At the cell voltages required to reach technologically viable current densities in proton-exchange membrane (PEM) electrolyzers, IrO2 catalysts are suspected to undergo a transformation to an amorphous hydrous form. Here, we present a systematic ab initio thermodynamics study analyzing the shape and stability of IrO2 nanoparticles in this potential range. Our results confirm a thermodynamic instability of the rutile crystal structure induced by the stabilization of highly oxidized O species at the surface already at onset potentials for the oxygen evolution reaction (OER). Intriguingly, this is preceded by a transformation of the equilibrium shape at even lower potentials. Instead of the well-studied IrO2(110) facets, this shape is dominated by IrO2(111) facets that have hitherto barely received attention. Our findings highlight the need to extend detailed characterization studies to this high-potential range, not the least to establish more suitable active-site models for the OER that may then serve as the basis for computational screening studies aimed at reducing the rare-metal content in future PEM OER catalysts.

The ionization potentials of electrolyte solutions provide important information about the electronic structure of liquids and solute-solvent interactions. We analyzed the positions of solute and solvent bands of aqueous hydroxide and the influence of the solvent environment on the ionization potential of hydroxide ions. We used the concept of a computational hydrogen electrode to define absolute band positions with respect to vacuum. We found that many-body perturbation theory in the G0 W0 approximation substantially improves the relative and absolute positions of the band edges of solute and solvent with respect to those obtained within Density Functional Theory, using semi-local functionals, yielding results in satisfactory agreement with recent experiments.

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Methods from Jahn-Teller theory and invariant theory have been combined for the construction of analytic diabatic potential-energy surfaces of triply degenerate states in tetrahedral molecules. The potentials of a threefold degenerate electronic state of T(2) symmetry, subject to the T(2)xt(2) or T(2)x(t(2)+t(2)) Jahn-Teller effect in a three-dimensional or six-dimensional space of nuclear coordinates, respectively, are considered. The permutation symmetry of four identical nuclei is taken into account in the polynomial expansion of the diabatic surfaces. Symmetry adapted polynomials up to high orders are explicitly given and a simple combinatorial scheme was developed to express terms of arbitrary order as products of a small number of polynomials which are invariant under the permutation of identical nuclei. The method is applied to the methane cation in its triply degenerate ground state. The parameters of the analytic surfaces have been fitted to accurate ab initio data calculated at the full-valence CASSCF/MRCI/cc-pVTZ level. A three-sheeted six-dimensional analytic potential-energy surface of the (2)T(2) ground state of CH(4) (+) is reported, which involves terms up to eighth order in the degenerate stretching coordinate, up to 12th order in the degenerate bending coordinate, and up to fourth order in the stretch-bend coupling.

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