Lanthanum Hydroxide Nanorod-Templated Graphitic Hollow Carbon Nanorods for Supercapacitors.

Lanthanum hydroxide nanorods were employed as both a template and catalyst for carbon synthesis by chemical vapor deposition. The resulting carbon possesses hollow nanorod shapes with graphitic walls. The hollow carbon nanorods were interconnected at some junctions forming a mazelike network, and the broken ends of the tubular carbon provide accessibility to the inner surface of the carbon, resulting in a surface area of 771 m2/g. The hollow carbon was tested as an electrode material for supercapacitors. A specific capacitance of 128 F/g, an energy density of 55 Wh/kg, and a power density of 1700 W/kg at 1 A/g were obtained using the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, as the electrolyte.

Introduction

Graphene-based materials with designed nanoarchitectures have been studied for a wide range of applications.[1,2] Many methodologies have been established to prepare three-dimensional or porous carbons with graphitic walls to achieve high surface area and high conductivity, which are of great importance for electrical double-layer capacitors.[3−7] The hydrophobic nature and high surface area of carbon-based materials also provide advantages when employed as catalyst supports, which allows better adsorption and diffusion of reactions.[8,9] Among these carbon materials, templated carbon has been prepared using hard templates, such as zeolites and mesoporous silica.[10−16] Unfortunately, most of them have amorphous carbon framework, which limited their use in energy storage as well as catalysis.[17] Graphene- and carbon nanotube (CNT)-based carbon electrodes with porous or three-dimensional structures have shown excellent performance in supercapacitors.[18−22] Therefore, it is of great interest to make templated carbon with graphene-like walls for energy storage and other applications. Mesoporous silica has been used with various organic precursors to produce carbons with improved graphitic content.[17] Recently, a lanthanum-exchanged zeolite was reported to be an excellent template for carbon synthesis.[23] The resulting carbon replicated the zeolite pore structure with graphene-like walls and conductivity comparable to gold. It is assumed that the carbon source reacts with La first to form LaC2, which is then turned to La(OH)3 by reacting with steam leading to the growth of graphitic carbon (Figure ). This work demonstrated the application of La as a catalyst for the synthesis of graphitic carbon with controlled morphology and structure. In the present study, pure lanthanum hydroxide nanorods (LaNR) were employed as templates to produce hollow carbon nanorods. Compared to the La-zeolite, the pure lanthanum hydroxide templates are removed by HCl, and the resulting solution can be used to make new nanorods.

Figure 1

Schematic illustration of the carbon formation mechanism on La(OH)3 surface.

Results and Discussion

The LaNRs were hydrothermally synthesized using a solution of La(NO3)3·6H2O and NaOH at 110 °C for 24 h followed by centrifugation, drying, and annealing at 200 °C for 1 h. The lanthanum hydroxide nanorod-templated carbon (LaNRTC) was synthesized by chemical vapor deposition (CVD) using acetylene and steam at 600 °C. Washing with concentrated HCl removes the La(OH)3 template leaving hollow replicas of the nanorods.

The LaNR shown in Figure a,d possesses a rod shape with an average diameter of 16 nm and an average length of 140 nm. The lattice fringes of LaNR (Figure b) have a d-spacing of 0.285 nm, corresponding to the (200) face of La(OH)3 (JCPDS 36-1481). Figure b,e shows the morphology of the templated carbon before removal of LaNR. The transmission electron microscopy (TEM) image shows that carbon covers the surface of the LaNR. The scanning electron microscopy (SEM) image (Figure b) shows better contrast compared to that of the pure LaNR since the carbon covered on the surface of LaNR is conductive. These images indicate that the carbon was grown only on the surface of LaNR without formation of amorphous carbon elsewhere. This demonstrates that the La(OH)3 serves as both a template and catalyst for carbon growth, which is consistent to our previous work that no carbon was formed under the same conditions if only mesoporous silica was placed in the reactor.[24] And after La(OH)3 was deposited on mesoporous silica, carbon was formed along the surface of the templates. After removal of the LaNR by using acid, the resulting carbon retains the rod shape morphology with similar particle size (Figure c), and the inner space is empty as shown in the TEM image (Figure f). The acid may access LaNR through defects in the carbon coating, leading to openings in the tubular carbon network. The edge of the resulting carbon exhibits lattice fringes with a d-spacing of 0.40 nm attributed to the (002) planes of graphitic carbon, indicating the graphitic nature of the lanthanum hydroxide nanorod-templated carbon. The d-spacing is bigger than graphite (0.34 nm) due to less than five graphene layers on the edge of the carbon, where the d-spacing has been reported to increase as the number of the graphene layers decreases.[25]

Figure 2

SEM and TEM images of (a, d) lanthanum hydroxide nanorods (LaNR); lanthanum hydroxide nanorod-templated carbon (b, e) before (C@LaNR) and (c, f) after (LaNRTC) template removal.

The LaNRTC after template removal shows a broad peak around 22° in the X-ray diffraction (XRD) and the corresponding d-spacing is calculated using Bragg equation, which is consistent to the d-spacing measured in the TEM image. This is attributed to the (002) planes of the carbon, whereas the LaNR and C@LaNR mainly exhibit XRD patterns for nanocrystalline La(OH)3 (JCPDS 36-1481) (Figure a). This is also consistent to the TEM images (Figure ), where the LaNRTC shows a few layers of carbon. The Raman spectra exhibit a strong G band (∼1600 cm–1) with a IG/ID ratio of 1.03. Previously, a La-zeolite-templated carbon was reported to possess five- or seven-member rings, which contributed to the peaks for disordered carbon (D band), but they are still sp2 hybridized.[23] In that case, the overall carbon material has graphitic framework with good conductivity. Combining the results from TEM and Raman, the LaNRTC possesses graphitic walls.

Figure 3

(a) Powder X-ray diffraction patterns of LaNR, C@LaNR, and LaNRTC; (b) Raman spectrum of LaNRTC; (c) N2 adsorption–desorption isotherms of LaNR and LaNRTC; (d) density functional theory (DFT) pore size distributions of LaNR and LaNRTC.

Figure c shows the N2 adsorption–desorption isotherms for the LaNR template and the resulting carbon, LaNRTC. The isotherm of the pure LaNR shows a type II isotherm with a low Brunauer–Emmett–Teller (BET) surface area of 54 m2/g, indicating no porosity but space between nanoparticles. In contrast, the LaNRTC shows a type IV isotherm corresponding to mesoporous features with a clear hysteresis between 0.4 and 1.0. The adsorption below 0.1 also indicates some microporosity. The BET surface area of the LaNRTC is much higher than that of the pure LaNR (771 vs 54 m2/g). The micropore volume and total pore volume of LaNRTC is much higher than LaNR, as shown in Table S1. Both LaNRTC and LaNR show similar DFT pore size distribution, which might due to the absence of uniform pore structure (Figure d). The large increase in surface area is due to the transformation of the morphology from rods to tubular network with accessibility to inner surface of the tubes. The TEM images in Figure show both closed ends and open ends. The open ends result in pore openings as large as 30 nm (Figure ). Micropores resulting from defects are not apparent in these images. Additionally, some of the tube junctions were fused together as interconnected L-shaped channels. It is assumed that the carbon grows from one rod to the other across the tightly packed junctions rather than just covering an individual rod. This arises because during the CVD process the carbon cannot completely coat the randomly stacked La(OH)3 nanorods. Thus, there are hollow nanorods and a complicated tubular carbon network. Overall, the material is composed of carbon tubes, tube stacks, and fused mazelike nano carbon tube networks. The large increase in the surface area from pure LaNR to LaNRTC is much higher than that of most CNTs (<500 m2/g).[7] The TEM images in Figure also represent the relationship between the nanorods template and the pore structure of resulting carbon. Since the synthesis of La(OH)3 nanorods with various sizes and shapes has not been well established, the pore structure is not controlled by different La(OH)3 templates here, but it is of great interest for future work. The hollow carbon tubes with different length and diameters are presented in Figure , and they are formed on La(OH)3 nanorods with various sizes. The size of La(OH)3 nanorods controls the resulting carbon’s dimensions and inner diameters, which affect the porosity of each carbon tube in the same batch. By using different La(OH)3 templates, it is possible to control the pore structure of the resulting carbon in bulk.

Figure 4

Schematic illustration of the formation and TEM images of broken ends and connected junctions in the LaNRTC.

The LaNRTC was tested as an electrode material for supercapacitors using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) as the electrolyte. Figure a shows the cyclic voltammograms (CVs) of the LaNRTC at different scan rates (10, 25, 50, 75, and 100 mV/s) in the range of −2.0–2.0 V.[26−29] The shape of the CV is nearly rectangular indicating ideal double-layer capacitive behavior. The capacitances of the electrodes were calculated from CV using the following equationwhere I is the current as a function of time, υ is the scan rate. The specific capacitance in different scan rates was calculated using following equationwhere m is mass of both electrodes. The highest specific capacitance obtained for the LaNRTC was 128 ± 2 F/g at 10 mV/s. Table summarizes the obtained specific capacitance values at different scan rates. To compare the LaNRTC with a mesoporous carbon templated by mesoporous silica, wrinkled mesoporous carbon was prepared according to reported procedure.[24] The capacitance of LaNRTC is higher than the mesoporous carbon templated by La2O3 supported on wrinkled mesoporous silica (Figure S1), which has a capacitance of 70 F/g. The capacitance of LaNRTC is among the best results for mesoporous graphenes,[20,30−36] and aligned CNTs[18,37] as well as activated carbons[38−41] using ionic liquids as the electrolyte. Unlike these carbons, the LaNRTC does not require complicated porous silica templates, lengthy preparation process, or additional activation steps. And the ionic liquids allow a higher operating voltage window, which is more practical for various applications.

Figure 5

Electrochemical characterizations of LaNRTC: (a) cyclic voltammograms; (b) charge–discharge curves from 1 to 6 A/g; (c) Nyquist plots; (d) Ragone plot; (e) specific capacitance retention as a function of cycle number.

Table 1

Specific Capacitance (F/g) in Different Scanned Rates

scan rates (mV/s)	specific capacitance (F/g)
10	128.1
25	126.2
50	125.0
75	122.6
100	120.2

Figure b shows the charge–discharge curves for the LaNRTC in different current densities (1–10 A/g). The JME cells were charged to 3.5 V and discharged completely at different current densities. Linear discharge curves show the ideal capacitive behavior. After charging up to 3.5 V, it is important to stabilize the coin cell before it starts to discharge. A plateau at 3.5 V is purposely introduced to investigate the stability of the coin cell. At 1 A/g, the IR drop is ∼0.2 V, which shows the good conductive behavior of the JME cell. Energy densities (Ed) and power densities (Pd) were calculated using following equationswhere t is the time taken to discharge to 0 V from the initial voltage (ΔV), subtracting IR drop at the beginning of discharge. The energy density and power density of the LaNRTC were found to be 55 Wh/kg and 1700 W/kg, respectively, at 1 A/g. In contrast, the mesoporous carbon templated by La2O3 supported on wrinkled mesoporous silica (Figure S1) has an energy density of 35.8 Wh/kg and a power density of 1499 W/kg at 1 A/g.

Figure c shows the Nyquist plots for the LaNRTC in EMITFSI electrolyte. Electrochemical impedance spectroscopy (EIS) can provide information such kinetics in the double-layer charging–discharging processes. The almost vertical increase in the lower frequency region is indicative of an ideal capacitance behavior. In the Nyquist plot, the x axis intercepts at the highest frequency region indicating bulk electrolyte resistance, which mainly depends on the electrolyte solution.

Figure d shows the Ragone plot of the LaNRTC, polyacrylonitrile (PAN) nanofibers,[42] and carbon nanotubes (CNTs) at different current densities 1–10 A/g by comparing energy densities and power densities. Energy densities were determined by discharging from 3.5 to 0 V. When the current density increases, the energy density decreased, but retained 90% of its original energy density at discharge rate of 10 A/g, whereas PAN and CNTs retained only 75 and 80%, respectively. The energy density of the LaNRTC is much higher than PAN nanofibers and the pure CNTs, as shown in Figure d.

Figure e shows the plot of specific capacitance retention against cycle number. After the 100th cycle, 98% of the specific capacitance was retained, whereas 97% of the capacitance was retained after 1000th cycle. Capacitance (89%) was retained after 5000th cycles, indicating the good cycling ability. This combined with the high energy density, and power density of the LaNRTC makes this carbon a good electrode material for supercapacitors.

Conclusions

In conclusion, the lanthanum hydroxide nanorods function as both catalyst and template for carbon growth, resulting in hollow carbon nanorods. The hollow carbon nanorods possess connected junctions and open ends, which result in a mazelike structure with high surface area and porosity. The LaNRTC shows excellent performance as an electrode material for supercapacitors taking advantage of its unique structure as well as low resistance. The templating method using lanthanum hydroxide nanoparticles also creates possibilities for the preparation of other graphitic carbons with controlled morphology.

Experimental Section

Synthesis of La(OH)3 Nanorods

In a typical synthesis, 5.77 g of La(NO3)3·6H2O in 10 mL of deionized water was mixed with a solution of 14.7 g of NaOH in 20 mL of water and heated to 110 °C in a 45 mL Teflon-lined autoclave for 24 h. The solid product was collected by centrifugation and washed with water several times then dried overnight at 100 °C. The product was then annealed at 200 °C for 1 h.

Synthesis of La(OH)3 Nanorod-Templated Carbon

The La(OH)3 nanorods (∼1 g) were placed in a ceramic boat centered in a horizontal quartz tube reactor. The reactor was heated to 600 °C under N2 (200 mL/min). Then, acetylene (30 mL/min) and steam (water is pumped at 5 mL/h) were fed into the reactor for 1.5 h followed by carbonization under N2 at 850 °C for 2 h. The resulting carbon/La(OH)3 composite was washed with concentrated HCl and water to remove the La(OH)3 template. For comparison, a mesoporous carbon was prepared with the same method, but by using La(OH)3 supported on wrinkled mesoporous silica as the template following reported procedure.[24]

Fabrication of Coin Cell-Type Supercapacitors

Symmetric JME cells were fabricated according to a previously reported procedure for JME packaging (Figure S2).[43] Electrodes were prepared by mixing the La(OH)3 nanorod-templated carbon with 2% poly(tetrafluoroethylene) binder. The carbon electrodes were immersed in the electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), for 1 h at room temperature under vacuum prior for assembling the coin cell. The two electrodes were separated by a Teflon film (0.02 mm) and constructed under inert environment.

Characterization

Scanning electron microscope (SEM) images were recorded on a Zeiss-LEO model 1530 scanning electron microscope. Transmission electron microscope (TEM) images were recorded using a JEOL 2100 transmission electron microscope. Powder X-ray diffraction (XRD) patterns were acquired on a Rigaku Ultima IV diffractometer using Cu Kα radiation. Raman spectra was obtained on a Jobin Yvon Horiba high-resolution LabRam Raman microscope. The N2 adsorption–desorption isotherms and BET surface areas were measured using a Quantachrome AS1 Autosorb. Electrochemical measurements were made using a BT2000 Arbin battery testing system. Electrochemical impedance spectroscopy (EIS) was measured on a EG&G Princeton Applied Research potentiostat/galvanostat.