The stability of carbon-supported electrocatalysts has been largely investigated in acidic electrolytes, but the literature is much scarcer regarding similar stability studies in alkaline medium. ...Herein, the degradation of Vulcan XC-72-supported platinum nanoparticles (noted Pt/C), a state-of-the-art proton exchange membrane fuel cell electrocatalyst, is investigated in alkaline medium by combining electrochemical measurements and identical location transmission electron microscopy; electrochemical surface area (ECSA) losses were bridged to electrocatalyst morphological changes. The results demonstrate that the degradation in 0.1 M NaOH at 25 °C is severe (60% of ECSA loss after only 150 cycles between 0.1 and 1.23 V vs RHE), which is about 3 times worse than in acidic media for this soft accelerated stress test. Severe carbon corrosion has been ruled out according to Raman spectroscopy and X-ray photoelectron spectroscopy measurements, and it seems that the chemistry of the carbon support (in particular, the interface (chemical bounding)) between the Pt nanoparticles and their carbon substrate does play a significant role in the observed degradations.
We have investigated the photophysical properties and intracellular behaviour of a series of hydrophilic conjugated porphyrin dimers. All the dimers exhibit intense linear absorption at 650-800 nm ...and high singlet oxygen quantum yields (0.5-0.9 in methanol), as required for an efficient sensitiser for photodynamic therapy (PDT). They also exhibit fluorescence at 700-800 nm, with fluorescence quantum yields of up to 0.13 in methanol, and show extremely large two-photon absorption maxima of 8,000-17,000 GM in the near-IR. The dimers aggregate in aqueous solution, but aggregation is reduced by binding to bovine serum albumin (BSA), as manifested by an increase in fluorescence intensity and a sharpening in the emission bands. This process can be regarded as a model for the interaction with proteins under physiological conditions. Confocal fluorescence microscopy of live cells was used to monitor the rate of cellular uptake, intracellular localisation and photostability. Porphyrin dimers with positively charged substituents partition into cells more efficiently than the negatively charged dimers. The photostability of these dimers, in living cells, is significantly better than that of the clinical photosensitiser verteporfin. Analysis of the photophysical parameters and intracellular imaging data indicates that these dimers are promising candidates for one-photon and two-photon excited PDT.
Amorphous CdSe nanoparticles were prepared by a base-catalyzed room-temperature reaction between cadmium nitrate and selenourea, with dodecanethiol as a capping ligand. The nanoparticle size could be ...controlled from 1.9 to 3.6 nm by increasing the water concentration in the reaction. When the nanoparticles were heated in a pyridine suspension, excitonic peaks appeared in the initially featureless optical absorption spectra. By changing the suspension solvent and the capping ligand and its concentration, it was shown that the dynamic surface exchange between the ligand and pyridine controls the crystallization process. This phenomenon was interpreted as a surface rigidity effect imposed by the ligand, whose importance was separately evidenced on the dried nanoparticles by the evolution of X-ray diffraction patterns and Raman spectra. In particular, both techniques showed that a threshold temperature is needed before crystallization occurs, and such a threshold was related to ligand desorption. The surface effect was directly visualized by high-resolution transmission electron microscopy observations of the amorphous particles, where crystallization under the electron beam was observed to start by the formation of a crystalline nucleus in the nanoparticle interior and then to extend to the whole structure.
This paper discusses the feasibility of tantalum electrodeposition carried out under potential control from 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide BMPyrTFSI room temperature ...ionic liquid electrolyte containing tantalum pentafluoride (TaF5). The influence of electrode nature, electrolyte temperature and electrolysis duration on the morphology, the adherence, the thickness and the crystalline properties of tantalum deposits is reported. The obtained tantalum deposits are analyzed by SEM, XRD, Raman spectroscopy and XPS surface analytical techniques. Pulsed potential electrodeposition is investigated in order to enhance deposit properties. Post electrodeposition thermal treatments are also reported.
One approach to increase the energy density of Li-ion batteries is to use high potential cathode material like LiNi0·5Mn1·5O4 (LNMO). However, it suffers from low coulombic efficiency, self-discharge ...and poor cyclability in carbonates-based electrolytes. Many mechanisms to explain degradation such as HF generation, surface catalytic activity and transition metals dissolution have been suggested to explain these behaviors. By comparison with a non-fluorinated environment, we demonstrated that hydrofluoric acid is not the main reason of capacity loss. A comparison of electrolyte degradation on model thin-film and composite electrodes proved that electrolyte oxidation is catalyzed on the active material surface of LNMO and not on the carbon. A Tafel like behavior of the electrolyte oxidation was obtained thanks to the measure of the steady state current at different potentials. The low coulombic efficiency is essentially related to the self-discharge mechanism. Finally, the capacity fading has been quantitatively correlated to the electrolyte oxidation: at 25 °C, about 4% of oxidized electrolyte molecules leads to the degradation of the material, probably due to the dissolution of surface transition metal. By lowering the operating temperature, the electrolyte degradation kinetics decreased, leading proportionally to better cycling stability. Perspectives of this work are also drawn.
Display omitted
Solutions made of tetraglyme (G4) containing Ca(TFSI)
2
have been studied as models to understand the solvation structure and the conductivity properties of multivalent ions in low dielectric ...constant ethereal electrolytes. These solutions have been characterised using electrochemical impedance spectroscopy, rheological measurement, and Raman spectroscopy. The ionic conductivity of these electrolytes shows an intriguing non-monotonic behaviour with temperature which deviates from the semi-empirical Vogel-Tammann-Fulcher equation at a critical temperature. This behaviour is observed for both Mg(TFSI)
2
and Ca(TFSI)
2
, but not LiTFSI, indicating a difference in the solvation structure and the thermodynamic properties of divalent ions compared to Li
+
. The origin of this peculiar behaviour is demystified using temperature-controlled Raman spectroscopy and first-principles calculations combined with a thermodynamic analysis of the chemical equilibrium of Ca
2+
ion-pairing
versus
solvation. As long-range electrostatic interactions are critical in solutions based on low dielectric ethereal solvents, a periodic approach is here proposed to capture their impact on the solvation structure of the electrolyte at different salt concentrations. The obtained results reveal that the thermodynamic and transport properties of Ca(TFSI)
2
/G4 solutions stem from a competition between enthalpic (ionic strength) and entropic factors that are directly controlled by the solution concentration and temperature, respectively. At high salt concentrations, the ionic strength of the solution favours the existence of free ions thanks to the strong solvation energy of the polydentate G4 solvent conjugated with the weak complexation ability of TFSI
. At elevated temperatures, the configurational entropy associated with the release of a coordinated G4 favours the formation of contact ion-pairs due to its flat potential energy surface (weak strain energy), offering a large configuration space. Such a balance between ion-pair association and dissociation not only rationalises the ionic conductivity behaviour observed for Ca(TFSI)
2
/G4 solutions, but also provides valuable information to extrapolate the ionic transport properties of other electrolytes with different M(TFSI)
n
salts dissolved in longer-chain glymes or even poly(ethylene oxide). These findings are essential for the understanding of solvation structures and ionic transport in low-dielectric media, which can further be used to design new electrolytes for Li-ion and post Li-ion batteries as well as electrocatalysts.
The solvation structure and ionic conductivity of Ca(TFSI)
2
/tetraglyme solutions were studied using a combination of experimental and theoretical approaches. A higher fraction of free ions was detected in solutions with higher molar concentrations.
Proton-conducting aromatic-based ionomers bearing superacid side chains are usually synthesized by polymer postmodification, which does not allow controlling ion exchange capacity and ionic group ...distribution along the ionomer and, thus, its chemical structure and functional properties. Bottom-up approach overcomes this problem. Here, we report the preparation of a novel ionic monomer and its polycondensation with commercial monomers. The obtained random ionomers are the first to show high phase separated organization at macro-, micro-, and nanoscale, common to the reference proton-conducting material Nafion. Additionally, membranes were cast from the solutions of ionomers in their Li+ and K+ forms in order to study the cation’s influence on both morphology and performance of the materials. The difference in ionic domain organization, depending on the initial cationic form of the ionomers, was reported for the first time. The proposed materials show superior proton conductivity than Nafion, especially at low relative humidity, which makes them potential substitute of the benchmarked Nafion for fuel cell application.
Display omitted
•Two new isoformular Cu(L1O) and Ni(L1O) complexes structures have been synthesized.•Cu(L1O) and Ni(L1O) complexes were characterized by single crystal X-ray, Raman, FTIR and ...UV–Visible techniques.•Hirshfeld surface analyses of the synthetized complexes were also performed.•For the two coordination complexes, magnetic investigations showed the presence of high antiferromagnetic interactions.
The reaction of the proligand thiosemicarbazido 2-pyrazoline (H2L2), derived from 2,4-pentanedione bis-thiosemicarbazone (H2L1) with Cu(II) and Ni(II) salts, under aerobic conditions, yielded the mononuclear title complexes M(L1O). The open form of the ligand, H2L1, is oxidized at the central CH2 of the backbone and doubly deprotonated. The obtained complexes were characterized by using single crystal X-ray diffraction, FT-IR and UV–Vis spectroscopic techniques. The structures of the title complexes were determined respectively in monoclinic and triclinic symmetry, where the copper complex Cu(L1O) and nickel complex Ni(L1O) crystallize in the centrosymmetric space groups C2/c and P1¯, respectively. The asymmetric unit is formed of MII atom occupying a general position, linked by two sulfur and two nitrogen atoms, to form MIIS2N2 square plane environment in both complexes. The crystal structure stabilization of both complexes is ensured by weak interactions hydrogen bonds D–H⋯A with (D = N, C and A = N, O, Sand / or C). This has been confirmed by the three dimensional Hirshfeld surface analysis and the two dimensional fingerprint plots that highlight the dominance of H⋯H, S⋯H/H⋯S and H⋯N/N⋯H intermolecular interactions. The magnetic study revealed that the magnetic chains of Cu and Ni transition metal atoms in square plane environment show two different magnetic interactions in both investigated complexes, with linear and undulating (on zig-zag) metallic chains in Cu and Ni complexes, with antiferromagnetic behaviour for the first complex and ferrimagnetic for the second one.
The bis(2,5-di(pyridin-2-yl)-1,3,4-thiadiazole-κ2N,N′)-bis(thiocyanato-κ1N)cobalt(II) complex has been synthetized from the reaction of 2,5-bis(pyridin-2-yl)-1,3,4-thiadiazole (L) with metallic salt ...CoCl2.6H2O and thiocyanate ion (SCN−) as coligand in H2O/CH3CN at room temperature. The synthetized complex (CoL2(SCN)2) has been fully characterized by single crystal X-ray diffraction, Hirshfeld surface analysis, as well as UV–Visible, FTIR, Raman, and NMR spectroscopy. TGA analysis and magnetic measurements were also performed. CoL2(SCN)2 crystallizes in monoclinic symmetry and P 21/c space group with two independent cobalt crystallographic sites, where each cobalt atom is localized in a distorted octahedral environment CoN6, with the thiadiazole molecules (L) as bidentate ligands in equatorial sites and terminal SCN− ions in axial positions. The crystal cohesion is assured by intermolecular hydrogen bonding, C–H⋯π and π–π stacking; in addition to the N–Cu coordination bonds. This has been confirmed by the three dimensional Hirshfeld surface analysis and the two dimensional fingerprint plots that highlight the dominance of intermolecular interactions C⋯H/H⋯C and S⋯H/H⋯S. The thermal analysis of CoL2(SCN)2 reveals that this complex is thermally stable up to 200 °C. Variable-temperature magnetic susceptibility measurements on CoL2(SCN)2 complex indicated an antiferromagnetic exchange between the two non-equivalent cobalt(II) ions with a ferrimagnetic behaviour.
•Bis(2,5-di(pyridin-2-yl)-1,3,4-thiadiazole-κ2N,N′)-bis(thiocyanato-κ1N)cobalt(II) complex has been synthesized.•CoL2(SCN)2 complex was characterized by X-ray, FTIR, Raman, UV–Visible and NMR techniques.•Magnetic measurements showed an antiferromagnetic exchange between the two non-equivalent cobalt(II) ions.
The Lewis and Brønsted acidic properties of the surface sites of a high surface area tin(IV) oxide sample were investigated by the CO adsorption at low temperature using IR spectroscopy. Two distinct ...cus (coordinatively unsaturated sites) Sn4+ cation sites having different Lewis acidic strength have been evidenced: (i) Sn4+I strongly bonding CO (νCO = 2210-2196 cm−1), (ii) Sn4+II (νCO = 2183-2177 cm−1), being less acidic. Several types of OH groups have been also evidenced: (i) OH occluded in the bulk and inaccessible to CO (νOH = 3445-3435 cm−1), (ii) non-acidic OH groups (νOH = 3740-3725 and 3660 cm−1) which give rise to a νCO band at 2155 cm−1 and νOH bands respectively at 3640 and 3570 cm−1 (ΔνOH = 90 cm−1) as CO adsorption occurs, (iii) slightly Brønsted acidic OH groups (νOH = 3625 cm−1) which lead to a νCO band at 2164 cm−1 and a νOH band at 3740 cm−1 (ΔνOH = 155 cm−1). Depending on the treatment in vacuum (at room temperature or at 773 K) prior to CO adsorption, the populations of these different sites may vary. An outgassing in vacuum at 773 K leads to the partial dehydroxylation of the SnO2 surface, increasing the number of Sn4+ Lewis acidic sites.
PDF