Reaction coordinate mapping of hydrogen evolution mechanism on Mg3N2 monolayer

In this work, we have envisaged the hydrogen evolution reaction (HER) mechanism on Mg₃N₂ monolayer based on electronic structure calculations within the framework of density functional theory (DFT) formalism. The semiconducting nature of Mg₃N₂ monolayer motivates us to investigate the HER mechanism on this sheet. We have constructed the reaction coordinate associated with HER mechanism after determining the hydrogen adsorption energy on Mg₃N₂ monolayer, while investigating all possible adsorption sites. After obtaining the adsorption energy, we subsequently obtain the adsorption free energy while adding zero point energy difference (ΔZPE) and entropic contribution (TΔS). We have not only confined our investigations to a single hydrogen, but have thoroughly observed the adsorption phenomena for increasing number of hydrogen atoms on the surface. We have determined the projected density of states (DOS) in order to find the elemental contribution in the valence band and conduction band regime for all the considered cases. We have also compared the work function value among all the cases, which quantifies the amount of energy required for taking an electron out of the surface. The charge transfer mechanism is also being investigated in order to correlate with the HER mechanism with amount of charge transfer. This is the first attempt on this material to the best of our knowledge, where theoretical investigation has been done to mapping the reaction coordinate of HER mechanism with the associated charge transfer process and the work function values, not only for single hydrogen adsorption, but also for increasing number of adsorbed hydrogen.     

A new hydrogen storage system based on efficient reversible catalytic hydrogenation/dehydrogenation of terphenyl

Noble metal catalysts on mesoporous SiO₂ and modified carbon supports were found to enhance the activities of terphenyl (TPh) hydrogenation and tercyclohexane (TCH) dehydrogenation without side reactions, such as cracking, hydrogenolysis, ring opening and/or coke formation. The noble metal catalysts could be used for a reversible hydrogen storage system. Five percent Pt/SiO₂ catalyst was highly active in TCH dehydrogenation without stirring, due to an easier diffusion of organic molecules to the small catalyst particles during dehydrogenation.     

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.     

Superdense state of the monolayer hydrogen on adsorbent under liquefied temperature

A systematic examination of the cryogenic H₂ adsorption properties below critical point of H₂ (T_c = 33 K) on various kind of adsorbents was carried out, then the density of adsorbed H₂ on each adsorbent and its temperature dependence were experimentally compared with the liquid H₂. Usually, H₂ adsorption capacities on porous materials were investigated at liquid nitrogen temperature (77 K) to realize its practical application for H₂ storage and most of reports have focused on the development of new porous materials. On the other hand, the density of adsorbed H₂ below critical point of H₂ was investigated for metal-organic frameworks, super-activated carbon, and graphene nanoplatelets in this study. We found the superdense H₂ adsorption as the monolayer state, having much higher density than liquid H₂.     

Improving the hydrogen storage capacity of metal organic framework by chemical functionalization

The hydrogen storage capacity of transition metal decorated terphenyl linkers was investigated using density functional theory based M05-2X, M06 and wB97XD methods. The –OH and –SH groups are used as anchors to bind various transition metals such as Sc, Ti, V, and Cr on terphenyl linker. It has been found that each transition metal can bind four hydrogen molecules through Kubas interaction. The correlation between electron density at the bond critical point corresponding to H–H bond and concomitant intermolecular distances between transition metal and hydrogen molecules has been used to illustrate the Kubas mechanism. Further, to estimate the bulk storage capacity, 42 hydrogen molecules are allowed to interact with the new metal organic framework fragment in all possible binding sites. The calculated interaction energy per hydrogen molecule is found to be −3.38 kcal/mol. Comparison of this value with previous reports shows that this energy is suitable for room temperature hydrogen storage applications.     

First principles study on desorption of chemisorbed hydrogen atoms from single-walled carbon nanotubes under external electric field

Based on first principles calculations, we suggest a new way to release chemisorbed hydrogen atoms from the single-walled carbon nanotubes. The transverse external electric field serves as the key source for this mechanism. As the electric field strength increases, there is a notable increase in the C–H bond lengths at the sharp highly curved sites of the carbon nanotube. The compression of the tube along the direction of electric field results in the release of hydrogen atoms from the sharp sites. The synergetic action of electric field and compression turns out the desorption process easier at favorable field strengths. This present investigation provides a new alternative way to release hydrogen atoms from the hydrogenated carbon nanotubes under the influence of external electric field that requires comparatively lower temperature suitable for hydrogen based fuel cells used in mobile applications.     

Enhancement of hydrogen binding affinity with low ionization energy Li2F coating on C60 to improve hydrogen storage capacity

The capacity of hydrogen storage for solid sorbents depends strongly on the binding affinity between hydrogen molecules and solid sorbents. By coating C₆₀ with a low ionization energy material (Li₂F), we obtained an enhanced binding energy and an improved electron transfer between H₂ and hosts. With the first-principles calculations and charge analysis, we found that the orbital interactions play a dominant role in this system and eventually 68H₂ molecules can be stably stored by a C₆₀(Li₂F)₁₂ cluster with a binding energy of 0.12 eV/H₂. The resulting gravimetric and volumetric density of H₂ stored on C₆₀(Li₂F)₁₂ are 10.86 wt% and the 59 g/L through calculations. Our investigation indicates that metals or metal clusters with lower ionization energies would be beneficial to enhance interactions between hydrogen and hosts, and thus, the hydrogen storage capacities for solid sorbents can be greatly improved.     

Progress of graphene and loaded transition metals on Mg-based hydrogen storage alloys

Mg-based hydrogen storage alloys have become a research hotspot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, its high thermodynamic stability and slow kinetics limit its application, so the modification of Mg-based hydrogen storage alloys has become the development direction of Mg-based alloys. Transition metals can be used as catalysts for the dehydrogenation of hydrogen storage alloys due to their excellent structural, electrical, and magnetic properties. Graphene, because of its unique sp² hybrid structure, excellent chemical stability, and a specific surface area of up to 2600 m²/g, can be used as a support for transition metal catalysts. In this paper, the internal mechanism of graphene as a catalyst for the catalysis of Mg-based hydrogen storage alloys was analyzed, and the hydrogen storage properties of graphene-catalyzed Mg-based hydrogen storage alloys were reviewed. The effects of graphene-supported different catalysts (transition metal, transition metal oxides, and transition metal compounds) on the hydrogen storage properties of Mg-based hydrogen storage alloys were also reviewed. The results showed that graphene played the roles of catalysis, co-catalysis, and inhibition of grain aggregation and growth in Mg-based hydrogen storage materials.     

Enhancement of hydrogen storage performance in shell and tube metal hydride tank for fuel cell electric forklift

Novel metal hydride (MH) hydrogen storage tanks for fuel cell electric forklifts have been presented in this paper. The tanks comprise a shell side equipped with 6 baffles and a tube side filled with 120 kg AB₅ alloy and 10 copper fins. The alloy manufactured by vacuum induction melting has good hydrogen storage performance, with high storage capacity of 1.6 wt% and low equilibrium pressure of 4 MPa at ambient temperature. Two types of copper fins, including disk fins and corrugated fins, and three kinds of baffles, including segmental baffles, diagonal baffles and hole baffles, were applied to enhance the heat transfer in metal hydride tanks. We used the finite element method to simulate the hydrogen refueling process in MH tanks. It was found that the optimized tank with corrugated fins only took 630 s to reach 1.5 wt% saturation level. The intensification on the tube side of tanks is an effective method to improve hydrogen storage performance. Moreover, the shell side flow field and hydrogen refueling time in MH tanks with different baffles were compared, and the simulated refueling time is in good agreement with the experimental data. The metal hydride tank with diagonal baffles shows the shortest hydrogen refueling time because of the highest velocity of cooling water. Finally, correlations regarding the effect of cooling water flow rate on the refueling time in metal hydride tanks were proposed for future industrial design.     

The effect of the electric field intensity on the hydrogen storage of B/N-co-doped graphdiyne nanosheet

The importance of substituting fossil fuels with clean and renewable energies, and the introduction of hydrogen as a promising option in this field is one of the most popular challenges for researchers. Here, we performed the spin-polarized DFT calculations to investigate the ability of the hydrogen adsorption and storage of modified graphdiyne (GDY) by B and N atoms (BN-GDY) under positive and negative external electric fields. Our findings show that the BN-GDY nanosheet has a weak interaction with the H₂ molecule in absence of the electric field, and the electric field can effectively improve this interaction and increase the adsorption energy of the H₂ molecule on the BN-GDY nanosheet. Also, the negative electric field has more effect relative to the positive one, and with increasing the intensity of the electric field, the adsorption energy has an upward trend. At the highest intensities of positive and negative applied fields (±0.046 V/Å), the BN-GDY nanosheet can store up to 4 and 8H₂ molecules with the average adsorption energies of ?0.253 and ?0.258 eV/H₂, and the H₂ storage capacity can reach up to 3.59 and 6.93 wt%, respectively. The preference of our work for practical application is the free metal promotion of H₂ adsorption and storage.     

Effect of hydrogen blending on the energy capacity of natural gas transmission networks

In this paper the effects of hydrogen on the transport of natural gas-hydrogen mixture in a high-pressure natural gas transmission system are investigated in detail. Our research focuses on the decrease in transferable energy content under identical operating conditions as hydrogen is blended in the gas transmission network. Based on the extensive literature review the outstanding challenges and key questions of using hydrogen in the natural gas system are introduced. In our research the transmissible energy factor - TEF - is defined that quantifies the relative energy capacity of the pipeline caused by hydrogen blending. A new equation is proposed in this paper to find the value of TEF at specific pressure and temperature conditions for different hydrogen concentrations. This practical equation helps the natural gas system operators in the decision-making process when hydrogen emerges in the gas transmission system. In this paper the change of the compression power requirement, which increases significantly with hydrogen blending, is investigated in detail.     

Techno-economic optimization of PV system for hydrogen production and electric vehicle charging stations under five different climatic conditions in India

India is one of the most populous countries in the world, and this has implications for its energy consumption. The country's electricity generation and road transport are mostly dominated by fossil fuels. As such, this study assessed the techno-economics and environmental impact of a solar photovoltaic power plant for both electricity and hydrogen production at five different locations in India (i.e., Chennai, Indore, Kolkata, Ludhiana, and Mumbai). The hydrogen load represents a refueling station for 20 hydrogen fuel cell vehicles with a tank capacity of 5 kg for each location. According to the results, the highest hydrogen production occurred at Kolkata with 82,054 kg/year, followed by Chennai with 79,030 kg/year. Ludhiana, Indore, and Mumbai followed with 78,524 kg/year, 76,935 kg/year and 74,510 kg/year, respectively. The levelized cost of energy (LCOE) for all locations ranges between 0.41 and 0.48 $/kWh. Mumbai recorded the least LCOH of 3.00 $/kg. The total electricity that could be generated from all five cities combined was found to be about 25 GWh per annum, which translates to an avoidable emission of 20,744.07 metric tons of CO₂e. Replacing the gasoline that could be used to fuel the vehicles with hydrogen will result in a CO₂ reduction potential of 2452.969 tons per annum in India. The findings indicate that the various optimized configurations at the various locations could be economically viable to be developed.     

Development of regulations,codes and standards on composite tanks for on-board gaseous hydrogen storage

Composite tanks for on-board gaseous hydrogen storage is one of key parts of the hydrogen fuel cell vehicle. Regulations, codes and standards (RC & S) are conducive to overcoming technological barriers to commercialization. This paper reviews the development of RC & S on composite tanks for on-board gaseous hydrogen storage and addresses their highlights on technical requirements. First, an overview of RC & S for composite tanks is introduced. Then, a comparative study on technical requirements of RC & S including service conditions, design requirements, materials, manufacture, qualification tests and management is presented. Finally, several major differences in RC & S, i.e., tank classification in ISO 19881 and penetration test method are discussed. Some issues for further research, such as initial burst pressure, material hydrogen compatibility and periodic inspection methods are proposed.     

Bounding material properties for automotive storage of hydrogen in metal hydrides for low-temperature fuel cells

Metal hydride material properties required for on-board hydrogen storage for use with automotive polymer electrolyte fuel cell systems are discussed. Thermodynamic relationships between enthalpy and entropy of sorption are determined such that the storage system can be thermally integrated with the fuel cell system and be refueled at reasonable H₂ supply pressures of 50–200 atm. Simple criteria are developed for specifying minimum discharge kinetic rates needed to satisfy hydrogen demand on automotive duty cycles. Simple criteria are also developed for specifying minimum charge kinetic rates needed to refuel metal hydride tanks in reasonable time. Accessible intrinsic capacity and bulk density of the metal hydride are determined for the storage system to achieve system level targets for gravimetric and volumetric capacities. Based on these analyses, it is recommended that the storage media properties be measured on samples prepared by mixing the metal hydride with a high thermal conductivity material, and compacted to 600 kg m⁻³ bulk density. The compact should have a minimum effective thermal conductivity of 8.5 W m⁻¹ K⁻¹.     

In-situ growth of CdS quantum dots on g-C3N4 nanosheets for highly efficient photocatalytic hydrogen generation under visible light irradiation

Well dispersed CdS quantum dots were successfully grown in-situ on g-C₃N₄ nanosheets through a solvothermal method involving dimethyl sulfoxide. The resultant CdS–C₃N₄ nanocomposites exhibit remarkably higher efficiency for photocatalytic hydrogen evolution under visible light irradiation as compared to pure g-C₃N₄. The optimal composite with 12 wt% CdS showed a hydrogen evolution rate of 4.494 mmol h⁻¹ g⁻¹, which is more than 115 times higher than that of pure g-C₃N₄. The enhanced photocatalytic activity induced by the in-situ grown CdS quantum dots is attributed to the interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and CdS, which leads to effective charge separation on both parts.     

The role of Li and Ni metals in the adsorbate complex and their effect on the hydrogen storage capacity of single walled carbon nanotubes coated with metal hydrides,LiH and NiH2

In this first principles study based on density functional theory, we report the hydrogen storage capability of (5, 5) single walled carbon nanotubes coated with Lithium hydride and Nickel hydride. The paper brings out the role of lightweight Li atom and heavy Ni atom in binding the respective hydrides and hydrogen molecules with the single walled carbon nanotubes. The investigation is carried out for half and full coverage of the adsorbates (metal hydrides) on the sidewalls of the carbon nanotubes. The clustering of the adsorbates is observed in full coverage case of both the systems and its effect on hydrogen storage capacity and binding energy is reported. The clustering patterns are different in each of the systems and dependent on the nature of the metal atom in the metal hydride. The storage capacity of single walled carbon nanotubes coated with heavy transition metal hydride is around 3 wt.% whereas it is around 6 wt.% in their counterparts coated with lightweight metal hydride.     

Effect of long range van der Waals interactions on hydrogen storage capacity and heat of adsorption in large pore silicas

The low temperature hydrogen adsorption capacity of mesopores silica aerogel was investigated and compared with that of other large pore silica based materials (MCM-41, HMS) within a range of surface area and large (2 nm) to very large (20 nm) pore sizes. The hydrogen uptake of the aerogel measured at pressure of 1 bar and a temperature of 77 K is around 2.5 times lower than that of MCM-41, although it has a comparable specific surface area (just 30% smaller). The explanation found is the relation between lower hydrogen heat of adsorption and larger pore size for the investigated materials, which leads to higher surface coverage in the smaller pores.     

Selective storage and evolution of hydrogen on nafion/NaCl/graphene quantum dot mixed matrix using tensammetry as power electrochemical technique

Hydrogen storage/evolution behavior of nafion/NaCl/graphene quantum dot (GQD) mixed matrix as selective hydrogen capacitor (power source) was evaluated in detail through an electrochemical process at two independent potential ranges. For this purpose, a three-electrode system included Pt disk as counter electrode, Ag/AgCl as reference electrode and GQD-based mixed matrix-modified Pt disk as working electrode. For hydrogen storage, the deposition potential and time were evaluated to ?1.0 V (vs. Ag/AgCl) and 120 s, respectively under high basic solution generated using NaOH (1.0 M) solution, followed by evolution of hydrogen at +0.8 V (vs. Ag/AgCl) during formation of hydrogen bubbles. The main advantage of this system was the occurrence of hydrogen storage and evolution at two independent potential windows. Both mass transfer and adsorption processes were estimated for the tensammetric peak during the evolution step. The mechanism of hydrogen storage and evolution was obeyed from diffusion and tensammetry, respectively. According to Randles–Sevcik equation using 1.0 mM ${{\text{Fe}\left(\text{CN}\right)}_6}^{3?/4?}$, the active surface area of nafion/NaCl/GQD mixed matrix was ～1906 m²g^?1. Based on the CHN analyses, pressure-concentration temperature as well as hydrogen temperature-programmed desorption, the capacity of the synthesized GQDs for hydrogen storage and evolution was estimated to at least 10.1 and 8.6 wt%, respectively. The stability of the electrode was also estimated during 7000s by chronoamperometry during applying at least 40 cycles in the range from ?1.0 to +1.3 V with reproducible tensammetric peak current (relative standard deviation: 2.54%).     

Research on the hydrogen production performance of methanol reforming microchannels with multi-scale structures

Methanol microreactors are of much application value in mobile hydrogen production (HP) thanks to their tiny volume, flexibility and safety and all that. Microchannels, the core of a reactor, provide a site and heat supply for the reaction. In this paper, a microchannel with multi-scale structures, i.e. submicro structure, corrugated structure, fin structure and matrix structure, is designed. Then the influence mechanism of these structures on the hydrogen production of methanol reforming is studied. Specifically, the influences of microstructures like submicro and corrugated structures on the performance of the catalyst in the microchannel as well as the influence of fin structure and matrix structure on the heat and mass transfer performance of the channel are studied. From the experimental research on the methanol conversion rate and H₂ flow rate of the microchannel with multi-scale structures, the influence rule of different structures on the HP performance of the channel is summarized. The experimental results show that these multi-scale structures not only improve the loading of the catalyst of the microchannel, but also its heat and mass transfer, which increases the methanol conversion rate of the microchannel with multi-scale structures by 33% and its H₂ flow rate by 0.266 mol/h.