Interfacial Acid-Base Equilibria and Electric Fields Concurrently Probed by In Situ Surface-Enhanced Infrared Spectroscopy

Electric Field Generated at the Millisecond Pulse-Polarized Interface Facilitates the Electrolytic Conversion of SO2 into H2S

Interfacial electric field holds significant importance in determining both the polar molecular configuration and surface coverage during electrocatalysis. This study introduces a methodology leveraging the varying electric dipole moment of SO2 under distinct interfacial electric field strengths to enhance the selectivity of the SO2 electroreduction process. This approach presented the first attempt to utilize pulsed voltage application to the Au/PTFE membrane electrode for the control of the molecular configuration and coverage of SO2 on the electrode surface. Remarkably, the modulation of pulse duration resulted in a substantial inhibition of the hydrogen evolution reaction (HER) (FEH2 < 3%) under millisecond pulse conditions (ta = 10 ms, tc = 300 ms, Ea = -0.8 V (vs Hg/Hg2SO4), Ec = -1.8 V (vs Hg/Hg2SO4)), concomitant with a noteworthy enhancement in H2S selectivity (FEH2S > 97%). A comprehensive analysis, incorporating in situ Raman spectroscopy, electrochemical quartz crystal microbalance, COMSOL simulations, and DFT calculations, corroborated the increased selectivity of H2S products was primarily associated with the inherently large dipole moment of the SO2 molecule. The enhancement of the interfacial electric field induced by millisecond pulses was instrumental in amplifying SO2 coverage, activating SO2, facilitating the formation of the pivotal intermediate product *SOH, and effectively reducing the reaction energy barrier in the SO2 reduction process. These findings provide novel insights into the influences of ion and molecular transport dynamics, as well as the temporal intricacies of competitive pathways during the SO2 electroreduction process. Moreover, it underscores the intrinsic correlation between the electric dipole moment and surface-molecule interaction of the catalyst.

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.

Electrically driven proton transfer promotes Brønsted acid catalysis by orders of magnitude

Electric fields play a key role in enzymatic catalysis and can enhance reaction rates by 100,000-fold, but the same rate enhancements have yet to be achieved in thermochemical heterogeneous catalysis. In this work, we probe the influence of catalyst potential and interfacial electric fields on heterogeneous Brønsted acid catalysis. We observed that variations in applied potential of ~380 mV led to a 100,000-fold rate enhancement for 1-methylcyclopentanol dehydration, which was catalyzed by carbon-supported phosphotungstic acid. Mechanistic studies support a model in which the interfacial electrostatic potential drop drives quasi-equilibrated proton transfer to the adsorbed substrate prior to rate-limiting C-O bond cleavage. Large increases in rate with potential were also observed for the same reaction catalyzed by Ti/TiOyHx and for the Friedel Crafts acylation of anisole with acetic anhydride by carbon-supported phosphotungstic acid.

Interfacial Tuning of Electrocatalytic Ag Surfaces for Fragment-Based Electrophile Coupling

Construction of C‒C bonds in medicinal chemistry frequently draws on the reductive coupling of organic halides with ketones or aldehydes. Catalytic C(sp3)‒C(sp3) bond formation, however, is constrained by the competitive side reactivity of radical intermediates following sp3 organic halide activation. Here, an alternative paradigm deploys catalytic Ag surfaces for reductive fragment-based electrophile coupling compatible with sp3 organic halides. We use in-situ spectroscopy, electrochemical analyses, and simulation to uncover the catalytic interfacial structure and guide reaction development. Specifically, Mg(OAc)2 outcompetes the interaction between Ag and the aldehyde, thereby tuning the Ag surface for selective product formation. Data are consistent with an increased population of Mg-bound aldehyde facilitating the addition of a carbon-centered radical (product of Ag-electrocatalyzed organic halide reduction) to the carbonyl. Electron transfer from Ag to the resultant alkoxy radical yields the desired alcohol. Molecular interfacial tuning at reusable catalytic electrodes will accelerate development of sustainable organic synthetic methods.

In Situ Spectroscopic Elucidation of the Electrochemical Potential Drop at Polyelectrolytes/Au Interfaces

Polyelectrolytes have been widely applied in electrochemical devices. Understanding the polyelectrolyte/electrode interfaces is pivotal for polyelectrolyte-based applications. Here, we measured the electrochemical potential drop and the local activity of the mobile ion of H+ or OH- at the polyelectrolytes/Au interfaces by in situ electrochemical surface-enhanced Raman spectroscopy and voltammetry in three-electrode cells. We found that the potential dependences of the electrochemical potential drop in polyelectrolytes were smaller than that in conventional electrolyte solutions. The interfacial activity of H+ or OH- was much lower than that of bulk polyelectrolytes. The potential-dependent molecular dynamics simulations showed that the mobility of ionomers of polyelectrolytes in an electrostatic field was limited by a polymer matrix. These results suggested a characteristically thicker compact layer in the electrical double layer of a polyelectrolyte/electrode interface due to the accumulation of mobile H+ or OH- with a thicker hydration layer and immobile ionomers.

Anion-π catalysis on carbon allotropes

Anion-π catalysis, introduced in 2013, stands for the stabilization of anionic transition states on π-acidic aromatic surfaces. Anion-π catalysis on carbon allotropes is particularly attractive because high polarizability promises access to really strong anion-π interactions. With these expectations, anion-π catalysis on fullerenes has been introduced in 2017, followed by carbon nanotubes in 2019. Consistent with expectations from theory, anion-π catalysis on carbon allotropes generally increases with polarizability. Realized examples reach from enolate addition chemistry to asymmetric Diels-Alder reactions and autocatalytic ether cyclizations. Currently, anion-π catalysis on carbon allotropes gains momentum because the combination with electric-field-assisted catalysis promises transformative impact on organic synthesis.

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.

Electric field-assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

The vision to control the charges migrating during reactions with external electric fields is attractive because of the promise of general catalysis, emergent properties, and programmable devices. Here, we explore this idea with anion-π catalysis, that is the stabilization of anionic transition states on aromatic surfaces. Catalyst activation by polarization of the aromatic system is most effective. This polarization is induced by electric fields. The use of electrochemical microfluidic reactors to polarize multiwalled carbon nanotubes as anion-π catalysts emerges as essential. These reactors provide access to high fields at low enough voltage to prevent electron transfer, afford meaningful effective catalyst/substrate ratios, and avoid interference from additional electrolytes. Under these conditions, the rate of pyrene-interfaced epoxide-opening ether cyclizations is linearly voltage-dependent at positive voltages and negligible at negative voltages. While electromicrofluidics have been conceived for redox chemistry, our results indicate that their use for supramolecular organocatalysis has the potential to noncovalently electrify organic synthesis in the broadest sense.

Electrode/Electrolyte Synergy for Concerted Promotion of Electron and Proton Transfers toward Efficient Neutral Water Oxidation

Neutral water oxidation is a crucial half-reaction for various electrochemical applications requiring pH-benign conditions. However, its sluggish kinetics with limited proton and electron transfer rates greatly impacts the overall energy efficiency. In this work, we created an electrode/electrolyte synergy strategy for simultaneously enhancing the proton and electron transfers at the interface toward highly efficient neutral water oxidation. The charge transfer was accelerated between the iridium oxide and in situ formed nickel oxyhydroxide on the electrode end. The proton transfer was expedited by the compact borate environment that originated from hierarchical fluoride/borate anions on the electrolyte end. These concerted promotions facilitated the proton-coupled electron transfer (PCET) events. Due to the electrode/electrolyte synergy, Ir-O and Ir-OO- intermediates could be directly detected by in situ Raman spectroscopy, and the rate-limiting step of Ir-O oxidation was determined. This synergy strategy can extend the scope of optimizing electrocatalytic activities toward more electrode/electrolyte combinations.

Phenol as a Tethering Group to Gold Surfaces: Stark Response and Comparison to Benzenethiol

Understanding the adsorption of organic molecules on metals is important in numerous areas of surface science, including electrocatalysis, electrosynthesis, and biosensing. While thiols are commonly used to tether organic molecules on metals, it is desirable to broaden the range of anchoring groups. In this study, we use a combined spectroelectrochemical and computational approach to demonstrate the adsorption of 4-cyanophenols (CPs) on polycrystalline gold. Using the nitrile stretching vibration as a marker, we confirm the adsorption of CP on the gold electrode and compare our results with those obtained for the thiol counterpart, 4-mercaptobenzonitirle (MBN). Our results reveal that CP adsorbs on the gold electrode via the OH linker, as evidenced by the similarity in the direction and magnitude of the nitrite Stark shifts for CP and MBN. This finding paves the way for exploring new approaches to modify electrode surfaces for controlled reactivity. Furthermore, it highlights adsorption on metals as an important step in the electroreactivity of phenols.

Non-Covalent Interactions Mimic the Covalent: An Electrode-Orthogonal Self-Assembled Layer

Charge-transfer events central to energy conversion and storage and molecular sensing occur at electrified interfaces. Synthetic control over the interface is traditionally accessed through electrode-specific covalent tethering of molecules. Covalent linkages inherently limit the scope and the potential stability window of molecularly tunable electrodes. Here, we report a synthetic strategy that is agnostic to the electrode's surface chemistry to molecularly define electrified interfaces. We append ferrocene redox reporters to amphiphiles, utilizing non-covalent electrostatic and van der Waals interactions to prepare a self-assembled layer stable over a 2.9 V range. The layer's voltammetric response and in situ infrared spectra mimic those reported for analogous covalently bound ferrocene. This design is electrode-orthogonal; layer self-assembly is reversible and independent of the underlying electrode material's surface chemistry. We demonstrate that the design can be utilized across a wide range of electrode material classes (transition metal, carbon, carbon composites) and morphologies (nanostructured, planar). Merging atomically precise organic synthesis of amphiphiles with in situ non-covalent self-assembly at polarized electrodes, our work sets the stage for predictive and non-fouling synthetic control over electrified interfaces.

Fractional Charge Density Functional Theory and Its Application to the Electro-inductive Effect

We employed the chemical potential equalization principle to demonstrate that fractional electrons are involved in the electro-inductive effect as well as the vibrational Stark effect. By the chemical potential model, we were able to deduce that the frontier molecular orbitals of immobilized molecules can provide valuable insight into these effects. To further understand and quantify these findings, we introduced fractional charge density functional theory (FC-DFT), a canonical ensemble approach for open systems. This method allows for the calculation of electronic energies, nuclear gradients, and the Hessian matrix of fractional electronic systems. To correct the spurious delocalization error commonly found in approximate density functionals for small systems, we imposed the Perdew-Parr-Levy-Balduz (PPLB) condition through linear interpolation of two adjacent integer points (LI-FC-DFT). Although this approach is relatively simple in terms of molecular modeling, the results obtained through LI-FC-DFT calculations predict the same trend seen in experimental reactivity and the frequency change of immobilized molecules.

Inductive Effect Alone Cannot Explain Lewis Adduct Formation and Dissociation at Electrode Interfaces

Understanding breaking and formation of Lewis bonds at an electrified interface is relevant to a large range of phenomena, including electrocatalysis and electroadsorption. The complexities of interfacial environments and associated reactions often impede a systematic understanding of this type of bond at interfaces. To address this challenge, we report the creation of a main group classic Lewis acid-base adduct on an electrode surface and its behavior under varying electrode potentials. The Lewis base is a self-assembled monolayer of mercaptopyridine and the Lewis acid is BF₃, forming a Lewis bond between nitrogen and boron. The bond is stable at positive potentials but cleaves at potentials more negative of approximately -0.3 V vs Ag/AgCl without an associated current. We also show that if the Lewis acid BF₃ is supplied from a reservoir of Li⁺BF₄^- electrolyte, the cleavage is completely reversible. We propose that the N-B Lewis bond is affected both by the field-induced intramolecular polarization (electroinduction) and by the ionic structures and ionic equilibria near the electrode. Our results indicate that the second effect is responsible for the Lewis bond cleavage at negative potentials. This work is relevant to understanding the fundamentals of electrocatalytic and electroadsorption processes.

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.

Correlating the Experimentally Determined CO Adsorption Enthalpy with the Electrochemical CO Reduction Performance on Cu Surfaces

CO binding energy has been widely employed as a descriptor for effective catalysts in the electrochemical CO₂ and CO reduction reactions (CO₍₂₎ RR), however, it has yet to be determined experimentally at electrochemical interfaces due to the lack of suitable techniques. In this work, we developed a method to determine the standard adsorption enthalpy of CO on Cu surfaces with quantitative surface enhanced infrared absorption spectroscopy. On dendritic Cu at -0.75 V vs. SHE, the standard adsorption enthalpy, entropy and Gibbs free energy were determined to 1.5±0.5 kJ mol^-1 , ≈37.9±13.4 J/(mol K), and ≈-9.8±4.0 kJ mol^-1 , respectively. Comparison of the standard adsorption enthalpy of oxide-derived Cu and dendritic Cu, as well as their CORR activities, suggests the presence of stronger binding sites on OD Cu, which could favor multicarbon products. The method developed in this work will help establish the correlation between the CO binding energy and the CO₍₂₎ RR activity.

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.

Interfacial electric fields catalyze Ullmann coupling reactions on gold surfaces

The electric fields created at solid-liquid interfaces are important in heterogeneous catalysis. Here we describe the Ullmann coupling of aryl iodides on rough gold surfaces, which we monitor in situ using the scanning tunneling microscope-based break junction (STM-BJ) and ex situ using mass spectrometry and fluorescence spectroscopy. We find that this Ullmann coupling reaction occurs only on rough gold surfaces in polar solvents, the latter of which implicates interfacial electric fields. These experimental observations are supported by density functional theory calculations that elucidate the roles of surface roughness and local electric fields on the reaction. More broadly, this touchstone study offers a facile method to access and probe in real time an increasingly prominent yet incompletely understood mode of catalysis.

Sub-Nanometer Mapping of the Interfacial Electric Field Profile Using a Vibrational Stark Shift Ruler

The characterization of electrical double layers is important since the interfacial electric field and electrolyte environment directly affect the reaction mechanisms and catalytic rates of electrochemical processes. In this work, we introduce a spectroscopic method based on a Stark shift ruler that enables mapping the electric field strength across the electric double layer of electrode/electrolyte interfaces. We use the tungsten-pentacarbonyl(1,4-phenelenediisocyanide) complex attached to the gold surface as a molecular ruler. The carbonyl (CO) and isocyanide (NC) groups of the self-assembled monolayer (SAM) provide multiple vibrational reporters situated at different distances from the electrode. Measurements of Stark shifts under operando electrochemical conditions and direct comparisons to density functional theory (DFT) simulations reveal distance-dependent electric field strength from the electrode surface. This electric field profile can be described by the Gouy-Chapman-Stern model with Stern layer thickness of ∼4.5 Å, indicating substantial solvent and electrolyte penetration within the SAM. Significant electro-induction effect is observed on the W center that is ∼1.2 nm away from the surface despite rapid decay of the electric field (∼90%) within 1 nm. The applied methodology and reported findings should be particularly valuable for the characterization of a wide range of microenvironments surrounding molecular electrocatalysts at electrode interfaces and the positioning of electrocatalysts at specific distances from the electrode surface for optimal functionality.

Nitrile Infrared Intensities Characterize Electric Fields and Hydrogen Bonding in Protic, Aprotic, and Protein Environments

Nitriles are widely used vibrational probes; however, the interpretation of their IR frequencies is complicated by hydrogen bonding (H-bonding) in protic environments. We report a new vibrational Stark effect (VSE) that correlates the electric field projected on the -C≡N bond to the transition dipole moment and, by extension, the nitrile peak area or integrated intensity. This linear VSE applies to both H-bonding and non-H-bonding interactions. It can therefore be generally applied to determine electric fields in all environments. Additionally, it allows for semiempirical extraction of the H-bonding contribution to the blueshift of the nitrile frequency. Nitriles were incorporated at H-bonding and non-H-bonding protein sites using amber suppression, and each nitrile variant was structurally characterized at high resolution. We exploited the combined information available from variations in frequency and integrated intensity and demonstrate that nitriles are a generally useful probe for electric fields.

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) plays an essential role in a wide range of electrocatalytic processes. A vast array of theoretical and computational methods have been developed to study electrochemical PCET. These methods can be used to calculate redox potentials and pK_a values for molecular electrocatalysts, proton-coupled redox potentials and bond dissociation free energies for PCET at metal and semiconductor interfaces, and reorganization energies associated with electrochemical PCET. Periodic density functional theory can also be used to compute PCET activation energies and perform molecular dynamics simulations of electrochemical interfaces. Various approaches for maintaining a constant electrode potential in electronic structure calculations and modeling complex interactions in the electric double layer (EDL) have been developed. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET spanning the adiabatic, nonadiabatic, and solvent-controlled regimes have been developed and provide analytical expressions for the rate constants and current densities as functions of applied potential. The quantum mechanical treatment of the proton and inclusion of excited vibronic states have been shown to be critical for describing experimental data, such as Tafel slopes and potential-dependent kinetic isotope effects. The calculated rate constants can be used as input to microkinetic models and voltammogram simulations to elucidate complex electrocatalytic processes.