Activation Barrier For Diffusion Research Articles

First-principles calculations to investigate the impact of fluorine doping on electrochemical properties of Li-rich Li2MnO3 layered cathode materials.

Li-rich layered oxides are promising candidates for high-capacity Li-ion battery cathode materials. In this study, we employ first-principles calculations to investigate the effect of F doping on Li-rich Li2MnO3 layered cathode materials. Our findings reveal that both Li2MnO3 and Li2MnO2.75F0.25 exhibit significant volume changes (greater than 10%) during deep delithiation, which could hinder the cycling of more Li ions from these two materials. For Li2MnO3, it is observed that oxygen ions lose electrons to compensate for charge during the delithiation process, leading to a relatively high voltage plateau. After F doping, oxidation occurs in both the cationic (Mn) and anionic (O) components, resulting in a lower voltage plateau at the beginning of the charge, which can be attributed to the oxidation of Mn3+ to Mn4+. Additionally, F doping can somewhat suppress the release of oxygen in Li2MnO3, improving the stability of anionic oxidation. However, the increase of the activation barriers for Li diffusion can be observed after F doping, due to stronger electrostatic interactions between F- and Li+, which adversely affects the cycling kinetics of Li2MnO2.75F0.25. This study enhances our understanding of the impact of F doping in Li2MnO3 based on theoretical calculations.

First principles study on the performance of Sn&F double-doped LiCoO2 in lithium-ion batteries

The demand for LIBs using LiCoO2 as a cathode material with higher energy density and improved safety is urgent. We have carried out a theoretical investigation of Sn&F double-doped LiCoO2 using first-principles calculations. The Sn&F double-doped LiCoO2 system, LiCo0.96Sn0.04O0.98F0.02, was modelled using a 3 × 3 × 1 supercell of LiCoO2 containing a pair of Co-O vacancies, with one O atom and one Co atom replaced by F and Sn, respectively. Our study shows that Sn&F doping contributes to a reduction in volume variation and inhibits Co migration during delithiation. Furthermore, Sn&F doping is shown to reduce the average activation barrier for Li diffusion.

A density functional theory study of defective and doped structures of MgB2 and their interaction with hydrogen

The LiBH4+MgH2 system exhibits promising potential for solid-state hydrogen storage, yet the sluggish rehydrogenation of MgB2 poses a significant challenge. In this study, we utilize density functional theory (DFT) simulations to investigate the energetics of hydrogen in pure, defective, and doped MgB2 in equilibrium with molecular hydrogen. Our findings reveal that the B-Frenkel defect process is the thermodynamically most favorable in MgB2. The calculated solution energy for hydrogen at the interstitial site indicates low hydrogen solubility, even at mild temperatures. Incorporating hydrogen into a pre-existing B vacancy enhances the process, facilitated by doping of O on the B site. Additionally, Li doping on the Mg site enhances incorporation by forming strong Li–H bonds, predicting lower activation barriers for hydrogen dissociation and diffusion. Doping MgB2 with O and F at the B site significantly enhances hydrogen solubility. These dopants make MgB2 a promising material for high-capacity and fast-kinetics hydrogen storage applications, and further research into these effects can lead to advancements in energy storage technology.

Regulation of d‐Orbital Electron in Fe‐N4 by High‐Entropy Atomic Clusters for Highly Active and Durable Oxygen Reduction Reaction

AbstractSimultaneously improving activity and stability is a crucial yet challenge in the development of metallic single‐atom‐based catalysts. In current work, a novel approach is introduced to address this issue by combining post‐adsorption and secondary pyrolysis techniques to create a synergistic catalytic system, in which the single atoms (SAs) Fe sites played in the NC matrix (Fe─NC) are coupled with high‐entropy atomic clusters (HEACs). Theoretical calculations reveal that the incorporation of HEACs lead to a rehybridization of the 3d orbital configuration of Fe‐N4, which helps to balance the adsorption/desorption energy of oxygenated intermediates. In situ spectroscopy further reveals that the rate‐limiting step of OH* desorption on HEAC/Fe─NC in oxygen reduction reaction (ORR) is more facile compared to atomic Fe─NC, implying a higher ORR activity. Moreover, the synergistic effect of diffusion activation barriers and configuration entropy contributes to the structural stability of HEAC/Fe─NC, resulting in remarkable durability. Consequently, this unique catalyst exhibits half‐wave potentials of 0.927 and 0.828 V in an aqueous solution of KOH (0.1 m) and HClO4 (0.1 m), respectively, along with excellent durability. The findings propose a novel strategy for modulating the electronic structure of metallic SAs catalysts and enhancing their stability through strong interactions between SAs and HEACs.

Hydrogen absorption in an epitaxial thin film of high-entropy perovskite oxide

We synthesized an epitaxial film of high-entropy perovskite oxide (HEPO) consisting of three elements (Ca, Sr, Ba) in the A site and 12 elements (Si, Ti, Cr, Mn, Fe, Co, Ni, Ge, Zr, Sn, Ce, Hf) in the B site of ABO3, and investigated hydrogen absorption properties in the HEPO film. The hydrogen depth profile was measured by nuclear reaction analysis via the 1H(15N,αγ)12C reaction, showing the hydrogen absorption in the HEPO film with a maximum atomic concentration of 0.3 in the ABO3 unit. The diffusion coefficient of H in the film was analyzed from the H depth profile, and the activation barrier for diffusion was estimated to be (0.54±0.13) eV. An absorption band was observed at 3290 cm−1 in the infrared absorption spectrum, which corresponds to the O–H stretching vibration. Simultaneous measurements of nuclear reaction and resistance revealed that the HEPO film remained electrically insulating regardless of the hydrogen concentration. We demonstrated that hydrogen is incorporated in HEPO forming OH species while keeping HEPO electrically insulating.

First-principles evaluation of transition metal dichalcogenide-graphene pairs functionalized with oxygen-containing groups for sodium-ion battery anodes.

Composites of reduced graphene oxides (rGOs) with transition metal dichalcogenides have garnered considerable attention as promising anode materials for sodium-ion batteries (SIBs) because of their superior theoretical capacity and long-term stability compared with pure graphene. However, the underlying mechanism of how the oxygen functional groups improve the functionality of rGO remains unclear. In this study, we investigated the roles of functional groups in rGO-based heterogeneous bilayers using first-principles density functional theory calculations. The thermodynamic affinities to Na atoms, kinetic diffusion, and working potential behaviors of the Na atoms in various models, such as MoS2/graphene (Gr), MoS2/Gr-O, MoS2/Gr-OH, and MoS2/Gr-COOH, were accurately evaluated. It is clearly demonstrated that the noncovalent bonding nature is a predominant descriptor of Na affinity rather than the charge distribution around the intercalated Na atom. The activation barriers for Na atomic diffusion in the MoS2/Gr, MoS2/Gr-O, MoS2/Gr-OH, and MoS2/Gr-COOH models were estimated to be 0.24, 0.27, 0.35, and 0.31 eV, respectively. This indicated that the functional groups slightly delayed the Na motion. Notably, the obtained results demonstrated that the -COOH group not only enhanced the affinity towards Na intercalation but also induced a low working voltage at approximately 1 V. Therefore, the carboxyl functional group exhibits high material stability, making rGO a promising candidate for SIB anode materials.

Diffusion of Lanthanum/Cerium/Manganese in Rare‐Earth Heavy Rail Steel at the Refractory Interface

One of the key factors determining the degree of erosion in the outlet parts of refractory interfaces is diffusion. By studying the representative diffusion behavior of lanthanum, cerium, and manganese in molten rare‐earth high‐carbon heavy rail steel using different refractory interfaces, the diffusion kinetics mechanism of each alloying element is illustrated. The results show that the concentration of diffused lanthanum, cerium, and manganese in rare‐earth high‐carbon heavy rail steel imparts minor changes on the two aluminum–magnesium refractories used in our experiments, whereas the other four refractories show unstable behavior. Furthermore, it is revealed that the smaller the porosity of a refractory material, the greater the decrease in the diffusion channel of each element in molten steel, which in turn increases the diffusion activation barrier and results in slower diffusion. Overall, the suitability degree of the refractory materials tested in experiments for processing molten rare‐earth high‐carbon heavy rail steel with regard to cost, adhesion strength, and erosion degree is as follows: Al2O3–MgO > Al2O3–MgO–SiO2 coating > Al2O3–SiO2–SiO2 coating > Al2O3–SiO2 > Al2O3 > ZrO2.

Diffusion zone evolution around growing crystals in combeite glass

AbstractThe precipitation of nonstoichiometric crystals (of different composition from the parent glass) motivated us to study the stoichiometric Na2O·2CaO·3SiO2 ( N2CS3) glass‐forming system comprehensively. The concentration profiles of Na and Ca in the diffusion zone around Na4+2xCa4−x[Si6O18], , crystals in an N2C3S glass were characterized by energy‐dispersive spectroscopy and were then theoretically described. We show that the diffusion zone is formed because of the rejection of part of calcium by the growing crystal. The radial variation of the Ca concentrations and the crystal nucleation rate through the diffusion zone were analyzed simultaneously. We found that the nucleation rate in the diffusion zone near the crystal–glass interface drastically decreases due to the increase in the work of critical cluster formation resulting from the composition change. In contrast, the diffusion activation barrier remains practically constant. Moreover, the Ca diffusion coefficients, DCa, in Na2O·2CaO·3SiO2 and CaO·SiO2 glasses show the same Arrhenius dependence, demonstrating that (surprisingly) DCa does not depend on the glass composition in the CaO·SiO2–Na2O·SiO2 metasilicate joint. This work provides the quantitative spatial distribution of chemical components in a partially crystallized glass, shedding light on solid solution crystallization in a glass‐forming liquid and its consequences.

Understanding Ionic Diffusion Mechanisms in Li2S Coatings for Solid-State Batteries: Development of a Tailored Reactive Force Field for Multiscale Simulations.

In order to investigate Li2S as a potential protective coating for lithium anode batteries using superionic electrolytes, we need to describe reactions and transport for systems at scales of >10,000 atoms for time scales beyond nanoseconds, which is most impractical for quantum mechanics (QM) calculations. To overcome this issue, here, we first report the development of the reactive analytical force field (ReaxFF) based on density functional theory (DFT) calculations on model systems at the PBE0/TZVP and M062X/TZVP levels. Then, we carry out reactive molecular dynamics simulations (RMD) for up to 20 ns to investigate the diffusion mechanisms in bulk Li2S as a function of vacancy density, determining the activation barrier for diffusion and conductivity. We show that RMD predictions for diffusion and conductivity are comparable to experiments, while results on model systems are consistent with and validated by short (10-100 ps) ab initio molecular dynamics (AIMD). This new ReaxFF for Li2S systems enables practical RMD on spatial scales of 10-100 nm (10,000 to 10 million atoms) for the time scales of 20 ns required to investigate predictively the interfaces between electrodes and electrolytes, electrodes and coatings, and coatings and electrolytes during the charging and discharging processes.

Microscopic Diffusion of Atomic Hydrogen and Water in HER Catalyst MoS2 Revealed by Neutron Scattering.

The design of novel and abundant catalytic materials for electrolysis is crucial for reaching carbon neutrality of the global energy system. A deliberate approach to catalyst design requires both theoretical and experimental knowledge not only of the target reactions but also of the supplementary mechanisms affecting the catalytic activity. In this study, we focus on the interplay of hydrogen mobility and reactivity in the hydrogen evolution reaction catalyst MoS2. We have studied the diffusion of atomic hydrogen and water by means of neutron and X-ray photoelectron spectroscopies combined with classical molecular dynamics simulations. The observed interaction of water with single-crystal MoS2 shows the possibility of intercalation within volume defects, where it can access edge sites of the material. Our surface studies also demonstrate that atomic hydrogen can be inserted into MoS2, where it then occupies various adsorption sites, possibly favoring defect vicinities. The motion of H atoms parallel to the layers of MoS2 is fast with D ≈ 1 × 10-9 m2/s at room temperature and exhibits Brownian diffusion behavior with little dependence on temperature, i.e., with a very low diffusion activation barrier.

Thermodynamics and Kinetics of the Cathode-Electrolyte Interface in All-Solid-State Li-S Batteries.

Lithium–sulfur batteries (LSBs) are among the most promising energy storage technologies due to the low cost and high abundance of S. However, the issue of polysulfide shuttling with its corresponding capacity fading is a major impediment to its commercialization. Replacing traditional liquid electrolytes with solid-state electrolytes (SEs) is a potential solution. Here, we present a comprehensive study of the thermodynamics and kinetics of the cathode–electrolyte interface in all-solid-state LSBs using density functional theory based calculations and a machine learning interatomic potential. We find that among the major solid electrolyte chemistries (oxides, sulfides, nitrides, and halides), sulfide SEs are generally predicted to be the most stable against the S8 cathode, while the other SE chemistries are predicted to be highly electrochemically unstable. If the use of other SE chemistries is desired for other reasons, several binary and ternary sulfides (e.g., LiAlS2, Sc2S3, Y2S3) are predicted to be excellent buffer layers. Finally, an accurate moment tensor potential to study the S8|β-Li3PS4 interface was developed using an active learning approach. Molecular dynamics (MD) simulations of large interface models (>1000s atoms) revealed that the most stable Li3PS4(100) surface tends to form interfaces with S8 with 2D channels and lower activation barriers for Li diffusion. These results provide critical new insights into the cathode–electrolyte interface design for next-generation all-solid-state LSBs.

Low coverage disordered decanethiol monolayers on Au(001): A conjecture regarding the formation of Au-adatom-molecule complexes

We present a scanning tunneling microscopy study of decanethiol on Au(001) in the low coverage regime. As expected, the hex reconstruction is lifted, however, no ordered decanethiol phases form. We observe large areas free of Au islands and covered with a disordered decanethiol phase. I(t) spectroscopy measurements suggest that this disordered phase is dynamic and most likely comprises diffusing Au adatoms, decanethiol molecules, and/or Au-adatom-decanethiol molecule complexes. We have performed density functional theory calculations and show that the activation barrier for diffusion is lower when Au-adatom-molecule complexes are considered in comparison to the case of bare molecule. These findings suggest that although no vacancy pits form on Au(001), Au-adatoms expelled during the lifting of the hex reconstruction may be still important for the diffusion of thiol molecules on this surface.

Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites

Mass transport plays an important role in zeolite catalyzed reactions and catalyst deactivation, yet experimental measurement of mass transport, particularly ultra-slow diffusion processes (e.g., of bulky aromatics), is limited because of time scale restraints. Here, we use density functional theory to overcome these limitations and calculate diffusion barriers of benzene and all twelve C 7 –C 12 methylbenzenes through the straight and sinusoidal channels of silicalite-1 (MFI framework). Straight and sinusoidal diffusion barriers are well-predicted by a critical diameter describing the minimum width of the molecule, where benzene, toluene, and para -xylene (all 6.6 Å) diffuse through both channels with barriers 200 kJ mol −1 lower than those of pentamethylbenzene (8.2 Å). The MFI framework distorts to accommodate species with larger critical diameters and this distortion correlates to activation barriers where smaller molecules, such as benzene, distort the framework to smaller extents compared to larger species, such as pentamethylbenzene. Diffusing through the straight channel of MFI is always more facile than via the sinusoidal channel, by an average of 39 kJ mol −1 because the tortuosity of the sinusoidal channels forces larger framework distortions than straight channel diffusion. We show that DFT-calculated straight channel diffusion activation barriers agree well with those reported by frequency response experiments and can be used to calculate self-diffusivities of molecules, with appropriate entropy corrections. Examining all aromatics provides insights to the role of molecule size, channel tortuosity, and entropy during intracrystalline diffusion to provide a reference point for the species that can reasonably diffuse through both channels (e.g., benzene, toluene, xylenes, durene), through straight channels only (e.g., 1,2,3-trimethylbenzene), or simply are ‘stuck’ within intersections (e.g., pentamethylbenzene) in MFI. • Diffusion barriers trend with critical diameter where aromatics with smaller critical diameters diffuse with lower barriers. • MFI distorts to accommodate aromatic diffusion and the extent of this distortion is predicted by critical diameter. • Diffusing through the straight channel occurs with barriers, on average, 39 kJ mol −1 lower than sinusoidal diffusion.

Sorption and Diffusion of Methane, Carbon Dioxide, and Their Mixture in Amorphous Polyethylene at High Pressures and Temperatures

Molecular dynamics (MD) simulations are performed to study the sorption and transport properties of CH4 and CO2 in amorphous polyethylene at temperatures from 350 to 600 K and pressures up to 500 bar. The uptake of CH4 and CO2 by polyethylene generally increased with increasing pressure and decreasing temperature. However, at high pressures, for example, the uptake of methane by polyethylene increases with temperature. The self-diffusion coefficients of methane and carbon dioxide generally increase with pressure. These results are, in general, consistent with the swelling behavior of the polymer. Interestingly, for the penetrants, the activation barrier of diffusion decreases with pressure. MD simulations are also carried out for the CH4/CO2 mixture in amorphous polyethylene. Here, the overall sorption and transport properties were similar to those reported for pure CH4 and pure CO2 in polyethylene. The sorption selectivity of CO2/CH4 decreases with increasing pressure and temperature and was mostly independent of the bulk mole fraction of methane. Importantly, at high pressures, the mobility of methane found here is higher than that of the corresponding pure methane in polyethylene and the opposite trend is observed in the case of carbon dioxide. These results might be due to the fact that the swelling of the polymer in the presence of carbon dioxide is significantly higher than that in the presence of methane, especially at high pressures. The diffusion and membrane selectivities of carbon dioxide/methane show a similar trend to the sorption selectivity data. Furthermore, the simulation data were in good agreement with the theoretical calculations based on the PC-SAFT equation of state.

Effects of structural characteristics of Cu grain boundaries on graphene growth

The grain structure of graphene significantly affects its properties; therefore, controlling the grain structure is crucial for the application of graphene. Previous investigations have shown inconsistent results regarding the correlation between the grain structures of graphene and the underlying Cu. We investigated the origin of this discrepancy by analyzing the effects of the structural characteristics of the Cu grain boundaries on graphene growth in polycrystalline Cu micropatterns defined by a lithographically patterned SiO2 mask layer. The number densities of the graphene grains are statistically characterized to be smaller than those of Cu grains, suggesting that graphene nuclei grow across some Cu grain boundaries without changing their orientations. It is shown that such growth is more likely to occur across low-angle and Σ3 twin boundaries, while random high-angle grain boundaries can significantly affect the growth rate and edge orientation of graphene islands, presumably leading to the correlation between the grain structures of Cu and graphene. It is suggested that this difference is the origin of the abovementioned discrepancy and is explained in terms of deeper grooves with a large number of atomic steps in random high-angle grain boundaries, which increase the diffusion distance and activation barrier for the surface diffusion of carbon.

Influence of interactions between hydrogen and [formula omitted] twin boundary on hydrogen embrittlement in α-Ti

Ti and Ti alloys are used in several areas thanks to their good mechanical properties, however, the large affinity of hydrogen during industrial application would limit their service life. This study serves as a comprehensive investigation of the embrittlement effects induced by additional hydrogen in detail by means of first-principles calculations with the Pseudopotential Plane-wave method within the framework of the Rice-Wang thermodynamic model. The existence of the twin boundary markedly influences segregation energy, the activation barriers for diffusion between sites, and intergranular embrittlement. We demonstrate that segregation energy and the barriers in the region near the boundary are consistently lower than in bulk, indicating that the sites located at the boundary are more attractive to hydrogen than bulk. Further, the intergranular strength of a Ti grain boundary is reduced with hydrogen segregation. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the weakening effect of hydrogen is due to the breaking of the interfacial Ti-Ti bonds, which significantly decrease twin boundary adhesion and its resistance to cracking.

Stability of heterogeneous single-atom catalysts: a scaling law mapping thermodynamics to kinetics

Heterogeneous single-atom catalysts (SACs) hold the promise of combining high catalytic performance with maximum utilization of often precious metals. We extend the current thermodynamic view of SAC stability in terms of the binding energy (Ebind) of single-metal atoms on a support to a kinetic (transport) one by considering the activation barrier for metal atom diffusion. A rapid computational screening approach allows predicting diffusion barriers for metal–support pairs based on Ebind of a metal atom to the support and the cohesive energy of the bulk metal (Ec). Metal–support combinations relevant to contemporary catalysis are explored by density functional theory. Assisted by machine-learning methods, we find that the diffusion activation barrier correlates with (Ebind)2/Ec in the physical descriptor space. This diffusion scaling-law provides a simple model for screening thermodynamics to kinetics of metal adatom on a support.

Nb-TiCo multiphase alloys: The significant impact of Ti/Co ratio on solidification path, microstructure and hydrogen permeability

Nb-TiCo multiphase alloys are prospective membrane materials for hydrogen permeation. Their composition, such as the initial Nb content, has a great influence on their hydrogen permeability (Φ). What the research hasn’t actually mentioned, however, is whether other elements (e.g. Ti or Co) could affect their hydrogen permeability. To address this question, the effects of the Ti/Co ratio on their solidification path, microstructure and hydrogen permeability have been comprehensively investigated. The results show that the Ti/Co ratio has a significant effect on the solidification direction along the eutectic valley but has little effect on the solidification structure. As the Ti/Co ratio decreases, the volume fraction of the BCC-Nb phase decreases whereas the eutectic phases increase. With these changes, the Φ value increases gradually, which is closely connected to the increasing hydrogen diffusion coefficient. Typically, the Φ value of Nb45Ti25Co30 is 1.03 (∼1.08) times higher than that of Nb45Ti27.5Co27.5 (Nb45Ti30Co25). This can be attributed to a decrease in the hydrogen diffusion activation barrier and a smaller hydrogen adsorption energy. The present work demonstrates that Nb-TiCo multiphase alloys with higher Φ values can still be exploited by a proper optimum design for the composition, such as the Ti/Co ratio.

Mixed Ionic–Electronic Conductor of Perovskite LixLayMO3−δ toward Carbon‐Free Cathode for Reversible Lithium–Air Batteries

AbstractMixed ionic–electronic conductors (MIECs) can play a pivotal role in achieving high energies and power densities in rechargeable batteries owing to their ability to simultaneously conduct ions and electrons. Herein, a new strategy is proposed wherein late 3d transition metals (TMs) are substituted into a perovskite Li‐ion conductor to transform it into a Li‐containing MIEC. First‐principles calculations show that perovskite LixLayMO3 with late 3d TMs have a low oxygen vacancy formation energy, implying high electron carrier concentrations corresponding to high electronic conductivity. The activation barriers for Li diffusion in LixLayMO3 (M = Ti, Cr, Mn, Fe, and Co) are below 0.411 eV, resulting in high Li‐ion conductivity. The designed perovskites of Li0.34La0.55MnO3−δ experimentally prove to have high electronic (2.04 × 10−3 S cm−1) and Li‐ion (8.53 × 10−5 S cm−1) conductivities, and when applied in a carbon‐free cathode of a Li–air cell, they deliver superior reversibility at 0.21 mAh cm−2 over 100 charge/discharge cycles while avoiding the degradation associated with carbonaceous materials. This strategy enables the effective design of Li‐conducting MIEC and reversible Li–air batteries.

Effects of Oxygen Adsorption on Morphological Evolution of Epitaxial Ag Island Films Grown on Si

This study presents results showing that oxygen adsorption can significantly affect the morphological evolution of silver island films epitaxially grown on Si(100) and Si(111) substrates. The effects of oxygen adsorption are characterized by analyzing the morphologies formed during the thermal annealing of the island films under different annealing gas compositions. Silver island films are shown to evolve into an isotropic spherical shape in the presence of oxygen, suggesting that oxygen adsorption decreases the surface energy anisotropy of silver, which is supported by our density function theory calculation results. Oxygen is also observed to enhance the rate of dewetting and coarsening of the films, leading to a smaller areal coverage and smaller number density of silver particles. This is explained by the decrease in the activation barrier for the surface diffusion of silver adatoms and their clusters on silver and silicon surfaces with chemisorbed oxygen.