Interplay between Structure and Relaxations in Perfluorosulfonic Acid Proton Conducting Membranes

This study focuses on changes in the structure of ionomer membranes, provided by the 3M Fuel Cells Component Group, as a function of the equivalent weight (EW) and the relationship between the structure and the properties of the membrane. Wide-angle X-ray diffraction results showed evidence of both non-crystalline and crystalline ordered hydrophobic regions in all the EW membranes except the 700 EW membrane. The spectral changes evident in the vibrational spectra of the 3M membranes can be associated with two major phenomena: (1) dissociation of the proton from the sulfonic acid groups even in the presence of small amounts of water; and (2) changes in the conformation or the degree of crystallinity of the poly­(tetrafluoroethylene) hydrophobic domains both as a function of EW and membrane water content. All the membranes, regardless of EW, are thermally stable up to 360 °C. The wet membranes have conductivities between 7 and 20 mS/cm at 125 °C. In this condition, the conductivity values follow VTF behavior, which suggests that the proton migration occurs via proton exchange processes between delocalization bodies (DBs) that are facilitated by the dynamics of the host polymer. The conductivity along the interface between the hydrophobic and hydrophilic domains makes a larger contribution in the smaller EW membranes likely due to the existence of a greater number of interfaces in the membrane. The larger crystalline domains present in the higher EW membranes provide percolation pathways for charge migration between DBs, which reduces the probability of charge transfer along the interface. Therefore, at higher EWs although there is charge migration along the interface within the hydrophobic–hydrophilic domains, the exchange of protons between different DBs is likely the rate-limiting step of the overall conduction process

Interplay between Composition, Structure, and Properties of New H₃PO₄‑Doped PBI₄N–HfO₂ Nanocomposite Membranes for High-Temperature Proton Exchange Membrane Fuel Cells

Polybenzimidazole (PBI) has become a popular polymer of choice for the preparation of membranes for potential use in high-temperature proton exchange membrane polymer fuel cells. Phosphoric acid-doped composite membranes of poly­[2,2′-(<i>m</i>-phenylene)-5,5′-bibenzimidazole] (PBI4N) impregnated with hafnium oxide nanofiller with varying content levels (0–18 wt %) have been prepared. The structure–property relationships of both the undoped and acid-doped composite membranes are studied using thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the presence of nanofiller improves the thermal and mechanical properties of the undoped membranes and facilitates a greater level of acid uptake. The degree of acid dissociation within the acid-doped membranes is found to increase with increasing nanofiller content. This results in a conductivity, at 215 °C and a nanofiller level <i>x</i> ≥ 0.04, of 9.0 × 10^–2 S cm^–1 for [PBI4N­(HfO₂)_<i>x</i>]­(H₃PO₄)_<i>y</i>. This renders nanocomposite membranes of this type as good candidates for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs)

Influence of Anions on Proton-Conducting Membranes Based on Neutralized Nafion 117, Triethylammonium Methanesulfonate, and Triethylammonium Perfluorobutanesulfonate. 2. Electrical Properties

The electrical properties of a Nafion proton exchange membrane change dramatically when neutralized and then doped with a proton-conducting ionic liquid (PCIL). Broadband electric spectroscopy elucidates the molecular relaxation and polarization phenomena of neutralized Nafion (nN117) doped with triethylammonium methanesulfonate (TMS) and triethylammonium perfluorobutanesulfonate (TPFBu) ionic liquids. These data, coupled with those of part 1 suggest proton conduction mechanisms for both the pure PCILs and in PCIL-doped nN117. At 130 °C, the PCILs have conductivities of σ_TMS = 1.4 × 10^–2S/cm and σ_TPFBu = 9 × 10^–3S/cm, while correspondingly doped nN117 have conductivities of σ_NTMS = 6.1 × 10^–3 S/cm and σ_NTPFBu = 1.8 × 10^–3S/cm. The pure PCILs show three interfacial polarizations associated with proton transfer mechanisms above the melting point. PCIL-doped nN117 also has three interfacial polarizations that depend on the nanostructure characteristics of the PCIL sorbed within the nN117 polar domains. Below the PCIL melting point, doped nN117 has two dielectric relaxations, α and β, associated with dipolar relaxations involving both the sorbed PCILs and the ionomer matrix. The data indicate a long-range charge transfer process that occurs through proton exchange between cationic clusters. Segmental motion of the polymer chains and the molecular dimensions of the ionic liquid nanoaggregates mediate this charge transfer

Interplay between Structural and Dielectric Features of New Low k Hybrid Organic–Organometallic Supramolecular Ribbons

The synthesis and characterization of low k one-dimensional (1D) hybrid organic–organometallic supramolecular ribbons <b>3a</b>,<b>b</b>, through halogen-bond driven co-crystallization of <i>trans</i>-[Pt­(PCy₃)₂(CC-4-py)₂] (<b>1</b>) with 1,4-diiodotetrafluorobenzene (<b>2a</b>) and <i>trans</i>-1,2-bis-(2,3,5,6-tetrafluoro-4-iodophenyl)-ethylene (<b>2b</b>), are reported. The co-crystals <b>3a</b>,<b>b</b> have been obtained by isothermal evaporation of a chloroform solution containing the corresponding starting materials at room temperature. X-ray structure determinations show that noncovalent interactions other than halogen bonds help in the construction of the crystal packing; these interactions are stronger in <b>3b</b>, thus reducing the chain mobility with respect to <b>3a</b>. Accordingly, the broadband dielectric spectroscopic determinations, carried out from 10^–2 to 10⁷ Hz and at a temperature ranging from 25 to 155 °C, showed that both <b>3a</b> and <b>3b</b> materials exhibit a real component of dielectric permittivity (ε′) significantly lower than SiO₂. In particular in the case of <b>3b</b>, the rigidity of the 1D chain explains the observed ε″ and tan δ values. A permittivity value that is significantly lower than that of the silica reference, tan δ values lower than 0.02 in the entire investigated temperature range, and less than 0.004 at <i>T</i> < 100 °C make <b>3b</b> a very promising low k hybrid organic–organometallic material for application as dielectric films in next generation microelectronics

Highly Conducting 3D-Hybrid Polymer Electrolytes for Lithium Batteries Based on Siloxane Networks and Cross-Linked Organic Polar Interphases

The development of polymer electrolytes with high ionic conductivity, high lithium transference number, and high electrochemical stability is one of the main aims in the field of lithium battery research. In this work, we describe the synthesis and the characterization of new electrolyte systems, composed of three-dimensional hybrid inorganic–organic networks doped with LiClO₄. The preparation route comprises only three steps, namely a sol–gel reaction, salt dissolution, and an epoxide polymerization reaction. The lithium concentration, and thus the lithium transference number, was modulated by adding lithium hydroxide in the sol–gel step. In this way, seven electrolytes with varying salt concentrations were prepared. The hybrid electrolytes are characterized by good ionic conductivities (up to 8·10^–5 S/cm at room temperature) and high thermo-mechanical and electrochemical stabilities. Stability tests versus lithium metal via galvanostatic polarization showed that this material is superior with respect to reference poly­(ethylene oxide) based electrolytes

Interplay Between Hydroxyl Density and Relaxations in Poly(vinylbenzyltrimethylammonium)‑<i>b</i>‑poly(methylbutylene) Membranes for Electrochemical Applications

Anion-exchange membranes (AEMs) consisting of poly­(vinyl benzyl trimethylammonium)-<i>b</i>-poly­(methylbutylene) of three different ion exchange capacities (IECs), 1.14, 1.64, and 2.03 mequiv g^–1, are studied by High-Resolution Thermogravimetry, Modulated Differential Scanning Calorimetry, Dynamic Mechanical Analysis, and Broadband Electrical Spectroscopy in their OH^– form. The thermal stability and transitions are elucidated, showing a low temperature <i>T</i>_g and a higher temperature transition assigned to a disorder–order transition, <i>T</i>_δ, which depends on the IEC of the material. The electric response is analyzed in detail, allowing the identification of three polarizations (only two of which contribute significantly to the overall conductivity, σ_EP and σ_IP,1) and two dielectric relaxation events (β₁ and β₂), one associated with the tolyl side groups (β₁) and one with the cationic side chains (β₂). The obtained results are integrated in a coherent picture and a conductivity mechanism is proposed, involving two distinct conduction pathways, σ_EP and σ_IP,1. Importantly, we observed a reordering of the ion pair dipoles which is responsible for the <i>T</i>_δ at temperatures higher than 20 °C, which results in a dramatic decrease of the ionic conductivity. Clustering is highly implicated in the higher IEC membrane in the hydroxyl form, which reduces the efficiency of the anionic transport

Toward Pt-Free Anion-Exchange Membrane Fuel Cells: Fe–Sn Carbon Nitride–Graphene Core–Shell Electrocatalysts for the Oxygen Reduction Reaction

We report on the development of two new <i>Pt-free</i> electrocatalysts (ECs) for the oxygen reduction reaction (ORR) process based on graphene nanoplatelets (GNPs). We designed the ECs with a <i>core–shell</i> morphology, where a GNP <i>core</i> support is covered by a carbon nitride (CN) <i>shell.</i> The proposed ECs present ORR active sites that are not associated with nanoparticles of metal/alloy/oxide but are instead based on Fe and Sn subnanometric clusters bound in <i>coordination nests</i> formed by carbon and nitrogen ligands of the CN <i>shell</i>. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new <i>Pt-free</i> ECs for the ORR having ample room for further performance improvement beyond the state of the art