Bifunctional nanoparticles decorated Ni1‐xMnxCo2O4 ultrathin nanoflakes‐like electrodes for supercapacitor and overall water splitting

Summary Synthesizing triple transition metal oxide (TTMO) is an extraordinary strategy to develop electrodes for efficient energy storage and conversion devices, owing to their unique nanostructure with high porosity and specific surface area. The cobalt‐based mixed‐valence oxides have attracted great attention due to their facile synthesis, low cost, and excellent electrochemical performance. However, less attention is paid to investigating the effect of different substitutions on the physico‐chemical properties of TTMO. In this study, nanoparticles (NPs) decorated ultrathin Ni1‐xMnxCo2O4 nanoflakes (NPs@NFs) are synthesized by tuning the molar ratio between Mn and Ni via facile deep eutectic solvents (DESs) method. Unique and highly porous NPs@NFs nanostructures aid to increase the overall surface area of the materials, whereas Mn, Ni, and Co ions participate in their redox‐active capacity, improving the electrochemical activity of the material. This Ni0.8Mn0.2Co2O4 hybrid nanostructure exhibited excellent supercapacitive performance with a high specific capacity (Cs) of 761 mAh g−1 at a higher current density of 30 mA cm−2 and superior cycling retention of 92.86% after 10 000 cycles. Further, a hybrid asymmetric supercapacitor (Ni0.8Mn0.2Co2O4//AC) device exhibited an extended potential window of 1.5 V, which results in an ultrahigh energy density of 66.2 W kg−1 by sustaining a power density of 1519 Wh kg−1. The electrocatalytic activity of the optimized Ni0.8Mn0.2Co2O4 shows the outstanding performance toward hydrogen evolution reaction (HER) (150 mV/ 161 mV dec−1) and oxygen evolution reaction (OER) (123 mV/47 mV dec−1) with a lower voltage of 1.51 V (@10 mA cm−2) for overall water splitting, with outstanding stability up to 25 hours. These results indicate that chemically synthesized ultrathin NPs@NFs‐like nanostructure is a capable electrode for multiple applications, such as supercapacitors, and overall water splitting. Highlights Facial synthesis of nanoflakes of Ni1‐XMnXCo2O4 thin films by a DES method. Fully decorated NPs on the interconnected NFs provided a higher surface area. Nanostructures with a maximum specific capacity of 761 mAh g−1 at 30 mA cm−2, with excellent stability of 92.86%. The solid‐state device shows an excellent energy density of 66.2 Wh kg−1 at a power density of 1519 W kg−1. The assembled Ni0.8Mn0.2Co2O4 /Ni‐based water splitting exhibited low voltage (1.51 V) and superb stability up to 25 hours. Specific capacity, HER, OER