Article pubs.acs.org/est
Enhanced Activated Carbon Cathode Performance for Microbial Fuel Cell by Blending Carbon Black Xiaoyuan Zhang,† Xue Xia,‡ Ivan Ivanov,† Xia Huang,‡ and Bruce E. Logan*,† †
Department of Civil & Environmental Engineering, Penn State University, 231Q Sackett Building, University Park, Pennsylvania 16802, United States ‡ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *
ABSTRACT: Activated carbon (AC) is a useful and environmentally sustainable catalyst for oxygen reduction in air-cathode microbial fuel cells (MFCs), but there is great interest in improving its performance and longevity. To enhance the performance of AC cathodes, carbon black (CB) was added into AC at CB:AC ratios of 0, 2, 5, 10, and 15 wt % to increase electrical conductivity and facilitate electron transfer. AC cathodes were then evaluated in both MFCs and electrochemical cells and compared to reactors with cathodes made with Pt. Maximum power densities of MFCs were increased by 9−16% with CB compared to the plain AC in the first week. The optimal CB:AC ratio was 10% based on both MFC polarization tests and three electrode electrochemical tests. The maximum power density of the 10% CB cathode was initially 1560 ± 40 mW/m2 and decreased by only 7% after 5 months of operation compared to a 61% decrease for the control (Pt catalyst, 570 ± 30 mW/m2 after 5 months). The catalytic activities of Pt and AC (plain or with 10% CB) were further examined in rotating disk electrode (RDE) tests that minimized mass transfer limitations. The RDE tests showed that the limiting current of the AC with 10% CB was improved by up to 21% primarily due to a decrease in charge transfer resistance (25%). These results show that blending CB in AC is a simple and effective strategy to enhance AC cathode performance in MFCs and that further improvement in performance could be obtained by reducing mass transfer limitations.
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INTRODUCTION Microbial fuel cells (MFCs) are being explored as a technology for energy recovery and wastewater treatment based on electricity generation from wastewater organics using exoelectrogenic bacteria.1−3 Air cathodes are used in MFCs to produce high power from readily available oxygen in air, without the need for wastewater aeration.4,5 Catalysts are needed to reduce the overpotential for oxygen reduction, and Pt is commonly used in lab-scale reactors. However, Pt is very expensive and a precious metal, and its catalytic performance can significantly decrease over time due to chemical and biological fouling.6,7 Various alternatives to Pt have been proposed that can primarily be separated into three types.8 The first is transition metal macrocyclic compounds, such as cobalt tetramethylphenylporphyrin (CoTMPP),9 Co-naphthalocyanine (CoNPc),10 and iron(II) phthalocyanine (FePc).11 The second is non-noble metallic oxides, such as manganese dioxides (MnO2)12,13 and lead dioxide (PbO2).14 However, these two types of alternative catalysts have cost or performance limitations. The third and most promising alternative is activated carbon (AC)-based catalysts due to their low cost and good performance.8,15−20 AC cathodes are typically constructed using certain types of carbon-based materials that have good catalytic activity, but the electrical conductivity of the AC is poor compared to that of carbon cloth. Therefore, the AC is usually pressed onto a stainless steel mesh that provides structural support and © 2014 American Chemical Society
functions as a current collector. Poly(tetrafluoroethylene) (PTFE) is normally used as a binder, as opposed to Nafion with Pt, which greatly reduces the cost of the cathode materials. AC cathodes can be made with a PTFE binder by batch18,20 or continuous (rolling) cold-pressing of the AC onto the stainless steel mesh.16,17,21 The mass of AC used per area of cathode can affect performance, with an optimal performance determined in one study of 0.43 kg/m2 (tested range of 0.07 to 1.71 kg/m2).20 AC cathodes can produce power densities similar to those of Pt in MFCs, but there is still considerable interest in further improving cathode performance and longevity. Selection of the material used to make the AC is important for power production. Peat-based and coal-based carbons have been shown to produce higher power densities than coconut shellbased, hardwood-based, and phenolic resin-based carbons.18 The AC can be chemically modified to improve performance as well. Pyrolysis of AC with iron ethylenediaminetetraacetic acid (FeEDTA) was shown to improve maximum power densities in MFCs by 10% compared to plain AC.6 High temperature ammonia gas treatment can also improve AC performance, but the procedure is energy-intensive.22 Pt cathodes have been Received: Revised: Accepted: Published: 2075
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shown to be reduced in performance by ∼20% to >50% over one year, depending on what substrate is used.23 In one study, the performance of the AC was reduced by ∼20−40% after one year (same time frame as Pt).24 MFC performance is mainly limited by oxygen reduction kinetics at the cathode.25,26 While it is important to improve cathode performance and longevity, such improvements must be made using methods that do not substantially increase cathode materials and manufacturing costs. In order to improve cathode performance we examined the addition of carbon black (CB) particles with AC during cathode construction. While CB has poor performance as an oxygen reduction catalyst,15,27 it primarily is used in Pt cathodes as a catalyst support due to its high electrical conductivity. CB powder is relatively inexpensive and can easily be incorporated into the AC cathode due to its much smaller particle size compared to most AC particles. Catalyst materials for oxygen reduction should have both high catalytic activity and conductivity. We hypothesized that adding CB into AC would facilitate electron transfer and thus would improve overall catalytic performance. To determine the impact of CB on cathode performance, different ratios of CB were added to AC. The performance of these cathodes was examined in both electrochemical tests and in MFCs over time.
other chamber facing the air as a working electrode, and a Ag/AgCl reference electrode (3 M KCl, +0.21 V versus a standard hydrogen electrode; RE-5B; BASi, West Lafayette, USA) was placed close to the cathode. A 50 mM phosphate buffer solution (PBS) was used as medium, which contained 4.57 g/L Na2HPO4, 2.45 g/L NaH2PO4·H2O, 0.31 g/L NH4Cl, and 0.13g/L KCl. Chronoamperometry tests were carried out by setting a potential in a stepwise manner after the reactor operated in open circuit condition for 3 h. Each potential (0.2, 0.1, 0, −0.1, and −0.2 V versus Ag/AgCl) was applied for 2 h. A rotating disc electrode (RDE) can be used to reduce mass transfer limitations to catalysts, allowing measurement of the limiting current due to reaction kinetics. RDE tests were therefore conducted to evaluate the catalyst activity of the AC materials compared to that of Pt. Catalyst ink was prepared by adding 40 mg of the powdered sample (10% Pt on Vulcan XC-72, plain AC powder, or AC powder with 10 wt % CB) to 0.8 mL of isopropyl alcohol, followed by ultrasonication for 15 min. Nafion (5 wt % solution, 0.2 mL) was added to the suspensions, with ultrasonication for an additional 15 min. The ink suspension (40 μL) was drop coated onto a 1.3-cm diameter graphite carbon disk and allowed to dry in the air overnight, leading to a sample loading of 1.2 mg/cm2. The catalyst ink samples were used for kinetic studies and electrochemical impedance spectroscopy (EIS) analysis. To better simulate catalyst conditions in MFCs, small cathodes (1.3-cm diameter) were fabricated following the same procedure described above, but without the diffusion layer, allowing the catalyst layer to direct face the solution during RDE tests. The effective projected area during the tests was 0.283 cm2 (0.6 cm in diameter) for both two types of samples. RDE tests were carried out using a modulated speed rotator (MSR rotator, PINE Instruments, USA) and a medium of 50 mM PBS sparged with pure oxygen. Before the tests, solutions were sparged for at least 30 min with oxygen, and cyclic voltammetry (CV) was run at 20 mV/s between 0.3 to −0.8 V vs Ag/AgCl until the current response was the same from cycle to cycle. The reproducibility demonstrated an absence of contaminants on the electrode surface or excess oxygen trapped in the pores of the sample.18 Chronoamperometry tests were then run by setting a potential in a stepwise manner (from 0 to −0.6 V, with 30 min at each potential), at rotation rates between 100 and 2500 rpm. The average number of electrons transferred (n) and kinetic current (iK) for the oxygen reduction reaction were calculated based on the Koutecky−Levich analysis using29
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MATERIALS AND METHODS Air-Cathode Material and Fabrication. AC (Norit SX plus, Norit Americas Inc., USA) was selected on the basis of its better performance compared to other types of readily available carbons18 and applied at a constant optimal loading of 0.43 kg/m2 (300 mg for each 7 cm2 cathode). CB powder (Vulcan XC-72, Cabot Corporation, USA) was added into AC, and the content was varied at weight ratios of CB:AC = 0% (pure AC control), 2%, 5%, 10%, and 15%. Cathodes were fabricated using a batch, cold-press process.6,20 The AC (300 mg) and CB (0−45 mg) powders were added into a vial and mixed by vortexing for 0.5 min. The binder solution was prepared by adding 37.8 μL of 60% PTFE and 700 μL of DI water into a beaker, followed by ultrasonication for 1 min. Then the AC and CB powders were transferred into the beaker with the binder and mixed in a blender for 0.5 min to form a paste. The paste was then ultrasonicated for 1 min and spread using a spoon onto one side of a stainless steel mesh (50 × 50, type 304, McMaster-Carr, USA) that served as the support material and current collector. Diffusion layers were prepared by applying two layers of poly(dimethylsiloxane) (PDMS) solution28 onto a textile material (Amplitude Prozorb, Contec Inc., USA), with the textile placed onto the cathode side facing the AC. Then, the cathode and the textile were pressed together at 40 MPa for 20 min (Model 4386, Carver Inc., USA) and dried at 80 °C in an oven overnight before use.6 Pt cathodes were also prepared to benchmark performance compared to the AC cathodes by applying a Pt catalyst layer (5 mg/cm2 10% Pt on Vulcan XC-72 and Nafion binder) on one side of a stainless steel mesh and two diffusion layers of PDMS on the other side of the mesh.28 Electrochemical Analysis. Electrochemical tests were conducted using a potentiostat (VMP3Multichannel Workstation, BioLogic Science Instruments, USA). Cathodes were examined in an abiotic and electrochemical reactor that was cubic shaped, with two 2-cm cylindrical chambers bolted together with an anion exchange membrane (AEM; AMI-7001, Membrane International Inc., USA) in the middle.5 A high purity platinum plate (99.99%, 1 cm2) was placed in the middle of one chamber as a counter electrode, a cathode was placed on one side of the
⎛ ⎞ −1/2 1 1 1 ⎟ω = +⎜ 2/3 1/6 − ⎝ 0.620nFAD v i iK C⎠
(1)
where i is the measured current, F Faraday’s constant, A the effective projected area of the disk electrode (0.283 cm2), D the diffusion coefficient of oxygen (2.7 × 10−5 cm2/s), v is the kinematic viscosity (8.08 × 10−3 cm2/s), C is the concentration of oxygen in the solution (2.3 × 10−7 mol/cm3), and ω is the rotation rate of the electrode. The average number of electrons transferred (n) for Pt, AC, and AC with 10% CB was calculated using these constants (D, v, and C) based on typical values of seawater with a solution conductivity of 7.5 mS/cm, which was similar to that of 50 mM PBS. EIS was carried out for the catalyst ink at the highest rotation rate of 2500 rpm, when the mass transfer limitations were significantly reduced. Impedance measurements were conducted at 2076
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−0.1 V vs Ag/AgCl over a frequency range of 10,000 to 0.002 Hz with a sinusoidal perturbation of 10 mV amplitude. EIS data were analyzed by fitting the spectra to an equivalent circuit model (Supplementary Figure S1).20 MFC Experiments. Cubic-shape single-chamber MFCs were constructed using a 4-cm length Lexan block with a 3-cm diameter inner cylindrical chamber, as previously described.30 The anode was a graphite fiber brush (2.5 cm in both diameter and length) with a core of two twisted titanium wires that functioned as a current collector. The anode was heat treated in a muffle furnace at 450 °C for 30 min and then placed horizontally in the middle of the cylindrical chamber. The cathode was placed at the other side of the reactor, with the diffusion layers facing the air. All MFCs were inoculated with the effluent of MFCs that were operated for over 1 year. The medium was 1 g/L sodium acetate in 50 mM PBS amended with 12.5 mL/L minerals and 5 mL/L vitamins.31 All MFCs were operated in batch mode with a 1000 Ω external resistor (except as noted) in a 30 ± 1 °C room. Initially, a standard Pt/C carbon cloth cathode4 was used for all the MFCs during start-up to ensure that the anodes achieved the same performance during acclimation. After 1 week, a repeatable cycle of voltage was produced by the MFCs. To ensure all the anode biofilms were fully acclimated, the MFCs were operated for 1 month, and then the Pt/C cathodes were removed and replaced with the new AC based cathodes or new Pt/C stainless steel cathodes. All MFC tests were conducted in duplicate. Voltages (U) were recorded across an external resistance (R) every 20 min using a multimeter with a computerized data acquisition system (2700, Keithley Instrument, USA). Polarization curves were obtained using a multicycle method, by applying different external resistors, with each resistance used for a complete cycle.32 Polarization tests were conducted after the MFCs had been operated after 1 week, 3.5 months, and 5 months, by varying the external resistances from 1000 to 20 Ω. Current densities (J) and power densities (P) were normalized by air-cathode projected area (A = 7 cm2), using J = U/RA and P = JU.32 Anode potentials were reported versus an Ag/AgCl reference electrode (+0.21 V versus a standard hydrogen electrode). Coulombic efficiencies (CEs) were calculated at each external resistance as previously described.32
Figure 1. Current−potential curves of different cathodes in electrochemical cells.
MFC Performance and Durability. Power generation was enhanced in MFCs by adding CB into the AC, with the same optimum ratio of 10% as that obtained in abiotic tests. After 1 week of operation, the MFCs with 10% ratio cathodes produced the highest power density of 1560 ± 40 mW/m2, followed by the 5% (1510 ± 10 mW/m2), 15% (1510 ± 10 mW/m2), and 2% ratio cathodes (1460 ± 10 mW/m2) (Figure 2A). The power density of the MFCs with the 10% ratio cathode was 16% higher than those with the plain AC cathodes (1340 ± 120 mW/m2) and 7% higher than MFCs with a Pt cathode (1460 ± 10 mW/m2). Anode potentials were essentially the same at the same current densities in all MFCs, indicating that the cathode potentials were responsible for the differences in power generation (Figure 2B). After 3.5 months of operation, the MFCs with 10% CB still produced the highest maximum power density of 1500 ± 210 mW/m2, which was 29% higher than the plain AC cathodes (1160 ± 120 mW/m2). The cathodes with the 5% and 15% ratios still produced similar maximum power densities of 1340 ± 40 mW/m2 (5% ratio) and 1330 ± 40 mW/m2 (15% ratio). The MFCs with the Pt cathodes had greatly reduced power production, with a 55% reduction in maximum power to 650 ± 10 mW/m2. This large decrease with Pt was consistent with previous reports6 as further discussed below. The AC with a 10% CB:AC ratio had the lowest decrease in performance of 4% in the maximum power densities, compared to a 13% decrease for plain AC. The CEs of MFCs with different cathodes increased with the current densities (Figure 3), also consistent with previous reports.33 The MFCs with 10% CB achieved the highest CE of 74% at the current density of 9.9 A/m2 (Figure 3). After 5 months, the MFCs with the 10% ratio continued to produce the highest power density of 1450 ± 10 mW/m2, which was now 150% higher than Pt (570 ± 30 mW/m2) and 14% higher than plain AC (1270 ± 80 mW/m2). The maximum power densities for this 10% ratio cathode were decreased by only 7% compared to the first week, demonstrating that AC based cathodes sustained high power generation. The slight decrease of AC cathode performance over time might be due to the clogging of micropores in the AC.24 There could also be small amounts of loss of the AC from the cathode support, but this might not affect the overall performance since the loading of AC was relatively high (86 times that of Pt).20 In contrast, maximum power of the Pt cathode decreased by 61% compared to that after 1 week. This decrease was likely due to biofouling and Pt catalyst losses (Supplementary Figure S2). When carbon cloth (fuel cell grade) was used as the current collector with the Pt catalyst, the durability was similar to that of the AC cathodes,20 but the cost
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RESULTS AND DISCUSSION Electrochemical Performance of Different Cathodes. Chronoamperometry tests on the abiotic cathodes in the electrochemical reactor showed that the cathode performance was enhanced by blending CB with AC, with an optimal ratio of 10% (Figure 1). This cathode had an open circuit potential (OCP) of 0.25 V, which was higher than the one without CB (0.20 V) or the other ratios (2%, 0.22 V; 5%, 0.23 V; 15%, 0.23 V). The Pt cathode produced the highest OCP of 0.32 V. The cathode with the 10% CB ratio generated the highest current density of 8.7 A/m2 at −0.2 V, which was 23% higher than that produced without CB (7.1 A/m2) and 12% higher than Pt (7.8 A/m2). All cathodes with CB showed improved performance compared to plain AC. The current densities at −0.2 V were 8.0−8.7 A/m2, increasing with the CB:AC ratio from 2% to 10%, and were enhanced by 13% to 23% compared to plain AC (7.1 A/m2). Adding more CB (15%) produced a current density of 8.5 A/m2, which was 20% higher than that of plain AC but was not more than that obtained with 10% CB (Figure 1). 2077
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Figure 2. (A−C) Power densities and (D−F) electrode potentials as a function of current density in MFCs using different cathodes after 1 week, 3.5 months, and 5 months of operation.
plain AC (0.28 mA). The Pt/C ink generated the highest limiting current of 0.40 mA (Figure 4E). The kinetic current of the AC with 10% CB, based on the K-L analysis, was 2.39 mA compared to 1.07 mA for plain AC and 8.39 mA for Pt/C (Figure 4F). The average number of electron transferred was 3.2 for Pt and was 2.9 for the 10% CB, compared to 2.8 for the plain AC. These numbers for Pt are much lower than those previously reported for Pt-catalyzed electrodes (n = 4),18 suggesting that the constants chosen for eq 1 might not be adequate for the solution conditions used here. EIS tests were conducted for the catalyst inks at the highest rotation rate of 2500 rpm. The use of 10% CB decreased charge transfer resistances by 25% to 576 Ω compared to plain AC (766 Ω) (Figure 5 and Supplementary Table S1). This reduction in charge transfer resistance therefore demonstrated our hypothesis that adding CB would facilitate electron transfer and enhance the overall catalytic performance of AC based catalysts. The Pt/C had the highest performance, with 237 Ω of ohmic resistance and 315 Ω of charge transfer resistance (Figure 5 and Supplementary Table S1). To simulate cathode conditions more similar to that in MFCs with the same catalyst and binder loading and the same current collector, but with controlled mass transfer, RDE samples were also prepared following the above mentioned fabrication procedure
Figure 3. Coulombic efficiency as a function of current density in MFCs using different cathodes after 3.5 months of operation.
of carbon cloth (fuel cell grade, $500−$600/m2, www. fuelcellearth.com) is much higher than that of stainless steel mesh ($10−$30/m2, www.alibaba.com). Catalyst Activity and Kinetics. Catalyst performance was further evaluated using RDE with two different methods of sample preparation. In the RDE tests using catalyst ink, the limiting current of AC was improved with 10% CB at all rotation rates (Figure 4). At the highest rotation rate (2500 rpm) the mixture with 10% CB was 0.34 mA, 21% higher than that of 2078
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Figure 4. Current−potential curves of catalyst inks of Pt/C (A), plain AC (B), and AC with 10% CB (C) in the RDE tests. Current comparison of different catalyst inks at rotation rates of 100 rpm (D) and 2500 rpm (E). Koutecky−Levich plots for oxygen reduction on different catalyst inks (F).
but without diffusion layers. Blending 10% CB into AC enhanced the current generation among all rotation rates (Figure 6). At the lowest rotation rate of 100 rpm, the Pt/C produced current densities similar to those with plain AC and AC with 10% CB (Figure 6). At the higher rotation rates, Pt/C showed higher current production at potentials that were more negative than −0.3 V. For example, at 900 rpm, the current was 1.42 mA for Pt/C at −0.6 V, which was much higher than that of plain AC (0.90 mA) or AC with CB (0.96 mA) (Figure 6). However, in the MFCs, the maximum power densities were normally obtained among the cathode potential range between −0.05 to −0.15 V (around −0.1 V in most cases) (Figure 2). At −0.1 V, at the highest rotation rate of 2500 rpm, the current for Pt/C was 0.58 mA, compared to 0.57 mA for plain AC, and 0.63 mA with 10% CB. This trend showing slightly better performance of the 10% CB loading was similar to results obtained on the basis of polarization tests in MFCs. Taken together, these results demonstrated that blending 10% CB into AC was a simple and effective strategy to improve the AC cathode performance. Implications of Catalyst Activities for MFC Performance. By blending 10% CB into AC, the catalyst activity was improved and the maximum power densities of MFCs with 10% CB cathodes were increased by 16% in the first week and by 29% after 3.5 months, compared to those of the plain AC. The cathodes with the 10% CB also showed much better longevity compared to the Pt/C cathodes. The cost of CB is
Figure 5. (A) Nyquist plots using catalyst ink consisting of Pt/C, plain AC, or AC with 10% CB at a rotation rate of 2500 rpm in RDE tests. (B) Nyquist plots with Z′ axis shifted to omit ohmic resistance for comparison of charge transfer resistances. 2079
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Figure 6. Current−potential curves of the cathodes without diffusion layers using Pt/C (A), plain AC (B), and AC with 10% CB (C) as catalysts in the RDE tests. Current comparison of different samples at rotation rates of 100 rpm (D), 900 rpm (E), and 2500 rpm (F).
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only ∼$1 per kg (www.alibaba.com). The AC loading in this study is 0.43 kg/m2, and therefore the cost for a 10% CB loading (0.043 kg/m2) is negligible (∼$0.043/m2) compared to other cathode costs. Recently, AC modified by pyrolyzed with iron ethylenediaminetetraacetic acid (FeEDTA), at a weight ratio of FeEDTA:AC = 0.2:1, also showed an enhanced catalyst activity in MFC systems, producing 10% higher maximum power than that of plain AC in the first week.6 The performance and durability of these different approaches and the factors determining the lifetime of the catalysts will therefore need to be considered in future studies. Binders, diffusion layers, and fabrication methods can also affect AC based cathode performances, so there will need to additional considerations given to these components. At this point, peat-based AC (0.43 kg/m2) blended with 10% CB appears to be the most cost-effective catalyst mixture for making MFC air cathodes using cold-pressing procedures.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (1) 814-863-7908. Fax: (1)814-863-7304. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors thank Valerie Watson and Marta Hatzell for the help with RDE and EIS analyses and David Jones for laboratory support. This research was supported by the Strategic Environmental Research and Development Program (SERDP) and Award KUS-I1-003-13 from the King Abdullah University of Science and Technology (KAUST). The authors thank the anonymous reviewers for their instructive comments.
ASSOCIATED CONTENT
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S Supporting Information *
Additional figures, a table that contains EIS fitting results, an equivalent circuit for EIS data analysis, and photos of the Pt cathodes. This material is available free of charge via the Internet at http://pubs.acs.org. 2080
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dx.doi.org/10.1021/es405029y | Environ. Sci. Technol. 2014, 48, 2075−2081
