AB5-type Hydrogen Storage Alloys Research Articles

Function mechanism of Fe in improving cycle stability and plateau characteristics of AB5-type hydrogen storage alloys

Rare-earth AB5-type alloys have great application potential in solid-state hydrogen storage. To further improve their plateau characteristics and cycling life, the effects of Fe on the long-term hydrogen storage properties of LaNi5-xFex (x = 0, 0.5, 1) alloys are studied, and the function mechanisms are revealed. Fe induces a (Fe,Ni) phase layer with good ductility around LaNi5 main phase, which plays a buffer role to relieve the microstrain during hydrogen absorption/desorption, thus stabilizing the crystal structure and improving the cycling performance. Further, Fe with larger atomic radius than Ni relieves the compression from 12o to 6 m site of the LaNi5 phase, and suppresses the plateau splitting phenomenon, resulting in a single and long hydrogen absorption/desorption plateau throughout 1000 cycles. This study promotes the understanding and application of cheap and effective Fe in improving the hydrogen storage properties of AB5-type alloys during long-term cycling.

Effect of Surface Coating on Electrochemical Properties of Rare Earth-Based AB5-type Hydrogen Storage Alloys

Effect of Surface Coating on Electrochemical Properties of Rare Earth-Based AB5-type Hydrogen Storage Alloys

Properties of AB5 type hydrogen storage alloy after α-AlH3 alloying

AB5 type hydrogen storage alloys have been widely used in Ni-MH batteries due to their high electrochemical capacity, excellent rate performance, and low pollution. However, the high activation difficulty limited their development in the solid-state hydrogen storage field. Mechanical alloying to synthesize composites is an effective way to reduce activation energy. In this paper, the structure and hydrogen storage properties of the LaNi5+nAlH3 (n = 0, 1, 3 wt.%) composites were investigated. The results show that with the incremental of AlH3 alloying content, the lattice constant a, b, and c and the volume (V) of the LaNi5 unit cell tend to increase, the hydrogen absorption platform pressure of the composite material gradually decreased, and the hydrogen release platform pressure gradually increased. It resulted in a reduction in the hysteresis factor (Hf) of the composite from 0.66 to 0.35. This will reduce energy waste during energy conversion. In addition, the activation property of the alloy was improved after AlH3 alloying. This will facilitate the application of AB5 type hydrogen storage alloys.

Enhanced cycling stability and reduced hysteresis of AB5-type hydrogen storage alloys by partial substitution of Sn for Ni

Rare-earth AB5-type alloys have been widely studied due to their great application potential in gaseous hydrogen storage, but their overall hydrogen storage properties still need to be further improved for more extensive applications. In this work, the effect of Sn partial substitution for Ni on the plateau characteristics and cycling performance of the LaNi5-xSnx (x = 0, 0.25, 0.5, 0.75, 1) alloys are systematically studied. It is found that the segregation effect caused by Sn addition leads to the multi-CaCu5 phase structure with different cell parameters which may have a positive effect on stabilizing the alloys’ structure during cycling by playing a buffer role. Also, the replacement of Sn element results in a higher anisotropic c/a value, which reduces microstrain and improves the cycle life. The capacity retention after 1000 cycles increases from 83.2% (x = 0) to 95.8% (x = 0.75). Moreover, the addition of Sn significantly reduces the hysteresis of the alloys from 0.212 (x = 0) to 0.023 (x = 0.5) at 383 K, owing to the reduction of the microstrain during hydrogen absorption/desorption.

Enhancement in hydrogenation properties of ball-milled AB5-type hydrogen storage alloy through catalyst

Ball-milling of the hydrogen storage alloy has been shown to improve many hydrogenation properties under specific ball-milling parameters. The present investigation is based on further improvement in the hydrogenation properties through the addition of a catalyst. Catalysts are known to increase the rate of reaction. In the present investigation, rigorous studies have been performed on the various factors affecting the hydrogenation reaction through the addition of catalyst during ball-milling. The basic idea of the mechanism of catalyst is the availability of the free valencies behaving like active centres for promoting the reaction rate. Free valencies are more crowded at corners, peaks and cracks on the catalyst. The rough surface of the catalyst and finely dispersed catalyst are more effective in promoting the reaction rate. In the present investigation, various factors like spillover effect; defects introduced to the catalyst and hydrogen storage alloy during ball-milling; elasticity of metal hydride matrix; nature, size, hardness, number of unpaired electrons and variable valency of catalyst; the electronegative difference between catalyst and metal hydride; electron distribution and orbital structure; the ability of the catalyst to transfer stable diatomic hydrogen into the desired ionic configuration; binding between H atoms and catalyst and lowering of activation energy due to catalyst have been discussed in terms of their effect on hydrogenation properties. It has been found that all these factors help in lowering the activation energy for the hydrogenation process and improve the hydrogenation properties.

The catalytic oxidation properties for BH4− and electrochemical properties of the Ag-decorated AB5-type hydrogen storage alloy

Anode catalysts for direct borohydride fuel cells (DBFCs) are one of the key factors controlling the performance of such cells. Hydrogen storage alloys are efficient and inexpensive DBFC anode catalysts. However, when hydrogen storage alloys are used as DBFC anode catalysts, their catalytic oxidation capacity is weakened due to their easy passivation in alkaline electrolytes. In order to improve the catalytic oxidation ability of AB5-type hydrogen storage alloy, Ag-decorated AB5-type alloy catalysts with different Ag contents were prepared by vibration ball milling method in this study. The microstructures, catalytic properties on BH4− and electrochemical properties of pure Ag, AB5-type and Ag-decorated AB5-type alloy samples were studied in detail. The results show that the Ag-decorated AB5-type alloys are composed of a single LaNi5 phase. Some of Ag powders cover the surface of the AB5-type alloy particles. Among the Ag-decorated AB5-type alloys, the Ag-decorated AB5-type alloy with an Ag content of 2 wt.% exhibits the best electrochemical performance and catalytic performance for BH4−. Its maximum discharge time in 6 M KOH solution is 303 min, which is 43 min longer than that of the AB5-type alloy. The number of electrons transferred by the Ag-decorated AB5-type alloy with 2 wt.% is the largest, reaching 1.45. The improvement of the electrochemical and catalytic properties of the alloy is mainly attributed to the proper Ag powders coating on the surface of the AB5-type alloy particles, which protects the electrode from oxidation and corrosion, and enhances the conductivity of the alloy electrode.

Preparation of A2B7-type La-Mg-Ni Hydrogen Storage Alloy and Its Catalytic Oxidation Performance for BH4 -

The La-Mg-Ni hydrogen storage alloys with A2B7/AB5 double phase were successfully prepared by vacuum melting combined with ball milling. The phase structure, electrochemical properties and the catalytic oxidation properties for BH4- of the (Mm1-xMgx)Ni2.485Co0.525Mn0.28Al0.21 (x=0.1-0.5 )alloys were investigated by XRD and electrochemical measurements. The results showed that (Mm1-xMgx)Ni2.485Co0.525Mn0.28Al0.21 alloys are composed of LaNi5 phase and (La, Mg)2Ni7 phase. Among the studied alloys, the electrochemical properties of the alloy with x=0.3 is the best, and the electrochemical properties of the alloy with x=0.3 is significantly better than that of the AB5-type hydrogen storage alloy, which is mainly due to the formation of (La, Mg)2Ni7 phase in the alloy. At the same time, the alloy with x=0.3 also shows a better catalytic oxidation effect on BH4-, which is also better than that of the AB5- type hydrogen storage alloy. The catalysis of (Mm1-xMgx)Ni2.485Co0.525Mn0.28Al0.21 alloy catalyst is related to its electrochemical performance. The catalysis of hydrogen storage alloys may have a positive correlation with its own electrochemical performance.

Effect of Mn on the long-term cycling performance of AB5-type hydrogen storage alloy

Rare-earth AB5-type hydrogen storage alloys are widely studied due to their extensive application potentials in hydrogen compressors, heat pump, Ni–MH batteries etc. However, their shortcomings such as plateau splitting and capacity degradation during hydrogen absorption/desorption hinder their practical applications. In this paper, we study the effect of Mn partial substitution for Ni on the plateau characteristics and long-term cycling performance of LaNi5-xMnx alloys. It is found that Mn addition expands the lattice interstitial for hydrogen accommodation, thus prohibiting the plateau splitting phenomenon. In addition, the substitution of Mn for Ni stabilizes the crystal structure of the alloys against hydrogen absorption/desorption, thus relieving the capacity degradation. The capacity retention of the alloys at the 1000th cycle (S1000) increases from 83.2% (x = 0) to 94.0% (x = 0.75). But when x reaches 1, the hydrogen desorption reversibility is reduced due to the low plateau pressure, resulting in a slight decrease in capacity retention.

Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB5-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB5-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB5-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB5-type hydrogen storage alloy.

Long-term hydrogen absorption/desorption properties of an AB5-type LaNi4.75Mn0.25 alloy

AB5-type hydrogen storage alloys have been extensively studied due to their wide applications, but their degradation process is still controversial, hindering the improvement in their cycling performance. In this work, the degradation mechanism of an AB5-type LaNi4.75Mn0.25 alloy with single CaCu5-type phase structure has been investigated based on its long-term hydrogen storage properties and the structural changes during cycling. It is found that within 1000 hydrogen absorption/desorption cycles in the temperature range of 343–383 K, the P-C-T curves remain a single plateau, but the plateau slope becomes steeper and the hydrogen storage capacity decreases. These degradation phenomena are caused by the structural evolution of the alloy including particle pulverization and crystal lattice damage. The internal driving force of the structural changes is found to be the microstrains generated during hydrogen absorption/desorption. But with cycling, the isotropic parameter c/a increases, decreasing the hysteresis and slowing down the degradation process.

Correlation between partial inhibition of hydrogen evolution using thiourea and catalytic activity of AB5-type hydrogen storage alloy towards borohydride electrooxidation

Direct borohydride fuel cells (DBFCs) are devices which directly convert the chemical energy stored in the borohydride ion and oxidant into electrical energy as a result of redox reactions. Unfortunately, a significant amount of fuel is lost as a result of the undesirable hydrolysis reaction. The selection of an efficient borohydride hydrolysis inhibitor requires detailed knowledge regarding the interaction mechanism between the inhibitor molecule and electrode surface. In this study, various amounts of thiourea additives (0.011–1.600 mM) were tested to select the best fuel composition for DBFC application. When AB5-type anode was used, only partial inhibition of sodium borohydride hydrolysis was a desirable phenomenon. Partially released hydrogen results in the improvement of catalytic properties of the alloy. The addition of 0.016 mM thiourea does not inhibit the oxidation of borohydride, on the contrary, it increases the practical capacity from 27% to 41% of the theoretical value. Moreover, we indicate that the addition of thiourea prevents corrosion as well as degradation of the electrode surface. Pressure measurements confirmed the effectiveness of thiourea in relation to hydrogen evolution, while X-ray photoelectron spectroscopy and Raman spectroscopy revealed that the electrode surface was not poisoned.

On structural model of AB5-type multi-element hydrogen storage alloy

The present study is focused on the development of a structural model for multi−element AB5−type alloy which can correlate the hydrogenation characteristics with structural parameters. At present no such model is available, which can predict the trend of hydrogenation properties like hydrogen storage capacity, heat of formation and plateau pressure of multi−element hydrogen storage material. It is done by trial and error method. In present investigation efforts are made to correlate atomic radius of substituted elements with heat of formation of hydride, plateau pressure and hydrogen storage capacity by calculating equivalent radius of B (rB*) in multi−element composition, contraction in A−B bond, radius of voids and ratio of rA with rB*. The heat of formation decreases and hydride stability increases with increasing value of rB* and contraction in A−B bond. While hydrogen storage capacity decreases with increasing value of rB* and contraction in A−B bond. Based on this, correlation between thermodynamic and structural properties has been established and structural model has been developed.

Hybrid nickel-metal hydride/hydrogen battery

High capacity, high efficiency and resource-rich energy storage systems are required to store large scale excess electrical energy from renewable energy. We proposed “Hybrid Nickel-Metal Hydride/Hydrogen (Ni-MH/H2) Battery” using high capacity AB5-type hydrogen storage alloy and high-pressure H2 gas as negative electrode active materials. It was experimentally confirmed that hydrogen gas can be utilized as an active material of negative electrode by the presence of the AB5-type hydrogen storage alloy. The experimental average cell voltage suggested that H2 gas passed through the alloy in the form of atoms. The calculated gravimetric energy density of this hybrid battery increased up to 1.5 times of the conventional Ni-MH battery with low content of rare-earth element which is 32 wt% of the Ni-MH battery.

Long-life Ni-MH batteries with high-power delivery at lower temperatures: Coordination of low-temperature and high-power delivery with cycling life of low-Al AB5-type hydrogen storage alloys

Although a low-Al or an Al-free design is an efficient way to develop low-temperature and high-rate metal hydride alloys, the cycling life of these alloys is poor. Our strategy is to employ B-side anti-corrosion elements (e.g., Fe, Si, Sn, Cu) to coordinate the low-temperature and high-power delivery with cycling life. We confirmed that excellent electrochemical kinetics (i.e., surface catalytic and bulk H-diffusion ability) is the primary condition for low-temperature and high-rate delivery, while it reverses with anti-corrosion ability. As a result, the low-temperature dischargeability (LTD), high-rate dischargeability (HRD) and peak power (Ppeak) progressively decrease with Ni, Si, Cu, Fe, Sn and Al substitution, but the cycling stability successively increases with Ni, Si, Fe, Sn, Cu and Al substitution. Based on the thermodynamics and the coordination of the LTD, the HRD and Ppeak with the cycling life, the (LaCe)1.0(NiCoMn)4.85Al0.05Cu0.1 alloy presents the best overall electrochemical properties. Notably, when using an as-designed Cu-doped anode, the assembled commercial 100 Ah prismatic Ni-MH batteries present excellent power delivery at −40 °C.

Effect of Bismuth on hydrogen storage properties of melt-spun LaNi4.7-x Al0.3Bix (x = 0.0, 0.1, 0.2, 0.3) ribbons

The LaNi5 intermetallic compound is an AB5 type hydrogen storage alloy which exhibits low operating temperature, easy activation, low pressure and tolerance to impurities. In this study, LaNi4.7-x Al0.3Bix (x = 0.0, 0.1, 0.2, 0.3) alloys were produced by melt-spinning technique and the effects of Al and Bi additions on the microstructure, thermal and hydrogen storage properties of LaNi5 were investigated. The results showed that substitution of Ni with Al led to a desired decrease in absorption/desorption plateau pressure and hysteresis without a decrease in hydrogen storage capacity. In contrast, Bi substitution with Ni increased the absorption/desorption plateau pressure, reduced the hydrogen capacity and increased pulverization resistance of the alloy due to the formation of BiLa and AlNi3 intermetallic phases at the grain boundaries.

Investigations on calculation of heat of formation for multi-element AB5-type hydrogen storage alloy

With development in hydrogen energy research, more and more applications of hydrogen storage materials have been put forward. This requires synthesis of new materials for specific purpose. In context to designing of metal hydride bed, thermodynamic parameter ‘Heat of formation’ (ΔH) for hydrogen storage alloy is very important. Theoretical calculation of ΔH for binary compound or ternary hydride is accomplished by well known ‘Miedema's Rule of Reverse Stability’. Experimentally ΔH may be determined using Van't Hoff Equation. So far, theoretical calculation of ΔH for multi-element alloy is not known. In the present investigation simple phenomenological formulae have been proposed to calculate ΔH for multi-element alloy including AHm, BHm, ABn, ABnH2m, ABn-xCx, ABn-xCxH2m, ABn-x-yCxDy, ABn-x-yCxDyH2m and so on. The calculated values of ΔH in present investigation have been compared with the experimental reported value or calculated by any other model reported in literature. An excellent agreement has been observed between the two.

Influence of co-milling with palladium black on hydrogen sorption performance and poisoning tolerance of surface modified AB5-type hydrogen storage alloy

This work is focused on the effect of Pd introduction into AB5-type hydrogen storage alloy through ball milling, in combination with conventional electroless plating technique, on hydrogen sorption performance and poisoning tolerance of the material. The AB5 substrate alloy was milled with small (0–5 wt%) amounts of Pd black. The effect of co-milling of the AB5 substrate with Pd on structure/phase composition, morphology, and hydrogen storage performance of the material was investigated.It was shown that co-milling of the AB5-type alloy powder with ≤1 wt% of Pd black does not result in the improvement of the material activation performance and poisoning tolerance while increasing Pd additive content produced insignificant improvements. At the same time, ball milled samples, further subjected to a standard procedure of electroless autocatalytic deposition of Pd, exhibited similar (co-milling with <1 wt% Pd) or better (co-milling with ≥1 wt% Pd) hydrogen uptake kinetic performances at lower Pd surface loading as compared to the non-milled AB5 alloy powder plated with Pd at similar conditions. The observed features were credited to the higher specific surface area of the ball milled material where Pd species introduced in the course of the ball milling played a role as nucleation centres during subsequent deposition of Pd. Conversely, the Pd-plated material obtained by co-milling of AB5 with >2 wt% of Pd black exhibited lower maximum hydrogen sorption capacities, as compared to the non-milled surface modified reference material. The most probable reason for that is in the distortion and disproportionation of the parent structure of the AB5 substrate during its co-milling with Pd black.

A solar rechargeable battery based on hydrogen storage mechanism in dual-phase electrolyte

Solar water splitting is an effective approach to hydrogen production and application of solar energy. However, the photo-generated hydrogen should be initially stored in high pressure cylinder and subsequently applied in hydrogen-oxygen fuel cells. Herein, a solar rechargeable battery is proposed based mainly on hydrogen storage mechanism in dual-phase electrolyte. Specifically, the hydrogen production, storage and utilization are integrated into a hybrid system of the dye-sensitized solar cell and electrochemical cell with the dye-sensitized TiO2 as photo-anode, LiI as the cathode active material in organic electrolyte, AB5-type hydrogen storage alloy as anode in alkaline solution, and PEDOT-modified Nafion membrane as separator. Here, the photo-generated electrons in organic electrolyte pass to the AB5-type hydrogen storage alloy to split water in alkaline aqueous electrolyte for generating hydrogen, which is in situ stored into AB5-type hydrogen storage alloy. Subsequently, the hydrogen stored in the AB5-type hydrogen storage alloy can be oxidized by electrochemical way to generate electricity, coupled with LiI cathode in organic electrolyte. The solar rechargeable battery demonstrates a new solution of the solar energy conversion, hydrogen production, storage, and utilization, achieving the new energy conversion and storage from solar energy to chemical energy, and further to electrical energy.

Improvement in low-temperature and instantaneous high-rate output performance of Al-free AB5-type hydrogen storage alloy for negative electrode in Ni/MH battery: Effect of thermodynamic and kinetic regulation via partial Mn substituting

Abstract A series of Al-free Mn-modified AB 5 -type hydrogen storage alloys have been designed and the effects of thermodynamic stability and electrochemical kinetics on electrochemical performance via Mn substituting have been investigated. Compared with high-Al alloys, the Al-free alloys in this study have better low-temperature performance and instantaneous high-rate output because of the higher surface catalytic ability. After partial substitution of Ni by Mn, both the hydrogen desorption capacity and plateau pressure decrease, and correspondingly results in an improved thermodynamic stability which is adverse to low-temperature delivery. Additionally, with the improvement of charge acceptance ability and anti-corrosion property via Mn substitution, the room-temperature discharge capacity and cycling stability increase slightly. However, Mn adversely affects the electrochemical kinetics and deteriorates both the surface catalytic ability and the bulk hydrogen diffusion ability, leading to the drop of low-temperature dischargeability, high-rate dischargeability and peak power ( P peak ). Based on the thermodynamic and kinetic regulation and overall electrochemical properties, the optimal composition is obtained when x = 0.2, the discharge capacity is 243.6 mAh g −1 at −40 °C with 60 mA g −1 , and the P peak attains to 969.6 W kg −1 at −40 °C.

Investigations on AB3-, A2B7- and A5B19-type La[sbnd]Y[sbnd]Ni system hydrogen storage alloys

The structure and properties of new LaYNi system alloys with high hydrogen-storing capacity were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), solid-H2 reactions (P-C-I curves) and electrochemical measurements. The LaY2Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-type) hydrogen storage alloys were prepared with the induction-melting rapid-quenching method and annealed at 1148 K for 16 h. The LaYNiMnAl alloys were also compared with commercial AB5-type hydrogen storage alloy with high capacity. Similarly to LaMgNi system hydrogen storage alloy, LaYNi system alloys are multiphase structures and Y element in the LaYNi alloys avoid or delay the hydrogen-induced amorphous (HIA) of the alloys in the hydrogenation/dehydrogenization process. The hydrogen storage capacities of the A2B7- and A5B19-type alloys at 313 K are 1.48 wt.% and 1.45 wt.%, respectively, which are larger than that of the AB5-type alloy (1.38 wt.%). The maximum discharge capacities of the A2B7- and A5B19-type alloy electrodes at 298 K are 385.7 mAh g−1 and 362.1 mAh g−1, respectively, which are larger than that of the AB5-type alloy (356.1 mAh g−1). The maximum discharge capacity of the A2B7-type alloy exceeds the theoretical capacity (372 mAh g−1) of the AB5-type alloy. The A2B7- and A5B19-type alloy electrodes have better cycling ability than the AB5-type alloy.