Nanoarchitectonics of Mesoporous M₂P₂O₇ (M = Mn(II), Co(II), and Ni(II)) and M_2–xCo_xP₂O₇ 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-M 2 P 2 O 7 ) particles and to fabricate graphite rodcoated (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 •nH 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.

Manganese oxide-based mesoporous thin-film electrodes: manganese disproportionation reaction in alkaline media

“…We previously showed that the graphite rod is a better substrate for the fabrication of mesoporous CaFe 2 O 4 and M 2 P 2 O 7 (fabricated using the MASA method); the mesoporous CaFe 2 O 4 and M 2 P 2 O 7 electrodes undergo mechanical degradation in an alkaline media over the FTO surface, but the same electrodes that are fabricated over the graphite surface are robust and unaffected during the electrochemical test. 39,44,45 However, this is not the case in the m -LMO electrodes, which undergo disproportionation reactions and display a similar degradation over both substrates. The only advantage of using a graphite substrate might be a smaller IR-drop in the OER potentials and better overpotential values.…”

“…S28‡) undergo a chemical transformation into layered β-M(OH) 2 in alkaline media. 44,45 The characterization and transformation processes of Mn 2 P 2 O 7 have been extensively investigated in our previous publication. 45 Therefore, we will focus on the disproportionation reaction on the graphite rod-coated mesoporous Mn 2 P 2 O 7 electrode, which is already transformed into β-Mn(OH) 2 before any electrochemical tests upon dipping the electrode into a 3.0 M KOH electrolyte solution.…”

“…44,45 The characterization and transformation processes of Mn 2 P 2 O 7 have been extensively investigated in our previous publication. 45 Therefore, we will focus on the disproportionation reaction on the graphite rod-coated mesoporous Mn 2 P 2 O 7 electrode, which is already transformed into β-Mn(OH) 2 before any electrochemical tests upon dipping the electrode into a 3.0 M KOH electrolyte solution. Since β-Mn(OH) 2 is a layered material, the Mn( vi ) disproportionation reaction is significantly enhanced and also supports our mechanism in Scheme 1.…”

Hierarchical Porous Carbon Supported Co2P2O7 Nanoparticles for Oxygen Evolution and Oxygen Reduction in a Rechargeable Zn–Air Battery

“…However, the catalyst exhibited only good ORR activity. It is therefore postulated that the phosphating of the active center may endow the catalyst with bifunctionality. − The potential center of cobalt pyrophosphate (Co 2 P 2 O 7 ) allows for the maintenance of several intermediate states of cobalt geometry, including tetrahedral, trigonal bipyramidal, and low-octahedral configurations . In addition, the flexibility between the PO 4 3– groups and Co promotes the flexible coordination of PO 4 3– groups with transition metals, which can easily change their local positions to ensure effective redox changes of the transition metals .…”