  Metal-oxide surfaces act as both Br{\o}nsted acids and bases, which allows
the exchange of protons with the electrolyte solution and generates either
positive or negative proton charges depending on the environmental pH. These
interfacial proton charges are then compensated by counter-ions from the
electrolyte solution, which leads to the formation of the electric double layer
(EDL). Because the EDL plays a crucial role in electrochemistry, geochemistry
and colloid science, understanding the structure-property relationship of the
EDL in metal-oxide systems from both experimental and theoretical approaches is
necessary. This chapter focuses on the physical chemistry of the protonic
double layer at the metal-oxide/electrolyte interface. In particular,
determinations of the EDL capacitance and the double-layer potential from
potentiometric titration experiments, electrochemical methods,
surface-sensitive vibrational spectroscopy and X-ray photoelectron spectroscopy
are summarized. This is followed by discussions from the atomistic modelling
aspect of the EDL, with an emphasis on the density-functional theory-based
molecular dynamics simulations. A conclusion and outlook for future works on
this topic are also given.
