Electric double layer of Pt(111): Known unknowns and unknown knowns

The electric double layer is one of the most fundamental concepts in electrochemistry. Nevertheless, its exact structure is still often unresolved. In this contribution, we discuss what we, at present, believe to know and what we believe not to know about the structure of the Pt(111) double layer close to the potential of zero charge (pzc). To this end, we discuss and compare several models put forward in the last few years in order to explain recent (and less recent) experimental observations. Through a discussion of several models, we conclude that water adsorption at the interface likely plays an important role in correctly interpreting capacitance features observed close to the pzc. The capacitance features at low electrolyte concentration, which show important deviations from Gouy-Chapman diffuse layer theory, are, to date, not yet resolved beyond doubt, but we critically analyze and compare the various models that have been put forward to model the interface. We thereby point out weaknesses in the models as well as similarities and differences between the models.