Mg2−xTixNi (x = 0, 0.5) alloys prepared by mechanical alloying for electrochemical hydrogen storage: Experiments and first-principles calculations

Mg_2−xTi_xNi (x = 0, 0.5) electrode alloys have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of substitutional doping of Ti in Mg₂Ni phase have been investigated by first-principles density functional theory calculations. XRD analysis results indicate that Ti substitution for Mg in Mg₂Ni-type alloys results in the formation of TiNi (Pm-3m) and TiNi₃ intermetallics. With the increase of milling time, the TiNi phase captures Ni from Mg₂Ni to further form TiNi₃ phase and the MgO phase increases. The calculated results of enthalpy of formation indicate that the most preferable site of Ti substitution in Mg₂Ni lattice is Mg(6i) position and the stability of phase gradually decreases along the sequence TiNi₃ phase > TiNi phase > Mg₉Ti_3Mg(6i)Ni₆ Ti-doped phase > Mg₂Ni phase. SEM observations show that the average particle sizes of Mg₂Ni and Mg_1.5Ti_0.5Ni milled alloys decrease and increase, respectively with increasing the milling time. The TEM analysis results reveal that TiNi and Mg₂Ni coexist as nanocrystallites in the Mg_1.5Ti_0.5Ni alloy milled for 20 h. Electrochemical measurements indicate that the maximum discharge capacities of Mg₂Ni and Mg_1.5Ti_0.5Ni alloys rise and decline, respectively with the prolongation of milling time. The Mg_1.5Ti_0.5Ni alloy milled for 20 h shows the highest discharge capacity among all milled alloys. The capacity retaining rate of Mg_1.5Ti_0.5Ni milled alloys is better than that of Mg₂Ni milled alloys.     

Structural characterization and electrochemical hydrogen storage properties of Ti2−xZrxNi (x = 0, 0.1, 0.2) alloys prepared by mechanical alloying

Nominal Ti₂Ni was synthesized under argon atmosphere at room temperature using a planetary high-energy ball mill. The effect of milling time and Zr substitution for Ti on the microstructure was characterized by XRD, SEM and TEM, and the discharge capacities of Ti_2−xZr_xNi (x = 0, 0.1, 0.2) were examined by electrochemical measurements at galvanostatic conditions. XRD analysis shows that amorphous phase of Ti₂Ni can be elaborated by 60 h of milling, whereas Zr substitution hinders amorphization process of the system. The products of ball milling nominal Ti_2−xZr_xNi (x = 0.1, 0.2) were austenitic (Ti, Zr)Ni and partly TiO, despite the fact that the operation was carried out under argon atmosphere. By comparing the SEM micrographs, it is found that the amorphous phase of Ti₂Ni was formed in the stage of cold-welding during milling, while with Zr substitution particles were flaky and finer, inhomogeneous in size distribution with massive agglomeration. TEM analysis was carried out and confirmed the observations via XRD. In the electrochemical tests, amorphous Ti₂Ni shows the best discharge capacity at 102 mAh/g at a current density of 40 mA/g. Without need of activation, it exhibits extraordinary cycling stability under room temperature. On the other hand, the effect of Zr substitution on the electrochemical property of Ti₂Ni is tricky, as superficially the discharge capacity drops drastically with Zr substitution, but with increase of Zr content (from x = 0.1 to x = 0.2), the discharge capacity increases generally, which credits to larger unit-cell-volume provided by ZrNi compared to TiNi. It is also found that the Ti–Ni system becomes significantly susceptible to oxidation when Zr is introduced to the initial powders as mechanical alloying is deployed as a synthesis method.     

Electrochemical hydrogen storage characteristics of Mg1.5Al0.5−xZrxNi (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) alloys synthesized by mechanical alloying

Mg_1.5Al_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) type alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that Zr facilitated the amorphization of Mg₂Ni phase, while Al retarded the amorphization of this phase. The increase in the Zr content was observed to bring about significant improvement in the discharge capacities at all the ball milling durations. The stepwise replacement of Al with Zr, however, caused considerable reduction in the initial discharge capacities of the alloys. Despite the adverse effect of Al on the initial discharge capacity, it prevented the rapid degradation of Mg₂Ni phase with the charge/discharge cycles. When the beneficial effects of Zr and Al were combined by designing Mg_1.5Al_0.5−xZr_xNi type alloys, Mg_1.5Al_0.2Zr_0.3Ni alloy was found to have the highest discharge capacity at almost all the charge/discharge cycle steps. Among the obtained capacity retaining rates, Mg_1.5Al_0.4Zr_0.1Ni alloy had the best performance. This alloy has kept at least 50% of its initial discharge capacity at 20th cycle. The analysis by the electrochemical impedance spectroscopy revealed that the charge transfer resistances of Al-rich alloys were low at high depth of discharges. This observation was attributed to the formation of the porous unstable Mg(OH)₂ layer due to the intercalation of Al₂O₃ layers, which have the high rate of solubility in strongly basic solutions, and thus the exposition of the underlying electrocatalytically active Ni sites.     

Synergistic effects of multiwalled carbon nanotubes and Al on the electrochemical hydrogen storage properties of Mg2Ni-type alloy prepared by mechanical alloying

Mg_2−xAl_xNi (x = 0, 0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by XRD, SEM and TEM. XRD analysis results indicate that Al substitution results in the formation of AlNi-type solid solution that can interstitially dissolve hydrogen atoms. In contrast, the addition of MWCNTs hardly affects the XRD patterns. SEM observations show that after co-milling with 5 wt. % MWCNTs, the particle sizes of both Mg₂Ni and Mg_1.75Al_0.25Ni milled alloys are decreased explicitly. The TEM images reveal that ball milling is a good method to cut long MWCNTs into short ones. These MWCNTs aggregate along the boundaries and surfaces of milled alloy particles and play a role of lubricant to weaken the adhesion of alloy particles. The majority of MWCNTs retain their tubular structure after ball milling except a few MWCNTs whose tubular structure is destroyed. Electrochemical measurements indicate that all milled alloys have excellent activation properties. The Mg_1.75Al_0.25Ni-MWCNTs composite shows the highest discharge capacity due to the synergistic effects of MWCNTs and Al on the electrochemical hydrogen storage properties of Mg₂Ni-type alloy. However, the improvement on the electrode cycle stability by adding MWCNTs is unsatisfactory.     

Microstructure and electrochemical hydrogen storage characteristics of La0.67Mg0.33−xCaxNi2.75Co0.25 (x = 0-0.15) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of La_0.67Mg_0.33−xCa_xNi_2.75Co_0.25 (x = 0, 0.05, 0.10 and 0.15) alloys are investigated. The results show that all alloys mainly consist of (La, Mg)Ni₃ and LaNi₅ phases, besides a small amount of (La, Mg)₂Ni₇ phase. The cycle stability (S₈₀) after 80 charge/discharge cycles of all alloy electrodes first increases from 60.1% (x = 0) to 64.2% (x = 0.05), then decreases to 45.9% (x = 0.15). The high rate dischargeability of all alloy electrodes first increases from 52.6% (x = 0) to 61.4% (x = 0.10), then decreases to 57.2% (x = 0.15). Moreover, the charge-transfer resistance (R_ct) first decreases from 168.2 mΩ (x = 0) to 125.7 mΩ (x = 0.10), then increases to 136.6 mΩ (x = 0.15). All the results indicate that the substitution of Mg with a certain amount of Ca can improve the overall electrochemical characteristics.     

Electrochemical hydrogen storage characteristics of nanocrystalline and amorphous Mg20Ni10-xCox(x=0−4) alloys prepared by melt spinning

Nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂₀Ni_10-xCo_x (x = 0, 1, 2, 3, 4) were synthesized by melt-spinning technique. The microstructures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The electrochemical hydrogen storage characteristics of the as-cast and spun alloys were measured. The obtained results show that the substitution of Co for Ni does not change the major phase of Mg₂Ni, but it leads to the formation of secondary phase MgCo₂ and Mg. No amorphous phase forms in the as-spun alloy (x = 0), whereas the as-spun alloy (x = 4) holds a nanocrystalline and amorphous structure, confirming that the substitution of Co for Ni significantly heightens the glass forming ability of the Mg₂Ni-type alloy. The substitution of Co for Ni and melt spinning significantly improve the electrochemical hydrogen storage performances of the alloys. When Co content x increases from 0 to 4, the maximum discharge capacity of the as-cast alloy increases from 30.3 to 113.3 mAh/g, and from 135.5 to 402.5 mAh/g for as-spun (30 m/s) alloy. The capacity retaining rate of the as-cast alloy after 20 cycles rises from 36.71 to 37.04%, and from 27.06 to 83.35% for as-spun (30 m/s) alloy, respectively.     

Electrochemical hydrogen storage properties of Mg2−xAlxNi (x = 0, 0.3, 0.5, 0.7) prepared by hydriding combustion synthesis and mechanical milling

Mg_2−xAl_xNi (x = 0, 0.3, 0.5, 0.7) hydrogen storage alloys used as the negative electrode in a nickel–metal hydride (Ni–MH) battery were successfully prepared by means of hydriding combustion synthesis (HCS) and the selected alloy Mg_1.5Al_0.5Ni was further modified by mechanical milling (MM). The structural and electrochemical hydrogen storage properties of Mg_2−xAl_xNi alloys have been investigated in detail. XRD results show that a new phase Mg₃AlNi₂ that possesses an excellent cycling stability is observed with the substitution of Al for Mg. A short-time mechanical milling has a significant effect on improving the discharge capacity of the HCS product of Mg_1.5Al_0.5Ni. The maximum discharge capacity of Mg_1.5Al_0.5Ni ascends with increasing mechanical milling time and reaches the maximum 245.5 mAh/g when milled for 10 h. The alloy milled for 5 h shows the best electrochemical kinetics, which is due to its smaller mean particle size and uniform distribution of the particles. Further increasing in mechanical milling time could not bring about better electrochemical kinetics, which might be attributed to the agglomeration of the alloy particles and thus the charge-transfer reaction and hydrogen diffusion are restrained. It is suggested that the novel method of HCS + MM is promising to prepare ternary Mg-based intermetallic compound for electrochemical hydrogen storage.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.     

Electrochemical performances of the as-melt La0.75−xMxMg0.25Ni3.2Co0.2Al0.1 (M = Pr,Zr; x = 0, 0.2) alloys applied to Ni/metal hydride (MH) battery

The partial replacement of La by M (M = Pr, Zr) has been performed in order to ameliorate the electrochemical hydrogen storage performances of La–Mg–Ni-based A₂B₇-type electrode alloys. For this purpose, we adopt melt spinning technology to prepare the La_0.75−xM_xMg_0.25Ni_3.2Co_0.2Al_0.1 (M = Pr, Zr; x = 0, 0.2) electrode alloys. Then systemically investigate the effects that the preparation methods and M (M = Pr, Zr) substitution have on the structures and electrochemical hydrogen storage characteristics of the alloys. The analysis of XRD and TEM reveals that the as-cast and spun alloys hold a multiphase structure, containing two main phases (La, Mg)₂Ni₇ and LaNi₅ as well as a trace of residual phase LaNi₂. Besides, the as-spun (M = Pr) alloy displays an entire crystalline structure, while an amorphous-like structure is detected in the as-spun (M = Zr) alloy, implying the replacement of La by Zr facilitates forming amorphous phase. Based upon electrochemical measurements, an impact engendered by melt spinning on the electrochemical performances of the alloys appears to be evident. The cycle stabilities monotonously augment with the growing of the spinning rate. The discharge capacity and high rate discharge ability (HRD), however, exhibit difference. For the (M = Pr) alloy, they first mount up and then fall with the rising of the spinning rate, whereas for the (M = Zr) alloy, they always decline as the spinning rate elevates. Furthermore, the replacement of La by M (M = Pr, Zr) considerably enhances the cycle stability of the alloys and the replacement of La by Pr clearly increases the discharge capacity, but the Zr replacement results in an adverse impact.     

Improved hydrogen storage behaviours of nanocrystalline and amorphous Mg2Ni-type alloy by Mn substitution for Ni

The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt spinning technique. The structures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that the as-spun (x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun (x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The hydrogen absorption capacity of the alloys first increases then decreases with rising Mn content, but the hydrogen desorption capacity of the alloys grows with increasing Mn content. Furthermore, the substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving both the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn from 0 to 0.4, the discharge capacity of as-spun (30 m/s) alloy grows from 116.7 to 311.5 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle rises from 36.7 to 78.7%.     

Effect of substituting Mn for Ni on the hydrogen storage and electrochemical properties of ReNi2.6−xMnxCo0.9 alloys

ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys were prepared by induction melting. The effects of partially substituting Mn for Ni on the phase structure and electrochemical properties of the alloys were investigated systematically. In the alloys, (La, Ce)₂Ni₇ phase with a Ce₂Ni₇-type structure, (Pr, Ce)Co₃ phase with a PuNi₃-type structure, and (La, Pr)Ni₅ phase with a CaCu₅-type structure were the main phases. The (La,Pr)Ni phase appeared when x increased to 0.45, and the (La, Pr)Ni₅ phase disappeared with further increasing x (x > 0.45). The hydrogen-storage capacity of the ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys initially increased and reached a maximum when Mn content was x = 0.45, and then decreased with further increasing Mn content. The ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloy exhibited a hydrogen-storage capacity of 0.81, 0.98, 1.04, 0.83 and 0.53 wt.%, respectively. Electrochemical studies showed that the maximum discharge capacity of the alloy electrodes initially increased from 205 mAh/g (x = 0.0) to 352 mAh/g (x = 0.45) and then decreased to 307 mAh/g (x = 90). The hydrogen absorption rate first increased and then decreased with addition of Mn element. The ReNi_2.15Mn_0.45Co_0.9 alloy showed faster hydrogen absorption kinetics than that of the other alloys. The presence of Mn element slowed hydrogen desorption kinetics.     

Microstructure and electrochemical investigations of La0.76−xCexMg0.24Ni3.15Co0.245Al0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys

The effects of substitution of Ce for La on the microstructure and electrochemical performance of La_0.76−xCe_xMg_0.24Ni_3.15Co_0.245Al_0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses showed that the main phases of the alloys consist of (La, Mg)Ni₃ phase (PuNi₃-type rhombohedral structure), LaNi₅ phase (CaCu₅-type hexagonal structure) and (La, Mg)₂Ni₇ phase (Ce₂Ni₇-type hexagonal structure). The cell volume of the (La, Mg)Ni₃ phase, (La, Mg)₂Ni₇ phase and LaNi₅ phase decreased monotonously with increasing Ce content. Electrochemical investigations showed a decrease in the discharge capacity, while high rate dischargeability (HRD) first increased and then decreased with increasing Ce content. The Ce substitution for La slightly enhanced the cyclic stability of the alloy electrodes. The pressure–composition (P–C) isotherms showed that the plateau region was broadened with Ce content increased in the alloys, meanwhile, two plateaus appeared and pressure of the hydrogen absorption and desorption increased accordingly.     

Effect of Al content on structure and electrochemical properties of LaNi4.4 − xCo0.3Mn0.3Alx hydrogen storage alloys

The structure and electrochemical properties of LaNi_4.4 − xCo_0.3Mn_0.3Al_x hydrogen storage alloys have been investigated by XRD and simulated battery test, including maximum capacity, cyclic stability, self-discharge, high-rate dischargeability (HRD). Samples A, B, C and D were used to represent alloys LaNi_4.4Co_0.3Mn_0.3Al, LaNi_4.3Co_0.3Mn_0.3Al_0.1, LaNi_4.2Co_0.3Mn_0.3Al_0.2 and LaNi_4.1Co_0.3Mn_0.3Al_0.3 respectively. The results indicated that as-prepared LaNi_4.4 − xCo_0.3Mn_0.3Al_x alloys are all single-phase alloys with hexagonal CaCu₅ type structure. The maximum discharge capacity is 330.4 mAh g⁻¹ (Alloy C). With the increase of Al content from A to D, cycle life of alloy electrode has been improved. Higher capacity retention of 89.29% (after 50 charge/discharge cycles) has been observed for electrode D, while with a smaller capacity loss of 12.5% in its self-discharge test. Better high-rate charge/discharge behaviors have been observed in electrode B, and the maximum data is 54.7% at charge current of 900 mA/g) and 68.54% at discharge current of 1800 mA/g). Furthermore, the electrochemical impedance spectroscopy (EIS) analysis shown that the reaction of alloy electrode is controlled by charge-transfer step. The addition of Al results in the formation of protective layer of aluminum oxides on the surface of the alloy electrode, which is good for the improvement of electrode properties in alkaline solution and is detrimental for the charge-transfer process. Therefore, a suitable addition of Al is needed to improve its electrode properties.     

Structure and hydrogen storage properties of Mg2Cu1−xNix (x = 0–1) alloys

The effect of Ni-substitution on the structure and hydrogen storage properties of Mg₂Cu_1−xNi_x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg₂Cu with face-centered orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg₂Cu and Mg₂Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 573 K under 50 bar H₂. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH₂, MgCu₂ and Mg₂NiH₄ while the discharged samples contained ternary alloys of Mg–Cu–Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg–Cu and Mg–Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH₂ and Mg₂NiH₄ destabilization, leading to a decrease of the desorption temperature of these two phases.     

Effect of Cr doping on the structural,electrochemical properties of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Crx]O2 (x = 0, 0.02, 0.04, 0.06, 0.08) as cathode materials for lithium secondary batteries

Prospective positive-electrode (cathode) materials for a lithium secondary battery, viz., Li[Li_0.2Ni_0.2−x/2Mn_0.6−x/2Cr_x]O₂ (x = 0, 0.02, 0.04, 0.06, 0.08), were synthesized using a solid-state pyrolysis method. The structural and electrochemical properties were examined by means of X-ray diffraction, cyclic voltammetry, SEM and charge–discharge tests. The results demonstrated that the powders maintain the α-NaFeO₂-type layered structure regardless of the chromium content in the range x ≤ 0.08. The Cr doping of x = 0.04 showed improved capacity and rate capability comparing to undoped Li[Li_0.2Ni_0.2Mn_0.6]O₂. ac impedance measurement showed that Cr-doped electrode has the lower impedance value during cycling. It is considered that the higher capacity and superior rate capability of Cr-doping samples would be ascribed to the reduced resistance of the electrode during cycling.     

Effect of Ni content on the hydrogen storage behavior of ZrCo1−xNix alloys

ZrCo_1−xNi_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared and their hydrogen storage behavior were studied. ZrCo_1−xNi_x alloys of compositions with x = 0, 0.1, 0.2 and 0.3 prepared by arc-melting method and characterized by X-ray diffraction analysis. XRD analysis showed that the alloys of composition with x = 0, 0.1, 0.2 and 0.3 forms cubic phase similar to ZrCo with traces of ZrCo₂ phase. A trace amount of an additional phase similar to ZrNi was found for the alloy with composition x = 0.3. Hydrogen desorption pressure–composition–temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus and the hydrogen desorption pressure–composition isotherms (PCIs) were generated for all the alloys in the temperature range of 523–603 K. A single sloping plateau was observed for each isotherm and the plateau pressure was found to increase with increasing Ni content in ZrCo_1−xNi_x alloys at the same experimental temperature. A van't Hoff plot was constructed using plateau pressure data of each pressure–composition isotherm and the thermodynamic parameters were calculated for desorption of hydrogen in the ZrCo_1−xNi_x–H₂ systems. The enthalpy and entropy change for desorption of hydrogen were calculated. In addition, the hydrogen absorption–desorption cyclic life studies were performed on ZrCo_1−xNi_x alloys at 583 K up to 50 cycles. It was observed that with increasing Ni content the durability against disproportionation of alloys increases.     

Cycle life and hydrogen storage properties of mechanical alloyed Ca1−xZrxNi5−yCry; (x = 0, 0.05 and y = 0, 0.1)

CaNi₅–based alloys have been synthesized by mechanical alloying followed by isothermal annealing. The formation of the CaNi₅ structure occurred when the milled powders were heated at 800 °C under vacuum for 3 h. The abundance of CaNi₅ phase in the alloys ranges from 60 to 70 wt.%. Replacement of Zr into the Ca site reduces the unit cell volume of CaNi₅ whilst replacement of Cr into the Ni site slightly increases the unit cell volume. The hydrogen storage capacity of all substituted alloys is decreased and the hydrogen sorption plateau regions are narrowed compared to those of pure CaNi₅. Substitution of Zr into the Ca site extinguishes the flat plateau region unlike replacement of Cr into the Ni site where a flat plateau is maintained. The reaction enthalpy ΔH for both absorption and desorption are directly proportional to the unit cell volume of the alloys. The hydrogen storage capacity of all alloys rapidly decays for the first 50 cycles at 85 °C followed by a more gradual decline after 50 further cycles. The hydrogen storage capacity of the alloys after 200 cycles is in the range of 65–75% of the initial capacity.     

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.     

A study on the hydrogen-storage properties of La2−xTixMgNi9 (x = 0.1, 0.2, 0.3, 0.4) alloys

The La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys were prepared by magnetic levitation melting under Ar atmosphere. The effects of partial substitution Ti for La on the phase structures, hydrogen-storage properties and electrochemical characteristics of the alloys were investigated systematically. For La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys, LaNi₅, LaNi₃ and LaMg₂Ni₉ are the main phases, the maximum hydrogen-storage capacity is 1.51, 1.36, 1.35 and 1.22 wt%, respectively. The absorption–desorption plateau pressure of the alloys first decreases and then increases with increase of Ti content, and the La_1.8MgTi_0.2Ni₉ alloy has the lowest absorption–desorption plateau pressure. The discharge voltage of the alloy electrodes rises with increasing the amount of Ti content. The La_1.8Ti_0.2MgNi₉ alloy electrode presents good electrochemical performance.     

Kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg₂NiH₄-x wt.% MmNi_3.8Co_0.75Mn_0.4Al_0.2 (x = 5, 10, 20, 40) composites prepared by mechanical milling have been investigated in this paper. XRD results indicate that the as-milled Mg₂NiH₄ shows nanocrystalline or amorphous-like structure, and it does not react with MmNi_3.8Co_0.75Mn_0.4Al_0.2 during mechanical milling. As the amount of MmNi_3.8Co_0.75Mn_0.4Al_0.2 increases, the maximum discharge capacity decreases initially from 508 mAh/g (x = 5) to 440 mAh/g (x = 10) and then increases to 509 mAh/g (x = 40). Meanwhile, the capacity retention (R₁₀) increases from 12.8% (x = 5) to 23.4% (x = 40), and the corrosion potential of electrode (E_corr) increases from −0.930 V to −0.884 V (vs. Hg/HgO). Especially, the more MmNi_3.8Co_0.75Mn_0.4Al_0.2 content the composite contains, the higher high rate dischargeability (HRD) the electrode exhibits, which could be attributed to the catalytic reaction and reduction of the Mg₂NiH₄ grain size brought by MmNi_3.8Co_0.75Mn_0.4Al_0.2. The improvement in electrode kinetics has been depicted from the bulk hydrogen diffusion coefficient (D), the exchange current density (I₀) and the charge transfer resistance (R_ct) on the alloy surface.