Nanoarchitectonics of Mesoporous MPO (M = Mn(II), Co(II), and Ni(II)) and MCoPO and Transformation to Their Metal Hydroxides with Decent Charge Capacity in Alkali Media.

A general synthetic method has been developed to synthesize spherical mesoporous metal pyrophosphate (-MPO) particles and to fabricate graphite rod-coated (GR-MPO) electrodes, which are important as energy storage materials. The clear aqueous solution of the ingredients (namely, M(HO), HPO, water, and P123) assembles, upon excess water evaporation, into a mesostructured MHPO(NO)·HO-P123 semisolid that is calcined to produce the spherical -MPO (where M is Ni, Co, Mn, Ni/Co, or Mn/Co) particles, coated over GR, and calcined to fabricate the GR-MPO electrodes. The mesostructured and mesoporous materials are characterized using diffraction (XRD), spectroscopy (ATR-FTIR, XPS, and EDX), N adsorption-desorption, and imaging (SEM and TEM) techniques. The electrochemical/chemical investigations showed that the GR-MPO electrodes transform to β-M(OH) in alkali media. The spherical -NiPO particles transform into spherical ultrathin nanoflakes of β-Ni(OH). However, the -MnPO and -CoPO particles transform to much thicker β-Mn(OH) and β-Co(OH) plate-like nanoparticles, respectively. The size and morphology of the β-M(OH) particle depend on the of the MPO and determine the charge capacity (CC) and specific capacitance (SC) of the electrodes. The β-Ni(OH) and β-NiCo(OH) electrodes display high CC (129 and 170 mC/cm, respectively) and SC (234.5 and 309 mF/cm, respectively) values. However, these values are almost 10× smaller in β-Mn(OH), β-Co(OH), β-MnCo(OH), and cobalt-rich β-NiCo(OH) electrodes.