No abstract

Potassium-ion batteries (KIBs) are promising electrochemical energy storage systems because of their low cost and high energy density. However, practical exploitation of KIBs is hampered by the lack of high-performance cathode materials. Here we report a potassium manganese hexacyanoferrate (K2Mn[Fe(CN)6]) material, with a negligible content of defects and water, for efficient high-voltage K-ion storage. When tested in combination with a K metal anode, the K2Mn[Fe(CN)6]-based electrode enables a cell specific energy of 609.7 Wh kg−1 and 80% capacity retention after 7800 cycles. Moreover, a K-ion full-cell consisting of graphite and K2Mn[Fe(CN)6] as anode and cathode active materials, respectively, demonstrates a specific energy of 331.5 Wh kg−1, remarkable rate capability, and negligible capacity decay for 300 cycles. The remarkable electrochemical energy storage performances of the K2Mn[Fe(CN)6] material are attributed to its stable frameworks that benefit from the defect-free structure.

Electrochemical carbon dioxide reduction reaction (CO2RR) represents a promising way to generate fuels and chemical feedstock sustainably. Recently, studies have shown that two‐dimensional metal carbides and nitrides (MXenes) can be promising CO2RR electrocatalysts due to the alternating −C and −H coordination with intermediates that decouples scaling relations seen on transition metal catalysts. However, further by tuning the electronic and surface structure of MXenes it should still be possible to reach higher turnover number and selectivities. To this end, defect engineering of MXenes for electrochemical CO2RR has not been investigated to date. In this work, first‐principles modelling simulations are employed to systematically investigate CO2RR on M2XO2‐type MXenes with transition metal and carbon/nitrogen vacancies. We found that the −C‐coordinated intermediates take the form of fragments (e. g., *COOH, *CHO) whereas the −H‐coordinated intermediates form a complete molecule (e. g., *HCOOH, *H2CO). Interestingly, the fragment‐type intermediates become more strongly bound when transition‐metal vacancies are present on most MXenes, while the molecule‐type intermediates are largely unaffected, allowing the CO2RR overpotential to be tuned. The most promising defective MXene is Hf2NO2 containing Hf vacancies, with a low overpotential of 0.45 V. More importantly, through electronic structure analysis it could be observed that the Fermi level of the MXene changes significantly in the presence of vacancies, indicating that the Fermi level shift can be used as an ideal descriptor to rapidly predict the catalytic performance of defective MXenes. Such an evaluation strategy is applicable to other catalysts beyond MXenes, which could enhance high throughput screening efforts for accelerated catalyst discovery.

A novel photoluminescent supramolecular hyperbranched polymer (SHP) without conventional chromophores was constructed for the first time by inclusion complexation between α-cyclodextrin and diethylenetriamine. The SHP showed wide-band fluorescence dependent upon the excitation wavelength.

Graphene/metal oxides (G/MO) composite materials have attracted much attention as the anode of sodium ion batteries (SIBs), because of the high theoretical capacity. However, most metal oxides operate based on the conversion mechanism and the alloying mechanism has changed to Na 2 O after the first cycle. The influence of G/Na 2 O (G/N) on the subsequent sodiation process has never been clearly elucidated. In this work, we report a systematic investigation on the G/N interface from both aspects of theoretical simulation and experiment characterization. By applied first-principles simulations, we find that the sluggish kinetics in the G/MO materials is mainly caused by the high diffusion barrier (0.51 eV) inside the Na 2 O bulk, while the G/N interface shows a much faster transport kinetics (0.25 eV) via unique double-interstitialcy mechanism. G/ N interface possesses an interfacial storage of Na atom through the charge separation mechanism. The experimental evidence confirms that high interfacial ratio structure of G/N greatly improves the rate performance and endows G/MO materials the interfacial storage. Furthermore, the experimental investigation finds that the high interfacial ratio structure of G/N also benefits from the reversible reaction between SnO 2 and Sn during cycling. Lastly, the effects of (N, O, S) doping in graphene systems at the G/N interface were also explored. This work provides a fundamental comprehension on the G/MO interface structure during the sodiation process, which is helpful to design energy storage materials with high rate performance and large capacity.