- Email: [email protected]
Supported ultrafine ruthenium oxides with specific capacitance up to 1099 F g−1 for a supercapacitor
Accepted Manuscript Title: Supported ultrafine ruthenium oxides with specific capacitance up to 1099 F g−1 for a supercapacitor Author: Pengfei Wang Hui Liu Yuxing Xu Yunfa Chen Jun Yang Qiangqiang Tan PII: DOI: Reference:
S0013-4686(16)30364-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.02.089 EA 26706
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-12-2015 28-1-2016 15-2-2016
Please cite this article as: Pengfei Wang, Hui Liu, Yuxing Xu, Yunfa Chen, Jun Yang, Qiangqiang Tan, Supported ultrafine ruthenium oxides with specific capacitance up to 1099Fgminus1 for a supercapacitor, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.02.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Supported
ultrafine
ruthenium
oxides
with
specific
capacitance up to 1099 F g-1 for a supercapacitor
Pengfei Wanga,b, Hui Liua,c, Yuxing Xua, Yunfa Chena, Jun Yanga,*, Qiangqiang Tana,*
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing, China 100190. Tel: 86-10-8254 4915; Fax: 86-10-8254 4915; E-mail: [email protected] (J.Y.); Tel: 86-10-6252 9377; Fax: 86-10-8254 5008; E-mail: [email protected] (Q.T.) b
University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, China
c
Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences, Beijing
100190, China
1
Graphic abstract
50 nm
-1
RuO2/rGO
Specific capacity (F g )
1200 800 RuO2/C
400
RuO2/CNT RuO2/rGO Commercial RuO2
0 0
400
800
Cycle
1200
1600
2000
15 RuO2/C
RuO2/rGO -Z'' ()
RuO2/CNT
10
RuO2/rGO Commercial RuO2
5 0
10 nm
0
1
2
3
4
5
6
7
Z' ()
2
Highlights
Ru nanoclusters with an average size of 1.7 nm were prepared in aqueous phase.
The Ru nanoclusters were loaded on various carbon-based substrates.
The Ru clusters were converted into ultrafine RuO2 on the carbon-based substrates.
The supported ultrafine RuO2 exhibit high specific capacitances up to 1099 F g-1.
3
Abstract Reducing the particle size is a straightforward way to increase the specific surface area of ruthenium oxide, which usually translates to the high specific capacitance for a supercapacitor. Herein, we report a facile strategy to fabricate ultrafine ruthenium oxides supported on various carbon-based substrates (carbon powders, carbon nanotubes, or reduced graphene oxides) as excellent electrode materials for a supercapacitor. The novelty of this work lies in its synthetic approach, which involves an aqueous synthesis of ruthenium nanoclusters under the control of pH value, and an air oxidation-based conversion process. In particular, owing to their ultrafine particle size, the as-prepared carbon-, carbon nanotube-, or reduced graphene oxide-supported ruthenium oxides exhibit specific capacitance as high as 879.1 F g-1, 966.8 F g-1 and 1099.6 F g-1, respectively, for a supercapacitor at a current density of 0.5 A g-1. The specific capacitance maintains 98.4% (for carbon supports), 98.0% (for carbon nanotube supports) and 98.4% (for reduced graphene oxide supports) at current density of 1 A g-1 with good cycling stability. The remarkable simplicity and environmental friendliness of this synthesis may provide a liable quantity production route to produce ruthenium oxides as highly efficient electrode materials for a supercapacitor. Keywords: Ruthenium oxide; Supercapacitor; Ultrafine; Ruthenium nanoclusters; Specific capacitance
4
1. Introduction Fast-growing markets for portable power sources with instantaneous high-power density have stimulated great interest in research of supercapacitors, also known as ultracapacitors or electrochemical capacitors in recent years [17]. As a promising candidate electrode material used in supercapacitors, ruthenium oxide (RuO2) plays an important role in the applications of supercapacitors because of its proton-electron mixed conductive nature, remarkably high theoretical specific capacitance (up to 2200 F g-1), wide potential window, highly reversible redox reactions, good thermal stability, long cycle life, and high rate capability [815]. Increasing the specific surface areas is an effective way to improve the performance of RuO2 as electrode materials for supercapacitors, capable of providing more active centers for the multiple redox reactions, which usually translate to the high specific capacitance. In comparison with a number of strategies, e.g. depositing RuO2 films on substrates with a rough surface [1618], and coating a thin RuO2 film on high-surface-area materials [19,20], reducing the particle size is a straightforward approach to increase the specific surface area of RuO2 electrode. Smaller sized particles not only can shorten the diffusion distance but also can facilitate proton transport in the bulk of RuO2. The smaller the particle size, the higher the gravimetric capacitance and utilization efficiency. Therefore, one of the key points is to cut RuO2 materials down to a nanosize while maintain their high electrical and protonic conduction. Recently, a number of synthetic methods, e.g. sol-gel method [21], reduction or oxidation method [2224], and hydrothermal method [4,25,26] have 5
been developed to generate RuO2 or its composites with ultrafine sizes as electrode materials for supercapacitors. Typically, as reported by Rojo and coworkers, a nanometer-sized crystalline RuO2 anchored on carbon nanotubes (CNTs) using a supercritical fluid deposition method has shown a specific capacitance of up to 900 F g-1 [27]. However, in these strategies, additive agents such as surfactants, polymers, quaternary ammonium salts, or organic ligands are usually needed to protect the ultrafine RuO2 particles, and their remaining on the particle surface may result in abatement of active sites for the redox reactions, leading to low capacitive behavior of the electrode materials. For example, Hu and coworkers synthesized a three-dimensional porous framework using a one-step hydrothermal method [26], in which RuO2 particles with an average size of 23 nm are loaded uniformly on reduced graphene oxide (rGO) hydrogels. The as-prepared electrode materials only exhibit specific capacitance of 345 F g-1 at the mass loading of RuO2 of 15%. Alternatively, Amir et al. prepared rGO-RuO2 hybrid materials as supercapacitor electrodes by an in situ sol-gel deposition method [22]. Although their sol–gel route results in ultrafine, hydrated amorphous RuO2 particles with sizes of only 1.0–2.0 nm, which uniformly decorated the surfaces of rGO sheets, the obtained rGO–RuO2 electrodes exhibit a specific capacitance of 500 F g-1 in a 1 M H2SO4 electrolyte at a current density of 1.0 A g-1, far from the theoretical value. Herein, we report a facile, economical, and environmental friendly route for the production of ultrafine RuO2 particles supported on various carbon-based substrates (carbon powders, carbon nanotubes, or reduced graphene oxides) as excellent 6
electrode materials for a supercapacitor. This strategy involves the synthesis of Ru nanoclusters in aqueous phase under the control of pH value (no additional protective agents were used to stabilize the Ru nanoclusters), and subsequent loading on carbon-based substrates. Then the Ru nanoclusters are converted into ultrafine RuO2 particles on the same supports by a thermal treatment in air. We will demonstrate that the supported RuO2 particles exhibit high specific capacitance of up to 1099 F g-1 for a supercapacitor at a current density of 0.5 A g-1. The specific capacitance could maintain 98.0% or higher at current density of 1 A g-1 with good cycling stability, showing that the ultrafine sizes of RuO2 particles are favorable for the enhancement in their capacitive behavior. 2. Experimental 2.1. General Materials The chemical reagents, including ruthenium(III) chloride (RuCl3, Ru content 4555 wt%), hydrazine hydrate (N2H4·H2O, 80 wt%), polytetrafluoroethylene (PTFE, 60 wt% aqueous dispersion) and CYL2-60 carbon nanotube (CYL2-60 with diameter of 4060 nm, length of 12 μm, and special surface area of 100150 m2 g-1) from Aladdin Reagents, conductive carbon black EC-300J (cell grade with special surface area of 800 m2 g-1 and specific resistance of 0.52 ohm cm-1) from Shanghai Tengmin Industrial co., Ltd., Graphene Oxide (1 mg mL-1 aqueous dispersion) from Dalian Melone biotechnology Co., Ltd., Vulcan XC-72 carbon powders (XC-72C with BET surface area of 250 m2 g-1 and average particle size of 4050 nm) from Cabot 7
Corporation, sulphuric acid (H2SO4, 9598 wt%) and ethanol (99.5%) from Beijing Chemical Works, titanium gauze (30 mesh woven from 0.102 mm diameter wire) and commercial RuO2·xH2O (54% Ru) with product number of 346088 from J&K Scientific Ltd., and sodium borohydride (NaBH4, ≥98%) from Tianjin Fuchen chemical reagent, were used as received. Deionized water was distilled by a Milli-Q Ultrapure-water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious washing with de-ionized water before drying in an oven. 2.2. Synthesis of Ru nanoclusters The Ru nanoclusters were prepared in aqueous phase with the control of pH value lower than 4.9. Typically, 41.5 mg of RuCl3 was dissolved into 200 mL of DI water with vigorous stirring. Meanwhile, 37.9 mg of NaBH4 was dissolved into 100 mL of DI water. Freshly prepared NaBH4 aqueous solution was cautiously added dropwise to RuCl3 solution under stirred continuously, while a real-time measurement for the pH of the reaction mixture was carried out. The volume of the NaBH4 solution was carefully controlled to maintain the pH value of the reaction system to be always lower than 4.9. After stopping the addition of NaBH4 solution, the mixture was further stirred for 5 min to gain stable Ru hydrosol. 2.3. Loading the Ru nanoclusters on carbon-based substrates For the loading of the Ru nanoclusters on the Vulcan XC-72 carbon powders (C), CYL2-60 carbon nanotubes (CNT), or reduced graphene oxides (rGO), 28.2 mg of C
8
or CNT or 15 mL of aqueous graphene oxide dispersion was added into 250 mL of the as-prepared Ru hydrosol. The mixture was first ultrasonically treated for 10 min, and then magnetically stirred for 10 h. The precipitates (Ru/C, Ru/CNT, Ru/GO) were collected by centrifugation, washed with DI water thrice to remove the water-soluble ions and dried in air. In special, the Ru/GO was further treated in mixture of 11 L of hydrazine hydrate (80 wt%) and 175 L of ammonium hydroxide (28 wt%) in 95C for 1 h to transform the graphene oxide (GO) to reduced graphene oxide (rGO), and then washed and collected by centrifugation. 2.4. Conversion of Ru into RuO2 on the various carbon-based substrates These Ru/C, Ru/CNT, and Ru/rGO samples were heated at 150C in air for 2 h to convert the Ru into RuO2 for the generation of final ultrafine RuO2/C, RuO2/CNT, and RuO2/rGO nanocomposites. 2.5. Sample characterizations The pH value was measured using a pH meter (S220-USP/EP with pH-resolution of 0.001/0.01 and pH-relative accuracy of ±0.002) from Mettler Toledo Co., Ltd. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed on the JEOL JEM-2100 electron microscope operating at 200 kV with supplied software for automated electron tomography. For the TEM measurements, a drop of the nanoparticle solution was dispensed onto a 3-mm carbon-coated copper grid. Excessive solution was removed by an absorbent paper, and the sample was dried under vacuum at room temperature. The percentage of RuO2 on the 9
carbon-based substrates was measured using inductively coupled plasma atomic emission spectrophotometry (ICP-AES, Optima 5300DV, Perkin Elmer). X-ray photoelectron spectroscopy (XPS) was conducted on a VG ESCALAB MKII spectrometer. The Brunauer–Emmett–Teller (BET) surface areas of the as-synthesized samples were determined by the isothermal N2 adsorption/desorption method at 77 K in the range of 0.00–1.00 relative pressure via Micromeritics Adsorption Instrument attached by an automatic surface analyzer ( SSA-7300). Prior to measurements, the as-synthesized samples (20 mg) were degassed under vacuum at 110°C for 10h. 2.6. Fabrication of electrode and electrochemical measurements The working electrodes were fabricated by mixing the as-prepared RuO2/C, RuO2/CNT, RuO2/rGO nanocomposites or commercial RuO2 powders, carbon black, and polytetrafluoroethylene (PTFE) with a mass ratio of 85/10/5. The percentage of RuO2 in RuO2/C, RuO2/CNT and RuO2/rGO are 33.6%, 32.8% and 35.5%, respectively. These mixtures were dispersed in small amount of ethanol for the formation of paste, which was pressed into thin tablets and dried at 100C for 10 h in vacuum. Subsequently, the tablets were coated onto the titanium gauzes under the pressure of 10 MPa to obtain the electrode. The mass of as-prepared nanocomposites coated on the titanium gauze was 2 mg. All of the electrochemical measurements were carried out using a coin-type (CR2025) two electrode system. These coin cells consisted of two symmetrical working electrodes, polypropylene film (MPF with thickness of 45 μm from Nihon Kaiheiki Kogyo) as separator, and 1 M aqueous H2SO4 solution as electrolyte. The 10
titanium gauzes that coated with RuO2/C, RuO2/CNT, RuO2/rGO, or commercial RuO2 powders were used as the working electrodes. Cyclic voltammetry (CV), galvanostatic charge/discharge test, and electrochemical impedance spectroscopy (EIS) measurements were obtained using a CHI650 electrochemical workstation. The CV tests were done between 0 and 1.2 V at scan rates of 10, 20, 50, 100 and 200 mV s-1, respectively. EIS measurements were conducted over the frequency range from 100 kHz to 10 mHz at 0.5 V with 5 mV of ac perturbation. The specific capacitance of the electrodes can be calculated using an equation in term of C = It/ΔVm, where C is the specific capacitance (F g-1), I is the response current density (A g-1), t is the discharge time (s), ΔV is the potential (V) and m is the mass of the electroactive materials in the electrodes (g). 3. Results and discussion The overall strategy in this study was schematically depicted in Fig. 1a, which involves the synthesis Ru nanoclusters in aqueous solution by NaBH4 reduction of Ru3+ ion precursors under the control of pH value (step i), the subsequent loading the Ru clusters on the surface of carbon-based substrates (step ii), and the conversion of Ru clusters to ultrafine RuO2 particles via a air oxidation process on the same substrates (step iii). A notable advantage of this strategy is no additional capping agent on the surface of final ultrafine RuO2 products, capable of providing high surface area for the redox reactions taken place on the electrode materials.
11
The Ru nanoclusters were prepared in aqueous phase using a method we developed previously with slight modification [28]. NaBH4 is a common reducing agent used to prepare metal particles from metal ions [29,30], in this study, by means of the dropwise addition of aqueous NaBH4 solution to the aqueous RuCl3 solution, the Ru3+ ions are reduced following an overall reaction indicated below:
8Ru3+ + 3BH4 + 12H2O = 8Ru + 3B(OH)4 + 24H+
After the reaction, the dark aqueous RuCl3 solution (photo I in Fig. 1b) turns into a brown transparent hydrosol of Ru without any perceivable precipitation (photo II in Fig. 1b). At a pH value lower than 4.9, the Ru hydrosol prepared by NaBH4 reduction could be stabilized by surface adsorption of hydronium ions (H3O+) or other hydrated protons such as H5O2+ and H7O3+, which would impart positive charges to the Ru nanoclusters and stabilize them against agglomeration by electrostatic repulsion [28]. Fig. S1 in Supplementary Materials (SM) shows the TEM and HRTEM images of the as-prepared Ru nanoclusters together with a histogram for the particle size distribution. The average particle size of 1.70 nm is accompanied by a narrow size distribution with standard deviation 0.28 nm. By precipitating out the Ru nanoclusters from the hydrosol using a 1 M aqueous NaOH solution, the yield of nanoparticles was estimated to be ca. 95%. Centrifugation loss and cluster attachment to the container wall are the likely causes. After aging the mixture of Ru hydrosol and carbon-based substrates (carbon 12
powers, carbon nanotubes, or graphene oxides) under vigorous stirring for 6 h at room temperature, the Ru nanoclusters could be efficiently loaded on the carbon-based substrates, leading to the formation of Ru/C, Ru/CNT, and Ru/GO nanocomposites and leaving behind a clear aqueous phase. Subsequently, the Ru/GO was further treated with a mixture hydrazine hydrate and ammonium hydroxide in 95C to reduce the GO in the nanocomposites into rGO [31]. The representative TEM and HRTEM images in SM Fig. S2 reveal that the Ru nanoclusters are dispersed very well on the surface of various carbon-based substrates (SM Fig. S2a and b for Ru/C, SM Fig. S2c and d for Ru/CNT, and SM Fig. S2e and f for Ru/rGO, respectively). The Ru on the surface of various carbon-based substrates was examined by XPS to confirm its chemical state. Because the Ru 3d3/2 peak ovelaps with the C 1s peak in XPS spectra, preventing an unambiguous analysis of the nanoparticle surface, the Ru 3p XPS peak was used instead. As shown by Fig. 2a, c and e, the doublets at 462.2 and 484.45 eV for Ru/C (Fig. 2a), at 462.05 and 484.2 eV for Ru/CNT (Fig. 2c), and at 462.1 and 484.35 eV for Ru/rGO (Fig. 2e), respectively, are signatures of Ru metal in the zero valent state [15,32,33]. The XPS spectra of O 1s in the nanocomposites were also analyzed. As exhibited in SM Fig. S3a, c, and e for Ru/C, Ru/CNT, and Ru/rGO samples, respectively, the O 1s peaks of in these as-prepared nanocomposites have three overlapping components, including lattice oxygen O2- (530.2 eV), which is indiscernible in Ru/rGO sample (SM Fig. S3e), hydroxyl (532.0 eV), and bound H2O (533.9 eV), which are all consistent with the previous investigations [34]. The existence of bound H2O indicates that the Ru exists in a hydrate form. 13
However, after further calcination in air, as displayed by Fig. 2b, d, and f, the doublets at 463. 5 and 485.6 eV for Ru/C (Fig. 2b), at 463.5 and 485.5 eV for Ru/CNT (Fig. 2d), and at 463.3 and 485.5 eV Ru/rGO (Fig. 2f), respectively, which reflect the Ru at oxidized state, e.g. RuO2 [32,3537], can fit for the XPS spectra very well, indicating that the RuO2 is the dominant product upon thermal treatment of Ru/C, Ru/CNT and Ru/rGO in air at 150C. The transformation from Ru to RuO2 on various carbon-based substrates could be further suggested by the XPS analyses of O 1s after thermal treatment. The O 1s XPS spectra after thermal treatment in air were shown in SM Fig. S3b, d, and f. As displayed, after thermal treatment, the intensity of O 1s peaks at 530.2 eV (lattice oxygen O2-) is greatly enhanced, indicating the aforementioned oxidation transformation. The presence of bound H2O (the peak at 533.9 eV in XPS spectra) verifies that the obtained RuO2 remains a hydrate form after thermal treatment. In addition, the transformation from Ru to RuO2 on the carbon-based substrates could be further confirmed by XRD analyses. The XRD patterns of Ru/C, Ru/CNT, and Ru/rGO before and after calcination in air were shown in SM Fig. S4. As displayed, after calcination, the hexagonal Ru in Ru/C and Ru/CNT (SM Fig. S4a, b, and c, JCPDS Card File 060663) has been converted into tetragonal RuO2 phase supported on corresponding carbon-based substrates (SM Fig. S4d, e, and f, JCPDS Card File 401290). The mass percentages of RuO2 in the final RuO2/C, RuO2/CNT, and RuO2/rGO was determined using inductively coupled plasma atomic emission spectrophotometry 14
(ICP-AES), which are 33.6%, 32.8%, and 35.5%, respectively. The TEM (Fig. 3a, c and e) and HRTEM images (Fig. 3b, d and f) of the as-obtained RuO2/C, RuO2/CNT and RuO2/rGO were exhibited in Fig. 3, which manifest that the resulting ultrafine RuO2 in the final composite products have the comparable size and size distribution in comparison with those of the Ru nanoclusters. Moreover, the well dispersion of ultrafine RuO2 particles on various carbon-based substrates is also maintained after thermal treatment in air.
The as-prepared RuO2/C, RuO2/CNT and RuO2/rGO nanocomposites were examined as electrode materials for a supercapacitor and benchmarked against the commercial RuO2·xH2O from J&K Scientific Ltd. Fig. 4a, b, c, and d show the cyclic voltammograms (CVs) of the as-prepared ultrafine RuO2 particles supported on various carbon-based substrates obtained in 1 M H2SO4 with a potential range of 01.2 V at scan rates of 10, 20, 50, 100, and 200 mV s-1, respectively. For the same mass loading, the CV loops have different areas for different electrode materials, indicating different levels of stored charge. Observed from these tested CV curves, the more rectangle shapes suggest better capacitive behaviors for RuO2/C, RuO2/CNT and RuO2/rGO nanocomposites in comparison with that of commercial RuO2, which might be attributed to the large surface area of ultrafine size of RuO2 particles accessible for the multiple redox reactions. We conducted the nitrogen adsorption-desorption isotherms of RuO2 clusters supported on different carbon-based 15
substrates to evaluate their specific surface area (SM Fig. S5). The calculated BET (Brunauer-Emmett-Teller)
surface
area
of
RuO2/C,
RuO2/CNT,
RuO2/rGO,
commercial RuO2 were summarized in SM Table S1, and as exhibited, the specific surface areas of RuO2 clusters supported on carbon-based substrates (159.9, 149.6, and 131.5 m2 g-1 for RuO2/C, RuO2/CNT, and RuO2/rGO, respectively) are much higher than that of commercial RuO2 (10.2 m2 g-1). In addition, the relatively short diffusion distance for the charges in tiny RuO2 particles would also be helpful for enhancing the electro-active sites.
In special, The CV curves of RuO2/rGO nanocomposites (Fig. 4c) exhibit more rectangular shape compared with those of RuO2/C and RuO2/CNT nanocomposites (Fig. 4a and b), revealing that the ultrafine RuO2 particles supported on rGO have better electrochemical capacitance than that of RuO2 supported on other carbon-based substrates. The high capacitive performance of ultrafine RuO2/rGO compared with that of RuO2/C or RuO2/CNT probably because the rGO has high electrical conductivity and huge specific surface area. In addition, the better dispersion of ultrafine RuO2 particles on the surface of rGO substrates, as evinced by Fig. 3e and f, may also have positive effect on their capacitive behavior. Further, as displayed by Fig. 4e, f, g, and h for the galvanostatic chargedischarge curves of RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 at different current densities, the almost triangular shape indicates their ideal capacitive behavior because of the high degree of symmetry in charge and discharge. In accordance with the results obtained 16
from the CVs, the galvanostatic charge-discharge curves of ultrafine RuO2 particles supported on various carbon-based substrates (Fig. 4e, f, and g) exhibit longer discharge time compared with that of commercial RuO2 (Fig. 4h), suggesting the former has higher specific capacitance than latter due to the tiny size induced enhancement for specific surface area. Fig. 5a shows the specific capacitances of RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 samples at the current densities of 0.5, 1, 2, 5 and 10 A g-1, respectively, which are calculated based on the mass of RuO2. The specific capacitances measured at different current densities for ultrafine RuO2 particles supported on various carbon-based substrates and commercial RuO2 were listed in SM Table S2. Again, the ultrafine RuO2/rGO nanocomposites exhibit higher capacitances at all current densities (1099.6, 1022.5, 958.5, 907.2, 813.2 F g-1 at current density of 0.5, 1, 2, 5, and 10 A g-1, respectively) than those of RuO2 particles supported on other carbon-based substrates (879.1, 815.6, 701.3, 628.2, 520.2 F g-1 for RuO2/C and 966.8, 886.2, 827, 764.4, 672.1 F g-1 for RuO2/CNT at current density of 0.5, 1, 2, 5, and 10 A g-1, respectively) as well as commercial RuO2 (735.1, 662.7, 534.4, 442.8, 383.2 F g-1 at current density of 0.5, 1, 2, 5, and 10 A g-1, respectively). However, as manifested by the data in SM Table S2, all the supported ultrafine RuO2 particles show superior specific capacitances to the commercial RuO2 sample. The ultrafine size, which imparts high surface area for the redox reactions, the interaction between RuO2 and carbon-based substrates, the good conductivity of the substrates, and the uniform distribution of RuO2 particles over the substrates may account for the 17
superior specific capacitance for the supported samples.
The capacitances for 2000 cycles at constant current density of 1 A g-1 were used to evaluate the long-term cycling performance of electrode materials. The comparison of the long-term cycling performance was shown in the Fig. 5b. The specific capacitance retention ratios after 2000 cycles are 98.4% for RuO2/C, 98.0% for RuO2/CNT, 98.4% for RuO2/rGO, and 97.6% for commercial RuO2, respectively, indicating that the nanocomposites have excellent cycling stability. Fig. 5c shows the Nyquist plots of the RuO2/C, RuO2/CNT, RuO2/rGO and commercial RuO2 measured by electrochemical impedance spectroscopy (EIS), which illustrate the frequency response of the electrode/electrolyte system. In the low-frequency area, the line slope in the EIS plots indicates Warburg impedance, which is caused by ionic diffusion, and the more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor. As shown in the Nyquist plots, in the low-frequency area, the curves of ultrafine RuO2 particles supported on various carbon-based substrates are more vertical to the x axis than that of commercial RuO2, indicating a significant enhancement of the transport of electrolyte ions in supported RuO2 electrodes. In the high-frequency area, the interception of the curve in the real part Z’ indicates the equivalent series resistance (ESR), which represents the resistance caused by the transfer of ions across the interface of the electrolyte solution and electrode. Again, the supported ultrafine RuO2 particles display lower equivalent 18
series resistance (0.71, 0.87, and 0.59 for RuO2/C, RuO2/CNT, and RuO2/rGO, respectively) than that of commercial RuO2 (0.91 ). 4. Conclusions In summary, we have demonstrated a facile and economical aqueous route with environmental friendliness for the production of ultrafine RuO2 particles supported on various carbon-based substrates as excellent electrode materials for a supercapacitor. We first synthesized Ru nanoclusters with an average size of 1.7 nm in aqueous phase under the control of pH value lower than 4.9, and then loaded them on carbon-based substrates. Subsequently, we converted the Ru nanoclusters into ultrafine RuO2 particles on the same supports via a thermal treatment in air. The as-prepared RuO2/C, RuO2/CNT and RuO2/rGO nanocomposites for a supercapacitor adopting the H2SO4 electrolyte exhibit high specific capacitances of 879.1 F g-1, 966.8 F g-1 and 1099.6 F g-1, respectively, at a current density of 0.5 A g-1. The specific capacitance maintains 98.4% (for RuO2/C), 98.0% (for RuO2/CNT) and 98.4% (for RuO2/rGO) at current density of 1 A g-1 with good cycling stability. We believe that the remarkable simplicity of this synthetic strategy may provide a liable quantity production route to produce RuO2 as highly efficient electrode materials for a supercapacitor. Acknowledgments Financial support from the National Natural Science Foundation of China (Grant Nos.: 21173226, 21376247, 21573240, 21506225), Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences (COM2015A001), and the 19
Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No.: KGCX2-YW-341) is gratefully acknowledged. References 1. U.M. Patil, S.B. Kulkarni, V.S. Jamadade, C.D. Lokhande, Chemically synthesized hydrous RuO2 thin films for supercapacitor application, J. Alloy. Compd. 509 (2011) 1677. 2. B.K. Balan, H.D. Chaudhari, U.K. Kharul, S. Kurungot, Carbon nanofiber-RuO2-poly (benzimidazole) ternary hybrids for improved supercapacitor performance, RSC Adv. 3(2013) 2428. 3. M.R. Gao, Y.F. Xu, J. Jiang, S.H. Yu, Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices, Chem. Soc. Rev. 42 (2013) 2986. 4. X. Wu, Y. Zeng, H. Gao, J. Su, J. Liu, Z. Zhu, Template synthesis of hollow fusiform RuO2·xH2O nanostructure and its supercapacitor performance, J. Mater. Chem. A 1 (2013) 469. 5. J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Hang, D. Chen, G. Shen, Flexible asymmetric supercapacitors based upon Co9S8 nanorod//Co3O4@RuO2 nanosheet arrays on carbon cloth, ACS Nano 7 (2013) 5453. 6. S.N. Pusawale, P.R. Deshmukh, J.L. Gunjakar, C.D. Lokhande, SnO2–RuO2 composite films by chemical deposition for supercapacitor application, Mater. Chem. Phys. 139 (2013) 416. 7. A.-Y. Lo, Y. Jheng, T.-C. Huang, C.-M. Tseng, Study on RuO2/CMK-3/CNTs composites for high power and high energy density supercapacitor, Appl. Energ. 153 (2015) 15. 20
8. J.P. Zhang, P.J. Cygan, T.R. Jow, Hydrous ruthenium oxide as an electrode material for electrochemical capacitors, J. Electrochem. Soc. 142 (1995) 2699. 9. N. Soin, S.S. Roy, S.K. Mitra, T. Thundat, J.A. McLaughlin, Nanocrystalline ruthenium oxide dispersed Few Layered Graphene (FLG) nanoflakes as supercapacitor electrodes. J. Mater. Chem. 22 (2012) 14944. 10. L.Y. Chen, Y. Hou, J.L. Kang, A. Hirata, T. Fujita, M.W. Chen, Toward the theoretical capacitance of RuO2 reinforced by highly conductive nanoporous gold, Adv. Energy Mater. 3 (2013) 851. 11. A. Ananth, S. Dharaneedharan, M.S. Gandhi, M.-S. Heo, Y.S. Mok, Novel RuO2 nanosheets Facile synthesis, characterization and application, Chem. Eng. J. 223 (2013) 729. 12. J.-Y.
Kim,
K.-H.
Kim,
H.K.
Kim,
S.-H.
Park,
K.-Y.
Chung,
K.B.
Kim,
Nanosheet-assembled 3D nanoflowers of ruthenium oxide with superior rate performance for supercapacitor applications, RSC Adv. 4 (2014) 16115. 13. F. Yang, L. Zhang, A. Zuzuarregui, K. Gregorczyk, L. Li, M. Beltrán, C. Tollan, J. Brede, C. Rogero, A. Chuvilin, M. Knez, Functionalization of defect sites in graphene with RuO2 for high capacitive performance, ACS Appl. Mater. Interfaces 7 (2015) 20513. 14. K.S. Yang, C.H. Kim, B.-H. Kim, Preparation and electrochemical properties of RuO2-containing activated carbon nanofiber composites with hollow cores, Electrochim. Acta 174 (2015) 290. 15. P. Wang, Y. Xu, H. Liu, Y. Chen, J. Yang, Q. Tan, Carbon/carbon nanotube-supported RuO2 nanoparticles with a hollow interior as excellent electrodematerials for supercapacitors, 21
Nano Energy 15 (2015) 116. 16. F. Pico, J. Ibañez, M.A. Lillo-Rodenas, A. Linares-Solano, R.M. Rojas, J.M. Amarilla, J.M. Rojo, Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrodes, J. Power Sources 176 (2008) 417. 17. Z.-S. Wu, D.-W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li, H.-M. Cheng, Anchoring hydrous RuO2 on graphene sheets for high-performance electrochemical capacitors, Adv. Funct. Mater. 20 (2010) 3595. 18. J. Zhang, J. Jiang, H. Li, X.S. Zhao, A high-performance asymmetric supercapacitor fabricated with graphene-based electrodes, Energy Environ. Sci. 4 (2011) 4009. 19. H.-T. Fang, M. Liu, D.-W. Wang, X.-H. Ren, X. Sun, Fabrication and supercapacitive properties of a thick electrode of carbon nanotube-RuO2 core-shell hybrid material with a high RuO2 loading, Nano Energy 2 (2013) 1232. 20. Z. Zhou, Y. Zhu, Z. Wu, F. Lu, M. Jing, X. Ji, Amorphous RuO2 coated on carbon spheres as excellent electrode materials for supercapacitors, RSC Adv. 4 (2014) 6927. 21. Y. Takasu, S. Onoue, K. Kameyama, Y. Murakami, K. Yahikozawa, Preparation of ultrafine RuO2-IrO2-TiO2 oxide particles by a sol-gel process, Electrochim. Acta 39 (1994) 1993. 22. F.Z. Amir, V.H. Pham, J.H. Dickerson, Facile synthesis of ultra-small ruthenium oxide nanoparticles anchored on reduced graphene oxide nanosheets for high-performance supercapacitors, RSC Advances 5(2015) 67638. 23. Z. Hou, M. Li, M. Han, J. Zeng, S. Liao, Aqueous phase synthesis and characterizations of Pt nanoparticles by a modified citrate reduction method assisted by inorganic salt stabilization for PEMFCs, Electrochim. Acta 134(2014) 187. 22
24. L.S. Sarma, T.D. Lin, Y.-W. Tsai, J.M. Chen, B.J. Hwang, Carbon-supported Pt–Ru catalysts prepared by the Nafion stabilized alcohol-reduction method for application in direct methanol fuel cells, J. Power Sources 139(2005) 44. 25. C.-M. Chuang, C.-W. Huang, H. Teng, J.-M. Ting, Hydrothermally synthesized RuO2/Carbon nanofibers composites for use in high-rate supercapacitor electrodes, Compos. Sci. Technol. 72 (2012) 1524. 26. Y. Yang, Y. Liang, Y. Zhang, Z. Zhang, Z. Li, Z. Hu, Three-dimensional graphene hydrogel supported ultrafine RuO2 nanoparticles for supercapacitor electrodes. New. J. Chem. 39 (2015) 4035. 27. F. Pico, E. Morales, J.A. Fernandez, T.A. Centeno, J. Ibañez, R.M. Rojas, J.M. Amarilla, J.M. Rojo, Ruthenium oxide/carbon composites with microporous or mesoporous carbon as support and prepared by two procedures. A comparative study as supercapacitor electrodes, Electrochim. Acta 54 (2009) 2239. 28. J. Yang, J.Y. Lee, T.C. Deivaraj, H.-P. Too, Preparation and characterization of positively charged ruthenium nanoparticles, J. Colloid Interface Sci. 271 (2004) 308. 29. G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjapanayis, Borohydride reductions of metal ions. A new understanding of the chemistry leading to nanoscale particles of metals, borides, and metal borates, Langmuir 8 (1992) 771. 30. G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjapanayis, Borohydride Reduction of Cobalt Ions in Water. Chemistry Leading to Nanoscale Metal, Boride, or Borate Particles, Langmuir 9 (1993) 162. 31. A. Achari, K.K.R. Datta, M. De, V.P. Dravid, M. Eswaramoorthy, Amphiphilic 23
aminoclay-RGO hybrids: a simple strategy to disperse a high concentration of RGO in water, Nanoscale 5 (2013) 5316. 32. J.F. Moulder, J. Chastain, Handbook of x-ray photoelectron spectroscopy. A reference book of standard spectra for identification and interpretation of XPS data, Physical Electronics Division, Perkin-Elmer Corp., Eden Prairie, Minnesota, ©1992. 33. P. Wang, H. Liu, Q. Tan, J. Yang, Ruthenium oxide-based nanocomposites with high specific surface area and improved capacitance as a supercapacitor, RSC Adv. 4 (2014) 42839. 34. C. Wang, C. Hu, The capacitive performance of activated carbon–ruthenium oxide composites for supercapacitors: Effects of ultrasonic treatment in NaOH and annealing in air, Mater. Chem. Phys. 83 (2004) 289. 35. Z. Liu, J.Y. Lee, M. Han, W. Chen, L.M. Gan, Synthesis and characterization of PtRu/C catalysts from microemulsions and emulsions, J. Mater. Chem. 12 (2002) 2453. 36. X. Zhang, K.-Y. Chen, Water-in-oil microemulsion synthesis of platinum-ruthenium nanoparticles, their characterization and electrocatalytic properties, Chem. Mater. 15 (2003) 451. 37. F. Ye, H. Liu, J.H. Yang, H. Cao, J. Yang, Morphology and structure controlled synthesis of ruthenium nanoparticles in oleylamine, Daiton Trans. 42 (2013) 12309.
24
Fig. 1. (a) Schematic illustration to show the fabrication of ultrafine RuO2 particles supported on various carbon-based substrates as electrode materials for a supercapacitor; (b) Photos to show the aqueous RuCl3 solution before (I) and after addition of NaBH4 (II).
25
(a)
(b) Ru3p3/2
(d)
Intensity (a.u.)
Ru3p3/2 Ru3p1/2
(e)
4+
Ru 3p1/2
Ru3p1/2
4+
Ru 3p3/2
Intensity (a.u.)
(c)
4+
Ru 3p3/2
(f)
Ru3p3/2
4+
Ru 3p1/2
4+
Ru 3p3/2 4+
Ru 3p1/2
Ru3p1/2
450
460
470
480
490
450
460
470
480
490
Binding energy (eV)
Binding energy (eV)
Fig. 2. XPS spectra of as-prepared Ru/C (a,b), Ru/CNT (c,d), and Ru/rGO (e,f) before (a,c,e) and after thermal treatment in air (b,d,f), respectively.
26
Fig. 3. TEM images (a,c,e) and HRTEM images (b,d,f) of ultrafine RuO2/C (a,b), RuO2/CNT (c,d), and RuO2/rGO nanocomposites (e,f) converted from their corresponding Ru nanoclusters supported on various carbon-based substrates.
27
-1
10mV s -1 100mV s
-1
20mV s -1 200mV s
50mV s
-1
-1
-1
-1
1Ag
0.5 A g -1
2Ag -1
5Ag
10 A g
0.12
(a)
(e)
1.0
0.06 0.00
0.5
-0.06 0.12
0.0
(b)
Potential (V)
0.00
Current (A)
(f)
1.0
0.06
-0.06 0.12
(c) 0.06 0.00
0.5
0.0
(g)
1.0
0.5
-0.06 0.12
0.0
(d)
(h)
1.0
0.06 0.00
0.5
-0.06 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
200
Potential (V)
400
600
800
1000 1200 1400 1600
Time (s)
Fig. 4. Cyclic voltammograms of ultrafine RuO2/C (a), RuO2/CNT (b), RuO2/rGO (c), and commercial RuO2 (d) at different scan rates; galvanostatic charge-discharge curves of ultrafine RuO2/C (e), RuO2/CNT (f), RuO2/rGO (g), and commercial RuO2 (h) at different current densities.
28
-1
Specific capacity (F g )
-1
1200
0.5 A g
1Ag
-1 -1
2Ag
5Ag
-1
10 A g
-1
800 RuO2/C
400
RuO2/CNT RuO2/rGO
(a)
Commercial RuO2
0 0
20
40
60
80
100
1200
-1
Specific capacity (F g )
Cycle Number
800
RuO2/C
400
RuO2/CNT RuO2/rGO
(b)
Commercial RuO2
0 0
400
800
1200
1600
2000
Cycle Number 15 RuO2/C RuO2/CNT RuO2/rGO
-Z'' ()
10
Commercial RuO2
5
(c) 0 0
1
2
3
4
5
6
7
Z' ()
Fig. 5. Plots of specific capacitance for ultrafine RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 at different current densities (a); comparison of the specific capacitance for ultrafine RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 at current density of 1 A g-1 (b); comparison of Nyquist plots for ultrafine RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 (c).
29
S0013-4686(16)30364-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.02.089 EA 26706
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-12-2015 28-1-2016 15-2-2016
Please cite this article as: Pengfei Wang, Hui Liu, Yuxing Xu, Yunfa Chen, Jun Yang, Qiangqiang Tan, Supported ultrafine ruthenium oxides with specific capacitance up to 1099Fgminus1 for a supercapacitor, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.02.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Supported
ultrafine
ruthenium
oxides
with
specific
capacitance up to 1099 F g-1 for a supercapacitor
Pengfei Wanga,b, Hui Liua,c, Yuxing Xua, Yunfa Chena, Jun Yanga,*, Qiangqiang Tana,*
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing, China 100190. Tel: 86-10-8254 4915; Fax: 86-10-8254 4915; E-mail: [email protected] (J.Y.); Tel: 86-10-6252 9377; Fax: 86-10-8254 5008; E-mail: [email protected] (Q.T.) b
University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, China
c
Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences, Beijing
100190, China
1
Graphic abstract
50 nm
-1
RuO2/rGO
Specific capacity (F g )
1200 800 RuO2/C
400
RuO2/CNT RuO2/rGO Commercial RuO2
0 0
400
800
Cycle
1200
1600
2000
15 RuO2/C
RuO2/rGO -Z'' ()
RuO2/CNT
10
RuO2/rGO Commercial RuO2
5 0
10 nm
0
1
2
3
4
5
6
7
Z' ()
2
Highlights
Ru nanoclusters with an average size of 1.7 nm were prepared in aqueous phase.
The Ru nanoclusters were loaded on various carbon-based substrates.
The Ru clusters were converted into ultrafine RuO2 on the carbon-based substrates.
The supported ultrafine RuO2 exhibit high specific capacitances up to 1099 F g-1.
3
Abstract Reducing the particle size is a straightforward way to increase the specific surface area of ruthenium oxide, which usually translates to the high specific capacitance for a supercapacitor. Herein, we report a facile strategy to fabricate ultrafine ruthenium oxides supported on various carbon-based substrates (carbon powders, carbon nanotubes, or reduced graphene oxides) as excellent electrode materials for a supercapacitor. The novelty of this work lies in its synthetic approach, which involves an aqueous synthesis of ruthenium nanoclusters under the control of pH value, and an air oxidation-based conversion process. In particular, owing to their ultrafine particle size, the as-prepared carbon-, carbon nanotube-, or reduced graphene oxide-supported ruthenium oxides exhibit specific capacitance as high as 879.1 F g-1, 966.8 F g-1 and 1099.6 F g-1, respectively, for a supercapacitor at a current density of 0.5 A g-1. The specific capacitance maintains 98.4% (for carbon supports), 98.0% (for carbon nanotube supports) and 98.4% (for reduced graphene oxide supports) at current density of 1 A g-1 with good cycling stability. The remarkable simplicity and environmental friendliness of this synthesis may provide a liable quantity production route to produce ruthenium oxides as highly efficient electrode materials for a supercapacitor. Keywords: Ruthenium oxide; Supercapacitor; Ultrafine; Ruthenium nanoclusters; Specific capacitance
4
1. Introduction Fast-growing markets for portable power sources with instantaneous high-power density have stimulated great interest in research of supercapacitors, also known as ultracapacitors or electrochemical capacitors in recent years [17]. As a promising candidate electrode material used in supercapacitors, ruthenium oxide (RuO2) plays an important role in the applications of supercapacitors because of its proton-electron mixed conductive nature, remarkably high theoretical specific capacitance (up to 2200 F g-1), wide potential window, highly reversible redox reactions, good thermal stability, long cycle life, and high rate capability [815]. Increasing the specific surface areas is an effective way to improve the performance of RuO2 as electrode materials for supercapacitors, capable of providing more active centers for the multiple redox reactions, which usually translate to the high specific capacitance. In comparison with a number of strategies, e.g. depositing RuO2 films on substrates with a rough surface [1618], and coating a thin RuO2 film on high-surface-area materials [19,20], reducing the particle size is a straightforward approach to increase the specific surface area of RuO2 electrode. Smaller sized particles not only can shorten the diffusion distance but also can facilitate proton transport in the bulk of RuO2. The smaller the particle size, the higher the gravimetric capacitance and utilization efficiency. Therefore, one of the key points is to cut RuO2 materials down to a nanosize while maintain their high electrical and protonic conduction. Recently, a number of synthetic methods, e.g. sol-gel method [21], reduction or oxidation method [2224], and hydrothermal method [4,25,26] have 5
been developed to generate RuO2 or its composites with ultrafine sizes as electrode materials for supercapacitors. Typically, as reported by Rojo and coworkers, a nanometer-sized crystalline RuO2 anchored on carbon nanotubes (CNTs) using a supercritical fluid deposition method has shown a specific capacitance of up to 900 F g-1 [27]. However, in these strategies, additive agents such as surfactants, polymers, quaternary ammonium salts, or organic ligands are usually needed to protect the ultrafine RuO2 particles, and their remaining on the particle surface may result in abatement of active sites for the redox reactions, leading to low capacitive behavior of the electrode materials. For example, Hu and coworkers synthesized a three-dimensional porous framework using a one-step hydrothermal method [26], in which RuO2 particles with an average size of 23 nm are loaded uniformly on reduced graphene oxide (rGO) hydrogels. The as-prepared electrode materials only exhibit specific capacitance of 345 F g-1 at the mass loading of RuO2 of 15%. Alternatively, Amir et al. prepared rGO-RuO2 hybrid materials as supercapacitor electrodes by an in situ sol-gel deposition method [22]. Although their sol–gel route results in ultrafine, hydrated amorphous RuO2 particles with sizes of only 1.0–2.0 nm, which uniformly decorated the surfaces of rGO sheets, the obtained rGO–RuO2 electrodes exhibit a specific capacitance of 500 F g-1 in a 1 M H2SO4 electrolyte at a current density of 1.0 A g-1, far from the theoretical value. Herein, we report a facile, economical, and environmental friendly route for the production of ultrafine RuO2 particles supported on various carbon-based substrates (carbon powders, carbon nanotubes, or reduced graphene oxides) as excellent 6
electrode materials for a supercapacitor. This strategy involves the synthesis of Ru nanoclusters in aqueous phase under the control of pH value (no additional protective agents were used to stabilize the Ru nanoclusters), and subsequent loading on carbon-based substrates. Then the Ru nanoclusters are converted into ultrafine RuO2 particles on the same supports by a thermal treatment in air. We will demonstrate that the supported RuO2 particles exhibit high specific capacitance of up to 1099 F g-1 for a supercapacitor at a current density of 0.5 A g-1. The specific capacitance could maintain 98.0% or higher at current density of 1 A g-1 with good cycling stability, showing that the ultrafine sizes of RuO2 particles are favorable for the enhancement in their capacitive behavior. 2. Experimental 2.1. General Materials The chemical reagents, including ruthenium(III) chloride (RuCl3, Ru content 4555 wt%), hydrazine hydrate (N2H4·H2O, 80 wt%), polytetrafluoroethylene (PTFE, 60 wt% aqueous dispersion) and CYL2-60 carbon nanotube (CYL2-60 with diameter of 4060 nm, length of 12 μm, and special surface area of 100150 m2 g-1) from Aladdin Reagents, conductive carbon black EC-300J (cell grade with special surface area of 800 m2 g-1 and specific resistance of 0.52 ohm cm-1) from Shanghai Tengmin Industrial co., Ltd., Graphene Oxide (1 mg mL-1 aqueous dispersion) from Dalian Melone biotechnology Co., Ltd., Vulcan XC-72 carbon powders (XC-72C with BET surface area of 250 m2 g-1 and average particle size of 4050 nm) from Cabot 7
Corporation, sulphuric acid (H2SO4, 9598 wt%) and ethanol (99.5%) from Beijing Chemical Works, titanium gauze (30 mesh woven from 0.102 mm diameter wire) and commercial RuO2·xH2O (54% Ru) with product number of 346088 from J&K Scientific Ltd., and sodium borohydride (NaBH4, ≥98%) from Tianjin Fuchen chemical reagent, were used as received. Deionized water was distilled by a Milli-Q Ultrapure-water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious washing with de-ionized water before drying in an oven. 2.2. Synthesis of Ru nanoclusters The Ru nanoclusters were prepared in aqueous phase with the control of pH value lower than 4.9. Typically, 41.5 mg of RuCl3 was dissolved into 200 mL of DI water with vigorous stirring. Meanwhile, 37.9 mg of NaBH4 was dissolved into 100 mL of DI water. Freshly prepared NaBH4 aqueous solution was cautiously added dropwise to RuCl3 solution under stirred continuously, while a real-time measurement for the pH of the reaction mixture was carried out. The volume of the NaBH4 solution was carefully controlled to maintain the pH value of the reaction system to be always lower than 4.9. After stopping the addition of NaBH4 solution, the mixture was further stirred for 5 min to gain stable Ru hydrosol. 2.3. Loading the Ru nanoclusters on carbon-based substrates For the loading of the Ru nanoclusters on the Vulcan XC-72 carbon powders (C), CYL2-60 carbon nanotubes (CNT), or reduced graphene oxides (rGO), 28.2 mg of C
8
or CNT or 15 mL of aqueous graphene oxide dispersion was added into 250 mL of the as-prepared Ru hydrosol. The mixture was first ultrasonically treated for 10 min, and then magnetically stirred for 10 h. The precipitates (Ru/C, Ru/CNT, Ru/GO) were collected by centrifugation, washed with DI water thrice to remove the water-soluble ions and dried in air. In special, the Ru/GO was further treated in mixture of 11 L of hydrazine hydrate (80 wt%) and 175 L of ammonium hydroxide (28 wt%) in 95C for 1 h to transform the graphene oxide (GO) to reduced graphene oxide (rGO), and then washed and collected by centrifugation. 2.4. Conversion of Ru into RuO2 on the various carbon-based substrates These Ru/C, Ru/CNT, and Ru/rGO samples were heated at 150C in air for 2 h to convert the Ru into RuO2 for the generation of final ultrafine RuO2/C, RuO2/CNT, and RuO2/rGO nanocomposites. 2.5. Sample characterizations The pH value was measured using a pH meter (S220-USP/EP with pH-resolution of 0.001/0.01 and pH-relative accuracy of ±0.002) from Mettler Toledo Co., Ltd. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed on the JEOL JEM-2100 electron microscope operating at 200 kV with supplied software for automated electron tomography. For the TEM measurements, a drop of the nanoparticle solution was dispensed onto a 3-mm carbon-coated copper grid. Excessive solution was removed by an absorbent paper, and the sample was dried under vacuum at room temperature. The percentage of RuO2 on the 9
carbon-based substrates was measured using inductively coupled plasma atomic emission spectrophotometry (ICP-AES, Optima 5300DV, Perkin Elmer). X-ray photoelectron spectroscopy (XPS) was conducted on a VG ESCALAB MKII spectrometer. The Brunauer–Emmett–Teller (BET) surface areas of the as-synthesized samples were determined by the isothermal N2 adsorption/desorption method at 77 K in the range of 0.00–1.00 relative pressure via Micromeritics Adsorption Instrument attached by an automatic surface analyzer ( SSA-7300). Prior to measurements, the as-synthesized samples (20 mg) were degassed under vacuum at 110°C for 10h. 2.6. Fabrication of electrode and electrochemical measurements The working electrodes were fabricated by mixing the as-prepared RuO2/C, RuO2/CNT, RuO2/rGO nanocomposites or commercial RuO2 powders, carbon black, and polytetrafluoroethylene (PTFE) with a mass ratio of 85/10/5. The percentage of RuO2 in RuO2/C, RuO2/CNT and RuO2/rGO are 33.6%, 32.8% and 35.5%, respectively. These mixtures were dispersed in small amount of ethanol for the formation of paste, which was pressed into thin tablets and dried at 100C for 10 h in vacuum. Subsequently, the tablets were coated onto the titanium gauzes under the pressure of 10 MPa to obtain the electrode. The mass of as-prepared nanocomposites coated on the titanium gauze was 2 mg. All of the electrochemical measurements were carried out using a coin-type (CR2025) two electrode system. These coin cells consisted of two symmetrical working electrodes, polypropylene film (MPF with thickness of 45 μm from Nihon Kaiheiki Kogyo) as separator, and 1 M aqueous H2SO4 solution as electrolyte. The 10
titanium gauzes that coated with RuO2/C, RuO2/CNT, RuO2/rGO, or commercial RuO2 powders were used as the working electrodes. Cyclic voltammetry (CV), galvanostatic charge/discharge test, and electrochemical impedance spectroscopy (EIS) measurements were obtained using a CHI650 electrochemical workstation. The CV tests were done between 0 and 1.2 V at scan rates of 10, 20, 50, 100 and 200 mV s-1, respectively. EIS measurements were conducted over the frequency range from 100 kHz to 10 mHz at 0.5 V with 5 mV of ac perturbation. The specific capacitance of the electrodes can be calculated using an equation in term of C = It/ΔVm, where C is the specific capacitance (F g-1), I is the response current density (A g-1), t is the discharge time (s), ΔV is the potential (V) and m is the mass of the electroactive materials in the electrodes (g). 3. Results and discussion The overall strategy in this study was schematically depicted in Fig. 1a, which involves the synthesis Ru nanoclusters in aqueous solution by NaBH4 reduction of Ru3+ ion precursors under the control of pH value (step i), the subsequent loading the Ru clusters on the surface of carbon-based substrates (step ii), and the conversion of Ru clusters to ultrafine RuO2 particles via a air oxidation process on the same substrates (step iii). A notable advantage of this strategy is no additional capping agent on the surface of final ultrafine RuO2 products, capable of providing high surface area for the redox reactions taken place on the electrode materials.
11
The Ru nanoclusters were prepared in aqueous phase using a method we developed previously with slight modification [28]. NaBH4 is a common reducing agent used to prepare metal particles from metal ions [29,30], in this study, by means of the dropwise addition of aqueous NaBH4 solution to the aqueous RuCl3 solution, the Ru3+ ions are reduced following an overall reaction indicated below:
8Ru3+ + 3BH4 + 12H2O = 8Ru + 3B(OH)4 + 24H+
After the reaction, the dark aqueous RuCl3 solution (photo I in Fig. 1b) turns into a brown transparent hydrosol of Ru without any perceivable precipitation (photo II in Fig. 1b). At a pH value lower than 4.9, the Ru hydrosol prepared by NaBH4 reduction could be stabilized by surface adsorption of hydronium ions (H3O+) or other hydrated protons such as H5O2+ and H7O3+, which would impart positive charges to the Ru nanoclusters and stabilize them against agglomeration by electrostatic repulsion [28]. Fig. S1 in Supplementary Materials (SM) shows the TEM and HRTEM images of the as-prepared Ru nanoclusters together with a histogram for the particle size distribution. The average particle size of 1.70 nm is accompanied by a narrow size distribution with standard deviation 0.28 nm. By precipitating out the Ru nanoclusters from the hydrosol using a 1 M aqueous NaOH solution, the yield of nanoparticles was estimated to be ca. 95%. Centrifugation loss and cluster attachment to the container wall are the likely causes. After aging the mixture of Ru hydrosol and carbon-based substrates (carbon 12
powers, carbon nanotubes, or graphene oxides) under vigorous stirring for 6 h at room temperature, the Ru nanoclusters could be efficiently loaded on the carbon-based substrates, leading to the formation of Ru/C, Ru/CNT, and Ru/GO nanocomposites and leaving behind a clear aqueous phase. Subsequently, the Ru/GO was further treated with a mixture hydrazine hydrate and ammonium hydroxide in 95C to reduce the GO in the nanocomposites into rGO [31]. The representative TEM and HRTEM images in SM Fig. S2 reveal that the Ru nanoclusters are dispersed very well on the surface of various carbon-based substrates (SM Fig. S2a and b for Ru/C, SM Fig. S2c and d for Ru/CNT, and SM Fig. S2e and f for Ru/rGO, respectively). The Ru on the surface of various carbon-based substrates was examined by XPS to confirm its chemical state. Because the Ru 3d3/2 peak ovelaps with the C 1s peak in XPS spectra, preventing an unambiguous analysis of the nanoparticle surface, the Ru 3p XPS peak was used instead. As shown by Fig. 2a, c and e, the doublets at 462.2 and 484.45 eV for Ru/C (Fig. 2a), at 462.05 and 484.2 eV for Ru/CNT (Fig. 2c), and at 462.1 and 484.35 eV for Ru/rGO (Fig. 2e), respectively, are signatures of Ru metal in the zero valent state [15,32,33]. The XPS spectra of O 1s in the nanocomposites were also analyzed. As exhibited in SM Fig. S3a, c, and e for Ru/C, Ru/CNT, and Ru/rGO samples, respectively, the O 1s peaks of in these as-prepared nanocomposites have three overlapping components, including lattice oxygen O2- (530.2 eV), which is indiscernible in Ru/rGO sample (SM Fig. S3e), hydroxyl (532.0 eV), and bound H2O (533.9 eV), which are all consistent with the previous investigations [34]. The existence of bound H2O indicates that the Ru exists in a hydrate form. 13
However, after further calcination in air, as displayed by Fig. 2b, d, and f, the doublets at 463. 5 and 485.6 eV for Ru/C (Fig. 2b), at 463.5 and 485.5 eV for Ru/CNT (Fig. 2d), and at 463.3 and 485.5 eV Ru/rGO (Fig. 2f), respectively, which reflect the Ru at oxidized state, e.g. RuO2 [32,3537], can fit for the XPS spectra very well, indicating that the RuO2 is the dominant product upon thermal treatment of Ru/C, Ru/CNT and Ru/rGO in air at 150C. The transformation from Ru to RuO2 on various carbon-based substrates could be further suggested by the XPS analyses of O 1s after thermal treatment. The O 1s XPS spectra after thermal treatment in air were shown in SM Fig. S3b, d, and f. As displayed, after thermal treatment, the intensity of O 1s peaks at 530.2 eV (lattice oxygen O2-) is greatly enhanced, indicating the aforementioned oxidation transformation. The presence of bound H2O (the peak at 533.9 eV in XPS spectra) verifies that the obtained RuO2 remains a hydrate form after thermal treatment. In addition, the transformation from Ru to RuO2 on the carbon-based substrates could be further confirmed by XRD analyses. The XRD patterns of Ru/C, Ru/CNT, and Ru/rGO before and after calcination in air were shown in SM Fig. S4. As displayed, after calcination, the hexagonal Ru in Ru/C and Ru/CNT (SM Fig. S4a, b, and c, JCPDS Card File 060663) has been converted into tetragonal RuO2 phase supported on corresponding carbon-based substrates (SM Fig. S4d, e, and f, JCPDS Card File 401290). The mass percentages of RuO2 in the final RuO2/C, RuO2/CNT, and RuO2/rGO was determined using inductively coupled plasma atomic emission spectrophotometry 14
(ICP-AES), which are 33.6%, 32.8%, and 35.5%, respectively. The TEM (Fig. 3a, c and e) and HRTEM images (Fig. 3b, d and f) of the as-obtained RuO2/C, RuO2/CNT and RuO2/rGO were exhibited in Fig. 3, which manifest that the resulting ultrafine RuO2 in the final composite products have the comparable size and size distribution in comparison with those of the Ru nanoclusters. Moreover, the well dispersion of ultrafine RuO2 particles on various carbon-based substrates is also maintained after thermal treatment in air.
The as-prepared RuO2/C, RuO2/CNT and RuO2/rGO nanocomposites were examined as electrode materials for a supercapacitor and benchmarked against the commercial RuO2·xH2O from J&K Scientific Ltd. Fig. 4a, b, c, and d show the cyclic voltammograms (CVs) of the as-prepared ultrafine RuO2 particles supported on various carbon-based substrates obtained in 1 M H2SO4 with a potential range of 01.2 V at scan rates of 10, 20, 50, 100, and 200 mV s-1, respectively. For the same mass loading, the CV loops have different areas for different electrode materials, indicating different levels of stored charge. Observed from these tested CV curves, the more rectangle shapes suggest better capacitive behaviors for RuO2/C, RuO2/CNT and RuO2/rGO nanocomposites in comparison with that of commercial RuO2, which might be attributed to the large surface area of ultrafine size of RuO2 particles accessible for the multiple redox reactions. We conducted the nitrogen adsorption-desorption isotherms of RuO2 clusters supported on different carbon-based 15
substrates to evaluate their specific surface area (SM Fig. S5). The calculated BET (Brunauer-Emmett-Teller)
surface
area
of
RuO2/C,
RuO2/CNT,
RuO2/rGO,
commercial RuO2 were summarized in SM Table S1, and as exhibited, the specific surface areas of RuO2 clusters supported on carbon-based substrates (159.9, 149.6, and 131.5 m2 g-1 for RuO2/C, RuO2/CNT, and RuO2/rGO, respectively) are much higher than that of commercial RuO2 (10.2 m2 g-1). In addition, the relatively short diffusion distance for the charges in tiny RuO2 particles would also be helpful for enhancing the electro-active sites.
In special, The CV curves of RuO2/rGO nanocomposites (Fig. 4c) exhibit more rectangular shape compared with those of RuO2/C and RuO2/CNT nanocomposites (Fig. 4a and b), revealing that the ultrafine RuO2 particles supported on rGO have better electrochemical capacitance than that of RuO2 supported on other carbon-based substrates. The high capacitive performance of ultrafine RuO2/rGO compared with that of RuO2/C or RuO2/CNT probably because the rGO has high electrical conductivity and huge specific surface area. In addition, the better dispersion of ultrafine RuO2 particles on the surface of rGO substrates, as evinced by Fig. 3e and f, may also have positive effect on their capacitive behavior. Further, as displayed by Fig. 4e, f, g, and h for the galvanostatic chargedischarge curves of RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 at different current densities, the almost triangular shape indicates their ideal capacitive behavior because of the high degree of symmetry in charge and discharge. In accordance with the results obtained 16
from the CVs, the galvanostatic charge-discharge curves of ultrafine RuO2 particles supported on various carbon-based substrates (Fig. 4e, f, and g) exhibit longer discharge time compared with that of commercial RuO2 (Fig. 4h), suggesting the former has higher specific capacitance than latter due to the tiny size induced enhancement for specific surface area. Fig. 5a shows the specific capacitances of RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 samples at the current densities of 0.5, 1, 2, 5 and 10 A g-1, respectively, which are calculated based on the mass of RuO2. The specific capacitances measured at different current densities for ultrafine RuO2 particles supported on various carbon-based substrates and commercial RuO2 were listed in SM Table S2. Again, the ultrafine RuO2/rGO nanocomposites exhibit higher capacitances at all current densities (1099.6, 1022.5, 958.5, 907.2, 813.2 F g-1 at current density of 0.5, 1, 2, 5, and 10 A g-1, respectively) than those of RuO2 particles supported on other carbon-based substrates (879.1, 815.6, 701.3, 628.2, 520.2 F g-1 for RuO2/C and 966.8, 886.2, 827, 764.4, 672.1 F g-1 for RuO2/CNT at current density of 0.5, 1, 2, 5, and 10 A g-1, respectively) as well as commercial RuO2 (735.1, 662.7, 534.4, 442.8, 383.2 F g-1 at current density of 0.5, 1, 2, 5, and 10 A g-1, respectively). However, as manifested by the data in SM Table S2, all the supported ultrafine RuO2 particles show superior specific capacitances to the commercial RuO2 sample. The ultrafine size, which imparts high surface area for the redox reactions, the interaction between RuO2 and carbon-based substrates, the good conductivity of the substrates, and the uniform distribution of RuO2 particles over the substrates may account for the 17
superior specific capacitance for the supported samples.
The capacitances for 2000 cycles at constant current density of 1 A g-1 were used to evaluate the long-term cycling performance of electrode materials. The comparison of the long-term cycling performance was shown in the Fig. 5b. The specific capacitance retention ratios after 2000 cycles are 98.4% for RuO2/C, 98.0% for RuO2/CNT, 98.4% for RuO2/rGO, and 97.6% for commercial RuO2, respectively, indicating that the nanocomposites have excellent cycling stability. Fig. 5c shows the Nyquist plots of the RuO2/C, RuO2/CNT, RuO2/rGO and commercial RuO2 measured by electrochemical impedance spectroscopy (EIS), which illustrate the frequency response of the electrode/electrolyte system. In the low-frequency area, the line slope in the EIS plots indicates Warburg impedance, which is caused by ionic diffusion, and the more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor. As shown in the Nyquist plots, in the low-frequency area, the curves of ultrafine RuO2 particles supported on various carbon-based substrates are more vertical to the x axis than that of commercial RuO2, indicating a significant enhancement of the transport of electrolyte ions in supported RuO2 electrodes. In the high-frequency area, the interception of the curve in the real part Z’ indicates the equivalent series resistance (ESR), which represents the resistance caused by the transfer of ions across the interface of the electrolyte solution and electrode. Again, the supported ultrafine RuO2 particles display lower equivalent 18
series resistance (0.71, 0.87, and 0.59 for RuO2/C, RuO2/CNT, and RuO2/rGO, respectively) than that of commercial RuO2 (0.91 ). 4. Conclusions In summary, we have demonstrated a facile and economical aqueous route with environmental friendliness for the production of ultrafine RuO2 particles supported on various carbon-based substrates as excellent electrode materials for a supercapacitor. We first synthesized Ru nanoclusters with an average size of 1.7 nm in aqueous phase under the control of pH value lower than 4.9, and then loaded them on carbon-based substrates. Subsequently, we converted the Ru nanoclusters into ultrafine RuO2 particles on the same supports via a thermal treatment in air. The as-prepared RuO2/C, RuO2/CNT and RuO2/rGO nanocomposites for a supercapacitor adopting the H2SO4 electrolyte exhibit high specific capacitances of 879.1 F g-1, 966.8 F g-1 and 1099.6 F g-1, respectively, at a current density of 0.5 A g-1. The specific capacitance maintains 98.4% (for RuO2/C), 98.0% (for RuO2/CNT) and 98.4% (for RuO2/rGO) at current density of 1 A g-1 with good cycling stability. We believe that the remarkable simplicity of this synthetic strategy may provide a liable quantity production route to produce RuO2 as highly efficient electrode materials for a supercapacitor. Acknowledgments Financial support from the National Natural Science Foundation of China (Grant Nos.: 21173226, 21376247, 21573240, 21506225), Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences (COM2015A001), and the 19
Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No.: KGCX2-YW-341) is gratefully acknowledged. References 1. U.M. Patil, S.B. Kulkarni, V.S. Jamadade, C.D. Lokhande, Chemically synthesized hydrous RuO2 thin films for supercapacitor application, J. Alloy. Compd. 509 (2011) 1677. 2. B.K. Balan, H.D. Chaudhari, U.K. Kharul, S. Kurungot, Carbon nanofiber-RuO2-poly (benzimidazole) ternary hybrids for improved supercapacitor performance, RSC Adv. 3(2013) 2428. 3. M.R. Gao, Y.F. Xu, J. Jiang, S.H. Yu, Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices, Chem. Soc. Rev. 42 (2013) 2986. 4. X. Wu, Y. Zeng, H. Gao, J. Su, J. Liu, Z. Zhu, Template synthesis of hollow fusiform RuO2·xH2O nanostructure and its supercapacitor performance, J. Mater. Chem. A 1 (2013) 469. 5. J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Hang, D. Chen, G. Shen, Flexible asymmetric supercapacitors based upon Co9S8 nanorod//Co3O4@RuO2 nanosheet arrays on carbon cloth, ACS Nano 7 (2013) 5453. 6. S.N. Pusawale, P.R. Deshmukh, J.L. Gunjakar, C.D. Lokhande, SnO2–RuO2 composite films by chemical deposition for supercapacitor application, Mater. Chem. Phys. 139 (2013) 416. 7. A.-Y. Lo, Y. Jheng, T.-C. Huang, C.-M. Tseng, Study on RuO2/CMK-3/CNTs composites for high power and high energy density supercapacitor, Appl. Energ. 153 (2015) 15. 20
8. J.P. Zhang, P.J. Cygan, T.R. Jow, Hydrous ruthenium oxide as an electrode material for electrochemical capacitors, J. Electrochem. Soc. 142 (1995) 2699. 9. N. Soin, S.S. Roy, S.K. Mitra, T. Thundat, J.A. McLaughlin, Nanocrystalline ruthenium oxide dispersed Few Layered Graphene (FLG) nanoflakes as supercapacitor electrodes. J. Mater. Chem. 22 (2012) 14944. 10. L.Y. Chen, Y. Hou, J.L. Kang, A. Hirata, T. Fujita, M.W. Chen, Toward the theoretical capacitance of RuO2 reinforced by highly conductive nanoporous gold, Adv. Energy Mater. 3 (2013) 851. 11. A. Ananth, S. Dharaneedharan, M.S. Gandhi, M.-S. Heo, Y.S. Mok, Novel RuO2 nanosheets Facile synthesis, characterization and application, Chem. Eng. J. 223 (2013) 729. 12. J.-Y.
Kim,
K.-H.
Kim,
H.K.
Kim,
S.-H.
Park,
K.-Y.
Chung,
K.B.
Kim,
Nanosheet-assembled 3D nanoflowers of ruthenium oxide with superior rate performance for supercapacitor applications, RSC Adv. 4 (2014) 16115. 13. F. Yang, L. Zhang, A. Zuzuarregui, K. Gregorczyk, L. Li, M. Beltrán, C. Tollan, J. Brede, C. Rogero, A. Chuvilin, M. Knez, Functionalization of defect sites in graphene with RuO2 for high capacitive performance, ACS Appl. Mater. Interfaces 7 (2015) 20513. 14. K.S. Yang, C.H. Kim, B.-H. Kim, Preparation and electrochemical properties of RuO2-containing activated carbon nanofiber composites with hollow cores, Electrochim. Acta 174 (2015) 290. 15. P. Wang, Y. Xu, H. Liu, Y. Chen, J. Yang, Q. Tan, Carbon/carbon nanotube-supported RuO2 nanoparticles with a hollow interior as excellent electrodematerials for supercapacitors, 21
Nano Energy 15 (2015) 116. 16. F. Pico, J. Ibañez, M.A. Lillo-Rodenas, A. Linares-Solano, R.M. Rojas, J.M. Amarilla, J.M. Rojo, Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrodes, J. Power Sources 176 (2008) 417. 17. Z.-S. Wu, D.-W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li, H.-M. Cheng, Anchoring hydrous RuO2 on graphene sheets for high-performance electrochemical capacitors, Adv. Funct. Mater. 20 (2010) 3595. 18. J. Zhang, J. Jiang, H. Li, X.S. Zhao, A high-performance asymmetric supercapacitor fabricated with graphene-based electrodes, Energy Environ. Sci. 4 (2011) 4009. 19. H.-T. Fang, M. Liu, D.-W. Wang, X.-H. Ren, X. Sun, Fabrication and supercapacitive properties of a thick electrode of carbon nanotube-RuO2 core-shell hybrid material with a high RuO2 loading, Nano Energy 2 (2013) 1232. 20. Z. Zhou, Y. Zhu, Z. Wu, F. Lu, M. Jing, X. Ji, Amorphous RuO2 coated on carbon spheres as excellent electrode materials for supercapacitors, RSC Adv. 4 (2014) 6927. 21. Y. Takasu, S. Onoue, K. Kameyama, Y. Murakami, K. Yahikozawa, Preparation of ultrafine RuO2-IrO2-TiO2 oxide particles by a sol-gel process, Electrochim. Acta 39 (1994) 1993. 22. F.Z. Amir, V.H. Pham, J.H. Dickerson, Facile synthesis of ultra-small ruthenium oxide nanoparticles anchored on reduced graphene oxide nanosheets for high-performance supercapacitors, RSC Advances 5(2015) 67638. 23. Z. Hou, M. Li, M. Han, J. Zeng, S. Liao, Aqueous phase synthesis and characterizations of Pt nanoparticles by a modified citrate reduction method assisted by inorganic salt stabilization for PEMFCs, Electrochim. Acta 134(2014) 187. 22
24. L.S. Sarma, T.D. Lin, Y.-W. Tsai, J.M. Chen, B.J. Hwang, Carbon-supported Pt–Ru catalysts prepared by the Nafion stabilized alcohol-reduction method for application in direct methanol fuel cells, J. Power Sources 139(2005) 44. 25. C.-M. Chuang, C.-W. Huang, H. Teng, J.-M. Ting, Hydrothermally synthesized RuO2/Carbon nanofibers composites for use in high-rate supercapacitor electrodes, Compos. Sci. Technol. 72 (2012) 1524. 26. Y. Yang, Y. Liang, Y. Zhang, Z. Zhang, Z. Li, Z. Hu, Three-dimensional graphene hydrogel supported ultrafine RuO2 nanoparticles for supercapacitor electrodes. New. J. Chem. 39 (2015) 4035. 27. F. Pico, E. Morales, J.A. Fernandez, T.A. Centeno, J. Ibañez, R.M. Rojas, J.M. Amarilla, J.M. Rojo, Ruthenium oxide/carbon composites with microporous or mesoporous carbon as support and prepared by two procedures. A comparative study as supercapacitor electrodes, Electrochim. Acta 54 (2009) 2239. 28. J. Yang, J.Y. Lee, T.C. Deivaraj, H.-P. Too, Preparation and characterization of positively charged ruthenium nanoparticles, J. Colloid Interface Sci. 271 (2004) 308. 29. G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjapanayis, Borohydride reductions of metal ions. A new understanding of the chemistry leading to nanoscale particles of metals, borides, and metal borates, Langmuir 8 (1992) 771. 30. G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjapanayis, Borohydride Reduction of Cobalt Ions in Water. Chemistry Leading to Nanoscale Metal, Boride, or Borate Particles, Langmuir 9 (1993) 162. 31. A. Achari, K.K.R. Datta, M. De, V.P. Dravid, M. Eswaramoorthy, Amphiphilic 23
aminoclay-RGO hybrids: a simple strategy to disperse a high concentration of RGO in water, Nanoscale 5 (2013) 5316. 32. J.F. Moulder, J. Chastain, Handbook of x-ray photoelectron spectroscopy. A reference book of standard spectra for identification and interpretation of XPS data, Physical Electronics Division, Perkin-Elmer Corp., Eden Prairie, Minnesota, ©1992. 33. P. Wang, H. Liu, Q. Tan, J. Yang, Ruthenium oxide-based nanocomposites with high specific surface area and improved capacitance as a supercapacitor, RSC Adv. 4 (2014) 42839. 34. C. Wang, C. Hu, The capacitive performance of activated carbon–ruthenium oxide composites for supercapacitors: Effects of ultrasonic treatment in NaOH and annealing in air, Mater. Chem. Phys. 83 (2004) 289. 35. Z. Liu, J.Y. Lee, M. Han, W. Chen, L.M. Gan, Synthesis and characterization of PtRu/C catalysts from microemulsions and emulsions, J. Mater. Chem. 12 (2002) 2453. 36. X. Zhang, K.-Y. Chen, Water-in-oil microemulsion synthesis of platinum-ruthenium nanoparticles, their characterization and electrocatalytic properties, Chem. Mater. 15 (2003) 451. 37. F. Ye, H. Liu, J.H. Yang, H. Cao, J. Yang, Morphology and structure controlled synthesis of ruthenium nanoparticles in oleylamine, Daiton Trans. 42 (2013) 12309.
24
Fig. 1. (a) Schematic illustration to show the fabrication of ultrafine RuO2 particles supported on various carbon-based substrates as electrode materials for a supercapacitor; (b) Photos to show the aqueous RuCl3 solution before (I) and after addition of NaBH4 (II).
25
(a)
(b) Ru3p3/2
(d)
Intensity (a.u.)
Ru3p3/2 Ru3p1/2
(e)
4+
Ru 3p1/2
Ru3p1/2
4+
Ru 3p3/2
Intensity (a.u.)
(c)
4+
Ru 3p3/2
(f)
Ru3p3/2
4+
Ru 3p1/2
4+
Ru 3p3/2 4+
Ru 3p1/2
Ru3p1/2
450
460
470
480
490
450
460
470
480
490
Binding energy (eV)
Binding energy (eV)
Fig. 2. XPS spectra of as-prepared Ru/C (a,b), Ru/CNT (c,d), and Ru/rGO (e,f) before (a,c,e) and after thermal treatment in air (b,d,f), respectively.
26
Fig. 3. TEM images (a,c,e) and HRTEM images (b,d,f) of ultrafine RuO2/C (a,b), RuO2/CNT (c,d), and RuO2/rGO nanocomposites (e,f) converted from their corresponding Ru nanoclusters supported on various carbon-based substrates.
27
-1
10mV s -1 100mV s
-1
20mV s -1 200mV s
50mV s
-1
-1
-1
-1
1Ag
0.5 A g -1
2Ag -1
5Ag
10 A g
0.12
(a)
(e)
1.0
0.06 0.00
0.5
-0.06 0.12
0.0
(b)
Potential (V)
0.00
Current (A)
(f)
1.0
0.06
-0.06 0.12
(c) 0.06 0.00
0.5
0.0
(g)
1.0
0.5
-0.06 0.12
0.0
(d)
(h)
1.0
0.06 0.00
0.5
-0.06 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
200
Potential (V)
400
600
800
1000 1200 1400 1600
Time (s)
Fig. 4. Cyclic voltammograms of ultrafine RuO2/C (a), RuO2/CNT (b), RuO2/rGO (c), and commercial RuO2 (d) at different scan rates; galvanostatic charge-discharge curves of ultrafine RuO2/C (e), RuO2/CNT (f), RuO2/rGO (g), and commercial RuO2 (h) at different current densities.
28
-1
Specific capacity (F g )
-1
1200
0.5 A g
1Ag
-1 -1
2Ag
5Ag
-1
10 A g
-1
800 RuO2/C
400
RuO2/CNT RuO2/rGO
(a)
Commercial RuO2
0 0
20
40
60
80
100
1200
-1
Specific capacity (F g )
Cycle Number
800
RuO2/C
400
RuO2/CNT RuO2/rGO
(b)
Commercial RuO2
0 0
400
800
1200
1600
2000
Cycle Number 15 RuO2/C RuO2/CNT RuO2/rGO
-Z'' ()
10
Commercial RuO2
5
(c) 0 0
1
2
3
4
5
6
7
Z' ()
Fig. 5. Plots of specific capacitance for ultrafine RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 at different current densities (a); comparison of the specific capacitance for ultrafine RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 at current density of 1 A g-1 (b); comparison of Nyquist plots for ultrafine RuO2/C, RuO2/CNT, RuO2/rGO, and commercial RuO2 (c).
29