Insight Into The Important Electrochemical Reactions From First-Principles Study

Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects. A large part of this field deals with the study of chemical changes caused by the passage of an electric current and the production of electrical energy by chemical reactions. In fact, the field of electrochemistry encompasses a huge array of different phenomena (e.g., electrophoresis and corrosion), devices (electro-analytical sensors, batteries, and fuel cells), and technologies (the electroplating of metals and the large-scale production of aluminum and chlorine). Recently, the growing awareness of the risks to human-induced climate has considerably raised the necessity of alternative energy production. The conversion of chemical energy into electricity has become increasingly important and thus, clean and efficient devices such as fuel cells are now attracting a great deal of attention from academic and industrial communities.The interface that forms between an aqueous solution and a metal surface creates a unique reaction environment that can markedly influence the reactivity of molecules within the interface. A collective understanding of the synergy between the effects of solution, applied potential and electronic structure of the metal would be invaluable for understanding elementary processes that govern the electrochemical and electrostatic phenomena. While well-defined spectroscopic characterization of surfaces via sum frequency generation, surface enhanced Raman spectroscopy, and diffuse reflectance infrared spectroscopy, as well as scanning tunneling microscopy, are beginning to provide atomic scale resolution. There is a strong need for theory to complement these experimental efforts and help provide a fundamental understanding of potential-dependent interfacial phenomena including changes in molecular structure, chemisorption, water activation, and surface reconstruction.The first-principles methods have been widely utilized to describe and predict significant quantities in chemistry, e.g. the geometrical structure, the bonding energy and the reaction barrier of elementary reactions. Much progress has been made in the understanding of catalytic reactions at the solid/gas interface. Electrocatalysis at the solid/liquid interface remains a highly challenging field where the first-principles simulation is increasingly utilized. Despite the intrinsic complexity of the many-body problem in electrocatalysis (water, ions, adsorbates and electrodes), theoretical simulation methods based on first-principles theory have been developed for investigating the reactions under electrochemical conditions. Here we developed and applied a new theoretical approach based on periodic DFT calculations to model electrochemical reactions where both effects due to the electrochemical potential and the solvation are taken into account. In our approach, the surface is explicitly polarized by adding/subtracting charges and the counter charges are placed as Gaussian-distributed plane charges in vacuum. The electrochemical potential can be calculated through correcting the calculated work function, which is then related to the work function of the standard hydrogen electrode (SHE). Next, the DFT-calculated energy must be corrected to compare the total energy of phases with different charges. Two extra energy contributions must be removed from DFT total energy, namely, (i) the energy of the countercharge itself (ECQ) and its electrostatic interaction with the charged-slab (ECQ-slab) and (ii) the energy of the excess charge in the slab (EuQ). For reactions involving the releasing of proton and electron, the reaction energy can be computed by referencing to the normal hydrogen electrode in a manner proposed by Nφrskov group. This is governed by Gproton+electron= G(1/2H2)-neU, where e presents the transfer electron, n means the number of electron and U is electrochemical potential.The effect of the water environment on the phase diagram has been examined through a continuum solvation model by solving the Poisson-Boltzmann equation numerically in the periodic slab as implemented recently. In our studies, we introduce a large vacuum region (30 A) along the Z axis that separates two adjacent slabs. In the middle of the vacuum region, we define a potential zero plane as the boundary condition for the integration of the Poisson-Boltzmann equation, which can be solved via the finite-difference multigrid method. We solve the Poisson-Boltzmann equation twice in each electronic SCF loop with (Vsol) and without (Vvac) the implicit solution to obtain the excess potentialΔV(Vsol-Fvac) due to solvation.ΔV is then added to the total potential for solving the Kohn-Sham equation in the self-consistent loop.The characteristic parameters of the interface (differential capacity (Cd) and potential of zero charge (PZC)) can be evaluated quantitatively by using Gaussian plane model and modified Poisson-Boltzmann continuum solvation model. For the Pt(111) surface, we found that the Cd (6.7μF/cm2) from vacuum calculation is smaller than the experimental values. By adding the continuum solvation model, the Cd has been examined and increased to the 16.2μF/cm2, which agrees to the experimental results. Then the continuum sovaltion model with first water shell has also been checked. We found that the first shell water plays only a minor role in affecting the Cd (18.4μF/cm2). As to the CO/Pt(111) system, the calculated value of Cd and PZC are 14μF/cm2 and 1.08 V, respectively, which are consistent with the experiment (Cd and PZC are 15μF/cm2 and 1.10±0.04 V respectively).Oxygen evolution reaction (H2O→1/2O2+2H++2e-) as encountered in water electrolysis is one of the most important anodic reactions. Because the reaction causes the major energy loss in water electrolysis and is also involved in many applications concerning energy storage/conversion, better anode materials for more efficient oxygen evolution have been consistently pursued for years. It is known generally that the oxidative species (such as OH and O) dissociated from H2O appear on electrode at certain positive potentials, and by further increasing the potential, O2 evolves originated from these surface oxidative species. Oxygen coupling reaction on Pt metal surfaces is systematically investigated in this work by combining periodic density functional theory calculations with the new theoretical approach to mimic the electrochemical environment. With this method the surface phase diagrams for both the closed-packed Pt(111) and stepped (211) are determined, which demonstrates that stepped surface sites can better accumulate oxidative species and thus reach to a higher local O coverage compared to Pt(111) at a given potential. The water environment is proved to affect the phase diagram marginally. By fully exploring the possible oxygen coupling channels on Pt surfaces, we show that the oxygen coupling reaction is kinetically difficult on metallic Pt surfaces below 1.4 V. There is no facile O coupling channels on Pt(111) as the barriers are no less than 1 eV. Although an O+ OH→OOH reaction can eventually occur at the stepped sites with the increase of local O coverage and the calculated barrier is lower than 0.7 eV at 1.4 V (NHE), at such high potentials the (111) surface can already undergo surface oxidation due to the penetration of oxygen into subsurface. The theory thus indicates that oxygen evolution on Pt anode occurs on Pt surface oxides as dictated by thermodynamics, and also demonstrates that the local surface structure and coverage can be more important in affecting the barrier of surface reactions than the electric fields.Notoriously, at the elevated potentials above 1 V (vs. SHE) as for oxygen reduction reactions, surface Pt atoms were believed to exchange with the surface OH or O species from H2O dissociation, which eventually lead to the corrosion of the electrode and a remarkable decrease in catalytic activity. Due to the great difficulty to resolve the surface structure in situ, the oxidation process of Pt electrode remains poorly understood at the atomic level. A microscopic picture on the corrosion kinetics is urgently called for towards the rational design of novel electrode materials. This work represents the first theoretical attempt to elucidate the oxidation mechanism of Pt electrode under electrochemical conditions by exploring the oxidation kinetics of differently-structured Pt surfaces, including Pt(111), Pt(211) and Pt(100). We show that the most abundant and close-packed (111) surface in Pt metal can be oxidized due to the presence of surface OH. The corrosion is self-accelerated kinetically once the defects are created as demonstrated by the low kinetic barrier of oxidation on Pt(211). By contrast, the open Pt(100) facet is very inert towards surface oxidation. Apart from the revealed surface-structure sensitivity, Pt corrosion is also strongly affected by the local oxygen coverage as pinned by the electrochemical potential. For Pt(111), the subsurface oxygen formation occurs only above 0.5 ML oxygen coverage. We find that the presence of Au on Pt can effectively reduce the local oxygen coverage and thus prevent the surface oxidation. The kinetics model for the surface oxidation proposed in this work provides new insights for designing the next-generation of anticorrosion electrode materials in fuel cell.Finally, we have studied the water electrolysis on the oxide (i.e. RuO2) anode. This work represents the first theoretical attempt to elucidate the origin of Tafel line and OER reaction mechanism at RuO2(110). The surface phase diagrams and potential energy surface for RuO2(110) are determined. The effect of water environment on surface phase diagram is quantitatively evaluated and found to be small. From our results, it can be seen that at low potential the OH-covered surface (OHBR+OHTOP) is the most stable phase. Above the potential 1.55V, the full oxygen overlayer is preferred thermodynamically (OBR+OTOP). These results are consistent with the previous phase diagram calculated for RuO2(110), where the OH covered and O covered surface phases have been reported. It is interesting to note that another O/OH mixed phase has emerged between 1.33 V and 1.55 V, where one hydroxyl adsorbs on BR and the rest of actives sites are occupied by O adatoms (OHBR+O(TOP+BR)). All oxygen coupling reaction channels on RuO2 surfaces at different potentials are then examined. We found that the electric field induced by the excess surface charge does not affect significantly the barrier of the oxygen coupling reaction. Instead, the barrier of water dissociation has reduced with the applied potentials linearly and the Tafel line can be determined (Tafel slope:105 mV), whose value is close to the experiment (118 mV). Our results shed new insights into the fundamentals of electrochemistry and provide important clues for the design of better anode catalysts in water electrolysis.With the aim to establish a predictive basis for the design and the optimization of electrocatalysts, we have developed a new theoretical model for metal/solution interface and introduced its applications in electrocatalysis. Two representative reactions, namely water electrolysis (Pt and RuO2 anodes) and Pt corrosion, are selected to illustrate how the theoretical methods are applied to electrocatalytic reactions. The microscopic nature of these electrochemical reactions under the applied potentials is described and the understanding of the reactions is summarized. The thermodynamics and kinetics of the electrochemical reactions affected by the interplay of the electrochemical potential, the bonding strength and the local surface structure are addressed at the atomic level.