The effect of substitution of Mn or Al on the hydrogen sorption characteristics of CeNi5

The hydrides of the ternary alloys of CeNi_5?xM_x (M = Mn or Al and x = 0.5 and 0.75) have been prepared and investigated. The absorption plateau pressure of the system CeNi₅—H is reduced 8- to 75-fold, by the replacement of Ni by Mn (or Al), without significantly impairing its hydrogen capacity. This substitution has great influence in reducing the hysteresis effect associated with the hydrogen absorption and desorption. The hysteresis and plateau-sloping factors are very low compared with that in Mischmetal-Ni₅ hydride. The enthalpies and entropies of hydrogenation and dehydrogenation for CeNi_5?xM_x are computed and found to be in the range 20–25 kJ(mole H₂)^?1) and ～90–100 J(K mole H₂)^?1, respectively. The high effective entropies of the hydride (high configurational entropy of H in the lattice) are attributed to extensive hydrogen disorder in the interstitial sites of the CeNi_5?xM_x lattice. Desorption of the hydrogen in the two-phase region (α + β) for CeNi_4.25Mn_0.75-H follows first-order kinetics with an activation energy of ～33 ± 1 kJ(mole)^?1. The favorable physiochemical properties of CeNi_5?xMn_x-H system make it very attractive for applications.     

Hydrogenation of acetylene and propyne over hydrogen storage ErNi5-xAlx alloys and the role of absorbed hydrogen

The hydrogen storage properties of ErNi_5-xAl_x (x = 0, 0.5, 0.75, 1, 1.25, and 1.5) alloys were investigated by pressure-composition isotherms and in situ X-ray diffraction measurements under a hydrogen atmosphere. Catalytic reactivities toward the hydrogenation of alkynes (acetylene and propyne) over ErNi_5-xAl_x (x = 0, 1, and 1.5) alloys were also studied and the contribution of absorbed hydrogen to hydrogenation is discussed. All ErNi_5-xAl_x alloys possess a hexagonal structure (CaCu₅-type) with the space group P6/mmm. The substitution of Al for Ni facilitated hydrogen absorption at lower hydrogen pressures by the formation of larger interstitial spaces. ErNi_3.5Al_1.5H_n with absorbed hydrogen showed higher reactivities for the catalytic hydrogenation of acetylene and propyne than ErNi₅ and ErNi₄Al without absorbed hydrogen. The reason for this was concluded to be that absorbed hydrogen activates adsorbates (acetylene and hydrogen) that are supplied from the gas phase.     

New insights into the hydrogen storage performance degradation and Al functioning mechanism of LaNi5-xAlx alloys

Rare-earth based AB₅-type alloys have wide application prospect in many areas such as gaseous hydrogen storage, hydrogen compressing, Ni/MH batteries etc. But opinions on their cycling degradation mechanism remain controversial, hindering them from further improvement. In this study, the hydrogen storage degradation mechanism of LaNi_5-xAl_x (x = 0, 0.25, 0.5) alloy system is studied using EXAFS as one of the main methods, and meanwhile the functioning mechanism of Al is illustrated. It is found that the hydrogen absorption/desorption plateau becomes tilted and the hydrogen storage capacity is decreased with cycling. These degradation phenomena are caused by the crystal damage due to the lattice strains as well as the mis-occupation of metal atoms during hydrogen absorption/desorption. Al with larger atomic radius stabilizes LaNi₅ lattice structure by decreasing the lattice volume expansion rate upon hydrogenation and detaining the atomic migration during cycling, thus significantly improving the alloys' cycling stability from 89.1% (LaNi₅) to 98.2% (LaNi_4.5Al_0.5) after 1000 cycles.     

Study on hydrogen storage properties of LaNi3.8Al1.2−xMnx alloys

Effects of the Mn substitution on microstructures and hydrogen absorption/desorption properties of LaNi_3.8Al_1.2−xMn_x (x = 0.2, 0.4, 0.6) hydrogen storage alloys were investigated. The pressure-composition (PC) isotherms and absorption kinetics were measured in a temperature range of 433 K ≤ T ≤ 473 K by the volumetric method. XRD analyses showed that with the increase of the Mn content in the LaNi_3.8Al_1.2−xMn_x alloys, the lattice parameter a was decreased, c increased and the unit cell volume V reduced. It was found that the absorption/desorption plateau pressure was increased and the hydrogen storage capacity was enhanced with the increase of Mn content. The absorption/desorption plateau pressure of the alloys was linearly changed with the Mn content x and the lattice parameter a, while the hydrogen storage capacity was linearly increased with the increase of c/a ratio. It was also found that the slope factor S_f was closely correlated with the lattice strain of the alloys.     

Formation of AlNi phase and its influence on hydrogen absorption kinetics of Mg77Ni23-xAlx alloys at intermediate temperatures

In order to reduce the obstacle influence of coarse Mg₂Ni phase on hydrogen absorption kinetics in Mg–Ni alloys, aluminum was doped and Mg₇₇Ni_23-xAl_x (x = 0, 3, 6, 9) alloys were prepared. The results show that AlNi phase was formed when Al was added, the size of primary Mg₂Ni phase decreases with increasing Al content till 6 at.%, while primary Mg₂Ni phase was diminished and primary Mg phase was formed when Al content increased to 9 at.%. The initial hydrogenation rates of Mg₇₇Ni_23-xAl_x alloys were increased, which is resulted from the refined primary Mg₂Ni and the catalytic AlNi phase. More importantly, the hydrogenation rates and capacities were significantly improved at 150 °C, especially for the Mg₇₇Ni₁₇Al₆ alloy. The apparent activation energy of the Mg₇₇Ni₁₇Al₆ alloy for hydrogenation was reduced to 73.68 kJ/mol from 102.27 kJ/mol of the Mg₇₇Ni₂₃ alloy. Its enthalpy changes for hydrogenation at low and high platforms are 72.3 kJ/mol and 53.9 kJ/mol, respectively. The multiple channels and short distance for hydrogen atoms diffusion provided by refined primary Mg₂Ni phase, the solid dissolution of Al in Mg₂Ni lattice, and catalytic effect of AlNi on hydrogenation, leading to the improvement of the hydrogen storage properties.     

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 LaNi_5-xMn_x 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.     

Effect of interstitial boron and carbon on the hydrogenation properties of Ti25V35Cr40 alloy

Doping of interstitial elements B or C into a BCC-type Ti₂₅V₃₅Cr₄₀ alloy to raise effective desorption hydrogenation capacity was investigated. Ti₂₅V₃₅Cr₄₀M_x alloys (M = B or C and x = 0, 0.1, 1, or 5) were prepared by arc-melting followed by homogenization treatment. X-ray diffraction shows that the as-cast specimens have a BCC structure, but they contain some amount of precipitates that increases with the doping concentration of B and C. Doping-induced precipitates can be greatly eliminated by annealing treatment at 1200 °C, indicating that B or C might have been partially dissolved into the interstitial sites in the BCC lattice of matrix phase of specimens. With the doping of C, the second plateau pressure of annealed specimens in the PCI curves at T = 30 °C significantly increases with the amount of C, but the maximum hydrogenation capacity is reduced. On the other hand, the second plateau pressure and maximum hydrogenation capacity are only slightly affected by the B doping. Under optimum doping conditions, the effective hydrogen desorption capacities are increased from 0.80 H/M of the sample without doping to 0.86 H/M and 0.87 H/M for Ti₂₅V₃₅Cr₄₀B₁ and Ti₂₅V₃₅Cr₄₀C_0.1, respectively. The improvement might be ascribed to the increase in second plateau pressure caused by less stable hydrogen atoms at the lattice sites of Ti₂₅V₃₅Cr₄₀ containing interstitial B or C.     

Structural characteristics and hydrogen storage properties of Ca3.0−xMgxNi9 (x = 0.5, 1.0, 1.5 and 2.0) alloys

The effect of Mg content on the structural characteristics and hydrogen storage properties of the Ca_3.0−xMg_xNi₉ (x = 0.5, 1.0, 1.5 and 2.0) alloys was investigated. The lattice parameters and unit cell volume of the PuNi₃-type (Ca, Mg)Ni₃ main phase decreased with increasing Mg content. The 6c site of PuNi₃-type structure was occupied by both Ca and Mg atoms. Moreover, the occupation factor of Ca on the 6c site decreased with the increase of Mg content. The hydrogen absorption capacity of the alloys decreased due to higher Mg content. However, the thermodynamic properties of hydrogen absorption and desorption were improved and the plateau pressures were increased. When x = 1.5–2.0, the Ca_3.0−xMg_xNi₉ alloys had favorable enthalpy (ΔH) and entropy (ΔS) of hydride formation.     

Structural change and hydrogenation behavior of (Sr,Ca)2Al alloys

The phase structure and hydrogenation behavior of the (Sr_1−xCa_x)₂Al (x = 0.5, 0.6, 0.7, 0.8 and 0.9) alloys were investigated. It was found that the (Sr_1−xCa_x)₂Al alloys have different phase components with the change of Ca content. When x = 0.6 and 0.7, the single (Sr, Ca)₂Al phase can be obtained. The lattice parameters of (Sr, Ca)₂Al compound in the (Sr_1−xCa_x)₂Al alloys decrease gradually with the increase of Ca content. The (Sr, Ca)₂Al phase can absorb hydrogen even at 303 K. During hydrogenation, the (Sr, Ca)₂Al compound transforms into distinct phases at different temperatures.     

Properties of La0.2Y0.8Ni5−xMnx alloys for high-pressure hydrogen compressor

Hydrogen absorption/desorption properties of La_0.2Y_0.8Ni_5−xMn_x (x = 0.2, 0.3, 0.4) alloys for high-pressure hydrogen compression application were investigated systematically. The Pressure–Composition isotherms and absorption kinetics were measured at 293, 303 and 313 K by the volumetric method. XRD analyses showed that all the investigated alloys presented CaCu₅ type hexagonal structure and the unit cell volume increased in both a and c lattice axes with Mn substitution. Hydrogen absorption/desorption measurements revealed that Mn could lower the plateau pressure effectively, improve the hydrogen storage capacity and absorption kinetics but slightly increase the slope of the pressure plateau and hysteresis. The study results suggest that La_0.2Y_0.8Ni_4.8Mn_0.2 is suitable for the high-pressure stage compression of the hydrogen compressor and the other two alloys, La_0.2Y_0.8Ni_4.7Mn_0.3 and La_0.2Y_0.8Ni_4.6Mn_0.4, for the preliminary stage.     

Correlation study between hydrogen absorption property and lattice structure of Mg-based BCC alloys

Nanostructured Mg₆₀Ni₅Co_mX_35 − m (X = Co, B, Al, Cr, V, Pd and Cu) body centered cubic (BCC) alloys were synthesized by mechanical alloying method. These Mg-based alloys with different lattice parameters can show significantly different hydrogen absorption properties. The BCC alloys with lattice parameter in the range of 0.300∼0.308 nm absorb large amount of hydrogen at 373 K and the BCC alloys with the parameter larger than 0.313 nm have difficulty to absorb hydrogen at this temperature. Geometric effect is thought to be one of the dominant factors to affect the hydrogen absorption property of interstitial alloys. Nanostructure, fresh surface area and defects produced during mechanical alloying process are also important facts that make Mg-based alloys absorb hydrogen at 373 K.     

Structure and hydrogen storage properties of La1-xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys prepared by melt spinning

Melt spinning technology was applied to prepared La_1-xPr_xMgNi_3.6Co_0.4 (x = 0–0.4) alloys, and phase composition, micro-structure, morphology and hydrogen storage properties were systematically investigated. The results show that the alloys contain two phases, LaMgNi₄ and LaNi₅ which have been detected by XRD and SEM. The grain of the alloys is refined by increasing Pr content and the phase abundance changed obviously. The hydrogen absorption capacity (wt%) of the alloys is 1.663, 1.659, 1.60, 1.593 and 1.566, corresponding the Pr substation of x from 0 to 0.4. The hydrogenation cycle stability indicates that the hydrogen capacity declined severely with the hydrogenation cycles. It is attributed to the hydrogen-induced amorphization which is confirmed by the XRD results after hydrogenation cycles. In order to recover the hydrogen storage capacity after cycles, the annealing treatment at 673 K for 3 h was carried out. And the XRD and HRTEM results show that the amorphization structure after hydrogen absorption/desorption cycles is re-crystallized by annealing treatment.     

Effect of Cr on the hydrogen storage and electronic properties of BCC alloys: Experimental and first-principles study

Inventing an effective method to store large amounts of hydrogen at room temperature is one of the key challenges in developing a hydrogen-based economy. Metal hydrides have attracted attention owing to their promising hydrogen storage capabilities. We have systematically studied the structural and electronic properties of mechanically synthesized Ti_0.5V_1.5-xCr_x (0 ≤ x ≤ 0.3) alloys and investigated the influence of the addition of Cr atoms on the hydrogen storage properties of vanadium-rich body-centered-cubic (V-BCC) alloys. X-ray diffraction (XRD) results indicate that all alloys are composed of BCC main phase, with the lattice parameters exhibiting no change following chemical modification. The kinetic measurements have revealed that Cr-containing alloys exhibit improved hydrogen uptake. X-ray photoelectron spectroscopy (XPS) measurements have shown that the addition of Cr has a significant effect on the anti-oxidation properties of V-BCC alloys, increasing their chemical activity and thus enhancing the hydrogen storage properties. Moreover, XPS results elucidate the role of activation of the studied materials. Additionally, the electrochemical properties of the negative electrodes (as part of Ni-MH_x secondary batteries) made of Ti_0.5V_1.4-xNi_0.1Cr_x (0 ≤ x ≤ 0.3) system have been studied by cyclic charge-discharge and demonstrate that doping of the V-BCC alloys with Cr can significantly improve the cycle-life stability of anode that exhibits similar discharge performance up to 50 cycles. First principles simulations are used to analyse the changes in the electronic density of states close to the Fermi level, as a function of Cr concentration, as well as binding energies and structural changes upon hydrogen absorption. Furthermore, ab initio studies confirmed that H absorption is favoured with increasing Cr-content. Our study highlights the importance of the addition of Cr to V-BCC alloys on both solid-gas and electrochemical hydrogenation reactions.     

Study on hydrogen storage property of (ZrTiVFe)xAly high-entropy alloys by modifying Al content

(ZrTiVFe)_xAl_y high-entropy alloys are potential hydrogen storage materials because of their intermediate properties of high hydrogen uptake capacity and fast kinetics. In this study, equimolar and non-equimolar (ZrTiVFe)₈₀Al₂₀ and (ZrTiVFe)₉₀Al₁₀ alloys were prepared, and the effect of Al content on the microstructure, element distribution, and hydrogen storage properties of (ZrTiVFe)_xAl_y alloys were investigated. The results show that both alloys are composed of C14 Laves phase, a small amount of tetragonal and HCP phases. With the increase of Al content, the content and the size of C14 Laves phase decrease, the V element content in C14 Laves phase also decreases, which is resulted from the contents of Zr, V, and Fe element constituting the C14 Laves phase decrease. The Ti element can combine with the excessive Al to form a tetragonal phase around the C14 phase, and the growth of the tetragonal phase causes the refinement of the C14 phase. The (ZrTiVFe)₉₀Al₁₀ alloy absorbed 1.3 wt. % H at room temperature, which indicates the better hydrogenation capacity and kinetics. The improvement of hydrogen storage property is resulted from the increased C14 Laves phase, more V element in C14 Laves phase and the severe lattice distortion of (ZrTiVFe)₉₀Al₁₀ alloy. It is found that the (ZrTiVFe)₈₀Al₂₀ alloy can be activated easily with only three cycles, which is caused by the refined Laves phase. After hydrogenated, H atoms are dissolved into the lattice space of C14 Laves phase for both alloys, and crystalline structure is not changed.     

Effects of partial substitutions of cerium and aluminum on the hydrogenation properties of La(0.65−x)CexCa1.03Mg1.32Ni(9−y)Aly alloy

Hydrogen in metal hydrides could be one of the promising energy storage mediums to address the intermittent nature of renewable energy. To convert the hydrogen energy to electricity, the storage system has to be coupled with a fuel cells system. Hence, it is important to design a hydrogen storage system that meets the operating requirements for a fuel cell system. In this work, the effects of partial substitution of both cerium and aluminum on the hydrogenation properties of La_(0.65−x)Ce_xCa_1.03Mg_1.32Ni_(9−y)Al_y alloys were investigated simultaneously using factorial design. Both Ce and Al additions greatly improved the reversibility of hydrogen storage capacity. However, the maximum hydrogen storage capacity and absorption kinetics can be reduced by the additions. As Ce and Al gave opposite effects on the absorption and desorption plateaus, they could be used to tune the properties of the alloys to the desired operating conditions for fuel cell applications.     

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V₄₈Fe₁₂Ti_15-xCr₂₅Al_x (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

Study on the eutectic formation and its correlation with the hydrogen storage properties of Mg98Ni2-xLax alloys

Transition metals and rare-earth elements have excellent catalytic effects on improving the de-/hydrogenation properties of Mg-based alloys. In this study, a small amount of La is used to substitute the Ni in Mg₉₈Ni₂ alloy, and some Mg₉₈Ni_2-xLa_x (x = 0, 0.33, 0.67, and 1) alloys show the better overall hydrogen storage properties. The effects of La on the solidification and de-/hydrogenation behaviors of the alloys are revealed. The results indicate that different factors dominate the processes of hydrogen absorption and desorption. The Mg₉₈Ni_1·67La_0.33 alloy absorb 7.04 wt % hydrogen at 300 °C, with the highest isothermal absorption rate, the Mg₉₈Ni_1·33La_0.67 hydride show the highest isothermal desorption rates and the lowest peak desorption temperature of 327 °C. The La addition can increase the driving force of hydrogenation, thus the hydrogenation rates and capacities of the Mg₉₈Ni_1·67La_0.33 and Mg₉₈Ni_1·33La_0.67 alloys are improved. The formation of refined eutectic structures is a key factor that facilitates the desorption processes of the Mg₉₈Ni_2-xLa_x hydrides with x = 0.67 and 1. High-density LaH₃ nanophses are in-situ formed from the LaMg_x (8.5 < x < 12) phase, which results in the improved de-/hydrogenation properties. The further La addition deteriorates the hydrogen storage properties of Mg₉₈Ni_2-xLa_x alloy.     

Advanced interpretation of hydrogen absorption process in LaMgNi3.6M0.4 (M = Ni,Mn, Al,Co, Cu) alloys using statistical physics treatment

To obtain good economic and environmental benefits, LaMgNi_3.6M_0.4 (M = Al, Mn, Ni, Co, Cu) alloys are investigated for the hydrogen storage. The absorption data of hydrogen in the tested alloys are measured experimentally at 373 K. The hydrogen absorption isotherms are analyzed using three models derived from statistical physics formalism. The adequate model permits to discover significant details about the absorption phenomenon via determining the density of the interstitial sites (D_m), the number of hydrogen atoms per site (n) and the energetic parameter ΔE. The results indicate that multi-atomic (n > 1) and multi-linking (n < 1) phenomena are feasible for hydrogen absorption in LaMgNi_3.6M_0.4 (M = Al, Mn, Ni, Cu, Co) metals. The effects of the substitutions of Ni with Mn, Co, Cu and Al on the hydrogen absorption capacity are investigated. The interaction hydrogen/metal is analyzed by the calculation of the absorption energies. The chemical interaction is the responsible for the hydrogen absorption phenomenon. The contribution of this work is to provide advanced investigations of the hydrogen absorption mechanism in LaMgNi_3.6M_0.4 (M = Al, Mn, Ni, Co, Cu) metals, which are promising alloys for the hydrogen storage.     

Effect of introducing manganese as additive on microstructure,hydrogen storage properties and rate limiting step of Ti–Cr alloy

TiCr₂ with adding different amount of Mn (0, 2, 4 and 8 wt.%) alloys have been investigated. All alloys have C14-type main phase (gray color in SEM) and Ti minor phase (dark gray color in SEM). Rietveld fitting results proved that the lattice parameter a and cell volume of C14-type phase decreased with increasing Mn content. The first hydrogenation measurement manifest that all alloys have best activation properties and could be activated without any prior heat treatment and hydrogen exposure. However, introducing Mn led to the decrease of the first hydrogen absorption rate of TiCr₂ alloy, which may be due to the decrease of cell volume of C14-type main phase. The first hydrogenation properties at low temperature and effect of air exposure of the alloy were discussed. The results showed that the maximum hydrogen absorption capacity at 0 °C was obviously higher than that at room temperature. In addition, TiCr₂ alloy doped with minor amounts of Mn after long-time air exposure showed better hydrogenation performance. This may be due to the Mn additive acting as a deoxidizer. Finally, the first hydrogenation kinetic mechanisms of all alloys at different temperature were also studied by using the rate limiting step.