Oxygen Evolution and Reduction on Two-Dimensional Transition Metal Dichalcogenides

Motivated by the need to find good electrocatalysts for water oxidation and O2 reduction, composed of nontoxic and earth-abundant elements, a systematic screening of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is performed. To identify compounds that are intrinsically active and can fully take advantage of the high surface area of 2D catalysts, this study focuses on the properties of the ideal basal planes of 2D TMDCs, in the 2H, 1T, and 1T′ phases. Over two hundred materials proposed in computational databases are studied by means of first-principles-based simulations coupled with continuum embedding models to account for the presence of electrochemical environments. The best candidates with overpotentials for the oxygen evolution and reduction reactions (OER and ORR) lower than 0.5 V under acidic conditions and higher stability against degradation in electrochemical environments are selected. For OER, the designed workflow identifies one active and thermodynamically stable material, and seven materials that are metastable at the oxidative potentials and acidic pH. On the other hand, for ORR, we identify 20 materials with overpotentials less than 0.5 V. Among these compounds, six bifunctional materials have been experimentally reported, with 1T-NbTe2 and 1T′-MoTe2 being the best performing catalysts for OER and ORR, respectively

Overlap of Radial Dangling Orbitals Controls the Relative Stabilities of Polyhedral B n H n-X Isomers (n = 5-12, x = 0 to n-1)

The removal of H atoms from polyhedral boranes results in the formation of dangling radial orbitals with one electron each. If there is a requirement of electrons for skeletal bonding to meet the Wade's rule, these are provided from the exohedral orbitals. Additional electrons occupy a linear combination of the dangling orbitals. Stabilization of these molecular orbitals depends on their overlap. The lateral (sideways) overlap of dangling orbitals decreases with the decreasing cluster size from 12 to 5 boron atoms as the orbitals become more and more splayed out. Thus, as the number of dangling orbitals increases, the destabilization of their combinations increases at a higher rate for smaller polyhedral boranes, leading to flat structures with the removal of a fewer number of hydrogens. Though exohedral orbitals form better overlap in larger polyhedral clusters, the increase of electrons with the removal of H atoms results in occupancy of antibonding skeletal orbitals (beyond Wade's rules) and leads to flat structures. The reverse happens when hydrogens are added to a flat cluster. Substitution of BH by Si does not change structural patterns. © Copyright © 2019 American Chemical Society