Electric Fields at Metal-Surfactant Interfaces: A Combined Vibrational Spectroscopy and Capacitance Study

Measuring the Electric Fields of Ions Captured in Crown Ethers

Controlling reactivity with electric fields is a persistent challenge in chemistry. One approach is to tether ions at well-defined locations near a reactive center. To quantify fields arising from ions, we report crown ethers that capture metal cations as field sources and a covalently bound vibrational Stark shift probe as a field sensor. We use experiments and computations in both the gas and liquid phases to quantify the vibrational frequencies of the probe and estimate the electric fields from the captured ions. Cations, in general, blue shift the probe frequency, with effective fields estimated to vary in the range of ∼0.2-3 V/nm in the liquid phase. Comparison of the gas and liquid phase data provides insight into the effects of mutual polarization of the molecule and solvent and screening of the ion's field. These findings reveal the roles of charge, local screening, and geometry in the design of tailored electric fields.

Cation effects in hydrogen evolution and CO2-to-CO conversion: A critical perspective

The rates of many electrocatalytic reactions can be strongly affected by the structure and dynamics of the electrochemical double layer, which in turn can be tuned by the concentration and identity of the supporting electrolyte's cation. The effect of cations on an electrocatalytic process depends on a complex interplay between electrolyte components, electrode material and surface structure, applied electrode potential, and reaction intermediates. Although cation effects remain insufficiently understood, the principal mechanisms underlying cation-dependent reactivity and selectivity are beginning to emerge. In this Perspective, we summarize and critically examine recent advances in this area in the context of the hydrogen evolution reaction (HER) and CO2-to-CO conversion, which are among the most intensively studied and promising electrocatalytic reactions for the sustainable production of commodity chemicals and fuels. Improving the kinetics of the HER in base and enabling energetically efficient and selective CO2 reduction at low pH are key challenges in electrocatalysis. The physical insights from the recent literature illustrate how cation effects can be utilized to help achieve these goals and to steer other electrocatalytic processes of technological relevance.

Selective and Efficient CO2 Electroreduction to Formate on Copper Electrodes Modified by Cationic Gemini Surfactants

Electroreduction of CO2 into valuable chemicals and fuels is a promising strategy to mitigate energy and environmental problems. However, it usually suffers from unsatisfactory selectivity for a single product and inadequate electrochemical stability. Herein, we report the first work to use cationic Gemini surfactants as modifiers to boost CO2 electroreduction to formate. The selectivity, activity and stability of the catalysts can be all significantly enhanced by Gemini surfactant modification. The Faradaic efficiency (FE) of formate could reach up to 96 %, and the energy efficiency (EE) could achieve 71 % over the Gemini surfactants modified Cu electrode. In addition, the Gemini surfactants modified commercial Bi2 O3 nanosheets also showed an excellent catalytic performance, and the FE of formate reached 91 % with a current density of 510 mA cm-2 using the flow cell. Detailed studies demonstrated that the double quaternary ammonium cations and alkyl chains of the Gemini surfactants played a crucial role in boosting electroreduction CO2 , which can not only stabilize the key intermediate HCOO* but also provide an easy access for CO2 . These observations could shine light on the rational design of organic modifiers for promoted CO2 electroreduction.

Electrochemical carbon-carbon coupling with enhanced activity and racemate stereoselectivity by microenvironment regulation

Enzymes are characteristic of catalytic efficiency and specificity by maneuvering multiple components in concert at a confined nanoscale space. However, achieving such a configuration in artificial catalysts remains challenging. Herein, we report a microenvironment regulation strategy by modifying carbon paper with hexadecyltrimethylammonium cations, delivering electrochemical carbon-carbon coupling of benzaldehyde with enhanced activity and racemate stereoselectivity. The modified electrode-electrolyte interface creates an optimal microenvironment for electrocatalysis-it engenders dipolar interaction with the reaction intermediate, giving a 2.2-fold higher reaction rate (from 0.13 to 0.28 mmol h-1 cm-2); Moreover, it repels interfacial water and modulates the conformational specificity of reaction intermediate by facilitating intermolecular hydrogen bonding, affording 2.5-fold higher diastereomeric ratio of racemate to mesomer (from 0.73 to 1.82). We expect that the microenvironment regulation strategy will lead to the advanced design of electrode-electrolyte interface for enhanced activity and (stereo)selectivity that mimics enzymes.

Electro-inductive Effect Dominates Vibrational Frequency Shifts of Conjugated Probes on Gold Electrodes

Interfacial electric fields play a critical role in electrocatalysis and are often characterized by using vibrational probes attached to an electrode surface. Understanding the physical principles dictating the impact of the applied electrode potential on the vibrational probe frequency is important. Herein, a comparative study is performed for two molecular probes attached to a gold electrode. Both probes contain a nitrile (CN) group, but 4-mercaptobenzonitrile (4-MBN) exhibits continuous conjugation from the electrode through the nitrile group, whereas this conjugation is interrupted for 2-(4-mercaptophenyl)acetonitrile (4-MPCN). Periodic density functional theory calculations predict that the CN vibrational frequency shift of the 4-MBN system is dominated by induction, which is a through-bond polarization effect, leading to a strong potential dependence that does not depend significantly on the orientation of the CN bond relative to the surface. In contrast, the CN vibrational frequency shift of the 4-MPCN system is influenced less by induction and more by through-space electric field effects, leading to a weaker potential dependence and a greater orientation dependence. These theoretical predictions were confirmed by surface-enhanced Raman spectroscopy experiments. Balancing through-bond and through-space electrostatic effects may assist in the fundamental understanding and design of electrocatalytic systems.

Electroreductive Defluorination of Unsaturated PFAS by a Quaternary Ammonium Surfactant-Modified Cathode via Direct Cathodic Reduction

Remediation of per- and polyfluoroalkyl substances (PFAS) in groundwater remains a technological challenge due to the trace concentrations of PFAS and the strength of their C-F bonds. This study investigated an electroreductive system with a quaternary ammonium surfactant-modified cathode for degrading (E)-perfluoro(4-methylpent-2-enoic acid) (PFMeUPA) at a low cathodic potential. A removal efficiency of 99.81% and defluorination efficiency of 78.67% were achieved under -1.6 V (vs Ag/AgCl) at the cathode modified by octadecyltrimethylammonium bromide (OTAB). The overall degradation procedure started with the adsorption of PFMeUPA onto the modified cathode. This adsorption process was promoted by hydrophobic and electrostatic interactions between the surfactants and PFMeUPA, of which the binding percentage, binding mode, and binding energy were determined via molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The step-wise degradation pathway of PFMeUPA, including reductive defluorination and hydrogenation, was derived. Meanwhile, C-F bond breaking with direct electron transfer only was achieved for the first time in this study, which also showed that the C═C bond structure of PFAS facilitates the C-F cleavage. Overall, this study highlights the crucial role of quaternary ammonium surfactants in electron transfer and electrocatalytic activities in the electroreductive system and provides insights into novel remediation approaches on PFAS-contaminated groundwater.

Distinguishing between the Electrostatic Effects and Explicit Ion Interactions in a Stark Probe

Vibrational Stark probes are incisive tools for measuring local electric fields in a wide range of chemical environments. The interpretation of the frequency shift often gets complicated due to the specific interactions of the probe, such as hydrogen bonding and Lewis bonding. Therefore, it is important to distinguish between the pure electrostatic response and the response due to such specific interactions. Here we report a molecular system that is sensitive to both the Stark effect from a single ion and the explicit Lewis bonding of ions with the probe. The molecule consists of a crown ether with an appended benzonitrile. The crown captures cations of various charges, and the electric field from the ions is sensed by the benzonitrile probe. Additionally, the lone pair of the benzonitrile can engage in Lewis interactions with some of the ions by donating partial charge density to the ions. Our system exhibits both of these effects and therefore is a suitable test bed for distinguishing between the pure electrostatic and the Lewis interactions. Our computational results show that the electrostatic influence of the ion is operative at large distances, while the Lewis interaction becomes important only within distances that permit orbital overlap. Our results may be useful for using the nitrile probe for measuring electrostatic and coordination effects in complex ionic environments such as the electrode-electrolyte interfaces.

Voltage-Dependent FTIR and 2D Infrared Spectroscopies within the Electric Double Layer Using a Plasmonic and Conductive Electrode

Strong electric fields exist between the electric double layer and charged surfaces. These fields impact molecular structures and chemistry at interfaces. We have developed a transparent electrode with infrared plasmonic enhancement sufficient to measure FTIR and two-dimensional infrared spectra at submonolayer coverages on the surface to which a voltage can be applied. Our device consists of an infrared transparent substrate, a 10-20 nm layer of conductive indium tin oxide (ITO), an electrically resistive layer of 3-5 nm Al₂O₃, and a 3 nm layer of nonconductive plasmonic gold. The materials and thicknesses are set to maximize the surface number density of the monolayer molecules, electrical conductivity, and plasmonic enhancement while minimizing background signals and avoiding Fano line shape distortions. The design was optimized by iteratively characterizing the material roughness and thickness with atomic force microscopy and electron microscopy and by monitoring the plasmon resonance enhancement with spectroscopy. The design is robust to repeated fabrication. This new electrode is tested on nitrile functional groups using a monolayer of 4-mercaptobenzonitrile as well as on CO and CC stretching modes using 4-mercaptobenzoic acid methyl ester. A voltage-dependent Stark shift is observed on both monolayers. We also observe that the transition dipole strength of the CN mode scales linearly with the applied voltage, providing a second way of measuring the surface electric field strength. We anticipate that this cell will enable many new voltage-dependent infrared experiments under applied voltages.

Theoretical Investigation of the X-ray Stark Effect in Small Molecules

We have studied the Stark effect in the soft x-ray region for various small molecules by calculating the field-dependent x-ray absorption spectra. This effect is explained in terms of the response of molecular orbitals (core and valence), the molecular dipole moment, and the molecular geometry to the applied electric field. A number of consistent trends are observed linking the computed shifts in absorption energies and intensities with specific features of the molecular electronic structure. We find that both the virtual molecular orbitals (valence and/or Rydberg) as well as the core orbitals contribute to observed trends in a complementary fashion. This initial study highlights the potential impact of x-ray Stark spectroscopy as a tool to study electronic structure and environmental perturbations at a submolecular scale.

Controlling Water Delivery to an Electrochemical Interface with Surfactants

Most electrochemical reactions require delivery of protons, often from water, to surface-adsorbed species. However, water also acts as a competitor to many such processes by directly reacting with the electrode, which necessitates using water in small amounts. Controlling the water content and structure near the surface is an important frontier in directing the reactivity and selectivity of electrochemical reactions. Surfactants accumulate near surfaces, and therefore, they can be used as agents to control interfacial water. Using mid-IR spectro-electrochemistry, we show that a modest concentration (1 mM) of the cationic surfactant CTAB in mixtures of 10 M water in an organic solvent (dDMSO) has a large effect on the interfacial water concentration, changing it by up to ∼35% in the presence of an applied potential. The major cause of water content change is displacement due to the accumulation or depletion of surfactants driven by potential. Two forces drive the surfactants to the electrode: the applied potential and the hydrophobic interactions with the water in the bulk. We have quantified their competition by varying the water content in the bulk. To our knowledge, for the first time, we have identified the electrochemical equivalent of the hydrophobic drive. For our system, a change in applied potential of 1 V has the same effect as adding a 0.55 mole fraction of water to the bulk. This work illustrates the significance of surfactants in the partitioning of water between the bulk and the surface and paves the way toward engineering interfacial water structures for controlling electrochemical reactions.

Automated Variable Electric-Field DFT Application for Evaluation of Optimally Oriented Electric Fields on Chemical Reactivity

Recent theoretical work and experiments at molecular junctions have provided a strong conceptualization for the effects of oriented electric fields (OEFs) on organic reactions. Depending on the axis of application, OEFs can increase (or decrease) the reaction rate or distinguish between isomeric pathways. Despite the conceptual elegance of OEFs, which may be applied externally or induced locally, as tools for catalyzing organic reactions, implementation in synthetically relevant systems has been hampered by inefficiencies in evaluating reaction sensitivity to field effects. Herein, we describe the development of the Automated Variable Electric-Field DFT Application (A.V.E.D.A.) for streamlined evaluation of a reaction's susceptibility to OEFs. This open-source software was designed to be accessible for nonexpert users of computational and programming tools. Following initiation by a single command (and with no subsequent intervention) the Linux workflow manages a series of density functional theory calculations and mathematical manipulations to optimize local-minimum and transition-state structures in oriented electric fields of increasing magnitude. The resulting molecular and reaction dipole moments, field-perturbed geometries, and net effective activation energies are compiled for user interpretation. Ten representative pericyclic reactions that showcase the development and evaluation of A.V.E.D.A. are described.

Dynamically Formed Surfactant Assembly at the Electrified Electrode-Electrolyte Interface Boosting CO2 Electroreduction

Electrocatalytic reactions occur in the nanoscale space at the electrified electrode-electrolyte interface. It is well known that the electrode-electrolyte interface, also called as interfacial microenvironment, is difficult to investigate due to the interference of bulk electrolytes and its dynamic evolution in response to applied bias potential. Here, we employ electrochemical co-reduction of CO₂ and H₂O on commercial Ag electrodes as a model system, in conjunction with quaternary ammonium cationic surfactants as electrolyte additives. We probe bias-potential-driven dynamic response of the interfacial microenvironment as well as the mechanistic origin of catalytic selectivity. By virtue of comprehensive in situ vibrational spectroscopy, electrochemical impedance spectroscopy, and molecular dynamics simulations, it is revealed that the structure of surfactants is dynamically changed from a random distribution to a nearly ordered assembly with increasing bias potential. The nearly ordered surfactant assembly regulates the interfacial water environment by repelling isolated water and suppressing water orientation into an ordered structure as well as promotes CO₂ enrichment at the electrified interface. Eventually, the formed hydrophobic-aerophilic interface microenvironment reduces the activity of water dissociation and increases the selectivity of CO₂ electroreduction to CO. These results highlight the importance of regulating the interfacial microenvironment by organic additives as a means of boosting the electrochemical performance in electrosynthesis and beyond.

The Solvation-Induced Onsager Reaction Field Rather than the Double-Layer Field Controls CO2 Reduction on Gold

The selectivity and activity of the carbon dioxide reduction (CO₂R) reaction are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in situ-generated CO on Au in the presence of alkali cations, we quantify the total electric field present at catalytic active sites and deconvolute this field into contributions from (1) the electrochemical Stern layer and (2) the Onsager (or solvation-induced) reaction field. Contrary to recent theoretical reports, the CO₂R kinetics does not depend on the Stern field but instead is closely correlated with the strength of the Onsager reaction field. These results show that in the presence of adsorbed (bent) CO₂, the Onsager field greatly exceeds the Stern field and is primarily responsible for CO₂ activation. Additional measurements of the cation-dependent water spectra using vibrational sum frequency generation spectroscopy show that interfacial solvation strongly influences the CO₂R activity. These combined results confirm that the cation-dependent interfacial water structure and its associated electric field must be explicitly considered for accurate understanding of CO₂R reaction kinetics.

Guiding CO2RR Selectivity by Compositional Tuning in the Electrochemical Double Layer

The electrochemical conversion of carbon dioxide to value-added chemicals provides an environmentally benign alternative to current industrial practices. However, current electrocatalytic systems for the CO₂ reduction reaction (CO₂RR) are not practical for industrialization, owing to poor specific product selectivity and/or limited activity. Interfacial engineering presents a versatile and effective method to direct CO₂RR selectivity by fine-tuning the local chemical dynamics. This Account describes interfacial design strategies developed in our laboratory that use electrolyte engineering and porous carbon materials to modify the local composition at the electrode-electrolyte interface.Our first strategy for influencing surface reactivity is to perturb the electrochemical double layer by tuning the electrolyte composition. We approached this investigation by considering how charged molecular additives can organize at the electrode surface and impact CO₂ activation. Using a combination of advanced electrochemical techniques and in situ vibrational spectroscopy, we show that the surfactant properties (the identity of the headgroup, alkyl chain length, and concentration) as well as the electrolyte cation identity can affect how surfactant molecules assemble at a biased electrode. The interplay between the electrolyte cations and the surfactant additives can be regulated to favor specific carbon products, such as HCOO^-, and suppress the parasitic hydrogen evolution reaction (HER). Together, our findings highlight how molecular assemblies can be used to design selective electrocatalytic systems.In addition to the electrolyte design, the local spatial confinement of reaction intermediates presents another strategy to direct CO₂RR selectivity. We were interested in uncovering the role of porous carbon-supported catalysts toward selective carbon product formation. In our initial study, we show that carbon porosity can be optimized to enhance C₂H₄ and CO selectivity in a series of Cu catalysts embedded in a tunable carbon aerogel matrix. These results suggested that local confinement of the active surface plays a role in CO₂ activation and motivated an investigation into probing how this phenomenon can be translated to a planar Cu electrode. Our findings show that carbon modifiers facilitated surface reconstruction and regulated CO₂ diffusion to suppress HER and improve the C_2-3 product selectivity. Given the ubiquity of carbon materials in catalysis, this work demonstrates that carbon plays an active role in regulating selectivity by restricting the diffusion of substrate and reaction intermediates. Our work in tuning the composition of the electrochemical double layer for increased CO₂RR selectivity demonstrates the potential versatility in boosting catalytic performance across an array of catalytic systems.

Electric-field-induced coupling of aryl iodides with a nickel(0) complex

External electric fields can influence the reactivity of organometallic complexes in solution. This effect is demonstrated to induce carbon–carbon bond forming chemistry with a kinetically inert nickel complex at room temperature.

Characterization of Acetonitrile Isotopologues as Vibrational Probes of Electrolytes

Acetonitrile has emerged as a solvent candidate for novel electrolyte formulations in metal-ion batteries and supercapacitors. It features a bright local C≡N stretch vibrational mode whose infrared (IR) signature is sensitive to battery-relevant cations (Li⁺, Mg²⁺, Zn²⁺, Ca²⁺) both in pure form and in the presence of water admixture across a full possible range of concentrations from the dilute to the superconcentrated regime. Stationary and time-resolved IR spectroscopy thus emerges as a natural tool to study site-specific intermolecular interactions from the solvent perspective without introducing an extrinsic probe that perturbs solution morphology and may not represent the intrinsic dynamics in these electrolytes. The metal-coordinated acetonitrile, water-separated metal-acetonitrile pair, and free solvent each have a distinct vibrational signature that allows their unambiguous differentiation. The IR band frequency of the metal-coordinated acetonitrile depends on the ion charge density. To study the ion transport dynamics, it is necessary to differentiate energy-transfer processes from structural interconversions in these electrolytes. Isotope labeling the solvent is a necessary prerequisite to separate these processes. We discuss the design principles and choice of the CD₃¹³CN label and characterize its vibrational spectroscopy in these electrolytes. The Fermi resonance between ¹³C≡N and C-D stretches complicates the spectral response but does not prevent its effective utilization. Time-resolved two-dimensional (2D) IR spectroscopy can be performed on a mixture of acetonitrile isotopologues and much can be learned about the structural dynamics of various species in these formulations.

Emergence of Electric Fields at the Water-C12E6 Surfactant Interface

We study the properties of the interface of water and the surfactant hexaethylene glycol monododecyl ether (C12E6) with a combination of heterodyne-detected vibrational sum frequency generation (HD-VSFG), Kelvin-probe measurements, and molecular dynamics (MD) simulations. We observe that the addition of the hydrogen-bonding surfactant C12E6, close to the critical micelle concentration (CMC), induces a drastic enhancement in the hydrogen bond strength of the water molecules close to the interface, as well as a flip in their net orientation. The mutual orientation of the water and C12E6 molecules leads to the emergence of a broad (∼3 nm) interface with a large electric field of ∼1 V/nm, as evidenced by the Kelvin-probe measurements and MD simulations. Our findings may open the door for the design of novel electric-field-tuned catalytic and light-harvesting systems anchored at the water-surfactant-air interface.

Electric Fields Influence Intramolecular Vibrational Energy Relaxation and Line Widths

Intramolecular vibrational energy relaxation (IVR) is fundamentally important to chemical dynamics. We show that externally applied electric fields affect IVR and vibrational line widths by changing the anharmonic couplings and frequency detunings between modes. We demonstrate this effect in benzonitrile for which prior experimental results show a decrease in vibrational line width as a function of applied electric field. We identify three major channels for IVR that depend on electric field. In the dominant channel, the electric field affects the frequency detuning, while in the other two channels, variation of anharmonic couplings as a function of field is the underlying mechanism. Consistent with experimental results, we show that the combination of all channels gives rise to reduced line widths with increasing electric field in benzonitrile. Our results are relevant for controlling IVR with external or internal fields and for gaining a more complete interpretation of line widths of vibrational Stark probes.

Local Electric Fields in Aqueous Electrolytes

Vibrational Stark shifts were explored in aqueous solutions of organic molecules with carbonyl- and nitrile-containing constituents. In many cases, the vibrational resonances from these moieties shifted toward lower frequency as salt was introduced into solution. This is in contrast to the blue-shift that would be expected based upon Onsager's reaction field theory. Salts containing well-hydrated cations like Mg²⁺ or Li⁺ led to the most pronounced Stark shift for the carbonyl group, while poorly hydrated cations like Cs⁺ had the greatest impact on nitriles. Moreover, salts containing I^- gave rise to larger Stark shifts than those containing Cl^-. Molecular dynamics simulations indicated that cations and anions both accumulate around the probe in an ion- and probe-dependent manner. An electric field was generated by the ion pair, which pointed from the cation to the anion through the vibrational chromophore. This resulted from solvent-shared binding of the ions to the probes, consistent with their positions in the Hofmeister series. The "anti-Onsager" Stark shifts occur in both vibrational spectroscopy and fluorescence measurements.

Vibrational Stark shift spectroscopy of catalysts under the influence of electric fields at electrode-solution interfaces

External control of chemical processes is a subject of widespread interest in chemical research, including control of electrocatalytic processes with significant promise in energy research. The electrochemical double-layer is the nanoscale region next to the electrode/electrolyte interface where chemical reactions typically occur. Understanding the effects of electric fields within the electrochemical double layer requires a combination of synthesis, electrochemistry, spectroscopy, and theory. In particular, vibrational sum frequency generation (VSFG) spectroscopy is a powerful technique to probe the response of molecular catalysts at the electrode interface under bias. Fundamental understanding can be obtained via synthetic tuning of the adsorbed molecular catalysts on the electrode surface and by combining experimental VSFG data with theoretical modelling of the Stark shift response. The resulting insights at the molecular level are particularly valuable for the development of new methodologies to control and characterize catalysts confined to electrode surfaces. This Perspective article is focused on how systematic modifications of molecules anchored to surfaces report information concerning the geometric, energetic, and electronic parameters of catalysts under bias attached to electrode surfaces.

Heterogeneous electrocatalysis: characterization of interfacial electric field within the electrochemical double layer.

Mechanistic Insights about Electrochemical Proton-Coupled Electron Transfer Derived from a Vibrational Probe

Proton-coupled electron transfer (PCET) is a fundamental step in a wide range of electrochemical processes, including those of interest in energy conversion and storage. Despite its importance, several mechanistic details of such reactions remain unclear. Here, we have combined a proton donor (tertiary ammonium) with a vibrational Stark-shift probe (benzonitrile), to track the process from the entry of the reactants into the electrical double layer (EDL), to the PCET reaction associated with proton donation to the electrode, and the formation of products. We have used operando vibrational spectroscopy and periodic density functional theory under electrochemical bias to assign the reactant and product peaks and their Stark shifts. We have identified three main stages for the progress of the PCET reaction as a function of applied potential. First, we have determined the potential necessary for desolvation of the reactants and their entry into the polarizing environment of the EDL. Second, we have observed the appearance of product peaks prior to the onset of steady state electrochemical current, indicating formation of a stationary population of products that does not turn over. Finally, more negative of the onset potential, the electrode attracts additional reactants, displacing the stationary products and enabling steady state current. This work shows that the integration of a vibrational Stark-shift probe with a proton donor provides critical insight into the interplay between interfacial electrostatics and heterogeneous chemical reactions. Such insights cannot be obtained from electrochemical measurements alone.

Interfacial Polarization and Ionic Structure at the Ionic Liquid-Metal Interface Studied by Vibrational Spectroscopy and Molecular Dynamics Simulations

Ionic liquids (ILs) have both fundamental and practical value in interfacial science and electrochemistry. However, understanding their behavior near a surface is challenging because of strong Coulomb interactions and large and irregular ionic sizes, which affect both their structure and energetics. To understand this problem, we present a combined experimental and computational study using a vibrational probe molecule, 4-mercaptobenzonitrile, inserted at the junction between a metal and a variety of ILs. The vibrational frequency of the nitrile in the probe molecule reports on the local solvation environment and the electrostatic field at this junction. Within the ethylmethyl imidazolium (EMIM⁺) cation family of ILs, we varied the anions over a range of sizes and types. Complementing our surface spectroscopy, we also ran molecular dynamics simulations of these interfaces to better understand the ionic structures that produced the measured fields. The magnitude of the frequency shifts, and thereby fields, shows a general correlation with the size of anions, with larger anions corresponding to smaller fields. We find that the source of this correlation is partial intercalation of smaller anions into the probe monolayer, resulting in tighter packing of ionic layers near the surface. Larger anions reduce the overall lateral ion packing density near the surface, which reduces the net charge per unit area and explains the smaller observed fields. The insight from this work is important for developing a fundamental picture of concentrated electrolytes near interfaces and can help with designing ILs to create tailored electric fields near an electrode.

Strong Propensity of Ionic Liquids in Their Aqueous Solutions for an Organic-Modified Metal Surface

Understanding ionic structure and electrostatic environments near a surface has both fundamental and practical value. In electrochemistry, especially when room temperature ionic liquids (ILs) are involved, the complex ionic structure near the interface is expected to crucially influence reactions. Here we report evidence that even in dilute aqueous solutions of several ILs, the ions aggregate near the surface in ways that are qualitatively different from simple electrolytes. We have used a vibrational probe molecule, 4-mercaptobenzonitrile (MBN), tethered to a metal surface to monitor the behavior of the ionic layers. The characteristic nitrile vibrational frequency of this molecule has distinct values in the presence of pure water (∼2232 cm^-1) and pure IL (for example, ∼2226 cm^-1 for ethylmethylimidazolium tetrafluoroborate, [EMIM][BF₄]). This difference reflects the local electrostatic field and the hydrogen-bonding variations between these two limiting cases. We tracked this frequency shift as a function of IL concentration in water all the way from pure water to pure IL. We report two important findings. First, only one nitrile peak is observed for the entire concentration range, indicating that at least on the length scale of the probe molecule water and ILs do not phase separate within the interface, and no heterogeneously distinct electrostatic environments are formed. Second, and more importantly, we find that even up to a significant mole fraction of bulk water (x ∼ 0.95), the nitrile frequency does not change from that indicative of a pure IL for [EMIM][BF₄], indicating preferential aggregation of the ions near the surface. Because this behavior is very similar to surfactants, we chose an imidazolium cation with a longer side chain which resulted in behavior expected from a surfactant, with a preferential layer of the ions on the surface even in dilute water solutions (x ∼ 0.995). This observation indicates that even those ILs that are not nominally categorized as surfactants have a strong tendency to aggregate at the surface. Because ILs serve as electrolytes in a range of electrochemical reactions, including those requiring water, our results are likely useful for mechanistic understanding and tuning of such reactions.

Reorganization of Interfacial Water by an Amphiphilic Cationic Surfactant Promotes CO2 Reduction

The presence of cetyltrimethylammonium bromide (CTAB) near the surface of a Cu electrode promotes the electrochemical reduction of CO₂ to fuels. CTAB increases the CO₂ reduction rate by as much as 10× and decreased the HER rate by 4×, leading to ∼75% selectivity toward the reduction of CO₂. Surface enhanced infrared absorption spectroscopy (SEIRAS) was used to probe the effects of CTAB adsorption on the structure of interfacial water and CO₂ reduction intermediates. HER suppression was found to arise from the displacement of interfacial water molecules from CTAB adsorption within the double layer. The enhanced CO₂ reduction rate can be correlated to an increased population of atop-bound CO and the emergence of a low frequency atop-CO band. These results unravel the role of additives in improving CO₂-to-fuels electrocatalysis and establishing this as a powerful methodology for directing product selectivity.