Mechanistic insights into the competition between electrochemical CO2 reduction and hydrogen evolution on Ag-based electrocatalysts via operando Raman spectroscopy
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Mechanistic insights into the competition between electrochemical CO 2 reduction and hydrogen evolution on Ag-based electrocatalysts via operando Raman spectroscopy Kinran Lau, †Muhammad Adib Abdillah Mahbub, †Nini Zhang, Anirudha Shekhawat, Xin Wang, Sabine Seisel, Ridha Zerdoumi and Wolfgang Schuhmann * To establish electrochemical CO 2 reduction (CO 2 RR) as a viable industrial route for fuel and chemical production, it is crucial to sustain CO 2 RR over the competing hydrogen evolution reaction (HER) even at high current densities. However, the underlying mechanism of HER dominance at higher overpotentials remains poorly understood. Here, using operando Raman spectroscopy, we first probe the CO 2 -to-CO pathway on Ag catalysts modified with alkaline earth metals (AgMg, AgCa, AgSr, AgBa) in a Na + - containing electrolyte. These modified catalysts exhibit more pronounced Raman features than pure Ag, enabling the detection of key CO 2 RR intermediates. Notably, AgBa shows the clearest progression of intermediates with increasing cathodic potential: CO 2 /*COO − /*COOH /*CO, providing direct spectroscopic evidence for the proposed CO formation mechanism. At potentials more negative than −0.3 V vs. RHE, CO 2 RR-related signals diminish, but this is accompanied by the emergence of a broad band at ∼532 cm −1 , which is assigned to the libration of interfacial water. This feature strongly correlates with the visible occurrence of the HER current, suggesting its role in HER initiation. We propose that an increasingly negatively charged electrode drives the reorientation of interfacial water molecules into an “H-down”configuration, creating a favorable geometry to trigger HER. The accumulation of this ordered interfacial water structure may represent the molecular origin of HER dominance at high overpotentials. We hope that these insights provide a framework for designing strategies to suppress HER and promote CO 2 RR by controlling interfacial water reorientation. Introduction The urgent need to tackle rising atmospheric CO 2 levels has driven extensive research into the electrochemical CO 2 reduction reaction (CO 2 RR) as a sustainable route for fuel and chemical production. 1 For practical applications, CO 2 RR has to be operated at industrially relevant current densities (hundreds of mA cm −2 ) with high selectivity and long-term stability for the generation of valuable products such as CO, ethylene, or ethanol. 2,3 However, the competing hydrogen evolution reaction (HER) oen undermines CO 2 RR efficiency. 4,5 To enhance CO 2 mass transport to the catalyst interface and suppress HER, gasfed ow cells using gas diffusion electrodes (GDEs) have largely replaced conventional liquid-fed H-cells. 6,7 Despite this advancement, HER oen outcompetes CO 2 RR at higher overpotentials and eventually becomes the dominant reaction, which is usually accompanied by ooding of the GDE. 8,9 When ooding happens, electrolyte penetrates the hydrophobic gas diffusion layer (GDL), reducing CO 2 availability at the catalyst interface further promoting HER over CO 2 RR. 10 Hence, to design better CO 2 RR catalysts, it is crucial to understand the exact mechanism driving this gradual transition from the desirable CO 2 RR to the competitive HER. However, probing this dynamic process requires techniques capable of characterizing the electrode interface under operating conditions, which is oen not trivial. To this end, operando Raman spectroscopy has demonstrated itself as a powerful, non-invasive tool to directly observe key catalytic species during electrochemical reactions such as CO 2 RR and HER. 11–13 A distinct advantage of Raman spectroscopy is its broad spectral range, spanning from low-frequency M–O and M–C vibrations (<1000 cm −1 , where M stands for metal) to midrange COO − and CO stretches (∼1350–2150 cm −1 ), and highfrequency OH stretches from water molecules (∼3400– 3600 cm −1 ). 14 This wide coverage enables a more Analytical Chemistry—Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universits¨ atsstr. 150, Bochum, 44780, Germany. E-mail: wolfgan[email protected] †These authors contributed equally to this work. Cite this: Chem. Sci.,2025,16,23160 All publication charges for this article have been paid for by the Royal Society of Chemistry Received 28th June 2025 Accepted 27th October 2025 DOI: 10.1039/d5sc04774a rsc.li/chemical-science 23160 |Chem. Sci.,2025,16,23160–23173 © 2025 The Author(s). Published by the Royal Society of Chemistry Chemical Science EDGE ARTICLE
comprehensive view of the catalytic interface compared to surface-enhanced infrared absorption spectroscopy (SEIRAS), which typically cannot access vibrations below ∼1000 cm −1 . 15 Due to the relatively small scattering cross-section of water, 16 Raman spectroscopy is also well-suited for examining aqueous systems without the bulk water signals overwhelming the spectrum. Moreover, common CO 2 RR catalysts such as Ag, Au, and Cu exhibit surface-enhanced Raman scattering (SERS) on rough surfaces, greatly amplifying the Raman signals near the interface up to a factor of 10 5 –10 6 , 17 which is benecial for detecting reaction intermediates. For Ag nanoparticles, it has been estimated that the SERS effect can be probed up to 5 nm away from the surface. 18 Overall, these advantages make operando Raman spectroscopy a valuable method for elucidating the competition between CO 2 RR and HER under reaction conditions. In this study, contrary to the popular choice of Cu capable of yielding C 2+ products, 19 we focus on Ag as a simple model system for operando Raman analysis. Unlike Cu, which produces a complex mixture of products and may contain Cu + species that complicate mechanistic interpretation, 20–23 Ag follows a well-dened CO 2 RR pathway (CO 2 /*COO − / *COOH /*CO, where *denotes an adsorbed species), with CO being the dominant product: 24–27 *þCO2! e*COO ! H2O OH *COOH ! e OH *CO/*þCO This provides a straightforward readout, where the gradual transition from CO 2 RR to HER at more cathodic potentials is marked by a decrease in CO production and a corresponding increase in H 2 . However, a key challenge of using Ag is its weak CO binding energy, which is even lower than that of Au and Cu, 26,28,29 making it potentially difficult to detect CO 2 RR intermediates. To address this problem, we draw inspiration from Cu-based catalysts, where the incorporation of alkaline earth metals has been demonstrated as an effective strategy to increase the Raman detectability of CO and improve CO 2 RR performance. 30–33 For instance, Xie et al. screened 109 Cu-based bimetallic combinations and identied Cu–Mg as the most active catalyst with up to 80% C 2+ faradaic efficiency (FE) at −1A cm −2 . 30 On a similar note, Xu et al. reported a Cu/BaO catalyst achieving 61% FE for C 2+ alcohols at −400 mA cm −2 , which was attributed to the metal/oxide interface stabilizing the hydroxylcontaining CO 2 RR intermediates. 32 Motivated by these ndings, we investigate whether depositing small amounts of Group 2 metals (Mg, Ca, Sr, Ba) onto Ag can similarly strengthen CO binding and thereby enhance CO 2 RR intermediate detection by Raman spectroscopy. Notably, while the Group 2-modied catalysts do not surpass pure Ag in overall CO 2 RR activity, they signicantly improve the Raman visibility of surface intermediates. On pristine Ag, CO 2 RR species are barely detectable, whereas AgBa shows distinct peaks corresponding to *COOH and *CO, with additional shoulder features attributable to *COO − . Interestingly, these CO 2 RR signals become negligible beyond −0.3 V vs. RHE, but this disappearance coincides with the emergence of a broad peak at ∼532 cm −1 , which is assigned to the librational mode of interfacial water. This spectroscopic signature aligns with a sharp rise in current associated with HER. Taken together, we propose that as the electrode becomes increasingly negatively charged, more interfacial water molecules reorient their hydrogen atoms towards the surface into an “H-down”conguration, creating a favorable geometry for HER initiation. This interfacial water reorganization may represent the molecular origin behind the gradual transition from CO 2 RR to the competitive HER. Overall, our work highlights the capability of operando Raman spectroscopy to directly probe surface intermediates and offer mechanistic insights into the interplay between CO 2 RR and HER. Results and discussion Synthesis and characterization of modied Ag catalysts Group 2-modied Ag catalysts (AgMg, AgCa, AgSr, AgBa) were prepared via a simple precipitation method. As depicted schematically in Fig. 1a, commercial Ag nanoparticles (SigmaAldrich, 99.5%, <100 nm) were rst dispersed in an aqueous solution of metal chlorides (MgCl 2 , CaCl 2 , SrCl 2 , BaCl 2 ) with a metal-to-Ag molar ratio of 2%. An excess of K 2 CO 3 was then added dropwise to precipitate the respective metal carbonates onto the Ag surface. The resulting materials were washed, centrifuged, and dried before collection (see Experimental section in SI for details). Unlike common coprecipitation methods, where Ag and dopant precursors are precipitated together, we start with the same batch of commercial Ag particles, which minimizes the variations in particle size across samples. Scanning electron microscopy (SEM) analysis shows that unmodied Ag particles have an average diameter of 57 ± 20 nm (Fig. 1b). Aer Group 2 addition, the primary particle size only increases slightly, reaching 69 ±22 nm for AgBa (Fig. 1c), while other modied Ag samples also fall within a similar range of 61–69 nm (Table S1 and Fig. S1–S3). We chose carbonate rather than hydroxide precipitation for two main reasons. First, metal hydroxides readily convert to carbonates upon exposure to CO 2 during electrochemical testing. But more importantly, the solubility product (K sp ) of Group 2 hydroxides increases dramatically down the series, spanning eight orders of magnitude from Mg(OH) 2 (5.61 ×10 −12 mol 3 L −3 ) to Ba(OH) 2 (2.55 × 10 −4 mol 3 L −3 ) (Table S2). This means that signicantly higher OH − concentrations would be required to precipitate Ba(OH) 2 compared to Mg(OH) 2 . In contrast, Group 2 carbonates have consistently low solubility, with K sp values of 6.82 ×10 −6 mol 2 L −2 for MgCO 3 and roughly 10 −9 to 10 −10 mol 2 L −2 for CaCO 3 , SrCO 3 , and BaCO 3 (Table S2), which allows for more comparable precipitation conditions across all samples. Despite this modication, X-ray diffraction (XRD) patterns (Fig. 1d) show no distinct reections for the metal carbonates, likely due to their low loading. Only reections corresponding to Ag are observed, along with signals from PTFE (2q=18°) and carbon paper (2q= 26°). 34 However, inductively coupled plasma mass spectrometry (ICP-MS) and X-ray photoelectron spectroscopy (XPS) conrm the successful deposition of Group 2 elements onto the Ag particles. ICP-MS analysis (Table S3) consistently shows 0.01– © 2025 The Author(s). Published by the Royal Society of Chemistry Chem. 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0.04 mol% Group 2 content relative to Ag for all samples, except for AgSr, which exhibits a notably higher value (0.9 mol% Sr). This high level of Sr likely reects the lowest solubility constant of SrCO 3 (5.6 ×10 −10 mol 2 L −2 , Table S2) among Group 2 carbonates, though it remains lower than the nominal 2% initially added. XPS measurements (Fig. S4–S6) further reveal the distribution between bulk and surface. While Mg and Ca were not detected likely due to their trace amount, both Sr and Ba were clearly observed. As a representative example, AgBa records a bulk Ba content of 0.04 mol% by ICP-MS, but a signicantly higher surface-sensitive Ba/Ag ratio of 0.55% by XPS, suggesting the presence of Ba at the surface following carbonate precipitation. Electrochemical CO 2 reduction Electrochemical CO 2 RR measurements were performed using a custom ow cell setup illustrated in Fig. 2a. As mentioned in the introduction, a GDE was employed to enhance CO 2 mass transport and availability, where CO 2 was supplied from the back side of the GDE and reached the catalyst layer at the electrolyte interface. The working electrode consisted of the Group 2-modied Ag catalysts drop-cast onto the GDE, while Ni foam served as the counter electrode. A 1 M NaOH solution was used as electrolyte, with the catholyte and anolyte compartments separated by an anion exchange membrane. Gaseous products were quantied using online gas chromatography (GC), while liquid products were sampled at each current density and analysed by high-performance liquid chromatography (HPLC). Chronopotentiometric measurements were carried out in a current density range from −25 mA cm −2 to −400 mA cm −2 for all catalysts (Ag, AgMg, AgCa, AgSr, AgBa), and only three main products were detected: CO, formate, and H 2 (Fig. S7–S16). Using AgBa as a representative example, Fig. 2b presents the evolution of product distribution as the current density increases. At low current densities (up to −100 mA cm −2 ), CO is the predominant product (>90% FE), with only minor amounts of formate and H 2 formed. However, at higher current densities, FE CO declines while FE formate and FE H2 increase, suggesting that CO 2 RR becomes limited by mass transport, while the competing HER starts to take over. At −400 mA cm −2 ,FE H2 reaches up to 32% for AgBa. The CO 2 RR performance of all catalysts is summarized in Fig. 2c comparing their product selectivity at −300 mA cm −2 , which is the current density when HER becomes evident. A higher FE CO and a lower FE H2 indicate more favorable CO 2 RR performance, and vice versa. Among the modied catalysts, AgMg, AgCa, and AgBa with comparable Group 2 loading (Table S3) exhibit similar CO 2 RR activity at −300 mA cm −2 , with a FE CO at around 55%, a FE formate of close to 25%, and a FE H2 below 20%. In contrast, AgSr exhibits worse selectivity, achieving only 41% FE CO and 38% FE H2 , likely due to the excessively high Sr content compared to other samples (Table S3), which appears to be detrimental to CO 2 RR activity. Notably, all Group 2-modied catalysts demonstrate decreased CO 2 RR activity compared to pure Ag, which delivers a FE CO of 67% and only a FE H2 of 7% at the same current density. This inferior performance of the modied catalysts is likely due to the non-conductive nature of the deposited metal Fig. 1 (a) Schematic of the synthesis of Group 2-modified Ag catalysts via carbonate precipitation. SEM images and particle size distributions of (b) Ag and (c) AgBa catalysts. (d) XRD patterns of Group 2-modified Ag catalysts drop-cast on carbon paper. 23162 |Chem. Sci.,2025,16,23160–23173 © 2025 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article
carbonates, which contribute to ohmic losses and partially block the active surface sites of Ag. The current density vs. potential (iR-corrected vs. RHE) curves in Fig. 2d reveal a similar trend, where pure Ag has the lowest overpotential, followed by AgMg, AgCa, and AgBa with nearly identical potential proles, while the worst performing AgSr displays the highest overpotential. Qualitative analysis of Raman spectra To gain deeper insights into the CO 2 RR mechanism, we investigate the catalysts using operando Raman spectroscopy. As illustrated in Fig. 3a, the setup uses a 532 nm green laser and a water immersion objective to probe the catalyst-coated GDE immersed in an electrolyte, with CO 2 supplied from the bottom of the GDE. Although these Raman measurements are performed under CO 2 ow and cathodic potentials, we emphasize that the conditions are not directly comparable to those in the ow cell in Fig. 2. The rst key difference is that the Raman setup employs a static electrolyte with both the working electrode (catalyst-coated GDE) and counter electrode (Ni foam) immersed in the same solution, whereas the ow cell has enhanced electrolyte ow, with the catholyte and anolyte compartments separated by a membrane. In addition, to avoid damaging the objective lens, the Raman electrolyte (pH 13, 0.1 M NaOH + 0.5 M Na 2 SO 4 ) is less alkaline than the ow cell conditions (pH 14, 1 M NaOH). For clarity, we structure our discussion in two parts: Fig. 3 presents a qualitative overview of how the Raman spectra evolve with potential, while Fig. 4 dives into specic spectral regions for more quantitative analysis. Fig. 3b and c show the potential-dependent Raman spectra for pure Ag and AgBa, respectively, from before applying potential (dry and OCP) to aer cathodic bias (−0.05 V to −0.5 V vs. RHE at pH 13). Spectral acquisition beyond −0.5 V proves challenging, as bubble accumulation on the GDE signicantly reduces the signal-to-noise ratio. Although the exact conditions and potentials of the Raman setup are not directly comparable to those in the electrochemical cell, complementary ow-cell measurements at low current density conrm that this regime corresponds to CO production rather than HER (Fig. S17). Hence, the operando Raman measurements remain valuable for providing a qualitative view of potential-dependent processes. The Raman spectra for other samples (AgMg, AgCa, AgSr) are presented in Fig. S18. Notably, common to all the samples, there are two regions exhibiting pronounced changes with potential. The lowfrequency region (∼300–600 cm −1 , orange) is dominated by Ag– OH vibrations that grow in intensity under increasingly negative Fig. 2 (a) Schematic of the flow cell used for electrochemical CO 2 reduction on catalyst-coated GDEs in 1 M NaOH (pH 14). (b) Faradaic efficiencies of CO, formate, and H 2 for the AgBa catalyst at current densities in the range from −25 to −400 mA cm −2 . (c) Faradaic efficiencies at −300 mA cm −2 for all Group 2-modified catalysts (AgMg, AgCa, AgSr, AgBa) compared to pure Ag. (d) Current densities and iR-corrected potentials for all samples. Data beyond −300 mA cm −2 are not shown here due to vigorous bubble formation interfering with accurate resistance measurements. Error bars represent ±1 standard deviation from triplicate measurements. © 2025 The Author(s). Published by the Royal Society of Chemistry Chem. Sci.,2025,16,23160–23173 | 23163 Edge Article Chemical Science
potentials. The higher-frequency region (∼1350–2050 cm −1 , blue) contains signals associated with CO 2 reduction intermediates (e.g. COO − , COOH, and CO), which typically diminish as the potential becomes more cathodic. However, a direct comparison between Ag (Fig. 3b) and AgBa (Fig. 3c) reveals a striking difference: AgBa shows much stronger signals for CO 2 RR intermediates. At OCP and under dry conditions, neither sample shows peaks for COOH (1760 cm −1 ) or CO (∼1900–2000 cm −1 ). But as soon as cathodic potential is applied at just −0.05 V, AgBa already exhibits a strong COOH peak together with a weaker CO signal, which are both much more prominent than on pure Ag. These features gradually fade with more negative potentials and become nearly undetectable at −0.3 V. COO − species (1404 and 1550 cm −1 ) are also detected as shoulders within the CH 2 and H 2 O bending regions in the same potential window, which will be discussed further in Fig. 4. Lastly, we observe a prominent Ag–O peak (∼230 cm −1 ), which may originate from surface oxides 35 or binding with polyvinylpyrrolidone (PVP) stabilizers, 36,37 together with several strong PVP-related bands across 200–1700 cm −1 (Table S4) under dry and OCP conditions. These signals, however, rapidly diminish once a cathodic bias is applied (−0.05 V, Fig. S19), suggesting that PVP largely desorbs under reductive conditions. The disappearance of these features appears to be crucial for catalyst activation, enabling strong and intense Raman signals for following CO 2 RR intermediates. Pure Ag follows a similar trend of declining CO 2 RR-related species (COO − , COOH, and CO) under the cathodic sweep, but all the signals are much weaker throughout. Interestingly, all Group 2-modied samples (AgMg, AgCa, AgSr, AgBa) show enhanced COOH and CO signals relative to pure Ag, with AgBa displaying the strongest intensity. The stronger signals could arise from either physical SERS effects or chemical stabilization of intermediates that increase their surface coverage. Surface roughness from the deposited carbonates may contribute to some degree, 38 but pyridine adsorption experiments (Fig. S20–S22) show that Ag and AgBa have comparable SERS activity under cathodic conditions, suggesting that physical effects alone cannot account for the difference. Thus, while both physical and chemical factors may Fig. 3 (a) Schematic of the operando Raman setup under CO 2 flow, recorded from −0.05 to −0.5 V vs. RHE in 0.1 M NaOH + 0.5 M Na 2 SO 4 (pH 13). Operando Raman spectra at different applied potentials for (b) Ag and (c) AgBa catalysts. 23164 |Chem. Sci.,2025,16,23160–23173 © 2025 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article
play a role, it is likely that Group 2 species help stabilize CO 2 RR intermediates, making them more detectable by Raman spectroscopy. Similar observations have been reported for Group 2modied Cu catalysts (e.g. Cu–Mg, Cu–Ba), where stronger CO Raman signals and higher FEs for C 2+ products were attributed to increased *CO coverage, which in turn facilitated C–C coupling. 30–33 By analogy, Group 2 metals on Ag may also enhance *CO binding similarly. However, unlike Cu, CO 2 reduction on Ag typically terminates at CO as the main product. In this case, stronger *CO binding may hinder CO desorption and thus reduce the overall CO 2 RR performance. This may explain the lower FE CO observed for Group 2-modied catalysts compared to pure Ag (Fig. 2c), apart from the metal carbonates covering some active surface areas of Ag. Quantitative analysis of Raman spectra Although Fig. 3 provides an overview of species present across the chosen potential range, it does not allow us to follow their emergence or disappearance quantitatively. To enable meaningful comparison across spectra with varying absolute intensities, the same normalization and baseline correction procedure was applied to all raw spectra. Prior to the Fig. 4 Normalized Raman intensities of AgBa measured in the potential range from −0.05 to −0.5 V vs. RHE for (a) *COO − , (b) *COOH, (c) *CO, (d) water OH, and (e) Ag–OH, with their corresponding integrated areas shown in (f–j). The shaded backgrounds in (a–e) indicate the range for integration. For clarity, only smoothed signals are plotted here. © 2025 The Author(s). Published by the Royal Society of Chemistry Chem. Sci.,2025,16,23160–23173 | 23165 Edge Article Chemical Science
measurement, the objective lens was carefully adjusted to nd the optimum working distance that yielded the highest intensity at 100 cm −1 (Rayleigh scattering tail), and the position was xed during subsequent potential-dependent measurements. The normalization of each spectrum was performed by setting its maximum intensity at 100 cm −1 to 1 and the minimum intensity across the full recorded range (100–3700 cm −1 )to0. Additionally, to avoid artifacts from manual tting, we subtracted the baseline of all the spectra using the automated Statistics-sensitive Non-linear Iterative Peak-clipping (SNIP) algorithm with a xed half-window of 55 cm −1 . 39,40 Fig. S23–S27 show the spectra before and aer baseline subtraction. Further details can be found in the SI. Fig. 4 shows the normalized Raman spectra and their integrated intensities as a function of applied potential for AgBa, which is selected as a representative example due to its strongest spectral features. Panels a–e highlight the potentialdependent evolution of different species involving COO − , COOH, CO, H 2 O, and Ag–OH, while panels f–j display the integrated areas for the corresponding regions. Results for other samples (Ag, AgMg, AgCa, AgSr) are shown in Fig. S28– S31. While absolute intensities may vary between samples, the qualitative trends discussed below are generally observed across all cases unless otherwise noted. As established in Fig. 3, signals associated with hydrogen-containing species (H 2 O, Ag–OH) increase with more negative potentials, while carboncontaining species (COO − , COOH, CO) tend to rst emerge at mildly cathodic potentials before diminishing aerwards. COO − region (Fig. 4a and f). The symmetric (n s ) and antisymmetric (n as ) stretching vibrations of COO − appear as shoulders at ∼1404 and ∼1550 cm −1 , respectively, although their interpretation is complicated by neighbouring bands from CH 2 deformation (∼1440–1450 cm −1 ) and H 2 O bending (∼1620 cm −1 ). 41 The assignments are consistent with literature values, including the work by Firet and Smith who reported similar COO − features during CO 2 reduction on Ag using attenuated total reection Fourier transform infrared spectroscopy (ATR-FTIR). 42 As the potential sweeps from −0.05 V to more negative values, both COO − shoulders diminish. Integration of the 1380–1410 cm −1 and 1530–1570 cm −1 regions (shaded blue in Fig. 4a) conrms this trend (Fig. 4f): n as (COO − ) declines sharply and plateaus around −0.2 V, while n s (COO − ) decreases more gradually. The slower decay of n s (COO − ) may result from additional contributions by CO 2 , which has a similar Raman shifor its symmetric stretch (CO 2 : 1388 cm −1 ; COO − : 1404 cm −1 ). 43 We note that formate COO − stretches may also overlap with n s (COO − ) and n as (COO − ) signals used here (Table S5), but since formate is only a minor product (<10% before HER dominates, Fig. 2b), its inuence on quantifying the COO − intermediate is expected to be negligible. Due to its minimal spectral overlap and higher reliability, n as (COO − )is used for quantication in further analysis (Fig. 6). COOH region (Fig. 4b and g). The C]O stretch of COOH appears as a distinct peak at 1760 cm −1 , which diminishes steadily as the potential becomes more negative, eventually disappearing at −0.4 V. Integration over 1725–1800 cm −1 (Fig. 4g) conrms this decay. The 1760 cm −1 feature likely corresponds to monomeric COOH, which matches well with the C]O stretching frequency of aliphatic carboxylic acids. 14 In contrast, when carboxylic acids form dimers or are hydrogen-bonded (Hbonded), their C]O vibration shis to lower frequencies. 14 Hence, we attribute the shoulder at 1660 cm −1 (Fig. 4a) as dimeric or H-bonded COOH species, which was also observed by Firet and Smith. 42 CO Region (Fig. 4c and h). The CO stretching band initially grows in intensity from −0.05 V and peaks at −0.2 V, before disappearing at around −0.3 V (Fig. 4h). Interestingly, during the cathodic sweep, the CO peak gradually shis to higher wavenumbers (right axis of Fig. 4h), suggesting a transition from more highly coordinated to less coordinated adsorption sites. In general, CO bound to more metal atoms exhibits lower vibrational frequencies: n(CO) on 3-fold hollow < 2-fold bridge < 1-fold atop sites. 44 However, the exact values could vary considerably depending on the local coordination environment. Experimentally, CO vibrations on Ag have been observed between ∼1800–2100 cm −1 . 45–50 Density functional theory (DFT) calculations predict CO stretching frequency on Ag surfaces to be in the range of ∼1895–1961 cm −1 for hollow sites, ∼1935– 2002 cm −1 for bridge sites, and ∼2050–2123 cm −1 for atop sites (Table S6). 44,51–53 Moreover, the CO stretching frequency can be red-shied in the presence of an electric eld by the electrochemical Stark effect, due to an increasing amount of back donation from Ag d states into the CO 2p*antibonding orbital, which weakens the C–O bond and reduces its frequency. 54–56 At −0.05 V, the CO peak rst appears at 1926 cm −1 (Fig. 4c and h), likely arising from CO adsorbed at higher-coordinated sites (hollow/bridge). However, as the potential is swept to the negative direction, the CO signal intensies and shisto 1948 cm −1 by −0.15 V, accompanied by the emergence of a shoulder at 1988 cm −1 . This suggests a growing contribution from lower-coordination sites (bridge). At −0.2 V, the spectrum shows a clear doublet at 1946 and 1987 cm −1 with comparable intensity, indicating the presence of at least two distinct adsorption environments. At −0.25 V, the higher-frequency component becomes dominant with a main peak at 1985 cm −1 , suggesting a signicant presence of bridge CO. Beyond −0.3 V, the CO signals diminish and become too weak for reliable interpretation. Overall, it appears that CO undergoes a shifrom more highly coordinated sites (hollow/bridge) to less coordinated congurations (bridge) as the potential becomes more negative. We hypothesize that this transition is driven by the competitive adsorption from other species (e.g.*H, *OH, *H 2 O), potentially displacing CO from its original sites, particularly *H which is known to favourably adsorb on hollow sites. 53,57,58 Similar competitive adsorption behaviors have been reported in the literature. Schmitt and Gewirth only observed hollow/bridgebound CO on pure Ag using SERS, but upon introducing a triazole ligand, they observed the emergence of atop and even physisorbed CO. 47 However, we note that CO may also exhibit coverage-dependent preferences for adsorption sites in the absence of competitive adsorption. 59,60 H 2 O region (Fig. 4d and i). The O–H stretching band (3150– 3700 cm −1 ) grows with increasing cathodic potential, reaching 23166 |Chem. Sci.,2025,16,23160–23173 © 2025 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article
a maximum at −0.35 V before declining thereaer (Fig. 4i). However, for pure Ag, the water intensity continues to increase past −0.35 V with no signs of dropping (Fig. S28). Apart from the intensity changes, the peak position shis gradually to lower frequencies from 3486 cm −1 at −0.1 V to 3456 cm −1 at −0.5 V (right axis of Fig. 4i). This indicates a growing population of Hbonded water molecules during the cathodic sweep, which is a common observation even in the absence of CO 2 . 61–63 More interestingly, the sudden redshito ∼3460 cm −1 at −0.25 V coincides with the sharp increase in water intensity, which also aligns with the spike in the current response (Fig. 5a). Taken together, these observations suggest that this abrupt rise in Hbonded water from −0.25 V is closely related to the onset of HER, which would be discussed extensively in the next section. Ag–OH region (Fig. 4e and j). The four peaks at 356, 418, 486, and 532 cm −1 in the low-frequency region demonstrate two distinct groups of behaviors: the rst three already appear at less negative potentials, while the 532 cm −1 peak only emerges beyond −0.25 V. The 356–486 cm −1 peaks are tentatively assigned to Ag–OH species, as they fall within the expected range of Ag–O modes, 64–68 while the higher-frequency 532 cm −1 peak is attributed to interfacial water (vide infra). To better separate their contributions, we integrated the 315–532 cm −1 region to represent Ag–OH, and the 532–640 cm −1 range to isolate the higher-frequency feature (Fig. 4j). For the rst group of peaks (356, 418, 486 cm −1 ), which is denoted as Ag–OH, another plausible assignment could be Ag– CO modes. However, DFT studies predict Ag–CO vibrations to appear near 200 cm −1 (Table S6), 44,51,53 which would be too low to account for these features. Due to the weak binding of CO on Ag, direct experimental observation of Ag–C vibrations is relatively scarce, with a notable exception from Abe et al. who reported a peak at around 160 cm −1 . 45 To further rule out COrelated origins, we conducted a control experiment in CO 2 - free, N 2 -purged electrolyte (Fig. S32), where similar peaks (345, 418, 490 cm −1 ) remain observable in the low-frequency region. Interestingly, the integrated intensity of the Ag–OH region (315– 532 cm −1 , Fig. 4j) closely follows the trend of CO (Fig. 4h): both rising from −0.05 V to −0.2 V, then decreasing until −0.3 V. This correlation can be rationalized by the fact that CO and OH − are co-products of *COOH reduction (*COOH + e − /*CO + OH − ), further supporting our assignment of Ag–OH. In contrast to other Raman features, the 532 cm −1 peak exhibits a distinctly different behavior. As shown by the integrated intensity from 532–640 cm −1 (Fig. 4j), this band only begins to emerge at around −0.25 V and continues to rise throughout the cathodic sweep. To visualize the emergence of this feature more clearly, the Raman spectra at each potential are plotted separately in Fig. S33. Notably, this onset at −0.25 V coincides with the abrupt increase in the water OH stretching band (Fig. 4i) and the spike in current (Fig. 5a), strongly suggesting a link between this species and the accumulation of Hbonded water as HER begins. Initially, in the absence of an applied potential, interfacial water molecules are more randomly distributed. However, as the electrode becomes negatively polarized, they reorient into a more ordered structure, with their hydrogen atoms pointing towards the electrode surface, resulting in an “H-down”conguration. 69–74 When these interfacial water molecules experience strong electrostatic interactions near the negatively charged electrode, their movement is restricted and can result in frustrated rotation (“libration”), giving rise to a broad Raman band between ∼400– 700 cm −1 . Such features have been repeatedly observed in CO 2 - free electrolytes on Ag 64,74–76 as well as on electrodes such as Au and Pd. 70,71 Notably, Chen et al. observed a broad peak at around 515 cm −1 on Ag in 1 M Na 2 SO 4 in the potential range from −1.30 to −1.60 V vs. SCE (−0.29 to −0.59 V vs. RHE at pH 13). Upon deuterium substitution, this peak shied to ∼370 cm −1 by a factor of 1.39, conrming its involvement of hydrogen atoms. 74 In our N 2 -purged control of AgBa without CO 2 supply Fig. 5 (a) Current and (b) interfacial water area (532–640 cm −1 ) as a functional of potential for all samples (Ag, AgMg, AgCa, AgSr, AgBa). (c) Correlation between log(jcurrentj) and log(interfacial water area). © 2025 The Author(s). Published by the Royal Society of Chemistry Chem. Sci.,2025,16,23160–23173 | 23167 Edge Article Chemical Science
(Fig. S32), we also observe a similar broad feature near 450– 600 cm −1 appearing at more negative potentials, while no appreciable signals are detected in the CO range (1850– 2050 cm −1 ), supporting the assignment that its origin is not related to CO 2 . Notably, this 532 cm −1 feature appears broader and more pronounced on unmodied Ag (Fig. S28) than on carbonate-modied samples, likely due to the greater polarizability of pure metal to attract more interfacial water under cathodic bias. To further rule out the possibility that this 532 cm −1 feature arises from carbonate residues, we measured potentialdependent Raman spectra on bare Ag in 1 M Na 2 CO 3 (Fig. S34), which only show the characteristic carbonate C–O stretch at around 1070 cm −1 , with no observable signals in the 400–700 cm −1 region. In addition, Raman spectra of solid Group 2 carbonates are not expected to display peaks in this range. 77 More importantly, the emergence of this interfacial water band has been correlated with the onset of HER current. 71,74,76 To illustrate this, we plot the current (Fig. 5a) and the integrated intensity between 532–640 cm −1 (Fig. 5b) as a function of potential. For all samples (Ag, AgMg, AgCa, AgSr, AgBa), both the current and the 532 cm −1 band begin to rise at around −0.25 V, aligning well with the HER onset. Fig. 5c shows the direct relationship between log(current) and log(interfacial water area), with R 2 values of at least 0.8 across all samples. Given its strong correlation with HER current, we assign this broad 532 cm −1 band to the libration of interfacial water molecules. In this H-down conguration, the shortened distance between the electrode and the hydrogen atoms promotes electron transfer into the antibonding orbital of water, facilitating the Volmer step (*+H 2 O+e − /*H+OH − ) and initiating HER. 71 While *H is not directly observed here presumably due to its low polarizability, 78,79 this increased population of interfacial water could serve as a spectroscopic proxy for growing *H coverage. Since *H preferentially binds to hollow sites, 57 its accumulation may displace *CO to lowercoordination sites, potentially explaining the CO shito higher wavenumbers observed in Fig. 4h. To further examine the relationship between interfacial water and HER, we compared Na + with K + electrolytes. Previous studies using scanning tunneling microscopy (STM) and SEIRAS demonstrated that cations have a strong impact on the interfacial water structure of Au electrodes: “structure-making” cations such as Li + remain more solvated and stay further away from the surface, resulting in an ordered, ice-like layer of interfacial water, whereas “structure-breaking”cations such as K + or Cs + are less hydrated and can approach closer to the electrode, thereby disrupting the H-bonding network of interfacial water. 63 This ion-specic interfacial water structure correlates well with HER activity, which decreases from Li + to Cs + . Based on this, we expect K + to suppress both HER and water-related Raman signals. Indeed, electrochemical ow cell measurements with AgBa in 1 M KOH show that HER remains minor until −600 mA cm −2 (40% FE, Fig. S35), whereas in 1 M NaOH HER already dominates at −400 mA cm −2 (32% FE, Fig. 2b). Operando Raman spectra recorded in 0.1 M KOH + 0.5 M K 2 SO 4 (Fig. S36–S37) further reveal that the 400–700 cm −1 features are strongly diminished relative to Na + . Similar suppression of the water libration band has also been reported for Cs + electrolytes. 75,76 Together, these observations suggest that disrupting the H-bonded interfacial water makes HER less favorable. Raman-derived mechanism for CO 2 RR and HER Having examined the emergence of each Raman-active species individually for AgBa, we now compile their potentialdependent proles into a unied plot to facilitate direct comparison (Fig. 6a). The integrated intensity for each species is scaled to 0–1 to give its relative area. Similar plots for other samples (Ag, AgMg, AgCa, AgSr) are shown in Fig. S38. Fig. 6b summarizes the proposed mechanism consistent with our data and the established literature. We begin with CO 2 reduction in Fig. 6a, where a clear progression of the intermediates is observed: CO 2 /*COO − / *COOH /*CO. Both *COO − and *COOH appear from the initial potential of −0.05 V, but their intensities decline with further cathodic bias. Notably, *COO − diminishes at −0.2 V, while *COOH persists until −0.3 V. This trend supports a stepwise mechanism, where CO 2 is rst activated to form *COO − via electron transfer (ET), followed by proton transfer (PT) from water to yield *COOH (Fig. 6b). In contrast, *CO does not show a substantial presence initially at −0.05 V. It gradually increases and peaks at −0.2 V before declining until −0.3 V. Importantly, the decrease in *COO − /*COOH between −0.05 and −0.2 V mirrors the rise in *CO, providing a strong conrmation that CO is formed by further reducing *COOH (*COOH + e − /*CO +OH − ) (Fig. 6b). Beyond −0.3 V, all CO 2 RR-related species only show negligible signals in the spectra, likely due to the growing dominance of the competitive HER, as indicated by the simultaneous spike in current (Fig. 5a). While CO 2 RR may still occur past −0.3 V, its contribution is likely minor on the several-second timescale of our Raman measurements. We complement this CO 2 RR to CO mechanism with additional insights from the literature (Fig. 6b). First, the activation of CO 2 to *COO − is generally considered the ratedetermining step (RDS) at most potential windows (<−0.7 V vs. SHE for Au). 26,80 Notably, this ET step only involves electrons but not protons as in concerted proton-coupled electron transfer (PCET), implying a weak pH dependence of CO 2 reduction to CO. This is consistent with electrochemical studies showing that CO current densities on Ag and Au remain nearly constant across acidic and neutral pH when plotted on the SHE scale. 80,81 Second, at alkaline conditions where water is expected to be the primary proton donor, the subsequent PT step (*COO − /*COOH) likely proceeds via an Eley–Rideal mechanism, as *COO − canengageinhydrogen bonding with nearby water molecules via its O atom. 24,82 Since CO formation only requires solvent water as its main hydrogen source (as opposed to *H), this explains why CO is the dominant product at low overpotentials before HER becomes competitive (which requires *H). 23168 |Chem. Sci.,2025,16,23160–23173 © 2025 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article