In situ growth CNT@MOFs core–shell structures enabling high specific supercapacitances in neutral aqueous electrolyte

Conductive metal-organic frameworks (c-MOFs) are promising active electrode materials for electrochemical double-layer capacitors with a performance that already exceeds most carbon-based materials. However, their excellent supercapacitance is primarily based on organic or alkaline electrolytes, which largely impede their broad applications and sustainabilities. In this work, we propose a new synthesis approach for fabricating carbon nanotubes and c-MOFs (CNT@MOFs) core–shell structures, which result in high supercapacitance in neutral aqueous electrolytes. We identify that CNTs provide abundant active sites to ensure high capacitance, and Ni₃(2,3,6,7,10,11-hexaiminotriphenylene (HITP))₂ nanoarrays that in situ grow on the surface of CNTs bundles can significantly improve the conductivity and provide enough ion transport pathways in aqueous electrolytes. Specifically, using CNT@MOFs core–shell structures as an electrode, we obtained a high initial capacitance of 150.7 F·g⁻¹ at 0.1 A·g⁻¹ in 1 M Na₂SO₄ solution and good capacity retention of 83.5% after 10,000 cycles at 4 A·g⁻¹. We also found that the carboxyl groups on the surface of CNTs provide better anchor sites for the in situ growth of c-MOF, which promotes the uniform growth of c-MOF shells on the CNT surface and improves aqueous electrolyte accessibility. We believe that the high supercapacitance in aqueous electrolytes reported in this work would provide a good prospect for deploying c-MOF based energy storage devices into biomedical and other healthcare electronic applications.