Single-crystalline Ni(OH)2 nanosheets vertically aligned on a three-dimensional nanoporous metal for high-performance asymmetric supercapacitors
Transition-metal hydroxides (TMHOs) or oxides (TMOs) with layered crystalline structures are attractive electrode materials for high-density charge storage in electrochemical supercapacitors. However, their randomly stacked nanostructures on conductive reinforcements, typically carbon materials, exhibit only modest enhancement of rate capability because of poor electron and ion transports that are limited by highly anisotropic conductivity, excessive grain boundaries and weak TMHO or TMO/C interfaces. Here we report a hybrid electrode design to tackle all three of these problems in layered Ni(OH)2 for high-performance asymmetric supercapacitors, wherein the single-crystalline Ni(OH)2 nanosheets are vertically aligned on a three-dimensional bicontinuous nanoporous gold skeleton with epitaxial Au/Ni(OH)2 interfaces (NP Au/VA Ni(OH)2). As a result of the unique nanoarchitecture, the pseudocapacitive behavior of Ni(OH)2 is dramatically enhanced for ensuring a volumetric capacitance as high as ∼2911 F cm−3 (∼2416 F g−1 for the constituent Ni(OH)2) in the NP Au/VA Ni(OH)2 electrode with excellent rate capability. Asymmetric supercapacitors assembled with this NP Au/VA Ni(OH)2 electrode and activated carbon have a high gravimetric energy of 31.4 W h kg−1 delivered at an exceptionally high power density of 100 kW kg−1 with excellent cycling stability.