A DFT study on enhanced adsorption of H2 on Be-decorated porous graphene nanosheet and the effects of applied electrical fields

The adsorption of the hydrogen molecule on the pure porous graphene nanosheet (P-G) or the one decorated with Be atom (Be-G) was investigated by the first-principle DFT calculations. The Be atom was adsorbed on the P-G with a binding energy of ?1.287 eV to successfully establish the reasonable Be-G. The P-G was a poor substrate to interact weakly with the H₂, whereas the Be-G showed a high affinity to the adsorbed H₂ with an enhanced adsorption energy and transferred electrons of ?0.741 eV and 0.11 e, respectively. A molecular dynamics simulation showed that the H₂ could also be adsorbed on the Be-G at room temperature with a reasonable adsorption energy of ?0.707 eV. The interaction between the adsorbed H₂ and the Be-G was further enhanced with the external electrical fields. The applied electrical field of ?0.4 V/Å was found to be the most effective to enhance the adsorption of H₂ on the Be-G with the modified adsorption energy and the improved transferred electrons being ?0.708 eV and 0.17 e, respectively. Our study shows that the Be-G is a promising substrate to interact strongly with the H₂ and could be applied as a high-performance hydrogen gas sensor, especially under the external electrical field.     

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH₂ hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH₂ crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH₂, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH₂ host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH₂ are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH₂-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.     

Promoted photoinduced charge separation and directional electron transfer over dispersible xanthene dyes sensitized graphene sheets for efficient solar H2 evolution

Directional electron transfer and effective charge separation facilitated by graphene sheets have provided an inspiring approach to enhance the efficiencies of photoelectric conversion and photocatalysis. Herein, we demonstrated the feasibility of constructing a high-performance of the dye-sensitized H₂ evolution system using dispersible graphene sheets as both efficient electron transfer carrier and catalyst scaffold. Among the xanthene dyes sensitized H₂ evolution catalysts in this study, photocatalyst of Rose Bengal (RB) sensitized graphene decorated with Pt is the most active one and exhibits the highest apparent quantum efficiency (AQE) of 18.5% at wavelength of 550 nm and rather long-term stability for H₂ evolution. Dispersible graphene sheets can not only capture electrons from the excited dye and then transfer them to the decorated catalysts efficiently for improving charge separation with a small energy loss, but also afford large interfaces for highly dispersing catalyst nanoparticles with more active sites, thereby significantly enhancing the H₂ evolution efficiency than graphite oxide (GO) and multiwall carbon nanotubes (MWCNTs). This work proposes a potential strategy to develop efficient photocatalytic systems for solar-energy-conversion and provides a new insight into mechanistic study of photoinduced electron transfer by effective synergetic combination of dispersible graphene sheets with an efficient dye and a H₂ evolution catalyst.     

Co(OH)2 nanoparticles deposited on reduced graphene oxide nanoflake as a suitable electrode material for supercapacitor and oxygen evolution reaction in alkaline media

In this work, cobalt hydroxide nanoparticles are simply synthesized (size is about 50 nm) and deposited on the reduced graphene oxide nanoflake by the hydrothermal method. Then, the ability of glassy carbon electrode modified with this low-cost nanocomposite is examined as a supercapacitor and oxygen evolution electrocatalysts in 2.0 mol L^?1 KOH by a three-electrode system. The modified electrode as a pseudocapacitor with potential windows of 0.35 V, exhibits a powerful specific capacitance (235.20 F g^?1 at 0.1 A g^?1 current density), energy density, stability (about 90% of the initial capacitance value maintain after 2000 cycles at 1.0 A g^?1) and fast charge/discharge ability. Furthermore, the modified electrode displays a good electrocatalytic activity for oxygen evolution reaction with a current density of 10.0 mA cm^?2 at 1.647 V, small Tafel slope of 56.5 mV dec^?1, good onset potential of 1.521 V vs. RHE and suitable durability.     

Ab initio study of hydrogen adsorption on Zn2(NDC)2(diPyTz) metal-organic framework decorated with alkali and alkaline earth metal cations

We have studied effect of alkali and alkaline earth metal cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺) decoration on hydrogen adsorption of the organic linker of Zn₂(NDC)₂(diPyTz) by employing three cluster models: diPyTz:mLi⁺ (m = 1–4), diPyTz:mLi⁺:nH₂ (m = 0,1,2 and n = 1–5) and diPyTz:1M⁺:1H₂ (M⁺ = Na⁺, K⁺, Be²⁺, Mg²⁺) complexes, using density functional theory (DFT) and second-order Moller–Plesset perturbation theory (MP2). The calculated binding energies show that decoration of the organic linker with alkali and alkaline earth metal cations enhanced H₂ interaction with diPyTz when compared with the pristine diPyTz. The atomic charges were derived by Mulliken, ChelpG and ESP methods. Finally, the atoms in molecules theory (AIM) were also applied to get more insight into the nature of the interaction of diPyTz and Li⁺. Results of AIM analysis show that N–Li⁺ bond in diPyTz organic linker's complex appears as shared electron interaction.     

Impact of the modification method of Ni/ZrO2 catalyst by alkali and alkaline earth metals on its activity in thermo-chemical conversion of cellulose

The goal of this work is to determine an impact of the modification method of Ni/ZrO₂ catalyst by alkali and alkaline earth metals on its activity in thermo-chemical conversion of cellulose to hydrogen-rich gas. Me_xO-ZrO₂ supports (where Me = Ca, Mg, Na or K) were prepared by impregnation, precipitation and sol-gel methods. The obtained results reveal that an introduction of dopants to the zirconia support considerably enhances the H₂ yield in comparison to unmodified catalyst. An increase in the hydrogen formation is accompanied by a rise in the total volume of the produced gases. It is demonstrated that the highest amount of hydrogen is formed in the presence of the catalysts containing CaO-ZrO₂ support followed by Na doped materials. This phenomenon can be attributed to more efficient incorporation of Ca²⁺ and Na⁺ cations in the zirconia lattice making it more stable in the reaction conditions. Moreover, it is observed that an activity order of the investigated catalysts is consistent with the changes in the basic character of their surface.