Enrico Negro

Structure-Relaxation Interplay of a New Nanostructured Membrane Based on Tetraethylammonium Trifluoromethanesulfonate Ionic Liquid and Neutralized Nafion 117 for High-Temperature Fuel Cells

In this report, the electrical performance at T > 100 degrees C and low relative humidity of proton-conducting Nafion-based membranes was improved by preparing new materials based on Nafion 117 (N117) neutralized with triethylammonium (TEA(+)) and doped with the ionic liquid (IL) trifluoromethanesulfonate of triethylammonium (TEA-TF). In particular, a new two-step protocol for the preparation of [N117(x-)(TEA(+))(x)/(TEA-TF)(y)] is proposed. [N117(x-)(TEA(+))(x)/(TEA-TF)(y)] membrane is composed of ca. 30 wt % of TEA-TF. The structure of the different nanophases composing the materials and their interactions were investigated by FT-IR ATR and micro-Raman spectroscopy. The thermal stability, water uptake, and mechanical properties of the membranes were studied by thermogravimetric analysis and dynamic mechanical analysis measurements. With respect to pristine N117, the thermal and mechanical properties of the proposed materials were improved. The electric response of [N117(x-)(TEA(+))(x)/(TEA-TF)(y)] was studied by broad band dielectric spectroscopy in the frequency range from 10(-2) Hz to 10 MHz and for temperatures between 5 and 155 degrees C. In comparison to the N117 reference, the following was observed: (a) the stability range of conductivity (SRC) of the [N117(x-)(TEA(+))(x)] membrane increases up to 155 degrees C, while its sigma(DC) at T = 100 degrees C is lowered by ca. 2 orders of magnitude; (b) the SRC of [N117(x-)(TEA(+))(x)/(TEA-TF)(y)] is similar to that of [N117(x-)(TEA(+))(x)], while the sigma(DC) at 145 degrees C decreases in the order 7.3 x 10(-3) > 6.1 x 10(-3) > 9.7 x 10(-4) S x cm(-1) for [N117(x-)(TEA(+))(x)/(TEA-TF)(y)], N117, and [N117(x-)(TEA(+))(x)] membranes, respectively. In conclusion, the lower water uptake, the improved thermal and mechanical stability, and the good conductivity make [N117(x-)(TEA(+))(x)/(TEA-TF)(y)] a promising membrane to improve for application in proton exchange membrane fuel cells operating under anhydrous conditions at T > 100 degrees C.

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.

Pt and Ni Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

A new synthesis route aimed at the preparation of electrocatalysts to be used for fuel cells is described. A precursor material is prepared starting from a platinum chloride and a nickel cyanometalate complex in the presence of sucrose, which acts as an organic binder. The most critical steps for the preparation of the electrocatalysts are the thermal decomposition of the precursor, which was studied in the 400-700°C temperature range, and the procedure for activating the products. The resulting materials were extensively characterized via inductively coupled plasma atomic emission spectroscopy, elemental analysis, high-resolution thermogravimetry, vibrational spectroscopy in the middle and far infrared, X-ray photoelectron spectrosopy, X-ray diffraction, scanning electron microscopy, and cyclic voltammetry. These investigations yielded information regarding the influence of the preparation conditions on the structure of the final materials and their electrocatalytic performance. The electrochemical efficiency in the oxygen reduction reaction of the proposed electrocatalysts proved to be much higher than that shown by standard materials having a similar platinum content. This synthesis route appears very promising due to the ease with which materials having the desired metal composition can be obtained and the promising electrochemical performance of the resulting electrocatalysts.

A Key concept in Magnesium Secondary Battery Electrolytes.

A critical roadblock toward practical Mg-based energy storage technologies is the lack of reversible electrolytes that are safe and electrochemically stable. Here, we report on high-performance electrolytes based on 1-ethyl-3-methylimidazolium chloride (EMImCl) doped with AlCl3 and highly amorphous δ-MgCl2 . The phase diagram of the electrolytes reveals the presence of four thermal transitions that strongly depend on salt content. High-level density functional theory (DFT)-based electronic structure calculations substantiate the structural and vibrational assignment of the coordination complexes. A 3D chloride-concatenated dynamic network model accounts for the outstanding redox behaviour and the electric and magnetic properties, providing insight into the conduction mechanism of the electrolytes. Mg anode cells assembled using the electrolytes were cyclically discharged at a high rate (35 mA g(-1) ), exhibiting an initial capacity of 80 mA h g(-1) and a steady-state voltage of 2.3 V.

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.

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.

New Nanocomposite Hybrid Inorganic–Organic Proton‐Conducting Membranes Based on Functionalized Silica and PTFE

Two types of new nanocomposite proton-exchange membranes, consisting of functionalized and pristine nanoparticles of silica and silicone rubber (SR) embedded in a polytetrafluoroethylene (PTFE) matrix, were prepared. The membrane precursor was obtained from a mechanical rolling process, and the SiO₂ nanoparticles were functionalized by soaking the membranes in a solution of 2-(4-chlorosulfonylphenyl)ethyl trichlorosilane (CSPhEtCS). The membranes exhibit a highly compact morphology and a lack of fibrous PTFE. At 125 °C, the membrane containing the functionalized nanoparticles has an elastic modulus (2.2 MPa) that is higher than that of pristine Nafion (1.28 MPa) and a conductivity of 3.6×10⁻³  S cm⁻¹ despite a low proton-exchange capacity (0.11 meq g⁻¹). The good thermal and mechanical stability and conductivity at T>100 °C make these membranes a promising low-cost material for application in proton-exchange membrane fuel cells operating at temperatures higher than 100 °C.

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.

New nanocomposite proton conducting membranes based on a core–shell nanofiller for low relative humidity fuel cells

New hybrid inorganic–organic proton conducting membranes containing a ZrTa nanofiller dispersed in a Nafion® matrix are described. The ZrTa nanofiller exhibits a “core–shell” morphology, where the harder ZrO2 forms the “core”, which is covered by a “shell” of the softer Ta2O5. The hybrid membranes are thermally stable up to 170 °C. Interactions between the polymer matrix and the nanofiller increase the thermal stability of both the –SO3H groups and the fluorocarbon polymer backbone. In comparison with Nafion, the hybrid membranes have a lower water uptake (W.U.) that depends on the concentration of nanofiller. The residual water, which is approximately 4 wt%, is likely located at the Nafion–nanofiller interface. Infrared results indicate that the nanofiller does not neutralize all of the R–SO3H groups in the hybrid membrane and the small amount of residual water in the material does not cause the dissociation of the R–SO3H protons. Fuel cell tests show that the maximum power density yielded by the membrane electrode assembly (MEA) containing the hybrid membrane is better than that of the MEA containing Nafion, particularly at low values of relative humidity. The hybrid membranes require much less water to conduct protons effectively and are more efficient at retaining water than Nafion at low water activities.

Synthesis of Nanocomposites from Pd0 and a Hyper-Cross-Linked Functional Resin Obtained from a Conventional Gel-Type Precursor

Hyper-cross-linked resins stemming from a gel-type poly-chloromethylated poly(styrene-co-divinylbenzene) resin (GT) have been investigated by a multi-methodological approach based on elemental analysis, scanning electron microscopy, X-ray microanalysis, and solvent absorption. The hyper-cross-linking of the parent resin was accomplished by Friedel-Crafts alkylation of the phenyl rings of the resins with the chloromethyl groups. This produced a permanent pore system comprising both micropores (<2.0 nm in diameter) and mesopores (2.2 nm). The chloromethyl groups that did not react in the hyper-cross-linking step were transformed into methylmercaptan groups and the latter were then converted into sulfonic groups by oxidation with hydrogen peroxide. By this procedure the extensive permanent porosity of the parent unsulfonated hyper-cross-linked polymer (HGT) was retained by the sulfonated polymer (HGTS). The final exchange capacity of HGTS was determined to be 0.36 mmol g(-1). HGTS was easily metalated with Pd(II) and the subsequent reduction of the metal centers with either aqueous sodium borohydride, formaldehyde, or dihydrogen produced three Pd(0)/HGTS nanocomposites. The metal nanoparticles had diameters in the 1-6 nm range for all the nanocomposites, as determined by TEM, but with somewhat different distributions. When formaldehyde was used, more than 90% of the nanoparticles were less than 3 nm and their radial distribution throughout the polymer beads was quite homogeneous. These findings show that with this reducing agent the metal nanoparticles are generated within the pore system of the polymer matrix, hence their size is controlled by the dimensions of the pores of the polymeric support.