Probing the Actives Sites of Oxide Encapsulated Electrocatalysts with Controllable Oxygen Evolution Selectivity
Electrocatalysts encapsulated by nanoscopic overlayers can catalyze 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 (SiOx) or titanium oxide (TiOx) overlayer. The performance of SiOx|Ir and TiOx|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. SiOx and TiO¬x overlayers were found to be permeable to H2O and O2 such that the OER can occur at the MOx|Ir (M = Ti, Si) buried interface, which was further supported with molecular dynamics simulations. In contrast, Fe(II)/Fe(III) redox reactions occur to the same degree irrespective of whether electrocatalysts are bare, have TiOx overlayers with thicknesses less than 4 nm, or have SiOx overlayers with thicknesses less than 2 nm. This observation is attributed to facile electronic transport between the buried interface and outer surface of the overlayer, as measured with through-plane conductivity and ionic permeability 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.