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In this study, colloidal silver and silver-alloys (Ag–Pt, Ag–Au, Ag–Ir, and Ag–Pd) prepared by the Bonneman technique were evaluated as anode catalysts for sodium borohydride oxidation using cyclic voltammetry (CV), chronoamperometry (CA), chronopotentiometry (CP) and rotating disk electrode (RDE) voltammetry. The CV results show that the colloidal Ag-alloys were electrochemically active towards borohydride oxidation with oxidation potentials ranging between - 0.7 and 0.4 V vs. Hg / HgO (MOE). The most negative oxidation potential was recorded on Ag–Pt. CA results show that the steady state current density was highest on Ag–Pt, followed by Ag–Ir, Ag–Au, and Ag–Pd. The lowest overpotential was recorded on Ag–Ir for a current step change of 10 mA cm - 2 . A significant temperature effect and a small rotation speed effect were found in the rotating disc voltammetry for all the investigated colloids. The highest peak current was recorded on Ag–Au, while the most negative peak potential was recorded on Ag–Ir.

Bipolar plates are a key component of PEM fuel cells, and they constitute a large portion of the weight and total cost of a fuel cell stack. In order to reduce both the cost and weight of the bipolar plates, considerable attention is being paid to developing metallic bipolar plates to replace non-porous graphite. In this study, TiN was coated on a martensitic stainless steel (SS410) using a PVD technology (plasma enhanced reactive evaporation) to increase the corrosion resistance of the base metal. XRD, SEM, EIS and potentiodynamic tests were used to characterize the TiN-coated SS410. In order to investigate the suitability of these coated materials as the anodes and cathodes in a PEMFC, potentiostatic tests were conducted under both simulated cathode and anode conditions. The simulated anode environment was - 0.1 V vs SCE purged with H 2 and the simulated cathode environment was 0.6 V vs SCE purged with O 2 . The dense TiN coatings formed on SS410 much improved the corrosion resistance of SS410 and, thus, these coated materials could potentially be used in PEMFCs as a bipolar plate material provided they also satisfy the other physical and mechanical property requirements such as interfacial contact resistance, light weight, high mechanical strength and manufacturability.

The oxidation of methanol and sodium borohydride (NaBH4) on supported colloidal Os and Os-alloys (i.e. Os–Sn, Os–Mo and Os–V) have been studied by voltammetry and chrono-techniques (i.e., potentiometry, amperometry). The electrolytes were: 0.5 M H2SO4 for methanol oxidation and 2 M NaOH (without and with 1.5 × 10 3 M thiourea (TU)) for borohydride oxidation, respectively. Voltammetry results showed that the investigated Os-based materials were catalytically inactive with respect to methanol oxidation, contradicting therefore, the theoretical suggestion by Kua and Goddard [J Am Chem Soc 1999; 121:10928–41] indicating electrocatalytic activity for Os toward methanol oxidation. Regarding the oxidation of borohydride, for direct borohydride fuel cells, preliminary results showed a voltammetric wave, with oxidation potentials between 0.1 and 0.3 V vs. Ag/AgCl, KClstd, which were attributed to borohydride and TU oxidation. Chronopotentiometry results of Os-alloys showed a more positive operating potentials compare to colloidal Os catalysts.

The factors that affect the self-discharge characteristics of a metal hydride (MH) anode are investigated, which include the state of charge (SOC), temperature, storage time and microencapsulation of the alloy with a thin copper (Cu) layer. It is found that the self-discharge rate of the MH electrode decreases with decreasing temperature and storage time. Microencapsulation of the alloy with a thin Cu layer is found to greatly decrease the self-discharge rate. Also, it is found that the self-discharge rate of the Ni-MH cells is not correlated with SOC. It is believed that the main mechanism for the self-discharge of the Ni-MH cells is related to the hydrogen release from the MH anode.

The performance of a Nickel/Metal Hydride (Ni/MH) battery closely depends on the characteristics of the negative MH electrode. Exchange current density, high-rate dischargeability, discharge potential and apparent activation energy of a MH electrode are very important properties, among which the high-rate dischargeability and discharge potential of a MH electrode determine the specific energy and specific power of electric vehicles (EVs) when Ni/MH batteries are applied to EVs. Significant improvements in exchange current density, high-rate dischargeability and discharge potential of a MH electrode have been observed for a 9.0 wt % copper coated LaNi4.7Al0.3 MH electrode. The high-rate dischargeabilities were determined to be 88.4% for the LaNi4.7Al0.3 electrode and 99.4% for Cu-coated LaNi4.7Al0.3 electrode. The discharge potential for the Cu-coated LaNi4.7Al0.3 electrode is lower (i.e. more negative) than that for the LaNi4.7Al0.3 electrode, especially at a large discharge current density (i.e. 200 mA g −1 ). The discharge potentials of the Cu-coated LaNi4.7Al0.3 electrode are almost the same value (i.e. −0.930 V vs. Hg/Hgo) at both 20 and 200 mA g −1 discharge current densities. There is no significant difference between the two apparent activation energies for the electrode reactions for the electrodes with and without the microencapsulation of the MH powders at the same hydrogen concentration.

In this paper, a review is given of new technologies designed to improve the performance of advanced Ni/MH and Ni/Zn batteries. The typical reaction process of the Zn electrode, and a special Zn electrode manufacturing process, provide useful methods for improving advanced Ni/Zn batteries. The reaction mechanism of the Zn electrode, which is related to the charge- and mass-transfer reactions, is different from that of the MH electrode and nickel hydroxide electrode. Thus, it is necessary to add wetting and binding materials in the pasted Zn electrode. The stability of the Zn electrode is dependent on these wetting and binding materials. A Zn electrode with a long cycle lifetime has been developed for an experimental Ni/Zn cell (1.2 A h ) . The experimental cell has been cycled over 120 times.

The characteristics and mechanisms of enhanced electrochemical properties for a Mg 2 Ni alloy produced by a ball-milling treatment were investigated by electrochemical measurements and XRD analysis. The particle size decreases, the internal strain increases, and the alloy gradually becomes amorphous on ball-milling. With the accompanying increased diffusion rate of hydrogen into the ball-milled Mg 2 Ni alloy, the limiting current density, i crit , is decreased and the exchange current density, i 0 , is increased. As a result, the specific discharge capacity is significantly increased compared to that of the as-received Mg 2 Ni alloy and reaches a maximum for a ball-milling time of 25 h .

In this paper, we report electrochemical charge/discharge of hydrogen for a pellet metal-hydride electrode using an MmNi5−XMX alloy as active material. Strong dependence of temperature and discharge current was found for electrochemical applications. Pellet electrode showed good stability over 250 charge/discharge cycles. Self-discharge studies were carried out on completely charged electrode. The capacity of this electrode to absorb/desorb hydrogen could be defined as a stochastic function of some variables like temperature, cycling and discharge current conditions. More studies are in progress for calculating electrochemical parameters in these metal-hydride electrode systems.

Hydrogen storage alloys are a group of new functional intermetallics which can be used in heat pumps, catalysts, hydrogen sensors and Ni/MH batteries. The development of Ni/MH (Metal Hydride) batteries based on MH negative electrodes has seen considerable activity in recent years. Batteries based on such hydride materials have some major advantages over the more conventional lead–acid and nickel–cadmium systems. These advantages include: high-energy density; high-rate capability; tolerance to overcharge and over-discharge; the lack of any poisonous heavy metals; and no electrolyte consumption during charge/discharge cycling. The most important electrochemical characteristics of the hydrogen storage compounds used in these batteries include capacity, cycle lifetime, exchange current density and equilibrium potential. These characteristics can be changed by designing the composition of the hydrogen storage alloy to provide optimum performance of the Ni/MH batteries. The electrochemical behaviour of such intermetallics depends on the types of intermetallics (mainly AB 2 and AB 5 ), microstructure, the nature and amount of each element in the intermetallic compound, and the electrochemical process(es) taking place. The addition of some highly electrocatalytic materials for the hydrogen evolution reaction (h.e.r.) are beneficial in generating optimum performance for the MH electrodes. In this paper, we present some recent results on the electrochemical behaviour of such compounds and the mechanisms of the electrochemical reactions.

Hydrogen diffusion in the metal hydride dominates the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. In this study, an AB5-type hydrogen-absorbing alloy was used as the metal hydride electrode material. The oxidation and the rate-determining mass transfer reactions in the metal hydride electrode were studied and analyzed using anodic polarization measurements. The diffusion coefficient of hydrogen in the metal hydride alloy was determined using both anodic polarization measurements at varying hydrogen concentrations and a galvanostatic technique. The diffusion coefficient of hydrogen in the MH alloy is of the order of 10 −11 cm 2 s −1 assuming that the average radius of the alloy particles is 5 μm . The anodic polarization performance of an electrode without hydrogen in the alloy is related both to the oxidation of the alloy powder and to the formation of a passive layer on the surface of the alloy powder.

Charge transfer reaction across the electrode/electrolyte interface and hydrogen diffusion in the negative MH alloy electrode dominate the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. The mass transfer process in the MH electrode mainly involves hydrogen diffusion in the bulk MH alloy. The charge transfer reaction in the negative electrode reflects the capability of hydrogen reduction and oxidation reactions at the surface of the MH alloy powder. In this study, an AB 5 -type hydrogen-absorbing alloy was used as the negative electrode material. The rate-determining mass transfer process in the bulk MH alloy electrode was studied and analyzed using anodic polarization measurements. The exchange current density, which is related to the charge transfer reaction, was analyzed by using the hydrogen equilibrium pressure. The estimation of hydrogen diffusion coefficient in the MH alloy is strongly dependent on the value of the effective reaction area of charge transfer reaction at the surface of the alloy powder.

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 and positive electrodes and the composition of the hydrogen-absorbing alloy. The metal hydride (MH) alloy powder for the negative electrode of the Ni/MH battery was first pulverized and oxidized by electrochemically charging/discharging for a number of cycles. A multicomponent AB5-based alloy Mm0.95Ti0.05Ni3.85Co0.45Mn0.35Al0.35 was developed as the hydrogen-absorbing alloy. The discharge characteristics of the negative electrode, including discharge capacity, cycle lifetime and exchange current density, were studied by means of the electrochemical experiments and analysis in the experimental cell. The polarization measurements show that the exchange current density of the MH electrode increases with increasing number of charge/discharge cycles and then it almost remains constant after 30 cycles. A microcracking activation, resulting from an increase in reaction surface area and an improvement in the electrode surface activation, decreases the polarization resistance and increases the hydrogen exchange current densities. Measurement of the hydrogen diffusivities for the Mm0.95Ti0.05Ni3.85Co0.45Mn0.35Al0.35 alloy shows that the ratio D/a2 (D=hydrogen diffusivity; a=sphere radius) increases with increasing number of cycles up to 20 cycles and it remains constant after further cycling.

In order to better understand the hysteresis phenomena in metal–hydrogen systems, tests were made on two intermetallic compounds using a new technique, the half-cycle technique. It was seen that the portion of the materials converted to the β phase (hydride) in the half cycle is then converted much easier the second time in the first part of a subsequent complete cycle. This can be explained in terms of the sample condition changes in the initial half cycle when a portion of the sample disintegrates into a fine powder. The new surface produced makes it easier for the hydrogen dissociation and hydride (metal) nucleation processes to take place, resulting in less energy dissipation. Therefore, the magnitude of the hysteresis is reduced when a half cycle is followed by a complete cycle.

Two mathematical models have been applied to AB2-type hydrogen-absorbing alloys. The first model is for the calculation of hydride formation enthalpy and the second model allows for the calculation of P-C-T curves. Certain physical parameters (activity coefficient of hydrogen (γ), partial molar volume of hydrogen (VH), solution heat of hydrogen (ΔHs), enthalpy (ΔH) and entropy (ΔS) of formation of a hydride, slope factor (fs) of a plateau and the variation rate (k) of slope factor with respect to temperature in a plateau region of P-C-T curves) for these intermetallic compounds and their hydrides are estimated from these models. From the second model, the relationship between the hysteresis factor (RT ln Pa/Pd) and temperature, hydrogen concentration and slope factor of the plateau region for the P-C-T curves has been obtained.

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 (Mm0.95Ti0.05Ni3.85 Co0.45Mn0.35Al0.35) 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.

Based on the static equilibrium of electrochemical reaction kinetics, i.e. the same static reaction current density of forward and backward reactions, a relationship has been established between the equilibrium potential during discharge (Ee) and hydrogen concentration (CH) in a hydride alloy (Mm0.4Ml0.6Ni3.8Co0.5Al0.3Mn0.4) at an equilibrium discharge condition. The theoretical results are in an agreement with the experimental electrochemical data in the two-phase (α-β) region of the hydrogen-absorbing alloy. The enthalpy (ΔH) and entropy (ΔS), which is a function of hydrogen concentration in the electrode alloy, were obtained by fitting the curve of the equilibrium potential vs hydrogen concentration.

The hydrogen storage capacity of an air-exposed LaNi4.7Al0.3 alloy powder was investigated using hydrogen absorption/desorption experiments. The LaNi4.7Al0.3 alloy powder was first exposed in air for up to 90 days producing an oxidized surface layer which was about 50 nm in depth of the grain particle surface. The hydrogen storage capacity decreased with increasing exposure to air. However the slope of the P-C-T plateau remained approximately constant for the different exposure times.

In an attempt to understand the hysteresis phenomena better, tests were made on a series of VxNb1 − x (x = 0.1, 0.2, 0.3), (Nb1 − xFex)1 − yCry (x, y = 0.15, 0.14 and 0.20, 0.08) and (Nb1 − xFex)1 − xCrx (x = 0.05, 0.07) solid solution alloys and Zr(FexCr1 − x)2 (x = 0.55, 0.75) intermetallic compounds. A pronounced hysteresis was observed for all the alloys used. It was seen that, as the temperature increases the hysteresis effect as measured by the ratio ( P f P d ), where Pf is the plateau pressure for hydride formation and Pd is the plateau pressure for hydride decomposition, decreases. The hysteresis effect was also affected by the number of hydriding/dehydriding cycles. The hysteresis becomes smaller as the number of cycles increases. The hysteresis effect in the solid solution alloys can be explained in terms of the elastic strain energy. The change in hysteresis in the intermetallic compounds upon cycling is explained in terms of attrition (i.e. decreasing particle size) of the Zr(FexCr1 − x)2.

The negative electrodes of an experimental sealed battery (AA size, 1Ah capacity) were made from a multicomponent alloy (Mm 0.95 Ti 0.05 Ni 3.85 Co 0.45 Mn 0.35 Al 0.35 ) powder coated with 10 wt% nickel, polytetrafluoroethylene (PTFE) and nickel powder. The Ohmic resistivity of the electrode increases with increasing PTFE content. The average discharge voltage of the battery decreases with increasing discharge current and increases marginally with increasing temperature. The overpotential of the negative electrode decreases with increasing temperature.

Pressure-composition isotherms for hydrogen absorption and desorption are determined for (Nb1 −xFex)1 −xCrx alloys (x = 0.05, 0.07) over the temperature range 298–353 K. Pronounced hysteresis was observed at all temperatures. The average enthalpies ([H]/[M]= 0.2–0.7) for hydride formation are −14.9 and −145.4 kJ (mol H)−1 and for hydride decomposition are 33.6 and 33.4 kJ (mol H)−1 forx-values of 0.05 and 0.07, respectively.

Hysteresis in metal—hydrogen systems is affected by many different factors including temperature, alloy composition and cycling. In an attempt to better understand these phenomena, tests were made on a series of Vx-Nb1−x solid solution alloys (x = 0.1, 0.2 and 0.3) using a Sieverts' type technique to determine PCT relationships. The PC isotherms were determined at temperatures 318, 353 and 373 K and for varying number of hydriding/dehydriding cycles. A large hysteresis effect was observed for x = 0.3 but as the value of x decreases the hysteresis becomes smaller. The Vx-Nb1−x alloys behave like most metal—hydrogen systems in that as the temperature increases, the hyteresis effect as measured by the ratio (Pf/Pd), where Pf is the plateau pressure for hydride formation and Pd is the plateau pressure for hydride decomposition, decreases. The hysteresis effect also becomes smaller at the second and third hydride/dehydriding cycles, but no further change is seen after the third cycle. The hysteresis in these alloys can be explained in terms of strains introduced into the solvent lattice (Nb) by the smaller radii solute atoms (V).

The pressure-composition isotherms for the (Nb1 − xFex)1 − xCrx-H systems (where x = 0.05 and 0.07) were determined at three different temperatures, 298, 318 and 353K, using a Sieverts-type technique. Pronounced hysteresis was observed. The pressure required for hydride formation, Pf, exceeded the decomposition pressure, Pd, by a factor of 20 at room temperature. The (Nb1 − xFex)1 − xCrx alloys behave like most metal-hydrogen systems in that as the temperature increases, the hysteresis effect decreases. The hysteresis effect is explained in terms of small radii of the solute (Fe and Cr) atoms which introduce strains into the solvent (Nb) lattice.

Hysteresis in metal-hydrogen systems is affected by many different factors including temperature, alloy composition and cycling. In an attempt to better understand this phenomenon, the pressure-composition isotherms for the (Nb1−xFex)1−yCry-H systems (where x = 0.20 and 0.15) were determined at five different temperatures (255, 269, 318, 353 and 370 K) and over a varying number of hydriding/dehydriding cycles. Pronounced hysteresis effects are observed at all temperatures. The hysteresis becomes smaller as the amount of iron decreases and as the temperature increases. The number of hydriding/dehydriding cycles has little effect on reducing the hysteresis. The hysteresis in this alloy can be explained in terms of elastic strain energy effects.

The group V transition metal (Nb, V, Ta)-hydrogen systems are reviewed. Consideration is given to the different types of hydrides formed, their crystallography, thermodynamic data and hysteresis effects on hydriding/dehydriding. The group V transition metals do not themselves react with hydrogen at room temperature, but quite readily react with hydrogen when alloyed with other bcc metals of smaller atomic radii. There are uncertainties regarding the structures of hydrides formed by this group of metals. These uncertainties mainly arise because of the limited number of neutron diffraction studies of the hydrides. Monohydrides have been reported for Nb, V and Ta, while dihydrides have been reported only for Nb and V. Thermodynamic data show that the niobium-hydrogen solid solution is more stable than the vanadium- or tantalum-hydrogen solid solutions. Hysteresis have been seen in metal-H systems in this group and, as yet there is no complete explanation for its occurrence.

The elastic and plastic accommodation during hydride formation and decomposition and their effects on hysteresis are examined under the experimental conditions where there can be both pressure hysteresis and solvus hysteresis. It is shown that both the elastic and the plastic accommodation energies in both hydride formation and decomposition will contribute to hysteresis. The accommodation energies are generally not equal for hydride formation and hydride decomposition. The relative contributions of elastic or plastic energies to hysteresis are different for different types of materials. For practically important intermetallic compounds-hydrogen systems, elastic strain energy is the major cause of hysteresis.

The pressure-composition isotherms for the V x Nb 1 − x -H systems (where x = 0.1, 0.2 and 0.3) were determined at three different temperatures, 318, 353 and 373 K, using a Sieverts' type technique. A large hysteresis was observed for x = 0.3 and as the value of x decreases this hysteresis becomes smaller. The V x Nb 1 − x alloys behave like most metal hydrogen systems in that as the temperature increases, the hysteresis effect decreases. The hysteresis effect is explained in terms of small radius of the solute (V) atoms which introduce strains into the solvent (Nb) lattice.

Experimental observations of, and theoretical models for, hysteresis in metal-hydrogen systems are reviewed. Hysteresis can be manifested in three common ways, namely: pressure hysteresis, solvus hysteresis and thermal hysteresis. Hysteresis is of primary importance in metal hydride applications such as heat pumps and refrigeration systems because it represents a loss of thermodynamic efficiency. The magnitude of hysteresis is shown to be affected by many different factors including temperature, alloy composition, hydrogen aliquot size, annealing of metal sample, number of absorption/desorption cycles and particle size. Although it is accepted that plastic deformation during both hydride formation and decomposition contributes to hysteresis, no existing theory is adequate to account for the complete range of experimentally observed behaviour.

One group of potential hydrogen storage compounds, i.e. Zr(Fe x Cr 1− x ) 2 intermetallics, are investigated. These intermetallics are predominantly single-phased, identified as the hexagonal Laves phase. Hydrogen is absorbed quite readily, with no special activation treatment. Hydrogen absorption results in the formation of a distinct hydride phase, with the same crystal structure as the parent compound. Of these intermetallics, Zr(Fe 0.75 Cr 0.25 ) 2 demonstrates the best overall energy storage characteristics, i.e. a relatively high sorption capacity (H/M = 1) and low hydride stability ( ΔH = −25 kJ mol −1 H 2 ). This composition compares quite favorably to LaNi 5 in terms of hydrogen capacity, hydride stability and reaction kinetics.