Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO2 reduction (CO2R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO2R mechanism. CO2R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO2R performance.


Sum frequency generation spectroscopy setup
The VSFG system has been described in detail previously; 1 however, we give a brief description of the system here. The VSFG system uses a femtosecond Ti:Sapphire regenerative amplifier (Spectra Physics-Solstice) that produces 90 fs pulses centered at 800 nm with an average power of 3.5 W and a 2 kHz repetition rate. Seventy percent of the output was used to pump an OPA (Topaz Prime) with non-collinear difference-frequency generation. The OPA produced a tunable, broadband infrared (IR) beam. To probe the CO stretch, the IR beam was centered at ∼2100 cm −1 with an energy of ∼5 µJ. The remaining thirty percent of the output beam was spectrally narrowed to ∼10 cm −1 using an air-spaced etalon. The narrowed visible beam had an energy of about 10 µJ and a 0.128 mm 2 beam area. The visible and IR beams were overlapped spatially onto the sample in a Kretschmann configuration at a 56 • incident angle. The envelope of the non-resonant peak with complete temporal overlap of the visible and IR beams is given in Figure S1a. To collect CO spectra, the visible field was time delayed 730 fs relative to the IR field and the VSFG signal was integrated for 2 min. Figure S1b shows the non-resonant suppression by increasing the time delay of the visible field. The signal was integrated for 120 s to show that the non-resonant signal is fully suppressed under experimental conditions.  A BioLogic SP-150 potentiostat was used to control the applied potential. Potential steps from -0.1 V to -0.9 V vs. Ag/AgCl were applied to the Au electrode, staying at each potential step for 1 min before VSFG CO spectra were collected. The resulting CO spectra were fitted with a Lorentizan function to obtain the peak position.

Kinetics Measurement
The kinetics measurement for the CO Faradaic efficiency of CO has been described elsewhere, 3 but we provide a summary here. The measurement was made on a 100 nm polycrystalline Au sample that was deposited on a microscope slide with a 20 nm Cr adhesion layer.
The electrochemical H-cell was connected to an Agilent 7890B gas chromatograph through a custom headspace sampling system. Two compartments of the H-cell were separated by a Nafion membrane, which was activated in 10 % H 2 SO 4 at 80 • C for 10 min before each experiment. A Ag/AgCl electrode was used as the reference electrode while a Pt thin film electrode was used as the counter electrode. The electrolyte was purged with CO 2 for 15 min at a rate of 0.35 L/min to convert Na 2 CO 3 into NaHCO 3 at the beginning of the measurement.
During the measurement, a constant potential, i.e. -1.2 V vs. Ag/AgCl, was applied to the Cr/Au working electrode by a Biologic SP-50 potentiostat, while simultaneously sampling from the headspace and injecting the sample gas into the GC every 20 min. The process was repeated three times for both unpurified and purified (in the presence of 3.4 µM EDTA) electrolyte. The results were averaged to obtain the FE of CO. After the measurement, the working electrodes were tested via XPS to determine trace metal deposition and the upper limit active site density.

XPS trace metal deposition and LODs
XPS measurements to detect trace metal deposition were collected on thin film Au (as described in kinetics measurements) before and after electrolysis conditions. Electrolysis was performed in a CO 2 purged 0.1M NaHCO 3 with -1.2 V vs. Ag/AgCl. For XPS measurements corresponding to electrode deactivation, kinetics were run simultaneously to confirm Au deactivation. For purified samples that use EDTA, 3.4µM of EDTA was added to the electrolyte soltuion. XPS spectra were collected with a Kratos Axis Ultra X-ray photo-electron spectrometer (monochromatic Al Kα X-ray source, E photon = 1486.6 eV, pass energy 20 eV).
The binding energy scale was calibrated using the C 1s peak (BE = 284.5 eV) from typical surface contamination on the Au electrode. All peak areas were integrated using Casa XPS software and adjusted for atomic sensitivity accordingly.
High resolution spectra of the most common trace metal impurities reported in the literature (Zn, Cu, Fe, and Pb) 4-6 were collected on each sample. The Pt 4f region was also collected to evaluate Pt leaching onto the Au surface during electrocatalysis. Below in Figure   S2 are the the high resolution spectra of Fe 2p, Pb 4f, and Pt 4f which are not included in the main text. The non-zero signal seen here for Pt 4f is due to the overlap of the Au 5p 3/2 feature in this region, confirmed in our previous work. 7 The surface was also probed after 16 h of electrolysis and is given in Figure S3 The XPS LOD for each trace metal on a Au surface was calculated using the method outlined by Shard 8 for a homogeneously mixed trace element within a matrix element. The background signal of a clean (pre-electrolysis) Au electrode was collected within the binding energy ranges of each trace metal impuritites. These results can be see in the main text (for Cu and Zn) and below (for Fe and Pb).
The LOD for each possible trace element was calculated using Eq. 1 below: To account for the trace metals deposited on the Au surface opposed to homogeneously mixed throughout, the Au 4f area was adjusted by taking into account the surface-to-bulk intensity ratio of the XPS signal. The inelastic mean free path (λ) for Au was calculated to be 2.01 nm using Eq. 2, 9 giving a probe depth of 6.03 nm. The Au lattice spacing of a (111) surface (0.238 nm) was used to determine that approximately 26 layers of Au are probed during the XPS measurements.
The surface-to-bulk intensity ratio (calculated using Eq.  Table S1. These values represent the atomic LODs for the trace metals probed in this study. The high LOD for Pt is due to the interference of the Au 5p in this region.
The feature leads to a large σ B term in Eq. 1, raising the calculated LOD for Pt 4f.

CO spectra and Stark tuning slopes
The VSFG measurement taken under CO purging conditions were repeated a couple of times for each cation. The Stark tuning slopes shown in the manuscript and used for further calculations are the averaged result of these trials. Here we show one set of data for each of the cations except Rb + , already shown in the manuscript.

Disentangling dipolar-coupling effects
The field-induced Stark effect, chemical shift effects, and coverage-induced dipolar-coupling effects could all potentially contribute to the observed frequency shift of adsorbed molecules.
An applied potential will cause a variation of bonding between adsorbed molecules and the electrode and a field-dependent Stark shift, simultaneously. It is not trivial to fully resolve the contributions from each of these effects. However, it is possible to elucidate dipolarcoupling effects from the observed frequency shifts. In this section, we seek to disentangle the contribution from dipolar-coupling effects by treating the Stark effect and chemical shift as a whole, while as described in the manuscript and following Section 8, we calibrate the Stark tuning rate to address the chemical shift effect.
The frequency (ω(φ, Θ)) can be expressed as a function of applied potential (φ) and coverage (Θ), as described by Eq. 4: 10 K φ is the intrinsic Stark tuning slope, and K Θ is the slope-constant associated with the adsorbate surface coverage. w 0 is the reference frequency at reference potential (φ 0 ). To derive the apparent Stark tuning slope (K app ), which was obtained from VSFG measurement for COSS, the total derivative in φ of Eq. 4 can be determined as: To disentangle the contribution of the dipolar-coupling term (K Θ · dΘ dφ ) from the apparent Stark tuning slope, the potential-jump experiment, as previously described, 10 was first conducted to resolve K Θ in 0.1 M NaHCO 3 under CO 2 purging. A series of potential steps were first applied to the Au electrode ( -0.1 V to -0.9 V vs. Ag/AgCl, holding for 1 min at each potential), consistent with VSFG measurements. The bias was switched and held at -0.3 V vs. Ag/AgCl, where COSS adsorption is usually at a maximum. VSFG spectra were continuously collected with 5 s integration for each spectra ( Figure S9a) The COSS intensity increases significantly in the first 20 s and then stabilizes, indicating the CO reaches a saturated coverage, while the frequency blue-shifts in the same time period as the COSS intensity increase before stabilizing ( Figure S9b). Since the applied potential remained consistent throughout this time period, the blue-shift is solely induced by the dipolar-coupling effect resulting from the increase in COSS coverage on the Au surface. A simple way to explain the observed blue-shift is that the external field induced by the parallel dipole field from neighboring adsorbed CO could accelerate the vibrational frequency, although a more complete explanation can be derived by introducing a mutual energy term into the system's total energy expression. 11,12 VSFG intensity is quadratically correlated with the adsorbate's number density, 13 thus, the relative coverage of adsorbed CO was calculated by the square root of the COSS signal intensity normalized to the square root of the VSFG intensity after the frequency (and coverage) stabilization. A linear relationship between the frequency and relative coverage is then obtained ( Figure S9c) with a slope of K Θ .
We then calculate the potential-dependent relative coverage of COSS for each cation used here ( Figure S9d), and fit with a linear function to obtain dΘ dφ , as given in the first row of Table S2. The dipolar-coupling term (K Θ · dΘ dφ ) is then calculated, and subtracted from the apparent Stark tuning slope (K app ) to get the intrinsic Stark tuning slope (K φ ) (Table S2) for each cation. The dipolar-coupling term is about 10% of K app , indicating the dipole-dipole coupling effect caused by CO coverage change is not dominating the observed frequency shift.
The cation dependent trend for the intrinsic Stark tuning slopes still increases from Li + to Cs + , which further confirms that the Stern layer thickness decreases for larger alkali cation. Figure S9: a) Time-dependent CO spectra obtained at -0.3 V vs. Ag/AgCl following a potential jump from -0.9 V; b) COSS frequency shift and relative coverage change versus time; c) The linear relationship between the CO frequency shift and relative coverage; d) Potential-dependent CO coverage change for each cation. The intensity of COSS peak was normalized to the intensity at CO maximum adsorption potential, and taken the square root to calculate the relative coverage. The relative coverage is assume to be 1 at maximum absorption potential. The error bar results from averaging multiple trials conducted for each cation. S15 Figure S10: Double layer capacitance data obtained from fits of electrochemical impedance data for alkali cations. Error bars represent the standard deviation of multiple measurement.

Stark tuning slopes and Stern layer thicknesses
The Stern layer thicknesses are derived from the Stark tuning data and measured PZC using the Gouy-Chapman-Stern (GCS) model, as described previously. 7 More specifically, assuming the value (3.5Å) for the OHP of Cs + , we calibrate the Stark tuning rate for COAS and COSS, i.e. 1.40 cm −1 /(MV/cm) and 4.29 cm −1 /(MV/cm), respectively. Then the Stern layer thicknesses are derived from the intrinsic Stark tuning data after excluding dipolarcoupling effect for COSS. Table S3 shows the intrinsic Stark tuning slopes and derived Stern layer thicknesses for COSS and COAS from the procedure outlined here. The experimentally obtained apparent Stark tuning slopes and the process of calibrating dipolar-coupling effect is already shown in Table S2.