Hierarchical MIL-88A/NiCo2O4@N-CNT Hybrid for Asymmetric Supercapacitors and Hydrogen Evolution Reaction

The development of multifunctional hybrid electrode materials with high-performance energy storage and electrocatalytic activity is highly desirable for advanced energy technologies. Herein, a MIL-88A/NiCo2O4@N-CNT ternary composite was prepared through ultrasonic-assisted assembly of porous MIL-88A, redox-active NiCo2O4, and conductive nitrogen-doped carbon nanotubes (N-CNTs). The synergistic interaction among these components provides abundant active sites, improved electrolyte accessibility, and enhanced charge transfer pathways. An increase in the BET surface area was observed in the case of hybrid composite (176.43 m2 g−1), compared to MIL-88A (99.35 m2 g−1) and NiCo2O4 (145.52 m2 g−1). The electrode based on MIL-88A/NiCo2O4@N-CNT was obtained with a specific capacity of 2036 C g−1 at 10 mV s−1 and 2209 C g−1 at 2.0 A g−1 in a three-electrode cell system. The specific capacity for asymmetric supercapacitors with the MIL-88A/NiCo2O4@N-CNT//AC structure was found to be 412 C g−1 from cyclic voltammetry (CV) and 432 C g−1 from galvanostatic charge–discharge (GCD), and the energy density was 98.6 Wh kg−1, and the power density was 1727 W kg−1. The electrochemical stability was also confirmed, as after 10,000 charge–discharge cycles, the hybrid supercapacitor still maintained 87.66% capacity with a coulombic efficiency of 93.60%. Moreover, the fabricated material proved to be a suitable catalyst for the hydrogen evolution reaction (HER) process, showing low overpotential (98 mV at 10 mA cm−2), a Tafel slope of 46.20 mV dec−1, and current retention of 95.3% after 20 h. This synergistic effect between the porous framework of MOF, multivalent active NiCo2O4 centers, and conducting N-CNT structure provides the above improved properties, which are effective in providing electrons.