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Electricity generation of microbial fuel cell with waterproof breathable membrane cathode
Journal of Power Sources 300 (2015) 491e495
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Electricity generation of microbial fuel cell with waterproof breathable membrane cathode Defeng Xing a, b, *, Yu Tang a, b, Xiaoxue Mei a, b, Bingfeng Liu a, b a b
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A fast and inexpensive method of making cathode was developed. Cathode was fabricated via assembling stainless steel mesh with waterproof breathable membrane. SSM/Pt@WBM showed better stability than Pt@SSM/WBM after longterm operation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2015 Received in revised form 25 September 2015 Accepted 28 September 2015 Available online xxx
Simplification of fabrication and reduction of capital cost are important for scale-up and application of microbial electrochemical systems (MES). A fast and inexpensive method of making cathode was developed via assembling stainless steel mesh (SSM) with waterproof breathable membrane (WBM). Three assemble types of cathodes were fabricated; Pt@SSM/WBM (SSM as cathode skeleton, WBM as diffusion layer, platinum (Pt) catalyst applied on SSM), SSM/Pt@WBM and Pt@WBM. SSM/Pt@WBM cathode showed relatively preferable with long-term stability and favorable power output (24.7 W/m3). Compared to conventional cathode fabrication, air-cathode was made for 0.5 h. The results indicated that the novel fabrication method could remarkably reduce capital cost and simplify fabrication procedures with a comparable power output, making MFC more prospective for future application. © 2015 Elsevier B.V. All rights reserved.
Keywords: Microbial fuel cell Cathode Cathode fabrication Waterproof breathable membrane
1. Introduction Microbial fuel cells (MFCs) are devices that use bacteria as catalyst to oxidize a wide range of biodegradable matters with simultaneous bioenergy generation and have become one
* Corresponding author. School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin, Heilongjiang Province 150090, China. E-mail addresses: [email protected] (D. Xing), [email protected] (Y. Tang), [email protected] (X. Mei), [email protected] (B. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.09.106 0378-7753/© 2015 Elsevier B.V. All rights reserved.
promising technology for wastewater treatment [1,2]. To date, the main query and discussion on this attractive technology for future application focuses on increase of electron transfer and reduction of the capital cost [3,4]. Single-chamber air-cathode MFC, using readily available air as sustainable oxidant, has gradually become the preferential choice due to its practicability, sustainability and high power density [5e8]. Components of conventional cathode fabrication including carbon cloth (CC), platinum catalyst, polytetrafluoroethylene (PTFE), Nafion binder are widely used in an air-cathode MFC system regardless of extremely high expense [9]. To reduce the capital cost
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D. Xing et al. / Journal of Power Sources 300 (2015) 491e495
for future application, some cost-effective substitutes of catalyst, diffusion layer, cathode and anode materials etc. were investigated. Non-noble metal, metal oxides and carbon could be used as an alternative to platinum (Pt) catalyst in MFCs [10]. The carbon cloth as base material of the cathode has advantages of good conductivity, air permeability and excellent corrosion resistance, but imposes cost on MFC systems. In recent years, substitutes such as carbon felt, carbon mesh, stainless steel mesh (SSM), graphite brush, etc. have been tested and shown great performance as electrode materials [11e15]. Fabricating cathode is one of the most important process for constructing MFC, and it is as well in continual trial and improvement. An ultrafiltration hydrophilic tubular membrane coated with graphite and Co-tetra-methylphenylporphyrin (CoTMPP) was used as cathode instead of the conventional cathode structure in the previous study [16]. Similarly, anion (AEM) and cation (CEM) exchange membranes were tried as supporting material with the same catalyst [17]. Using carbon mesh instead of carbon cloth, cathodes with other diffusion layers such as poly (dimethylsiloxane) (PDMS), Goretex cloth etc. were as well tested [12,18]. These cathode fabrication methods differed greatly from the conventional way, saving capital cost and simultaneously showing favorable power generation. However, the materials used in the procedures were still not quite cheap, and the skeleton materials (e.g. AEM, CEM and carbon mesh) were not firm enough for supporting large cathode when scaling up MFC. Serving as electrode material, SSM showed not only similar superiority as CC but also had strong structure, which was considered rather important for further scaling up. As previously described, another approach for making cathode was investigated by applying polydimethylsiloxane (PDMS) (diffusion layer) onto SSM with series of operation and showed competitive performance [19]. An one-step method was developed by applying polypyrrole/anthraquinone-2-sulfonate (PPy/AQS) onto SSM via electropolymerization [20]. In spite of low cost and acceptable performance, the complexity of methods with multiple steps was still exposed in these studies as another prospective challenge. In this study, a fast, uncomplicated and inexpensive method for making cathode was developed by assembling SSM with waterproof breathable membrane (WBM). The electrochemical performances of assemble and conventional fabrication methods were compared. 2. Materials and methods 2.1. MFC configuration and operation Single-chamber air-cathode MFCs (cylindrical chamber, 28 ml) with different types of cathode (7 cm2 of surface area) were constructed. The cathodes were fabricated via assembling stainless steel mesh (SSM, type SUS304) with building-used waterproof breathable membrane (WBM) that was thermally laminated by two outer layers of polypropylene and an inner layer of microporous polyolefin membrane) (BREAWARF, Ningbo SHANQUAN Building Material Co., Ltd., CHN) (detailed description in Supporting Information). For the first type of cathode, SSM (mesh of No. 40, 60 and 80) spread with Pt catalyst (0.5 mg/cm2) was assembled with WBM as diffusion layer (designated as Pt@SSM40/WBM, Pt@SSM60/WBM and Pt@SSM80/WBM). For the second type of cathode, WBM spread with Pt catalyst was assembled with SSM (mesh of No. 40) as the current collector (SSM40/Pt@WBM). For third type of cathode, Pt catalyst was directly spread on WBM without SSM (Pt@WBM) (Figure S1 and Figure S2 in Supporting Information). For control tests, the cathodes were made with one carbon/PTFE layer and three diffusion layers of PTFE based on
carbon cloth (CC, type B, E-TEK, 30% wet-proofed) (Pt@CC/PTFE) and SSM (Pt@SSM40/PTFE) spread with 0.5 mg/cm2 Pt catalyst as previous descriptions [9,21,22]. All the anodes were identically made of titanium-core graphite brushes (2.5 cm length 2.5 cm outer diameter) as previous described [15]. All MFCs were fed with nutrient solution, which contained 2 g sodium acetate, 11.55 g Na2HPO4$12H2O, 2.77 g NaH2PO4$2H2O, 0.31 g NH4Cl and 0.13 g KCl (per liter of DI water). The activated sludge from the secondary sedimentation tank of a local wastewater treatment plant (Harbin, China) was mixed with 2 g/L sodium acetate in volume ratio of 1:5, and was then added into the reactors as inoculum. All MFCs were enriched and operated with an external resistor of 1000 U in fed-batch mode in a temperaturecontrolled room at 30 C. 2.2. Calculation and analyses The voltage across a 1000 U resistor in external circuit of the MFC was monitored at 30 min intervals using multichannel data acquisition system (Model 2700 with 7702 module, Keithly Instruments Inc., USA) connected to a personal computer via PCI interface. Then the current and power were calculated as previous description [7]. Polarization and power density curves were measured by changing external resistors from 2000 to 75 U. 3. Results and discussion 3.1. Electrochemical performance and stability Cathodes fabricated according to the method by eliminating carbon/PTFE layer, replacing diffusion layers with WBM, and brushing catalyst on SSM (Pt@SSM/WBM) were based on SSM with mesh number of 40 (Pt@SSM40/WBM), 60 (Pt@SSM60/WBM) and 80 (Pt@SSM80/WBM), respectively (Fig. 1). In contrast to Pt@SSM40/PTFE, Pt@SSM40/WBM showed a slightly lower power density of ~27.0 W/m3, while power densities of Pt@SSM60/WBM (~35.4 W/m3) and Pt@SSM80/WBM (~36.7 W/m3) were respectively 16.1% and 20.3% higher than that of Pt@SSM40/PTFE (Fig. 1a). For two control groups, the power density of MFC (~927.5 mW/m2, ~30.5 W/m3) with Pt@SSM40/PTFE cathode was similar with that obtained by MFC with Pt@CC/PTFE cathode (~912.3 mW/m2, ~30.0 W/m3 (Fig. 1a), which were still acceptable in comparison with previous studies [14,23]. The maximum power density of Pt@SSM80/WBM was slightly lower than that obtained in the MFCs with cathode coating of PDMS (47 W/m3) [19], presumably due to different operation conditions and configuration of MFCs. Pt@SSM80/WBM and Pt@SSM60/WBM showed higher voltage outputs versus current densities with open circuit voltage (OCV) of 775 mV and 747 mV (Fig. 1b). Other cathodes obtained slightly different voltage curves with OCV of 723e736 mV except Pt@WBM with OCV of 719 mV. The chemical oxygen demand (COD) removal of all MFCs ranged from 93.8% to 96.6% and no large difference could be observed among them (Figure S3 in Supporting Information). Although Pt@SSM40/PTFE groups showed favorable power generation in the early period, its power output stability unstoppably became increasingly serious as time went by. During long term operation, the MFCs (SSM40 Later) could merely produce a maximum power density of 6.5 W/m3 power at later stage, which was only one quarter of that produced by the same reactors (27.0 W/m3, SSM40 Early) (Figure S4 in Supporting Information), and the variation on power generation over time happened similarly to Pt@SSM60/WBM and Pt@SSM80/WBM as well. In order to overcome the instability weakness on account of choosing SSM as improper catalyst carrier, some tiny transformation was made
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stability among different MFCs were highlighted and easy to observe (Fig. 2). In the early period, All MFCs performed favorably on electricity generation with high voltage output except for Pt@WBM, which uniformly matched the results obtained previously. As operation time lasted, Pt@SSM/WBM groups gradually showed their weakness on operation stability and the electricity generation went to a rather low level. However, SSM/Pt@WBM, Pt@WBM and two control groups were time-independent and could maintain their respective voltage outputs. Generally, power generation of the MFC fabricated in conventional method would decrease with the SSM mesh number [23]. However for SSM/WBM cathode, limit to cathode electrochemical reaction was no longer air transportation as carbon/PTFE layer was removed and WBM substituted PTFE. For this cathode structure, the keys would be the amount of stabilized catalyst on SSM and the efficiency of electron conduction nearby the SSM. The SSM with larger mesh number possessed higher surface area to adhere catalyst as well as finer contact area for electron conduction, which consequently enhanced the cathode electrochemical reaction. 3.2. Long-term power generation stability In this study, All MFCs with SSM/WBM cathodes inevitably suffered the problem of power generation instability. Generally, a Pt@SSM/WBM MFC after long-term operation could only generated 1/4 power of the highest power density in early stage. As the SSM was woven by SS wire with smooth surface, it was rather difficult to have catalyst brushed and adhered strongly on it for a long period. The quantity of catalyst on SSM decreased gradually with every pouring and refilling substrate in batch mode, which probably became the key to the instability of power output. To avoid the problem, cathode of SSM/Pt@WBM was made by brushing catalyst on WBM, a type of soft membrane with relatively rough surface. As tested in long-term operation, it could reliably ensure the stability of power and electricity generation. Besides, SSM/Pt@WBM could as well solve the problem though with quite low power density. 3.3. Improvement on cathode fabrication method
Fig. 1. Power density (a) and potential curves (b) of MFCs with different cathodes of CC, SSM, SSM/WBM (40, 60 and 80), SSM/Pt@WBM (40) and Pt@WBM. CC, SSM and WBM represent carbon cloth, stainless steel mesh and waterproof breathable membrane, respectively. The number behind SSM represents the No. of mesh of SSM. (Error bars represent standard deviation based on measurements from duplicate reactors in three cycles).
through brushing the catalyst directly on WBM but not on SSM (SSM40/Pt@WBM). Compared to 27.0 W/m3 of MFC with Pt@SSM40/WBM, MFC with SSM40/Pt@WBM still reached an acceptable power density of ~24.7 W/m3 (Fig. 1). As significant reward, the stability of long-term operating could be greatly enhanced and guaranteed to perform equally satisfactory as carbon cloth (CC) or SSM. Despite the simplicity and cheapness of SSM40/ Pt@WBM, an alternative plan was as well taken by eliminating SSM and brushing catalyst directly on WBM (Pt@WBM) for further simplification. Thus, the steps of making cathode were further reduced and the cost was simultaneously cut down. As sacrifice, Pt@WBM performed poorly on power generation due to its high ohmic resistance (Figure S5 in Supporting Information) with only ~7.6 W/m3 (Fig. 1), which was far lower than other testes and unacceptable. From the voltage production curves, the differences of operation
According to conventional method to make MFC cathode, a carbon base layer (or carbon/PTFE layer) should be first applied onto the cathode skeleton, air-dried and heated, then the same work for 3e4 diffusion layer (60% PTFE); finally, the other side of the cathode was brushed with catalyst manually [9]. The total fabricating time usually lasts 5e8 h, and no doubt was that the
Fig. 2. Voltage curves (across external resistance of 1000 U) of MFCs with different cathodes.
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fabrication was kind of time-consuming work and unrealistic for further practical application. As for Pt@SSM/WBM or SSM/ Pt@WBM, the method became much easier and saved time (usually takes 0.5 h) due to eliminating carbon/PTFE layer and replacing diffusion layers with available WBM, and so was the Pt@WBM. In comparison with unconventional and novel methods in previous studies, our method still held an evident advantage on time and material consuming [13,14,17e19]. In addition, eliminating carbon/PTFE layer reduced the cost (~40 USD/m2) for making cathode, and WBM (~1 USD/m2) cost much less than PTFE (at least 180 USD/m2) when serving as diffusion layer material. The conventional method and the method in this study consumed the same catalyst of equal amount for MFC cathodes, we could estimate the capital cost of each MFC cathode in comparison (Fig. 3). After improving conventional method by replacing CC with SSM, the capital cost of fabricating MFC cathode was greatly diminished. Compared to Pt@SSM/PTFE, Pt@SSM/WBM or SSM/Pt@WBM saved 25.9% of capital cost. In further, the cathode made by the new method even cost nothing except for tiny part of SSM (~15 USD/m2) and WBM (~1 USD/m2), making MFC cathode inexpensive and competitive in comparison with both conventional method and previous neoteric methods of cathode fabrication [16,17,19,20]. 3.4. Outlook The experiments have investigated three types of MFCs with different cathodes fabricated by a novel method using WBM as diffusion layer: Pt@SSM/WBM showed the most exciting power density yet with serious problems of long-term power output instability; Pt@WBM was the cheapest and simplest, but the power generation was far beyond satisfaction. By contrast, SSM/Pt@WBM was considered as the most appropriate and promising one with 24.7 W/m3 power output and low capital cost of ~16 USD/m2 (without considering catalyst). Besides, the method of MFC cathode
fabrication was sharply simplified and improved, consuming less than 0.5 h for the whole procedures due to eliminating carbon/PTFE layer and replacing diffusion layers with available WBM. Therefore, the building-used WBM makes MFC cathode both uncomplicated and inexpensive. As we have discussed, MFCs with Pt@SSM/WBM cathode generates higher power as the SSM number increases, and this is likely to happen on SSM/Pt@WBM. Finer SSM can ensure better conductivity (lower ohmic resistance) around cathode in spite of adding no more surface area to carry catalyst, which probably leads to higher power generation. Therefore, further investigation on SSM number optimization is considered to be necessary. Besides, as Pt/C catalyst is rare and particularly expensive, some other tested catalysts such as iron-based catalysts, CoTMPP, polypyrrole/carbon black composite, MnO2 etc. in previous studies [24e27] remain to be applied in this cathode system so as to further lower the capital cost. Furthermore, great inspiration has been given by making cathode with easy stacking piece of WBM and SSM. Generally, a MFC anode is made using high-expense carbon cloth, graphite brush etc. with rough surface and good conductivity, which are yet the primary characteristics for cathode material. Since the assembly of WBM and SSM works favorably on cathode, it is reasonable to believe that the method can be transplanted to anode as well, leading to significant reduction in anode capital cost. Moreover, the results hereinbefore have indicated that the WBM is feasible and cost-effective to serve as cathode diffusion layer, proposing a very practical MFC cathode fabrication method for further scaling up and application, and more work on scaling-up with WBM should be finished to test its structure strength. 4. Conclusions In this study, a fast and inexpensive method of air-cathode fabrication was developed via assembling stainless steel mesh (SSM) with waterproof breathable membrane (WBM). Power densities of MFCs with different cathodes were compared, Pt@SSM80/ WBM showed a maximum power density of 36.7 W/m3. The power output of MFC with Pt@SSM/WBM cathode decreased obviously after long-term operation. SSM/Pt@WBM cathode showed relatively preferable with long-term stability and favorable power output. The results indicated that the novel fabrication method could reduce capital cost and simplify fabrication procedures. Acknowledgments This study was supported by National Natural Science Foundation of China (Nos. 31270004, 51422805), the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2013DX13), the Fundamental Research Funds for the Central Universities (No. HIT.BRETIV. 201319). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.106. References [1] [2] [3] [4] [5] [6]
Fig. 3. Evaluation on fabrication cost for platinum catalyst, SSM, SSM/WBM, SSM/ Pt@WBM and Pt@WBM cathodes. Capital costs were normalized to geometric area of 1 m2.
B.E. Logan, J.M. Regan, Environ. Sci. Technol. 40 (2006) 5172e5180. D.R. Lovley, Curr. Opin. Biotechnol. 17 (2006) 327e332. B.E. Logan, M. Elimelech, Nature 488 (2012) 313e319. B.E. Logan, K. Rabaey, Science 337 (2012) 686e690. Y. Fan, H. Hu, H. Liu, J. Power Sources 171 (2007) 348e354. H. Liu, R. Ramnarayanan, B.E. Logan, Environ. Sci. Technol. 38 (2004) 2281e2285. €der, J. Keller, S. Freguia, [7] B.E. Logan, B. Hamelers, R. Rozendal, U. Schro P. Aelterman, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006)
D. Xing et al. / Journal of Power Sources 300 (2015) 491e495 5181e5192. [8] H. Liu, B.E. Logan, Environ. Sci. Technol. 38 (2004) 4040e4046. [9] S. Cheng, H. Liu, B.E. Logan, Electrochem. Commun. 8 (2006) 489e494. [10] Y. Ahn, I. Ivanov, T.C. Nagaiah, A. Bordoloi, B.E. Logan, J. Power Sources 269 (2014) 212e215. [11] Q. Deng, X. Li, J. Zuo, A. Ling, B.E. Logan, J. Power Sources 195 (2010) 1130e1135. [12] Y. Luo, F. Zhang, B. Wei, G. Liu, R. Zhang, B.E. Logan, J. Power Sources 196 (2011) 9317e9321. [13] X. Wang, S. Cheng, Y. Feng, M.D. Merrill, T. Saito, B.E. Logan, Environ. Sci. Technol. 43 (2009) 6870e6874. [14] S. You, X. Wang, J. Zhang, J.-Y. Wang, N. Ren, X. Gong, Biosens. Bioelectron. 26 (2011) 2142e2146. [15] B. Logan, S. Cheng, V. Watson, G. Estadt, Environ. Sci. Technol. 41 (2007) 3341e3346. [16] Y. Zuo, S. Cheng, D. Call, B.E. Logan, Environ. Sci. Technol. 41 (2007) 3347e3353. [17] Y. Zuo, S. Cheng, B.E. Logan, Environ. Sci. Technol. 42 (2008) 6967e6972.
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[18] Y. Luo, F. Zhang, B. Wei, G. Liu, R. Zhang, B.E. Logan, Biochem. Eng. J. 73 (2013) 49e52. [19] F. Zhang, T. Saito, S. Cheng, M.A. Hickner, B.E. Logan, Environ. Sci. Technol. 44 (2010) 1490e1495. [20] C. Feng, Q. Wan, Z. Lv, X. Yue, Y. Chen, C. Wei, Biosens. Bioelectron. 26 (2011) 3953e3957. [21] J. Jia, Y. Tang, B. Liu, D. Wu, N. Ren, D. Xing, Bioresour. Technol. 144 (2013) 94e99. [22] Y. Zhang, M.D. Merrill, B.E. Logan, Int. J. Hydrogen Energy 35 (2010) 12020e12028. [23] F. Zhang, M.D. Merrill, J.C. Tokash, T. Saito, S. Cheng, M.A. Hickner, B.E. Logan, J. Power Sources 196 (2011) 1097e1102. [24] L. Birry, P. Mehta, F. Jaouen, J.P. Dodelet, S.R. Guiot, B. Tartakovsky, Electrochim. Acta 56 (2011) 1505e1511. [25] S. Cheng, H. Liu, B.E. Logan, Environ. Sci. Technol. 40 (2005) 364e369. [26] Y. Yuan, S. Zhou, L. Zhuang, J. Power Sources 195 (2010) 3490e3493. [27] L. Zhang, C. Liu, L. Zhuang, W. Li, S. Zhou, J. Zhang, Biosens. Bioelectron. 24 (2009) 2825e2829.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Electricity generation of microbial fuel cell with waterproof breathable membrane cathode Defeng Xing a, b, *, Yu Tang a, b, Xiaoxue Mei a, b, Bingfeng Liu a, b a b
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A fast and inexpensive method of making cathode was developed. Cathode was fabricated via assembling stainless steel mesh with waterproof breathable membrane. SSM/Pt@WBM showed better stability than Pt@SSM/WBM after longterm operation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2015 Received in revised form 25 September 2015 Accepted 28 September 2015 Available online xxx
Simplification of fabrication and reduction of capital cost are important for scale-up and application of microbial electrochemical systems (MES). A fast and inexpensive method of making cathode was developed via assembling stainless steel mesh (SSM) with waterproof breathable membrane (WBM). Three assemble types of cathodes were fabricated; Pt@SSM/WBM (SSM as cathode skeleton, WBM as diffusion layer, platinum (Pt) catalyst applied on SSM), SSM/Pt@WBM and Pt@WBM. SSM/Pt@WBM cathode showed relatively preferable with long-term stability and favorable power output (24.7 W/m3). Compared to conventional cathode fabrication, air-cathode was made for 0.5 h. The results indicated that the novel fabrication method could remarkably reduce capital cost and simplify fabrication procedures with a comparable power output, making MFC more prospective for future application. © 2015 Elsevier B.V. All rights reserved.
Keywords: Microbial fuel cell Cathode Cathode fabrication Waterproof breathable membrane
1. Introduction Microbial fuel cells (MFCs) are devices that use bacteria as catalyst to oxidize a wide range of biodegradable matters with simultaneous bioenergy generation and have become one
* Corresponding author. School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin, Heilongjiang Province 150090, China. E-mail addresses: [email protected] (D. Xing), [email protected] (Y. Tang), [email protected] (X. Mei), [email protected] (B. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.09.106 0378-7753/© 2015 Elsevier B.V. All rights reserved.
promising technology for wastewater treatment [1,2]. To date, the main query and discussion on this attractive technology for future application focuses on increase of electron transfer and reduction of the capital cost [3,4]. Single-chamber air-cathode MFC, using readily available air as sustainable oxidant, has gradually become the preferential choice due to its practicability, sustainability and high power density [5e8]. Components of conventional cathode fabrication including carbon cloth (CC), platinum catalyst, polytetrafluoroethylene (PTFE), Nafion binder are widely used in an air-cathode MFC system regardless of extremely high expense [9]. To reduce the capital cost
492
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for future application, some cost-effective substitutes of catalyst, diffusion layer, cathode and anode materials etc. were investigated. Non-noble metal, metal oxides and carbon could be used as an alternative to platinum (Pt) catalyst in MFCs [10]. The carbon cloth as base material of the cathode has advantages of good conductivity, air permeability and excellent corrosion resistance, but imposes cost on MFC systems. In recent years, substitutes such as carbon felt, carbon mesh, stainless steel mesh (SSM), graphite brush, etc. have been tested and shown great performance as electrode materials [11e15]. Fabricating cathode is one of the most important process for constructing MFC, and it is as well in continual trial and improvement. An ultrafiltration hydrophilic tubular membrane coated with graphite and Co-tetra-methylphenylporphyrin (CoTMPP) was used as cathode instead of the conventional cathode structure in the previous study [16]. Similarly, anion (AEM) and cation (CEM) exchange membranes were tried as supporting material with the same catalyst [17]. Using carbon mesh instead of carbon cloth, cathodes with other diffusion layers such as poly (dimethylsiloxane) (PDMS), Goretex cloth etc. were as well tested [12,18]. These cathode fabrication methods differed greatly from the conventional way, saving capital cost and simultaneously showing favorable power generation. However, the materials used in the procedures were still not quite cheap, and the skeleton materials (e.g. AEM, CEM and carbon mesh) were not firm enough for supporting large cathode when scaling up MFC. Serving as electrode material, SSM showed not only similar superiority as CC but also had strong structure, which was considered rather important for further scaling up. As previously described, another approach for making cathode was investigated by applying polydimethylsiloxane (PDMS) (diffusion layer) onto SSM with series of operation and showed competitive performance [19]. An one-step method was developed by applying polypyrrole/anthraquinone-2-sulfonate (PPy/AQS) onto SSM via electropolymerization [20]. In spite of low cost and acceptable performance, the complexity of methods with multiple steps was still exposed in these studies as another prospective challenge. In this study, a fast, uncomplicated and inexpensive method for making cathode was developed by assembling SSM with waterproof breathable membrane (WBM). The electrochemical performances of assemble and conventional fabrication methods were compared. 2. Materials and methods 2.1. MFC configuration and operation Single-chamber air-cathode MFCs (cylindrical chamber, 28 ml) with different types of cathode (7 cm2 of surface area) were constructed. The cathodes were fabricated via assembling stainless steel mesh (SSM, type SUS304) with building-used waterproof breathable membrane (WBM) that was thermally laminated by two outer layers of polypropylene and an inner layer of microporous polyolefin membrane) (BREAWARF, Ningbo SHANQUAN Building Material Co., Ltd., CHN) (detailed description in Supporting Information). For the first type of cathode, SSM (mesh of No. 40, 60 and 80) spread with Pt catalyst (0.5 mg/cm2) was assembled with WBM as diffusion layer (designated as Pt@SSM40/WBM, Pt@SSM60/WBM and Pt@SSM80/WBM). For the second type of cathode, WBM spread with Pt catalyst was assembled with SSM (mesh of No. 40) as the current collector (SSM40/Pt@WBM). For third type of cathode, Pt catalyst was directly spread on WBM without SSM (Pt@WBM) (Figure S1 and Figure S2 in Supporting Information). For control tests, the cathodes were made with one carbon/PTFE layer and three diffusion layers of PTFE based on
carbon cloth (CC, type B, E-TEK, 30% wet-proofed) (Pt@CC/PTFE) and SSM (Pt@SSM40/PTFE) spread with 0.5 mg/cm2 Pt catalyst as previous descriptions [9,21,22]. All the anodes were identically made of titanium-core graphite brushes (2.5 cm length 2.5 cm outer diameter) as previous described [15]. All MFCs were fed with nutrient solution, which contained 2 g sodium acetate, 11.55 g Na2HPO4$12H2O, 2.77 g NaH2PO4$2H2O, 0.31 g NH4Cl and 0.13 g KCl (per liter of DI water). The activated sludge from the secondary sedimentation tank of a local wastewater treatment plant (Harbin, China) was mixed with 2 g/L sodium acetate in volume ratio of 1:5, and was then added into the reactors as inoculum. All MFCs were enriched and operated with an external resistor of 1000 U in fed-batch mode in a temperaturecontrolled room at 30 C. 2.2. Calculation and analyses The voltage across a 1000 U resistor in external circuit of the MFC was monitored at 30 min intervals using multichannel data acquisition system (Model 2700 with 7702 module, Keithly Instruments Inc., USA) connected to a personal computer via PCI interface. Then the current and power were calculated as previous description [7]. Polarization and power density curves were measured by changing external resistors from 2000 to 75 U. 3. Results and discussion 3.1. Electrochemical performance and stability Cathodes fabricated according to the method by eliminating carbon/PTFE layer, replacing diffusion layers with WBM, and brushing catalyst on SSM (Pt@SSM/WBM) were based on SSM with mesh number of 40 (Pt@SSM40/WBM), 60 (Pt@SSM60/WBM) and 80 (Pt@SSM80/WBM), respectively (Fig. 1). In contrast to Pt@SSM40/PTFE, Pt@SSM40/WBM showed a slightly lower power density of ~27.0 W/m3, while power densities of Pt@SSM60/WBM (~35.4 W/m3) and Pt@SSM80/WBM (~36.7 W/m3) were respectively 16.1% and 20.3% higher than that of Pt@SSM40/PTFE (Fig. 1a). For two control groups, the power density of MFC (~927.5 mW/m2, ~30.5 W/m3) with Pt@SSM40/PTFE cathode was similar with that obtained by MFC with Pt@CC/PTFE cathode (~912.3 mW/m2, ~30.0 W/m3 (Fig. 1a), which were still acceptable in comparison with previous studies [14,23]. The maximum power density of Pt@SSM80/WBM was slightly lower than that obtained in the MFCs with cathode coating of PDMS (47 W/m3) [19], presumably due to different operation conditions and configuration of MFCs. Pt@SSM80/WBM and Pt@SSM60/WBM showed higher voltage outputs versus current densities with open circuit voltage (OCV) of 775 mV and 747 mV (Fig. 1b). Other cathodes obtained slightly different voltage curves with OCV of 723e736 mV except Pt@WBM with OCV of 719 mV. The chemical oxygen demand (COD) removal of all MFCs ranged from 93.8% to 96.6% and no large difference could be observed among them (Figure S3 in Supporting Information). Although Pt@SSM40/PTFE groups showed favorable power generation in the early period, its power output stability unstoppably became increasingly serious as time went by. During long term operation, the MFCs (SSM40 Later) could merely produce a maximum power density of 6.5 W/m3 power at later stage, which was only one quarter of that produced by the same reactors (27.0 W/m3, SSM40 Early) (Figure S4 in Supporting Information), and the variation on power generation over time happened similarly to Pt@SSM60/WBM and Pt@SSM80/WBM as well. In order to overcome the instability weakness on account of choosing SSM as improper catalyst carrier, some tiny transformation was made
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stability among different MFCs were highlighted and easy to observe (Fig. 2). In the early period, All MFCs performed favorably on electricity generation with high voltage output except for Pt@WBM, which uniformly matched the results obtained previously. As operation time lasted, Pt@SSM/WBM groups gradually showed their weakness on operation stability and the electricity generation went to a rather low level. However, SSM/Pt@WBM, Pt@WBM and two control groups were time-independent and could maintain their respective voltage outputs. Generally, power generation of the MFC fabricated in conventional method would decrease with the SSM mesh number [23]. However for SSM/WBM cathode, limit to cathode electrochemical reaction was no longer air transportation as carbon/PTFE layer was removed and WBM substituted PTFE. For this cathode structure, the keys would be the amount of stabilized catalyst on SSM and the efficiency of electron conduction nearby the SSM. The SSM with larger mesh number possessed higher surface area to adhere catalyst as well as finer contact area for electron conduction, which consequently enhanced the cathode electrochemical reaction. 3.2. Long-term power generation stability In this study, All MFCs with SSM/WBM cathodes inevitably suffered the problem of power generation instability. Generally, a Pt@SSM/WBM MFC after long-term operation could only generated 1/4 power of the highest power density in early stage. As the SSM was woven by SS wire with smooth surface, it was rather difficult to have catalyst brushed and adhered strongly on it for a long period. The quantity of catalyst on SSM decreased gradually with every pouring and refilling substrate in batch mode, which probably became the key to the instability of power output. To avoid the problem, cathode of SSM/Pt@WBM was made by brushing catalyst on WBM, a type of soft membrane with relatively rough surface. As tested in long-term operation, it could reliably ensure the stability of power and electricity generation. Besides, SSM/Pt@WBM could as well solve the problem though with quite low power density. 3.3. Improvement on cathode fabrication method
Fig. 1. Power density (a) and potential curves (b) of MFCs with different cathodes of CC, SSM, SSM/WBM (40, 60 and 80), SSM/Pt@WBM (40) and Pt@WBM. CC, SSM and WBM represent carbon cloth, stainless steel mesh and waterproof breathable membrane, respectively. The number behind SSM represents the No. of mesh of SSM. (Error bars represent standard deviation based on measurements from duplicate reactors in three cycles).
through brushing the catalyst directly on WBM but not on SSM (SSM40/Pt@WBM). Compared to 27.0 W/m3 of MFC with Pt@SSM40/WBM, MFC with SSM40/Pt@WBM still reached an acceptable power density of ~24.7 W/m3 (Fig. 1). As significant reward, the stability of long-term operating could be greatly enhanced and guaranteed to perform equally satisfactory as carbon cloth (CC) or SSM. Despite the simplicity and cheapness of SSM40/ Pt@WBM, an alternative plan was as well taken by eliminating SSM and brushing catalyst directly on WBM (Pt@WBM) for further simplification. Thus, the steps of making cathode were further reduced and the cost was simultaneously cut down. As sacrifice, Pt@WBM performed poorly on power generation due to its high ohmic resistance (Figure S5 in Supporting Information) with only ~7.6 W/m3 (Fig. 1), which was far lower than other testes and unacceptable. From the voltage production curves, the differences of operation
According to conventional method to make MFC cathode, a carbon base layer (or carbon/PTFE layer) should be first applied onto the cathode skeleton, air-dried and heated, then the same work for 3e4 diffusion layer (60% PTFE); finally, the other side of the cathode was brushed with catalyst manually [9]. The total fabricating time usually lasts 5e8 h, and no doubt was that the
Fig. 2. Voltage curves (across external resistance of 1000 U) of MFCs with different cathodes.
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fabrication was kind of time-consuming work and unrealistic for further practical application. As for Pt@SSM/WBM or SSM/ Pt@WBM, the method became much easier and saved time (usually takes 0.5 h) due to eliminating carbon/PTFE layer and replacing diffusion layers with available WBM, and so was the Pt@WBM. In comparison with unconventional and novel methods in previous studies, our method still held an evident advantage on time and material consuming [13,14,17e19]. In addition, eliminating carbon/PTFE layer reduced the cost (~40 USD/m2) for making cathode, and WBM (~1 USD/m2) cost much less than PTFE (at least 180 USD/m2) when serving as diffusion layer material. The conventional method and the method in this study consumed the same catalyst of equal amount for MFC cathodes, we could estimate the capital cost of each MFC cathode in comparison (Fig. 3). After improving conventional method by replacing CC with SSM, the capital cost of fabricating MFC cathode was greatly diminished. Compared to Pt@SSM/PTFE, Pt@SSM/WBM or SSM/Pt@WBM saved 25.9% of capital cost. In further, the cathode made by the new method even cost nothing except for tiny part of SSM (~15 USD/m2) and WBM (~1 USD/m2), making MFC cathode inexpensive and competitive in comparison with both conventional method and previous neoteric methods of cathode fabrication [16,17,19,20]. 3.4. Outlook The experiments have investigated three types of MFCs with different cathodes fabricated by a novel method using WBM as diffusion layer: Pt@SSM/WBM showed the most exciting power density yet with serious problems of long-term power output instability; Pt@WBM was the cheapest and simplest, but the power generation was far beyond satisfaction. By contrast, SSM/Pt@WBM was considered as the most appropriate and promising one with 24.7 W/m3 power output and low capital cost of ~16 USD/m2 (without considering catalyst). Besides, the method of MFC cathode
fabrication was sharply simplified and improved, consuming less than 0.5 h for the whole procedures due to eliminating carbon/PTFE layer and replacing diffusion layers with available WBM. Therefore, the building-used WBM makes MFC cathode both uncomplicated and inexpensive. As we have discussed, MFCs with Pt@SSM/WBM cathode generates higher power as the SSM number increases, and this is likely to happen on SSM/Pt@WBM. Finer SSM can ensure better conductivity (lower ohmic resistance) around cathode in spite of adding no more surface area to carry catalyst, which probably leads to higher power generation. Therefore, further investigation on SSM number optimization is considered to be necessary. Besides, as Pt/C catalyst is rare and particularly expensive, some other tested catalysts such as iron-based catalysts, CoTMPP, polypyrrole/carbon black composite, MnO2 etc. in previous studies [24e27] remain to be applied in this cathode system so as to further lower the capital cost. Furthermore, great inspiration has been given by making cathode with easy stacking piece of WBM and SSM. Generally, a MFC anode is made using high-expense carbon cloth, graphite brush etc. with rough surface and good conductivity, which are yet the primary characteristics for cathode material. Since the assembly of WBM and SSM works favorably on cathode, it is reasonable to believe that the method can be transplanted to anode as well, leading to significant reduction in anode capital cost. Moreover, the results hereinbefore have indicated that the WBM is feasible and cost-effective to serve as cathode diffusion layer, proposing a very practical MFC cathode fabrication method for further scaling up and application, and more work on scaling-up with WBM should be finished to test its structure strength. 4. Conclusions In this study, a fast and inexpensive method of air-cathode fabrication was developed via assembling stainless steel mesh (SSM) with waterproof breathable membrane (WBM). Power densities of MFCs with different cathodes were compared, Pt@SSM80/ WBM showed a maximum power density of 36.7 W/m3. The power output of MFC with Pt@SSM/WBM cathode decreased obviously after long-term operation. SSM/Pt@WBM cathode showed relatively preferable with long-term stability and favorable power output. The results indicated that the novel fabrication method could reduce capital cost and simplify fabrication procedures. Acknowledgments This study was supported by National Natural Science Foundation of China (Nos. 31270004, 51422805), the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2013DX13), the Fundamental Research Funds for the Central Universities (No. HIT.BRETIV. 201319). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.106. References [1] [2] [3] [4] [5] [6]
Fig. 3. Evaluation on fabrication cost for platinum catalyst, SSM, SSM/WBM, SSM/ Pt@WBM and Pt@WBM cathodes. Capital costs were normalized to geometric area of 1 m2.
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