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)

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