  Semipermeable silica membranes are attractive as protective coatings for
metal electrocatalysts such as platinum but their impact on the catalytic
properties has not been fully understood. Here, we develop a first principles
formalism to investigate how silica membranes interact with the surface of
platinum metal electrocatalysts to develop a better understanding of the
membrane-metal interplay. By generalizing the concept of Pourbaix diagrams to
electrochemical solid-solid interfaces, we establish which bonds are formed
between the SiO$_2$ membrane and the Pt(111) surface in aqueous electrolytes
for different pH values and potential biases. We find that the membrane
termination changes as a function of the pH and the potential, which affects
the adhesion strength and the energy requirements for partial membrane
detachment, controlling the Pt surface area that is accessible for reactant
species. The charge-transfer between the Pt surface and the SiO$_2$ membrane is
also pH and potential dependent and results in changes of the Pt surface d-band
states, which are known to correlate with catalytic activity. Our analysis
reveals the complex response of a buried interface to the electrochemical
environment and identifies trends that are expected to apply also to other
membrane-coated electrocatalysts.
