Poly(arylenesulfonic acids) with Frozen-In Free Volume as Hydrogen Fuel Cell Membrane Materials

Abstract

A new class of proton conducting membranes based on rigid-rod, liquid crystalline, aromatic sulfonic acid polymers, and copolymers was developed at Case Western Reserve University over the past 15 years. Sulfonic acid crowding or incorporation of bulky comonomers forced the chains apart and induced nanoporosity (called frozen in free volume, FFV) that helped in water retention at low external humidity. Very high proton conductivity of these materials made them excellent candidates for fuel cell membranes, especially for operation at elevated temperature and low humidity. Initial work focused on poly(naphthalene bis-tetracarboximides), which were found to be hydrolytically unstable. Then the focus shifted to chemically stable poly(phenylene sulfonic acids), including their copolymers, grafts, and crosslinked derivatives. This paper tracks the study of these remarkable polymers from initial work on polyimides to recent results where polyphenylene membranes were fabricated and tested in a hydrogen/air fuel cell.

On the evolution of sulfonated polyphenylenes as proton exchange membranes for fuel cells

“…al. 38,39,44 would use their synthetic approach to report on various other poly(paraphenylene)s as proton exchange membranes for PEMFCs. The sulfonated polymers PPDSA, PBPDSA, and BXPY (see Figure 7) were prepared through copper-mediated Ullman coupling reactions between pre-functionalized acid-bearing monomers.…”

Free volume enhanced proton exchange membranes from sulfonated triptycene poly(ether ketone)

“…Nafion's chemical stability and excellent ionic transport properties are well known and extensively reviewed, [1,2] but high cost of production and reduced proton conductivities after exceeding temperatures 80 °C are significant limitations to this material. [3][4][5][6][7][8][9][10] One of the promising alternative polymeric systems to solve those problems is sulfonated poly aryl ethers. [3][4][5][6][7] Sulfonated poly aryl ethers can be polymerized through reactions with low cost reactants and are thermally and chemically stable at temperatures above 80 °C.…”

“…One way to address the loss of water at low humidity and high temperature is to increase intrinsic free volume within the membrane. Intrinsic free volume can be introduced by adding bulky groups to sulfonated poly aryl ethers through rigid side chains, [9,20,21] chain branching, [22] or by introducing bulky groups into the polymer backbone. [23][24][25][26] When the polymer membrane is completely dried, these rigid bulky groups prevent packing and produce nanopores.…”

“…However, this is accompanied by stress to the material that results in distorted bond angles and an increase in internal energy. [9] This internal energy is relieved when the membrane is exposed to small molecule solvents that can fill the pores and plasticize the polymer. Since water molecules are plasticizers for sulfonate polymers that are hydrophilic, it is energetically favorable for polymers with bulky groups to retain more water molecules at low humidity to prevent the energetic penalty from drying the membrane and thus retain a higher proton conductivity.…”

Atomistic simulation study of the hydrated structure and transport dynamics of a novel multi acid side chain polyelectrolyte membrane

“…Nafion is a widely used membrane material for flow batteries [7] and PEMFCs [11]. A standard variety of Nafion (Nafion-117) 1 has good proton conductivity (0.01 S/cm for 50% Relative Humidity (RH)) [12] and good mechanical stability at room temperature, but loses water upon increase of temperature. Due to this loss of water, the proton conductivity of Nafion degrades [11] [13].…”