Xianwei Li,Zhenqian Wu,Xudong Ma,Ling Xin,Xiao Yu,Yong Liu
Carbonaceous materials , such as hard carbon and porous carbon , have gained great attentions as the anode materials for Na-ion batteries (NIBs). However, their defects and disordered structure cause the low initial coulombic efficiency (ICE), high-potential slope region and small charge capacity below 0.5 V. Herein, a structurally stable MOF contained 1,3,5-triazine-2,4,6-triamine moieties, belonging to the tetragonal space group I 4/m, is synthesized. On this basis, MOF-derived crystalline carbon with graphite-like crystal, instead of amorphous porous carbon, can be obtained due to slow pyrolysis of MOF framework along with timely and sufficient reassembly of the pyrolyzed carbon atoms into graphite-like crystal under graphite-induced condition. The graphite-like crystal has a geometrically similar unit cell to graphite crystal but with a magnification ratio of 1.25, yielding proportionally expanded d-spacing of (002) plane (4.2 Å), which enables the reversible Na + intercalation/de-intercalation. When applied in NIBs, it exhibits a very high ICE of 96.0% and reversible charge capacity of 356.2 mAh g −1 , reaching the advanced level of graphite applied in LIBs . More importantly, its charge capacity below 0.5 V can account for 90% of the total charge capacity, which is also the key parameter to effectively increase the energy density of full batteries.
Haomiao Tang,Wenbin Wang,Jun Zhou,Tiantian Li,Zhu Shu
Polymeric carbon nitride (PCN), as a metal-free photocatalyst, has drawn wide attention in the photocatalytic H2 evolution. However, the photocatalytic activity of directly synthesized PCN is limited by its low crystallinity. Currently, regulating the melon-based PCN into tri-s-triazine-based crystalline PCN to further optimize its structure has been proved to effectively improve its photocatalytic activity. The heptazine-based crystalline carbon nitride, potassium poly(heptazine imide) (abbreviated as K-PHI), has been used in photocatalytic H2 evolution benefiting from its high crystallinity, as the high crystallinity narrows the bandgap and increases the light capture efficiency and increases the charge mobility. Nevertheless, the effect of synthesis temperature on crystallinity has not yet been reported. In this work, the effect of temperature on the crystallinity of heptazine-based crystalline carbon nitride was studied by one-step synthesis at different temperatures. It shows that the heptazine-based crystalline structure appears when the temperature exceeds 540°C. Additionally, the crystallinity of all samples is gradually improved with increasing temperature until the sample begins to decompose beyond 630°C. The sample synthesized at 630°C demonstrates the highest photocatalytic H2 evolution rate of 1.798 mmol h−1 g−1 under visible light irradiation, which is 31.5 times that of bulk PCN. Based on systematic material characterizations, the mechanism of the effect of synthesis temperature on crystallinity and the contribution of crystallinity to photocatalytic efficiency were revealed.
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