Yazar "Putz, Barbara" seçeneğine göre listele

Listeleniyor 1 - 4 / 4
  • [ X ]
    Öğe
    Effective electrocatalytic methanol oxidation of Pd-based metallic glass nanofilms
    (Royal Soc Chemistry, 2020) Sarac, Baran; Karazehir, Tolga; Ivanov, Yurii P.; Putz, Barbara; Greer, A. Lindsay; Sarac, A. Sezai; Eckert, Juergen
    Compared to their conventional polycrystalline Pd counterparts, Pd79Au9Si12 (at%) - metallic glass (MG) nanofilm (NF) electrocatalysts offer better methanol oxidation reaction (MOR) in alkaline medium, CO poisoning tolerance and catalyst stability even at high scan rates or high methanol concentrations owing to their amorphous structure without grain boundaries. This study evaluates the influence of scan rate and methanol concentration by cyclic voltammetry, frequency-dependent electrochemical impedance spectroscopy and a related equivalent circuit model at different potentials in Pd-Au-Si amorphous NFs. Structural and compositional differences for the NFs are assessed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray (EDX) mapping and X-ray diffraction (XRD). The ratio of the forward to reverse peak current density i(pf)/i(pb) for the MG NFs is similar to 2.2 times higher than for polycrystalline Pd NFs, evidencing better oxidation of methanol to carbon dioxide in the forward scan and less poisoning of the electrocatalysts by carbonaceous (e.g. CO, HCO) species. Moreover, the electrochemical circuit model obtained from EIS measurements reveals that the MOR occurring around -100 mV increases the capacitance without any significant change in oxidation resistance, whereas CO2 formation towards lower potentials results in a sharp increase in the capacitance of the Faradaic MOR at the catalyst interface and a slight decrease in the corresponding resistance. These results, together with the high i(pf)/i(pb) = 3.37 yielding the minimum amount of carbonaceous species deposited on the thin film during cyclic voltammetry and stability in the alkaline environment, can potentially make these amorphous thin films potential candidates for fuel-cell applications.
  • [ X ]
    Öğe
    Enhancement of Interfacial Hydrogen Interactions with Nanoporous Gold-Containing Metallic Glass
    (Amer Chemical Soc, 2021) Sarac, Baran; Ivanov, Yurii P.; Micusik, Matej; Karazehir, Tolga; Putz, Barbara; Dancette, Sylvain; Omastova, Maria
    Contrary to the electrochemical energy storage in Pd nanofilms challenged by diffusion limitations, extensive metalhydrogen interactions in Pd-based metallic glasses result from their grain-free structure and presence of free volume. This contribution investigates the kinetics of hydrogen-metal interactions in goldcontaining Pd-based metallic glass (MG) and crystalline Pd nanofilms for two different pore architectures and nonporous substrates. Fully amorphous MGs obtained by physical vapor deposition (PVD) co-sputtering are electrochemically hydrogenated by chronoamperometry. High-resolution (scanning) transmission electron microscopy and corresponding energydispersive X-ray analysis after hydrogenation corroborate the existence of several nanometer-sized crystals homogeneously dispersed throughout the matrix. These nanocrystals are induced by PdHx formation, which was confirmed by depth-resolved X-ray photoelectron spectroscopy, indicating an oxide-free inner layer of the nanofilm. With a larger pore diameter and spacing in the substrate (Pore40), the MG attains a frequency-independent impedance at low frequencies (similar to 500 Hz) with very high Bode magnitude stability accounting for enhanced ionic diffusion. On the contrary, on a substrate with a smaller pore diameter and spacing (Pore25), the MG shows a larger low-frequency (0.1 Hz) capacitance, linked to enhanced ionic transfer in the near-DC region. Hence, the nanoporosity of amorphous and crystalline metallic materials can be systematically adjusted depending on AC- and DC-type applications.
  • [ X ]
    Öğe
    Metallic Glass Films with Nanostructured Periodic Density Fluctuations Supported on Si/SiO2as an Efficient Hydrogen Sorber
    (Wiley-V C H Verlag Gmbh, 2020) Sarac, Baran; Ivanov, Yury; Karazehir, Tolga; Putz, Barbara; Greer, A. Lindsay; Sarac, A. Sezai; Eckert, Juergen
    Nanostructured metallic glass films (NMGF) can exhibit surface and intrinsic effects that give rise to unique physical and chemical properties. Here, a facile synthesis and electrochemical, structural, and morphologic characterization of Pd-Au-Si based MGs of approximately 50 nm thickness supported on Si/SiO(2)is reported. Impressively, the maximum total hydrogen charge stored in the Pd-Au-Si nanofilm is equal to that in polycrystalline Pd films with 1 mu m thickness in 0.1 mH(2)SO(4)electrolyte. The same NMGF has a volumetric desorption charge that is more than eight times and 25 % higher than that of polycrystalline PdNF and Pd-Cu-Si NMGF with the same thickness supported on Si/SiO2, respectively. A significant number of nanovoids originating from PdH(x)crystals, and an increase in the average interatomic spacing is detected in Pd-Au-Si NMGF by high-resolution TEM. Such a high amount of hydrogen sorption is linked to the unique density fluctuations without any chemical segregation exclusively observed for this NMGF.
  • [ X ]
    Öğe
    Synergistic enhancement of hydrogen interactions in palladium-silicon-gold metallic glass with multilayered graphene
    (Royal Soc Chemistry, 2023) Sarac, Baran; Ivanov, Yurii P.; Putz, Barbara; Karazehir, Tolga; Mitterer, Christian; Greer, A. Lindsay; Sarac, A. Sezai
    Amorphous PdSiAu-based metallic glass thin films (MGTFs) obtained by physical vapor deposition were deposited on multilayered graphene (MLGR) supported on Si/SiO2, where the MLGR is carried to the top by upward pressure of the deposited atomic layer passing through the crystal lattice of graphene. Samples were electrochemically hydrogenated by chronoamperometry and characterized by cyclic voltammetry in 0.1 M H2SO4. MLGR-containing samples have a prominent Raman peak at 1415 cm-1. This sample shows & SIM;2.6 times larger hydrogen desorption charge and & SIM;4.5 times larger electrocatalytic hydrogen activity compared to the MLGR-free counterparts. Furthermore, the capacitance retrieved from the simulation of electrochemical impedance data indicates a & SIM;2.6 times increase upon MLGR inclusion. High-resolution (scanning) transmission electron microscopy after hydrogenation corroborates the existence of nm-sized PdHx crystals around the MGTF-Si/SiO2 interface and the presence of a graphene layer on top of the MGTF due to bond breaking between the MLGR and Si/SiO2. The enhanced hydrogen activity due to the synergistic effect of MLGR and MGTF layer-by-layer nanostructure reveals itself in the diffusion kinetics, where 50% faster hydrogen ion transfer into the MGTF is obtained when the MLGR top layer is present. The areal and volumetric hydrogen desorption charge exceed almost all the considered Pd-based counterparts, especially when comparing systems with similar thicknesses. Hence, the developed hybrid nanostructure can be envisaged as an alternative ultra-high hydrogen charger for small-scale applications. Presence of only a few layers of graphene boosts hydrogen intake of Pd-based metallic glass thin films by 2.6 times with 4.5 times higher electrocatalytic hydrogen evolution reaction activity, a tremendous improvement in metal-hydrogen interactions.