Reversible Hydrogen Storage Capacity Research Articles

Exploring microstructure variations and hydrogen storage characteristics in TiVNbCrNi high-entropy alloys with different Ni incorporation

In recent years, high-entropy alloys (HEAs), as a novel class of hydrogen storage materials, have been deemed highly promising due to their extensive compositional tunability and the unique high entropy effects. This study aims to enhance the comprehensive hydrogen storage performance of the TiVNbCr-based HEA by introducing Ni. The effects of Ni addition on the microstructural evolution of the alloy and its impact on activation properties, hydrogen absorption and desorption kinetics, thermodynamics, and cycling performance under the complex physicochemical environment of multiple principal elements was systematically investigated. The findings indicate that Ni addition shortens the activation incubation period of the alloy but more Ni reduces the intrinsic hydrogen absorption kinetics. In the Ti25V30Nb10Cr33Ni2 alloy, a high reversible hydrogen storage capacity of up to 2.2 wt% at room temperature was achieved, surpassing most of the currently reported body-centered cubic (BCC)-structured high-entropy hydrogen storage alloys. Furthermore, the influence of Ni on the alloy's hydrogen desorption kinetics and cycling hydrogenation performance was studied. Results from 110 cycles of testing reveal that the introduction of Ni-induced Laves phases can act to relax stress, thereby reducing strain accumulation during the hydrogen absorption and desorption cycling process. This could provide guidance for the design of high-entropy hydrogen storage alloys with extended cycling lifetimes.

Improving the plateau performance of the TiZrFeMnCrV high-entropy alloy by partial substitution of V with Fe, Mn and Cr

High-entropy alloys (HEAs) have shown desirable hydrogen storage performance owing to the lattice distortion effect and favorable phase structure. However, their pressure-composition-temperature (PCT) curves are usually lack of a well-defined hydrogen ab-/desorption plateau. Herein, we reported that by substituting 0.5 at.% Fe, Mn and Cr for the equivalent amount of V in the Ti1.0Zr1.0Fe1.0Mn1.0Cr1.0V1.0 (defined as V1.0 alloy) alloy, the plateau performance could be considerably improved. The non-equiatomic Ti1.0Zr1.0Fe1.5Mn1.0Cr1.0V0.5, Ti1.0Zr1.0Fe1.0Mn1.5Cr1.0V0.5 and Ti1.0Zr1.0Fe1.0Mn1.0Cr1.5V0.5 HEAs (defined as Fe1.5, Mn1.5 and Cr1.5 alloy, respectively) with a single C14 Laves phase rapidly absorb 1.82 wt%, 1.90 wt% and 1.89 wt% of hydrogen at 30 °C, respectively. Compare with the V1.0 alloy, the Fe1.5, Mn1.5 and Cr1.5 alloys show clearly visible hydrogen ab-/desorption plateau. Among all the alloys, the Fe1.5 alloy own the highest plateau pressure and hysteresis with values of 0.18/0.11 MPa and 0.41, followed by the Cr1.5 alloy with values of 0.17/0.05 MPa and 0.38, and the Mn1.5 alloy with values of 0.10/0.08 MPa and 0.28. Moreover, all the alloys exhibit good hydrogen ab-/desorption cycling performance, especially for the V1.0, Fe1.5 and Mn1.5 alloys, retaining 98, 99 and 97% reversible hydrogen storage capacity after 15 cycles, respectively.

Mg-MOF-74 Derived Defective Framework for Hydrogen Storage at Above-Ambient Temperature Assisted by Pt Catalyst.

Metal-organic frameworks (MOFs) are promising candidates for room-temperature hydrogen storage materials after modification, thanks to their ability to chemisorb hydrogen. However, the hydrogen adsorption strength of these modified MOFs remains insufficient to meet the capacity and safety requirements of hydrogen storage systems. To address this challenge, a highly defective framework material known as de-MgMOF is prepared by gently annealing Mg-MOF-74. This material retains some of the crystal properties of the original Mg-MOF-74 and exhibits exceptional hydrogen storage capacity at above-ambient temperatures. The MgO5 knots around linker vacancies in de-MgMOF can adsorb a significant amount of dissociated and nondissociated hydrogen, with adsorption enthalpies ranging from -22.7 to -43.6kJmol-1, indicating a strong chemisorption interaction. By leveraging a spillover catalyst of Pt, the material achieves a reversible hydrogen storage capacity of 2.55wt.% at 160°C and 81bar. Additionally, this material offers rapid hydrogen uptake/release, stable cycling, and convenient storage capabilities. A comprehensive techno-economic analysis demonstrates that this material outperforms many other hydrogen storage materials at the system level for on-board applications.

Effect of cobalt substitution for nickel on microstructural evolution and hydrogen storage properties of La0.66Mg0.34Ni3.5–xCox alloys

Superlattice hydrogen storage alloys offer a compelling advantage with rapid hydriding rate and high storage capacity. However, its practical applications face challenges including complex structure, low dehydriding capacity, and cyclic instability. In this work, we successfully prepared La0.66Mg0.34Ni3.5–xCox superlattice hydrogen storage alloys with enhanced dehydriding capacity and stability by partially substituting Co for Ni. X-ray diffraction (XRD) refinements analysis reveals the presence of (La,Mg)3Ni9, (La,Mg)5Ni19, and LaNi5 phases within the alloy. Following Co substitution in the La0.66Mg0.34Ni3.4Co0.1 alloy, there is a significant increase in content of the (La, Mg)3Ni9 phase and a reduction in the hysteresis factor, resulting in an improved reversible hydrogen storage capacity from 1.45 wt% to 1.60 wt%. The dehydriding kinetics of the alloy is controlled by diffusion model with an activation energy of 8.40 kJ/mol. Furthermore, the dehydriding enthalpy value of the Co-substituted alloy decreases from 30.84 to 29.85 kJ/mol. Impressively, the cycling performance of the alloy after Co substitution exhibits excellent stability, with a capacity retention rate of 92.3% after 100 cycles. These findings provide valuable insights for the development of cost-effective hydrogen storage materials.

Effect of in-situ formation of CeH2.73/CeO2-x and V2O3 on hydrogen storage performance of MgH2

MgH2, one of the most promising lightweight hydrogen storage mediums, has aroused intensive attention due to its outstanding reversible hydrogen storage capacity. However, it suffers from the intrinsic high operating temperature and sluggish kinetics from de-/hydrogenation, hindering its further industrial application. Herein, we successfully synthesized a novel rare earth Ce co-doping with V element forming bimetallic oxide additive, CeVO4 nanoparticles, which exhibits an excellent catalytic effect for enhancing the hydrogen storage performance in MgH2. With an optimization addition of 10 wt% CeVO4, the initial dehydrogenation temperature of the composite decreased from 286 °C to 212 °C. The composite can desorb 6.03 wt% H2 within 6 min at 300 °C and absorb 1.9 wt% H2 within 20 min even at a low temperature of 50 °C. The apparent dehydrogenated activation energy of the composite was obtained as 61.38 ± 3.9 kJ mol−1. The catalyst mechanism analysis revealed that the “hydrogen pumping” effect of the in-situ formed CeH2·73/CeO2-x and V2O3 play a crucial role in boosting the de-/hydrogenation properties of CeVO4-catalyzed MgH2. This work is helpful for further rational designs of novel catalysts in promoting the commercialization of MgH2.

Effect of industrial grade raw materials and its associated impurities on first hydrogenation of BCC solid solution alloys

This study explores the effects of using industrial-grade raw materials on the hydrogen storage properties of body-centered cubic (BCC) solid solution alloys. Industrial scrap metals were chosen as the raw material, and the desired alloys were synthesized using a vacuum arc melting furnace. Instead of using pure vanadium (V), different inexpensive industrial substitutes such as low-grade vanadium (V), vanadium nitride (VN), vanadium oxide (VO), and ferrovanadium (FeV) were utilized. The crystal structure, morphology, first hydrogenation, thermodynamics, and cyclability of the prepared alloys were analyzed by using XRD, SEM-EDS, and XPS methods. The crystal structure and morphology analysis revealed multiphase alloys in all the prepared alloys and a Ti rich third phase in the VN sample, which was absent otherwise. The first hydrogenation was performed at room temperature under a hydrogen pressure of 2 MPa without any prior heat treatment. The VO alloy displayed negligible hydrogen absorption during first hydrogenation, while the V, VN, and FeV alloys showed maximum hydrogen storage capacities of 3.49 wt%, 2.47 wt%, and 2.18 wt%, respectively. The presence of oxygen hinders the activation of the VO alloy and requires two activation cycles to activate the sample. A short-term cyclability analysis showed a reversible hydrogen storage capacity of 2 wt%. The change in enthalpy of hydrogen absorption and desorption for V alloy were calculated to be −36.73 ± 8.6 and 57.34 ± 8.3 kJ/mol H2, respectively. One of the prominent advantages of utilizing scrap raw materials was the substantial reduction in overall raw material costs, which saw an impressive 95% decrease when compared to using pure raw materials. These results highlight the economic benefits of incorporating recovered industrial scrap into hydrogen storage alloys.

Composition optimization and hydrogen storage properties of Ti–V–Mn–Fe alloys

Ti–V-based hydrogen storage alloys have garnered interest owing to their salient advantages of reversible hydrogen absorption/desorption at room temperature and excellent kinetic properties. However, the complicated smelting technology of high-purity V metal renders expensive prices, hampering their large-scale production and application. Herein, to overcome this hurdle, abundant and inexpensive Mn and Fe are added to obtain Ti–V–Mn–Fe hydrogen storage alloys. The novel alloy compositions were obtained using a uniform design method. The hydrogen absorption/desorption behaviors of alloys prepared using arc melting technology were analyzed in detail. Systematic investigations show that the activation, kinetics, and thermodynamic properties of alloys are closely correlated with the Ti/(Mn + Fe) ratios. The designed alloy compositions have a multiphase structure with a body-centered cubic (BCC) main phase. The lattice parameters of the BCC phase increase with increasing Ti/(Mn + Fe) ratio, and the large lattice parameters of alloys are beneficial to facilitate the diffusion of hydrogen atoms, resulting in fast kinetics. A reasonable Ti/(Mn + Fe) ratio positively affects the hydrogen storage properties. Among them, the Ti40V40Mn2Fe18 alloy with Ti/(Mn + Fe) = 2 achieved a hydrogen absorption capacity of 3.472 wt% at 298 K and an effective reversible hydrogen storage capacity of 1.485 wt% above 0.1 MPa at 363 K. This study provides a novel insight for designing Ti–V-based alloy compositions via the uniform design method.

Enhanced hydrogen storage properties of Ti40-xV45Zr15Crx alloys by dual-phase nanostructures of eutectic and spinodal decomposition

Multi-principal elements alloys (MPEAs) with body center cubic (BCC) structure have shown great potential in hydrogen storage. However, the multi-cycle activation process and the high desorption temperature have inhibited their applications. In this study, the Ti40-xV45Zr15Crx (x = 1, 2, 3 at. %) alloys with three different BCC phases are synergized, and high hydrogenation capacities of over 2 wt% are achieved in the first hydrogen absorption. Owing to the formation of spinodal decomposition nanostructure in Ti38V45Zr15Cr2 alloy, 90 % of total hydrogen capacity is reached in 30s at 250 °C under 1 MPa H2, which indicates no incubation time is needed for activation. The Ti38V45Zr15Cr2 alloy also shows the highest reversible hydrogen storage capacity with fastest kinetics at 200 °C. The hydrogenation reaction is controlled by the 1D diffusion process of H atoms according to JMAK model. The spinodal decomposition structure can accelerate the decomposition of H2 molecules and the diffusion of H atoms by providing extra energy from large lattice distortion. The high-density interfaces in nano-eutectic structure can serve as active sites for the nucleation of hydrides and fast paths for the diffusion of H atoms.

Enhanced hydrogen storage properties of MgH2 with the co-addition of LiBH4 and YNi5 alloy

MgH2, as one of the typical solid-state hydrogen storage materials, has attracted extensive attention. However, the slow kinetics and poor cycle stability limit its application. In this work, LiBH4 and YNi5 alloy were co-added as additives to MgH2 via ball milling, thereby realizing an excellent dehydrogenation performance and good cycle stability at 300 °C. The MgH2-0.04LiBH4-0.01YNi5 composite can release 7 wt.% of hydrogen in around 10 min at 300 °C and still have a reversible hydrogen storage capacity of 6.42 wt.% after 110 cycles, with a capacity retention rate as high as 90.3 % based on the second dehydrogenation capacity. The FTIR results show that LiBH4 can reversibly absorb and desorb hydrogen throughout the hydrogen ab/desorption process, which contributes a portion of the reversible hydrogen storage capacity to the MgH2-0.04LiBH4-0.01YNi5 composite. Due to the small amount of LiBH4 and YNi5, the dehydrogenation activation energy of MgH2 did not decrease significantly, nor did the dehydrogenation enthalpy (∆H) change. However, the MgNi3B2 and in-situ formed YH3 during the hydrogen absorption/desorption cycles is not only beneficial to the improvement of the kinetics performance for MgH2 but also improves its cycle stability. This work provides a straightforward method for developing high reversible hydrogen capacity on Mg-based hydrogen storage materials with moderate kinetic performance.

Thermodynamic improvement of lithium hydrides for hydrogen absorption and desorption by incorporation of porous silicon

Lithium is a popular lightweight material in the field of energy storage because of its hydrogen-binding properties and electrochemical advantages. The high hydrogen uptake capacity (∼12.6 wt%) of lithium hydride (LiH) is limited by the major thermodynamic constraint of requiring a higher temperature (∼700 °C) for desorption at 0.1 MPa. Incorporating a third element that creates Li phases during LiH dehydrogenation can help overcome thermodynamic constraints. This study involves modifying the hydrogen storage properties and thermodynamic characteristics of LiH through mechanical alloying with porous silicon (PS). Pressure composition isotherms measure the reversible hydrogen storage capacity (∼3.39 wt%) of LiH-PS alloy at different temperatures. The energy of hydrogen interaction is quantified by isosteric heat of absorption, which provides the enthalpy change in the reaction system. Hydrogen absorption and desorption enthalpies of 94.5 kJ (mol H2)−1 and 114.9 kJ (mol H2)−1 demonstrate the lowest energy demand among previously reported LiH alloys. The energy of hydrogen interaction is quantified by isosteric heat of absorption, which provides the enthalpy change in the reaction system. The hydride decomposition of the alloy indicates the possible range of hydrogen desorption.

Tailoring hydrogenation and anti-oxidation properties of titanium - iron - chromium alloys by regulating zirconium content at room temperature

The effects of Fe substitution by Zr on the microstructure, hydrogen storage properties and oxidation resistance of the TiFe0.85-xCr0.15Zrx (x = 0, 0.05, 0.10, 0.15) alloys are studied in this work. It is shown that all the alloys are consisted of TiFe phase, MgZn2-type phase and minor Ti phase. The introduction of Zr causes no change for the TiFe main phase. All the alloys absorb hydrogen directly under mild conditions without any activation process. As Zr content increases, the maximum hydrogen storage capacity of the first hydrogenation increases while the reversible hydrogen storage capacity decreases. The marked change of the first hydrogenation properties may be due to the formation of the bright phase (MgZn2-type phase). The decrease of the reversible hydrogen storage capacity is due to the formation of the stable hydrides Cr2ZrH3 and TiH2, which start to decompose at around 700 °C. During the air exposure test, the alloys with higher Zr content show good oxidation resistance. For the PCT measurement, the alloys with higher Zr show lower hydrogen absorption and desorption platform pressure, which means more stable hydrides are formed during the first hydrogenation. Moreover, the rate limiting step model of the first and fifth hydrogenation processes of all the alloys has also been investigated in detail.

Computational evaluation of lithium functionalized Penta-BN2 as promising reversible hydrogen storage

Finding advanced hydrogen storage materials with high capacity and high efficiency is a key issue for hydrogen transportation and application. In this study, the first-principles calculation method is used to examine the potential of 2D pentagonal BN2 (Penta-BN2) as an excellent hydrogen storage material. The results indicate that Penta-BN2 has superior metallic and structural stable properties. The pristine Penta-BN2 is found to have weak adsorption energy for H2 molecules, which cannot meet the requirements of hydrogen storage. However, the Li-functionalized Penta-BN2 substrate is found to be relatively excellent to adsorb H2 molecules. A Li atom is bound closely to Penta-BN2 with large adsorption energy reaching more than 3.79 eV, and the 2 × 2 × 1 Penta-BN2 supercell can be suitably accommodated 4 Li atoms. 4Li-functionalized Penta-BN2 stores up to 16H2 molecules with a hydrogen gravimetric density of 8.70 wt%. The adsorption and desorption capacity of the system for H2 molecules under practical temperature and pressure conditions are investigated with semi-empirical calculations. The maximum reversible hydrogen storage capacity is up to 7.92 wt% between the practical condition of 30 atm/233 K for adsorption to 3 atm/358 K for desorption, which is better than other 2D pentagonal materials and boron-nitrogen compounds. The proposed investigations of this work illustrate that Li-functionalized Penta-BN2 is a promising hydrogen storage material with high efficiency, reversibility, and high-capacity characteristics.

Enhancing Mg-Li alloy hydrogen storage kinetics by adding molecular sieve via friction stir processing

Mg-Li alloy is a lightweight hydrogen storage material with high hydrogen capacity, but its poor kinetics limited its practical applications. In this work, MCM-22 molecular sieve was added to Mg-Li alloy by friction stir processing (FSP) as the catalyst to enhance the kinetic properties of Mg-Li alloy (denoted as Mg-Li-MCM-22). The resulting Mg-Li-MCM-22 possesses the reversible hydrogen storage capacity of ca. 6 wt.% and can release 5.62 wt.% hydrogen within 50 min at 623 K, showing improved kinetics. The Chou model and Johnson–Mehl–Avrami–Kolmogorov (JMAK) model calculations reveal that the lattice defects generated by FSP improve the kinetics of hydrogen adsorption/desorption. The pinning effect of MCM-22 particles produces more grain boundaries and dislocations, thus, increasing the diffusion rate of hydrogen atoms and providing more nucleation sites, therefore, reducing the dehydrogenation activation energy. This work provides a new strategy for the preparation of hydrogen storage materials.

Tuning the hydrogen thermodynamics of NaAlH4 by encapsulation within a titanium shell

The use of NaAlH4 as a practical hydrogen storage material has seen extensive progress. However, it is still difficult to control the hydrogen storage properties of NaAlH4 and in particular its hydrogen thermodynamics. Herein, we demonstrate the potential of core-shell NaAlH4@Ti nanostructures in altering the hydrogen storage properties of NaAlH4. To this aim, a facile solvent evaporation method was developed to enable the making of freestanding NaAlH4 nanoparticles and their individual encapsulation within a Ti shell. These core-shell nanostructures enabled: (i) an effective confining environment for NaAlH4 and its dehydrogenation products, and (b) fast hydrogen kinetics due to a weakening of the Al–H bond and the shorter diffusion lengths. At 180 °C, a reversible hydrogen storage capacity of 4.4 mass% was observed. More remarkably, this core-shell approach enabled a shift in the equilibrium plateau pressure, therefore demonstrating the possibility to control the hydrogen thermodynamics of NaAlH4 at the nanoscale.

Evaluating some design criteria for TiFe-based ternary hydrogen storage alloys

Elemental substitution is widely practised to fabricate TiFe-based alloys with enhanced hydrogen storage characteristics. While some hydrogen storage properties can be improved others are usually adversely affected by this method. Hence, establishing sensible alloying design criteria is important for further developing Ti-Fe-based alloys. Here, the effects of substitutional elements in Ti-Fe-based ternary alloys are assessed based on several factors such as hydrogen bond dissociation energy, lattice parameters, interstitial site size, and intermetallic phase formation. Our calculations show that all the substitutional elements studied in this work (except Ni and Co) increase the lattice parameter compared to the TiFe alloy, while they have a diverse effect on the interstitial site sizes. The thermodynamic stability of the alloys could not be adequately described based on the lattice parameter and factors such as interstitial site sizes along the local atomic environment should be also considered. Hydrogen storage capacity of the Ti-Fe-based ternary alloys also appears to be influenced by the metal-hydrogen bond dissociation energy of the substitutional elements and their capability to form intermetallic phases with Ti. TiFe-based ternary alloys were modified by V and Zr substitutions according to the above criteria. The modified alloys were mostly composed of the B2 TiFe phase, offering significantly easier activation and a higher reversible hydrogen storage capacity compared to the binary alloy.

A significantly improved hydrogen storage performance of nanocrystalline Ti–Fe–Mn–Pr alloy

Improving the activation performance and increasing the hydrogen storage capacity are the main problems faced by TiFe-based alloys. In this paper, alloys with chemical composition Ti1.1-xPrxFe0.8Mn0.2 (x = 0, 0.02, 0.04, 0.06, 0.08) are designed to achieve a reversible hydrogen storage capacity of 1.81 wt% in 15 min, which is close to the theoretical hydrogen storage capacity of TiFe alloy of 1.86 wt%. Alloys with Pr content x ≥ 0.04 can be directly activated without the incubation period. Transmission electron microscopy analysis found that doping Pr introduced TiFe (111) crystal plane with smaller surface energy, makes it easy for hydrogen atoms to diffuse from the surface to interior. Moreover, the addition of Pr significantly refines the grains and introduces a large number of grain boundaries and defects, improving the activation rate and kinetics. The physisorption results show that the specific surface area of the TiFe-based alloy after hydrogen absorption is significantly increased by doping Pr.

Synergistic Catalytic Effects of AlH3–TiF3 Composites on the Hydrogen Storage Performance of MgH2

We report the synergistic catalytic effects of AlH3–TiF3 composites that can improve the kinetic properties of Mg-based hydrogen storage materials. The resulting MgH2–AlH3–TiF3 composites (MA-F) exhibit 5.55 wt % reversible hydrogen storage capacity with a reduced activation energy of 100.85 kJ/mol H2. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements revealed that the in situ generated Al* reacts with TiF3 and Mg, forming Al3Ti, AlF3, and Mg17Al12 during the heating process. Al3Ti and AlF3 act as the actual catalytically active centers for the hydrogen desorption and absorption processes, accelerating the reversible conversion between Mg and MgH2. Moreover, the presence of Mg17Al12 can narrow the free energy gap between the hydrogenated and dehydrogenated states, favoring the improvement of thermodynamic properties. These discoveries demonstrate the importance of the interaction between Mg/MgH2 and transition metal composites to improve the hydrogen storage functionalities.

Solar-Driven Reversible Hydrogen Storage.

The lack of safe and efficient hydrogen storage is a major bottleneck for large-scale application of hydrogen energy. Reversible hydrogen storage of light-weight metal hydrides with high theoretical gravimetric and volumetric hydrogen density is one ideal solution but requires extremely high operating temperature with large energy input. Herein, taking MgH2 as an example, a concept is demonstrated to achieve solar-driven reversible hydrogen storage of metal hydrides via coupling the photothermal effect and catalytic role of Cu nanoparticles uniformly distributed on the surface of MXene nanosheets (Cu@MXene). The photothermal effect of Cu@MXene, coupled with the "heat isolator" role of MgH2 indued by its poor thermal conductivity, effectively elevates the temperature of MgH2 upon solar irradiation. The "hydrogen pump" effect of Ti and TiHx species that are in situ formed on the surface of MXene from the reduction of MgH2 , on the other hand, plays a catalytic role in effectively alleviating the kinetic barrier and hence decreasing the operating temperature required for reversible hydrogen adsorption and desorption of MgH2 . Based on the combination of photothermal and catalytic effect of Cu@MXene, a reversible hydrogen storage capacity of 5.9wt% is achieved for MgH2 after 30 cycles using solar irradiation as the only energy source.

Reversible hydrogen storage capacity of vanadium decorated small boron clusters (BnV2, n = 6–10): A dispersion corrected density functional study

We present our theoretical investigation on hydrogen storage capacity of vanadium decorated small boron clusters (BnV2, n = 6–10) employing dispersion-corrected density functional theory. Stabilities of the clusters during H2 adsorption are confirmed from the global reactivity parameters. BnV2 clusters are found to adsorb up to ten H2 molecules in quasi-molecular form via Kubas-Niu-Rao-Jena kind of interactions with average adsorption energy in the range of 0.17–0.26 eV/H2. ADMP molecular dynamics simulation reveals the thermal stability, structural integrity and reversibility of BnV2, n = 6–10 at room temperature (300 K). The maximum practical hydrogen storage capacity at temperature up to 80 K and pressure ranges of 1–60 bar are found to store up to 8.75–10.78 wt% which is well above the target set by US-DOE (5.5 wt% by 2025). The results obtained from our investigations assure the potential of vanadium decorated small boron clusters for reversible hydrogen storage.

Reversible hydrogen storage capacity of Li and Sc doped novel C 8 N 8 cage: Insights from density functional theory

In this work, we designed and addressed the hydrogen storage capacities of Li and Sc doped novel C8N8 cages using dispersion corrected density functional theory (DFT-D3). The stabilities of C8N8Li2 and C8N8Sc2 cages were confirmed by chemical hardness, HOMO-LUMO gaps, and molecular dynamic simulations. Both C8N8Li2 and C8N8Sc2 could adsorb 5H2 molecules with an average hydrogen adsorption energy of 0.187 and 0.291 eV/H2, keeping the original structure intact. The H2 molecules interacted with the Li/Sc via Niu-Rao-Jena and Kubas type interactions, and the charge re-distribution in H2 molecules in C8N8Li2-nH2 and C8N8Sc2-nH2 showed that H2 molecules were adsorbed in a quasi-molecular fashion. The molecular dynamics simulation revealed the thermal stability and structural integrity of Li and Sc decorated C8N8 cages at a relatively high temperature of 400 K, and at 300 K most of the H2 molecules were desorbed from the host material, keeping their initial structure intact, which confirmed their reversibility. The practical H2 storage capacities of C8N8Li2 and C8N8Sc2 cages at temperature and pressure ranges of 40 to 160 K, 40 to 180 K, and 10 to 60 bar were found to be 8.32 wt%, and 6.33 wt%, which were fairly above the target of US-DOE (5.5 wt% by 2025). At the temperature and pressure range of 220 to 280 K and 30 to 60 bar, the gravimetric density of both the cages was approximately 5.5 wt%. Hence, the newly designed C8N8Li2 and C8N8Sc2 cages can be considered as potential candidates for reversible hydrogen storage systems.