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Fe-N-C (iron-nitrogen-carbon) electrocatalysts have emerged as potential alternatives to precious metal-based materials for the oxygen reduction reaction (ORR). However, the structure of these materials under electrochemical conditions is not well understood, and their poor stability in acidic environments poses a formidable challenge for successful adoption in commercial fuel cells. To provide molecular-level insights into these complex phenomena, we combine periodic density functional theory (DFT) calculations, exhaustive treatment of coadsorption effects for ORR reaction intermediates, including O and OH, and comprehensive analysis of solvation stabilization effects to construct voltage-dependent ab initio thermodynamic phase diagrams that describe the in situ structure of the active sites. These structures are further linked to activity and stability descriptors that can be compared with experimental parameters such as the half-wave potential for ORR and the onset potential for carbon corrosion and CO2 evolution. The results indicate that pyridinic Fe sites at zigzag carbon edges, as well as other edge sites, exhibit high activity for ORR compared to sites in the bulk. However, edges neighboring the active sites are prone to instability via overoxidation and consequent site loss. The results suggest that it could be beneficial to synthesize Fe-N-C catalysts with small sizes and large perimeter edge lengths to enhance ORR activity, while voltage fluctuations should be limited during fuel cell operation to prevent carbon corrosion of overoxidized edges.

Strain and support effects play a crucial role in heterogeneous catalysis, which has been intensively studied over metal-based catalysts. In contrast, there is little discussion about the two effects in oxide systems. In this work, using an ultrathin ZnO film as an example, we investigate strain and support effects on the structure and surface reactivity of oxide catalysts through density functional theory calculations. Our results suggest that tensile strain increases the surface reactivity of ZnO films as indicated by enhanced CO and NH3 adsorptions and compressive strain renders an early phase transition from an inert graphene-like phase to a more reactive wurtzite-like phase. The support (Au, Pt, and Ru) can promote the phase transition and surface reactivity concurrently, which exhibits a larger effect on the reactivity than the strain. The support effect can be ascribed to the increasing rumple and polarization of ZnO films through the strong ZnO–substrate interaction, which enhances the surface reactivity. The insight helps us to develop advanced oxide-based catalysts through the strain and/or substrate engineering. [ABSTRACT FROM AUTHOR]

NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs. NiFe and CoFe layered double hydroxides are among the most active electrocatalysts for the alkaline oxygen evolution reaction. Here, by combining operando experiments and rigorous DFT calculations, the authors unravel their active phase, the reaction center and the catalytic mechanism. [ABSTRACT FROM AUTHOR]

Tuning surface strain is a powerful strategy for tailoring the reactivity of metal catalysts. Traditionally, surface strain is imposed by external stress from a heterogeneous substrate, but the effect is often obscured by interfacial reconstructions and nanocatalyst geometries. Here, we report on a strategy to resolve these problems by exploiting intrinsic surface stresses in two-dimensional transition metal nanosheets. Density functional theory calculations indicate that attractive interactions between surface atoms lead to tensile surface stresses that exert a pressure on the order of 105 atmospheres on the surface atoms and impart up to 10% compressive strain, with the exact magnitude inversely proportional to the nanosheet thickness. Atomic-level control of thickness thus enables generation and fine-tuning of intrinsic strain to optimize catalytic reactivity, which was confirmed experimentally on Pd(110) nanosheets for the oxygen reduction and hydrogen evolution reactions, with activity enhancements that were more than an order of magnitude greater than those of their nanoparticle counterparts. [ABSTRACT FROM AUTHOR]

Abstract: Density Functional Theory calculations with both standard GGA and hybrid functionals are performed on Au adatoms, steps, and low index surfaces with coordination numbers (CNs) varying from 3 to 9. The results are used to study adsorption thermodynamics and reactivity of CO on Au nanoparticles. We find that the hybrid functional improves calculated site preferences and predicts CO top site adsorption, regardless of the Au CN, in good agreement with experiments. The calculated adsorption energies vary monotonically with respect to Au CNs, and the results from the hybrid functional are around 20% smaller than the corresponding values from the GGA–PBE functional. A comparison with experimental adsorption energies suggests that these functionals may bound the true CO–Au interaction strength, and seven-coordinated Au atoms may be the active low-coordinated sites on many Au single crystal surfaces. However, thermodynamic analysis on Wulff-like Au particles at ambient temperatures shows that, even though the number of 6-coordinated corner Au atoms is much less than the number of 7-coordinated edge Au atoms and of higher-coordinated Au atoms, they are the dominant sites for CO adsorption on Au nanoparticles with sizes up to 10nm. In addition, we find that CO adsorption is not influenced by the shape of Au nanoparticles, but the CO oxidation reaction may be. [Copyright &y& Elsevier]

Dynamic electrochemical surface stress response during the oxygen reduction reaction (ORR) on Pt and Pd cantilever electrodes in HClO 4 and KOH was examined to elucidate surface binding configurations during O 2 reduction electrocatalysis. Upon reduction of O 2 , the surface of Pt exhibits a compressive surface stress response, ΔStress, in both acid and base electrolytes due to adsorption of the ORR reactant and intermediates (O 2 , O, and OH). The magnitude of compressive ΔStress on Pt is greater in acid relative to base. On the other hand, the surface of Pd exhibits a negligible ΔStress in acid and a slight compressive ΔStress in base. Thus, magnitudes of the compressive ΔStress (surface expansion) during the ORR follow the order of Pt (acid) > Pt (base) > Pd (base) > Pd (acid) ∼ 0. Density functional theory (DFT) calculations of adsorbate-induced excess surface stress on Pt(111) and Pd(111) surfaces imply a greater compressive surface stress induced on Pt(111) for nearly all adsorbate geometries examined. This trend, which agrees with the experimental observations, can be correlated to a greater tensile intrinsic surface stress of Pt(111) relative to Pd(111) resulting from difference in bond strength and bulk modulus of two metals. On stepped Pt(221) and Pd(221) surfaces, both the intrinsic tensile stress of the clean surface and the adsorbate-induced excess compressive stress are significantly reduced due to the presence of less coordinated, flexible step sites. Moreover, this difference between surface stress at terrace and step sites is more pronounced on Pt, which exhibits a greater intrinsic surface stress. [ABSTRACT FROM AUTHOR]

• Pt/CoO x H y interfaces dominate WGS activity in PtCo bimetallic nanoparticles. • Activity of metal/oxyhydroxide interfaces are higher than the alloy and platinum. • Results suggest a new way to interpret WGS reactivity on alloy metal nanoparticles. A series of cobalt-promoted Pt catalysts supported on multi-walled carbon nanotubes was synthesized, and their performance was evaluated for the water-gas shift (WGS) reaction. Compared to the monometallic Pt catalyst, the WGS turnover rate (TOR) at 300 °C was promoted by a factor of 10 at a Pt:Co molar ratio of 1:3. X-ray absorption spectroscopy and XRD showed the presence of a Pt 3 Co alloy along with a partially oxidized cobalt and a free cobalt metal phase after reduction pretreatment. In order to determine the dominant active site over the Co-promoted catalysts, selective leaching of partially oxidized Co (designated as CoO x H y) and Co metal was performed with a 5% wt. acetic acid solution, while preserving the Pt-rich phases. The WGS TOR at 300 °C for the Co-promoted catalysts after leaching was observed to be even lower than that of the monometallic Pt catalyst. Thus, the alloy formation was determined to be inconsequential towards promotion in the WGS TOR, while the dominant active site was determined to be a PtCo alloy in intimate contact with the CoO x H y phase. Combined Density Functional Theory (DFT) calculations and ab-initio thermodynamic phase diagrams point to a monolayer of CoOH being the most stable Co phase on Pt(1 1 1) under WGS conditions. Calculations of OH binding energies on Pt(1 1 1), Pt 3 Co(1 1 1), and at the interface between CoOH overlayers and Pt(1 1 1) show that trends in the WGS activity of these catalysts are linked to the strength of OH binding, with the strongest OH binding found at the interface between CoOH and Pt, supporting the conclusion that a similar interface is the source of enhanced WGS activity in the PtCo bimetallic system. [ABSTRACT FROM AUTHOR]