Probing the active sites of oxide encapsulated electrocatalysts with controllable oxygen evolution selectivity†

Abstract

Electrocatalysts encapsulated by nanoscopic overlayers can control the rate of redox reactions at the outer surface of the overlayer or at the buried interface between the overlayer and the active catalyst, leading to complex behavior in the presence of two competing electrochemical reactions. This study investigated oxide encapsulated electrocatalysts (OECs) comprised of iridium (Ir) thin films coated with an ultrathin (2–10 nm thick) silicon oxide (SiO_x) or titanium oxide (TiO_x) overlayer. The performance of SiO_x|Ir and TiO_x|Ir thin film electrodes towards the oxygen evolution reaction (OER) and Fe(II)/Fe(III) redox reactions were evaluated. An improvement in selectivity towards the OER was observed for all OECs. Overlayer properties, namely ionic and electronic conductivity, were assessed using a combination of electroanalytical methods and molecular dynamics simulations. SiO_x and TiO_x overlayers were found to be permeable to H₂O and O₂ such that the OER can occur at the MO_x|Ir (M = Ti, Si) buried interface, which was further supported with molecular dynamics simulations of model SiO₂ coatings. In contrast, Fe(II)/Fe(III) redox reactions occur to the same degree with TiO_x overlayers having thicknesses less than 4 nm as bare electrocatalyst, while SiO_x overlayers inhibit redox reactions at all thicknesses. This observation is attributed to differences in electronic transport between the buried interface and outer overlayer surface, as measured with through-plane conductivity measurements of wetted overlayer materials. These findings reveal the influence of oxide overlayer properties on the activity and selectivity of OECs and suggest opportunities to tune these properties for a wide range of electrochemical reactions.