Nickel–cobalt phosphate nanoparticle-layer shielded in-situ grown copper–nickel molybdate nanosheets for electrochemical energy storage

Transition metal molybdates and phosphates are prominent electroactive materials for energy storage devices due to their high theoretical capacity, multiple valence states, good electrical conductivity, and natural abundance. Herein, we report in-situ grown high redox-active copper-nickel molybdate (CuNiMo) on copper foam (CF) via a one-pot solvothermal approach and the influence of reaction time on its morphology was studied in detail. The CuNiMo electrode prepared at a reaction time of 9 h (CuNiMo-9/CF) exhibited ultrathin nanosheets (UTNSs) that are vertically grown from CF substrate with the interconnected network. Owing to the advanced morphological features, the CuNiMo-9/CF electrode delivered a superior areal capacity of 387.5 µAh cm⁻² (at 5 mA cm⁻²) to the other CuNiMo electrodes obtained at the reaction times of 6 and 12 h. In addition, the bare CF, CuNi-9/CF, and CuMo-9/CF electrodes were investigated for comparison. Next, Co-Ni phosphate (CoNiP) nanoparticle-layer was decorated on CuNiMo-9/CF UTNSs by a solution-immersion technique to enhance the redox chemistry. Exploiting the synergistic features of mixed metal molybdates and phosphates, the CoNiP@CuNiMo-9/CF composite material exhibited an exalted areal capacity of 666 µAh cm⁻² at the same current density. Furthermore, the hybrid cell (HC) was assembled with the CoNiP@CuNiMo-9/CF as the positive electrode to explore its practical applicability. The assembled HC also exhibited maximum energy and power densities of 38.2 Wh kg⁻¹ (1.09 mWh cm⁻²) and 2327.5 W kg⁻¹ (66.5 mW cm⁻²), respectively. Also, the HC device demonstrated good cycling stability with a notable capacity retention of 87.7% after 3000 cycles. The real-time viability of HC was also tested by powering various electronic components.