Electrochemical investigations with single crystal surfaces have yielded foundational insights in the modern electrochemistry and electrocatalysis . Single crystal electrodes offer unparalleled certainty in the structure of electrode surface which facilitates the interpretation of experimental observations of complex surface-mediated electrochemical processes. Practical implications of developments in electrochemistry and electrocatalysis have been increasingly appreciated as renewable electricity is recognized as a key element in the global transition away from the fossil fuel powered society . Low temperature electrochemical devices such as fuel cells and electrolyzers typically contain complex electrode/electrolyte interfaces with polymer electrolytes, i.e., the membrane-electrode assembly (MEA), while mechanistic studies were mostly conducted at the interface between electrode and liquid electrolytes . Such stark difference naturally raises the question whether mechanistic insights gained at solid/liquid interface could be smoothly transferred to practical solid/polymer interfaces, which has not been satisfactorily addressed due to the complexity of the solid/polymer interfaces and characterization challenges in electrochemical devices . Interfaces between a single crystal surface and a polymer electrolyte could significantly reduce the complexity of solid/polymer interfaces, and thus could be informative model systems.