Application of a.c. impedance technique to the study of the proton diffusion process in the porous MnO2 electrode

The proton diffusion coefficient in γ-MnO₂ at various stages of discharge of an electrolytic manganese dioxide electrode has been determined taking into consideration the true molar volume and the electrochemically accessible surface area of MnO₂. The unique electrochemical cell which allows the electrode expansion during the discharge to be monitored in situ was reported. The a.c. impedance technique and the transmission line equivalent circuit were used. Electrode ohmic resistance, Faradaic resistance, and double-layer capacitance were also obtained by means of numerical fitting of the impedance data using the transmission line model.     

Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na₂SO₄ and fumed silica (SiO₂). The MnO₂ electrode showed a maximum specific capacitance of 151 F g⁻¹ after 1000 cycles at 100 mV s⁻¹ when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g⁻¹ obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.     

Interfacial synthesis of porous MnO2 and its application in electrochemical capacitor

In this paper, the porous manganese dioxide (MnO₂₎ was prepared by an interfacial reaction of potassium permanganate in water/ferrocene in chloroform. The surface area and pore distribution of MnO₂ can be controlled by simply adjusting the reaction time and the content of surfactant in the aqueous phase. The electrochemical performance of the prepared MnO₂ was evaluated as an electrode material for supercapacitors by the means of cyclic voltammetry and galvanostatic charge-discharge tests. Electrochemical tests results indicated that the pore size plays an important role at high charge-discharge rate, the sample with a large pore size shows a better rate capability, while the sample with a small pore size but large surface area delivers a large capacitance at low current rate.     

A study of lithium insertion into MnO2 containing TiS2 additive a battery material in aqueous LiOH solution

The electrochemical behavior and surface characterization of manganese dioxide (MnO₂) containing titanium disulphide (TiS₂) as a cathode in aqueous lithium hydroxide (LiOH) electrolyte battery have been investigated. The electrode reaction of MnO₂ in this electrolyte is shown to be lithium insertion rather than the usual protonation. MnO₂ shows acceptable rechargeability as the battery cathode. The influence of TiS₂ (1, 3 and 5 wt%) additive on the performance of MnO₂ as a cathode has been determined. The products formed on reduction of the cathode material have been characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), fourier transform infrared spectroscopy (IR) and transmission electron microscopy (TEM). It is found that the presence of TiS₂ to ≤3 wt% improves the discharge capacity of MnO₂. However, increasing the additive content above this amount causes a decrease in its discharge capacity.     

MnO2/MCMB electrocatalyst for all solid-state alkaline zinc-air cells

Nanostructured MnO₂/mesocarbon microbeads (MCMB) composite has been prepared successfully for use in zinc-air cell as electrocatalyst for oxygen reaction. The scanning electron microscope (SEM) images showed that the MnO₂ nanorods were formed and covered on the surface of MCMB in bird’s nest morphology. X-ray diffraction (XRD) pattern indicated that the MnO₂ has the hollandite structure with a composition approximating KMn₈O₁₆. By the cathodic polarization curve tests, the nanostructured material demonstrated excellent electrocatalytic activity as a kind of oxygen electrode electrocatalyst compared with electrolytic MnO₂. An all solid-state zinc-air cell has been fabricated with this material as electrocatalyst for oxygen electrode and potassium salt of cross-linked poly(acrylic acid) as an alkaline polymer gel electrolyte. The cell has good discharge characteristics at room temperature.     

Coaxial carbon nanofibers/MnO2 nanocomposites as freestanding electrodes for high-performance electrochemical capacitors

Carbon nanofibers (CNFs)/MnO₂ nanocomposites were prepared as freestanding electrodes using in situ redox deposition and electrospinning. The electrospun CNFs substrates with porosity and interconnectivity enabled the uniform incorporation of birnessite-type MnO₂ deposits on each fiber, thus showing unique and conformal coaxial nanostructure. CNFs not only provided considerable specific surface area for high mass loading of MnO₂ but also offered reliable electrical conductivity to ensure the full utilization of MnO₂ coatings. The effect of MnO₂ loading on the electrochemical performances was investigated by cyclic voltammetry (CV), impedance measurements and Galvonostatic charging/discharging technique. The results showed that an ultrathin MnO₂ deposits were indispensable to achieve better electrochemical performance. The maximum specific capacitance (based on pristine MnO₂) attained to 557 F/g at a current density of 1 A/g in 0.1 M Na₂SO₄ electrolyte when the mass loading reached 0.33 mg/cm². This freestanding electrode also exhibited good rate capability (power density of 13.5 kW/kg and energy density of 20.9 Wh/kg at 30 A/g) and long-term cycling stability (retaining 94% of its initial capacitance after 1500 cycles). These characteristics suggested that such freestanding CNFs/MnO₂ nanocomposites are promising for high-performance supercapacitors.     

Morphological characteristics of mechanically processed ZnO and MnO2 powder mixtures

The evolution of structural-morphological characteristics of ZnO + 1 wt.% MnO₂ and ZnO + 10 wt.% MnO₂ samples during prolonged mechanical treatment (MT) was investigated by X-ray, AFM, LPS, SEM and IR methods. Changes in crystallite size due to mechanical activation were determined from X-ray measurements. These results were correlated with changes in particle size followed by SEM and AFM. Particle and aggregate size distributions were determined from LPS measurements. Interpretation of the evolution of IR-absorption peaks during mechanical activation using the theory of average dielectric constants enabled analysis of changes of particle and agglomerate shapes. All these results enabled establishment of a relationship between crystallites, particles, aggregates of particles, and superficial processes that occur during mechanical activation.     

Synthesis and electrical properties of nanostructured Ba2SnYO5.5 proton conductor

Ba₂SnYO_5.5 nanopowders were synthesized by a gel polymerization method. In this process, a gel containing Ba, Sn and Y cations has been obtained by the polymerization of acrylic acid using N,N′-methylene bis-acrylamide as a cross-linking reagent and hydrogen peroxide as an igniting reagent. The gel was calcined at 1200 °C, giving rise to the Ba₂SnYO_5.5 single-phase nanopowders with the grain size ranging from 50 nm to 70 nm. Nanopowders were sintered at 1150 °C by spark plasma sintering (SPS) to obtain dense nanostructure materials (> 95%) containing grains whose size ranges between 70 nm and 100 nm. Nanostructured Ba₂SnYO_5.5 shows a good chemical stability in wet atmosphere. However, its protonic conductivity decreases compared with that of microcrystallin Ba₂SnYO_5.5 ceramics.     

A comparative electrochemical study of LiMn2O4 spinel thin-film and porous laminate

Novel Electrostatic Spray Deposition (ESD) technique was used to fabricate LiMn₂O₄ spinel thin-films. Cyclic voltammograms of both the ESD and porous laminate films show the double peaks in the 4.0 V range characteristic of the LiMn₂O₄ spinel materials. The porous laminates exhibit two semicircles in the impedance spectra while the ESD films show only one single semicircle. The diffusion time constant in the laminate films was typically one order of magnitude larger than that in the ESD thin-films. The apparent lithium-ion chemical diffusion coefficient in LiMn₂O₄ was found to be of the order of 10⁻⁹ cm²/s for both the porous laminate film and the ESD films despite the difference in the diffusion time constants.     

Structural modification of Li[Li0.27Co0.20Mn0.53]O2 by lithium extraction and its electrochemical property as the positive electrode for Li-ion batteries

In our previous report, we have synthesized Li₂MnO₃-LiCoO₂ solid solutions and have investigated electrochemical properties [J.-M. Kim, T. Sho, N. Kumagai, Electrochem. Commun. 9 (2007) 103]. These materials have showed a long charge plateau at above 4.5 V during the first charge, which disappears with the subsequent cycles. This phenomenon is usually observed in Li₂MnO₃ and Li₂MnO₃-LiMeO₂ system (Me = Ni_1/2Mn_1/2 [Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid State Lett. 4 (2001) 191], Co [K. Numata, C. Sakaki, S. Yamanaka, Solid State Ionics 117 (1999) 257; Y.J. Park, Y.-S. Hong, X. Wu, M.K. Kim, K.S. Ryu, S.H. Chang, J. Electrochem. Soc. 151 (2004) A720], Fe [M. Tabuchi, A. Nakashima, H. Shigemura, K. Ado, H. Kobayashi, H. Sakaebe, H. Kageyama, T. Nakamura, M. Kohzaki, A. Hirano, R. Kanno, J. Electrochem. Soc. 149 (2002) A509], or Cr [B. Ammundsen, J. Paulsen, Adv. Mater. 13 (2001) 943]). In this study, we investigate the relationship between the first lithium extraction process and the electrochemical property of the synthesized Li[Li_0.27Co_0.2Mn_0.53]O₂ material. The crystal structure and electrochemical performance of the synthesized Li[Li_0.27Co_0.20Mn_0.53]O₂ are modified by the Li⁺ extraction.     

A novel LiTi2(PO4)3/MnO2 hybrid supercapacitor in lithium sulfate aqueous electrolyte

In this work, carbon-coated lithium-ion intercalated compound LiTi₂(PO₄)₃ and MnO₂ have been synthesized and they deliver a capacity of 90 and 60 mAh/g in 1 M Li₂SO₄ neutral aqueous electrolyte within safe potentials without O₂ and H₂ evolution, respectively. The novel hybrid supercapacitor in which MnO₂ was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode was assembled and the LiTi₂(PO₄)₃/MnO₂ hybrid supercapacitor showed a sloping voltage profile from 0.7 to 1.9 V, at an average voltage near 1.3 V, and delivers a capacity of 36 mAh/g and an energy density of 47 Wh/kg based on the total weight of the active electrode materials. It exhibits a desirable profile and maintains over 80% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibit an excellent rate capability, even at a power density of 1000 W/kg, it has a specific energy 25 Wh/kg compared with 43 Wh/kg at the power density about 200 W/kg.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

On the fractal study of LiMn2O4 electrode surface

Fractal dimension of a LiMn₂O₄ electrode prepared by sol-gel method was determined using electrochemical techniques based on the phenomenon of “diffusion towards electrode surface”. A simple discussion was made on the methodology to understand what is really estimated as the fractal dimension. It was demonstrated that the value of fractal dimension determined based on electrochemical methods is strongly dependent on the electrochemical system situation. This is generally true for all real electrodes involving insertion/extraction processes. This comes from the fact that surface morphology of the electrode is subject of significant changes during the electrochemical experiment.     

Influence of structural parameters on proton insertion in γ-MnO2

The proton insertion/deinsertion behavior of γ-MnO₂ samples of various oxygen contents and various structural parameters has been studied. The analysis of the voltammograms observed during the discharges and charges of the different samples is presented, and leads to a classification of the samples according to their structural parameters. The proton insertion capacities and the cycling properties of the different samples have been compared and correlated to their structural properties.     

XANES study of polyaniline-V2O5 and sulfonated polyaniline-V2O5 nanocomposites

In this work, two materials for secondary lithium battery cathodes formed by polyaniline-V₂O₅ and sulfonated polyaniline-V₂O₅, which have a higher charge capacity than the V₂O₅ xerogel, were synthesized. X-ray absorption and Fourier transform infrared spectroscopies were employed to analyze the short-range interactions in these materials. Based on these experiments, it was possible to observe significant differences in the symmetry of the VO₅ units, and this was attributed to the intimate contact between V₂O₅ and the polymers, and to some flexibility of the VO₅ square pyramids due to the low range order of the nanocomposites.     

The study of lithium ion de-insertion/insertion in LiMn2O4 and determination of kinetic parameters in aqueous Li2SO4 solution using electrochemical impedance spectroscopy

In this work, we report a basic study on the mechanism of lithium ion de-insertion/insertion process from/into LiMn₂O₄ cathode material in aqueous Li₂SO₄ solution using electrochemical impedance spectroscopy (EIS). An equivalent circuit distinguishing the kinetic parameters of lithium ion de-insertion/insertion is used to simulate the experimental impedance data. The fitting results are in good agreement with the experimental results and the parameters of the kinetic process of Li⁺ de-insertion and insertion in LiMn₂O₄ at different potentials during charge and discharge are obtained using the same circuit. The results indicate that the de-insertion/insertion behavior of lithium ions at LiMn₂O₄ cathode in Li₂SO₄ aqueous solution is similar to that reported in the organic electrolytes. The charge transfer resistance (R_ct), warburg resistance, double layer capacitance and chemical diffusion coefficient (DLi+) vary with potentials during de-insertion/insertion processes. R_ct is lowest at the CV peak potentials and the important kinetic parameter, DLi+ exhibits two distinct minima at potentials corresponding to CV peaks during de-insertion–insertion and it was found to be between 10⁻⁸ and 10⁻¹⁰ cm² s⁻¹during lithium de-insertion/insertion processes.     

Al2O3-coated SnO2/TiO2 composite electrode for the dye-sensitized solar cell

A novel Al₂O₃-coated SnO₂/TiO₂ composite electrode has been applied to the dye-sensitized solar cell. In such an electrode, two kinds of energy barriers (SnO₂/TiO₂ and TiO₂/Al₂O₃) were designed to suppress the recombination processes of the photo-generated electrons and holes. After the SnO₂ was modified by colloid TiO₂, the photoelectric conversion efficiency of the SnO₂/TiO₂ composite cell increased to 2.08% by a factor of 2.8 comparing with that of the SnO₂ cell. The Al₂O₃ layer on the SnO₂/TiO₂ composite electrode further suppressed the generation of the dark current, resulting in 37% improvement in device performance comparing with the SnO₂/TiO₂ cell.     

Investigation on electrochemical reduction process of Nb2O5 powder in molten CaCl2 with metallic cavity electrode

The direct electrochemical reduction process of Nb₂O₅ powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb₂O₆ was formed upon addition of solid Nb₂O₅ into molten CaCl₂ when CaO was present. During the electrolysis solid Nb₂O₅ was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V (vs. Ag/AgCl), which was more positive than the reduction potential of Ca²⁺. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V (vs. Ag/AgCl). Nb₂O₅ pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb₂O₅ powder in a molten system.     

Effect of pore structure on anomalous behaviour of the lithium intercalation into porous V2O5 film electrode using fractal geometry concept

The effect of pore structure on anomalous behaviour of the lithium intercalation into porous V₂O₅ film electrode has been investigated in terms of fractal geometry by employing ac-impedance spectroscopy combined with N₂ gas adsorption method and atomic force microscopy (AFM). For this purpose, porous V₂O₅ film electrodes with different pore structures were prepared by the polymer surfactant templating method. From the analysis of N₂ gas adsorption isotherms and the triangulation analysis of AFM images, it was found that porous V₂O₅ surfaces exhibited self-similar scaling properties with different fractal dimensions depending upon amount of the polymer surfactant in solution and the spatial cut-off ranges. All the ac-impedance spectra measured on porous V₂O₅ film electrodes showed the non-ideal behaviour of the charge-transfer reaction and the diffusion reaction, which resulted from the interfacial capacitance dispersion and the frequency dispersion of the diffusion impedance, respectively. From the comparison between the surface fractal dimensions by using N₂ gas adsorption method and AFM, and the analysis of ac-impedance spectra by employing a constant phase element (CPE), it is experimentally confirmed that the lithium intercalation into porous V₂O₅ film electrode is crucially influenced by the pore surface irregularity and the film surface irregularity.     

Experimental study of bluff-body stabilized LPG-H2 jet diffusion flame with preheated reactant

The effects of hydrogen, H₂ addition and preheated reactants on bluff-body stabilized LPG-H₂ diffusion flame for two cases namely, (I) preheated air and (II) preheated air and fuel are reported in the present paper. Results confirm that the H₂ addition leads to a reduction in flame length. Besides this, the flame length is also observed to be reduced with increasing reactant temperature and lip thickness of the bluff-body. The soot free length fraction (SFLF) for both cases is observed to be increased with H₂ addition to fuel stream, which might have caused due to decrease in the C/H ratio in the flame. Interestingly, the SFLF is observed to be reduced with increasing lip thickness and reactants temperature, which can be attributed to the attenuation in induction period of soot formation and enhanced soot volume fraction, respectively. The NO_x emission level is found to be decreased in coaxial burner with hydrogen addition for both case I and II. In contrast, it is observed to be enhanced in bluff-body stabilized flame. The former can be ascribed to the reduction in residence time of gas mixture, whereas the latter can be explained on the basis of enhanced flame temperature. Besides this, emission index of NO_x (EINO_x) is also found to be enhanced with increase in lip thickness and reactant temperature which may be caused due to both enhanced residence time and thermal effect, respectively.