Parallel oxygen and chlorine evolution on Ru1−xNixO2−y nanostructured electrodes

Nanocrystalline materials with chemical composition corresponding to formula Ru_1−xNi_xO_2−y (0.02 < x < 0.30) were prepared by sol-gel approach. Substitution of Ru by Ni has a minor effect on the structural characteristics extractable from X-ray diffraction patterns. The electrocatalytic behavior of Ru_1−xNi_xO_2−y with respect to parallel oxygen (oxygen evolution reaction, OER) and chlorine (chlorine evolution reaction, CER) evolution in acidic media was studied by voltammetry combined with differential electrochemical mass spectrometry (DEMS). The DEMS data indicate a significant decrease of the over-voltage for chlorine evolution with respect to that of pure RuO₂. The oxygen evolution is slightly hindered. The increasing Ni content affects the electrode material activity and selectivity. The overall material's activity increases with increasing Ni content. The activity of the Ru-Ni-O oxides towards Cl₂ evolution shows a distinguished maximum for material containing 10% of Ni. Further increase of Ni content results in suppression of Cl₂ evolution in favor of O₂ evolution. A model reflecting the cation-cation interactions resulting from Ni-doping is proposed to explain the observed trends in electrocatalytic behavior.     

Hydrothermal synthesis of LiNixCo1−xO2 cathode materials

Ultrafine powders of LiCoO₂, nonstoichiometric LiNiO₂ and LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/Co, Li/Ni and Li/(Ni + Co) was studied. The final products were investigated by XRD, TEM and EDS. To synthesize a stoichiometric LiNiO₂ under mild hydrothermal conditions was found to be a big challenge. Transmission electron microscopies (TEM) revealed the formation of well-crystallized LiCoO₂ and LiNi_0.9Co_0.1O₂ with average size of 100 nm and 10 nm, respectively.     

New NiFe2−xCrxO4 spinel films for O2 evolution in alkaline solutions

Spinel-type ternary ferrites with composition NiFe_2−xCr_xO₄ (0 ≤ x ≤ 1) were synthesized by a precipitation method and their physicochemical and electrocatalytic properties have been investigated using IR, XRD, BET surface area, XPS, impedance and Tafel polarization techniques. The study indicated that substitution of Cr from 0.2 to 1.0 mol in the spinel matrix increased the apparent electrocatalytic activity of the base oxide towards the O₂ evolution reaction in 1 M KOH at 25 °C. The apparent electrocatalytic activity of the oxide with 0.8-1.0 mol Cr was found to be the greatest among the present series of oxides investigated. It is noteworthy that the electrocatalytic activity of the oxide with x = 0.8-1.0 was also greater than those of other spinel/perovskite O₂ evolving electrocatalysts reported in literature.     

The effects of decomposition products of electrolytes on the thermal stability of bare and TiO2-coated delithiated Li1−xNi0.8Co0.2O2 cathode materials

In this work, the effects of decomposition products of electrolytes on the thermal stability of bare and TiO₂-coated Li_1−xNi_0.8Co_0.2O₂ (1 > x ≥ 0) cathode material have been investigated by means of thermoanalytical, thermokinetic and temperature-programmed desorption-mass spectroscopy (TPD-MS) techniques. It is shown clearly that the decomposition products of the electrolytes such as carboxylates have distinctive effects on the thermal stability of the electrode materials. Firstly, the thermoanalytical and TPD-MS results indicate that surface coating can suppress the amount of oxygen release from the delithiated cathode material. The thermokinetic analytical results show that the reaction of oxygen release (i.e. oxygen loss) from delithiated Li_1−xNi_0.8Co_0.2O₂ material can be promoted by carboxylate salts supported on the electrode surface due to the decrease of initiated activation energy E_a of the reaction. Finally, the amount of carboxylate salts and length of carbon chains in carboxylates have different promotional effects on the thermal properties of the electrode materials.     

Structural, electrochemical and thermal properties of LiNi0.8−yTiyCo0.2O2 as cathode materials for lithium ion battery

Multiple substitution compounds with the formula LiNi_0.8−yTi_yCo_0.2O₂ (0≤y≤0.1) were synthesized by sol-gel method using citric acid as a chelating agent. The effects of titanium substitution on the structural, electrochemical and thermal properties of the cathode materials are investigated. A solid solution phase (R-3m) is observed in the range of 0≤y≤0.1 for the titanium-doped materials. X-ray photoelectron spectroscopy (XPS) shows that there are Ni³⁺, Ni²⁺, Co³⁺, Co²⁺ and Ti⁴⁺ five transition metal ions in titanium-doped materials. Rietveld refinement of X-ray diffraction (XRD) patterns indicates that titanium substitution changes the materials’ structure with different cationic distribution. An increase of the Ni/Co amount in the 3a Li site is found with the addition of titanium amount. An improved cycling performance is observed for titanium-doped cathode materials, which is interpreted to a significant suppression of phase transitions and lattice changes during cycling. The thermal stability of titanium-doped materials is also improved, which can be attributed to its lower oxidation ability and enhanced structural stability at delithiated state.     

Electrochemical and structural study of Ce0.8Sm0.2−xLaxO1.9 electrolyte materials for SOFC

Ce_0.8Sm_0.2−xLa_xO_1.9 powders, denoted as La_xSDC (for x=0, 0.01, 0.03, 0.05, 0.07 and 0.1), were synthesized via the mechanical milling reaction method. The La³⁺ doping content has a remarkable influence on structural and electrical properties. The phase identification and morphology were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameters were calculated by the Rietveld method. It was observed that the lattice parameter values in Ce_0.8Sm_0.2−xLa_xO_1.9 systems obey Vegard's law. The pellets were then sintered at 1500 °C in air for 7 h. The relative densities of these pellets were over 93.7%.The electrical conductivity was studied using two-probe impedance spectroscopy and results showed that the conductivity of Ce_0.8Sm_0.2−xLa_xO_1.9 first increased and then decreased with La dopant content x. Results also showed that Ce_0.8Sm_0.17La_0.03O_1.9 had the highest electrical conductivity, σ₇₀₀ °C equal to 3.8×10⁻² Scm⁻¹ and an activation energy equal to 0.77 eV. It was therefore concluded that co-doping with the appropriate amount of La can further improve the electrical properties of ceria electrolytes.     

Studies on electrical properties of La0.8Sr0.2Ga0.8Mg0.2O2.80 (LSGM) and LSGM-SrSn1−xFexO3 (x = 0.8; 0.9) composites and their chemical reactivity

We report the electrical conductivity properties of solid-state synthesized perovskite-like La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.80 (LSGM) and LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites. LSGM exhibits both bulk and grain-boundary contribution in the ac impedance plots. The grain-boundary conductivity (σ_gb) is slightly (≤half-order of magnitude) higher than that of the bulk oxide ion conductivity (σ_bulk). Powder XRD study reveals that no chemical reaction occurs between LSGM and SrSn_1−xFe_xO₃ (1:1 wt.%) at 1000 °C (48 h) and forms a single-phase perovskite-like compound at 1300 °C (48 h) in air, while in hydrogen atmosphere, at 800 °C for 48 h, a growth of LaSrGaO₄ and LaSrGa₃O₇ impurity phases and formation of metallic Fe was observed. The LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites show a single or part of semicircle in air at low-temperature regime. The electrical conductivity of the composites were found to be much higher compared to pure LSGM and lower about an order of magnitude than those of pure Sn-doped SrFeO₃ perovskite.     

MnxCu1−xCo2O4 used as bifunctional electrocatalyst in alkaline medium

Composite film electrodes containing mechanically mixed Mn_xCu_1−xCo₂O₄ (0 ≤ x ≤ 1) particles, carbon black Vulcan XC72R and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were formed on the glassy carbon disk surface of a rotating ring-disk electrode (RRDE) and studied for the oxygen reduction and evolution reactions (ORR and OER, respectively) in 1 M KOH solution. The electrocatalytic activities for both reactions were observed to depend strongly on the Mn content in CuCo₂O₄. An opposite trend was observed for the apparent and intrinsic electrocatalytic activities for the ORR; the simultaneous presence of Cu and Mn was found to be detrimental to the intrinsic charge density, but beneficial to the geometric charge density with a maximum for Mn_0.6Cu_0.4Co₂O₄. The latter was characterized by the highest total number of electrons exchanged per O₂ molecule, n, close to 4, greater k₁ (4e⁻ process)/k₂ (2e⁻ process) ratios, and by a unique and low Tafel slope (−41 mV dec⁻¹). The results obtained for the OER showed that the intrinsic electrocatalytic activity is determined by the number of active sites (Co⁴⁺) electrochemically formed at the oxide surface prior to the OER, from Co³⁺ cations. The partial substitution of Cu by Mn in CuCo₂O₄ was found to decrease the OER activity.     

Electrical conductivity and redox stability of La2Mo2−xWxO9 materials

A series of compounds La₂Mo_2−xW_xO₉ (x = 0-2) were synthesized using a freeze-dried precursor method at relatively low temperatures (673-823 K). These materials were characterised by thermogravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetric (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM) and dilatometric measurements. Oxygen stoichiometry was evaluated by coulometric titration and thermogravimetric analysis at 873-1273 K. The ionic and electronic conductivities of these materials were analysed by impedance spectroscopy and a Hebb-Wagner ion-blocking method under moderately reducing conditions. The presence of W⁶⁺ leads to an increase of the stability range (about 10⁻¹⁶ Pa for La₂Mo_0.5W_1.5O₉ at 1073 K) and prevents oxygen loss and amorphisation. Within the stability range, the electronic conductivity increases gradually as the temperature increases and as the oxygen partial pressure reduces. This indicates that the electronic transport is mainly n-type as a result of the oxygen-content decreasing in the molybdate lattice. Further reduction of the oxygen partial pressure gave rise to the decomposition of La₂Mo_2−xW_xO₉, leading to the formation of new phases with molybdenum in lower oxidation states, which further enhances the electronic conductivity. The results of the coulometric titration and the thermogravimetric studies under a dry 5% H₂/Ar flow suggest that tungsten doped lanthanum molybdate materials can be used as electrolyte only at low temperature and under moderate reducing conditions.     

Dielectric and ferroelectric properties of Ba0.8Sr0.2Ti1−5x/4NbxO3 ceramics

Ba_0.8Sr_0.2Ti_1−5x/4Nb_xO₃ ceramics, x = 0, 0.01, 0.05, 0.10, were fabricated by conventional solid-state reaction. With increasing niobium content the ferroelectric phase transition temperature decreases linearly, and the dispersivity of the transition increases. Niobium B-site decreases transition temperature more pronounced than Sr²⁺ at A-site. The heterovalent substitution of Nb⁵⁺ in low content causes local defect dipole, while more substitutions introduce disorder to disturb the long-range dipole correlation. Ba_0.8Sr_0.2Ti_1−0.5/4Nb_0.1O₃ ceramic shows weak ferroelectric loop at room temperature far from its transition temperature, 153 K.     

Effects of surface modification on the cycling stability of LiNi0.8Co0.2O2 electrodes by CeO2 coating

A high-performance LiNi_0.8Co_0.2O₂ cathode was successfully fabricated by a sol-gel coating of CeO₂ to the surface of the LiNi_0.8Co_0.2O₂ powder and subsequent heat treatment at 700 °C for 5 h. The surface-modified and pristine LiNi_0.8Co_0.2O₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), slow rate cyclic voltammogram (CV), and differential scanning calorimetry (DSC). Unlike pristine LiNi_0.8Co_0.2O₂, the CeO₂-coated LiNi_0.8Co_0.2O₂ cathode exhibits no decrease in its original specific capacity of 182 mAh/g (versus lithium metal) and excellent capacity retention (95% of its initial capacity) between 4.5 and 2.8 V after 55 cycles. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.     

Amorphous Ru1−yCryO2 loaded on TiO2 nanotubes for electrochemical capacitors

Amorphous Ru_1−yCr_yO₂/TiO₂ nanotube composites were synthesized by loading different amount of Ru_1−yCr_yO₂ on TiO₂ nanotubes via a reduction reaction of K₂Cr₂O₇ with RuCl₃·nH₂O at pH 8, followed by drying in air at 150 °C. Cyclic voltammetry and galvanostatic charge/discharge tests were applied to investigate the performance of the Ru_1−yCr_yO₂/TiO₂ nanotube composite electrodes. For comparison, the performance of amorphous Ru_1−yCr_yO₂ was also studied. The results demonstrated that the three dimensional nanotube network of TiO₂ offered a solid support structure for active materials Ru_1−yCr_yO₂, allowed the active material to be readily available for electrochemical reactions, and increased the utilization of active materials. A maximum specific capacitance 1272.5 F/g was obtained with the proper amount of Ru_1−yCr_yO₂ loaded on the TiO₂ nanotubes.     

Molecular dynamics simulation on ionic conduction process of oxygen in Ce1−xMxO2−x/2

Molecular dynamics simulation of CeO₂ doped with M³⁺ (a trivalent cation) with ionic radii ranging from 1.019 Å (Y³⁺) to 1.160 Å (La³⁺) were performed to examine the effects of the dopant cation size on ionic conductivity. Interatomic potential parameters were empirically fitted with equilibrium properties and energy barriers from ab initio calculations. Vacancy trapping and edge blocking mechanisms were studied. Analysis on vacancy trapping showed that the effect was more pronounced in La- and Y-doped ceria than those doped with Gd and Sm. Analysis of the edge blocking effect showed that larger-sized dopants would limit the available pathways for vacancy hopping. The combined effects satisfactorily explained the influence of the dopant cation size on the ionic conductivity of heavily doped ceria.     

A novel method for the preparation of submicron-sized LiNi0.8Co0.2O2 cathode material

A novel method has been employed to synthesize layered LiNi_0.8Co_0.2O₂ cathode material by calcination of Ni–Co hydroxide–carbonate precursor prepared by a route involving separate nucleation and aging steps (SNAS) together with LiOH under air atmosphere. Thermogravimetry (TG) and differential thermal analysis (DTA) combined with on-line evolved gas mass spectrometry (EGMS) analysis were employed to study the reaction process. The synthesized material was characterized by means of X-ray diffraction (XRD), laser particle size distribution analysis, field emission scanning electron microscope (FE-SEM) and galvanostatic charge/discharge cycling. The synthesized LiNi_0.8Co_0.2O₂ presents a narrow distribution of submicron-sized particles and exhibits a good electrochemical property with initial discharge specific capacity of 194.8 mAh g⁻¹ in the voltage range 2.75–4.5 V (versus Li/Li⁺). The novel method for the preparation of submicron-sized LiNi_0.8Co_0.2O₂ material has the particular advantage of simple synthesis process and low synthesis cost.     

Micro- and macro-indentation behaviour of Ba0.5Sr0.5Co0.8Fe0.2O3−d perovskite

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−d (BSCF) is a candidate material for the application as oxygen separation membrane. However, the requisite mechanical reliability needs to be warranted. Indentation tests on dense BSCF yielded data for hardness, stiffness and fracture toughness up to a temperature of 340 °C. Complementary to this, the fracture toughness was also evaluated up to 800 °C based on an indentation-strength method.Up to 200 °C, the values of all characteristic mechanical parameters decreased. At high temperatures they increased. The morphology of the indentation cracks depended on the applied indentation load. This was taken into account while selecting suitable expressions for calculating indentation toughness. The temperature dependence of the normalised fracture toughness as determined by indentation technique and indentation-strength method matched quite well. They revealed a good agreement with the temperature dependence of previously reported normalised fracture stresses. In addition to this, the effect of annealing on the mechanical properties of the material was also studied.     

Re-examination of bulk and grain boundary conductivities of Ce1−xGdxO2−δ ceramics

Powders of gadolinium-doped ceria solid solutions, Ce_1−xGd_xO_2−δ (x = 0.05, 0.1, 0.2, 0.3 and 0.4), were prepared by a freeze-drying precursor route. Dense ceramic pellets with average grain sizes in the range of several microns were obtained after sintering at 1600 °C. Cobalt nitrate was added to the powders to obtain dense ceramic samples with grain sizes in the submicrometer range at 1150 °C. The ionic conduction was analysed by impedance spectroscopy in air, to de-convolute the bulk and grain boundary contributions. The bulk conductivity at low temperature clearly decreases with increasing content of Gd whereas the activation energy increases. An alternative method is proposed to analyse the extent of defect interactions on conduction. For samples without addition of Co, the specific grain boundary conductivity increases with increasing Gd content. Addition of cobalt does not alter the bulk properties but produces an important increase in the specific grain boundary conductivity, mainly in samples with lower Gd-concentration (x = 0.05 and 0.1). Segregation of Gd and its strong interaction with charge carriers may explain the blocking effects of grain boundaries.     

Mechanical characterization of porous Ba0.5Sr0.5Co0.8Fe0.2O3−d

The mechanical stability of porous Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−d (BSCF) material was investigated using depth-sensitive microindentation and ring-on-ring biaxial bending tests. The porous BSCF was characterized as potential substrate material for the deposition of a dense membrane layer. Indentation tests yielded values for hardness and fracture toughness up to a temperature of 400 °C, while bending tests permitted an assessment of elastic modulus and fracture stress up to 800 °C. In addition the fracture toughness was evaluated up to 800 °C measuring in bending tests the fracture stress of pre-indented specimens. The results proof that the indentation-strength method can be applied for the determination of the fracture toughness of this porous material. In comparison to dense material the values of the mechanical parameters were as expected lower but the temperature dependences of elastic modulus, fracture strength and toughness were similar to those reported for dense BSCF.     

Synthesis and electrochemical properties of submicron LiNi0.8Co0.2O2 by a polymer-pyrolysis method

A polymer-pyrolysis method was used to synthesize LiNi_0.8Co_0.2O₂, which has potential application in lithium ion batteries. The effect of calcination temperature and time on the structure and electrochemical performance of the material was investigated. XRD analysis showed that the powders obtained by calcination at 750 °C for 3 h had the best-ordered hexagonal layer structure. SEM image showed these powders were fine, narrowly distributed with platelet morphology. The charge-discharge tests demonstrated these powders had the best electrochemical properties, with an initial discharge capacity of 189 mAh/g and capacity retention of 95.2% after 50 cycles when cycled at 50 mA/g between 3.0 and 4.3 V. Besides, these powders also had exhibited excellent rate capability.     

In situ XAFS analysis of the LixNi0.8Co0.2O2 cathode during cycling in lithium batteries

The layered LiNi_0.8Co_0.2O₂ system has drawn interest as a cathode material for lithium battery high-power applications. In order to determine the charge compensation mechanism and structural perturbations occurring in the system during cycling, in situ battery X-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted on a cell cycled at a moderate rate and typical Li-ion battery operating voltages (3.0-4.1 V). The XAFS data collected at the Ni and Co edges approximately every 30 min during cycling revealed details about the response of the cathode to Li insertion and extraction. These measurements on the Li_xNi_0.8Co_0.2O₂ cathode (0.29<x<0.78) demonstrated that the material retains excellent structural short-range order leading to superior cycling. Interestingly, the Co and Ni atoms behaved differently in response to Li insertion/extraction. This study corroborates previous work that explains the XAFS of the Ni atoms in terms of a Ni³⁺ Jahn-Teller ion. An analysis of the metal-metal distances suggests, contrary to a qualitative analysis of the X-ray absorption near-edge structure (XANES), that Co³⁺ is oxidized to the maximum extent possible (within the Li content range of this experiment) at x=0.47±0.04, while Ni³⁺ is oxidized in equal and linear increments proportional to the battery's state-of-charge. XAFS results on discharge show an almost completely reversible process with charge compensation through Co⁴⁺ and Ni⁴⁺ site reduction and a return to the original structural state.     

Electronic transport in Ce0.8Sm0.2O1.9−δ ceramics under reducing conditions

Ce_0.8Sm_0.2O_1.9−δ powders were prepared by a freeze drying method and used to obtain ceramic disks. These samples were used to study the electronic transport properties of this material. A Hebb-Wagner method was used to obtain the electronic conductivity under ion blocking conditions. Typical values of electronic conductivity measured for this material at 800 °C were about 0.37 S m⁻¹ at Po₂=10⁻¹⁶ atm and 0.58 S m⁻¹ at P_O2=10⁻¹⁸ atm. These values are significantly lower than results reported for ceria-based materials with different trivalent additives. A coulometric titration method was used to estimate the charge carrier concentrations, and the mobility of carriers was obtained on combining the results of conductivity and concentration. Typical values of mobility show weak temperature dependence and decrease with increasing oxygen deficiency, suggesting a limiting value of about 0.5×10⁻⁷ m². V⁻¹ s⁻¹ for relatively high δ.