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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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    Origin of Electrocatalytic Activity in Amorphous Nickel-Metalloid Electrodeposits
    (American Chemical Society, 2021) Sarac, Baran; Karazehir, Tolga; Micusik, Matej; Halkali, Celine; Gutnik, Dominik; Omastova, Maria; Sarac, A. Sezai
    In transition metal-based alloys, the nonlinearity of the current at large cathodic potentials reduces the credibility of the linear Tafel slopes for the evaluation of electrocatalytic hydrogen activity. High-precision nonlinear fitting at low current densities describing the kinetics of electrochemical reactions due to charge transfer can overcome this challenge. To show its effectiveness, we introduce a glassy alloy with a highly asymmetric energy barrier: amorphous NiP electrocoatings (with different C and O inclusions) via changing the applied DC and pulsed current and NaH2PO2 content. The highest hydrogen evolution reaction (HER) activity with the lowest cathodic transfer coefficient ? = 0.130 with high J0 = -1.07 mA cm-2 and the largest surface areas without any porosity are observed for the pulsed current deposition. The calculated ? has a direct relation with morphology, composition, chemical state and coating thickness defined by the electrodeposition conditions. Here, a general evaluation criterion with practicality in assessment and high accuracy for electrocatalytic reactions applicable to different metallic alloy systems is presented. © 2021 American Chemical Society.