Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts

Sha, Yao; Yu, Ted H.; Merinov, Boris V.; Shirvanian, Pezhman; Goddard, William A.

doi:10.1021/jz101753e

Cited by 112 publications

(195 citation statements)

References 33 publications

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“…15 Free energies of activation for O 2 dissociation (between steps 2 → 3) on both supported (TS1) and unsupported Pt 13 (TS2) nanoparticles are 0.16 and 0.37 eV, respectively, as shown in Figure 3. The defective graphene support significantly lowers the O 2 dissociation barrier compared to the energy barriers of the free Pt 13 nanoparticle and flat Pt surfaces, which range in DFT gas-phase calculations between 0.24 and 0.76 eV depending on O 2 coverage (e.g., 0.12−0.50 ML) on Pt(111), 48 0.69 and 0.90 eV on Pt(111), 49 0.8 and 0.9 eV on Pt(211), 49 0.44 eV on Pt(111), 50 and between 0.15 and 0.30 eV on Pt(111). 51 The O 2 dissociation barrier energy on the free Pt 13 nanoparticle is close to the experimental value of 0.32 eV on Pt(111) in the gas-phase.…”

Section: Resultsmentioning

confidence: 99%

Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles

2012

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The mechanisms of the oxygen reduction reaction (ORR) on defective graphene-supported Pt 13 nanoparticles have been investigated to understand the effect of defective graphene support on the ORR and predict details of ORR pathways. We employed density functional theory (DFT) predictions using the projector-augmented wave (PAW) method within the generalized gradient approximation (GGA). Free energy diagrams for the ORR over supported and unsupported Pt 13 nanoparticles were constructed to provide the stability of possible intermediates in the electrochemical reaction pathways. We demonstrate that the defective graphene support may provide a balance in the binding of ORR intermediates on Pt 13 nanoparticles by tuning the relatively high reactivity of free Pt 13 nanoparticles that bind the ORR intermediates too strongly subsequently leading to slow kinetics. The defective graphene support lowers not only the activation energy for O 2 dissociation from 0.37 to 0.16 eV, but also the energy barrier of the rate-limiting step by reducing the stability of HO* species. We predict the ORR mechanisms via direct four-electron and series two-electron pathways. It has been determined that an activation free energy (0.16 eV) for O 2 dissociation from adsorbed O 2 * at a bridge site on the supported Pt 13 nanoparticle into O* + O* species (i.e., the direct pathway) is lower than the free energy barrier (0.29 eV) for the formation of HOO* species from adsorbed O 2 * at the corresponding atop site, indicating that the direct pathway may be preferred as the initial step of the ORR mechanism. Also, it has been observed that charge is transferred from the Pt 13 nanoparticle to both defective graphene and the ORR intermediate species.

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Section: Resultsmentioning

confidence: 99%

Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles

2012

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“…Quantum Mechanics (QM) calculations have provided new insights into the critical steps and how they depend on alloying, but the extrapolation to develop new catalysts and solvent conditions is not obvious. [3][4][5][6][7][8][9][10][11][12] Even so, determining the atomistic mechanisms underlying the electrochemical transformations should provide guidance to achieve a significant acceleration in making the water energy cycle (1) and (2) efficient and competitive with respect to traditional carbonbased approaches. 13 Unfortunately, in situ experimental characterization of the mechanisms, including the state of the interfaces, reactants, intermediates, reaction paths, and energy barriers under realistic operating reaction conditions is extremely difficult because of the transient character of the species involved and the difficulty in measuring atomistic details.…”

Section: Introductionmentioning

confidence: 99%

Optimizing the oxygen evolution reaction for electrochemical water oxidation by tuning solvent properties

et al. 2015

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a Electrochemical water-based energy cycles provide a most promising alternative to fossil-fuel sources of energy. However, current electrocatalysts are not adequate (high overpotential, lack of selectivity toward O 2 production, catalyst degradation). We propose here mechanistic guidelines for experimental examination of modified catalysts based on the dependence of kinetic rates on the solvent dielectric constant.To illustrate the procedure we consider the fcc(111) platinum surface and show that the individual steps for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) change systematically with the polarizability of the medium. Thus changing this environmental variable can be used to tune the rate determining steps and the barriers, providing a means for screening and validating new systems to optimize the rate determining steps for the ORR and OER reaction pathways.

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“…This process can be realized via two reactions: direct OH formation or O hydration 4 The new index is the perpendicular distance from the pure metal line and an indicator of the exothermicity of reactions starting with O and ending with OH. The reason for not using the exothermicity explicitly, such as BE O − BE OH , is that they are very different for pure metals; for example, this value is 0.61 eV for Au and 1.74 eV for Fe.…”

Section: Resultsmentioning

confidence: 99%

“…Pure Pt appears to have a too low oxo-asymmetry index value and appears to be limited in process II as it was shown in our previous work. 3,4 Pt-Pd-Pt, Pt-Ir-Pt, and Pt-Rh-Pt need to be moved more to the upper left which also means improving process II of the ORR. Pt-Os-Pt needs to be moved more down which means process III, H 2 O formation, limits the ORR.…”

Section: Overlap = D-dos(-593)+d-dos(-435)+d-dos(-377)+d-dos(-067mentioning

confidence: 99%

Elucidating challenges of reactions with correlated reactant and product binding energies on an example of oxygen reduction reaction

Torres

Merinov

et al. 2016

Journal of Molecular Catalysis A: Chemical

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Using density functional theory (DFT), Pt-based sandwich catalysts have been studied to identify a strategy for improving the energetically unfavorable O hydration catalytic reaction (O + H 2 O → 2OH) in fuel cells. The challenge for this type of reaction is that the reactant, O, and product, OH, have correlated binding energies, making the improvement of the overall energetics of the reaction problematic. We screened 28 different transition metals as the Pt-M-Pt sandwich middle layer and developed a new index that specifically describes the difficulty of the reaction which involves adsorbed atomic O as the reactant and adsorbed OH as the product. This index is found 2 to predict well the barrier of the O hydration. In order to understand how the index can be optimized, we further studied the electronic density of states (DOS) to elucidate the DOS changes for the different Pt-M-Pt sandwiches. This gives insight on strategies that might be applied to improve the catalytic reactions where the reactant and product have correlated binding energies, which is in fact a common challenge in heterogeneous catalysis.

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Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts

Cited by 112 publications

References 33 publications

Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles

Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles

Optimizing the oxygen evolution reaction for electrochemical water oxidation by tuning solvent properties

Elucidating challenges of reactions with correlated reactant and product binding energies on an example of oxygen reduction reaction

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