Fabrication of mesoporous nickel pyrophosphate electrodes and their transformation to nickel hydroxide with decent capacitance in alkaline media

Abstract

The development of high-energy-density electrodes is paramount for the advancement of renewable and clean energy storage materials. In this study, we have devised a synthetic approach to fabricate mesoporous Ni₂P₂O₇ (m-NiPP) electrodes with a decent charge capacity. The method involves the formation of a liquid crystalline mesophase from an aqueous solution containing nickel nitrate hexahydrate salt (Ni(II)), pyrophosphoric acid (PPA), and a non-ionic surfactant (P123). The mesophase solidifies through the polymerization of Ni(II) ions and PPA, ultimately forming a mesostructured Ni₂H_xP₂O₇(NO₃)_x·nH₂O semi-solid, which can be subsequently calcined to yield mesoporous Ni₂P₂O₇ (m-NiPP). The gelation and polymerization process can be monitored using gravimetric, ATR-FTIR, XRD, and POM techniques as water evaporates during the transformation. The results reveal that the reaction between the Ni(II) ion and PPA initiates in the solution phase, continues in the gel phase, and concludes upon gentle heating. The same clear aqueous solution can be coated onto a substrate, such as FTO or graphite rods, and then calcined at various temperatures to produce the m-NiPP electrodes, composed of spherical mesoporous NiPP particles. These electrodes remain amorphous over a wide temperature range, but crystallize at approximately 700 °C while retaining their porous structure. However, when exposed to a 3 M KOH solution, the spherical m-NiPP particles undergo a transformation into β-Ni(OH)₂ particles. These transformed particles are approximately 1.5 nm thick, equivalent to 3–4 layers, and 7 nm wide, all while maintaining their spherical morphology. This transformation process occurs rapidly for amorphous m-NiPP and proceeds more slowly in the case of crystalline m-NiPP. The resulting electrodes exhibit a substantial charge capacity of 422 C g⁻¹ and an impressive specific capacitance of over 1407 F g⁻¹.