Electrocatalysis is central to electrified energy conversion technologies, including the production of fuels and chemicals from renewable electricity. In recent years, atomistic modeling based on density functional theory (DFT) has become an indispensable tool for catalysis researchers because it can provide a molecular-level perspective into catalytic reactivity that is otherwise inaccessible to experimental measurements. Computational electrocatalysis model inputs are intended to capture fundamental understanding of the complex interfacial environment of the catalytic active site, but rely on practical approximations that can affect the reliability of insights gained from DFT. This Perspective outlines a framework for rigorous and reproducible integration of theory and experiment in heterogeneous electrocatalysis research. We discuss state-of-the-art computational methods and sources of error in model development and interpretation. We describe opportunities for validation of theoretical models and experimental interpretations as it pertains to the structure of active sites, reaction energetics, and microkinetic modeling. We provide examples to help computational researchers develop models that move beyond obtaining qualitative agreement with reactivity trends, and toward the development of rigorous mechanistic insights that can be leveraged to predict promising new catalysts or operating conditions. Rigor and reproducibility are enhanced when theory and experiment have multiple nodes of connection, which enhances the longevity and robustness of conclusions and insights about electrocatalytic reactivity.

Propylene oxide, a key commodity of the chemical industry for a wide range of consumer products, is synthesized through sequential propane dehydrogenation and epoxidation reactions. However, the lack of a direct catalytic route from propane to propylene oxide reduces efficiency and represents a major challenge for catalysis science. Herein, we report the discovery of a highly active and selective catalyst, made of alumina-supported subnanometer copper clusters, which can directly convert propane to propylene oxide at temperatures as low as 150 °C. Moreover, at higher temperatures, on the same catalysts, the selectivity is switched to propylene. Accompanying theoretical calculations indicate that partially oxidized and/or hydroxylated clusters have low activation energies for both propane dehydrogenation and propylene epoxidation pathways, enabling direct conversion with very high selectivity for propylene oxide. The discovery of a low-temperature catalyst that can convert propane directly to propylene oxide provides an important opportunity for the development of energy-efficient and economic catalysts for this industrially critical process. Similarly, when operating at higher temperatures, these catalysts are posed as potent oxidative dehydrogenation catalysts.

Quantitative measurements of heterogeneous electron transfer kinetics for facile reactions and/or under high viscosity conditions, such as those encountered in concentrated hydrogen-bonded electrolytes (CoHBEs), present significant challenges, including iR drop and limited diffusional flux. These issues lead to a limited kinetic window, which could be potentially insufficient for ideal kinetic fitting. Herein, we introduce the application of scanning electrochemical microscopy (SECM) for determining electron transfer (ET) kinetics of quinones in aqueous and CoHBEs media. A systematic approach was incorporated to quantitatively analyze approach curves with transient convection effects through a series of velocity- and viscosity-dependent experiments, combined with lattice Boltzmann method (LBM) simulations. SECM facilitates the measurement of kinetics with a broader overpotential window compared to conventional methods such as the rotating disk electrode method. The kinetic behaviors were interpreted using the Butler-Volmer (BV) and Marcus-Hush-Chidsey (MHC) models, with the latter providing the reorganization energy (λ) of the redox reaction. We investigated the apparent proton-coupled electron transfer (PCET) reaction rate constant (k) of benzoquinone and anthraquinone-2,7-disulfonic acid disodium salt (2,7-AQDS), with further discussion on how proton concentration, viscosity, and electrode material modulate the standard rate constant (k°), transfer coefficient (α), and λ. In general, PCET reactions exhibit larger λ and smaller k° compared to pure ET processes. The SECM-based kinetic measurement method additionally enables experiments with spatial resolution, allowing for the exploration of reaction heterogeneity. This work highlights the unique versatility and applicability of SECM in studying heterogeneous ET under various conditions. Figure 1

Direct electrochemical reduction of carbon dioxide (CO2) capture species, i.e., carbamate and (bi)carbonate, can be promising for CO2 capture and conversion from point-source, where the energetically demanding stripping step is bypassed. Here, we describe a class of atomically dispersed nickel (Ni) catalysts electrodeposited on various electrode surfaces that are shown to directly convert captured CO2 to methane (CH4). A detailed study employing X-ray photoelectron spectroscopy (XPS) and electron microscopy (EM) indicate that highly dispersed Ni atoms are uniquely active for converting capture species to CH4, and the activity of single-atom Ni is confirmed using control experiments with a molecularly defined Ni phthalocyanine catalyst supported on carbon nanotubes. Comparing the kinetics of various capture solutions obtained from hydroxide, ammonia, primary, secondary, and tertiary amines provide evidence that carbamate, rather than (bi)carbonate and/or dissolved CO2, is primarily responsible for CH4 production. This conclusion is supported by 13C nuclear magnetic resonance (NMR) spectroscopy of capture solutions as well as control experiments comparing reaction selectivity with and without CO2 purging. These findings are understood with the help of density functional theory (DFT) calculations showing that single-atom nickel (Ni) dispersed on gold (Au) is active for the direct reduction of carbamate, producing CH4 as the primary product. This is the first example of direct electrochemical conversion of carbamate to CH4, and the mechanism of this process provides new insight on the potential for integrated capture and conversion of CO2 directly to hydrocarbons.

Proton-coupled electron transfer (PCET) reactions on semiconducting metal oxide surfaces often involve charged defects in the form of electron or hole polarons. Herein, vibronically nonadiabatic PCET theory is used to model rate constants for the PCET reaction between a reduced anatase TiO2(101) surface and 4-MeO-TEMPO, where electron polarons on the TiO2 surface directly participate in the PCET reaction. This modeling strategy treats the transferring proton as well as all electrons quantum mechanically and includes the effects of excited vibronic states. The rate constant expression depends on the reorganization energy, as well as the reaction free energies and vibronic couplings for different pairs of vibronic states, and accounts for proton donor-acceptor motion. Hybrid functional periodic density functional theory (DFT) is used to calculate the parameters in the rate constant expression, and a Hubbard alpha-based constrained DFT approach is used to enforce charge constraints consistent with the two electronically diabatic states for the PCET reaction. This modeling strategy is applied to compute the PCET rate constants and kinetic isotope effects for reactions involving five-coordinate and six-coordinate Ti3+ defects on the TiO2(101) surface, showing that excited vibronic states contribute significantly to the rate constant for both defects, especially for deuterium. This study highlights the importance of hydrogen tunneling and excited vibronic states in interfacial PCET reactions. Such modeling strategies can be used to further understand and tailor the reactivity of metal oxide surfaces for energy conversion.

Electrode surfaces modified with peptides or other biomolecules are of great interest for applications in catalysis and separations. At the electrochemical interface, the structure of biomolecular adsorbates may be sensitive to the applied potential and the distribution of solvent and ions near the electrode surface. Herein, periodic density functional theory (DFT) calculations are used to describe changes in the adsorption structure of the l-cysteine amino acid on Au(111) as a function of applied potential. This theoretical study reveals the fundamental mechanisms of potential-dependent rearrangement of cysteine on electrode surfaces. These systems are analyzed using a hybrid quantum-classical computational approach that combines constant-potential periodic DFT with a classical representation of the liquid electrolyte. In agreement with experimental measurements, grand canonical thermodynamic analyses suggest that the cysteine exists primarily in its zwitterionic form over a wide range of applied potentials. The structure of adsorbed zwitterionic cysteine is dictated by the cationic ammonium and anionic carboxylate functional groups, where these charged moieties experience competing Coulombic interactions with the charged Au(111) surface and the electrolyte ions within the electric double layer. These competing interactions drive the rearrangement of cysteine with applied potential, which in turn determines the nature of ion structuring at the interface. The potential-dependent free energies of cysteine zwitterions are also significantly influenced by the ionic strength of the electrolyte because of the interactions between charged zwitterion functional groups and oppositely charged electrolyte ions. Understanding the interplay between adsorption structure, applied potential, and electrolyte ion structuring can guide the assembly of structured biomolecules on solid surfaces. The impact of zwitterionic amino acids and peptides on near-surface electrolyte composition may be further exploited to tailor microenvironments for various applications of interfacial electrochemistry.

Proton-coupled electron transfer (PCET) is an elementary reaction that plays a pivotal role in various electrochemical energy conversion and storage processes. PCET reactions involving nonaqueous proton donors are of interest for applications in energy storage such as redox flow batteries, as well as in CO 2 and O 2 reduction catalysis. Yet, there remains much to be understood about the molecular reactivity of these systems. In this talk, I will describe periodic density functional theory (DFT) studies of interfacial PCET reactions involving different imidazolium proton donors on Pt(111) electrode surfaces. The imidazole conjugate bases adsorb strongly to the electrode surface; however, this adsorption behavior varies with isomers that feature distinct proton positions and with different functional substituent groups. Because the adsorption of imidazole conjugate bases could affect reactivity, these models are used as a foundation to understand the intrinsic PCET kinetics of these systems. The Volmer and Heyrovsky steps of hydrogen evolution reaction are two of the most basic heterogeneous electrochemical PCET reactions and are therefore chosen as model interfacial PCET reactions in this study. Potential-dependent reaction energies and activation barriers are calculated using the charge extrapolation method, where the electrode potential is tuned by modifying proton coverage for different-sized unit cells. This approach is used to develop relationships between PCET reaction thermodynamics and kinetics through Brønsted–Evans–Polanyi (BEP) relationships for different imidazoles, where BEP slopes are analogs to charge transfer coefficients in the Butler–Volmer equation. Investigating reaction parameters influencing charge transfer kinetics at the electrode-electrolyte interface is crucial for designing efficient energy storage systems, impacting overall device performance. These studies show trends in the kinetic behavior of different functionalized imidazoles, which can aid in the design of catalytic and energy storage systems.

Electric fields affect interfacial structure and reactivity during electrochemical reactions. First-principles modeling can be used to elucidate the electrostatic potential profiles at the electrode–electrolyte interface, where these potentials can be highly inhomogeneous due to the presence of electrolyte ions and molecular adsorbates. In this presentation, periodic density functional theory (DFT) calculations are used to understand field-driven proton-coupled electron transfer reactivity and adsorbate reorientation at heterogeneous electrochemical interfaces. These calculations show that field-driven protonation of graphite-conjugated organic acids is enabled by continuous electronic conjugation between solid carbon electrodes and molecular surface sites. The redox potentials for proton-coupled electron transfer in these systems are predicted computationally, showing close agreement with experimental voltammetry measurements. Constant-potential DFT calculations are also used to understand the adsorption structure of L-cysteine on Au(111) electrode surfaces. Using a classical electrolyte model, these calculations elucidate the interactions between the cysteine adsorbate, the electrode surface, and the electrolyte ions at different applied potentials. These studies shed light on the effects of interfacial electric fields on electrochemical reactivity and adsorption geometries at solid–liquid electrochemical interfaces.

The roles of the ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), and water in controlling the mechanism, energetics, and electrocatalytic activity of CO2 reduction to CO on silver in nonaqueous electrolytes were investigated. The first electron transfer occurs to CO2 at reduced overpotentials when it is trapped between the planes of the [EMIM]+ ring and the electrode surface due to cation reorientation as determined from voltammetry, in situ surface-enhanced Raman spectroscopy, and density functional theory calculations. Within this interface, water up to 0.5 M does not induce significant Faradaic activity, opposing the notion of it being a free proton source. Instead, water acts as a hydrogen bond donor, and the proton is sourced from [EMIM]+. Furthermore, this study demonstrates that alcohols with varying acidities tune the hydrogen bonding network in the interfacial microenvironment to lower the energetics required for CO2 reduction. The hydrogen bonding suppresses the formation of inactive carboxylate species, thus preserving the catalytic activity of [EMIM]+. The ability to tune the hydrogen bonding network opens new avenues for advancing IL-mediated electrocatalytic reactions in nonaqueous electrolytes.

The hydrogen evolution reaction (HER) is an important electrocatalytic reaction used for the electrochemical production of hydrogen gas. Its reverse reaction, the hydrogen oxidation reaction, is used to produce protons used in electrocatalytic reduction reactions for various electrochemical energy conversion and storage applications. While many fundamental studies have analyzed interfacial reactivity in aqueous HER, analogous mechanistic understanding of HER reactivity in nonaqueous media remains limited. In this presentation, periodic density functional theory (DFT) calculations are applied to mechanistic studies of HER on Pt(111) using various imidazolium proton donors. These studies incorporate analyses of imidazole adsorption and potential-dependent activation energies. The conjugate bases of imidazole adsorb strongly to the electrode surface. Yet, the adsorption geometries and energetics are sensitive to substituent effects and isomeric configurations at the interface. The potential-dependent binding geometries and adsorption energies of different imidazoles adsorbed to Pt(111) are analyzed in a dielectric continuum implicit solvent. These calculations demonstrate how imidazole derivatization and electrode surface charge impact the adsorption of imidazole conjugate bases during HER. To gain insights into the kinetics of the elementary Volmer and Heyrovsky steps of HER, potential-dependent reaction energies and activation barriers were calculated using the charge extrapolation method. Energetics were calculated for different electrode potentials by modifying the coverage of protons. This approach was used to develop relationships between reaction thermodynamics and kinetics via Brønsted–Evans–Polanyi relationships for various imidazoles, where this relationship gives charge transfer coefficients in the Butler–Volmer equation. The goal of assessing these reaction parameters is to enhance understanding of charge transfer processes and molecular interactions at the electrode-electrolyte interface, which is essential for the design of high-efficiency energy conversion and storage devices. The potential-dependent adsorption behavior and kinetics of elementary proton-coupled electron transfer steps are used to understand the reaction kinetics of hydrogen evolution in a nonaqueous imidazole-based electrolyte through comparisons to voltammetry measurements. Through these analyses, we identify kinetic trends across functionalized imidazoles, providing valuable perspectives for the development of catalytic systems and energy storage technologies.

Proton-coupled electron transfer (PCET) is a fundamental class of chemical reactions that is central to many electrochemical energy conversion and storage processes. In this talk, I will discuss recent theoretical model development for interfacial and intermolecular PCET reactions. First, I will describe a rate constant model for the reaction between reduced anatase TiO 2 surfaces and a 4-MeO-TEMPO nitroxyl radical. The rate constants are modeled using vibronically nonadiabatic PCET theory, where the transferring proton and electron are treated quantum mechanically. Hybrid functional periodic density functional theory (DFT) calculations are used to derive model parameters such as driving forces, reorganization energies, and proton vibrational wavefunctions. This modeling strategy highlights the role of inner-sphere bond reorganization and excited vibronic states on the PCET reactivity of metal oxides. Next, I will show how similar computational approaches can be used to predict whether electrochemical PCET is likely to proceed through concerted electron–proton transfer or sequential electron transfer and proton transfer steps. Using PCET reactions between redox-active quinones and functionalized imidazole bases as an example, mechanistic predictions are informed by DFT-calculated potential energy surfaces. These PCET modeling strategies have broad applicability to kinetic studies of electrochemical energy conversion and storage processes.

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.

Proton-coupled electron transfer (PCET) is a critical elementary step in many (photo)electrocatalytic transformations. PCET reactions on semiconducting metal oxide surfaces involve proton transfer that is charge compensated by electronic defects, such as electrons or holes. When highly localized as small polarons, these electronic defects are coupled with local bond distortions that accompany the excess or depleted charge. Using anatase TiO 2 as a model system, I will describe recent first-principles modeling studies of interfacial PCET on metal oxide surfaces. The PCET reaction free energies involving the cleavage of O–H bonds on TiO 2 surfaces varies widely depending on whether the charge-compensating electronic defects are conduction d-band electrons or valence p-band holes. Differences in PCET thermochemistry are accounted for using a Marcus theory framework based on defect energy levels and inner-sphere reorganization energies. The PCET rate constants associated with the cleavage of O–H bonds in the presence of a nitroxyl radical oxidant are calculated using vibronically nonadiabatic PCET theory, where both the transferring electron and proton are treated quantum mechanically. Input parameters to the rate constant expressions are calculated using hybrid periodic density functional theory calculations. These studies highlight the effects of local electronic defects on the PCET reactivity of semiconductor surfaces.

Understanding the charge transfer processes at solid oxide fuel cell (SOFC) electrodes is critical to designing more efficient and robust materials. Activation losses at SOFC electrodes have been widely attributed to the ambipolar migration of charges at the mixed ionic-electronic conductor-gas interface. Empirical Butler-Volmer kinetics based on the transition state theory is often used to model the current-voltage relationship, where charged particles transfer classically over an energy barrier. However, the hydrogen oxidation/water electrolysis reaction H2(g) + O2- ⇌ H2O(g) + 2e- must be modeled through concerted electron and proton tunneling events, where we unify the theory of the electrostatic surface potential with proton-coupled electron transfer kinetics. We derive a framework for the reaction rate that depends on the electrostatic surface potential, adsorbate dipole moment, the electronic structure of the electron donor/acceptor, and vibronic states of the hydrogen species. This theory was used to study the current-voltage characteristics of the Ni/gadolinium-doped ceria electrode in H2/H2O(g), where we find excellent validation of this novel model. These results yield the first reported quantification of the solvent reorganization energy for an SOFC material and suggest that the three-phase boundary mechanism is the dominant pathway for charge transfer at cermet electrodes.

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 pKa 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.

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, EPCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy εLUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, εLUS is obtained from only a single DFT calculation but can predict EPCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.

The systematic improvement of Fe-N-C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe-N-C materials is the formation of the FeIII-OOH intermediate. They also show that an active site model with a water molecule axially ligated to the iron center throughout the catalytic cycle produces results that are consistent with the experimental measurements. In particular, reliable prediction of the ORR onset potential and the Fe(III/II) redox potential associated with the conversion of FeIII-OH to FeII and desorbed H2O requires an axial H2O co-adsorbed to the iron center. The observation of a five-coordinate rather than four-coordinate active site has significant implications for the thermodynamics and mechanism of ORR. These findings highlight the importance of solvent-substrate interactions and surface charge effects for understanding the PCET reaction mechanisms and transition-metal redox couples under realistic electrochemical conditions.