Weberite-type sodium transition metal fluorides (Na2M2+M'3+F7) have emerged as potential high-performance sodium intercalation cathodes, with predicted energy densities in the 600-800 W h/kg range and fast Na-ion transport. One of the few weberites that have been electrochemically tested is Na2Fe2F7, yet inconsistencies in its reported structure and electrochemical properties have hampered the establishment of clear structure-property relationships. In this study, we reconcile structural characteristics and electrochemical behavior using a combined experimental-computational approach. First-principles calculations reveal the inherent metastability of weberite-type phases, the close energetics of several Na2Fe2F7 weberite polymorphs, and their predicted (de)intercalation behavior. We find that the as-prepared Na2Fe2F7 samples inevitably contain a mixture of polymorphs, with local probes such as solid-state nuclear magnetic resonance (NMR) and Mössbauer spectroscopy providing unique insights into the distribution of Na and Fe local environments. Polymorphic Na2Fe2F7 exhibits a respectable initial capacity yet steady capacity fade, a consequence of the transformation of the Na2Fe2F7 weberite phases to the more stable perovskite-type NaFeF3 phase upon cycling, as revealed by ex situ synchrotron X-ray diffraction and solid-state NMR. Overall, these findings highlight the need for greater control over weberite polymorphism and phase stability through compositional tuning and synthesis optimization.
Electrochemical CO 2 reduction (ECR) has received significant attention with the rising emphasis on renewable energy. However, high Faradaic efficiency (FE), big current density and high product selectivity are still the challenge for ECR. Selecting ionic liquids (ILs) as electrolytes for ECR has the advantages of enhancing the Faradaic efficiency and improving the current density. Herein, in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is employed for the first time to study the ECR process on nickel foam (NF) in imidazolium ILs. Results show that the hydrophobicity of imidazolium ILs is very important to the ECR, and 1-Butyl-3-methylimidazolium chloride (BmimCl) presents the best performance towards ECR. The FE of CO is as high as 94.6%, with a bigger current density of 72 mA cm -2 . Particularly, the FE of CO can keep around 82% after 15 h continuous electrolysis, indicating the good stability. Structural details of the BmimCl-NF electrode interface are obtained by in situ SHINERS along with density functional theory (DFT) calculations, indicating that the adsorption configuration change of imidazolium cations in BmimCl on NF electrode is favorable for ECR. Moreover, the strong interaction between Cl anions in BmimCl and CO 2 could bend and activate the CO 2 molecules. The reaction intermediate CO 3 2– confirmed by in situ SHINERS helps to determine the reaction pathway. Particularly, the DFT calculation showed the priority of the ECR reaction on the Ni surface for the CO product. This work presents a promising method for in situ studying the mechanism and kinetics of CO 2 electroreduction in ionic liquids.
The solar energy is the most promising energy to solve energy crisis and environmental problem. Quantum dot can be applied to solar cells in two structures of QDSC to improve the energy conversion efficiency. The two structures are p-i-n type QDSC and quantum dots sensitized solar cells. The energy conversion efficiency of p-i-n type QDSC may increase up to 45%. Both CdSe and CdS quantum dot can be used as the sensitizer of the QDSSC and each of them has its demerits and merits, but the conversion efficiency of QDSSC is low if they were used respectively. Thus, in order to overcome their demerits respectively, we could try to combine their merits. QDSC is the most promising technique to solve the problems of solar cell. But before large-scale application their efficiency and stability should be improved.
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