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Electron transfer reactions are the most important processes at electrochemical interfaces. They are determined by the interplay between the interaction of the reactant with the solvent and the electronic levels of the electrode surface. Theoretical treatments only based on Density Functional Theory calculations are not sufficient. This review emphasizes mainly the effect of the electronic structure of the electrode material on electron transfer under different kinetic regimes. Our goal is to understand experimental results in the framework of a theory valid for arbitrary strengths of electronic coupling.

Following a renaissance from the 1980s, electrochemistry has developed into sophisticated interdisciplinary science integrating solid-state and surface science and further, biological sciences. Most remarkably, even the single-molecule can now be addressed. Single-molecule electrochemistry covers transition metal complexes, organic redox molecules, fragile biomolecules, and molecular-scale hybrids between metallic nanoparticles and complex molecules, supported by new theoretical frames. We overview here selected areas of molecular scale electrochemistry. After a theoretical minimum, we address complex molecules in electron transfer, enzyme catalysis, and nanoparticle catalysis. Our focus is on issues not so much previously highlighted, such as competition between superexchange and sequential conduction, and resonance features in the transition between the two limits. Another aspect is coherent multi-electron transfer for large bias voltages often needed to drive enough current through solute molecular junctions. We note finally some single-molecule perspectives relating to DNA-based molecules and to spin transitions via chiral molecules.

The effect of a gold underlayer on the outer-sphere non-adiabatic electron transfer on a graphene surface is investigated theoretically using both periodic and cluster DFT calculations. We propose a model that describes the alignment of energy levels and charge redistribution at the metal/graphene/redox electrolyte interface. Model calculations were performed for the [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ redox couples. It is shown that the gold support increases the rate constant of electron transfer. Gold electronic states hybridize with graphene wave functions, which provides an effective overlap with reactant orbitals outside the graphene layer and favors an increasing reaction rate. Although the Fermi level shift relative to the Dirac point in graphene depends significantly on the redox couple, this weakly affects the electron transfer kinetics at the Au(111)/graphene/electrolyte interface due to a small contribution of graphene states to the rate constant as compared to gold ones.

We report a combined experimental and computational study on the heterogeneous electron transfer kinetics for a simple one electron transfer reaction (ferrocene/ferrocenium Fc+/Fc couple) in a series of molecular solvents and ionic liquids. We focus on the diagnostics of the electron transfer regime (adiabatic vs nonadiabatic) and assess the parameters of the quantum mechanical electron transfer theory, which determine the observed tendencies in the solvent effect on the electron transfer rates. The applicability of the linear plots of the electron transfer rate constant vs longitudinal relaxation time (or solvent viscosity) for distinguishing between different ET kinetic regimes is analyzed. Classical molecular dynamics simulations were performed to calculate the potential of mean force for Fc and Fc+. The structure of the reaction layer derived from molecular dynamics is thoroughly investigated. The experimental dielectric spectra for the both type of solvents are used for quantum corrections of the out...

Nickel is a promising alternative to noble metal catalysts for hydrogen electrocatalysis in alkaline membrane fuel cells and electrolyzers. To boost its electrocatalytic activity, it is often associated with other metals, like Cu. In this work, we combine density functional theory (DFT), Monte Carlo calculations and kinetic modeling in order to shed light on the origin of the experimentally observed activity enhancement on bimetallic NiCu nanoparticles containing ca. 5% of Cu compared to pure Ni (Topics in Catalysis, 58, (2015), 1181–1192; J. Electroanal. Chem., 783, (2016), 146–151). It is shown that the bifunctional HOR mechanism, where H ad and OH ad recombine to form water, is not viable on the monometallic Ni electrode because of the strong hydrogen adsorption and the high activation energy barrier of the recombination reaction. On bimetallic NiCu surfaces, DFT calculations predict a significant weakening of H ad adsorption energy as well as a decrease of the activation barrier of the recombination reaction for ca. 50% Cu surface coverage. Monte Carlo simulations reveal that the surface of NiCu nanoparticles is enriched in Cu atoms even for a small ( c a . 5%) Cu atomic fraction in the bulk. Kinetic modeling suggests that the experimental data for NiCu nanoparticles can be well reproduced either considering fast recombination step (bifunctional mechanism) or fast (10-fold enhancement compared to pure Ni) Volmer step (Heyrovsky-Volmer mechanism).

We investigate current–voltage curves for describing electron transfer across the basal plane of a graphene electrode, considering a wide range of electronic coupling. Emphasis is put on the specif...

Since the discovery of graphene, this material is in the focus of intensive research in the field of electrochemical energy storage devices. Numerous experimental works demonstrate, however, contradictory results on the electron transfer kinetics at a graphene electrode, in particular, on electrocatalytic properties of graphene edges. In this work, we explore the spatially resolved electrochemistry of the basal plane and edge sites in the outer-sphere electron transfer step at the oxygen reduction reaction in acetonitrile solutions using theories and computations. We employ a quantum mechanical theory and explore the graphene–O2 orbital overlap resting on the results of density functional theory calculations and some data obtained earlier from molecular dynamics simulations. The zigzag graphene edges provide ca. 4 times faster electron transfer (in comparison with the basal plane) at long separations, >4 A, i.e., in the diabatic regime. At the same time, effective rate constants obtained by integrating ov...

Structural mapping of intermediate size and large molecules and biomolecules at ultra-high resolution using single-crystal electrodes and in situ scanning tunnelling microscopy continues to disclose surprising findings. In situ scanning tunnelling spectroscopy has also recently disclosed new electrochemical conductivity features at the level of the single molecule. We overview briefly elements of this development over the last few years, with focus on three recent discoveries: (1) a new packing mode of a core monolayer target thiol, the amino acid cysteine (Cys) on Au(100)-electrodes, quite different from Cys packing on Au(111)- and Au(110)-electrodes; (2) transition of a core ferrocene in situ scanning tunnelling spectroscopy probe from stochastic single-molecule to macroscopic behaviour, a concept at the heart of nanoscience; and (3) unexpected behaviour of the large molybdenum enzyme sulfite oxidase, when going from macroscopic to single-molecule electrochemistry. We compare these studies with other recent discoveries of single-molecule protein conductivity and molecular scale inorganic nanostructures.

Electrochemical electron transfer (ET) of transition metal complexes or redox metalloproteins can be catalyzed by more than an order of magnitude by molecular scale metallic nanoparticles (NPs), often rationalized by concentration enhancement of the redox molecules in the interfacial region, but collective electronic AuNP array effects have also been forwarded. Using DFT combined with molecular electrochemical ET theory we explore here whether a single molecular scale Au nanocluster (AuC) between a Au (111) surface and the molecular redox probe ferrocene/ferricinium (Fc/Fc+) can trigger an ET rate increase. Computational challenges limit us to Au n Cs (n up to 147), which are smaller than most electrocatalytic AuCs studied experimentally. AuC-coating thiols are addressed both as adsorption of two S atoms at the structural Au55 bridge sites and as superexchange of variable-size AuCs via a single six-carbon alkanethiyl bridge. Our results are guiding, but enable comparing many AuC surface details (apex, ridge, face, direct vs superexchange ET) with a planar Au(111) surface. The rate-determining electronic transmission coefficients for ET between Fc/Fc+ and AuC are highly sensitive to subtle AuC electronic features. The transmission coefficients mostly compete poorly with direct Fc/Fc+ ET at the Au(111) surface, but Fc/Fc+ 100 face-bound on Au79 and Au147 and ridge bound on Au19 leads to a 2- or 3-fold rate enhancement, in different distance ranges. Single AuCs can thus indeed cause rate enhancement of simple electrochemical ET, but additional, possibly collective AuNC effects, as well as larger clusters and more complete coating layers, also need to be considered.

Ion pairing can strongly affect the rates of electron transfer reactions. To explain this effect we propose a model Hamiltonian which describes the interactions between the pairing ion and the reactant, solvent and inner sphere reorganization, and bond breaking. Explicit expressions for the energies of the initial and final states, and for the energy of activation are derived in the weak adiabatic limit. The model is applied to the reduction of Cu(II) in the presence of chloride ions. For this purpose the pertinent system parameters are obtained from density functional theory. Our model explains, why chloride enhances the rate of the first electron transfer in copper deposition. Fil: Nazmutdinov, Renat. Kazan National Research Technological University; Rusia Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina Fil: Colombo, Estefanía. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina Fil: Santos, Elisabeth. Universitat Ulm. Faculty Of Natural Sciences. Institute Of Theoretical Physics; Alemania Fil: Schmickler, Wolfgang. Universitat Ulm. Faculty Of Natural Sciences. Institute Of Theoretical Physics; Alemania

The data on the rate constants for ferrocene/ferrocenium (Fc/Fc + ) electron transfer in ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF 4 ]) are reported for a series of -S-(CH 2 ) n -Fc adlayers (n = 8…16) on polycrystalline gold. The values and trends are compared to the previously published data for similar series in aqueous medium, as well as to the results for ‘free’ Fc at -S-(CH 2 ) n − 1 -CH 3 modified gold in the same ionic liquid. For the latter case, the trend is opposite as compared to that in the available aqueous series (attached and ‘free’ ruthenium ammine complex): the ratio of rate constants for attached and ‘free’ reactants decreases with electrode-reactant distance in the former case, and increases in the latter. The presented comparison discovers the role of reactant and product work terms, which are essential for attached reactants as well because of conformational diversity. Corresponding contributions are estimated from molecular modeling and considered in terms of reaction volume.

The two-electron reduction of some Ni(II) complexes with α-diimine ligands ([Ni(bpy)3]2+ and [Ni(pybox)2]2+) at a platinum electrode from acetonitrile (AN) and dimethylformamide (DMF) solutions is investigated by cyclic voltammetry (CV). Digisim simulations are employed to determine kinetic parameters. For [Ni(bpy)3]2+ in AN the first electron transfer (ET) step was found to be rate-controlling. The rate constants of the first and second steps (k1 and k2) are practically the same in the DMF solution. In contrast, the second ET step is rate controlling for the [Ni(pybox)2]2+ complex in AN, while the difference between k1 and k2 for this complex reactant in DMF becomes less noticeable. Molecular modeling based on the quantum mechanical theory of electron transfer and on density functional theory (DFT) calculations is performed for the first time to predict the activation barriers of the two-step heterogeneous redox processes and to elucidate the role of the ligands and solvents in the kinetics of these processes. Three-dimensional reaction free energy surfaces are constructed along the solvent and intramolecular coordinates. The electronic transmission coefficients are estimated as well. The results of model calculations are in qualitative agreement with the experimental data.

The structure of bimetallic NiCu nanoparticles (NP) is investigated as a function of their composition and size by means of Lattice MonteCarlo (LMC) and molecular dynamics (MD) simulations. According to our results, copper segregation takes place at any composition of the particles. We found that this feature is not size-dependent. In contrast, nickel segregation depends on the NP size. When the size increases, Ni atoms tend to remain in the vicinity of the surface and deeper. For smaller NPs, Ni atoms are located at the surface as well. Our results also showed that most of the metal atoms segregated at the surface area were found to decorate edges and/or form islands. Our findings agree qualitatively with the experimental data found in the literature. In addition, we comment on the reactivity of these nanoparticles. Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Belletti, Gustavo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina Fil: Shermukhamedov, S. A.. Kazan National Research Technological University; Rusia Fil: Glukhov, D. V.. Kazan National Research Technological University; Rusia Fil: Santos, Elizabeth del Carmen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Institute of Theoretical Chemistry; Alemania. Universitat Ulm; Alemania Fil: Schmickler, Wolfgang. Universitat Ulm; Alemania. Institute of Theoretical Chemistry; Alemania Fil: Nazmutdinov, Renat. Kazan National Research Technological University; Rusia

Cysteine (Cys) is an essential amino acid with a carboxylic acid, an amine and a thiol group. We have studied the surface structure and adsorption dynamics of L-cysteine adlayers on Au(100) from aqueous solution using electrochemistry, high-resolution electrochemical scanning tunnelling microscopy (in situ STM), and molecular modelling. Cys adsorption on this low-index Au-surface has been much less studied than Cys adsorption on Au(111)- and Au(110)-electrode surfaces. Chronopotentiometry was employed to monitor the adsorption dynamics at sub-second resolution and showed that adsorption is completed in 30 minutes at Cys concentrations above 100 μM. Two consecutive steps could be fitted to these data. Two separate reductive desorption peaks of Cys adlayers on Au(100) with a total coverage of 2.52 (±0.15) × 10-10 mol/cm2 were observed. In situ STM showed that adsorbed Cys is organized in stripes with “fork-like” features which co-exist in (11×2)-2Cys and (7×2)-2Cys lattices, quite differently from Cys adsorption on Au(111)-electrode surfaces. Stripe structures with bright STM contrast in the center suggest that a second Cys adlayer on top of a first adlayer is formed, supporting the dual-peak reductive desorption of Cys adlayers. In addition, monolayers of both pure L-Cys, pure D-Cys and a 1:1 racemic mixture of L- and D-Cys on Au(100) were studied. Virtually identical macroscopic electrochemical features were found, but in situ STM discloses many more defects for the racemic mixture than for the pure enantiomers due to structural mismatch of L- and D-Cys. Density functional theory (DFT) calculations combined with a cluster model for the Au(100) surface were carried out to investigate the adsorption energy and geometry of adsorbed monomer and dimer Cys species in different orientations, with detailed attention to the chirality effects. Optimized DFT geometries were used to construct model STM images, and kinetic Monte Carlo simulations undertaken to illuminate the growth of adsorbate rows and the mechanism of the adlayer formation as well as the Cys adsorption patterns specific to the Au(100)-electrode surface.

Cerium redox couple is widely investigated in aqueous flow batteries due to its high redox potential. While mixed supporting electrolytes of methanesulfonic acid (MSA) and sulfuric acid have been adopted to increase the solubility and reversibility of the cerium redox reaction, the influence of such mixed ligands on the electron transfer is absent. Herein, the density functional theory is applied to study the activation energy of the electron transfer, which is in agreement with the reaction kinetics measured by electrochemical impedance spectroscopy. Cerium complexes with different ratios of methanesulfonate and sulfate ligands are assumed with optimized geometries. The standard redox potentials of these cerium complexes are calculated, which follows the same trend as the open circuit potentials of the cerium electrolyte. The exchange current densities of cerium redox couples in the electrolytes with different ratios of MSA and sulfuric acid are 0.16–0.28 mA cm−2 on glassy carbon electrode, qualitatively conforming the small variation of activation energies.

Theoretical treatments of electrochemical reactions at semiconductors are usually based on theories that presume a weak interaction between the reactant and the electrode. Herein a theory is developed that is valid for arbitrary interaction strengths, and its consequences are explored within a simple coupling scheme. This model can be used as a framework for the investigation of specific catalytic reactions including photoelectrocatalysis.

Outer-sphere electron transfer (ET) between optically active transition-metal complexes and either other transition-metal complexes or metalloproteins is a prototype reaction for kinetic chirality. Chirality as the ratio between bimolecular rate constants of two enantiomers mostly amounts to 1.05-1.2 with either the Λ or Δ form the more reactive, but the origin of chirality in ET parameters such as work terms, electronic transmission coefficient, and nuclear reorganization free energy has not been addressed. We report a study of ET between the Λ-/Δ-[Co(Ox)3](3-) pair (Ox = oxalate) and horse heart cytochrome c (cyt c). This choice is prompted by strong ion-pair formation that enables separation into inter-reactant interaction (chiral "recognition") and ET within the ion pair ("stereoselectivity"). Chiral selectivity was first addressed experimentally. Λ-[Co(Ox)3](3-) was found to be both the more strongly bound and faster reacting enantiomer expressed respectively by the ion-pair formation constant KX and ET rate constant kET(X) (X = Λ and Δ), with KΛ/KΔ and kET(Λ)/kET(Δ) both ≈1.1-1.2. rac-[Co(Ox)3](3-) behavior is intermediate between those of Λ- and Δ-[Co(Ox)3](3-). Chirality was next analyzed by quantum-mechanical ET theory combined with density functional theory and statistical mechanical computations. We also modeled the ion pair K(+)·[Co(Ox)3](3-) in order to address the influence of the solution ionic strength. The complex structure of cyt c meant that this reactant was represented solely by the heme group including the chiral axial ligands L-His and L-Met. Both singlet and triplet hemes as well as hemes with partially deprotonated propionic acid side groups were addressed. The computations showed that the most favorable inter-reactant configuration involved a narrow distance and orientation space very close to the contact distance, substantiating the notion of a reaction complex and the equivalence of the binding constant to a bimolecular reaction volume. The reaction is significantly diabatic even at these short inter-reactant distances, with electronic transmission coefficients κel(X) = 10(-3)-10(-2). The computations demonstrated chirality in both KX and κel(X) but no chirality in the reorganization and reaction free energy (driving force). As a result of subtle features in both KX and κel(X) chirality, the "operational" chirality κET(Λ)KΛ/κET(Δ)KΔ emerges larger than unity (1.1-1.2) from the molecular modeling as in the experimental data.

Mixtures of ethylene glycol with water are a prominent example of media with variable viscosity. Classical molecular dynamics simulations at room temperature were performed for mixtures of ethylene glycol (EG) and water with EG mole fractions of xE = 0.0, 0.1, 0.2, 0.4, 0.6, 0.9, 1.0. The calculated dielectric loss spectra were in qualitative agreement with experiment. We found a slightly overestimated slowdown of the dynamics with increasing EG concentration compared to experimental data. Statistics of the hydrogen bond network and hydrogen bond lifetimes were derived from suitable time correlation functions and also show a slowdown in the dynamics with increasing xE. A similar picture is predicted for the time scales of EG conformer changes and for molecular reorientation. A slight blue shift was obtained for the power spectra of the molecular center of mass motion. The results were used to give a qualitative interpretation of the origin of three different relaxation times that appear in experimental complex dielectric spectra and of their change with xE.

The capacitive behaviour of interfaces between aqueous solutions of non-specifically adsorbing electrolytes and Hg, Bi(111) and Ga electrodes has been studied on the basis of a quantum chemical approach and using a simple lattice model in the mean field approximation. Dependencies of the compact layer differential capacitance (CH) on the electrode surface charge density (σ) have been constructed and compared with the experimental data. A significant difference in the CH(σ) dependence observed in the experiment with different metal electrodes has been explained in terms of the interplay between contributions from metal electrons and adsorbed solvent molecules. New data elucidating the metal–water adsorption bond have been reported as well.

A possibility to synthesize heteronuclear compounds from aqueous solutions containing the Fe(II), Fe(III), Al(III), Cl− and OH− ions is explored. The XRF analysis, IR-spectroscopy, a model based on the potentiometric titration data, as well as molecular modeling and density functional theory are employed to determine the elemental and phase composition and the structure of synthesized compounds. The most probable forms of the heteronuclear complexes are proposed on the basis of a combined quantum chemical and spectroscopic analysis.

State-of-the-art in the area of quantum-chemical modeling of electron transfer (ET) processes at metal electrode/electrolyte solution interfaces is reviewed. Emphasis is put on key quantities which control the ET rate (activation energy, transmission coefficient, and work terms). Orbital overlap effect in electrocatalysis is thoroughly discussed. The advantages and drawbacks of cluster and periodical slab models for a metal electrode when describing redox processes are analyzed as well. It is stressed that reliable quantitative estimations of the rate constants of interfacial charge transfer reactions are hardly possible, while predictions of qualitatively interesting effects are more valuable. © 2015 Wiley Periodicals, Inc.

Effects of adding varied amounts of copper to carbon-supported nickel particles on their structure, composition and electrocatalytic activity for the hydrogen oxidation and evolution reactions in alkaline medium have been explored. Ni1-x Cu x /C catalysts were prepared by the incipient wetness impregnation. Comprehensive characterization of the catalysts included X-ray powder diffraction, X-ray photoelectron spectroscopic, transmission electron microscopic and cyclic voltammetric analyses, while atomistic Monte Carlo simulations have been undertaken to obtain further insights into the structure of the bimetallic NiCu nanoparticles. We found that compared to monometallic Ni, NiCu nanoparticles show lower propensity towards oxidation under ambient conditions. Furthermore, we report that adding Cu allows increasing the surface-weighted electrocatalytic activity, and the specific surface area of Ni1-x Cu x /C electrodes, both contributing to a ca four-fold enhancement of the mass-weighted activity. The nature of the synergistic interactions between Ni and Cu is proposed on the basis of the analysis of experimental data and Monte Carlo structural modelling results.

Ionic and molecular permeability of monolayers of alkanethiols on the Au(111) surface in an aqueous NaF solution is investigated using the molecular dynamics (MD) method. The surface charge-driven penetration of the electrolyte components into the monolayer was found to have a threshold. The mechanism of ion permeation comprises two steps, (1) the formation of a polar channel of water molecules which are drawn into the nonpolar monolayer due to local electric field fluctuations (2) transport of ions through the polar channel to the Au(111) surface. It is concluded that the defect-free monolayers are impermeable for the electrolyte components at electrode potentials related to the stability region of the monolayer. The monolayer of hexadecanethiols (—SC16H33) is less permeable with that of hexanethiols (—SC6H13), this is explained in terms of a closer packing structure for longer hydrocarbon tails.

A quantum mechanical theory is employed to describe heterogeneous ferrocene (Fc)/ferrocenium (Fc+) electron transfer across Au(111)/[bmim][BF4] and Au(111)/acetonitrile interfaces. Classical molecular dynamics simulations were performed to calculate the potential of mean force for Fc and Fc+ and to estimate the solvent reorganization energy. The structure of the reaction layer and the solvent shell of Fc and Fc+ as derived from molecular dynamics are thoroughly investigated. The molecular structure of ferrocene and ferrocenium species, as well as the reactant–electrode orbital overlap are addressed on the basis of a quantum chemical approach. The experimental dielectric spectra for both types of solvents are used for quantum corrections of the outer-sphere reorganization energy as well as for estimations of the effective frequency factor in the limit of strong and weak electronic coupling. The dependence of the electronic transmission coefficient on the electrode–reactant distance is calculated for severa...

The mobilities and activities of ions in NaCl-glucose-water mixtures are probed by means of conductometry and potentiometry. A positive deviation of the limiting conductance from the Stokes-Einstein (SE) relation is observed. This striking effect is discussed in terms of ‘excluded volume’ concept for strongly heterogeneous liquids. A drastic difference between sodium cations and chloride anions is argued resting on measurements with ion selective electrodes. The microscopic structure of water-glucose solutions is thoroughly addressed by classical molecular dynamics (MD) method. The MD simulations reproduce qualitatively the main experimental findings; trends in the behavior of the Na+ and Cl− ions are attributed to a noticeable difference in the structure and dynamics of their solvation shells.

A kinetic model is developed to describe the heating and evaporation of a spherical nanoparticle under the influence of a scanning tunneling microscope (STM). Simulations were performed for silicon nanoparticles of different sizes and for different STM parameters. Different kinetic features of evaporation are predicted and discussed. The lifetime of the nanoparticles is estimated and compared with original experimental data obtained for layers of silicon nanoparticles formed upon magnetron sputtering and deposited on a highly oriented pyrolytic graphite surface (HOPG).

Adsorption of a H2O molecule on Bi, Ga and Hg electrode surfaces is studied in the framework of cluster model at the density functional theory (DFT) level. At bismuth(111) single crystal plane the hollow site is energetically more preferable for the H2O adsorption (− 31.1 kJ mol− 1), while the adsorption at top site of Hg and Ga metal surfaces is confirmed to be energetically the most preferable (− 35.6 and − 24.7 kJ mol− 1, respectively). The calculations for Bi(111), Hg, and Ga are further extended to include the effect of external electrical field, and data analysis is completed with the help of the mean field approximation in order to model adsorbed water behaviour in the H2O molecules' bilayer. An associate of 13 H2O molecules is modelled in order to address the influence of lateral interactions in a water bilayer. The Ga surface is argued to be more hydrophilic than the Bi(111) and Hg surfaces. Despite the weaker adsorption energy of a single H2O molecule at the Ga surface, water molecules at the Ga/water interface are additionally stabilized by stronger hydrogen bonds. We stress the important role of the H2O bilayer at a metal electrode surface, which depends on the atomic corrugation of a metal surface.

We investigate oxygen reduction in alkaline solution with the focus on gold and silver electrodes. On the basis of theoretical considerations we propose a detailed mechanism, which we support by explicit calculations. In accordance with previous suggestions we conclude that the first step is an outer-sphere electron transfer to the oxygen molecule. The breaking of he oxygen-oxygen bond involves adsorption of OH on the electrode surface. This is much stronger on Au(100) than on Au(111), which explains why the former is a better catalyst. Fil: Ignaczak, Anna. University of Lodz; Polonia Fil: Nazmutdinov, Renat. Kazan National Research Technological University; Rusia Fil: Goduljan, Aleksej. Universitat Ulm; Alemania Fil: Moreira de Campos Pinto, Leandro. Universidade Federal do Mato Grosso do Sul; Brasil. Universitat Ulm; Alemania Fil: Juarez, Maria Fernanda. Universitat Ulm; Alemania Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; Argentina Fil: Santos, Elizabeth del Carmen. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Schmickler, Wolfgang. Universitat Ulm; Alemania

The hydrogen absorption into overlayers of Pd deposited on Au(111) has been investigated by density functional theory (DFT). Hydrogen concentrations, absorption environments, and geometrical and electronic effects have been analyzed, seeking for a better understanding of the general principles governing the process and the effect of foreign supports. The results show that the absorption is more favored than in pure Pd leading to lower absorption energies and less repulsive interactions due to the surface expansion induced by the gold larger lattice constant. Our findings also suggest that the hydrogen absorption process is more favorable for a less number of Pd overlayers. This situation changes gradually until the substrate influence is no longer detected and the pure palladium nature appears. An entangled combination of repulsive forces, strain effect, structural ordering and chemical affinity has been found. The kinetics of hydrogen absorption has been studied as well. Two cases were explored: (1) the absorption of an adsorbed hydrogen atom and (2) the bond-breaking and penetration of a H2 molecule. Fil: Quaino, Paola Monica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina Fil: Nazmutdinov, Renat. Kazan National Research Technological University; Rusia Fil: Peiretti, Leonardo Federico. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina Fil: Santos, Elizabeth del Carmen. Ulm University; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina

Density functional theory (DFT) and molecular dynamics (MD) techniques are used to study proton transfer from an aqueous solution to an Ag(1 1 1) surface. DFT is applied to study Ag–water and Ag–hydronium interactions as well as proton transfer for small systems based on the cluster model. The data gained are then used to adjust an empirical Ag–water interaction potential and to reparametrize an empirical valence-bond (EVB) model, which has been successfully applied for the study of proton transfer to a Pt(1 1 1) surface before. Employing these force fields in MD simulations enables dynamic modeling of the electrolyte–metal interface on a scale large enough to give realistic results. Results from a MD trajectory study on Ag(1 1 1) are reported and compared to the analogous study for platinum. Low discharge rates on Ag(1 1 1) are observed, and the potential range for hydrogen evolution can be estimated. The different behavior relative to Pt(1 1 1) can be traced to features of the respective potential energy surfaces and to the different structural properties of the aqueous/metallic interfaces.

The adsorption of a water molecule on a basal Bi(111) electrode surface, crystallising in the rhombohedral system, has been studied in the framework of cluster model. The quantum chemical calculations were performed at the Density Functional Theory (DFT) level and the electrical double layer effects were analysed by using an external electric field. In contrast to computational predictions reported previously for other metal surfaces, crystallising in the face-centred cubic or hexagonal close-packed systems, a hollow site for Bi(111) was found to be energetically the most preferable; the water adsorption energy amounts to − 28 kJ mol− 1. In a wide range of surface charge densities the water molecule is bound preferentially through the O atom in orientation perpendicular to the surface plane. The Bi(111) hydrophilic properties are compared with those for other metals. Some adsorption characteristics of a hydrogen atom and a hydroxyl group at Bi(111) are reported as well, which give evidence in favour of the non-dissociative adsorption of water molecules.

The kinetics of single electron heteroatom reduction in Ce(IV)-decatungstate anion is demonstrated to be strongly dependent on the nature of the supporting electrolyte cation. DC polarography reveals an increase of both reduction rate at a positively charged mercury electrode and a limiting diffusion current in the Li + …Cs + sequence. Both effects look anomalous in the framework of traditional concepts of electrode kinetics. To clarify the nature of these observations, ionic association of Ce(IV)-decatungstate with various alkali metal cations was addressed experimentally by means of the potentiometric technique. Finally, our self-consistent interpretation of various cation nature effects rests on a difference in the number of bound cations and their location around the polyanion. The ionic association is modeled by using the results of DFT calculations of the CeW 10 anion, non-local electrostatics and an extended version of the Bjerrum model.

Recently, while studying the oxygen reduction reaction (ORR) in alkaline medium on Mn oxides with various crystal structures, we documented [1] an unparalleled activity of Mn2O3 oxide with bixbyite structure. Then we also observed a clear correlation of the electrocatalytic activity with the formal potential of Mn(IV/III) interfacial redox transition and suggested the ORR mechanism involving this transition as a mediator step. To extend our understanding of this complex mechanism, we also compared the behavior of various oxides in peroxide oxidation/reduction reactions [2] and in respect to the peroxide yield during the ORR. All materials were studied in the form of composites with Sibunit carbon (weight ratio 1:1). This carbon binder provides a high utilization of the oxide surface in electrocatalysis [3]. The voltammetry in deairated 1M NaOH solution and rotating disc electrode (RDE) measurements in the same but O2-saturated solution were used to determine the total charge and the ORR kinetic currents. Rotating ring disc electrode (RRDE) was applied to compare the peroxide yields on various Mn oxides. All the values were compared after normalizing them to the BET surface area. The number of electrons involved in the oxygen reduction was close to 4 and 3 for more and less active oxides, respectively. Less active oxides demonstrated significantly higher peroxide yields and slower peroxide reduction, with a convection-independent limiting current which documents a slow chemical step in a wide potential region. The independent experimental facts which can be considered for comparison with simulations are as follows: surface concentration of the Mn active centers (estimated from the total charge in Mn(IV/III) transition region); specific activity of pure carbon generating additional peroxide, which further reacts on the oxide component in carbon-oxide composites (see in Ref.[4] about the dual role of carbon); ORR polarization curves; polarization curves for peroxide reactions; peroxide yield during the ORR. This wide set of data provides a valuable input for multi-parametric simulations, which significantly decreases the number of possible solutions. The modeling involved the rate constants for five reaction steps as well as the parameter for interactions of the adsorbed oxygen species involved in the Mn(IV/III) transition (in terms of Frumkin isotherm). We found that the complete set of data can be interpreted only assuming the structural dependence of at least two rate constants: one at the initial stage (peroxide formation) and another at the final stage (peroxide reduction). Another intriguing possibility to explain the very high Mn2O3 activity in the ORR is to assume a direct ORR pathway for this oxide, and to consider a series pathway for all other oxides. To elucidate whether the latter hypothesis is reasonable, we addressed the O-O bond break on Mn2O3 and MnOOH (typical examples of high and low activities, respectively) in the framework of a quantum chemical approach resting on the cluster model of the oxide surface and the DFT level of theory. The computational results were used to map the effective reaction path. We found that for both model surfaces the direct bond cleavage at the oxide surface is hardly feasable both for the O2 molecule and most likely for peroxide intermediates. This finding makes it possible to exclude the direct pathway and to focus on molecular reasons of the structural effects on the rate constant for other steps such as the reduction of the adsorbed peroxide intermediate. The work is supported by EraNetRus program, project (#270 NANO-Morf). [1] A.S. Ryabova et al, Electrochimica Acta 187 (2016) 161–172. [2] A.S. Ryabova et al, ChemElectroChem 3 (2016) 1667 – 1677. [3] A.S. Ryabova et al, Electrochimica Acta 246 (2017) 643–653. [4] T. Poux et al, Catalysis Today 189 (2012) 83– 92.

The reduction of anions (considering hexacyanoferrate (III) as example) at charged conducting cylinders of nano-size, i.e., metal wires or carbon tubes, in contact with electrolyte solution is explored over a wide range of overpotentials. Two key parameters contributing to the current (solvent reorganization energy and work terms) are addressed in the framework of a quantum mechanical theory of electron transfer. It is argued that double layer effects play a crucial role and entail a significant rise of the current density as compared with plain metal electrodes. The stationary diffusion to a nanocylinder was found to proceed much faster, which results in an additional enhancement of the current. Some qualitative effects of the electron transfer across a conducting nanocylinder are discussed (in part, the appearance of an inverted Arrhenius plot).

The electrochemical reactivity of Co(III), Cr(III), and Ru(III) ammine complexes is discussed within the framework of a quantum chemical approach. The electronic structure of oxidized and reduced complex forms is addressed at the density functional theory level; intramolecular and solvent contributions to the Franck−Condon barrier, as well as electronic transmission coefficients are computed. Standard electrode potentials are estimated for certain redox pairs. The charge distribution in the reactants is employed to calculate electrostatic interactions with a mercury electrode surface and to make double layer corrections more realistic as compared to point/spherical reactant models. The model predictions are employed to explain the rate constant and transfer coefficient. Also discussed are trends in the available experimental data, including new original results on a comparative study of Co(III) and Cr(III) hexaammine complexes.

We have used L-cysteine (Cys) as a model system to study the surface electronic structures of single molecules at the submolecular level in aqueous buffer solution by a combination of electrochemical scanning tunneling microscopy (in situ STM), electrochemistry including voltammetry and chronocoulometry, and density functional theory (DFT) computations. Cys molecules were assembled on single-crystal Au(110) surfaces to form a highly ordered monolayer with a periodic lattice structure of c(2x2) in which each unit contains two molecules; this conclusion is confirmed by the results of calculations based on a slab model for the metal surface. The ordered monolayer offers a platform for submolecular scale electronic mapping that is an issue of fundamental interest but remains a challenge in STM imaging science and surface chemistry. Single Cys molecules were mapped as three electronic subunits contributed mainly from three chemical moieties: thiol (-SH), carboxylic (-COOH), and amine (-NH2) groups. The contrasts of the three subunits depend on the environment (e.g., pH), which affects the electronic structure of adsorbed species. From the DFT computations focused on single molecules, rational analysis of the electronic structures is achieved to delineate the main factors that determine electronic contrasts in the STM images. These factors include the molecular orientation, the chemical nature of the elements or groups in the molecule, and the interaction of the elements with the substrate and tip. The computational images recast as constant-current-height profiles show that the most favorable molecular orientation is the adsorption of cysteine as a radical in zwitterionic form located on the bridge between the Au(110) atomic rows and with the amine and carboxyl group toward the solution bulk. The correlation between physical location and electronic contrast of the adsorbed molecules was also revealed by the computational data. The present study shows that cysteine packing in the adlayer on Au(110) from the liquid environment is in contrast to that from the ultrahigh-vacuum environment, suggesting solvent plays a role during molecular assembly.

An empirical valence-bond (EVB) model is developed to describe the transfer of a proton from aqueous solution bulk to a metal electrode surface. The results of density functional calculations are used for parametrizing the model for a Pt(111) surface, which was chosen as a model system. Thereby, the metal surface is addressed in terms of the cluster model. The developed EVB model makes it possible to perform large-scale molecular dynamics (MD) simulations for a metal/electrolyte solution interface. Using MD we explore the proton transfer to a Pt(111) electrode surface with different negative surface charge densities. Preliminary conclusions are reported to elucidate some aspects of the proton transfer mechanism.

Mechanism of copper underpotential deposition at stepped faces of platinum single crystals Pt(hkl) is studied using cyclic voltammetry, scanning probe microscopy, and quantum-chemical modelling. It is shown that the first stage of UPD is one-dimensional decoration of the (100)- or (110)-orientated steps, then copper monolayer forms at (111)-terraces. The final stage is the secondary step decoration. Quantum-chemical modelling, with the using of long-distance potentials of the Cu-Pt and Cu-Cu pair interactions, allows estimating the energy of copper adsorption at different structure elements of the substrate (steps, kinks, terraces) and revealing the succession of the adatom monolayer formation; it also provides additional information for the identifying of the nature of voltametric peaks for different stages of the copper adsorption-desorption.

Solvent dynamics effects on the electroreduction of [PtCl3(H2O)]− at a mercury electrode are explored in the framework of Sumi–Marcus model using an efficient computational scheme. According to results of density functional calculations, the second electron transfer step may be regarded as rate controlling. The nonmonotonous influence of solvent viscosity on the reaction rate is predicted and explained in terms of the saddle point avoidance. The results of model calculations are employed to interpret experimental data reported earlier in the literature.

The mechanism of discharge of a hydronium ion at a mercury electrode is explored using a combined microscopic approach. The quantum mechanical aspects of the proton and electron tunneling are addressed in the framework of Dogonadze–Kuznetsov–Levich theory. The cluster model is employed to describe the electrode surface; quantum chemical calculations are performed at the density functional theory level. The classical molecular dynamics simulations are performed for a hydronium ion at the mercury/water molecules interface, in order to judge about the distance dependent work term. The results of simulations indicate that the hydronium ion does not specifically adsorb at the uncharged electrode surface. The proton energy surfaces are built at several values of the H3O+-electrode distances using a quantum chemical approach, which allows for non-equilibrium solvent effects. The electron transfer is found to be adiabatic; the proton tunneling probability is estimated by means of an effective computational scheme. The Tafel plots are constructed in a wide range of the electrode potentials; their origin is ascribed mostly to the complex interplay between the partial contributions to the resulting current. An attempt is also made to model the kinetic isotope effect.