Outer Helmholtz Plane of the Electrical Double Layer Formed at the Solid Electrode–Liquid Interface

Abstract

In electrolyte solutions containing alkali-metal cations, the hydrated cations approach to the electrode surface through noncovalent and electrostatic interactions. The cations next to the electrode form a plane parallel to the electrode surface as shown in Figure 1. The plane is called the outer Helmholtz plane (OHP). The microscopic area above the electrode surface including the OHP is named the electrical double layer. In an electrochemical reaction, a reactant must cross the electrical double layer to reach the electrode surface; the structure and physical properties of the electrical double layer are important for fundamental electrochemistry and electrochemical applications such as capacitors and fuel cells. A huge electric field is induced between an electrode surface and the OHP by the localization of ionic species at the OHP, becoming the driving force for a reaction at the electrode. Several models for the electrical double layer were proposed by the pioneers of electrochemistry. [1][2][3][4][5] The Graham and Bockris-Devanathan-Muller (BDM) models are currently accepted for the case where ions are adsorbed specifically on an electrode.[5] Specifically adsorbed ions also form a plane which is called the inner Helmholtz plane (IHP). The BDM model consists of three parts: IHP, OHP, and the diffuse layer. For the molecular orientation of adsorbed water, the modified double layer model is proposed based on vibrational spectroscopy. [6] The specifically adsorbed species at the IHP has been widely studied using scanning probe microscopy and vibrational spectroscopy. In contrast, the microscopic structure of ions in the OHP has not been reported due to the limitation of in situ analytical methods. Recently, Marković et al. reported that the activities for the oxygen reduction and the methanol oxidation reactions depend on the alkali metal cations in the OHP.[7] Activities of the oxygen reduction and the methanol oxidation reactions give the following order on platinum electrodes:They suggested that hydrated Li + and Na + , which interact with adsorbed OH strongly, block the surface active sites for these reactions. Interaction of alkali metal cations with the electrode surface also affects adlayer formation and the structural transition of specifically adsorbed species.[8] Figure 2 shows cyclic voltammograms (CVs) of Ag (100) in CsBr and LiBr. The pair peaks around À0.8 V and the broad peaks at À1.1 V vs Ag/AgCl have been assigned to the order-disorder transition of the adsorbed Br (Br ad ) and the reorientation of surface water, respectively.[9-11] The peak potentials depend on the alkali metal cations significantly. The cations in the OHP, which are not adsorbed on the electrode, definitely affect the electrochemical processes.The nature of the water in the electrical double layer differs from that in the bulk solution. The molecular orientation of adsorbed water depends on the electrode potentials and electrode materials.[12] However, the detailed structural and physical properties of the water above the fi...

Structure of the electrical double layer on Ag(100): Promotive effect of cationic species on Br adlayer formation

“…In the case of Cs on c(2 × 2)-2Br, we report that OHP-Cs moves away from the surface at positive potentials. 4 This result indicates that OHP-Cs has a positive charge. Figure 2 shows the specular rod profiles on Ag(100) between −1.2 and −0.6 V in LiBr and CsBr.…”

“…21 The detailed procedure for the structural analysis is given in previous reports. 4,18 Figure 1 shows CVs of Ag(100) in AMBr (AM = Cs, K, and Li). The pair peaks around −0.8 V and the broad peaks at −1.1 V have been assigned to the order-disorder transition and the adsorption-desorption of Br ad , respectively.…”

“…Our previous result for the c(2 × 2)-2Br layer on Ag(100) shows that the specular rod in CsBr differs from that in LiBr above −0.6 V significantly. 4 Localization of the heavy atom (Cs) in the EDL causes a typical change in the rod profile. However, at −1.3 V, the profile in CsBr becomes very similar to that in LiBr, indicating that the vertically ordered structure of Cs disappears at −1.3 V. Detailed structural analysis reveals that neither Br nor Cs are layered in the EDL.…”

“…4 X-ray diffraction can reveal the structure of both the adsorbed and nonadsorbed species in the EDL. 4,18,19 In this paper, the Cs structure on Ag(100) during Br adlayer formation was investigated using in-situ measurement of the x-ray specular rod. We report the structural effects of Cs on the Br adsorption and the order-disorder transition.…”

“…Cyclic voltammograms (CVs) in different alkali metal (AM) ion solutions show that these steps depend on the cationic species. 3,4 Nonspecifically adsorbed cations are known to affect the catalytic activity of fuel cell reactions through noncovalent interaction. 5,6 Noncovalent interactions between adsorbed and nonadsorbed species often result in a structural gap between electrochemical and ultrahigh vacuum (UHV) conditions.…”