MOF-derived NiCo-LDH Nanocages on CuO Nanorod Arrays for Robust and High Energy Density Asymmetric Supercapacitors.

Layered double hydroxide (LDH) is widely explored in supercapacitors on account of its high capacity, adjustable composition and easy synthesis process. Unfortunately, solitary LDH still has great limitations as an electrode material due to its shortcomings, such as poor conductivity and easy agglomeration. Herein, nanoflakes assembled NiCo-LDH hollow nanocages derived from a metal-organic framework (MOF) precursor are strung by CuO nanorods formed from etching and oxidation of copper foam (CF), forming hierarchical CuO@NiCo-LDH heterostructures. The as-synthesized CuO@NiCo-LDH/CF shows a large capacitance (5607 mF cm-2 at 1 mA cm-2 ), superior rate performance (88.3 % retention at 10 mA cm-2 ) and impressive cycling durability (93.1 % capacitance is retained after 5000 cycles), which is significantly superior to control CuO/CF, CuO@ZIF-67/CF, NiCo-LDH/CF and Cu(OH)2 @NiCo-LDH/CF electrodes. Besides, an asymmetrical supercapacitor consists of CuO@NiCo-LDH/CF and activated carbon displays a maximum energy density of 47.3 Wh kg-1 , and its capacitance only declines by 6.8 % after 10000 cycles, demonstrating remarkable cycling durability. The advantages of highly conductive and robust CuO nanorods, MOF-derived hollow structure and the core-shell heterostructure contribute to the outstanding electrochemical performance. This synthesis strategy can be extended to design various core-shell heterostructures adopted in versatile electrochemical energy storage applications.

A heterostructure of NiMn-LDH nanosheets assembled on ZIF-L-derived ZnCoS hollow nanosheets with a built-in electric field enables boosted electrochemical energy storage.

Supercapacitors (SCs) have emerged as an efficient technology toward the utilization of renewable energy, which demands high-performance electrode materials. Transition-metal sulfides (TMSs) and layered double hydroxides (LDHs) rich in active sites and valence states are very promising electrode materials, but they still suffer from inherent defects, such as low electric conductivity, sluggish reaction kinetics and large volume change during electrochemical reactions. In this work, NiMn-LDH nanosheets are assembled on the surfaces of ZnCoS hollow nanosheet arrays derived from a zeolitic imidazolate framework-L (ZIF-L) to form a ZnCoS@NiMn-LDH heterostructure (ZCS@LDH) with a built-in electric field. The unique structure gives rise to abundant exposed active sites and improved ion diffusion. More importantly, the built-in electric field can enhance conductivity and charge transfer by modulating the electronic structures. With these merits, the optimal ZCS@LDH-6 electrode displays outstanding specific capacitance (2102.2 F g-1 at 1 A g-1) and remarkable rate performance (68.1% at 10 A g-1). The assembled asymmetric supercapacitor (ASC) using the ZCS@LDH-6 electrode shows high energy storage capacity (41.7 W h kg-1 at 850.0 W kg-1), satisfactory cycle life (92.2% capacitance retention after 10 000 cycles) and high coulombic efficiency (95.8%). This work will shed light on designing high-performance electrode materials via heterostructure and morphology engineering.

CeO2 decorated on Co-ZIF-L-derived nickel-cobalt sulfide hollow nanosheet arrays for high-performance supercapacitors.

Transition metal sulfides have been widely explored as electrode materials for supercapacitors. Unfortunately, the slow redox reaction kinetics and severe volume changes during charge/discharge result in compromised electrochemical performance. In this work, a nickel-cobalt sulfide hollow nanosheet array decorated with cerium oxide nanoparticles (NiCoS/CeO2) has been constructed using a cobalt zeolitic imidazolate framework-L as the template coupled with subsequent solvothermal synthesis. Benefiting from the advantages of the hollow and hierarchical NiCoS nanosheets with a large number of accessible active sites and fast charge transport pathways, as well as CeO2 layers with rich oxygen vacancies and strong interaction between NiCoS and CeO2, NiCoS/CeO2 exhibits a high specific capacity of 1278.1 C g-1 (1 A g-1), which is approximately 1.9 times that of NiCoS. The asymmetric supercapacitor assembled using NiCoS/CeO2 and activated carbon shows an excellent storage capability (34.9 W h kg-1 at 850.0 W kg-1) and an impressive cycle life (91.9% capacitance retention after 10 000 charge and discharge cycles at 10 A g-1).