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

Molybdenum trioxide (MoO₃) is a well-known transition metal oxide that has drawn much attention as a functional material having numerous applications. However, a vast majority of studies have primarily focused on α-MoO₃, the thermodynamically stable polymorph of MoO₃. This present work encompasses the synthesis of single crystals of two metastable hexagonal MoO₃ described by the formulas {Mn_0.03Na_0.01}@[Mo_0.93^VIMo_0.07^VO₃] (1) and {Cu_0.01Na_0.01}@[Mo_0.97^VIMo_0.03^VO₃] (2), their comprehensive structural characterization by single-crystal X-ray crystallography, and routine spectral and microscopic studies. Interestingly, compound 1 acts as an efficient electrocatalyst for the hydrogen evolution reaction (HER) as well as an effective proton conductor in comparison to the performance of compound 2. The HER activity of compound 1 is characterized by an overpotential of 340 mV@1 mA cm^-2 and a low Tafel slope of 75 mV/decade. The catalyst (compound 1) displays a Faradaic efficiency of 88% with a turnover frequency of 2.9 s^-1. The proton conductivity value of this compound (1) is determined to be 4.9 × 10^-3 S cm^-1 at 55 °C under 98% relative humidity; the relevant proton conduction is operated by the Grotthuss mechanism with an activation energy of 0.17 eV.