Abstract:

Catalysts design has been one of the most relevant issues for the hydrogen economy during decades, especially for the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR). These catalytic processes are controlled by the electronic structure of the catalyst, that
can be modified using different techniques, e.g. addition of alloying elements, introduction of defects and/or surface orientation, and also by the introduction of elastic strains in the lattice. In this investigation, the effect of the applied elastic strains on the adsorption of H, O, and OH on the (111) surfaces of 8 fcc (Ni, Cu, Pd, Ag, Pt, Au, Rh, Ir) and on the (0001) surfaces
of 3 hcp (Co, Zn, Cd) transition metals was analyzed by means of density functional theory calculations. Surface slabs were subjected to different strain states (uniaxial, biaxial, shear, and a combination of them) up to strains dictated by the mechanical stability limits indicated by
phonon calculations. It was found that the adsorption energy followed the predictions of the d-band theory but -surprisingly- the variations in the adsorption energy only depended on the area of the adsorption hole and not on the particular elastic strain tensor applied to achieve this
area. The analysis of the electronic structure showed that the applied strains did not modify the shape of the Projected Density of States (PDOS) of the d-orbitals of the transition metals but only led to a shift in the energy levels. Moreover, the presence of the adsorbates on the surfaces
led to negligible changes in the PDOS and the adsorption energies were a function of the Fermi energy which in turn was associated to the change of the area of the adsorption through a general linear law that was valid for all metals. This information can be used to estimate the effect of any elastic strain on the adsorption energies of H, O, and OH in 11 transition metals
with more than half-filled d-orbitals. Additionally, the adsorption energies calculated for H, O, and OH were also used to determine the rate limiting steps for both the HER and the ORR in all metals and the catalytic activity as a function of the applied strain. This strategy allows an accurate determination of the catalytic activity for the HER and the ORR of any transition metal from first principles calculations and can be used to design new catalysts by means of elastic strain engineering.