Bullet-like microstructured nickel ammonium phosphate/graphene foam composite as positive electrode for asymmetric supercapacitors

Abstract

Unique microstructured nickel ammonium phosphate Ni(NH₄)₂(PO₃)₄·4H₂O and Ni(NH₄)₂(PO₃)₄·4H₂O/GF composite were successfully synthesized through the hydrothermal method with different graphene foam (GF) mass loading of 30, 60 and 90 mg as a positive electrode for asymmetric supercapacitors. The crystal structure, vibrational mode, texture and morphology of the samples were studied with X-ray diffraction (XRD), Raman spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis and scanning electron microscopy (SEM). The prepared materials were tested in both 3-and 2-electrode measurements using 6 M KOH electrolyte. The composite material Ni(NH₄)₂(PO₃)₄·4H₂O/60 mg exhibited a remarkable gravimetric capacity of 52 mA h g⁻¹, higher than the 34 mA h g⁻¹ obtained for the Ni(NH₄)₂(PO₃)₄·4H₂O pristine sample, both at 0.5 A g⁻¹. For the fabrication of the asymmetric device, activated carbon from pepper seed (ppAC) was used as a negative electrode while Ni(NH₄)₂(PO₃)₄·4H₂O/60 mg GF was adopted as the positive electrode. The Ni(NH₄)₂(PO₃)₄·4H₂O/60 mg GF//ppAC asymmetric device delivered a specific energy of 52 Wh kg⁻¹ with an equivalent specific power of 861 W kg⁻¹ at 1.0 A g⁻¹ within a potential range of 0.0–1.5 V. Moreover, the asymmetric device displayed a capacity retention of about 76% for over 10 000 cycles at a high specific current of 10.0 A g⁻¹.