Highly Oxidized Graphene Anchored Ni(OH)2 Nanoflakes as Pseudocapacitor Materials for Ultrahigh Loading Electrode with High Areal Specific Capacitance

Abstract

The contact between Ni(OH)2 and graphene oxide (GO) determines the specific capacitance, high-rate performance, and stability of Ni(OH)2-GO composites when they were used as capacitive materials with high/ultrahigh material loading. To improve this contact, the exfoliated GO from Hummers’ method is oxidized twice for anchoring Ni(OH)2 nanoflakes. The X-ray photoelectron spectroscopy (XPS) results reveal that the further oxidation process increases the carbonyl (C–O) groups and oxygen content on the GO surface. The Ni(OH)2-GO composites were obtained through a simple hydrothermal process. Morphology and microstructure characterizations indicate that the further oxidation of GO improves the affinity of Ni(OH)2 and GO via the increased surface groups on the GO. Due to the high conductivity and suitable structure, the Ni(OH)2 anchored on the treated GO (Ni(OH)2/TGO) exhibits good capacitive performance and high areal specific capacitance. The Ni(OH)2/TGO exhibits high specific capacitance of 1236.4 F g–1 at 1.0 mV s–1 and 1374.8 F g–1 at 0.1 A g–1, respectively, which is higher than that of Ni(OH)2 on the untreated GO. The capacitance retention of Ni(OH)2/TGO is 52.2% even at 10 A g–1, which is higher than that of Ni(OH)2/GO (48.8%). For the high conductivity, the specific capacitance is still 996.2 F g–1 at 1.0 A g–1 even with ultrahigh material loading of 12.48 mg cm–2, which can be transferred to 12.06 F cm–2 calculated by areal specific capacitance. Furthermore, low deterioration is observed in Ni(OH)2/TGO (8.8% loss) after 1000-cycle charge–discharge test at 1.0 A g–1, which is lower than that of Ni(OH)2/GO (19.5% loss). The asymmetric supercapacitor, using the Ni(OH)2/TGO and activated carbon as the positive material and negative material, respectively, exhibits high energy density of 22.5 Wh kg–1 at 86.3 W kg–1 and 17.8 Wh kg–1 even at 4.05 kW kg–1.