Hierarchical mesoporous NiO nanoarrays with ultrahigh capacitance for aqueous hybrid supercapacitor

Abstract Hybrid supercapacitors (HSCs), which usually involve faradaic or pseudocapacitive positive materials and electric double-layer capacitive negative materials, have demonstrated great potentials with enhanced energy density outdistancing traditional electrical double-layer capacitors. To endow materials with higher energy density and power density, the rational design and synthesis of electrodes with hierarchical and mesoporous structure are highly desired. In this work, we report the fabrication of hierarchical mesoporous NiO nanoarrays (NiO-HMNAs) as a battery-type electrode for hybrid supercapacitor with an ultrahigh specific capacitance (3114 F g −1 at the current density of 5 mA cm −2 ), which is beyond the theoretical faradaic capacitance value of NiO. NiO-HMNAs were prepared by a self-generated sacrificial template approach, which involves the preparation of hierarchical ZnO/NiO composites by co-deposition of Zn 2+ and Ni 2+ and the removal of ZnO by an alkali etching process to construct mesoporous structure. The ultrahigh capacitance of NiO-HMNAs is ascribed to the nearly full redox reaction of NiO in the unique hierarchical mesoporous architecture, and the raised electrical double-layer capacitance at the enlarged surface of nanoarrays. Moreover, the optimized HSC fabricated by using NiO-HMNAs as the positive electrode and macroporous graphene monoliths (MGMs) as the negative electrode has demonstrated a high energy density of 67.0 W h kg −1 at a power density of 320 W kg −1 with a maximum voltage of 1.6 V and outstanding cycleability (capacitance retention of 89.6% after 6000 cycles).