PFSA Polymers with Short Side Chains

2 PFSA Polymers with Short Side Chains 

The so-called Dow membrane with short side chains once attracted mueh attention, because it showed excellent performance in fuel ceU operation compared with Nation membranes [53-55]. The polymer stmcture is shown in Fig. 2.5. The Dow membranes are prepared by the copolymerization of TFE with PSVE, which is shown in Fig. 2.6. 

An important difference between the Dow and Nation membranes was considered to be membrane resistivity, which depends on membrane thickness and lEC [55]. 

6 Structure of the perfluorosulfonyl monomer of the short side chain membrane. 

Originally, the monomer in Fig. 2.5 was prepared by DuPont by the synthetic scheme shown in Fig. 2.7 [58]. Thermolysis of the acyl fluoride in Fig. 2.7 did not give a desired monomer, but gave a cyclo compound. In order to prevent the cyclization, a new synthetic route was developed as shown in Fig. 2.8, which was applied to the synthesis of Dow membranes [59]. A chlorine atom was introduced to the acyl fluoride to improve the selectivity of vinyl ether formation. The Dow membrane was also developed for brine electrolysis, but was not commercialized probably because of its high cost. Difficulty in the preparation of the acyl fluoride in Fig. 2.8 is one of the causes. Recently, new synthetic processes for the short side chain monomer were developed, as represented in Fig. 2.9. 

---

A modification of the polymer that has been adopted by various groups is to have shorter side chains as compared to Nafion. Short side chains increase the crystallinity of the PFSA, thus reducing the solubility. Solvay Solexis has developed Aquivion, a membrane based on Hyflon, which is a copolymer of Teflon and sulfonyl fluoride vinyl ether with low EW (790-870) and good crystallinity, with proton conductivity values in the order of 30 mS cm at 120°C, 30% RH [22]. A similar approach is followed by 3M, who have shown 580 EW membranes approaching 100 mS cm at 120°C and RH 50% [23]. Gore recently reported values >50 mS cm at 30% RH and > 100 mS cm 50% RH with a new, undisclosed ionomers [24]. DuPont recently presented results on MEAs with new ionomer that showed a much reduced dependence on the RH as compared to Nafion-based membranes [17]. 

Aquivion E87-12S short-side chain perfluorosulfonic acid (SSC-PFSA) membrane with equivalent weight (EW) of 870 g eq and 120 pm thickness produced by Solvay Specialty Polymers was tested in a polymer electrolyte membrane water electrolyser (PEMWE) and compared to a benchmark Nation N115 membrane (EW 1100 g eq ) of similar thickness [27]. Both membranes were tested in conjunction with in-house prepared unsupported Ir02 anode and carbon-supported Pt cathode electrocatalyst. The electrocatalysts consisted of nanosized Ir02 and Pt particles (particle size 2-4 nm). The electrochemical tests showed better water splitting performance for the Aquivion membrane and ionomer-based membrane-electrode assembly (MEA) as compared to Nafion (Fig. 2.21). Lower ohmic drop constraints and smaller polarization resistance were observed for the electrocatalyst-Aquivion ionomer interface indicating a better catalyst-electrolyte interface. A current density of 3.2 A cm for water... 

---