Elsevier

Journal of Power Sources

Volume 152, 1 December 2005, Pages 27-33
Journal of Power Sources

Nafion-layered sulfonated polysulfone fuel cell membranes

https://doi.org/10.1016/j.jpowsour.2005.03.214Get rights and content

Abstract

Sulfonated polysulfone (SPSU) with high ion exchange capacity (IEC) and ion conductivity was synthesized through deep sulfonation of polysulfone using the trimethysilyl chlorosulfonate as sulfonation agent. The silicon-containing compounds formed during the synthesis of SPSU were completely removed from the SPSU by simple evaporation. Water swelling, ion exchange capacity, conductivity and fuel cell performance were measured for the SPSU membranes. SPSU membranes with IEC twice that of Nafion-115 were prepared. The conductivity of the SPSU increased exponentially with the relative humidity (RH), achieving conductivities of 0.1 S cm−1 for RH > 70%. A Nafion-layered SPSU PEM fuel cell membrane was synthesized through pressing a thin Nafion-115 layer on to both sides of SPSU membrane. The Nafion layers on the SPSU prevented the water-soluble SPSU from being washed out of MEA, and the membrane was stable during the fuel cell operation up to 120 °C.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) have attracted the attention because of their high-energy conversion efficiency, zero pollution emission and low operation temperature suitable for transportation and residential applications. Nafion (DuPont) [1], a perfluorosulfonic acid polymer, is the most widely used as proton conductor in the PEMFCs, due to its high ionic conductivity, good thermal stability, mechanical strength and chemical stability. However, its high price (presently, $800 m−2) [2] and the difficulty involved in its disposal [3] have spurred investigations for low-cost non-perfluorinated ionomer membranes, which are environmentally disposable. Proton conducting membranes investigated to date include polysulfonated triflurostyrene [4], polyether ether ketone (PEEK) [5], [6], poly(arylene ether sulfone) (PSU) [7], [8], [9], PVDF-graft styrene [10], [11], [12], acid-doped polybenzimidazole (PBI) [13], [14], polyphosphazene [15], [16] and ion resin/polystyrene sulfonate composite materials [17]. The ideal fuel cell membranes should have low-cost (less than $10 kW−1 in a PEMFC), high chemical stability, high mechanical strength and high conductivity. To date, none of the proposed materials have satisfied the desired specification.

PEEK, PSU and polyphosphazene are cheap and chemically stable [6], [8], [17]. To achieve good proton conductivity these materials must be highly sulfonated; as a result, these polymers swell substantially with water uptake and some are even soluble in water. Sulfonated poly(arylene ether sulfone) (SPSU), shown in Fig. 1, contains many polar groups (e.g. single bondSO2single bond(sulfone) and single bondOsingle bond) along the backbone chain which contribute to the water uptake, but contribute little to proton conductivity.

Cross-linking may be employed to decrease the water swelling of these membranes. The Cross-linking has usually involved multivalent cations bonding to sulfonic acid groups on different molecules [8], [18]. This improves the mechanical strength of the polymer but results in decreased proton conductivity. Literature reports of the conductivity of SPSU are circa 0.01 S cm−1 [7], one magnitude order lower than that of Nafion-115.

We report here the preparation of layered Nafion/SPSU/Nafion membranes. The Nafion keeps the SPSU from being washed out of the membrane electrode assembly (MEA). The layered membranes have two thin Nafion membranes (∼10 μm) pressed on to both sides of a SPSU membrane. Because the SPSU molecules are large and carry the same kind of charges as Nafion, SPSU cannot penetrate through the Nafion layers, preventing the SPSU from being washed out of the MEA. Fuel cell performance of the layered membranes was comparable to that of Nafion, much better than that previously reported in the literature.

Section snippets

Materials

Poly(arylene ether sulfone) (PSU), P-3500, was obtained from Solvay Advanced Polymers Company. The weight average molecular weight is about 77 000. 1,2-Dichloroethane, trimethysilyl chlorosulfonate, N,N-dimethylacetamide and methanol were purchased from Aldrich. Nafion-1100 solution (5 wt.%) was purchased from Ion Power Inc.

Synthesis of SPSU

The sulfonation process of the SPSU was similar to that used by Lufrano et al. [7] and Nolte et al. [8]. However, the highly sulfonated SPSU we prepared is soluble in the

Synthesis and silicon analysis of the SPSU

The sulfonation reaction conditions are shown in Table 1. Excess sulfonation agent was employed to obtain highly sulfonated PSU. The degree of sulfonation of PSU, (defined as the mole ratio of sulfonic acid group per monomer), was determined by IEC (results summarized in Table 1). With a large excess of sulfonation agent more than 100% degree of sulfonation was achieved, indicating that more than sulfonic acid residue was added per monomer unit. The conversion of trimethysilyl chlorosulfonate

Conclusions

Sulfonated polysulfone (SPSU), with IEC more than twice that of Nafion-115, can be synthesized through deep sulfonation of polysulfone using the trimethysilyl chlorosulfonate as sulfonation agent. The silicon-containing compounds formed during the synthesis of SPSU can be completely removed from the SPSU by simple evaporation. The proton conductivity of the SPSU increases with the relative humidity (RH) and exceeds the conductivity of Nafion-115 above 80% RH. The higher the IEC, the higher is

Acknowledgement

The research work was supported by Doctor Program Foundation of the China Education Ministry (Grant No: 20040425006). We also thank the National Science Foundation (DMR-0212707 through the Materials Research and Science Engineering Center at Princeton) for partial support of this work.

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