Results 1 - 10 of 534
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[en] Highlights: • Preparation of structurally well-defined Ag70Pt30/Pt(111) surface alloy electrodes. • Surface de-alloying of Ag70Pt30/Pt(111) monolayer surface alloy on an atomic scale. • Potential dependent change from 2D surface dissolution to 3D restructuring. • Ag surface atoms with different stability towards electrochemical corrosion. • Improved ORR performance of the corroded electrodes compared to Pt(111). - Abstract: Aiming at a better understanding of surface de-alloying processes on an atomic scale and its impact on the electrocatalytic properties of bimetallic electrodes in the oxygen reduction reaction (ORR), we have investigated the electrochemical de-alloying of a Ag70Pt30 monolayer surface alloy on Pt(111), in electrochemical potential step measurements and the performance of the resulting electrodes in the ORR. Structurally well-defined electrode surfaces were prepared and characterized by scanning tunneling microscopy (STM) before and after the electrochemical (EC) measurements under ultrahigh vacuum (UHV) conditions. Potential step experiments were performed in a dual thin-layer flow cell in 0.5 M H2SO4 supporting electrolyte, using a UHV-EC transfer system which allowed electrode transfer without intermediate contact to air. STM imaging reveals an increasing, selective removal of Ag from the surface layer in the potential range 0.95–1.05 VRHE, resulting in the formation of monolayer vacancy islands. The last 10% of Ag were removed only at the onset of Pt corrosion and 3D surface restructuring at 1.10 VRHE. Correlating these findings with the total amount of dissolved Ag determined in electrochemical measurements we propose a possible corrosion mechanism. The corroded surfaces, in particular the more strongly corroded surfaces, showed an improved ORR performance, with a lower overpotential, by up to 100 mV, and a lower H2O2 yield than Pt(111). The higher stability of the surface alloys compared to a Ag(111) as well as the ORR activity are discussed in terms of electronic ligand and strain effects, for the latter structural effect have to be considered as well.
[en] Highlights: • Diffusion of ferrocene in dialkylimidazolium ionic liquids is well described by the Stokes-Einstein equation with slip boundary conditions. • Diffusion is faster in a symmetric than in a non-symmetric IL with same number of alkyl carbons N, with the difference decreasing with increasing N. • The Fc0/+ redox couple is an ideal reference model for voltammetric studies in ILs. - Abstract: The diffusion of ferrocene (Fc) molecules in ionic liquids (ILs) was studied using cyclic voltammetry. The symmetric ILs 1,3-dialkylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(CN/2)2im][NTf2] with N = 4, 6, 8, and 10) and non-symmetric ILs 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CN−1C1im][NTf2] with N = 3, 4, 6, 8, and 10) were used to examine the effect of the symmetry of alkyl substitution on the cation and the role of alkyl chain length on the diffusion of Fc. The diffusion coefficient D of Fc was determined by applying the Randles-Sevcik equation to the peak current in the cyclic voltammograms. The diffusion coefficient was found to be higher in a symmetric IL than in a non-symmetric IL with the same number of alkyl carbon atoms N, with the difference decreasing with increasing N. The diffusion of Fc in these ILs is well described by the Stokes-Einstein equation with slip boundary conditions, but with an effective hydrodynamic radius of 0.23 ± 0.01 nm, which is less than the 0.27 nm crystallographic radius of Fc, in agreement with previous studies of the diffusion of solutes in ILs that show the hydrodynamic radius to be less than the van der Waals radius of the solute.
[en] Ordered mesoporous carbons (OMCs) is one of the most promising electrode material for supercapacitor. However, pure OMCs have low specific capacitance due to its simplex storage mechanism based on electric double layer. In this work, anthraquinone (AQ) is used as a modifier to boost the capacitance of OMCs. The modified materials (AQ/OMCs) can not only provide mesoporous channels facilitating rapid ion diffusion, but also generate extra pseudocapacitance improving specific capacitance greatly. As evidenced by electrochemical measurements, AQ/OMCs can exhibit specific capacitance as high as 346 F g−1 in 1 M H2SO4 electrolyte at the current density of 0.5 A g−1. Besides, the AQ/OMCs also possess excellent rate performance with capacitance retention ratio of up to 84.3% even at a very high current density of 30 A g−1. The outstanding capacitive performance of AQ/OMCs can be ascribed to the synergic effect between OMCs and AQ, in which ordered mesoporous channels facilitate rapid ion diffusion, and AQ generates large pseudocapacitance. In addition, asymmetric supercapacitor is assembled using AQ/OMCs and OMC as negative and positive electrode, respectively, which can deliver a very high energy density of 14.51 Wh kg−1 and excellent long-term cycle stability, retaining 96.3% of initial capacitance, after 10,000 cycles.
[en] Highlights: • Monophasic Sb1-xBix compositions are obtained by high-energy mechanical alloying. • Chemical bonding with bismuth promotes the electrochemical magnesiation of antimony. • The biphasic alloying magnesiation was followed by operando XRD and 25Mg solid-state NMR. • The low Sb cycling performance also arises from the stability of the Mg3Sb2 alloy. - Abstract: Despite strong physical and chemical similarities between antimony and bismuth, a distinct behaviour is observed in the electrochemical magnesiation of their micrometric powders. Bismuth undergoes a complete and highly reversible alloying reaction, whereas antimony displays no electrochemical activity. Taking advantage of the complete SbBi solid solution, monophasic compositions Sb1-xBix were prepared by high-energy mechanochemical synthesis and characterized by X-ray diffraction and solid-state 25Mg nuclear magnetic resonance spectroscopy. The electrochemical magnesiation at low current rate shows a full alloying process of Sb1-xBix-based electrodes leading to monophasic Mg3(Sb1-xBix)2. This chemical association of antimony and bismuth enables a positive effect on the electrochemical magnesiation of the electrode and enables higher specific capacities compared to Bi-based electrodes. However, this synergy only operates in the nominal discharge since an irreversible capacity loss, which scales with the antimony content, is observed in the subsequent charge. Operando XRD reveals a complex segregation process leading to pure bismuth and Mg3Sb2 at the end of charge which is further rationalized by density functional theory calculations as an instability of the Mg3(Sb1-xBix)2 solid solution.
[en] A model of a non-polarizable interface is developed and considered in terms of the specific adsorption of charged redox species and the nonspecific adsorption of the supporting electrolyte. The adsorption coefficients of charged redox species depend on their charges and the potential at the inner Helmholtz plane. The adsorbed amounts of the redox species and supporting electrolyte are estimated using the Frumkin isotherm and Gouy–Chapman theory, respectively. The potential dependence of the adsorbed amount of either one or both of the oxidized and reduced forms of the redox species is maximized near its standard potential. The electrocapillary equation at a non-polarizable liquid metal–solution interface under an externally controlled potential is revisited. The derived electrocapillary equation facilitates calculating the electrocapillary curve of the non-polarizable interface based on the quantity of adsorbed ions. The electrocapillary curve calculated based on the model may have a concave part, which would indicate that the interface is thermodynamically forbidden, near the standard redox potential of the redox couple.
[en] In this study, a symmetrical poly (3, 4-ethylenedioxythiophene) (PEDOT) coated on poly (vinyl alcohol) (PVA)-graphene oxide (GO)-manganese oxide (MnO2) microfibers (PVA-GO-MnO2/PEDOT) supercapacitor was successfully prepared using a combination of two facile techniques; electrospinning and electropolymerisation. The FESEM analysis revealed the uniform distribution of manganese oxide nanoparticles on the surface of cross-linking PVA-GO microfibers and a cauliflower-like morphology was observed upon deposition of PEDOT on the surface of PVA-GO-MnO2 microfibers. The chemical composition of PVA-GO-MnO2/PEDOT and oxidation state of manganese were characterised using Raman and X-Ray photoelectron spectroscopies. The inclusion of MnO2 and PEDOT in the microcomposite proved the enhancement of specific capacitance where PVA-GO-MnO2/PEDOT exhibited a specific capacitance of 144.66 F/g compared to PVA-MnO2/PEDOT (107.22 F/g), PVA-GO/PEDOT (94.73 F/g) and PEDOT (62.86 F/g). A wide potential window (1.8 V) was achieved for PVA-GO-MnO2/PEDOT with an excellent capacitance retention of 91.18% suggesting an ideal capacitive behaviour and good cycling stability. PVA-GO-MnO2/PEDOT microcomposite also showed an improved specific energy and specific power with small equivalent series resistance (34.5 Ω) and charge transfer resistance (0.62 Ω). This demonstrated that symmetric electrode of PVA-GO-MnO2/PEDOT can offer a great promise in producing high-performance supercapacitors.
[en] Lithium Phosphorus Oxynitride glass (LiPON) is the most used electrolyte for lithium solid-state microbatteries. The current main challenge is improving its ionic conductivity without deteriorating its very low electronic conductivity, wide electrochemical window and stability with Li. While the chemical mechanisms of ionic conduction have been intensively studied, they still are controversial and not fully understood to date. Therefore, it is particularly important to well define and understand the influence of the keys parameters acting on the ionic conductivity. In this study, LiPON is deposited by reactive radio frequency (RF) sputtering of Li3PO4 under nitrogen by two ways. LiPON deposited with a non-standard method exhibits an exceptionally high ionic conductivity (6.7 × 10−6 S cm−1) compared to the standard method (1.4 × 10−6 S cm−1). The chemical composition and structure are compared using RBS/NRA and FTIR, respectively. The electrical properties as ionic and electronic conductivities, charge mobility and carrier concentration are measured by impedance spectroscopy. It is demonstrated the positive influence of the disorder created by the phosphate groups on the mobility. This high mobility creates a depletion area at the interfaces which can be suppressed by adding a buffer layer. The interfacial properties are also investigated from both chemical and electrical point of view using ToF-SIMS and impedance spectroscopy.
[en] Highlights: • The SrSc0.175Nb0.025Co0.8O3-δ perovskite was evaluated for proton conducting SOFC. • Polarization resistance of SSNC decreased obviously via introducing H2O in gas phase. • The symmetrical cell showed the lowest Rp of 0.26 Ω cm−2 at 600 °C in 3% H2O-air. • This study suggests that in situ creation of H+ is possible at the SSNC cathode. - Abstract: Proton-conducting solid oxide fuel cells (H+-SOFCs) have attracted considerable interest recently. However, the overall cell performance of H+-SOFCs is still low due to the lack of a promising cathode material. In this study, SrSc0.175Nb0.025Co0.8O3-δ (SSNC) was synthesized for evaluation as a cathode material in H+-SOFCs based on a BaZr0.1Ce0.7Y0.2O3-δ (BZCY) electrolyte. The chemical compatibility and stability of the SSNC cathode with the BZCY electrolyte in humidified air were studied. In addition, the electrochemical behavior of the SSNC cathode on the BZCY electrolyte was investigated using SSNC/BZCY/SSNC symmetrical cells at 600 °C in dry air and humidified air at various H2O partial pressures. Promising electrocatalytic activity was observed for the SSNC cathode in humidified air. The area specific resistance obtained on symmetrical cells at 600 °C in a 10% H2O-air atmosphere was 0.26 Ω cm2. A promising peak power density of 498 mW cm−2 was obtained using an anode-supported cell with a 46 μm-thick BZCY electrolyte layer at 700 °C.
[en] Present investigation deals with electrochemical determination of copper(II), lead(II) and mercury(II) ions, using ethylenediaminetetraacetic acid (EDTA) chelating ligand modified polyaniline (PANI) and singe walled carbon nanotubes (SWCNTs) based nanocomposite (PANI/SWNCTs). Stainless steel (SS) electrode was modified with PANI and SWCNTs based nanocomposite. PANI/SWCNTs nanocomposite was electrochemically synthesized using potential cycling technique. Further it was modified with EDTA in the presence of 1-ethyl-3(3-(dimethylamin propyl)carbodiimide (EDC) as activating agent, using dip coating technique at room temperature. The EDTAPANI/SWCNTs/SS electrode was characterized by cyclic voltammetry in 0.5 M H2SO4, which was complemented with electrochemical impedance spectroscopy (EIS). AFM and SEM analysis was applied for the morphological studies of EDTAPANI/SWCNTs nanocomposite structure. FTIR analysis was applied for the structural and compositional analysis of EDTAPANI/SWCNTs nanocomposite. All the characterizations were performed before and after the modification of PANI/SWCNTs nanocomposite structure with chelating ligand. Differential pulse voltammetry (DPV) was used for the determination of Cu(II), Pb(II) and Hg(II) ion concentrations. Analytical characteristic such as selectivity and sensitivity of here above-mentioned metal ions was studied. The limit of detection the EDTAPANI/SWCNTs/SS toward Cu(II), Pb(II) and Hg(II) was determined as 0.08 μM, 1.65 μM and 0.68 μM respectively.