Double-layer structure of the Pt(111)–aqueous electrolyte interface

We present detailed measurements of the double-layer capacitance of the Pt(111)–electrolyte interface close to the potential of zero charge (PZC) in the presence of several different electrolytes consisting of anions and cations that are considered to be nonspecifically adsorbed. For low electrolyte concentrations, we show strong deviations from traditional Gouy–Chapman–Stern (GCS) behavior that appear to be independent of the nature of the electrolyte ions. Focusing on the capacitance further away from PZC and the trends for increasing ion concentration, we observe ion-specific capacitance effects that appear to be related to the size or hydration strength of the ions. We formulate a model for the structure of the electric double layer of the Pt(111)–electrolyte interface that goes significantly beyond the GCS theory. By combining two existing models, namely, one capturing the water reorganization on Pt close to the PZC and one accounting for an attractive ion–surface interaction not included in the GCS model, we can reproduce and interpret the main features the experimental capacitance of the Pt(111)–electrolyte interface. The model suggests a picture of the double layer with an increased ion concentration close to the interface as a consequence of a weak attractive ion–surface interaction, and a changing polarizability of the Pt(111)–water interface due to the potential-dependent water adsorption and orientation.

A molecular-level understanding of the electric double layer is important in order to understand many electrochemical processes and interfacial phenomena (1). Being an important catalytic material, platinum is one of the best-studied electrode materials, but detailed studies of its double-layer structure are remarkably scarce. The reason for the absence of detailed studies may lie in the fact that, except for Pt(111), none of the low-index planes of platinum exhibit a double-layer window, in which the metal is bare of adsorbates and can be considered ideally polarizable (2). The absence of a double-layer window, in which neither hydrogen or hydroxyl are adsorbed to the surface, renders the capacitance a combination of capacitive and pseudocapacitive contributions that is difficult to unequivocally separate, impeding detailed quantitative studies. Although the Pt(111) surface exhibits a small double-layer window in the range of 0.4 V to 0.6 V versus the reversible hydrogen electrode (RHE), in which the interface is considered free of adsorbates and ideally polarizable (2), the available capacitance data shows that under these conditions, even Pt(111) does not follow the expected textbook behavior (3).In the absence of any specific adsorption, the capacitance of the electric double layer of an ideally polarizable interface is expected to follow the Gouy–Chapman–Stern (GCS) model (4, 5). The GCS theory divides the total capacitance of the electric double layer Ctot into an “inner layer” or “Stern” capacitance Ci and the diffuse Gouy–Chapman capacitance C_GC:1CGCS=1Ci+1CGC .[1]The Gouy–Chapman capacitance CGC is computed by deriving the concentration of ions in the double layer from a Poisson–Boltzmann distribution including only electrostatic interactions. In the case of a symmetric electrolyte, this gives rise to the following expression for CGC:CGC(E)=(2z2e2εsε0ckBT)12cosh(ze(E−Epzc)2kBT),[2] where E is the potential at the Stern layer, Epzc is the potential of zero charge (PZC) of the interface, z is the charge number of the electrolyte ions, e is the unit of charge, εs is the dielectric constant of the solvent, ε0 is the vacuum permittivity, c is the bulk concentration of the electrolyte, kB is the Boltzmann constant, and T is the temperature. This expression should be accurate in the limit of (very) low electrolyte concentration close to the PZC, and it should be the dominant contribution to the capacitance at low electrolyte concentrations.Classical work by Grahame on the double layer of a mercury electrode in various electrolytes has confirmed the applicability of the GCS model at low electrolyte concentration (4, 6). However, in recent experiments, we have shown that the Pt(111)–perchloric acid interface, which is traditionally considered to be ideally polarizable, has a much higher differential capacitance than predicted by GCS theory and that the concentration-dependent capacitance does not follow the predictions of the GCS theory (3). In our original publication, we tentatively attributed this behavior to an (attractive) interaction between the ions and the electrode, which is not accounted for in GCS theory. More recently, we also developed a mean-field model (7), in which we showed that relatively weak ion–surface interaction strengths are sufficient to reproduce the observed experimental observations. A similar conclusion was reached in a recent theory paper by Schmickler (8).To improve our understanding of the nature of this interaction and the associated anomalous behavior of the diffuse double layer of Pt(111), we extend our capacitance measurements here to a range of electrolytes with different monovalent anions and cations, which are all considered to be nonspecifically adsorbed (Li⁺, Na⁺, K⁺, Cs⁺, ClO₄⁻, F⁻, and CH₃SO₃⁻). Combining these experimental investigations with theoretical modeling allows us to put forward a more refined model for the double-layer structure of Pt(111), in which both an attractive ion–surface interaction and field-dependent water adsorption to the interface play a central role. We believe that the significance of our model likely goes beyond the Pt(111)–electrolyte interface, as the interactions are not specific to that interface. Therefore, our results are an important step toward resolving the true structure of the electric double layer.