Grand Canonical Density Research Articles

Understanding the Unique Reactivity of Cu for Electrochemical CO2 Reduction with a 3-Site Model.

Cu-based catalysts for the electrochemical reduction of CO2 and CO exhibit a perplexingly unique reactivity toward multicarbon based products compared to other studied electrocatalysts. Here we use insights gained from a recent phenomenological 3-site microkinetic model and grand-canonical density functional theory calculations to clarify the importance of an underemphasized aspect critical to Cu's unique reactivity: a population of so-called "reservoir" sites. Using model Cu surface motifs, we discuss how these types can be represented by undercoordinated structural defects like step edges and grain boundaries which form a network of highly anisotropic migration channels. These pathways are found to be amenable for feeding *CO over time to reactive sites like Cu adatoms more active toward C-C coupling. These results highlight an often overlooked aspect of catalyst optimization: reservoir site engineering, which exploits surface mobility and presents an equally important avenue for electrocatalyst engineering oversimply maximizing active site densities.

Probing the Electric Double-Layer Capacitance to Understand the Reaction Environment in Conditions of Electrochemical Amination of Acetone.

To elucidate interfacial dynamics during electrocatalytic reactions, it is crucial to understand the adsorption behavior of organic molecules on catalytic electrodes within the electric double layer (EDL). However, the EDL structure in aqueous environments remains intricate when it comes to the electrochemical amination of acetone, using methylamine as a nitrogen source. Specifically, the interactions of acetone and methylamine with the copper electrode in water remain unclear, posing challenges in the prediction and optimization of reaction outcomes. In this study, initial investigations employed impedance spectroscopy at the potential of zero charge to explore the surface preconfiguration. Here, the capacitance of the EDL was utilized as a primary descriptor to analyze the adsorption tendencies of both acetone and methylamine. Acetone shows an increase in the EDL capacitance, while methylamine shows a decrease. Experiments are interpreted using combined grand canonical density functional theory and ab initio molecular dynamics to delve into the microscopic configurations, focusing on their capacitance and polarizability. Methylamine and acetone have larger molecular polarizability than water. Acetone shows a partial hydrophobic character due to the methyl groups, forming a distinct adlayer at the interface and increasing the polarizability of the liquid interface component. In contrast, methylamine interacts more strongly with water due to its ability to both donate and accept hydrogen bonds, leading to a more significant disruption of the hydrogen bond network. This disruption of the hydrogen network decreases the local polarizability of the interface and decreases the effective capacitance. Our findings underscore the pivotal role of EDL capacitance and polarizability in determining the local reaction environment, shedding light on the fundamental processes important for electro-catalysis.

Do the Hydrogen Evolution Reaction and Oxygen Reduction Reaction Really Need To Be Suppressed in O2-Tolerant CO2 Electroreduction Catalyzed by DAE-BPy-CoPor and Co-TAPP?

This letter chooses DAE-BPy-CoPor [DAE, 1,2-bis(5'-formyl-2'-methylthien-3'-yl)cyclopentene; BPy, 2,2'-bipyridine-5,5'-dicarbaldehyde; and CoPor, two-dimensional cobalt porphyrin-based covalent organic framework] and Co-TAPP [10,15,20-tetrakis(4-aminophenyl)porphinatocobalt] as two model catalysts to investigate the effect of the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in the O2-tolerant CO2 electroreduction reaction (CO2RR). Both catalysts have been proven to have CO2RR activity at a potential range from -0.60 to -1.10 V when CO2 and O2 are co-feeding (Zhu, H.-J.; Si, D.-H.; Guo, H.; Chen, Z.; Cao, R.; Huang, Y.-B. Oxygen-tolerant CO2 electroreduction over covalent organic frameworks via photoswitching control oxygen passivation strategy. Nat. Commun. 2024, 15, 1479, DOI: 10.1038/s41467-024-45959-9). The calculated results by grand canonical density functional theory (GC-DFT) and microkinetic (MK) simulation show that the effect of HER could be categorized into two groups: (1) Positive *H formation energy: HER would have no effect on the turnover frequency (TOF) of the CO2RR. (2) Negative *H formation energy: HER would also have no effect to the TOF of CO2RR when the calculated limiting potential for HER is larger than that for CO2RR. Otherwise, all active sites would be covered by *H and poisoned before CO2RR can happen. The effect of ORR could be categorized into three types: (1) ORR would have no effect to the TOF of the CO2RR when ORR happened with the CO2RR at the same time. (2) ORR is suppressed; the active site is covered by an intermediate state (such as *OH); and the CO2RR is also suppressed because of the poisoned active site. (3) ORR is suppressed; the active site is not covered; and the TOF of the CO2RR is not affected.

Origin of Copper Dissolution in Electrocatalytic Reduction Conditions Involving Amines

Cu dissolution has been identified as the dominant process that causes cathode degradation and losses even under cathodic conditions involving . Despite extensive experimental research, our fundamental and theoretical understanding of the atomic-scale mechanism for Cu dissolution in electrochemical conditions, eventually coupled with surface restructuring processes, is limited. Here, driven by the observation that the working Cu electrode is corroded using mixtures of acetone and methylamine even under reductive potential conditions (-0.75V vs RHE), we employed Grand Canonical density functional theory to understand this dynamic process under potential from a microscopic perspective. We show that amine ligands in solution directly adsorb on the electrode, coordinate with the metal center, drive the rearrangement of the copper surface by extracting Cu atoms in low coordination positions, where other ligands can coordinate and stabilize a surface copper-ligands complex, finally forming a detached Cu-tetramine di-cation complex in solution, even at negative potential conditions. Calculations predict that dissolution would occur for a potential of -0.7 V vs RHE or above. Our work provides a fundamental understanding of Cu dissolution facilitated by surface restructuring in amine solutions under electroreduction conditions, which is required for the rational design of durable Cu-based cathodes for electrochemical amination or other amine involving reduction processes.

Influence of Simple Salts on Solvent Reduction Stability at Mg-Alloy Anodes Interface: A Potential-Dependent DFT Study

Using the grand canonical density functional theory (GC-DFT) approach, the investigation focuses on the significance of incorporating anion species into electrolyte solvation structures, particularly with doubly charged Mg2+ ions. Our work extends previous methodologies by examining the thermodynamic stability of acetonitrile (AN) at the interface with Mg3Bi2 and Mg2Sn. Based on energy comparisons, two distinct anions, TFSI− and ClO−, are strategically introduced.Despite the known chemical compatibility of alloy anodes with the electrolyte solution, our research discovers a novel form of solvent degradation, which has also been noted with pure Mg anodes and conventional electrolytes. Notably,a the AN molecule near anion species exhibits less susceptibility to reduction (-0.8 — -0.4 V vs. Mg2+/Mg) in the lower potential range, compared to the fully dissociated solvation structure.Charge density difference and density of states analyses detail how solvent molecules become electrophilic, causing the LUMO to overlap with the Fermi level at lower potentials when the electrostatic interaction with anion species is considered. Experimental studies using nuclear magnetic resonance (NMR) spectroscopy and linear sweep voltammetry (LSV) validate the theoretical outcomes, providing a comprehensive understanding. By augmenting prior approaches, our methodology offers valuable guidance for electrolyte composition based on predominant solvation structures in multivalent solutions.

Structure Sensitivity and Catalyst Restructuring for CO2 Electro-Reduction on Copper

Cu is the most promising metal catalyst for CO2 electroreduction (CO2RR) to multi-carbon products, but the structure sensitivity of the reaction and the stability versus restructuring of the catalyst surface under reaction conditions are still controversial. Here, atomic scale simulations of surface energies and reaction pathway kinetics supported by experimental evidence unveil that CO2RR does not take place on perfect planar Cu(111) and Cu(100) surfaces but rather on steps or kinks defects, and that these planar surfaces tend to restructure in reaction conditions to the active stepped surfaces. By combining basin hopping global sampling and grand canonical density functional theory, we show that the extremely low CO coverage on (111) and (100) surfaces, originating from sluggish CO2 conversion and unfavorable CO binding, limits the ability of these surfaces to reduce CO2 to multi-carbon products. Steps and kinks at surfaces, despite the lack of decrease in C-C coupling barriers on these sites, exhibit a significant increase in activity arising from beneficial CO2 activation and higher CO coverage. Notably, the square motifs adjacent to defects, not the defects themselves, are the active sites for CO2RR via synergistic effect. In addition, the strong binding of CO on defective sites acts as a thermodynamic driving force for the restructuring of planar surfaces to active stepped terminations under reactive conditions. We evaluate these mechanisms against experiments of CO2RR on UHV-prepared ultraclean Cu surfaces. Overall, our findings highlight the structural sensitivity in steering CO2RR and elucidate the origin of in situ restructuring of Cu catalysts during the reaction. We furthermore feature that the active sites for CO2RR are created under reaction conditions.

Spiro-linked hanging group cobalt phthalocyanine for CO2-to-methanol electrocatalysis unveiled by grand canonical density functional theory

Heterogeneous cobalt phthalocyanine (CoPc) is one of the few favorable molecular catalysts for the electrocatalytic reduction of carbon dioxide to methanol. In-plane substituent modification of planar conjugated phthalocyanine ligands is a key way to establish the structure-activity relationship and enhance the catalytic performance of cobalt phthalocyanines. While steric hanging substituents inspired by efficient enzymes' catalytic architectures are prevalent in metalloporphyrin systems, their application in metal phthalocyanines remains underexplored. Herein, we carried out a systematic constant potential theoretical study on heterogeneous hanging group substituted CoPc electrocatalytic carbon dioxide reduction reaction. Notably, a phenol-based hanging group-modified tetra amino cobalt phthalocyanine was found to have excellent activity and selectivity for the methanol production. Intriguingly, the hanging group substituent connects to the ligand through a spirocyclic structure (rather than the conventional C-N single bond), which can reduce spatial steric hindrance and maintain hydrogen bonding. The phenolic H atoms in the hanging group form effective hydrogen bonds with the O atoms in the key structures of *CHO and *CH2O, resulting in a strong stabilizing effect that facilitates the selectivity-determining *CO → *CHO and rate-determining *CHO → *CH2O over a wide potential range. Our findings offer a theoretically effective hanging group substituent for metal phthalocyanine catalysts, broadening the scope and complexity of substituent design.

Overlooked Role of Electrostatic Interactions in HER Kinetics on MXenes: Beyond the Conventional Descriptor ΔG ∼ 0 to Identify the Real Active Site.

Understanding the atomic-level mechanism of the hydrogen evolution reaction (HER) on MXene materials is crucial for developing affordable HER catalysts, while their complex surface terminations present a substantial challenge. Herein, employing constant-potential grand canonical density functional theory calculations, we elucidate the reaction kinetics of the HER on MXenes with various surface terminations by taking experimentally reported Mo2C as a prototype. We observe a contradictory scenario on Mo2C MXene when using conventional thermodynamic descriptor ΔGH* (hydrogen binding energy). Both competing surface phases that emerge close to the equilibrium potential meet the ΔGH* ∼ 0 criterion, while they exhibit distinctly different reaction kinetics. Contrary to previous studies that identified surface *O species as active sites, our research reveals that these *O sites are kinetically inert for producing H2 but are easily reduced to H2O. Consequently, the surface Mo atoms, exposed from the rapid reduction of the surface *O species, serve as the actual active sites catalyzing the HER via the Volmer-Heyrovsky mechanism, as confirmed by experimental studies. Our findings highlight the overlooked role of electrostatic repulsion in HER kinetics, a factor not captured by thermodynamic descriptor ΔGH*. This work provides new insights into the HER mechanism and emphasizes the importance of kinetic investigations for a comprehensive understanding of the HER.

Dynamic Adaptation of Active Site Driven by Dual-side Adsorption in Single-Atomic Catalysts During CO2 Electroreduction.

Single-atom iron embedded in N-doped carbon (Fe-N-C) is among the most representative single-atomic catalysts (SACs) for electrochemical CO2 reduction reaction (CO2RR). Despite the simplicity of the active site, the CO2-to-CO mechanism on Fe-N-C remains controversial. Firstly, there is a long debate regarding the rate-determining step (RDS) of the reactions. Secondly, recent computational and experimental studies are puzzled by the fact that the CO-poisoned Fe centers still remain highly active at high potentials. Thirdly, there are ongoing challenges in elucidating the high selectivity of hydrogen evolution reaction (HER) over CO2RR at high potentials. In this work, we introduce a novel CO2RR mechanism on Fe-N-C, which was inspired by the dynamic of active sites in biological systems. By employing grand-canonical density functional theory and kinetic Monte-Carlo, we found that the RDS is not fixed but changes with the applied potential. We demonstrated that our proposed dual-side mechanisms could clarify the reason behind the high catalytic activity of CO-poisoned metal centers, as well as the high selectivity of HER over CO2RR at high potential. This study provides a fundamental explanation for long-standing puzzles of an important catalyst and calls for the importance of considering the dynamic of active sites in reaction mechanisms.

Cu Based Dilute Alloys for Tuning the C2+ Selectivity of Electrochemical CO2 Reduction.

Electrochemical CO2 reduction is a promising technology for replacing fossil fuel feedstocks in the chemical industry but further improvements in catalyst selectivity need to be made. So far, only copper-based catalysts have shown efficient conversion of CO2 into the desired multi-carbon (C2+) products. This work explores Cu-based dilute alloys to systematically tune the energy landscape of CO2 electrolysis toward C2+ products. Selection of the dilute alloy components is guided by grand canonical density functional theory simulations using the calculated binding energies of the reaction intermediates CO*, CHO*, and OCCO* dimer as descriptors for the selectivity toward C2+ products. A physical vapor deposition catalyst testing platform is employed to isolate the effect of alloy composition on the C2+/C1 product branching ratio without interference from catalyst morphology or catalyst integration. Six dilute alloy catalysts are prepared and tested with respect to their C2+/C1 product ratio using different electrolyzer environments including selected tests in a 100-cm2 electrolyzer. Consistent with theory, CuAl, CuB, CuGa and especially CuSc show increased selectivity toward C2+ products by making CO dimerization energetically more favorable on the dominant Cu facets, demonstrating the power of using the dilute alloy approach to tune the selectivity of CO2 electrolysis.

Revisiting the role of foreign atoms in [formula omitted] reduction on CuSn single-atom surface alloys

The electrochemical CO2 reduction is a promising process for capturing the emitted carbon dioxide by converting CO2 into value-added chemicals. Although copper-based catalysts have a unique ability to produce multi-carbon products, they suffer from poor selectivity. Copper-based alloys and, in particular, single atom alloys, hold promise for fine-tuning the selectivity of CO2 reduction reaction. In this study, we applied the grand canonical density functional theory in combination with the implicit solvent to model CO2 electroreduction with the formation of CO and HCOO− on copper and highly diluted copper-tin alloys. We demonstrate that the insertion of a substitutional Sn atom introduces a destabilization effect for all intermediates and completely disrupts the adsorption positions of the CO, H, and H/CO2 co-adsorption. However, the influence of Sn on intermediate adsorption energies is primarily localized in its immediate vicinity. We demonstrate that sparsely distributed single Sn atoms do not account for the experimentally observed significant reduction in formate and hydrogen generation across the entire Sn-substituted Cu surface. This discrepancy underscores the need to reevaluate the proposed surface structure and the nature of the active sites on CuSn single-atom surface alloys.

Adsorption grand potential of OH on metal oxide surfaces.

Describing chemical processes at solid-liquid interfaces as a function of a fixed electron chemical potential presents a challenge for electronic structure calculations and is essential for understanding electrochemical phenomena. Grand Canonical Density Functional Theory (GCDFT) allows treating solid-liquid interfaces in such a way that studying the influence of a fixed electron potential arises naturally. In this work, GCDFT is used to compute the adsorption grand potential (AGP), a key parameter for understanding and predicting the behavior of adsorbates on surfaces. We focused on the adsorption of an OH molecule on three metallic surfaces commonly used in electrochemical processes, such as the oxygen evolution reaction (OER). Our study aims to offer insights into how AGP can be used to compare adsorption strengths under different fixed electron chemical potentials, which is crucial for designing efficient electrode materials. By determining the average number of electrons self-consistently under varying chemical potentials, we showed how one can distinguish between electron acquisition and depletion during the adsorption process, offering a deeper understanding of the adsorbate-surface interactions. The approach used in this work employs the Kohn-Sham-Mermin formulation of the Grand Canonical Density Functional Theory. The computations were performed using the periodic open-source density functional theory software, JDFTx, with the Garrity-Bennett-Rabe-Vanderbilt library of ultrasoft pseudopotentials. Calculations were made using truncated Coulomb potentials and the auxiliary Hamiltonian method with the PBE exchange-correlation functional, along with DFT-D2 long-range dispersion corrections. The implicit solvation model CANDLE was used to describe the electrolyte with a 1M concentration.

Potential-dependent kinetics of oxygen chemisorption as the crucial step of oxygen reduction reaction: GCDFT study

Nitrogen-doped carbon materials (NCMs) are widely regarded as promising alternatives to expensive platinum-based electrocatalysts for the oxygen reduction reaction (ORR). While NCMs exhibit considerable electrochemical activity in alkaline media, their performance in acidic environments remains a significant challenge. However, acidic conditions are commercially desirable for ORR’s catalysis in proton-exchange membrane fuel cells (PEMFCs). The dramatic pH dependence of NCM effectiveness has sparked ongoing debate, with several factors under consideration, including surface protonation, variations in hydrogen binding energy, differences in proton donors, and interface structure. In this work, we present a grand canonical density functional theory (GCDFT) study of the chemisorption step on pristine and nitrogen-doped graphene. Through nudged elastic band (NEB) calculations at various electrode potentials, we propose a potential-dependent (and thus pH-dependent) mechanism of oxygen chemisorption at graphitic nitrogen (Ngr) defects, offering new insights into the pH dependency of the onset potential in NCM catalysts.

Modelling the activity trend of the hydrogen oxidation reaction under constant potential conditions.

A microkinetic model is constructed for the electrocatalytic alkaline hydrogen oxidation reaction based on grand canonical density functional theory calculations and linear relationships with the adsorption energies of hydrogen and hydroxide as descriptors. Using this model, the activity trend suitable for efficient catalyst screening has been identified.

Using ligand regulation, metal replacement, and ligand doping strategies on Zr-FUM to improve methane separation from coalbed gas

Developing adsorbents suitable for industrial applications that can effectively enhance the separation of methane (CH4) from nitrogen (N2) in coalbed gas is crucial to improve energy recovery and mitigate greenhouse gas emissions. In this study, three modification strategies were implemented on Zr-FUM, including ligand regulation, metal replacement, and ligand doping, to synthesize Zr-FDCA, Al-FUM, and Zr-FUM-FA, with the aim of improving the performance of CH4/N2 separation under humid conditions. The results demonstrated that the promotion of robust orbital overlap and strengthened electrovalent bonding on adsorbents can selectively enhance CH4 adsorption. As a result, Zr-FUM-FA achieved a saturated CH4 adsorption capacity of 1.37 mmol/g, a CH4 working window of 307 s, and a CH4/N2 sorbent selection parameter (Ssp) of 47.31, exceeding the performance of most reported adsorbents. Analyses of the pore structure, surface morphology, and functional groups revealed that the presence of an ultramicropore proximity to CH4, reduced static resistance, and enhanced electrovalent bond were key factors for CH4 separation. Grand Canonical Monte Carlo and Density Functional Theory studies indicated that the introduction of -C-H- in FA played a crucial role in enhancing CH4 adsorption. Optimization of adsorption parameters using the Aspen adsorption package showed that in a dual-adsorbent bed system, the recovery and purity of CH4 in Zr-FUM-FA reach 99.5% and 97.3%, respectively, providing important theoretical support for the improvement of CH4 recovery in the pressure swing adsorption process from coalbed gas.

Theoretical investigation of mixed-metal metal-organic frameworks as H2 adsorbents: insights from GCMC and DFT simulations

ABSTRACT Molecular hydrogen ( H 2 ) is a renewable energy carrier, however, its practical applications are limited due to the challenges of developing safe and efficient H 2 storage devices. Metal Organic Frameworks (MOFs) containing at least two different metal ions in their structures are called as mixed-metal MOFs (MM-MOFs) and they could adsorb H 2 in higher amounts compared to structures containing single metal nodes. We theoretically examined the H 2 storage capacities of 26 MM-MOFs having various physical and chemical properties applying Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) simulations. H 2 adsorption isotherms were calculated using a five-site anisotropic H 2 model. QIXSOG, YOMVIG, OSOYUR, Cu-Mg-BTC, Fe-Mg-BTC, and Cr-Mg-BTC were selected as top-performing MM-MOFs maximising H 2 adsorption gravimetrically and volumetrically at near-ambient conditions (233 K and 100 bar), approaching the DOE targets. YOMVIG has the largest H 2 adsorption enthalpy, calculated as − 9.93 kJ/mol at 233 K and 100 bar. DFT simulations have been conducted to analyse preferable H 2 adsorption sites as well as identify guest-host interactions. Electron density difference analysis showed that adsorbed H 2 molecules in the OSOYUR, Cr-Mg-BTC, Cu-Mg-BTC, and Fe-Mg-BTC are polarised. Our study challenges existing literature by identifying promising MM-MOFs as potential next-generation hydrogen storage adsorbents at near-ambient conditions.

Origin of copper dissolution under electrocatalytic reduction conditions involving amines.

Cu dissolution has been identified as the dominant process that causes cathode degradation and losses even under cathodic conditions involving methylamine. Despite extensive experimental research, our fundamental and theoretical understanding of the atomic-scale mechanism for Cu dissolution under electrochemical conditions, eventually coupled with surface restructuring processes, is limited. Here, driven by the observation that the working Cu electrode is corroded using mixtures of acetone and methylamine even under reductive potential conditions (-0.75 V vs. RHE), we employed Grand Canonical density functional theory to understand this dynamic process under potential from a microscopic perspective. We show that amine ligands in solution directly chemisorb on the electrode, coordinate with the metal center, and drive the rearrangement of the copper surface by extracting Cu as adatoms in low coordination positions, where other amine ligands can coordinate and stabilize a surface copper-ligand complex, finally forming a detached Cu-amine cationic complex in solution, even under negative potential conditions. Calculations predict that dissolution would occur for a potential of -1.1 V vs. RHE or above. Our work provides a fundamental understanding of Cu dissolution facilitated by surface restructuring in amine solutions under electroreduction conditions, which is required for the rational design of durable Cu-based cathodes for electrochemical amination or other amine involving reduction processes.

Influence of Simple Salts on Solvent Reduction Stability at Mg‐Alloy Anodes Interface: A Potential‐Dependent DFT Study

AbstractThe significance of incorporating anion species into electrolyte solvation structures, particularly with doubly charged Mg2 + ions, is investigated using the grand canonical density functional theory (GC‐DFT) approach. In an extension of previously established methodology, the work explores the thermodynamic stability in acetonitrile (AN) at the interface with Mg3Bi2 and Mg2Sn. Two different anions, TFSI− and , are strategically incorporated based on energy comparisons. Despite the known chemical compatibility of alloy anodes with the electrolyte solution, the research reveals a novel form of solvent degradation, which is also reported in the case of pure Mg anode with conventional electrolytes. Notably, the AN molecule adjacent to anion species exhibits reduced susceptibility to reduction (−0.8 – −0.4 V vs Mg2 +/Mg) in the lower potential range in comparison with the solvation structure of full dissociation. Charge density difference and density of states analyses detail solvent molecules becoming electrophilic, with the LUMO overlapping with the Fermi level at lower potentials when the electrostatic interaction with anion species are considered. Experimental studies using nuclear magnetic resonance (NMR) spectroscopy and linear sweep voltammetry (LSV) validate the theoretical results, providing a comprehensive understanding. This methodology, augmenting prior approaches, provides valuable guidance for electrolyte composition based on predominant solvation structures in multivalent solutions.

Destabilization of Oxygen Reduction Reaction Intermediates on a PGM-Free Electrocatalyst By Axial Spectator Ligands

Platinum-group free Fe-N-C catalysts have been shown to be a promising class of electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. However, typical synthesis methods can result in a variety of FeN4 sites embedded in graphene, including bulk-hosted and edge-hosted FeN4. The planar structure of the FeN4 active site allows two additional ligands to adsorb to Fe on either side of the graphene, which can be present from ORR reaction intermediates and other solution phase species. Here, we studied the impact of axially adsorbed ligands on bulk-hosted and edge-hosted FeN4 sites by screening over all pairs of H, O, OH, OOH, O2, H2O, NO, SO4, HSO4, and ClO4 axial ligands using solvated grand canonical density functional theory (GCDFT) to self-consistently account for applied potential. The first axial Fe adsorbate can strongly influence the binding of a second axial adsorbate and increase its adsorption energy by up to 2 eV, with more destabilization occurring for a more positive applied potential and the O* ORR intermediate being affected most. However, GCDFT calculations on these same sites without axial spectator ligands show that O* and OH* are both thermodynamically persistent, suggesting that axial spectator ligands might play a role in explaining this catalyst’s high experimental activity. Additionally, we show how the combined effects of potential, nature of the FeN4 site, and axial ligand impact O2 adsorption favorability and availability of sites in the presence of competing molecules. This work highlights the need for the continued development of advanced electrocatalyst models that account for a wide range of active site structures to facilitate comparison with experiment.

Rationalizing Acidic Oxygen Evolution Reaction over IrO2: Essential Role of Hydronium Cation.

The development of active, stable, and more affordable electrocatalysts for acidic oxygen evolution reaction (OER) is of great importance for the practical application of electrolyzers and the advancement of renewable energy conversion technologies. Currently, IrO2 is the only catalyst with high stability and activity, but a high cost. Further optimization of the catalyst is limited by the lack of understanding of catalytic behaviors at the acid-IrO2 interface. Here, in strong interaction with the experiment, we develop an explicit model based on grand-canonical density function theory (GC-DFT) calculations to describe acidic OER over IrO2. Compared to the explicit models reported previously, hydronium cations (H3O+) are introduced at the electrochemical interface in the current model. As a result, a variation in stable IrO2 surface configuration under the OER operating condition from previously proposed complete *O-coverage to a mixture coverage of *OH and *O is revealed, which is well supported by in situ Raman measurements. In addition, the accuracy of predicted overpotential is increased in comparison with the experimentally measured. More importantly, an alteration of the potential limiting step from previously identified *O → *OOH to *OH → *O is observed, which opens new opportunities to advance the IrO2-based catalysts for acidic OER.