Baohua Yue

The modulation of metal–insulator transition temperature of vanadium dioxide: a density functional theory study

The modulation of metal–insulator transition (MIT) temperature and phase stability of thermochromic materials based on all the transition metal doped VO2 were systematically studied using density functional theory (DFT) calculations. The free energies, formation enthalpies, and Fermi energies of transition metal doped VO2 were evaluated from DFT calculations; the cell volumes and bulk moduli were obtained by fitting the free energies to the Birch–Murnaghan equation of states; and the decomposition enthalpies and entropies of the transition metal doped VO2 were calculated using both experimental data and DFT calculations. Based on these results, the MIT temperature was associated with lattice distortion of VO2 (M1) upon doping, the expansion of cell volume and the decrease in β-angle were associated with the decrease in MIT temperature, and the shrinkage of cell volume and the increase in β-angle were associated with the increase in MIT temperature. And it was also concluded that VO2 (M1) doped with high valence cations is more stable than those doped with low valence cations. These conclusions are consistent with experimental facts that W-, Mo-, and Re- are the most studied and the most effective dopants for the reduction of MIT temperature, and La-, Hg-, and Ag-doped VO2 undergoes phase separation. In addition, DFT calculations without spin-polarization were also carried out, and the influence of spin-polarization was evaluated. Finally, scandium was proposed as a potential dopant for VO2 in view of balanced comprehensive performance.

Proton Transport Pathways in an Acid–Base Complex Consisting of a Phosphonic Acid Group and a 1,2,3-Triazolyl Group

The proton transport pathways in an acid-base complex consisting of a phosphonic acid group and a 1,2,3-triazolyl group were studied using density functional theory (DFT) calculations in terms of stable configurations and transition states of the molecular or ionic dimers and trimers and verified by proof-of-concept experiments including experimental measurements of overall conductivity and (1)H NMR and FTIR spectroscopy of the methylphosphonic acid (MPA) and 1,2,3-triazole (Tri) complex as well as overall proton conductivity of polymeric blend of poly(vinylphosphonic acid) (PVPA) and poly(4-vinyl-1H-1,2,3-triazole) (PVTri). From the DFT calculations of dimers and trimers composed of ethylphosphonic acid (EPA), Tri, and their deprotonated counterparts, it was concluded that the intermolecular hydrogen bonds of the transition states corresponding to proton transport are much shorter than those of stable configurations, but the O-H and N-H bonds are much longer than those of stable configurations. The tautomerization activation energy decreases from 0.927-1.176 eV in Tri-Tri dimers to 0.336-0.444 eV in the EPA-Tri dimers. From the proof-of-concept experiments, about a 50 fold increase in overall conductivity was observed in the MPA-Tri complex consisting of 10% (molar ratio) MPA compared to pure Tri, and the calculated activation energy is consistent with the experimental activation energy evaluated from temperature dependence of proton conductivity of pure Tri and the MPA-Tri complex. In addition, the fast proton exchange between MPA and Tri, consistent with the DFT calculations, was verified by (1)H NMR and FTIR spectroscopy. Finally, a polymeric blend of PVPA and PVTri was prepared, and its proton conductivity at about 2.1 mS·cm(-1) in anhydrous state at 100 °C was observed to be significantly higher than that of PVPA or of poly(VPA-co-1-vinyl-1,2,4-triazole). The proton conductivity of the polymeric PVPA and PVTri blend in humidity state is in the same range as that of NAFION 117.

Synergetic proton conducting effect in acid–base composite of phosphonic acid functionalized polystyrene and triazolyl functionalized polystyrene

The synergetic proton conducting effect with three orders of magnitude improvement in proton conductivity was observed in an acid–base composite composed of phosphonic acid functionalized polystyrene (PS-PA) and triazolyl functionalized polystyrene (PS-Tri). In addition, a new method for the development of proton conducting materials by the combination of different acidic and basic polymers is proposed. The PS-PA was synthesized by the bromination of polystyrene on the para-position of the phenyl ring followed by phosphonation and hydrolysis. The PS-Tri was synthesized by the chloromethylation of polystyrene on the para-position of the phenyl ring followed by azidation and 1,3-dipolar cycloaddition or ‘click’ reaction. A maximum proton conductivity of 11.2 mS cm−1, which is three orders of magnitude higher than that of pristine PS-PA or PS-Tri, a tensile strength of 16.3 MPa, and a minimum water uptake of 15.1% (90 °C, 90% RH) were observed in the PS-PA/PS-Tri composite composed of 66.7% PS-PA. Finally, a mosaic-like morphology model and space-charge effects were proposed to explain the synergetic proton conducting effect.

Hydrogen Bond and Proton Transport in Acid–Base Complexes and Amphoteric Molecules by Density Functional Theory Calculations and 1H and 31P Nuclear Magnetic Resonance Spectroscopy

Intermolecular and intramolecular hydrogen bond (H-bond) and proton transport in acid-base complexes and amphoteric molecules consisting of phosphonic acid groups and nitrogenous heterocyclic rings are investigated by density functional theory calculations and (1)H NMR and (31)P NMR spectroscopy. It is concluded that a phosphonic acid group can act both as H-bond donor and H-bond acceptor, while an imine nitrogen atom can only act as H-bond acceptor and an amine group as H-bond donor. And the intramolecular H-bond is weaker than the intermolecular H-bond attributing to configurational restriction. In addition, the strongest H-bond interaction is observed between a phosphonic acid and a 1H-indazole because of the formation of double H-bonds. The (1)H NMR and (31)P NMR chemical shifts for the acid-base complexes are consistent with the density functional theory calculations. From the (1)H NMR chemical shifts, fast proton exchange is observed between a phosphonic acid and 1H-benzimidazole or 1H-indazole. Finally, it is proposed that polymeric material tethered with 1H-benzimidazole or 1H-indazole rings is a favorable component for high-temperature proton exchange membranes based on acid-base complexes or acid-base amphoteric molecules.

Hydrogen transfer reaction in polycyclic aromatic hydrocarbon radicals.

Density functional theory calculations have been successfully applied to investigate the formation of hydrocarbon radicals and hydrogen transfer pathways related to the chemical vapor infiltration process based on model molecules of phenanthrene, anthra[2,1,9,8-opqra]tetracene, dibenzo[a,ghi]perylene, benzo[uv]naphtho[2,1,8,7-defg]pentaphene, and dibenzo[bc,ef]ovalene. The hydrogen transfer reaction rate constants are calculated within the framework of the Rice-Ramsperger-Kassel-Marcus theory and the transition state theory by use of the density functional theory calculation results as input. From these calculations, it is concluded that the hydrogen transfer reaction between two bay sites can happen almost spontaneously with energy barrier as low as about 4.0 kcal mol(-1), and the hydrogen transfer reactions between two armchair sites possess lower energy barrier than those between two zigzag sites.

Maleimide: a potential building block for the design of proton exchange membranes studied by ab initio molecular dynamics simulations

Ab initio molecular dynamics (AIMD) simulations are applied to the study of proton transport in solid state maleimide. The AIMD simulations reproduce correctly the structural and energetic characteristics. The simulations reveal the direct proton hopping between two maleimide molecules, and the proton hopping frequency is evaluated. The temperature dependence of proton hopping frequency obeys the Arrhenius activation process with an activation energy of 4.39 kcal mol−1. Finally, it is proposed that maleimide is a potential building block for the design of high-temperature proton exchange membranes.

Finite element analysis of the Poisson–Boltzmann equation coupled with chemical equilibriums: redistribution and transport of protons in nanophase separated polymeric acid–base proton exchange membranes

Abstract The finite element analysis is applied to the study of the redistribution and transport of protons in model nanophase separated polymeric acid–base composite membranes by the Poisson–Boltzmann equation coupled with the acid and base dissociation equilibriums for the first time. Space charge redistribution in terms of proton and hydroxide redistributions is observed at the interfaces of acidic and basic domains. The space charge redistribution causes internal electrostatic potential, and thus, promotes the macroscopic transport of protons in the acid–base composite membranes.