S. Suda

Anodic Oxidation of Alkali Borohydrides Catalyzed by Nickel

Borohydrides in alkaline media are potential fuels for fuel cells due to their high energy and power density. In this work, we studied the anodic oxidation of borohydrides on a nickel electrode. The open-circuit potential was found to be about 0.15-0.2 V more negative than the hydrogen potential, depending on the concentration of BH - 4 . The results of polarization measurements indicated that a high power density can be achieved for the BH - 4 /Ni system. However, the coulombic efficiency was found to be 50% or less due to hydrogen evolution. A further investigation showed that hydrogen gas was not only generated from the hydrolysis reaction, but also from the electrochemical reaction. The actual anodic reaction of borohydride on the Ni electrode was proved to be a four-electron process rather than an eight-electron one.

Effects of lattice strain on the hysteresis of pressure-composition isotherms for the LaNi5H2 system

Abstract The effects of annealing on the hysteresis of pressure-composition isotherms for the LaNi 5 H 2 system were examined. It was found that the higher was the annealing temperature before absorption, the higher was the pressure of the subsequent absorption isotherm and eventually the larger was the hysteresis. The dehydrided samples, which were annealed at various temperatures, were subjected to X-ray powder diffraction line profile analysis using the Warren-Averbach method. It was concluded from the results obtained for the crystallite domain size and the lattice strain that the predominant factor affecting the hysteresis is the microstrain in the direction perpendicular to the hexagonal c axis.

Kinetics of the reaction between MgNi alloys and H2

The kinetics of the hydriding and dehydriding reaction of Mg2Ni and Mg-10 wt % Ni was investigated. The reaction rate at various hydride compositions was measured by the pressure change in a constant volume. In the case of Mg2Ni, at the beginning of the reaction the amount of absorbed/desorbed hydrogen can be expressed by n = k(p0 – peq) (ns – n0), where n is the ratio of H to Mg in the solid, p0 and peq are the initial hydrogen pressure of the system and the equilibrium pressure, and n0 and ns are the values of n at the start of the reaction and a constant around 2, respectively. After a transient period of approximately 1 min, a diffusion-controlled stage took place as dn/dt = k′(p - peq)/t, where p is the pressure at time t. In the case of Mg-10 wt % Ni, throughout a run the absorption/desorption rate was expressed by dn/dt = k″ (p - peq)1.25.

Reaction kinetics of metal hydrides and their mixtures

Reaction rates in the hydriding and the dehydriding processes were studied experimentally for hydriding alloys such as LaNi5, MmNi5(Mm  mischmetall), aluminium-substituted mischmetall nickels, TiMn10.5 and Ti0.8Zr0.2Cr0.8Mn1.2. We also studied binary mixtures of these hydriding alloys to examine the effects of mixing on the kinetic properties. A term N was introduced for the evaluation of the pressure and temperature dependences of the reaction rate.

Effective thermal conductivity of metal hydride beds

Abstract An experimental investigation aimed at improving the effective thermal conductivity of activated metal hydride is reported. Of the many material configurations which have been used to enhance heat transmission through metal hydride beds, a three-dimensional structure of a porous aluminium composite has shown the best results giving transmissions a factor of 9 or 10 greater than those of the unmodified hydride bed. The experimental results were used to derive equations for estimating the effective thermal conductivity as a function of the system pressure, the temperature, the hydride composition and the void fraction of the materials used.

Operating characteristics of a metal hydride heat pump for generating cooled air

Abstract A metal hydride heat pump was constructed and the dependence of its operating characteristics on such variables as the pressure-temperature-concentration and kinetic properties of paired hydrides, the heat transfer performance of the heat exchangers, the amount of hydrogen gas with which the system was charged, the cycle time, the mass flow rate of the air supply and the operating temperature was investigated. The optimum operation conditions and parameters were evaluated and will be used as guidelines for future heat pump designs.

Experimental measurements of thermal conductivity

Abstract Two experimental apparatuses — one based on a steady state method and the other based on a non-steady state method — were constructed for measuring the effective thermal conductivity of metal hydrides. The experimental and analytical procedures for the steady state method are described in detail and data are presented for the TiMn1.5 hydride as a function of hydrogen gas pressure, temperature and hydride composition.

Study of the hydriding kinetics of LaNi4.7Al0.3H system by a step-wise method

Abstract The hydriding kinetics of activated LaNi4.7Al0.3H were determined under isochoric and variable pressure conditions by applying step-wise changes of the hydrogen concentration. Experiments were performed in three different phases from the α to β phases under isothermal conditions. A rate equation was derived by taking account of the reversible nature of the hydriding and dehydriding reactions dn dt = k( P P e ) a {1−( P f P ) a ( n n f ) b } Reactions were observed to proceed differently in the three phases, i.e. in the α, α + β and β phases. Reaction rates in the α and β phases are much faster than those in the two-phase (α + β) region. The reaction orders with respect to hydrogen pressure and hydrogen concentration H/M were found to be considerably different in each phase. It is concluded that the rate-controlling step in the hydriding reaction changes with the phase changes. In the α, α + β and β phase regions, the rate-controlling step was characterized as the surface reaction, nucleation and growth, and diffusion of hydrogen atoms to the hydride layer respectively.

Development of a double-stage heat pump: experimental and analytical surveys

Abstract A double-stage heat pump using three different kinds of metal hydrides was developedto demonstrate the practical applicability of the hydriding-dehydriding reactions to the generation of high temperature heat through the dehydriding reaction, under lower temperature conditions, with high operational performance. Extensive research was carried out on the design, construction and operation,and the operating performance of two experimental demonstration units with different capacities (7.72 kW and 77 kW) was analysed. The emphasis of this work was the generation of high temperature steam in thetemperature range 120–150°C using a low grade heat source at a temperature of 80°C and ambient air as the cooling medium. Experimental and analytical results of the operational performance of thetesting unit was 7.72kW capacity are described in detail.

Reaction kinetics of hydrogen-metal hydride systems

Abstract An overview of past papers is presented, which is related to the reaction kinetics of metal hydrides. The experimental procedures, designs of reaction cells, rate equations, reaction mechanisms and thermal effects are discussed. The reaction rates are affected considerably by the design of reaction cell, experimental procedure, history of sample preparation, and number of hydriding/dehydriding cycles. The temperature change of the metal hydride bed caused by exothermal and endothermal reactions is the most influential factor to the intrinsic kinetic behavior. In this paper two technical approaches are proposed to alleviate those thermal effects by designing a highly sensitive reaction cell acting against the abrupt change of the reaction heat and by providing an appropriate experimental technique.