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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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    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.
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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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    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.