Influence of Anion/Cation Substitution (Sr²⁺ → Ba²⁺, Al³⁺ → Si⁴⁺, N^3– → O^2–) on Phase Transformation and Luminescence Properties of Ba₃Si₆O₁₅:Eu²⁺ Phosphors

A series of promising cyan, green, and yellow emission (Ba, Sr)₃(Si, Al)₆(O, N)₁₅:Eu²⁺ phosphors were synthesized by a Pechini-type sol–gel ammonolysis method. Variations in luminescence properties and crystal structure caused by the modification of phosphor composition were studied in detail. The prefired temperatures of the precursors play a key role in the process of forming the final products. Under UV light excitation, the as-prepared Ba₃Si₆O₁₅:Eu²⁺ phosphor presents a strong cyan emission located at 498 nm. Moreover, the as-prepared oxynitride phosphors, Eu²⁺-activated (Ba_1–<i>y</i>Sr_<i>y</i>)₃Si_6–<i>x</i>Al_<i>x</i>O_15−μN_δ (<i>x</i> = 0–1.2, <i>y</i> = 0–0.6), display a broader excitation band covering the entire visible region. Under blue light excitation, Ba₃Si_6–<i>x</i>Al_<i>x</i>O_15−μN_δ:Eu²⁺ phosphors show a intense and narrow green emission at 520 nm, and the luminescent intensity can be enhanced by increasing Al content within a certain range. However, (Ba_1–<i>y</i>Sr_<i>y</i>)₃Si₆O_15−μN_δ:Eu²⁺phosphors exhibit green (520 nm) to yellow (554 nm) emission with increasing Sr content. Unexpectedly, Eu²⁺ doped Ba₃Si₆O₉N₄-type Ba₃Si₆O_15−μN_δ–1300 °C phosphor exhibits a bluish green emission and strong thermal quenching behavior. The (Ba_1–<i>y</i>Sr_<i>y</i>)₃Si_6–<i>x</i>Al_<i>x</i>O_15−μN_δ:Eu²⁺ phosphors exhibit a small thermal quenching, and the quantum yields measured under 460 nm excitation could reach up to 89% for green Ba₃Si_6–<i>x</i>Al_<i>x</i>O_15−μN_δ:Eu²⁺ phosphor and 71% for yellow (Ba_1–<i>y</i>Sr_<i>y</i>)₃Si_6<i>x</i>O_15−μN_δ:Eu²⁺ phosphor. White LEDs with tunable color temperature and higher color rendering index were fabricated by combining the prepared cyan Ba₃Si₆O₁₅:Eu²⁺/green Ba_2.91Eu_0.09Si_6–<i>x</i>Al_<i>x</i>O_15−μN_δ (<i>x</i> = 0.06)/yellow (Ba_{0.97–<i>y</i>}Sr_<i>y</i>)₃Eu_0.09Si₆O_15−μN_δ (<i>y</i> = 0.4) phosphor and a red phosphor with a UV or blue LED chip, indicating that they are promising phosphors for white LEDs

High Energy Density in Combination with High Cycling Stability in Hybrid Supercapacitors

Hybrid supercapacitors are considered the next-generation energy storage equipment due to their superior performance. In hybrid supercapacitors, battery electrodes need to have large absolute capacities while displaying high cycling stability. However, enhancing areal capacity via decreasing the size of electrode materials results in reductions in cycling stability. To balance the capacity–stability trade-off, rationally designed proper electrode structures are in urgent need and still of great challenge. Here we report a high-capacity and high cycling stability electrode material by developing a nickel phosphate lamination structure with ultrathin nanosheets as building blocks. The nickel phosphate lamination electrode material exhibits a large specific capacity of 473.9 C g–1 (131.6 mAh g–1, 1053 F g–1) at 2.0 A g–1 and only about 21% capacity loss at 15 A g–1 (375 C g–1, 104.2 mAh g–1, 833.3 F g–1) in 6.0 M KOH. Furthermore, hybrid supercapacitors are constructed with nickel phosphate lamination and activated carbon (AC), possessing high energy density (42.1 Wh kg–1 at 160 W kg–1) as well as long cycle life (almost 100% capacity retention after 1000 cycles and 94% retention after 8000 cycles). The electrochemical performance of the nickel phosphate lamination structure not only is commensurate with the nanostructure or ultrathin materials carefully designed in supercapacitors but also has a longer cycling lifespan than them. The encouraging results show the great potential of this material for energy storage device applications

Engineering Oxygen Vacancies on Mixed-Valent Mesoporous α‑MnO₂ for High-Performance Asymmetric Supercapacitors

Intrinsically poor conductivity and sluggish ion-transfer kinetics limit the further development of electrochemical storage of mesoporous manganese dioxide. In order to overcome the challenge, defect engineering is an effective way to improve electrochemical capability by regulating electronic configuration at the atomic level of manganese dioxide. Herein, we demonstrate effective construction of defects on mesoporous α-MnO2 through simply controlling the degree of redox reaction process, which could obtain a balance between Mn3+/Mn4+ ratio and oxygen vacancy concentration for efficient supercapacitors. The different structures of α-MnO2 including the morphology, specific surface area, and composition are successfully constructed by tuning the mole ratio of KMnO4 to Na2SO3. The electrode materials of α-MnO2-0.25 with an appropriate Mn3+/Mn4+ ratio and abundant oxygen vacancy showed an outstanding specific capacitance of 324 F g–1 at 0.5 A g–1, beyond most reported MnO2-based materials. The asymmetric supercapacitors formed from α-MnO2-0.25 and activated carbon can present an energy density as high as of 36.33 W h kg–1 at 200 W kg–1 and also exhibited good cycle stability over a wide voltage range from 0 to 2.0 voltage (kept at approximately 98% after 10 000 cycles in galvanostatic cycling tests) and nearly 100% Coulombic efficiency. Our strategy lays a foundation for fine regulation of defects to improve charge-transfer kinetics