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

Abstract

Today, in modern material research, molybdenum trioxide (MoO 3 ), 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 MoO 3 , 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 MoO 3 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 {Co II (H 2 O) 6 } 2+ into α-MoO 3 layers, leading to the isolation of pink-colored water-soluble crystals of [Mo 2 VI O 6 (CH 3 COO){Co II (H 2 O) 6 } 0.5 ]•H 2 O (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 @MoO 3−x (2). Compound 2, representing a new class of molybdenum bronzes, intercalates β-Co(OH) 2 into the α-MoO 3 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%.

Hexagonal Mo Bronze: Single Crystal Structures, Electrocatalytic Hydrogen Evolution, and Proton Conductivity

“…Molybdenum oxide-based materials, like MoO x and MoO x S y , have also been used to carry out surface modification of liquid metal particles, resulting in the modulation of their electronic band structures, which have potential applications in the field of sensing, photocatalysis, optics, and electronics . Our group was the pioneer in reporting single crystals of α-MoO 3 intercalating transition metal complexes, {Ni­(H 2 O) 6 } 2+ and {Co­(H 2 O) 6 } 2+ , whose electrodeposition led to electrically conducting molybdenum bronze compounds [HMo 3 VI Mo V O 12 (CH 3 COO)­{Ni II (H 2 O) 5 (OH)}] and Co­(OH) 2 @MoO 3– x , exhibiting excellent electrocatalytic HER activity. , The bronze compounds having both Mo­(V) and Mo­(VI) valence states are electrically conducting due to the presence of free electrons, making them a suitable candidate for the electrocatalytic HER . Porous materials are of particular interest in electrocatalytic water splitting applications owing to their large surface area that can accommodate small molecules …”

Synergy of {Co(H2O)6}2+ with a Polyoxometalate Leads to Aqueous Homogeneous Hydrogen Evolution: Experiments and Computations

“…Therefore, the major target of the modern research is to develop highly efficient and robust electrocatalysts. Nature has gifted several attractive metal (Fe and Ni) hydrogenase catalysts, which catalyze proton reduction to produce molecular H 2 . − Over the years, numerous efficient and stable heterogeneous electrocatalysts for hydrogen evolution reaction (HER) have been developed. − However, the homogeneous catalysts are more important than the heterogeneous catalysts not only because of their simple reaction mechanisms but also because of the fact that it is much easier to handle a homogeneous catalyst (user-friendly) . Moreover, most of the naturally occurring catalysts are homogeneous in nature.…”