C. Witham

Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes

Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes (SWNT) was found to exceed 8 wt.%, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes.

Hydrogen Desorption and Adsorption Measurements on Graphite Nanofibers

Graphite nanofibers were synthesized and their hydrogen desorption and adsorption properties are reported for 77 and 300 K. Catalysts were made by several different methods including chemical routes, mechanical alloying, and gas condensation. The nanofibers were grown by passing ethylene and H2 gases over the catalysts at 600 °C. Hydrogen desorption and adsorption were measured using a volumetric analysis Sieverts apparatus, and the graphite nanofibers were characterized by transmission electron microscopy and Brunauer–Emmett–Teller surface area analysis. The absolute level of hydrogen desorption measured from these materials was typically less than the 0.01 H/C atom, comparable to other forms of carbon.

Electrochemical Studies on LaNi5 − x Sn x Metal Hydride Alloys

Electrochemical studies were performed on LaNi(sub 5-x)Sn(sub x) with 0(less than or equal to)x(less than or equal to)0.5. We measured the effect of the Sn substituent on the kinetics of charge transfer and diffusion during hydrogen absorption and desorption, and the cyclic lifetimes of LaNi(sub 5-x)Sn(sub x) electrodes in 250 mAh laboratory test cells. We report beneficial effects of making small substitutions of Sn for Ni in LaNi(sub 5) on the performance of metal hydride alloy anode in terms of cyclic lifetime, capacity and kinetics. The optimal concentration of Sn in LaNi(sub 5-x)Sn(sub x) alloys for negative electrodes in alkaline rechargable secondary cells was found to lie in the range 0.25(less than or equal to)x(less than or equal to)0.3.

The effect of tin on the degradation of LaNi5−ySny metal hydrides during thermal cycling

Abstract Three LaNi 5− y Sn y alloys with y = 0.0, 0.1 and 0.2 have been subjected to hundreds of hydrogen absorption-desorption reactions during thermal cycling from room temperature to over 500 K. Both LaNi 5 H x and LaNi 4.9 Sn 0.1 H x were cycled until their reversible hydrogen storage capacities had decreased by about 60%. X-ray diffractometry and transmission electron microscopy showed that the resulting degraded hydrides had partially disproportionated into nanocrystalline f.c.c. LaH x and Ni metal. The substitution of only a small amount of tin suppressed the rate of degradation by more than a factor of 3 for y = 0.1, and a factor of 20 for y = 0.2. For all three alloys there was a reasonably consistent correlation between the degree of disproportionation and the degradation in reversible hydrogen capacity. The present study verifies that partial Sn substitution for Ni in LaNi 5 produces alloys that are very resistant to intrinsic disproportionation.

Electrochemical Properties of LaNi5–xGex Alloys in Ni-MH Batteries

Electrochemical studies were performed on LaNi5–xGex metal hydride alloys with 0 <= x <= 0.5. We carried out single-electrode studies to understand the effects of the Ge substituent on the hydrogen absorption characteristics, the electrochemical capacity, and the electrochemical kinetics of hydrogen absorption and desorption. The electrochemical characteristics of the Ge-substituted alloys are compared to those of the Sn-substituted alloys reported earlier. LaNi5–xGex alloys show compositional trends similar to LaNi5–xSnx alloys, but unlike the Sn-substituted alloys, Ge-substituted alloys continue to exhibit facile kinetics for hydrogen absorption/desorption at high solute concentrations. Cycle lives of LaNi5–xGex electrodes were measured in 300 mAh laboratory test cells and were found to be superior to the Sn-substituted LaNi5 and comparable to a Mm(Ni,Co,Mn,Al)5 alloy. The optimum Ge content for LaNi5–xGex metal hydride alloys in alkaline rechargeable cells is in the range 0.4 <= x <= 0.5.

Hydriding behavior of gas-atomized AB5 alloys

Abstract The hydriding characteristics of some AB 5 alloys produced by high pressure gas atomization (HPGA) have been examined during reactions with hydrogen gas, and in electrochemical cells. The hydrogen storage capacities and the equilibrium pressures for HPGA processed LaNi 5 , LaNi 4.75 Sn 0.25 , and MmNi 3.5 Co 0.8 Al 0.4 Mn 0.3 alloys (where Mm denotes Mischmetal) are found to be nearly identical to annealed alloys produced as ingots. The large discontinuous volume change across the α–β plateau region for gas-atomized LaNi 5 H x was seen to produce extensive fracturing in all but the smallest alloy spheres. However, only the largest spheres of the gas-atomized MmNi 3.5 Co 0.8 Al 0.4 Mn 0.3 and LaNi 4.75 Sn 0.25 H x alloys exhibited any discernible fracturing. The maximum electrochemical storage capacities of the gas-atomized LaNi 4.75 Sn 0.25 and MmNi 3.5 Co 0.8 Al 0.4 Mn 0.3 alloys were found to be smaller than the capacities of annealed alloys prepared from ingots.

Electrochemical Evaluation of La‐Ni‐Sn Metal Hydride Alloys

A detailed electrochemical evaluation of Sn-modified LaNi[sub 5] was performed to evaluate its performance as a negative electrode in alkaline rechargeable cells. Substituting small amounts of Sn for Ni provides a large improvement in the initial capacity and cycle lifetime of the electrode, and also serves to improve the kinetics of hydrogen absorption-desorption processes.

Distributions of hydrogen and strains in LaNi5 and LaNi4.75Sn0.25

Abstract Hydrogen distributions and internal strains that accompany hydriding of binary LaNi5 were compared to those of the ternary alloy LaNi4.75Sn0.25, which is known to have a cycle life superior to that of LaNi5 in electrochemical cells and in gas storage applications. X-ray diffractometry shows that the unit cell volume of the hydride phase changes more continuously with hydrogen concentration in LaNi4.75Sn0.25 than in binary LaNi5. Gas-phase isotherms show that the Sn atoms make significant changes to the local chemical potential of hydrogen atoms. Using generic hydrogen–solute interactions in Monte Carlo simulations and physical arguments, it is shown that normal coarsening of hydride zones will be altered, or even arrested, by hydrogen–solute interactions. Small-angle neutron scattering shows that the distribution of deuterium in partially-deuterated LaNi4.75Sn0.25 is more homogeneous than in partially-deuterated LaNi5, at least on the spatial scales around 100 A. It is suggested that the more homogeneous deuterium distribution in LaNi4.75Sn0.25 suppresses the strain gradients that cause decrepitation of the metal hydride.

Gas-phase H2 absorption and microstructural properties of LaNi5−xGex alloys

Abstract Metal hydride alloys were prepared based on the LaNi 5 formula by substituting 0–10 at.% of Ge for Ni. Crystal lattice parameters were obtained from X-ray diffraction patterns of each alloy. Hydrogen pressure–composition–temperature isotherms were measured for these LaNi 5− x Ge x (0≤ x ≤0.5) alloys at temperatures ranging 296 to 348 K. These isotherms were used to determine thermodynamic parameters for hydrogen absorption and desorption. The unit cell volumes increase, and the plateau pressures decrease, with increasing Ge substitution. Isotherms and capacities from gas-phase absorption are comparable to those of LaNi 5− x Sn x .

Electrochemical Studies on LaNi(sub 5-x)Sn(sub x) Metal Hydride Alloys

Electrochemical studies were performed on LaNi(sub 5-x)Sn(sub x) with 0(less than or equal to)x(less than or equal to)0.5. We measured the effect of the Sn substituent on the kinetics of charge transfer and diffusion during hydrogen absorption and desorption, and the cyclic lifetimes of LaNi(sub 5-x)Sn(sub x) electrodes in 250 mAh laboratory test cells. We report beneficial effects of making small substitutions of Sn for Ni in LaNi(sub 5) on the performance of metal hydride alloy anode in terms of cyclic lifetime, capacity and kinetics. The optimal concentration of Sn in LaNi(sub 5-x)Sn(sub x) alloys for negative electrodes in alkaline rechargable secondary cells was found to lie in the range 0.25(less than or equal to)x(less than or equal to)0.3.