Browsing by Author "Wang, Guofeng"

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    Atomic Structure Evolution of Pt–Co Binary Catalysts: Single Metal Sites versus Intermetallic Nanocrystals
    (Wiley, 2021-12) Li, Xing; He, Yanghua; Cheng, Shaobo; Li, Boyang; Zeng, Yachao; Xie, Zhenhua; Meng, Qingping; Qingping, Lu; Kisslinger, Kim; Tong, Xiao; Hwang, Sooyeon; Yao, Siyu; Li, Chenzhao; Qiao, Zhi; Shan, Chongxin; Zhu, Yimei; Xie, Jian; Wang, Guofeng; Wu, Gang; Su, Dong; Mechanical and Energy Engineering, School of Engineering and Technology
    Due to their exceptional catalytic properties for the oxygen reduction reaction (ORR) and other crucial electrochemical reactions, PtCo intermetallic nanoparticle (NP) and single atomic (SA) Pt metal site catalysts have received considerable attention. However, their formation mechanisms at the atomic level during high-temperature annealing processes remain elusive. Here, the thermally driven structure evolution of Pt–Co binary catalyst systems is investigated using advanced in situ electron microscopy, including PtCo intermetallic alloys and single Pt/Co metal sites. The pre-doping of CoN4 sites in carbon supports and the initial Pt NP sizes play essential roles in forming either Pt3Co intermetallics or single Pt/Co metal sites. Importantly, the initial Pt NP loadings against the carbon support are critical to whether alloying to L12-ordered Pt3Co NPs or atomizing to SA Pt sites at high temperatures. High Pt NP loadings (e.g., 20%) tend to lead to the formation of highly ordered Pt3Co intermetallic NPs with excellent activity and enhanced stability toward the ORR. In contrast, at a relatively low Pt loading (<6 wt%), the formation of single Pt sites in the form of PtC3N is thermodynamically favorable, in which a synergy between the PtC3N and the CoN4 sites could enhance the catalytic activity for the ORR, but showing insufficient stability.
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    Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells
    (Springer Nature, 2022) Liu, Shengwen; Li, Chenzhao; Zachman, Michael J.; Zeng, Yachao; Yu, Haoran; Li, Boyang; Wang, Maoyu; Braaten, Jonathan; Liu, Jiawei; Meyer, Harry M., III; Lucero, Marcos; Kropf, A. Jeremy; Alp, Esen E.; Gong, Qing; Shi, Qiurong; Feng, Zhenxing; Xu, Hui; Wang, Guofeng; Myers, Deborah J.; Xie, Jian; Cullen, David A.; Litster, Shawn; Wu, Gang; Mechanical and Energy Engineering, Purdue School of Engineering and Technology
    Nitrogen-coordinated single atom iron sites (FeN4) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. However, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface. We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN4 sites into highly stable pyridinic N-coordinated FeN4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mgPt cm−2). The encouraging stability improvement represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications.
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    Atomically dispersed single iron sites for promoting Pt and Pt3Co fuel cell catalysts: performance and durability improvements
    (RSC, 2021-09) Qiao, Zhi; Wang, Chenyu; Li, Chenzhao; Zeng, Yachao; Hwang, Sooyeon; Li, Boyang; Karakalos, Stavros; Park, Jaehyung; Kropf, A. Jeremy; Wegener, Evan C.; Gong, Qing; Xu, Hui; Wang, Guofeng; Myers, Deborah J.; Xie, Jian; Spendelow, Jacob S.; Wu, Gang; Mechanical and Energy Engineering, School of Engineering and Technology
    Significantly reducing platinum group metal (PGM) loading while improving catalytic performance and durability is critical to accelerating proton-exchange membrane fuel cells (PEMFCs) for transportation. Here we report an effective strategy to boost PGM catalysts through integrating PGM-free atomically-dispersed single metal active sites in the carbon support toward the cathode oxygen reduction reaction (ORR). We achieved uniform and fine Pt nanoparticle (NP) (∼2 nm) dispersion on an already highly ORR-active FeN4 site-rich carbon (FeN4–C). Furthermore, we developed an effective approach to preparing a well-dispersed and highly ordered L12 Pt3Co intermetallic nanoparticle catalyst on the FeN4–C support. DFT calculations predicted a synergistic interaction between Pt clusters and surrounding FeN4 sites through weakening O2 adsorption by 0.15 eV on Pt sites and reducing activation energy to break O–O bonds, thereby enhancing the intrinsic activity of Pt. Experimentally, we verified the synergistic effect between Pt or Pt3Co NPs and FeN4 sites, leading to significantly enhanced ORR activity and stability. Especially in a membrane electrode assembly (MEA) with a low cathode Pt loading (0.1 mgPt cm−2), the Pt/FeN4–C catalyst achieved a mass activity of 0.451 A mgPt−1 and retained 80% of the initial values after 30 000 voltage cycles (0.60 to 0.95 V), exceeding DOE 2020 targets. Furthermore, the Pt3Co/FeN4 catalyst achieved significantly enhanced performance and durability concerning initial mass activity (0.72 A mgPt−1), power density (824 mW cm−2 at 0.67 V), and stability (23 mV loss at 1.0 A cm−2). The approach to exploring the synergy between PGM and PGM-free Fe–N–C catalysts provides a new direction to design advanced catalysts for hydrogen fuel cells and various electrocatalysis processes.
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    COMPUTATIONAL STUDY OF SURFACE-SEGREGATED PT ALLOY CATALYSTS FOR OXYGEN REDUCTION REACTION
    (2010-07-27T19:21:50Z) Xiao, Chan; Chen, Rongrong; EI-Mounayri, Hazim; Wang, Guofeng
    In this thesis two research objectives have been accomplished using computational simulation techniques. (1) The surface segregation phenomena in the surfaces of (111), unreconstructed (110) and reconstructed (1x2) missing row (110) surfaces of Pt-Ni and Pt-Co disordered alloys have been accurately predicted using Monte Carlo (MC) simulation method, and (2) the configuration and energy of the adsorption of O, O2, OH, and H2O molecules which are presented in oxygen reduction reaction (ORR) on the surface of pure Pt and surface-segregated Pt-binary alloys (i.e., Pt-Ni, Pt-Co and Pt-Fe) have been determined using density functional theory (DFT) calculations. This thesis yields some guiding principles for designing novel catalysts for proton exchange membrane fuel cells. The Pt concentration profiles of the surfaces of Pt-Ni and Pt-Co alloys were attained from the MC simulations in which the system energy was evaluated through the developed modified embedded atom method (MEAM) for Pt-Ni and Pt-Co alloys. It was found from our simulations that the Pt atoms strongly segregate to the outermost layer and the Ni atoms segregate to the second sub-layer in the (111) surface of both Pt-Ni and Pt-Co alloys. When Pt concentration is higher than 75 at.%, pure Pt top layer could be formed in the outermost layer (111) surface of both alloys. Moreover, segregation reversal phenomenon (Ni atoms segregating to the outermost layer while Pt atoms to the second sub-layer) was observed in our MC simulations of unreconstructed (110) surface of Pt-Ni alloys. In contrast, a Pt enriched outermost surface layer was found in a Pt-Ni reconstructed (1x2) missing row (110) surface. Our MC simulation results agree well with published experimental observations. In addition, adsorption of atomic and molecular oxygen, water and hydroxyl on the (111) and (100) surfaces of pure Pt and Pt-based alloys (Pt-Ni, Pt-Co and Pt-Fe) were studied using spin DFT method and assuming a coverage of 0.25 monolayer. Both the optimized configurations and the corresponding adsorption energies for each species were obtained in this study. In particular, we elucidated the influence of the adsorption energies of atomic oxygen and OH on the activity for ORR on Pt binary alloy catalysts in acidic environment. The calculated adsorption energies of atomic oxygen on the (111) surfaces of pure Pt, Pt-Ni, Pt-Co and Pt-Fe are -3.967 eV, -3.502 eV, -3.378 eV and -3.191 eV, respectively. The calculated adsorption energies of hydroxyl on the (111) surfaces of pure Pt, Pt-Ni, Pt-Co and Pt-Fe are -2.384 eV, -2.153 eV, -2.217 eV and -2.098 eV, respectively. The interaction between the adsorbed atomic and hydroxyl and the corresponding (111) surface becomes weaker for the surface-segregated alloys compared to pure Pt catalyst. The same results were obtained for the (100) surfaces.
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    Molecular Dynamics Simulations of the Mechanical Deformation Behavior of Face-Centered Cubic Metallic Nanowires
    (2010-05-05T14:41:16Z) Heidenreich, Joseph David; Wang, Guofeng; Chen, Jie; Jones, Alan
    Nanoscale materials have become an active area of research due to the enhanced mechanical properties of the nanomaterials in comparison to their respective bulk materials. The effect that the size and shape of a nanomaterial has on its mechanical properties is important to understand if these materials are to be used in engineering applications. This thesis presents the results of molecular dynamics (MD) simulations on copper, gold, nickel, palladium, platinum, and silver nanowires of three cross-sectional shapes and four diameters. The cross-sectional shapes investigated were square, circular, and octagonal while the diameters varied from one to eight nanometers. Due to a high surface area to volume ratio, nanowires do not have the same atomic spacing as bulk materials. To account for this difference, prior to tensile loading, a minimization procedure was applied to find the equilibrium strain for each structure size and shape. Through visualization of the atomic energy before and after minimization, it was found that there are more than two energetically distinct areas within the nanowires. In addition, a correlation between the anisotropy of a material and its equilibrium strain was found. The wires were then subjected to a uniaxial tensile load in the [100] direction at a strain rate of 108 s-1 with a simulation temperature of 300 K. The embedded-atom method (EAM) was employed using the Foiles potential to simulate the stretching of the wires. The wires were stretched to failure, and the corresponding stress-strain curves were produced. From these curves, mechanical properties including the elastic modulus, yield stress and strain, and ultimate strain were calculated. In addition to the MD approach, an energy method was applied to calculate the elastic modulus of each nanowire through exponential fitting of an energy function. Both methods used to calculate Young’s modulus qualitatively gave similar results indicating that as diameter decreases, Young’s modulus decreases. The MD simulations were also visualized to investigate the deformation and yield behavior of each nanowire. Through the visualization, most nanowires were found to yield and fail through partial dislocation nucleation and propagation leading to {111} slip. However, the 5 nm diameter octagonal platinum nanowire was found to yield through reconstruction of the {011} surfaces into the more energetically favorable {021} surfaces.
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    TENSILE DEFORMATION BEHAVIOR AND MECHANICAL PROPERTY STUDY OF SIX FCC METALS
    (Office of the Vice Chancellor for Research, 2010-04-09) Heidenreich, Joseph; Wang, Guofeng
    Nanomaterials have enhanced mechanical properties in comparison to their respective bulk materials. To understand the effect of the size and shape on the mechanical properties of nanomaterials, we used molecular dynamics (MD) methods to simulate the deformation process of copper, gold, nickel, palladium, platinum, and silver nanowires of three cross-sectional shapes (quare, circular, and octagonal) and four diameters (varied from one to eight nanometers). In this work, the nanowires were subjected to a uniaxial tensile load in the [100] direction at a strain rate of 108 s-1 at a simulation temperature of 300 K. The embedded-atom method was employed to describe the many-body atomic interaction energy in metallic systems. The nanowires were stretched to failure and the corresponding stress-strain curves were produced. From these curves, mechanical properties including the elastic modulus, yield stress and strain, and ultimate strain were calculated. In addition to the MD approach, an energy method was applied to calculate the elastic modulus of each nanowire through exponential fitting of an energy function. Both methods used to calculate Young’s modulus qualitatively gave similar results indicating that as diameter decreases, Young’s modulus decreases. The atomic structures generated from MD simulations were examined in details to investigate the deformation and yield behavior of each nanowire. It was found that most nanowires yield and fail through partial dislocation nucleation and propagation leading to {111} slip. However, the octagonal platinum nanowire, whose diameter is 5 nm, was found to yield through reconstruction of the {011} surfaces into the more energetically favorable {111} surfaces.