Geometrical effect in Mg-based metastable nano alloys with BCC structure for hydrogen storage

Mg-based materials are thought to be promising candidates for future hydrogen storage applications due to the low cost, abundant resources and large hydrogen storage capacity. However, they suffer from the challenges of sluggish kinetics and large volume change after hydriding/dehydriding (H/D) process. In order to address the problems, we successfully synthesized the Mg-based Body-Centered Cubic (BCC) metastable nano alloys with much improved kinetics while almost no obvious structure change after H/D process. In this work, the obtained Mg₅₅Co₄₅ metastable alloy with BCC structure can reach a hydrogen storage capacity of 3.24 wt% (hydrogen per metal or H/M = 1.28, H/Mg = 2.33) at −15 °C and this absorption temperature in Mg-based BCC structure is the lowest temperature reported for Mg-based materials to absorb hydrogen. Importantly, the BCC structure is maintained without obvious metal lattice change after H/D process. Nevertheless, the potential uptake of about 20 wt% theoretical hydrogen capacity (H/M = 9) for this unique BCC structure cannot be reached up to now. Herein, we discuss the mechanism from the geometrical effect aspect to figure out the difference between the experimental hydrogen storage capacity (H/M = 1.28) and the theoretical one (H/M = 9).     

Perspectives of high entropy alloys as hydrogen storage materials

Storage is a challenging issue that cuts across distribution, delivery, and safe end-uses of hydrogen as fuel. All the fuel cell vehicles are equipped with inefficient and unsafe high-pressure hydrogen cylinders. It is well known that storing such a highly flammable gas at high pressure is not safe. Only hydrogen can be stored safely as a form of metal hydrides, and all the investigated metal hydrides are inefficient in one way or another. Four essential hydrogen parameters for solid-state storage for fuel cell applications are high volumetric storage capacity, excellent heat transfer, and recharge time and feasible charging discharging temperatures. The available metal tanks have good gravimetric storage capacity but did not satisfy the prescribed criterion for good volumetric capacity necessary for mobile applications. Recently, some promising reports are published on the hydrogen storage properties of newly discovered High Entropy Alloys (HEAs). HEAs provide vast composition selection freedom for the formation of favorable simple solid solution phase for hydrogen storage. The four core effects of these alloys may also play a vital role in hydrogen storage properties. Here we reviewed and summarized the published results on hydrogen storage properties of HEAs to date. We underlined different essential aspects for the future development of HEAs as hydrogen storage materials. This review article discusses and describes the perspectives of HEAs in regards to the hydrogen storage applications of these alloys and will provide insight into the future development of hydrogen storage HEAs.     

A study on crystal structure and chemical state of TiCrVMn hydrogen storage alloys during hydrogen absorption-desorption cycling

The evolution of crystal structure and chemical state of the V-based hydrogen storage alloy (Ti_0.32Cr_0.46V_0.22)₉₆Mn₄ during hydrogen absorption/desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Reasons for the degradation in cycling capacity of the alloy are presented and discussed. One reason is the continuous reduction of the V-based cell volume during cycling, which cannot hold further hydrogen atoms. The decrease in cycling capacity can also be attributed to the oxidation of Ti, V, and Cr elements during cycling.     

Exploration of Ti substitution in AB2-type YZrFe based hydrogen storage alloys

To improve the hydrogen storage properties of YZrFe alloys, the alloying with Ti was carried out to obtain Y_0.7Zr_(0.3-x)Ti_xFe₂ (x = 0.03, 0.09, 0.1, 0.2) alloys by different processes. It was expected that Ti would substitute Zr and decrease the lattice constant of YFe₂-based C15 Laves phase. All YZrTiFe quaternary alloys consist of the main Y(Zr)Fe₂ phase and the minor YFe₃ phase. Despite the large solubility of Ti in Zr or Zr in Y, the Ti incorporation into YZrFe alloys results in the inhomogeneity of Y and the segregation of Ti, and thus decreases the hydrogen storage capacity. Only the alloy Y_0.7Zr_0.27Ti_0.03Fe₂ containing very few Ti shows the substitution of Ti to Zr and the resultant improvement in the dehydriding equilibrium pressure.     

Effect of rapid solidification on hydrogen solubility in Mg-rich Mg-Ni alloys

The hydrogenation properties of Mg_100−xNi_x alloys (x = 0.5, 1, 2, 5) produced by melt spinning and subsequent high-energy ball milling were studied. The alloys were crystalline and, in addition to Mg matrix, contained finely dispersed particles of Mg₂Ni and metastable Mg₆Ni intermetallic phases. The alloys exhibited excellent hydrogenation kinetics at 300 °C and reversibly absorbed about 6.5 mass fraction (%) of hydrogen. At the same temperature, the as prepared Mg_99.5Ni_0.5 and Mg₉₅Ni₅ powders dissolved about 0.6 mass fraction (%) of hydrogen at the pressures lower than the hydrogen pressure corresponding to the bulk Mg-MgH₂ two-phase equilibrium, exhibiting an extended apparent solubility of hydrogen in Mg-based matrix. The hydrogen solubility returned to its equilibrium value after prolonged hydrogenation testing at 300 °C. We discuss this unusually high solubility of hydrogen in Mg-based matrix in terms of ultrafine dispersion of nanometric MgH₂ precipitates of different size and morphology formed on vacancy clusters and dislocation loops quenched-in during rapid solidification.     

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

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

Reversible hydrogen storage in electrospun polyaniline fibers

Due to its many physisorption sites as well as chemisorption sites, polyaniline (PANI) has been investigated for hydrogen storage purposes. The PANI was produced in house via traditional chemical synthesis methods and then electrospun to produce fibers. These PANI fibers were investigated and compared with standard bulk PANI and found to be stable up to 150 °C. When investigating PANI fibers, using PCT measurements, it was found that a reversible hydrogen storage capacity of ∼3–10 wt.% could be obtained at different temperatures. Hydrogen kinetic sorption measurements in prolonged cycles (up to 66 cycles) reveal an uptake and release of >6–10 wt.% on these PANI materials. The importance of the type of measurement is discussed as to its effect on the morphology and structure of the PANI nanofibers. The surface morphologies before and after hydrogen sorption on these PANI fibers encompass significant changes in the microstructure (nanofibrallar swelling effect). Detailed chemical and physical characterization of the PANI fibers is reported as part of this work.     

A layered porous Ni structure contributes to superior low-temperature performance of hydrogen storage alloys

The poor low-temperature performance of negative electrode materials— hydrogen storage alloys (HSAs) has impeded applications of nickel metal hydride batteries in new energy vehicles. Here, we propose a simple and effective strategy to improve low-temperature properties of the commercial HSA with HCl etching and heat treatment. The layered porous Ni surface structure formed during this process improves the electrochemical reaction kinetics, and thus results in a larger discharge capacity of 102.63 mAh g⁻¹ at 233 K compared with those of bare (11.52 mAh g⁻¹) and acid-corroded (43.00 mAh g⁻¹) alloys. This method could be extended to enhance the low-temperature performance of other HSA systems.     

Design of hydrogen storage alloys in view of chemical bond between atoms

The chemical interactions operating in hydrogen storage alloys are simulated by the DV-Xα molecular orbital method, using tetrahedral or octahedral model clusters. It is found that hydrogen interacts more strongly with hydride non-forming elements, B (e.g., Ni, Fe) than hydride forming elements, A (e.g., La, Ti, Mg), in agreement with our previous calculations of hydrogen storage alloys (e.g., LaNi₅, TiFe, Mg₂Ni). However, it is noted that such a B–H interaction is not dominant unless A elements exist in the neighborhood, so that both A and B are indeed essential elements for hydrogen storage alloys. Also, it is shown that the A/B compositional ratio of hydrogen storage alloys can be understood in terms of a simple parameter, 2 Bo(A–B)/[Bo(A–A)+Bo(B–B)], where the Bo(A–B), Bo(A–A) and the Bo(B–B) are the bond strengths between atoms given in the parentheses.     

An approach to design single BCC Mg-containing high entropy alloys for hydrogen storage applications

Mg is a lightweight element that can increase the gravimetric hydrogen storage capacity of high entropy alloys (HEAs). This work presents a new approach to design single-phase Mg-containing HEAs with attractive hydrogen storage properties. The design method is based on four calculated parameters (?, VEC, $\overline{\Delta {\text{H}}_{\infty }}$ and $\overline{\Delta {\text{H}}_{\text{f}}^{\text{o}}}$) that allow us to find alloy compositions that form single body-centered cubic (BCC) solid solutions with high hydrogen affinity. The method was tested in the Mg–Al–Ti–Mn–Nb system and the Mg₁₂Al₁₁Ti₃₃Mn₁₁Nb₃₃ alloy was selected among 1326 calculated compositions. This alloy was produced by high energy ball milling resulting in a homogeneous single-phase BCC alloy that absorbed 1.7 wt.% of H by forming a BCC monohydride. Despite its H uptake being H/M = 1, the gravimetric capacity of the lightweight Mg₁₂Al₁₁Ti₃₃Mn₁₁Nb₃₃ alloy was comparable to refractory BCC-HEAs with H uptake of H/M = 2.     

Effect of rare earth (RE) elements on V-based hydrogen storage alloys

In this work, the effect of RE additives on the properties of _{V55Ti22.5Cr16.1Fe6.4}${\mathrm{V}}_{55}{\mathrm{Ti}}_{22.5}{\mathrm{Cr}}_{16.1}{\mathrm{Fe}}_{6.4}$ alloy (RE=La$\mathrm{RE}=\mathrm{La}$, Pr, Ce and Nd, separately) was discussed. It was demonstrated that RE additives improve the activation property rather than kinetics during cycling, absorption capacity and the plateau pressure. Two phases, including BCC main phase and Ce second one, were found in Ce-containing alloy. It is inferred that RE element offers a route for hydrogen to enter the alloys more easily, which leads to the improvement of activation property of the alloys.     

A low-cost BCC alloy prepared from a FeV80 alloy with a high hydrogen storage capacity

A V₃₀Ti₃₂Cr₃₂Fe₆ alloy prepared from a FeV80 master alloy is reported. It has a high hydrogen absorption/desorption capacity, good activation performance and kinetics. Heat-treatment at 1673 K is an effective way to increase the capacity and flatten the plateau due to the homogenization of the compositions in the alloy and the disappearance of Laves phase after heat-treatment. The heat-treated alloy can absorb 3.76 wt.%H at 298 K. It desorbs 2.35 wt.%H at 298 K and 2.56 wt.%H at 373 K. The development of this alloy could be of great significance to the application of V-based BCC hydrogen storage alloys.     

Effects of Zr addition on electrochemical characteristics of MgTiMnNi hydrogen storage alloys

The electrochemical hydrogen storage properties of 25 h milled Mg_0.80Ti_0.175Mn_0.025Zr_xNi_1-x (x = 0, 0.025, 0.05, 0.1) quinary alloys were investigated. The substitution of Zr for Mg or Ni leads to an increase in structural disorder and amorphization. Thus, the maximum discharge capacity and the cycling stability of MgNi-based alloys can be enhanced. The x-ray diffraction patterns indicate that all additive elements are entirely dissolved in the synthesized alloys, and amorphous structure was successfully obtained by 25 h milling. Among the milled alloys, the Mg_0.80Ti_0.175Mn_0.025Zr_0.10Ni_0.90 alloy exhibited the best discharge capacity of 604 mA h g⁻¹ at the initial charge/discharge cycle. The obtained results demonstrate that using multi-component compositions is beneficial for enhancing the structural and cyclic stability of MgNi-based alloys. Therefore, substituting additive elements for Mg or Ni may offer impressive performance for efficient hydrogen storage applications.     

Magnesium-based alloys for solid-state hydrogen storage applications: A review

Magnesium hydrides (MgH₂) have attracted extensive attention as solid-state H₂ storage, owing to their low cost, abundance, excellent reversibility, and high H₂ storage capacity. This review comprehensively explores the synthesis and performance of Mg-based alloys. Several factors affecting their hydrogen storage performance were also reviewed. The metals addition led to destabilization of Mg–H bonds to boost the dehydrogenation process. A unique morphology could provide more available active sites for the dissociation/recombination of H₂ and allow the activation energy reduction. Also, an appropriate support prevent the agglomeration of Mg particles, hence, sustains their nanosize particles. Moreover, the perspective of avenues for future research presented in this review is expected to act as a guide for the development of novel Mg-based H₂ storage systems. New morphological shape of catalysts, more unexplored and highly potential waste materials, and numerous synthesis procedures should be explored to further enhance the H₂ storage of Mg-based alloys.     

Hydrogen storage properties of Zr2Co crystalline and amorphous alloys

To further explore the application feasibility of Zr₂Co alloy in tritium-related fields, hydrogenation/dehydrogenation properties of this material of crystalline or amorphous structure, prepared by arc melting or melt spinning, were studied by pressure-composition temperature measurement, X-ray diffraction, differential scanning calorimeter, thermal desorption spectroscopy. It was found that the two kinds of Zr₂Co alloys can absorb hydrogen in a close full concentration of ~9 mmol/g, and may have similar equilibrium hydrogen pressure in the order of 10^?6 Pa at room temperature. In their hydrogenated samples various hydrides were observed to form, including ZrH₂, Zr₂CoH₅, ZrCoH₃ and an amorphous one with gradually decreasing general thermostability. The amorphous alloy exhibited easier hydrogen induced disproportionation caused by highly stable ZrH₂ and much slower hydrogen absorption kinetics. This disproportionation behavior of the crystalline alloy was found to be entirely suppressed by changing heating process. The results firmly indicate that crystalline Zr₂Co alloy could be more favorable for tritium treatment due to very low equilibrium pressure and the feasibility of eliminating the disproportionation.     

Electrochemical properties of magnesium-based hydrogen storage alloys improved by transition metal boride and silicide additives

Transition metal borides and silicides prepared by mechanical alloying (MA) and chemical reduction methods (CR) were introduced to improve the corrosion resistance of magnesium-based hydrogen storage alloys. The additive of FeB prepared by MA can remarkably enhance the discharge capacity and cycling stability which has initial discharge capacity of 355.9 mA h g⁻¹ and keeps 224 mA h g⁻¹ after 100 cycles, and the exchange density I₀ of MgNi–NiB(CR) electrodes is 344.80 mA g⁻¹ but MgNi is only 67.6 mA g⁻¹ which leads to the better rate capability of the composite alloys. The results of SEM characterization, cyclic charge–discharge tests, potentiodynamic polarization, linear polarization and AC impedance experiment show that the corrosion inhibition property of MgNi in alkaline is improved by transition metal boride and silicide additives.     

Mg-based metastable nano alloys for hydrogen storage

Mg-based materials have been widely researched for hydrogen storage development due to the low price of Mg, abundant resources of Mg element in the earth's crust and the high hydrogen capacity (ca. 7.7 mass% for MgH₂). However, the challenges of poor kinetics, unsuitable thermodynamic properties, large volume change during hydrogen sorption cycles have greatly hindered the practical applications. Here in this review, our recent achievements of a new research direction on Mg-based metastable nano alloys with a Body-Centered Cubic (BCC) lattice structure are summarized. Different with other metals/alloys/complex hydrides etc. which involve significant lattice structure and volume change from hydrogen introduction and release, one unique nature of this kind of metastable nano alloys is that the lattice structure does not change obviously with hydrogen absorption and desorption, which brings interesting phenomenon in microstructure properties and hydrogen storage performances (outstanding kinetics at low temperature and super high hydrogen capacity potential). The synthesis results, morphology and microstructure characterization, formation evolution mechanisms, hydrogen storage performances and geometrical effect of these metastable nano alloys are discussed. The nanostructure, fresh surface from ball milling process and fast hydrogen diffusion rate in BCC lattice structure, as well as the unique nature of maintaining original BCC metal lattice during hydrogenation result in outstanding hydrogen storage performances for Mg-based metastable nano alloys. This work may open a new sight to develop new generation hydrogen storage materials.     

Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

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

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

Effect of processing parameter on hydrogen storage characteristics of as quenched Ti45Zr38Ni17 quasicrystalline alloys

The present study deals with the microstructural changes with respect to the processing parameter (quenching rate) and their correlation with hydrogen storage characteristics of Ti₄₅Zr₃₈Ni₁₇ quasicrystalline alloys. The ribbons of the alloy have been synthesized at different quenching rates obtained through different wheel speeds (35, 40, 45 and 50 m/s) and investigated for their hydrogen storage characteristics. The lower cooling rate obtained through low wheel speed (35 m/s) produces, i-phase grains whose size ranges from 300-350 nm, whereas higher cooling rates obtained through high wheel speed (45 and 50 m/s) promote the formation of grains with size ranges from 100-150 nm in Ti₄₅Zr₃₈Ni₁₇ ribbons. It has been found that the ribbons synthesized at 35 m/s absorbed ∼2.0 wt%, whereas ribbons synthesized at 50 m/s absorbed ∼2.84 wt. % of hydrogen. Thus the hydrogen storage capacity of ribbon increases for the ribbons produced at higher quenching rate. One of the salient features of the present study is that the improvement of hydrogen storage capacity obtained through higher quenching rates (∼45 to 50 m/s wheel speed) leading to the formation of lower grain size.