Co(OH)2 Intercalated in MoO3 by Molecular Electrodeposition Resulting in a Conductive Material: A Molybdenum Bronze for Efficient Electrocatalytic Hydrogen Evolution

Today, in modern material research, molybdenum trioxide (MoO3), a versatile and smart metal oxide, is placed as one of the topmost advanced materials as far as its publication record goes. Specially, the intercalation science of MoO3, intercalating any organic and inorganic species into its lamellar space, has a huge impact not only in materials chemistry but also in overall chemical science. This is because such host–guest materials have enormous potential toward electrocatalysis. Surprisingly, the transition-metal-aqua complex intercalated in MoO3 was not known until we had reported recently (as a communication) by characterizing it using single-crystal X-ray crystallography, even though the applications of transition metals are unbeatable in any field of research. In this work, we have successfully intercalated {CoII(H2O)6}2+ into α-MoO3 layers, leading to the isolation of pink-colored water-soluble crystals of [Mo2 VIO6(CH3COO)­{CoII(H2O)6}0.5]·H2O (1), synthesized in a one-step aqueous-green synthesis and characterized by single-crystal X-ray crystallography. The homogeneous electrochemistry of compound 1 in its acidic aqueous solution results in the concomitant electrochemical deposition of Co­(OH)2@MoO3–x (2). Compound 2, representing a new class of molybdenum bronzes, intercalates β-Co­(OH)2 into the α-MoO3 layers. The bronze-like properties of compound 2 have been validated by measuring its electrical conductivity values. Compound 2 exhibits a modest electrical conductivity of 1.41 × 10–5 S cm–1 and a low activation energy (Ea) value of 390 meV at 25 °C. The electrical conductivity of compound 2 increases with increasing temperature. A conductive material is known to be a good electrocatalyst. Compound 2 acts as an efficient heterogeneous electrocatalyst for hydrogen evolution reaction (HER) at a low overpotential of 168 mV to achieve 10 mA cm–2 current density with a Tafel slope value of 98 mV dec–1. The catalytic performance of compound 2 is demonstrated by its long-term electrolysis stability for 10 h with its Faradic efficiency of 91%.