The practical application of metal–organic frameworks (MOFs) for energy storage is faced with great challenges, such as poor structural stability and limited active sites. Herein, we have co-designed a three-dimensional (3D) self-assembled hexagonal zeolitic imidazolate framework-L (ZIF-L) structure with a 3D conformation that greatly reduces the self-aggregation of two-dimensional (2D) layered materials. Due to the rational design of the specific morphology and atomically different coordination abilities of Ni2+ and Co2+ in the framework, the micro-nano electric field is constructed, and the structural stability and electrochemistry reaction activity of ZIF-L are obviously improved. Moreover, the consecutive hollow structure is also formed by regulating the Ni–Co ratio, which can significantly enhance the specific capacitance and cycling stability of the Ni-ZIF-L electrode through the formation of fast electrolyte ions transfer channels. Consequently, the Ni-ZIF-L-40 electrode exhibits a high specific capacity (568.9 F·g−1 at 0.5 A·g−1) and long cycle stability (89.5% retention after 5000 cycles at 5 A·g−1). In addition, the Ni-ZIF-L-40//activated carbon (AC) asymmetric supercapacitor assembled using AC also shows an excellent cycling stability (91.1% retention after 4000 cycles at 5 A·g−1). This study may open a new window for the practical application of intrinsic MOFs-based electrodes for energy storage and conversion.
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Research Article
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Although lithium-ion batteries (LIBs) dominate the current application of energy storage devices, they are not suitable for further large-scale applications due to the limited resources and increasing cost of lithium. Compared with lithium, sodium shows abundant resources, low cost, and similar physical–chemical properties, and sodium-ion batteries (SIBs) are regarded as the most impressive alternative to LIBs. However, several challenges hinder the practical application of SIBs, particularly the current cathode materials that usually exhibit poor cycling performance and low rate capability. Therefore, the rational design of excellent cathode materials with both high capacity and long cycling stability is of great imperative. With the rapid development of technology, many universities have now established majors in new energy materials and devices and have set up comprehensive experiments related to LIBs. However, the current experimental teaching rarely emphasizes SIBs and, particularly, the cathode materials. Therefore, a comprehensive research experiment on the double-metal doped Prussian blue cathode has been designed to help students build a systematic professional knowledge of energy storage systems.
In this experiment, a double-metal doped Prussian blue was synthesized through a facile coprecipitation method. The precise structure of NiMnHCF is systematically verified by various characterization methods, which shows a Prussian blue single- crystal structure. In addition, the electrochemical performance of NiMnHCF as a cathode of SIBs is deeply investigated in the half coin cell.
The experimental results are explained as follows: 1) The lattice parameters of NiMnHCF were slightly lower than those of MnHCF, which can be attributed to the Ni2+ radius being less than Mn2+. Due to changes in nucleation rate and nucleus composition, the addition of Ni2+ affected the morphology and particle size of the precipitate. 2) The redox peak pattern of NiMnHCF was wider, and the CV curve area was larger. Compared with MnHCF, NiMnHCF has a higher specific capacity, indicating that the improvement of the bimetal regulation strategy can reduce the structural perturbation of its electrochemical process and significantly improve cycling stability. 3) The charge transfer resistance of NiMnHCF was lower than that of MnHCF. This indicates that the addition of Ni improved the electrochemical conversion process of MnHCF. 4) The discharge-specific capacity of NiMnHCF was significantly increased at various current rates. Compared to MnHCF, they have better rate capability and long-term cyclic stability.
In conclusion, the double- metal codoped PBA of NiMnHCF shows obvious improved rate capability and cycling performance as the cathode of SIBs, owing to the modified nano/microstructure. The simple strategy is inspiring for further application of large-scale commercial SIBs. This comprehensive experimental project belongs to the current research of new energy, which can help in stimulating the scientific research interest and industrialization thinking of students and providing a comprehensive talent pool to meet the strategic needs of national energy storage.
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