Three-dimensional electrode of Ni/Co layered double hydroxides@NiCo2S4@graphene@Ni foam for supercapacitors with outstanding electrochemical performance

Abstract

ABSTRACT Great challenge for the fabrication of free-standing three-dimensional electrode still remains to simultaneously achieve high specific capacitance, rate performance and cycle stability. The paper reprted a new three-dimensional (3D) electrode of Ni/Co layered double hydroxide@NiCo2S4@graphene@Ni foam (Ni/Co-LDH@NiCo2S4@G) for supercapacitors. The as-prepared 3D electrode offers an unique architecture, which create an efficient conduction network and maximum utilization of space and interface. The graphene acts as well-knit and conductive skin coated on the skeleton of Ni foam for growing NiCo2S4. The conductive NiCo2S4 array serves as bridge between Ni/Co-LDH and graphene, leading to ultrafast electron transfer and electrolyte transport. A slew of splits and holes existing in the NiCo2S4 array play one role as the ion-reservoir to contain host of electrolyte ions. To evaluate the feasibility of 3D electrode’s application in supercapacitors, the electrochemical performance was investigated by using the three-electrodes test system and two-electrodes test system, respectively. The 3D electrode exhibits high specific capacitance (2001.2 F g−1), big areal capacitance (15.21 F cm−2), high rate capability (1645.6 F g−1 at the current density of 8 A g−1) and prominent synergetic effect (more than 1.6-fold that of the theoretical capacitance) in a three-electrode test system with 3 M KOH electrolyte. Interestingly, the addition of K3Fe(CN)6 into the KOH electrolyte further enhances the pseudocapacitance via both directly contributing pseudocapacitance to the Ni/Co-LDH@NiCo2S4@G and promoting electron gain and loss of Co and Ni ions. The specific capacitance increases to 6282.6 F g−1 at the current density of 6 A g−1. The asymmetric supercapacitor of Ni/Co-LDH@NiCo2S4@G/activated carbon provides the energy density of 102.8 W h kg−1 at the power density of 800 W kg−1 in the two-electrodes test system, which is close to that of lithium ion battery. Such a large capacitive performance make it can be used as promising electrode materials for next-generation high-performance supercapacitors.