Nanoarchitectonics of mesoporous M2P2O7 (M = Mn(II), Co(II), and Ni(II)) and M2–xCoxP2O7 and transformation to their metal hydroxides with decent charge capacity in alkali media

Abstract

A general synthetic method has been developed to synthesize spherical mesoporous metal pyrophosphate (m-M2P2O7) particles and to fabricate graphite rod-coated (GR-M2P2O7) electrodes, which are important as energy storage materials. The clear aqueous solution of the ingredients (namely, [M(H2O)6](NO3)2, H4P2O7, water, and P123) assembles, upon excess water evaporation, into a mesostructured M2HxP2O7(NO3)x·nH2O–P123 semisolid that is calcined to produce the spherical m-M2P2O7 (where M is Ni, Co, Mn, Ni/Co, or Mn/Co) particles, coated over GR, and calcined to fabricate the GR-M2P2O7 electrodes. The mesostructured and mesoporous materials are characterized using diffraction (XRD), spectroscopy (ATR-FTIR, XPS, and EDX), N2 adsorption–desorption, and imaging (SEM and TEM) techniques. The electrochemical/chemical investigations showed that the GR-M2P2O7 electrodes transform to β-M(OH)2 in alkali media. The spherical m-Ni2P2O7 particles transform into spherical ultrathin nanoflakes of β-Ni(OH)2. However, the m-Mn2P2O7 and m-Co2P2O7 particles transform to much thicker β-Mn(OH)2 and β-Co(OH)2 plate-like nanoparticles, respectively. The size and morphology of the β-M(OH)2 particle depend on the Ksp of the M2P2O7 and determine the charge capacity (CC) and specific capacitance (SC) of the electrodes. The β-Ni(OH)2 and β-Ni0.67Co0.33(OH)2 electrodes display high CC (129 and 170 mC/cm2, respectively) and SC (234.5 and 309 mF/cm2, respectively) values. However, these values are almost 10× smaller in β-Mn(OH)2, β-Co(OH)2, β-Mn1–xCox(OH)2, and cobalt-rich β-Ni1–xCox(OH)2 electrodes.