Robust MOF-on-MOF heterostructures as efficient cathode candidates for next-generation supercapacitors

Metal–organic frameworks (MOFs) have shown great potential as cathode in supercapacitor applications. However, they can also be used as a platform for designing a new class of materials. Herein, a MOF-on-MOF heterostructure, Ni-BTC@ZIF-67, has been synthesized via a solvothermal method followed by the in situ growth of ZIF-67 on Ni-BTC and was well-characterized through various techniques (PXRD, FT-IR, BET, SEM, TEM, and XPS). From SEM, TEM, and XPS analysis, it is clear that ZIF-67 was successfully deposited over the Ni-BTC surface via N–O bonding interaction. One potential application of Ni-BTC@ZIF-67 is as an electro-active material in the construction of supercapacitors. This material demonstrated an impressive specific capacitance of 1063 F g−1 when subjected to a 4 A g−1 current. The robust nature of both MOFs helped attain a cyclic retention capacity of about ∼98% after 5500 cycles. Moreover, an asymmetric supercapacitor was assembled with activated carbon and Ni-BTC@ZIF-67 in an aqueous system, which delivered an energy density of 22 W h Kg−1 at a power density of 1075 W Kg−1 with a cyclic retention of ∼83% after 1000 cycles. Thus, the MOF-on-MOF architecture can be a new pathway for supercapacitor applications in the future.