Cobalt induced growth of hollow MOF spheres for high performance supercapacitors

The development of highly-efficient metal organic framework (MOF) based supercapacitors has attracted much attention. In this work, hollow structure Ni/Co-MOFs have been facilely synthesized by using Co ions as structure-directing agents. Various structures of MOFs, such as hollow, yolk-shell and solid spheres, can be easily obtained through adjusting the molar ratio of Ni/Co. The introduction of an appropriate amount of Co ions can induce the formation of hollow spheres, where the hollow structure can offer rich redox reaction sites and provide fast transport pathways. Moreover, adsorption simulations and density of states calculations illustrate that the adsorbability of OH- and the electrochemical activity on the mixed-metallic MOFs are improved after introducing the Co ions, which can promote the reversible redox reaction activity of the electrode materials. The as-obtained hollow Ni/Co-MOF with the optimized amount of Co ions yields a high capacitance of 1498 F g(-1) at 1 A g(-1), an excellent rate capability of 67% capacitance retention at 30 A g(-1) and a good cycling stability of 71% after 3000 cycles at 5 A g(-1). This work may provide useful guidance for tuning the structure and electronic properties of MOFs to enhance the electrochemical performance for supercapacitors.