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2. A first principles analysis of potential-dependent structural evolution of active sites in Fe-N-C catalysts.

Author

Morankar, Ankita, Deshpande, Siddharth, Zeng, Zhenhua, Atanassov, Plamen, and Greeley, Jeffrey

Subjects
Fe-N-C catalysts, density functional theory, fuel cells, oxygen reduction reaction
Abstract

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.

Published
2023

9. A Combined First-Principles and Data-Driven Computational Framework to Analyze the Surface Structure, Composition, and Stability of Binary Alloy Catalysts

Author

Deshmukh, Gaurav, Ghanekar, Pushkar, and Greeley, Jeffrey

Abstract

Pt-based bimetallic alloys are considerably more active than Pt in the oxygen reduction reaction (ORR). This increased activity has been attributed to the weakening of the adsorption of ORR intermediates due to the presence of Pt “skins.” Density functional theory (DFT) calculations have, in turn, pointed to the importance of surface segregation energies and Pt leaching on the formation and stability of the skins on close-packed surfaces of Pt alloys. The generalizability of these insights across different chemical environments, surface compositions, and facets, however, remains a subject of active research and is the focus of this work. We present a generalized computational framework combining DFT calculations and data-driven methods to predict the stability of different Pt3X(X= Ni, Co, Fe, and Cu) alloy facets under vacuum conditions and in the presence of an electrochemical environment, wherein we analyze the combined effect of segregation, intrasurface phase separation, leaching, and surface oxidation as a function of electrode potential. The analysis reveals that a subtle interplay of these factors influences Pt skin formation and stability, with Pt segregation being a strong function of the surface structure and continuous base metal dissolution being thermodynamically, although not always kinetically, favored at ORR-relevant voltages.

Published
2024
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16. Chiral Pinwheel Heterojunctions Self Assembled from C60 and Pentacene

Author

Smerdon, Joseph A., Rankin, Rees B., Greeley, Jeffrey, Guisinger, Nathan, and Guest, Jeffrey

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

We demonstrate the self-assembly of C$_{60}$ and pentacene (Pn) molecules into acceptor-donor heterostructures which are well-ordered and -- despite the high degree of symmetry of the constituent molecules -- {\it chiral}. Pn was deposited on Cu(111) to monolayer coverage, producing the random-tiling ($R$) phase as previously described. Atop $R$-phase Pn, post-deposited C$_{60}$ molecules cause rearrangement of the Pn molecules into domains based on chiral supramolecular `pinwheels'. These two molecules are the highest-symmetry achiral molecules so far observed to coalesce into chiral heterostructures. Also, the chiral pinwheels (composed of 1 C$_{60}$ and 6 Pn each) may share Pn molecules in different ways to produce structures with different lattice parameters and degree of chirality. High-resolution scanning tunneling microscopy (STM) results and knowledge of adsorption sites allow the determination of these structures to a high degree of confidence. The measurement of chiral angles identical to those predicted is a further demonstration of the accuracy of the models. Van der Waals density functional theory calculations reveal that the Pn molecules around each C$_{60}$ are torsionally flexed around their long molecular axes and that there is charge transfer from C$_{60}$ to Pn in each pinwheel.

Published
2013
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30. In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution

Author

Dionigi, Fabio, Zeng, Zhenhua, Sinev, Ilya, Merzdorf, Thomas, Deshpande, Siddharth, Lopez, Miguel Bernal, Kunze, Sebastian, Zegkinoglou, Ioannis, Sarodnik, Hannes, Fan, Dingxin, Bergmann, Arno, Drnec, Jakub, Araujo, Jorge Ferreira de, Gliech, Manuel, Teschner, Detre, Zhu, Jing, Li, Wei-Xue, Greeley, Jeffrey, Cuenya, Beatriz Roldan, and Strasser, Peter

Published
2020
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31. Origin of Stability and Activity Enhancements in Pt‐based Oxygen Reduction Reaction Catalysts via Defect‐Mediated Dopant Adsorption.

Author

Sawant, Kaustubh J., Zeng, Zhenhua, and Greeley, Jeffrey P.

Subjects
OXYGEN reduction, PLATINUM alloys, DOPING agents (Chemistry), TRANSITION metal alloys, ADSORPTION (Chemistry), ELECTROCATALYSTS, HYDROGEN evolution reactions
Abstract

Platinum alloys are highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in acidic conditions. However, these alloys are susceptible to metal loss through leaching and degradation, leading to reduced catalyst stability and activity. Recently, it has been shown that doping with oxophilic elements can significantly alleviate these problems, with a prominent example being Mo‐doped Pt alloys. Here, to achieve atomic scale understanding of the exceptional activity and stability of these alloys, we present a detailed density functional theory description of the dopants' structures and impact on electrocatalyst properties. Beginning with the Mo/Pt system, we demonstrate that Mo can be stabilized in the form of low‐dimensional oxyhydroxide moieties on Pt defects. The resulting structures enhance stability and activity via distinct physical processes, with the Mo moieties both directly inhibiting Pt dissolution at defects and indirectly enhancing ORR activity by generation of strain fields on surrounding Pt terraces. We then generalize these analyses to other metal dopant elements, and we demonstrate that similar low‐dimensional oxyhydroxide structures control the electrocatalytic properties through an intricate interplay of the structures' acid stability, intrinsic activity for the ORR, and ability to induce ORR‐promoting strain fields on Pt. [ABSTRACT FROM AUTHOR]

Published
2024
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