Yazar "Ivanov, Yurii P." seçeneğine göre listele

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    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.
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    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.
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    Multilayer crystal-amorphous Pd-based nanosheets on Si/SiO2 with interface-controlled ion transport for efficient hydrogen storage
    (Elsevier Ltd, 2022) Sarac, Baran; Ivanov, Yurii P.; Karazehir, Tolga; Mühlbacher, Marlene; Sarac, A. Sezai; Greer, A. Lindsay; Eckert, Jürgen
    This contribution shows an unusually high hydrogen storage of multilayer amorphous (A)-crystalline (C) Pd–Si based nanosheets when stacked in the right order. Samples with A/C/A/C/A stacking sequence exhibit 40 and 12 times larger hydrogen sorption than monolithic crystalline and amorphous samples, respectively. The maximum capacitance calculated from the fitting of electrochemical impedance measurements of the same sample is twice larger than that of the conventional polycrystalline Pd films of similar thickness. Five times higher diffusion coefficient calculated from modified Cottrell equation is obtained compared to specimens with C/A/C/A/C stacking. For the A/C/A/C/A multilayers, nanobubbles with diameters of 1–2 nm are homogeneously distributed at Si/SiO2 interface, and PdHx crystal formation in these regions confirms hydrogen-metal interactions. Furthermore, corrosion-resistant amorphous top layer permits larger amounts of hydrogen ion transfer to inner layers. Thus, hydrogen storage and production can be enhanced by smart design of multilayers targeted for proton exchange membrane electrolysis or fuel cells. © 2021 Hydrogen Energy Publications LLC
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    Surface-governed electrochemical hydrogenation in FeNi-based metallic glass
    (Elsevier, 2020) Sarac, Baran; Zadorozhnyy, Vladislav; Ivanov, Yurii P.; Kvaratskheliya, Askar; Ketov, Sergey; Karazehir, Tolga; Gumrukcu, Selin
    The hydrogenation and oxide formation behavior of Fe-Ni-based metallic glasses (MGs), where measurements by the conventional gas-solid reaction method are difficult, is analyzed by a two-step approach: chronoamperometry followed by cyclic voltammetry (CA + CV). We introduce a concept of effective volume by measuring the thickness of the region where the hydrogen and hydroxyl ion interactions with Fe-based MG take place, which is characterized by high-angle annular dark-field scanning transmission electron microscopy. A very constant film thickness influenced by the OH- and H+ is confirmed by TEM, where the chemical homogeneity is maintained within this region. The weight percent of hydrogen and the corresponding hydrogen-to-metal ratio are determined as 1.16% and 0.56, respectively. When compared to previous studies conducted by the electrochemical- permeation method, the H/M ratio is found to be an order of magnitude larger. Electrochemical impedance spectroscopy (EIS) and subsequent equivalent circuit modeling (ECM) of the tested ribbons resolve the surface-diffusion processes for hydride formation and oxidation kinetics. This contribution provides a different perspective for the design and study of low-cost and high-performance amorphous nanofilms for hydrogenenergy applications, particularly when the common gas-adsorption methods are problematic.
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    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.
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    Ultrahigh hydrogen-sorbing palladium metallic-glass nanostructures
    (Royal Society of Chemistry, 2019) Sarac, Baran; Ivanov, Yurii P.; Karazehir, Tolga; Mühlbacher, Marlene; Kaynak, Baris; Greer, A. Lindsay; Sarac, A. Sezai
    Pd-Based amorphous alloys can be used for hydrogen energy-related applications owing to their excellent sorption capacities. In this study, the sorption behaviour of dc magnetron-sputtered and chronoamperometrically-saturated Pd-Si-Cu metallic-glass (MG) nanofilms is investigated by means of aberration-corrected high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and electrochemical techniques. The volume expansion of ?V = 10.09 Å3 of a palladium hydride unit cell obtained from HRTEM images due to the hydrogenation of the Pd-MG nanofilms is 1.65 times larger than ?V of the Pd-polycrystalline counterpart loaded under the same conditions. Determined by scanning transmission electron microscopy-high annular dark-field imaging and electron energy loss spectroscopy, the huge difference between the two Pd-based systems is accounted for by the "nanobubbles" originating from hydrogenation, which generate active sites for the formation and expansion of spatially dispersed palladium hydride nanocrystals. A remarkable difference in the hydrogen sorption capacity is measured by electrochemical impedance spectroscopy compared to the Pd polycrystal nanofilms particularly in the ? and ? regions, where the maximum hydrogen to palladium ratio obtained from a combination of chronoamperometry and cyclic voltammetry is 1.56 and 0.61 for the MG and Pd-polycrystal nanofilms, respectively. The findings place Pd-MGs among suitable material candidates for future energy systems. © The Royal Society of Chemistry.