Enhanced manganese dioxide supercapacitor electrodes produced by electrodeposition

Electrodeposited thin films of manganese dioxide, prepared using chronoamperometry on a platinum substrate in an electrolyte of MnSO₄ in H₂SO₄, possess a significantly higher capacitance compared to the literature materials (>2000 F g⁻¹ which is at least a 250% increase in performance) when cycled over a 0.8 V potential window in an aqueous electrolyte of 0.5 M Na₂SO₄. This excellent performance is discussed in terms of the manganese dioxide electrodeposition mechanism, in particular the growth mechanism under the preferred slow mass transport of electro-active species, and its effects on morphology. Furthermore, the origin of the enhanced capacitance is discussed, in which case we have proposed arises from contributions made by hydroxyl groups on the manganese dioxide nano-particulate surface, in addition to the fast redox reactions that are necessary for pseudo-capacitance.     

Construction of composite electrodes comprising manganese dioxide nanoparticles distributed in polyaniline-poly(4-styrene sulfonic acid-co-maleic acid) for electrochemical supercapacitor

This work demonstrated a novel and simple route for preparing a composite comprising of manganese oxide (MnO₂) nanoparticles and polyaniline (PANI) doped poly(4-styrene sulfonic acid-co-maleic acid) (PSSMA) by “electrochemical doping-deposition”. The PANI-PSSMA-MnO₂ composite was characterized by scanning electron microscopy (SEM)), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). SEM images revealed a uniform dispersion of MnO₂ nanoparticles in the porous structure of PANI-PSSMA structure. XRD measurements showed the distortion of the crystal structure of β-MnO₂ after deposition of MnO₂ in PANI-PSSMA structure. Thus, the XRD pattern of PANI was predominating. Cyclic voltammetry and chronopotentiometry were employed in 0.5 M Na₂SO₄ to evaluate the capacitor properties. The results showed a significant improvement in the specific capacitance of the composite electrode. The specific capacitance of PANI-PSSMA-MnO₂ (50.4 F g⁻¹) had improvement values of 172% compared to that of PANI (18.5 F g⁻¹). When only the MnO₂ mass was considered, the composite had a specific capacitance of 556 F g⁻¹.     

Supercapacitor behavior of new substituted manganese dioxides

Non-substituted γ-MnO₂ and γ-Mn_1−yA_yO_2−δ (A = Co, Al) compounds synthesized by the electrochemical–hydrothermal route have been studied as active electrode materials for aqueous electrochemical supercapacitors. Advantage of the Mn to Co or Al substitution was taken in order to increase the surface area of γ-Mn_1−yA_yO_2−δ materials, thus leading to enhanced supercapacitive behavior. The effect of the surface area on the specific capacitance value and its origin, i.e. double layer or faradic, is discussed.     

Effect of electrodeposition conditions on the electrochemical capacitive behavior of synthesized manganese oxide electrodes

Galvanostatic electrodeposition techniques were applied for the preparation of novel electroactive manganese oxide electrodes. The effects of supersaturation ratio on the morphology and crystal structure of electrodeposited manganese oxide were studied. Manganese oxide electrodes were synthesized by anodic deposition from acetate-containing aqueous solutions on Au coated Si substrates through the control of nucleation and growth processes. By changing deposition parameters, a series of nanocrystalline manganese oxide electrodes with various morphologies (continuous coatings, rod-like structures, aggregated rods and thin sheets) and an antifluorite-type crystal structure was obtained. Detailed chemical and microstructural characterization of as-deposited electrodes was conducted using SEM, TEM and AAS. Manganese oxide thin sheets show instantaneous nucleation and single crystalline growth, rods have a mix of instantaneous/progressive nucleation and polycrystalline growth and continuous coatings form by progressive nucleation and polycrystalline growth.In addition, the electrochemical behavior was investigated by cyclic voltammetry. The experimental results show that manganese oxide electrodes, with rod-like and thin sheet morphology, exhibited enhanced electrochemical performance. The highest specific capacitance (∼230 F g⁻¹) and capacitance retention rates (∼88%) were obtained for manganese oxide thin sheets after 250 cycles in 0.5 M Na₂SO₄ at 20 mV s⁻¹.     

Anodic deposition of manganese oxide electrodes with rod-like structures for application as electrochemical capacitors

The electrochemical properties of nanocrystalline manganese oxide electrodes with rod-like structures were investigated to determine the effect of morphology, chemistry and crystal structure on the corresponding electrochemical behavior of manganese electrodes. Manganese oxide electrodes of high porosity composed of 1-1.5 μm diameter rods were electrochemically synthesized by anodic deposition from a dilute solution of Mn(CH₃COO)₂ (manganese acetate) onto Au coated Si substrates without any surfactants, catalysts or templates under galvanostatic control. The morphology of the electrodes depended on the deposition current density, which greatly influenced the electrochemical performance of the capacitor. Electrochemical property and microstructure analyses of the manganese oxide electrodes were conducted using cyclic voltammetry and microstructural techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The synthesized rod-like manganese oxide electrodes at low current densities exhibited a high specific capacitance due to their large surface areas. The largest value obtained was 185 F g⁻¹ for deposits produced at .5 mA cm⁻². Specific capacity retention for all deposits, after 250 charge-discharge cycles in an aqueous solution of 0.5 M Na₂SO₄, was about 75% of the initial capacity.     

Preparation of various manganese dioxide composites and their desulfurization performance

The performance of desulfurization materials plays a key role in desulfurization technology. In this study, different types of manganese dioxide (MnO₂) composites were prepared to improve desulfurization performance. These composites were characterized intensively via SEM, XRD, XPS, and BET. Desulfurization performance was measured through thermogravimetry (TG), and the desulfurization mechanism of different types of MnO₂ composite was investigated. Results showed that the desulfurization performances of MnO₂ composites are determined by the combined effects of the materials’ pore structure, specific surface area, active components and Mn valence contents. The desulfurization performances of high specific surface area MnO₂ and porous MnO₂ were enhanced on account of their excellent physical structures. The desulfurization performance of alkali metal additive LiOH doped MnO₂ improved through the addition of active components. The desulfurization performance of bimetallic oxide MnO₂/CeO₂ improved through the synergistic effect of bimetallic oxides. The desulfurization performance of carrier type MnO₂/NaY improved through the dispersion of MnO₂ particles. Among the composites obtained, porous MnO₂ revealed the best desulfurization performance, this composite demonstrated an average SO₂ capture rate of 0.283 g_SO2/g_material·h within the first hour of reaction, and its SO₂ capture capacity was 0.633 g_SO2/g_material.     

Electrochemical properties of nanosized hydrous manganese dioxide synthesized by a self-reacting microemulsion method

Manganese dioxide has been synthesized by a new simple self-reacting microemulsion method. The synthesized MnO₂ has been found to be amorphous structure containing a moderate amount of water by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Particles in a spherical shape with about 4 nm in diameter have been observed by transmission electron microscopy. Cyclic voltammetic tests have been performed between −0.5 and 0.5 V versus Hg/Hg₂SO₄ in 1 mol L⁻¹ Na₂SO₄ solution at sweep rates up to 50 mV s⁻¹. A specific capacitance value as high as 246.2 F g⁻¹ was obtained, which was much higher than 146.5 F g⁻¹ of MnO₂ prepared by chemical co-precipitation. After 600 cycles, only 6% decrease of specific capacitance was measured which indicated that such a material possesses good cycling property.     

The effect of oxygen reduction on activated carbon electrodes loaded with manganese dioxide catalyst

The statistical design-of-experiment method was used to identify the significant factors for making a manganese oxide-loaded activated carbon matrix. The carbon matrixes, which were made by reacting KMnO₄ with carbon material, were tested as gas-diffusion electrodes for oxygen reduction. Three factors—KMnO₄ concentration, reaction temperature, and reaction duration were tested in a two-level full-factorial design-of-experiment. The modification of carbon morphology and its effect on the performance of oxygen reduction are discussed. Temperature, KMnO₄ concentration, and the interaction between temperature and reaction time were found to have a significant influence on the catalytic activity of the manganese oxide-loaded carbon electrode.     

Supercapacitive properties of hybrid films of manganese dioxide and polyaniline based on active carbon in organic electrolyte

This is the first report about supercapacitive performance of hybrid film of manganese dioxide (MnO₂) and polyaniline (PANI) in an organic electrolyte (1.0 M LiClO₄ in acetonitrile). In this work, a high surface area and conductivity of active carbon (AC) electrode is used as a substrate for PANI/MnO₂ film electro-codeposition. The redox properties of the coated PANI/MnO₂ thin film exhibit ideal capacitive behaviour in 1 M LiClO₄/AN. The specific capacitance (SC) of PANI/MnO₂ hybrid film is as high as 1292 F g⁻¹ and maintains about 82% of the initial capacitance after 1500 cycles at a current density of 4.0 mA cm⁻², and the coulombic efficiency (η) is higher than 95%. An asymmetric capacitor has been developed with the PANI/MnO₂/AC positive and pure AC negative electrodes, which is able to deliver a specific energy as high as 61 Wh kg⁻¹ at a specific power of 172 W kg⁻¹ in the range of 0-2.0 V. These results indicate that the organic electrolyte is a promising candidate for PANI/MnO₂ material application in supercapacitors.     

Charge storage mechanism of manganese dioxide for capacitor application: Effect of the mild electrolytes containing alkaline and alkaline-earth metal cations

Effect of the electrolyte particularities on manganese dioxide in the neutral electrolytes containing alkaline (Li⁺, Na⁺, or K⁺) and alkaline-earth (Mg²⁺, Ca²⁺, or Ba²⁺) metal cations for electrochemical capacitor is investigated by cyclic voltammetry and electrochemical impedance spectroscopy. A very high capacitance of 357.9 F g⁻¹ is measured when Ca²⁺ ion is used as electrolyte cation. The results show that cations of the electrolyte instead of proton or anions are responsible for the pseudo-capacitive behavior of manganese dioxide. The capacitance of manganese dioxide is found to depend strongly on the electrolyte particularities, for example, pH value, cation and anion species, and salt concentrations. A logarithmic dependence of capacitance of MnO₂ on cation activity is obtained for all alkaline and alkaline-earth metal cations. The capacitance of MnO₂ can be doubled by replacing univalent cation with bivalent cation at a close cation activity.     

Cycle life improvement of alkaline batteries via optimization of pulse current deposition of manganese dioxide under low bath temperatures

Pulse current electrodeposition (PCD) method has been applied to the preparation of novel electrolytic manganese dioxide (EMD) in order to enhance the cycle life of rechargeable alkaline MnO₂–Zn batteries (RAM). The investigation was carried out under atmospheric pressure through a systematic variation of pulse current parameters using additive free sulfuric acid–MnSO₄ electrolyte solutions. On time (t_on) was varied from 0.1 to 98.5 ms, off time (t_off) from 0.25 to 19.5 ms, pulse frequencies (f) from 10 to 1000 Hz and duty cycles (θ) from 0.02 to 0.985. A constant pulse current density (I_p) of 0.8 A dm⁻² and average current densities (I_a) in the range of 0.08–0.8 A dm⁻² were applied in all experiments. Resultant materials were characterized by analyzing their chemical compositions, X-ray diffractions (XRD) and scanning electron microscopy (SEM). Electrochemical characterizations carried out by charge/discharge cycling of samples in laboratory designed RAM batteries and cyclic voltammetric experiments (CV). It has been proved that specific selection of duty cycle, in the order of 0.25, and a pulse frequency of 500 Hz, results in the production of pulse deposited samples (pcMDs) with more uniform distribution of particles and more compact structure than those obtained by direct current techniques (dcMDs). Results of the test batteries demonstrated that, in spite of reduction of bath temperature in the order of 40 °C, the cycle life of batteries made of pcMDs (bath temperature: 60 °C) was rather higher than those made of conventional dcMDs (boiling electrolyte solution). Under the same conditions of EMD synthesis temperature of 80 °C and battery testing, the maximum obtainable cycle life of optimized pcMD was nearly 230 cycles with approximately 30 mAh g⁻¹ MnO₂, compared to that of dcMD, which did not exceed 20 cycles. In accordance to these results, CV has confirmed that the pulse duty cycle is the most influential parameter on the cycle life than the pulse frequency. Because of operating at lower bath temperatures, the presented synthetic mode could improve its competitiveness in economical aspects.