Hydrous RuO2 Research Articles

Synthesis of Amorphous Hydrous Ruthenium Dioxide for Electrochemical Capacitors

Abstract Amorphous hydrous ruthenium dioxide RuO2·xH2O was synthesized by a modified neutralization reaction and heat treatment process, and electrochemical performances of the synthesized product were investigated using the graphite plate with high conductivity as current collector. In the experiment a home-made sprayer and sodium dodecyle sulfate (SDS) were used as an assistant technique and dispersive agent, respectively. Results indicate that amorphous and high performance electrode materials with great specific area of 218 m2/g and fluffy and heavy black color morphology can be obtained after the precursor of the synthesized product is annealed at 175 °C. Cyclic voltammetry (CV) results show the prepared product is featured with good average specific capacitance (995 F/g at 1 mV/s) and excellent electrochemical rate. It is further proved by electrochemical impedance spectra (EIS) test that the prepared product has lower equivalent serial resistance of −25 mΩ and good electrochemical rate performance.

Screen-Printable and Flexible RuO2 Nanoparticle-Decorated PEDOT:PSS/Graphene Nanocomposite with Enhanced Electrical and Electrochemical Performances for High-Capacity Supercapacitor.

PSS), graphene, and hydrous ruthenium(IV) oxide (RuO2) nanoparticles for use in high-performance electrochemical capacitors. As a polymeric binder, PSS allows stable dispersion of graphene and hydrous RuO2 nanoparticles (NPs) in an aqueous PSS system through electrostatic stabilization, ensuring better utilization of the three components. Additional PSS molecules were added to optimize the solution viscosity to obtain screen-printed electrodes. The effects of graphene and hydrous RuO2 NPs on the electrical and electrochemical properties of PSS were systematically investigated. The graphene sheets greatly enhanced the charge-transport properties, such as the doping level and conjugation length, through strong π-π stacking interactions with the PEDOT structure. The hydrous RuO2 NPs anchored to the PSS/graphene surfaces facilitated redox reactions with the surrounding electrolyte, and significantly enhanced the specific capacitance of the electrode materials. The resulting RuO2/PEDOT:PSS/graphene electrode with a thickness of ∼5 μm exhibited high conductivity (1570 S cm(-1)), a large specific capacitance (820 F g(-1)), and good cycling stability (81.5% after 1000 cycles).

Atomic Layer Deposition of Metal Oxides on Activated Carbons for High Energy Density and High Performance Supercapacitors

Nanostructured ceramic oxides have shown to increase specific capacitance of supercapacitors via Faradaic redox reactions [1]. Many research and development efforts have focused on transition metal oxides as alternatives to expensive hydrous RuO2. Examples of these activities include hydrothermal Co3O4 on carbon fiber papers [2], Co3O4/MnO2 nanowires [3], electrodeposition of MnO2 on carbon nanotube (CNT)-textile [4], wet chemistry of MnO2 in CNT [5], hydrothermal Ni(OH)2 on graphene [6], electrochemical anodization of NiO-TiO2 nanotubes [7], and solution synthesis of SnO2-graphenen composites [8]. More recently, atomic layer deposition (ALD) has been used to deposit V2O5 on CNT [9] and amorphous TiO2 on graphene [10]. While these studies have shown improved performance to various degrees, they either involve the use of expensive materials, such as CNT or graphene, or processes that are not easy to implement for large scale production. Here we present an alternative and promising method to prepare high energy density and extremely high performance active materials for supercapacitors (or pseudocapacitors) by direct ALD oxide coatings onto high surface area, activated carbons (AC). Figure 1 shows discharge voltage profiles of cells containing ALD oxide coated electrodes, overlaid with a control cell without oxide coating. The cells were built in symmetric configuration with a conventional Li-ion electrolyte. The cells were cycled between 0 to 2.5V under a constant current of 0.15A/g at room temperature. Both NiO and V2O5 coated cells delivered a specific capacitance of 114 F/g (based on single electrode active weight), representing > 50% increase over the control cell (75 F/g). The enhanced specific capacitance of the ALD oxide coated cells is a result of pseudocapacitive contribution from the ALD oxides. Figure 2 shows rate capability test of the symmetric supercapacitor cells. The discharge capacitance is normalized to that of a low current density (0.15A/g). The ALD oxide coated cells showed excellent rate capability, with > 87% and 92% capacitance retention for V2O5 and NiO coated cells, respectively, when discharged at 15 A/g current. Both oxide coated cells showed superior rate capability performance to that of the control cell. The remarkable rate capability demonstrated by the ALD oxide coated cells is attributed to nano-sized oxide coatings afforded by ALD, which results in facile surface redox reactions. Figure 3 shows cycle life test where the symmetric supercapacitor cells were cycled between 0 to 2.5V at a constant current density of 1.5 A/g. The ALD NiO cell showed enhanced specific capacitance while maintaining extremely stable, long cycle performance with > 90% capacitance retention after 30,000 cycles. Such extremely stable cycle life suggests that the nano-structured oxides enabled by ALD are stress/stain free during charge and discharge cycles involving Faradaic charge transfer (redox) reactions. We will show that ALD enabled oxide coatings on AC allow for high operating voltages, leading to increased capacitance and energy density while demonstrating superior electrochemical performance stability to samples without ALD oxide coating. Our results in this study suggest pseudocapacitive oxide coating via ALD on AC is a promising material processing methodology for supercapacitor energy storage devices with significantly enhanced specific capacitance, energy density and electrochemical stability while maintaining exceptionally high rate capability and extremely stable cycle life. Such methodology is scalable economically when high throughput and high volume ALD processes are implemented. References Avinash Balakrishnan and K. R. V. Subramanian, Nanostructured Ceramic Oxides for Supercapacitor Applications, CRC Press, New York (2014). Lei Yang, Shuang Cheng, Yong Ding, Xingbao Zhu, Zhong Lin Wang, and Meilin Liu, Nano Lett. 12, 321 (2012). Jinping Liu, Jian Jiang, Chuanwei Cheng, Hongxing Li, Jixuan Zhang, Hao Gong, and Hong Jin Fan, Adv. Mater. 23, 2076 (2011). Liangbing Hu, Wei Chen, Xing Xie, Nian Liu, Yuan Yang, Hui Wu, Yan Yao, Mauro Pasta, Husam N. Alshareef, and Yi Cui, ACS Nano, 5(11), 8904 (2011). Wei Chen, Zhongli Fan, Lin Gu, Xinhe Bao and Chunlei Wang, Chem. Commun., 46, 3905 (2010). Hailiang Wang, Hernan Sanchez Casalongue, Yongye Liang and Hongjie Dai, J. Am. Chem. Soc., 132 (21), 7472 (2010). Jae-Hun Kim, Kai Zhu, Yanfa Yan, Craig L. Perkins, and Arthur J. Frank, Nano Lett. 10, 4099 (2010). Hun Park and Tae Hee Han, Bull. Korean Chem. Soc., 34(11), 3269 (2013). Sofiane Boukhalfa, Kara Evanoff and Gleb Yushin, Energy Environ. Sci., 5, 6872 (2012). Chunmei Ban, Ming Xie, Xiang Sun, Jonathan J Travis, GongkaiWang, Hongtao Sun, Anne C Dillon, Jie Lian and Steven M George, Nanotechnology 24, 424002 (2013). Figure 1

Three-dimensional crisscross porous manganese oxide/carbon composite networks for high performance supercapacitor electrodes

Manganese oxide/carbon (MnOx/C) composites have been successfully prepared via a high temperature heat treatment method followed by the electrochemical oxidation. The presence of carbon not only enhances the electronic conductivity of manganese oxides (MnOx), but also provides more active sites for the transformation of manganese monoxide (MnO) during the galvanostatic charge–discharge process. Simultaneity, the interconnected porous structures of MnOx/C samples are believed to provide a continuous channel for the diffusion of electrolyte ion and shorten the diffusion length of ions involved in the charge/discharge cycling processes. Consequently, these advantages endow the MnOx/C electrode a better capacitance performance, a superior long-term cyclic stability and outstanding rate capability compared with pristine MnOx. More importantly, the composites show a fascinating capacitance of 807Fg−1 at 1Ag−1, which is much higher than the reported hydrous RuO2 electrodes. It can be easily speculated that MnOx/C composites will act as a promising electrode materials for designing high-performance supercapacitors.

Preparation and Electrochemical Properties of Polyaniline Composite Electrodes Prepared by In-Situ Polymerization in Hydrous Ruthenium Oxide Dispersed Aqueous Solution.

Polyaniline (PANI)/ruthenium oxide (RuO2) composites were prepared by a chemical oxidation polymerization with various amount of hydrous RuO2. In the PANI/RuO2 composites, RuO2 particles were surrounded by PANI. The morphology and structure of the PANI/RuO2 composites were examined by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Analysis by TEM revealed that the aggregations of the composites were increased with the amounts of hydrous RuO2. Electrochemical performances of prepared composites were characterized by cyclic voltammetry (CV). The composites prepared with 20 wt% of hydrous RuO2 per aniline monomer showed the highest specific capacitance of 710 F/g at 5 mV/s among the prepared composites. However, the specific capacitance of the composites with excess amounts of hydrous RuO2 is lower than the highest one. The large aggregation of RuO2 particles prevents the participation in electrochemical reaction due to the long pathway from the electrolyte and a decrease of effective surface area.

Hydrous RuO2 nanoparticles as an efficient NIR-light induced photothermal agent for ablation of cancer cells in vitro and in vivo.

Metal oxides are receiving an incremental attention in recent years for their potential applications in ablation of cancer cells due to their efficient photothermal conversion and good biocompatibility, but the large sizes and poor photo-stability will seriously limit their practical application. Herein, hydrous RuO2 nanoparticles were synthesized by a facile hydrothermal treatment and surface-modified with polyvinylpyrrolidone (PVP) coating. PVP-coated RuO2 nanoparticles exhibit a well dispertion in saline solution, strong characteristic plasmonic absorption in NIR region, enhanced photothermal conversion efficiency of 54.8% and remarkable photo-stability under the irridation of an 808 nm laser. The nanoparticles were further employed as a new photothermal ablation agent for cancer cells which led rapidly to cellular deaths both in vitro and in vivo.

Towards Implantable Bio-Supercapacitors: Pseudocapacitance of Ruthenium Oxide Nanoparticles and Nanosheets in Acids, Buffered Solutions, and Bioelectrolytes

Metal oxides, in particular ruthenium-based oxides, are promising electrode materials for aqueous pseudocapacitors. Strong acids or bases are favored over neutral electrolytes owing to the higher capacitance. Here we explore the pseudocapacitive behavior of ruthenium oxide nanoparticles and nanosheets in near neutral pH as an environmentally benign electrolyte. The pseudocapacitive charge storage in poorly-crystalline hydrous RuO2 nanoparticles, and highly-crystalline RuO2 nanosheets were investigated in acetic acid-lithium acetate (AcOH-AcOLi) buffered solutions. It is shown that capacitance values as high as 1,038 F g−1 can be achieved in AcOH-AcOLi buffered solutions with RuO2 nanosheets, which is 44% higher than the benchmark RuO2·nH2O in H2SO4 electrolyte (720 F g−1). Furthermore, comparable performance was obtained in phosphate buffered saline and fetal bovine serum. The mechanism of the pseudocapacitive properties is discussed based on the difference in the surface redox behavior of different RuO2 nanomaterials in acid, neutral, buffered solutions, and in weak acid.

Enhanced electrochemical performance of hydrous RuO2/mesoporous carbon nanocomposites via nitrogen doping.

Hydrous RuO2 nanoparticles have been uniformly deposited onto nitrogen-enriched mesoporous carbons (NMCs) via a facile hydrothermal method. The nitrogen doping in the carbon framework not only provides reversible pseudocapacitance but also guides uniform deposition of RuO2 nanoparticles. As a result, an extremely high specific capacitance of 1733 F/g per RuO2, comparable to the theoretic capacitance of RuO2, is reached when 4.3 wt % of RuO2·1.25H2O is loaded onto the NMCs. Systematic studies show that either nitrogen-free or excess nitrogen doping result in RuO2 clusters formation and worsen the electrochemical performances. With intermediate nitrogen and RuO2 content (8.1 wt % N, 29.6 wt % of RuO2·1.25H2O), the composites deliver excellent power performance and high specific capacitance (402 F/g) with reversible capacitive response at 500 mV/s. The unique properties of nitrogen in textual, morphological, and electrochemical aspects may also provide further understanding about the effects of nitrogen doping and metal oxide deposition on supercapacitor performance.

SYNTHESIS OF HYDROUS RUTHENIUM OXIDE NANOPARTICLES IN SUB- AND SUPERCRITICAL WATER AND THEIR CAPACITIVE PROPERTIES

Hydrous ruthenium oxide (RuO2 · nH2O) nanoparticles with various particle sizes and water contents were synthesized in sub- and supercritical water in a very short reaction time of 15 min. The particle size, surface area, morphology, crystalline structure, and electrochemical properties were analyzed and compared with those of commercial RuO2 particles. Ultrafine spherical RuO2 · 0.6H2O nanoparticles with an average size of 4.2 nm were produced in subcritical water (250°C, 300 bar), while larger and more highly crystalline rod-shaped RuO2 · (0.3–0.5)H2O particles were produced in supercritical water (400°C, 300 bar). The use of NaOH under the supercritical water conditions resulted in a decrease in particle size. The hydrous RuO2 nanoparticles synthesized in subcritical water exhibited a much higher specific capacitance (255 F g−1) at a scan rate of 10 mV s−1 than those synthesized in supercritical water (77 F g−1) and commercial RuO2 (8 F g−1).

Enhanced Rate Performance of Mesoporous Co3O4Nanosheet Supercapacitor Electrodes by Hydrous RuO2Nanoparticle Decoration

Mesoporous cobalt oxide (Co3O4) nanosheet electrode arrays are directly grown over flexible carbon paper substrates using an economical and scalable two-step process for supercapacitor applications. The interconnected nanosheet arrays form a three-dimensional network with exceptional supercapacitor performance in standard two electrode configuration. Dramatic improvement in the rate capacity of the Co3O4 nanosheets is achieved by electrodeposition of nanocrystalline, hydrous RuO2 nanoparticles dispersed on the Co3O4 nanosheets. An optimum RuO2 electrodeposition time is found to result in the best supercapacitor performance, where the controlled morphology of the electrode provides a balance between good conductivity and efficient electrolyte access to the RuO2 nanoparticles. An excellent specific capacitance of 905 F/g at 1 A/g is obtained, and a nearly constant rate performance of 78% is achieved at current density ranging from 1 to 40 A/g. The sample could retain more than 96% of its maximum capacitance even after 5000 continuous charge-discharge cycles at a constant high current density of 10 A/g. Thicker RuO2 coating, while maintaining good conductivity, results in agglomeration, decreasing electrolyte access to active material and hence the capacitive performance.

Nanosized Mn–Ru binary oxides as effective bifunctional cathode electrocatalysts for rechargeable Li–O2batteries

Mn–Ru binary oxides have been synthesized via a hydrothermal approach and their performance as bifunctional catalysts for the air electrode is evaluated in non-aqueous Li–O2 secondary batteries. Characterization of the catalysts by X-ray diffractometry and transmission electron microscopy confirms that the as-prepared oxides contain γ-MnO2 and hydrous RuO2 with the morphology of fusiform nanorods and nanoparticles, respectively. Linear scanning voltammetric measurements reveal that the binary oxides exhibit remarkable electrocatalytic activities towards both the oxygen reduction and oxygen evolution reactions. Li–O2 batteries with Mn–Ru oxides as cathode catalysts show a higher discharge voltage plateau and a higher specific capacity of 6500 mA h gcarbon−1 than those with Ketjen black (KB) alone, and the subsequent charge voltage is ca. 500 mV lower than that with KB. Moreover, much enhanced cyclability of Li–O2 batteries is achieved with a capacity retention ratio of 73.2% after 5 cycles. The cyclability is maintained for 50 cycles without sharp decay under a limited discharge depth of 1100 mA h gcarbon−1, suggesting that such a bifunctional electrocatalyst is a promising candidate for the air electrode in Li–air batteries.

Quasi-solid-state pseudocapacitors using proton-conducting gel polymer electrolyte and poly(3-methyl thiophene)–ruthenium oxide composite electrodes

We report the studies on a flexible quasi-solid-state configuration of the redox supercapacitors (pseudocapacitors) assembled with an ionic liquid-based proton conducting non-aqueous gel polymer electrolyte (ILGPE) and composite electrodes of conducting polymer [poly-3-methyl thiophene (pMeT)] and hydrous ruthenium dioxide (RuO2.xH2O). The presence of RuO2.xH2O in the composite electrodes has been confirmed by X-ray diffraction and thermogravimetric analysis. The ILGPE films, prepared with the solution of an ionic liquid (1-ethyl 3-methyl imidazolium trifluoromethanesulfonate) and a salt ammonium trifluoromethanesulfonate entrapped in a host polymer poly(vinylidene fluoride-co-hexafluoropropylene), have been characterized. The symmetrical pseudocapacitors have been assembled and characterized using electrochemical impedance spectroscopy, cyclic voltammetry, and charge–discharge tests. The composite electrodes with the ∼13 wt.% hydrous RuO2 loading in pMeT film has shown a maximum specific capacitance value of ∼118 F g−1 (of the composite electrode material). The corresponding maximum specific energy and power density have been found to be ∼12.8 W kg−1 and ∼513 W kg−1, respectively. With further increase in the content of RuO2.xH2O, a slight decrease in specific capacitance value has been observed, which indicates the reduction in utilization rate of RuO2.xH2O. The composite electrodes show stable capacitance values up to 5,000 charge–discharge cycles.

Synthesis of nanosized MnO2 prepared by the polyol method and its application in high power supercapacitors

Over the last years, the different polymorphs of MnO2 have been intensely studied as alternative compounds to amorphous hydrous RuO2 as supercapacitor electrode materials. In the present work, nanosized birnessite-type MnO2 platelets were synthesized via the polyol method followed by a subsequent ligand removal with NaOH. The resulting compound is easily dispersible in polar solvents, thus allowing the preparation of stable dispersions (2–3 days) that could be used in thin electrode preparation. The capacitance of the synthesized product was 130 F g−1 in a potential window of 0.8 V at a scan rate of 2 mV s−1. The synthesized material was also studied using a cavity microelectrode to evaluate the electrochemical performance of the nanostructured oxide at high scan rates. Cyclic voltammetry measurements were successfully carried out both in the previous potential window between 0.1 and 0.9 V (vs. SHE) and in a larger potential window between −0.6 and 1.1 V (vs. SHE) from 0.1 and up to 1,000 mV s−1. Therefore, herein prepared material could be potentially used for high power applications, although further work should be carried out to upscale such compound without compromising the performance.

Supercapacitive properties of electrodeposited RuO2 electrode in acrylic gel polymer electrolytes

Hydrous ruthenium oxide (RuO2) is prepared by electrodeposition on a platinum substrate and its supercapacitive properties are characterized adopting acrylic gel polymer electrolytes, such as poly(acrylic acid) (PAA), potassium polyacrylate (PAAK), and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS). The electrodeposited hydrous RuO2 exhibits an amorphous compact stratified morphology with a higher loading (0.15 mg cm−2) than that of a previous report, and shows broad redox peaks on both cathodic and anodic scans in the cyclic voltammetry. In particular, the RuO2 electrode for supercapacitor adopting the PAMPS electrolyte shows the highest specific capacitance of 642 F g−1 at 20 mV s−1. This is due to the efficient utilization of active RuO2 species and greater proton accommodation toward the negative oxygen sites of PAMPS's side chain. In addition, it is possible to improve sustainability against high-rate current with the RuO2 electrode with the PAMPS electrolyte, due to the crosslinks of the gel electrolyte, which support the mechanical strength.

Fabrication and supercapacitive properties of a thick electrode of carbon nanotube–RuO2 core–shell hybrid material with a high RuO2 loading

A core-shell hybrid material of amorphous hydrous RuO2-coated carbon nanotubes (CNT–RuO2) with a RuO2 loading as high as 82.4wt% was prepared by a solution method using RuCl3 and NaHCO3 aqueous solutions. The effect of preparation conditions, especially the dripping speed of the NaHCO3 solution, on the formation of the core-shell structure was investigated, and the corresponding mechanism was discussed. Supercapacitive properties of the CNT–RuO2 and amorphous hydrous RuO2 electrodes with a thickness of over 200μm were studied and the crucial factors to govern their rate capability were analyzed. For the thick CNT–RuO2 electrode, a comparison of its specific capacitance before and after subtracting the effect of the voltage drop of discharge curves caused by the inner resistance of the CNT–RuO2 symmetrical supercapacitor indicates that electronic conductivity is more important than proton diffusion in determining its rate capability.

Ruthenia-Based Electrochemical Supercapacitors: Insights from First-Principles Calculations

Electrochemical supercapacitors (ECs) have important applications in areas wherethe need for fast charging rates and high energy density intersect, including in hybrid and electric vehicles, consumer electronics, solar cell based devices, and other technologies. In contrast to carbon-based supercapacitors, where energy is stored in the electrochemical double-layer at the electrode/electrolyte interface, ECs involve reversible faradaic ion intercalation into the electrode material. However, this intercalation does not lead to phase change. As a result, ECs can be charged and discharged for thousands of cycles without loss of capacity. ECs based on hydrous ruthenia, RuO2·xH2O, exhibit some of the highest specific capacitances attained in real devices. Although RuO2 is too expensive for widespread practical use, chemists have long used it as a model material for investigating the fundamental mechanisms of electrochemical supercapacitance and heterogeneous catalysis. In this Account, we discuss progress in first-principles density-functional theory (DFT) based studies of the electronic structure, thermodynamics, and kinetics of hydrous and anhydrous RuO2. We find that DFT correctly reproduces the metallic character of the RuO2 band structure. In addition, electron-proton double-insertion into bulk RuO2 leads to the formation of a polar covalent O-H bond with a fractional increase of the Ru charge in delocalized d-band states by only 0.3 electrons. This is in slight conflict with the common assumption of a Ru valence change from Ru(4+) to Ru(3+). Using the prototype electrostatic ground state (PEGS) search method, we predict a crystalline RuOOH compound with a formation energy of only 0.15 eV per proton. The calculated voltage for the onset of bulk proton insertion in the dilute limit is only 0.1 V with respect to the reversible hydrogen electrode (RHE), in reasonable agreement with the 0.4 V threshold for a large diffusion-limited contribution measured experimentally. DFT calculations also predict that proton diffusion in RuO2 is hindered by a migration barrier of 0.8 eV, qualitatively explaining the observed strong charging rate-dependence of the diffusion-limited contribution. We found that reversible adsorption of up to 1.5 protons per Ru on the (110) surface contributes to the measured capacitive current at higher voltages. PEGS-derived models of the crystal structure of hydrated ruthenia show that incorporation of water in Ru vacancies or in bulk crystals is energetically much more costly than segregation of water molecules between slabs of crystalline RuO2. These results lend support to the so-called "water at grain boundaries" model for the structure of hydrous RuO2·xH2O. This occurs where metallic nanocrystals of RuO2 are separated by grain boundary regions filled with water molecules. Chemists have attributed the superior charge storage properties of hydrous ruthenia to the resulting composite structure. This facilitates fast electronic transport through the metallic RuO2 nanocrystals and fast protonic transport through the regions of structural water at grain boundaries.

Structural analysis of carbon-supported Ru-decorated Pt nanoparticles synthesized using forced deposition and catalytic performance toward CO, methanol, and ethanol electro-oxidation

Carbon-supported Ru-modified Pt nanoparticles (Ru(Pt)/C), prepared using forced deposition under H2 stream, were tested as anodic fuel cell electrocatalysts. Ru nanoislands, mainly composed of RuO, RuO2, and hydrous RuO2 (RuOxHy), were deposited on the Pt nanoparticles. The coverage of Pt by Ru species (θ) depended on the Ru salt concentration and the treatment duration, the deposition process following an adsorption-controlled Elovich-type kinetics. The CO oxidation kinetics was optimal for θ≈0.50, whereas for both, methanol and ethanol, the highest activity was found for θ≈0.15–0.20. The observed promotional effect was attributed to the reaction of hydroxylated Ru species with strongly adsorbed CO and related intermediates. The forced deposition technique appears to be a suitable simple method for the preparation of Ru(Pt) nanoparticles with improved fuel oxidation performance. The Tafel analysis allowed correlating the reaction pathway and the limiting step with the coverage by Ru.

Miniature asymmetric ultracapacitor of patterned carbon nanotubes and hydrous ruthenium dioxide

A symmetric ultracapacitor CNT_CNT and an asymmetric ultracapacitor CNT_hRuO2 of mini size have been prepared with patterned carbon nanotubes (CNT) and hydrous ruthenium dioxide. Galvanostatic charge/discharge results indicate that CNT_hRuO2 is the superior one in both power and energy densities. In a potential window 2.0 V, the CNT_hRuO2 cell displays an energy density of 24.0 W h kg−1 at a power density of 22.9 kW kg−1. Its power density can be raised to 41.1 kW kg−1 at the expense of the energy density, which drops to 6.8 W h kg−1. On the other hand, CNT_CNT performs at a lower level, delivering 5.2 W h kg−1 at 5.5 kW kg−1. The favorable charge/discharge performance of CNT_hRuO2 is attributed to hydrous RuO2, whose pseudocapacitance drives the other electrode of the vertical CNT array to work harder and makes more use of its double-layer capacitance. The analysis of individual electrode capacitance indicates that the high capacitance of hRuO2 also causes a disproportion in voltage partition, which restricts the low limit of cycling current in an extended potential window. On energy cycling, CNT_hRuO2 demonstrates sufficient stability in 10 000 cycles, after an initial 13% drop in capacitance.

Tuning the Functionality of a Carbon Nanofiber–Pt–RuO2 System from Charge Storage to Electrocatalysis

Chemical-functionalization-induced switching in the property of a hybrid system composed of a hollow carbon nanofiber (CNF) and Pt and RuO(2) nanoparticles from charge storage to electrocatalysis is presented. The results of this study show how important it is to have a clear understanding of the nature of surface functionalities in the processes involving dispersion of more than one component on various substrates including carbon nanomorphologies. When pristine CNF is used to decorate Pt and RuO(2) nanoparticles, random dispersion occurs on the CNF surface (C-PtRuO(2)). This results in mainly phase-separated nanoparticles rich in RuO(2) characteristics. In contrast to this, upon moving from the pristine CNF to those activated by a simple H(2)O(2) treatment to create oxygen-containing surface functional groups, a material rich in Pt features on the surface is obtained (F-PtRuO(2)). This is achieved because of the preferential adsorption of RuO(2) by the functionalized surface of CNF. A better affinity of the oxygen-containing functional groups on CNF toward RuO(2) mobilizes relatively faster adsorption of this moiety, leading to a well-controlled segregation of Pt nanoparticles toward the surface. Further reorganization of Pt nanoparticles leads to the formation of a Pt nanosheet structure on the surface. The electrochemical properties of these materials are initially evaluated using cyclic voltammetric analysis. The cyclic voltammetric results indicate that C-PtRuO(2) shows a charge storage property, a typical characteristic of hydrous RuO(2), whereas F-PtRuO(2) shows an oxygen reduction property, which is the characteristic feature of Pt. This clear switch in the behavior from charge storage to electrocatalysis is further confirmed by galvanostatic charge-discharge and rotating-disk-electrode studies.

Anodic composite deposition of hydrous RuO2–TiO2 nanocomposites for electrochemical capacitors

This work employs a simple one-step method, anodic composite deposition, to fabricate hydrous RuO2–TiO2 (denoted as RuO2·xH2O–TiO2) composite films from an aqueous bath containing RuCl3·xH2O and TiO2 nanoflowers for the electrochemical capacitor (EC) application. The composition of composites can be simply controlled by varying the concentration of TiO2 nanoflowers of the rutile phase in the deposition solution. The highly porous morphologies of various RuO2·xH2O–TiO2 composites are characterized with a field emission scanning electron microscope (FE-SEM). The pseudocapacitive performances (e.g., electrochemical reversibility, specific capacitance, electrochemical impedance spectroscopic responses) of relatively thick RuO2·xH2O–TiO2 composites (0.9±0.1mgcm−2) are systematically investigated. The high-power capacitive behavior of RuO2·xH2O–TiO2 composites, high utilization of RuO2·xH2O and the simple one-step method for electrode preparation indicate the unique merits of anodic composite deposition.