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

Molybdenum trioxide (MoO 3 ) 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-MoO 3 , the thermodynamically stable polymorph of MoO 3 . This present work encompasses the synthesis of single crystals of two metastable hexagonal MoO 3 described by the formulas {Mn 0.03 Na 0.01 }@[Mo 0.93 VI Mo 0.07 V O 3 ] ( 1 ) and {Cu 0.01 Na 0.01 }@[Mo 0.97 VI Mo 0.03 V O 3 ] ( 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.