Synergistic effect of Ni and Co ions on molybdates for superior electrochemical performance

Nickel cobalt (Ni–Co) molybdates with different stoichiometric nickel and cobalt ratios have been synthesized by a facile hydrothermal method. The hydrothermal method is robust enough to synthesis of Ni–Co molybdates with same crystal structure and similar nanorods morphology at different ratios of Ni and Co. The electrochemical performance of Ni–Co molybdates is measured as positive electroactive material in a three-electrode configuration, which demonstrates typical faradaic redox behaviors of Ni–Co molybdates that consistent with battery-type materials. Owing to the synergistic effect of Ni and Co ions, the electrochemical performance in terms of specific capacity, rate capability and cycling stability can be readily tuned by varying the Ni and Co content. In particular, the Ni0.67Co0.33MoO4 demonstrates the highest specific capacity of 441C g−1 at 1 A g−1, superior rate capability of 71% capacity retention after 50 times increase in current density. In addition, the Ni0.67Co0.33MoO4 is used to assemble hybrid supercapacitors with reduced graphene oxide, which shows high specific capacity (119C g−1 at 1 A g−1), high specific energy (25.6 Wh kg−1 at 775 W kg−1) and high specific power (7750 W kg−1 at 13.2 Wh kg−1).