Letter pubs.acs.org/JPCL
Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries Shuangyin Wang, Xinsheng Zhao, Thomas Cochell, and Arumugam Manthiram* Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Nitrogen-doped carbon nanotubes have been grown, for the first time, on graphite felt (N-CNT/GF) by a chemical vapor deposition approach and examined as an advanced electrode for vanadium redox flow batteries (VRFBs). The unique porous structure and nitrogen doping of N-CNT/GF with increased surface area enhances the battery performance significantly. The enriched porous structure of NCNTs on graphite felt could potentially facilitate the diffusion of electrolyte, while the N-doping could significantly contribute to the enhanced electrode performance. Specifically, the N-doping (i) modifies the electronic properties of CNT and thereby alters the chemisorption characteristics of the vanadium ions, (ii) generates defect sites that are electrochemically more active, (iii) increases the oxygen species on CNT surface, which is a key factor influencing the VRFB performance, and (iv) makes the NCNT electrochemically more accessible than the CNT. SECTION: Energy Conversion and Storage; Energy and Charge Transport edox flow batteries have attracted much interest as a promising energy storage device due to their long life, high energy efficiency, and low maintenance cost.1−3 They have emerged as one of the promising approaches for efficiently storing and utilizing intermittent renewable energy sources like solar and wind.4 Of them, the all-vanadium redox flow batteries (VRFBs), which utilize vanadium-containing chemicals as positive V(IV)/V(V) and negative V(II)/V(III) electrolytes, have become attractive to minimize crossover contamination problems.2−5 In the VRFB system, carbon-based materials are widely used in the electrodes to catalyze the electrode reactions.6−9 The most popular electrode in the VRFB is graphite felt (GF), which consists of carbon fibers with a large diameter.10,11 Even though they have a wide operating potential range, good stability, and low cost, GFs still have serious disadvantages, such as low surface area and low electrocatalytic activity (poor kinetics and reversibility). It is of critical importance to develop materials with high electrocatalytic activity toward the redox reactions in the VRFB system. To address this issue, various approaches including heat treatment, acid oxidation, and modification with metals of GF have been pursued by several groups.4,7,10−12 Carbon nanotubes (CNTs) with unique physical and chemical properties have attracted great interest in applications such as batteries, hydrogen storage, supercapacitors, and fuel cells.13−16 More recently, CNTs or functionalized CNTs have been considered as good electrode materials for redox flow batteries.7,8 On the other hand, nitrogen-doped carbon materials including nitrogen-doped mesoporous carbon (NMPC) and nitrogen-doped GF (N-GF) have been discovered as electrode materials for VRFBs.6,10 Compared with N-MPC and N-GF, nitrogen-doped carbon nanotubes (N-CNTs) may
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© 2012 American Chemical Society
be more interesting due to their unique physical and chemical properties such as excellent electronic conductivity and high thermal/chemical stability. Accordingly, N-CNTs have been recognized as advanced electrode materials in the energy storage field with improved activity and stability.17 Besides, the growth methods of N-CNTs have been well-established, which ensures large-scale production of N-CNTs as electrode materials for energy devices. However, the utilization of NCNTs for VFRBs has not been investigated yet. We present here, for the first time, the successful growth of N-CNTs on GF by a chemical vapor deposition (CVD) method. The ideal porous GF substrate provides an excellent porous framework for the growth of carbon nanotubes (undoped and doped with nitrogen). The CNTs (or NCNTs) on the surface of GF could significantly increase the electrochemical surface area of the carbon materials due to their relatively small size (∼30 nm in diameter and several μm in length), thus resulting in higher battery performance in VRFBs. The enriched porous structures of CNTs or N-CNTs on GF could potentially facilitate the diffusion of electrolyte in the VRFB system. In addition, the N-doping could further improve the electrode performance because of the modified electronic and surface properties of CNTs on GF. Figure 1A shows the scanning electron microscopy (SEM) image of the pristine GF, in which the relatively smooth surface can be seen. The porous architecture of the GF, as shown in the inset of Figure 1A, can facilitate mass transport of the electrolyte during operation. The N-CNTs on GF were Received: July 3, 2012 Accepted: July 27, 2012 Published: July 27, 2012 2164
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Figure 1. SEM images of (A) pristine GF with the inset showing the photograph of a typical GF, (B) CNT grown on GF, (C) N-CNT grown on GF, and (D) the TEM image of N-CNT after posttreatment.
grown by a single-injection CVD process using ferrocene (2.5 wt %) as a growth catalyst dissolved in ethylenediamine (ED) as an N-CNT source solution.18 The pristine GF was placed in the center of a quartz tube and heated to 800 °C under Ar gas flow, followed by the injection of the ferrocene/ED solution slowly. After the reaction was allowed to proceed, the reactor chamber was cooled to 400 °C. For a comparison, undoped carbon nanotubes on graphite felt (CNT/GF) were also prepared by a similar CVD process using toluene as the CNT solution source.19 Figure 1B and C shows the SEM images of CNT- and N-CNT-modified GFs, respectively. As seen, microporous nanotube networks have been successfully grown on the surface of the GF. The transmission electron microscopy (TEM) image (Figurea S1 (Supporting Information) and 1D) of N-CNT confirms the formation of the tubular structure. Figure S1 (Supporting Information) shows the TEM image of the N-CNT sample without any post-treatment after the CVD growth. It could be observed that the N-CNT growth catalyst nanoparticles (Fe, decomposed from ferrocene) exist in the as-grown N-CNT samples. A simple immersion of the asprepared samples into HCl solution could effectively remove the Fe nanoparticles, as evidenced in Figure 1D. The diamter of the N-CNT is ∼30 nm, which is much smaller than that of the carbon fibers in the GF (∼10 μm), resulting in a high surface area for the electrocatalytic reactions. The surface areas of CNTs and GF were measured to be 106.1 and 11.5 m2 g−1, respectively, by the Brunauer−Emmett−Teller method, indicating the advantage of CNTs for electrochemical reactions. In addition, the N-CNT and CNT networks are uniformly and densely distrubuted onto the surface of the GF and have a very porous structure (inset of Figure 1C), which would favor the mass transport of the electrolyte during the charge/discharge process, leading to significantly enhanced battery performance, as discussed below. In order to confirm the successful incorporation of nitrogen and to investigate the types of nitrogen in the carbon lattice network, X-ray photoelectron spectra (XPS) were recorded with the N-CNT. To exactly know the percentage of N in the CNT and to avoid the C contribution from the GF, the XPS data were collected on the N-CNT on copper foil, which was grown under the same experimental conditions as that on the GF. The XPS survey spectrum in Figure 2B shows the presence of C, N, O, and Fe, in which C is dominant and nitrogen with a
Figure 2. (A) XPS survey spectrum and (B) fine-scanned N1s spectrum of the N-CNT.
N/C atomic ratio of around 5/95 incorporated into the carbon network is seen. Besides, the oxygen species observed in the XPS survey spectrum are due to the adsorption of oxygen onto the carbon surface, and the other peaks in the survey spectrum are from the residual iron (catalyst for CNT growth). These catalyst nanoparticles could be removed by immersing the sample into HCl solution before the electrochemical testing.18 Figure 2B shows the fine-scanned N1s XPS spectrum of the NCNT sample. In order to determine the types of nitrogen present in the N-CNT, the spectrum was deconvoluted into three subpeaks, which could be attributed to pyridinic N (398.2 eV), pyrrolic N (400.1 eV), and graphitic N (401.3 eV). It can be clearly seen that pyridinic nitrogen is dominant compared to the other two types of nitrogen. It has been proposed that pyridinic nitrogen species cause structural deformation in the N-CNT samples and expose planar edges or defect sites where they reside.6 Nitrogen-doped carbon samples have been widely investigated for electrochemical applications, especially for the oxygen reduction reaction in fuel cells,20−23 and it has been concluded that the edge plane sites could facilitate the adsorption of oxygen and could efficiently improve the electrocatalytic activity of carbon materials. Therefore, it can be concluded that nitrogen-doped carbon with dominant pyridinic nitrogen species would be electrochemically more active. The effect of N-doping on the structure of the CNT on GF was studied by Raman spectroscopy,18,24 as shown in Figure 3. Two distinct peaks were observed for both the CNT/GF and N-CNT/GF samples. The D band, observed at approximately 1311 cm−1 for the two samples, is due to the disorder induced by the structural defects on the graphitic plane of the CNTs. The G band, observed at approximately 1583 cm−1, is commonly found with all graphitic structures and is attributed to the E2g vibrational mode present in the sp2-bonded graphitic carbons. The intensity ratio of the D band to the G band (ID/ IG) is an indicator of the amount of defects present in the 2165
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the apearance of the more porous structure of the CNT (and N-CNT) network on the surface of the GF. More importantly, N-CNT/GF shows much higher capacity than the undoped CNT/GF, illustrating the advantage of nitrogen doping. The Coulombic efficiency (CE), voltage efficiency (VE), and overall energy efficiency (EE) values of the VRFBs obtained with the different electrode materials are summarized in Table S1 (Supporting Information) and plotted in Figure 4B. Because the same battery geometry and membrane were used, the differences in the energy efficiency would reflect the differences in the electrocatalytic activity of the electrode materials. It could be observed in Table S1 (Supporting Information) and Figure 4B that the energy efficiencies on CNT/GF and N-CNT/GF are, respectively, 4.7 and 8.8% higher compared to that on pristine GF. Also, the performane of the cell with N-CNT/GF as the electrode is higher than that with CNT/GF, demonstrating the advantageous role of nitrogen doping. In fact, nitrogen-doped carbon materials (CNT, graphene, etc.) have been demonstrated to exhibit excellent activity for various electrochemical reactions. The doped nitrogen atoms in the carbon lattice network appear to modify/alter the electronic properties of the carbon nanotubes and introduce lots of defect sites on the CNT, as suggested by the Raman data (Figure 3). The modified electronic properties of the N-CNT could have different adsorption behavior with various species, such as the vanadium ions in the present system,6 and the carbon atoms around the defect sites are electrochemically more active. It has been demonstrated that pristine carbon-based electrode materials including GF and CNTs show poor activity for VRFB.7,11,12 In order to improve the electrode performance, much attention has been paid toward the modification of the electrode. Most typically, various surface treatment methods have been developed. For example, Yan et al.7 have treated the carbon nanotubes with strong acid, which generates defect sites and oxygen-containing groups on the CNT surface and improves their performance for VRFBs; they concluded that oxygen functional groups are the key factor in catalyzing the redox reactions.7,11,12 In our present work, in order to investigate the effect of nitrogen doping on the oxygen adsorption properties of the CNT, we collected the XPS survey spectra of the N-CNT (Figure 2) and CNT (Figure S2, Supporting Information). It can be seen that the nitrogen doping significantly increases the oxygen species on the NCNT. The increased oxygen species may facilitate catalyzing the redox reactions in the VRFB system. We also compared the fundamental electrochemical properties of the N-CNT and CNT by investigating their capacitance behavior. Figure S3 (Supporting Information) shows the cyclic voltammetry (CV) curves of the N-CNT and CNT in 1 M H2SO4. While the CNT shows only the double-layer capacitance, the N-CNT shows additional pseudocapacitance due to nitrogen doping. More interestingly, the N-CNT shows higher double-layer capacitance than the CNT in addition to the extra pseudocapacitance. This finding indicates that the N-CNT is electrochemically more accessible for electrode reactions, resulting in improved VRFB performance compared to the CNT. Finally, we investigated the durability of the as-prepared NCNT/GF in comparison to GF in the VRFB system. For the VRFB system, the performance decays are mainly due to the degradation of the membrane materials. Figure S4A (Supporting Information) shows the cycling charge−discharge curves for 50 cycles, and Figure S4B (Supporting Information) shows the capacity decay with cycle number. It can be seen that both the
Figure 3. Raman spectra of CNT/GF and N-CNT/GF.
carbon materials and provides a quantitative measurement of the edge plane exposure. The ID/IG ratio of 1.53 for the NCNT/GF sample is much higher than that for CNT/GF (1.15). The higher ID/IG ratio for N-CNT/GF indicates more defects and edge plane exposure, caused by the heterogeneous nitrogen-atom doping onto the graphite layers of CNTs, which agrees well with the XPS data. The performances of the VRFBs with the fabricated electrodes were evaluated by the charge−discharge experiments, which were performed using a small-scale cell with the GF-based electrodes, in which 20 mL of 1 M V(IV) + 2 M H2SO4 and 20 mL of 1 M V(III) + 2 M H2SO4 solutions were pumped into the positive and negative sides, respectively, as the catholyte and anolyte. All of the cells were charged to 1.7 V and discharged to 0.8 V at a constant current density of 10 mA cm−2. Figure 4A presents the charge−discharge curves of the cells. It can be seen that the CNT (and N-CNT) grown on the GF surface significantly improves the capacity compared to the unmodified pristine GF due to the increased surface area and
Figure 4. (A) Charge−discharge curves of the VRFBs with pristine GF (black), CNT/GF (red), and N-CNT/GF (blue) electrodes and (B) CE, VE, and overall EE values for the three electrode materials. 2166
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N-CNT/GF and GF electrodes decay fast initially and level off subsequently with a similar decay trend, indicateing that the NCNT/GF has reasonable stability as the pristine GF. In summary, we have directly grown, for the first time, undoped and nitrogen-doped CNTs on GF via a CVD method. VRFBs assembled with CNT/GF and N-CNT/GF as electrodes show significantly enhanced performance in terms of capacity and energy efficiency due to the unique porous structures with higher surface area. More interestingly, NCNT/GF shows better battery performance than CNT/GF, confirming the beneficial role of nitrogen doping. Overall, the nitrogen doping modifies the electronic properties of the CNT, increases the oxygen species on the CNT surface, and makes the N-CNT electrochemically more accessible than the CNT. In addition to offering significant improvements in the performance of VRFBs, we believe that these materials could also find applications in other electrochemical reactions such as fuel cells and lithium air batteries.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and more experimental characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397.
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