An oriented Ni–Co-MOF anchored on solution-free 1D CuO: a p–n heterojunction for supercapacitive energy storage

Herein, we propose an effective strategy to enhance the electrochemical activity of a metal organic framework-based (MOF) electrode material for electrochemical capacitors. The fabrication involves the synthesis of CuO nanowires on a Cu substrate through a facile solution-free dry oxidation route followed by the deposition of an oriented Ni–Co-zeolitic imidazolate framework (Ni–Co-ZIF) on 1D CuO. This synthesis strategy benefitted from the highly exposed redox active sites of the aligned Ni–CoZIF, an "ion and electrolyte repository", to assist the diffusion of electrolyte ions, and a p–n heterojunction between CuO and the Ni–Co-ZIF. ZIFs represent an emerging and unique class of MOFs. The oriented pseudocapacitive Ni–Co-ZIF@CuO composite electrode yielded excellent electrochemical merits including a high gravimetric capacitance which is 3.3- and 2.1-fold higher than those of the selfsupported CuO and bulk MOF, respectively. Furthermore, we employed first principles density functional theory calculations to study the enhanced electronic conductivity and reduced work function of Ni–CoZIF@CuO systems upon CuO doping, which reinforced the experimental findings. Moreover, an asymmetric supercapacitor (ASC) device was assembled to evaluate the application of the as-fabricated electrode material for electrochemical capacitors. The gadget delivered a maximum energy density of 43 W h kg^-1 , with improved cycling stability after 10 000 cycles. The oriented Ni–Co-ZIF@CuO with remarkable electrochemical activity and mechanical flexibility inspires for next-generation MOF-based electrode materials with superior electrochemical attributes.