Structural Investigation of Sulfonated Polyphenylene Ionomers for the Design of Better Performing Proton-Conductive Membranes

Abstract

To achieve high-performance proton-exchange membranes (PEMs), understanding of the polymer structure/ property relationship is crucial. In particular, the structure of water clusters (number, size, interdomain distance, interconnectivity, etc.) and hydrophobic domains dominates important membrane properties, such as proton conductivity and mechanical strength, which can be adjusted by the monomer sequence in the polymer chains. In the present paper, we have prepared three sulfonated polyphenylene-based copolymers (SPP-MP, SPP-BP, and SPP-QP) whose main chain components were the same but their sequence differed by the use of different hydrophobic monomers (monophenylene, -MP; biphenylene, -BP; and quinquephenylene, -QP, respectively). Careful investigation of the proton nuclear magnetic resonance ( 1 H NMR) spectra suggested that the randomness of the hydrophilic component (sulfophenylene unit) was dominated by the hydrophobic component: 51% for -MP, 32% for -BP, and 19% for -QP, respectively. Transmission electron microscopy (TEM) observation of the three polyphenylene ionomer membranes revealed that the lower randomness of the hydrophilic component caused a larger hydrophilic domain size in their phase-separated morphology under dry conditions. Small-angle X-ray scattering (SAXS) measurements suggested that SPP-QP, with the lowest randomness of the hydrophilic component, possessed the most pronounced periodic structure under humidified conditions. The connectivity of water clusters, estimated by the small-angle neutron scattering (SANS) measurements, was in the order SPP-QP > SPP-MP > SPP-BP with a minor humidity dependence. The proton conductivity and elongation at break increased with increasing connectivity of the water clusters or decreasing randomness of the hydrophilic component. These results suggest that the sequence of the hydrophobic component strongly affected the hydrophilic component, and accordingly, the membrane morphology and properties.

Proton Exchange Membrane Fuel Cells (PEMFCs): Advances and Challenges

“…Another limiting factor that has reduced its use in comparison with perfluorinated-based membranes is the membrane lifetime, as its durability is inferior to that of Nafion under similar fuel cell operating conditions. In this context, SPEEK appears to be unsuitable for PEM because of limited membrane lifetime, but similar polymers, such as sulfonated aromatic polymers, e.g., polyphenylene, have shown improved stability under working conditions [ 67 , 68 , 69 ].…”

“…The sulfonation of PEEK to develops SPEEK, which maintains the good mechanical, thermal, and chemical stability of PEEK and, interestingly, displays high proton conductivity values (0.10-0.18 S/cm), however, SPEEK conductivity decreases at temperatures over 120 • C, and therefore, the optimal operating temperature is generally limited to 80-120 • C. Another limiting factor that has reduced its use in comparison with perfluorinated-based membranes is the membrane lifetime, as its durability is inferior to that of Nafion under similar fuel cell operating conditions. In this context, SPEEK appears to be unsuitable for PEM because of limited membrane lifetime, but similar polymers, such as sulfonated aromatic polymers, e.g., polyphenylene, have shown improved stability under working conditions [67][68][69].…”

Fully Hydrocarbon Membrane Electrode Assemblies for Proton Exchange Membrane Fuel Cells and Electrolyzers: An Engineering Perspective

“…enable well‐connected hydrophilic domains [ 161 ] to guarantee the high proton conductivity (≈0.1 S cm −1 at <60% RH), [ 120,174 ]…”

Protocol for synthesis and characterization of ePTFE reinforced, sulfonated polyphenylene in the application to proton exchange membrane fuel cells

“…This protocol allows for the preparation of the sulfonated aromatic polymers and their reinforced membranes (composites with porous, expanded polytetrafluoroethylene: ePTFE), and their application in proton exchange membrane fuel cells. As aromatic polymers, sulfonated polyphenylenes solely composed of phenylene and sulfonic acid groups are preferable in terms of high proton conductivity and chemical stability ( Miyake et al., 2017 ; Shiino et al., 2019 , 2020 ). Furthermore, tetrafluorophenylene groups are incorporated to provide polymers (SPP-TP-f) with high concentration of the sulfonic acid groups, solubility in lower alcohols (ethanol and isopropanol), and enhanced compatibility with ePTFE substrate ( Long and Miyatake, 2021a ).…”