Chemically softened boundary of metal/vacuum/solid-electrolyte from first principles

We carried out electronic structure calculations on a Ni/vacuum/ZrO ₂ triple phase boundary (TPB) and found that the TPB shows two distinct chemical/physical properties and responses depending on the contact structure between Ni and ZrO₂ surfaces. To classify the properties of TPB in a systematic way, we propose a new concept of a chemically softened boundary. In the concept, the highly (poorly) softened boundary shows (i) extremely ordered (disordered) structure at the TPB, (ii) metallic (nonmetallic) density of states induced by Ni atoms at the TPB, and (iii) promotive (capacitive) response in charge redistributions by oxygen removal which mimics the discharge reaction in solid oxide fuel cell (SOFC). On the basis of the above results obtained using a relatively large TPB model which has both the highly and poorly softened boundaries, we propose smaller TPB models composed of a Ni-tip with a pyramidal shape and ZrO₂ layers (atomic contact TPB models). S ingle-atom-contact and three-atom-contact: TPBs show chemical/physical properties and responses of poorly and highly softened boundaries, respectively.