A cost-effective, classical united-atom (UA) force field for ionic liquids (ILs) was proposed, which can be used in simulations of ILs composed by 1-alkyl-3-methyl-imidazolium cations ([C(n)mim](+)) and seven kinds of anions, including tetrafluoroborate ([BF(4)](-)), hexafluorophosphate ([PF(6)](-)), methylsulfate ([CH(3)SO(4)](-)), trifluoromethylsulfonate ([CF(3)SO(3)](-)), acetate ([CH(3)CO(2)](-)), trifluoroacetate ([CF(3)CO(2)](-)), and bis(trifluoromethylsulfonyl)amide ([NTf(2)](-)). The same strategy in our previous work (J. Phys. Chem. B 2010, 114, 4572) was used to parametrize the force field, in which the effective atom partial charges are fitted by the electrostatic potential surface (ESP) of ion pair dimers to account for the overall effects of polarization in ILs. The total charges (absolute values) on the cation/anion are in the range of 0.64-0.75, which are rescaled to 0.8 for all kinds of ions by a compromise between transferability and accuracy. Extensive molecular dynamics (MD) simulations were performed over a wide range of temperatures to validate the force field, especially on the enthalpies of vaporization (ΔH(vap)) and transport properties, including the self-diffusion coefficient and shear viscosity. The liquid densities were predicted very well for all of the ILs studied in this work with typical deviations of less than 1%. The simulated ΔH(vap) at 298 and 500 K are also in good agreement with the measured values by different experimental methods, with a slight overestimation of about 5 kJ/mol. The influence of ΔC(p) (the difference between the molar heat capacity at constant pressure of the gas and that of liquid) on the calculation of ΔH(vap) is also discussed. The transport coefficients were estimated by the equilibrium MD method using 20-60 ns trajectories to improve the sampling. The proposed force field gives a good description of the self-diffusion coefficients and shear viscosities, which is comparable to the recently developed polarizable force field. Although slightly lower dynamics is found in simulations by our force field, the order of magnitude of the self-diffusion coefficient and viscosity are reproduced for all the ILs very well over a wide temperature range. The largest underestimation of the self-diffusion coefficient is about one-third of the experimental values, while the largest overestimation of the viscosity is about two times the experimental values.

Our recently developed improved united atom force field shows a good quality to reproduce both the static and transport properties of neat ionic liquids (ILs). Combined with the TIP4P-Ew water model, the force field is used to simulate the mixture of 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]) and water without further optimization to adjust any cross parameters. Liquid densities of the mixture are well predicted over the entire concentration range at temperatures from 298.15 to 353.15 K. Simulations also reproduce the positive values of excess volumes and excess enthalpies, as well as their increase with temperature. The simulated viscosities are in good agreement with experimental values, especially in the water-rich region. We found three distinct regions by analyzing the concentration dependent self-diffusion coefficients via Stokes-Einstein (SE) relation, indicating the mixture experiences significant microheterogeneity with the adding of water. This observation is well connected to the structure features obtained in simulations, such as radial distribution functions (RDFs), spatial distribution functions (SDFs) and water clustering analysis. At the water mole fraction (x(2)) less than 0.2, most of the water molecules are isolated in the polar cation-anion network in ionic liquids. With the increase of x(2) from 0.2 to 0.8, large water cluster forms and eventually percolates the whole system. When x(2) > 0.8, ionic liquids show a moderate degree of aggregation (with maximum around 0.9 to 0.95) before the cations and anions are fully dissolved in water.

The modulating degree of defect and graphitization carbon coupled metal oxide composite is extremely important because these materials have been applied in various fatal fields, especially in energy storage/conversion and catalysis. Herein, we reported the in situ formation of long defect-rich nitrogen-doped carbon nanotubes (NCNTs) coupled with a porous TiO 2 @NC composite (TiO 2 @NC−NCNTs) derived from spindletype bimetallic (Ti/Co) MOF microrods. Thanks to the rational composition and unique structure, the as-prepared Pt/TiO 2 @NC−NCNTs-7 can be used as a bifunctional fuel cell electrocatalyst. For MOR, the optimized catalyst exhibits superior mass activity of 577 mA mg Pt −1 (in acidic medium) and 3100 mA mg Pt −1 (in alkaline medium), which is 2.02 and 4.08 times superior than the commercial Pt/C catalyst, and shows excellent CO tolerance and better cyclic stability. This state-of-the-art nanocatalyst also exhibits a more positive onset potential (1.01 V) and upper limiting current density (5.03 mA cm −2 ) than commercial Pt/C (0.96 V and 4.75 mA cm −2 ) toward oxygen reduction reaction. The optimized degree of defect and graphitization in N-doped carbon (including NC and NCNTs) of the nanocatalyst not only provides large amounts of active sites to Pt NP deposition but also is positive for the charge transfer improvement of the overall nanocatalyst. The strong-coupled Pt−NCNTs(NC)−TiO 2 can further boost the catalytic activity and CO-tolerance. This work not only opens a new approach to develop a novel carbon/metal oxide composite with the desired morphology and properties but also provides new insights into the design and construction of state-ofthe-art nanomaterials for fuel cells, water splitting, and photocatalysis.

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