Perovskite-type oxides La1−xSrxMnO3 for cathode catalysts in direct ethylene glycol alkaline fuel cells

Carbon-supported La_1−xSr_xMnO₃ (LSM/C) was prepared by reversible homogeneous precipitation method, and its catalytic activities for oxygen reduction under the existence of ethylene glycol (EG) were investigated by using rotating disk electrode. LSM/C exhibited the high activity for oxygen reduction irrespective with the presence of EG, indicating that EG is not oxidized by LSM/C at the cathode side in the present system. Consequently, LSM/C can serve as a cathode catalyst in alkaline direct alcohol fuel cells with no crossover problem. Performance test for fuel cells operation also supported these results and showed cathodic polarization curves were not affected by the concentration of EG supplied to anode even at 5 mol dm⁻³.     

Thermal stability of Li1−yNixMn(1−x)/2Co(1−x)/2O2 layer-structured cathode materials used in Li-Ion batteries

In order to develop safe lithium-ion batteries using Ni-based cathode active materials, such as LiNi_xMn_(1−x)/2Co_(1−x)/2O₂, thermal stability is one of the most important requirements. We used XRD and TDS-MS in the first step of our study to elucidate the thermal stability and to improve it under anomalous high temperature conditions. We investigated the relationship between the thermal stability and cathode composition, especially for that of the nickel and lithium content. The XRD indicated that the crystal structure of electrochemically delithiated materials changed from a layered into a spinel structure followed by a rock-salt structure as the temperature rose. The TDS-MS indicated that these changes coincided with the release of oxygen from the cathode materials. We found that decreasing the lithium content and increasing the nickel content made the temperature of the crystal structure change and oxygen release lower, and thus, influenced the cathode composition.     

Structural and electrochemical properties of Ti1.5Zr5.5VxNi10−x

Quaternary alloys with the formula Ti_1.5Zr_5.5V_xNi_10−x (x between 0 and 3.0) were studied as a potential replacement for Laves phase alloys used as the negative electrode active material in nickel metal hydride batteries. The V-containing alloys all show multi-phase structures. The major phase shifts from a Zr₇Ni₁₀ structure to a Zr₉Ni₁₁ structure and finally to a C14 structure as the vanadium content increases. Other minor phases with C15 and ZrNi crystal structures are also present. The solubility of vanadium is high in AB2 phases (both C14 and C15), moderate for the ZrNi phase and very low for Zr₇Ni₁₀ and Zr₉Ni₁₁ phases. The bulk hydrogen transport property of the alloys is dominated by synergetic effects between major and minor phases. Electrochemical testing shows that the highest discharge capacity, 357 mAh/g, was obtained from an alloy with a chemical composition of Ti_1.5Zr_5.5V_2.5Ni_7.5 and mainly C14 structure. Testing also shows the high rate dischargeability is controlled by the surface reaction and Ti_1.5Zr_5.5V_0.5Ni_9.5 has the best high rate dischargeability.     

LaFeyNi1−yO3 supported nickel catalysts used for steam reforming of ethanol

LaFe_yNi_1−yO₃ perovskite-type oxide supported highly dispersed NiO catalysts were prepared by one-step citric-complexing method, and applied to the steam reforming of ethanol for hydrogen production. NiO/LaFeO₃ prepared by impregnation was also presented for comparison. The XRD and TEM results indicate that one-step citric-complexing method is a simple as well as an effective way for producing well-dispersed NiO particles supported on perovskite oxides. The dispersive NiO particles tend to interact with the perovskite oxide and partially incorporate into the perovskite structure, leading to the formation of LaFe_yNi_1−yO₃ and some resultantly separated Fe ions onto the perovskite surface. The smaller the NiO particles are, the easier the incorporation is. The catalystic performance tests showed that the high activities of NiO/LaFe_yNi_1−yO₃ were attributed to the metallic Ni with high dispersion. The CH₄ selectivity was sensitive to the particle sizes of supported Ni, and the smaller nickel particles favor the lower amount of methane formed. Characterizations of used catalysts indicated that the sintering of nickel particles was not significant even at the high reaction temperature. The LaFe_yNi_1−yO₃ supported nickel catalysts exhibited very good carbon deposition resistance, which could be ascribed to the highly dispersed Ni particles and the formation of oxygen vacancies in LaFe_yNi_1−yO₃ due to the partial substitution of Ni ions for Fe ions.     

Alloying effects on the hydride formation of Zr(Mn1−xCox)2

To study the alloying effects on ZrMn₂-H system, thermodynamic properties of Zr(Mn_1−xCo_x)₂ hydride were measured by volumetric method. ZrMn₂ gave a single plateau region in the pressure-composition isotherm. On the other hand, double plateaus were clearly observed in Zr(Mn_0.7Co_0.3)₂ and Zr(Mn_0.6Co_0.4)₂-H systems. The appearance of the double plateau characteristics would be explained in view of the hydrogen binding in the tetrahedral occupation sites in Zr(Mn_1−xCo_x)₂. Since the hydrogen binding in the tetrahedral 2ZrMnCo site would be less stable than that in the 2Zr2Mn site, the equilibrium pressure increases with increasing cobalt content. The appearance of the first plateau was ascribed the increase in the bonding of Mn-H in 2Zr2Mn site adjoining the 2ZrMnCo site.     

Structural characteristics and electrochemical performance of layered Li[Mn0.5−xCr2xNi0.5−x]O2 cathode materials

Li[Mn_0.5−xCr_2xNi_0.5−x]O₂ (0 < 2x <0.2) (Mn/Ni = 1) cathode materials have been synthesized by a solution method. X-ray diffraction patterns of the as-prepared materials were fitted based on a hexagonal unit cell (α-NaFeO₂ layer structure). The extent of Li/Ni intermixing decreased, and layering of the structure increased, with increasing Cr content. Electrochemical cycling of the oxides, at 30 °C in the 3–4.3 V range vs. Li/Li⁺, showed that the first charge capacity increased with increasing Cr content. However, maximum discharge capacity (∼143 mAh g⁻¹) was observed for 2x = 0.05. X-ray absorption near edge spectroscopic (XANES) measurements on the K-edges of transition metals were carried out on pristine and delithiated oxides to elucidate the charge compensation mechanism during electrochemical charging. The XANES data revealed simultaneous oxidation of both Ni and Cr ions, whereas manganese remains as Mn⁴⁺ throughout, and does not participate in charge compensation during oxide delithiation.     

Hydrogen storage properties of Mg1−xCaxNi2−y defect solid solutions

Ternary Mg_1−xCa_xNi_2−y solid solutions were synthesized by powder sintering. The phase structures and hydrogen storage properties of the sintered samples were investigated. In a certain range of x and y values, the samples are a single C15 Laves phase with various types of defects. The reduction of Ni content leads to the formation of omission solid solution with vacancies on the sites of Ni. These vacancies increase the hydrogen storage capacity, but decrease the reversibility of hydrogen absorption and desorption.     

Novel FexCr2−x(MoO4)3 electrocatalysts for oxygen evolution reaction

Transition metal mixed oxides of Fe, Cr and Mo with nominal compositional formula, Fe_xCr_2−x(MoO₄)₃ (x = 0, 0.25, 0.50 and 0.75) have been obtained by a co-precipitation method and investigated for their structural and electrocatalytic properties by XRD, TEM, XPS, BET, electrochemical impedance spectroscopy and anodic Tafel polarization. Results show that introduction of Fe for Cr from 0.25 to 0.75 mol into the Cr₂(MoO₄)₃ matrix improved the electrocatalytic activity toward the O₂ evolution reaction (OER) in 1 M KOH considerably; the magnitude of improvement being maximum with 0.5 mol Fe. Values of the Tafel slope were close to 35 mV at low and 2.303RT/F at high overpotentials on Fe-substituted oxides. The OER follows nearly second order kinetics in OH⁻ concentration at low overpotentials.     

Physical and electrochemical properties of spherical Li1+x(Ni1/3Co1/3Mn1/3)1−xO2 cathode materials

A (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor with an uniform, spherical morphology was prepared by coprecipitation using a continuously stirred tank reactor method. The as-prepared spherical (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor served to produce dense, spherical Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ (0 ≤ x ≤ 0.15) cathode materials. These Li-rich cathodes were also prepared by a second synthesis route that involved the use of an M₃O₄ (M = Ni_1/3Co_1/3Mn_1/3) spinel compound, itself obtained from the carbonate (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor. In both cases, the final Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ products were highly uniform, having a narrow particle size distribution (10-μm average particle size) as a result of the homogeneity and spherical morphology of the starting mixed-metal carbonate precursor. The rate capability of the Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ electrode materials, which was significantly improved with increased lithium content, was found to be better in the case of the denser materials made from the spinel precursor compound. This result suggests that spherical morphology, high density, and increased lithium content were key factors in enabling the high rate capabilities, and hence the power performances, of the Li-rich Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ cathodes.     

Hydrogen absorption properties of Zr(V1−xFex)2 intermetallic compounds

The Zr(V_1−xFe_x)₂ (x = 0.02, 0.05, 0.10, 0.15, 0.25) alloys were prepared by the arc-melt method and annealed at 1273 K for 168 h in an argon atmosphere. Phase structure investigations of the as-cast and annealed Zr(V_1−xFe_x)₂ alloys indicate the annealing treatment can eliminate the minority phases originating from the non-equilibrium solidification of as-cast alloys. The ZrV₂-type phase becomes the dominant one in each annealed alloy. The substitution of Fe in V sites leads to the contraction of their lattice. For annealed Zr(V_1−xFe_x)₂ alloys, the P–t and PCT curves obtained between 673 K and 823 K give the evidence that the absorption process is controlled by a rate-controlling hydrogen diffusion. With the increase of iron, the equilibrium pressure and the plateau slope increase while the hydrogenation capacity and the absolute value of enthalpy and entropy decrease accordingly. The stability of metal hydride reduces gradually as the Fe content varies from x = 0.02 to 0.25 which promotes the hydrogen release and favors the practical applications of the Zr(V_1−xFe_x)₂ alloys.     

RuxNb1−xO2 catalyst for the oxygen evolution reaction in proton exchange membrane water electrolysers

Bimetallic catalyst system of ruthenium oxide (RuO₂) and niobium oxide (Nb₂O₅) was prepared using the Adams method and the hydrolysis method. Physical and electrochemical characterizations of the catalysts were studied using X-ray diffraction (XRD), Scanning electron microscopy (SEM), cyclic voltammogram (CV) and polarization measurements. Nb₂O₅ addition to RuO₂ was found to increase the stability of RuO₂. In Adams method the sodium nitrate was found to be forming complex with Nb₂O₅ at high temperature reaction. This makes Adams method unsuitable for the synthesis of RuO₂–Nb₂O₅ bimetallic system. Hydrolysis method on other hand does not have this problem. But a proper mixture of two oxides was not obtained in hydrolysis method. A lower crystallite size for bimetallic system was obtained with Adams method compared to hydrolysis method. RuO₂ prepared by Adams method had higher activity compared to the hydrolysis counterpart in electrolyzer operation with nafion membrane. A cell voltage of 1.62 V was obtained with RuO₂ (A) at 1 A/cm². A higher stability for Ru_0.8Nb_0.2O₂(A) compared to RuO₂(A) was observed in continuous cyclic voltammogram and electrolyzer cell test.     

Activity of perovskite La1−xSrxMnO3 catalysts towards oxygen reduction in alkaline electrolytes

The behaviour of the perovskite-based series of compounds La_1−xSr_xMnO₃ (where x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) towards oxygen reduction in an ambient temperature alkaline 1 M KOH electrolyte is presented. Within this series, the intermediate compound La_0.4Sr_0.6MnO₃ exhibits the greatest catalytic activity, approaching that of the considerably more expensive fuel cell grade Pt-black examined under the same conditions. The origin of this activity is discussed in terms of material structure and morphology, which exists in the structural transition region between cubic LaMnO₃ and hexagonal SrMnO₃. The small crystallite size and relatively large BET surface area of this material reflect this high level of structural disorder. Furthermore, these features enable this compound to exhibit the greatest proportion of direct four-electron oxygen reduction (preferred) compared to the less efficient two-electron reduction to peroxide.     

Effect of Ni content on the hydrogen storage behavior of ZrCo1−xNix alloys

ZrCo_1−xNi_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared and their hydrogen storage behavior were studied. ZrCo_1−xNi_x alloys of compositions with x = 0, 0.1, 0.2 and 0.3 prepared by arc-melting method and characterized by X-ray diffraction analysis. XRD analysis showed that the alloys of composition with x = 0, 0.1, 0.2 and 0.3 forms cubic phase similar to ZrCo with traces of ZrCo₂ phase. A trace amount of an additional phase similar to ZrNi was found for the alloy with composition x = 0.3. Hydrogen desorption pressure–composition–temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus and the hydrogen desorption pressure–composition isotherms (PCIs) were generated for all the alloys in the temperature range of 523–603 K. A single sloping plateau was observed for each isotherm and the plateau pressure was found to increase with increasing Ni content in ZrCo_1−xNi_x alloys at the same experimental temperature. A van't Hoff plot was constructed using plateau pressure data of each pressure–composition isotherm and the thermodynamic parameters were calculated for desorption of hydrogen in the ZrCo_1−xNi_x–H₂ systems. The enthalpy and entropy change for desorption of hydrogen were calculated. In addition, the hydrogen absorption–desorption cyclic life studies were performed on ZrCo_1−xNi_x alloys at 583 K up to 50 cycles. It was observed that with increasing Ni content the durability against disproportionation of alloys increases.     

Hydrogen storage properties of the pseudo binary laves phase (Sc1−xZrx)(Co1−yNiy)2 system

The (Sc_1−xZr_x)(Co_1−yNi_y)₂-H_z system has been studied using both experimental techniques and ab initio calculations. The material was synthesised through high temperature synthesis and characterised using powder XRD. Hydrogen absorption and desorption was studied in-situ using synchrotron radiation. Maximal storage capacity increased when Co replaced Ni and substitution of Sc for Zr increased the equilibrium pressure. Density functional based calculations reproduce the experimental trends in terms of cell parameters both for the non-hydrogenated systems as well as for the hydrogenated systems, and helped to quantitatively understand the observed hydrogen uptake properties.     

Synthesis and characterization of doped Li[Mn0.5−x/2Ni0.5−x/2Cox]O2 positive electrode materials

Layered positive electrode materials for rechargeable lithium-ion batteries of the general formula Li[Mn_0.5−x/2Ni_0.5−x/2Co_x]_1−yM_yO₂ (x ≤ 1/3, 0 ≤ y ≤ 0.05) were synthesized by a solid state route. The effect of doping elements M on the electrochemical performance was investigated. It was found that doping with niobium or tantalum has a positive effect on the cycling stability compared to the undoped parent compounds (y = 0). High discharge capacities, excellent cycling stabilities and high rate capabilities were achieved.     

Hydrogen storage properties and microstructures of Ti0.7Zr0.3(Mn1−xVx)2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys

Hydrogen storage properties, activation performance and thermodynamics of Ti_0.7Zr_0.3(Mn_1−xV_x)₂ (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys and associated microstructures and surface chemical states were investigated by hydrogenation measurements and relevant structure and surface characterization methods. The results showed that the phase composition of the alloy changed from single C14 Laves phase (x ≤ 0.2) to coexistent Laves phase and V-based BCC solid solution phase with increasing V content (x ≥ 0.3). The V in the alloys catalyzed hydrogen dissociation and improved resistivity to oxygen poisoning, so that the alloys could be easily and quickly activated at 293 K even after being exposed in air for a long time. The hydrogen storage capacity of the alloy increased and the plateau pressure decreased with increasing V content. The x   = 0.2 and 0.3 alloys exhibited the best reversible hydrogen storage capacities of above 1.8 wt% at 1 kPa–4 MPa and 293 K. The relative partial molar enthalpy |ΔH|$\left|\Delta H\right|$ increased but the relative partial molar entropy |ΔS|$\left|\Delta S\right|$ decreased with increasing V content, and deviated from the linear relationship for x = 0.4 and 0.5 alloys due to coexisted BCC phase in the alloys.     

Structural and transport properties of layered Li1+x(Mn1/3Co1/3Ni1/3)1−xO2 oxides prepared by a soft chemistry method

In this work structural and transport properties of layered Li_1+x(Mn_1/3Co_1/3Ni_1/3)_1−xO₂ oxides (x = 0; 0.03; 0.06) prepared by a “soft chemistry” method are presented. The excessive lithium was found to significantly improve transport properties of the materials, a corresponding linear decrease of the unit cell parameters was observed. The electrical conductivity of Li_1.03(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ composition was high enough to use this material in a form of a pellet, without any additives, in lithium batteries and characterize structural and transport properties of deintercalated Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ compounds. For deintercalated samples a linear increase of the lattice parameter c together with a linear decrease of the parameter a with the increasing deintercalation degree occurred, but only up to 0.4-0.5 mol of extracted lithium. Further deintercalation showed a reversal of the trend. Electrical conductivity measurements performed of Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ samples (y = 0.1; 0.3; 0.5; 0.6) showed an ongoing improvement, almost two orders of magnitude, in relation to the starting composition. Additionally, OCV measurements, discharge characteristics and lithium diffusion coefficient measurements were performed for Li/Li⁺/Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ cells.     

Effects of Mg substitution on crystal structure and hydrogenation properties of Pr1−xMgxNi3

The effects of substitution of Pr by Mg in PrNi₃ with a PuNi₃-type structure were investigated using pressure–composition (P–C) isotherm measurements and X-ray diffraction. The unit cell of Pr_0.68Mg_0.32Ni_3.04 contracted anisotropically in comparison to that of PrNi₃. The maximum hydrogen capacity of PrNi₃ reached 1.25 H/M in the first absorption. A plateau region was observed between 0.82 H/M and 1.04 H/M in the first absorption cycle. However, 0.85 H/M of hydrogen remained in the sample after the first full desorption. Pr_0.68Mg_0.32Ni_3.04 showed reversible hydrogenation properties. The maximum hydrogen capacity was 1.22 H/M. The plateau region of Pr_0.68Mg_0.32Ni_3.04 was between 0.08 H/M and 0.87 H/M, which was wider than that of PrNi₃. Pr_0.68Mg_0.32Ni_3.04 retained the PuNi₃-type structure after hydrogenation, whereas the crystal structure of PrNi₃ changed from that of PuNi₃-type to an unknown structure. The structural change in PrNi₃ during hydrogenation was evidently different from that in Pr_0.68Mg_0.32Ni_3.04.     

Visible-light-driven photocatalytic H2 evolution from water splitting with band structure tunable solid solution (AgNbO3)1−x(SrTiO3)x

(AgNbO₃)_1−x(SrTiO₃)_x samples were successfully employed as photocatalysts for photocatalytic hydrogen evolution under visible light. The samples were characterized by a series of techniques, including X-ray diffractometry, scanning electron microscopy, UV–Vis spectrophotometry, and electrochemistry technology. The band gaps of (AgNbO₃)_1−x(SrTiO₃)_x solid solutions can be tuned continuously from 3.21 to 2.65 eV and the flat-band potentials (V_fb) can be shifted positively from −0.79 to −0.31 V vs. SHE when x decreased from 1 to 0. Band positions of (AgNbO₃)_1−x(SrTiO₃)_x samples were further testified by density functional theory, suggesting that the band gap narrowing of the solid solutions derived from the hybridization of (Ti 3d and Nb 4d) and (O 2p and Ag 4d) orbital. The photocatalytic activities of samples for H₂ evolution with Pt cocatalyst were evaluated in aqueous methanol solution under visible light irradiation. The highest photocatalytic activity was obtained at (AgNbO₃)_0.25(SrTiO₃)_0.75. Photocatalytic activity in hydrogen evolution of these solid solutions proved to be closely dependent on band structures.     

Photocatalytic hydrogen production from aqueous methanol solutions under visible light over Na(BixTa1−x)O3 solid-solution

A series of equivalent substitution solid-solution Na(Bi_xTa_1−x)O₃ (x = 0–0.10) was prepared by a simple hydrothermal method using Ta₂O₅ and NaBiO₃ as precursors. The Na(Bi_0.08Ta_0.92)O₃ photocatalyst exhibited the highest performance of H₂ evolution (59.48 μmol h⁻¹ g⁻¹) under visible-light irradiation (λ > 400 nm) without co-catalyst, whereas no H₂ evolution is observed for NaTaO₃ under the same conditions. The UV-Vis spectra indicate that the Na(Bi_0.08Ta_0.92)O₃ powders can absorb not only ultraviolet light like pure NaTaO₃ powder but also the visible-light spectrum. The absorption edge corresponds to a band gap of 2.88 eV. The results of density functional theory calculation illuminate that the visible-light absorption bands in the Na(Bi_xTa_1−x)O₃ catalysts are attributed to the band transition from the O2p to the Bi2s + 2p + Ta5d hybrid orbital.