Synthesis and Evaluation of Polyphenylene-Based Copolymers with Superacid Groups (IV)-Effect of Superacid Structure
Authors: Takahashi Rio , Fujita Masahiro , Takeoka Yuko , Rikukawa Masahiro 
Published: December 2024

In order to improve the performance of polymer electrolyte fuel cells (PEFCs), polyelectrolytes with higher proton conductivity under low humidity and high temperature conditions will be required. It is well known that hydrocarbon polyelectrolytes with superacid groups show high proton conductivity at low relative humidity (RH) conditions compared to typical sulfonated polyelectrolytes due to the strong acidity providing a high efficiency of carrier generation. In this study, random copolymers consisting of the hydrophilic poly(4’-phenoxybenzoyl-1,4-phenylene) (SNF-PPBP) unit with sulfonyl(trifluoromethyl sulfonyl)imidate as a super acid group, and the hydrophobic poly(benzoyl-1,4-phenylene) (PBP) unit were synthesized. The effect of the hydrophilic structure and phase separation on electrolyte membrane properties was investigated.

Two types of random copolymers, PPBP
m
-PBP
n
with different copolymerization ratios (m : n) were synthesized by Ni(0) coupling reaction. The copolymerization ratios of PPBP
m
-PBP
n
were 1.2 : 1.0 and 2.3 : 1.0, respectively. The number-average molecular weights were determined by GPC to be 2.22×104 g mol-1 (PPBP1.2-PBP1.0) and 3.03×104 g mol-1 (PPBP2.3-PBP1.0), respectively. After chlorosulfonation of PPBP
m
-PBP
n
s (SC-PPBP-PBPs), trifluoromethyl-bis(sulfonyl)imide (SNF) groups were introduced by using trifluoromethansulfonamide. FT-IR measurements suggested that the sulfonate groups were also introduced as by-products during the synthesis of SC-PPBP-PBP. The introduction rate of sulfonyl chloride groups (X
SC) and sulfonate groups (X
SO3H) to PPBP units of SC-PPBP-PBP and that of SNF groups (Y
SNF) and sulfonate groups (Y
SO3H) were estimated from elemental analysis data. The X
SC and X
SO3H for PPBP units of SC-PPBP-PBPs were 75% and 62% (SC-PPBP1.2-PBP1.0), and 61% and 51% (SC-PPBP2.3-PBP1.0), respectively. The Y
SNF and Y
SO3H for PPBP units of SNF-PPBP-PBPs were 75% and 62% (SNF-PPBP1.2-PBP1.0), and 62% and 50% (SNF-PPBP2.3-PBP1.0). The ion exchange capacity (IEC) of SNF-PPBP-PBPs determined by back titration was 2.41 meq g-1 (SNF-PPBP1.2-PBP1.0) and 2.23 meq g-1 (SNF-PPBP2.3-PBP1.0). TG-MS measurements exhibit that the 5% weight loss temperature (T
d-5%) of both polymers was about 230℃, indicating that both polymers have sufficient thermal resistance for PEFC applications. Despite the high IEC value, SNF-PPBP1.2-PBP1.0 showed higher thermal resistance because of the dense introduction of ion exchange groups and these electron-withdrawing properties. The proton conductivity was evaluated in the range of 30 - 80%RH at 80oC, and SNF-PPBP-PBPs showed higher humidity dependence and proton conductivity compared with Nafion®211 membrane under high humidification conditions. SNF-PPBP-PBPs also showed higher proton conductivity than that of the homopolymer SNF-PPBP (IEC = 2.39 meq g-1). These results suggest that the superacid groups have high carrier generation ability, especially in high humidity regions, and the random copolymer structure contributes to the improvement of proton conduction. The PEFC performance was evaluated at 80oC and 90%RH using membrane assembly electrodes (MEAs) fabricated with SNF-PPBP-PBPs, Nafion®211, and SNF-PPBP (IEC = 2.51 meq g-1) membranes. The limiting current densities (I
lim) of MEAs with SNF- PPBP1.2-PBP1.0, SNF-PPBP2.3-PBP1.0, Nafion®211, and SNF-PPBP membranes were 1825, 1904, 1924, and 1429 mA cm-2, respectively. The membrane resistance (R
m) of MEAs with SNF- PPBP1.2-PBP1.0, SNF-PPBP2.3-PBP1.0, and Nafion®211 membranes were 29, 17, and 21 mΩ, respectively. SNF-PPBP1.2-PBP1.0 and SNF-PPBP2.3-PBP1.0 MEAs showed high power generation properties comparable to Nafion MEAs regardless of the copolymerization ratio.