Adaptive Electric Fields Embedded Electrochemical Barrier Calculations

Surface Amorphization of Bismuth for Efficient Acidic CO2 Electrolysis

The electrochemical conversion of CO2 into valuable chemicals under acidic conditions provides a promising solution to challenges, such as carbon loss and catalyst instability caused by carbonate precipitation. However, acidic CO2 electroreduction remains severely constrained by intense competition from the hydrogen evolution reaction (HER) and sluggish CO2 activation kinetics. Here, we report a bismuth (Bi) nanoparticle catalyst with an amorphous surface layer (a-Bi), which demonstrates high catalytic activity and selectivity toward formic acid (HCOOH) formation in acidic electrolytes. The catalyst achieves impressive Faradaic efficiencies for HCOOH production, exceeding 90% over a wide current density range (-100 to -1000 mA cm-2) with corresponding potentials ranging from -1.24 to -1.75 V versus the reversible hydrogen electrode (vs RHE). Notably, the partial current density for an HCOOH reaches an impressive value of more than -900 mA cm-2 at -1.75 V vs RHE. Furthermore, the a-Bi catalyst exhibited stability for over 52 h at high production rates (-500 mA cm-2) alongside a single-pass carbon efficiency of approximately 85%. In situ spectroscopy and theoretical simulation revealed that surface amorphization significantly enhances the adsorption of CO2 and lowers the hydrogenation barrier, thereby accelerating the CO2RR kinetics while effectively suppressing the HER. This work presents a facile crystallization engineering strategy to address critical carbon loss challenges, thereby advancing the sustainability and scalability of acidic CO2 electroreduction processes.

A First Principle Study to Understand the Importance of Edge-exposed and Basal Plane Defective MoS2 Towards Nitrogen Reduction Reaction

Nitrogen reduction reaction (NRR) as a promising approach to ammonia synthesis has received much attention in recent years. Molybdenum disulfides (MoS2), as one of the most potential candidates for NRR, are extensively investigated. However, the inert basal plane limits the application of MoS2. Herein, by using density functional theory (DFT) calculations, we constructed edge-exposed MoS2 and different kinds of basal plane defects, including anti-site, sulfur vacancy and pore defects, to systematically investigate their influence on the NRR performance. The thermodynamically calculated results revealed that the NRR on edge-exposed MoS2, anti-site defects, sulfur vacancy with three sulfur atoms missing (S3V) and porous defect (D) exhibit great catalytic activity with low limiting potentials. The calculated limiting potentials are -0.43 and -0.47 V at armchair and zigzag edge MoS2, -0.42 and -0.44 V at anti-site defects, -0.49 and -0.67 V at S3V and D. However, by inspecting the thermodynamic properties of the hydrogen evolution reaction, we proposed that the zigzag-end MoS2 and anti-site defects exhibit a better NRR selectivity compared to armchair-end MoS2, S3V and D. Electronic structure calculations reveals that the edge-exposed and basal plane defective MoS2 can improve the conductivity of the material by reducing the band gap. Donation-backdonation mechanism can effectively promote the activation of nitrogen molecule. Our results pave the way to understanding the defective effects of the MoS2 inertness plane for NRR and designing high-performance NRR catalysts.

A charge calibration strategy for describing the charge transfer during the electrochemical elementary step

Constant potential modeling of electrocatalytic processes remains a significant challenge in the field of computational catalysis, primarily due to the difficulty in simultaneously considering the influence of constant potential conditions, explicit solvent environment, and the double-layer structure. In this work, we propose a charge calibration strategy for electrocatalytic processes. This strategy accounts for charge transfer in systems with explicit solvation and ions during constant-potential free energy modeling. In our strategy, interfacial counter-ions are employed to model the Helmholtz layer and determine the surface charge density, which defines the electrode potential. During the simulation of electrochemical reactions, extra charges are introduced/extracted to/from the system to compensate for electron transfer between the electrode and the reaction species and keep a constant surface charge density along the reaction profile. Our method showcases the impact of potential-dependent solvent reorganization on reaction kinetics and underscores the importance of constant potential kinetics. We anticipate that the strategy presented here will inspire further theoretical and experimental studies for electrochemistry interfaces.

Dynamic Structural Evolution of Active Sites and Effect on Heterogeneous Catalysis: Theoretical Insights

The exploration of structure-activity relationships is one of the key subjects for the rational design of heterogeneous catalysts. Advancements in characterization techniques have unveiled significant geometric and electronic structural changes at active sites under working conditions. This dynamic nature of active sites presents a formidable challenge in determining their structure-activity relationships. Over the past decade, considerable theoretical studies have concentrated on deciphering the complex catalytic processes that occur at these dynamic active sites under working conditions, with much success in methodology, models, and mechanisms. Herein, we review key factors influencing active site evolution, such as surface adsorbates, temperature, and electrochemical potential. We also provide an overview of recent progress in understanding the effects of active site evolution on their catalytic performance. A better understanding to the dynamic evolution of active sites and corresponding effects on the catalysis under working conditions can help in the rational design of novel heterogeneous catalysts.

Understanding the Selectivity Differences of NO Electroreduction on Ag and Au Electrodes

Although noble metals Ag and Au have similar chemical reactivities, their catalytic selectivity for NO electroreduction is significantly different. Namely, hydroxylamine is often considerably produced on Ag while not observed on the Au electrode. In this study, first-principles calculations and the electric field controlling constant potential (EFC-CP) method are adopted to unveil the underlying reasons. We first reveal a distinct NO* adsorption configuration, vertical on Ag and inclined on Au, leading to different reduction pathways to NOH* and HNO*, respectively. Via complete electrochemical barrier calculations and detailed kinetic analysis, we find the hydroxylamine selectivity difference between Ag and Au is mainly induced by adsorption strength of NH2OH*. On Ag, the obtained NH2OH* prefers to desorb and produce hydroxylamine, while NH2OH* is bonded strongly to Au and favors further reduction to ammonia. The study advances our understanding of factors regulating product selectivity, providing crucial insights for designing NO electroreduction catalysts toward hydroxylamine production.

Toward improved descriptors by refining the complex reaction network in electrocatalysis

Electrocatalysis is one of the key technologies for developing sustainable and fossil resource free routes to produce fuels and chemicals. The limiting potential (UL), defined by the reaction free energy of the most difficult electrochemical step in a given pathway, is an effective descriptor for establishing the activity trend of a set of electrocatalysts, allowing high throughput screening of new catalysts. However, the reaction network of electrocatalytic processes is rather complex, especially for the reactions with necessary thermochemical steps, e.g., the synthesis of valuable C-N bond-containing chemicals. Thermochemical steps cannot be significantly enhanced by electrode potentials, where kinetics is a non-negligible issue at even high overpotentials. This makes it challenge by using limiting potential to accurately describe activity trends for the reactions with necessary thermochemical steps. To this end, we propose an effective scheme to determine an improved descriptor. We suggest refining the rather complex reaction network at first. In particular, it is suggested to decouple electro- and thermochemical steps and exclude the unfavorable pathways with an excessively high thermochemical barrier. Then, a global comparison among the other pathways can be made, to determine the optimal pathway and the improved descriptor (the reaction free energy of the most difficult step of the optimal pathway, defined as ΔGrRPD-limiting). In addition, the studies on reaction kinetics are also suggested to understand the exception of the best catalysts and provide the direction of experimental optimization. This scheme is of a great compromise between practical efficiency and the accuracy toward the rational design of electrocatalysts.

Non-Nernstian Effects in Theoretical Electrocatalysis

Electrocatalysis is one of the principal pathways for the transition to sustainable chemistry, promising greater energy efficiency and reduced emissions. As the field has grown, our theoretical understanding has matured. The influence of the applied potential on reactivity has developed from the first-order predictions based on the Nernst equation to the implicit inclusion of second-order effects including the interaction of reacting species with the interfacial electric field. In this review, we explore these non-Nernstian field effects in electrocatalysis, aiming to both understand and exploit them through theory and computation. We summarize the critical distinction between Nernstian and non-Nernstian effects and outline strategies to address the latter in theoretical studies. Subsequently, we examine the specific energetic contributions of the latter on capacitive and faradaic processes separately. We also underscore the importance of considering non-Nernstian effects in catalyst screening and mechanistic analysis. Finally, we provide suggestions on how to experimentally unravel these effects, offering insights into practical approaches for advancing the field.

Exploring Nitrogen Reduction Reaction Mechanisms with Graphyne-Confined Single-Atom Catalysts: A Computational Study Incorporating Electrode Potential and pH

This study reconciles discrepancies between practical electrochemical conditions and theoretical density functional theory (DFT) frameworks, evaluating three graphyne-confined single-atom catalysts (Mo-TEB, Mo@GY, and Mo@GDY). Using both constant charge models in vacuum and constant potential models with continuum implicit solvation, we closely mimic real-world electrochemical environments. Our findings highlight the crucial role of explicitly incorporating electrode potential and pH in the constant potential model, providing enhanced insights into the nitrogen reduction reaction (NRR) mechanisms. Notably, the superior NRR performance of Mo-TEB is attributed to the d-band center's proximity to the Fermi level and enhanced magnetic moments at the atomic center. This research advances our understanding of graphyne-confined single-atom catalysts as effective NRR platforms and underscores the significance of the constant potential model for accurate DFT studies of electrochemical reactions.

Electrochemical Reduction of N2 to Ammonia Promoted by Hydrated Cation Ions: Mechanistic Insights from a Combined Computational and Experimental Study

Alkali ions, major components at the electrode-electrolyte interface, are crucial to modulating reaction activity and selectivity of catalyst materials. However, the underlying mechanism of how the alkali ions catalyze the N2 reduction reaction (NRR) into ammonia remains elusive, posing challenges for experimentalists to select appropriate electrolyte solutions. In this work, by employing a combined experimental and computational approach, we proposed four essential roles of cation ions at Fe electrodes for N2 fixation: (i) promoting NN bond cleavage; (ii) stabilizing NRR intermediates; (iii) suppressing the competing hydrogen evolution reaction (HER); and (iv) modulating the interfacial charge distribution at the electrode-electrolyte interface. For N2 adsorption on an Fe electrode with cation ions, our constrained ab initio molecular dynamic (c-AIMD) results demonstrate a barrierless process, while an extra 0.52 eV barrier requires to be overcome to adsorb N2 for the pure Fe-water interface. For the formation of *NNH species within the N2 reduction process, the calculated free energy barrier is 0.50 eV at the Li+-Fe-water interface. However, the calculated barrier reaches 0.81 eV in pure Fe-water interface. Furthermore, experiments demonstrate a high Faradaic efficiency for ammonia synthesis on a Li+-Fe-water interface, reaching 27.93% at a working potential of -0.3 V vs RHE and pH = 6.8. These results emphasize how alkali metal cations and local reaction environments on the electrode surface play crucial roles in influencing the kinetics of interfacial reactions.

Towards rational design in electrochemical denitrification by analyzing pH dependence

A small fraction of NOx (<1%) always exists in CO2 feedstock (e.g. exhausted gas), which can significantly reduce the efficiency of CO2 electroreduction by ∼30%. Hence, electrochemical denitrification is the precondition of CO2 electroreduction. The pH effect is a key factor, and can be used to tune the selectivity between N2 and N2O production in electrochemical denitrification. However, there has been much controversy for many years about the origin of pH dependence in electrocatalysis. To this end, we present a new scheme to accurately model the pH dependence of the electrochemical mechanism. An extremely small pH variation from pH 12.7 to pH 14 can be accurately reproduced for N2O production. More importantly, the obviously different pH dependence of N2 production, compared to N2O, can be attributed to a cascade path. In other words, the N2 was produced from the secondary conversion of the as-produced N2O molecule (the major product), instead of the original reactant NO. This is further supported by more than 35 experiments over varying catalysts (Fe, Ni, Pd, Cu, Co, Pt and Ag), partial pressures (20%, 50% and 100%) and potentials (from -0.2 to 0.2 V vs. reversible hydrogen electrode). All in all, the insights herein overturn long-lasting views in the field of NO electroreduction and suggest that rational design should steer away from catalyst engineering toward reactor optimization.

Computational Insights on Structural Sensitivity of Cobalt in NO Electroreduction to Ammonia and Hydroxylamine

It has been reported that it was selective to produce ammonia on metallic cobalt in the electrocatalytic nitric oxide reduction reaction (eNORR), where hexagonal close-packed (hcp) cobalt outperforms face-centered cubic (fcc) cobalt. However, hydroxylamine is more selectively produced on a cobalt single-atom catalyst (Co-SAC). Herein, we uncover the structural sensitivity over hcp-Co, fcc-Co, and Co-SAC in eNORR by employing a recently developed constant potential simulation method and microkinetic modeling. It was found that the superior activity for ammonia production on hcp-Co can be attributed to its facile electron and proton transfer and a stronger lateral suppression effect from NO* over fcc-Co. The exceptional hydroxylamine selectivity on Co-SAC is due to the modified electronic structure, namely, a positively charged active center. It was found that it is more favorable to produce NOH* over hcp-Co and fcc-Co, while HNO* is more preferable on Co-SAC, which are firmly correlated with the vertical and strong NO adsorption on the former and the moderate adsorption on the latter. In other words, a key factor for selectivity control is the first step of NO* protonation. Therefore, the local structure and electronic structure of the catalysts can be critical in regulating the activity and selectivity in eNORR.

Computational electrocatalysis beyond conventional hydrogen electrode model: CO2 reduction to C2 species on copper facilitated by dynamically formed solvent halide ions at the solid-liquid interface

The reduction of CO2 into value-added chemicals and fuels has been actively studied as a promising strategy for mitigating carbon dioxide emissions. However, the dilemma for the experimentalist in choosing an appropriate reaction medium and neglecting the effect of solvent ions when using a simple thermochemical model, normally leads to the disagreement between experimental observations and theoretical calculations. In this work, by considering the effects of both the anion and cation, a more realistic CO2 reduction environment at the solid-liquid interface between copper and solvent ions has been systematically studied by using ab initio molecular dynamics and density functional theory. We revealed that the co-occurrence of alkali ions (K+) and halide ions (F-, Cl-, Br-, and I-) in the electric double layer (EDL) can enhance the adsorption of CO2 by more than 0.45 eV compared to that in pure water, and the calculated energy barrier for CO-CO coupling also decreases 0.32 eV in the presence of I ion on a negatively charged copper electrode. The hydrated ions can modulate the distribution of the charge near the solid-liquid interface, which significantly promotes CO2 reduction and meanwhile impedes the hydrogen evolution reaction. Therefore, our work unveils the significant role of halide ions at the electrode-electrolyte interface for promoting CO2 reduction on copper electrode.

Modeling Interfacial Dynamics on Single Atom Electrocatalysts: Explicit Solvation and Potential Dependence

ConspectusSingle atom electrocatalysts, with noble metal-free composition, maximal atom efficiency, and exceptional reactivity toward various energy and environmental applications, have become a research hot spot in the recent decade. Their simplicity and the isolated nature of the atomic structure of their active site have also made them an ideal model catalyst system for studying reaction mechanisms and activity trends. However, the state of the single atom active sites during electrochemical reactions may not be as simple as is usually assumed. To the contrary, the single atom electrocatalysts have been reported to be under greater influence from interfacial dynamics, with solvent and electrolyte ions perpetually interacting with the electrified active center under an applied electrode potential. These complexities render the activity trends and reaction mechanisms derived from simplistic models dubious.In this Account, with a few popular single atom electrocatalysis systems, we show how the change in electrochemical potential induces nontrivial variation in the free energy profile of elemental electrochemical reaction steps, demonstrate how the active centers with different electronic structure features can induce different solvation structures at the interface even for the same reaction intermediate of the simplest electrochemical reaction, and discuss the implication of the complexities on the kinetics and thermodynamics of the reaction system to better address the activity and selectivity trends. We also venture into more intriguing interfacial phenomena, such as alternative reaction pathways and intermediates that are favored and stabilized by solvation and polarization effects, long-range interfacial dynamics across the region far beyond the contact layer, and the dynamic activation or deactivation of single atom sites under operation conditions. We show the necessity of including realistic aspects (explicit solvent, electrolyte, and electrode potential) into the model to correctly capture the physics and chemistry at the electrochemical interface and to understand the reaction mechanisms and reactivity trends. We also demonstrate how the popular simplistic design principles fail and how they can be revised by including the kinetics and interfacial factors in the model. All of these rich dynamics and chemistry would remain hidden or overlooked otherwise. We believe that the complexity at an electrochemical interface is not a curse but a blessing in that it enables deeper understanding and finer control of the potential-dependent free energy landscape of electrochemical reactions, which opens up new dimensions for further design and optimization of single atom electrocatalysts and beyond. Limitations of current methods and challenges faced by the theoretical and experimental communities are discussed, along with the possible solutions awaiting development in the future.

Importance of the Walden Inversion for the Activity Volcano Plot of Oxygen Evolution

Since the birth of the computational hydrogen electrode approach, it is considered that activity trends of electrocatalysts in a homologous series can be quantified by the construction of volcano plots. This method aims to steer materials discovery by the identification of catalysts with an improved reaction kinetics, though evaluated by means of thermodynamic descriptors. The conventional approach for the volcano plot of the oxygen evolution reaction (OER) relies on the assumption of the mononuclear mechanism, comprising the * OH, * O, and * OOH intermediates. In the present manuscript, two new mechanistic pathways, comprising the idea of the Walden inversion in that bond-breaking and bond-making occurs simultaneously, are factored into a potential-dependent OER activity volcano plot. Surprisingly, it turns out that the Walden inversion plays an important role since the activity volcano is governed by mechanistic pathways comprising Walden steps rather than by the traditionally assumed reaction mechanisms under typical OER conditions.

Activity Trend and Selectivity of Electrochemical Ammonia Synthesis in Reverse Artificial Nitrogen Cycle

Ammonia is important for modern agriculture and food production as it is a major source of fertilizer. Electrochemical ammonia synthesis (EAS) with sustainable energy generated electricity and decentralized reactors has been considered as environmentally friendly process. Several nitrogen sources have been considered and intensively studied in experiments and computations. Recently, it has been proposed and demonstrated that nitrogen oxides (NOx ) electroreduction for selective ammonia production is feasible. Fundamental insights on experimental observation are necessary for more rational design of catalysts and reactors in the future. In this concept, we review the theoretical and computational insights of electrochemical nitrogen oxides reduction, particularly, the activity trend over diverse transition metal catalysts and products selectivity at varying potentials. Finally, we address the opportunities and challenges in the reverse artificial nitrogen cycle, as well as fundamental issues in electrochemical reaction modelling.

Electric Double Layer Effects in Electrocatalysis: Insights from Ab Initio Simulation and Hierarchical Continuum Modeling

Structures of the electric double layer (EDL) at electrocatalytic interfaces, which are modulated by the material properties, the electrolyte characteristics (e.g., the pH, the types and concentrations of ions), and the electrode potential, play crucial roles in the reaction kinetics. Understanding the EDL effects in electrocatalysis has attracted substantial research interest in recent years. However, the intrinsic relationships between the specific EDL structures and electrocatalytic kinetics remain poorly understood, especially on the atomic scale. In this Perspective, we briefly review the recent advances in deciphering the EDL effects mainly in hydrogen and oxygen electrocatalysis through a multiscale approach, spanning from the atomistic scale simulated by ab initio methods to the macroscale by a hierarchical approach. We highlight the importance of resolving the local reaction environment, especially the local hydrogen bond network, in understanding EDL effects. Finally, some of the remaining challenges are outlined, and an outlook for future developments in these exciting frontiers is provided.

Approximating constant potential DFT with canonical DFT and electrostatic corrections

The complexity of electrochemical interfaces has led to the development of several approximate density functional theory (DFT)-based schemes to study reaction thermodynamics and kinetics as a function of electrode potential. While fixed electrode potential conditions can be simulated with grand canonical ensemble DFT (GCE-DFT), various electrostatic corrections on canonical, constant charge DFT are often applied instead. In this work, we present a systematic derivation and analysis of the different electrostatic corrections on canonical DFT to understand their physical validity, implicit assumptions, and scope of applicability. Our work highlights the need to carefully address the suitability of a given model for the problem under study, especially if physical or chemical insight in addition to reaction energetics is sought. In particular, we analytically show that the different corrections cannot differentiate between electrostatic interactions and covalent or charge-transfer interactions. By numerically testing different models for CO₂ adsorption on a single-atom catalyst as a function of the electrode potential, we further show that computed capacitances, dipole moments, and the obtained physical insight depend sensitively on the chosen approximation. These features limit the scope, generality, and physical insight of these corrective schemes despite their proven practicality for specific systems and energetics. Finally, we suggest guidelines for choosing different electrostatic corrections and propose the use of conceptual DFT to develop more general approximations for electrochemical interfaces and reactions using canonical DFT.