Improving the Durability and Performance of Sulfonated Poly(arylene ether)s by Introducing 9,10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide Structure for Fuel Cell Application

Polymer electrolyte membranes in which the hydrophilic and hydrophobic domains phase separate exhibit improved properties and stability. Such a phase separation of hydrophilic and hydrophobic domains can be achieved by polymerizing a 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide-bisphenol A (DOPO-BPA) and 1,4-bis(4-fluorobenzoyl)benzene (1,4-FBB) monomer. In this work, sulfonated polymer membranes with various degrees of sulfonation (DS) were prepared and their physicochemical and electrochemical properties were studied. In addition, the effect of molecular structure on the durability of the copolymers was investigated. The sulfonated copolymers were characterized by Fourier-transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy. Then, sulfonated membranes were prepared using these copolymers by the solvent casting method, and their morphologies were investigated by atomic force microscopy. The effect of DS on the thermal, mechanical, and oxidative stabilities, water uptake behavior, and ion-exchange capacity of the membranes was determined. The results showed that compared with the commercially available Nafion 212 polymer electrolyte membrane, the electrolyte membrane based on DOPO-BPA and 1,4-FBB exhibited a lower water uptake and excellent dimensional stability despite having a relatively high ion-exchange capacity. The low water uptake is an important characteristic that ensures the stability of the polymer electrolyte membrane in fuel cell applications.

Ladder-type sulfonated poly(arylene perfluoroalkylene)s for high performance proton exchange membrane fuel cells

Sulfonated poly(arylene perfluoroalkylene)s containing a sulfone-bonded ladder structure (SPAF-P-Lad) were synthesized by treating the precursor SPAF-P polymers with oleum as a novel proton exchange membrane for fuel cells. SPAF-P-Lad membranes had excellent solubility in polar organic solvents and high molecular weight (M _n = 145.4-162.9 kDa, M _w = 356.9-399.1 kDa) to provide bendable membranes with ion exchange capacity (IEC) ranging from 1.76 to 2.01 meq. g^-1. SPAF-P-Lad membranes possessed higher proton conductivity than that of the precursor SPAF-P membranes because of the stronger water affinity. Compared with SPAF-P membranes (T _g: 72-90 °C, Young's modulus: 0.08-0.42 GPa; yield stress: 5.7-15.1 MPa), SPAF-P-Lad membranes showed better mechanical stability to humidity and temperature and improved tensile properties (Young's modulus: 0.51-0.59 GPa; yield stress: 23.9-29.6 MPa). The selected membrane, SPAF-mP-Lad, exhibited improved fuel cell performance, in particular, under low humidity with air; the current density at 0.5 V was 0.56 A cm^-2, while that for SPAF-pP was 0.46 A cm^-2. The SPAF-mP-Lad membrane endured an open circuit voltage hold test for 1000 h with average decay of as small as 70 μV h^-1. A series of post-analyses including current-voltage characteristics, molecular structure, molecular weight, and IEC suggested very minor degradation of the membrane under the accelerated testing conditions.

Sulfonated Aromatic Polymers as Proton-Conducting Solid Electrolytes for Fuel Cells: a Short Review

This review describes main strategies for the development of sulfonated aromatic polymers (SAP) with optimal properties for medium temperature (90–120 ºC) polymer electrolyte membrane fuel cell applications. SAP are promising economical polymer electrolytes, but there still exist some challenges about these materials due mainly to excessive swelling in water, poor mechanical strength and low dimensional stability. Here, the state-of-the-art of SAP is reviewed and the main focus will be directed to properties of SAP, including proton conductivity, water uptake, mechanical strength, permeability. Especially three approaches to improve the performances are addressed: the formation of copolymers, the formation of hybrid materials and the polymer reticulation.