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Seawater electrolysis is an attractive approach for producing clean hydrogen fuel in scenarios where freshwater is scarce and renewable electricity is abundant. However, chloride ions (Cl−) in seawater can accelerate electrode corrosion and participate in the undesirable chlorine evolution reaction (CER). This problem is especially acute in acidic conditions that naturally arise at the anode as a result of the desired oxygen evolution reaction (OER). Herein, we demonstrate that ultrathin silicon oxide (SiOx) overlayers on model platinum anodes are highly effective at suppressing the CER in the presence of 0.6 M Cl− in both acidic and unbuffered pH-neutral electrolytes by blocking the transport of Cl− to the catalytically active buried interface while allowing the desired oxygen evolution reaction (OER) to occur there. The permeability of Cl− in SiOx overlayers is 3 orders of magnitude less than that of Cl− in a conventional salt-selective membrane used in reverse osmosis desalination. The overlayers also exhibit robust stability over 12 h in chronoamperometry tests at moderate overpotentials. SiOx overlayers demonstrate a promising step toward achieving selective and stable seawater electrolysis without the need to adjust the pH of the electrolyte. 

Encapsulation of an active electrocatalyst with a permeable overlayer is an attractive approach to simultaneously enhance its stability, activity, and selectivity. However, the structure–property relationships that govern the performance of encapsulated electrocatalysts are poorly understood, especially those describing the electrocatalytic behavior of the buried interface between the overlayer and active electrocatalyst. Using planar silicon oxide (SiO x )-encapsulated platinum (Pt)/titanium (Ti) bilayer thin films as model electrodes, the present study investigates the physical and electrochemical properties of the SiO x |Pt buried interface. Through a combination of X-ray photoelectron spectroscopy and electroanalytical measurements, it is revealed that a platinum oxide (PtO x ) interlayer can exist between the SiO x overlayer and Pt thin film. The thickness and properties of the PtO x interlayer can be altered by modifying (i) the thickness of the SiO x overlayer or (ii) the thickness of the Pt layer, which may expose the buried interface to oxophilic Ti. Importantly, SiO x |Pt electrodes based on ultrathin Pt/Ti bilayers possess thinner PtO x interlayers while exhibiting reduced permeabilities for Cu 2+ and H + and enhanced stability during cycling in 0.5 M H 2 SO 4 . These findings highlight the tunability of buried interfaces while providing new insights that are needed to guide the design of complex electrocatalysts that contain them.