Constructing highly-efficient electron transport channels in the 3D electrode materials for high-rate supercapacitors: The case of NiCo2O4@NiMoO4 hierarchical nanostructures

It's demonstrated that transport channels of electrons are very crucial to the performances of supercapacitor electrodes and different morphologies of nanomaterials usually imply different properties on electron transport in them. Hence, we constructed two types of NiCo2O4@NiMoO4 hierarchical core-shell nanostructures, in which NiCo2O4 scaffolds are in form of uninterrupted nanosheet arrays (UNSAs) or nanoneedle arrays (NNAs) and NiMoO4 hierarchies in form of nanosheets, and investigated electron transport properties of their resultant electrodes. Results showed that NiCo2O4-UNSA@NiMoO4 and NiCo2O4-NNA@NiMoO4 electrodes respectively exhibit high areal capacitances of 7.29 F cm−2 and 5.96 F cm−2 (current density of 2 mA cm−2), both of which are much improved compared with the previous work. And more interestingly, the capacitances from NiCo2O4-UNSA@NiMoO4 electrodes are enhanced by 22-39% compared to those from NiCo2O4-NNA@NiMoO4 ones at various current densities. And theoretical simulations and electrochemical impedance spectroscopy results confirmed that compared to the NNA ones, the UNSA scaffolds can provide more accessible and efficient electron transport channels (especially at high-rate charge-discharge processes), which leads to a much lower charge-transfer resistance and superior rate capability. Furthermore, the assembled asymmetric supercapacitors of NiCo2O4-UNSA@NiMoO4//active carbon show a high energy density (52.6 Wh kg−1 at 332.4 W kg−1) and a high power density (2632.8 W kg−1 at 36.9 Wh kg−1).