Yttrium doped covalent triazine frameworks as promising reversible hydrogen storage material: DFT investigations

Through Density Functional Theory (DFT) simulations, we have explored the possibility of yttrium (Y) doped Triazine (Covalent Triazine Frameworks i.e., CTF-1) to be a promising material for reversible hydrogen storage. We have found that Y atom strongly bonded on Triazine surface can adsorb at the most 7H₂ molecules with an average binding energy of ?0.33 eV/H₂. This boosts the storage capacity of the system to 7.3 wt% which is well above the minimum requirement of 6.5 wt% for efficient storage of hydrogen as stipulated by the US Department of Energy (DoE). The structural integrity over and above the desorption temperature (420 K) has been entrenched through Molecular Dynamics simulations and the investigation of metal-metal clustering has been corroborated through diffusion energy barrier computation. The mechanism of interactions between Y and Triazine as well as between H₂ molecules and Y doped Triazine has been explored via analyses of the partial density of states, charge density, and Bader charge. It has been perceived that the interplay of H₂ molecules with Y on Triazine is Kubas-type of interaction. The above-mentioned analysis and outcomes make us highly optimistic that Y doped Triazine could be employed as reversible hydrogen storage material which can act as an environmentally friendly alternate fuel for transport applications.     

High-capacity hydrogen storage in zirconium decorated zeolite templated carbon: Predictions from DFT simulations

The hydrogen (H₂) storage capacity of Zirconium (Zr) decorated zeolite templated carbon (ZTC) has been investigated using sophisticated density functional theory (DFT) simulations. The analysis shows that the Zr atom gets bonded with ZTC strongly with binding energy (BE) of ?3.92 eV due to electron transfer from Zr 4d orbital to C 2p orbital of ZTC. Each Zr atom on ZTC can attach 7H₂ molecules with average binding energy of ?0.433 eV/H₂ providing gravimetric wt% of 9.24, substantially above the limit of 6.5 wt% set by the DoE of the United States of America. The H₂ molecules are involved via Kubas interaction with Zr atom, which involves the charge transfer between Zr 4d orbital and H 1s orbital with interaction energy higher than physisorption but lower than chemisorption. The structural integrity of the system is confirmed via molecular dynamics (MD) simulations at room temperature and at highest desorption temperature of 500 K. We have investigated the chances of metal clustering by computing diffusion energy (E_D) barrier for the movement of Zr atom, and we obtained via calculation, we can infer that the presence of E_D barrier of ～2.36 eV may prevent the possibility. As the system ZTC has been synthesized, Zr doped ZTC is stable, existence of sufficient diffusion barrier prevents the clustering and adsorption energy and wt% of H₂ are within the range prescribed by DoE, we feel that Zr decorated ZTC can be fabricated as promising hydrogen storage material for fuel cell applications.     

High hydrogen storage ability of a decorated g-C3N4 monolayer decorated with both Mg and Li: A density functional theory (DFT) study

In this work, the hydrogen storage properties of a g-C₃N₄ monolayer decorated with both Mg and Li were thoroughly investigated by performing density functional theory (DFT) calculations. Along these lines, the projected densities of states (PDOS) and the Bader Charge analysis showed that both Mg and Li atoms can transfer their electronic charges to the g-C₃N₄ monolayer. Interestingly, the latter is transformed from a semiconductor material to a metallic conductor configuration, while a local electric field is formed around it. On top of that, the formed local electric field polarized hydrogen molecules and as a result, led to an enhanced hydrogen adsorption ability. Mg atoms have more outmost electrons, and more charges can be transferred to the monolayer, which leads to the creation of a stronger local electric field to adsorb an elevated number of hydrogen molecules than Li atoms. On the other hand, Li atoms are lighter, more active and easier to lose outmost electrons than Mg atoms. By considering these advantages, a g-C₃N₄ monolayer decorated with one unit of both Mg and Li was investigated, which has the ability to adsorb 10 hydrogen molecules, leading thus to a high hydrogen storage capacity of 10.01 wt %. Our work paves the way for the development of novel material configurations for improved hydrogen storage applications.     

Ultrahigh reversible hydrogen storage in K and Ca decorated 4-6-8 biphenylene sheet

By applying density functional theory (DFT) and ab-initio molecular dynamics (AIMD) simulations, we predict the ultrahigh hydrogen storage capacity of K and Ca decorated single-layer biphenylene sheet (BPS). We have kept various alkali and alkali-earth metals, including Na, Be, Mg, K, Ca, at different sites of BPS and found that K and Ca atoms prefer to bind individually on the BPS instead of forming clusters. It was found that 2?2?1 supercell of biphenylene sheet can adsorb eight K, or eight Ca atoms, and each K or Ca atom can adsorb 5H_2, leading to 11.90% or 11.63% of hydrogen uptake, respectively, which is significantly higher than the DOE-US demands of 6.5%. The average adsorption energy of H₂ for K and Ca decorated BPS is ?0.24 eV and ?0.33 eV, respectively, in the suitable range for reversible H₂ storage. Hydrogen molecules get polarized in the vicinity of ionized metal atoms hence get attached to the metal atoms through electrostatic and van der Waals interactions. We have estimated the desorption temperatures of H₂ and found that the adsorbed H₂ can be utilized for reversible use. We have found that a sufficient energy barrier of 2.52 eV exists for the movement of Ca atoms, calculated using the climbing-image nudged elastic band (CI-NEB) method. This energy barrier can prevent the clustering issue of Ca atoms. The solidity of K and Ca decorated BPS structures were investigated using AIMD simulations.     

Li decorated penta-silicene as a high capacity hydrogen storage material: A density functional theory study

The H₂ adsorption characteristics of Li decorated single-sided and double-sided penta-silicene are predicted via density functional theory (DFT). The orbital hybridization results in Li atom strongly bind onto the surface of the penta-silicene with a large binding energy and it keeps the decorated Li atoms from aggregation. Moreover, Li decorated double-sided penta-silicene can store up to 12H₂ molecules with the average hydrogen adsorption energy of ?0.220 eV/H₂ and hydrogen uptake capacity of 6.42 wt%, respectively. The ab initio molecular dynamics (AIMD) simulations demonstrate the H₂ molecules are released gradually from the substrate material with the increasing simulation time and the calculated desorption temperature T_D is 281 K in the suitable operating temperature range. Our explorations confirm that Li decorated penta-silicene can be regarded as a promising hydrogen storage candidate for hydrogen storage applications.     

Release of chemisorbed hydrogen from carbon nanotubes: Insights from ab-initio molecular dynamics simulations

The dynamics and energetics related to the release of chemisorbed hydrogen from small-diameter single-walled carbon nanotubes is investigated by first-principles molecular dynamics simulations. Our results suggest a possible route for thermally-activated desorption of hydrogen from the nanotube sidewall, leading to formation of molecular H₂, and shed light on the basic mechanisms of the reversible storage of hydrogen in carbon nanotubes. In agreement with recent experiments, simulations indicate carbon nanotubes as suitable materials for the reversible storage of hydrogen. Moreover, calculations point to the restoration of the π bond patterning of the sidewall as the driving force for the desorption of hydrogen from carbon nanotubes.     

Hydrogen storage in scandium decorated triazine based g-C3N4: Insights from DFT simulations

Hydrogen is being considered a ‘fuel of the future,’ a viable alternative to fossil fuels in fuel cell vehicles. Using Density Functional Theory simulations, reversible, onboard hydrogen storage in Sc-decorated triazine-based graphitic carbon nitride (g-C₃N₄) has been explored. Sc atom binds strongly on the g-C₃N₄ structure with a binding energy of ?7.13 eV. Each Sc atom can reversibly bind 7 molecules of hydrogen, giving a net gravimetric storage capacity of 8.55 wt%, an average binding energy of ?0.394 eV per H₂, and a corresponding desorption temperature of 458.28 K, fulfilling the criteria prescribed by the US Department of Energy. The issue of transition metal clustering has been investigated by computing the diffusion energy barrier (2.79 eV), which may be large enough to hinder the clustering tendencies. The structural integrity of Sc-g-C₃N₄ has been verified through ab-initio Molecular Dynamics simulations. The interaction mechanism of Sc over g-C₃N₄ and H₂ over Sc-g-C₃N₄ has been explored using density of states and charge transfer analysis. A flow of charge from valence 3d orbitals of Sc towards vacant orbitals of g-C₃N₄ during the binding of Sc over g-C₃N₄ is observed. The binding of H₂ on Sc-g-C₃N₄ may be via Kubas type of interactions which is stronger than physisorption due to net charge gain by H 1s orbital from Sc 3d orbital. Our systematic investigations indicate that Sc-decorated g-C₃N₄ may be a high-performance material for reversible hydrogen storage applications.     

A single tiny Tin(O2)n cluster decorated an ultra-small boron nitride nanotube for hydrogen storage material: A density functional theory study

The adsorption of hydrogen (H₂) on tiny titanium dioxide Ti_n(O₂)_n clusters where n = 1, 2 and 3 decorated a (5, 5) ultra-small boron nitride nanotube (BNNT) is studied theoretically using the density functional theory calculation. Ti_n(O₂)_n/BNNT is very stable and it can hold a large number of H₂ molecules while maintaining its stability. That H₂ adsorption on Ti_n(O₂)_n/BNNT/BNNT shifts the geometry of Ti_n(O₂)_n/BNNT as the number of H₂ molecules adsorbed on its surface increased. For example, the bond between N–O increases while the bond between the H atoms in the H₂ molecules shortens. Furthermore, the local density of states (LDOS), crystal orbital overlaps population (COOP), and charge distribution analysis all confirm that H₂ formed a bond with TiO₂/BNNT. Thus, we can conclude that Ti_n(O₂)_n/BNNT is a promising material for hydrogen storage.     

Simultaneously engineering K-doping and exfoliation into graphitic carbon nitride (g-C3N4) for enhanced photocatalytic hydrogen production

Doping and exfoliation are effective strategies to improve the photocatalytic activity of bulk graphitic carbon nitride (g-C₃N₄). Therefore, it can be inferred that engineering element-doping and exfoliation into g-C₃N₄ would further enhance the photocatalytic performance. Herein, we demonstrated a KOH-assisted hydrothermal-reformed melamine strategy for achieving the simultaneous K-doping and exfoliation of g-C₃N₄. The as-synthesized K-doped g-C₃N₄ ultrathin nanosheets displayed much enhanced photocatalytic hydrogen evolution rate (HER) of about 13.1 times higher than that of the bulk g-C₃N₄ under visible-light irradiation, achieving an apparent quantum efficiency of 6.98% at 420 nm. The improved photocatalytic HER can be attributed to the high surface area offering numerous photocatalytic active sites, enlarged conductive band edge optimizing photoreduction potential, and K-doping promoting charge generation and separation as well as the long life-time of photogenerated carriers. This work would provide a promising way to integrate co-doping and exfoliation into new gC₃N₄based materials.     

Geometric structures and hydrogen storage properties of M2B7 (M=Be,Mg, Ca) clusters

Based on the density functional theory, we investigate the electronic properties of the clusters M₂B₇ (M = Be, Mg, Ca) and their hydrogen storage properties systematically in this paper. Extensive global search results show that the global minimal structures of the three systems (Be₂B₇, Mg₂B₇ and Ca₂B₇) are heptagonal biconical structure, and the two alkaline earth metals are located at the top of the biconical. Chemical bonding analyses show that M₂B₇ clusters have 6σ and 6π delocalized electrons, which are doubly aromatic. At the wB97XD level, the three systems have good hydrogen storage capabilities. The hydrogen storage density of Be₂B₇ is as high as 23.03 wt%, while Mg₂B₇ and Ca₂B₇ also far exceed the hydrogen storage target set by the U.S. Department of Energy in 2017. Their average adsorption energies of H₂ molecules all ranged from 0.1 eV/H₂ to 0.48 eV/H₂, which is fall in between physisorption and chemisorption. Extensive Born Oppenheimer molecular dynamics (BOMD) simulations show that the H₂ molecules of the three systems can be completely released at a certain temperature. Therefore, M₂B₇ systems can achieve reversible adsorption of H₂ molecules at normal temperature and pressure. It can be seen that the B₇ clusters modified by alkaline earth metals may become a promising new nano-hydrogen storage material.     

Computational exploration of magnesium-decorated carbon nitride (g-C3N4) monolayer as advanced energy storage materials

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C₃N₄, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C₃N₄ monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from ?0.25 eV to ?0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C₃N₄ is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.     

An efficient Z-scheme (Cr,B) codoped g-C3N4/BiVO4 photocatalyst for water splitting: A hybrid DFT study

A systematic theoretical research on the geometrical, electronic, optical, charge transfer, and photocatalytic mechanisms of pure, Cr-doped, B-doped, and (Cr, B) codoped g-C₃N₄/BiVO₄ heterostructures using a hybrid density functional approach has been carried out. The face-to-face g-C₃N₄/BiVO₄ composed of two-dimensional materials of g-C₃N₄ and BiVO₄ (010) surface, can introduce a built-in electric field, which promotes interface charge transfer and prevents the electron-hole pair recombination, and causing g-C₃N₄ monolayer with negative charge and BiVO₄ (010) surface with positive charge. Under visible light irradiation, electrons are excited to the conduction band minimum (CBM) of the BiVO₄ (010) surface undergoing the hydrogen evolution reaction (HER), while the holes remain in the valence band maximum (VBM) of g-C₃N₄ monolayer aiding the oxygen evolution reaction (OER). The band edge potentials of BiVO₄ (010) surface is higher than that of g-C₃N₄ monolayer, which ensures a stronger redox reaction potential and therefore belongs to a typical Z-scheme heterostructure. In addition, the Cr or/and B (co)doping introduces the Cr-3d or/and B-2p states to reduce the bandgap and generate impurity levels, thus enhancing solar energy utilization rate and expanding the optical absorption in the visible-light range. The optical absorption intensity of the (Cr, B) codoped g-C₃N₄/BiVO₄ is superior to pure and Cr or B doped g-C₃N₄/BiVO₄, confiriming the synergistic effect of Cr-3d and B-2p states. Thus, this research is helpful to design a novel and potential Z-scheme photocatalyst useful for the photocatalytic water splitting.     

CaH2-assisted structural engineering of porous defective graphitic carbon nitride (g-C3N4) for enhanced photocatalytic hydrogen evolution

The practical applications of graphitic carbon nitride (g-C₃N₄) for photocatalytic hydrogen evolution is strictly hindered by the low surface area, poor light harvesting capability and detrimental recombination of photoexcited charge carriers. Herein, using melamine as precursor and metal hydride (i.e., CaH₂) as active agent, we facilely incorporate various types of defects (i.e., nitrogen (N) vacancies (V_N), cyano groups (CN) and surface absorbed oxygen species(O_abs)) into g-C₃N₄ within a single step. The as-prepared material (denoted as MM-H) exhibits narrowed bandgap, promoted photoexcited electron-hole separation rate and facilitated charge transfer kinetics with enlarged BET surface area and massive porosity. As a result, a prominently enhanced photocatalytic H₂ productivity efficiency (1305.9 μmol h⁻¹g⁻¹) is shown on MM-H. This performance is better than that of g-C₃N₄ with CaH₂ post-treatment (617.3 μmol h⁻¹g⁻¹) and raw bulk-C₃N₄ (178.2 μmol h⁻¹g⁻¹). This work opens up a new dimension for designing high performance g–C₃N₄–based catalysts targeting various photocatalytic processes.     

Remarkable improvement in hydrogen storage capacities of two-dimensional carbon nitride (g-C3N4) nanosheets under selected transition metal doping

We have performed DFT simulations to quest for an optimal material for onboard hydrogen (H₂) storage applications. Using first-principles calculations, we established that the selected transition metals (M: Sc, Ti, Ni, V) decorated two-dimensional (2D) g-C₃N₄ sheets as optimal materials with reversible and significantly high H₂ gravimetric densities. By effectively avoiding metal-metal (M-M) clustering effect in case of mono doping, up to four molecules of H₂ per dopant could be adsorbed with an average binding energy of around 0.30–0.6 eV/H₂, which is ideal for practical applications. Decorating the g-C₃N₄ sheet with (M-M) dimers, the systems are found to be even more efficient for H₂ binding than single dopant decoration. The stability of these M decorated g-C₃N₄ sheets have been confirmed with ab-initio molecular dynamics simulations. We have further calculated the H₂ desorption temperatures of metal decorated g-C₃N₄ sheets, which confirms the practical application of these metal decorated sheets at ambient working conditions.     

Single transition metal atom anchored on g-C3N4 as an electrocatalyst for nitrogen fixation: A computational study

Electrocatalytic nitrogen reduction reaction (NRR) provides a green and sustainable way to produce ammonia at ambient conditions. The key to realize highly efficient NRR is the catalysts. To design highly active electrocatalysts for NRR, the multistep mechanism involved in NRR must be clearly unraveled. Herein, single V atoms anchored on g-C₃N₄ is identified to be an efficient electrocatalyst for NRR by screening single 3d transition metal (TM = Sc to Zn) atoms anchored by g-C₃N₄ (TM@g-C₃N₄) through density functional theory calculations. NRR takes place on V@g-C₃N₄ preferentially through distal path with a relatively low limiting potential of ?0.55 V. The outstanding NRR performance of V@g-C₃N₄ is found from the peculiar electronic structure of V after anchored in the six-fold cavity of g-C₃N₄ and the good transmitter role of V for electron transfer between N_xH_y species and g-C₃N₄. Moreover, the formation energy and dissolution potential indicate that V@g-C₃N₄ is thermodynamically and electrochemically stable and the aggregation of V atoms is unfavorable thermodynamically, signifying that the synthesis of V@g-C₃N₄ is feasible in experiments. Our work screens out a superior noble metal-free NRR electrocatalyst and will be helpful for the development of ambient artificial nitrogen fixation.     

Superior hydrogen storage capacity of Vanadium decorated biphenylene (Bi+V): A DFT study

Herein, the hydrogen storage competency of vanadium-decorated biphenylene (Bi+V) has been investigated using Density Functional Theory simulations. The metal atom interacts with biphenylene with a binding energy value of −2.49 eV because of charge transfer between V 3d and C 2p orbitals. The structure and electronic properties are studied in terms of adsorption energy values, the spin-polarized partial density of states (PDOS), band structure plots, and charge transfer analysis. The Kubas-type interactions lead to average hydrogen adsorption energy values of −0.51 eV/H₂ which fulfills DOE-US criteria (0.2–0.7 eV/H₂). The diffusion energy barrier value of 1.75 eV lowers the chances of metal clustering. The complex binds 5H₂ on each V-atom resulting in a storage capacity of 7.52 wt% with an average desorption temperature of 595.96 K. The ab-initio molecular dynamics (AIMD) and phonon dispersions validates structural integrity at higher temperatures suggesting the excellent storage properties of this material at room temperature.     

K and halogen binary-doped graphitic carbon nitride (g-C3N4) toward enhanced visible light hydrogen evolution

Water splitting driven by solar energy to produce hydrogen, which is highly dependent on the designing of semiconductor photocatalyst, is an efficient technology to address energy shortage problems and environment issues simultaneously. Here, the halogen and potassium binary-doped graphitic carbon nitride (named as X-K-C₃N₄, X = F, Cl, Br, I) photocatalysts were synthetized via simply one pot thermal polymerization method, which shown optimized band structure, enhanced optical absorption, higher separation rate of photogenerated carriers, and thus improved photocatalytic performance under visible light irradiation. As result, F–K–C₃N₄ is demonstrated to be highly efficient in the separation and transfer of carriers owing to the existence of C–F bond, CN triple bond and K junction. The F–K–C₃N₄ shows a highest H₂ evolution rate of 1039 μmol g⁻¹ h⁻¹ and a remarkable stability under visible light irradiation (λ ≥ 420 nm), which is about 8.5 times higher than that of pristine g-C₃N₄.     

Vanadium (V) and Niobium (Nb) as the most promising co-catalysts for hydrogen sulfide splitting screened out from 3d and 4d transition metal single atoms

The options of transition metals as co-catalysts for photocatalytic H₂S splitting are restricted to some noble metals and related compounds which have noticeable achievements despite their high prices. Substituting with cheap transition metals and downsizing the size to single atom level are economic ways to lower the cost. Herein, the s-triazine graphite-like carbon nitride sheet g-C₃N₄ (001) is chosen as the model to study the performances of 3d and 4d transition metal single atoms (TMSA = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd) in H₂S splitting based on density functional theory (DFT) calculations. It is found that low-cost transition metals with industrial relevance (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Tc, Cd) are completely comparable with noble metals (Ru, Rh, Pd, Ag). Among them, V and Nb are the most promising co-catalysts with good thermodynamic stabilities, favorable responses to visible light, high photoinduced electron-hole separation efficiencies, sufficient potentials for H₂S splitting, and low energy barriers for H₂S dissociation into H₂ and S. The noticeable improved activities of V/g-C₃N₄ and Nb/g-C₃N₄ are attributed to the formation of strong interfacial chemical bonds which could promote electrons transferring to H₂S derivates. In addition, the introduction of photoinduced electrons could further improve the activities of V/g-C₃N₄ and Nb/g-C₃N₄ with more electrons transferring to H₂S derivates. It is expected that this work could provide a helpful guidance to choose appropriate TMSA co-catalysts as references for H₂S splitting.     

Dehydrogenation of a liquid organic hydrogen carrier compound 1-(3-cyclohexylpropyl)-3-ethylcyclohexane: A density functional theory study

As a compound for liquid organic hydrogen carrier (LOHC) applications, 1-(3-cyclohexylpropyl)-3-ethylcyclohexane was designed and its dehydrogenation reaction was investigated using density functional theory calculations. To check how this compound could be stable, vibrational frequency analysis and formation energy calculations were conducted. Our findings revealed that this LOHC compound was dynamically and chemically stable. Using Mulliken population analysis, the dehydrogenation process was clearly explained. To reduce the dehydrogenation energy, different substituents, such as N, Cl, and Br were used. Our results suggested that N-substitution could be potentially suitable to lower the dehydrogenation energy. Reaction barriers of pristine and N-substituted systems for dehydrogenation reactions were investigated through nudged elastic band methods. In addition, the gap between HOMO and LUMO was calculated to check chemical reactivity.