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Hierarchical composite films grown on current collectors are popularly reported to be directly used as electrodes for supercapacitors. Highly dense and conductive NiCo2O4 nanowires are ideal backbones to support guest materials. In this work, low crystalline MnO2 nanoflakes are electrodeposited onto the surface of NiCo2O4 nanowire films pre-coated on nickel foam. Each building block in the composite films is a NiCo2O4−MnO2 core−shell nanowire on conductive nickel foam. Due to the co-presence of MnO2 and NiCo2O4, the MnO2@NiCo2O4@Ni electrode exhibits higher specific capacitance and larger working voltage than the NiCo2O4@Ni electrode. It can have a high specific capacitance of 1186 F·g−1 at 1 A·g−1. When the core−shell NiCo2O4−MnO2 composite and activated carbon are assembled as a hybrid capacitor, it has the highest energy density of 29.6 Wh·kg−1 at a power density of 425 W·kg−1 with an operating voltage of 1.7 V. This work shows readers an easy method to synthesize composite films for energy storage.

The research on supercapacitors has been attractive due to their large power density, fast charge/discharge speed and long lifespan. The electrode materials for supercapacitors are thus intensively investigated to improve the electrochemical performances. Various transition metal layered double hydroxides (LDHs) with a hydrotalcite-like structure have been developed to be promising electrode materials. Earth-abundant metal hydroxides are very suitable electrode materials due to the low cost and high specific capacity. This is a review paper on NiMn LDHs for supercapacitor application. We focus particularly on the recent published papers using NiMn LDHs as electrode materials for supercapacitors. The preparation methods for NiMn LDHs are introduced first. Then, the structural design and chemical modification of NiMn LDH materials, as well as the composites and films derived from NiMn LDHs are discussed. These approaches are proven to be effective to enhance the performance of supercapacitor. Finally, the reports related to NiMn LDH-based asymmetric supercapacitors are summarized. A brief discussion of the future development of NiMn LDHs is also provided.