Deciphering\nthe Capacitance of the Pt(111)/Water Interface:\nA Micro- to Mesoscopic Investigation by AIMD and Implicit Solvation

We use <i>ab initio</i> molecular dynamics\nsimulations\nbased on density-functional theory to revisit the enigmatic capacitance\npeak of the electrified Pt(111)/water interface around the potential\nof zero charge. We demonstrate that counterbalancing the electronic\nexcess charges with partially charged hydrogen atoms constitutes a\ncomputationally efficient approach to converged interfacial water\nstructures. The thus enabled detailed analysis of the interfacial\nwater response clarifies that the peak in the capacitance is predominantly\ndue to structural reorientation, although its magnitude is significantly\nincreased by strong internal electronic polarization, also known as\ncharge transfer (CT). We find that CT is more complex than previously\nthought, resulting from the interplay between chemisorbed water and\ndepolarization effects from the surrounding water. Finally, we demonstrate\nthat quantitative agreement with the experimental peak can be achieved\nthrough inclusion of the interfacial response into an implicit solvent\nmodel for the extended part of the double layer. This suggests that\nsuch models can accurately reproduce screened interfacial fields as\na function of potential, despite their notoriously small native capacitance.