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    Corrosion prevention of mild steel in acidic medium by 2-Pyrrolidi-n-1-yl-1,3-thiazole-5-carboxylic acid: Theoretical and experimental approach
    (Elsevier, 2022) Karazehir, Tolga; Mert, Mehmet Erman; Mert, Basak Dogru
    The inhibition efficiency of 2-Pyrrolidin-1-yl-1,3-thiazole-5-carboxylic acid (PTCA) against mild steel (MS) corrosion was investigated in acidic solution by using quantum chemical calculations based on Density Functional Theory (DFT) method and electrochemical measurements. The electrochemical impedance spectroscopy (EIS), potentiodynamic, potential zero charge (pzc) analysis and electrochemical noise (EN) measurements at various concentrations (from 0.1 to 10 mM) and immersion times were utilized in experimental part. The surface analysis was achieved scanning electron microscope (SEM) and contact angle measurements in the absence and presence of 10 mM PTCA. According to DFT results, PTCA exhibited 3.737 eV band gap and 8.130 Debye dipole moment which were a signal of potentially convenient corrosion inhibitor properties. PTCA has a remarkable corrosion inhibition capability to mild steel, which inhibited both anodic and cathodic corrosion rates, relying on it's physically adsorption on the metal solution interface and protection ability was increased with increasing PTCA concentration. The obtained adsorption equilibrium constant was 11.11 x 103 M-1 and calculated standard free energy of adsorption was -33.03 kJ mol-1. The determined activation energy values were 55.58 kJ mol-1 and 96.86 kJ mol-1 in 0.5 M HCl in the absence and presence of 10 mM PTCA, respectively. PTCA demonstrated a strong inhibition efficiency of 98.3%, after 168 h immersion, according to the EIS results. As a consequently, we recommend that PTCA is a convenient inhibitor in 0.1 M HCl for mild steel protection against corrosion.
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    Corrosion prevention of mild steel in acidic medium by 2-Pyrrolidi-n-1-yl-1,3-thiazole-5-carboxylic acid: Theoretical and experimental approach
    (Elsevier, 2022) Karazehir, Tolga; Mert, Mehmet Erman; Mert, Basak Dogru
    The inhibition efficiency of 2-Pyrrolidin-1-yl-1,3-thiazole-5-carboxylic acid (PTCA) against mild steel (MS) corrosion was investigated in acidic solution by using quantum chemical calculations based on Density Functional Theory (DFT) method and electrochemical measurements. The electrochemical impedance spectroscopy (EIS), potentiodynamic, potential zero charge (pzc) analysis and electrochemical noise (EN) measurements at various concentrations (from 0.1 to 10 mM) and immersion times were utilized in experimental part. The surface analysis was achieved scanning electron microscope (SEM) and contact angle measurements in the absence and presence of 10 mM PTCA. According to DFT results, PTCA exhibited 3.737 eV band gap and 8.130 Debye dipole moment which were a signal of potentially convenient corrosion inhibitor properties. PTCA has a remarkable corrosion inhibition capability to mild steel, which inhibited both anodic and cathodic corrosion rates, relying on it's physically adsorption on the metal solution interface and protection ability was increased with increasing PTCA concentration. The obtained adsorption equilibrium constant was 11.11 x 103 M-1 and calculated standard free energy of adsorption was -33.03 kJ mol-1. The determined activation energy values were 55.58 kJ mol-1 and 96.86 kJ mol-1 in 0.5 M HCl in the absence and presence of 10 mM PTCA, respectively. PTCA demonstrated a strong inhibition efficiency of 98.3%, after 168 h immersion, according to the EIS results. As a consequently, we recommend that PTCA is a convenient inhibitor in 0.1 M HCl for mild steel protection against corrosion.
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    Effect of Gd-doping in Ni/NiO core/shell magnetic nanoparticles (MNPs) on structural, magnetic, and hydrogen evolution reaction
    (Aip Publishing, 2022) Adanur, Idris; Karazehir, Tolga; Dogru Mert, Basak; Akyol, Mustafa; Ekicibil, Ahmet
    In this study, Gd-x-doped Ni/NiO MNPs (x: 0.0%, 2.5%, 5.0%, and 10.0%) with a protective polyvinylpyrrolidone (PVP) layer have been synthesized via a polyol reduction process. The x-ray diffraction patterns revealed that samples have a cubic structure with Fm3m space group and no change in the crystallite structure was observed with doping Gd3+ ions. The crystallite size (D-c) decreases from 2.70 to 1.27 nm when Gd is doped into Ni/NiO MNPs. Transmission electron microscopy analysis revealed that the Ni/NiO MNPs with Gd(5%) concentration are formed as spherical multicore-like shape core/shell MNPs with a protective PVP layer. The magnetic hysteresis measurements taken at 10 and 300 K show that the saturation magnetization (M-s) decreases with increasing Gd3+ ions in the structure. The highest effective magnetic moment (mu(eff)) was obtained as 10.34 mu(B) in the NG-2 sample. We ascribe that the high mu(eff) value in this sample is due to the increase in d-f exchange interaction between Ni(3d(7)) and Gd(4f(7)) and the contribution of the dipole moment of PVP molecules. The electrochemical measurements showed that the current density values were 0.294 and 0.319 mA/cm(2) at-1.3 V in the absence of Gd (NG-0) and Gd(5%) doped (NG-2) samples, respectively. beta c was 159 and 132 mV/dec for NG-0 and NG-2 samples, respectively. The diminishing of beta c and the charge resistance (Rct) proved that the Gd doped catalyst enhanced the hydrogen evolution activity and the Gd(5%) doped sample exhibited the highest catalyst performance.
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    Effect of Supporting Electrolyte on Capacitance and Impedance Properties of Electrodeposited PEDOT/ERGO Electrodes for Supercapacitor
    (2023) Karazehir, Tolga
    In this study, the ability of Poly (3,4-ethylenedioxythiophene)/electrochemically reduced graphene oxide (PEDOT/ERGO) electrodes for supercapacitors to store electrical energy is studied. PEDOT/ERGO electrodes are produced using a simple two-step electrochemical method that involves electrochemically reduction of graphene oxide, and then PEDOT is electrochemically deposited onto the ERGO in different electrolyte solutions. Electrochemical techniques such as cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS) are utilized to investigate the electrochemical characteristics of PEDOT/ERGO electrodes produced in different electrolyte systems including 0.1 M lithium perchloride/acetonitrile (LiClO4/ACN), tetraethylammoniumtetrafluoroborate/acetonitrile (Et4NBF4/ACN), and tetrabutylammonium hexaflorophosphate/acetonitrile (Bu4NPF6/ACN). The electrolyte type has a significant impact on electrochemical results. PEDOT/ERGO manufactured in LiClO4/ACN demonstrates a high specific capacitance (Csp) and low charge transfer resistance (Rct). According to results, the Csp value is decreased in the following order; 26.48 mFcm-2 for LiClO4, 20.58 mFcm-2 for Et4NBF4, 8.96 mFcm-2 for Bu4NPF6.
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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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    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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    Electrodeposited Pd nanoparticles on polypyrole/nickel foam for efficient methanol oxidation
    (Pergamon-Elsevier Science Ltd, 2023) Karazehir, Tolga
    The present work describes the Ni foam (Ni-F)/polypyrrole (PPy)/palladium (Pd) (Ni-F/PPy/ Pd) multilayered catalysts via a facile electrochemical technique. Potentiostatic deposition of PPy on the surface of Ni-F is followed by galvanostatic deposition of Pd nanoparticles on Ni-F/PPy acted as supports for electrochemical deposition of Pd nanoparticles. The pro-duced catalysts are utilized for electrocatalytic methanol oxidation in alkaline media. Chronoamperometry (CA), cyclic voltammetry (CVs), and electrochemical impedance spectroscopy (EIS) techniques are used to examine the electrocatalytic performance of Ni-F/PPy/Pd based electrodes for methanol oxidation. The polypyrrole modification on Ni-F leads to an improvement in the electrocatalytic activity of the Ni-F/PPY-Pd catalysts toward methanol oxidation. As an open-pored, porous metal with high electrical conductivity, nickel foam produces a substantial amount of active area during the modification of Pd and polypyrrole, which results in significant catalytic activity and a rapid rate charge transfer reaction kinetics on methanol oxidation. The Ni-F/PPy/Pd10 catalyst exhibits enhanced specific activity than its counterparts and a reduced onset potential for methanol oxida-tion, as well as a low Tafel slope. Based on these results, Ni-F/PPy/Pd10 is suggested as a good material for the anode in the electrocatalytic oxidation of methanol. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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    Electrosorption of Hydrogen in Pd-Based Metallic Glass Nanofilms
    (Amer Chemical Soc, 2018) Sarac, Baran; Karazehir, Tolga; Muehlbacher, Marlene; Kaynak, Baris; Gammer, Christoph; Schoeberl, Thomas; Sarac, A. Sezai
    As an efficient potential hydrogen storage and conversion system, hydrogen electrosorption and evolution mechanisms in Pd-based metallic glass thin films (MGTFs) are investigated. In this study, thin films of 55 nm thickness were deposited by dc magnetron sputtering. The amorphous structure of MGTFs and the atomically smooth interface between the MGTF and substrate were confirmed by transmission electron microscopy, whereas the composition-dependent surface roughness was obtained via atomic force microscopy. The shifts in the broad diffraction maxima for the Si and Cu additions were evaluated by X-ray diffraction. The Pd thin film (PdTF) and MGTF working electrodes were chronoamperometrically saturated in 0.5 M H2SO4 solution. The formation of palladium hydride (PdHx) in the MGTFs was investigated by X-ray photoelectron spectroscopy. Cyclic voltammograms were subsequently recorded (between -0.2 and 1.4 V) at sweep rates of 0.02 V s(-1). Electrochemical impedance spectroscopy of MGTFs and PdTF was performed in full spectrum including sorption, desorption, and evolution of hydrogen in a conventional three-electrode configuration. Electrochemical circuit modeling provided the relationship between the composition-dependent hydrogen evolution and H absorption/adsorption processes. The adsorption capacitance parameter Y-ad corresponding to alpha- and beta-hydride formation in the case of Pd0.79Si0.16Cu0.05 MGTF is similar to 5 times higher than that of the crystalline Pd thin film which is in line with the decrease in the charge-transfer resistance R-ct. Addition of Cu disturbs the symmetry of the glass formers, leading to remarkable changes in interfacial hydrogen bonding and diffusion of hydrogen into sublayers. Compared to other Pd-based micron-sized materials, our findings show excellent volumetric hydrogen storage capacity 4 times higher than that of the traditional counterparts of several microns, and 50% higher than the Pd thin films of the same thickness, together with high tunable capacitance, charge-transfer resistance, and diffusivity depending on the glass-forming characteristics of the nanosized MGTF.
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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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    Experimental and theoretical study on hydrogen production by using Ag nanoparticle-decorated graphite/Ni cathode
    (Wiley, 2021) Yildiz, Resit; Dogru Mert, Basak; Karazehir, Tolga; Gurdal, Yeliz; Toprak Doslu, Serap
    In this study, graphite (G) electrode was coated with nickel and decorated with silver nanoparticles (G/Ni/Ag) with the help of galvanostatic method, and electrodes were used as a cathode in alkaline water electrolysis system. The characterization was achieved using X-ray diffraction and field emission scanning electron microscopy. Hydrogen evolution performance of electrodes was investigated via cyclic voltammetry, chronoamperometry, cathodic polarization curves, and electrochemical impedance measurements. Electrochemical results showed that hydrogen production efficiency significantly increased and charge transfer resistance decreased via G/Ni/Ag. The electrochemical water splitting performance of G/Ni/Ag, was established in a joint experimental and computational effort. Water and proton adsorption on Ag-decorated Ni surface were investigated using density functional theory. Electronic structure calculations identified the role of Ag adatom and Ni surface on water and proton adsorptions. From the computational studies, O in water was more reliable to adsorb at the bridge position of the Ag and Ni atoms, leading improved orbital overlap between H and Ni atoms and maximized chemical and physical interactions between the H2O molecules. Therefore, the Ag-decorated Ni(111) surface provides preferable adsorption site for the O atom in water and direct interactions between water Hs and available surface Ni atoms promote water dissociation.
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    Experimental and theoretical study on hydrogen production by using Ag nanoparticle-decorated graphite/Ni cathode
    (Wiley, 2021) Yildiz, Resit; Dogru Mert, Basak; Karazehir, Tolga; Gurdal, Yeliz; Toprak Doslu, Serap
    In this study, graphite (G) electrode was coated with nickel and decorated with silver nanoparticles (G/Ni/Ag) with the help of galvanostatic method, and electrodes were used as a cathode in alkaline water electrolysis system. The characterization was achieved using X-ray diffraction and field emission scanning electron microscopy. Hydrogen evolution performance of electrodes was investigated via cyclic voltammetry, chronoamperometry, cathodic polarization curves, and electrochemical impedance measurements. Electrochemical results showed that hydrogen production efficiency significantly increased and charge transfer resistance decreased via G/Ni/Ag. The electrochemical water splitting performance of G/Ni/Ag, was established in a joint experimental and computational effort. Water and proton adsorption on Ag-decorated Ni surface were investigated using density functional theory. Electronic structure calculations identified the role of Ag adatom and Ni surface on water and proton adsorptions. From the computational studies, O in water was more reliable to adsorb at the bridge position of the Ag and Ni atoms, leading improved orbital overlap between H and Ni atoms and maximized chemical and physical interactions between the H2O molecules. Therefore, the Ag-decorated Ni(111) surface provides preferable adsorption site for the O atom in water and direct interactions between water Hs and available surface Ni atoms promote water dissociation.
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    Functionalized highly electron-rich redox-active electropolymerized 3,4-propylenedioxythiophenes as precursors and targets for bioelectronics and supercapacitors
    (Royal Soc Chemistry, 2021) Karazehir, Tolga; Sarac, Baran; Gilsing, Hans-Detlev; Gumrukcu, Selin; Eckert, Jurgen; Sarac, A. Sezai
    In order to combine capacitive properties with processability, e.g. solubility in organic solvents, poly(3,4-propylenedioxythiophene) derivatives containing different functional groups like oxyphenyl methanol (-OPhCH2OH), oxybenzyl (-OBz), bromide (-Br) and tosyl (-OTs) were synthesized and electropolymerized as thin films from acetonitrile (ACN) using Et4NBF4 as an electrolyte. Multifunctionality in the substitution pattern of the polymer exhibits a similar trend between monomer oxidation potentials and specific capacitance (C-sp) vs. crystal size. The presence of pi-pi stacking interactions in the polymer structures plays an important role in building the crystal structures. The same order of flat band potential and C-sp values are observed for -OBz < -Br < -OTs < -OPhCH2OH substitutions. The structures of PProDOT-OBz and PProDOT-OPhCH2OH resemble each other much more than those of PProDOT-Br and PProDOT-OTs. Impedance measurements were conducted at different applied biases in order to define a Mott-Schottky analysis revealing the dependence of the semiconducting properties on the type of substituent present in the PProDOT derivative.
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    Low-Symmetry Phthalocyanine Cobalt Bis(dicarbollide) Conjugate for Hydrogen Reduction
    (Wiley-V C H Verlag Gmbh, 2018) Nar, Ilgin; Atsay, Armagan; Gumrukcu, Selin; Karazehir, Tolga; Hamuryudan, Esin
    The synthesis and characterization of two new A(3)B type unsymmetrical zinc phthalocyanines, one bearing a hydroxyl group and the other bearing a cobalt bis(dicarbollide) unit, are reported here. The introduction of diethylaminophenoxy moieties to the phthalocyanine fragments yields outstanding solubilities in organic solvents, as well as providing electropolymerization on the surface of the electrode. These modified electrodes are found to be effective for the electrocatalytic reduction of hydrogen ions in acidic media. The enhanced hydrogen-evolution reaction (HER) performance for the bis(dicarballide)-functionalized phthalocyanine-modified electrode can be simultaneously observed. This is attributed to its redox-active metal center of cobalt bis(dicarbollide), creating a synergistic effect, to facilitate many more electrocatalytic active sites, resulting in improved HER performance. The catalyst shows an onset potential of 163 mV at a current density of 8.79 mu Acm(-2), and a Tafel slope as low as 28 mVdec(-1).
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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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    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.