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

Unique microstructured nickel ammonium phosphate Ni(NH 4 ) 2 (PO 3 ) 4 ·4H 2 O and Ni(NH 4 ) 2 (PO 3 ) 4 ·4H 2 O/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(NH 4 ) 2 (PO 3 ) 4 ·4H 2 O/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(NH 4 ) 2 (PO 3 ) 4 ·4H 2 O 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(NH 4 ) 2 (PO 3 ) 4 ·4H 2 O/60 mg GF was adopted as the positive electrode. The Ni(NH 4 ) 2 (PO 3 ) 4 ·4H 2 O/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 .