating iron oxides onto carbon electrodes reduced acclimation times needed to produce power in MFCs using mixed cultures [32]. Similarly, adding Mn4+ [14], Fe3O4, or Fe3O4 and Ni2+ to graphite anodes [27] also increased power production. It is also well known that the presence of positively charged compounds to naturally occurring surfaces such as quartz and sand increases the adhesion of negatively-charged bacteria due to electrostatic attraction of the cells to the surface [33–35]. We reasoned based on these studies that inefficient attachment of bacterial nanowires to carbon cloth electrodes could be limiting power production. In this paper, we show that increasing the positive charge on the carbon surface using an ammonia gas treatment reduces the enrichment time and increases power production in a mixed culture MFC. 2. Materials and methods 2.1. Electrodes The anode was either ammonia-treated or plain carbon cloth (non-wet proofed, type A, E-TEK). Ammonia gas treatment on carbon cloth was conducted using a thermogravimetric analyzer (TGA) [36]. The furnace temperature was ramped up to 700 C at 50 C/min using nitrogen gas (70 mL/min) before switching the gas feed to 5% NH3 in helium gas. The sample was then held at 700 C for 60 min, before being cooled back to room temperature under nitrogen gas (70 mL/min) over 120 min. The carbon cloth cathode contained a Pt catalyst (0.5 mg cm 2 Pt) and four diffusion layers was prepared as previously described [24]. Both electrodes had a projected surface area of 7 cm2. 2.2. MFC inoculation and operation Single chamber air-cathode MFCs were constructed as previous described with the electrode spacing set at 2 cm [15]. The MFCs were inoculated with domestic wastewater (50/50 v/v) collected from the primary clarifier of the Pennsylvania State University Wastewater Treatment Plant and a phosphate buffered nutrient solution (PBS, 50 mM; [15]) containing 1 g/L sodium acetate. This solution was replaced until the similar output voltage produced over two consecutive cycles, typically requiring five or more solution changes over 120 h (1 kX fixed external resistance). The solution was then switched to a feed solution containing sodium acetate (1 g L 1) and a higher concentration of PBS (200 mM). The 200 mM PBS solution contained: NH4Cl (0.31 g L 1);
493
NaH2PO4 Æ H2O (19.88 g L 1); Na2HPO4 Æ H2O (11 g L 1); KCl (0.13 g L 1), and a metal (12.5 mL) and vitamin (5 mL) solution [23]. The feed solution was replaced when the voltage dropped below 20 mV, forming one complete cycle of operation. Polarization curves were obtained by measuring the stable voltage generated at various external resistances, and then used to evaluate the maximum power density [37]. At each external resistance, the MFC ran for at least two complete operation cycles. All tests were conducted in a 30 C temperature-controlled room. 2.3. Analytical measurements and calculations Cell voltage across an external resistor was recorded using a multimeter with a data acquisition system (2700, Keithly). Current density was calculated as i = V/RA, where V (mV) is the voltage, R (X) the external resistance, and A (cm2) the geometric surface area of the anode electrode. Power density (mW m 2) was calculated as P = 10iV (10 is used for unit conversions), and Coulombic efficiency was calculated as Ec = Cp/Cth · 100%, where Cp (C) is the total Coulombs calculated by integrating the current over time, and Cth is the theoretical amount of Coulombs available from the oxidation of acetate. Surface charge was conducted on a Mettler Toledo DL53 titrator (Mettler Toledo Inc., Columbus, OH) according to the method presented by Chen et al. [38] using a 0.01 M NaCl electrolyte. Carbon cloth (about 0.15 g) was cut to small pieces (5 mm · 5 mm) before adding the electrolyte (200 mL). Titrations were conducted with a pseudo-equilibration time of 10 min, with each sample analyzed in duplicate. 3. Results 3.1. Acclimation time Following inoculation, the MFC containing the untreated carbon cloth anode required 150 h before reaching the first maximum power production (Fig. 1). The reactor was then refueled five times before the cell voltages became reproducible in terms of maximum voltages and duration of current generation. Using the ammoniatreated carbon cloth anode, the first maximum power cycle was reduced to 60 h, with a reproducible cycle of voltage production requiring a number of refueling cycles similar to that obtained with the untreated anode. The reduction of time needed to produce the initial maximum voltage using the ammonium-treated anode was found to be reproducible in additional tests (data not shown). These results suggest that bacterial attachment to the anode electrode was greatly improved using the ammonium treatment process. 3.2. Reactor performance The maximum power density and coulombic efficiency were both increased as a result of increased phosphate
494
S. Cheng, B.E. Logan / Electrochemistry Communications 9 (2007) 492–496
70
0.6 Treated Untreated
50
0.4 0.3
40 30 20
0.2
10
0.1
0
0 0
100
200
Fig. 1. Enrichment with a mixed solution of phosphate buffered nutrient medium (50 mM) containing 1 g/L sodium acetate with domestic wastewater (50/50 v/v) for MFCs with different anodes. Each spike in power generation was followed by re-fueling of the reactor with new substrate, resulting in the next cycle of power generation.
concentration and ammonium gas treatment of the anode. The maximum power density of the MFC with a 200 mM phosphate buffer (untreated anode) was 1640 mW m 2 (Fig. 2) versus 1330 m 2 previously found by increasing solution conductivity using NaCl [25]. Using an ammonium-treated anode, the maximum power density increased to 1970 mW m 2 and volumetric power density increased to 115 W/m3 (Fig. 2). This is the highest power density obtained to date using an air-cathode MFC, and represents an increase of 48% based on surface area or volume compared to previous results using this reactor operated in a fed batch mode (1330 mW m 2, 77 W m 2; [25]). The Coulombic efficiency (CE) with the ammonia-treated anode ranged from 30% to 60% depending on the current density, with values approximately 20% higher than those obtained with untreated anode and the phosphate buffer (Fig. 3). These CEs with phosphate buffer are similar to those previously obtained using NaCl to increase system performance (25–61%; [25]). The increased performance of the anode can be attributed to the increased surface charge of the carbon cloth. Ammonia treatment of the carbon cloth increased the surface charge from 0.38 meq m 2 to 3.99 meq m 2 at pH 7. 2000 -2
Power density (mW m )
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
1500 1000 V-Treated V-Untreated P-Treated P-Untreated
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500
0.6
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-2
Current density (mA cm ) Fig. 2. Power density for MFCs with different anodes, 200 mM phosphate buffered nutrient medium containing 1 g/L sodium acetate.
0
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1
-2
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Time (h)
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Fig. 3. Coulombic efficiency for MFCs with different anodes, 200 mM phosphate buffered nutrient medium containing 1 g/L sodium acetate.
The increase in positive charge was due to the formation of nitrogen-containing surface functional groups on the carbon cloth surface during the ammonium treatment, which has been previously shown by elemental analysis of the surface [38]. 4. Discussion Ammonia treatment of the carbon anode, in concert with several other recently developed techniques to increase power in MFCs, resulted here in a maximum power density of 1970 mW m 2 based on anode surface area, or 115 W/m3 based on reactor volume. This power density larger by a factor of 7.5 times as much power as first achieved with this basic reactor system (292 mW m 2, [12]). Increases in power density have resulted from removal of a proton exchange membrane bonded to the cathode (494 mW m 2 [12]), reduced electrode spacing (1210 mW m 2 [13]), and increases in solution conductivity (1330 mW m 2 [13]). The higher initial power production of the untreated anode measured here (1640 mW m 2) than that previously obtained in the same system (1330 mW m 2 [13]) was due to the use of a different electrolyte to increase solution conductivity. In the previous study, NaCl was used to increase solution ionic strength. Increasing the Cl concentration can affect the oxygen reduction activity of the cathode [39,40]. Here we used a phosphate buffer which not only increased solution conductivity but also eliminated this adverse effect of Cl on the cathode. Furthermore, a high concentration of PBS provided additional buffering capacity at the electrodes to reduce the effect of pH increases at the cathode due to the rapid consumption of protons at higher power densities [41]. The improvement in power generation with the ammonium-treated anode could result from two different factors: increased adhesion of bacteria during reactor start up, and an increase in the efficiency of electron transfer from the bacteria to the surface. The substantial reduction in acclimation time demonstrates that bacterial adhesion to the surface was important. The higher power density of the system suggests that electron transfer was also improved due to the ammonium treatment. At higher power densities
S. Cheng, B.E. Logan / Electrochemistry Communications 9 (2007) 492–496
(>0.5 mA cm 2; Fig. 2), we observed from the polarization curve that the cell voltage remained high, suggesting that the presence of functional groups at the surface improved electron flow to the electrode surface. Polarization curves of the ammonia-treated and the untreated anodes were identical at lower current densities (<0.3 mA cm 2), indicating that the activation energy needed for electron transfer to the surface was unaffected. This increase in power is not due to ammonia as a fuel, as it has been shown that electricity is not generated from ammonia in an MFC [42]. This increase in MFC performance was due to nearly an order-of-magnitude increase in the surface charge (3.99 meq m 2 at pH 7) of the electrode. In preliminary experiments, the carbon cloth anode was treated with H2 and then CH4 gas under the same temperature conditions. However, this treatment reduced the charge of carbon cloth surface, and MFC tests with this material showed a decrease in the maximum power density (data not shown). Thus, these results indicate that the improved performance obtained here with ammonia gas treatment was due to increased surface charge, and not simply from high temperature treatment of the carbon cloth electrode. Ammonium treatment of carbon electrodes should be a widely applicable method for increasing power generation with different anode materials. For example, other anode materials used in previous MFC studies such as carbon paper [13], graphite granules [43], graphite felt [5], and reticulated vitreous carbon (RVC, [44]) could all be treated using a similar ammonium gas treatment. Treatment of these materials can be expected to facilitate bacterial adhesion and increased electron transfer to the anode surface. 5. Conclusions Ammonium treatment of the anode increased the surface charge of carbon cloth electrodes and improved MFC performance. The increase in power density to 1970 mW m 2 (115 W m 3) resulted both improved bacterial adhesion and the improved performance of the system at higher current densities (0.5 to 0.9 mA cm 2). The results demonstrate a new approach for improving power generation in MFCs. Acknowledgments The authors thank Dr. Weifang Chen and Adam Redding for help with surface charge measurements and ammonium gas treatment. This research was supported by National Science Foundation Grant BES-0401885. References [1] H.J. Kim, H.S. Park, M.S. Hyun, I.S. Chang, M. Kim, B.H. Kim, Enzyme Microbiol. Technol. 30 (2002) 145. [2] B.H. Kim, H.J. Kim, M.S. Hyun, D.H. Park, J. Microbiol. Biotechnol. 9 (1999) 127. [3] H.J. Kim, M.S. Hyun, I.S. Chang, B.H. Kim, J. Microbiol. Biotechnol. 9 (1999) 365.
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