ZIF-67-derived edge-oriented graphene clusters coupled with carbon nanotubes containing encapsulated Co nanoparticles for high-frequency electrochemical capacitors

There is a strong interest in increasing the frequency response of electrochemical capacitors (ECs) from typically less than 1 Hz to the hundreds or the kilo Hz range, so that such high-frequency ECs (HF-ECs) could replace conventional capacitors for AC line-frequency filtering and other capacitor applications. The development of such HF-ECs is hindered by their typically low capacitance density and operation voltage. Herein, by treating ZIF-67 particulate films in CH₄/H₂ plasma, edge-oriented graphene (EOG) formed around the carbonized ZIF-67 particulate skeleton, and this EOG was coupled with carbon nanotubes (CNTs) that were grown with the aid of Co catalyst nanoparticles, which were generated by reducing the Co²⁺ ions associated with ZIF in the plasma. Used as electrodes, these EOG/CNT/carbonized ZIF-67 composites exhibited a large electrode areal capacitance of 1.0 mF cm^-2 and an excellent frequency response of -84° phase angle at 120 Hz in aqueous electrolyte cells, whereas values of 0.67 mF cm^-2 and -78° for the electrode areal capacitance and the phase angle at 120 Hz, respectively, were obtained in organic electrolyte cells with the operation voltage of 2.5 V. Using three pairs of electrodes stacked together, a single integrated cell operating at 7.5 V and having a characteristic frequency of ∼3.8 kHz at -45° phase angle, was demonstrated. These results suggest the potential to use this EOG/CNT/carbonized ZIF-67 composite structure for developing HF-ECs.