Durability of Sulfonated Phenylene Poly(Arylene Ether Ketone) Semiblock Copolymer Membrane in Wet-Dry Cycling for PEFCs

Abstract

The mechanical durability of sulfonated poly (phenylene) (SPP) membrane, used for polymer electrolyte fuel cells (PEFCs), is evaluated by the United States Department of Energy (USDOE) stress protocol involving wet-dry cycling, and the degradation is analyzed specifically by comparing with sulfonated poly(arylene ether ketone) (SPK) membrane. Initially, the SPP membrane exhibits 2-fold higher stiffness and 50% lower dimensional change ratio than the SPK membrane. In durability cycling, the SPP membrane lasts more than 20,000 wet-dry cycles without mechanical failure, which is more than 5-fold better durability than that for the SPK membrane. Higher mechanical strength and lower dimensional change can reduce both irreversible membrane deformation and mechanical stress attributed to the membrane swelling and shrinking. In post-test analyses, the SPP membrane is found to rupture in the peripheral region of the membrane electrode assemblies. The SPP membrane maintains only 10% of the elongation at break in the peripheral region but 50% in the electrode region, compared with the pristine condition. It is most likely that the membrane deteriorates in the peripheral region due to stress concentration by cell compression and membrane deformation during wet-dry cycling. For the commercial use of the polymer electrolyte fuel cells (PEFCs), hydrocarbon (HC) membranes are expected to be nextgeneration membrane alternatives to conventional perfluorosulfonic acid (PFSA) membranes (e.g., Nafion), due to their low manufacturing cost, low through-membrane gas permeability, flexibility in molecular design and environmental compatibility.1,2 However, it is necessary to improve proton conductivity, chemical durability and mechanical durability to the same levels as those for the PFSA membranes under practical fuel cell operating conditions. To meet these targets, HC membranes have been developed intensively during the past decade. For example, polymer composition and morphology have been modified to maintain high proton conductivity, even under low humidified conditions. Acid-functionalized aromatic polymers such as poly (arylene ether) The oxidative stability of the HC membrane is also a key issue for the chemical durability. Oxidative dissolution of the membrane results in gas leakage through the membrane, and the degradation products of the membrane specifically adsorb on the Pt surface and block the oxygen reduction reaction, resulting in voltage losses. 17The oxidative stability of the HC membrane is enhanced by increasing the numbers of hydrophobic groups, decreasing water absorbing capacity 18 and introducing electron-withdrawing sulfone or ketone groups, as well as sulfonic acid groups, into the hydrophilic regions. 19Based on these backgrounds, we proposed an HC membrane that was composed of a sulfonated benzophenone poly(arylene ether ketone) (SPK) semiblock copolymer (Fig. 1a). 20,21 This membrane exhibited higher oxidative stability than those of conventional HC membranes, due to the introduction of chemically stable co...

Remarkable Reinforcement Effect in Sulfonated Aromatic Polymers as Fuel Cell Membrane

“…In the post-test SPK membrane, a macroscopic crack through the cathode, membrane, and anode was observed at the edge of the electrode region both in the photographic (Figure S2a) and in the cross-sectional SEM (Figure a) images. The reason why the macroscopic crack was observed only at this region is the stress concentration, which was well-described in the literature. , In contrast, in the post-test NF-reinforced SPK membrane, a macroscopic crack was not observed in the photographic image (Figure S3). In the cross-sectional SEM images (Figure c and d), no crack of any size was detected.…”

“…Consequently, the wet−dry cycle number (N) of the NF-reinforced SPK cell was 18 320, which was more than 60 times higher than that of the SPK cell (N = 300) with similar cell configuration but slightly different tested conditions (the United States Department of Energy (USDOE) protocol) reported in the literature. 24,25 Thus, quantitative comparison between two cells should be avoided; however, the NF-reinforced SPK cell possessed much higher mechanical durability than the parent SPK membrane without the use of extra components such as a subgasket film and/or soft gas diffusion layer. 24,25 As a reference, it was reported that the Nafion XL membrane, reinforced by a microporous PTFE-rich mechanical support layer, survived in a similar humidity cycling test for 20 000 cycles.…”

Design of flexible polyphenylene proton-conducting membrane for next-generation fuel cells

“…It has been reported that hydrocarbon ionomer membranes exhibited lower wet-dry cycle durability than those for PFSA membranes ( 32 ), mainly due to the lower dimensional stability (or higher water absorbability) of hydrocarbon ionomer membranes. Thus, the wet-dry cycle durability of the SPP-QP membrane was evaluated by means of the U.S. Department of Energy (DOE) protocol ( 33 , 34 ). As a preliminary result, the SPP-QP membrane lasted more than 6000 wet-dry cycles without mechanical failure (without increase in gas permeability).…”

“…The wet-dry cycle durability was evaluated by the DOE protocol. Details on the evaluation conditions were described in our previous paper ( 34 ).…”

ePTFE reinforced, sulfonated aromatic polymer membranes enable durable, high-temperature operable PEMFCs

“…However, aromatic ionomers with high IECs generally show high water uptake and swelling and thus, suffer from mechanical failure in practical fuel cells, in particular, when the humidity changes frequently ( Ishikawa et al., 2017 ). One of the effective approaches is to reinforce the aromatic ionomers with mechanically robust, non-ionic porous substrate.…”