Pinit Kidkhunthod

Charge storage mechanisms of manganese oxide nanosheets and N-doped reduced graphene oxide aerogel for high-performance asymmetric supercapacitors

Although manganese oxide- and graphene-based supercapacitors have been widely studied, their charge storage mechanisms are not yet fully investigated. In this work, we have studied the charge storage mechanisms of K-birnassite MnO2 nanosheets and N-doped reduced graphene oxide aerogel (N-rGOae) using an in situ X-ray absorption spectroscopy (XAS) and an electrochemical quart crystal microbalance (EQCM). The oxidation number of Mn at the MnO2 electrode is +3.01 at 0 V vs. SCE for the charging process and gets oxidized to +3.12 at +0.8 V vs. SCE and then reduced back to +3.01 at 0 V vs. SCE for the discharging process. The mass change of solvated ions, inserted to the layers of MnO2 during the charging process is 7.4 μg cm−2. Whilst, the mass change of the solvated ions at the N-rGOae electrode is 8.4 μg cm−2. An asymmetric supercapacitor of MnO2//N-rGOae (CR2016) provides a maximum specific capacitance of ca. 467 F g−1 at 1 A g−1, a maximum specific power of 39 kW kg−1 and a specific energy of 40 Wh kg−1 with a wide working potential of 1.6 V and 93.2% capacity retention after 7,500 cycles. The MnO2//N-rGOae supercapacitor may be practically used in high power and energy applications.

Hybrid energy storage of battery-type nickel hydroxide and supercapacitor-type graphene: redox additive and charge storage mechanism

Herein, hybrid energy storages (HESs) of battery-type Ni(OH)2 and supercapacitor-type electrochemically reduced graphene oxide (ERGO) were fabricated using potassium ferricyanide (K3Fe(CN)6) as a redox additive in KOH electrolyte for high specific energy and power applications. The as-fabricated HES of Ni(OH)2//ERGO in a single coin cell (CR2016) size in 4 mM K3[Fe(CN)]6 in 1 M KOH provides a wide working voltage up to 1.6 V and exhibits a maximum specific energy of 85 W h kg−1 at the specific power of 726 W kg−1 with a high capacity retention over 88% after 10 000 cycles, while the HES in 1 M KOH provides a lower maximum specific energy of 61 W h kg−1. A Fe(CN)63−/Fe(CN)64− redox couple has a great electrochemical reversibility in nature since Fe(CN)63− can obtain electrons from Ni(OH)2 through the reduction process and Fe(CN)64− can donate electrons to NiOOH for the oxidation process. The HES reported herein may be practically used for high energy applications.

Improved giant dielectric properties of CaCu3Ti4O12 via simultaneously tuning the electrical properties of grains and grain boundaries by F− substitution

A novel concept to simultaneously modify the electric responses of the grain and grain boundaries of CaCu3Ti4O12 ceramics was proposed, involving doping with F− anions to improve the giant dielectric properties. The grain growth rate of CaCu3Ti4O12 ceramics was enhanced by doping with F− anions, which were found to be homogeneously dispersed in the microstructure. Substitution of F− anions can cause an increase in the resistance of the insulating grain boundary and a decrease in the grain resistance. The former originated from the ability of the F− dopant to enhance the Schottky barrier height at the grain boundaries, leading to a great decrease in the dielectric loss tangent by a factor of 5 (tanδ < 0.1). The latter was primarily attributed to the increase in Ti3+ and Cu+ concentrations due to charge compensation, resulting in a significantly enhanced intensity of space charge polarization at the grain boundaries. This is the primary cause of the increase in dielectric permittivity from ≈104 to ≈105. The giant dielectric and electrical properties were well described by the Maxwell–Wagner polarization relaxation based on the internal barrier layer capacitor model of Schottky barriers at the grain boundaries.

A structural study of praseodymium gallate glasses by combined neutron diffraction, molecular dynamics and EXAFS techniques

Neutron diffraction and Extended X-ray Absorption Fine Structure (EXAFS) studies have been carried out on a range of praseodymium gallate glasses (Pr2O3-Ga2O3) prepared by aerodynamic levitation and laser heating. The short and intermediate range ordering around the rare-earth have been obtained by a multi-technique method. The results show that simple molecular dynamics simulations give a good representation of the coordination structure around the rare-earth in these glasses.

Temperature dependent local structure in BaTiO3 single crystal

ABSTRACT The temperature dependent local structure of BaTiO3 single crystal was investigated by Synchrotron X-ray Absorption Spectroscopy (XAS) technique. From the X-ray Absorption Near-Edge Structure (XANES) measurements at Ti K-edge, it was seen that the local structure around Ti atom gradually changed with increasing temperature, showing the phase transition from tetragonal to cubic. A comparison between the measured and calculated XANES spectra revealed the gradual change of local structure as a result of the decreasing c/a ratio.

Effect of ethanol assistance on chemical structure of ferroelectric bismuth vanadate via solvothermal method

ABSTRACT Bismuth vanadate (BiVO4) was synthesized via ethanol-assisted solvothermal method performing at heating temperature of 200°C for 2–6 h. Characteristics of BiVO4 were investigated by X-ray diffraction, field emission-scanning microscopy and Brunauer, Emmett and Teller technique. Single monoclinic phase with T-shape like particles was obtained and average crystalline sizes as well as surface area were improved by a prolonged reaction time. Electron transitions in the structure were described by diffuse reflectance of the BiVO4. Moreover, X-ray absorption spectra at vanadium K-edge were presented in more information of chemical structure of vanadiums oxidation state and bond distance between vanadium and oxygen.

Effect of nanograin–boundary networks generation on corrosion of carburized martensitic stainless steel

Martensitic stainless steel parts used in carbonaceous atmosphere at high temperature are subject to corrosion which results in a large amount of lost energy and high repair and maintenance costs. This work therefore proposes a model for surface development and corrosion mechanism as a solution to reduce corrosion costs. The morphology, phase, and corrosion behavior of steel are investigated using GIXRD, XANES, and EIS. The results show formation of nanograin–boundary networks in the protective layer of martensitic stainless steel. This Cr2O3–Cr7C3 nanograin mixture on the FeCr2O4 layer causes ion transport which is the main reason for the corrosion reaction during carburizing of the steel. The results reveal the rate determining steps in the corrosion mechanism during carburizing of steel. These steps are the diffusion of uncharged active gases in the stagnant–gas layer over the steel surface followed by the conversion of C into C4− and O into O2− at the gas–oxide interface simultaneously with the migration of Cr3+ from the metal-oxide interface to the gas-oxide interface. It is proposed that previous research on Al2O3 coatings may be the solution to producing effective coatings that overcome the corrosion challenges discussed in this work.

Structure and electrochemical properties of acid-leached LiMn 2 − x Ti x O 4

To gain deeper understanding on the relationships between crystal structure and electrochemical properties of manganese oxide electrodes for supercapacitors, acid-leached LiMn2 − xTixO4 were investigated. Based on both Reitveld refinement on X-ray diffraction patterns and X-ray absorption near edge structure, it is proposed that replacing some Mn with Ti not only enlarges the unit cells but also alters the crystal structure, specifically the occupancy of each element in the lattice. While the tetrahedral 8a position and octahedral 16c position are both vacant in leached MnO2, those in leached LiMn2 − xTixO4 with x > 0 are occupied by Ti and Mn, respectively. The differences in the unit cell dimension and the crystal structure affect the electrochemical behaviors of the compounds. Although the total specific capacitance generally decreases with Mn active species, trend in %Cins is opposite in samples with x = 0.25–0.75. Despite having some cations in the positions which should hinder the ion diffusion, the enlarged cavity size resulting from Ti substitution promotes the diffusion process and enhances the insertion capacitance in these samples.

Effect of electric field on local structure of PZT single crystal studied by X-ray absorption spectroscopy technique

ABSTRACT The aim of this work is to investigate the local structure of PbZr1−xTixO3 (PZT) single crystal with x = 0.44 under the application of electric field employing Synchrotron X-ray Absorption Near-Edge Structure (XANES) technique. In this experiment, PZT single crystal was subjected to 0.35 kV/mm and 0.70 kV/mm electric fields. The Ti K-edge XANES spectra reflect the significant change of the local structure of PZT unit cell before and after static electrical loading. From the XANES results, it was clearly seen that XANES spectral intensity gradually changed when the electrical load was applied. This can be caused by the shift of Ti atoms in the unit cell. The clear agreement between the measured and simulated XANES spectra was the supporting evidence of Ti off-center displacement in the PZT unit cell.

Phase formation investigation in PZT materials by Synchrotron X-ray absorption spectroscopy techniques

ABSTRACT PbZr1-xTixO3 (PZT) powders (x = 0.45–0.54) were synthesized by conventional solid state reaction method. The phase information was investigated by a combination of X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. The XRD measurements indicated the global phase formation of the prepared powders was the mixture of rhombohedral and tetragonal phases. However, the local phase formation around titanium atom was revealed using X-ray Absorption Near-Edge Structure (XANES), indicating the existence of monoclinic phase within the compositional range studied.