Layered NH₄Co _x Ni_1−x PO₄·H₂O (0 ≦ x ≦ 1) nanostructures finely tuned by Co/Ni molar ratios for asymmetric supercapacitor electrodes

The synergetic effects of mixed transition metal compounds have demonstrated great potential in improving the electrochemical performance of materials. In this work, large scale of ammonium/bimetallic phosphates (NH₄Co _x Ni_1−x PO₄·H₂O) with layered nanostructures are synthesized by a facile one-step solvothermal method. The tuning effects of Co/Ni molar ratios on the crystal structure and morphology of NH₄Co _x Ni_1−x PO₄·H₂O are systematically studied by XRD, FTIR, SEM, TEM. It is revealed that the synergistic effects of cobalt and nickel may significantly influence the crystal structure and morphologies of NH₄Co _x Ni_1−x PO₄·H₂O, exposing more CoO₆ and NiO₆ octahedra active sites for redox reactions, improving the surface coverage of the redox species. Electrochemistry tests prove that the sample with a Co/Ni ratio of 2:3 which has layered nanoplate structure and abundant uniform pores, and exhibits the highest specific capacitance of 1567 F g⁻¹ at 1 A g⁻¹ and long-term cycling stability of 97.6 % compared with single-component ammonium nickel or cobalt phosphate hydrate. An asymmetric cell is constructed using NH₄Co _x Ni_1−x PO₄·H₂O (Co/Ni 2:3) and activated carbon with an operation potential from 0 to 1.65 V. This asymmetric device presented a high energy density of 37.4 Wh kg⁻¹ at a power density of 826 W kg⁻¹. More importantly, the device also displayed a good long-term cycling stability with 91.2 % capacity retention after 5000 cycles.