Ab initio calculations of free-energy reaction barriers

From 2e- to 4e- pathway in the alkaline oxygen reduction reaction on Au(100): Kinetic circumvention of the volcano curve

We report the free energy barriers for the elementary reactions in the 2e- and 4e- oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e- pathway is clearly favored, while the barrier for the O-O dissociation channel is significantly higher at 0.5 eV. Above 0.7 V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O-O dissociation channel, leading to the 4e- pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0-1.2 V region (RHE). In contrast, the O-O dissociation barrier on Au(111) is significantly higher, at close to 0.9 eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements.

Catalytic Activity and Electrochemical Stability of Ru1-xMxO2 (M = Zr, Nb, Ta): Computational and Experimental Study of the Oxygen Evolution Reaction

We use computations and experiments to determine the effect of substituting zirconium, niobium, and tantalum within rutile RuO2 on the structure, oxygen evolution reaction (OER) mechanism and activity, and electrochemical stability. Calculated electronic structures altered by Zr, Nb, and Ta show surface regions of electron density depletion and accumulation, along with anisotropic lattice parameter shifts dependent on the substitution site, substituent, and concentration. Consistent with theory, X-ray photoelectron spectroscopy experiments show shifts in binding energies of O-2s, O-2p, and Ru-4d peaks due to the substituents. Experimentally, the substituted materials showed the presence of two phases with a majority phase that contains the metal substituent within the rutile phase and a second, smaller-percentage RuO2 phase. Our experimental analysis of OER activity shows Zr, Nb, and Ta substituents at 12.5 atom % induce lower activity relative to RuO2, which agrees with computing the average of all sites; however, Zr and Ta substitution at specific sites yields higher theoretical OER activity than RuO2, with Zr substitution suggesting an alternative OER mechanism. Metal dissolution predictions show the involvement of cooperative interactions among multiple surface sites and the electrolyte. Zr substitution at specific sites increases activation barriers for Ru dissolution, however, with Zr surface dissolution rates comparable to those of Ru. Experimental OER stability analysis shows lower Ru dissolution from synthesized RuO2 and Zr-substituted RuO2 compared to commercial RuO2 and comparable amounts of Zr and Ru dissolved from Zr-substituted RuO2, aligned with our calculations.

Role of Surface Features on the Initial Dissolution of CH3NH3PbI3 Perovskite in Liquid Water: An Ab Initio Molecular Dynamics Study

The degradation of CH3NH3PbI3 (MAPbI3) hybrid organic inorganic perovskite (HOIP) by water has been the major issue hampering its use in commercial perovskites solar cells (PSCs), as MAPbI3 HOIP has been known to easily degrade in the presence of water. Even though there have been numerous studies investigating this phenomenon, there is still no consensus on the mechanisms of the initial stages of dissolution. Here, we attempt to consolidate differing mechanistic interpretations previously reported in the literature through the use of the first-principles constrained ab initio molecular dynamics (AIMD) to study both the energetics and mechanisms that accompany the degradation of MAPbI3 HOIP in liquid water. By comparing the dissolution free energy barrier between surface species of different surficial types, we find that the dominant dissolution mechanisms of surface species vary widely based on the specific surface features. The high sensitivity of the dissolution mechanism to surface features has contributed to the many dissolution mechanisms proposed in the literature. In contrast, the dissolution free energy barriers are mainly determined by the dissolving species rather than the type of surfaces, and the type of surfaces the ions are dissolving from is inconsequential toward the dissolution free energy barrier. However, the presence of surface defects such as vacancy sites is found to significantly lower the dissolution free energy barriers. Based on the estimated dissolution free energy barriers, we propose that the dissolution of MAPbI3 HOIP in liquid water originates from surface defect sites that propagate laterally along the surface layer of the MAPbI3 HOIP crystal.

Density functional theory and ab initio molecular dynamics reveal atomistic mechanisms for carbonate clumped isotope reordering

Carbon (¹³C) and oxygen (¹⁸O) isotopes in carbonates form clumped isotope species inversely correlated with temperature, providing a valuable paleothermometer for sedimentary carbonates and fossils. However, this signal resets ("reorders") with increasing temperature after burial. Research on reordering kinetics has characterized reordering rates and hypothesized the effects of impurities and trapped water, but the atomistic mechanism remains obscure. This work studies carbonate-clumped isotope reordering in calcite via first-principles simulations. We developed an atomistic view of the isotope exchange reaction between carbonate pairs in calcite, discovering a preferred configuration and elucidating how Mg²⁺ substitution and Ca²⁺ vacancies lower the free energy of activation (ΔA^‡) compared to pristine calcite. Regarding water-assisted isotopic exchange, the H⁺-O coordination distorts the transition state configuration and reduces ΔA^‡. We proposed a water-mediated exchange mechanism showing the lowest ΔA^‡ involving a reaction pathway with a hydroxylated four-coordinated carbon atom, confirming that internal water facilitates clumped isotope reordering.

Anharmonic Correction to Free Energy Barriers from DFT-Based Molecular Dynamics Using Constrained Thermodynamic Integration

For the calculation of anharmonic contributions to free energy barriers, constrained thermodynamic λ-path integration (λ-TI) from a harmonic reference force field to density functional theory is presented as an alternative to the established Blue Moon ensemble method (ξ-TI), in which free energy gradients along the reaction coordinate ξ are integrated. With good agreement in all cases, the λ-TI method is benchmarked against the ξ-TI method for several reactions, including the internal CH₃ group rotation in ethane, a nucleophilic substitution of CH₃Cl, a retro-Diels-Alder reaction, and a proton transfer in zeolite H-SSZ-13. An advantage of λ-TI is that one can use virtually any reference state to compute anharmonic contributions to reaction free energies or free energy barriers. This is particularly relevant for catalysis, where it is now possible to compute anharmonic corrections to the free energy of a transition state relative to any reference, for example, the most stable state of the active site and the reactants in the gas phase. This is in contrast to ξ-TI, where free energy barriers can only be computed relative to an initial state with all reactants coadsorbed. Finally, the Bennett acceptance ratio method combined with λ-TI is demonstrated to reduce the number of required integration grid points with tolerable accuracy, favoring thus λ-TI over ξ-TI in terms of computational efficiency.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO₂ catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO₂ support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO₂. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO₂, which can be crucial for supported metal catalysts in biomass conversion.

Uncovering the Electrolyte-Dependent Transport Mechanism of LiO2 in Lithium-Oxygen Batteries

Lithium-oxygen batteries (LOBs) offer extremely high theoretical energy density and are therefore strong contenders for bringing conventional batteries into the next generation. To avoid deactivation and passivation of the electrode due to the gradual covering of the surface by discharge products, electrolytes with high donor number (DN) are becoming increasingly popular in LOBs. However, the mechanism of this electrolyte-assisted discharge process remains unclear in many aspects, including the lithium superoxide (LiO₂) intermediate transportation mechanism and stability at both electrode/electrolyte interfaces and in bulk electrolytes. Here, we performed a systematic Born-Oppenheimer molecular dynamics (BOMD)-level investigation of the LiO₂ solvation reactions at two interfaces with high- or low-DN electrolytes (dimethyl sulfoxide (DMSO) or acetonitrile (CH₃CN), respectively), followed by examinations of stability and condensation once the LiO₂ monomers are solvated. Release of partial discharge product LiO₂ is found to be energetically favorable into DMSO from the Co₃O₄ cathode with a small energy barrier. However, in the presence of CH₃CN electrolyte, the release of LiO₂ from the electrode surface is found to be energetically unfavorable. Dissolved LiO₂(sol) clusters in bulk DMSO solvents are found to be more favorable to dimerize and agglomerate into a toroidal shape rather than to decompose, which avoids the emergence of strong oxidant ions (O₂^-) and preserves the system stability. This study provides two complete molecular-level pathways (solution and surface) from first-principles understanding of LOBs, offering guidance for future selection and design of electrode catalysts and solvents.

OH-···Au Hydrogen Bond and Its Effect on the Oxygen Reduction Reaction on Au(100) in Alkaline Media

Using ab initio molecular dynamics simulations with fully solvated ions, we demonstrate that solvated OH^- forms a stable hydrogen bond with Au(100). Unlike the hydrogen bond between H₂O and Au reported previously, which is more favorable for negatively charged Au, the OH^-···Au interaction is stabilized when a small positive charge is added to the metal slab. For electro-catalysis, this means that while OH₂···Au plays a significant role in the hydrogen evolution reaction, OH^-···Au could be a significant factor in the oxygen reduction reaction in alkaline media. It also points to a fundamental difference in the mechanism of oxygen reduction between gold and platinum electrodes.

Cross-Sphere Electrode Reaction: The Case of Hydroxyl Desorption during the Oxygen Reduction Reaction on Pt(111) in Alkaline Media

Hydroxide ion is a common electrolyte when electrode reactions take place in alkaline media. In the case of oxygen reduction reaction on Pt(111), we demonstrate by ab initio molecular dynamics calculations that the desorption of hydroxyl (OH*) from the electrode surface to form a solvated OH^- is a cross-sphere process, with the OH* reactant in the inner sphere and the OH^- product directly generated in the aqueous outer sphere. Such a mechanism is distinct from the typical inner sphere and outer sphere reactions. It is dictated by the strong hydrogen bonding interactions between a hydroxide ion and water molecules and is facilitated by proton transfer through solvation layers. It should play a significant role whenever OH* desorption, or its reverse, OH^- adsorption, is involved in an electrochemical reaction.

Mechanistic Study of IrO2 Dissolution during the Electrocatalytic Oxygen Evolution Reaction

Understanding the mechanistic interplay between the activity and stability of water splitting electrocatalysts is crucial for developing efficient and durable water electrolyzers. Ir-based materials are among the best catalysts for the oxygen evolution reaction (OER) in acidic media, but their degradation mechanisms are not completely understood. Here, through first-principles calculations we investigate iridium dissolution at the IrO₂(110)/water interface. Simulations reveal that the surface-bound IrO₂OH species formed upon iridium dissolution should be thermodynamically stable in a relatively wide potential window undergoing transformations into Ir^VI (as IrO₃) at high anodic potentials and Ir^III (as Ir(OH)₃) at low anodic potentials. The identified high-valence surface-bound dissolution intermediates of Ir are determined to display greater OER activities than the pristine IrO₂(110) surface in agreement with the experimentally observed high activity of an amorphous hydrated IrO_x surface layer. Combined with recent experimental results, our simulations illuminate the mechanistic details of the degradation mechanism of IrO₂ and how it couples to electrocatalytic OER.

Ab Initio Modeling of Transition Metal Dissolution from the LiNi0.5Mn1.5O4 Cathode

Irreversible dissolution of transition metals (TMs) from cathode materials in lithium-ion batteries (LIBs) represents a serious challenge for the application of high-energy-density LIBs. Despite substantial improvements achieved by Ni doping of the LiMn₂O₄ spinel, the promising high-voltage LiNi_0.5Mn_1.5O₄ (LNMO) cathode material still suffers from the loss of electro-active materials (Mn and Ni). This process contributes to the formation of solid-electrolyte interfaces and capacity loss severely limiting the battery life cycle. Here, we combine static and ab initio molecular dynamics free energy calculations based on the density functional theory to investigate the mechanism and kinetics of TM dissolution from LNMO into the liquid organic electrolyte. Our calculations help deconvolute the impact of various factors on TM dissolution rates such as the presence of surface protons and oxygen vacancies and the nature of TMs and electrolyte species. The present study also reveals a linear relationship between adsorption strength of the electrolyte species and TM dissolution barriers that should help design electrode/electrolyte interfaces less vulnerable to TM dissolution.

Carbon dioxide capture in 2,2'-iminodiethanol aqueous solution from ab initio molecular dynamics simulations

The reaction of carbon dioxide (CO₂) with aqueous 2,2'-iminodiethanol (trivial name is diethanolamine: DEA) has been investigated using both blue moon ensemble and metadynamics approaches combined with ab initio molecular dynamics (AIMD) simulations. A spontaneous direct proton transfer from DEA zwitterion (DEAZW) to DEA but not to H₂O has been observed in straightforward AIMD simulation in the time scale of ps. The ab initio free-energy calculations reproduced the overall free-energy difference, predicting the ionic products DEA carbamate ion (DEAC) and the protonated DEA (DEAH). The computed free-energy barrier for the first reaction step, which is the CO₂ binding (48 kJ mol^-1), is found to agree reasonably well with the available experimental data (52-56 kJ mol^-1). By contrast, the barriers for the next step, the deprotonation of zwitterion realized either via reaction with DEA or H₂O, are underestimated by 25-35 kJ mol^-1 compared to the experimental reference. A part of this error is attributed to the neglected reversible work needed to bring two reactants together, which might significantly contribute to the free-energy of activation of bimolecular reactions in a dilute solution. The computed free-energy profile is compared with our results [Y. Kubota et al., J. Chem. Phys. 146, 094303 (2017)] for the same reaction in 2-aminoethanol (trivial name is monoethanolamine: MEA).

Water Adsorption and Insertion in MOF-5

The high surface areas and tunable properties of metal-organic frameworks (MOFs) make them attractive materials for applications in catalysis and the capture, storage, and separation of gases. Nevertheless, the limited stability of some MOFs under humid conditions remains a point of concern. Understanding the atomic-scale mechanisms associated with MOF hydrolysis will aid in the design of new compounds that are stable against water and other reactive species. Toward revealing these mechanisms, the present study employs van der Waals-augmented density functional theory, transition-state finding techniques, and thermodynamic integration to predict the thermodynamics and kinetics of water adsorption/insertion into the prototype compound, MOF-5. Adsorption and insertion energetics were evaluated as a function of water coverage, while accounting for the full periodicity of the MOF-5 crystal structure, that is, without resorting to cluster approximations or structural simplifications. The calculations suggest that the thermodynamics of MOF hydrolysis are coverage-dependent: water insertion into the framework becomes exothermic only after a sufficient number of H₂O molecules are coadsorbed in close proximity on a Zn-O cluster. Above this coverage threshold, the adsorbed water clusters facilitate facile water insertion via breaking of Zn-O bonds: the calculated free-energy barrier for insertion is very low, 0.17 eV at 0 K and 0.04 eV at 300 K. Our calculations provide a highly realistic description of the mechanisms underlying the hydrolysis of MOFs under humid working conditions.

Ab initio calculations for industrial materials engineering: successes and challenges

Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO(2) junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.