Opening size optimization of metal matrix in rolling-pressed activated carbon air–cathode for microbial fuel cells

Applied Energy 123 (2014) 13–18

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Opening size optimization of metal matrix in rolling-pressed activated carbon air–cathode for microbial fuel cells Xiaojing Li, Xin Wang ⇑, Yueyong Zhang, Ning Ding, Qixing Zhou ⇑ MOE Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, No. 94 Weijin Road, Nankai District, Tianjin 300071, China

h i g h l i g h t s  A power density of 2151 mW m

2

was achieved using an optimized matrix of 40 mesh. 0.2 to 0 V.  An optimized matrix can significantly decrease the charge transfer resistance.  Mesh density had a better correlationship with power density than opening size.

 Power exhibited the same trend as that of linear sweep voltammetry at

a r t i c l e

i n f o

Article history: Received 21 November 2013 Received in revised form 17 February 2014 Accepted 18 February 2014 Available online 12 March 2014 Keywords: Microbial fuel cell Activated carbon Air–cathode Stainless steel mesh Electrochemical impedance spectrum

a b s t r a c t Stainless steel mesh (SSM) with four opening sizes (20–80 M) were investigated as matrixes of activated carbon air–cathodes in microbial fuel cells (MFCs). The highest power density of 2151 ± 109 mW m 2 (at 5.54 ± 0.14 A m 2) was obtained using 40 M, with a value 45% higher than 1485 ± 18 mW m 2 of 80 M. The trend of linear sweep voltammetries were in accordant with power output over a cathodic potential range from 0.2 to 0 V. The differences in performance were attributed to the internal resistances. Charge transfer resistance (Rct) was the dominant internal resistance in most of air–cathodes, with the lowest value of 2 X in 40 M. Density of metal mesh exhibited a more significant correlationship with maximum power densities (R2 = 0.9222) compared to opening size (R2 = 0.7068), demonstrated that the density of metal current collector was vital to the performance of cathodes. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Microbial fuel cells (MFCs) are recognized as a green power generation technology because any sources of biodegradable organic matters, including simple molecules (such as carbohydrates and proteins) as well as complex mixtures (such as brewery wastewater and corn stover) [1–6], can be theoretically used in MFCs for power generation based on the catalysis of living microorganisms [7,8]. In addition to improving the catalytic activity of the anaerobic bio-anode and reducing the ohmic resistance of the electrolyte to a minimum [9], to improve the performance of the cathode is a relevant way to significantly increase the power of MFCs. The combination of a refined microbial anode and a robust cathode together with an optimized fuel cell configuration can be a realizable design for high power MFCs.

⇑ Corresponding authors. Tel.: +86 22 23507800; fax: +86 22 23501117. E-mail addresses: [email protected] (X. Wang), [email protected] (Q. Zhou). http://dx.doi.org/10.1016/j.apenergy.2014.02.048 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

Activated carbon air–cathode (ACAC) is such a robust and highly efficient cathode with advantages of low cost (30 $ m 2 compared to 1600 $ m 2 of Pt–Nafion cathode) and high reproducibility [10,11]. This cathode is promising to be applied in a large scale, compared to those made of expensive catalysts (typically Pt) and binders (usually Nafion) [12–16]. Gas diffusion layer (GDL), catalyst layer (CL) and current collector (CC) are main elements of an ACAC. Both GDL and CL had been systematically investigated previously to maximize the performance under minimum losses [17–19]. However, the CC in ACAC had not been thoroughly studied so far. To reduce electrode ohmic losses, electrodes (such as graphite and carbon) need to be supported by a highly conductive metal CC. Several kinds metal mesh had been applied, such as Ni mesh [20], Ni foam [21], Al-alloy mesh [22], Ti mesh [23], copper mesh [24] and stainless steel mesh (SSM) [25–28]. The stainless steel mesh was usually used as the CC in air–cathodes because it is inexpensive and relatively anticorrosion. The properties of SSM, such as mesh (opening size) and density, was closely related to the

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performance by affecting the oxygen transfer, ion transfer and electrical conductivity between the CL and metal surface. Besides, an overview of the estimated capital costs of bioelectrochemical systems based on materials currently used in laboratory systems showed that the cost proportion of CC was 4.7%, while this percentage was predicted to be 40% based on inexpensive materials in the future [7]. While in our ACAC, the cost of CC approximately takes 65% of total cost. Thus the CC need to be paid more attention. The effect of different mesh on power generation has been examined in a cathode using Pt catalyst, poly(dimethylsiloxane) (PDMS) binder and brushing method [29]. They found that the power density increased with an increase of mesh opening size, as the result of best performance of 30 M and 50 M. However, these results are obtained in a different cathode and therefore they cannot be directly translatable to the performance of our newly developed rolling-pressed ACAC. Here the performance of ACACs made of different mesh SSM with activated carbon catalyst, polytetrafluoroethylen (PTFE) binder and rolling method was investigated. The electrochemical characteristics of different cathodes were compared and analyzed, in terms of linear sweep voltammetry (LSV), Tafel plot and electrochemical impedance spectrum (EIS). The performance including maximum power densities (MPDs), polarization curve, anodes/cathodes potentials and Coulombic Efficiency (CE) was also evaluated.

Cathodes were soaked in 50 mM PBS in abiotic reactors for at least 24 h before electrochemical tests. The target cathode (project area = 7 cm2) and a platinum sheet of 1 cm2 were used as the working electrode and the counter electrode. The reference electrode was saturated calomel electrodes (3 M KCl, 0.241 V versus standard hydrogen electrode). In order to avoid the undesirable losses on measurements, all the electrochemical tests were performed in the same reactor using a firmly fixed reference electrode. Reactors were refilled each time with the same electrolyte. LSV was conducted from 0.3 to 0.2 V with a scan rate of 0.1 mV s 1. Tafel plots (log |current density|, A m 2, versus |overpotential|, V) were recorded by sweeping the overpotential (|g|, mV) from 0 to 100 mV at 0.1 mV s 1, where g = 0 is the open circuit potential (OCP). EIS was performed over a frequency range of 100 kHz to 10 mHz with a sinusoidal perturbation signal amplitude of 10 mV using a potentiostat (Autolab PGSTAT 302N, Metrohm, Switzerland). In order to evaluate the performance of each electrode with different overpotentials, the initial potential was set at the overpotential of 0 V, 0.1 V, 0.2 V and 0.3 V in turn as previously used by us [28]. The Nyquist plots were used to interpret the spectra. A least-squares fitting program (ZsimpWin 3.10) was employed to simulate plots according to an equivalent circuit (Fig. S2). 3. Results

2. Materials and methods 3.1. Performance of the four different air–cathodes 2.1. Cathodes Six cycles after inoculation, maximum voltages were obtained in all MFCs. MPDs decreased as follow: 40 M (2151 ± 109 mW m 2 at 5.54 ± 0.14 A m 2) >60 M (1993 ± 53 mW m 2 at 5.34 ± 0.07 A m 2) >20 M (1970 ± 137 mW m 2 at 7.50 ± 0.26 A m 2)

Voltage (V)

600

Cell voltages across 1 kX were recorded every minute using a data acquisition system (PISO-813, ICP DAS Co., Ltd.). Polarization curves were performed at the 3rd cycle when anodes were well acclimated. Polarization and power density curves were obtained by varying the external resistance from 1000 to 30 X with a time interval of 25 min to ensure a stable voltage.

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MFCs were single-chamber cubic-shaped reactors constructed as previously described (4 cm long and 3 cm in diameter) [28]. Acetone cleaned carbon fiber brush (25 mm in diameter) was employed as the anode [30]. All reactors were inoculated by the effluent from an MFC operated for over one year. The medium was a phosphate buffer nutrient solution (PBS) containing: Na2HPO4
12H2O, 10.317 g L 1; NaH2PHO4
2H2O 3.321 g L 1; NH4Cl 0.31 g L 1; KCl 0.13 g L 1; trace minerals (12.5 mL L 1) and vitamins (5 mL L 1) contained 1.0 g L 1 of sodium acetate as the fuel [28]. Reactors were refilled each time when the voltage decreased to less than 50 mV at 30 °C, forming a complete cycle.

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Cathodes were made by rolling-press method according to procedures previously described [28]. All the cathodes were consisted of a SSM with a CL rolled on the water facing side and a GDL rolled on the air facing side. The mesh opening sizes of SSM were: 20, 40, 60 and 80 (type 304, Detiannuo Commercial Trade Co. Ltd., Tianjin, China). The corresponding air–cathodes were marked as 20 M, 40 M, 60 M and 80 M. The GDL was made of carbon black (Jinqiushi Chemical Co. Ltd., Tianjin, China) and PTFE emulsion (60 wt%, Hesen, Shanghai, China) with a mass ratio of 3:7, followed by heating at 340 °C for 20 min. The CL was made of activated carbon (Xinsen Carbon Co. Ltd., Fujian, China) and PTFE emulsion with a mass ratio of 6:1. The air–cathodes sheets were dried at room temperature for least 24 h before installed in MFCs.

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Current density (A⋅m-2 ) Fig. 1. Polarization and power density curves (A) and electrode potentials (B) of four air–cathodes using different matrix.

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3.2. LSV and Tafel plots of the four different air–cathodes According to abiotic LSVs, the 40 M exhibited a higher current density than other cathodes when the potential was lower than 0.09 V (Fig. 2). The highest current density of 5.5 A m 2 was obtained by 40 M among four different cathodes at the potential of 0 V (versus saturated calomel electrode, SCE). At the potential of 0.2  0 V, current densities decreased as 40 M > 60 M  20 M > 80 M. Since the maximum power densities of different cathodes were obtained at a cathodic potential around 0.13 V (versus SCE), the highest current density of 8.9 A m 2 was observed in 40 M, which was 37%, 22% and 19% higher than those of 80 M (6.5 A m 2), 20 M (7.3 A m 2) and 60 M (7.5 A m 2) respectively, following the similar trend as power densities. Based on the Tafel plots showed in Fig. 3, the exchange current density, j0 (A m 2), was calculated and listed in Table S1 according to the linear region of the plots (overpotential ranged from 60 to 80 mV). Exchange current is a parameter to show the current density of each electrode when the overpotential is zero. It closely related with the electrochemical activity of the cathode at equilibrium state. j0 was increased from 20 M to 80 M, with values

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Potential (V) Fig. 2. LSVs of the four different air–cathodes in 50 mM PBS (pH = 7) at room temperature. A saturated calomel electrode (SCE) was used as the reference electrode.

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lg|Current density| (A⋅m )

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>80 M (1485 ± 18 mW m 2 at 6.51 ± 0.04 A m 2) (Fig. 1A). The maximum power density of 40 M was 45% higher than that of 80 M. The MPDs of 20 M, 40 M and 60 M were all observed at external resistance of 50 X except 100 X of 80 M, indicated that the internal resistance of 80 M could be higher than other cathodes. Anode potentials of the four samples were similar, indicated that the voltage losses were mainly due to cathodes. However, the OCPs of cathodes decreased as 80 M (280 mV) > 60 M (264 mV)>40 M (253 mV) > 20 M (243 mV). The voltage (616 mV) of 80 M was the highest at external resistance of 1000 X (Fig. 1B). However, a fast voltage drop was observed on 80 M with the increase of current density, resulting in the lowest voltage when the current density was higher than 1.3 A m 2. Thus, the cathodic polarization of 80 M was much more serious than other cathodes at a high current. The maximum cell voltages during four continuous cycles were showed in supporting information (Fig. S1). The maximum voltage of 607 ± 4 V (868 ± 7 mA m 2) was achieved by 40 M, while the voltages of 20 M, 60 M and 80 M were 597 ± 5 V (853 ± 8 mA m 2), 600 ± 4 V (857 ± 5 mA m 2) and 604 ± 9 V (862 ± 12 mA m 2) respectively. However, the duration of 60 M (71 ± 2 h) and 80 M (70 ± 4 h) were longer than those of 40 M (66 ± 5 h) and 20 M (53 ± 6 h).

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|Overpotential| (V) Fig. 3. Tafel plots of the four air–cathodes using different matrix. The inserted figure is the linear fit of the Tafel plots over the overpotential range from 60 to 80 mV.

negatively related to mesh opening size. The exchange current density of 80 M (1.28 A m 2) was only 2% higher than that of 60 M (1.26 A m 2), but with a value 39% and 12% higher than those of 20 M (0.92 A m 2) and 40 M (1.14 A m 2), suggesting that cathodes made of 80 M had a potential high capacity of oxygen reduction reaction (ORR) current, but the activity was limited by factors such as internal resistances. All these j0 were comparable with those reported (1.0 A m 2) by us previously [28]. 3.3. EIS of the four different air–cathodes The internal resistance decreased with the increase in overpotential from 0.1 to 0.3 V (Figs. 4 and 5). At the overpotential of 0.3 V, the 40 M cathode had the smallest internal resistance of 13.8 X, while 80 M had the largest resistance of 110 X. In summary, the internal resistance decreased as follow: 40 M (14 X) > 60 M (56 X) > 20 M (64 X) > 80 M (127 X). As expected, the solution resistances (Rs) were all similar for the cathodes at different overpotentials, due to the use of the same cell, fixed reference electrode and PBS solution in EIS tests. Differences in total resistance were mainly contributed by the charge transfer resistance (Rct). The maximum Rct of 109 X was observed in 80 M, which was approximately 2.5, 54.5 and 2.1 times higher than those of 60 M (43 X), 40 M (2 X) and 20 M (52 X) respectively. The 40 M had the lowest Rct and therefore the highest power densities. The Rct of 60 M was similar to those of 20 M. The Rct of different cathodes showed a similar trend as power densities (40 M > 60 M  20 M > 80 M). The Warburg impedance (W) of 40 M was obviously observed in EIS tests (Fig. 4D), but not significant in 20 M, 60 M and 80 M (Fig. 4C–F), probably because the Rct cathodes of 20 M, 60 M and 80 M were too large to exhibit the small W. W is a common diffusion circuit element to model semi-infinite linear diffusion and unrestricted diffusion to the electrode, which is related to the diffusion resistance. W appeared only when the Rct was not the dominant resistance. The Rct tended to slightly decrease with the increase of overpotentials on each cathode. The double layer capacitances (Cdl) of 40 M had the largest value at 0 V (3.5  10 4 X 1 sn, Table S2). Cdl is used to describe the charge storage characteristics of the Helmholtz double layer at the interface between the cathode surface and the electrolyte. Cdl decreased with an increase of overpotential from 0.1 to 0.3 V, with values 3 times higher than those of 20 M, 60 M and 80 M. The changes of Cdl at different overpotentials in 20 M, 60 M and 80 M were insignificant when taking the fitting error into consideration (<13%, Table S2).

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Fig. 4. Nyquist plots of four air–cathodes using different matrix at the overpotentials of 0 V, 0.1 V, 0.2 V and 0.3 V.

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it was determined by both thickness and the diameter of stainless steel wire.

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Planar density of the four meshes were decreased as 32.76 ± 0.33 mg/cm2 (60 M) > 27.58 ± 0.40 mg/cm2 (40 M) >19.42 ± 0.66 mg/cm2 (20 M) > 10.83 ± 0.02 mg/cm2 (80 M). The planar density did not increase linearly with the decrease of opening size because

The highest MPD of 2151 mW m 2 was achieved with SSM cathode of 40 M. This power density was 33% and 28% higher than 1616 mW m 2 (Nafion binder) and 1680 mW m 2 (PDMS binder) with SSM air–cathodes [24,29], primarily due to the differences in GDL (PTFE/carbon black versus PDMS/carbon black) and CL (PTFE/activated carbon versus Nafion/Pt or PDMS/Pt) , but also the use of different methods of cathodes making (rolling-press and brushing). Using the same ACAC made according to the same maternal and rolling-press method, the MPD obtained with 60 M increased by 84% from 1086 [31] to 1993 mW m 2 only when the anode was replaced from heated carbon mesh (two dimensional material) to an acetone cleaned carbon fiber brush (three dimensional material). This great increase of power density was due to the enlarged surface area between anode and electrolyte as well as the decreased spacing between the anode and cathode [32]. In additional, if the cathode of 40 M was pretreated by

X. Li et al. / Applied Energy 123 (2014) 13–18

alkaline, the MPD could be further increased owing to the increased micro-pore area/volume as demonstrated previously [33]. The best performance of 40 M was also demonstrated by the electrochemical characteristics of cathode. LSVs over the potential range from 0.2 to 0 V (versus SCE) exhibited the same trend as the power densities, indicated that the performance of different air–cathodes in MFCs is critical at this potential range. Although the cathode of 80 M had a larger exchange current density of 1.28 A m 2 and therefore a higher ORR activity at equilibrium state than 1.14 A m 2 of 40 M (Table S1), the power density was significantly lower than that of 40 M, which was possibly due to the largest Rct caused by the finest mesh. The lowest Rct resulted in the smallest internal resistance of 40 M, possibly attributed to the coarse wire diameter as well as the relatively larger amount of metal mesh in 40 M. Thus more catalysts were contacted with metal surface so that electrons were transferred more efficient from active sites to the external circuit. Rct was the largest contributor to the internal resistance (Fig. 5), suggesting that cathode reaction was primarily kinetically limited under our operating condition. Rct decreased with the increase of overpotential, because larger driving force for electron transfer is needed to generate a higher current and therefore has a higher overpotential of oxygen reduction. W appeared only when the Rct was not the dominant resistance, indicating that the diffusion resistance was not a dominant factor of power output unless the charge transfer resistance was very small. The Cdl of 40 M (2.3  10 4 X 1 sn) were approximately 3 times higher than those of 20 M (7.5  10 5 X 1 sn), 60 M (8.1  10 5 X 1 sn) and 80 M (8.3  10 5 X 1 sn), showing that the relative high power output may be partly from the large capacitance in double layer. As reported by us very recently, the capacitance of an anode is critical to eliminate the power overshoot since the capacitance is a reservoir of electrons as well as a stabilizer when the current is high [34]. Here for the air–cathodes, it can be roughly explained that the 40 M had a 3 times higher capability to ensure a high current output and therefore sustained the highest current density under the same cathodic potential in MFCs (Fig. 1B). Different from previous results [29], we found the linear fitting correlationship (R2 = 0.7068, Fig. S3) between mesh opening size and maximum power density was less significant than that (R2 = 0.9222, Fig. 6) between the planar density of metal mesh and maximum power density, probably due to the differences in catalytic materials and binders as well as the method for electrode making. Although the optimized mesh opening size was comparable (30–50 M), the planar density of metal mesh compared to mesh opening size was more accuracy and credible to explain the difference of performance in this study. Certainly, the mesh opening size was also an important parameter for assessing cathode capacity.

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Density of metal mesh (mg⋅cm ) Fig. 6. Fitting relationship between MPD and mesh planar density (mg/cm2).

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This was demonstrated by the relative high linear fitting correlationship between mesh opening size and CE (R2 = 0.9827, Fig. S4), and between mesh opening size and j0 (R2 = 0.8780 and Fig. S5) in this work. 5. Conclusions Four stainless steel meshes with different opening sizes (from 20 M to 80 M) were compared as matrixes in activated carbon air–cathodes. The highest power density of 2151 mW m 2 was obtained using 40 M, which was attributed to the smallest resistance and the largest capacitance. Main differences in the internal resistance was from the charge transfer resistance, indicated that the opening size of the matrix initially had a significant effect on the charge transfer process. Compared to the mesh opening size, the density of metal mesh had a more significant correlationship with maximum power densities (R2 = 0.9222 > 0.7068), showing that the density is one of the most important properties when we select the matrix for a high performance air–cathode in the future. Acknowledgments This research work was financially supported by the Ministry of Science and Technology as a 863 major project (Grant No. 2013AA06A205), National Natural Science Foundation of China as a young scholar project (No. 21107053) and as a key project (No. 21037002), Tianjin Research Program of Application Foundation and Advanced Technology (No. 13JCQNJC08000) and MOE Innovative Research Team in University (IRT13024). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy. 2014.02.048. References [1] Logan BE, Rabaey K. Conversion of wastes into bioelectricity and chemicals by using microbial electrochemical technologies. Science 2012;337:686–90. [2] Wang X, Feng Y, Lee H. Electricity production from beer brewery wastewater using single chamber microbial fuel cell. Water Sci Technol 2008;57:1117–22. [3] Wang X, Feng Y, Wang H, Qu Y, Yu Y, Ren N, et al. Bioaugmentation for electricity generation from corn stover biomass using microbial fuel cells. Environ Sci Technol 2009;43:6088–93. [4] He Z, Minteer SD, Angenent LT. Electricity generation from artificial wastewater using an upflow microbial fuel cell. Environ Sci Technol 2005;39:5262–7. [5] Sevda S, Dominguez-Benetton X, Vanbroekhoven K, De Wever H, Sreekrishnan T, Pant D. High strength wastewater treatment accompanied by power generation using air cathode microbial fuel cell. Appl Energy 2013;105:194–206. [6] Chen JY, Li N, Zhao L. Three-dimensional electrode microbial fuel cell for hydrogen peroxide synthesis coupled to wastewater treatment. J Power Sources 2014;254:316–22. [7] Rozendal RA, Hamelers HV, Rabaey K, Keller J, Buisman CJ. Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol 2008;26:450–9. [8] Liang P, Wei J, Li M, Huang X. Scaling up a novel denitrifying microbial fuel cell with an oxic-anoxic two stage biocathode. Frontiers Environ Sci Eng 2013;7:913–9. [9] Peng X, Yu H, Wang X, Zhou Q, Zhang S, Geng L, et al. Enhanced performance and capacitance behavior of anode by rolling Fe3O4 into activated carbon in microbial fuel cells. Bioresour Technol 2012;121:450–3. [10] Dong H, Yu H, Wang X. Catalysis kinetics and porous analysis of rolling activated carbon-PTFE air–cathode in microbial fuel cells. Environ Sci Technol 2012;46:13009–15. [11] Zhang F, Pant D, Logan BE. Long-term performance of activated carbon air cathodes with different diffusion layer porosities in microbial fuel cells. Biosens Bioelectron 2011;30:49–55. [12] Park DH, Zeikus JG. Improved fuel cell and electrode designs for producing electricity from microbial degradation. Biotechnol Bioeng 2003;81:348–55. [13] Zhao F, Harnisch F, Schröder U, Scholz F, Bogdanoff P, Herrmann I. Application of pyrolysed iron (II) phthalocyanine and CoTMPP based oxygen reduction

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Glossary AC: activated carbon ACAC: activated carbon air–cathode CC: current collector CE: Coulombic efficiency CL: catalyst layer GDL: gas diffusion layer MFCs: microbial fuel cells MPDs: maximum power densities OCP: open circuit potential ORR: oxygen reduction reaction PTFE: polytetrafluoroethylene SSM: stainless steel mesh