We conduct a comparative study to evaluate several lattice Boltzmann (LB) models for solving the near incompressible Navier-Stokes equations, including the lattice Boltzmann equation with the multiple-relaxation-time (MRT), the two-relaxation-time (TRT), the single-relaxation-time (SRT) collision models, and the entropic lattice Boltzmann equation (ELBE). The lid-driven square cavity flow in two dimensions is used as a benchmark test. Our results demonstrate that the ELBE does not improve the numerical stability of the SRT or the lattice Bhatnagar-Gross-Krook (LBGK) model. Our results also show that the MRT and TRT LB models are superior to the ELBE and LBGK models in terms of accuracy, stability, and computational efficiency and that the ELBE scheme is the most inferior among the LB models tested in this study, thus is unfit for carrying out numerical simulations in practice. Our study suggests that, to optimize the accuracy, stability, and efficiency in the MRT model, it requires at least three independently adjustable relaxation rates: one for the shear viscosity ν (or the Reynolds number Re), one for the bulk viscosity ζ, and one to satisfy the criterion imposed by the Dirichlet boundary conditions which are realized by the bounce-back-type boundary conditions.

Developing cheap, highly efficient and stable electrocatalysts for both oxygen and hydrogen evolution reactions (OER and HER) is extremely meaningful to realize large-scale implementation of water splitting technology. Herein, we report the phase and composition controlled synthesis of cobalt sulfide (CoS) hollow nanospheres (HNSs) and their catalytic efficiencies for hydrogen and oxygen evolution reactions in alkaline media. Three CoS compounds, i.e., CoS, CoS, and CoS HNSs, were precisely synthesized by simply adjusting the molar ratio of carbon disulfide to cobalt acetate using a facile solution-based strategy. Electrochemical results reveal that the as-prepared CoS HNSs exhibit superior OER and HER catalytic performance to CoS and CoS HNSs in 1.0 M KOH, with overpotentials of 290 mV for the OER and 193 mV for the HER at 10 mA cm, and the corresponding Tafel slopes of 57 and 100 mV dec, respectively. In addition, the CoS HNSs exhibit remarkable long-term catalytic durability, which is even superior to precious metal catalysts of RuO and Pt/C. Moreover, an alkaline electrolyzer assembled using CoS HNSs as both anode and cathode materials can achieve 10 mA cm at a low cell voltage of 1.54 V at 60 °C with a faradaic efficiency of 100% for overall water splitting. Further analysis demonstrates that the surface morphology, crystallographic structure and coordination environment of Co active sites in combination determine the HER/OER activities in the synthesized binary CoS series, which would provide insight into the rational design of transition metal chalcogenides for highly efficient hydrogen and oxygen-involved electrocatalysis.

New types of polyurethanes were prepared by reacting soybean oil-based polyol and different isocyanates. The polyurethanes can be used as foams, elastomers, coatings, adhesives, etc. Their properties strongly depend on crosslinking density and the structure of isocyanates. Aromatic triisocyanates impart the highest density, glass transition, modulus, and tensile strength, but have the lowest elongation at break, swelling in toluene, and impact resistance. Aliphatic triisocyanates and diisocyanates give rubbery materials with the highest elongation at break, highest swelling, and the lowest tensile strength. Polyurethanes with aromatic and cycloaliphatic diisocyanates were similar in properties, with values between those of the two groups.

Vegetable oils are very heterogeneous materials with a wide distribution of triacylglycerol structures and double-bond contents. The hydrogenation of epoxidized soybean oil (ESO) produces polyols having a functionality distribution related to that of soybean oil. Therefore, these polyols are convenient substances for studying the impact of structural heterogeneity on network formation and properties. Polyols of hydroxyl numbers ranging from 225 to 82 mg KOH/g and weight-average functionalities ranging from 4.4 to 2.7 were obtained by the variation of the time of hydrogenation of ESO. An analysis of the functionality distribution in polyols shows that gel points with diisocyanates vary from 54 to 76% conversion. The molecular weights of the network chains of polyurethanes prepared from these polyols and diphenyl methane diisocyanate varied from 688 to 1993. Polyols with hydroxyl numbers above 200 mg KOH/g gave glassy polymers, whereas those below that value gave rubbers. The heterogeneity of polyols had a negative effect on the elastic properties only at low crosslinking densities.

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