Hydrogen Energy Materials

Hydrogen energy，as a clean and renewable energy source，presents a promising pathway to accelerate the achievement of "Carbon neutrality" and "Carbon peak" targets. The application of hydrogen energy from generation to final use included four links，namely the preparation，storage，transportation，and application of hydrogen. Storage and transportation contributed to 30%-50% of the overall cost. Therefore，the development of safe and efficient hydrogen storage and transportation technologies was critical to unlocking the full potential of hydrogen energy applications. Among the various hydrogen storage materials，MgH₂ was one of the hydrogen storage materials with the most development potential，due to its abundant resources，low price，high hydrogen storage capacity （theoretical hydrogen storage capacity was 7.6%），and good cycling performance. However，the hydrogen release temperature was high，and the hydrogen release kinetic performance was poor，which limited its large-scale application. Catalyst doping was a simple and effective method to improve the hydrogen storage performance of Mg/MgH₂ system，which could provide active sites for H adsorption，dissociation，and diffusion of H₂ molecules in the process of hydrogen absorption and release of Mg/MgH₂，so as to improve its kinetic performance. In addition to catalyst doping，activation also played a key role in improving the hydrogen absorption and desorption reaction rates and cycling stability. Microwave was an emerging activation method in which not only was the energy absorbed by the dipole molecules and conductive electrons in a material and converted into heat，but also excited the surface electrons of the catalyst into a higher energy state which could change the path of the surface chemical reaction，such as promoting H dissociation. The microwave field could also reorient polar molecules，increasing the adsorption of reactants on the catalyst surface，thereby increasing the reaction rate. Therefore，the doping of catalysts with strong microwave absorption properties for modification could not only provide an active site for hydrogen absorption and release，but also use the various effects induced by its coupling with microwave to activate magnesium-based hydrogen storage materials and strengthen the dehydrogenation reaction process. In this study，graphene （G）-supported Fe₃O₄ nanoparticle （Fe₃O₄@G）was prepared and doped into magnesium powder via mechanical ball milling，followed by activation using microwave irradiation. The doping of Fe₃O₄/graphene composite in magnesium nanomaterials not only ensured effective dispersion of the catalyst and MgH₂ on graphene but also facilitated activation through the strong interaction between Fe₃O₄/graphene and microwaves. Furthermore，graphene could construct efficient thermal transport channels for the rapid dehydrogenation process due to its high thermal conductivity. The effects of different catalyst dosages，Fe₃O₄/graphene ratios，and microwave activation time on the hydrogen adsorption and dehydrogenation of Mg/MgH₂ were systematically studied. When studying the effect of microwave activation on the hydrogen absorption performance of the Mg-Fe₃O₄@G composite material，the activation time was investigated，and it was found that the hydrogen absorption kinetic performance and the maximum hydrogen storage capacity were remarkably improved by about 15% by microwave activation for 150 s. Compared with Fe₃O₄ and graphene single doping，Fe₃O₄@G composite doping could significantly improve the hydrogen storage performance of Mg. The best hydrogen storage performance was achieved when the doping amount of Fe₃O₄@G was 10 wt%，especially when the mass ratio of Fe₃O₄ to graphene was 6∶4. Mg-6Fe₃O₄@4G showed a high hydrogen storage capacity of 6.18 wt%. Johnson-Mehl-Avrami-Kohnogorov （JMAK）model was used to calculate the activation energy of hydrogen absorption and dehydrogenation to 56.7 and 79.89 kJ·mol⁻¹，respectively，which were 30%-50% lower than commercial Mg/MgH₂. The hydrogenation and dehydrogenation cycling performance of Mg-6Fe₃O₄@4G at 300 ℃ was investigated，and it was found that the capacity was maintained well after 10 cycles，with the capacity retention rate of 96%，demonstrating that the composite had good cyclic performance. The dehydrogenation process of MgH₂-6Fe₃O₄@4G could be greatly enhanced by microwave heating，in which the dehydrogenation rate could amount to 91% after 30 s and 95% at 35 s under 1500 W microwave irradiation. The results of four conventional hydrogenation followed by microwave dehydrogenation cycles showed that the average hydrogen storage capacity retention rate of Mg-6Fe₃O₄@4G composite was 97.5%，and the average dehydrogenation rate was 95.5%. The Mg-6Fe₃O₄@4G composite had good cyclic stability，and the dehydrogenation efficiency could be significantly enhanced by microwave radiation. In order to study the principle of improving the dehydrogenation performance by microwave heating，SEM analysis was conducted on the prepared composite material before and after microwave irradiation. It was found that the composite material was broken into smaller particles after microwave heating，and the surface of the particles became rougher，showing more pores and cracks，which not only reduced the particle size of the material，but also formed cracks. The formation of a new surface accelerated the adsorption and diffusion of hydrogen，thereby improving the kinetic properties of the material. This study provided a systematic reference to improving the hydrogen absorption and desorption kinetics of Mg/MgH₂ and developing a rapid dehydrogenation method which was featured by the enhancement of microwave heating.

The dangers posed by global warming and climate change have spurred scientists to intensify efforts in developing economically efficient methods to reduce atmospheric carbon dioxide （CO₂） accumulation. One particularly promising approach is catalytic conversion of CO₂ to methanol （CH₃OH） using renewable energy sources. This method not only mitigates CO₂ emissions but also plays a pivotal role in hydrogen storage and release processes，potentially driving advancements in the chemical and energy sectors. Such technology could promote more sustainable resource utilization，aligning with the dual objectives of environmental protection and sustainable development. Palladium （Pd）-based heterogeneous catalysts have emerged as prominent subjects of study in the realm of hydrogenation catalysis due to their exceptional hydrogen adsorption capabilities and impressive resistance to deactivation. In the domain of CO₂ hydrogenation to methanol，monometallic Pd catalysts are critical focal points of research due to their significant value and potential impact. These catalysts typically consist of active monometallic Pd supported on specific carriers，such as CeO₂，SiO₂，ZnO，and Ga₂O₃. These oxide carriers serve as support materials，providing a foundation for the dispersion of active metal Pd and fostering strong metal-oxide carrier interactions （SMSI）. This interaction significantly influencs the dispersion of active metal Pd and the formation of metal-carrier interface sites，both of which play crucial roles in the rate and selectivity of catalytic reactions. CeO₂，functioning as a catalyst carrier，prossesses remarkable CO₂ adsorption capacity and oxygen mobility. For instance，rod-like structures of CeO₂ （CeO₂-R） exhibit higher oxygen vacancy density and quantity，as well as enhanced oxygen mobility in CO₂ hydrogenation to methanol. By introducing an appropriate amount of Pd，CeO₂-R could enhance CO₂ reactivity and increase methanol yield. Using high-surface-area mesoporous SiO₂ support materials like MCM-41 and SBA-15 is beneficial in terms of improving the catalytic performance of CO₂ hydrogenation to methanol by facilitating the better dispersion of Pd nanoparticles. Additionally，the addition of alkaline promoters like Ca significantly increases the activity and selectivity of Pd catalysts. ZnO，serving as a carrier，could promote Pd particle dispersion and stability，consequently enhancing catalytic activity. The addition of MgO as a promoter could bolster reducibility of Pd catalysts，leading to improve activity and methanol selectivity in CO₂ hydrogenation. Similarly，Ga₂O₃ as a carrier also plays a pivotal function in CO₂ hydrogenation reactions. Through in-situ fourier-transform infrared spectroscopy （FT-IR） studies，it was revealed that Pd/β-Ga₂O₃ catalysts effectively convert CO₂ into for mate intermediates，which were subsequently hydrogenated to methanol，demonstrating outstanding selectivity. Despite advancements with monometallic Pd catalysts for CO₂ hydrogenation to methanol，they still face significant challenges related to activity，product selectivity，and catalyst stability. Therefore，researchers actively explored bimetallic catalysts as a strategy to enhance catalytic performance. By introducing metal oxides such as ZnO，Ga₂O₃，and In₂O₃ to form PdZn，PdGa，and PdIn alloys or metal intermetallic compounds，catalyst performance could be enhanced. These alloying processes could alter the chemical properties of the metal surface，provide new active sites，increase the metal surface area，and induce defect effects，thereby promoting CO₂ adsorption and activation，as well as methanol synthesis. In the case of PdZn alloy，ZnO formed PdZn alloys with Pd，increasing metal surface area and catalytic activity. Pd particles of varying sizes displayed distinct SMSI effects on ZnO carriers. Larger Pd particles showcased a stronger SMSI with ZnO carrier，enhancing both catalyst activity and methanol selectivity. Doping ZnO with Pd to create PdZn alloy catalysts significantly improved CO₂ hydrogenation to methanol efficiency，particularly on TiO₂ and Al₂O₃ carriers where PdZn catalysts performed exceptionally well. The addition of Ga₂O₃ promoted the formation of PdGa alloys，aiding in enhanced methanol selectivity. Pd-modified carriers like carbon nanotubes （CNT），promoted strong interactions between Pd and Ga₂O₃，resulting in the formation of PdGa alloys，thus enhancing methanol synthesis activity. Ga-doped SiO₂ carriers effectively suppressed the reverse water-gas shift （RWGS） reaction while improving CO₂ hydrogenation selectivity. PdIn alloys and intermetallic compounds also exhibited excellent catalytic performance. By introducing In₂O₃ to Pd catalysts to form PdIn alloys，catalysts' adsorption，CO₂ activation，and C-O bond cleavage capabilities could be enhanced. Furthermore，other bimetallic alloys like Pd-Cu alloys showed promising catalytic performance on various carriers，demonstrating high methanol selectivity and yield. The formation of alloy catalysts promoted H₂ dissociation and methanol production，enhancing CO₂ hydrogenation efficiency. By controlling oxide carriers，optimizing active sites，and improving metal-carrier interactions to form alloys or intermetallic compounds，catalytic performance in CO₂ hydrogenation to methanol reactions could be effectively enhanced. The development of monometallic Pd catalysts supported on specific carriers，such as CeO₂，SiO₂，ZnO，and Ga₂O₃，harnessed the unique properties of these support materials，including CO₂ adsorption and oxygen migration capabilities，to enhance the catalytic process. Despite some progress in monometallic Pd catalysts，significant challenges still remained，including activity，selectivity，and stability. To address these challenges，scientists investigated bimetallic catalysts，where Pd was combined with metal oxides to form alloys or intermetallic compounds. These bimetallic catalysts，through alterations in the chemical properties of the metal surface，the provision of new active sites，and the increase in metal surface area，exhibited enhanced performance. Research on Pd catalysts for CO₂ hydrogenation to methanol held immense potential for advancing the goals of mitigating climate change and developing sustainable energy sources. The insights gained from these studies were crucial for the ongoing optimization of catalyst design and its industrial application.

The white paper "China's Energy Development in the New Era" directly pointed out that energy saving，low-carbon，and a diversified new type of clean enterprise system is a major development orientation for the future，promising to achieve China's carbon neutrality goal by 2060. Within this strategic framework，clean power becomes more and more important，with hydrogen being a noteworthy efficient and clean energy carrier at home and abroad. Among clean energy technologies，the production of hydrogen from water electrolysis is attractive because it is sustainable and can be used on a massive scale. The key is electrode preparation technology，determining the high rate and low cost of the whole hydrogen production process. By researching the hydrogen adsorption free energy （ΔG_H*）of electrode materials，researchers have concluded that precious metal-based electrodes （Pt，Rh，Re，Pd and Ir）are excellent for hydrogen evolution efficiency，with Pt being the best material. However，due to limited reserves and high costs，their wide application is restricted. As a result，non-precious metal catalysts have attracted attention in recent years. Ni is also considered as a non-precious metal material，due to its better reserves，relatively low price and high performance for alkaline water hydrogen production. Currently，common methods for preparing hydrogen evolution reaction （HER）electrodes include liquid-phase synthesis，chemical vapor deposition （CVD），electrodeposition，plasma spraying，magnetron sputtering，high-energy ball milling，and powder sintering，etc. 1）Liquid-phase synthesis technology：This includes solvothermal/hydrothermal methods，redox reaction methods，sol-gel processes，hydrolysis reactions，and precipitation （including direct precipitation，co-precipitation，and homogeneity precipitation）. Disadvantages are also obvious：the operation is complicated，requires higher control of reaction conditions，and may take too long. 2）CVD：The chemical composition in CVD process is complex，often involving a combination of gas-phase and surface reactions. Volatile precursors are delivered to the reaction zone using carrier gas and/or diffusion，and precursors can react with each other or decompose in the gas phase （homogeneous reactions），or adsorb upon a substrate surface and react with reactive gases （heterogeneous reactions）. Both reactions produce solid films and gas-state by-products on the substrate. This chemical process differentiates CVD from other physical deposition processes like evaporation，sputtering，and molecular beam epitaxy，with advantages like simple equipment operation and maintenance，and control over coating density. However，it has disadvantages like high reaction temperature，complex preparation process，frequent cleaning of the deposition chamber，and potential environmental pollution from exhaust gas requiring proper disposal. 3）Electrochemical deposition：Electrodeposition is a straightforward and low-cost catalyst layer preparation method，which features strong catalytic activity，simple operation，good controllability，and fast deposition rate for obtaining non-equilibrium structures. However，it requires large electrical energy consumption，and the high internal stress in the coating layer often leads to poor performance of the finished product. 4）Plasma spraying：Depending on the environment，it includes vacuum plasma spraying （VPS），low-pressure plasma spraying （LPPS），and atmospheric plasma spray （APS）. It offers good bonding strength and high efficiency and quality，but has low powder usage efficiency and expensive equipment. Under the action of an electric field，water molecules in the solution decompose to form hydrogen ions （H⁺）and hydroxyl ions （OH⁻）through a redox reaction between the anode and cathode. Hydrogen is produced at the cathode and oxygen at the anode. Hydrogen ions receive electrons to reduce into hydrogen at the cathode，while hydroxyl ions release electrons to oxidize into oxygen and water at the anode. Electrochemical hydrogen evolution occurs in three steps：Volmer step，Heyrovsky step and Tafel step. HER kinetics includes two steps：the hydrolysis of hydroxyl ions （OH⁻）and adsorption of hydrogen atoms is called Volmer step，consisting of electron reduction on catalyst surface M for example adsorption H⁺，producing adsorbed hydrogen atoms （H^*）where basic electrolytes contain H₂O as proton source，while acidic electrolytes contain H₃O⁺. The next step is the production of H₂ by Heyrovsky manner，Tafel step，or a combination of both. The formation and conversion of adsorbed hydrogen is key to HER，involving both chemical adsorption and desorption due to hydrogen adsorption free energy. This free energy describes HER reaction rate performance，with a perfect HER catalyst having moderate hydrogen adsorption free energy to ensure efficient and smooth step processes. If the free energy is higher than zero，the reaction rate is lower，a negative value helps binding and facilitates Volmer step. Too strong negative forces inhibit further reaction （Heyrovsky or Tafel steps）. Tafel slope，associated with the rate-controlling step，relates to electrode reaction rate and overpotential. In basic HER，Volmer step is typically the slowest link，influencing the total reaction rate. Controlling reaction conditions or using appropriate catalysts to lower hydrogen adsorption free energy near 0 accelerates Volmer step，improving HER rate. While non-precious metals like cobalt and mammothine have advantages in hydrogen adsorption free energy compared to nickel，storage capacity and price restrict large-scale application. Ni-based electrodes have progressed in HER research and widely studied. Future trends include combining methods to construct heterojunction interfaces or synthesize composite catalysts. Machine learning technology applied to catalytic materials design and discovery has accelerated this field，significantly contributing to the development and optimization of non-PGM （Platinum-group metals）catalysts.

Molybdenum metal powder with suitable size，morphology，purity，and dispersityis of immense importance for alloy preparation by powder metallurgy. In the work，the temperature-programmed reduction （TPR）reaction between hexagonal-shaped h-MoO₃ and high-purity H₂ was investigated in the range of room temperature to 1273 K，and various technologies such as Thermogravimetry-differential thermal analysis （TG-DTA），X-ray diffraction （XRD）and field emission scanning electronic microscope （FESEM）were adopted to analyze the phase transition law and morphological evolution behavior during the reaction process. The results of TG curves showed that there were five main stages in the whole reaction process，among those the mass loss produced by Stages I and II was 5.84%，which was independent of the reducing atmosphere，and the mass loss in the two stages were mainly due to the removal of adsorbed water and ammonium ion existed on the sample surface. The mass loss of Stages III and IV was 12.32%，which was close to the theoretical mass loss of the reduction of MoO₃ to MoO₂ （11.1%），that was，the main reaction took place in the stages was the transformation of MoO₃ to MoO₂. The mass loss of Stage V was 20.65%，which was close to the theoretical mass loss of MoO₂ to Mo （22.22%），so the main reaction took place in the stage was the transition from MoO₂ to Mo. In addition，it was found that the required temperature for each mass loss stage was increased with the increase of heating rate，and the main reason was due to the hysteresis phenomenon produced by the high heating rate. The results of DTA curves also supported this view. When the reaction extent （defined as the ratio of the instantaneous mass loss to the maximum mass loss）was less than α₃=0.1443，the existed phase in the sample was still h-MoO₃. When the reaction extent increased to α₄=0.2510，the residual sample was transformed into the mixture of α-MoO₃ and Mo₄O₁₁，further increased the reaction extent to α₅=0.3088，the reaction product was still composed of α-MoO₃ and Mo₄O₁₁；however，the content of Mo₄O₁₁ in the sample at this time was significantly higher than that at α₄=0.2510. When the reaction extent was increased to α₆=0.4129，it could be found that α-MoO₃ disappeared completely and MoO₂ began to be generated，and the obtained products were mainly composed of Mo₄O₁₁ and MoO₂. While when the reaction extent was increased to α₇=0.4753，the reaction product was only MoO₂. With the continuous increase of reaction temperature，the reduction reaction of MoO₂ to Mo began，and during the following stage，the content of MoO₂wasgradually decreased，while that of metallic Mo gradually increased. When the reaction reached a plateau，the reaction product was only metallic Mo. The total reaction paths of h-MoO₃→α-MoO₃+Mo₄O₁₁→MoO₂→Mo was concluded. According to FESEM images，it was found that the original sample first changed from a smooth hexagonal prismatic structure to a rough step-like structure，then to a smooth and irregular layered shape，and finally evolved into fine smooth particles，with the continue process of the reaction. In this case，the average particle size of the as-prepared Mo powder was measured to be 369.81 nm. The work also found that the chemical vapor phase transport mechanism dominated during the reduction process of MoO₃ to MoO₂；as to the reduction process ofMoO₂ to Mo，however，the pseudomorphic transformation mechanism dominated，and the obtained metallic Mo basically maintained the same sheet morphology as MoO₂.

As the world confronts escalating energy demands and heightened environmental concerns，the imperative for innovative and sustainable energy conversion technologies has never been more acute. Water electrolysis，a process that cleaves water into hydrogen and oxygen，stands out as a beacon of clean energy potential due to its prospect of high efficiency and minimal environmental impact. Within this dynamic landscape，metal-organic frameworks （MOFs），characterized by their versatile structures and multifunctional properties，have risen to prominence as a transformative class of materials poised to revolutionize the field of catalytic water splitting. This review embarked on a detailed exploration of the synthesis strategies of MOFs and delved into the myriad ways in which these porous materials could be meticulously engineered. The hydrothermal and solvothermal methods were known for their ability to generate highly crystalline MOFs. In addition，more recent developments，such as microwave-assisted synthesis and electrochemical synthesis，offered rapid synthesis with controlled morphology. Also，this review introduced other synthesis methods，such as vapor deposition，ultrasonic assisted synthesis，and so on. This in-depth analysis not only provided a comprehensive understanding of the current state of MOF synthesis but also set the stage for future innovations in material design. This review also focused on the application of MOFs in catalyzing the hydrogen evolution reaction （HER）and the oxygen evolution reaction （OER）and illuminated the remarkable strides made in enhancing the catalytic efficiency of these materials. The mechanism of reactions was dissected to provide a clearer understanding of the role MOFs play. In the case of HER，it needed the Volmer，Heyrovsky，and Tafel steps from acidic to alkaline media. OER，in comparison，was a more complex four-electrons transfer process，and its kinetics was a primary area of focus for improving the efficiency of water electrolysis. This review highlighted the advancements in enhancing MOFs' catalytic performance through strategic structural design and functional modifications. Integrating suitable metal centers and organic ligands into MOFs has been shown to significantly improve their activity and stability in alkaline conditions，a crucial factor for effective water splitting. Despite their potential，MOFs faced several challenges in practical applications，primarily in stability，catalytic activity，and electrical conductivity，particularly in alkaline environments. This review underscored the importance of addressing these challenges to advance the practical application of MOFs in water electrolysis. The need for in-depth research into the structure-performance relationship of MOFs and their catalytic mechanisms in water splitting was also stressed. Such understanding was pivotal for the development of more efficient MOF-based electrocatalysts. One of the most promising developments in this field was the exploration of bifunctional catalysts that were capable of efficiently performing both HER and OER，pointing towards a significant direction for MOFs in overall water splitting. This review showcased various MOFs that had been explored for this purpose，demonstrating their potential in enhancing water splitting efficiency. These examples highlighted the progress in developing dual-function MOF catalysts and underscored their significance in comprehensive water splitting applications. This review also discussed the future perspectives of MOFs in energy conversion. It emphasized the need for continuous research and innovation in MOFs，particularly in terms of structural optimization，functionality，stability，scalability，and theoretical understanding. Researchers were encouraged to delve into the microstructure of MOFs，optimizing parameters like pore size，surface area，and distribution of active sites to improve catalytic efficiency. Furthermore，functionalizing MOFs by incorporating specific organic ligands or metal active centers could expand their performance and applicability. Under high current densities and long-term usage，the stability and durability of MOFs remained areas for improvement. Future research should focus on material design and post-synthesis treatments to enhance structural stability. To achieve large-scale production and application in real-world energy systems，challenges related to cost，fabrication efficiency，and device compatibility need to be addressed. Future research should explore more economical and effective synthesis methods and develop MOFs-based electrode materials that were suitable for practical energy conversion systems. In conclusion，this review presented MOFs as a significant class of materials with tremendous potential in catalyzing water splitting. It highlighted the latest advancements in their synthesis strategies and applications in HER and OER，providing a roadmap for future research and applications in energy-related fields. This review not only underscored the immense potential of MOFs in catalyzing water splitting but also charted a path for future innovations，aiming to harness their full potential in addressing global energy challenges.

Electrochemical water splitting into H₂ and O₂ is a promising technique for decarbonizing energy and hydrogen production. Among various water electrolysis techniques, proton exchange membrane （PEM）water electrolyzers have attracted growing attention owing to high current density，excellent voltage efficiency，and ultrahigh gas purity，which are perfectly 0%~100% adaptive for transforming and storing renewable energy sources such as solar，wind，and hydro energy. At present，the development of hydrogen production by PEM electrolysis is mainly focused on reducing the number of precious metals，improving the energy conversion efficiency of electrolyzer，and prolonging the service life. The anodic oxygen evolution reaction （OER）is the rate control step of water electrolysis reaction. The activity，stability，and cost of anodic catalyst have become important factors restricting the large-scale development of PEM water electrolysis hydrogen production. Therefore，it is urgent to find an effective method to reduce the cost and improve the performance of catalyst at the same time. On the one hand，limited by the requirements of the strong acidic anode environment，high anode potential，good electrical conductivity and so on of PEM electrolyzer，while ensuring the performance and life of the electrolyzer，the anodic oxygen evolution electrocatalyst can only use a few precious metals such as Ir and Ru or their oxides as catalyst materials，which are resistant to oxidation and corrosion，but the use of precious metal materials has greatly increased the preparation cost. Therefore，there is an urgent need to further improve OER catalytic performances in terms of atomic activities and utilization rates to maximize the turnover frequency values. On the other hand，developing high-performance electrocatalysts at high current densities is a prerequisite for the commercial application of PEM electrolyzer technology. There are two key challenges to make robust acidic OER catalysts at high current densities. One is strong anodic polarization which destroys the active sites，causing a loss of catalytic activity. The other is the high local concentration of protons near catalytic surface which corrodes the active sites，resulting in poor durability of catalysts. This is a bottleneck affects the efficiency and lifetime of PEM electrolyzers and hinders their industrial use development of catalysts for anodic oxygen evolution reaction at high current. In order to facilitate the practical industrial application of PEM electrolyzers，it is imperative to develop highly active OER electrocatalysts that can operate robustly in strongly acidic media. However，OER activity and stability are often in a dilemma in electrocatalyst design. Therefore，the main challenge at present is how to develop efficient catalysts with less precious metal content，high intrinsic activity，and high stability without affecting the catalytic performance，so as to minimize the cost of precious metal catalyst and accelerate the commercialization process of PEM water hydrogen production. In this paper，from the aspects of reducing precious metals，reducing oxygen evolution overpotential，and increasing catalyst life，the latest research progress of oxygen evolution catalysts for water hydrogen production by PEM electrolysis at home and abroad in recent years were reviewed，mainly including alloying and heteroatom doping，nanostructured and supported catalysts. These electrocatalysts had high mass activity and durability and low noble metal content. Alloying or doping Ru or Ir and its oxides with some transition metals （such as Cu，Fe，Ni，Co，W，Cr，etc.）could change the electronic or geometric structure of the catalytic material and effectively improve the intrinsic activity of the catalyst active site. Through reasonable regulation and design of the size，shape，composition，and structure of the catalyst，excellent nanostructure could be obtained，thus significantly improving the surface area of the material，especially improving the utilization rate of precious metal materials，increasing the active site of the catalytic material，and improving the electron and proton transport. The use of carrier stabilization effect was also of great significance to achieve high activity and stability of OER catalyst，because the electronic interaction between catalyst and carrier could not only stabilize and disperse the active site，but also adjust the interface interaction，so as to optimize the physicochemical properties of catalyst/carrier. Including load capacity，active specific surface area，electronic structure，corrosion resistance，etc.，these strategies were used to develop OER catalysts with low load，high activity，and high durability. Finally，the future developments direction of low-load high-performance PEM electrolytic water oxygen evolution catalyst were proposed：1）Designing functional carrier by using multi-component electronic coordination effect and multi-dimensional structural epitaxy effect，the low-cost and high-stability carrier was prepared to enhance the durability of the catalyst；2）Through surface doping，defect engineering modification，and structural regulation to accelerate the reaction kinetics process and improve the intrinsic activity and utilization rate of the catalyst；3）Deeply understanding the structure-activity relationship of the catalyst，designing and regulating the chemical composition，crystal structure，and surface topography of the catalyst，optimizing the number and distribution of surface active sites，so as to develop efficient water electrolytic catalysts with high activity，stability and selectivity.

Hydrogen，as one of the most promising energy carriers in the 21st century，it has many properties such as being clean，abundant and sustainable，there has three segments of hydrogen energy industry，include hydrogen production，storage and transportation of hydrogen，usage of hydrogen. The intermediate part is the most urgent to be developed，owing to hydrogen is difficult to be stored by a low-cost and reliable storage method. Nowadays，except for storage in liquid organic hydrogen carriers （LOHCs） or hydrides，hydrogen can be stored in hydrogen slurry （HS） which composed of LOHCs with hydrides，HS has attracted interests of avoids the drawback of hydrides which were pulverize to plug the gas pipe，improved the sluggish reaction rate of LOHCs，can be transported with exist fuels infrastructure. Currently，dibenzyltoluene （DBT） is considered as one of the most promising LOHCs，has the advantages of high boiling point，high hydrogen storage capacity （6.2%），non-toxic，non-flammable and cheap，but it also has slowly hydrogenation rate. In order to accelerate the rate of hydrogenation，magnesium-based hydride （16%LaNi₅-Mg hydride，replaced by LM） was chosen as the hydrogenation catalyst of DBT，the kinetics of hydrogenation reaction of HS （LM-DBT） were studied systematically. The properties of MH_x were characterized by X-ray diffraction （XRD），meanwhile，the fluctuation of weight and heat change of MH_x during programly heating were captured by thermalgravimetric analysis-differential scanning calorimetry （TG-DSC），the hydrogenation productions of DBT （H_n-DBT） were analyzed by ¹H nuclear magnetic resonance （¹H NMR）. The effect of reaction temperature was studied，and the results showed that the kinetics of hydrogenation reaction of DBT was continue to accelerate with rising reaction temperature，the kinetics of hydrogenation reaction of HS （LM-DBT） increased first and then slowed down with the rising temperature，the optimal hydrogenation temperature of HS （LM-DBT） was 300 ℃，hydrogenation absorption could reach 2.9% in 10 h. The effect of solid-liquid ration in HS （LM-DBT） was studied，the kinetics of hydrogenation reaction of HS （LM-DBT） increased first and then slowed down with the more and more addition of LM，the optimal solid-liquid ratio in HS （LM-DBT） was 1∶15，the hydrogenation absorption of HS （LM-DBT） was up to 4.8% in 10 h. LM was composed of MgH₂，Mg₂NiH₄ and LaH₃，the main catalytic hydrogenation phase in LM was Mg₂NiH₄，when reaction temperature was higher than critical dehydrogenation temperature of LM and Mg₂NiH₄，they could promote the hydrogenation of DBT at hydrogen atmosphere. DBT had three unsaturated rings （side ring-middle ring-side ring），there existed five pathways during hydrogenation progress： in order of middle ring，side ring，side ring；in order of side ring，middle ring，side ring；in order of side ring，side ring，middle ring；hydrogenation occurred at all three rings at the same time or hydrogenation in random. Hydrogenation productions of DBT were analyzed by ¹H NMR，and the results showed that hydrogenation pathway of DBT was constantly in the order of the side ring，side ring and middle ring which was not influenced by reaction temperature，solid-liquid ratio and component of HS. The consequences provided that when the reaction temperature was higher than critical dehydrogenation temperature of MH_x，MH_x （LM and Mg₂NiH₄ that contained transition metal element-Ni） could catalyze the hydrogenation of DBT at hydrogen atmosphere. A hypothesis could be put forward was that MH_x （contained transition metal elements such as Mn，Co，Fe，Pt，Pd，et al.） could catalyze the hydrogenation of LOHCs at suitable reaction temperature and hydrogen atmosphere. DBT had three benzene rings，and the results showed that the hydrogenation order was always at the side ring，side ring and middle ring，at the same time，DBT had not been completely hydrogenated during these experiment，compared with the saturated hydrogenation capacity of 6.2%，there was still a definite gap. It was speculated that the spatial structure of benzene rings at both ends changed after hydrogenation，owing to the hydrogenation progress contained two steps： DBT was activated by MH_x and MH_x provided active H atom to promote the hydrogenation of DBT，two side hydrogenation rings provided additional steric hindrance effect for the hydrogenation process of intermediate benzene ring. It was assumed that when the diameter of MH_x particles were small to several nanometer，MH_x could fully contact with DBT in slurry，at this time，the kinetics of hydrogenation of HS would be improved and DBT could be hydrogenated completely. The dehydrogenation of HS was endothermic reaction，dehydrogenation progress was accompanied with rising system pressure which meant when system pressure was too high that HS would hardly to dehydrogenate completely. When system pressure was too low，the metal or alloy was difficult to combine with H atom which released from H_n-DBT to keep form hydrides. Therefore，the dehydrogenation catalyst for slurry was MH_x with high temperature and a low platform pressure.

Group 5B metals （Nb，V and Ta） and related alloys are considered as promising hydrogen permeation membranes to replace commercial Pd and its alloys. Vanadium-based alloys，as a new substitute for Pd membranes，not only have lower cost but also higher hydrogen permeability. Despite all this，huge challenges remain-especially the poor mechanical stability and high susceptibility to hydrogen embrittlement （HE） which is closely related to high absorbed hydrogen concentration of vanadium. Among group 5B alloys，hydrogenation （thermal-hydrogen treatment technology） has obvious advantages in improving the properties of V-based alloys.However，no relevant research has been carried out on V-TiFe alloys so far. Based on this，the hydrogenation behavior of （Ti₆₅Fe₃₅）_100-xV_x （x=0，2.5，5，10，15，25），x is the percentage of atoms of V） alloys with low cost and high hydrogen permeability were explored. In the solid state，two operating processes of static hydrogenation and dynamic hydrogenation were used，besides，the effects of hydrogen flow，hydrogenation time and hydrogenation temperature （350 and 450 ℃） on the hydrogenation behavior of the above alloys with different V content were systematically studied by the control variable method. The solidification path，microstructure evolution and mechanical properties of the alloy were analyzed by thermodynamic calculation software （Thermo-Calc），scanning electron microscope （SEM），energy spectrometer （EDS），X-ray diffraction meter （XRD），and Vickers hardness tester. First，the composition-temperature curve obtained from the thermodynamic solidification data of Thermo-Calc using the origin software found that： when 0<x<8，the primary phase was TiFe；when x=8，the alloy appeared fully eutectic；and when 8<x<25，the primary phase was body-centered cubic （bcc）-V.With the decrease of solidification temperature，binary and ternary eutectic reactions occurred successively until the end of solidification. Subsequently，the phase analysis of （Ti₆₅Fe₃₅）_100-xV_x cast alloys by XRD，SEM and EDS showed that： when 0<x<10，the alloy was composed of the primary TiFe phase and {bcc-（V，Ti）+TiFe} phase；when x=10，the alloy consisted of a divorced eutectic {β-Ti+TiFe}；when x>10，the alloy consistd of a primary bcc-（V，Ti） solid solution phase and a small amount of TiFe phase. In addition，exploring the changes of hydrogenation amount of （Ti₆₅Fe₃₅）_100-xV_x alloys caused by the difference in hydrogen flow，hydrogenation time and hydrogenation temperature，it was found that： when V content was constant，the hydrogenation effect of alloys was proportional to the variable parameters under any single variable parameter such as hydrogen flow，hydrogenation time and hydrogenation temperature，in other words，when the hydrogenation temperature and hydrogen flow kept a constant，the hydrogenation amounts of the alloy grew with the increase of hydrogenation time. After the hydrogenation time reached a certain level，the hydrogenation amount reached the maximum and tends to be stable，the alloy composition also had a decisive influence on the hydrogenation amount of the alloy besides，that was to say，it would cause an increase in hydrogenation with the increase of V content. Finally，the mechanical properties of all alloys of the system were studied，and the hardness of the cast microstructure of （Ti₆₅Fe₃₅）_100-xV_x cast alloys and the hardness after hydrogenation at 350 and 450 ℃ were measured using Vickers hardness tester. The hardness of the alloy of the hydrogenated V-TiFe system was significantly enhanced compared with the alloy sample before hydrogenation，that was，Vickers hardness of （Ti₆₅Fe₃₅）₈₅V₁₅ alloy after hydrogenation at 450 ℃ was HV_0.3 720.4，which was equivalent to 1.12 times before hydrogenation，and the hardness increased with the increase of hydrogenation temperature，which could be attributed to the solid solution strengthening of H atom inside the alloy and the precipitation of hydrides.

Hydrogen storage technology, as an intermediate link in the manufacture, storage and use of hydrogen energy, has become a bottleneck in the large-scale utilisation of hydrogen energy, and the development of solid-state hydrogen storage materials and systems is expected to be the most effective way to solve this problem. In solid-state hydrogen storage, lightweight hydrogen storage alloys represented by rare earth-magnesium-based metal hydrides are characterised by high hydrogen storage density, fast reaction rate of hydrogen absorption and discharge, and good cyclic stability. Meanwhile, powder alloy compaction can increase the volumetric hydrogen storage density. Therefore, magnesium-rich Mg-Ni-（La, Ce) alloys were determined as the research carriers, and the moulding density and volumetric hydrogen storage density of the powder alloys were improved by moulding pressure. Mg₈₇Ni₁₀（La, Ce）₃ and Mg₉₂Ni₄（La, Ce）₄ alloys pressed plates were prepared at pressures of 760, 1140 and 1520 MPa and their hydrogen storage properties were investigated by using a hydrogen storage property tester (PCT) in order to investigate the effect of molding pressure on the hydrogen storage properties of Mg-Ni-RE alloys. X-ray diffraction (XRD) was used to analyze the longitudinal slices of the alloy after hydrogen absorption, scanning electron microscopy (SEM) was used to study the longitudinal slices of the alloy's pressed slices in terms of macroscopic and microscopic pore structures, and to explore the mechanism of the change in the hydrogen storage properties of the alloy after pressing. The results showed that the higher the forming pressure, the higher the number of cycles to complete the activation of the alloy, in which Mg₈₇Ni₁₀（La, Ce）₃ powder alloy could be activated by four cycles of hydrogen absorption and release, and the maximum forming pressure of 1520 MPa under the piezoelectric sheet alloy needed seven cycles of hydrogen absorption and release in order to complete the activation. Activation of Mg₉₂Ni₄（La, Ce）₄ powdered alloy was completed with two cycles of hydrogen uptake and discharge, whereas four cycles of hydrogen uptake and discharge were required to complete the activation of the pressed sheet alloy at a maximum forming pressure of 1520 MPa. At the initial activation stage, the higher the moulding pressure, the lower the mass hydrogen storage density of the alloy, but the effect of the moulding pressure on the mass hydrogen storage density of the alloy decreased as the number of cycles increased. Among them, Mg₈₇Ni₁₀（La, Ce）₃ had a hydrogen uptake capacity of around 5.20% at different pressures for both the pressed and powdered alloys after the eighth cycle of hydrogen uptake, and Mg₉₂Ni₄（La, Ce）₄ had a hydrogen uptake capacity of around 4.78% for both the powdered and pressed alloys after four cycles of hydrogen uptake. The volumetric hydrogen storage densities of Mg₈₇Ni₁₀（La, Ce）₃ and Mg₉₂Ni₄（La, Ce）₄ alloys reached the maximum values of 115.0 and 99.23 g·L⁻¹, respectively, at a moulding pressure of 1520 MPa, which were more than double the volumetric hydrogen storage densities in the powder state. SEM images showed that after the alloy powder press moulding, the particles were broken and deformed, and the specific surface area and porosity of the materials were reduced, which limited the hydrogen diffusion path and increased the barriers to hydrogen diffusion, and ultimately led to a slowing down of the initial hydrogen storage kinetics of the alloys and a significant decrease in the amount of hydrogen stored, but the effect of the moulding pressure on the alloy's hydrogen uptake kinetics gradually decreased with the increase in the number of cycles. XRD analysis showed that after saturation hydrogen absorption in the press-formed alloy, metal hydride phases were formed from the surface layer to the interior, with no unhydrogenated metal phases remaining.

As the world energy crisis and environmental pollution continue to intensify，the promotion of the clean energy revolution has become the direction of the world energy industry reform. Hydrogen energy is recognized as one of the clean energy sources to reduce carbon emissions and address fossil energy depletion. Hydrogen energy is the breakthrough of the energy revolution，and its development and utilization will certainly bring about significant changes in the energy structure. At present，hydrogen storage is the bottleneck in the application of clean hydrogen energy. Solid-state hydrogen storage has the advantages of high mass hydrogen storage density and low hydrogen storage pressure，which makes it an ideal way to store hydrogen. Solid rare earth alloy LaNi₅ has the advantages of good toxicity resistance，easy activation，and fast hydrogen absorption and deposition at low pressure and room temperature，but the mass hydrogen storage density is lower than that of other room-temperature solid hydrogen storage alloys. AB₅ solid hydrogen storage container，which has low and uniform stress，was effectively adapted to the heavy truck structure，realizing the special-shape of the hydrogen storage container and its integration with the carrier's chassis frame. At the same time，lightweight alloys were used to design the hydrogen storage container，realizing the lightweight of the integrated hydrogen storage system.Reducing the negative impact of low mass hydrogen storage density of AB₅-type hydrogen storage alloys on integrated hydrogen storage systems.Under the same working condition，solid hydrogen storage containers made of Q345R，titanium alloy，6061 Al and LA103Z to be integrated with the heavy truck's chassis framewere designed. The integrated hydrogen storage system reduced thecentral core of the heavy truck by mounting solid-state hydrogen storage containers in the track's chassis frame. The barrel thickness of Q345R，titanium alloy，6061 Al and LA103Z solid-state hydrogen storage containers was 6，4，7 and 11 mm，respectively. The containers' weight was 24.4，9.2，9.9 and 8.7 kg，respectively. The mass hydrogen storage density of the system was 1.04%，1.23%，1.22% and 1.24%，respectively. Among them，the integrated hydrogen storage system with the tank made of LA103Z had the lightest mass and the highest hydrogen storage density，and the rare-earth hydrogen storage alloy with low mass hydrogen storage density had the least adverse effect on the hydrogen storage density of the integrated system. The integrated hydrogen storage designed mounts the hydrogen storage container in an unused space in the heavy truck chassis，taking up no additional cargo space. Without the integrated design，the solid-state hydrogen storage device saved the truck only 38.1 L·kg⁻¹ H₂ of cargo space compared to the 35 MPa cylinder. The integrated hydrogen storage system saved 62.5 L·kg⁻¹ H₂ of cargo space for the heavy truck compared to the 35 MPa cylinder，increasing the cargo space by 64.4%. Comparative analysis of integrated hydrogen storage systems based on rare earth lanthanide-nickel，titanium-iron，titanium-manganese，and vanadium-based solid solution materials forhydrogen storage at room temperature was carried out for the heavy Q345R tank. The integrated hydrogen storage system based on titanium-manganese alloy had the highest hydrogen storage capacity （30.5 kg）and hydrogen storage density （1.36%），and the integrated hydrogen storage system based on vanadium-based solid solution alloy was the most conducive to lightweight design. Hydrogen absorption and expansion of hydrogen storage materials generated large stresses （expansion stress） in the solid-state hydrogen storage container. This expansion stress was characterized by low weekly stress with long cycle time and high stress，which might lead to plastic deformation or even fatigue failure of the hydrogen storage container after many cycles. Therefore，it was necessary to consider the effect of expansion stress on the hydrogen storage system during hydrogen absorption and deposition. The stress-strain model of the hydrogen storage taking into account the volumetric and thermal expansion of the hydrogen-absorbing material in the tank was created. The stress-strain analysis of the solid-state hydrogen storage container was carried out using the finite element method，which comprehensively considered the hydrogen pressure inside the hydrogen storage container，the stress generated by the thermal and volumetric expansion of the hydrogen storage alloy in the process of hydrogen absorption and desorption，and the stress on the external surface of the hydrogen storage container. Stress concentration occurredbecause of discontinuity between the receiver and the head，and the maximum stress occurredat the connection between the receiver and the head. The analytical results showed that the maximum values of equivalent stresses of Q345R，titanium alloy，6061 Al and LA103Z hydrogen storage containers were 116.09，173.16，130.19 and 100.42 MPa，respectively，which did not exceed the tensile strength and yield strength of the material. To obtain specific stress values for the solid-state hydrogen storage container along the axial and radial directions and visualize the stress changes，paths were created in the radial and diameter directions. The maximum values of the equivalent elastic strain of Q345R，titanium alloy，6061 Al and LA103Z hydrogen storage container were 0.000599，0.00156，0.00213and 0.00246 mm·mm⁻¹，respectively，and the strain maxima were in the micrometer scale.