Mesoporous Octagonal Microplates Co _{2– <i>x</i>} Ni _<i>x</i> P ₂ O ₇ ( <i>x</i> = 0.00, 0.50, 1.00, 1.50, and 2.00) Metal Pyrophosphates for High-Performance Asymmetric Supercapacitors

Monoclinic Co_2-<i>x</i> Ni _<i>x</i> P₂O₇ (<i>x</i> = 0.00-2.00) pyrophosphates were synthesized and composition-tuned to reveal an optimal morphology/porosity at <i>x</i> = 1.00 that delivered high-performance supercapacitor electrodes. Across the series, X-ray diffraction (XRD) results confirmed a pure phase of Co_2-<i>x</i> Ni _<i>x</i> P₂O₇ (<i>P</i>21/c), with Ni substitution providing acceptable crystallite-size shifts and a systematic lattice shrinkage. Field emission scanning electron microscopy (FE-SEM) showed that <i>x</i> = 1.00 specimen formed well-faceted octagonal microplates with the highest specific surface area (11.381 m²/g) and mesoporous surfaces with average pore sizes of ∼10 nm and mesopore volume of 0.0909 cm³/g, as revealed by Brunauer-Emmett-Teller/Barett-Joyner-Halenda (BET/BJH) analysis. X-ray photoelectron spectroscopy (XPS) identified Co²⁺, Ni²⁺, and P⁵⁺, which is consistent with OH^--coupled M²⁺/M³⁺ pseudocapacitance observed using cyclic voltammetry (CV)/galvanostatic charge-discharge (GCD) in 3 M KOH. The <i>x</i> = 1.00 electrode achieved 654 F/g at 0.5 A/g and maintained 84.6% of its initial state after a 3000-cycle GCD test at 5 A/g. An asymmetric device (Co_1.00Ni_1.00P₂O₇//rGO) delivered 56.68 Wh/kg at 938.36 W/kg. Electrochemical enhancement resulted from a combination of mixed-metal redox centers and optimized meso-porosity/microstructure, as evidenced by CV, GCD, and electrochemical impedance spectroscopy (EIS). The findings demonstrated that compositional control presented an effective strategy for controlling mesoporousity and enhancing redox utilization in Co-Ni pyrophosphate electrodes.