(Invited Plenary) Aromatic Ionomers As Alternative Fuel Cell Membranes: Issues and Possibilities
Authors: Miyatake Kenji 
Published: February 2020

Background and objective
One of the important challenges in the current fuel cell research is to develop an alternative membrane to state-of-the-art perfluorinated sulfonic acid (PFSA) ionomers. While the PFSA ionomer membranes are highly proton conductive and chemically and physically stable, high gas permeability, high cost, and environmental incompatibility of the fluorinated materials are drawbacks for the wide-spread dissemination of fuel cells. In the past couple of decades, a variety of proton conductive materials have been proposed as alternative membranes. Nonfluorinated hydrocarbon polymers, acid-doped polymers, inorganic/organic nanohybrids, solid acids with superprotonic phase transition, and acid/base ionic liquids fall into this category. Some of them are claimed to exhibit high proton conductivity, very low gas permeability, and reasonable stability. However, none of them can compete with PFSAs under a wide range of fuel cell operating conditions. The most critical issues of these alternative membranes are insufficient durability and significant dependence of the proton conductivity upon humidity. In addition, interfaces between these emerging proton conductive materials and the catalyst layers have not been well explored. In this communication, current issues (in particular, chemical and mechanical stabilities and interfacial problems) and future possibilities of aromatic ionomer membranes will be discussed.
Stability issues
In pursuit of better performing and more stable aromatic ionomer membranes, we have investigated the effect of a simple structure, sulfo-1,4-phenylene unit, as a hydrophilic component on the membrane properties. Unlike typical aromatic ionomer membranes, the newly designed ionomer membranes exhibited reduced water uptake and excellent mechanical stability under humidified conditions due to the absence of polar groups such as ether, ketone, or sulfone groups in the hydrophilic component. The membrane has survived several tens of thousands cycles in humidity cycling test of US DOE protocol. The high local ion concentration in the hydrophilic segments contributed to an increase in the proton conductivity especially at low humidity. Consequently, the high ion exchange capacity (IEC = 2.67 meq/g) membrane exhibited high proton conductivity under low humidity (20% RH, 7.3 mS/cm) conditions at 80 °C, which are one of the highest values reported thus far for aromatic ionomer membrane.1)

Interfacial issues
In the literature, a number of hydrocarbon ionomer membranes that exhibit comparable proton conductivity to PFSA membranes have been reported, however, they underperformed in operating fuel cells compared to PFSA membranes in most cases. This is partly because of the cathode performance was lower even when the same cathode catalyst layers were used. We have revealed that the double ionomer membrane composed of thin layer Nafion attached on our aromatic ionomer membrane showed higher fuel cell performance than the parent (single layer) ionomer membrane.2,3) The Nafion inter-layer improved the interfacial contact between the aromatic ionomer membrane and the catalyst layer. Based on the results, we have then designed and synthesized novel ionomer membranes composed of perfluoroalkyl and the sulfonated phenylene groups. The resulting membranes could support good electrocatalytic performance of the catalyst layer at the membrane-electrode interface due to the well-controlled finely phase-separated morphology.
Acknowledgement
This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through the SPer-FC Project.
References
1) J. Miyake, T. Mochizuki, K. Miyatake, ACS Macro Lett., 4, 750-754 (2015).
2) T. Mochizuki, K. Kakinuma, M. Uchida, S. Deki, M. Watanabe, K. Miyatake, ChemSusChem, 7, 729-733 (2014).
3) T. Mochizuki, M. Uchida, H. Uchida, M. Watanabe, K. Miyatake, ACS Appl. Mater. Interfaces, 6, 13894-13899 (2014).