Advanced materials for improved PEMFC performance and life

Abstract

Physical and functional attributes are reviewed for recently developed Nafion^® products that satisfy emerging fuel cell requirements—including stronger, more durable membranes, and polymer dispersions of higher quality and consistency for catalyst inks and film formation. Size exclusion chromatography (SEC) analysis has confirmed that dispersion viscosity is related to an “apparent” molar mass, resulting from a molecular aggregate structure. Membranes produced with solution-casting and advanced extrusion technologies exhibit improved water management and mechanical durability features, respectively. Additionally, DuPont has shown that experimentally modified Nafion^® polymer exhibits 56% reduction in fluoride ion generation, which is considered a measure of membrane lifetime.

Introduction

DuPont introduced Nafion^® perfluorinated polymer [1] in the mid-1960s. Nafion^® is a copolymer of tetrafluoroethylene or “TFE”, and perfluoro(4-methyl-3,6-dioxa-7-octene-1-sulfonyl fluoride) or “vinyl ether”, as shown in Fig. 1. Nafion^® polymer is a thermoplastic resin that can be melt-formed into typical shapes such as beads, film, and tubing. The perfluorinated composition of the copolymer imparts chemical and thermal stability rarely available with non-fluorinated polymers. The ionic functionality is introduced when the pendant sulfonyl fluoride groups (SO₂F) are chemically converted to sulfonic acid (SO₃H). The copolymer’s acid capacity is related to the relative amounts of co-monomers specified during polymerization, and can range from 0.67 to 1.25 meq. g⁻¹ (1500–800EW, respectively).

The unique functional properties of Nafion^® PFSA polymer have enabled a broad range of applications. Initially, Nafion^® membranes were used for spacecraft fuel cells; however, by the early-1980s, membrane electrolysis production of chlorine and sodium hydroxide from sodium chloride emerged as the largest application for Nafion^® membranes. Other important industrial applications include production of high purity oxygen and hydrogen, recovery of precious metals, and dehydration/hydration of gas streams. In addition, Nafion^® super-acid catalysts are used to produce fine chemicals. Starting in 1995, DuPont began a series of process and product development programs specific to PEM fuel cell applications.

The traditional extrusion-cast membrane manufacturing process was developed for “thick” films, typically greater than 125 μm. The extruded polymer film must be converted from the SO₂F to the SO₃K form using an aqueous solution of potassium hydroxide and dimethyl sulfoxide, followed by an acid exchange with nitric acid to the final SO₃H form [2].

Technical advances in fuel cell design and performance have increased demand for thin membranes produced at production rates that will meet the lower conversion cost goals required for fuel cell applications. Furthermore, there is a growing demand for larger production lot sizes, increased roll lengths and improved physical appearance. To meet this need, DuPont developed a solution-casting process for supplying high-volume, low-cost membrane to the fuel cell industry that was planning automated processes for membrane electrode assemblies [3], [4].

DuPont’s new membrane process uses typical solution-casting technology and equipment, as shown in Fig. 2. A base film [1] is unwound and measured for thickness [2]. Polymer dispersion is applied [3] to the base film, and both materials enter a dryer section [4]. The composite membrane/backing film is measured for total thickness [5], with the membrane thickness the difference from the initial backing film measurement. The membrane is inspected for defects [6], protected with a coversheet [7], and wound on a master roll [8]. The membrane is produced in a clean room environment [9]. Master rolls are slit into product rolls, which are individually sealed and packaged for shipment.

This process has several key advantages: (1) pre-qualification of large dispersion batches for quality (e.g., free of contamination) and expected performance (e.g., acid capacity); (2) increased overall production rates for H⁺ membrane from solution-casting as compared to polymer extrusion followed by chemical treatment; and (3) improved thickness control and uniformity, including the production capability of very thin membranes (e.g., 12.7 μm).

Two patented high-pressure processes, solvent-based [5] and water-based [6], are used to convert Nafion^® polymer (sulfonic acid form) into polymer dispersions having solids contents ranging from 5 to 20% by weight. These dispersions are formulated into carbon inks and catalyst coatings, and used either “as supplied” or with modifiers [7], and/or reinforcement materials to fabricate electrode coatings and membranes [8], [9], [10].

The manufacture of polymer dispersions has undergone considerable change since first introduced by DuPont, with the recent “second generation” dispersions exhibiting more stable and consistent viscosity, improved acid capacity and reduced metal ion content. These features enable more predictable coating formulations, consistent processing, and improved fuel cell performance. A “third generation” dispersion is in the final R&D stages, and will provide broader formulation capabilities for both solvent and polymer content. It will also allow further process simplification for preparing coatings, casting membranes and fabrication of membrane electrode assemblies.

The useful lifetime of a membrane is related to the chemical stability of the ionomer. While Nafion^® PFSA polymer has demonstrated highly efficient and stable performance in fuel cell applications, evidence of membrane thinning and fluoride ion detection in the product water indicates that the polymer is undergoing chemical attack. The fluoride loss rate is considered an excellent measure of the health and life expectancy of the membrane [11]. Peroxide radical attack on polymer endgroups [12] with residual H-containing terminal bonds is generally believed to be the principal degradation mechanism.

In this degradation mechanism, cross-over oxygen from the cathode side, or air bleed on the anode side, provides the oxygen needed to react with hydrogen from the anode side and produce H₂O₂, which can decompose to give OH or OOH radicals. These radicals can then attack any H-containing terminal bonds present in the polymer. Peroxide radical attack on H-containing endgroups is generally believed to be the principal degradation mechanism. This form of chemical attack is most aggressive in the presence of peroxide radicals at low relative humidity conditions and temperatures exceeding 90 °C.

Hydroxy or peroxy radicals resulting from the decomposition of hydrogen peroxide in the fuel cell attack the polymer at the endgroup sites and initiate decomposition. The reactive endgroups can be formed during the polymer manufacturing process and may be present in the polymer in small quantities. An example of attack on an endgroup such as CF₂X, where X=COOH, is shown below.

Several proposed mechanisms include the following sequential reactions: abstraction of hydrogen from an acid endgroup to give a perfluorocarbon radical, carbon dioxide and water (step 1). The perfluorocarbon radical can react with hydroxy radical to form an intermediate that rearranges to an acid fluoride and one equivalent of hydrogen fluoride (step 2). Hydrolysis of the acid fluoride generates a second equivalent of HF and another acid endgroup (step 3).

$$\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{CF}{}_2\text{COOH}\text{+}\text{OH}\text{→}\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{CF}{}_2\text{+}\text{CO}{}_2\text{+}\text{H}{}_2\text{O}$$

$$\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{CF}{}_2\text{+}\text{OH}\text{→}\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{CF}{}_2\text{OH}\text{→}\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{COF}\text{+}\text{HF}$$

$$\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{COF}\text{+}\text{H}{}_2\text{O}\text{→}\text{R}{}_{\mathrm{<mtext>f</mtext>}}\text{−}\text{COOH + HF}$$