Synergy of inside doped metals–Outside coated graphene to enhance hydrogen storage in magnesium-based alloys

Grain growth of magnesium(Mg)and its hydride is one of the main reasons for kinetic and capacity degradation during the hydrogen absorption and desorption cycles.To solve this problem,herein we propose a novel method involving synergistic effect of inside embedded metals and outside coated graphene to limit the growth of Mg and its hydride grains.The graphene coated Mg-Y-Al alloys were selected as a model system for demonstrating this positive effect where the Mg_(91)Y_(3)Al_(6)alloy was first prepared by rapidly solidified method and then high-pressure milled with 5 wt%graphene upon 5 MPa hydrogen gas for obtaining in-situ formed YAl_(2)and YH_(3)embedded in the MgH_(2)matrix with graphene shell(denoted as MgH_(2)-Y-Al@GR).In comparison to pure MgH_(2),the obtained MgH_(2)-Y-Al@GR composites deliver much better kinetics and more stable cyclic performance.For instance,the MgH_(2)-Y-Al@GR can release about 6.1 wt%H_(2)within 30 min at 300℃ but pure MgH_(2)only desorbs∼1.5 wt%H_(2).The activation energy for desorption of MgH_(2)-Y-Al@GR samples is calculated to be 75.3±9.1 kJ/mol that is much lower than approximately 160 kJ/mol for pure MgH_(2).Moreover,its capacity retention is promoted from∼57%of pure MgH_(2)to∼84%after 50th cycles without obvious particle agglomeration and grain growth.The synergistic effect of outside graphene coating with inside embedded metals which could provide a huge number of active sites for catalysis as well as inhibit the grain growth of Mg and its hydride is believed to be responsible for these.

Modification of BCC phase and the enhanced reversible hydrogen storage properties of Ti-V-Fe-Mn alloys with varied V/Fe ratios

Ti-V-based alloys are proved of huge potential in storing hydrogen,but the incomplete reversible hydrogen storage capacity caused by overstability of V hydride has limited the large-scale application.In this study,Ti_(32)V_(40+x)Fe_(23-x)Mn_(5)(x=0,4,8,12,at.%)alloys were designed,and the effects of V/Fe ratio on phase constitution and hydrogen storage properties were investigated.The main phase of the alloys is body-centered cubic(BCC)phase,and the lattice constants of the BCC phase decrease with the decrease of V/Fe ratio.Moreover,C14 Laves phase exists in alloys with a Fe content of 19at.%to 23at.%.For hydrogenation,the C14 Laves phase can accelerate the hydrogen absorption rate,but the hydrogen absorption capacity is reduced.With the decrease of V/Fe ratio,the hydride gradually destabilizes.Owing to its large lattice constant and high hydrogen absorption phase content,the Ti_(32)V_(52)Fe_(11)Mn_(5)alloy shows the most enhanced hydrogen storage properties with hydrogenation and dehydrogenation capacities of 3.588wt.%at 298 K and 1.688wt.%at 343 K,respectively.The hydrogen absorption capacity of this alloy can be reserved to 3.574wt.%after 20 cycles of hydrogen absorption and desorption.

Electrochemical hydrogen storage characteristics of nanocrystalline and amorphous Mg_2Ni-type alloys prepared by melt-spinning

The nanocrystalline and amorphous Mg2Ni-type alloys with nominal compositions of Mg2Ni1-xMnx （x=0, 0.1, 0.2, 0.3, 0.4） were synthesized by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are obtained. The structures of the as-spun alloy ribbons were characterized by XRD and HRTEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were measured by an automatic galvanostatic system. The electrochemical impedance spectrums （EIS） were plotted by an electrochemical workstation. The hydrogen diffusion coefficients （D） in the alloys were calculated by virtue of potential-step measurement. The results show that all the as-spun （x=0） alloys hold 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 substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn substitution from 0 to 0.4, the discharge capacity of the as-spun （20 m/s） alloy increases from 96.5 to 265.3 mA·h/g, and its capacity retaining rate （S20） at the 20th cycle increases from 31.3% to 70.2%. Furthermore, the high rate dischargeability （HRD）, electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with raising the amount of Mn substitution.

Phase structure and electrochemical properties of La_(0.7)Ce_(0.3)Ni_(3.75)Mn_(0.35)Al_(0.15)Cu_(0.75-x)Fe_x hydrogen storage alloys

La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-xFex （x=0-0.20） hydrogen storage alloys were synthesized by induction melting and subsequent annealing treatment, and phase structure and electrochemical characteristics were investigated. All alloys consist of a single LaNi5 phase with CaCu5 structure, and the lattice constant a and the cell volume （V） of the LaNi5 phase increase with increasing x value. The maximum discharge capacity gradually decreases from 319.0 mA？h/g （x=0） to 291.9 mA？h/g （x=0.20） with the increase in x value. The high-rate dischargeability at the discharge current density of 1200 mA/g decreases monotonically from 53.1% （x=0） to 44.2% （x=0.20）. The cycling stability increases with increasing x from 0 to 0.20, which is mainly ascribed to the improvement of the pulverization resistance.

Electrochemical hydrogen storage characteristics of as-cast and annealed La_(0.8-x)Nd_xMg_(0.2)Ni_(3.15)Co_(0.2)Al_(0.1)Si_(0.05)(x=0-0.4)alloys

The La-Mg-Ni-based A2B7-type La0.8-xNdxMg0.2Ni3.15Co0.2Al0.15 （x=0, 0.1, 0.2, 0.3, 0.4） electrode alloys were prepared by casting and annealing. The influences of partial substitution of Nd for La on the structure and electrochemical performance of the as-cast and annealed alloys were investigated. It was found that the experimental alloys consist of two major phases, （La, Mg）2Ni7 phase with the hexagonal Ce2Ni7-type structure and LaNi5 phase with the hexagonal CaCu5-type structure, as well as some residual phase LaNi3 and NdNi5. The discharge capacity and high rate discharge ability （HRD） of the as-cast and annealed alloys first increase and then decrease with Nd content growing. The as-cast and annealed alloys （x=0.3） yield the largest discharge capacities of 380.3 and 384.3 mA·h/g, respectively. The electrochemical cycle stability of the as-cast and annealed alloys markedly grows with Nd content rising. As the Nd content increase from 0 to 0.4. The capacity retaining rate （S100） at the 100th charging and discharging cycle increases from 64.98% to 85.17% for the as-cast alloy, and from 76.60% to 96.84% for the as-annealed alloy.

Phase structure and electrochemical properties of La_(1.7+x)Mg_(1.3-x)(NiCoMn)_(9.3)(x=0-0.4) hydrogen storage alloys

The phase structure and electrochemical properties of La1.7＋xMg1.3-x（NiCoMn）9.3（x=0-0.4） alloys were investigated. The XRD analysis reveals that the alloys consist of LaNi5 phase and other phases, such as LaMg2Ni9 phase （PuNi3 structure） and La4MgNi19 phases （Ce5Co19＋Pr5Co19 structure, namely A5B19 type）. With the increase of the x value, the LaMg2Ni9 phase fades away and La4MgNi19 phases appear, while the abundance of LaNi5 phase firstly increases and then decreases. At the same time, the cell volume of LaNi5 phase and LaMg2Ni9 phase decreases. The electrochemical measurement shows that alloy electrodes could be activated in 4-5 cycles, and with the increase of the x value, the maximum discharge capacity gradually increases from 330.9 mA-h/g （x=0） to 366.8 mA-h/g （x=0.4）, but the high-rate dischargeability （HRD） and cyclic stability （S） decrease somewhat （x=0.4, HRD600=82.32%, S100=73.8%）. It is found that the HRD is mainly controlled by the electrocatalytic activity on the alloy electrode surface, and the decline of cyclic stability is due to the appearance of A5B19 type phase with larger hydrogen storage capacity, which leads to larger volume expansion and more intercrystalline stress and then easier pulverization during charging/discharging.

Preparation of LaMgNi_(4-x)Co_x alloys and hydrogen storage properties

The LaMgNi4xCox （x=0, 0.3, 0.5） compounds were prepared by the method of levitation melting and a subsequent heat treatment at 1073 K for 10 h. XRD analysis shows that the obtained LaMgNia-xCox alloys consist of a single phase with the structure of cubic SnMgCu4 （AuBe5 type）. The hydrogen absorption/desorption properties of LaMgNi4 were investigated by PCI measurement at various temperatures （T=373, 398, 423 K） and the results show that the maximum absorbed hydrogen capacity reaches 1.45% （5.79H/M） under a hydrogen pressure of 4.3 MPa at 373 K. The XRD patterns during absorbing procedure at 373 K indicate the phase structure changing from cubic （a-LaMgNi4） to orthorhombic （fl-LaMgNiaH3.41） and after hydrogenation finally back to cubic （y-LaMgNiaH4.87）, and a partial desorption was also observed under this condition. With increasing temperature, a slight decrease of the absorbed hydrogen content was observed and the number of plateaus reduces from two to one, but the hydrogen absorption kinetics improves. The electrochemical properties of the LaMgNiaxCox were measured by simulated battery test, which shows that the discharge capacity of the alloys significantly improves with the increase of Co content.

Key technology and application of AB_(2) hydrogen storage alloy in fuel cell hydrogen supply system

At present,there is limited research on the application of fuel cell power generation system technology using solid hydrogen storage materials,especially in hydrogen-assisted two-wheelers.Considering the disadvantages of low hydrogen storage capacity and poor kinetics of hydrogen storage materials,our primary focus is to achieve smooth hydrogen ab-/desorption over a wide temperature range to meet the requirements of fuel cells and their integrated power generation systems.In this paper,the Ti_(0.9)Zr_(0.1)Mn_(1.45)V_(0.4)Fe_(0.15) hydrogen storage alloy was successfully prepared by arc melting.The maximum hydrogen storage capacity reaches 1.89 wt% at 318 K.The alloy has the capability to absorb 90% of hydrogen storage capacity within 50 s at 7 MPa and release 90% of hydrogen within 220 s.Comsol Multiphysics 6.0 software was used to simulate the hydrogen ab-/desorption processes of the tank.The flow rate of cooling water during hydrogen absorption varied in a gradient of(0.02 t x)m s^(-1)(x=0,0.02,0.04,0.06,0.08,0.1,0.12).Cooling water flow rate is positively correlated with the hydrogen absorption rate but negatively correlated with the cost.When the cooling rate is 0.06 m s^(-1),both simulation and experimentation have shown that the hydrogen storage tank is capable of steady hydrogen desorption for over 6 h at a flow rate of 2 L min^(-1).Based on the above conclusions,we have successfully developed a hydrogen-assisted two-wheeler with a range of 80 km and achieved regional demonstration operations in Changzhou and Shaoguan.This paper highlights the achievements of our team in the technological development of fuel cell power generation systems using solid hydrogen storage materials as hydrogen storage carriers and their application in twowheelers in recent years.

A review of classical hydrogen isotopes storage materials

Hydrogen storage alloys(HSAs)are attracting widespread interest in the nuclear industry because of the generation of stable metal hydrides after tritium absorption,thus effectively preventing the leakage of radioactive tritium.Commonly used HSAs in the hydrogen isotopes field are Zr2M(M=Co,Ni,Fe)alloys,metallic Pd,depleted U,and ZrCo alloy.Specifically,Zr2M(M=Co,Ni,Fe)alloys are considered promising tritium-getter materials,and metallic Pd is utilized to separate and purify hydrogen isotopes.Furthermore,depleted U and ZrCo alloy are well suited for storing and delivering hydrogen isotopes.Notably,all the aforementioned HSAs need to modulate their hydrogen storage properties for complex operating conditions.In this review,we present a comprehensive overview of the reported modification methods applied to the above alloys.Alloying is an effective amelioration method that mainly modulates the properties of HSAs by altering their local geometrical/electronic structures.Besides,microstructural modifications such as nano-sizing and nanopores have been used to increase the specific surface area and active sites of metallic Pd and ZrCo alloys for enhancing de-/hydrogenation kinetics.The combination of metallic Pd with support materials can significantly reduce the cost and enhance the pulverization resistance.Moreover,the poisoning resistance of ZrCo alloy is improved by constructing active surfaces with selective permeability.Overall,the review is constructive for better understanding the properties and mechanisms of hydrogen isotope storage alloys and provides effective guidance for future modification research.

Comparative study on the hydrogen storage performance of as-milled MgRENi rapid quenched alloy catalyzed by metal sulfides

The composites of Mg_(20)Pr_(1)Sm_(3)Y_(1)Ni_(10)as-quenched alloy and 3 wt.%M(M=CoS,CoS_(2),MoS_(2))catalyst were prepared by high-speed vibration ball mill.The effects of metal sulfides on the hydrogenation and dehydrogenation dynamics of alloys were compared.The results show that the as-milled composites contain a large number of amorphous embedded by a small amount of nanocrystals,and there are many point defects.After ball milling,the crystal grain size in the composites containing CoS is relatively larger,followed by CoS_(2)and MoS_(2)again.After hydrogenation,the amorphous phase is crystallized to form Mg_(2)NiH_(4),YH_(3),Pr_(8)H_(18.96),Sm_(3)H_7,Mg,Co or Mo phases,however,Mg_(2)Ni,YH_(2),PrH_(2)and Ni_(3)Y phases appeared after dehydrogenation.The maximum hydrogenation capacity of the composites containing CoS,CoS_(2)and MoS_(2)are 3.939,4.265 and 4.507 wt.%,respectively.The hydrogenation saturation ratio of composite containing MoS_(2)is higher than that of the composites containing CoS and CoS_(2).The dehydrogenation activation energy of the composites containing CoS,CoS_(2)and MoS_(2)is 107.76,68.43 and 63.28 kJ.mol^(-1).H_(2).On the improvement of hydrogen storage performance of Mg_(20)Pr_(1)Sm_(3)Y_(1)Ni_(10)alloy,the catalytic effect of MoS_(2)sulfide is better than that of CoS_(2)sulfide,and which is better than CoS sulfide.

Research Progress of Solid Hydrogen Storage Materials for Hydrogen Energy Storage and Transportation

With the rapid development of hydrogen energy, hydrogen storage alloys have attracted wide attention owing to their key advantages, such as high volume density, proper plateau pressure, environmental friendliness and good safety. In the present review, the research progress of the improvement in hydrogen storage alloys, including rare-earth-based alloys, Mg-based alloys, Ti/Zr-based alloys, V-based alloys and high entropy alloys are systematically summarized. The influences of elemental substitution, catalyst doping, preparation methods and nanotechnology on the crystal structure, hydrogen storage properties as well as their affecting mechanisms, are discussed. Furthermore, the development trend and future research directions are proposed, which are expected to bring novel research ideas and potentially applicable methods for the development of high-performance hydrogen storage alloys.

Properties of Ti-Based Hydrogen Storage Alloy

An efficient and safe hydrogen storage method is one of the important links for the large-scale development of hydrogen in the future. Because of its low price and simple design, Ti-based hydrogen storage alloys are considered to be suitable for practical applications. In this paper, we review the latest research on Ti-based hydrogen storage alloys. Firstly, the machine learning and density functional theory are introduced to provide theoretical guidance for the optimization of Ti-based hydrogen storage alloys. Then, in order to improve the hydrogen storage performance, we briefly introduce the research of AB type and AB2 type Ti-based alloys, focusing on doping elements and adaptive after treatment. Finally, suggestions for the future research and development of Ti-based hydrogen storage alloys are proposed. .

Effects of annealing treatment on the microstructure and electrochemical properties of low-Co hydrogen storage alloys containing Cu and Fe

The effects of annealing treatment on the microstructure and electrochemical properties of low-Co LaNi 3.55 Mn 0.35 Co 0.20 Al 0.20 Cu 0.75 Fe 0.10 hydrogen storage alloys were investigated. X-ray diffraction （XRD） analysis indicated that annealing treatment remarkably reduced the lattice strain and defects, and increased the unit-cell volume. The optical microscope analysis showed that the as-cast alloy had a crass dendrite microstructure with noticeable composition segregation, which gradually disappeared with increasing annealing temperature, and the micro-structure changed to an equiaxed structure after annealing the alloy at 1233 K. The electrochemical tests indicated that the annealed alloys demonstrated much better cycling stability compared with the as-cast one. The capacity retention at the 100th cycle increased from 90.0% （as-cast） to 94.7% （1273 K）. The annealing treatment also improved the discharge capacity. However, the high rate dischargeability （HRD） value of the annealed alloy slightly dropped, which was believed to be ascribed to the decreased exchange current density and the hydrogen diffusion coefficient in alloy bulk.

Effect of stoichiometry and Cu-substitution on the phase structure and hydrogen storage properties of Ml-Mg-Ni-based alloys

To improve the electrochemical properties of rare-earth-Mg-Ni-based hydrogen storage alloys, the effects of stoichiometry and Cu-substitution on the phase structure and thermodynamic properties of the alloys were studied. Nonsubstituted Ml0.80Mg0.20（Ni2.90Co0.50-Mn0.30Al0.30）x （x=0.68, 0.70, 0.72, 0.74, 0.76） alloys and Cu-substituted Ml0.80Mg0.20（Ni2.90Co0.50-yCuyMn0.30Al0.30）0.70 （y=0, 0.10, 0.30, 0.50） alloys were prepared by induction melting. Phase structure analysis shows that the nonsubstituted alloys consist of a LaNi5 phase, a LaNi3 phase, and a minor La2Ni7 phase;in addition, in the case of Cu-substitution, the Nd2Ni7 phase appears and the LaNi3 phase vanishes. Ther-modynamic tests show that the enthalpy change in the dehydriding process decreases, indicating that hydride stability decreases with in-creasing stoichiometry and increasing Cu content. The maximum discharge capacity, kinetic properties, and cycling stability of the alloy electrodes all increase and then decrease with increasing stoichiometry or increasing Cu content. Furthermore, Cu substitution for Co ame-liorates the discharge capacity, kinetics, and cycling stability of the alloy electrodes.

Study on High Rate Discharge Performance and Mechanism of AB_5 Type Hydrogen Storage Alloys

The effects of surface treatment, particle size distribution,rare earth composition and B additive on the high rate discharge performance of hydrogen storage alloys were investigated. It is found that the activity, discharge capacity and high rate dischargeability of the alloys are improved after physical and chemical modification as a result of the increase of the surface area and formation of the electrocatalysis layers, which increase both the electrochemical reaction rate on the alloy surface and H diffusion rate in the alloy bulk. It is also found that both the over-coarse and over-fine particle size increase the contact resistance of the electrode, resulting in a decrease of discharge capacity, deterioration of high rate dischargeability and lower discharge plateau. In another word, a suitable particle size distribution can enhance the alloy activity, discharge capacity and high rate dischargeability. In addition, the high rate dischargeability is enhanced by increasing La content and decreasing Ce content of the alloy composition because of enlargement of the unit cell volume and the improvement of the surface activity. Moreover, B additive resultes in the formation of the second phase, and makes the alloys easier pulverization, which greatly improves the activity, discharge capacity and high rate dischargeability.

Effect of praseodymium substitution for lanthanum on structure and properties of La_(0.65-x)Pr_xNd_(0.12)Mg_(0.23)Ni_(3.4)Al_(0.1)(x=0.00–0.20) hydrogen storage alloys

In order to investigate the effect of substituting La with Pr on structural and hydrogen storage properties of La-Mg-Ni system （AB3.5-type） hydrogen storage alloys, a series of La0.65-xPrxNd0.12Mg0.23Ni3.4Al0.1（x=0, 0.10, 0.15, 0.2） hydrogen storage alloys were prepared. X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersive spectrometer （EDS） analyses revealed that two alloys （x=0.0 and 0.10） were composed of （La,Mg）2（Ni,Al）7 phase, La（Ni,A1）5 phase and （La,Mg）Ni2 phase, while other alloys （x=0.15 and 0.20） consisted of （La,Mg）2（Ni,A1）7 phase, La（Ni,A1）5 phase, （La,Mg）Ni2 phase and （La,Mg）（Ni,A1）3 phase. All alloys showed, however, only one pressure plateau in P-C isotherms. The Pr/La ratio in alloy composition influenced hydrogen storage capacity and kinetics properties. Electrochemical studies showed that the discharge capacity decreased from 360 mAh/g （x=-0.00） to 335 mAh/g （x=-0.20） as x increased. But the high-rate dischargeability （HRD） of alloy electrodes increased from 26% （x=0.00） to 56% （x=-0.20） at a discharge current density of Id=1800 mA/g. Anode polarization measurements were done to further understand the electrochemical kinetics properties after Pr substitution.

Effect of Ti substitution on hydrogen storage properties of Zr_(1-x)Ti_xCo (x = 0, 0.1, 0.2, 0.3) alloys

Zr1-xTixCo(x = 0, 0.1, 0.2, 0.3) alloys were prepared by arc-melting method and the effect of Ti substitution on hydrogen storage properties was studied systematically. Hydrogen desorption pressure-composition-temperature(PCT) measurements were carried out using Sievert’s type volumetric apparatus for ZrCo(at 473 K, 573 K and 673 K) and Zr1-xTixCo alloys(at 673 K), respectively. Products after dehydrogenation were characterized by X-ray diffraction(XRD). In addition, the kinetics of Zr1-xTixCo hydride was investigated at 473 K and 673 K,respectively, under hydrogen pressure of 5 MPa. Results showed that Ti substitution for Zr did not change the crystal structure of ZrCo phase.With the increase of temperature from 473 K to 673 K, the extent of disproportionation for ZrCo alloy increased. With Ti content increasing at 673 K, the desorption equilibrium pressure of Zr1-xTixCo-H2 systems elevated and the disproportionation reaction of Zr1-xTixCo alloys was inhibited effectively. Ti substitution decreased the kinetics rate and the effective hydrogen storage capacity of Zr1-xTixCo alloys slightly.Generally speaking, it was found that Zr0.8Ti0.2Co alloy had better anti-disproportionation property with less decrease of effective hydrogen storage capacity which was beneficial to tritium application in the International Thermonuclear Experimental Reactor(ITER).

Research of heat treatment of low-Co AB_5 type hydrogen storage alloys for MH-Ni batteries

The effects of low-Co AB_5 type hydrogen storage alloys prepared by quenchingand annealing on the performances of MH-Ni batteries were investigated, and the characteristics ofthe low-Co AB_5 type hydrogen storage alloys were compared with those of the high-Co AB_5 typehydrogen storage alloy as well. The results showed that the faster the cooling of the low-Cohydrogen storage alloy is, the better homogeneity of the chemical composition for the alloy and thelonger cycle life of the battery are, but the electrochemical discharge capacity and high-ratedischarge ability are reduced. The high-rate discharge ability and charge retention of MH-Nibatteries for the conventional as-cast annealed low-Co hydrogen storage alloy were superior to thosefor the rapidly quenched low-Co hydrogen storage alloy and the high-Co hydrogen storage alloy, buta little inferior in the cycle life.

Effect of Heat-Treatment Process on Properties of Rare Earth Mg-Based System Hydrogen Storage Alloys with AB_3-Type

The effect of heat-treatment process on the properties of Mm0.8Mg0.2(NiCoAlMn)3.5 hydrogen storage alloy was discussed . The electrochemical properties such as cycling stability, activation property, and the plateau voltage of the alloy which was heat-treated in various temperatures and times had different changes during the cycle process, the optimum heat-treatment conditions of this alloy were determined by this work.

Laves phase hydrogen storage alloys for super-high-pressure metal hydride hydrogen compressors

Ti-Cr- and Ti-Mn-based alloys were prepared to be low- and high-pressure stage metals for a double-stage super-high-pressure metal hydride hydrogen compressor. Their crystallographic characteristics and hydrogen storage properties were investigated. The alloy pair Ti0.9Zr0.1Mn1.4- Cr0.35V0.2Fe0.05/TiCr1.55Mn0.2Fe0.2 was optimized based on the comprehensive performance of the studied alloys. The product hydrogen with a pressure of 100 MPa could be produced from 4 MPa feed gas when hot oil was used as a heat reservoir.