Sandra Lavina

Vibrational Studies and Properties of Hybrid Inorganic-Organic Proton Conducting Membranes Based on Nafion and Hafnium Oxide Nanoparticles

In this report, we will describe the effect of different concentrations of HfO2 nanopowders on the structure and properties of [Nafion/(HfO2)n] membranes with n = 0, 3, 5, 9, 11, 13, and 15 wt %, respectively. Films were prepared by a solvent casting procedure using HfO2 oxoclusters and Nafion. Seven new homogeneous membranes were obtained with thicknesses ranging from 200 to 350 microm. Each membrane is characterized by a rough HfO2-rich surface and a smooth Nafion-rich surface, with different physical-chemical properties. Membrane characterization was accomplished by means of thermogravimetric analysis (TGA), morphological measurements (environmental scanning electron microscopy) and vibrational spectroscopy (Fourier transform infrared attenuated total reflectance spectroscopy and Fourier transform Raman spectroscopy). These systems can be described in terms of five types of water domains, Nafion-HfO2 species with well-defined stoichiometry surrounded by Nafion and hydrated hafnia. The highest conductivity at 125 degrees C (3.2 x 10-2 S x cm(-1)) was measured on the [Nafion/(HfO2)5] film by electrical spectroscopy, with a stability range of conductivity between 5 and 115 degrees C.

Synthesis, characterization and ionic conductivity of poly[(oligoethylene oxide) ethoxysilane] and poly[(oligoethylene oxide) ethoxysilane]/(EuCl3)0.67

Poly[(oligoethylene oxide) ethoxysilane)] (I) and poly[(oligoethylene oxide) ethoxysilane)]/(EuCl3)0.67 (II) were synthesized by reacting tetraethoxysilane with oligo(ethylene glycol) of molecular weight 400 and oligo(ethylene glycol)400/(EuCl3)0.317, respectively. The products so obtained are very transparent and rubbery. By Fourier transform infrared and Raman spectroscopy studies and by using analytical results it was concluded that these products are crosslinked macromolecular materials where the Si atom is bonded to one OEt group and to three poly(ethylene oxide) 400 chains. Scanning electronic microscopy studies showed that the presence of EuCl3 in polymer host significantly affects the morphology of the material. Laser luminescence investigations on (II) showed that Eu3+ ion in the polymer host is accommodated in two different types of sites having a distorted C2v symmetry. Moreover, the ionic conductivity of these systems was investigated and the data were satisfactorily fitted by the empirical Vogel Tamman Fulcher equation. At 70°C the conductivities of (I) and (II) were 9 × 10−6 and 14.3 × 10−6 Ω−1 cm−1 respectively.

The first lithium zeolitic inorganic–organic polymer electrolyte based on PEG600, Li2PdCl4 and Li3Fe(CN)6: part I, synthesis and vibrational studies

The synthesis of Li 2 PdCl 4 and Li 3 Fe(CN) 6 precursors, and the preparation of the first lithium Z-IOPE material obtained by reacting these precursors with poly(ethylene glycol)600 (PEG600) is reported. This new material has been obtained through a sol ⇒ gel and gel ⇒ plastic transition. FIR and MIR spectroscopy studies, Raman laser and UV-vis investigations and detailed compositional data allowed us to propose a structural hypothesis and to detect the interactions between ions and the coordinating segments of the host network. It has been concluded that: (1) this compound is a mixed inorganic-organic network in which clusters formed by palladium and iron complexes are bonded together by PEG bridges; (2) the conformation of polyether chains in the bulk material is of the TGT (T = trans, G = gauche) type. The conductivity of the proposed [Fe x Pd y (CN) z Cl v (C 2n H 4n+2 O n+1 )Li l ] is 5.3 × 10 -5 S cm -1 at 35.1 °C.

Zeolitic inorganic–organic polymer electrolytes: synthesis, characterization and ionic conductivity of a material based on oligo(ethylene glycol) 600, (CH3)2SnCl2 and K4Fe(CN)6

This paper reports the synthesis of a new Z-IOPE material based on poly(ethylene glycol) 600, (CH3)2SnCl2 and K4Fe(CN)6. This material was synthesized via a sol-gel transition. FIR and MIR spectroscopy studies together with detailed compositional data allowed us to propose a final structure for this Z-IOPE material. It was concluded that this compound is a mixed inorganic-organic network in which clusters formed by tin and iron complexes are bonded together by PEG 600 bridges. The conformation of polyethereal chains in the bulk material is of the TGT (T=trans, G=gauche) type. Impedance spectroscopy measurements revealed that the material has a conductivity of 4.77.10−5 S cm−1 at 21.3°C.

Vibrational studies of the ion–polymer interactions in α-hydro-ω-oligo(oxyethylene)hydroxy-poly[oligo(oxyethylene)oxydimethylsililene]/δ-MgCl2

Abstract Polymer–polymer and salt–polymer interactions of seven electrolytic complexes based on α-hydro-ω-oligo(oxyethylene)hydroxy-poly[oligo(oxyethylene)oxydimethylsililene] and δ-MgCl 2 were studied. This aim was pursued by means of an accurate medium and far FT-IR spectroscopic analysis. By using the decomposition and difference spectroscopy techniques, intensity and frequency of terminal OH and CO stretchings were evaluated in order to detect the presence of anion clusters build up by the coordination of Cl − with -OH groups. The number of chlorine anions per chain coordinated magnesium was established. This analysis allowed us to gain a complete structural picture for these materials, pointing out three possible coordinations of magnesium by polyethereal chains.

Dielectric Relaxations and Conductivity Mechanism of Nafion: Studies Based on Broadband Dielectric Spectroscopy

INTRODUCTION Over the past 30 years, perfluorinated polymer electrolytes such as Nafion, Aciplex, Flemion and Dow membranes were the most promising materials for application in polymer electrolyte membrane fuel cells (PEMFCs) due to their chemical stability, high degree of proton conductivity and remarkable mechanical properties. The major drawbacks to large-scale commercial use of these systems involve the high cost and low proton conductivities at high temperatures and low humidity [1,2]. To overcome these drawbacks, there is a need to perform fundamental studies on the correlations existing between the mechanism of charge transport and the structure, the molecular relaxations and the water uptake of ionomer membranes. The wide variety of investigations performed in order to fulfil these aims disclosed the extremely complex nature of Nafion and resulted in a widely discussed assignment of the transition/relaxation phenomena observed in both DSC and DMA studies [1-5]. Recently, some interesting studies have been reported [3-7] which: a) reconcile the differences previously observed in the thermal behaviour of Nafion as measured by DSC and DMA; and b) provide more precise explanations on the molecular origins of endothermic DSC transitions and mechanical relaxations by vibrational spectroscopy, SAXS and F solid-state NMR analyses. To complete these knowledges it is of crucial importance to understand the electrical response of Nafion and its molecular origins in terms of dielectric relaxations. In this report, Broad-Band Dielectric Spectroscopic (BDS) measurements were carried out on Nafion samples with different water compositions (dry and wet). The aim of this study was to correlate: a) the mechanism of proton migration in Nafion materials in terms of ion migration events and host medium reorganizations vs. temperature and membrane composition; b) the transition/relaxation phenomena observed in both DSC and DMA studies [1-5] to those detected by BDS.

Synthesis and structure of electrolytic complexes based on α-hydro-ω-oligo(oxyethylene)hydroxy-poly[oligo(oxyethylene)oxydimethylsililene] and δ-MgCl2

Abstract An alternated copolymer with the formula α-hydro-ω-oligo(oxyethylene)hydroxy-poly[oligo(oxyethylene)oxydimethylsililene] and a molecular weight of 9860 was synthesized. Doping of this polymer with the anhydrous salt δ-MgCl 2 resulted in a new magnesium electrolytic complex poly[PEG400-alt-DEOS]/(MgCl 2 ) 0.26 . The structural hypothesis for the polymer was proposed on the basis of elemental analyses and molecular weight. Detailed 1 H-, 13 C-, and 29 Si-NMR spectral investigations fully confirmed the structure of poly[PEG400-alt-DEOS]. Mid-infrared region (MIR) FT-IR studies of the polymer showed that it presents (1) a sufficient number of terminal hydroxyl groups to confer a substantial degree of dissolution and salt-dissociation in the polymer complex; and (2) polyethereal fragments in a conformational geometry close to TGT (T= trans , G= gauche ). The conductivity against temperature plot for this very amorphous magnesium polymer electrolyte demonstrated that the material conducts ionically by means of two types of charge migration mechanisms.

6 – Hybrid inorganic–organic polymer electrolytes

Abstract: Polymer electrolytes (PEs) are macromolecular systems capable of transporting charged species such as ions or protons. The main application of PEs is in energy conversion and storage devices such as batteries and fuel cells. The chapter overviews the synthesis, structure, physical and electrical properties of three classes of hybrid inorganic–organic PEs: three-dimensional hybrid inorganic–organic networks as polymer electrolytes (3D-HION-APE), zeolitic inorganic–organic polymer electrolytes (Z-IOPEs) and hybrid gel electrolytes (HGEs). The basic structure of the materials consists of organic macromolecules bridging inorganic clusters or species. The chapter also includes an overview of the methods used in the characterization of the structure and of the electrical conductivity of PEs, with a particular reference to the jump relaxation model.

Platinum-free Carbon Nitride Electrocatalysts for PEMFCs Based on Pd, Co and Ni: Effect of Nitrogen on the Structure and Electrochemical Performance

INTRODUCTION In recent years, polymer electrolyte membrane fuel cells (PEMFCs), characterized by a very good efficiency and little or negligible polluting emissions at low operating temperatures (T < 130°C), were proposed as power sources for applications in the automotive field, in portable electronics and low-output stationary plants. However, the widespread application of this technology is hindered by important issues such as durability and high cost of the materials. In particular, the operation of a PEMFC is promoted by electrochemical reactions catalysed by expensive platinum-based materials where the active metal sites are supported on carbons with a very large surface area. Recent studies have proved that palladium can be used instead of platinum in the preparation of catalysts with good performance both in the oxygen reduction reaction (ORR) and in the hydrogen oxidation reaction (HOR) [1-4]. By combining metals such as Pd or Pt with first-row transition elements, such as Fe, Co and Ni, catalysts with active sites supported on graphites or carbon nitrides are prepared, endowed with an enhanced performance in the ORR [1-7]. This study reports: a) the preparation of new carbon nitride electrocatalysts based on Pd, Co and Ni by pyrolysis of plastic precursors; and b) the investigation of the effect of the support and catalytic site composition on the structure and electrochemical performance of prepared materials. Two groups of electrocatalysts with general formula Kc[PdxCoyNizCaNb] are prepared. Group I is characterized by Pd-Co-Ni active sites supported on carbon nitride matrixes with a nitrogen concentration <5 wt%, while Group II by N≈15 wt%. I is obtained starting from a ZIOPE-like precursor prepared using sucrose as the organic binder [3], while II is prepared with a precursor synthesized by coordinating the desired metals with polyacrylonitrile (PAN) macromolecular ligand [4]. The final catalysts are obtained by pyrolysis processes of the precursors.

Conductivity, thermal stability and morphology of a new Z-IOPE inorganic-organic network with the formula [FexSny(CH3)2y(CN)zClv(C2nH4n+2On+1)Kl]

This paper reports accurate studies on the morphology, thermal stability and electrical spectroscopy of a zeolitic inorganic-organic polymer electrolyte (Z- IOPE) with the formula [Fe x Sn y (CH 3 ) 2y (CN) z Cl v (C 2n - H 4n+2 O n+1 )K l ]. This material was prepared by means of a sol-gel process. A possible mechanism of the sol-gel process is proposed. Scanning electron microscopy showed that the Z-IOPE resembles a gummy paste with a rough texture and grains on the surface of the bulk material. Thermogravimetric investigations indicated that the material is thermally stable up to approximately 200 °C. A detailed study of the mechanism of ion conduction in this system was carried out using impedance spectroscopy in the 20 Hz to 1 MHz range. The analysis of real and imaginary components of conductivity spectra indicated that a full characterization of the AC electrical response for this Z-IOPE system requires an equivalent circuit analysis for frequencies lower than 10 kHz and correlated ionic motion analysis based on the Universal Power Law for frequencies higher than 10 kHz. These studies demonstrated that the Z-IOPE material conducts ionically by a mechanism mainly regulated by segmental motion of the host material, and that charge migration by ion hopping between equivalent coordination sites is not to be completely excluded in the host network. Finally, the conductivity of 4.77 × 10 -5 S.cm -1 at 25 °C classifies this hybrid inorganic-organic network as a good ionic conductor.