Continuum models of the electrochemical diffuse layer in electronic-structure calculations

Continuum electrolyte models represent a practical tool to account for the presence of the diffuse layer at electrochemical interfaces. However, despite the increasing popularity of these in the field of materials science, it remains unclear which features are necessary in order to accurately describe interface-related observables such as the differential capacitance (DC) of metal electrode surfaces. We present here a critical comparison of continuum diffuse-layer models that can be coupled to an atomistic first-principles description of the charged metal surface in order to account for the electrolyte screening at electrified interfaces. By comparing computed DC values for the prototypical Ag(100) surface in an aqueous solution to experimental data, we validate the accuracy of the models considered. Results suggest that a size-modified Poisson-Boltzmann description of the electrolyte solution is sufficient to qualitatively reproduce the main experimental trends. Our findings also highlight the large effect that the dielectric cavity parameterization has on the computed DC values.

Voltage effects on the stability of Pd ensembles in Pd-Au/Au(111) surface alloys

The catalytic performance of multimetallic electrodes is often attributed to a beneficial combination of ligand, strain, and ensemble effects. Understanding the influence of the electrochemical environment on the stability of the alloy surface structure is thus a crucial component to the design of highly active and durable electrocatalysts. In this work, we study the effects of an applied voltage to electrocatalytic Pd-Au/Au(111) surface alloys in contact with a model continuum electrolyte. Using planewave density functional theory, two-dimensional cluster expansions are parameterized and used to simulate dilute Pd-Au surface alloys under electrochemical conditions via Metropolis Monte Carlo within an extended canonical ensemble. While Pd monomers are stable at all potentials considered, different extents of surface electrification are observed to promote the formation of Pd dimers and trimers, as well as clusters of Pd monomers. We find that the relative proportion of monomer, dimer, and trimer surface fractions is in good agreement with in situ scanning tunneling microscopy measurements. The further development and refinement of the approaches described herein may serve as a useful aid in the development of next-generation electrocatalysts.

Estimation of electric field effects on the adsorption of molecular superoxide species on Au based on density functional theory

Superoxide species are key intermediates in the oxygen reduction reactions (ORR) that occur at the cathodes of aprotic metal-air batteries. Herein we report a DFT study of the effects of an externally applied electric field (ε) on the stability of various molecular superoxide species, including MO₂ (M = Li, Na, K) and O₂^-, on gold surfaces, which shows that the stability of such species on Au electrodes can be materially affected by the presence of an electric field and solvent molecules, suggesting that such effects should be included in the first-principles modeling of cathode reactions in metal-O₂ cells. In the ε range of ±0.4 V Å^-1, the stability of MO₂ species is found to vary by up to |0.4| eV on Au(111) and |0.2| eV on Au(211) in vacuo, which is larger than the field effects on the stability of O and OH, key intermediates in the ORR by hydrogen. An aprotic solvent such as dimethyl sulfoxide (DMSO), considered here via the inclusion of explicit DMSO molecules above the Au surfaces, stabilizes all three MO₂ species at zero fields and amplifies the field effects on the stability of MO₂, on both Au surfaces. The variations in the stability of the molecular MO₂ species with ε, which have small polarizabilities, are closely approximated by the first-order Stark effect (μ₀·ε, where μ₀ is the static surface dipole moment induced by adsorption at ε = 0 V Å^-1). The superoxide anion (O₂^-) that has been identified on an under-coordinated Au site has a larger polarizability than the MO_x species and a μ₀ that is opposite in sign to those of the metal MO₂ species, which results in larger errors by the first-order approximation, although its stability varies only moderately under positive electric fields of up to 0.4 V Å^-1.