Enhanced Electrochemical Performance of Ca-Doped Na3V2(PO4)2F3/C Cathode Materials for Sodium-Ion Batteries.

Na3V2(PO4)2F3 (NVPF) with a NASICON structure has garnered attention as a cathode material owing to its stable 3D structure, rapid ion diffusion channels, high operating voltage, and impressive cycling stability. Nevertheless, the low intrinsic electronic conductivity of the material leading to a poor rate capability presents a significant challenge for practical application. Herein, we develop a series of Ca-doped NVPF/C cathode materials with various Ca2+ doping levels using a simple sol-gel and carbon thermal reduction approach. X-ray diffraction analysis confirmed that the inclusion of Ca2+ does not alter the crystal structure of the parent material but instead expands the lattice spacing. Density functional theory calculations depict that substituting Ca2+ ions at the V3+ site reduces the band gap, leading to increased electronic conductivity. This substitution also enhanced the structural stability, preventing lattice distortion during the charge/discharge cycles. Furthermore, the presence of the Ca2+ ion introduces two localized states within the band gap, resulting in enhanced electrochemical performance compared to that of Mg-doped NVPF/C. The optimal NVPF-Ca-0.05/C cathode exhibits superior specific capacities of 124 and 86 mAh g-1 at 0.1 and 10 C, respectively. Additionally, the NVPF-Ca-0.05/C demonstrates satisfactory capacity retention of 70% after 1000 charge/discharge cycles at 10 C. These remarkable results can be attributed to the optimized particle size, excellent structural stability, and enhanced ionic and electronic conductivity induced by the Ca doping. Our findings provide valuable insight into the development of cathode material with desirable electrochemical properties.

DFT calculation in design of near-infrared absorbing nitrogen-doped graphene quantum dots.

The near-infrared light (NIR) absorption of nitrogen-doped graphene quantum dots (NGQDs) containing different N-doping sites is systematically investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations with Perdew-Burke-Ernzerhof (PBE) functionals. The results show that the ultra-small HOMO-LUMO gaps (0.3-1.0 eV) of various N-doping structures (graphitic, amino, and pyridinic at center, and graphitic at edge) are attributed to the spin-polarization of the energy states, which effectively enhances the NIR absorption for NGQDs. Overall, the graphitic N-doping structure exhibits the best NIR absorption. Moreover, the electron attraction effect of the different N-sites is found to be crucial for the LUMO level, where stronger electron attraction lowers the LUMO energy. This work provides critical insight in further design of NGQDs for NIR absorption.

First principles calculations on lithium diffusion near the surface and in the bulk of Fe-doped LiCoPO4.

The olivine phosphate LiCoPO4 is a prospective cathode material in high-voltage lithium-ion batteries. During lithium diffusion, the ions must overcome the diffusion energy barrier near the surface and in the bulk. Experimental studies have shown that Fe doping can enhance the electrochemical performance of LiCoPO4 with a doping concentration of x = 0.2 (LiFe0.2Co0.8PO4). DFT calculations can provide detailed understanding of the lithium diffusion mechanism, structural stability, and electronic properties for Fe-doped LiCoPO4 and elucidate the origins for this improvement from a microscopic viewpoint. In this study, the electronic structure of Fe-doped LiCoPO4 was calculated via first principles and compared with that of pristine LiCoPO4. To investigate the surface properties of LiCoPO4, surface energies with low indices were calculated. The results showed that the (010) surface has the lowest surface energy. Minimum energy diffusion pathways and energy barriers were calculated using the NEB method. Our calculations showed that the energy barrier for lithium-ion diffusion can be reduced by Fe doping modification. Furthermore, we investigated the diffusion processes of polarons and lithium ions migrating simultaneously. This study has implications for further application of LiCoPO4 as a cathode material.

Application of zeolitic imidazolate framework for hexavalent chromium removal: A feasibility and mechanism study.

The mechanisms and effectiveness of using zeolitic imidazolate framework (ZIF-8) [a sub-family of metal-organic framework (MOF)] particles on hexavalent chromium [Cr(VI)] removal were evaluated. The ultrasonic mixing method was applied for the preparation of ZIF-8, and chemicals used for ZIF-8 synthesis included ammonium hydroxide, zinc nitrate hexahydrate, and 2-methylimidazole. ZIF-8 particle had a clear rhombic dodecahedron morphology shape and a strong peak intensity with high crystallinity. The adsorption capacity (AC) of ZIF-8 was 30.3 mg of Cr(VI)/g of ZIF-8 [Cr(VI) = 50 mg/L]. The AC of Cr(VI) raised to 34.3 mg/g under acidic conditions (pH = 5), and the AC dropped to below 13.7 mg/g with a pH range from 7 to 11. It could be because of the competitive effects between CrO4 2- and hydroxide ions for adsorption locations of ZIF-8. Cr(VI) removal relied on the amount of Cr(VI) adsorbed on the particles of ZIF-8, and the mechanisms of Cr(VI) adsorption by ZIF-8 included chemical/physical processes and the rate-limiting step was the chemical adsorption. A fraction of sorbed Cr(VI) was reduced to Cr(III), and thus, ZIF-8 could serve as a reducing agent during Cr(VI) reduction. Cr(VI) was removed effectively from the water phase by ZIF-8 via adsorption and reduction mechanisms. PRACTITIONER POINTS: ZIF-8 particles had an adsorption capacity of 30.33 mg of Cr(VI)/g of ZIF-8. Cr(VI) sorption by ZIF-8 has chemical (rate-limiting step) and physical processes. ZIF-8 can serve as a reducing agent for Cr(VI) reduction. Cr(VI) can be removed by ZIF-8 via the adsorption and reduction mechanisms.

The origins of charge separation in anisotropic facet photocatalysts investigated through first-principles calculations.

It was recently discovered that the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can be completed on the {110} and {001} facets, respectively, of a 18-facet SrTiO3 mono-crystal. The effective charge separation is attributed to the facet junction at the interface between two arbitrary anisotropic crystal planes. Theoretical estimation of the built-in potential at the facet junction can greatly improve understanding of the mechanism. This work employs density functional theory (DFT) calculations to investigate such potential at the (110)/(100) facet junction in SrTiO3 crystals. The formation of the facet junction is verified by a calculated work function difference between the (110) and (100) planes, which form p-type and n-type segments of the junction, respectively. The built-in potential is estimated at about 2.9 V. As a result, with the ultra high built-in potential, electrons and holes can effectively transfer to different anisotropic planes to complete both photo-oxidative and photo-reductive reactions.

A soft-chemistry assisted strong metal-support interaction on a designed plasmonic core-shell photocatalyst for enhanced photocatalytic hydrogen production.

Engineering photocatalysts based on gold nanoparticles (AuNPs) has attracted great attention for the solar energy conversion due to their multiple and unique properties. However, boosting the photocatalytic performance of plasmonic materials for H2 generation has some limitations. In this study, we propose a soft-chemistry method for the preparation of a strong metal-support interaction (SMSI) to enhance the photocatalytic production of H2. The TiO2 thin overlayer covering finely dispersed AuNPs (forming an SMSI) boosts the photocatalytic generation of hydrogen, compared to AuNPs deposited at the surface of TiO2 (labelled as a classical system). The pathway of the charge carriers' dynamics regarding the system configuration is found to be different. The photogenerated electrons are collected by AuNPs in a classical system and act as an active site, while, unconventionally, they are injected back in the titania surface for an SMSI photocatalyst making the system highly efficient. Additionally, the adsorption energy of methanol, theoretically estimated using the density functional theory (DFT) methodology, is lower for the soft-chemistry SMSI photocatalyst accelerating the kinetics of photocatalytic hydrogen production. The SMSI obtained by soft-chemistry is an original concept for highly efficient photocatalytic materials, where the photon-to-energy conversion remains a major challenge.

Rational Tuning of Zirconium Metal-Organic Framework Membranes for Hydrogen Purification.

The effect of organic ligands on the separation performance of Zr based metal-organic framework (Zr-MOF) membranes was investigated. A series of Zr-MOF membranes with different ligand chemistry and functionality were synthesized by an in situ solvothermal method and a coordination modulation technique. The thin supported MOF layers (ca. 1 µm) showed the crystallographic orientation and pore structure of original MOF structures. The MOF membranes show excellent selectivity towards hydrogen owing to the molecular sieving effect when the bulkier linkers were used. The molecular simulation confirmed that the constricted pore apertures of the Zr-MOFs which were formed by the additional benzene rings lead to the decrease in the diffusivity of larger penetrants while hydrogen was not remarkably affected. The gas mixture separation factors of the MOF membranes reached to H2 /CO2 =26, H2 /N2 =13, H2 /CH4 =11.

Effect of nitrogen-doping configuration in graphene on the oxygen reduction reaction.

In this study, we investigate the oxygen reduction reaction (ORR) reactivity of nitrogen-doped graphene by using density functional theory (DFT), a computational quantum mechanical technique. Four doping configurations and five models were comprehensively studied: quaternary nitrogen (NQ), pyrrolic nitrogen (N5), two forms of pyridinic nitrogen (N6, N6nH) and three-pyridinic nitrogen (3N6). Models for possible sites during each step of ORR were set up and visualized to provide a platform to calculate the free energy of the reaction pathway to determine the suitability of each doping scenario. Associative mechanisms were displayed by all models except N5, which showed dissociative mechanism. The calculated free energy pathways demonstrate that the ranking of the reactivity for ORR by different nitrogen configurations from most reactive to least reactive is N6, NQ, N6nH, 3N6, and N5. Spin density and charge density aid in describing levels of reactivity.

Computational studies on the adsorption of CO2 in the flexible perfluorinated metal-organic framework zinc 1,2-bis(4-pyridyl)ethane tetrafluoroterephthalate.

Carbon dioxide adsorption sites within the flexible metal-organic framework (MOF) zinc 1,2-bis(4-pyridyl)ethane tetrafluoroterephthalate (Znbpetpa) were investigated using density functional theory (DFT) and canonical Monte Carlo (MC) calculations. Two types of sites with different heats of adsorption were found by using DFT and confirmed by the MC results. Expansion of the cavities occurred simultaneously with gas uptake and the process of "breathing" within the MOF was identified. The presence of such a mechanism makes the understanding of this structure useful in tuning the design of MOFs for permanent trapping of gases.

Van der Waals forces in the perfluorinated metal-organic framework zinc 1,2-bis(4-pyridyl)ethane tetrafluoroterephthalate.

Traditional density functional theory (DFT) and dispersion-corrected DFT calculations are performed to investigate the metal-organic framework zinc 1,2-bis(4-pyridyl)ethane tetrafluoroterephthalate (Znbpetpa). Without dispersion correction, straightening of the zigzag C-O-Zn chain connecting the secondary building units across the diagonal of the unit cell is observed, accompanied by a large anisotropic expansion of the structure along one cell parameter. The results show that van der Waals dispersion forces and specifically Zn-C equatorial interactions and the resulting effects on the zigzag chain play an important role in maintaining key structural features which match with experimental observations. It is suggested that the pore volume of the framework could be controlled by substituting the Zn metal centre with another transition element of different polarizability, while maintaining functional linkers.