Article pubs.acs.org/JPCC
Unveiling the Origin of Unusual Pseudocapacitance of RuO2·nH2O from Its Hierarchical Nanostructure by Small-Angle X‑ray Scattering Noboru Yoshida,† Yuki Yamada,† Shin-ichi Nishimura,† Yojiro Oba,‡ Masato Ohnuma,‡ and Atsuo Yamada*,† †
Department of Chemical System Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Quantum Beam Center, National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan
‡
S Supporting Information *
ABSTRACT: Hydrous ruthenium oxide (RuO2·nH2O) has inherent proton−electron mixed-conductive nature and offers huge pseudocapacitance (>700 F g−1), having attracted the attention of many capacitor engineers. However, the origin of the anomalous pseudocapacitance, exhibiting a strong maximum at a specific narrow optimum annealing temperature of ca. 150 °C, has yet to be understood. Here we show a longawaited explanation for this mystery based on its hierarchical nanostructure unveiled by small-angle X-ray scattering (SAXS). The striking contrast in X-ray atomic scattering factors enables SAXS to exclusively probe heavy RuO2 in subnanoto nanoscale, dispersed in confined water. We demonstrate that the surface area of the first aggregate of subnano primary RuO2 particles dominates the accessible number of proton and hence pseudocapacitance, providing critical insights into the nanoarchitectural design of high-performance electrodes for electrochemical capacitors.
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INTRODUCTION Electrochemical energy storage is a key technology for the broad implementation of a sustainable energy-saving society with electric vehicles and smart grids.1 Major requirements for energy-storage devices are high energy and power densities. Even among cutting-edge technologies, there are no devices that can satisfy both of these requirements. Although lithium ion batteries are among the most widespread energy-storage devices that provide high energy density, their power density is limited due to sluggish solid-state lithium diffusion in the electrodes. To compensate for their low power density, electrochemical capacitors (ECs), which employ rapid ion adsorption/desorption at electrodes, have garnered increasing attention as a supplementary or alternative energy-storage device that offers high power density.2−7 Considerable effort has been made to significantly increase the energy density of ECs without sacrificing their intrinsic high power density to make them suitable for potential applications in power sources for hybrid electric vehicles and backup systems for transmission grids. One strategy to enhance the energy density is to utilize rapid Faradaic reactions in combination with simple ion adsorption for charge storage.8−11 Such ECs that employ Faradaic reactions are referred to as pseudocapacitors or redoxcapacitors. Currently, there are mainly two types of electrode materials used for pseudocapacitors: (i) transition metal oxides (e.g., RuO2, IrO2, and MnO2) and (ii) conductive polymers (e.g., polyaniline, polypyrrole, and polythiophene). The specific capacitance of these pseudocapacitive materials far exceeds those of conventional carbon-based electrodes (ca. 200 F g−1), © XXXX American Chemical Society
which has led to extensive R&D on the pseudocapacitance observed for various electrode materials. RuO2-based materials are some of the most promising and interesting electrodes for pseudocapacitors12−21 that utilizes the fast redox reaction of Ru accompanied by the electroadsorption of protons:2,14,16 RuOa (OH)b + δ H+ + δ e− ↔ RuOa − δ (OH)b + δ
(1)
The pseudocapacitive behavior of anhydrous, crystalline RuO2 was first reported by Trasatti and Buzzanca in 1971,12 which initiated further research to understand the charge-storage mechanism and enhance the specific capacitance. In 1995, an important innovative breakthrough was achieved by Zheng et al., 18,22 who reported that hydrous ruthenium oxide (RuO2·nH2O) synthesized by a sol−gel method exhibits a significantly high specific capacitance of over 700 F g−1, which is much higher than that of anhydrous RuO2 (ca. 400 F g−1). They further reported that the specific capacitance of RuO2·nH2O is strongly dependent on the annealing temperature. The highest specific capacitance of 720 F g−1 was obtained for a confined water content of n ∼ 0.5 by annealing at around 150 °C, while annealing below or above 150 °C results in smaller specific capacitance.18,22 McKeown et al. and Dmowski et al. pointed out that the varying pseudocapacitance is due to changes in the proton−electron mixed conduction in Received: April 6, 2013 Revised: May 14, 2013
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dx.doi.org/10.1021/jp403402k | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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RuO 2 ·nH 2 O with the confined water content n. 23,24 RuO2·nH2O is composed of rutile-like RuO2 nanocrystals dispersed in confined water, where RuO2 and confined water facilitate electron and proton conduction, respectively, and their percolation networks are optimized by annealing at 150 °C.24 The importance of electron and proton conduction to achieve maximum capacitance has been demonstrated by Fu et al.,25 Hu et al.,26 and Sugimoto et al.27,28 Despite vigorous studies on the nature of RuO2·nH2O, the origin of its pseudocapacitance is yet to be fully explained. For instance, the contribution of doublelayer capacitance and Faradaic reaction to the overall capacitance remains unclear. Behind this situation lies the difficulty in quantitative analysis of the size and surface area of RuO2 nanodomains, due to the presence of confined water. There are several techniques that are employed to observe the nanostructure of materials. The Brunauer, Emmett, and Teller (BET) method,29 based on nitrogen adsorption isotherms, is generally used to evaluate the surface area and morphology of electrodes for double-layer capacitors. However, the presence of confined water in RuO2·nH2O hinders the adsorption of nitrogen on each RuO2 domain; therefore, the BET method cannot be used to provide the true surface area of RuO2.30 Although electron microscopy is one of the best methods to observe the size and shape of materials at the nanoscale, it is not a simple task to obtain representative nanostructural parameters, due to the limitations of the observation area and to damage caused by the electron beam. In contrast, small-angle X-ray scattering (SAXS) is a powerful technique to nondestructively determine the average and representative sizes of nanomaterials.31 More importantly, the striking contrast of the X-ray atomic scattering factors for RuO2 and H2O can be exploited to easily differentiate RuO2 nanodomains from surrounding confined water, which validates the use of SAXS for the analysis of RuO2·nH2O. However, there have been no studies on the application of SAXS to RuO2·nH2O, and the influence of the nanostructure of RuO2·nH2O on the pseudocapacitance remains unclear. In the present work, we determine the origin of pseudocapacitance for RuO2·nH2O by quantitative evaluation of the surface area of RuO2 nanodomains using SAXS. Samples of RuO2·nH2O with controlled value of n were prepared by changing annealing temperatures, and the relation between their nanostructures and specific capacitances was investigated. We demonstrate that the surface area and aggregate structure of the RuO2 nanodomains are dependent on the annealing temperature and closely correlated with the observed capacitance. In addition, detailed analysis on the origin of the pseudocapacitance has led to quantitative information regarding the contribution of double-layer capacitance and Faradaic reaction to the overall specific capacitance.
Figure 1. (a) Confined water content (n) and (b) observed gravimetric capacitance (Cexp) of RuO2·nH2O annealed at various temperatures. The original sample is plotted at an annealing temperature of 25 °C. The capacitance was measured using constant-current charge−discharge measurements in the potential range of 0−1.0 V (vs Ag/AgCl) at 0.1 and 1 A g−1.
dm−3 H2SO4 aqueous solution as the electrolyte. The detailed conditions for electrochemical characterization are described in the Supporting Information. Figure 1b shows the gravimetric capacitance of RuO2·nH2O annealed at various temperatures, as determined by constantcurrent charge−discharge measurements (Supporting Information, Figure S2), where the weight of the active material is based on the RuO2 content with exclusion of confined water. Under a charge−discharge operation at 1 A g−1, a maximum capacitance of 722 F g−1 was obtained for the sample annealed at 130 °C, whereas annealing above or below 130 °C resulted in smaller capacitance, as determined either by charge−discharge or by cyclic voltammetry measurements (Supporting Information, Figure S3). This is the same trend as that reported by Zheng et al.18,22 The amount of confined water influences proton mobility, which kinetically influences the observed specific capacitance.23,24 However, even under lower rate operation at 0.1 A g−1, the same significant decrease of specific capacitance was observed for samples annealed above 130 °C, which indicates that there is a nonkinetic factor that dominates the specific capacitance. Considering that the origin of the pseudocapacitance is a surface phenomenon, we postulated that the surface area of RuO2 nanodomains that are accessible by protons should be a determining factor of the specific capacitance. To gain a basic insight into the nanostructure of RuO2·nH2O, scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray diffraction (XRD) were conducted. SEM images (Supporting Information, Figure S4) indicated no clear difference in the morphology of RuO2·nH2O samples annealed at various temperatures. A TEM image (Figure 2a) of the sample annealed at 110 °C shows nano- or subnanodomains of
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RESULTS AND DISCUSSION RuO2·nH2O powders (Wako Pure Chemical Industries, Ltd.) were annealed in air at various temperatures for 6 h. Figure 1a shows the confined water content n for RuO2·nH2O annealed at various temperatures, which was determined from the weight loss at 450 °C measured with thermogravimetry (TG) (Supporting Information, Figure S1). The confined water content of the original RuO2·nH2O sample is n = 1.66 (plotted for an annealing temperature of 25 °C), which monotonically decreases with increasing annealing temperature due to the loss of confined water. A three-electrode beaker-type cell was constructed for electrochemical characterization with 0.5 mol B
dx.doi.org/10.1021/jp403402k | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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scattering source (i.e., RuO2 in this case).31 The shoulder is gradually shifted to the low-q region with an increase in the annealing temperature, which suggests that the average size of RuO2 primary particles increases with the annealing temperature; this is consistent with the XRD patterns and TEM observations. A region with weak q dependence is observed adjacent to the shoulder. In this area, the scattering intensity can be described using the Guinier approximation:31−33 ⎛ R 2 ⎞ g 2 q ⎟⎟ I(q) ≈ I(0) exp⎜⎜ − 3 ⎝ ⎠
(2)
Consequently, the average size of RuO2 domains can be obtained from the gradient of ln(intensity) vs q2. Assuming that the RuO2 domains are spherical, based on TEM observations, the radius of the spherical particles R, can be expressed as
Figure 2. TEM images and SAED patterns (insets) for RuO2·nH2O annealed at (a) 110 °C, (b) 160 °C, (c) 200 °C, and (d) 300 °C.
RuO2 (dark part) dispersed in unobservable confined water. As the annealing temperature is increased (Figure 2b−d), the RuO2 domains grow and finally form spherical aggregates with diameters of ca. 10 nm, as observed for the sample annealed at 300 °C. The SAED patterns shown as insets in Figure 2 reveal Debye rings and bright spots that gradually appear with increasing annealing temperature, which suggests the growth of RuO2 crystalline domains. In addition, the uniform dispersion and growth of RuO2 were clearly confirmed from dark-field TEM images (Supporting Information, Figure S5). XRD patterns (Supporting Information, Figure S6) showed a set of diffraction peaks that could be fully indexed with the rutile structure, and the intensity of these peaks increased with the annealing temperature, which indicates the crystal growth of rutile-RuO2 during annealing. SAXS profiles were measured for a quantitative evaluation of the size of RuO2 nanodomains in the presence of surrounding confined water and are presented in Figure 3, where q is the
R=
⎛ 5 ⎞0.5 ⎜ ⎟ R ⎝3⎠ g
(3) 2
Figure 4 shows ln(intensity) vs q plots generated from the SAXS profiles, after the contribution of large powders is
Figure 4. ln(intensity) vs q2 plots (Guinier plot) generated from SAXS profiles of RuO2·nH2O annealed at various temperatures in the range of (a) q2 < 4 nm−2 and (b) q2 < 40 nm−2. The contribution of large powders is subtracted as a background using a q−4 scattering function. The original sample and sample annealed and 110 °C show two Guinier regions (i.e., linear regions), originating from primary particles and aggregates of primary particles.
subtracted as a background, using q−4 as the scattering function based on Porod’s law.31 The estimated representative diameters of RuO2 domains are summarized in Figure 5a. For the original (25 °C) sample and sample annealed at 110 °C, the ln(intensity) vs q2 plot shows two linear regions at q2 < 1 nm−2 and 10 nm−2 < q2 < 30 nm−2, which suggests that there are two size regimes for RuO2 (i.e., primary particles and aggregates of primary particles). In contrast, the samples annealed above 130 °C show one linear region at q2 < 2 nm−2, which indicates strong aggregation of RuO2 primary particles with increasing annealing temperature. To validate the above analyses, we investigated the aggregation of RuO2 primary particles based on another kind of information in SAXS profiles; the intensity in the high-q region represents the surface area of primary RuO2 particles. In order to evaluate the difference between the surface area of primary particles and that of aggregates obtained from Guinier approximation, excess interface parameter (PEI) is defined
Figure 3. SAXS profiles for RuO2·nH2O annealed at various temperatures in the q range from 0.08 to 10 nm−1.
magnitude of momentum transfer and is equal to 4π(sin θ)/λ, θ is half the scattering angle, and λ is the wavelength of the incident X-ray (Mo Kα, λ = 0.0709 nm). The range of measured q was from 0.08 to 10 nm−1. In the low-q region, the scattering intensity decreases linearly with the relationship q−4, which is the scattering from large powder grains and should be subtracted for further analysis.31 A shoulder is observed in a given q region, which corresponds to the size of the primary C
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where R is the representative radius of RuO2 (m), and ρ is the specific gravity of RuO2 (g m−3). The ρg value for crystalline RuO2 (6.97 × 106 g m−3) is used, because the local structural analysis previously determined using the pair distribution function with total neutron scattering showed that RuO2 has an ordered rutile-like structure, even at low annealing temperature.24 The estimated SSAXS values are summarized in Figure 5c, together with the observed specific capacitance (Cexp). The surface areas of primary particles and aggregates are shown for both the original sample and the sample annealed at 110 °C. The Cexp plot below 110 °C follows the trend of SSAXS for RuO2 aggregates, which strongly suggests that the pseudocapacitance of samples annealed at
