Ab initio molecular dynamics simulations of the oxygen reduction reaction on a Pt(111) surface in the presence of hydrated hydronium (H3O)(+)(H2O)2: direct or series pathway?

Charge carrier dynamics in conducting polymer PEDOT using ab initio molecular dynamics simulations

As conducting polymers become increasingly important in electronic devices, understanding their charge transport is essential for material and device development. Various semi-empirical approaches have been used to describe temporal charge carrier dynamics in these materials, but there have yet to be any theoretical approaches utilizing ab initio molecular dynamics. In this work, we develop a computational technique based on ab initio Car-Parrinello molecular dynamics to trace charge carrier temporal motion in archetypical conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). Particularly, we analyze charge dynamics in a single PEDOT chain and in two coupled chains with different degrees of coupling and study the effect of temperature. In our model we first initiate a positively charged polaron (compensated by a negative counterion) at one end of the chain, and subsequently displace the counterion to the other end of the chain and trace polaron dynamics in the system by monitoring bond length alternation in the PEDOT backbone and charge density distribution. We find that at low temperature (T = 1 K) the polaron distortion gradually disappears from its initial location and reappears near the new position of the counterion. At the room temperature (T = 300 K), we find that the distortions induced by polaron, and atomic vibrations are of the same magnitude, which makes tracking the polaron distortion challenging because it is hidden behind the temperature-induced vibrations. The novel approach developed in this work can be used to study polaron mobility along and between the chains, investigate charge transport in highly doped polymers, and explore other flexible polymers, including n-doped ones.

Discovering surface reaction pathways using accelerated molecular dynamics and network analysis tools

We present an automated method that maps surface reaction pathways with no experimental data and with minimal human interventions. In this method, bias potentials promoting surface reactions are applied to enable statistical samplings of the surface reaction within the timescale of ab initio molecular dynamics (MD) simulations, and elementary reactions are extracted automatically using an extension of the method constructed for gas- or liquid-phase reactions. By converting the extracted elementary reaction data to directed graph data, MD trajectories can be efficiently mapped onto reaction pathways using a network analysis tool. To demonstrate the power of the method, it was applied to the steam reforming of methane on the Rh(111) surface and to propane reforming on the Pt(111) and Pt₃Sn(111) surfaces. We discover new energetically favorable pathways for both reactions and reproduce the experimentally-observed materials-dependence of the surface reaction activity and the selectivity for the propane reforming reactions.

We present an automated method that maps surface reaction pathways with no experimental data and with minimal human interventions.

Direct correlation of oxygen adsorption on platinum-electrolyte interfaces with the activity in the oxygen reduction reaction

The oxygen reduction reaction (ORR) on platinum catalysts is essential in fuel cells. Quantitative predictions of the relative ORR activity in experiments, in the range of 1 to 50 times, have remained challenging because of incomplete mechanistic understanding and lack of computational tools to account for the associated small differences in activation energies (<2.3 kilocalories per mole). Using highly accurate molecular dynamics (MD) simulation with the Interface force field (0.1 kilocalories per mole), we elucidated the mechanism of adsorption of molecular oxygen on regular and irregular platinum surfaces and nanostructures, followed by local density functional theory (DFT) calculations. The relative ORR activity is determined by oxygen access to platinum surfaces, which greatly depends on specific water adlayers, while electron transfer occurs at a similar slow rate. The MD methods facilitate quantitative predictions of relative ORR activities of any platinum nanostructures, are applicable to other catalysts, and enable effective MD/DFT approaches.

Oxygen Reduction Reaction on N-Doped Graphene: Effect of Positions and Scaling Relations of Adsorption Energies

The goal of this study is to provide insight into the mechanism of the oxygen reduction reaction (ORR) on N-doped graphene surfaces. Using density functional theory and a computational hydrogen electrode model, we studied the energetics of the ORR intermediates, the effect of the position of the reaction site, and the effect of the position of the N modification relative to the active site on model graphene surfaces containing one or two N atoms. We found that scaling relations can be derived for N-doped graphenes as well, but the multiplicity of the surface should be taken into account. On the basis of the scaling relations between intermediates OOH* and OH*, the minimal overpotential is 0.33 V. Analysis of the data showed that N atoms in the meta position usually decrease the adsorption energy, but those in the ortho position aid the adsorption. The outer position on the zigzag edge of the graphene sheet also promotes the adsorption of oxygenated species, while the inner position hinders it. Looking at the most effective active sites, our analysis shows that the minimal overpotential can be approached with various doping arrangements, which also explains the contradicting results in the literature. The dissociative pathway was also investigated, but we found only one possible active site; therefore, this pathway is not really viable. However, routes not preferred thermodynamically pose the possibility of breaking the theoretical limit of the overpotential of the associative pathway.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

O2 activation by core-shell Ru13@Pt42 particles in comparison with Pt55 particles: a DFT study

The reaction of O₂ with a Ru₁₃@Pt₄₂ core-shell particle consisting of a Ru₁₃ core and a Pt₄₂ shell was theoretically investigated in comparison with Pt₅₅. The O₂ binding energy with Pt₅₅ is larger than that with Ru₁₃@Pt₄₂, and O-O bond cleavage occurs more easily with a smaller activation barrier (E _a) on Pt₅₅ than on Ru₁₃@Pt₄₂. Protonation to the Pt₄₂ surface followed by one-electron reduction leads to the formation of an H atom on the surface with considerable exothermicity. The H atom reacts with the adsorbed O₂ molecule to afford an OOH species with a larger E _a value on Pt₅₅ than on Ru₁₃@Pt₄₂. An OOH species is also formed by protonation of the adsorbed O₂ molecule, followed by one-electron reduction, with a large exothermicity in both Pt₅₅ and Ru₁₃@Pt₄₂. O-OH bond cleavage occurs with a smaller E _a on Pt₅₅ than on Ru₁₃@Pt₄₂. The lower reactivity of Ru₁₃@Pt₄₂ than that of Pt₅₅ on the O-O and O-OH bond cleavages arises from the presence of lower energy in the d-valence band-top and d-band center in Ru₁₃@Pt₄₂ than in Pt₅₅. The smaller E _a for OOH formation on Ru₁₃@Pt₄₂ than on Pt₅₅ arises from weaker Ru₁₃@Pt₄₂-O₂ and Ru₁₃@Pt₄₂-H bonds than the Pt₅₅-O₂ and Pt₅₅-H bonds, respectively. The low-energy d-valence band-top is responsible for the weak Ru₁₃@Pt₄₂-O and Ru₁₃@Pt₄₂-OH bonds. Thus, the low-energy d-valence band-top and d-band center are important properties of the Ru₁₃@Pt₄₂ particle.

Antiviral Effect of Visible Light-Sensitive CuxO/TiO2 Photocatalyst

Photocatalysis is an effective technology for preventing the spread of pandemic-scale viruses. This review paper presents an overview of the recent progress in the development of an efficient visible light-sensitive photocatalyst, i.e., a copper oxide nanoclusters grafted titanium dioxide (CuxO/TiO2). The antiviral CuxO/TiO2 photocatalyst is functionalised by a different mechanism in addition to the photocatalytic oxidation process. The CuxO nanocluster consists of the valence states of Cu(I) and Cu(II); herein, the Cu(I) species denaturalizes the protein of the virus, thereby resulting in significant antiviral properties even under dark conditions. Moreover, the Cu(II) species in the CuxO nanocluster serves as an electron acceptor through photo-induced interfacial charge transfer, which leads to the formation of an anti-virus Cu(I) species and holes with strong oxidation power in the valence band of TiO2 under visible-light irradiation. The antiviral function of the CuxO/TiO2 photocatalyst is maintained under indoor conditions, where light illumination is enabled during the day but not during the night; this is because the remaining active Cu(I) species works under dark conditions. The CuxO/TiO2 photocatalyst can thus be used to reduce the risk of virus infection by acting as an antiviral coating material.

DFT Calculations of the Adsorption States of O2 on OH/H2O-Covered Pt(111)

The adsorption of O₂ on Pt(111) was studied with Density Functional Theory calculations. Various adsorbed states of O₂ were evaluated on clean and OH/H₂O-covered Pt(111) surfaces at the solid/gas and solid/liquid interfaces. The results reveal that the adsorption of O₂ on OH/H₂O-covered Pt(111) surface starts with the physical adsorption of O₂. Two other adsorption states are reachable from the physisorbed state, the end-on, and bridging chemisorbed O₂. Analysis of the energetics of these adsorption states shows that O₂ physically adsorbed at the OH/H₂O-covered Pt( 111) surface is a high energy state that requires activation to transition to the end-on chemisorbed O₂ state. On the other hand, the end-on chemisorbed state can transition to the bridging chemisorbed state with only a small activation energy when a nearby Pt adsorption site is available. Frequency analysis of the physisorbed, end-on, and bridging adsorption states shows that adsorbed O₂ stretching frequencies are close to 1400, 1300, and 900 cm^-1, respectively.

Theoretical Study on a Nitrogen-Doped Graphene Nanoribbon with Edge Defects as the Electrocatalyst for Oxygen Reduction Reaction

Both theory and experiment show that sp2 carbon nanomaterials doped with N have great potential as high-efficiency catalysts for oxygen reduction reactions (ORR). At present, there are theoretical studies that believe that C-sites with positive charge or high-spin density values have higher adsorption capacity, but there are always some counter examples, such as the N-doped graphene nanoribbons with edge defects (ND-GNR) of this paper. In this study, the ORR mechanism of ND-GNR was studied by density functional theory (DFT) calculation, and then the carbon ring resonance energy was analyzed from the perspective of chemical graph theory to elucidate the cause and distribution of active sites in ND-GNR. Finally, it was found that the overpotential of the model can be adjusted by changing the width of the model or dopant atoms while still ensuring proper adsorption energy (between 0.5 and 2.0 eV). The minimum overpotential for these models is approximately 0.36 V. These findings could serve as guidelines for the construction of efficient ORR carbon nanomaterial catalysts.

The Fe-N-C oxidase-like nanozyme used for catalytic oxidation of NOM in surface water

The removal of natural organic matter (NOM), particularly humic substances (HS) from surface waters during drinking water treatment is necessary to avoid various water quality problems in supply, such as the formation of disinfection by-products. As an alternative to conventional processes (e.g. coagulation), and in the light of the rapidly increasing applications of nanozyme in bio-catalysis, a novel Fe-N-C oxidase-like nanozyme (FeNZ) has been prepared and used to catalyze the oxidative degradation of NOM during simple aeration. Using humic acid (HA) as a model NOM it was found that the HA removal (as TOC) was increased by a factor of 6 with a low dose (10 mg/L) of FeNZ compared to an aerated solution without FeNZ. A variety of analytical methods was used to investigate the oxygen reduction reaction, including cyclic voltammetry, electron spin resonance, and density functional theory (DFT) simulation. Based on these studies, a catalytic oxidation mechanism described as "adsorption-activation-oxidation" was proposed. The enhanced NOM removal performance of FeNZ catalytic oxidation was confirmed with samples of natural surface water in terms of organic mineralization and conversion of hydrophobic to hydrophilic components. The results show great potential for the use of oxidase-like nano catalytic materials in the field of water treatment.

Observation of 18O/16O isotope effects at the cathode of a polymer electrolyte membrane fuel cell

¹⁸O/¹⁶O isotope effects were observed at the cathode of a polymer electrolyte membrane fuel cell at 25 and 35°C. Results of experiments in which the ¹⁸O/¹⁶O isotope ratios of the oxygen gases supplied to and exhausted from the cell were measured revealed that the lighter isotope ¹⁶O reacted more preferentially to form water molecules at the cathode than the heavier one, ¹⁸O. The value of the oxygen isotope separation factor, S₁, defined as the ratio of the ¹⁸O/¹⁶O isotope ratios of the oxygen gases supplied to and exhausted from the cell, ranged from 1.0030 to 1.0139, and tended to decrease with decreasing rate of oxygen utilisation (θ) and with increasing flow rate of the feed oxygen gas (D_F). The value of another separation factor, S₂, defined as the ratio of the ¹⁸O/¹⁶O isotope ratios of the exhausted oxygen gas and oxygen having reacted to form water molecules at the cathode, ranged from 1.0049 to 1.0304. The S₂ value was much less affected by the change in θ and D_F than the S₁ value with the majority of the S₂ value being in the range of 1.0240-1.0304.

Guiding Principles for Designing Highly Efficient Metal-Free Carbon Catalysts

Carbon nanomaterials are promising metal-free catalysts for energy conversion and storage, but the catalysts are usually developed via traditional trial-and-error methods. To rationally design and accelerate the search for the highly efficient catalysts, it is necessary to establish design principles for the carbon-based catalysts. Here, theoretical analysis and material design of metal-free carbon nanomaterials as efficient photo-/electrocatalysts to facilitate the critical chemical reactions in clean and sustainable energy technologies are reviewed. These reactions include the oxygen reduction reaction in fuel cells, the oxygen evolution reaction in metal-air batteries, the iodine reduction reaction in dye-sensitized solar cells, the hydrogen evolution reaction in water splitting, and the carbon dioxide reduction in artificial photosynthesis. Basic catalytic principles, computationally guided design approaches and intrinsic descriptors, catalytic material design strategies, and future directions are discussed for the rational design and synthesis of highly efficient carbon-based catalysts for clean energy technologies.

Covalent Organic Framework Electrocatalysts for Clean Energy Conversion

Covalent organic frameworks (COFs) are promising for catalysis, sensing, gas storage, adsorption, optoelectricity, etc. owning to the unprecedented combination of large surface area, high crystallinity, tunable pore size, and unique molecular architecture. Although COFs are in their initial research stage, progress has been made in the design and synthesis of COF-based electrocatalysis for the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and CO₂ reduction in energy conversion and fuel generation. Design principles are also established for some of the COF materials toward rational design and rapid screening of the best electrocatalysts for a specific application. Herein, the recent advances in the design and synthesis of COF-based catalysts for clean energy conversion and storage are presented. Future research directions and perspectives are also being discussed for the development of efficient COF-based electrocatalysts.

A post Gurney quantum mechanical perspective on the electrolysis of water: ion neutralization in solution

Electron fluxes crossing the interface between a metallic conductor and an aqueous environment are important in many fields; hydrogen production, environmental scanning tunnelling microscopy, scanning electrochemical microscopy being some of them. Gurney (Gurney 1931 Proc. R. Soc. Lond.134, 137 (doi:10.1098/rspa.1931.0187)) provided in 1931 a scheme for tunnelling during electrolysis and outlined conditions for it to occur. We measure the low-voltage current flows between gold electrodes in pure water and use the time-dependent behaviour at voltage switch-on and switch-off to evaluate the relative contribution to the steady current arising from tunnelling of electrons between the electrodes and ions in solution and from the neutralization of ions adsorbed onto the electrode surface. We ascribe the larger current contribution to quantum tunnelling of electrons to and from ions in solution near the electrodes. We refine Gurney's barrier scheme to include solvated electron states and quantify energy differences using updated information. We show that Gurney's conditions would prevent the current flow at low voltages we observe but outline how the ideas of Marcus (Marcus 1956 J. Chem. Phys.24, 966-978 (doi:10.1063/1.1742723)) concerning solvation fluctuations enable the condition to be relaxed. We derive an average barrier tunnelling model and a multiple pathways tunnelling model and compare predictions with measurements of the steady-state current-voltage relation. The tunnelling barrier was found to be wide and low in agreement with other experimental studies. Applications as a biosensing mechanism are discussed that exploit the fast tunnelling pathways along molecules in solution.

Capturing Solvation Effects at a Liquid/Nanoparticle Interface by Ab Initio Molecular Dynamics: Pt201 Immersed in Water

Solvation can substantially modify the adsorption properties of heterogeneous catalysts. Although essential for achieving realistic theoretical models, assessing such solvent effects over nanoparticles is challenging from a computational standpoint due to the complexity of those liquid/metal interfaces. This effect is investigated by ab initio molecular dynamics simulations at 350 K of a large platinum nanoparticle immersed in liquid water. The first solvation layer contains twice as much physisorbed water molecules above the terraces, than chemisorbed ones located only at edges and corners. The solvent stabilizes the binding energy of chemisorbates: 66% of the total gain comes from interactions with physisorbed molecules and 34% from the influence of bulk liquid.

Visible-Light-Sensitive Photocatalysts: Nanocluster-Grafted Titanium Dioxide for Indoor Environmental Remediation

Photocatalytic degradation of organic compounds requires photoexcited holes with strong oxidative power in the valence band (VB) of semiconductors. Although numerous types of doped semiconductors, such as nitrogen-doped TiO2, have been studied as visible-light-sensitive photocatalysts, the quantum yields of these materials were very low because of the limited oxidation power of holes in the nitrogen level above the VB. Recently, we developed visible-light-sensitive Cu(II) and Fe(III) nanocluster-grafted TiO2 using a facile impregnation method and demonstrated that visible-light absorption occurs at the interface between the nanoclusters and TiO2, as electrons in the VB of TiO2 are excited to the nanoclusters under visible-light irradiation. In addition, photogenerated holes in the VB of TiO2 efficiently oxidize organic contaminants, and the excited electrons that accumulate in nanoclusters facilitate the multielectron reduction of oxygen. Notably, Cu(II) and Fe(III) nanocluster-grafted TiO2 photocatalyst has the highest quantum yield among reported photocatalysts and has antiviral, self-cleaning, and air purification properties under illumination by indoor light fixtures equipped with white fluorescent bulbs or white light-emitting diodes.

Reaction sampling and reactivity prediction using the stochastic surface walking method

The new theoretical method demonstrates the ability of automated reaction sampling and activity prediction for complex organic reactions.

Coverage-dependent thermodynamic analysis of the formation of water and hydrogen peroxide on a platinum model catalyst

A DFT-based thermodynamic analysis of the adsorption properties of surface intermediates involved in the formation of water and hydrogen peroxide has been proposed at low and high coverages (353 K and 1 atm).

Ab initio and semi-empirical Molecular Dynamics simulations of chemical reactions in isolated molecules and in clusters

Recent progress in “on-the-fly” trajectory simulations of molecular reactions, using different electronic structure methods is discussed, with analysis of the insights that such calculations can provide and of the strengths and limitations of the algorithms available.

A density functional theory study on oxygen reduction reaction on nitrogen-doped graphene

Nitrogen (N)-doped carbons reportedly exhibit good electrocatalytic activity for the oxygen reduction reaction (ORR) of fuel cells. This work provides theoretical insights into the ORR mechanism of N-doped graphene by using density functional theory calculations. All possible reaction pathways were investigated, and the transition state of each elementary step was identified. The results showed that OOH reduction was easier than O-OH breaking. OOH reduction followed a direct Eley-Rideal mechanism (the OOH species was in gas phase, but H was chemisorbed on the surface) with a significantly low reaction barrier of 0.09 eV. Pathways for both four-electron and two-electron reductions were possible. The rate-determining step of the two-electron pathway was the reduction of O₂ (formation of OOH), whereas that of the four-electron pathway was the reduction of OH into H₂O. After comparing the barriers of the rate-determining steps of the two pathways, we found that the two-electron pathway was more energetically favored than the four-electron pathway.

New insights into the oxygen reduction reaction mechanism on Pt(111): a detailed electrochemical study

The oxygen reduction reaction (ORR) is undoubtedly the most important fuel-cell cathodic reaction. In this work, a detailed electrochemical analysis of the ORR on Pt(111) in nonadsorbing electrolytes was performed, which included the high-potential region Eup =1.15 V while ensuring the electrode surface structure stability. Our results suggest that the reduction of a soluble intermediate species formed during the ORR is the rate-determining step in the whole reaction mechanism. This species does not undergo any other electrochemical reaction at E>0.9 V and may accumulate close to the electrode surface. Together with dissolved O₂, this intermediate may modify the oxide-growth dynamics on Pt(111). Hence, both species interact with the electrode surface through complex catalytic networks. Under certain experimental conditions, oxygenated species from the oxidation of Pt(111) may enhance the overall ORR current. These results propose an alternative to explain the current state of the art for this fundamental process.

Effect of microstructure of nitrogen-doped graphene on oxygen reduction activity in fuel cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.

Hydrogen peroxide electrochemistry on platinum: towards understanding the oxygen reduction reaction mechanism

Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H(2)O(2) participates as an intermediate or not. The H(2)O(2) oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO(4). The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H(2)O(2) interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H(2)O(2) is oxidized to O(2) by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H(2)O(2) reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the "ORR-corrected mixed potential" (c-MP).

Electrolysis of water in the diffusion layer: first-principles molecular dynamics simulation

With Car-Parrinello molecular dynamics simulations the elementary reaction steps of the electrolysis of bulk water are investigated. To simulate the reactions occurring near the anode and near the cathode, electrons are removed or added, respectively. The study focuses on the reactions in pure water. Effects depending on a particular electrode surface or a particular electrolyte are ignored. Under anodic conditions, the reaction continues till molecular oxygen is formed, under cathodic conditions the formation of molecular hydrogen is observed. In addition the formation of hydrogen peroxide is observed as an intermediate of the anodic reaction. The simulations demonstrate that the electrochemistry of oxygen formation without direct electrode contact can be explained by radical reactions in a solvent. These reactions may involve the intermediate formation of ions. The hydrogen formation is governed by rapid proton transfers between water molecules.