J.R. Johnson

The correlation between composition and electrochemical properties of metal hydride electrodes

This paper is concerned with an overview of the properties of metal hydride electrodes used for battery applications. The emphasis is on the properties of AB5 electrodes but others are treated as well. The review begins with a brief discussion of the pertinent chemistry of hydrogen in metals, the properties of intermetallic hydrides and their relation to electrochemical behavior. Systematic guidelines which permit the modification of such properties for electrochemical applications are discussed. The electrochemical behaviors of certain specific AB5 alloy electrodes are covered in detail, emphasizing the effects of composition changes with respect to both the A and the B components. The consequences of electrode expansion and contraction with respect to hydride formation and decomposition are discussed quantitatively. Novel alloy compositions and phases are noted and evaluated. The attractive properties of cobalt-free, non-stoichiometric AB5+x electrodes are noted.

Alloys for hydrogen storage in nickel/hydrogen and nickel/metal hydride batteries

Since 1990, there has been an ongoing collaboration among the authors in the three laboratories (i) to prepare alloys of the AB5 and AB2 types, using arc-melting/annealing and mechanical alloying/annealing techniques; (ii) to examine their physiochemical characteristics (morphology, composition; (iii) to determine the hydrogen absorption/desorption behavior (pressure-composition isotherm as a function of temperature), and (iv) to evaluate their performance characteristics as hydride electrodes (charge/discharge, capacity retention, cycle life, high-rate capability). This review article presents the work carried out on representative AB5 and AB2 type modified alloys (by partial substitution of with small additives of other elements). The purpose of the modification was to optimize the thermodynamics and kinetics of the hydriding/dehydriding reactions and to enhance the stabilities of the alloys for the desired battery applications. The results of our collaboration, to date, demonstrate that: (i) alloys prepared by arc-melting/annealing and mechanical alloying/ annealing techniques exhibit similar morphology, composition and hydriding/dehydriding characteristics; (ii) alloys with the appropriate small amounts of substituent or additive elements — retain the single phase structure, improve the hydriding/dehydriding reactions for the battery applications, and enhance the stability in the battery environment — and (iii) the AB2 type alloys exhibit higher energy densities than the AB5 type alloy but the state-of-the-art, commercialized batteries are predominantly manufactured using AB5 type alloys.

Reaction of hydrogen with the high temperature (C14) form of TiCr2

Abstract At −78 °C the high temperature (C14) form of the intermetallic compound TiCr 2 will react directly and reversibly with hydrogen to form two non-stoichiometric hydride phases having the nominal compositions of TiCr 1.9 H 2.5 and TiCr 1.9 H 3.5 . The lower hydride phase is more stable than the higher. Standard free energies of formation of these phases at −78 °C are − 1.29 ± 0.04 and + 2.76 ± 0.02 kJ (g atom H 2 2 ) −1 with corresponding plateau pressures of approximately 0.2 and 30 atm. Relative partial molal thermodynamic quantities are also given for several other hydrogen compositions. At room temperature pressure-composition isotherms and X-ray diffraction data indicate the absence of any hydride phases: the starting (C14) intermetallic simply undergoes a lattice expansion with increasing hydrogen content without phase or structural changes. The critical temperature for the first miscibility gap region is approximately +15−20 ° C . Hysteresis is also absent in this intermetallic-hydrogen system.

Function of cobalt in AB5Hx electrodes

Abstract The role of cobalt in the behavior of AB 5 H x electrodes has been investigated. Alloy compositions were as follows, LaNi 4.3− x Co x Mn 0.4 Al 0.3 ( x =0, 0.2, 0.4, 0.75) and MmNi 4.3− x Co x Mn 0.4 Al 0.3 ( x =0, 0.75). Cobalt was found to decrease the molar volume of hydrogen, V H , in the hydride phase. Both LaNi 4.3− x Co x Mn 0.4 Al 0.3 and MmNi 4.3− x Co x Mn 0.4 Al 0.3 alloy electrodes were subjected to repeated electrochemical cycling and corrosion rates measured. The corrosion rate was found to be inversely proportional to the cobalt content of the electrode.

High-pressure studies of the TiCr1.8–H2 system Statistical thermodynamics above the critical temperature

Abstract Pressure–composition isotherms of the TiCr 1.8 –H 2 system were measured within the temperature range 298–433°K and over a wide pressure range up to 1000 atm H 2 . The above temperature range is well above the critical temperature, T C , of the system. Hence, partial molal enthalpies and entropies of formation were evaluated as a function of hydrogen composition. Both thermodynamic quantities obeyed a linear decrease (i.e. becoming more negative) with increasing H/M atomic composition ratio (with M=Ti+Cr atomic content). The experimental isotherms were compared to calculated expressions derived by a rigid-metal sublattice statistical thermodynamics model. Two approximations applied in solving the model, the Bragg–Williams (B.W.) and the Quasi-Chemical (Q.C.) were compared, respectively. The pairwise nearest neighbors H–H interaction parameter, η , was evaluated for each isotherm. For both approximations a similar temperature dependence of η ( T ) was obtained, with η changing from attractive (i.e. negative) to repulsive (i.e. positive) with increasing isotherms temperatures. A good agreement was obtained between the calculated T C values (derived from the η ( T ) parameters) and the experimental observations.

Irreversible effects in the FeTi-H system☆

The equilibrium hydrogen absorption isotherm using annealed strain-free FeTi exhibits only one plateau up to a composition of FeTiH1.95. However, the desorption isotherm exhibits the usual two plateaux. X-ray diffraction results of samples with compositions within the absorption plateau limits indicate the major phases present to be α and γ, although a small amount of β phase was always detected. Apparently the absorption isotherm represents the direct α → γ conversion and β phase precipitation is almost completely suppressed. The results are attributed to the influence of lattice strain upon the behavior of the β1 and β2 phases. A practical consequence of the effect is the distortion of the pressure-composition isotherm in the β-γ region as a function of hydriding-dehydriding cycles.

On the existence of f.c.c. TiCr1.8H5.3

Abstract An orthorhombic structure was originally assigned to the non-stoichiometric hydride TiCr1.8H3.6. Recent neutron and X-ray diffraction data indicated, however, that an alternative interpretation was possible, i.e. the solid consists of two hydride phases, an α′ Laves phase having a composition TiCr1.8H2.8 and an f.c.c. phase with a much higher hydrogen content. This proved to be the case and it was determined that the high concentration phase has a disordered fluorite structure with a 0 = 4.27 A . Its hydrogen content, as estimated from diffraction peak intensities, corresponds to TiCr1.8H5.3. We were not able to produce single-phase f.c.c. material but we prepared a mixed phase sample with an overall composition of TiCr1.8H4.8. The high pressure reaction leading to the formation of the fluorite phase is very sluggish and irreversible. Pressure-composition-temperature properties of this system were determined and are discussed and a revised phase diagram is proposed.

Lattice expansion as a measure of surface segregation and the solubility of hydrogen in α-FeTiHx

The lattice expansion of α-FeTiHx as a function of hydrogen content was found to be anomalously small. Samples of α-FeTiHx were prepared by absorbing hydrogen in previously activated strain-relieved FeTi with a specific surface area of 0.5 m2 g−1. The apparent partial molar volume VH of hydrogen was 0.1± 0.1 cm3 (g atom)−1 (FeTiH < 0.03); this is much lower than the accepted range of 2.2-1.4 cm3 (g atom)−1 previously reported for hydrogen-metal solutions. This behavior is attributed to preferential occupation by hydrogen of titanium-rich sites produced by surface segregation effects which are known to occur in activated FeTi. The α-(α + β) phase boundary is estimated to occur at an overall composition equivalent to FeTiH≈0.04. At this composition about 85% of the total hydrogen content is estimated to be associated with surface layers approximately 100 A in depth. Unactivated (low surface area) FeTi behaved quite differently and it is clear that, in the low dilution region, surface segregation effects play a dominant role in the behavior of the system. We also present evidence that strain and dislocations produced by a hydriding-dehydriding cycle increase the hydrogen solubility and shift the phase boundary towards higher hydrogen contents.

Behavior of hydrided and dehydrided LaNi5Hx as an hydrogenation catalyst

Abstract The catalytic properties of varying phase compositions of LaNi 5 H x were studied using the hydrogenation of 1-undecene as a model reaction. All experiments were carried out with the catalyst in liquid suspension at 308 K under varying pressures of hydrogen. The H content of the catalyst particles was tracked during the course of each experiment. Four types of catalytic behavior were observed and are catalogued in turn. While the metal solid solution phase (α-LaNi 5 H x ) exhibits catalytic properties, catalytic activity is significantly enhanced by the presence of the hydride phase (β-LaNi 5 H x ) in the particle bulk even though the surface composition of the particle is likely unchanged. It was concluded that several factors contribute to this enhancement: (1) the hydride phase acts as a reservoir of H atoms which communicate with the surface via microcracks and grain boundaries; (2) the substantial increase in bulk volume upon the conversion of the metal to the hydride phase enlarges these conduits and facilitates said communication; (3) weak chemisorption of hydrogen on the surface, due to the high hydrogen atom activity when the hydride phase is present, increases the amount of labile hydrogen.

Thermodynamic characterization and statistical thermodynamics of the TiCrMn–H2(D2) system

Pressure–composition (p–c) isotherms of the TiCrMn–H2(D2) system were measured over a wide temperature and pressure range (up to 1000 atm H2/D2) which included the super-critical range (above TC). The macroscopic thermodynamic parameters of the system and their isotope effects were evaluated. Also, hysteresis phenomenon between absorption and desorption isotherms were obtained as a function of temperature for the two hydrogen isotopes. The experimental p–c isotherms were compared to model calculated ones derived by simplified statistical thermodynamic treatments. From this comparison microscopic energy-related parameters were evaluated. These parameters included the average hydrogen–lattice interaction, the pairwise nearest-neighbor hydrogen–hydrogen interaction and the average vibrational frequency of H(D) at the interstitial sites.