Ab initio density functional theory is used to calculate the electrochemical phase diagram for the oxidation and reduction of water over the Pt(111) surface. Three different schemes proposed in the literature are used to calculate the potential-dependent free energy of hydrogen, water, hydroxyl, and oxygen species adsorbed to the surface. Despite the different foundations for the models and their different complexity, they can be directly related to one another through a systematic Taylor series expansion of the Nernst equation. The simplest model, which includes the potential only as a shift in the chemical potential of the electrons, accounts very well for the thermochemical features determining the phase-diagram.

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The sluggish kinetics associated with the oxygen reduction reaction (ORR) at the proton exchange membrane fuel cell cathode leads to high overpotentials and limits fuel cell performance. Although significant progress has been made in first-principles modeling of the ORR, the complexity of the electrified aqueous/metal interface has limited advances in the use of theory to elucidate the influence of electrode potential on the mechanism and kinetics. The first reduction step of adsorbed molecular oxygen has been speculated to be the rate-determining step in the ORR. Periodic density functional theoretical calculations are carried out with the double-reference method developed by Filhol and Neurock [ Angew. Chem. Int. Ed. , 45 , 402 (2006)] to determine the potential dependence of the reaction energy and activation barrier for the reduction of O2* to OOnormalH* on the fully hydrated Pt(111) surface. This method allows for tuning the electrode potential with a slab representation of the electrode surface. Electron transfer is found to precede the protonation of the adsorbed O2 molecule, occurring with the proton formally residing as an normalH3normalO+ species connected to the adsorbed O2 molecule by hydrogen bonding through two additional water molecules. The importance of the periodic representation of the metal electronic structure and the inclusion of extended solvation in considering the elementary kinetics is discussed.

The structure of active sites in Fe-based nonprecious metal oxygen reduction reaction catalysts remains unknown, limiting the ability to follow a rational design paradigm for catalyst improvement. Previous studies indicate that N-coordinated Fe defects at graphene edges are the most stable such sites. Density functional theory is used for determination of stable potential oxygen reduction reaction active sites. Clusters of Fe–N x defects are found to have N-coordination-dependent stability. Previously reported interedge structures are found to be significantly less stable than in-edge defect structures under relevant synthesis conditions. Clusters that include Fe–N3 defects are found to spontaneously cleave the O–O bond.

We report calculated oxygen reduction reaction energy pathways on multi-metal-atom structures that have previously been shown to be thermodynamically favorable. We predict that such sites have the ability to spontaneously cleave the O2 bond and then will proceed to over-bind reaction intermediates. In particular, the *OH bound state has lower energy than the final 2 H2O state at positive potentials. Contrary to traditional surface catalysts, this *OH binding does not poison the multi-metal-atom site but acts as a modifying ligand that will spontaneously form in aqueous environments leading to new active sites that have higher catalytic activities. These *OH bound structures have the highest calculated activity to date.

Electrochemical processes occurring in aqueous solutions are critically dependent upon the interaction between the metal electrode and the solvent. In this work, density functional theory is used to calculate the potentials for which molecular water and its activation products ͑adsorbed hydrogen and hydroxide͒ are stable when in contact with an immersed Ni͑111͒ electrode. The adsorption geometries of water and its dissociation products are also determined as functions of potential. At zero kelvin, water activates to form a surface hydroxide overlayer at potentials anodic of −0.5 V vs a normal hydrogen electrode ͑NHE͒. The cathodic activation of water to form a surface hydride occurs at potentials negative of −0.3 V NHE. There is a potential range at which both H and OH form on the surface, in agreement with inferences made from the experimental literature. The surface hydroxide/oxide phase transition occurs at 0.2 V NHE. The increased binding of oxygen to the surface at progressively anodic potentials correlates with weakening nickel-nickel interactions and the lifting of a metal atom above the surface plane. Thermodynamic extrapolations are made to ambient ͑300 K͒ and elevated ͑600 K͒ temperatures and Pourbaix diagrams calculated for the inert and activated surface phases of water on Ni͑111͒ and compared with experiment.

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