Electrochemical Reduction of Carbon Dioxide in an MFC–MEC

Article pubs.acs.org/est

Electrochemical Reduction of Carbon Dioxide in an MFC−MEC System with a Layer-by-Layer Self-Assembly Carbon Nanotube/ Cobalt Phthalocyanine Modified Electrode Huazhang Zhao,*,†,‡ Yan Zhang,†,‡ Bin Zhao,§,∥ Yingyue Chang,†,‡ and Zhenshan Li†,‡ †

Department of Environmental Engineering, Peking University, Beijing 100871, People's Republic of China The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, People's Republic of China § School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, People's Republic of China ∥ State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin 300160, People's Republic of China ‡

S Supporting Information *

ABSTRACT: Electrochemical reduction of carbon dioxide (CO2) to useful chemical materials is of great significance to the virtuous cycle of CO2. However, some problems such as high overpotential, high applied voltage, and high energy consumption exist in the course of the conventional electrochemical reduction process. This study presents a new CO2 reduction technique for targeted production of formic acid in a microbial electrolysis cell (MEC) driven by a microbial fuel cell (MFC). The multiwalled carbon nanotubes (MWCNT) and cobalt tetra-amino phthalocyanine (CoTAPc) composite modified electrode was fabricated by the layer-bylayer (LBL) self-assembly technique. The new electrodes significantly decreased the overpotential of CO2 reduction, and as cathode successfully reduced CO2 to formic acid (production rate of up to 21.0 ± 0.2 mg·L−1·h−1) in an MEC driven by a single MFC. Compared with the electrode modified by CoTAPc alone, the MWCNT/CoTAPc composite modified electrode could increase the current and formic acid production rate by approximately 20% and 100%, respectively. The Faraday efficiency for formic acid production depended on the cathode potential. The MWCNT/CoTAPc composite electrode reached the maximum Faraday efficiency at the cathode potential of ca. −0.5 V vs Ag/AgCl. Increasing the number of electrode modification layers favored the current and formic acid production rate. The production of formic acid was stable in the MFC−MEC system after multiple batches of CO2 electrolysis, and no significant change was observed on the performances of the modified electrode. The coupling of the catalytic electrode and the bioelectrochemical system realized the targeted reduction of CO2 in the absence of external energy input, providing a new way for CO2 capture and conversion.

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CO2 + 2H+ + 2e− ↔ HCOOH

INTRODUCTION Carbon dioxide (CO2) is now known to be a major cause of global warming, and the reduction of its atmospheric concentration has therefore become a critical issue.1,2 The electrochemical reduction of CO2 into valuable chemical materials can proceed under mild conditions of a moderate temperature and atmospheric pressure.3 The reduction products, such as formic acid (widely used in the field of medicine, chemical industries, as well as the leather industry), can also be targeted by selecting appropriate electrodes. Therefore, the electrochemical reduction of CO2 is considered as an effective carbon recycling method. Generally, a high overpotential exists in the electrochemical reduction of CO2. For example, the theoretical potential for electrochemically reducing CO2 to formic acid under standard conditions is −0.199 V (vs NHE) by the following reaction:1 © 2012 American Chemical Society

(1)

As a result of the high overpotential, a common metal electrode, when used as the cathode, requires a voltage of more than 4 V to decrease the cathode potential to below −1.5 V vs Ag/AgCl for effective CO2 reduction.4 Under such a high input voltage, hydrogen evolution reaction is inevitable, leading to high energy consumption for the reduction process. For practical application of the electrochemical reduction method, it is therefore necessary to seek alternative and inexpensive Received: Revised: Accepted: Published: 5198

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tometer (UV−vis), and a cyclic voltammetry (CV), respectively. Factors including the cathode potential, the number of modification layers and the stability of the electrode were investigated.

energy sources, and develop new technologies to reduce the overpotential. A microbial fuel cell (MFC) is a device that utilizes microorganisms to capture energy in bioconvertible substrates in the form of electricity.5−7 As a type of renewable energy, MFCs can be used in some electrochemical processes. However, the low voltage generated by an MFC, typically less than 0.8 V,8,9 cannot meet the low potential requirement in conventional electrochemical reduction of CO2. Although connecting MFCs in series can lead to a nearly additive increase in total voltage, an adverse voltage reversal behavior has been reported.9,10 Consequently, it is crucial to develop new electrolysis reactors and electrodes for reducing the overpotentials of the anode and the cathode in the electrochemical CO2 reduction process driven by a single MFC. A microbial electrolysis cell (MEC), developed on the base of an MFC, operates in a manner similar to an MFC.11,12 Instead of common electrolytic reactors, an MEC employs a biological anode, on which the substrate is oxidized by bacteria. The generated electrons at the anode are transferred through an external circuit to the cathode, while the generated protons diffuse through a proton exchange membrane to the cathode. Finally, these electrons and protons participate in the reduction reactions at the cathode.13 This feature enables an MEC to generate hydrogen12,14,15 or methane16,17 under much smaller voltage than that needed for common electrolytic reactors. The MEC and MFC coupled system has been used to produce hydrogen gas.12 To the best of our knowledge, no reports on MEC- and MFC-coupled system for CO2 reduction have been published to date. Phthalocyanine compounds are well-known electrode modifiers as a result of their excellent electrocatalytic properties. The planar 18π-conjugated skeleton with a strong π−π interaction in the phthalocyanine structure can promote the electron migration between reactants and catalytic active sites.18,19 Several metal phthalocyanines have been reported to be active toward the electro-reduction of CO2, among which cobalt phthalocyanine (CoPc) and its derivatives were shown to be the most active.18 Isaacs et al. electro-polymerized cobalt tetra-amino phthalocyanine (CoTAPc) on a glassy carbon electrode, which was used to catalyze the electro-reduction of CO2 to formic acid.20 The cathode potential for CO2 reduction was shifted positively to −1.0 V vs Ag/AgCl. However, carbon nanotubes (CNTs), a relatively new carbon nanostructured material, have high electrical conductivity and high surface area.21−23 CNTs and CoPc-modified electrodes have been prepared by some methods involving chemical coordination24 or physical adhesion.25 The introduction of CNTs to the CoPcmodified electrodes has been demonstrated to significantly reduce the overpotentials of some electrochemical reactions, such as O2 reduction,26 2-mercaptoethanol,24 and epinephrine25 oxidization. Until now, there is no report published on the application of such composite electrodes in CO2 electroreduction. This study aims to develop an electrochemical CO 2 reduction system, which can be driven by a single MFC. The MWCNT and CoTAPc composite electrode was fabricated by the layer-by-layer (LBL) self-assembly technique for the first time, and used as the MEC cathode for reducing CO2 to formic acid. The morphology, structures, and electrochemical behaviors of the modified electrode were characterized by a scanning electron microscope (SEM), an X-ray photoelectron spectroscopy (XPS), an ultraviolet and visible spectropho-

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EXPERIMENTAL SECTION Chemicals and Materials. MWCNTs (95%, 10−20 nm diameters) were purchased from Shenzhen Nanotech Port Co. Ltd., P.R. China. Before use, MWCNTs were purified and activated according to the protocol reported by Sun et al.27 CoTAPc (C32H20N12Co·2H2O) complex was synthesized and characterized according to established procedures.28 Polyethyleneimine (PEI) aqueous solution (50 wt %, M.W. 60 000, branched) was purchased from J&K Chemica, P.R. China. All other chemicals were of analytical grade and used without further purification. The indium tin oxide (ITO) electrode (Beijing Zhongjingkeyi Technology Co. Ltd., P.R. China) were sonicated in an acetone bath for 10 min, followed by washing with deionized water and absolute methanol and finally dried in N 2 atmosphere. The graphite electrode (GE, Tianjin Aidahengsheng Technology Co. Ltd., P.R. China) was gently polished to a mirror finish using a sandpaper (2000 mesh), followed by ultrasonic cleaning in ultrapure Milli-Q water. The modified electrodes with ITO as the substrate electrode were used for structure characterization, while the modified electrodes with GE as the substrate electrode were used for studying the electrochemical behaviors and electrolysis performance. Electrode Modification. The activated MWCNTs were dispersed in the DMF solvent by ultrasonication for 1 h to obtain a homogeneous solution (1 mg·mL−1), and 1 mg·mL−1 CoTAPc in DMF solvent was obtained through the same process. These two solutions were marked as MWCNT-DMF and CoTAPc-DMF, respectively. The multilayer films were fabricated on substrate electrodes as follows. First, both ITO and GE were submerged in a positively charged PEI aqueous solution (10 mg·mL−1) for 30 min, and then rinsed with water and dried under a N2 gas flow. Subsequently, the positively charged PEI-precoated substrate was alternately dipped in the negatively charged MWCNT-DMF and positively charged CoTAPc-DMF for 30 min, respectively. Between each dipping, the electrode was immersed into DMF solution for several minutes to thoroughly wash off the unabsorbed chemicals, and the electrode was dried in hot air (60 °C) to strengthen the newly formed layer. By repeating the above procedure, composite films with desired multiple layers could be obtained, marked as ITO-(MWCNT/CoTAPc)n or GE-(MWCNT/ CoTAPc)n (n = 1−5). Details of the electrode modification process are shown in Figure S1 (Supporting Information, SI). As the control, MWCNT-modified ITO electrode (ITOMWCNT) or GE electrode (GE-MWCNT) as well as CoTAPc-modified ITO electrode (ITO-CoTAPc) or GE electrode (GE-CoTAPc) were also prepared by dipping the substrate electrode into the respective solution. Reactor Construction. The system was composed of one MEC for CO2 reduction and one single-chambered MFC (5 × 5 × 5 cm3) for extra power supply. The MEC was transformed from a two-chambered MFC (5 × 5 × 5 cm3 for each compartment). Nonwet-proofed carbon fiber (3-mm thick, purchased from Beijing Evergrow Resources Co., Ltd.) was used as the anodes for the two MFCs and the cathode for the two-chambered MFC. Carbon paper containing a Pt catalyst (0.5 mg cm−2) on the water-facing side (Beijing LN-Power 5199

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of one molecule of formic acid from CO2 (n = 2 here); F is Faraday’s constant (96 485 C/mol of electrons); and I is the circuit current.

Sources Co. Ltd.) was used as the cathode for the singlechambered MFC. All electrodes used in the MFCs had a projected surface area of 16 cm2. The two compartments for the two-chambered MFC were separated by a proton exchange membrane (PEM, Nafion117, 5 cm diameter, Dupont, U.S.). The PEM was sequentially boiled in H2O2 (30%), deionized water, H2SO4 solution (0.5 M), and again in deionized water (each for 1 h) and then immersed in deionized water for use.29 MFC inoculation and some detailed operations are presented in the SI. When the two-chambered MFC became stable, it was transformed into MEC by replacing the catholyte with a CO2 saturated 0.1 M KHCO3 solution and also replacing the cathode with the test electrode (including the new electrode fabricated in this study). The MEC and the single-chambered MFC were connected in series with a 10 Ω resistor (to allow the circuit current measurement) when the electrolysis of CO2 was conducted. CO2 Electrolysis. CO2 was electro-reduced in the cathodic chamber of the MEC. An electrolysis time of 240 min was applied for each batch. The catholyte of KHCO3 solution (0.1 M) was saturated with CO2 (99.5%) before each electrolysis process, and CO2 gas was continuously aerated at a rate of 30 mL·min−1 during the electrolysis process. After each batch of electrolysis, samples of the catholyte were taken for analysis. By changing the serial resistor, effects of the cathode potential on the CO2 reduction were studied.30 Each batch of the CO2 electrolysis was repeated three times, and the average was used for data analysis. Analysis and Calculations. The surface morphology of the modified electrode with ITO substrate was investigated by an SEM (NOVA NANOSEM 430, FEI). XPS analysis was performed on a Kratos AXIS Ultra X-ray photoelectron spectrometer using a monochromated Al Kα excitation source (1486.7 eV). Quantitative analysis was performed with CasaXPS software. The UV−vis absorption spectra were recorded on a UV−vis-NIR spectrometer (UV 3100, Shimadzu). CV was carried out in an undivided conventional threeelectrode cell connected to a CHI 600B electrochemical workstation (Shanghai Chenhua Instruments Co., P.R. China). A modified electrode with graphite substrate (3 mm diameter) was employed as the working electrode. A Pt plate (2 × 7 mm2) and Ag/AgCl electrode (sat. KCl) were chosen as the counter and reference electrodes, respectively. All of the potential values are in reference to Ag/AgCl unless otherwise noted. The MFC characterizations were described in the SI. The products in the cathodic solution after electrolysis, including formic acid, were analyzed by ion chromatography (ICS−900 Dionex, AS23 column using 3.5 mM Na2CO3/1.0 mM NaHCO3 as the mobile phase). Gaseous products in the upper space of the cathodic chamber of the MEC were collected and analyzed by gas chromatography (7890A Agilent, 5 A molecular sieve column, TCD detector, and argon carrier gas). The Faraday efficiency for the formation of formic acid (FEHCOOH) was calculated as follows: FE HCOOH =

nHCOOH × n × F t

∫0 I dt

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RESULTS AND DISCUSSION Characterization of the Modified Electrodes. As shown in the SEM image of ITO-(MWCNT/CoTAPc)1 (Figure 1A),

Figure 1. SEM image (A) of the ITO-(MWCNT/CoTAPc)1 electrode and XPS spectra (B) of Co2p on the ITO-CoTAPc (a) and ITO(MWCNT/CoTAPc)1 (b) electrodes.

MWCNTs were evenly distributed on the modified electrode surface. The content of element Co of the modified electrode was measured to be about 0.73% by XPS analysis. CoTAPc particulates gathered into nanosized clusters (10−100 nm) dispersing among the MWCNTs. These results demonstrated that MWCNTs and CoTAPc were effectively introduced onto the electrode surface by the LBL self-assembly technique. The binding energy of Co2p in ITO-(MWCNT/CoTAPc)1 was higher than that in ITO-CoTAPc (shown in Figure 1B). This difference indicated that the introduction of MWCNTs reduced the electron density around the Co nuclei in CoTAPc. The occurrence of the electron transfer from CoTAPc to MWCNTs should be taken into consideration.31 The B and Q bands are the indicators for the presence of the phthalocyanine ring,32 which can be seen clearly in the UV−vis

× 100% (2)

where nHCOOH is the moles of the formic acid produced; n represents the number of electrons required for the formation 5200

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spectra of the CoTAPc-DMF solution. Due to the aggregation of CoTAPc on the modified electrodes, both B and Q bands in the spectra of ITO-CoTAPc and ITO-(MWCNT/CoTAPc)1 were broadened and weakened. With reference to ITOCoTAPc spectra, red shifts for B and Q bands appeared in the spectra of ITO-(MWCNT/CoTAPc)1 (B band: from 340 to 356 nm; Q-band: from 759 to 775 nm). The red shifts could be attributed to the electron transfer from the phthalocyanine ring to MWCNTs, which expanded the macrocyclic conjugated structure of CoTAPc and reduced the energy level difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).31,33 The interaction between MWCNTs and CoTAPc is supposed to benefit the electron transfer and the relevant electrochemical processes. The absorbance of modified electrodes was found to increase with the number of modification layers (Figure 2B),

Figure 3. Cyclic voltammograms of GE (a), GE-CoTAPc (b), GE(MWCNT/CoTAPc)1 (c) (A) and GE-(MWCNT/CoTAPc)n (n = 1, 3, 5) (B) in a CO2-saturated 0.1 M KHCO3 solution at pH 6.64 and scan rate of 50 mV·s−1. The insert in A shows CVs of a, b, and c electrodes in N2 atmosphere. The insert in B shows variations of the reduction peak current with increasing number of the modified layers at the potential of −0.5 V vs Ag/AgCl.

around the Co nuclei in CoTAPc, which expanded the macrocyclic conjugated structure of CoTAPc, and further increased the potential for CO2 reduction.31,33 Owing to the excellent electrical conductivity of the MWCNTs, the electron transfer was improved in the electrode modification films; and therefore the peak current for CO2 reduction increased. In addition, the peak current increased with the number of electrode modification layers, which was positively correlated with the MWCNT loading (Figure 3B). Electrochemical Reduction of CO2. The conversion from CO2 to formic acid requires the participation of both protons and electrons. MEC is a novel electrolytic cell using a biological anode. The biodegradation of organic compounds in the anode chamber can provide protons and electrons for the cathode reaction. Therefore, in an MEC, the electrolysis of water as well as high voltage input is not necessary.15 The open circuit potential of the MEC anode is around −0.3 V.11 Our investigation above indicated that, using the MWCNT/ CoTAPc-modified electrode fabricated by the LBL selfassembly technique, CO2 can be reduced at the cathode potential near −0.5 V. Therefore, only an extra voltage of 0.2 V is needed in theory to realize the CO2 reduction in a MEC. A single-chambered MFC was investigated to drive the MEC. The performance of the single-chambered MFC was shown in Figure 4. After a month’s cultivation and acclimation, a stable and repeatable peak voltage of 0.580 V of MFC (cathode potential 0.275 V, anode potential −0.305 V) across a 1000 Ω resistor was generated, which could last 4−5 h in each cycle. The open circuit voltage and maximum power density was 0.772 V and 880 mW·m−2, respectively. The performance of this MFC was therefore found to be comparable to those with similar configurations in other studies.13,29 The MFC and the MEC using GE-(MWCNT/CoTAPc)5 as cathode were

Figure 2. UV−vis spectra of 10−5 M CoTAPc in DMF solution (a), ITO-MWCNT (b), ITO-CoTAPc (c), and ITO-(MWCNT/CoTAPc)1 (d) electrodes (A) and ITO-(MWCNT/CoTAPc)n electrodes (B).

indicating the increase of MWCNTs and CoTAPc in the modification films. The multiple-layer modification would promote the catalytic effect of the modified electrodes. Electrochemical Behavior of the Modified Electrodes. CV was used to study the electrochemical behavior of the GE modified with MWCNT and CoTAPc in CO2-saturated 0.1 M KHCO3 solution (Figure 3A,B). The CVs of these electrodes in N2 atmosphere (in the absence of CO2) were used for comparison (the insert of Figure 3A). In the potential range from 0.6 to −1.0 V, almost no peak corresponding to CO2 reduction was observed on the bare GE (Figure 3A). It was demonstrated that the applied cathode potential was not low enough to reduce CO2 using the GE. For the GE-CoTAPc, the CO2 reduction peak appeared near −0.6 V. After introducing the MWCNTs (GE-(MWCNT/CoTAPc)1), the peak potential for CO2 reduction was shifted positively by 0.1 V to −0.5 V, and the peak current also increased markedly (Figure 3A). The π−π interaction between the phthalocyanine ring of CoTAPc and the sidewalls of MWCNTs reduced the electron density 5201

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improved by promoting the electrode selectivity and accurately controlling the cathode potential. As shown in Figure 6, the production rate of formic acid using an LBL self-assembly MWCNT/CoTAPc-modified

Figure 4. Polarization curve and power density curve of the power supply MFC.

connected in series with a 10 Ω resistor to allow the circuit current measurement. After 4 h of electrolyzing CO2 under the peak voltage, only formic acid was detected in the catholyte with a production rate of 21.0 ± 0.2 mg·L−1·h−1. The Faraday efficiency for formic acid production was 73.5 ± 0.6% (Figure 5). It can be concluded from our results that targeted reduction

Figure 6. Formic acid production rate and current in the CO2 electrochemical reduction process using GE-CoTAPc (A), GE(MWCNT/CoTAPc)1 (B), GE-(MWCNT/CoTAPc)3 (C), and GE(MWCNT/CoTAPc)5 (D) as cathode, respectively (a serial resistor of 80 Ω).

electrode (Column B) increased by nearly 100% compared with that using merely a CoTAPc-modified electrode (Column A). Two factors may have contributed to the improvement. First, the interaction between the MWCNTs and CoTAPc decreased the overpotential of reducing CO2 to formic acid. Second, the excellent electrical conductivity of the MWCNTs improved the electron transfer between CO2 and the active sites of CoTAPc and thereby increased the current (by about 20%). A larger amount of MWCNTs would be loaded in the modified electrode when multiple-layer modification was carried out. Consequently, the electron transfer would be further improved. As a result, both the current and formic acid production rate were increased. This result is in accordance with the CV results. Stability of the CO2 Electro-Reduction System. The production of formic acid was investigated under 10 successive batches of CO2 electrolysis in the MEC (using the GE(MWCNT/CoTAPc)5 cathode) driven by a single MFC. Each batch lasted 4 h. The results are shown in Figure S2 of the SI. The formic acid production rate remained at about 16.0 mg·L−1·h−1 (15.8 ± 0.6 mg·L−1·h−1 at a serial resistor of 80 Ω in Figure 5) and the Faraday efficiency at about 78%, indicating that no electrode “poisoning” occurred. No Co was detected in the catholyte after electrolysis. The voltammogram of the used GE-(MWCNT/CoTAPc)5 was almost the same as that of the fresh one (Figure S3 of the SI), which means that repeated use did not change the electrocatalytic activity of the modified electrode. In conclusion, the LBL self-assembly MWCNT/ CoTAPc modified electrode exhibited high stability in multiple uses and can guarantee the continuous production of formic acid by CO2 electrolysis. Outlook. The electrochemical reduction of CO2 to valuable chemical materials (formic acid in this work) is of great significance to the virtuous cycle of CO2. In this study, the MWCNT and CoTAPc composite modified electrode was fabricated by the LBL self-assembly technique. This new electrode can significantly reduce the CO2 reduction potential, and as a cathode can successfully reduce CO2 to formic acid in an MEC driven by a single MFC. This proposed technique saves energy without the need for external energy input. Existing industrial waste gases could provide CO2 sources for the transformation to valuable chemical materials. Besides, as

Figure 5. Variations of Faraday efficiency and formic acid production rate with cathode potential in the MFC−MEC [using GE-(MWCNT/ CoTAPc)5 as cathode] system for CO2 reduction. Values in brackets are the serial resistance.

of CO2 to formic acid was successfully realized in the MFC− MEC coupled system when the MWCNT and CoTAPcmodified electrode was used as the cathode in the MEC. The effects of cathode potentials on the CO2 reduction were further studied by changing the serial resistor in the MEC− MFC coupled system.30 Upon increasing the resistance, the cathode potential rose from −0.604 to −0.397 V, and the yield of formic acid decreased almost linearly. However, the Faraday efficiency for the formic acid production increased with the cathode potential and reached the maximum (77.4 ± 1.1%) at the cathode potential of −0.503 V followed by a decrease. The CV results of GE-(MWCNT/CoTAPc)5 shows that the CO2 reduction peak appeared at the cathode potential near −0.5 V (Figure 3B). When the cathode potential was more positive than −0.503 V, the reduction of CO2 to formic acid became more difficult, and correspondingly the Faraday efficiency decreased. When the cathode potential was more negative than −0.503 V, side reactions (such as hydrogen evolution reaction) were more likely to take place, which also lead to a decrease in the Faraday efficiency for formic acid formation. Gas products of the electrolysis at the cathode potential of −0.604 V were also collected and analyzed. CO and H2 accounted for 0.96% and 12.1% of the total gas, respectively. This proved that a lower cathode potential induced electron-consuming side reactions, thereby reducing the Faraday efficiency for the formic acid production. The Faraday efficiency can be further 5202

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(11) Call, D.; Logan, B. E. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environ. Sci. Technol. 2008, 42 (9), 3401−3406. (12) Sun, M.; Sheng, G. P.; Zhang, L.; Xia, C. R.; Mu, Z. X.; Liu, X. W.; Wang, H. L.; Yu, H. Q.; Qi, R.; Yu, T.; Yang, M. An MEC-MFCcoupled system for biohydrogen production from acetate. Environ. Sci. Technol. 2008, 42 (21), 8095−8100. (13) Cheng, S.; Liu, H.; Logan, B. E. Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environ. Sci. Technol. 2006, 40 (1), 364−369. (14) Cheng, S.; Logan, B. E. Sustainable and efficient biohydrogen production via electrohydrogenesis. Proc. Natl. Acad. Sci. U. S. A. 2007, 104 (47), 18871−18873. (15) Liu, H.; Grot, S.; Logan, B. E. Electrochemically assisted microbial production of hydrogen from acetate. Environ. Sci. Technol. 2005, 39 (11), 4317−4320. (16) Villano, M.; Aulenta, F.; Ciucci, C.; Ferri, T.; Giuliano, A.; Majone, M. Bioelectrochemical reduction of CO2 to CH4 via direct and indirect extracellular electron transfer by a hydrogenophilic methanogenic culture. Bioresour. Technol. 2010, 101 (9), 3085−3090. (17) Cheng, S. A.; Xing, D. F.; Call, D. F.; Logan, B. E. Direct biological conversion of electrical current into methane by electromethanogenesis. Environ. Sci. Technol. 2009, 43 (10), 3953−3958. (18) Vasudevan, P.; Phougat, N.; Shukla, A. K. Metal phthalocyanines as electrocatalysts for redox reactions. Appl. Organomet. Chem. 1996, 10 (8), 591−604. (19) Nyokong, T.; Bedioui, F. Self-assembled monolayers and electropolymerized thin films of phthalocyanines as molecular materials for electroanalysis. J. Porphyr. Phthalocyanines 2006, 10 (9−10), 1101−1115. (20) Isaacs, M.; Armijo, F.; Ramirez, G.; Trollund, E.; Biaggio, S. R.; Costamagna, J.; Aguirre, M. J. Electrochemical reduction of CO2 mediated by poly-M-aminophthalocyanines (M = Co, Ni, Fe): polyCo-tetraaminophthalocyanine, a selective catalyst. J. Mol. Catal A: Chem. 2005, 229 (1−2), 249−257. (21) Wang, J. Carbon-nanotube based electrochemical biosensors: A review. Electroanalysis 2005, 17 (1), 7−14. (22) Schnorr, J. M.; Swager, T. M. Emerging applications of carbon nanotubes. Chem. Mater. 2011, 23 (3), 646−657. (23) Yuan, Y.; Zhao, B.; Jeon, Y.; Zhong, S. K.; Zhou, S. G.; Kim, S. Iron phthalocyanine supported on amino-functionalized multi-walled carbon nanotube as an alternative cathodic oxygen catalyst in microbial fuel cells. Bioresour. Technol. 2011, 102 (10), 5849−5854. (24) Mugadza, T.; Nyokong, T. Synthesis and characterization of electrocatalytic conjugates of tetraamino cobalt (II) phthalocyanine and single wall carbon nanotubes. Electrochim. Acta 2009, 54 (26), 6347−6353. (25) Moraes, F. C.; Golinelli, D. L. C.; Mascaro, L. H.; Machado, S. A. S. Determination of epinephrine in urine using multi-walled carbon nanotube modified with cobalt phthalocyanine in a paraffin composite electrode. Sens. Actuator B-Chem. 2010, 148 (2), 492−497. (26) Xu, Z. W.; Li, H. J.; Cao, G. X.; Zhang, Q. L.; Li, K. Z.; Zhao, X. N. Electrochemical performance of carbon nanotube-supported cobalt phthalocyanine and its nitrogen-rich derivatives for oxygen reduction. J. Mol. Catal. A: Chem. 2011, 335 (1−2), 89−96. (27) Sun, J. J.; Zhao, H. Z.; Yang, Q. Z.; Song, J.; Xue, A. A novel layer-by-layer self-assembled carbon nanotube-based anode: Preparation, characterization, and application in microbial fuel cell. Electrochim. Acta 2010, 55 (9), 3041−3047. (28) Achar, B. N.; Fohlen, G. M.; Parker, J. A.; Keshavayya, J. Synthesis and structural studies of metal(II) 4,9,16,23-phthalocyanine tetraamines. Polyhedron 1987, 6 (6), 1463−1467. (29) Liu, H.; Logan, B. E. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol. 2004, 38 (14), 4040−4046. (30) Sun, M.; Sheng, G. P.; Mu, Z. X.; Liu, X. W.; Chen, Y. Z.; Wang, H. L.; Yu, H. Q. Manipulating the hydrogen production from acetate

anode reaction substrates in the MEC and MFC, the organic matters in wastewaters or wastes also released CO2 as well as electrons and protons during the biodegradation process.34 This part of CO2 can also be recycled for electrolysis, and as a result, CO2 cycle works virtuously while wastes or wastewaters can be effectively treated. Our research team is currently focusing on this respect. In addition, other new catalytic electrodes can also be introduced into the MEC−MFC coupled system to obtain other valuable reduction products of CO2, which makes the proposed technique more versatile in the CO2 capture and conversion.

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ASSOCIATED CONTENT

S Supporting Information *

More detailed information about the experimental methods (MFC inoculation and operation, MFC characterization) and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-62754292-815; fax: +86-10-62756526; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful for the financial support from the National Natural Science Fund (Grant No. 21077001) and the National Five-Year Technology Support Program (Grant No. 2011BAJ07B04) of China.

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