Electrocatalytic Mechanisms for an Oxygen Evolution Reaction at a Rhombohedral Boron Monosulfide Electrode/Alkaline Medium Interface.

Rhombohedral boron monosulfide (r-BS) with a layer stacking structure is a promising electrocatalyst for an oxygen evolution reaction (OER) within an alkaline solution. We investigated the catalytic mechanisms at the r-BS electrode/alkaline medium interface for an OER using hybrid solvation theory based on the first-principles method combined with classical solution theory. In this study, we elucidate the activities of the OER at the outermost r-BS sheet with and without various surface defects. The Gibbs free energies along the OER path indicate that the boron vacancies at the first and second layers of the r-BS surface (VB1 and VB2) can promote the OER. However, we found that the VB1 is easily occupied by the oxygen atom during the OER, degrading its electrocatalytic performance. In contrast, VB2 is suitable for the active site of the OER due to its structure stability. Next, we applied a bias voltage with the OER potential to the r-BS electrode. The bias voltage incorporates the positive excess surface charge into pristine r-BS and VB2, which can be understood by the relationship between the OER potential and potentials of zero charge at the r-BS electrode. Because the OH- ions are the starting point of the OER, the positively charged surface is kinetically favorable for the electrocatalyst owing to the attractive interaction with the OH- ions. Finally, we qualitatively discuss the flat-band potential at a semiconductor/alkaline solution interface. It suggests that p-type carrier doping could promote the catalytic performance of r-BS. These results explain the previous measurement of the OER performance with the r-BS-based electrode and provide valuable insights into developing a semiconductor electrode/water interface.

A Combined Reaction Path Search and Hybrid Solvation Method for the Systematic Exploration of Elementary Reactions at the Solid-Liquid Interface.

We present a combined simulation method of single-component artificial force induced reaction (SC-AFIR) and effective screening medium combined with the reference interaction site model (ESM-RISM), termed SC-AFIR+ESM-RISM. SC-AFIR automatically and systematically explores the chemical reaction pathway, and ESM-RISM directly simulates the precise electronic structure at the solid-liquid interface. Hence, SC-AFIR+ESM-RISM enables us to explore reliable reaction pathways at the solid-liquid interface. We applied it to explore the dissociation pathway of an H2O molecule at the Cu(111)/water interface. The reaction path networks of the whole reaction and the minimum energy paths from H2O to H2 + O depend on the interfacial environment. The qualitative difference in the energy diagrams and the resulting change in the kinematically favored dissociation pathway upon changing the solvation environments are discussed. We believe that SC-AFIR+ESM-RISM will be a powerful tool to reveal the details of chemical reactions in surface catalysis and electrochemistry.

First-Principles Investigation of Charged Germagraphene as a Cathode Material for Dual-Carbon Batteries.

As part of the concerted effort for development of energy storage technologies, dual-ion batteries (DIBs) or dual-carbon batteries (DCBs) are attracting interest, owing primarily to their eco-friendly active materials. The use of carbon as the active materials of DCBs brings about several challenges involving capacity and stability. This contribution aims to provide an in-depth understanding of the structural and electronic properties of Ge-doped graphene (Germagraphene) as a novel cathode material for DCBs. Density functional theory (DFT) calculations are combined with the effective screening medium (ESM) method for analyzing the electronic and band structure of PF6 - anion-adsorbed Germagraphene under a potential bias. These theoretical investigations indicate that the use of Ge as a dopant for graphene has a positive impact on the adsorption of the anion on the cathode under both neutral and electrically biased conditions.

Theoretical Analysis on the Stability of 1-Pyrenebutanoic Acid Succinimidyl Ester Adsorbed on Graphene.

The adsorbed structure of 1-pyrenebutanoic acid succinimidyl ester (PASE) on graphene was investigated based on density functional theory. We found two locally stable structures: a straight structure with the chainlike part of butanoic acid succinimidyl ester (BSE) lying down and a bent structure with the BSE part directed away from graphene, keeping the pyrene (Py) part adsorbed on graphene. Then, to elucidate the adsorption mechanism, we separately estimated the contributions of the Py and BSE parts to the entire PASE adsorption, and the adsorption effect of the BSE part was found to be secondary in comparison to the contribution of the Py. Next, the mobility of the BSE part at room temperature was confirmed by the activation energy barrier between straight and bent structures. To take account of the external environment, we considered the presence of amino acids and the hydration effect by a three-dimensional reference interaction site model. The contributions of glycine molecules and the solvent environment to stabilizing the bent PASE structure relative to the straight PASE structure were found. Therefore, the effect of the external environment around PASE is of importance when the standing-up process of the BSE part from graphene is considered.

Quantifying Uncertainties in Solvation Procedures for Modeling Aqueous Phase Reaction Mechanisms.

Computational quantum chemistry provides fundamental chemical and physical insights into solvated reaction mechanisms across many areas of chemistry, especially in homogeneous and heterogeneous renewable energy catalysis. Such reactions may depend on explicit interactions with ions and solvent molecules that are nontrivial to characterize. Rigorously modeling explicit solvent effects with molecular dynamics usually brings steep computational costs while the performance of continuum solvent models such as polarizable continuum model (PCM), charge-asymmetric nonlocally determined local-electric (CANDLE), conductor-like screening model for real solvents (COSMO-RS), and effective screening medium method with the reference interaction site model (ESM-RISM) are less well understood for reaction mechanisms. Here, we revisit a fundamental aqueous hydride transfer reaction-carbon dioxide (CO2) reduction by sodium borohydride (NaBH4)-as a test case to evaluate how different solvent models perform in aqueous phase charge migrations that would be relevant to renewable energy catalysis mechanisms. For this system, quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations almost exactly reproduced energy profiles from QM simulations, and the Na+ counterion in the QM/MM simulations plays an insignificant role over ensemble averaged trajectories that describe the reaction pathway. However, solvent models used on static calculations gave much more variability in data depending on whether the system was modeled using explicit solvent shells and/or the counterion. We pinpoint this variability due to unphysical descriptions of charge-separated states in the gas phase (i.e., self-interaction errors), and we show that using more accurate hybrid functionals and/or explicit solvent shells lessens these errors. This work closes with recommended procedures for treating solvation in future computational efforts in studying renewable energy catalysis mechanisms.

Toward Engineering of Solution Microenvironments for the CO2 Reduction Reaction: Unraveling pH and Voltage Effects from a Combined Density-Functional-Continuum Theory.

Engineering the electrolyte microenvironment represents an attractive route to tuning the selectivity of electrocatalytic reactions beyond catalyst composition and morphology. However, harnessing the full potential of this approach requires understanding the interplay between voltage, electrolyte composition, and adsorbate binding within the electrical double layer, which is absent from the usual theoretical approaches. In this work, we apply a recently developed density functional theory (DFT)-continuum approach based on the effective screening medium method and reference interaction site model (ESM-RISM) to explore electrolyte effects with an enhanced description of the electrochemical interface. Applying this method to the binding of CO adsorbates in potassium-containing electrolytes on copper, a problem of direct relevance to CO2 electroreduction to value-added products, we show that the interdependence of voltage and pH leads to an unexpected change in adsorption site preference on Cu(001) terraces. Our findings highlight the often-overlooked importance of the electrical double-layer structure for predicting catalyst operation.

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study.

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy HV, and an interstitial fluorine defect HI from the simple selection rule. The enthalpy relation of HI > H > -HV observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Specific ion effects at graphitic interfaces.

Improved understanding of aqueous solutions at graphitic interfaces is critical for energy storage and water desalination. However, many mechanistic details remain unclear, including how interfacial structure and response are dictated by intrinsic properties of solvated ions under applied voltage. In this work, we combine hybrid first-principles/continuum simulations with electrochemical measurements to investigate adsorption of several alkali-metal cations at the interface with graphene and within graphene slit-pores. We confirm that adsorption energy increases with ionic radius, while being highly dependent on the pore size. In addition, in contrast with conventional electrochemical models, we find that interfacial charge transfer contributes non-negligibly to this interaction and can be further enhanced by confinement. We conclude that the measured interfacial capacitance trends result from a complex interplay between voltage, confinement, and specific ion effects-including ion hydration and charge transfer.

Negative dielectric constant of water confined in nanosheets.

Electric double-layer capacitors are efficient energy storage devices that have the potential to account for uneven power demand in sustainable energy systems. Earlier attempts to improve an unsatisfactory capacitance of electric double-layer capacitors have focused on meso- or nanostructuring to increase the accessible surface area and minimize the distance between the adsorbed ions and the electrode. However, the dielectric constant of the electrolyte solvent embedded between adsorbed ions and the electrode surface, which also governs the capacitance, has not been previously exploited to manipulate the capacitance. Here we show that the capacitance of electric double-layer capacitor electrodes can be enlarged when the water molecules are strongly confined into the two-dimensional slits of titanium carbide MXene nanosheets. Using electrochemical methods and theoretical modeling, we find that dipolar polarization of strongly confined water resonantly overscreens an external electric field and enhances capacitance with a characteristically negative dielectric constant of a water molecule.

Origins and Implications of Interfacial Capacitance Enhancements in C60-Modified Graphene Supercapacitors.

Understanding and controlling the electrical response at a complex electrode-electrolyte interface is key to the development of next-generation supercapacitors and other electrochemical devices. In this work, we apply a theoretical framework based on the effective screening medium and reference interaction site model to explore the role of electrical double-layer (EDL) formation and its interplay with quantum capacitance in graphene-based supercapacitors. In addition to pristine graphene, we investigate a novel C60-modified graphene supercapacitor material, which promises higher charge-storage capacity. Beyond the expected enhancement in the quantum capacitance, we find that the introduction of C60 molecules significantly alters the EDL response. These changes in EDL are traced to the interplay between surface morphology and charge localization character and ultimately dominate the overall capacitive improvement in the hybrid system. Our study highlights a complex interplay among surface morphology, electronic structure, and interfacial capacitance, suggesting general improvement strategies for optimizing carbon-based supercapacitor materials.

Li deposition and desolvation with electron transfer at a silicon/propylene-carbonate interface: transition-state and free-energy profiles by large-scale first-principles molecular dynamics.

We report the result of a large-scale first-principles molecular dynamics simulation under different electric biases performed to understand the charge transfer process coupling with lithium deposition and desolvation processes. We applied the effective screening medium (ESM) method to control the bias across the electrode/solution interface, and simulated a series of Li de-solvation and Li-deposition reactions occurring under the bias. Solvated Li+ in the bulk propylene carbonate migrates to the Si electrode surface and gradually de-solvates through the transition state. Introducing the blue-moon ensemble method, we determined the possible structures and activation energies for the transition states.

Toward full simulation of the electrochemical oxygen reduction reaction on Pt using first-principles and kinetic calculations.

Present fuel cells must increase the activity of the oxygen reduction reaction (ORR) on platinum (or Pt alloy) electrodes. Detailed simulation analyses can direct future investigations by providing a better understanding of the ORR. We adopted a density functional theory (DFT)-based, first-principles molecular dynamics simulation for the elementary steps of the electrochemical ORR on Pt(111). The two-step process involves successive protonation of O and OH, which are adsorbed on Pt. The relevant redox potentials were estimated by changing the coverage of OH(ad) and O(ad). The reaction energy profiles were determined along the reaction coordinate using the Blue-Moon ensemble method and a constant-bias scheme in the DFT calculations. These profiles at different biases were then used to generate activation energies and symmetry factors. Cyclic voltammetry (CV) and linear sweep voltammetry profiles were then calculated from the Butler-Volmer rate, Nernst equilibrium, and mass diffusion equations using these obtained parameters, literature values and appropriate prefactors in the rate equations. The experimentally observed reversible and irreversible peaks in CV were obtained. The irreversibility of the protonation of O(ad), , attributed to its higher activation energy, affects the ORR potential and thus fuel cell performance. It is therefore necessary not only to tune the adsorption energy of the O(ad) and OH(ad) intermediates, which are the origin of the "volcano plot", but also to tune 's activation energy to elevate the performance above that of the volcano-top.

Polyanthraquinone as a Reliable Organic Electrode for Stable and Fast Lithium Storage.

In spite of recent progress, there is still a lack of reliable organic electrodes for Li storage with high comprehensive performance, especially in terms of long-term cycling stability. Herein, we report an ideal polymer electrode based on anthraquinone, namely, polyanthraquinone (PAQ), or specifically, poly(1,4-anthraquinone) (P14AQ) and poly(1,5-anthraquinone) (P15AQ). As a lithium-storage cathode, P14AQ showed exceptional performance, including reversible capacity almost equal to the theoretical value (260 mA h g(-1); >257 mA h g(-1) for AQ), a very small voltage gap between the charge and discharge curves (2.18-2.14=0.04 V), stable cycling performance (99.4% capacity retention after 1000 cycles), and fast-discharge/charge ability (release of 69% of the low-rate capacity or 64% of the energy in just 2 min). Exploration of the structure-performance relationship between P14AQ and related materials also provided us with deeper understanding for the design of organic electrodes.

Poly(benzoquinonyl sulfide) as a High-Energy Organic Cathode for Rechargeable Li and Na Batteries.

In concern of resource sustainability and environmental friendliness, organic electrode materials for rechargeable batteries have attracted increasing attentions in recent years. However, for many researchers, the primary impression on organic cathode materials is the poor cycling stability and low energy density, mainly due to the unfavorable dissolution and low redox potential, respectively. Herein, a novel polymer cathode material, namely poly(benzoquinonyl sulfide) (PBQS) is reported, for either rechargeable Li or Na battery. Remarkably, PBQS shows a high energy density of 734 W h kg-1 (2.67 V × 275 mA h g-1) in Li battery, or 557 W h kg-1 (2.08 V × 268 mA h g-1) in Na battery, which exceeds those of most inorganic Li or Na intercalation cathodes. Moreover, PBQS also demonstrates excellent long-term cycling stability (1000 cycles, 86%) and superior rate capability (5000 mA g-1, 72%) in Li battery. Besides the exciting battery performance, investigations on the structure-property relationship between benzoquinone (BQ) and PBQS, and electrochemical behavior difference between Li-PBQS battery and Na-PBQS battery, also provide significant insights into developing better Li-organic and Na-organic batteries beyond conventional Li-ion batteries.

Potential-induced electronic structure changes in supercapacitor electrodes observed by in operando soft X-ray spectroscopy.

The dynamic physiochemical response of a functioning graphene-based aerogel supercapacitor is monitored in operando by soft X-ray spectroscopy and interpreted through ab initio atomistic simulations. Unanticipated changes in the electronic structure of the electrode as a function of applied voltage bias indicate structural modifications across multiple length scales via independent pseudocapacitive and electric double layer charge storage channels.

A method of orbital analysis for large-scale first-principles simulations.

An efficient method of calculating the natural bond orbitals (NBOs) based on a truncation of the entire density matrix of a whole system is presented for large-scale density functional theory calculations. The method recovers an orbital picture for O(N) electronic structure methods which directly evaluate the density matrix without using Kohn-Sham orbitals, thus enabling quantitative analysis of chemical reactions in large-scale systems in the language of localized Lewis-type chemical bonds. With the density matrix calculated by either an exact diagonalization or O(N) method, the computational cost is O(1) for the calculation of NBOs associated with a local region where a chemical reaction takes place. As an illustration of the method, we demonstrate how an electronic structure in a local region of interest can be analyzed by NBOs in a large-scale first-principles molecular dynamics simulation for a liquid electrolyte bulk model (propylene carbonate + LiBF4).

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.

Gate-induced electron-state tuning of MoS2: first-principles calculations.

The electronic structure of electrostatically doped MoS2 thin films is investigated on the basis of first-principles total-energy calculations. We find that electron injection leads to a rapid downward shift in the energy of the unoccupied nearly free electron (NFE) state relative to other conduction bands. The NFE state finally crosses the Fermi level at an electron density of 0.81 × 10(14) cm(-2) that is attributable to the strong local electric field induced by charge accumulation near the surface. Electrons accommodated in the NFE state play an important role in determining the conducting properties of MoS2 thin films.

Effect of thermal motion on catalytic activity of nanoparticles in polar solvent.

In this study, we propose that electrode potential fluctuations due to the thermal motion of the solvent may serve to enhance the catalytic activity of nanostructures. The proposed model uses a simple, Marcus-type treatment of the statistical behavior of the solvent and the Butler-Volmer law for the instantaneous catalytic rate as a function of the electrode potential. The rapid development of probing techniques with high spatial and temporal resolution will help to further confirm and characterize the dynamical properties of nanostructures.

Charge manipulation in molecules encapsulated inside single-wall carbon nanotubes.

We report clear experimental evidence for the charge manipulation of molecules encapsulated inside single-wall carbon nanotubes (SWCNTs) using electrochemical doping techniques. We encapsulated ß-carotene (Car) inside SWCNTs and clarified electrochemical doping characteristics of their Raman spectra. C=C streching modes of encapsulated Car and a G band of SWCNTs showed clearly different doping behaviors as the electrochemical potentials were shifted. Electron extraction from encapsulated Car was clearly achieved. However, electrochemical characteristics of Car inside SWCNTs and doping mechanisms elucidated by calculations based on density-functional theory indicate the difficulty of charge manipulation of molecules inside SWCNTs due to the presence of strong on-site Coulomb repulsion energy at the molecules.