Alcohol-mediated self-assembly of mesoporous transition-metal pyrophosphates and phosphates: transformation to metal hydroxides and alkaline OER performance

Abstract

The sol–gel process offers a scalable route to mesoporous materials whose nanoarchitecture can be tuned by controlling the reaction medium. Here, methanol, ethanol, and butanol were used to regulate the reactivity and self-assembly of transition-metal nitrates, pyrophosphoric acid (PPA), and Pluronic® P123. Clear precursor solutions formed mesostructured (M₂H_xP₂O₇(NO₃)_x·yH₂O)_n–P123 intermediates (M = Mn, Co, Ni) via precipitation or gelation during solvent evaporation. The solution, precipitate, and supernatant fractions were isolated, characterized, and calcined to yield mesoporous products. Solution- and precipitate-derived samples produced amorphous mesoporous M₂P₂O₇ that crystallized at elevated temperatures, whereas the supernatant yielded mesoporous M₃(PO₄)₂. Mesoporous Ni₂P₂O₇ exhibited the highest surface area (410 m² g⁻¹) and most uniform pore size (5.3 nm), followed by Co₂P₂O₇, while Mn₂P₂O₇ was least favourable, reflecting solvent-dependent sol–gel behaviour. Electrodes fabricated on FTO and graphite and tested in 1 M KOH showed limited stability on FTO but good stability on graphite, except for Mn₂P₂O₇, which underwent Mn(VI) disproportionation. The most stable and OER-active electrodes originated from the Co(II) supernatant. However, all metal phosphate and metal pyrophosphate electrodes transformed into metal hydroxides under alkaline conditions, indicating that the observed electrochemical activity arises from the metal hydroxide phases.