Nanoscale surface morphology controls charge storage at stepped Pt-water interfaces

TL;DR

This work addresses how nanoscale Pt surface morphology governs interfacial charging at Pt–water interfaces by performing ab initio molecular dynamics with explicit electrode-potential control on stepped Pt surfaces that feature (111)×(111) and (111)×(100) edges. The approach resolves site-specific EDL structure, charge distribution, and local electrostatics, revealing that differential capacitance near the PZC mainly arises from terrace water chemisorption while step edges saturate with chemisorbed water below the PZC and accumulate excess positive charge, accompanied by elevated local potentials and a higher d-band centre at edge atoms. Key findings include a bilayer-like interfacial water structure with edge-specific saturation, a clear lateral charge gradient (edges positive, terraces negative), and edge electronic signatures consistent with d-band theory, all contributing to edge-enhanced reactivity. Collectively, these results explain the experimentally observed PZC shifts with step density and provide a predictive framework for optimizing interfacial charging in nanostructured Pt electrocatalysts.

Abstract

Platinum step edges dominate electrocatalytic activity in fuel cells and electrolysers, yet their atomistic electrochemical behaviour remains poorly understood. Here, we employ \textit{ab initio} molecular dynamics under controlled electrode potentials to model a realistic stepped Pt--water interface incorporating experimentally observed (111)$\times$(111) and (111)$\times$(100) edge motifs. This allows us to resolve, for the first time, the site-specific structure, charge distribution, and electrostatics of the electric double layer at a nanostructured Pt surface. We find that differential capacitance near the potential of zero charge (PZC) arises almost entirely from potential-dependent chemisorption of water on flat (111) terraces. In contrast, step edges are saturated with chemisorbed water even below the PZC and thus do not contribute to the capacitance. Instead, edges accumulate excess positive charge and exhibit a locally elevated electrostatic potential, as revealed by spatially resolved macroscopic potential profiles. This electrostatic asymmetry implies a greater barrier for electron accumulation at step sites compared to terraces, consistent with enhanced charge localisation and reactivity. Finally, the higher-in-energy d-band centre and sharper projected density of states at edge atoms further support their role as active, positively charged centres. Together, these results provide a mechanistic explanation for the observed experimental shift of the PZC with step density and establish a predictive framework for understanding and optimising interfacial charging in nanostructured Pt electrocatalysts.

Figures (7)

• Figure 1: Our model for the Interface between Pt and a pure aqueous electrolyte. (a) Representation of three replicas along the $x$-axis of the used supercell. Pt, O and H atoms are represented in blue, red and white respectively. Periodic boundaries are marked with black lines, and the cell is oriented with the $y$-axis directed into the page. The upper (111) terrace, lower (111) terrace, angled (111) facet, and angled (100) facet are indicated by red, green, magenta, and purple arrows, respectively. (b) Cross section of the supercell in the $xy$ plane showing the $6 \times 9$ replica of the orthorhombic unitary supercell (marked in cyan). (c) Planar-averaged mass density distribution along $z$ for the system solvated in a pure aqueous electrolyte. Distributions of water molecules, H atoms, and O atoms are shown in black, blue, and red, respectively. The average bulk density is indicated by the horizontal grey line and is computed within the central 5 Å of the cell, as indicated by the vertical grey lines.
• Figure 2: Topologically sensitive mass density distributions for a) water molecules, b) O atoms, and c) H atoms. Systems include pure water (blue) and three HF electrolytes with H:F ratios of 20:20 (black), 20:22 (red), and 18:22 (green). Distance $d$ is defined as the minimum distance to the surface plane
• Figure 3: Cylindrical mass density distributions centred about the (111)$\times$(111) (solid lines) and (111)$\times$(100) (dashed lines) edges for water (black), oxygen (red), and hydrogen (blue). Panels show a) water-only, and b–d) HF solutions with H:F ratios of 20:20, 20:22, and 18:22, respectively. See supplementary material section 1 and 2 for details.
• Figure 4: Total charge per area as a function of potential: (a) terrace (111), (b) edge (111)$\times$(111), and (c) edge (111)$\times$(100). The star denotes the potential of zero charge (0.52 V).
• Figure 5: The trajectory average Bader charge per atom as function of the lateral displacement from the average position of the (111)$\times$(111) edge Pt atoms across the terrace for the system at 0.52 V (20H:20F). The charges on (111)$\times$(111) edge atoms (blue circles), (111) terrace atoms (red circles) and (111)$\times$(100) edge atoms (gold circles) are shown, with the distance calculated by considering the displacement from the (111)$\times$(111) edge across the terrace. The average charge for each row of Pt atoms is indicated by diamonds with associated standard deviation.