Pseudocapacitive Characteristics Research Articles

Differences in the electrochemical behavior of ruthenium and iridium oxide in electrocatalytic coatings of activated titanium anodes prepared by the sol-gel procedure

The electrochemical characteristics of Ti0.6Ir0.4O2/Ti and Ti0.6Ru0.4O2/Ti anodes prepared by the sol-gel procedure from the corresponding oxide sols, obtained by force hydrolysis of the corresponding metal chlorides, were compared. The voltammetric properties in H2SO4 solution indicate that Ti0.6Ir0.4O2/Ti has more pronounced pseudocapacitive characteristics, caused by proton-assisted, solid state surface redox transitions of the oxide. At potentials negative to 0.0 VSCE, this electrode is of poor conductivity and activity, while the voltammetric behavior of the Ti0.6Ru0.4O2/Ti electrode is governed by proton injection/ejection into the oxide structure. The Ti0.6Ir0.4O2/Ti electrode had a higher electrocatalytical activity for oxygen evolution, while the investigated anodes were of similar activity for chlorine evolution. The potential dependence of the impedance characteristics showed that the Ti0.6Ru0.4O2/Ti electrode behaved like a capacitor over a wider potential range than the Ti0.6Ir0.4O2/Ti electrode, with fully-developed pseudocapacitive properties at potentials positive to 0.60 VSCE. However, the impedance characteristics of the Ti0.6Ir0.4O2/Ti electrode changed with increasing potential from resistor-like to capacitor-like behavior.

Pseudocapacitive Characteristics of Vanadium Oxide Deposits with a Three-Dimensional Porous Structure

A three-dimensional porous vanadium oxide was anodically deposited onto graphite substrates (denoted as ) at 0.7 V from an aqueous solution containing 25 mM and 5 mM . Through annealing at temperatures up to , the thermal stability of preserved its porous morphology and excellent capacitive performances in 3 M KCl (pH 2.4). X-ray photoelectron spectroscopic analysis revealed the mixed valence nature of oxy-/hydroxyl-vanadium species in which the amount of species was not significantly affected by varying the annealing temperature. A maximal specific capacitance of ca. measured at was obtained for this porous annealed between 150 and . Only 9–17% loss in specific capacitance was found for these when the scan rate of the cyclic voltammetry was increased from 25 to , demonstrating a typical high power property, which was never found in the -based supercapacitors.

X-ray Photoelectron Spectroscopy and in Situ X-ray Absorption Spectroscopy Studies on Reversible Insertion/Desertion of Dicyanamide Anions into/from Manganese Oxide in Ionic Liquid

Pseudocapacitive characteristics of manganese (Mn) oxide were recognized in aprotic ionic liquid (IL), namely, butylmethylpyrrolidinium−dicyanamide (BMP−DCA), within a potential window of 3 V. The electrochemical energy storage mechanism was examined using X-ray photoelectron spectroscopy and in situ X-ray absorption spectroscopy. It was confirmed that the DCA− anions, instead of the bigger BMP+ cations, were the working species that compensate the Mn valent state variation upon charging and discharging in the IL electrolyte. During oxidation of the Mn oxide electrode, to keep the charges balanced, the quasi-linear DCA− anions were inserted into the tunnels between the MnO6 octahedral units, expanding the structural framework. Importantly, a highly reversible process was observed during the subsequent reduction step. XPS depth profiling analyses showed that the electrochemical reaction thickness was approximately 50 nm.

Templated Nanocrystal-Based Porous TiO2 Films for Next-Generation Electrochemical Capacitors

The advantages in using nanoscale materials for electrochemical energy storage are generally attributed to short diffusion path lengths for both electronic and lithium ion transport. Here, we consider another contribution, namely the charge storage from faradaic processes occurring at the surface, referred to as pseudocapacitive effect. This paper describes the synthesis and pseudocapacitive characteristics of block copolymer templated anatase TiO(2) thin films synthesized using either sol-gel reagents or preformed nanocrystals as building blocks. Both materials are highly crystalline and have large surface areas; however, the structure of the porosity is not identical. The different titania systems are characterized by a combination of small- and wide-angle X-ray diffraction/scattering, combined with SEM imaging and physisorption measurements. Following our previously reported approach, we are able to use the voltammetric sweep rate dependence to determine quantitatively the capacitive contribution to the current response. Considerable enhancement of the electrochemical properties results when the films are both made from nanocrystals and mesoporous. Such materials show high levels of capacitive charge storage and high insertion capacities. By contrast, when mesoscale porosity is created in a material with dense walls (rather than porous walls derived from the aggregation of nanocrystals), insertion capacities comparable to templated nanocrystal films can be achieved, but the capacitance is much lower. The results presented here illustrate the importance of pseudocapacitive behavior that develops in high surface area mesoporous oxide films. Such systems provide a new class of pseudocapacitive materials, which offer increased charge storage without compromising charge storage kinetics.

Electrochemical and In Situ Optical Studies of Supported Iridium Oxide Films in Aqueous Solutions

Certain aspects of the dynamic behavior of electrochemically deposited hydrous Ir oxide films supported on Au microelectrodes during charge and discharge have been investigated by a combination of chronocoulometry and simultaneous in situ normalized reflectance spectroscopy techniques in aqueous solutions. Correlations between the reflectance spectra and the optical properties of the films in its various states of oxidation were sought from in situ transmission measurements for films supported on In-doped tin oxide on glass. The current transient response for microelectrodes following a potential step, within the voltage region in which the films display pseudocapacitive characteristics, was found to exhibit a well-defined peak, as opposed to a monotonic decay reported by other groups. Some features of this behavior can be attributed to changes in the conductivity of the film as a function of its state of charge, as has been proposed for electronically conducting polymers. Also presented in this work are data collected over the pH range 0.3–13, which confirm the much faster charge–discharge dynamics in basic compared to acidic media. A primitive model based on proton conductivity within the hydrated oxide lattice is presented which accounts grossly for this pH-induced effect.

Textural and pseudocapacitive characteristics of sol–gel derived RuO 2· xH 2O: Hydrothermal annealing vs. annealing in air

Hydrous ruthenium dioxide (RuO 2· xH 2O) prepared in a modified sol–gel process was subjected to annealing in air and water at various temperatures for supercapacitor applications. The textural and pseudocapacitive characteristics of RuO 2· xH 2O annealed in air and water were systematically compared to show the benefits of annealing in water (denoted as hydrothermal annealing). An important concept that hydrothermal annealing effectively restricts condensation of hydroxyl groups within nanoparticles, inhibits crystal growth, and maintains high water content of RuO 2· xH 2O is demonstrated in this work. The unique textural characteristics of hydrothermally annealed RuO 2· xH 2O are attributable to the high-pressured, water-enriched surroundings which restrain coalescence of RuO 2· xH 2O nanocrystallites. The crystalline, hydrous nature of hydrothermally annealed RuO 2· xH 2O favors the utilization of active species in addition to a merit of minor dependence of specific capacitance on the scan rate of CV for pseudocapacitors. As a result, RuO 2· xH 2O with hydrothermal annealing at 225 °C for 24 h exhibits 16 wt.% water, an average particle size of about 7 nm, and specific capacitance of ca. 390 F g −1.

Electrode Properties of Mn2O3 Nanospheres Synthesized by Combined Sonochemical/Solvothermal Method for Use in Electrochemical Capacitors

We report here an efficient single step combined sonochemical and solvothermal synthesis process to obtain bulk quantities of nanospherical particles of cubic Mn2O3 and characterized its pseudocapacitive characteristics in relevance to electrochemical capacitors for the first time. It has been found that quantitative determination of specific capacitance yielded a value of capacitance of ∼100 Fg−1 within 0–0.4 V (versus SCE) potential range in a 6 M KOH alkaline electrolyte. The as‐prepared nanopowders after being subjected to heat treatment at 400 ∘C were characterized by using XRD which shows a typical cubic single‐phase structure (space group Ia-3), the broad crystalline peaks indicating the presence of explicit nanostructure. Electron microscopic studies (FE‐SEM and TEM) revealed that the synthesized powders exhibit nanospherical morphology with uniform sphere‐like grains of ∼10–15 nm range. Two heat‐treated samples were studied in the context of crystallinity versus electrochemical capacitance using rate‐dependent cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three‐electrode system. The excellent well‐refined redox behavior corroborates with EIS measurements. The presence of near symmetric redox couple observed in CV has been attributed to pronounced one‐electron‐transfer process owing to the presence of facile Mn redox centers facilitating the reversible one‐electron transfer that accounts for its pseudocapacitance.

In situ X-ray absorption spectroscopic studies of anodically deposited binary Mn–Fe mixed oxides with relevance to pseudocapacitance

Binary Mn–Fe oxides with different Mn/Fe content ratios were prepared by anodic deposition. The deposited oxides were studied by in situ X-ray absorption spectroscopy (XAS) in 2 M KCl solution during the charging–discharging process. The experimental results clearly confirmed that the oxidation states of both Mn and Fe changed back and forth with adjusting the applied potential, contributing to the pseudocapacitive characteristics of the binary oxides. It was also found that, within a potential range of 1 V, because of Fe oxide addition the variation in the Mn oxidation state was increased from 0.70 (+3.25 to +3.95 for plain Mn oxide) to 0.81 (+3.12 to +3.93 for Mn90Fe10 oxide or +3.10 to +3.91 for Mn75Fe25 oxide), while the Fe oxide itself demonstrated an oxidation state shift of only 0.55. Accordingly, the optimum pseudocapacitance of the binary Mn–Fe oxide could be only achieved when the amount of Fe oxide was properly controlled. The highest specific capacitance of 255 F g −1 was obtained with a Mn/Fe atomic ratio of 90/10, while plain Mn oxide revealed a capacitance of only 205 F g −1.

Physicochemical properties and electrochemical behavior of binary manganese–cobalt oxide electrodes for supercapacitor applications

Binary Mn–Co oxide electrodes were prepared by anodic deposition at 25 °C in a mixture of manganese acetate and cobalt acetate aqueous solutions. It was found that the Mn-to-Co content ratio in the oxide could be easily controlled by adjusting the composition of the plating solution. The chemical states of the oxides produced were analyzed using an X-ray photoelectron spectroscope, and the deposition mechanism was thus explored in this study. Chronopotentiometry was also performed to evaluate the pseudocapacitive characteristics of the oxide electrodes. The experimental results indicated that an appropriate amount of Co addition could effectively inhibit the anodic dissolution of Mn and consequently improve the electrochemical reversibility and stability of the oxide electrode. However, it was also confirmed that high Co content caused a significant reduction in the specific capacitance of the oxide.

Correlating the Oxygen Redox Chemistry and Oxygen Nonstoichiometry at La1-xSrxMnO3 plus or minus delta SOFC Cathodes

This work uses the Wagner Factor as a qualitative means of explaining the effects that perturbing the oxygen content in La1-xSrxMnO3{plus minus}δ (LSM) cathodes has on their O2 redox activity and as well as the emergence of pseudo-capacitive characteristics under low pO2 atmospheres. The Wagner Factor correlates the rate constant of the O2 redox reaction in mixed conducting perovskites (kchem, Dchem) to changes in the oxide's oxygen concentration. The emergence of an oxygen under- stoichiometric (3-δ) state under low O2 partial pressures and/or at high cathodic overpotentials is believed to lead to the observed electrochemical capacitive behavior.

Anodic deposition of hydrous ruthenium oxide for supercapaciors: Effects of the AcO − concentration, plating temperature, and oxide loading

Effects of the sodium acetate (NaCH 3COO, denoted as NaAcO) concentration, plating temperature, and oxide loading on the pseudocapacitive characteristics of hydrous ruthenium oxide (denoted as RuO 2· xH 2O) films anodically plated from aqueous RuCl 3· xH 2O media were systematically investigated in this work. The electrochemical behavior of RuO 2· xH 2O with annealing in air at 200 °C for 2 h is approximately independent of the NaAcO concentration and plating temperature although a negative shift in the onset and peak potentials of deposition with rising the plating temperature is found. The morphologies and adhesion of RuO 2· xH 2O deposits strongly depend on the deposition rate which is obviously influenced by varying the above two deposition variables. The specific capacitance of RuO 2· xH 2O is monotonously decreased from 760 to 505 F g −1 when the oxide loading is gradually increased from 0.34 to 1.0 mg cm −2, due to the longer pathways of both electrons and protons during the redox transitions. The XRD and Raman spectroscopic analyses reveal the extremely localized crystalline nature of as-deposited RuO 2· xH 2O. All RuO 2· xH 2O deposits show the ideal pseudocapacitive characteristics, definitely illustrating the merits of RuO 2· xH 2O prepared by anodic deposition without considering the advantages of its simplicity, one-step, reliability, low cost, and versatility for electrode preparation.

Effect of Iron Doping on the Pseudo-Capacitive Characteristics of Manganese Oxide

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Microstructure and Pseudocapacitive Performance of Anodically Deposited Manganese Oxide with Various Heat-Treatments

Amorphous, hydrous manganese oxide was prepared by anodic deposition in manganese acetate solution. The effect of heat-treatments (up to 600°C) on the material characteristics of the oxides was investigated. The results indicated that the as-deposited oxide, which was fully amorphous, was transformed into a fibrous shape with nanocrystallinity after annealing at 200°C for 2 h. and were formed within the nanocrystalline oxide when heating at 400°C. Furthermore, by increasing the temperature over 500°C, the spherical particles became the only phase present. In addition, atomic force microscopy was also carried out to explore the surface morphology of the oxide electrodes. This characterization method recognized condensation, rearrangement, reconstruction, and growth of the deposited manganese oxide as a function of temperature. The corresponding electrochemical performances of the oxides were evaluated by chronopotentiometry. The pseudocapacitive characteristics, reversibility, and cyclic stability of the deposited manganese oxide were improved by introducing the proper heat-treatment. However, high-temperature heat-treatment promoted the formation of crystalline and and consequently resulted in the loss of the pseudocapacitive property of the oxides.

Electrochemical capacitors of RuO2 nanophase grown on LiNbO3(100) and sapphire(0001) substrates

The electrochemical properties of RuO2/LNO and RuO2/SA electrodes made of RuO2 rods and plates of nanometre size on LiNbO3(100) and sapphire(0001) substrates are investigated. Both rods and plates are grown vertically using chemical vapor deposition. The RuO2/LNO electrode has a higher electrolyte/solid surface area compared with the RuO2/SA electrode. When immersed in H2SO4 acid, voltammograms of both electrodes exhibit chemisorption and pseudocapacitive characteristics of the RuO2 single crystal. The chemisorption features decrease after repeated cyclic voltammetry (CV) sweeps, and the voltammograms become more mirror-like. The specific capacitance of the RuO2/LNO electrode measured in CV is 569 F g−1, that of RuO2/SA 357 F g−1. These values are reconfirmed in charging–discharging measurements. The measured capacitance decreases with the sweep rate, and the decreasing trend of RuO2/LNO is higher than that of RuO2/SA. Reduction of accessible charge at high sweep rates results from higher internal resistance of the RuO2/LNO electrode which is distributed because of its porous nature. The impedance spectrum of the RuO2/LNO electrode confirms its higher internal resistance. It also indicates the electrode is a nearly ideal capacitor below a knee frequency 200 Hz. Since the knee frequency is less than 1 Hz for most electrochemical capacitors, both RuO2/LNO and RuO2/SA are electrodes for fast charging–discharging applications.

Electrochemically deposited nanowhiskers of nickel oxide as a high-power pseudocapacitive electrode

High performance pseudocapacitive characteristics of potentiodynamically deposited NiO are reported here. Three-dimensional nanowhiskers of NiO were prepared by potentiodynamically depositing nickel hydroxide on stainless steel (SS) and heating the hydroxide in air at 300°C for 3h. The scan rate used for the deposition was 200mVs−1 and the electrolyte solution used was 100mM NiCl2∙6H2O at a pH8.0. The NiO-covered SS electrodes were characterized by cyclic voltammetry (CV) in 1M KOH electrolyte for the redox supercapacitor application. A maximum specific capacitance (SC) of 138Fg−1 was obtained from the CV at a scan rate of 100mVs−1. A SC value of 82Fg−1 was obtained even at the very high CV scan rate of 500mVs−1, demonstrating the high-power characteristics of NiO. Long cycle-life of the NiO nanowhiskers was also demonstrated. High specific capacitance, high power, high stability, and low cost of the electrode materials are favorable factors for commercial applications.