Simone Angioni

Polysulfonation of PBI-based membranes for HT-PEMFCs: a possible way to maintain high proton transport at a low H3PO4 doping level

Polybenzimidazoles are promising materials to replace Nafion™ as the electrolyte in HT-PEMFCs. One of their problems is striking the proper balance between the H3PO4 doping level, which controls the proton conductivity, and the long-term stability properties of the membrane. Monomer modification is a promising way to maintain high conductivity levels with reduced doping. Here, we reported a novel and facile approach to obtaining an easy modular and reproducible sulfonation degree. Some aryloxy-based polybenzimidazoles were synthesized and sulfonated with different amounts of –SO3H. We prepared many electrolyte membranes by doping the pristine polymers in solutions with different H3PO4 concentrations. The sulfonation degree greatly affected both acid uptake and conductivity. In particular, the membranes holding more protogenic groups absorbed less acid than the monosulfonated ones. However, polysulfonation was particularly efficient in improving proton conductivity at low relative humidity and doping level. We performed MEAs tests at 150 °C using H2 and air as the reactant gases, without any external humidification. We obtained power densities higher than 320 mW cm−2, with fuel cell performances of approximately 580 mV at 0.2 A cm−2, independent of the number of sulfonic groups. Preliminary durability tests did not show any membrane degradation over a 190 hour period. The reported membranes are therefore suitable for use in HT-PEMFCs.

Polymer and Composite Membranes for Proton-Conducting, High-Temperature Fuel Cells: A Critical Review

Polymer fuel cells operating above 100 °C (High Temperature Polymer Electrolyte Membrane Fuel Cells, HT-PEMFCs) have gained large interest for their application to automobiles. The HT-PEMFC devices are typically made of membranes with poly(benzimidazoles), although other polymers, such as sulphonated poly(ether ether ketones) and pyridine-based materials have been reported. In this critical review, we address the state-of-the-art of membrane fabrication and their properties. A large number of papers of uneven quality has appeared in the literature during the last few years, so this review is limited to works that are judged as significant. Emphasis is put on proton transport and the physico-chemical mechanisms of proton conductivity.

Operando electrochemical NMR microscopy of polymer fuel cells

The design of high-temperature polymer fuel cells (PEMFCs), e.g. those expected for automotive applications, requires a deep understanding of the electrochemical reactions occurring in the device during operation. Operando electrochemical nuclear magnetic resonance microscopy can constitute a powerful investigation tool to this aim. At present, however, some strong technical limitations, like low sensitivity to less mobile protons, and the limited temperature range of analysis, have bound its use to case models based on perfluorinated membranes operating at high relative humidity and low temperature. By means of a suitable design of the experimental set-up and the use of a new 3D acquisition protocol, we proved the feasibility of electrochemical NMR microscopy on low-water containing polybenzimidazole-based devices, thus allowing full operando characterization of high-temperature PEMFCs, and also paving the way for applications to other electrochemical devices, such as batteries, sensors, supercapacitors, etc.

Polybenzimidazoles with Enhanced Basicity: A Chemical Approach for Durable Membranes

A major issue of polybenzimidazole (PBI)-based membranes is the leaching loss of the doping acid during fuel cell operation. In this chapter, two approaches to improve the conductivity and its stability are presented by tailoring the polymer basicity: (a) modification of the polymer backbone by synthesis of new monomers and (b) fabrication of (nano)composite membranes with functionalized fillers. Chemical modification of the polymer backbone is likely the strategic choice for its significant effect, simplicity, reproducibility and cost.