Tuning the electronic conductance of REH_(x)(RE=Nd,Ce,Pr)by structural deformation

Hydride ion(H-)conductors have drawn much attention due to their potential applications in hydrideion-based devices.Rare earth metal hydrides(REH_(x))have fast H-conduction which,unfortunately,is accompanied by detrimental electron conduction preventing their application as ion conductors.Here,REH_(x)(RE=Nd,Ce,and Pr)with varied grain sizes,rich grain boundaries,and defects have been prepared by ball milling and subsequent sintering.The electronic conductivity of the ball-milled REH_(x)samples can be reduced by 2-4 orders of magnitude compared with the non-ball-milled samples.The relationship of electron conduction and miscrostructures in REH_(x)is studied and discussed based on experimental data and previously-proposed classical and quantum theories.The H-conductivity of all REH_(x)is about 10^(-4)to 10^(-3)S cm^(-1)at room temperature,showing promise for the development of H-conductors and their applications in clean energy storage and conversion.

NaH doped TiO_(2)as a high-performance catalyst for Mg/MgH_(2)cycling stability and room temperature absorption

This paper presents the catalytic effect of NaH doped nanocrystalline TiO_(2)(designated as NaTiOxH)in the improvement of MgH_(2)hydrogen storage properties.The catalyst preparation involves ball milling NaH with TiO_(2)for 3 hr.The addition of 5 wt%NaTiOxH powder into MgH_(2)reduces its operating temperature to∼185℃,which is∼110℃lower than the additive-free as-milled MgH_(2).The composite remarkably desorbs∼7.2 wt%H_(2)within 15 min at∼290℃and reabsorbs∼4.5 wt%H_(2)in 45 min at room temperature under 50 bar H_(2).MgH_(2)dehydrogenation is activated at 57 kJ/mol by the catalyst.More importantly,the addition of 2.5 wt%NaTiOxH catalyst aids MgH_(2)to reversibly produce∼6.1 wt%H_(2)upon 100 cycles within 475 hr at 300℃.Microstructural investigation into the catalyzed MgH_(2)composite reveals a firm contact existing between NaTiOxH and MgH_(2)particles.Meanwhile,the NaTiOxH catalyst consists of catalytically active Ti_(3)O_(5),and“rod-like”Na_(2)Ti_(3)O_(7)species liberated in-situ during preparation;these active species could provide multiple hydrogen diffusion pathways for an improved MgH_(2)sorption process.Furthermore,the elemental characterization identifies the reduced valence states of titanium(Ti<4+)which show some sort of reversibility consistent with H_(2)insertion and removal.This phenomenon is believed to enhance the mobility of Mg/MgH_(2)electrons by the creation and elimination of oxygen vacancies in the defective(TiO_(2-x))catalyst.Our findings have therefore moved MgH_(2)closer to practical applications.

Defect-rich potassium amide: A new solid-state potassium ion electrolyte

One of the major obstacles to the application of potassium-ion batteries in large-scale energy storage is the lack of safe and effective electrolytes.KNH_(2),a new potassium-ion solid electrolyte has been developed in this study.Its ionic conductivity reaches 4.84×10^(-5)S cm^(-1)at 150°C and can reach3.56×10^(-4)S cm^(-1)after mechanochemical treatment.The result from electron paramagnetic resonance(EPR) measurement shows that the increment of ionic conductivity is dependent on the concentration of nitrogen defects in the KNH_(2) electrolyte.To the best of our knowledge,this is the first report that adopts inorganic amide as an electrolyte for potassium-ion battery and initiates the search for a new amidebased solid electrolyte for an all-solid-state potassium-ion battery.

Hydrogen storage over alkali metal hydride and alkali metal hydroxide composites

Alkali metal hydroxide and hydride composite systems contain both protic （H bonded with O） and hydridic hydrogen. The interaction of these two types of hydrides produces hydrogen. The enthalpy of dehydrogenation increased with the increase of atomic number of alkali metals, i.e., -23 kJ/molnz for LiOH-LiH, 55.34 kJ/moln： for NaOH-NaH and 222 kJ/molH2 for KOH-KH. These thermodynamic calculation results were consistent with our experimental results. H2 was released from LiOH-LiH system during ball milling. The dehydrogenation temperature of NaOH-NaH system was about 150 ℃; whereas KOH and KH did not interact with each other during the heating process. Instead, KH decomposed by itself. In these three systems, NaOH-NaH was the only reversible hydrogen storage system, the enthalpy of dehydrogenation was about 55.65 kJ/molHz, and the corresponding entropy was ca. 101.23 J/（molHz .K）, so the temperature for releasing 1.0 bar H2 was as high as 518 ℃, showing unfavorable thermodynamic properties. The activation energy for hydrogen desorption of NaOH-NaH was found to be 57.87 kJ/mol, showing good kinetic properties.

Dinitrogen fixation mediated by lanthanum hydride

Dinitrogen fixation is one of the key reactions in chemistry, which is closely associated with food, environment, and energy. It has been recently recognized that the hydride materials containing negatively charged hydrogen(H~-) show promises for Nfixation and hydrogenation to ammonia. Herein, we report that rare earth metal hydrides such as lanthanum hydride can also fix Neither by heating to 200 °C or ball milling under ambient Npressure and temperature. The Nfixation by lanthanum hydride may proceed via an intermediate lanthanum hydride-nitride(La-H-N) structure to form the final lanthanum nitride product. The hydride ion functions as an electron donor, which provides electrons for Nactivation possibly mediated by the lanthanum atoms. It is observed that N–H bond is not formed during the Nfixation process, which is distinctly different from the alkali or alkaline earth metal hydrides. The hydrolysis of La-H-N to ammonia is feasible using water as the hydrogen source. These results provide new insights into the nitrogen fixation by hydride materials and more efforts are needed for the development of rare earth metal-based catalysts and/or nitrogen carriers for ammonia synthesis processes.

Mild-condition synthesis of A2ZnH4(A = K, Rb, Cs) and their effects on the hydrogen storage properties of 2LiH-Mg(NH2)2

In this paper, A2ZnH4(A = K, Rb and Cs) have been synthesized for the first time by a new approach involving in two-step reactions, in which the target samples can be produced under mild conditions(160 ℃ for 4 h). What’s more, the additive effects of A2ZnH4 on the hydrogen storage properties of 2LiH-Mg(NH2)2 composite have been investigated systematically. Experimental results show that K2ZnH4 has the best comprehensive modification effects among these hydrides. The 2LiH-Mg(NH2)2-0.1 K2ZnH4 sample shifts dehydrogenation peak temperature downwards by ca. 30 ℃ as compared to the pristine sample. In addition, about 70% extent of the theoretical hydrogen is able to desorb from the 0.1 K2ZnH4 doped sample at 140 ℃ within 2 h, however, only 20% extent of hydrogen is liberated from the pure sample under the same conditions. The improved desorption kinetics is indicated by the reduced dehydrogenation activation energy(Ea), the Ea of the 0.1 K2ZnH4 doped sample is around 68 ± 1.0 kJ mol-1 which is 28% lower than that of the pristine one. Furthermore, the dehydrogenation mechanism of the K2ZnH4 doped sample has been proposed.

The Roles of Alkali/Alkaline Earth Metals in the Materials Design and Development for Hydrogen Storage

CONSPECTUS:Hydrogen storage for onboard applications has been recognized as a grand challenge for the large-scale implementation of hydrogen fuel cell vehicles.Tremendous research efforts have thus been devoted to the design and development of hydrides of lightweight elements(HLEs).A prominent feature of these materials is the indispensable ingredient of alkali/alkaline earth cations.Alkali alkaline earth metals(AMs)are highly reactive and have a rich coordination chemistry.As a matter of fact,an AM cation can form a complete range of compounds with hydrogenous anions,such as H−,[NH2]−,[BH4]−,[AlH4]−,[NH2BH3]−,[TMHX]−,[R-CH2−NH]−,[R-CH2−O]−,etc.,and,thus,tune the Al−H,N−H,B−H,and C−H bond strengths for hydrogen storage.In this Account,our research efforts in the development of AM amide-hydride composites,AM amidoboranes,and metalorganic hydrides for hydrogen storage are reviewed.A partial substitution of the H in NH3 by AM gives rise to solid AM amides or imides.Those compounds can react with AM hydride(AMH)to produce H2.This is driven by the redox reaction between a protic H(N)and hydridic H(AM).A variety of amide-hydride composites holding promise for hydrogen storage were thus developed,including LiNH2-hydride,Mg(NH2)2-hydride,and complex amide-hydride composites.For amidoboranes,the substitution of H(N)in ammonia borane(AB)by AM transforms the molecular crystal AB into amidoboranes with an ionic crystal structure,leading to significant changes in terms of charge distribution,bond length,intermolecular forces,and so on,which in turn results in enhanced dehydrogenation properties.For metalorganic hydrides,through reacting AM compounds(usually AM hydride)with aliphatic,carbocyclic,or heterocyclic organic hydrides having“reactive protic H”,corresponding metalorganic hydrides are formed.Because of the electron-donating nature of AM,the strengths of the C−H bond in metal-organic hydrides can be modulated.For each material system,we will introduce the synthesis of materials,show their performances,correlate the hydrogen storage property to a crystal and/or electronic structure,and especially highlight the functions of AM in tuning the thermodynamic and/or kinetic properties of HLEs.At the end of the Account,challenges and a future research direction of the hydrogen storage field are discussed.