Research ArticleELECTROCHEMISTRY

Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface

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Science Advances  07 Oct 2020:
Vol. 6, no. 41, eabb1219
DOI: 10.1126/sciadv.abb1219
  • Fig. 1 Structure of electrified Pt(111)/water interface.

    (A) Schematic representation of the GCS model of the EDL. The EDL consists of a Helmholtz layer and Gouy-Chapman (diffuse) layer, and the interface potential distribution is shown by the red curve. (B) The EDL capacitance can be represented by the capacitors corresponding to the two layers (i.e., CH and CGC) connected in series. (C) Model of Pt(111)/water interface at PZC. There is significant interface electron redistribution along the surface normal z owing to water chemisorption, as indicated by the blue curve. (D) Profiles of water density (ρH2O) along the surface normal z at different potentials. The positions of water molecules are indicated by those of the oxygen atoms, and the zero in z coordinate indicates the position of the nuclei of the uppermost layer of Pt(111). All the potentials are referenced to the PZC of Pt(111). (E and F) Representative snapshots of the electrified Pt(111)/water interfaces at −0.93 and 0.84 V versus PZC. The Pt, Na, F, O, and H atoms are colored in gray, blue, violet, red, and white, respectively. The chemisorbed water is highlighted with the ball-and-stick model in comparison to other water with the stick model.

  • Fig. 2 Orientation and hydrogen bonding of interface water.

    Probability distribution profiles of angle φ (A) between the bisector of water and the surface normal and angle θ (B) between the O─H bond of water and the surface normal of the interface water at different applied potentials. Both angles are shown in the insets, and the interface water is taken as being within 4 Å̊ from the metal surface. The potentials are referenced to the PZC of Pt(111). (C) Number of hydrogen bond donors (pink circle) and acceptors (green diamond) of interface water molecules as a function of potential. A hydrogen bond is defined when the O─O distance is shorter than 3.5 Å and the O─O─H angle is less than 35°. The insets show the structural models of interface water at very negative and positive potentials. a.u., arbitrary units.

  • Fig. 3 Proposed model of Helmholtz layer.

    Potential profiles at the Pt(111)/water interface at different applied potentials (A) <PZC without water chemisorption, (B) PZC, and (C) >PZC. The Pt electrode and water solution are the regions colored by gray and light blue, respectively. The balls in red, white, blue, and violet stand for oxygen atoms, hydrogen atoms, cations, and anions, respectively. The interface potential change ∆ψ (blue) consists of the usual potential change Δψsol induced by the surface charge (green) and the potential ∆ψA caused by water chemisorption (red). The potential in bulk solution is set to zero. lA and lH denote the distance separation of the dipole induced by chemisorbed water and Helmholtz layer, respectively.

  • Fig. 4 AIMD data and model fitting.

    Plots of surface charge density σ (A) and surface coverage of chemisorbed water θA (B) as a function of the electrode potential U. The solid dots with error bars represent the computed data from AIMD simulations, and the black curves are the corresponding fits using the proposed theoretical model. The convergence of the computed U and the corresponding θA can found in figs. S3 and S7. The insets in (A) show the representative configurations of the chemisorbed water on Pt(111) at −0.93 V, PZC, and 0.84 V, respectively. The dashed lines in (B) indicate the respective θA at PZC and ∼0.1 V. The potentials are referenced to the PZC (UPZC).

  • Fig. 5 Capacitance of the Helmholtz layer.

    Decomposition of differential Helmholtz capacitance CH (blue) as a function of electrode potential U into two constituent components, the solvent capacitance Csol (green) and the capacitance CA (red) due to water chemisorption. The inset indicates that Csol and CA are connected in series. The potential window (∼0.2 V) of the double layer region of the Pt(111)/water interface at pH 4 is highlighted by light blue (38).

Supplementary Materials

  • Supplementary Materials

    Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface

    Jia-Bo Le, Qi-Yuan Fan, Jie-Qiong Li, Jun Cheng

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    • Sections S1 to S4
    • Figs. S1 to S10

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