Nanoarchitectonics of Mesoporous M₂P₂O₇ (M = Mn(II), Co(II), and Ni(II)) and M_2-<i>x</i> Co<i>_x</i>P₂O₇ 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 (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)center dot nH(2)O-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), N-2 adsorption-desorption, and imaging (SEM and TEM) techniques. The electrochemical/chemical investigations showed that the GR-M2P2O7 electrodes transform to beta-M(OH)(2) in alkali media. The spherical m-Ni2P2O7 particles transform into spherical ultrathin nanoflakes of beta-Ni(OH)(2). However, the m-Mn2P2O7 and m-Co2P2O7 particles transform to much thicker beta-Mn(OH)(2) and beta-Co(OH)(2) plate-like nanoparticles, respectively. The size and morphology of the beta-M(OH)(2) particle depend on the K-sp of the M2P2O7 and determine the charge capacity (CC) and specific capacitance (SC) of the electrodes. The beta-Ni(OH)(2) and beta-Ni0.67Co0.33(OH)(2) electrodes display high CC (129 and 170 mC/cm(2), respectively) and SC (234.5 and 309 mF/cm(2), respectively) values. However, these values are almost 10x smaller in beta-Mn(OH)(2), beta-Co(OH)(2), beta-Mn1-xCox(OH)(2), and cobalt-rich beta-Ni1-xCox(OH)(2) electrodes.