Solid State Ionics (v.178, #1-2)
Properties of solid state devices with mobile ionic defects. Part I: The effects of motion, space charge and contact potential in metal|semiconductor|metal devices
by Y. Gil; O.M. Umurhan; I. Riess (pp. 1-12).
The characteristics of solid state devices based on p-type semiconductors with mobile acceptors are discussed. The devices are basic ones of the form: metal|semiconductor|metal. The metal electrodes are assumed to be chemically inert and to block material exchange. The effect of the contact potentials as well as of the space charge are taken into consideration. The distribution of charge carriers (holes and acceptors) and the I–V relations are evaluated. These results are compared with those of a model in which the acceptors are immobile and with two approximations in which neutrality is assumed either at the boundary or throughout the whole semiconductor. The motion of the acceptors is found, in some cases, to introduce only minor changes in the I–V relations. This finding may be of significance for solid state devices of reduced scale. The I–V relations of samples much thicker than the equilibrium Debye length reduce to the ones obtained assuming local neutrality throughout the sample. The results also depend significantly on the reaction constant between the acceptors and holes to form neutral acceptors.
Keywords: Solid state device; Semiconductor; Mixed ionic electronic conductor; MIEC; I–V relations; Defect distribution
Location of deuterium atoms in BaSn0.5In0.5O2.75+ α by neutron powder diffraction at 10 K
by Tsuyoshi Ito; Takanori Nagasaki; Kouta Iwasaki; Masahito Yoshino; Tsuneo Matsui; Naoki Igawa; Yoshinobu Ishii (pp. 13-17).
This paper describes a neutron diffraction study that has located hydrogen atoms in a proton-conducting perovskite oxide more definitely than ever. The neutron diffraction data were collected on BaSn0.5In0.5O2.75 with and without dissolved D2O at 10 K. Data analysis by the Rietveld method and the maximum entropy method revealed that deuterium atoms were located at or very close to the 12 h site of the cubic perovskite structure (space group Pm3¯ m) with an O–D distance of 1.0 Å. This hydrogen position is similar to those suggested by the previous computational studies for proton-conducting perovskite oxides.
Keywords: Proton conductor; Perovskite oxide; Neutron diffraction
Microstructural analysis of doped-strontium cerate thin film membranes fabricated via polymer precursor technique
by Mohamed M. Elbaccouch; Satyajit Shukla; Nahid Mohajeri; Sudipta Seal; Ali T-Raissi (pp. 19-28).
Nanocrystalline thin film membranes of terbium (Tb)-doped strontium cerate (SrCeO3), which is of interest in the hydrogen (H2) separation and solid oxide fuel cells (SOFCs), was synthesized via polymer precursor technique. Continuous and dense thin film membranes of composition SrCe0.95Tb0.05O3− δ were prepared using spin-coating technique by utilizing ethylene glycol (EG)-based polymeric precursor. The polymeric precursor was deposited on silicon-based substrates, and converted to dense polycrystalline thin film ceramic membranes by sintering at relatively low temperatures. The number of spin-coating cycles and sintering temperatures were systematically varied to study their effect on the film morphology, thickness, and crystallite size within the membranes. Fourier transform infrared (FTIR) spectroscopy was utilized to study the changes in the polymer chemistry during the membrane processing. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to examine thermal decomposition and thermodynamics of the synthesized precursor, respectively. The scanning electron microscopy (SEM) analysis was used to study the surface morphology and estimate average particle size as a function of number of spin-coating cycles and sintering temperatures. Atomic force microscope (AFM) was utilized to determine the roughness and quality of the spin-coated films. The membrane thickness, crystal structure, and nanocrystallite size were determined using focused ion-beam (FIB) milling and X-ray diffraction (XRD) techniques. Furthermore, the surface chemistry of the thin film membranes was studied by means of X-ray photoelectron spectroscopy (XPS). This study demonstrated that by using the EG-based polymeric precursor, dense and continuous Tb-doped SrCeO3 thin film membranes, having thicknesses in the range of 0.2–2 μm and average nanocrystallite size of 8–70 nm, can be effectively synthesized by controlling the number of spin-coating cycles and sintering temperature.
Keywords: Hydrogen; Thin film; Membrane; Spin-coating; Characterization; SrCe; 0.95; Tb; 0.05; O; 3; −; δ
Preparation and electrical properties of NASICON-type structured Li1.4Al0.4Ti1.6(PO4)3 glass-ceramics by the citric acid-assisted sol–gel method
by Xiaoxiong Xu; Zhaoyin Wen; Jianguo Wu; Xuelin Yang (pp. 29-34).
Lithium-ion conducting Li1.4Al0.4Ti1.6(PO4)3 glass-ceramics with ultrapure NASICON-type phase were synthesized by a citric acid-assisted sol–gel method and characterized by TGA-DSC, XRD, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and complex impedance techniques. The influence of molar ratio of citric acid to cations as well as pH value on the formation of Li1.4Al0.4Ti1.6(PO4)3 sol was studied. Experimental results indicated that the citric acid-assisted sol–gel method made it possible to obtain well crystallized glass-ceramics of Li1.4Al0.4Ti1.6(PO4)3 at a much lower temperature within a shorter synthesis time in comparison to conventional solid-state reaction methods. The optimized conditions for citrate-based preparation process were at the molar ratio ( R) of [H3Cit+glycol]/[Li++Al3++Ti4+]=4 and pH=7, respectively. Highly pure and ultrafine powders were obtained by heating the amorphous powders got from the sol–gel process at a temperature between 800 and 1000 °C. The highest room temperature bulk and total Li+ conductivities were 2.09×10−3 S/cm and 6.13×10−4 S/cm, respectively. And the bulk activation energy as low as 0.29 eV was obtained for the same specimen prepared at 950 °C for 6 h. The high conductivity, good chemical stability and easy fabrication of the glass-ceramics suggest it to be a promising candidate as solid electrolyte for all-solid-state Li-ion rechargeable batteries.
Keywords: Li-ion conductor; Glass-ceramics; Ultrapure NASICON-type phase; Sol–gel method
Electrical conductivity of Li2TiO3 ceramics
by Th. Fehr; E. Schmidbauer (pp. 35-41).
The microstructure of Li2TiO3 ceramics, proposed as blanket material in deuterium–tritium fusion reactors, plays an important role for this application; grain size and grain shape determine to a considerable degree the properties in question. Electrical properties may reflect certain characteristics of the microstructure to some extent. In this communication impedance spectroscopy is applied to analyse electrical charge transport in ceramics of monoclinic β-Li2TiO3, synthesized in different conditions. Li+ ions are suggested to be the majority charge carriers. Typical activation energies for charge transport, deduced from the DC conductivity σDC, are EA∼0.6–0.9 eV and the bulk σDC (573 K)∼3×10−6 Ω−1cm−1. The DC and AC conductivities show some features of the microstructure. Anomalies in the DC conductivity at ∼570 K–900 K are indicative of a change in conduction which is tentatively assigned primarily to the formation of nuclei of the cubic γ-Li2TiO3 phase in this temperature region, well below the first order phase transition β→ γ at 1425 K–1485 K.
Keywords: Impedance spectroscopy; Ionic conduction; Lithium metatitanate; Microstructure; Fusion reactor blanket
Unexpected cationic distribution in tetrahedral/octahedral sites in nominal Li1+ xAl xGe2− x(PO4)3 NASICON series
by Pilar Maldonado-Manso; M.C. Martín-Sedeño; Sebastián Bruque; Jesús Sanz; Enrique R. Losilla (pp. 43-52).
Nominal Li1+ xAl xGe2− x(PO4)3 (0≤ x≤1.2) lithium fast ion conductors have been prepared as polycrystalline powders and characterised by Rietveld analysis of laboratory X-ray powder diffraction patterns (LXRPD). All samples display the ideal R 3¯c symmetry of compounds with NASICON structure. However,7Li,27Al,31P MAS–NMR and AEM analyses suggest an unusual cationic distribution where Al is located in both octahedral and tetrahedral sites, replacing germanium and phosphorus, respectively. In these compounds, Li ions are preferentially disposed at M1 sites, but additional Li incorporated to compensate the charge deficit of aluminium are located near M2 sites of the NASICON structure. Local mobility of Li is responsible for narrowing effects detected in7Li MAS–NMR experiments. Impedance spectroscopy measurements show that these materials are excellent lithium ionic conductors with bulk conductivity values close to 10−4 S cm−1 at room temperature.
Keywords: Lithium ionic conductor; NASICON; Rietveld analysis; 7; Li; 27; Al; 31; P MAS–NMR
The effect of oxygen vacancy concentration on the elastic modulus of fluorite-structured oxides
by Yanli Wang; Keith Duncan; Eric D. Wachsman; Fereshteh Ebrahimi (pp. 53-58).
The elastic modulus of fluorite-structured pure ceria, gadolinium doped ceria (GDC) and yttria stabilized zirconia (YSZ) with different levels of oxygen vacancy concentration were measured by the nanoindentation technique. The samples were heat treated at 800 °C under an oxygen partial pressure in the range of 0.21 to 10−25 atm for 15 h until equilibrium was reached at a sufficient depth. The defect concentrations were conserved to room temperature by fast cooling. The results reveal that the elastic modulus of both pure ceria and GDC decreases drastically in samples heat treated at very low partial pressures of oxygen. However, the elastic modulus change for YSZ, within the studied range, is insignificant. An analysis of the experimental results suggests that an increase in the point defect content weakens the attractive forces between atoms in fluorite-structured oxides.
Keywords: Elastic modulus; Fluorite structure; Nanoindentation; Oxygen vacancy
Effect of non-stoichiometry and synthesis temperature on the structure and conductivity of Ln2+ xM2− xO7− x/2 (Ln=Sm–Gd; M=Zr, Hf; x=0–0.286)
by A.V. Shlyakhtina; A.V. Knotko; M.V. Boguslavskii; S.Yu. Stefanovich; I.V. Kolbanev; L.L. Larina; L.G. Shcherbakova (pp. 59-66).
The effect of non-stoichiometry on the crystal structure and total conductivity of Ln2+ xZr2− xO7− x/2 (Ln=Sm–Gd; x=0–0.286) was investigated. The intensity of the pyrochlore superlattice reflections from Ln2+ xZr2− xO7− x/2 is shown to decrease with increasing Ln concentration. Within the homogeneity range of the pyrochlore phase of Ln2+ xZr2− xO7− x/2 (Ln=Sm–Gd), the activation energy of high-temperature conduction in samples prepared by 1600 °C sintering of mechanically activated oxide mixtures (Ln2O3 and ZrO2) is ∼0.87–1.04 eV. The highest conductivity among the Ln2+ xZr2− xO7− x/2 (Ln=Sm–Gd) materials is offered by stoichiometric Ln2Zr2O7 samples with a pyrochlore structure, which contain 5–8.1% LnZr+ZrLn anti-structure pairs, except for Gd2Zr2O7 (∼22%). The crystal structure of Ln2+ xHf2− xO7− x/2 (Ln=Sm–Gd) is investigated after sintering at 1000–1670 °C. The compounds Ln2.096Hf1.904O6.9 (Ln=Eu, Gd) prepared by 1200 °C sintering of mechanically activated oxides (Ln2O3 and HfO2) undergo a fluorite-type to pyrochlore phase transition above 1200 °C. The conductivity of Gd2Hf2O7 and Sm2.096Hf1.904O6.952 sintered at 1600 °C seems to be ionic above 780 °C, with an activation energy of 0.77 and 0.82 eV, respectively. In this work, using mechanical activation of starting mixtures, the conductivity of the Ln2+ xHf2− xO7− x/2 (Ln=Sm–Gd) hafnates was raised close to the level of Ln2+ xZr2− xO7− x/2 (Ln=Sm–Gd). The hafnates synthesized by the procedure in question are similar in structural disorder to Ln2+ xZr2− xO7− x/2 (Ln=Sm–Gd), and the disorder ensures high oxygen ion mobility and, accordingly, significant high-temperature conductivity.
Keywords: Pyrochlore; Fluorite; Oxide-ion conductor; Ceramics
Ionic conductivity and activation energy for oxygen ion transport in superlattices — The multilayer system CSZ (ZrO2+CaO)/Al2O3
by A. Peters; C. Korte; D. Hesse; N. Zakharov; J. Janek (pp. 67-76).
In multilayer systems consisting of an ionic conductor and an electric insulator the ionic current can flow both across the bulk phase and along the heterophase boundaries. Here we report the results of a study on CSZ (ZrO2+8.7 mol.% CaO)/Al2O3 multilayer systems, representing a system with incoherent interfaces as shown by HRTEM (high resolution transmission electron microscopy). In order to separate the interface contribution of the total conductivity from the bulk contribution, the thickness of the CSZ and Al2O3 layers have been varied systematically, and the oxygen ion conductivity was measured parallel to the interfaces as a function of temperature. The total conductivity of the CSZ increases by two orders of magnitude when the thickness of the individual CSZ layers is decreased from 0.78 μm to 40 nm. It depends linearly on the reciprocal thickness of the individual layers, i.e. on the number of CSZ/Al2O3 interfaces, indicating a parallel connection between individual conduction paths in the bulk and the interfacial regions. The activation energy for the ionic conductivity measured in the temperature range between 350 and 700 °C decreases from about 146 kJ mol−1 to 104 kJ mol−1 by reducing the CSZ layer thickness. The activation energy for the interfacial transport is evaluated as 70 kJ mol−1, suggesting a much higher ionic mobility in the disordered core regions of incoherent interfaces than in the bulk.
Keywords: PACS; 68.35.Fx; 66.30.Pa; 68.37.Lp; 68.37.Hk; 81.15.FgNanoionics; Solid electrolytes; Interfaces; Thin films; Multilayer structures; Pulsed laser deposition
Neutron powder diffraction study at high temperature of the Ruddlesden–Popper phase Sr3Fe2O6+ δ
by F. Prado; L. Mogni; G.J. Cuello; A. Caneiro (pp. 77-82).
The crystal and oxygen defect structure of the n=2 Ruddlesden–Popper phase Sr3Fe2O6+ δ have been studied by in situ high temperature neutron powder diffraction in the temperature range 20≤ T≤900 °C in air. The analysis of the neutron diffraction data revealed the presence of structural oxygen vacancies on both the O(1) sites linking the octahedra along the c axis and the O(3) sites in the FeO2 planes of the perovskite layers. The oxygen vacancies on the O(3) site increase with temperature up to ∼ 0.25 per formula unit at T=900 °C. This result supports previously proposed oxygen ion diffusion mechanism in Sr3Fe2O6+ δ that involves the migration of vacancies from an O(3) site to an adjacent O(1) site. The total linear expansion along the c axis α c=17.7(5) · 10− 6 K− 1 mainly affects the perovskite block while the width of the rock salt layers remains stable with temperature. The total volumetric expansion α V / 3=20(1) · 10− 6 K− 1 is around the average of the TEC values (14.8–27.1 K− 1) reported for the perovskite system La1 − xSr xCo1 − yFe yO3 − δ.
Keywords: Mixed conductor oxides; Sr; 3; Fe; 2; O; 6; +; δ; Ruddlesden–Popper phases; Oxygen defects; Ionic conductivity
Beneficial effect of order–disorder phase transition on oxygen sorption properties of perovskite-type oxides
by Qinghua Yin; Y.S. Lin (pp. 83-89).
A beneficial effect of oxygen vacancy disorder–order phase transition on oxygen sorption properties of perovskite-type oxides is reported in this paper. Three perovskite-type oxides, La0.1Sr0.9Co0.9Fe0.1O3− δ, SrCo0.8Fe0.2O3− δ and La0.5Sr0.5CoO3− δ were prepared by liquid citrate method. The phase transition temperatures from the brownmillerite structure to perovskite structure in He are 780 and 860 °C for SrCo0.8Fe0.2O3− δ and La0.1Sr0.9Co0.9Fe0.1O3− δ, respectively. In the sorption process (He–air), SrCo0.8Fe0.2O3− δ and La0.1Sr0.9Co0.9Fe0.1O3− δ exhibit larger oxygen sorption capacity than La0.5Sr0.5CoO3− δ below the phase transition point temperatures. Oxygen sorption kinetics on all the samples are fast, however, desorption kinetics on the samples are complex, depending on both the temperature and sorbent structure. An enhanced desorption rate is found for SrCo0.8Fe0.2O3− δ and La0.1Sr0.9Co0.9Fe0.1O3− δ at low temperatures with disorder–order phase transition involved in the process. The enhancement results from the lower isosteric heat of sorption on the brownmillerite structure, which implies that oxygen is easier to be released from the lattice structure as compared to the perovskite structure.
Keywords: Perovskite; Brownmillerite; Phase transition; Oxygen sorption; Kinetics
Preparation and properties of thin La1− xSr xCo1− yFe yO3− δ perovskitic membranes supported on tailored ceramic substrates
by O. Büchler; J.M. Serra; W.A. Meulenberg; D. Sebold; H.P. Buchkremer (pp. 91-99).
Mixed conducting La1− xSr xCo1− yFe yO3− δ (LSFC) perovskites are promising materials for oxygen separation membranes at high temperatures. Gastight 10–20 μm-thick perovskite layers were produced by vacuum slip-casting and screen printing on different porous substrates, i.e. Ce0.8Gd0.2O1.9 (CGO), LSFC and composites of both. Some of the substrates are doped with cobalt for enhancing the mechanical stability and the shrinkage rate of the substrate. The manufacturing of the substrates and the different perovskitic asymmetric membranes are described in detail. The microstructure, the chemical compatibility between substrate and membrane, and the stability of the sintered layers were analyzed by scanning electron microscopy (SEM, EDX), SIMS and XRD at room temperature. Also the shrinking rates, surface smoothness, porosity and air permeance of the different substrate compositions are characterized. SEM analysis of the sintered top-layers showed a thickness of 10–20 μm, only closed porosity after sintering at 1200 °C and an He-leak rate in the range between 10−4 to 10−6 mbar l s−1 cm−2. The thermal expansion coefficient and electrical conductivity of the membrane materials and the substrates are measured between 30 and 800 °C and oxygen flux measurements were carried out in the range 750–900 °C.
Keywords: Asymmetric membrane; LSFC; Oxygen separation; Perovskite; Mixed conducting membrane
Mixed conductivity and electrochemical behavior of (La0.75Sr0.25)0.95Cr0.5Mn0.5O3− δ
by V.V. Kharton; E.V. Tsipis; I.P. Marozau; A.P. Viskup; J.R. Frade; J.T.S. Irvine (pp. 101-113).
The electronic and oxygen–ionic transport in (La0.75Sr0.25)0.95Cr0.5Mn0.5O3− δ, a member of promising family of solid oxide fuel cell (SOFC) anode materials, was studied at 1023–1273 K in the oxygen partial pressure range from 10−20 to 0.5 atm. In oxidizing and moderately reducing atmospheres, this perovskite exhibits a predominant p-type electronic conductivity, which lies in the range 20–35 S/cm and is essentially p(O2)-independent. Reducing p(O2) below 10−16–10−12 atm leads to a drastic increase in the oxygen vacancy concentration, ionic conductivity and oxygen permeability, whilst the total conductivity decreases down to 1–3 S/cm. The ion transference numbers, calculated from the oxygen permeation data and measured by the faradaic efficiency technique controlling oxygen pressures at both sides of dense ceramic membranes, vary in the range 9×10−7 to 8×10−5 at 1223–1273 K, increasing with temperature. The average thermal expansion coefficients in air increases from 10.8×10−6 K−1 at 373–923 K up to 14.1×10−6 K−1 at 1223–1523 K. Under both oxidizing and reducing conditions, the electrochemical behavior of porous (La0.75Sr0.25)0.95Cr0.5Mn0.5O3-based electrodes applied onto (La0.9Sr0.1)0.98Ga0.8Mg0.2O3− δ solid electrolyte suggests a key role of electronic transport-related processes. As a result, the electrode performance can be significantly enhanced by optimizing current collector and/or by introducing an additional electronically-conductive component, such as metallic Ni or Ag. Further decrease of overpotentials may be achieved via incorporation of electrocatalytically active additions, including praseodymium oxide in oxidizing atmospheres and ceria at low p(O2).
Keywords: Perovskite; SOFC anode; Mixed conductivity; Oxygen permeation; Faradaic efficiency; Dilatometry; Electrode polarization
Si, Si/Cu core in carbon shell composite as anode material in lithium-ion batteries
by Ke Wang; Xiangming He; Li Wang; Jianguo Ren; Changyin Jiang; Chunrong Wan (pp. 115-118).
Si core in carbon shell (C/Si) composites were synthesized through an inverse emulsion polymerization of resorcinol–formaldehyde (RF) performed in the presence of silicon powders, followed by carbonization in an inert atmosphere. SEM, EDS and XRD analyses of the obtained carbon gel microspheres showed that the Si powders were encapsulated within the carbon gel microspheres. The C/Si composite exhibited a reversible lithium storage capacity of 910 mA h g−1 when used as anode in lithium-ion cells. Furthermore, the addition of Cu for Si core in carbon shell was also investigated to enhance the cycleability. It was shown that the addition of Cu further improved the cycling performance of the composite. The proposed process paves the way to prepare high performance anode materials for lithium-ion batteries.
Keywords: Inverse emulsion polymerization; Carbon gel microspheres; Si core in carbon shell; Si/Cu alloy
Fabrication of Li2O–B2O3–P2O5 solid electrolyte by aerosol flame deposition for thin film batteries
by Kihyun Cho; Teawon Lee; Jangwon Oh; Dongwook Shin (pp. 119-123).
Amorphous Li2O–B2O3–P2O5 thin films were prepared from an aqueous precursor solution of a LiNO3 and BCl3, POCl3 by Aerosol Flame Deposition. The aqueous precursor solution of the LiNO3 was first atomized with an ultrasonic vibrator (1.7 MHz), and B2O3 and P2O5 were synthesized from BCl3 and POCl3 by an oxygen–hydrogen flame. Their microstructure and electrical properties were investigated by SEM, XRD, Raman spectroscopy and impedance spectroscopy.XRD analysis revealed that a crystalline phase of LiCl was formed in the glass soot as well as amorphous boron and phosphorus oxides, and the crystalline LiCl found in deposited glass soot completely disappeared by heat treatment. The correlation between the structural modification of glass network, compositional variation and the conductivity was characterized by Raman spectroscopy. The glass electrolyte fabricated in this study exhibited a maximum conductivity of about 10−7S/cm at 30 sccm flow rate of BCl3 and POCl3.
Keywords: Thin film battery; Li; 2; O–B; 2; O; 3; –P; 2; O; 5; Aerosol Flame Deposition; Solid electrolyte
Effects of Sn doping on the structural and electrochemical properties of LiNi0.8Co0.2O2 cathode materials
by Xiaoling Ma; Chiwei Wang; Jinguo Cheng; Jutang Sun (pp. 125-129).
LiNi0.8Co0.2O2 and Sn-doped LiNi0.8Co0.2O2 cathode materials are synthesized via a rheological phase reaction method. They are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), particle size analysis and electrochemical tests. The effects of Sn doping on the structural and electrochemical properties are investigated. The Sn4+ ion occupies the Ni3+ (or Co3+) ion site to form SnNi∙(or SnCo∙) defects, at the same time, part of the Ni ions are reduced to form NiNi′ defects to make electric charge compensation in the crystal lattice, which is beneficial for stabilizing the structure, reducing the resistance and increasing conductivity of LiNi0.8Co0.2O2.
Keywords: Lithium-ion batteries; Sn doping; Rheological phase reaction method; Cathode materials; Defects
Effect of LiFePO4 coating on electrochemical performance of LiCoO2 at high temperature
by Hong Wang; Wei-De Zhang; Lun-Yu Zhu; Ming-Cai Chen (pp. 131-136).
LiFePO4 coating on LiCoO2 with a thickness ranging 10–100 nm improved the electrochemical performance of the cathode material at high temperature and high potential. At 60 °C and at a rate of 1 C, 5.0 wt.% LiFePO4-coated LiCoO2 showed better capacity retention (132 mAh/g) with 4.2 V charge-cutoff after 250 cycles, while the bare cathode (pure LiCoO2) showed 5% capacity retention under the same conditions after only 150 cycles. When cycled at 60 and 75 °C, the amount of Co dissolution from LiCoO2 was greatly reduced in the LiFePO4-coated cathode material. The coating layer of LiFePO4 not only protects the LiCoO2 but also serves as an active cathode material with good conductivity. The LiFePO4-coated LiCoO2 could be a high performance cathode material for Li-ion battery.
Keywords: Lithium-ion battery; Lithium cobalt oxide (LiCoO; 2; ); Lithium iron phosphates; Coating
Changes in electrochemical insertion of lithium into glass-like carbon affected by catalytic graphitization at 1000 °C
by J.M. Skowroński; K. Knofczyński; M. Inagaki (pp. 137-144).
Glass-like carbon spheres produced by activated carbonization of phenol resin in CO2 atmosphere demonstrated an XRD pattern typical for disordered carbons. This material was entirely useless as anode for lithium-ion cell because of both its extremely low discharge capacity (27 mAh/g) and cyclic reversibility for the first cycle (39%). Heat treatment of the original carbon at temperature 2700 °C appeared to be ineffective for obtaining graphite material. The product of such a treatment yielded discharge capacity of 74 mAh/g, which remained at an unacceptably low level. A simple heat treatment of disordered carbon with iron powder at relatively low temperature of 1000 °C allowed us to improve electrochemical behaviour of the carbon. Due to a partial transformation of glass-like carbon into graphite, discharge capacity increased to about 250 mA/g.
Keywords: Hard carbon; Catalytic graphitization; X-ray diffraction; Intercalation reactions; Anode material; Lithium-ion cell
Origin of electrochemical reactivity enhancement of post-annealed LiFePO4 thin films: Preparation of heterosite-type FePO4
by F. Sauvage; E. Baudrin; L. Laffont; J.-M. Tarascon (pp. 145-152).
We reported the dependence of the LiFePO4 thin films growth by Pulsed Laser Deposition (PLD) on their electrochemical properties. Whether the crystallized films are directly grown within the PLD chambers or post annealed, they present different textures and levels of surface carbon contamination, with the post-annealing films being the most contaminated. The post-annealed thermal treatment induces a drastic increase in the film electrochemical reactivity vs. Li+ (capacity 50 times higher). We show through combined cyclic voltametry and Raman spectroscopy measurements that the enhanced electrochemical activity lies in the films textural evolution. By purposely acting on the LiFePO4 film texturation we succeeded for the first time in preparing, through chemical oxidation, fully delithiated heterosite-type FePO4 thin films. Furthermore using three-electrode electrochemical impedance spectroscopy measurements on such films, we evidenced a noticeable decrease in the charge transfer resistance upon the de-insertion. Finally to account for the specific electrochemical/chemical reactivity of LiFePO4 thin films, a simple planar model is proposed.
Keywords: LiFePO; 4; FePO; 4; Thin film; Cathode; Pulsed Laser Deposition
Catalytic and electrocatalytic synthesis of NH3 in a H+ conducting cell by using an industrial Fe catalyst
by Martha Ouzounidou; Aglaia Skodra; Christos Kokkofitis; Michael Stoukides (pp. 153-159).
The catalytic and electrocatalytic synthesis of ammonia was studied in double-chamber and single-chamber proton conducting cells at 450–700 °C and at atmospheric total pressure. A Fe-based industrial catalyst was used as the working electrode. The kinetics of the catalytic reaction of ammonia synthesis were evaluated under open-circuit. An up to 80% increase in the reaction rate was observed when protons were electrochemically “pumped” to the Fe electrode. A weak NEMCA effect was observed with the Faradaic efficiency ( Λ) and the reaction rate enhancement ( ρ) remaining below 3.0 and 2.0, respectively.
Keywords: Ammonia synthesis; Solid state proton conductor; Fe catalyst-electrode; NEMCA
Investigation of copper dissolution in the presence of glyphosate using hydrodynamic voltammetry and chronoamperometry
by C.F.B. Coutinho; M.O. Silva; M.L. Calegaro; S.A.S. Machado; L.H. Mazo (pp. 161-164).
The copper dissolution was investigated in phosphate buffer in the presence of glyphosate using hydrodynamic voltammetry and chronoamperometry. The electrochemical studies showed that the addition of glyphosate in the phosphate buffer increases the electrochemical dissolution of copper electrode and this phenomenon was associated to the Cu(II) complexation by glyphosate. The apparent rate constant of the copper dissolution process (1.2×10−2 cm mol−1 s−1) and the diffusion coefficient of the complex (0.74×10−5 cm2 s−1), were obtained. It was observed that during the chronoamperometric experiments, in the absence of glyphosate, the electrode surface was covered by blue passive film, probably copper phosphate; nevertheless, in the presence of glyphosate, there was a gradual development of a blue color in the solution, indicating the copper dissolution and formation of the Cu(II)–glyphosate complex.
Keywords: Copper dissolution; Complex; Glyphosate; Passivation