Oxidation of Ni(H₂O)₆@MoO₃ to Ni₂O₃/MoO₃ Composites by Aerial Annealing: Electrocatalytic Hydrogen Evolution

Molybdenum trioxide (α-MoO₃) is a promising and inexpensive alternative to platinum group metals (PGMs), for electrocatalytic hydrogen evolution reaction (HER). However, to make it a viable candidate for electrocatalytic systems, we must address the hurdles associated with its inferior electrical conductivity and lack of active sites. Unlike Mo-based compounds such as MoS₂ and MoSe₂, which possess catalytically active edges, α-MoO₃ lacks inherent active sites for HER. Previous studies have employed various strategies to activate MoO₃ for HER, yet its activation in near-neutral conditions remain largely unexplored. In this study, a previously known α-MoO₃ intercalating {Ni(H₂O)₆}²⁺, [Mo^VI ₂O₆(CH₃COO){Ni^II(H₂O)₆}_0.5] ⋅ H₂O (Ni(H₂O)₆@MoO₃) is prepared via a simple and scalable room-temperature aqueous synthesis. In the subsequent aerial thermal annealing process at 300, 400 and 500 °C, Ni(H₂O)₆@MoO₃ acts as a self-sacrificial template, yielding mixed metal oxide composites of nickel and molybdenum (named as MoO₃-300, MoO₃-400 and MoO₃-500). The HR-TEM and XPS analyses confirm the formation of the Ni₂O₃ phase alongside the orthorhombic α-MoO₃. The annealing temperature plays a key role in the crystallinity, phase, morphology, and electrocatalytic performance of the resulting composites. The composite formed at 400 °C (MoO₃-400) shows the best electrocatalytic performance among them, showcasing a fivefold enhancement in the HER current density as compared to that shown by commercially available α-MoO₃ in mildly acidic acetate buffer. The enhanced performance towards HER by MoO₃-400 could be attributed to the nanostructured morphology attained via thermal treatment, which provides greater access to the surface sites and the synergistic interaction between the nickel oxide phases and MoO₃ structure, enabling an intermediate pH HER activity rarely reported for molybdenum oxide materials.