Electrochemical Discharge Capacity Research Articles

Concerning the transformations of Ti 3Ir alloy during high-energy ball-milling

Thermodynamically stable Ti 3Ir with the Cr 3Si (A15)-type crystal structure can gradually be transformed into a metastable Ti 3Ir alloy with a bcc crystal structure by high-energy ball-milling for 12.5 h. The long-term ball-milling for up to 112 h, however, is accompanied by a solid-state reaction between Ti 3Ir and the vial material leading to the formation of a nano-crystalline or amorphous MgZn 2-type phase with the Ti(Fe 1.85Ir 0.15) stoichiometry and a micro-crystalline thermodynamically stable CsCl-type phase. Both the CsCl- and MgZn 2-type phases were obtained in the crystalline form after annealing ball-milled Ti 3Ir. Properties of the A15 Ti 3Ir alloy changed considerably after ball-milling. The superconducting transition observed in the original A15 alloy at ∼4.3 K no longer exists in the ball-milled material. The electrochemically determined hydrogen storage capacity of the ball-milled alloy also decreases in comparison to the A15 material from 269 to 104 mAh/g. On the contrary, the electrochemical discharge capacity of Ti 3Ir increases from 45 to 104 mAh/g after ball-milling.

Preparation and electrochemical properties of some (Sc1−xTix)Ni alloys

A series of (Sc 1− x Ti x )Ni alloys and their hydrides (Sc 1− x Ti x )NiH y , where x =0, 0.1, 0.3, 0.5, and y =2.9, 2.58, 2.23 and 1.55, have been prepared. The ability to store hydrogen gradually decreases with increasing Ti content in line with the reduction of the unit cell volume from 32.16 Å 3 for ScNi to 29.68 Å 3 for (Sc 0.5 Ti 0.5 )Ni. The electrochemical discharge capacity, however, varies in a more complex manner with (Sc 0.7 Ti 0.3 )Ni composition displaying the highest value of 231 mAh/g. The discharge capacity losses after 10 electrochemical charge–discharge cycles also vary non-linearly between 46% for (Sc 0.9 Ti 0.1 )Ni and 5% for (Sc 0.5 Ti 0.5 )Ni.

Effect of annealing heat treatment on an atomized AB2 hydrogen storage alloy

The hydrogenation and electrochemical properties of atomized powders of a Ti–Zr based hydrogen storage alloy have been studied. The atomization process deteriorated the capability of hydrogenation in the P–C–I measurements, while its effect on the electrochemical discharge capacity was limited. Once the atomized powder was broken under pressure to create microcracks, the hydrogenation capacity was found to improve significantly. This indicates that the surface layer characteristics of the powder were responsible for the retardation of the hydrogenation reaction. A reason for this was the presence of a partially amorphous phase of the powder, which has low hydrogen diffusivity. Annealing heat treatment increased the degree of crystallization of the atomized powder, and the discharge capacity was markedly enhanced after annealing at 850°C for 4 h.

Improvement of the thermodynamical and electrochemical properties of multicomponent Laves phase hydrides by thermal annealing

The reversible hydrogen absorption and desorption behaviour of Zr1−xTixNi1/0.9Mn0.5/0.6Cr0.4V0.1 with xTi=0.1, 0.3, 0.4, 0.6 was investigated in solid/gas and electrochemical experiments at room temperature. One aspect of this work was to study the thermodynamical properties (Vol./AB2; Peq(plateau); Capmax) of the different samples depending on their Ti content. Another aspect was the analysis of the influence of the annealing treatment on the metallurgical properties and finally the hydrogen absorption/desorption properties of the different samples in solid–gas and electrochemical media. Zr0.7Ti0.3Ni0.9Mn0.6Cr0.4V0.1, for example, shows an increase in the maximum electrochemical discharge capacity by more than 30% to 355 mAhg−1 after appropriate annealing treatment.

Studies of electrochemical properties of TiNi alloy used as an MH electrode — I. Discharge capacity

The electrochemical properties of TiNi alloy used as an MH electrode were investigated. A mathematical model for the electrochemical discharge capacity of TiNi electrode was developed. The model was used to study the effects of various parameters on the discharge capacity, and cycle life. The predicted results from the model fit well with the experimental results. Increasing the discharge current density and charge/discharge cycling current density decreases the discharge capacity, but a high charge/discharge cycling current density increases the cycle life of the hydride electrode. Increasing the exchange current density, or the diffusion coefficient of hydrogen or reducing the particle size increases the discharge capacity. Upon increasing the discharge current density the controlling steps of discharge capacity change from hydrogen diffusion in α phase (including oxide film) to the charge-transfer reaction on the hydride electrode surface. However for the TiNi alloy electrode the discharge capacity still depends mainly on the diffusion ability of hydrogen even at high discharge current density (200 mA/g). There exists an optimum dissolved hydrogen content of the α phase, C αβ opt , at which the discharge capacity reaches a maximum value. Also C αβ opt increases with increasing discharge current density.

Effect of annealing treatment on electrochemical properties of Mm-based hydrogen storage alloys for Ni/MH batteries

Several multi-component Mm-based hydrogen storage alloys with cobalt content from 0.8–1.0 have been prepared. The hydrogen absorption–desorption characteristics in gas–solid reactions and the electrochemical properties as MH electrodes have been investigated. The addition of small amounts of Al effectively lowers the hydrogen equilibrium pressure and improves the cycling stability of the alloys. Electrochemical measurements show that the MmNi 3.4Co 1.0Mn 0.5Al 0.1 alloy exhibits a maximum electrochemical capacity of ∼322 mA h g −1 with a capacity decay of about 19.5% after 100 cycles. Annealing treatments flatten the plateau region and lower the hydrogen equilibrium pressure, which results in an increase of hydrogen uptake below 1 atm. The increasing trend of hydrogen storage capacity from the as-cast sample to the annealed sample in the gas–solid reactions is in good agreement with the electrochemical results. The electrochemical discharge capacity of the MmNi 3.4Co 1.0Mn 0.5Al 0.1 alloy increases to 334 mA h g −1 and 340 mA h g −1 after annealing for 3 h and 28 h, respectively, from 322 mA h g −1 in the as-cast condition. The electrochemical cycling stability of the annealed samples was also greatly improved. The capacity decay for both annealed samples is about 8.3% and 6.8%, respectively, after 100 charge–discharge cycles. It was suggested that annealing treatments enhance the compositional homogeneity and cause the secondary phase (separated phase) to dissolve in main phase, which result in the improvement of electrochemical cycling stability of the alloy electrodes.

Influence of amorphization on electrode performances of AB2 type hydrogen storage alloys

It is generally accepted that the electrochemical discharge capacity of a hydrogen storage alloy is closely related to its crystal characteristics. From our experiment, the discharge capacity of AB2 type Laves phase hydrogen storage alloy ZrCr0.4Mn0.2V0.1Ni1.3 decreased sharply from 324 mAh g−1 to 25 mAh g−1 after amorphization by mechanically milling the crystalline (as-cast) alloy with stainless steel balls. We believe that for the as-cast alloy, hydrogen atoms stored in the tetrahedral interstitial sites of Zr2B2 (B=Cr, V or Ni) all contribute to its electrochemical capacity. However for the amorphous alloy only hydrogen atoms in low-energy Zr3B and Zr4 tetrahedral interstitial sites contribute to its electrochemical capacity. We also believe that only half of the total sites in the crystalline or amorphous alloys can absorb and desorb hydrogen atoms reversibly, and that the process of ball milling manages to lower the barrier potential for the hydrogen discharge reaction by about 60 kJ (mol H)−1. We resolve the phenomenon into the finer particles including abundant hydrogen diffusion channels and the formation of a looser tetrahedral interstitial structure in the metal glass.

Effects of substitution of other elements for nickel in mechanically alloyed Mg 50Ni 50 amorphous alloys used for nickel—metal hydride batteries

It was recently found that MgNi amorphous alloys prepared by mechanical alloying exhibit a very high electrochemical discharge capacity, but because of corrosion those alloys show very bad durability during electrochemical cycling. In this paper, more than ten different amorphous alloys, in which nickel had been partially replaced by other elements, have been prepared to improve their electrochemical properties. Experimental results show that the partial substitution of Ti for Ni in Mg 50Ni 50 increases both the discharge capacity and degradation rate. The substitution of Zn for Ni shows no effect on the discharge capacity but increases the deterioration rate. The substitution of Fe, W, Cu, Mn, Cr, Al or C for Ni decreases both the deterioration rate and discharge capacity. The substitution of Se, Sb, Co, or Si decreases both the discharge capacity and cycling life. Optimum ternary alloys with reasonable discharge capacities and cycling lives are Mg 50Ni 45M 5 (MMn, Cu or Fe), with maximum capacities of 345 mAh·g −1, 343.3 mAh·g −1, and 273.3 mAh·g −1 respectively. The capacity degradation after 9 cycles is 57.5%, 51.4% and 40.5% respectively. This is far below the requirement for practical usage.

Effect of alloying with Ti, V, Mn on the electrochemical properties of ZrCrNi based Laves phase metal hydride electrodes

The main phase in Zr(Cr, Ni 1− x ) 2 Laves phase alloys (0.20⩽ x⩽0.60) changes from C15 to C14 at x = 0.45−0.50. Electrodes with Cr content in the range 0.30 ⩽ x⩽0.50 have favorable electrochemical capacities. The substitution effect of Cr or Ni by V or Mn in the form of Zr(Cr 4M y Ni 2 + y ) (M = V or Mn, x + y = 0.7−1.2) on the electrochemical discharge capacity was also studied. The alloys with x + y = 0.7−1.0/AB 2 formula have larger hydrogen storage and electrochemical discharge capacities. When Ti is used to partly substitute Zr in Ti y Zr 1− y M 2 ( y = 0.0−0.3) alloys, the plateau pressure of the hydride tends to increase with increasing y, however, its electrochemical capacity decreases as well due to lower reversible hydrogen storage density. The electrochemical cycling deterioration increases with increasing Ti content. Electrodes with no Ti or very low Ti content preserve 86% of their discharge capacities after 820 cycles under 65% depth of discharge, exhibiting excellent electrochemical stability.

The electrochemical charge-discharge properties of Zr[sbnd]Cr[sbnd]Ni hydrogen storage alloys

Zr(CrxNi1−x)2(0.15 ⩽ x ⩽ 0.65) alloys were prepared by arc melting in an argon atmosphere. Through X-ray diffraction, the main crystal structure of the alloys was found to change from Laves phase cubic C15 to hexagonal C14 as the CrtoNi ratio increased, and lattice parameters and hydrogen absorption capacities were mainly determined by the Cr content. The alloy Zr(Cr0.35Ni0.65)2, which was mainly of C15 Laves phase structure, was found to have maximum electrochemical discharge capacity of 305 mA h g−1 at 298 K. The activation behaviors of ZrCrNi-based hydride electrodes were also studied. Fine alloy powders, especially those pulverized by cycling in hydrogen gas and where the electrodes were pre-treated with HF-H2O2 solution, were highly activated. As both Zr and Cr have a high resistance against alkaline solution, it is reasonable for ZrCrNi alloy electrodes to have durable cycle lives. The chemical valence states and composition of electrode surfaces were also analyzed with X-ray photoelectron spectroscopy before and after charge-discharge cycling. An obvious variation in the Zr 3d5/2 peak position before and after cycling was detected. The discrepancy is ascribed to the chemical environment differences of Zr atom in primordial and activated surface layers.

Hydrogen absorption-desorption characteristics of Ti 0.35Zr 0.65Ni xV 2− x−yMn y alloys with C14 Laves phase for nickel/metal hydride batteries

The properties of Ti 0.35Zr 0.65Ni x V 2− x− y Mn y (1.0 ≤ x ≤ 1.2, 0.2 ≤ y ≤ 0.4) alloys including crystallographic parameters, the enthalpy for hydride formation, pressure-composition-temperature ( P-C-T) curves and electrochemical discharge capacities were investigated. Based on scanning electron microscopy results, with the exception of alloy A ( x = 1.0, y = 0.2) which forms two distinguishable phases, alloys B ( x = 1.0, y = 0.4), C ( x = 1.2, y = 0.2) and D ( x = 1.2, y = 0.4) all form single uniform phase. Structure contraction took place owing to partial replacement by nickel and/or manganese, but alloys B to D all exhibited C14 type Laves phases. The content of absorbed hydrogen increased as the d-electron concentration (DEC) of the alloys diminished. Although alloy A has the largest hydrogen absorption ability compared with the other alloys, this is not the case with respect to the hydrogen desorption ability of alloys. This can be attributed to the poor reversibility of hydriding-dehydriding of alloy A. It was found that the discharge capacities of alloys were affected not only by the content of absorbed hydrogen but also by the resistance to hydrogen release. Alloy C, Ti 0.35Zr 0.65Ni 1.2V 0.6Mn 0.2, possessing good hydriding-dehydriding properties in gas and liquid media, is a promising electrode material.

Activation of Zr 0.5Ti 0.5V 0.75Ni 1.25 alloy electrodes by hot alkaline solutions

Hot alkaline treatment of the multiphase alloy Zr 0.5Ti 0.5V 0.75Ni 1.25 results in significant surface corrosion, with at least some of the corrosion products being soluble in the treating solution. The corrosion process generates hydrogen that is largely absorbed by the alloy during treatment, resulting in heterogeneous particle cracking and significant electrochemical discharge capacity ( C 0) even before the first deliberate charge. The discharge capacity ( C 1) after the first deliberate charge is significantly increased by hot alkaline treatment. C 0 is a direct measure of alloy corrosion during the hot alkaline treatment and should be minimized in practice. C 1 is a measure of the electrochemical activation rate and should be maximized in practice. Another important parameter is the maximum capacity C max. The effects of treatment time and temperature as well as solution (KOH vs. NaOH) on C 0, C 1and C max are presented, along with some long-time cyclic capacity data.