Experimental and theoretical study of hydrogen absorption by LaNi3.6Mn0.3Al0.4Co0.7 alloy using statistical physics modeling

A monolayer model treated by statistical physics by means of the grand canonical ensemble has been developed, describing PCT isotherms for absorption of hydrogen by LaNi_3.6Mn_0.3Al_0.4Co_0.7 alloy. This model presents a high correlation with the experimental results. The experimental absorption isotherms are fitted at three temperature different (T = 293 K to T = 313 K). The physicochemical parameters involved in the model were determined from the experimental isotherms by numerical simulation. These parameters, such as two numbers of absorbed atoms per site n₁ and n₂, two receptor site densities N_1M and N_2M, and two energetic parameters, P₁ and P₂ are discussed in relationship with absorption process. The different thermodynamic functions which govern the absorption mechanism such as entropy (S_a), free enthalpy of Gibbs (G) and internal energy (E_int) are derived by statistical physics model calculations.     

Cyclic voltammetry and electrochemical impedance of MmNi3.6Co0.7Mn0.4Al0.3 alloy electrode before and after treatment with a hot alkaline solution containing reducing agent

The hydrogen storage alloy (MmNi_3.6Co_0.7Mn_0.4Al_0.3, Mm=Ce-rich mischmetal) electrodes were treated in an alkaline solution containing a reducing agent (KBH₄ or NaH₂PO₂). Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were applied to characterize the electrochemical properties of the alloy electrodes before and after surface treatment. The results show that the charging efficiency and electrochemical reaction activity of metal hydride (MH) electrode were markedly improved by the treating. The reaction of the untreated MH electrode was chiefly controlled by the charge transfer process at the interface of electrode/electrolyte, or by the mixture of the charge transfer and hydrogen diffusion processes, but the reaction of the treated electrode was mainly controlled by hydrogen atom's diffusion in the alloy bulk. The results of EIS measurements indicate that the charge transfer resistance of MH electrode was reduced and its specific surface area augmented after treatment.     

Microstructures and electrochemical characteristics of Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 hydrogen storage alloys prepared by mechanical alloying

The as-cast alloy with the composition of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 prepared by induction melting was milled for 6, 15, 40 and 50 h in this work. The microstructures of alloys were analyzed by XRD and DSC. Two broadening effects of XRD peak caused by crystallite and microstrain were separated by approximate function method and least square method. Crystallite sizes and microstrains of the alloys were calculated. The results show that alloys milled for 6 h or 15 h consist of nanocrystalline and polycrystalline phase. Lattice parameters (a, c) and volumes of alloy increase with the increasing milling time, whereas the ratio of c/a keep constant. Moreover, the crystallite sizes decrease and the microstrains in the alloys increase first and then decrease with the increase of milling time. The alloys milled for 40 h or 50 h transform partly into amorphous structures. The maximum discharge capacities of alloys decrease with the increase of milling time. The cycle stabilities of the milled alloys are better than those of the as-cast. And they increase with the increasing milling time.     

High temperature performance of La0.6Ce0.4Ni3.45Co0.75Mn0.7Al0.1 hydrogen storage alloy for nickel/metal hydride batteries

La_0.6Ce_0.4Ni_3.45Co_0.75Mn_0.7Al_0.1 hydrogen storage alloy has been prepared and its electrochemical characteristics and gas hydrogen absorption/desorption properties have been investigated at different temperatures. X-ray diffraction results indicated that the alloy consists of a single phase with CaCu₅-type structure. It is found that the investigated alloy shows good cycle performance and high-rate discharge ability, which display its promising use in the high-power type Ni-MH battery. The exchange current density and the diffusion coefficient of hydrogen in the bulky electrode increase with increasing temperature, indicating that increasing temperature is beneficial to charge-transfer reaction and hydrogen diffusion. However, the maximum discharge capacity, the charge retention and the cycling stability degrade with the increase of the temperature.     

Electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 hydrogen storage alloy

The electrochemical properties of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 hydrogen storage alloy have been studied through chronopotentiometric, chronoamperometric and cyclic voltammogram measurements. The maximum capacity value obtained was 265 mAh g⁻¹ at rate C/6 and the capacity decrease was recorded by 1.5% after 30 cycles. The values of the hydrogen diffusion coefficient D_H obtained through cyclic volammogram and chronoamperometric techniques were, respectively, 7.01 × 10⁻⁸ cm² s⁻¹ and 4.23 × 10⁻¹¹ cm² s⁻¹.     

Electrochemical reduction of La(Ni3.6Co0.7Al0.4Mn0.3) (LaMM) deuterides investigated by in situ neutron powder diffraction: Following the metal-hydride phase transition under technical operating conditions in a KOD electrolyte

We investigated the first charge cycle of LaNi_3.6Co_0.7Al_0.4Mn_0.3 (LaMM) during electrochemical reduction in a 6N KOD (potassium deuteroxide) electrolyte, corresponding to conditions of commercially used batteries by means of in situ neutron powder diffraction. Our measurement allowed to directly analyze the phase range of the α and β phases and the related volume change as a function of the charge transfer. The intercalation of hydrogen was followed in a home-made electrochemical cell, installed on the high intensity neutron powder diffractometer (DMC) at the Swiss continuous spallation neutron source. Compared to previous investigations following mostly in situ charging under pressure (following pressure–composition–temperature isotherms, PCT), our experimental conditions reflect closely the process as used in technical battery applications.     

Electrochemical properties of the Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 alloy mixed with Ni powder

Composites of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 alloy and Ni powders were mechanically synthesized and electrochemically tested in 6 M KOH electrolyte. In this work, the electrochemical properties of the alloys were greatly improved by mixing them with Ni, which plays a corrosion-resistance role in the alkali electrolyte and helps electron conduction. It has been shown that the numbers of activation cycles decreased compared with the alloys without Ni powder. All the alloys were activated after the second cycle. Improvements of the maximum discharge capacities were also found in this work. A maximum discharge capacity of 358 mAh g⁻¹ was measured in the Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 + 250 wt.% Ni composite. In addition to that, adding Ni was found to enhance the high-rate discharge ability of the alloys, which appear to be good candidates for the realization of MH battery electrodes.     

Pt and Ru dispersed on LiCoO2 for hydrogen generation from sodium borohydride solutions

Nano-sized platinum and ruthenium dispersed on the surface LiCoO₂ as catalysts for borohydride hydrolysis are prepared by microwave-assisted polyol process. The catalysts are characterized by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Very uniform Pt and Ru nanoparticles with sizes of <10 nm are dispersed on the surface of LiCoO₂. XRD patterns show that the Pt/LiCoO₂ and Ru/LiCoO₂ catalysts only display the characteristic diffraction peaks of a LiCoO₂ crystal structure. Results obtained from XPS analysis reveal that the Pt/LiCoO₂ and Ru/LiCoO₂ catalysts contain mostly Pt(0) and Ru(0), with traces of Pt(IV) and Ru(IV), respectively. The hydrogen generation rates using low noble metal loading catalysts, 1 wt.% Pt/LiCoO₂ and 1 wt.% Ru/LiCoO₂, are very high. The hydrogen generation rate using Ru/LiCoO₂ as a catalyst is slightly higher compared with that of Pt/LiCoO₂.     

Effects of pretreatment of LM-Ni3.9Co0.6Mn0.3Al0.2 alloy powders in a KOH/NaBH4 solution on the electrode characteristics and inner pressure of nickel-metal-hydride secondary batteries

The discharge capacities of lanthanum-rich mischmetal (LM)-Ni_3.9Co_0.6Mn_0.3Al_0.2 alloy electrodes are significantly degraded by an increase in the C rate. Nevertheless, the discharge capacity of alloy electrodes pretreated with KOH/NaBH₄ is maintained higher than that of raw alloy electrodes, with the difference in discharge capacities between the raw and pretreated alloy electrodes more prominent at higher C rates. The charge retention of the electrodes decreases with increasing rest time. In particular, the charge retention of the pretreated alloy electrode is lower than that of the raw alloy electrode due to the higher self-discharge rate. The overvoltage for hydrogen evolution of the pretreated alloy electrode is superior to that of the raw alloy electrode, particularly at higher temperatures. This phenomenon indicates that the charge efficiency of the electrode was significantly improved by the surface pretreatment, resulting from its high surface catalytic activity. Repeated charge-discharge increases the inner pressure of the battery. Nevertheless, due to its higher charge efficiency and faster recombination rate, the inner pressure of the battery made using the pretreated alloy electrode is much smaller than that of the battery made using a raw alloy electrode.     

Development of high power lithium-ion batteries: Layer Li[Ni0.4Co0.2Mn0.4]O2 and spinel Li[Li0.1Al0.05Mn1.85]O4

Layer Li[Ni_0.4Co_0.2Mn_0.4]O₂ and lithium excess spinel Li[Li_0.1Al_0.05Mn_1.85]O₄ were compared as positive electrode materials for high power lithium-ion batteries. Physical properties were examined by Rietveld refinement of X-ray diffraction pattern and scanning electron microscopic studies. From continuous charge and discharge tests at higher currents and different temperature environments using 3Ah class lithium-ion batteries, it was found that both materials presented plausible battery performances such as rate capability, cyclability at 60 °C at elevated temperature, and low temperature properties as well.     

Hydrogen evolution characteristics of Ni–Mn microencapsulated MlNi3.03Si0.85Co0.60Mn0.31Al0.08 alloys in 6 M KOH

Nickel–manganese alloys were coated from sulphate baths by electrodeposition with ‘Packed Bed’ technique on the surface of proprietary lanthanum rich non-stoichiometric MlNi_3.03Si_0.85Co_0.60Mn_0.31Al_0.08 (Ml = lanthanum rich misch metal) hydrogen storage alloy particles. The structure and nature of the microencapsulated alloys were characterized using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR). The hydrogen evolution reaction (HER) was investigated in 6 M KOH at 30 °C by galvnostatic cathodic polarisation technique. The effects of Ni/Mn ratio in the bath and deposition current density were studied. Among the investigated depositions, Ni₁₅₀Mn₁₀₀ (30) and Ni₁₅₀Mn₁₀ (60) (concentration of Ni and Mn salts in electrodeposition bath given in grams per liter; electrodeposition current density (CD) given within brackets in milliamphere per square centimeter) coated samples exhibited the highest activity towards the HER. It can be concluded that disordered paramagnetic coatings with Ni concentrations above 80 at.% exhibit higher catalytic activity towards HER. The Tafel mechanism is the easiest pathway for HER on most of the studied coatings. However, some of the Ni-rich coatings prefer the Volmer–Tafel path and one sample [Ni₁₅₀Mn₁₅₀ (80)] prefers the Heyrovsky–Volmer path.     

Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1−xCexNi2.8Co0.5 (x = 0-0.20) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of (La_0.7Mg_0.3)_1−xCe_xNi_2.8Co_0.5 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys have been investigated. The results show that all alloys consist of (La, Mg)Ni₃ and LaNi₅ phases. The cyclic stability (S₁₀₀) of the alloy electrodes increases from 58.7% (x = 0) to 69.8% (x = 0.20) after 100 charge/discharge cycles. The high rate dischargeability (HRD) increases from 66.8% (x = 0) to 69.6% (x = 0.10), then decreases to 65.1% (x = 0.20) at the discharge current density of 1200 mA/g. Moreover, the electrochemical kinetic characteristics of the alloy electrodes are also improved by increasing Ce content.     

Microstructure and electrochemical investigations of La0.76−xCexMg0.24Ni3.15Co0.245Al0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys

The effects of substitution of Ce for La on the microstructure and electrochemical performance of La_0.76−xCe_xMg_0.24Ni_3.15Co_0.245Al_0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses showed that the main phases of the alloys consist of (La, Mg)Ni₃ phase (PuNi₃-type rhombohedral structure), LaNi₅ phase (CaCu₅-type hexagonal structure) and (La, Mg)₂Ni₇ phase (Ce₂Ni₇-type hexagonal structure). The cell volume of the (La, Mg)Ni₃ phase, (La, Mg)₂Ni₇ phase and LaNi₅ phase decreased monotonously with increasing Ce content. Electrochemical investigations showed a decrease in the discharge capacity, while high rate dischargeability (HRD) first increased and then decreased with increasing Ce content. The Ce substitution for La slightly enhanced the cyclic stability of the alloy electrodes. The pressure–composition (P–C) isotherms showed that the plateau region was broadened with Ce content increased in the alloys, meanwhile, two plateaus appeared and pressure of the hydrogen absorption and desorption increased accordingly.