Aqueous amines are currently the most promising solution for large-scale CO2 capture from industrial sources. However, molecular design and optimization of amine-based solvents have proceeded slowly due to a lack of understanding of the underlying reaction mechanisms. Unique and unexpected reaction mechanisms involved in CO2 absorption into aqueous hydrazine are identified using 1H, 13C, and 15N NMR spectroscopy combined with first-principles quantum-mechanical simulations. We find production of both hydrazine mono-carbamate (NH2-NH-COO−) and hydrazine di-carbamate (−OOC-NH-NH-COO−), with the latter becoming more populated with increasing CO2 loading. Exchange NMR spectroscopy also demonstrates that the reaction products are in dynamic equilibrium under ambient conditions due to CO2 exchange between mono-carbamate and di-carbamate as well as fast proton transfer between un-protonated free hydrazine and mono-carbamate. The exchange rate rises steeply at high CO2 loadings, enhancing CO2 release, which appears to be a unique property of hydrazine in aqueous solution. The underlying mechanisms of these processes are further evaluated using quantum mechanical calculations. We also analyze and discuss reversible precipitation of carbamate and conversion of bicarbonate to carbamates. The comprehensive mechanistic study provides useful guidance for optimal design of amine-based solvents and processes to reduce the cost of carbon capture. Moreover, this work demonstrates the value of a combined experimental and computational approach for exploring the complex reaction dynamics of CO2 in aqueous amines.

2-Amino-2-methyl-1-propanol (AMP), a sterically hindered amine, exhibits a much higher CO2 absorption rate relative to tertiary amine diethylethanolamine (DEEA), while both yield bicarbonate as a major product in aqueous solution, despite their similar basicity. We present molecular mechanisms underlying the significant difference of CO2 absorption rate based on ab initio molecular dynamics simulations combined with metadynamics. Our calculations predict the free energy barrier for base-catalyzed CO2 hydration to be lower in aqueous AMP compared to DEEA. Further molecular analysis suggests that the difference in free energy barrier is largely attributed to entropic effects associated with reorganization of H2O molecules adjacent to the basic N site. Stronger hydrogen bonding of H2O with N of DEEA than AMP, in addition to the presence of bulky ethyl groups, suppresses the thermal rearrangement of adjacent H2O molecules, thereby leading to lower stability of the transition state involving OH− creation and CO2 polarization. Moreover, the hindered reorganization of adjacent H2O molecules is found to facilitate migration of OH− (created via proton abstraction by DEEA) away from the N site while suppressing CO2 approach. This leads us to speculate that catalyzed CO2 hydration in aqueous DEEA may involve OH− migration through multiple hydrogen-bonded H2O molecules prior to reaction with CO2, whereas in aqueous AMP it seems to preferentially follow the one H2O-mediated mechanism. This study highlights the importance of entropic effects in determining both mechanisms and rates of CO2 absorption into aqueous sterically hindered amines.

The lowest possible thermal conductivity of silicon–germanium (SiGe) bulk alloys achievable through alloy scattering, or the so-called alloy limit, is important to identify for thermoelectric applications. However, this limit remains a subject of contention as both experimentally-reported and theoretically-predicted values tend to be widely scattered and inconclusive. In this work, we present a possible explanation for these discrepancies by demonstrating that the thermal conductivity can vary significantly depending on the degree of randomness in the spatial arrangement of the constituent atoms. Our study suggests that the available experimental data, obtained from alloy samples synthesized using ball-milling techniques, and previous first-principles calculations, restricted by small supercell sizes, may not have accessed the alloy limit. We find that low-frequency anharmonic phonon modes can persist unless the spatial distribution of Si and Ge atoms is completely random at the atomic scale, in which case the lowest possible thermal conductivity may be achieved. Our theoretical analysis predicts that the alloy limit of SiGe could be around 1–2 W m−1 K−1 with an optimal composition around 25 at% Ge, which is substantially lower than previously reported values from experiments and first-principles calculations.

Electrochemical double layer capacitors, or supercapacitors, are high-power energy storage devices that consist of large surface area electrodes (filled with electrolyte) to accommodate ion packing in accordance with classical electric double layer (EDL) theory. Nanoporous carbons (NPCs) have recently emerged as a class of electrode materials with the potential to dramatically improve the capacitance of these devices by leveraging ion confinement. However, the molecular mechanisms underlying such enhancements are a clear departure from EDL theory and remain an open question. In this paper, we present the concept of ion reorganization kinetics during charge/discharge cycles, especially within highly confining subnanometer pores, which necessarily dictates the capacitance. Our molecular dynamics voltammetric simulations of ionic liquid immersed in NPC electrodes (of varying pore size distributions) demonstrate that the most efficient ion migration, and thereby largest capacitance, is facilitated by nonuniformity of shape (e.g., from cylindrical to slitlike) along nanopore channels. On the basis of this understanding, we propose that a new structural descriptor, coined as the pore shape factor, can provide a new avenue for materials optimization. These findings also present a framework to understand and evaluate ion migration kinetics within charged nanoporous materials.Keywords: constant potential; ion migration kinetics; ionic liquid; molecular dynamics; pore size factor; subnanometer confinement;

Over the past decade, interest in leveraging subnanometer pores for improved capacitance in electrochemical double layer capacitors (EDLCs) has readily grown. Correspondingly, many theoretical studies have endeavored to understand the mechanisms that dictate the capacitance enhancement once ions are confined within nanopores, typically within quasi-equilibrium conditions. However, a kinetic-based understanding of the capacitance may be important, especially since the dynamics of ion transport can exhibit dramatic differences under confinement compared to the bulk liquid phase; ion transport is driven by the competition between the electrostatic electrode–ion and ion–ion interactions, which can be comparable as the internal surface area to volume ratio increases. In this work, we study the relationship between the dynamics of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM/BF4) ionic liquid and the capacitance within two idealized cylindrical subnanometer pores with diameters of 0.81 and 1.22 nm using classical molecular dynamics simulations. By adjusting the voltage scan rate, we find that the capacitance is highly sensitive to the formation of an electroneutral ionic liquid region; with rapid charging, consolidated anion–cation contact pairs, which remain trapped within the pore, restrict the local accumulation of charge carriers and, thereby, the capacitance. These findings highlight potential kinetic limitations that can mitigate the benefits from electrodes with subnanometer pores.

The dielectric constant or relative permittivity (εr) of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms but also the specific atomic arrangement in the crystal lattice. In this Letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nanoflakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope is employed to simultaneously quantify the number of layers and local electrical properties. The measured εr increases monotonically as a function of the thickness and saturates to the bulk value at around 6–8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed by first-principles calculations. Our results of the dielectric response, being ubiquitously applicable to layered 2D semiconductors, are expected to be significant for this vibrant research field.

Through a combined density functional theory and in situ scanning electron microscopy study, the effects of presence of gold (Au) spreading on the lithiation process of silicon nanowire (SiNW) were systematically examined. Different from a pristine SiNW, an Au-coated SiNW (Au-SiNW) is lithiated in three distinct stages; Li atoms are found to be incorporated preferentially in the Au shell, whereas the thin AuSi interface layer may serve as a facile diffusion path along the nanowire axial direction, followed by the prompt lithiation of the Si core in the radial direction. The underlying mechanism of the intriguing stagewise lithiation behavior is explained through our theoretical analysis, which appears well-aligned with the experimental evidence.Keywords: density functional theory calculation; gold-coated silicon nanowire; in situ characterization; lithium ion battery; stagewise lithiation

AMP and its blends are an attractive solvent for CO2 capture, but the underlying reaction mechanisms still remain uncertain. We attempt to elucidate the factors enhancing bicarbonate production in aqueous AMP as compared to MEA which, like most other primary amines, preferentially forms carbamate. According to our predicted reaction energies, AMP and MEA exhibit similar thermodynamic favorability for bicarbonate versus carbamate formation; moreover, the conversion of carbamate to bicarbonate also does not appear more favorable kinetically in aqueous AMP compared to MEA. Ab initio molecular dynamics simulations, however, demonstrate that bicarbonate formation tends to be kinetically more probable in aqueous AMP while carbamate is more likely to form in aqueous MEA. Analysis of the solvation structure and dynamics shows that the enhanced interaction between N and H2O may hinder CO2 accessibility while facilitating the AMP + H2O → AMPH+ + OH− reaction, relative to the MEA case. This study highlights the importance of not only thermodynamic but also kinetic factors in describing CO2 capture by aqueous amines.

Aqueous monoethanolamine (MEA) has been extensively studied as a solvent for CO2 capture, yet the underlying reaction mechanisms are still not fully understood. Combined ab initio and classical molecular dynamics simulations were performed to revisit and identify key elementary reactions and intermediates in 25–30 wt% aqueous MEA with CO2, by explicitly taking into account the structural and dynamic effects. Using static quantum chemical calculations, we also analyzed in more detail the fundamental interactions involved in the MEA–CO2 reaction. We find that both the CO2 capture by MEA and solvent regeneration follow a zwitterion-mediated two-step mechanism; from the zwitterionic intermediate, the relative probability between deprotonation (carbamate formation) and CO2 removal (MEA regeneration) tends to be determined largely by the interaction between the zwitterion and neighboring H2O molecules. In addition, our calculations clearly demonstrate that proton transfer in the MEA–CO2–H2O solution primarily occurs through H-bonded water bridges, and thus the availability and arrangement of H2O molecules also directly impacts the protonation and/or deprotonation of MEA and its derivatives. This improved understanding should contribute to developing more comprehensive kinetic models for use in modeling and optimizing the CO2 capture process. Moreover, this work highlights the importance of a detailed atomic-level description of the solution structure and dynamics in order to better understand molecular mechanisms underlying the reaction of CO2 with aqueous amines.

Relatively low electron mobility has been thought to be a key factor that limits the overall photocatalytic performance of BiVO4, but the behavior of electrons has not been fully elucidated. We examine electron localization and transport in BiVO4 using hybrid density functional theory calculations. An excess electron is found to remain largely localized on one V atom. The predicted hopping barrier for the small polaron is 0.35 eV (with inclusion of 15% Hartree–Fock exchange), and tends to increase almost linearly with lattice constant associated with pressure and/or temperature changes. We also examine the interaction between polarons, and discuss the possible concentration-dependence of electron mobility in BiVO4.

Sodium-ion batteries (NIBs) have recently received great attention as a potential complement to existing lithium-ion battery (LIB) technology. Because of the difference between Na and Li in nature, what has been an attractive anode material for LIBs may or may not be utilized for NIBs. Using density functional theory calculations, we examine and compare the sodiation behaviors of Si, Ge, and Sn, in comparison also to their lithiation processes if needed. We evaluate single Na incorporation in the host matrices (M = Si, Ge, Sn) and also discuss the formation of Na–M alloys in terms of structural evolution and energetics, along with their mechanical and diffusion properties. While the alloy systems considered in this work appear to undergo similar transformation during sodiation and lithiation, the M networks tend to lose connectivity more rapidly in the former. At Na/Li:M = 1:1 ratio, the M networks in a-NaM alloys already disintegrate into compact isolated clusters while those in a-LiM still maintain extended connectivity via puckered conformation. This unique difference in their specific atomic arrangements contributes to the more rapid softening, larger volume expansion, and faster increase in Na diffusivity with sodiation in comparison to the case of lithiation.

We present the role of photogenerated charge carriers in the oxidation of CO by O2 on reduced, rutile TiO2(110) based on first-principles DFT calculations. Our calculations show that hole-trapped O2 at the O vacancy site adopts a tilted open ring configuration, while an additional electron preferentially localizes at the CO-bound Ti site. The electron–hole separated configuration likely converts to the O–O–C–O complex with a small barrier of around 0.1 eV. From the neutral intermediate state, CO2 is predicted to desorb off the surface with a barrier less than 0.2 eV if another hole is available. For comparison, we also look at both thermally activated and hole-mediated CO oxidation processes, but the predicted overall barriers of around 0.9 and 0.5 eV, respectively, appear to be high for facile CO oxidation at low temperatures. Our findings clearly highlight that excess electrons and holes can synergetically contribute to CO photooxidation, which is consistent with a recent experimental study by Petrik and Kimmel that provides evidence for involvement of multiple nonthermal reaction steps.Keywords: CO photooxidation; density functional theory calculation; nonthermal catalytic reaction; rutile TiO2(110); synergetic role of electrons and holes

Chemically doped graphene-based materials have recently been explored as a means to improve the performance of supercapacitors. In this work, we investigate the effects of 3d transition metals bound to vacancy sites in graphene with [BMIM][PF6] ionic liquid on the interfacial capacitance; these results are compared to the pristine graphene case with particular attention to the relative contributions of the quantum and electric double layer capacitances. Our study highlights that the presence of metal-vacancy complexes significantly increases the availability of electronic states near the charge neutrality point, thereby enhancing the quantum capacitance drastically. In addition, the use of metal-doped graphene electrodes is found to only marginally influence the microstructure and capacitance of the electric double layer. Our findings indicate that metal-doping of graphene-like electrodes can be a promising route toward increasing the interfacial capacitance of electrochemical double layer capacitors, primarily by enhancing the quantum capacitance.Keywords: density functional theory; electric double layer capacitance; ionic liquid; molecular dynamics; quantum capacitance; transition metal

Graphene-based materials have been proposed as promising electrodes for electric double layer capacitors. Recently, it has been found that one of the limitations of graphene electrodes is the finite quantum capacitance at low applied voltage. In this work, we investigate the impact of having point-like topological defects in graphene on the electronic structure and quantum capacitance. Our results clearly show that the presence of defects, such as Stone Wales, di-vacancies, and di-interstitials, can substantially enhance the quantum capacitance when compared to pristine graphene, which is found to be due to defect-induced quasi-localized states near the Fermi level. In addition, the charging behavior tends to be asymmetric around the neutrality point. We also discuss the possibility of tuning the electronic structure and capacitance through mixtures of these defects. Our findings suggest that graphene-based electrodes with topological defects may demonstrate noteworthy capacitance but should be carefully selected for use as either the positive or negative electrode.

•We examine and compare the lithiation behavior of Si and Ge using DFT calculations.•Li diffusivity is greater and less concentration-dependent in Ge as compared to Si.•Li diffusion is subject to Li–host interaction and host lattice rigidity/dynamics.•We reveal the origin of the superior rate performance of Ge-based anodes.•High performance anodes can be designed via fine-tuning of Si–Ge alloys.Silicon and germanium are both recognized as a promising anode material for high-energy lithium-ion batteries. Si is best known for its superior energy storage capacity, while Ge exhibits better rate capability and cycleability. To better understand the underlying reasons behind their lithiation behavior differences, particularly the enhanced Li transport in Ge, we examine and compare Li-host lattice interactions and dynamics using density functional theory calculations. At the onset of lithiation, an isolated Li interstitial is found to form polar covalent bonds with four nearest host atoms, while the degree of covalency is noticeably greater for Li–Si than Li–Ge bonds. The relatively stronger Li–Si interaction, along with the stiffer Si lattice tend to be responsible for the suppressed Li mobility (DLi = 10−13 cm2 s−1) in c-Si, as compared to the c-Ge case (DLi = 10−11 cm2 s−1). With continued lithiation, DLi in a-LixSi increases significantly from 10−12 to 10−7 cm2 s−1 (x = 0.14–3.57); contrarily, DLi in a-LixGe is around 10−7 cm2 s−1 and less concentration dependent. Our analysis shows that the rapid Li diffusion in a-LixGe is directly related to the facile atomic rearrangements of host Ge atoms even at the early stages of lithiation.

Correction for ‘Relative contributions of quantum and double layer capacitance to the supercapacitor performance of carbon nanotubes in an ionic liquid’ by Alexander J. Pak, Eunsu Paek and Gyeong S. Hwang, Phys. Chem. Chem. Phys., 2013, 15, 19741–19747.

The inherently large surface area and electrical conductivity of graphene-like electrodes have motivated extensive research for their use in supercapacitors. Although these properties are beneficial for the electric double layer (EDL) capacitance, the full utilization of graphene is curtailed by its intrinsically limited quantum capacitance due to the low density of electronic states near the neutrality point. While recent work has demonstrated that modifications to graphene can generally mitigate this limitation, a comprehensive analysis of the impact of graphene edges, which can be created during synthesis and post-treatment, has yet to be reported. Using a theoretical approach, we investigate the influence of graphene edges on both the quantum and EDL capacitances using edge-passivated zigzag graphene nanoribbons (ZGNRs) in [BMIM][PF6] ionic liquid as model systems. Our findings show that the presence of edges improves the quantum capacitance by increasing the electronic density of states, which is further amplified as the ZGNR width decreases. Our analysis also reveals that the EDL microstructure can be noticeably altered by the edges, which in turn increases the EDL capacitance. Through comparisons with pristine graphene electrodes, our study clearly highlights that edge defects in graphene-like electrodes can enhance supercapacitor performance by dramatically augmenting both EDL and quantum capacitances.

Silicon suboxides (SiOx, x < 2) have been recognized as a promising anode material for high-performance Li-ion batteries (LIBs), especially when the O content is relatively low. To better understand the lithiation behavior in partially oxidized silicon at the atomistic level, we perform density functional theory calculations to examine the structural evolution, bonding mechanism, mechanical property, and voltage profile of lithiated a-SiO1/3. With lithiation, the a-SiO1/3 host matrix gradually disintegrates as Li atoms are accommodated by both Si and O atoms. Interestingly, we find that the Si–Li coordination number (CN) monotonically increases up to CNSi–Li ≈ 10 in a-Li4SiO1/3, whereas CNO–Li tends to saturate far before full lithiation at CNO–Li ≈ 6; the formation mechanism of such intriguing Li6O complexes with Oh symmetry is investigated via detailed electronic structure analyses. Li incorporation in the a-SiO1/3 matrix is predicted to be highly favorable with a capacity comparable to that of fully lithiated Si (Li:Si ratio ≈ 4); additionally, the approximated lithiation voltage between 0.2 and 0.8 V is also well within the desirable range for LIB anode applications. Our study highlights the importance of controlling the Si:O ratio as well as O spatial distribution in order to tailor the desired lithiation properties; such a realization may benefit the rational design and development of high-performance silicon suboxide based anodes via fine-tuning of the oxidation conditions.Keywords: density functional theory; Li6O polycation; LIB anode; lithiation; silicon suboxide;

Motivated by promising demonstrations of carbon nanotube (CNT) electrodes in supercapacitors, we evaluate the capacitive performance of a (6,6) CNT in [BMIM][PF6] ionic liquid (IL), with particular attention to the relative contributions of the electric double layer (EDL) capacitance (CD) at the CNT/IL interface and the quantum capacitance (CQ) of the CNT. Our classical molecular dynamics simulations reveal that the use of the CNT improves CD when compared to planar graphene, which we discuss in terms of how the electrode curvature affects both the electric field strength and IL packing density. In addition, according to density functional theory calculations, the CQ of the CNT is constant and significantly larger than that of graphene near the Fermi level, which is a consequence of the larger number of available electron states in the CNT. Our study also shows that the relative performance of the CNT- and graphene-based electrodes can be a strong function of applied voltage, which we attribute to the shifting contributions of CQ and CD.

Based on density functional theory calculations, we present mechanisms underlying the improvement in the catalytic performance of Pd-based alloys for oxygen hydrogenation to water. As a model case, we consider the Pd/Pd3Co system where one or two Pd overlayers are located on top of the bimetallic substrate. Our calculations clearly demonstrate that the subsurface Co atoms assist in facilitating the oxygen reduction reaction by lowering the activation barriers for O/OH hydrogenation with a slight increase in the O2 scission barrier; however, we also find that the Co atoms lying below the subsurface have no significant contribution in altering the surface reactivity towards oxygen hydrogenation. The analysis of intra- and interlayer orbital interactions in the near-surface region elucidates the synergetic interplay between the surface electronic structure modification due to the underlying Co atoms (interlayer ligand effect) and the compressive strain caused by the Pd3Co substrate. This result also brings to light the significant contribution of the out of plane (dxz and dyz) states in altering the surface reactivity towards O hydrogenation.

Silicon anodes with excellent capacity retention and rate capability have been demonstrated utilizing nanoengineered structures, such as nanowires and nanoscale thin films. Here, we present a comparative study using density functional theory calculations to examine the surface effects on the composition, structural evolution, energetics and Li-ion mobility in amorphous LixSi alloys (0.42 ≤ x ≤ 3.57). When the Li content is sufficiently low, our calculations predict a slight Li surface enrichment as the presence of Li atoms contributes to the stabilization of the surfaces. As the Li content is further increased, the near-surface structure and alloy composition become similar to that in the bulk, except for the reduction in Si–Si connectivity within the outermost surface layer. The surface effects tend to be very shallow and only extend to the first couple of atomic layers; nonetheless, our ab initio molecular dynamics simulations highlight the improved Li mobility in the near-surface region. Additionally, our calculations show that Li mobility is extremely sensitive to the alloy composition, and Li diffusivity is enhanced by orders of magnitude in the highly lithiated stage.Highlights► We examine the surface effects on the lithiation behavior of amorphous silicon using density functional theory calculations. ► Li diffusivity is significantly enhanced in the near-surface region. ► When the Li content is sufficiently low, a slight Li surface enrichment contributes to the stabilization of the surfaces. ► As the Li content is further increased, the near-surface structure and alloy composition become similar to that in the bulk. ► The surface effects tend to be very shallow and only extend to the first couple of atomic layers.

We examine the lithiation behavior of silicon-graphene (Si-Gr) composites using density functional theory calculations. Our calculations demonstrate charge transfer from Li to both Si and C (in graphene); the excess electrons on graphene create an electric field, which attracts Li cations while repelling Si anions and thus results in a distinct alternative Li–Si layering structure near graphene. The interfacial Li ions exhibit substantially higher mobility along the Si/Gr interface in comparison to bulk diffusion in Si; such facile interfacial diffusion could contribute toward high performance anodes with fast charge/discharge rates. However, the presence of graphene tends to have no significant impact on the structural evolution of Si during lithiation, as Li atoms are mostly incorporated in the Si matrix rather than at the Si/Gr interface. Consequently, the theoretical capacity and voltage profile of the Si-Gr composite are predicted to be close to those of pure Si.

Graphene-based electrodes have been widely tested and used in electrochemical double layer capacitors due to their high surface area and electrical conductivity. Nitrogen doping of graphene has recently been demonstrated to significantly enhance capacitance, but the underlying mechanisms remain ambiguous. We present the doping effect on the interfacial capacitance between graphene and [BMIM][PF6] ionic liquid, particularly the relative changes in the double layer and electrode (quantum) capacitances. The electrode capacitance change was evaluated based on density functional theory calculations of doping-induced electronic structure modifications in graphene, while the microstructure and capacitance of the double layers forming near undoped/doped graphene electrodes were calculated using classical molecular dynamics. Our computational study clearly demonstrates that nitrogen doping can lead to significant enhancement in the electrode capacitance as a result of electronic structure modifications while there is virtually no change in the double layer capacitance. This finding sheds some insight into the impact of the chemical and/or mechanical modifications of graphene-like electrodes on supercapacitor performance.

Perovskite oxides ABO3 are important materials used as components in electronic devices. The highly compact crystal structure consists of a framework of corner-shared BO6 octahedra enclosing the A-site cations. Because of these structural features, forming a strong bond between A and B cations is highly unlikely and has not been reported in the literature. Here we report a pressure-induced first-order transition in PbRuO3 from a common orthorhombic phase (Pbnm) to an orthorhombic phase (Pbn21) at 32 GPa by using synchrotron X-ray diffraction. This transition has been further verified with resistivity measurements and Raman spectra under high pressure. In contrast to most well-studied perovskites under high pressure, the Pbn21 phase of PbRuO3 stabilized at high pressure is a polar perovskite. More interestingly, the Pbn21 phase has the most distorted octahedra and a shortest Pb—Ru bond length relative to the average Pb—Ru bond length that has ever been reported in a perovskite structure. We have also simulated the behavior of the PbRuO3 perovskite under high pressure by first principles calculations. The calculated critical pressure for the phase transition and evolution of lattice parameters under pressure match the experimental results quantitatively. Our calculations also reveal that the hybridization between a Ru:t2g orbital and an sp hybrid on Pb increases dramatically in the Pbnm phase under pressure. This pressure-induced change destabilizes the Pbnm phase to give a phase transition to the Pbn21 phase where electrons in the overlapping orbitals form bonding and antibonding states along the shortest Ru—Pb direction at P > Pc.

Carbon nanotube (CNT) electrodes in supercapacitors have recently demonstrated enhanced performance compared to conventional carbon-based electrodes; however, the underlying relationships between electrode curvature and capacitance remain unclear. Using computer simulations, we evaluate the capacitive performance of metallic (6,6), (10,10), and (16,16) CNTs in [BMIM][PF6] ionic liquid (IL), with particular attention to the relative contributions of the electric double layer (EDL) capacitance (CD) at the CNT/IL interface and the electrode quantum capacitance (CQ). Our classical molecular dynamics simulations reveal that CD improves with increasing electrode curvature, which we discuss in terms of how the curvature affects both the electric field strength and EDL microstructure. In addition, the CQ of the CNTs is constant near the Fermi level and increases with curvature, as also demonstrated by density functional theory calculations. Our study shows that the electrode curvature effect on the total interfacial capacitance can be a strong function of applied voltage, which we attribute to the shifting contributions of CQ and CD.

We present a theoretical explanation on how PdAu alloy catalysts can enhance the oxidation of CO molecules based on density functional theory calculations of CO adsorption and oxidation on AuPd/Pd(111) surfaces. Our study suggests that the enhanced activity is largely attributed to the possible existence of “partially-poisoned” Pd ensembles that accommodate fewer CO molecules than Pd atoms. Whereas the oxidation of preadsorbed CO is likely governed by O2 trapping, our study shows that small Pd ensembles such as dimers and compact trimers tend to provide more active sites than larger ensembles; CO adsorbed on a Pd monomer is found to react hardly with O2 to form CO2. In addition, we find the tendency of CO-induced Pd agglomeration, which may in turn facilitate CO oxidation by creating more dimers and compact trimers as compared with the adsorbate-free surface where monomers are likely prevailing.Keywords: CO oxidation; palladium−gold alloys; surface ensembles;

On the basis of density functional theory calculations, we first present a comparative study on the behavior of Li atoms in M (M = Si, Ge, and Sn) and evaluate how Li incorporation affects the electronic structure and bonding nature of the host lattices. We then discuss the energetics, structural evolution, and variations in electronic and mechanical properties of crystalline and amorphous Li–M alloys. Our calculations show that Li insertion is the least favorable in Si and the most favorable in Sn owing to its large effective interstitial space and softer matrix. Upon Li incorporation, the bonding strength of the host network is weakened, attributed to the transferred charge from Li. Li interstitials can migrate easily in all three host materials with a moderate migration barrier in Si and small barriers in Ge and Sn. Because of the cation repulsive interaction, Li atoms tend to remain isolated and well dispersed in M; also induced by this cationic nature is the charge redistribution toward Li, leading to the strong screening/shielding effect in Si, in which the excess charges are highly localized, and a relatively weaker effect in Sn. According to our mixing enthalpy calculations, alloying between Li and M is energetically favorable with Li–Sn alloys being the most stable, followed by Li–Ge and Li–Si alloys. On the basis of structural, electronic, and mechanical property analyses, we also demonstrate how the incorporation of Li atoms with increasing concentration leads to the disintegration of host networks and softening of the Li–M alloys, and associated with the more flexible lattices, the volume expansion at fully lithiated states are 434 (399)% (Si), 382 (353)% (Ge), and 305 (259)% (Sn) for amorphous (crystalline) Li–M alloys.

We have constructed model Hamiltonians for AuPt/Pt(100) and AuPd/Pd(100) surface alloys based on the cluster expansion method and density functional theory. The cluster expansion Hamiltonians were used in Monte Carlo simulations to study the equilibrium arrangements of surface atoms in these two systems for a range of compositions and temperatures. We report on and explain results from these simulations in terms of the differing interatomic interactions present in each alloy. In AuPt surface alloys, homonuclear Pt–Pt interactions are favored over heteronuclear Au–Pt interactions, whereas in AuPd the opposite is true. Accordingly, our simulations show that Pt prefers to agglomerate, whereas Pd prefers to form smaller contiguous ensembles, such as monomers and dimers. Our simulations also reveal that the AuPd surface alloy can adopt c(2 × 2) ordering at low temperatures and 50% Pd coverage and exhibits a tendency for Pd monomers to occupy sites at the second nearest-neighbor distance from one another. Finally, we compare experimental data available in the literature to our results and find them in good qualitative agreement.

Using a density functional theory approach, we examine the dielectric function (ε(ω)) optical spectra and electronic structure of various silicon nanowire (SiNW) orientations (⟨100⟩, ⟨110⟩, ⟨111⟩, and ⟨112⟩) with amorphous oxide sheaths (−a-SiOx) and compare the results against H-terminated reference SiNWs. We extend the same methods to investigate the effects of surface passivation on ⟨111⟩ SiNW properties using functional group termination (−H, −OH, and −F) and three different thicknesses of oxide sheath passivation. Oxide layer growth is evidenced in the spectra by concomitant appearance of tail oxide character with signatures of increased Si disorder. Suboxide contributions and increased Si disorder from oxidation average out the band structure dispersion observed in the reference SiNWs. Furthermore, we plot average Seraphin coefficients for ⟨111⟩ passivations that clearly distinguish functional group termination from surface oxidation and discuss the suboxide and disorder contributions on the characteristic intersection of these coefficients. The substantial difference in properties observed between ⟨111⟩−OH and ⟨111⟩−a-SiOx SiNWs emphasizes the importance of using realistic oxidation models to improve understanding of SiNW properties.Keywords: disorder; electronic structure; first-principles; optical property; oxidation; silicon nanowire

On the basis of density functional theory calculations, we present the energetics, structure, and electronic and mechanical properties of crystalline and amorphous Li−Si alloys. We also discuss the dynamic behavior of the alloys at finite temperatures based on ab initio molecular dynamics. When the Li content is sufficiently high, alloying between Li and Si is energetically favorable as evidenced by the negative mixing enthalpy; the alloy is most stable around 70 atom % Li in the crystalline phase and 70 ± 5 atom % Li in the amorphous phase. Our calculations unequivocally show that the incorporation of Li leads to disintegration of the tetrahedrally bonded Si network into small clusters of various shapes. Bader charge analysis shows that the charge state of Li remains nearly unchanged around +0.8, while that of Si varies approximately from −0.5 to −3.3 depending on the number of Si neighbors as can be understood as Zintl-like phases. Electronic structure analysis highlights that the charge transfer leads to weakening or breaking of Si−Si bonds with the growing splitting between 3s and 3p states, and consequently, the Li−Si alloys soften with increasing Li content.

On the basis of density functional theory calculations, we present the bonding and dynamic behavior of Li atoms in crystalline Si and how the incorporation of Li atoms affects the structure and stability of the host Si lattice. Our calculations clearly evidence that the inserted Li atom energetically prefers a tetrahedral interstitial site while exhibiting a shallow donor level. Because of their positive ionization, the interactions between neutral Li impurities are repulsive, suggesting that they favorably remain isolated, rather than clustered. We also find that the charge transferred from neutral Li is largely localized within the first nearest Si atoms, thereby effectively screening the positively ionized Li. In addition, our electronic structure analysis highlights that the charge transfer leads to a significant weakening of nearby Si−Si bonds by filling the antibonding sp3 states of Si. The mobility of Li interstitials is also estimated in the neutral and positive charge states.

We have constructed model Hamiltonians for AuPt/Pt(111) and AuPd/Pd(111) surface alloys based on the cluster expansion method and density functional theory. Using these cluster expansions in Monte Carlo simulations, we have calculated the size and shape distributions of Pt and Pd ensembles in these two materials for a range of compositions and temperatures. We report on and explain the results of our simulations in terms of the differing interatomic interactions present in each alloy. Through the use of electronic structure calculations, we find that in AuPt, homonuclear Pt−Pt interactions are favored over heteronuclear Au−Pt interactions, while in AuPd the opposite is true. Accordingly, our simulations show that Pd prefers to form small, isolated ensembles with extended shapes, and Pt prefers to agglomerate and form larger ensembles with compact shapes.

Using first principles calculations, we examine how the ensemble effect on the performance of bimetallic catalysts is affected by the change of surface electronic structure associated with their geometric parameters. We look at H2O2 formation from H2 and O2 based on three different Pd monomer systems including AuPd adlayers with a Pd monomer each on Pd(111) [AuPdM/Pd(111)] and Au(111) [AuPdM/Au(111)] and a 55-atom cluster with Au41Pd shell and Pd13 core [Au41Pd@Pd13]. Our calculations show that H2O2 selectivity tends to be significantly deteriorated in the Au41Pd@Pd13 and AuPdM/Au(111) cases, as compared to the AuPdM/Pd(111) case. This is largely due to enhancement of the activity of corresponding surface Pd and its Au neighbors, while isolated Pd surface sites surrounded by less active Au are responsible for the H2O2 formation by suppressing O−O cleavage. This study highlights that ensemble contributions in multimetallic nanocatalysts can be a strong function of their geometric conditions, particularly local strain and effective atomic coordination number at the surface, that are directly related to surface electronic states.

We examined the surface segregation behavior of Si in amorphous AuSi alloys using ab initio molecular dynamics simulations within density functional theory. For a thin Au70Si30 film, our simulations predict Si surface enrichment that leads to a stoichiometry close to Au60Si40 in the surface layer. The surface structure exhibits a rather ordered Au3Si2 phase, which somewhat differs from the bulk Au60Si40 structure that exhibits a tendency of random hard-sphere packing. We also discuss the origin of the Si surface segregation based on analysis of segregation-induced changes in the atomic and electronic structure of the AuSi alloy surface.

We present the role of Pd ensembles in the selective direct synthesis of H2O2 from H2 and O2 on a PdAu alloy surface based on periodic density functional theory calculations. Our calculations demonstrate that H2O2 formation is strongly affected by the spatial arrangement of Pd and Au surface atoms. In particular, Pd monomers surrounded by less active Au atoms that suppress O−O bond scission are primarily responsible for the significantly enhanced selectivity toward H2O2 formation on PdAu alloys compared to that on the monometallic Pd and Au counterparts.

Based on first principles quantum mechanics (DFT/GGA with pseudopotentials) calculations, we propose a new mechanism for monovacancy annihilation and single missing dimer creation. Our study shows that an isolated monovacancy can exist fairly stable, rather than liberating the remaining atom of the `defect' dimer readily. The liberation barrier is calculated to be 1.3 eV. However, the monovacancy can diffuse rapidly along a dimer row by overcoming a barrier of 0.3 eV, leading to vacancy–vacancy pairing at elevated temperatures. These results suggest that the vacancy–vacancy pairing may play a major role in creating a single missing-dimer vacancy (which is ≈1.8 eV more stable than two isolated monovacancies). We also present the pathways and barriers of (i) the remaining atom hopping between the buckled-up and down site of the `defect' dimer and (ii) the vacancy diffusion into the subsurface layer.

Using density functional theory calculations we investigate the function of subsurface boron in determining surface properties of Si(0 0 1). To demonstrate its effect on surface reactivity we compare the behaviors of water adsorption on the clean and B-modified surfaces. We find that subsurface boron brings about a significant change in surface chemical properties by altering charge polarization of Si(0 0 1) locally. As a consequence, water adsorption on the B-modified surface shows a distinctively different feature from that on the clean surface.

•Lithiation mechanisms and electrochemical properties of silicon-based nanomaterials.•Enhanced performance of Si nanostructures and composites for LIB anodes.•Molecular mechanisms for reductive electrolyte decomposition.•Structural and chemical evolution at the anode/electrolyte interface.Lithium ion batteries are currently a principal power source for small portable electronics. However, in order to extend their effective use as large-scale energy storage systems for electric vehicles and renewable energy, there is an imminent need to further increase the energy density, power density, and cycle life while retaining safety and cost at an affordable range. This fundamentally represents a knowledge and materials challenge that needs to develop a deeper understanding of electrode and electrolyte materials as well as their interfaces. Here, we briefly review recent progress in first-principles computational studies on the lithiation behavior of anode materials and the structural and chemical evolution of anode/electrolyte interfaces.

Using density functional theory and cluster expansion-based Monte Carlo simulations, we examine the effect of Pd dispersion on the energetics and barriers for the reaction of O2 with H atoms to form H2O and H2O2 on a AuPd/Pd(1 1 1) alloy surface. Our calculations show that this hydrogenation reaction is considerably affected by the distribution of Pd and Au atoms in the surface layer. In particular, on isolated Pd monomers surrounded by less active Au atoms, the activation barrier to form H2O2 is appreciably lowered due to the suppression of O–O bond cleavage. In contrast, the reactivity to H2O on the Pd dimer is predicted to be enhanced compared to pure Pd. Using Monte Carlo simulations we also predict Pd ensemble populations in the AuPd surface layer as a function of temperature and composition. Due to the favorability of Au–Pd interactions over Pd–Pd, we find that small ensembles, particularly monomers, preferentially exist. This study highlights how theoretical investigation of bimetallic alloys, particularly the surface arrangement of atoms and the influence of ensembles on reaction energetics, can offer insight into the design of catalysts and tailoring of reaction conditions.

Piperazine (PZ) and its blends have emerged as attractive solvents for CO2 capture, but the underlying reaction mechanisms still remain uncertain. Our study particularly focuses on assessing the relative roles of PZCOO− and PZH+ produced from the PZ + CO2 reaction. PZCOO− is found to directly react with CO2 forming COO−PZCOO−, whereas PZH+ will not. However, COO−PZCOO− appears very unlikely to be produced in thermodynamic equilibrium with monocarbamates, suggesting that its existence would predominantly originate from the surface reaction that likely occurs. We also find production of H+PZCOO− to be more probable with increasing CO2 loading, due partly to the thermodynamic favorability of the PZH+ + PZCOO− → H+PZCOO− + PZ reaction; the facile PZ liberation may contribute to its relatively high CO2 absorption rate. This study highlights an accurate description of surface reaction and the solvent composition effect is critical in thermodynamic and kinetic models for predicting the CO2 capture processes.

The lowest possible thermal conductivity of silicon–germanium (SiGe) bulk alloys achievable through alloy scattering, or the so-called alloy limit, is important to identify for thermoelectric applications. However, this limit remains a subject of contention as both experimentally-reported and theoretically-predicted values tend to be widely scattered and inconclusive. In this work, we present a possible explanation for these discrepancies by demonstrating that the thermal conductivity can vary significantly depending on the degree of randomness in the spatial arrangement of the constituent atoms. Our study suggests that the available experimental data, obtained from alloy samples synthesized using ball-milling techniques, and previous first-principles calculations, restricted by small supercell sizes, may not have accessed the alloy limit. We find that low-frequency anharmonic phonon modes can persist unless the spatial distribution of Si and Ge atoms is completely random at the atomic scale, in which case the lowest possible thermal conductivity may be achieved. Our theoretical analysis predicts that the alloy limit of SiGe could be around 1–2 W m−1 K−1 with an optimal composition around 25 at% Ge, which is substantially lower than previously reported values from experiments and first-principles calculations.

Aqueous monoethanolamine (MEA) has been extensively studied as a solvent for CO2 capture, yet the underlying reaction mechanisms are still not fully understood. Combined ab initio and classical molecular dynamics simulations were performed to revisit and identify key elementary reactions and intermediates in 25–30 wt% aqueous MEA with CO2, by explicitly taking into account the structural and dynamic effects. Using static quantum chemical calculations, we also analyzed in more detail the fundamental interactions involved in the MEA–CO2 reaction. We find that both the CO2 capture by MEA and solvent regeneration follow a zwitterion-mediated two-step mechanism; from the zwitterionic intermediate, the relative probability between deprotonation (carbamate formation) and CO2 removal (MEA regeneration) tends to be determined largely by the interaction between the zwitterion and neighboring H2O molecules. In addition, our calculations clearly demonstrate that proton transfer in the MEA–CO2–H2O solution primarily occurs through H-bonded water bridges, and thus the availability and arrangement of H2O molecules also directly impacts the protonation and/or deprotonation of MEA and its derivatives. This improved understanding should contribute to developing more comprehensive kinetic models for use in modeling and optimizing the CO2 capture process. Moreover, this work highlights the importance of a detailed atomic-level description of the solution structure and dynamics in order to better understand molecular mechanisms underlying the reaction of CO2 with aqueous amines.

Motivated by promising demonstrations of carbon nanotube (CNT) electrodes in supercapacitors, we evaluate the capacitive performance of a (6,6) CNT in [BMIM][PF6] ionic liquid (IL), with particular attention to the relative contributions of the electric double layer (EDL) capacitance (CD) at the CNT/IL interface and the quantum capacitance (CQ) of the CNT. Our classical molecular dynamics simulations reveal that the use of the CNT improves CD when compared to planar graphene, which we discuss in terms of how the electrode curvature affects both the electric field strength and IL packing density. In addition, according to density functional theory calculations, the CQ of the CNT is constant and significantly larger than that of graphene near the Fermi level, which is a consequence of the larger number of available electron states in the CNT. Our study also shows that the relative performance of the CNT- and graphene-based electrodes can be a strong function of applied voltage, which we attribute to the shifting contributions of CQ and CD.

Based on density functional theory calculations, we present mechanisms underlying the improvement in the catalytic performance of Pd-based alloys for oxygen hydrogenation to water. As a model case, we consider the Pd/Pd3Co system where one or two Pd overlayers are located on top of the bimetallic substrate. Our calculations clearly demonstrate that the subsurface Co atoms assist in facilitating the oxygen reduction reaction by lowering the activation barriers for O/OH hydrogenation with a slight increase in the O2 scission barrier; however, we also find that the Co atoms lying below the subsurface have no significant contribution in altering the surface reactivity towards oxygen hydrogenation. The analysis of intra- and interlayer orbital interactions in the near-surface region elucidates the synergetic interplay between the surface electronic structure modification due to the underlying Co atoms (interlayer ligand effect) and the compressive strain caused by the Pd3Co substrate. This result also brings to light the significant contribution of the out of plane (dxz and dyz) states in altering the surface reactivity towards O hydrogenation.

Correction for ‘Relative contributions of quantum and double layer capacitance to the supercapacitor performance of carbon nanotubes in an ionic liquid’ by Alexander J. Pak, Eunsu Paek and Gyeong S. Hwang, Phys. Chem. Chem. Phys., 2013, 15, 19741–19747.

AMP and its blends are an attractive solvent for CO2 capture, but the underlying reaction mechanisms still remain uncertain. We attempt to elucidate the factors enhancing bicarbonate production in aqueous AMP as compared to MEA which, like most other primary amines, preferentially forms carbamate. According to our predicted reaction energies, AMP and MEA exhibit similar thermodynamic favorability for bicarbonate versus carbamate formation; moreover, the conversion of carbamate to bicarbonate also does not appear more favorable kinetically in aqueous AMP compared to MEA. Ab initio molecular dynamics simulations, however, demonstrate that bicarbonate formation tends to be kinetically more probable in aqueous AMP while carbamate is more likely to form in aqueous MEA. Analysis of the solvation structure and dynamics shows that the enhanced interaction between N and H2O may hinder CO2 accessibility while facilitating the AMP + H2O → AMPH+ + OH− reaction, relative to the MEA case. This study highlights the importance of not only thermodynamic but also kinetic factors in describing CO2 capture by aqueous amines.

Relatively low electron mobility has been thought to be a key factor that limits the overall photocatalytic performance of BiVO4, but the behavior of electrons has not been fully elucidated. We examine electron localization and transport in BiVO4 using hybrid density functional theory calculations. An excess electron is found to remain largely localized on one V atom. The predicted hopping barrier for the small polaron is 0.35 eV (with inclusion of 15% Hartree–Fock exchange), and tends to increase almost linearly with lattice constant associated with pressure and/or temperature changes. We also examine the interaction between polarons, and discuss the possible concentration-dependence of electron mobility in BiVO4.

Electrolyte and electrode materials used in lithium-ion batteries have been studied separately to a great extent, however the structural and dynamical properties of the electrolyte–electrode interface still remain largely unexplored despite its critical role in governing battery performance. Using molecular dynamics simulations, we examine the structural reorganization of solvent molecules (cyclic ethylene carbonate:linear dimethyl carbonate 1:1 molar ratio doped with 1 M LiPF6) in the vicinity of graphite electrodes with varying surface charge densities (σ). The interfacial structure is found to be sensitive to the molecular geometry and polarity of each solvent molecule as well as the surface structure and charge distribution of the negative electrode. We also evaluated the potential difference across the electrolyte–electrode interface, which exhibits a nearly linear variation with respect to σ up until the onset of Li+ ion accumulation onto the graphite edges from the electrolyte. In addition, well-tempered metadynamics simulations are employed to predict the free-energy barriers to Li+ ion transport through the relatively dense interfacial layer, along with analysis of the Li+ solvation sheath structure. Quantitative analysis of the molecular arrangements at the electrolyte–electrode interface will help better understand and describe electrolyte decomposition, especially in the early stages of solid-electrolyte-interphase (SEI) formation. Moreover, the computational framework presented in this work offers a means to explore the effects of solvent composition, electrode surface modification, and operating temperature on the interfacial structure and properties, which may further assist in efforts to engineer the electrolyte–electrode interface leading to a SEI layer that optimizes battery performance.