The electrochemical behavior of Ni–MH battery,Ni(OH)2 electrode and metal hydride electrode

In the past years, the nickel–metal hydride rechargeable battery is already been widely used in many application fields, and is now replacing the nickel–cadmium battery. Electrochemical impedance spectroscopy technique is extensively used to reveal the electrode process kinetics, for example, Pb–H₂SO₄ battery, Ni–Cd battery. There are few reports on the EIS of Ni–MH battery. In this paper, an extensive study is focused on the electrochemical impedance spectroscopy of metal hydride electrode, β–Ni(OH)₂ electrode and Ni–MH battery with different states of charge.     

Hydrogenation and electrochemical studies of La–Mg–Ni alloys

La–Mg–Ni alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying with subsequent annealing under an argon atmosphere at 973 K for 0.5 h, were used to produce La_2-xMg_xNi₇ alloys (x = 0, 0.25, 0.5, 0.75, 1). Shaker type ball mill was used. An objective of the present study was to investigate an influence of amount of Mg in alloy on electrochemical, hydrogenation and dehydrogenation properties of La–Mg–Ni materials. X-ray diffraction analyses revealed formation of material with multi-phase structure. Obtained materials were studied by a conventional Sievert's type device at 303 K. It was observed that electrochemical discharge capacity and gaseous hydrogen storage capacity of La–Mg–Ni alloys increases with Mg content to reach maximum for La_1.5Mg_0.5Ni₇ alloy. Moreover, all of La–Mg–Ni alloys were characterized by improved hydrogen sorption kinetics in comparison to La–Ni alloy.     

Effect of Sm on hydrogen storage performance of La–Sm–Mg–Ni alloys

Abstract

In this study, Sm was adopted in order to completely replace the expensive Pr/Nd elements in the A2B7 type alloy. The results indicate that Sm is a favourable element for forming Ce2Ni7 type and Ce5Co19 type phases. With the increasing amount of Sm, the discharge capacity of the alloy retains a value of 283·3 mAh g^?1 at the current density of 1200 mA g^?1. The maximum discharge capacity of the alloys increases with the increasing Sm content when Mg content is relatively low. By optimising the composition and processing technology, the cycle life the alloy enhances from 74 cycles to more than 540 cycles, and the maximum discharge capacity also increases from 300 to 355 mAh g^?1.     

Effects of surface coating with polyaniline on electrochemical properties of La–Mg–Ni-based electrode alloys

The effects of surface coating with polyaniline on electrochemical properties of La_0.8Mg_0.2Ni_3.4Al_0.1 electrode alloys were studied in this paper. The flake-shaped polyaniline coatings were deposited on the surface of La_0.8Mg_0.2Ni_3.4Al_0.1 alloy powders by electrodeless deposition. Electrochemical studies showed that the discharge capacity increased to 391.8 mAh g⁻¹ after surface modification with polyaniline, compared to 382.5 mAh g⁻¹ for the bare alloys. The cyclic stability over 100 cycles improved from 81.6% to 87.5%. Also, the kinetic properties were investigated in detail.     

A paste type negative electrode using a MmNi5 based hydrogen storage alloy for a nickel–metal hydride (Ni–MH) battery

Different conducting materials (nickel, copper, cobalt, graphite) were mixed with a MmNi₅ type hydrogen storage alloy, and negative electrodes for a nickel–metal hydride(Ni–MH) rechargeable battery were prepared and examined with respect to the discharge capacity of the electrodes. The change in the discharge capacity of the electrodes with different conducting materials was measured as a function of the number of electrochemical charge and discharge cycles. From the measurements, the electrodes with cobalt and graphite were found to yield much higher discharge capacities than those with nickel or cobalt. From a comparative discharge measurements for an electrode composed of only cobalt powder without the alloy and an electrode with a mixture of cobalt and the alloy, an appreciable contribution of the cobalt surface to the enhancement of charge and discharge capacities was found.     

Effect of Mo–Ni treatment on electrochemical kinetics of La–Mg–Ni-based hydrogen storage alloys

In order to improve kinetic properties of La–Mg–Ni-based hydrogen storage alloys, Mo–Ni treatment was applied to La_0.88Mg_0.12Ni_2.95Mn_0.10Co_0.55Al_0.10 alloy powders. FESEM results showed that after Mo–Ni treatment some network-shaped substance with nano-size formed on the surface of the alloy particles. The EDS results revealed increase in Ni content and emerge of Mo element. EIS and Linear polarization showed that charge-transfer resistance decreased and exchange current density increased for the treated alloy electrode, and the high rate dischargeability (HRD) was consequently improved. HRD at 1500 mA/g increased from 22.5% to 39.5%. Mo- and Ni-single treatments were performed compared with the Mo–Ni treatment, and the results showed that the single treatment improved HRD slightly, far less than the Mo–Ni treatment.     

Ti–Ni alloys as MH electrodes in Ni–MH accumulators

In hydrogen solid–gas reaction at 300 K and 1 bar, the hydrogen content for Ti_3.87Ni_1.73Fe_0.7O_x (0.2≤ × ≤0.8) alloys was in range 1.93–0.05 (Cwt.H,%), and discharge capacity of 360–235 A h/kg was achieved accordingly. The ΔHH₂$\Delta H_{{\text{H}}_2}$ and ΔSH₂$\Delta S_{{\text{H}}_2}$ values of −32.29 kJ mol⁻¹ and −111.04 J mol⁻¹ K⁻¹, respectively, for Ti_3.87Ni_1.73Fe_0.7O_0.5 alloy were obtained using experimental PCT relations, where hysteresis effect was only slightly visible. The half-cell potentials (vs. Hg/HgO) of metal hydride (MH) electrodes based on Ti_3.87Ni_1.73Fe_0.7O_x (0.2≤ ×≤ 0.8) alloys were calculated.     

Structure and hydrogen storage performances of La–Mg–Ni–Cu alloys prepared by melt spinning

The (Mg₂₄Ni₁₀Cu₂)_100-xLa_x(x = 0, 5, 10, 15, 20) alloys were prepared adopting the method of melt spinning technology. Adding La brings on the formation of secondary phases of La₂Mg₁₇ and LaMg₃, while it does not change the major phase of Mg₂Ni. Originally, there already have nanocrystals and amorphous structures in the experimental alloys, and the addition of La is more conducive to the formation of glass. With adding La in as-spun alloys, the gaseous hydrogen absorption capacity was significantly reduced, but it markedly improved their hydriding rates. Adding La and melt spinning considerably enhanced the dehydriding rate, the reason for which is the decrease of activation energy incurred by adding La and melt spinning. In addition, the discharge capacity of the alloys were able to reach a maximum value during La content varying, and it obviously increased with spinning rate rising.     

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H₂ in 600 s and desorb 3.9 wt.% H₂ in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H₂, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.     

Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg₂Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg₂Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH₂ and Mg₂NiH₄ – a certain amount of Mg₂NiH_0.3 was always present. Similar as Mg₂NiH₄ phase, Mg₂NiH_0.3 islands were located on the surface of MgH₂ grains. Mg₂NiH_0.3 transformed into Mg₂NiH₄ at the expense of hydrogen from an adjoining MgH₂ grain. In such a way, a clean double layer (Mg)–Mg₂NiH₄ was formed, acting as a gate for easy hydrogen desorption from MgH₂. It was found that the Mg₂NiH₄ phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg₂NiH₄ desorption bridges and enrichment of Mg₂NiH₄ on LT1 during the ageing facilitated onset of rapid hydrogen desorption.     

Fabrication of Mg–Ni–Sn alloys for fast hydrogen generation in seawater

Mg-2.7Ni-x wt.% Sn(x = 0–2) alloys were fabricated to promote hydrogen generation kinetics of Mg-2.7Ni alloy. The Sn in Mg-2.7Ni-Sn alloys exists as Mg₂Sn phase at the grain boundary and solid solution at the Mg matrix. The Mg₂Sn at the grain boundary acts as the initiation site for pitting corrosion and the dissolved Sn in the alloy causes pitting corrosion by locally breaking the surface oxide film in the Mg matrix in seawater. The Mg-2.7Ni-1Sn alloy showed an excellent hydrogen generation rate of 28.71 ml min^?1 g^?1, which is 1700 times faster than that of pure Mg due to the combined action of galvanic and intergranular corrosion as well as pitting corrosion in seawater. As the solution temperature was increased from 30 to 70 °C, the hydrogen generation rate from the hydrolysis of the Mg-2.7Ni-1Sn alloy was dramatically increased from 34 to 257.3 ml min^?1 g^?1. The activation energy for the hydrolysis of Mg was calculated to be 43.13 kJ mol^?1.     

Hydrogen-absorbing alloys for the NICKEL–METAL hydride battery

In recent years, owing to the rapid development of portable electronic and electrical appliances, the market for rechargeable batteries has increased at a high rate. The nickel-metal hydride battery (Ni/MH) is one of the more promising types, because of its high capacity, high-rate charge/discharge capability and non-polluting nature. This type of battery uses a hydrogen storage alloy as its negative electrode. The characteristics of the Ni/MH battery, including discharge voltage, high-rate discharge capability and charge/discharge cycle lifetime are mainly determined by the construction of the negative electrode and the composition of the hydrogen-absorbing alloy. The negative electrode of the Ni/MH battery described in this paper was made from a mixture of hydrogen-absorbing alloy, nickel powder and polytetrafluoroethylene (PTFE). A multicomponent MmNi₅-based alloy (Mm_0.95Ti_0.05Ni_3.85 Co_0.45Mn_0.35Al_0.35) was used as the hydrogen-absorbing alloy. The discharge characteristics of the negative electrode, including discharge capacity, cycle lifetime, and polarization overpotential, were studied by means of electrochemical experiments and analysis. The decay of the discharge capacity for the Ni/MH battery (AA size, 1 Ah) was about 1% after 100 charge/discharge cycles and 10% after 500 charge/discharge cycles.     

Composition dependent microstructure evolution,activation and de-/hydrogenation properties of Mg–Ni–La alloys

In this work, in order to elucidate the effect of different alloying elements on the microstructure, activation and the de-/hydrogenation kinetics performance, the Mg–20La, Mg–20Ni and Mg–10Ni–10La (wt.%) alloys have been prepared by near equilibrium solidification combined with high-energy ball milling treatment to realize the internal optimization as well as particle refinement. The results show that the microstructures of the prepared alloys are significantly refined by forming different types and sizes of intermetallic compounds. Meanwhile, the effects of LaH₃ and Mg₂Ni within the activated samples on de-/hydrogenation kinetics are also discussed. It is observed that the alloy containing LaH₃ preserves stable hydrogenation behavior between 573 and 623 K, while the hydrogenation properties of the alloy containing Mg₂Ni is susceptible to temperature. A preferable hydrogenation performance is observed in Mg–10Ni–10La alloy, which can absorb as high as 5.86 wt% hydrogen within 15 min at 623 K and 3.0 MPa hydrogen pressure. Moreover, the desorption kinetics and the desorption activation energies are evaluated to illustrate the mechanism based on improved dehydrogenation performance. The addition of proper alloying elements Ni and La in combination with reasonable processing is an effective strategy to improve the de-/hydrogenation performance of Mg-based alloys.