Current Issue

2021, Volume 34,  Issue 2

2021, 34(2): i-ii.
2021, 34(2): iii-iv.
Solid oxide fuel cells (SOFCs) are regarded to be a key clean energy system to convert chemical energy (e.g. H2 and O2) into electrical energy with high efficiency, low carbon footprint, and fuel flexibility. The electrolyte, typically doped zirconia, is the "state of the heart" of the fuel cell technologies, determining the performance and the operating temperature of the overall cells. Yttria stabilized zirconia (YSZ) have been widely used in SOFC due to its excellent oxide ion conductivity at high temperature. The composition and temperature dependence of the conductivity has been hotly studied in experiment and, more recently, by theoretical simulations. The characterization of the atomic structure for the mixed oxide system with different compositions is the key for elucidating the conductivity behavior, which, however, is of great challenge to both experiment and theory. This review presents recent theoretical progress on the structure and conductivity of YSZ electrolyte. We compare different theoretical methods and their results, outlining the merits and deficiencies of the methods. We highlight the recent results achieved by using stochastic surface walking global optimization with global neural network potential (SSW-NN) method, which appear to agree with available experimental data. The advent of machine-learning atomic simulation provides an affordable, efficient and accurate way to understand the complex material phenomena as encountered in solid electrolyte. The future research directions for design better electrolytes are also discussed.
Assembling of a few particles into a cluster commonly occurs in many systems. However, it is still challenging to precisely control particle assembling, due to the various amorphous structures induced by thermal fluctuations during cluster formation. Although these structures may have very different degrees of aggregation, a quantitative method is lacking to describe them, and how these structures evolve remains unclear. Therefore a significant step towards precise control of particle self-assembly is to describe and analyze various aggregation structures during cluster formation quantitatively. In this work, we are motivated to propose a method to directly count and quantitatively compare different aggregated structures. We also present several case studies to evaluate how the aggregated structures during cluster formation are affected by external controlling factors, e.g., different interaction ranges, interaction strengths, or anisotropy of attraction.
The ring-polymer molecular dynamics (RPMD) was used to calculate the thermal rate coefficients of the multi-channel roaming reaction H+MgH→Mg+H2. Two reaction channels, tight and roaming, are explicitly considered. This is a pioneering attempt of exerting RPMD method to multi-channel reactions. With the help of a newly developed optimization-interpolation protocol for preparing the initial structures and adaptive protocol for choosing the force constants, we have successfully obtained the thermal rate coefficients. The results are consistent with those from other theoretical methods, such as variational transition state theory and quantum dynamics. Especially, RPMD results exhibit negative temperature dependence, which is similar to the results from variational transition state theory but different from the ones from ground state quantum dynamics calculations.
Diffusion of tracer particles in active bath has attracted extensive attention in recent years. So far, most studies have considered isotropic spherical tracer particles, while the diffusion of anisotropic particles has rarely been involved. Here we investigate the diffusion dynamics of a rigid rod tracer in a bath of active particles by using Langevin dynamics simulations in a two-dimensional space. Particular attention is paid to how the translation (rotation) diffusion coefficient $ D_{ \rm{T}} $ ($ D_{ \rm{R}} $) change with the length of rod $ L $ and active strength $ F_{ \rm{a}} $. In all cases, we find that rod exhibits superdiffusion behavior in a short time scale and returns to normal diffusion in the long time limit. Both $ D_{ \rm{T}} $ and $ D_{ \rm{R}} $ increase with $ F_{ \rm{a}} $, but interestingly, a nonmonotonic dependence of $ D_{ \rm{T}} $ ($ D_{ \rm{R}} $) on the rod length has been observed. We have also studied the translation-rotation coupling of rod, and interestingly, a negative translation-rotation coupling is observed, indicating that rod diffuses more slowly in the parallel direction compared to that in the perpendicular direction, a counterintuitive phenomenon that would not exist in an equilibrium counterpart system. Moreover, this anomalous (diffusion) behavior is reentrant with the increase of $ F_{ \rm{a}} $, suggesting two competitive roles played by the active feature of bath particles.
In order to study the effect of different modification methods on polysilsesquioxane (POSS) modified cellulose, a molecular dynamics method was used to establish a pure cellulose model and a series of modified models modified by polysilsesquioxane in different ways. And their thermodynamic properties were calculated. The results showed that the performance of cellulose models was better than that of unmodified model, and the modified effect was the best when two cellulose chains were grafted onto polysilsesquioxane by chemical bond (M2 model). Compared with pure cellulose model, the cohesive energy density and solubility parameters of M2 model are increased by 9%, and the values of tensile modulus, bulk modulus, shear modulus and Cauchy pressure increased by 38.6%, 29.5%, 41.1% and 29.5%, respectively. In addition, the free volume fraction and mean square displacement of each model were calculated and analyzed in this work. Compared with the pure cellulose model, the molecular chain entanglement of cellulose was increased due to the existence of the chemical bonds in the M2 model, which made the cellulose molecular chains occupy more free volume, so that the system had a smaller free volume fraction, inhibited the chain movement of cellulose chains, and thus improved the thermal stability of cellulose.
The lattice parameters, bulk modulus, first derivative of the bulk modulus, electronic band structures, phonon dispersion curves and phonon density of states calculations for Li2AlGa and Li2AlIn Heusler alloys are performed and compared in this study using density functional theory within the generalized gradient approximation. Computed lattice parameters display a good agreement with the literature. Obtained electronic band structures of both Heusler alloys show that they are in semi-metallic structure. Phonon dispersion curves and the phonon density of states graphs are also obtained in order to study the lattice dynamics of these Heusler alloys. It is noticed that Li2AlGa and Li2AlIn Heusler alloys are dynamically stable in the ground state.
The kinetics for hydrogen (H) adsorption on Ir(111) electrode has been studied in both HClO$ _4 $ and H$ _2 $SO$ _4 $ solutions by impedance spectroscopy. In HClO$ _4 $, the adsorption rate for H adsorption on Ir(111) increases from 1.74$ \times $10$ ^{-8} $ mol$ \cdot $cm$ ^{-2} $$ \cdot $s$ ^{-1} $ to 3.47$ \times $10$ ^{-7} $ mol$ \cdot $cm$ ^{-2} $$ \cdot $s$ ^{-1} $ with the decrease of the applied potential from 0.2 V to 0.1 V (vs. RHE), which is ca. one to two orders of magnitude slower than that on Pt(111) under otherwise identical condition. This is explained by the stronger binding of water to Ir(111), which needs a higher barrier to reorient during the under potential deposition of H from hydronium within the hydrogen bonded water network. In H$ _2 $SO$ _4 $, the adsorption potential is ca. 200 mV negatively shifted, accompanied by a decrease of adsorption rate by up to one order of magnitude, which is explained by the hindrance of the strongly adsorbed sulfate/bisulfate on Ir(111). Our results demonstrate that under electrochemical environment, H adsorption is strongly affected by the accompanying displacement and reorientation of water molecules that initially stay close to the electrode surface.
The photophysical and photochemical behaviors of thioxanthen-9-one (TX) in different solvents have been studied using nanosecond transient absorption spectroscopy. A unique absorption of the triplet state $ ^3 $TX$ ^* $ is observed, which involves two components, $ ^3 $n$ \pi $$ ^* $ and $ ^3 $$ \pi\pi^* $ states. The $ ^3 $$ \pi\pi^* $ component contributes more to the $ ^3 $TX$ ^* $ when increasing the solvent polarity. The self-quenching rate constant $ k_{ \rm{sq}} $ of $ ^3 $TX$ ^* $ is decreased in the order of CH$ _3 $CN, CH$ _3 $CN/CH$ _3 $OH (1:1), and CH$ _3 $CN/H$ _2 $O (1:1), which might be caused by the exciplex formed from hydrogen bond interaction. In the presence of diphenylamine (DPA), the quenching of $ ^3 $TX$ ^* $ happens efficiently via electron transfer, producing the TX$ ^\cdot $$ ^- $ anion and DPA$ ^{\cdot} $$ ^+ $ cation radicals. Because of insignificant solvent effects on the electron transfer, the electron affinity of the $ ^3 $n$ \pi $$ ^* $ state is proved to be approximately equal to that of the $ ^3 $$ \pi\pi^* $ state. However, a solvent dependence is found in the dynamic decay of TX$^{{ \cdot ^ - }}$ anion radical. In the strongly acid aqueous acetonitrile (pH = 3.0), a dynamic equilibrium between protonated and unprotonated TX is definitely observed. Once photolysis, $ ^3 $TXH$ ^{+*} $ is produced, which contributes to the new band at 520 nm.
Au@Au@Ag double shell nanoparticles were fabricated and characterized using TEM, STEM-mapping and UV-Vis methods. Using crystal violet as Raman probe, the surface-enhanced Raman scattering (SERS) activity of the as-prepared Au@Au@Ag nanoparticles was studied by comparing to Au, Au@Ag and Au@Au core-shell nanoparticles which were prepared by the same methods. Moreover, it can be found that the SERS activity was enhanced obviously by introduction of NaCl and the concentrations of NaCl played a key role in SERS detection. With an appropriate concentration of NaCl, the limit of detection as low as 10$ ^{-10} $ mol/L crystal violet can be achieved. The possible enhanced mechanism was also discussed. Furthermore, with simple sample pretreatment, the detection limit of 5 μg/g Rhodamine B (RhB) in chili powders can be achieved. The results highlight the potential utility of Au@Au@Ag for detection of illegal food additives with low concentrations.
The issues of low crystallinity and slow crystallization rate of poly(lactic acid) (PLA) have been widely addressed. In this work, we find that doping PLA with Zn(Ⅱ) ions can speed up the process of crystallization of PLA. Three kinds of Zn(Ⅱ) salts (ZnCl$ _2 $, ZnSt and ZnOAc) were tested in comparison with some other ions such as Mg(Ⅱ) and Ca(Ⅱ). The increased crystallinity and crystallization rate of PLA doping with Zn(Ⅱ) are reflected in FT-IR and variable temperature Raman spectroscopy. The crystallinity is further confirmed or measured with differential scanning calorimetry and X-ray diffraction. The crystallinity rate of the PLA/ZnSt-0.4 wt% material can reach 22.46% and the crystallinity rate of the PLA/ZnOAc-0.4 wt% material can reach 24.83%, as measured with differential scanning calorimetry.
g-C3N4 coupled with high specific area TiO2 (HSA-TiO2) composite was prepared by a simple solvothermal method, which was easy to operate with low energy consumption. Degradation of methyl orange test results showed that HSA-TiO2 effectively improved the photocatalytic activity effectively. Photoelectrochemical test results indicated that the separation of photo-generated carriers and the charge carrier migration speed of TiO2 were improved after combination with g-C3N4. g-C3N4/HSA-TiO2 showed strong photocatalytic ability. The degree of degradation of methyl orange by 6%-g-C3N4/HSA-TiO2 could reach up to 92.44%. Furthermore, it revealed good cycle performance. The photocatalytic mechanism of g-C3N4/HSA-TiO2 was proposed.
Four organic small-molecule hole transport materials ( D41 , D42 , D43 and D44 ) of tetraarylpyrrolo[3, 2-b]pyrroles were prepared. They can be used without doping in the manufacture of the inverted planar perovskite solar cells. Tetraarylpyrrolo[3, 2-b]pyrroles are accessible for one-pot synthesis. D42 , D43 and D44 possess acceptor-$ \pi $-donor-$ \pi $-acceptor structure, on which the aryl bearing substitutes of cyan, fluorine and trifluoromethyl, respectively. Instead, the aryl moiety of D41 is in presence of methyl with a donor-$ \pi $-donor-$ \pi $-donor structure. The different substitutes significantly affected their molecular surface charge distribution and thin-film morphology, attributing to the electron-rich properties of fused pyrrole ring. The size of perovskite crystalline growth particles is affected by different molecular structures, and the electron-withdrawing cyan group of D42 is most conducive to the formation of large perovskite grains. The D42 fabricated devices with power conversion efficiency of 17.3% and retained 55% of the initial photoelectric conversion efficiency after 22 days in dark condition. The pyrrolo[3, 2-b]pyrrole is efficient electron-donating moiety for hole transporting materials to form good substrate in producing perovskite thin film.
To address the limitations of the separate fluoride removal or detection in the existing materials, herein, amino-decorated metal organic frameworks NH$ _2 $-MIL-53(Al) have been succinctly fabricated by a sol-hydrothermal method for simultaneous removal and determination of fluoride. As a consequence, the proposed NH$ _2 $-MIL-53(Al) features high uptake capacity (202.5 mg/g) as well as fast adsorption rate, being capable of treating 5 ppm of fluoride solution to below the permitted threshold in drinking water within 15 min. Specifically, the specific binding between fluoride and NH$ _2 $-MIL-53(Al) results in the release of fluorescent ligand NH$ _2 $-BDC, conducive to the determination of fluoride via a concentration-dependent fluorescence enhancement effect. As expected, the resulting NH$ _2 $-MIL-53(Al) sensor exhibits selective and sensitive detection (with the detection limit of 0.31 $ \mu $mol/L) toward fluoride accompanied with a wide response interval (0.5-100 $ \mu $mol/L). More importantly, the developed sensor can be utilized for fluoride detection in practical water systems with satisfying recoveries from 89.6% to 116.1%, confirming its feasibility in monitoring the practical fluoride-contaminated waters.
Residues of tetracycline antibiotics (TCs) in environments may be harmful to human. Due to their high polarities, it is extremely challenging to efficiently enrich TCs with low concentrations in natural waters for analysis. In this work, a magnetic metal-organic framework Fe$ _3 $O$ _4 $@[Cu$ _3 $(btc)$ _2 $] was synthesized and applied as a dispersive micro-solid phase extraction adsorbent for TCs enrichment. Effects of dispersive micro-solid phase extraction conditions including extraction time, solution pH, and elution solvent on the extraction efficiencies of TCs were investigated. Results show that TCs could be enriched efficiently by Fe$ _3 $O$ _4 $@[Cu$ _3 $(btc)$ _2 $], and electrostatic interaction between TCs and Fe$ _3 $O$ _4 $@[Cu$ _3 $(btc)$ _2 $] dominated this process. Combined with liquid chromatography-tandem mass spectrometry, four TCs residues (oxytetracycline, tetracycline, chlortetracycline, and doxycycline) in natural waters were determined. The detection limits (LOD, $ S/N $ = 3) of the four antibiotics were 0.01-0.02 $ \mu $g/L, and the limits of quantitation (LOQ, $ S/N = $10) were 0.04-0.07 $ \mu $g/L. The recoveries obtained from river water and aquaculture water spiked with three TCs concentration levels ranged from 70.3% to 96.5% with relative standard deviations of 3.8%-12.8%. Results indicate that the magnetic metal-organic framework based dispersive micro-solid phase extraction is simple, rapid and high-loading for antibiotics enrichment from water, which further expand the practical application of metal-organic frameworks in sample pretreatment for environmental pollutant analysis.