How to efficiently oxidize H(2)O to O(2) (H(2)O → 1/2O(2) + 2H(+) + 2e(-)) is a great challenge for electrochemical/photo water splitting owing to the high overpotential and catalyst corrosion. Here extensive periodic first-principles calculations integrated with modified-Poisson-Boltzmann electrostatics are utilized to reveal the physical origin of the high overpotential of the electrocatalytic oxygen evolution reaction (OER) on RuO(2)(110). By determining the surface phase diagram, exploring the possible reaction channels, and computing the Tafel lines, we are able to elucidate some long-standing puzzles on the OER kinetics from the atomic level. We show that OER occurs directly on an O-terminated surface phase above 1.58 V vs NHE, but indirectly on a OH/O mixed phase below 1.58 V by converting first the OH/O mixed phase to the O-terminated phase locally. The rate-determining step of OER involves an unusual water oxidation reaction following a Eley-Rideal-like mechanism, where a water molecule from solution breaks its OH bond over surface Os with concurrent new O-OH bond formation. The free energy barrier is 0.74 eV at 1.58 V, and it decreases linearly with the increase of potential above 1.58 V (a slope of 0.56). In contrast, the traditionally regarded surface oxygen coupling reaction with a Langmuir-Hinshelwood mechanism is energetically less favored and its barrier is weakly affected by the potential. Fundamentally, we show that the empirical linear barrier~potential relation is caused by the linear structural response of the solvated transition state to the change of potential. Finally, the general strategy for finding better OER anode is also presented.

The Tafel equation is of fundamental importance in electrochemical kinetics, formulating a quantitative relation between the current and the applied electrochemical potential. The recent years have seen the rapid expansion and development in the application of first-principles density functional theory (DFT) simulation on electrocatalytic reactions that occur at the solid–liquid interface. This article reviews the current theoretical methods for electrochemistry modeling, in particular, those for the direct computation of Tafel kinetics of electrocatalytic reactions on surfaces based on DFT calculations. Representative reactions, namely, hydrogen evolution and oxygen evolution reactions, are selected to illustrate how the theoretical methods are applied to compute quantitatively the kinetics of multiple-step electrochemical reactions. We summarize in detail the computation procedure based on the first-principles periodic continuum solvation method for obtaining the charge transfer coefficient (CTC) and deducing the potential-dependent reaction rate. The theoretical results on the Tafel kinetics of electrochemical reactions are generalized and discussed.

Hydrogen evolution reaction (HER: H+ + e– → 1/2H2) on metals exhibits the characteristic kinetics of electrocatalytic process. Here a theoretical method based on the constant-charge first principles periodic continuum solvation model is proposed to resolve the potential-dependent reaction kinetics on Pt and Au surfaces, and the quantitative linkage is established between the Tafel kinetics (current vs potential) and the electrochemical condition, including the surface structure, the surface charging, and the coverage. The theoretical Tafel slopes for HER are determined to be 83 mV on Pt(111) and 70 mV on Pt(100), which are generally associated with the reactions involving the minority weakly adsorbed H, i.e. the atop H above 1 ML on Pt(111) and the bridging H above 1.5 ML on Pt(100). The mechanism and the contribution of each pathway (Volmer, Tafel, and Heyrovsky pathways) are determined quantitatively. It is revealed that HER at the minority surface steps has a much higher activity than at terraces, which is responsible for the overall activity on a typical Pt electrode. The theoretical model here paved the way toward the large-scale computational screening for both active and economic hydrogen electrode.

Electrochemical reactions catalyzed by metal electrode, despite their huge importance in industry, are not well understood at the atomic level. In relevance to water electrolysis, the oxygen coupling reaction on Pt metal surfaces is systematically investigated in this work by combining periodic density functional theory calculations with a new theoretical approach to mimic the electrochemical environment. In our approach, the surface is explicitly polarized by adding/subtracting charges and the counter charges are placed as Gaussiandistributed plane charges in a vacuum. With this method, the surface phase diagrams for both the closedpacked Pt(111) and stepped Pt(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 f OOH reaction can eventually occur at the stepped sites with an 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 sublayers. 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.

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Oxygen reduction is a critical reaction in the global energy cycle and a vital catalytic process in fuel cells. To date, the atomic level picture on how oxygen is electrocatalytically reduced on the traditional Pt catalyst is not established yet and the design of both active and economic catalysts remains a great challenge. Here first principles based theoretical methods can for the first time resolve the Tafel behavior and the polarization kinetics for oxygen reduction reaction (ORR) on Pt in aqueous soundings and reveal the origin of some key problems, mainly associated with the low intrinsic activity and the rapid poisoning of the electrocatalyst. The atomic level mechanism of ORR on Pt at the concerned potentials (∼0.8 V) is established, in which the critical surface coverage to achieve the reaction equilibrium is identified to be 0.25 ML O coverage. From the computed Tafel curve, the proton-coupled O−O bond breaking, i.e. H + + e + O 2ad → O + OH, is assigned to be the major O 2 reduction channel on Pt; and the reaction is quenched at the high potentials due to the presence of the surface O/OH/H 2 O network that prevents the adsorption of bidentate O 2 . We predict that a qualified ORR catalyst must allow bidentate O 2 adsorption under the equilibrium between adsorbed O and H 2 O in solution at the concerned potential.

The Fe/N/C catalysts have emerged recently as a representative class of non-Pt catalysts for oxygen electrocatalytic reduction, which could have a competitive catalytic performance to Pt. However, the nature of the catalyst remains elusive, especially on the active site structure and the electrocatalytic kinetics. Here we examine two kinds of Fe/N active sites for Fe/N/C catalysts, namely, the four-coordinated FeN4 and the five-coordinated Fe(CN)N4 centers embedded in graphene layers. By using large-scale first principles calculations with a periodic continuum solvation model based on the Modified-Poisson-Boltzmann equation (CM-MPB), we identified the four (4e) and two electron (2e) oxygen reduction pathways under acidic conditions. We find that both 4e and 2e pathways involves the formation of an OOH intermediate, which breaks its O-OH bond in the 4e pathway but is reduced to H2O2 in the 2e pathway. We show that at 0.8 V vs. SHE, the 4e pathway is preferred at both FeN4 and Fe(CN)N4 centers, but the 2e pathway is kinetically also likely on the Fe(CN)N4 center. The O-OH bond breaking of OOH is the key kinetic step, which has a similar free energy barrier to the OH reduction on the FeN4 center, and is the rate-determining step on the Fe(CN)N4 center. Due to the high adsorption energy of Fe towards the fifth ligand, such as OH and CN, we expect that the active site of the real Fe/N/C catalyst is the five coordinated Fe center. We found that the barrier of the O-OH bond breaking step is not sensitive to potential and a Tafel slope of 60 mV is predicted for the ORR on the Fe(CN)N4 center, which is consistent with experimental observation.

Record high activity and high stability revealed for hcp Pd2B nanosheets synthesized by a simple solvothermal method.