Synthesis and characterization of mesoporous Mn–Ni oxides for supercapacitors

Mesoporous Mn–Ni oxides with the chemical compositions of Mn_1-x Ni _x O _δ (x = 0, 0.2, and 0.4) were prepared by a solid-state reaction route, using manganese sulfate, nickel chloride, and potassium hydroxide as starting materials. The obtained Mn–Ni oxides, mainly consisting of the phases of α- and γ-MnO₂, presented irregular mesoporous agglomerates built from ultra-fine particles. Specific surface area of Mn_1–x Ni _x O _δ was 42.8, 59.6, and 84.5 m² g⁻¹ for x = 0, 0.2, and 0.4, respectively. Electrochemical properties were investigated by cyclic voltammetry and galvanostatic charge/discharge in 6 mol L⁻¹ KOH electrolyte. Specific capacitances of Mn_1-x Ni _x O _δ were 343, 528, and 411 F g⁻¹ at a scan rate of 2 mV s⁻¹ for x = 0, 0.2, and 0.4, respectively, and decreased to 157, 183, and 130 F g⁻¹ with increasing scan rate to 100 mV s⁻¹, respectively. After 500 cycles at a current density of 1.24 A g⁻¹, the symmetrical Mn_1–x Ni _x O _δ capacitors delivered specific capacitances of 160, 250, and 132 F g⁻¹ for x = 0, 0.2, and 0.4, respectively, retaining about 82%, 89%, and 75% of their respective initial capacitances. The Mn_0.8Ni_0.2O _δ material showed better supercapacitive performance, which was promising for supercapacitor applications.     

Electrochemical properties of perovskite and A<Subscript>2</Subscript>MO<Subscript>4</Subscript>-type oxides used as cathodes in protonic ceramic half cells

Three selected materials have been prepared and shaped as cathode of half cells using the proton-conducting electrolyte BaCe_0.9Y_0.1O_3 − δ (BCY10): two perovskite compounds, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) and La_0.6Sr_0.4Fe_0.8Co_0.2O_3 − δ (LSFC), and the praseodymium nickelate Pr₂NiO_4 + δ (PRN) having the K₂NiF₄-type structure. The electrochemical properties of these compounds have been studied under zero current conditions (two-electrode cell) and under polarization (three-electrode cell). Their measured area-specific resistances were about 1–2 Ω cm² at 600 °C. Under direct current polarization, it appears that the three compounds show almost similar values of current densities at 625 °C; however, at lower temperatures, BSCF appears to be the most efficient cathode material.     

Optimization on fabrication and performance of A-site-deficient La0.58Sr0.4Co0.2Fe0.8O3-δ cathode for SOFC

A-site-deficient perovskite cathode material La_0.58Sr_0.4Co_0.2Fe_0.8O_3 − δ (L58SCF) is coated on the yttria-stabilized zirconia electrolyte by screen-printing technique. Several key fabrication parameters including selection of additives (binder and pore former), effect of coating thickness, sintering temperature and time on the microstructure, and electrochemical performance of cathode are investigated by scanning electron microscopy and electrochemical impedance spectroscopy. We study the microstructure and the electrochemical property of the cathode with different kinds of additives. Results show that the cathode possesses fine microstructure, enough porosity, and ideal electrochemical property when polyvinyl butyral serves as both binder and pore former in the cathode. The cathode with three screen-printing coats (thickness 28 ± 7 μm, weight 6.07 ± 0.72 mg cm⁻²) sintering at 1,000 °C for 2 h shows lower polarization resistance of 0.183 Ω cm² at 800 °C. Based on the optimized parameters, the polarization resistances of the L58SCF–Ce_0.8Gd_0.2O_{1.9 – δ} composite cathode display the R _p values of 0.067 Ω cm² at 800 °C, 0.106 Ω cm² at 750 °C, 0.225 Ω cm² at 700 °C, and 0.550 Ω cm² at 650 °C.     

Electrochemical synthesis of ammonia using a cell with a Nafion membrane and SmFe0.7Cu0.3?xNixO3 (x = 0?0.3) cathode at atmospheric pressure and lower temperature

Samaria-doped ceria Ce_0.8Sm_0.2O_2−δ (SDC) and SmFe_0.7Cu_0.3−x Ni _x O₃ have been synthesized by the sol-gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The electrochemical synthesis of ammonia was investigated at atmospheric pressure and low temperature, using the SFCN materials as the cathode, a Nafion membrane as the electrolyte, nickel-doped SDC (Ni-SDC) as the anode and silver-platinum paste as the current collector. Ammonia was synthesized from 25 to 100°C when the SFCN materials were used as cathode, with SmFe_0.7Cu_0.1Ni_0.2O₃ giving the highest rates of ammonia formation. The maximum rate of evolution of ammonia was 1.13 × 10⁻⁸ mol·cm⁻²·s⁻¹ at 80°C, and the current efficiency reached as high as 90.4%. Supported by the National Natural Science Foundation of China (Grant No. 20863007)     

Preparation of BaCe0.8Gd0.2O3−δ by the citrate method and its application in the synthesis of ammonia at atmospheric pressure

BaCe_0.8Gd_0.2O_3– is a kind of high-temperature proton conductor. A precursor of BaCe_0.8Gd_0.2O_3– solid electrolyte was synthesized by the citrate method and characterized by thermal analysis (thermogravimetric analysis–differential thermal analysis), X-ray diffraction and scanning electron microscopy. Using the sintered samples as a solid electrolyte and silver–palladium alloy as electrodes, we synthesized ammonia from nitrogen and hydrogen at atmospheric pressure in the solid-state proton-conducting-cell reactor. The rate of evolution of ammonia was up to 3.09×10^–9 mol s^–1 cm^–2.     

Structural and electric properties of the Ce0.8(Sm1 ? xCax)0.2O2 ? δ system (x = 0.0–1.0)

CeO₂-based solid solutions with a fluorite structure are promising materials as electrolytes of medium-temperature electrochemical devices. This work presents the results of systematic studies of structural and electric properties and oxygen nonstoichiometry of the Ce_0.8(Sm_{1 − x} Ca _x )_0.2O_{2 − δ} system in a wide range of concentrations of 0 < x < 1 performed in order to establish the causes affecting the system conductivity and its behavior in a reducing medium. It is found that a single-phase solid solution of the fluorite type is formed in the whole concentration range. Parameters of its lattice cells decrease linearly at an increase in the concentration of Ca²⁺. Conductivity in air grows when calcium is added due to a decrease in the grain boundary resistance. The maximum conductivity in air was obtained for the composition of Ce_0.8(Sm_0.8Ca_0.2)_0.2O_{2 − δ} and is 13.71 × 10⁻³ S/cm at 873 K. Studies of the dependence of conductivity of the partial pressure of oxygen showed that electron conductivity is observed at a higher oxygen partial pressure at an increase in the temperature and calcium concentration. The critical partial pressure of oxygen ( p_O2 ^* )\left( {p_{O_2 }^* } \right) for the compositions of Ce_0.8(Sm_{1 − x} Ca _x )_0.2O_{2 − δ} with x = 0; 0.2, and 0.5 is 1.83 × 10⁻¹⁶, 1.73 × 10⁻¹³, and 3.63 × 10⁻¹³ atm at 1173 K, respectively, and 2.76 × 10⁻²¹, 5.05 × 10⁻¹⁸, and 1.31 × 10⁻¹⁸ atm at 1023 K.     

Synthesis of ammonia from natural gas at atmospheric pressure with doped ceria–Ca<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>–K<Subscript>3</Subscript>PO<Subscript>4</Subscript> composite electrolyte and its proton conductivity at intermediate temperature

A new type of oxide–salt composite electrolyte, yttrium doped ceria YDC–Ca₃(PO₄)₂–K₃PO₄, was developed and demonstrated for its promising use for ammonia synthesis. Using this composite electrolyte, ammonia was synthesized from nitrogen and natural gas at atmospheric pressure in the solid-state proton conducting cell reactor, and the optimal condition for ammonia production was determined . The evolved rate of ammonia is up to 6.95×10⁻⁹ mol s⁻¹ cm⁻².     

Oxygen ionic transport in Bi2O3-based oxides: The solid solutions Bi2O3–Nb2O5

The minimum concentration of niobium to stabilize the fluorite-type f.c.c. phase in the Bi₂O₃–Nb₂O₅ oxide system at temperatures below 996 K was ascertained to be about 10 mol%. Thermal expansion, electrical conductivity and crystal lattice parameters of the Bi(Nb)O_1.5+δ solid solutions decrease with increasing niobium content. Thermal expansion coefficients were calculated from the dilatometric data to be (10.314.5)×10⁻⁶ K⁻¹ at temperatures in the range 300–700 K and (17.526.0)×10⁻⁶ K⁻¹ at 700–1100 K. The conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics is predominantly ionic. The p-type electronic transference numbers of the Bi(Nb)O_1.5+δ solid solutions in air were determined to be less than 0.1. Annealing at temperatures below 900 K results in a sharp decrease in conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics. Received: 18 August 1997 / Accepted: 20 October 1997     

LiNi<Subscript>0.8</Subscript>Co<Subscript>0.2−x</Subscript>Ti<Subscript>x</Subscript>O<Subscript>2</Subscript> nanoparticles: synthesis,structure, and evaluation of electrochemical properties for lithium ion cell application

Layered Ti-doped lithiated nickel cobaltate, LiNi_0.8Co_{0.2 − x}Ti_xO₂ (where x = 0.01, 0.03, and 0.05) nanopowders were prepared by wet-chemistry technique. The structural properties of synthesized materials were characterized by X-ray diffraction (XRD) and thermo-gravimetric/differential thermal analysis (TG/DTA). The morphological changes brought about by the changes in composition of LiNi_0.8Co_{0.2 − x}Ti_xO₂ particles were examined through surface examination techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. Electrochemical studies were carried out using 2016-type coin cell in the voltage range of 3.0–4.5 V (vs carbon) using 1 M LiClO₄ in ethylene carbonate and diethyl carbonate as the electrolyte. Among the various concentrations of Ti-doped lithiated nickel cobaltate materials, C/LiNi_0.8Co_0.17Ti_0.03O₂ cell gives stable charge–discharge features.     

Physicochemical properties of non-stoichiometric oxides

One of the most important challenges with solid oxide fuel cells (SOFC) is to find cathode materials with high enough catalytic activity for the dissociation of the molecular oxygen. Oxide mixed conductors with the perovskite structure (ABO₃) and high Co content in the B site have been extensively studied to be used as cathode in SOFC. This is the second part of a review of high temperature properties of two mixed conductors systems. The first part was focused on the n = 2 Sr₃FeMO_6+δ (M = Fe, Co, Ni) Rudlesdden Popper phases, while in this paper we discuss the thermodynamic and transport properties of the perovskite solid solution Sr_1−x La _x Fe_0.2Co_0.8O_3−δ (0 ≤ x ≤ 0.4) in the temperature range 773 ≤ T ≤ 1173 K. In particular, the interest has been focused on the x = 0 sample, which exhibits large ionic conductivity values (σ_i ~1 S cm⁻¹), but suffers a structural transformation from cubic to orthorhombic symmetry because the ordering of the oxygen vacancies when the oxygen partial pressure decreases. Measurements of the oxygen chemical potential ( m_\textO2 \mu_{{{\text{O}}_{2} }} ) as function of oxygen content and temperature, coupled with high temperature X-ray diffraction data, permitted us to broaden the knowledge of the T–δ–p(O₂) phase diagram for the x = 0 sample. In addition, we have investigated the effects of the La incorporation on the stability range of the cubic phases of the Sr_1−x La _x Fe_0.2Co_0.8O_3−δ solid solution.     

Effect of preparative methods on electrical and electrochemical performance of lanthanum strontium manganite

The electrochemical performance of La_0.8Sr_0.2MnO₃:Ce_0.8Gd_0.2O₂ composite cathode was investigated for solid oxide fuel cell applications. Sol–gel, combustion, and solid-state syntheses yielded rhombohedral La_0.8Sr_0.2MnO₃, whereas mechanochemical process gave cubic structure. X-ray diffraction results established good chemical stability of La_0.8Sr_0.2MnO₃ with Ce_0.8Gd_0.2O₂ composite cathode. Combustion synthesis was found best among all preparative methods on the basis of lowest area specific resistance 0.70 Ω cm² at 800 °C. The activation energy E _a = 1.09 ± 0.01 eV indicated absorption of O₂ and was the rate-limiting process of cathode.     

A novel Et<Subscript>4</Subscript>NBF<Subscript>4</Subscript> and LiPF<Subscript>6</Subscript> blend salts electrolyte for supercapacitor battery

Electrolytes of 1 M blend salts (LiPF₆ and tetraethylammonium tetrafluoroborate, Et₄NBF₄) have been investigated in supercapacitor battery system with composite LiMn₂O₄ and activated carbon (AC) cathode, and Li₄Ti₅O₁₂ anode. The results obtained with the blend salts electrolytes are compared with those obtained with cells build using standard 1 M LiPF₆ dissolved in ethylene carbonate + dimethyl carbonate + ethyl (methyl) carbonate (EC + DMC + EMC, 1:1:1 wt.%) as electrolyte. It is found that the blend salts electrolyte performs better on both electrochemical and galvanostatic cycling stability, especially cycled at 4 C rate. When the concentration of LiPF₆ is 0.2 M and Et₄NBF₄ is 0.8 M, the capacity retention of the battery is 96.23% at 4 C rate after 5,000 cycles, much higher than that of the battery with standard 1 M LiPF₆ electrolyte, which is only 62.35%. These results demonstrate that the blend salts electrolyte can improve the galvanostatic cycling stability of the supercapacity battery. Electrolyte of 0.2 M LiPF₆ + 0.8 M Et₄NBF₄ in EC + DMC + EMC (1:1:1 wt.%) is a promising electrolyte for (LiMn₂O₄ + AC)/Li₄Ti₅O₁₂.     

Application of the van der Pauw Methodto Conductivity Relaxation Experimentson YBa2Cu3O6+δ

Summary.  The van der Pauw method has been applied to conductivity relaxation experiments on YBa₂Cu₃O_6+δ at 600°C in order to determine the chemical diffusion coefficient as a function of the oxygen partial pressure in the surrounding atmosphere (10⁰ > p _{O 2}/bar > 10⁻³). It is shown that the van der Pauw technique is suitable for monitoring the conductivity relaxation when the oxygen diffusion is perpendicular to the direct current flowing through the sample in accordance with the van der Pauw geometry using thin tablets as samples. The oxygen partial pressure is changed stepwise (generally Δlogp _{O 2} ≤ 0.5) by employing appropriate gas mixtures as well as an electrochemical oxygen pump device. An evaluation formula is given for the determination of the chemical diffusion coefficient neglecting slow surface processes. In addition, the electronic conductivity of YBa₂Cu₃O_6+δ has been measured at 600°C as a function of oxygen partial pressure of the ambient atmosphere (10⁰ > p _{O 2}/bar > 10⁻⁵) by means of the van der Pauw method applying the same experimental set-up. Typical values of the chemical diffusion coefficient are in the range of 10⁻⁶ cm²·s⁻¹; the results of the conductivity measurements are interpreted in terms of an appropriate defect model. Received May 30, 2000. Accepted June 8, 2000     

Catalytic properties of a (A=Ba, Ca, Sr and Y) modified lanthanum hexaaluminates for catalytic combustion of methane

A series of hexaaluminates, La_0.8A_0.2MnAl₁₁O_19−δ samples (A = Ba, Ca, Sr and Y) as new catalysts were prepared by carbonate precipitation and calcined at high temperature. The structure and properties of these samples were characterized by XRD, BET and XPS techniques. Upon calcination at 1200°C, the hexaaluminate structure was formed and it retained the specific surface area of 17∼20 m²g⁻¹. The La_0.8Ca_0.2MnAl₁₁O_19−δ catalyst has a surface area of 19.3 m²g⁻¹ and shows a good activity in CH₄ combustion.     

Electrochemical characteristics of cathodes based on perovskites modified by ceria

A system consisting of a solid oxide electrolyte of the Ce_0.9Gd_0.1O_{2 − x} (CGO) composition in contact with a two-layer cathode based on a nonstoichiometric composition (La_0.8Sr_0.2)_0.95MnO_{3 ± δ} (LSM1) and a stoichiometric perovskite La_0.8Sr_0.2MnO_{3 ± δ} (LSM2) is prepared by the tape-casting process. It was shown that the best electrochemical characteristics are achieved for a three-layer system LSM2/{CGO-LSM1}/CGO sintered at 1410°C. The use of Ce-modified perovskites La_0.8Sr_0.2MnO_{3 ± δ} and La_0.6Sr_0.6CoO_{3 ± δ} as the collector layer of two-layer electrodes allows the electrochemical characteristics at moderately high temperatures (600–750°C) to be improved.     

Oxalic acid-assisted preparation of LiFePO4/C cathode material for lithium-ion batteries

LiFePO₄/C composite is synthesized by oxalic acid-assisted rheological phase method. Fe₂O₃ and LiH₂PO₄ are chosen as the starting materials, sucrose as carbon sources, and oxalic acid as the additive. The crystalline structure and morphology of the products are characterized by X-ray diffraction and field emission scanning electron microscopy. The charge–discharge kinetics of LiFePO₄ electrode is investigated using cyclic voltammetry and electrochemical impedance spectroscopy. It is found that the introduction of appropriate amount of oxalic acid leads to smaller particle sizes, more homogeneous size distribution, and some Fe₂P produced in the final products, resulting in reduced polarization, impedance, and improved Li⁺ ion diffusion coefficient. The best cell performance is delivered by the sample with R = 1.5 (R of the molar ratio of oxalic acid to LiH₂PO₄). Its discharge capacity is 154 mAh g⁻¹ at 0.2 C rate and 120 mAh g⁻¹ at 5.0 C rate. At the same time, it exhibits an excellent cycling stability; no obvious decrease even after 1,000 cycles at 1.0 C rate.     

Electrophysical properties of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>2 − δ</Subscript> + <Emphasis Type="Italic">x</Emphasis> mol % Al<Subscript>2</Subscript>O<Subscript>3</Subscript> solid composite electrolytes

The influence of aluminum and cobalt oxides on the microstructure and the mechanical, electrical, and optical properties of the Ce_0.8Gd_0.2O_2−δ solid electrolyte was studied. Using nanosized Al₂O₃ in the synthesis allowed it to be uniformly distributed in the composite electrolyte and improved the microhardness of the resulting ceramics. The introduction of cobalt oxide made it possible to lower the synthesis temperature during the preparation of gasproof ceramics and hindered, to a certain extent, the formation of the GdAlO₃ phase during synthesis. Using the standard set of investigation methods allowed us to find solutions to several problems in using this electrolyte for solid-oxide fuel cells (SOFCs).     

Microstructure of the LaSrCuO<Subscript>4 − δ</Subscript>|Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface and its reversibility with respect to oxygen

The conditions of formation of electrode/electrolyte interfaces LaSrCuO_{4 − δ}|Ce_0.9Gd_0.1O_{2 − δ} are optimized. It is shown that electrode layers formed by the screen printing method have better developed surfaces and are more uniform and strong as compared with thick film layers applied by a brush. Symmetric LaSrCuO_{4 − δ}|Ce_0.9Gd_0.1O_{2 − δ}|LaSrCuO_{4 − δ} cells with porous electrodes are studied by impedance spectroscopy and cyclic voltammetry in the temperature range of 773–1173 K at the oxygen partial pressure of (28–2.1) × 10⁴ Pa. The oxygen process is shown to be limited by the charge transfer across the electrode/electrolyte interface. The exchange currents are calculated in the temperature range of 773–1173 K to amount from 1 × 10⁻³ to 3.5 × 10⁻² A/cm², which points to the high reversibility of the electrode/electrolyte interface with respect to oxygen.     

The effect of loading and particle size on the oxygen reaction in CGO impregnated Pt electrodes

Porous platinum electrodes impregnated with Gd _x Ce_1−x O_2−δ (CGO) are investigated to characterise how nano-sized CGO grains affect the oxygen reaction. Impedance measurements were performed at temperatures between 450 and 750 °C and at oxygen partial pressures of 0.2 and 5 × 10⁻⁵ bar for electrodes with various CGO loadings and electrodes annealed at various temperatures. The morphology was characterised by scanning electron microscopy and the CGO grain size was determined from X-ray diffraction peak broadening. The results showed that the polarisation resistance decreased with increasing CGO loading and increasing annealing temperature. CGO facilitates transport of oxygen ions thereby increasing the effective triple-phase boundary.     

Synthesis and electrochemical properties of spinel Li4Ti5O12−x Cl x anode materials for lithium-ion batteries

Li₄Ti₅O_12−x Cl _x (0 ≤ x ≤ 0.3) compounds were synthesized successfully via high temperature solid-state reaction. X-ray diffraction and scanning electron microscopy were used to characterize their structure and morphology. Cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge cycling performance tests were used to characterize their electrochemical properties. The results showed that the Li₄Ti₅O_12−x Cl _x (0 ≤ x ≤ 0.3) compounds were well-crystallized pure spinel phase and that the grain sizes of the samples were about 3–8 μm. The Li₄Ti₅O_11.8Cl_0.2 sample presented the best discharge capacity among all the samples and showed better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂. When the discharge rate was 0.5 C, the Li₄Ti₅O_11.8Cl_0.2 sample presented the superior discharge capacity of 148.7 mAh g⁻¹, while that of the pristine Li₄Ti₅O₁₂ was 129.8 mAh g⁻¹; when the discharge rate was 2 C, the Li₄Ti₅O_11.8Cl_0.2 sample presented the discharge capacity of 120.7 mAh g⁻¹, while that of the pristine Li₄Ti₅O₁₂ was only 89.8 mAh g⁻¹.