Accelerated OH– Transport in Activated Carbon Air Cathode by

Article pubs.acs.org/est

Accelerated OH− Transport in Activated Carbon Air Cathode by Modification of Quaternary Ammonium for Microbial Fuel Cells Xin Wang,† Cuijuan Feng,† Ning Ding,† Qingrui Zhang,‡ Nan Li,§ Xiaojing Li,† Yueyong Zhang,† and Qixing Zhou*,† †

MOE Key Laboratory of Pollution Processes and Environmental Criteria and 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 ‡ Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China § School of Environmental Science and Engineering, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, China S Supporting Information *

ABSTRACT: Activated carbon (AC) is a promising catalyst for the air cathode of microbial fuel cells (MFCs) because of its high performance and low cost. To increase the performance of AC air cathodes, the acceleration of OH− transport is one of the most important methods, but it has not been widely investigated. Here we added quaternary ammonium to ACs by in situ anchoring of a quaternary ammonium/epoxide-reacting compound (QAE) or ex situ mixing with anion exchange resins in order to modify ACs from not only the external surface but also inside the pores. In 50 mM phosphate buffer solution (PBS), the in situ anchoring of QAE was a more effective way to increase the power. The highest power density of 2781 ± 36 mW/m2, which is 10% higher than that of the control, was obtained using QAE-anchored AC cathodes. When the medium was switched to an unbuffered NaCl solution, the increase in maximum power density (885 ± 25 mW/m2) was in accordance with the anion exchange capacity (0.219 mmol/g). The highest power density of the anion exchange resin-mixed air cathode was 51% higher than that of the control, indicating that anion exchange is urgently needed in real wastewaters. Excess anchoring of QAE blocked both the mesopores and micropores, causing the power output to be inhibited.

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applications (from $1600/m2 to $30/m2).6,7 Highly reproducible AC air cathodes can be produced by the rolling-press method,8 and the maximum power density was increased by 35% from 802 to 1086 mW/m2 when the catalyst layer (CL) was not heated (using two-dimensional anodes).9 The electron transfer number was varied from 2.1 (hardwood-based) to 3.6 (peat-based), which was attributed to the differences in porous structure and the quantity of strong acid groups (pKa < 8).10 In addition to the ORR activity, ion transport in the air cathode is another key problem that must be addressed. Because of the neutral nature of the electrolyte in MFCs, the local pH inside the air cathode is alkaline. Therefore, the dominant transport process is anion (OH−) toward the bulk electrolyte (eq 1) rather than cation (H+) toward the CL (eq 2). Popat et al.11 revealed that in Pt-based air cathodes, up to

INTRODUCTION

Microbial fuel cells (MFCs) are a promising technology to directly recover electrical energy from waste biomass,1,2 especially for energy recovery from wastewaters.3 Electrons obtained from anodic oxidation of organic pollutants are transferred through external loading to the cathode and reacted with oxidant. Oxygen is one of the most applicable oxidants at the cathode because of its inexhaustible nature and relatively high redox potential. The air cathode is an advanced design that centralizes the electron transfer, ion transfer, and gas diffusion in the same electrode, which minimizes the operational cost by passive supplement of oxygen instead of aeration and increases the cathodic potential.4 In MFCs, the cathode is considered as the bottleneck of the performance.5 Despite its high cost, Pt is necessary to reduce the overpotential of the oxygen reduction reaction (ORR) in air cathodes and guarantee a high current. Recently, inexpensive activated carbon (AC) was demonstrated to be a highperformance substitute for Pt, which would allow the cost to be substantially decreased to an acceptable range for large-scale © 2014 American Chemical Society

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epoxy was then reacted with hydroxyl on the AC surface to form QAE-anchored AC (eq 4, in which R denotes polyaromatic graphene layers in AC). The QAE-anchored AC was rinsed using distilled water by suction filtration until the pH of the filtrate was neutral, and then the material (denoted as AN1) was dried in an oven at 40 °C for 24 h. Here we did not use dilute HCl to neutralize NaOH to avoid corrosion of the stainless steel matrix.16 C6H16ClNO2 was also produced as a byproduct by the subsidiary reaction shown in eq 5 and was considered to be removed from the AC during rinsing. In order to increase the efficiency of QAE anchoring, the epoxy intermediate in eq 3 (C6H14ClNO, 95%, Dongying J&M) with the same molarity as the QAE was also employed. The process of epoxy intermediate pretreatment was the same as the procedure described above. This sample was denoted as AN2. Fabrication of Air Cathodes. AC air cathodes were prepared by the rolling-press method as described previously (Figure S1 in the Supporting Information).8,9 Gas diffusion layers (GDLs) of each AC cathode were made in parallel by rolling a mixture of carbon black (Jinqiushi Chemical Co. Ltd., Tianjin, China) and polytetrafluoroethylene (PTFE, 60%, Horizon, Shanghai, China) with a mass ratio of 3:7 into a film. The thickness of each layer was controlled by the distance between the two roll shafts. The film was then roll-pressed onto a stainless steel mesh (60 × 60 mesh, type SUS304, Xinsilu Metal Product Co. Ltd., Shanghai, China) to form a film with a thickness of of 0.5 mm. After heating at 340 °C for 25 min, different CLs were roll-pressed down to the opposite side of different electrodes to reach a final thickness of 0.6 mm. Six AC air cathodes with different CLs were prepared as below (mass ratios are shown): (1) control, AC (SPC-02S, Xinsen):PTFE = 6:1; (2) CT-R, AC:cation exchange resin powder (005×7, Chemical Plant of Nankai University, China):PTFE = 6:1.25:1; (3) AN-R1, AC:anion exchange resin powder (205×7, Chemical Plant of Nankai University):PTFE = 6:0.5:1; (4) AN-R2, AC:anion exchange resin powder:PTFE = 6:1.25:1; (5) AN1:PTFE = 6:1; (6) AN2:PTFE = 6:1. The basic AC:PTFE ratio of 6:1 was previously determined by us.8 The ratios of anion exchange resin were determined by preliminary experiments to make the anion exchange capacities of AN-R1 and AN-R2 comparable to those of AN1 and AN2, allowing the effects of in situ anchoring and ex situ mixing of quaternary ammonium on the performance of the AC air cathode to be studied. The mass ratio of cation exchange resin (CT-R) was chosen to be the same as the mass ratio of anion exchange resin in AN-R2. MFC Configuration and Operation. Air cathode membraneless cubic MFCs with a volume of 28 mL were constructed as previously described.4 The anodes were acetonecleaned carbon fiber brushes.17 All of the anodes were preacclimated and operated at 1000 Ω for more than 2 months before the cathode was switched to one of the AC cathodes (7 cm2 in projected area). MFCs were operated for three cycles before polarization curves were measured. The medium contained acetate (1 g/L) as the electron donor, 50 mM PBS (Na2HPO4, 4.09 g/L; NaH2PO4·H2O, 2.93 g/L; KCl, 0.13 g/L; NH4Cl, 0.31 g/L), trace minerals (12.5 mL/L), and vitamins (5 mL/L).18 Each AC cathode was soaked in 50 mM PBS overnight before it was equipped on an MFC. All of the MFCs were operated in batch mode at 30 °C under 1000 Ω of external resistance and refilled when the voltage was lower than 50 mV. Polarization curves were measured in both PBS

>0.3 V of cathodic potential loss is due to the accumulation of OH− at a current density of 10 A/m2. In order to further increase the air cathode performance, one of the most applicable strategies is to decrease the overpotential by alleviating the OH− concentration gradient from the CL to the bulk electrolyte or through the cathodic biofilm.12 In Ptbased air cathodes, use of an anionomer binder (e.g., AS-4 produced by Tokuyama Corporation, Japan) had been reported to improve the potential by 81 mV at 20 A/m2 compared with that using a cationomer binder (e.g., Nafion).11 The quaternary anion exchange ionomer 1,4-diazabicyclo[2,2,2]octane (DABCO) polysulfone (QDPSU) also demonstrated higher performance than Nafion in wastewater.13 However, for the OH− produced or adsorbed inside pores, the ionomer binder may exhibit limited performance. Compared with Pt-based air cathodes, the OH− accumulation would be much more serious in AC-based air cathodes because the abundant pores in AC, especially the micro- and mesopores, would strongly adsorb OH−. However, so far as we know, this area, especially the in situ modification of ACs to accelerate anion transport, had not been investigated in AC-based air cathodes. anion toward bulk electrolyte

O2 + 4e− + 2H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 4OH−

(1)

cation toward catalyst layer

O2 + 4e− + 4H+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2H 2O

(2)

Quaternary ammonium is the most common functional group in anionomers such as AS-4 mentioned above because of its excellent capacity for anion exchange. Here we compared two methods of quaternary ammonium modification in CLs: simple mixing of anion exchange resin into the CL and in situ anchoring of a quaternary ammonium/epoxide-reacting compound (QAE) to the AC. Added cation exchange resin and blank AC cathodes were employed as the controls. Electrochemical analysis in an abiotic reactor and power production in MFCs were investigated as functions of the material properties.

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MATERIALS AND METHODS QAE-Anchored ACs. The QAE-anchored ACs were prepared according to the optimized conditions reported by Hou and co-workers.14,15 The QAE utilized here was C6H15Cl2NO (Dongying J&M Chemical Co., Ltd., Shandong, China) with a molecular weight of 188 g/mol. AC (3 g, SPC02S, Xinsen Carbon Co. Ltd., Fujian, China) was suspended in 14.5 mL of QAE solution (69%) with a AC/QAE mass ratio of 1:4, and the suspension was stirred for 24 h to achieve complete adsorption. Then the pH of the mixture was adjusted to 14 using NaOH, and the mixture was stirred at 35 °C for 48 h to form the epoxy intermediate, C6H14ClNO (eq 3). Part of the

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medium (conductivity of 6.5 ± 0.3 mS/cm) and NaCl medium (NaCl, 0.5 g/L; sodium acetate, 1g/L; mineral solution, 12.5 mL/L; vitamin solution, 5 mL/L; conductivity of 1.65 ± 0.02 mS/cm). Electrochemical and Material Analysis. Polarization curves and electrode potential curves were measured by varying the external resistance from 1000 to 30 Ω with a time interval of 30 min to ensure a stable voltage. Linear sweep voltammetry (LSV) was conducted using a potentiostat (Autolab PGSTAT 302N, Metrohm, Herisau, Switzerland) in 50 mM PBS (pH 6.77, conductivity = 5.93 mS/cm or pH 13.0, conductivity = 15.6 mS/cm) over a potential range from 0.3 to −0.2 V vs Ag/AgCl at a scan rate of 0.1 mV/s. The pH of 50 mM PBS was adjusted using a saturated NaOH solution. A Pt electrode (1 cm2) was used as the counter electrode. Each AC cathode was soaked in its electrolyte (pH 6.77 or 13.0) for at least 24 h before the measurement. Experiments were performed in duplicate to check reproducibility. All of the potentials reported in this work are versus Ag/AgCl except where noted. The ion exchange capacity of each CL was measured by the titration method (see the Supporting Information). Porous distributions of carbon powders were analyzed using a TriStar II 3020 analyzer (Micromeritics, Norcross, GA, USA), and an ASAP2010 analyzer (Micromeritics) was used for multipoint Brunauer−Emmett−Teller (BET) surface area measurements through nitrogen adsorption at 77 K as previously described.6 In order to investigate the in situ porous characteristics (including the effect of the rolling-press process on porous distribution), the porous structures of CLs with the stainless steel mesh were measured with a mercury porosimeter (Autopore IV, Micromeritics). 6 The surface elemental compositions of the carbon powders were analyzed using Xray photoelectron spectroscopy (XPS) (Axis Ultra DLD spectrometer, Kratos Analytical Ltd., Manchester, UK) with monochromatized Al Kα radiation as the X-ray source.16 Nitrogen distributions were investigated by transmission electron microscopy (TEM) on a Tecnai G2 F20 microscope (FEI, Hillsboro, OR, USA) equipped with an electron energy loss spectrometer. Surface morphologies of different CLs were examined by scanning electron microscopy (SEM) (LEO 1530VP, LEO Elektronenmikroskopie GmbH, Oberkochen, Germany).

Figure 1. (A, B) LSV curves of different cathodes in 50 mM PBS at (A) pH 6.77 and (B) pH 13.0. (C) Calculated cathodic local pH of AC cathodes. Potentials were measured using Ag/AgCl as the reference electrode.

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cathodes with anion exchange groups, whether QAE-anchored or from added anion exchange resin, can increase the current, especially over the potential range from the open-circuit potential (OCP) to −0.1 V (0.097 V vs SHE). The addition of cation exchange resin (CT-R) had a negative effect on the current production. The current loss compared with the control increased with decreasing cathode potential. Mixing different amounts of anion exchange resin with the AC cathode (AN-R1 and AN-R2) indeed enhanced the current output, but the amount of resin had no obvious effect on the current in the abiotic LSV curves. In situ anchoring of quaternary ammonium (AN1 and AN2) resulted in an obvious increase in performance at a current density lower than 5 A/m2. The highest current density of 13.2 A/m2 was obtained with AN1 at −0.2 V (−0.003 V vs SHE), with a value 25% higher than the value of 10.6 A/m2 with the control. However, the LSV curve of AN2 tended to be the same as the control curve at a current density higher than 9 A/m2. In order to eliminate the potential differences caused by OH− accumulation, LSV was performed in 50 mM PBS at pH 13.0 (Figure 1B). The onset potential was 0.168 ± 0.010 V

RESULTS Ion Exchange Performance. On the basis of the titration analysis, the anion exchange capacities (Qa) of the QAEmodified AC cathode (AN1) and the epoxy-intermediatemodified AC cathode (AN2) were 0.103 and 0.264 mmol/g, respectively, which were approximately 5−10 times higher than that of the control (0.0280 mmol/g). Addition of anion exchange resin also increased Qa, as the Qa values of AN-R1 (0.109 mmol/g) and AN-R2 (0.219 mmol/g) were comparable with those of A-1 and A-2 as designed previously. The cation exchange capacity (Qc) of the cathode with added cation exchange resin (CT-R) was 0.270 mmol/g, which was comparable with Qa of AN-R2 despite the positive or negative sign of the charges. LSV at pH 6.77 and pH 13.0. In an abiotic system, we first used LSV to evaluate the performance of different AC cathodes in 50 mM PBS at pH 6.77. All of these AC cathodes exhibited similar onset potentials of 0.265 ± 0.005 V (0.462 ± 0.005 V vs SHE) (Figure 1A). With a decrease in cathode potential, AC 4193

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Figure 2. SEM images of the catalyst layers in (A, B) the control, (C, D) CT-R, and (E, F) AN1 at enlargement factors of (A, C, E) 500 and (B, D, F) 3000. CT-R was the AC electrode with added cation exchange resin, and AN1 was the QAE-modified AC cathode.

(0.365 ± 0.010 V vs SHE) for all of the AC cathodes except AN2, which had an onset potential of 0.215 V (0.412 V vs SHE). With a decrease in cathode potential, the current density of AN2 was the highest until 7 A/m2, where the current densities of CT-R and AN-R2 dramatically increased, especially at potentials more negative than −0.1 V. Reversely, the current density of AN2 became the lowest at potentials more negative than −0.16 V (0.037 V vs SHE). With the assumptions of constant ORR electron transfer number and ignorable overpotential caused by oxygen transport, the cathodic local pH can be roughly estimated from the LSV data at pH 6.77 and 13.0 (Figure 1C; see “Calculation of cathodic local pH” in the Supporting Information). Because the current curves of CT-R and ANR2 were different from the others at pH 13.0, the overpotential from oxygen transport may affect the calculation, so the local pH values of these two electrodes were not compared. As shown in Figure 1C, the in situ anchoring of QAE can effectively alleviate the pH increase compared with the control.

For example, at a cathodic potential of −0.1 V (0.097 V vs SHE), the calculated local pH values of AN1 (9.7) and AN2 (8.3) decreased by 0.8 and 2.2 units compared with the control (10.5), showing that anchoring of quaternary ammonium to ACs exhibits excellent performance in conducting OH− and therefore decreasing the local pH. Adding more QAE (higher Qa) resulted in less pH variation. The addition of anion exchange resin to CL also alleviated OH− accumulation over most of the potential range. With a comparable Qa, the local pH of AN-R1 at −0.1 V was 10.2, which is 0.5 units higher than that of AN1, showing that the in situ anchoring of quaternary ammonium was a more effective method to decrease the cathodic local pH. Pore and Elemental Analysis. According to SEM images, the CL of the control consisted of irregular AC particles (from CT-R (750 ± 19 mW/ m2) > control (586 mW/m2) > AN2 (576 ± 19 mW/m2). At the same current density, the cathodic potentials decreased in the same trend, and more obvious potential differences were observed with NaCl solution than with 50 mM PBS (Figure 5B). 4196

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Natural Science Foundation of China (21107053 and 21037002), the Ministry of Science and Technology as an 863 major project (Grant 2013AA06A205), and the Tianjin Research Program of Application Foundation and Advanced Technology (13JCQNJC08000).

with an atomic N content of 0.75% (Qa = 0.103 mmol/g) increased both the cathodic current in abiotic tests and the power output in real MFCs. However, overloading of QAE to an atomic N content of 2.2% (Qa = 0.264 mmol/g) exhibited a negative effect on the performance of the AC cathode in all tests. For a two-chambered system without bacterial contamination on the surface of an air cathode, LSV in a neutral abiotic environment revealed that the in situ connection of quaternary ammonium with a small value of Qa (AN1) was a more effective method than mixing anion exchange resins (both AN-R1 and AN-R2) to increase current output, especially at a current density higher than 8 A/m2. AN2 had the highest current output when the current densitie was lower than 5 A/m2, but its good performance could not be extended to a wider potential window since parts of the ORR-active sites were blocked by excess QAE. Besides, the addition of anion exchange resin increased the performance of AC cathode, but the amount of resin had no obvious effect, showing that the acceleration of OH− transport was limited in a buffered system. At pH 13.0, a condition that theoretically would lead to no pH gradient inside and outside the CLs,11 CT-R and AN-R2 with the highest mass ratios of ion exchange resins compared with other AC cathodes had the highest current output, indicating that the interfaces introduced between resins and AC particles (Figure 2 and Table S3 in the Supporting Information) may become the leading factor in the performance. Therefore, the mixing of ion exchange resins into CLs had an additional benefit to increase the performance. In single-chambered membraneless MFCs, the differences were not as obvious as in the abiotic LSV data until the systems were tested in an unbuffered NaCl medium. Polarization tests in unbuffered NaCl medium revealed that the Qa became the most dominant factor for the MPD. The MPDs increased following the sequence of the Qa values, with the highest increase reaching up to 51% for AN-R2. However, AN-R2 did not achieve the highest MPD in 50 mM PBS. In other words, AN-R2 has the lowest decrease in MPD after buffer removal, confirming that the anion transport is much more important for an unbuffered system. Therefore, the addition of quaternary ammonium to the CL is necessary when MFCs are used to treat real wastewater in the future because the quaternary ammonium added in the AC air cathode can be an effective and sustainable supplement to the buffering capacity of wastewater.

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ASSOCIATED CONTENT

S Supporting Information *

Measurement of ion exchange capacity, calculation of cathodic local pH, and the three supporting tables and five supporting figures mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (86)22-23507800; fax: (86)22-23501117; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Liao Guan for his help with cathode preparation. This research was supported by the National 4197

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pretreatment methods on power production in microbial fuel cells. Environ. Sci. Technol. 2009, 43 (17), 6870−6874. (18) Lovley, D. R.; Phillips, E. J. P. Novel mode of microbial energy metabolism: Organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl. Environ. Microbiol. 1988, 54 (6), 1472−1480.

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