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    Effect of supporting electrolyte on capacitance and morphology of electrodeposited poly(3,4-propylenedioxythiophene) derivatives bearing reactive functional groups
    (Royal Soc Chemistry, 2022) Sarac, Baran; Karazehir, Tolga; Gilsing, Hans-Detlev; Eckert, Juergen; Sarac, A. Sezai
    The interactions between the electrolyte and electroactive species in redox active PProDOT derivatives can be enhanced with better-performing electrodes for electrochemical energy conversion and storage. Differently functionalized 3,4-propylenedioxythiophenes (ProDOTs) are electropolymerized from acetonitrile using Et4NBF4, Et4NPF6, or NaClO4 as an electrolyte to evaluate the electrochemical performance of the considered polymer derivatives, as well as under monomer-free conditions. We propose a new equation to assess the kinetics of the diffusion-limited adsorption process with two independent solved parameters: diffusion slope k and power exponent b. Electron-donating groups attached to the monomer stabilize radical cation and dication formation during electrogrowth. The ProDOT-Br structure has a low oxidation potential owing to the electron-rich group in the heterocyclic structure, which grants access to a radical cation intermediate and interaction with the ions of the electrolyte. Surface roughness and morphology are assessed by atomic force microscopy and scanning electron microscopy, respectively. The changes in the Fourier transform infrared spectrum are more pronounced with the selection of electrolyte type than the type of derivatives. Except the -OBz derivative, the largest specific capacitance calculated from the area under the final cyclic voltammetry curves is obtained for Et4NBF4, followed by Et4NPF6 and NaClO4. In all systems, the -OPhCH2OH derivative exhibited the lowest electrolyte diffusion. More pronounced diffusion is observed for the PProDOT derivatives measured in Et4NPF6 and Et4NBF4 electrolytes, whereas the lowest diffusion is obtained for the NaClO4 electrolyte. Findings in this study provide new insights into the factors requiring attention upon tailoring assemblies for organic electronic applications.
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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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    Effective Methanol Oxidation with Platinum Nanoparticles-Decorated Poly(2-bromomethyl-2-methyl-3,4-propylenedioxythiophene)-Coated Glassy Carbon Electrode
    (Electrochemical Soc Inc, 2021) Karazehir, Tolga; Sarac, Baran; Gilsing, Hans-Detlev; Eckert, Juergen; Sarac, A. Sezai
    Here, we developed a porous network of bromomethyl-substituted 3,4-propylenedioxythiophene polymer using a simple and efficient technique of electrochemical deposition used as conductive support for methanol oxidation. Platinum nanoparticles (PtNPs) are well dispersed and decorated on a high surface area of electrochemically deposited Poly(2-bromomethyl-2-methyl-3,4-propylenedioxythiophene (PProDOT-Br) on a glassy carbon electrode (GCE). A thin film of PProDOT-Br acts as a supporting matrix for deposition of PtNPs and improves the interfacial properties between electrode and electrolyte. The PtNPs-decorated PProDOT-Br (Pt/PProDOT-Br) samples were characterized by X-ray diffraction, Fourier transform infrared attenuated total reflectance spectroscopy, atomic force microscopy, and scanning electron microscopy. Furthermore, the electrocatalytic performance of Pt/PProDOT-Br on GCE for methanol oxidation was assessed by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy measurements. The findings suggest that the use of Pt/PProDOT-Br/GCE assemblies for efficient methanol oxidation in alkaline media with a small intermediate poisoning is promising for applications as anode material in DMFCs, which should be attributed to the PProDOT-Br support providing a larger surface area with porous nature and enabling the adsorption of more CH3OH for further oxidation. The developed porous network PProDOT-Br with high capacitance may also have large potential in supercapacitor applications.
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    Electrocatalytic Behavior of Hydrogenated Pd-Metallic Glass Nanofilms: Butler-Volmer, Tafel, and Impedance Analyses
    (Springer, 2020) Sarac, Baran; Karazehir, Tolga; Muehlbacher, Marlene; Sarac, A. Sezai; Eckert, Juergen
    Electrocatalytic activity and sorption behavior of hydrogen in nanosized Pd-Si-(Cu) metallic glass thin film and Pd thin film electrodes sputtered on a Si/SiO2 substrate were investigated by linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. The electrode MG4 (Pd69Si18Cu13) exhibits the best performance with the highest electrocatalytic activity in the hydrogen evolution region with less than half of the Tafel slope of Pd thin film of the same thickness and lowest overpotential at 10 mA cm(-2). A new approach has been adopted by a nonlinear fitting of the entire region of the polarization curve (far- and near-equilibrium cathodic and anodic regions) to the Butler-Volmer model. a parameter is lowest for the MG2 electrode (Pd79Si16Cu5), marking that nonequilibrium conditions change the reaction kinetics. Together with MG2, MG4 shows the lowest Bode magnitude values for hydrogen sorption and evolution regions, indicating that the bonding and release of hydrogen atoms to the electrode is easier. MG4 electrode shows a dramatic decrease of the overpotential after 100 cycles, yielding an increase in hydrogen activity. Besides, MG4 exhibits the sharpest current density drop in the HER region in cyclic voltammetry compared with other MG and Pd electrodes, indicating higher electrocatalytic activity towards hydrogen evolution. The findings highlight the influence of the selected metallic glasses for the design and development of metal catalysts with higher sorption kinetics and/or electrocatalytic turnover.
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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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    Nanoporous Pd-Cu-Si Amorphous Thin Films for Electrochemical Hydrogen Storage and Sensing
    (Amer Chemical Soc, 2021) Sarac, Baran; Karazehir, Tolga; Yuce, Eray; Muehlbacher, Marlene; Sarac, A. Sezai; Eckert, Juergen
    Increasing the efficiency of hydrogen storage and release using recent generation metallic glass nanofilms (MGNFs) offers green solutions for nanoscale energy applications. Contrary to flat nanofilms, enhanced electrochemical performance of Pd-Cu-Si MGNF assemblies for hydrogen interaction is obtained on different sizes and configurations of a nanoporous alumina support. In particular, 10 nm thick samples with pore diameters of 25 nm reach a high specific pseudocapacitance per unit mass of 637 F g(-1), which is more than an order of magnitude larger than for flat samples, surpassing the precious metal-based systems in the literature. The same electrode exhibits the highest double-layer capacitance calculated from the equivalent circuit model of the electrochemical impedance spectra, featuring its eligibility for hydrogen nanosensors. A rough and fully coated surface is attained for samples of 250 mu m thickness and above, while smoother and open-pore structures are observed for lower thicknesses, inducing a capillary pressure and turbulent flow effect for the latter case. The comparison of cyclic voltammetry (CV) profiles recorded in the region where hydrogen-metal interactions occur confirms a remarkable desorption charge difference, reaching 2.5 times higher values for the 50 nm thick 25 nm pore diameter than the 40 nm pore diameter and flat electrodes, and lower absolute impedance values near-DC range revealing their highly conductive behavior.
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    Oligoether Ester-Functionalized ProDOT Copolymers on Si/Monolayer Graphene as Capacitive Thin Film Electrodes
    (Electrochemical Soc Inc, 2020) Karazehir, Tolga; Sarac, Baran; Gilsing, Hans-Detlev; Eckert, Juergen; Sarac, A. Sezai
    In this study, electrochemical polymerization of 3,4-propylenedioxythiophene (ProDOT 1), ProDOT bearing oligoether ester (ProDOT-EO-ester 2) and their copolymerization onto homogeneously CVD coated nano-graphene/Si support is realized to attain graphene/ProDOT based copolymer hybrid nanostructures. By introducing oligoether side chain to ProDOT backbone and using different [ProDOT]/[ProDOT-EO-ester] molar ratios ensures a considerable decrease in oxidation potential of polymer allowing tunable properties to copolymers revealing improvement electrochemical capacitance and electrochemical activity which are clearly reflected by the experimental results. Capacitive behavior of copolymers is determined by electrochemical impedance spectroscopy, cyclic voltammetry. Moreover, The structural, morphological and spectroscopic characterization of the copolymers is investigated by XRD, AFM, SEM, EDX, FTIR, and Raman, respectively. By the increase of ProDOT in the copolymer composition, the higher dopant concentration is attained suggesting an enhanced conductivity agree well with the impedance and CV results, where the copolymerization of ProDOT 1 and ProDOT-EO-ester 2 in equal molarity results in the highest specific capacitance and redox activity. The adopted equivalent circuit model for polymers is in good agreement with the experimental data of impedance. Due to the difference in conjugated structure between ProDOT and ProDOT-EO-ester by the presence of the EO-ester group leads to a decrease in charge transfer resistance with increasing mole fraction of ProDOT-EO-ester. The charge transfer resistance of [ProDOT](0)/[ProDOT-EO-ester](0) = 1:1 coated Si/graphene is nearly 51 and 24 times lower value compared to those of PProDOT and P(ProDOT-EO-ester) homopolymers coated Si/graphene, respectively, confirming that the copolymerization improves the electron conduction. By Mott-Schottky measurements, increasing mole fraction of ProDOT-EO-ester 2 in copolymer composition results in the alteration of semiconducting behavior. The developed graphene-polymer hybrid electrodes can be a potential candidate for energy storage devices. (C) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.