Controlled growth of NiMoO4 center dot H2O nanoflake and nanowire arrays on Ni foam for superior performance of asymmetric supercapacitors

A facile hydrothermal method is developed for fabrication of large-scale NiMoO4 center dot H2O arrays with robust adhesion on Ni foam. Importantly, the morphology of NiMoO4 center dot H2O can be easily controlled to be nanoflake (H-NF) or nanowire (H-NW) arrays by using NH4F as additive. The obtained nanoflake morphology delivers better electrochemical activity than that of nanowire. The electrochemical performance of anhydrous NiMoO4 arrays obtained by annealing the NiMoO4 center dot H2O has also been investigated for comparison. It is believed that the presence of the structural water of NiMoO4 enhances the capacitive performance by making it a good ionic conductor. Furthermore, an asymmetric supercapacitor (ASC) is constructed using the as-prepared NiMoO4 center dot H2O nanoflake arrays as the positive electrode and activated carbon (AC) as the negative electrode. The optimized ASC with an extended operating voltage range of 0-1.6 V displays excellent electrochemical performance with a high energy density of 53.8 W h kg(-1) at a power density of 239 W kg(-1) in addition to superior rate capability. Moreover, the H-NF//AC ASC device exhibits remarkable cycling stability with 73.4% specific capacitance retention after 4000 cycles. Our result shows that this unique NiMoO4 center dot H2O nanoflake array is promising for electrochemical energy applications.