Stripping off of the hydration shells in the double layer formation: water networks matter

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation in operando processes. By THz spectroscopy we are able to follow the stripping off of the cation/anion hydration shells for a NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na + is attracted toward the electrode already at the smallest applied negative potentials, stripping-off of the Cl - hydration shell is observed only at higher potential values. These phenomena are directly measured by in operando THz spectroscopy with ultra-bright synchrotron light as a source and rationalized by accompanying molecular-dynamics simulations and electronic-structure calculations.


INTRODUCTION
One of the most challenging global frontiers is the economic transition towards renewable energy technologies and recycling of waste into valuable chemicals. Most routes to tackle these efforts and develop "green" processes involve electrochemistry. These range from battery, supercapacitor and fuel cell technologies (1), to corrosion and solar cells (2), to electrochemical water splitting, (3) all the way to using electric power for selective reduction of CO2 to form synthetic renewable fuels and valuable chemicals (4). Each of these applications involves electron transfer across the electrode/solution interface and is therefore governed by interfacial chemistry. Thus, all major efforts rely on increasing the speed and selectivity of interfacial reactions. This holds for electrocatalysis, as much as for batteries.
The fundamental principles governing interfacial chemistry have been established decades ago. Marcus theory and the Butler-Volmer formalism describe the general principles of electrochemical reaction kinetics, based either on looking at the solvation of reactants, intermediates and products or on the activated complex formed upon electron transfer, respectively. In parallel, the Mott-Schottky equation for the depletion layer allows to predict on macroscopic bases the capacitive behaviour at the semiconductor/electrolyte interface (5). In reality, however, very little is known at the molecular level on the structure and solvation state of the reacting species and the way they are activated directly at the electrode, due to lack of a microscopic description of the double layer arising at the solid/electrolyte interface. Several experiments (6)(7)(8) showed the validity of the Gouy-Chapman-Stern-Grahame model (GCS) for the description of solid/electrolyte interfaces at a macroscopic level. According to the GCS model (9), the charged electrode is in contact with the Inner Helmholtz Plane (IHP), composed of dehydrated immobile ions specifically bound to the surface. The Outer Helmholtz Plane (OHP), consisting of hydrated ions, follows the IHP, which is 4 the diffusive layer with mobile ions. Per contra, on the microscopic scale, the interfacial solvent structure and ion solvation in the IHP/OHP are not explicitly treated, and deviations from the GCS model have been reported (10,11). Whatever the electro-catalytic process, the reactants must approach the surface to react, i.e. they have to penetrate the double layer. A microscopic understanding of the double layer and the role of the solvent (water in most electrochemical applications as of today) in mediating ion-surface interactions is therefore of fundamental importance for future applications.
It is still an experimental challenge to probe interfaces under operando conditions, facing difficulties in regulating all the variables involved in electrochemical reactions (28,29), such as controlling surface structure and mass transport. Charge transfer at an unperturbed double layer has been characterized by an electrochemical probe (30). Using IR spectroscopy, Yamakata et al. (24) investigated modifications in the ion hydration shells on a CO-covered Pt electrode. Local and intramolecular mode probes have been used in Sum Frequency Generation experiments at aqueous interfaces in operando conditions (31) or in SERS/Stark effect spectroscopies (32).
Previous simulation studies further revealed that the hydration of metal surfaces induces the formation of a strongly bound adlayer where water molecules form a 2D-hydrogen bond network parallel to the surface. This adlayer interacts unfavourably with the adjacent water layer, which hence shows hydrophobic features similar to the water-air interface (33). When a negative 5 potential is applied, interfacial water reorients with H atoms toward the gold surface, slightly disrupting the hydrogen bonding network in the interfacial layer (21). These structural changes are expected to have an impact on the electric double layer formation, since the adsorption of ions at the metal surface was shown to require that an adlayer water molecule vacates its adsorption site (13).
In this work we focus on changes in the water network, such as the formation and stripping off of the hydration shell during the formation of the electric double layer (EDL). Any changes in the hydrogen bond network as well as ion hydration can be sensitively probed by THz spectroscopy, i.e. in the intermolecular fingerprint region between 10-700 cm -1 (34). The low frequency absorption spectrum of bulk water is well known (34)(35)(36)(37). Modes around 80 cm −1 are characterized by concerted motions involving the second solvation shell. Cross-correlating these modes with the local dipole demonstrates that their integrated contribution to the total IR activity is indeed very weak, which is consistent with the lack of any resonance around 80 cm -1 in the THz absorption spectrum, while the peak is present in Optical Kerr as well as Raman spectra. The hydrogen-bond stretching modes contribute to a peak centred at 200 cm −1 , which is dominated by first-shell dynamics. The frequency region above 500 cm −1 is dominated by water librational modes, i.e. hindered rotations.
We present the first reported fingerprint of the double-layer formation of an electrolyte solution at a gold surface under applied bias potential using the ultra-bright synchrotron Soleil as a low frequency radiation source. As sample, we used a 10 mM NaCl solution and applied a positive/negative potential at the gold (Au)-liquid interface. The low frequency spectra of aqueous NaCl have been extensively analysed in detail elsewhere (38,39). Any modification of the hydration layer of the two ions will be clearly identified by the change in their well-known THz 6 fingerprints.

RESULTS
Following the pioneering work of Nemes et al. (40), we adopted the electrochemical cell developed at AILES (Soleil Facility) (41), consisting of a Au grid as working electrode and a thin platinum foil as counter electrode. Since no Faradaic reactions were driven (see Fig. S1 in SI), we did not employ a reference electrode to prevent concentration alterations of the sample. The potential values reported here are referenced to a "reference potential" (Vref) at which a zero current is measured, equal to +0.03 V and +0.07 V for the positive and negative potential series, respectively (see Section S8 of the SI for conversion to a Ag/AgCl reference electrode). The exact position of the Au potential of zero charge (PZC) for our specific experimental setup cannot be precisely determined due to the polycrystalline nature of our electrode and the specific electrolyte composition and concentration. However, a recent study by Wang et al. (42) sets the PZC of a polycrystalline Au electrode in the range 0.17 V-0.22 V vs SHE by SECCM measurements, compatible with the voltage range probed in this present work. We recorded absorption THz spectra in the 50-350 cm -1 region, varying the potential in 20 mV steps from 0 V to +0.2 V for the positive voltages, and at the following values for the negative voltages: -2 mV, -4 mV, -8 mV, -10 mV, -20 mV, -50 mV, -150 mV. In order to confirm that background variations are negligible, the acquisition of each spectrum taken at the potential Vi was followed by acquisition of a spectrum at the reference potential Vref. Fig. 1 shows the difference spectra obtained by referencing each spectrum at potential Vi to the spectrum at the lowest or highest applied bias potential for the negative (A) or positive (B) potential series, respectively. This choice allows to better visualise the trends upon voltage application. A crucial advantage is that, in the difference spectra, any spectral contribution that does not change with the potential provides by construction a zero intensity. Therefore, the intensity of the difference spectra only arises from the region of the EDL (with a Debye length of 3 nm), where the potential is non-zero and decay with increasing distance from the surface, while the bulk does not provide spectral contribution, since it is not affected by the potential. The signal to noise ratio depends on the probed interfacial volume and it is estimated to be 210 -4 (cfr section S3 of the SI). All details about the experimental set-up are given in the SI (section S1-S8).
For negative bias potentials, the spectral intensity sharply decreases for potentials of a few millivolts (from -2 mV to -8 mV) and saturates around zero already at -50 mV. This implies that the spectral intensity does not change anymore from -50 mV up to the most negative potential. A different trend emerges for positive voltages: the spectral intensity initially increases with increasing potential from 0 mV to +80 mV and starts to decrease only at higher potential values.
In order to decipher the reason behind the different behaviour with negative and positive potentials and to unveil the spectral components responsible for the observed trends, we performed a Principal Component Analysis (PCA). Figure 1: Absorption spectra as a function of the applied potential. A: spectra acquired during negative potential series for 5 selected voltages. Each spectrum is referenced to the spectrum acquired at -150 mV. B: spectra acquired 8 during positive potential series for 5 selected voltages. Each spectrum is referenced to the spectrum acquired at +200 mV. The total set of spectra upon potential application is displayed in Fig. S7 and S9 of the SI.
In a nutshell, for both positive and negative potential series, a matrix M is constructed, each row containing one of the measured difference spectra, and subsequently diagonalised in order to obtain an orthonormal basis set carrying the fundamental effect of the applied potential on the spectral features. In practice, each spectrum is rewritten as a linear combination of potential-independent partial spectral components (Principal Components, PCs) weighted by the potential-dependent scores (eigenvalues). While a standard fitting procedure requires assumptions on the number of the bands in a given spectral region, their position and shape, the advantage of PCA is that it does not depend on any a priori knowledge. However, the extracted spectral components still need to be associated to meaningful physical observables. This is done in Fig. 2, by comparing the spectral components obtained from the PCA with theoretically reconstructed spectral components. The scores obtained from the PCA are reported in Figure S1 in the SI. The theoretical spectra are obtained from classical molecular-dynamics (MD) simulations of positively and negatively charged Au-electrolyte interfaces (17 simulations with surface charge, σ, values from -300 mC/m 2 to +300 mC/m 2 , see Methods Section for details).
As shown in Fig.2-A, a single spectral component is sufficient to describe the spectral changes with negative potentials, while two independent components are found for positive potentials (panels B and C). The residuals, obtained by subtracting the sum of the corresponding spectral components from the total spectrum (in Fig 1) at each potential (A, for negative potentials and B+C, for positive potentials), demonstrate that the low number of independent components is indeed sufficient to reproduce the full spectrum. In the following, we will analyze the results of the PCA by comparison with simulated spectral features. For each of the three PCs discussed above, we assign a calculated isolated spectral component and scale it by a surface charge dependent factor that is derived from either changes in the solvation environment of the ions or changes in the hydrogen bond network. This allows us to map the spectral changes to the atomistic processes at the EDL under applied bias.
The spectral component for negative potentials in Fig.2-A is compared and assigned to the theoretical THz spectrum of solvated Na + as a function of the Au surface charge () in panel D.
As detailed in the methods section, the spectra in panel D are obtained by multiplying the theoretical spectrum of solvated Na + (39) by a weighting factor, chosen to be the variation of the number of hydration water molecules with  in the first solvation shell of Na + in the MD simulations. The calculated spectrum of solvated Na + is therefore associated with the theoretical (-independent) principal component, and the variation of the number of hydration water molecules with  around Na + is used as a proxy for the -dependent score. Both experimental and theoretical spectra show a similar trend with negative applied potential/surface charge. Therefore, since the intensity in the theoretical spectra depends on the number of Na + hydration water molecules, an intensity increase/decrease in the experiments correlates directly with an increase/decrease in the number of hydration water around Na + cations at the interface.
We can hence assign the sharp intensity decrease detected in both the experimental and the theoretical spectra (panel A and D) to the (partial) depletion of the hydration layer of Na + at the interface, happening already at the smallest applied potentials/surface charges. When comparing the experimental spectral component in Fig. 2-A with the theoretical counterpart in Fig. 2-D we conclude that the ΔA (ν,V) maximum at ~250 cm -1 in Fig. 2-A does not arise from the hydration shell of Na + . It therefore has to be ascribed to a second contribution hidden in the experimental spectral component. This can occur when two (or more) spectral components share the same trend with the potential (i.e. same scores) and the PCA is not able to disentangle them. As discussed in more detail in the SI and since any ion contribution has been ruled out, this feature might be associated with the interfacial water network. Here, adlayer disordering at negative potentials (13) can induce a blue shift of the librational band (S3 in SI). In the methods section, this hypothesis is Concomitantly, a second component is observed for positive potentials (panel C in Fig. 2), and assigned to the water network at the interface (see also Figure S5 in the SI for comparison with the THz absorption spectrum of bulk water). Strikingly, the center frequency of the underlying peak (147±5 cm -1 , Figure S6 of the SI) resembles the hydration water band observed experimentally and theoretically in the hydration shell around hydrophobic alcohol chains (43).
This band arises from a wrapped hydrogen-bond (HB) interfacial-water network, (44) reminiscent of the planar 2-dimensional-HB-network formed at the hydrophobic air-water interface (45) and at other planar interfaces such as graphene-water (65), where interfacial water HBs preferentially orient parallel to the surface. A similar in-plane interfacial water orientation has been reported for Au-water interfaces (21,33). Importantly, this band is systematically red-shifted with respect to the 196 cm -1 band observed for bulk water (33,41,47). We base this assignment on the comparison with the spectra in panel F, which show the experimental spectrum of the HB stretching mode of water molecules hydrating a hydrophobic surface (41) Merging all the experimental results, we can infer that, for low positive potential values, the anions keep their full hydration layer and the interfacial HB-network is strengthened, while cations directly lose part of their hydration shell already at the lowest negative potential in our experimental window. This contrasts with what is described in a number of electrochemical models, in which Na + is expected to be more anchored to its hydration waters due to its higher 13 charge density compared to Cl - (48). At high positive/negative potentials, the classical electrochemical picture is restored, and the hydration shells of both Cland Na + are depleted.
In order to confirm and rationalize the asymmetric stripping off of the Cland Na + hydration shells at the molecular level, we use a combination of classical MD simulations and ab initio calculations  (21), concluding that the topmost interfacial layer at the Au-water interface is mostly oriented parallel to the surface at 0 V.
When applying an increasingly negative potential, water was found to reorient first with one OH, 15 and finally with both OH pointing toward the surface ( Figure S12). Such behaviour has been also described by MD simulations in Ref. (13), revealing that under a negative bias the re-orientation of water molecules with H atoms toward the gold surface disrupts the HB-network in the interfacial layer. Furthermore, our results on the in-plane orientation of water that persists longer under positive surface charging supports the idea of an asymmetric reorientation of water at positive/negative electrodes as described in Refs. (13,49). The novel result provided here is that the building-up of an in-plane oriented interfacial layer exclusively at positively charged gold surfaces causes anions and cations to migrate differently when positive/negative potentials are applied.  Table S2 of the SI. However, these calculations do not account for polarization and chemical changes induced by the ions and the applied bias. Energy decomposition analysis of charged gold cluster models ( Figure   4-A and section S16) demonstrates that polarization plays a role but is independent of the charge.
These calculations also reveal charge-transfer from the chloride ion to the gold cluster that is completely absent in the Na + -Au interaction. However, the fact that these effects are independent of cluster charge rules them out as the source of the different behavior of Na + and Clat the electrochemical double-layer. Grand-canonical (in electrons) periodic slab calculations on a Au(100) surface additionally confirm that the polarization of the electrode as measured by the change of number of electrons present in the metal slab is largely independent of the applied bias ( Figure 4-B and section S17) in the experimentally accessed range. In addition, a microsolvation analysis (section S18) demonstrates that Clprefers a surface-like asymmetric microsolvation pattern as is present in the hydrophobic OHP (cfr ref. 33), whereas Na + prefers to be symmetrically solvated which can be achieved at the IHP by replacing one or two water molecules with negatively charged Au atoms. All in all, the quantum-chemical calculations support the results obtained from experiment and the classical MD simulations and hint that the deficiencies of a classical fixed charge model do not lead to biased conclusions in the current study. In conclusion, by a novel THz spectro-electrochemical approach combined with molecular dynamics simulations, we have directly probed the solvation/desolvation processes at the Au surface under in-operando electrochemical conditions. Our results have dissected at the microscopic level the stripping off of the hydration shells of the electrolyte ions as well as the associated changes in the interfacial water HB-network. Our approach can now be used to investigate the crucial role of water in mediating other interfacial processes at metal as well as semiconductor/electrolyte interfaces. This new view will impact the understanding and optimization of electrochemical processes for technological applications.

A B
18

THz spectro-electrochemical measurement
THz-Far infrared (THz/FIR) absorption spectra of 10 mM NaCl (Sigma-Aldrich, purity ≥ 98%) were recorded in the frequency range 50-350 cm -1 with 2 cm -1 resolution. We employed a vacuum- show a similar trend as NNa-o. Therefore, PCA is not able to separate the possible spectral contribution of the change in the interfacial water network from the Na + hydration. This observation corroborates the hypothesis that the feature at around 250 cm -1 in the spectra of Figure   21 2-A arises from a water-related contribution with the same potential dependence as the Na + hydration fingerprint.

Electronic Structure Calculations
All density functional calculations for the cluster models were carried out with the Q-Chem electronic structure package (53). We applied the RPBE functional More details are found in the Supporting Information.