Effect of calcination on structural, morphological, magnetic and electrochemical properties of mesoporous Ni₂P₂O₇ microplates

The NH₄NiPO₄·H₂O (NNPO) precursor powder was prepared by the hydrothermal method at 200 °C for 8 h. In the study of calcination effect on the structural, morphological, magnetic and electrochemical properties of mesoporous Ni₂P₂O₇ microplates (NPOs), NNPO was calcined at different temperatures of 500, 600 and 700 °C each for 1 h and 3 h in argon atmosphere. XRD analysis confirmed the orthorhombic structure of NNPO with space group Pmnm(59), whereas all NPOs possessed the monoclinic crystal structure with space group B21/c(14). The calculated average crystallite size of NPOs (D_WH) by the Williamson–Hall equation was in a nanoscale with the increase of size due to the increasing calcination temperature. Images of all NPOs obtained by SEM and TEM techniques showed a thin plate-like morphology with mesoporous nature of pore size ranging from 2 to 16 nm. XANES results indicated the oxidation state of 2 + for Ni cation in all NPOs samples. The magnetic properties of all NPOs studied at room temperature using VSM techniques indicated a weak ferromagnetic behavior with the enhancement of coercive field (H_c) and unsaturated magnetization (M), suggesting to originate from the increase of particle size due to the increase of calcination temperature. Interestingly, electrochemical properties of active NPOs electrodes studied by CV, GCD and EIS tests in 3 M KOH electrolyte revealed the maximum specific capacitance (Csc) of 905.687 F g⁻¹ at a current density 1 A g⁻¹ with a good cycling stability 90.80% retention after 1000 cycle test in a cell, having NPO6_1 h (obtained by calcination NNPO at 600 °C for 1 h) as active electrode.