- Email: [email protected]
Pyrolyzed binuclear-cobalt-phthalocyanine as electrocatalyst for oxygen reduction reaction in microbial fuel cells
Bioresource Technology 193 (2015) 545–548
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Pyrolyzed binuclear-cobalt-phthalocyanine as electrocatalyst for oxygen reduction reaction in microbial fuel cells Baitao Li a, Mian Wang a, Xiuxiu Zhou a, Xiujun Wang a,⇑, Bingchuan Liu b, Baikun Li b a b
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, USA
h i g h l i g h t s Bi-CoPc pyrolyzed at different temperatures is examined as ORR catalyst. ORR activity and power generation were pyrolysis temperature dependent. Pyrolysis process modified the amounts of nitrogen on the catalyst surface. The content of pyrrolic-N was responsible for the improvement of ORR. Power density of SCMFC with Bi-CoPc-800 cathode was 604 mW m
a r t i c l e
i n f o
Article history: Received 3 April 2015 Received in revised form 27 May 2015 Accepted 28 May 2015 Available online 29 June 2015 Keywords: Microbial fuel cell Binuclear-cobalt-phthalocyanine Oxygen reduction reaction Pyrolysis Nitrogen doping
2
.
a b s t r a c t A novel platinum (Pt)-free cathodic materials binuclear-cobalt-phthalocyanine (Bi-CoPc) pyrolyzed at different temperatures (300–1000 °C) were examined as the oxygen reduction reaction (ORR) catalysts, and compared with unpyrolyzed Bi-CoPc/C and Pt cathode in single chamber microbial fuel cells (SCMFCs). The results showed that the pyrolysis process increased the nitrogen abundance on Bi-CoPc and changed the nitrogen types. The Bi-CoPc pyrolyzed at 800 °C contained a significant amount of pyrrolic-N, and exhibited a high electrochemical catalytic activity. The power density and current density increased with temperature: Bi-CoPc/C-800 > Bi-CoPc/C-1000 > Bi-CoPc/C-600 > Bi-CoPc/C-300 > Bi-CoPc/C. The SCMFC with Bi-CoPc/C-800 cathode had a maximum power density of 604 mW m 2. The low cost Bi-CoPc compounds developed in this study showed a potential in air-breathing MFC systems, with the proper pyrolysis temperature being chosen. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cell (MFC) is an emerging biotechnology capable of converting organic contaminants in wastewater into electricity using microorganisms as the biocatalyst. Platinum (Pt) is found so far the best cathodic catalyst due to its high oxygen reduction reaction (ORR) catalyzing capability and excellent chemical stability (Santoro et al., 2013a; Wang et al., 2013). However, Pt can easily get poisoned by a variety of chemicals (e.g. HS , Cl , CO) in wastewater (Santoro et al., 2013b; Zhao et al., 2009), and the high cost and low abundance limits its application in MFCs. Non-noble transition metal macrocycles, especially, cobalt phthalocyanine (CoPc) exhibits unique ORR capability (Jasinski, 1964). However, the stability of CoPc for ORR catalytic activity was low under acidic conditions (Coutanceau et al., 1995). ⇑ Corresponding author. Tel./fax: +86 20 87112943. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.biortech.2015.05.111 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Binuclear metal phthalocyanine (Bi-MePc or PcMe(II)–PcMe(II)) is advantageous over CoPc due to its symmetric disk shape and highly conjugated structure. Unlike the common MePc (e.g. CoPc, and FePc) containing only one metal ion at one valence, Bi-MePc contains two metallic ions capable of achieving the valence transformation (e.g. PcMe(II)–PcMe(II) to PcMe(I)–PcMe(III)) during the catalytic reactions. Bi-CoPc without pyrolysis exhibited higher power density than CoPc in SCMFCs (324 vs. 256 mW m 2) (Li et al., 2014). When Bi-CoPc was hybrid with NiO, the power density further increased to 400 mW m 2. However, the preparation of the hybrid catalyst involved the decomposition of Ni(NO3)26H2O at 400 °C, which made the synthesis procedure quite complicated (Li et al., 2014). It is critical to modify Bi-MePc cathodic catalysts in a relatively feasible approach to enhance the high ORR activity in MFCs. Preliminary studies have found that the ORR catalytic activity of nitrogen-containing catalysts could be enhanced by heat-treatment in inert atmosphere at high temperatures
546
B. Li et al. / Bioresource Technology 193 (2015) 545–548
(Bezerra et al., 2007). The objective of this study was to modify the carbon-supported Bi-CoPc catalysts (Bi-CoPc/C) with the pyrolysis process to improve both total nitrogen-doped content and nitrogen-functionalities, and determine the effect of heat-treatment temperatures (300–1000 °C) on the ORR activity of Bi-CoPc/C as the cathodic catalyst in single chamber MFCs (SCMFCs). There were three tasks in this study. First, the surface and chemical characterization of the Bi-CoPc/C catalysts heat-treated at different temperatures were conducted. Second, the electro-catalytic ORR activities of the catalysts were evaluated using the cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The physicochemical characterization and the electrochemical performance of the heat-treated catalysts were correlated. Third, the power generation and contaminant removal of Bi-CoPc/C based-cathodes were examined in SCMFCs treating municipal wastewater. 2. Method 2.1. Catalyst preparation 310 mg Bi-CoPc (NENU Industry of Fine Chem. Co.) was dissolved in 200 mL tetrahydrofuran (THF), followed by the addition of 620 mg carbon support (SBET = 1042 m2/g, Hesen Electric Co., Ltd., China). After the suspension was stirred at room temperature for 1 h, the excess THF solvent was removed using a rotary evaporator and the solid was dried overnight at 80 °C. The powder was pyrolyzed under flowing nitrogen at temperatures ranging from 300 to 1000 °C for 2 h. The catalysts were designated as Bi-CoPc/C-(T), with T denoting the heat-treatment temperature at 300, 600, 800, and 1000 °C. The catalyst without pyrolysis was denoted as Bi-CoPc/C. 2.2. SCMFC construction and inoculation The membrane-free SCMFCs (volume: 28 mL, depth: 4 cm, diameter: 3 cm) made of Perspex frames were operated at batch-mode which was described in our previous report (Li et al., 2014). The Bi-CoPc/C-(T) catalyst loading was 1.0 mg cm 2 on cathode. The Pt catalyst containing 20% of Pt/C (Hesen Electric Co., Ltd., China) was used as the positive control sample. The Pt loading of 0.5 mg cm 2 was used (Cheng et al., 2006). The wastewater taken from the influent section in a wastewater treatment plant (Guangzhou, China) was used as inoculants. The growth of anaerobic electrogenic bacteria was simulated according to the conventional method (Lovley and Phillips, 1988). The SCMFCs were operated at 30 ± 1 °C in an incubator, and each catalyst was examined in duplicate cycles. The external resistance (Rext) of 1000 X was used to connect the anodes and cathodes. The voltage over Rext was recorded every 0.5 h using a 32-channel USB data logging system (AD8233 h). 2.3. Surface chemistry characterizations and electrochemical measurement Detailed surface chemistry characterizations and electrochemical measurement were described in Supplementary materials. 3. Results and discussion 3.1. Morphology and structure characterization of Bi-CoPc/C-(T) catalysts The crystal structure and the composition of the synthesized catalysts changed with the pyrolysis temperature (Fig. S1). The
temperatures (T) applied during the heat treatment also clearly affected the morphology and particle size of Bi-CoPc/C-(T) catalysts (Figs. S2 and S3). The ORR catalytic performance is sensitive to the surface or subsurface composition of the catalysts, which was strongly dependent on the pyrolysis treatment temperature (Geng et al., 2011). Quantitative XPS analysis showed the clear difference in element composition among the pyrolyzed catalysts (Table S1). As the pyrolysis temperature increased, nitrogen content first kept stable, then sharply increased and reached the maximum value of 4.42% at 800 °C, and finally decreased to 4.11% at 1000 °C. Compared with Bi-CoPc/C, the catalyst pyrolyzed at 800 °C significantly increased the oxygen content and cobalt content by 2.2 and 1.8 times, respectively. Nitrogen functionality determined by high resolution N1s XPS spectra revealed the chemical state changes of nitrogen dopings on carbon support (Fig. S4). In the Bi-CoPc/C, N1s signal showed a main peak at 399.2 eV that could be ascribed to two chemically non-equivalent N atoms (Kera et al., 2006). The shoulder peak at 400.2 eV could be attributed to a shake-up as an excitation channel of the N1s peak. When the sample was heat treated at high temperature (>600 °C, Fig. S4C–E), two new peaks were observed. The peak at 398.8 eV could be assigned to the pyridinic-N on the edge of carbon support bonded with two carbon atoms, donating one p electron to the conjugated p system (Matter et al., 2006; Lee et al., 2009). The peak at 400.8 eV was attributable to the pyrrolic-N which had two adjacent carbon atoms and contributed to the p system with two p electrons (Lee et al., 2009). The fractions of the components obtained by the peak separation indicated that the pyrolysis temperature changed the content of each nitrogen component (Table S2). Co-N4 species were unstable at high temperature, which could partly convert to pyridinic-N and pyrrolic-N, leading to an increase in pyridinic-N and pyrrolic-N and a decrease in Co-N4. The content of pyridinic-N progressively increased from 13.73% at 600 °C to 48.85% at 1000 °C. A peculiar trend was observed for the pyrrolic-N content, which showed a non-linear change and the highest value of 38.13% was detected in Bi-CoPc/C-800. 3.2. Electro-catalytical performance of Bi-CoPc/C-(T) catalysts The ORR activity of Bi-CoPc/C after heat treatment was evaluated using CV (Fig. S5A) and LSV (Fig. S5B). The positive shift is the indicative of the enhanced ORR performance. The pyrolyzed Bi-CoPc/C-(T) catalysts showed higher activities than the unpyrolyzed Bi-CoPc/C, which was consistent with previous findings that heat-treatment effectively improved the ORR activity of catalysts (Bezerra et al., 2008; Lee et al., 2009). Unpyrolyzed Bi-CoPc/C had the lowest ORR potential of 120 mV. Pyrolysis at low temperature (300 °C) slightly increased the oxygen reduction potential to 31 mV. The peak potential for the catalyst heated at 800 °C continuously shifted to +82 mV, which was a remarkable improvement by 200 mV compared with Bi-CoPc/C. However, as the catalyst was heated at 1000 °C, a slight drop to +55 mV with a substantial decrease in current was observed, which might be caused by the reduction of surface area and the collapse of the carbon structures. Based on the XPS and electrochemical analysis, the relationship between the pyrrolic-N content and ORR catalytic activity can be established. The electrochemical reduction of oxygen is a multi-electron reaction with two possible pathways: one involving the gain of 4e to produce water, and the other involving a 2e pathway with HO2 as an intermediate. Electro-catalysts are expected to facilitate the 4e reduction to enhance the electricity generation of MFCs. The pyrrolic and graphitic nitrogen could catalyze the ORR through the 4e pathway, while pyridinic-N
547
B. Li et al. / Bioresource Technology 193 (2015) 545–548
F E D C B A
Voltage (V)
0.4
0.3
0.2
0.1
0.0 0
50
100
150
200
250 300 Time (h)
350
400
450
500
Fig. 1. The voltage generation of the batch-mode SCMFCs with different cathodic catalysts. (A) Bi-CoPc/C, (B) Bi-CoPc/C-300, (C) Bi-CoPc/C-600, (D) Bi-CoPc/C-800, (E) BiCoPc/C-1000, and (F) Pt/C.
The cathodes loaded with Bi-CoPc/C-(T) catalysts were installed in the SCMFCs and operated for 490 h (Fig. 1). All SCMFCs reached the maximum voltages in the 4th cycle (after 160 h). Compared with the unpyrolyzed catalyst Bi-CoPc/C, the pyrolyzed catalysts Bi-CoPc/C-(T) exhibited higher power generation. The SCMFCs with Bi-CoPc/C-800 cathode exhibited a higher voltage generation (383–395 mV) than that with Bi-CoPc/C-1000 (352–367 mV) and Bi-CoPc/C-600 (345–358 mV). Moreover, Bi-CoPc/C-(T) catalysts exhibited higher ORR rates than Bi-CoPc/C. The cycle durations of the SCMFCs with Bi-CoPc/C-(T) were shorter than that of Bi-CoPc/C cathodes (cycles 3rd–7th). Specifically, Bi-CoPc/C-800 (12–35 h) had shorter cycle duration than Bi-CoPc/C (50–76 h), implying Bi-CoPc/C-800 catalyst enhanced the ORR rate on cathode as well as the overall biochemical reactions in SCMFCs (Li et al., 2010). The power density and polarization behavior (Fig. S6) of SCMFCs were examined by steadily decreasing Rext from 30,000 to 20 O. The order for power density was: Pt/C > Bi-CoPc/C-800 > Bi-CoPc/C-1000 > Bi-CoPc/C-600 > Bi-CoPc/ C-300 > Bi-CoPc/C. Bi-CoPc/C without heat-treatment had a lower power density (244 mW m 2) than Bi-CoPc/C-300 (280 mW m 2), indicating that the heat treatment of the Bi-CoPc improved the ORR activity on the cathode. As the calcination temperatures were increased to 600 and 800 °C, the power densities steadily increased; but 1000 °C led to the decrease in power density. The maximum power density of Bi-CoPc/C-800 was 604 mW m 2, which was slightly lower than that of the Pt/C SCMFC (724 mW m 2). The enhanced power output was confirmed by EIS analysis (Nyquist plots) (Fig. S7). A close relationship between Rin and power output of SCMFCs was obtained: Bi-CoPc/C-800 delivered the highest power density and lowest Rin (154 X). Moreover, the Rin for Bi-CoPc/C-800 was only half of that for the hybrid
3.4. COD degradation and the coulombic efficiency of SCMFCs Bi-CoPc/C showed the lowest COD removal of 76.5% (Fig. 2). The COD removal slightly increased with the pyrolysis temperature, from 79.3% for Bi-CoPc/C-300 to 86.5% for Bi-CoPc/C-1000. The coulombic efficiency (CE) values indicated the conversion efficiency of organic substrates to electricity in MFCs. Bi-CoPc/C-800 catalyst with the highest voltage generation had a lower CE value (29.6%) than Pt (32.5%), even though it showed a higher COD removal. This was probably caused by the consumption of organic substrates by bacterial growth rather than by electricity generation (Li et al., 2010). 3.5. Significance for MFC applications Cathodic catalyst is critical for the cost and power generation of MFCs. The batch-mode SCMFC tests showed that the ORR activity of Bi-CoPc catalysts clearly enhanced with temperatures and
100 80 60 40 20
CO
D
al and CE (% )
3.3. Electricity generation in SCMFCs with different Bi-CoPc/C-(T) catalysts
Bi-CoPc/C-CoO catalyst previously developed (Li et al., 2014), which emphasized the importance of heat treatment for improving the ORR catalytic activity of MFC cathodes.
COD remov
contributed to the 2e pathway (Sidik et al., 2006; Ikeda et al., 2008). The hybrid Bi-CoPc catalyst (Bi-CoPc/C-NiO and Bi-CoPc/C-CoO) previously developed had no pyrrolic-N and pyridinic-N, since no pyrolysis treatment was applied in the synthesis (Li et al., 2014), whereas the pyrolyzed Bi-CoPc in this study showed remarkable amounts of pyrrolic-N and pyridinic-N.
P 0 Bi t/C -C -C oPc /C oP Bi -C c/C -10 Bi o P -C 80 00 c/C Bi oP -C -60 0 c / CoP 0 30 c/C 0 Bi
re
mo
va
l CE
Fig. 2. The COD removal and CE of SCMFCs with different cathodic catalysts.
548
B. Li et al. / Bioresource Technology 193 (2015) 545–548
reached the peak performance when treated at 800 °C. Although the Pt/C-catalyzed SCMFC outperformed all other SCMFCs, the cost ($1700 m 2) is 16 times higher than that of Bi-CoPc ($110 m 2) (Li et al., 2014). The much low cost of the Bi-CoPc catalysts makes them more favorable in practical applications. 4. Conclusions Heat-treated Bi-CoPc/C materials were developed as cathode catalysts in SCMFCs. The pyrolysis process modified the functional group, increased the amounts of nitrogen and changed the nitrogen types. The excellent electrocatalytic activity toward oxygen reduction reaction (ORR) for Bi-CoPc/C-800 was in line with the high amount of pyrrolic nitrogen configuration. The SCMFCs with Bi-CoPc/C-800 cathode produced a maximum power density of 604 mW m 2. Acknowledgements The authors are grateful to the financial supports from the National Natural Science Foundation of China (Project Nos. 21173086, 20975040 and U1301245) and Guangdong Natural Science Foundation (Project No. 2014A030313259). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.05. 111. References Bezerra, C.W.B., Zhang, L., Liu, H., Lee, K., Marques, A.L.B., Marques, E.P., Wang, H., Zhang, J., 2007. A review of heat-treatment effects on activity and stability of PEM fuel cell catalysts for oxygen reduction reaction. J. Power Sources 173 (2), 891–908. Bezerra, C.W.B., Zhang, L., Lee, K.C., Liu, H.S., Marques, A.L.B., Marques, E.P., Wang, H.J., Zhang, J.J., 2008. A review of Fe-N/C and Co-N/C catalysts for the oxygen reduction reaction. Electrochim. Acta 53 (15), 4937–4951.
Cheng, S., Liu, H., Logan, B.E., 2006. Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environ. Sci. Technol. 40 (1), 364–369. Coutanceau, C., El Hourch, A., Crouigneau, P., Leger, J., Lamy, C., 1995. Conducting polymer electrodes modified by metal tetrasulfonated phthalocyanines: preparation and electrocatalytic behaviour towards dioxygen reduction in acid medium. Electrochim. Acta 40 (17), 2739–2748. Geng, D., Chen, Y., Chen, Y., Li, Y., Li, R., Sun, X., Ye, S., Knights, S., 2011. High oxygenreduction activity and durability of nitrogen-doped graphene. Energy Environ. Sci. 4 (3), 760–764. Ikeda, T., Boero, M., Huang, S.F., Terakura, K., Oshima, M., Ozaki, J., 2008. Carbon alloy catalysts: active sites for oxygen reduction reaction. J. Phys. Chem. C 112 (38), 14706–14709. Jasinski, R., 1964. A new fuel cell cathode catalyst. Nature 201 (4925), 1212–1213. Kera, S., Casu, M., Bauchspieß, K., Batchelor, D., Schmidt, T., Umbach, E., 2006. Growth mode and molecular orientation of phthalocyanine molecules on metal single crystal substrates: a NEXAFS and XPS study. Surf. Sci. 600 (5), 1077–1084. Lee, K., Zhang, L., Lui, H., Hui, R., Shi, Z., Zhang, J., 2009. Oxygen reduction reaction (ORR) catalyzed by carbon-supported cobalt polypyrrole (Co-PPy/C) electrocatalysts. Electrochim. Acta 54 (20), 4704–4711. Li, B., Zhou, X., Wang, X., Liu, B., Li, B., 2014. Hybrid binuclear-cobalt-phthalocyanine as oxygen reduction reaction catalyst in single chamber microbial fuel cells. J. Power Sources 272, 320–327. Li, X., Hu, B., Suib, S., Lei, Y., Li, B., 2010. Manganese dioxide as a new cathode catalyst in microbial fuel cells. J. Power Sources 195 (9), 2586–2591. Lovley, D.R., Phillips, E.J.P., 1988. Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl. Environ. Microbiol. 54 (6), 1472–1480. Matter, P.H., Zhang, L., Ozkan, U.S., 2006. The role of nanostructure in nitrogencontaining carbon catalysts for the oxygen reduction reaction. J. Catal. 239 (1), 83–96. Santoro, C., Li, B.K., Cristiani, P., Squadrito, G., 2013a. Power generation of microbial fuel cells (MFCs) with low cathodic platinum loading. Int. J. Hydrogen Energy 38 (1), 692–700. Santoro, C., Stadlhofer, A., Hacker, V., Squadrito, G., Schröder, U., Li, B., 2013b. Activated carbon nanofibers (ACNF) as cathode for single chamber microbial fuel cells (SCMFCs). J. Power Sources 243, 499–507. Sidik, R.A., Anderson, A.B., Subramanian, N.P., Kumaraguru, S.P., Popov, B.N., 2006. O-2 reduction on graphite and nitrogen-doped graphite: experiment and theory. J. Phys. Chem. B 110 (4), 1787–1793. Wang, X., Santoro, C., Cristiani, P., Squadrito, G., Lei, Y., Agrios, A.G., Pasaogullari, U., Li, B., 2013. Influence of electrode characteristics on coulombic efficiency (CE) in microbial fuel cells (MFCs) treating wastewater. J. Electrochem. Soc. 160 (7), G3117–G3122. Zhao, F., Slade, R.C.T., Varcoe, J.R., 2009. Techniques for the study and development of microbial fuel cells: an electrochemical perspective. Chem. Soc. Rev. 38 (7), 1926–1939.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Pyrolyzed binuclear-cobalt-phthalocyanine as electrocatalyst for oxygen reduction reaction in microbial fuel cells Baitao Li a, Mian Wang a, Xiuxiu Zhou a, Xiujun Wang a,⇑, Bingchuan Liu b, Baikun Li b a b
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, USA
h i g h l i g h t s Bi-CoPc pyrolyzed at different temperatures is examined as ORR catalyst. ORR activity and power generation were pyrolysis temperature dependent. Pyrolysis process modified the amounts of nitrogen on the catalyst surface. The content of pyrrolic-N was responsible for the improvement of ORR. Power density of SCMFC with Bi-CoPc-800 cathode was 604 mW m
a r t i c l e
i n f o
Article history: Received 3 April 2015 Received in revised form 27 May 2015 Accepted 28 May 2015 Available online 29 June 2015 Keywords: Microbial fuel cell Binuclear-cobalt-phthalocyanine Oxygen reduction reaction Pyrolysis Nitrogen doping
2
.
a b s t r a c t A novel platinum (Pt)-free cathodic materials binuclear-cobalt-phthalocyanine (Bi-CoPc) pyrolyzed at different temperatures (300–1000 °C) were examined as the oxygen reduction reaction (ORR) catalysts, and compared with unpyrolyzed Bi-CoPc/C and Pt cathode in single chamber microbial fuel cells (SCMFCs). The results showed that the pyrolysis process increased the nitrogen abundance on Bi-CoPc and changed the nitrogen types. The Bi-CoPc pyrolyzed at 800 °C contained a significant amount of pyrrolic-N, and exhibited a high electrochemical catalytic activity. The power density and current density increased with temperature: Bi-CoPc/C-800 > Bi-CoPc/C-1000 > Bi-CoPc/C-600 > Bi-CoPc/C-300 > Bi-CoPc/C. The SCMFC with Bi-CoPc/C-800 cathode had a maximum power density of 604 mW m 2. The low cost Bi-CoPc compounds developed in this study showed a potential in air-breathing MFC systems, with the proper pyrolysis temperature being chosen. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cell (MFC) is an emerging biotechnology capable of converting organic contaminants in wastewater into electricity using microorganisms as the biocatalyst. Platinum (Pt) is found so far the best cathodic catalyst due to its high oxygen reduction reaction (ORR) catalyzing capability and excellent chemical stability (Santoro et al., 2013a; Wang et al., 2013). However, Pt can easily get poisoned by a variety of chemicals (e.g. HS , Cl , CO) in wastewater (Santoro et al., 2013b; Zhao et al., 2009), and the high cost and low abundance limits its application in MFCs. Non-noble transition metal macrocycles, especially, cobalt phthalocyanine (CoPc) exhibits unique ORR capability (Jasinski, 1964). However, the stability of CoPc for ORR catalytic activity was low under acidic conditions (Coutanceau et al., 1995). ⇑ Corresponding author. Tel./fax: +86 20 87112943. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.biortech.2015.05.111 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Binuclear metal phthalocyanine (Bi-MePc or PcMe(II)–PcMe(II)) is advantageous over CoPc due to its symmetric disk shape and highly conjugated structure. Unlike the common MePc (e.g. CoPc, and FePc) containing only one metal ion at one valence, Bi-MePc contains two metallic ions capable of achieving the valence transformation (e.g. PcMe(II)–PcMe(II) to PcMe(I)–PcMe(III)) during the catalytic reactions. Bi-CoPc without pyrolysis exhibited higher power density than CoPc in SCMFCs (324 vs. 256 mW m 2) (Li et al., 2014). When Bi-CoPc was hybrid with NiO, the power density further increased to 400 mW m 2. However, the preparation of the hybrid catalyst involved the decomposition of Ni(NO3)26H2O at 400 °C, which made the synthesis procedure quite complicated (Li et al., 2014). It is critical to modify Bi-MePc cathodic catalysts in a relatively feasible approach to enhance the high ORR activity in MFCs. Preliminary studies have found that the ORR catalytic activity of nitrogen-containing catalysts could be enhanced by heat-treatment in inert atmosphere at high temperatures
546
B. Li et al. / Bioresource Technology 193 (2015) 545–548
(Bezerra et al., 2007). The objective of this study was to modify the carbon-supported Bi-CoPc catalysts (Bi-CoPc/C) with the pyrolysis process to improve both total nitrogen-doped content and nitrogen-functionalities, and determine the effect of heat-treatment temperatures (300–1000 °C) on the ORR activity of Bi-CoPc/C as the cathodic catalyst in single chamber MFCs (SCMFCs). There were three tasks in this study. First, the surface and chemical characterization of the Bi-CoPc/C catalysts heat-treated at different temperatures were conducted. Second, the electro-catalytic ORR activities of the catalysts were evaluated using the cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The physicochemical characterization and the electrochemical performance of the heat-treated catalysts were correlated. Third, the power generation and contaminant removal of Bi-CoPc/C based-cathodes were examined in SCMFCs treating municipal wastewater. 2. Method 2.1. Catalyst preparation 310 mg Bi-CoPc (NENU Industry of Fine Chem. Co.) was dissolved in 200 mL tetrahydrofuran (THF), followed by the addition of 620 mg carbon support (SBET = 1042 m2/g, Hesen Electric Co., Ltd., China). After the suspension was stirred at room temperature for 1 h, the excess THF solvent was removed using a rotary evaporator and the solid was dried overnight at 80 °C. The powder was pyrolyzed under flowing nitrogen at temperatures ranging from 300 to 1000 °C for 2 h. The catalysts were designated as Bi-CoPc/C-(T), with T denoting the heat-treatment temperature at 300, 600, 800, and 1000 °C. The catalyst without pyrolysis was denoted as Bi-CoPc/C. 2.2. SCMFC construction and inoculation The membrane-free SCMFCs (volume: 28 mL, depth: 4 cm, diameter: 3 cm) made of Perspex frames were operated at batch-mode which was described in our previous report (Li et al., 2014). The Bi-CoPc/C-(T) catalyst loading was 1.0 mg cm 2 on cathode. The Pt catalyst containing 20% of Pt/C (Hesen Electric Co., Ltd., China) was used as the positive control sample. The Pt loading of 0.5 mg cm 2 was used (Cheng et al., 2006). The wastewater taken from the influent section in a wastewater treatment plant (Guangzhou, China) was used as inoculants. The growth of anaerobic electrogenic bacteria was simulated according to the conventional method (Lovley and Phillips, 1988). The SCMFCs were operated at 30 ± 1 °C in an incubator, and each catalyst was examined in duplicate cycles. The external resistance (Rext) of 1000 X was used to connect the anodes and cathodes. The voltage over Rext was recorded every 0.5 h using a 32-channel USB data logging system (AD8233 h). 2.3. Surface chemistry characterizations and electrochemical measurement Detailed surface chemistry characterizations and electrochemical measurement were described in Supplementary materials. 3. Results and discussion 3.1. Morphology and structure characterization of Bi-CoPc/C-(T) catalysts The crystal structure and the composition of the synthesized catalysts changed with the pyrolysis temperature (Fig. S1). The
temperatures (T) applied during the heat treatment also clearly affected the morphology and particle size of Bi-CoPc/C-(T) catalysts (Figs. S2 and S3). The ORR catalytic performance is sensitive to the surface or subsurface composition of the catalysts, which was strongly dependent on the pyrolysis treatment temperature (Geng et al., 2011). Quantitative XPS analysis showed the clear difference in element composition among the pyrolyzed catalysts (Table S1). As the pyrolysis temperature increased, nitrogen content first kept stable, then sharply increased and reached the maximum value of 4.42% at 800 °C, and finally decreased to 4.11% at 1000 °C. Compared with Bi-CoPc/C, the catalyst pyrolyzed at 800 °C significantly increased the oxygen content and cobalt content by 2.2 and 1.8 times, respectively. Nitrogen functionality determined by high resolution N1s XPS spectra revealed the chemical state changes of nitrogen dopings on carbon support (Fig. S4). In the Bi-CoPc/C, N1s signal showed a main peak at 399.2 eV that could be ascribed to two chemically non-equivalent N atoms (Kera et al., 2006). The shoulder peak at 400.2 eV could be attributed to a shake-up as an excitation channel of the N1s peak. When the sample was heat treated at high temperature (>600 °C, Fig. S4C–E), two new peaks were observed. The peak at 398.8 eV could be assigned to the pyridinic-N on the edge of carbon support bonded with two carbon atoms, donating one p electron to the conjugated p system (Matter et al., 2006; Lee et al., 2009). The peak at 400.8 eV was attributable to the pyrrolic-N which had two adjacent carbon atoms and contributed to the p system with two p electrons (Lee et al., 2009). The fractions of the components obtained by the peak separation indicated that the pyrolysis temperature changed the content of each nitrogen component (Table S2). Co-N4 species were unstable at high temperature, which could partly convert to pyridinic-N and pyrrolic-N, leading to an increase in pyridinic-N and pyrrolic-N and a decrease in Co-N4. The content of pyridinic-N progressively increased from 13.73% at 600 °C to 48.85% at 1000 °C. A peculiar trend was observed for the pyrrolic-N content, which showed a non-linear change and the highest value of 38.13% was detected in Bi-CoPc/C-800. 3.2. Electro-catalytical performance of Bi-CoPc/C-(T) catalysts The ORR activity of Bi-CoPc/C after heat treatment was evaluated using CV (Fig. S5A) and LSV (Fig. S5B). The positive shift is the indicative of the enhanced ORR performance. The pyrolyzed Bi-CoPc/C-(T) catalysts showed higher activities than the unpyrolyzed Bi-CoPc/C, which was consistent with previous findings that heat-treatment effectively improved the ORR activity of catalysts (Bezerra et al., 2008; Lee et al., 2009). Unpyrolyzed Bi-CoPc/C had the lowest ORR potential of 120 mV. Pyrolysis at low temperature (300 °C) slightly increased the oxygen reduction potential to 31 mV. The peak potential for the catalyst heated at 800 °C continuously shifted to +82 mV, which was a remarkable improvement by 200 mV compared with Bi-CoPc/C. However, as the catalyst was heated at 1000 °C, a slight drop to +55 mV with a substantial decrease in current was observed, which might be caused by the reduction of surface area and the collapse of the carbon structures. Based on the XPS and electrochemical analysis, the relationship between the pyrrolic-N content and ORR catalytic activity can be established. The electrochemical reduction of oxygen is a multi-electron reaction with two possible pathways: one involving the gain of 4e to produce water, and the other involving a 2e pathway with HO2 as an intermediate. Electro-catalysts are expected to facilitate the 4e reduction to enhance the electricity generation of MFCs. The pyrrolic and graphitic nitrogen could catalyze the ORR through the 4e pathway, while pyridinic-N
547
B. Li et al. / Bioresource Technology 193 (2015) 545–548
F E D C B A
Voltage (V)
0.4
0.3
0.2
0.1
0.0 0
50
100
150
200
250 300 Time (h)
350
400
450
500
Fig. 1. The voltage generation of the batch-mode SCMFCs with different cathodic catalysts. (A) Bi-CoPc/C, (B) Bi-CoPc/C-300, (C) Bi-CoPc/C-600, (D) Bi-CoPc/C-800, (E) BiCoPc/C-1000, and (F) Pt/C.
The cathodes loaded with Bi-CoPc/C-(T) catalysts were installed in the SCMFCs and operated for 490 h (Fig. 1). All SCMFCs reached the maximum voltages in the 4th cycle (after 160 h). Compared with the unpyrolyzed catalyst Bi-CoPc/C, the pyrolyzed catalysts Bi-CoPc/C-(T) exhibited higher power generation. The SCMFCs with Bi-CoPc/C-800 cathode exhibited a higher voltage generation (383–395 mV) than that with Bi-CoPc/C-1000 (352–367 mV) and Bi-CoPc/C-600 (345–358 mV). Moreover, Bi-CoPc/C-(T) catalysts exhibited higher ORR rates than Bi-CoPc/C. The cycle durations of the SCMFCs with Bi-CoPc/C-(T) were shorter than that of Bi-CoPc/C cathodes (cycles 3rd–7th). Specifically, Bi-CoPc/C-800 (12–35 h) had shorter cycle duration than Bi-CoPc/C (50–76 h), implying Bi-CoPc/C-800 catalyst enhanced the ORR rate on cathode as well as the overall biochemical reactions in SCMFCs (Li et al., 2010). The power density and polarization behavior (Fig. S6) of SCMFCs were examined by steadily decreasing Rext from 30,000 to 20 O. The order for power density was: Pt/C > Bi-CoPc/C-800 > Bi-CoPc/C-1000 > Bi-CoPc/C-600 > Bi-CoPc/ C-300 > Bi-CoPc/C. Bi-CoPc/C without heat-treatment had a lower power density (244 mW m 2) than Bi-CoPc/C-300 (280 mW m 2), indicating that the heat treatment of the Bi-CoPc improved the ORR activity on the cathode. As the calcination temperatures were increased to 600 and 800 °C, the power densities steadily increased; but 1000 °C led to the decrease in power density. The maximum power density of Bi-CoPc/C-800 was 604 mW m 2, which was slightly lower than that of the Pt/C SCMFC (724 mW m 2). The enhanced power output was confirmed by EIS analysis (Nyquist plots) (Fig. S7). A close relationship between Rin and power output of SCMFCs was obtained: Bi-CoPc/C-800 delivered the highest power density and lowest Rin (154 X). Moreover, the Rin for Bi-CoPc/C-800 was only half of that for the hybrid
3.4. COD degradation and the coulombic efficiency of SCMFCs Bi-CoPc/C showed the lowest COD removal of 76.5% (Fig. 2). The COD removal slightly increased with the pyrolysis temperature, from 79.3% for Bi-CoPc/C-300 to 86.5% for Bi-CoPc/C-1000. The coulombic efficiency (CE) values indicated the conversion efficiency of organic substrates to electricity in MFCs. Bi-CoPc/C-800 catalyst with the highest voltage generation had a lower CE value (29.6%) than Pt (32.5%), even though it showed a higher COD removal. This was probably caused by the consumption of organic substrates by bacterial growth rather than by electricity generation (Li et al., 2010). 3.5. Significance for MFC applications Cathodic catalyst is critical for the cost and power generation of MFCs. The batch-mode SCMFC tests showed that the ORR activity of Bi-CoPc catalysts clearly enhanced with temperatures and
100 80 60 40 20
CO
D
al and CE (% )
3.3. Electricity generation in SCMFCs with different Bi-CoPc/C-(T) catalysts
Bi-CoPc/C-CoO catalyst previously developed (Li et al., 2014), which emphasized the importance of heat treatment for improving the ORR catalytic activity of MFC cathodes.
COD remov
contributed to the 2e pathway (Sidik et al., 2006; Ikeda et al., 2008). The hybrid Bi-CoPc catalyst (Bi-CoPc/C-NiO and Bi-CoPc/C-CoO) previously developed had no pyrrolic-N and pyridinic-N, since no pyrolysis treatment was applied in the synthesis (Li et al., 2014), whereas the pyrolyzed Bi-CoPc in this study showed remarkable amounts of pyrrolic-N and pyridinic-N.
P 0 Bi t/C -C -C oPc /C oP Bi -C c/C -10 Bi o P -C 80 00 c/C Bi oP -C -60 0 c / CoP 0 30 c/C 0 Bi
re
mo
va
l CE
Fig. 2. The COD removal and CE of SCMFCs with different cathodic catalysts.
548
B. Li et al. / Bioresource Technology 193 (2015) 545–548
reached the peak performance when treated at 800 °C. Although the Pt/C-catalyzed SCMFC outperformed all other SCMFCs, the cost ($1700 m 2) is 16 times higher than that of Bi-CoPc ($110 m 2) (Li et al., 2014). The much low cost of the Bi-CoPc catalysts makes them more favorable in practical applications. 4. Conclusions Heat-treated Bi-CoPc/C materials were developed as cathode catalysts in SCMFCs. The pyrolysis process modified the functional group, increased the amounts of nitrogen and changed the nitrogen types. The excellent electrocatalytic activity toward oxygen reduction reaction (ORR) for Bi-CoPc/C-800 was in line with the high amount of pyrrolic nitrogen configuration. The SCMFCs with Bi-CoPc/C-800 cathode produced a maximum power density of 604 mW m 2. Acknowledgements The authors are grateful to the financial supports from the National Natural Science Foundation of China (Project Nos. 21173086, 20975040 and U1301245) and Guangdong Natural Science Foundation (Project No. 2014A030313259). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.05. 111. References Bezerra, C.W.B., Zhang, L., Liu, H., Lee, K., Marques, A.L.B., Marques, E.P., Wang, H., Zhang, J., 2007. A review of heat-treatment effects on activity and stability of PEM fuel cell catalysts for oxygen reduction reaction. J. Power Sources 173 (2), 891–908. Bezerra, C.W.B., Zhang, L., Lee, K.C., Liu, H.S., Marques, A.L.B., Marques, E.P., Wang, H.J., Zhang, J.J., 2008. A review of Fe-N/C and Co-N/C catalysts for the oxygen reduction reaction. Electrochim. Acta 53 (15), 4937–4951.
Cheng, S., Liu, H., Logan, B.E., 2006. Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environ. Sci. Technol. 40 (1), 364–369. Coutanceau, C., El Hourch, A., Crouigneau, P., Leger, J., Lamy, C., 1995. Conducting polymer electrodes modified by metal tetrasulfonated phthalocyanines: preparation and electrocatalytic behaviour towards dioxygen reduction in acid medium. Electrochim. Acta 40 (17), 2739–2748. Geng, D., Chen, Y., Chen, Y., Li, Y., Li, R., Sun, X., Ye, S., Knights, S., 2011. High oxygenreduction activity and durability of nitrogen-doped graphene. Energy Environ. Sci. 4 (3), 760–764. Ikeda, T., Boero, M., Huang, S.F., Terakura, K., Oshima, M., Ozaki, J., 2008. Carbon alloy catalysts: active sites for oxygen reduction reaction. J. Phys. Chem. C 112 (38), 14706–14709. Jasinski, R., 1964. A new fuel cell cathode catalyst. Nature 201 (4925), 1212–1213. Kera, S., Casu, M., Bauchspieß, K., Batchelor, D., Schmidt, T., Umbach, E., 2006. Growth mode and molecular orientation of phthalocyanine molecules on metal single crystal substrates: a NEXAFS and XPS study. Surf. Sci. 600 (5), 1077–1084. Lee, K., Zhang, L., Lui, H., Hui, R., Shi, Z., Zhang, J., 2009. Oxygen reduction reaction (ORR) catalyzed by carbon-supported cobalt polypyrrole (Co-PPy/C) electrocatalysts. Electrochim. Acta 54 (20), 4704–4711. Li, B., Zhou, X., Wang, X., Liu, B., Li, B., 2014. Hybrid binuclear-cobalt-phthalocyanine as oxygen reduction reaction catalyst in single chamber microbial fuel cells. J. Power Sources 272, 320–327. Li, X., Hu, B., Suib, S., Lei, Y., Li, B., 2010. Manganese dioxide as a new cathode catalyst in microbial fuel cells. J. Power Sources 195 (9), 2586–2591. Lovley, D.R., Phillips, E.J.P., 1988. Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl. Environ. Microbiol. 54 (6), 1472–1480. Matter, P.H., Zhang, L., Ozkan, U.S., 2006. The role of nanostructure in nitrogencontaining carbon catalysts for the oxygen reduction reaction. J. Catal. 239 (1), 83–96. Santoro, C., Li, B.K., Cristiani, P., Squadrito, G., 2013a. Power generation of microbial fuel cells (MFCs) with low cathodic platinum loading. Int. J. Hydrogen Energy 38 (1), 692–700. Santoro, C., Stadlhofer, A., Hacker, V., Squadrito, G., Schröder, U., Li, B., 2013b. Activated carbon nanofibers (ACNF) as cathode for single chamber microbial fuel cells (SCMFCs). J. Power Sources 243, 499–507. Sidik, R.A., Anderson, A.B., Subramanian, N.P., Kumaraguru, S.P., Popov, B.N., 2006. O-2 reduction on graphite and nitrogen-doped graphite: experiment and theory. J. Phys. Chem. B 110 (4), 1787–1793. Wang, X., Santoro, C., Cristiani, P., Squadrito, G., Lei, Y., Agrios, A.G., Pasaogullari, U., Li, B., 2013. Influence of electrode characteristics on coulombic efficiency (CE) in microbial fuel cells (MFCs) treating wastewater. J. Electrochem. Soc. 160 (7), G3117–G3122. Zhao, F., Slade, R.C.T., Varcoe, J.R., 2009. Techniques for the study and development of microbial fuel cells: an electrochemical perspective. Chem. Soc. Rev. 38 (7), 1926–1939.