Nanoarchitectonics of Mesoporous M 2 P 2 O 7 (M = Mn(II), Co(II), and Ni(II)) and M 2- x Co x P 2 O 7 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 -M 2 P 2 O 7 ) particles and to fabricate graphite rod-coated (GR-M 2 P 2 O 7 ) electrodes, which are important as energy storage materials. The clear aqueous solution of the ingredients (namely, [M(H 2 O) 6 ](NO 3 ) 2 , H 4 P 2 O 7 , water, and P123) assembles, upon excess water evaporation, into a mesostructured M 2 H x P 2 O 7 (NO 3 ) x · n H 2 O-P123 semisolid that is calcined to produce the spherical m -M 2 P 2 O 7 (where M is Ni, Co, Mn, Ni/Co, or Mn/Co) particles, coated over GR, and calcined to fabricate the GR-M 2 P 2 O 7 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-M 2 P 2 O 7 electrodes transform to β-M(OH) 2 in alkali media. The spherical m -Ni 2 P 2 O 7 particles transform into spherical ultrathin nanoflakes of β-Ni(OH) 2 . However, the m -Mn 2 P 2 O 7 and m -Co 2 P 2 O 7 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 K sp of the M 2 P 2 O 7 and determine the charge capacity (CC) and specific capacitance (SC) of the electrodes. The β-Ni(OH) 2 and β-Ni 0.67 Co 0.33 (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 10× smaller in β-Mn(OH) 2 , β-Co(OH) 2 , β-Mn 1- x Co x (OH) 2 , and cobalt-rich β-Ni 1- x Co x (OH) 2 electrodes.