Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers

Abstract

This paper presents an overview of the synthesis, chemical and electrochemical properties, and polymer electrolyte fuel cell applications of new proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Due to their chemical stability, high degree of proton conductivity, and remarkable mechanical properties, perfluorinated polymer electrolytes such as Nafion^®, Aciplex^®, Flemion^®, and Dow membranes are some of the most promising electrolyte membranes for polymer electrolyte fuel cells. A number of reviews on the synthesis, electrochemical properties, and fuel cell applications of perfluorinated polymer electrolytes have also appeared during this period. While perfluorinated polymer electrolytes have satisfactory properties for a successful fuel cell electrolyte membrane, the major drawbacks to large-scale commercial use involve cost and low proton-conductivities at high temperatures and low humidities. Presently, one of the most promising ways to obtain high performance proton-conducting polymer electrolyte membranes is the use of hydrocarbon polymers for the polymer backbone. The present review attempts for the first time to summarize the synthesis, chemical and electrochemical properties, and fuel cell applications of new proton-conducting polymer electrolytes based on hydrocarbon polymers that have been made during the past decade.

Introduction

The idea of using an organic cation exchange membrane as a solid electrolyte in electrochemical cells was first described for a fuel cell by Grubb in 1959. At present the polymer electrolyte fuel cell (PEFC) is the most promising candidate system of all fuel cell systems in terms of the mode of operation and applications. As shown in Fig. 1, a PEFC consists of two electrodes and a solid polymer membrane, which acts as an electrolyte. The polymer electrolyte membrane is sandwiched between two platinum-porous electrodes such as carbon paper and mesh. Some single cell assemblies can be mechanically compressed across electrically conductive separators to fabricate electrochemical stacks. In general, PEFCs require humidified gases, hydrogen and oxygen (or air) as a fuel for their operation. The electrochemical reactions that occur at both electrodes are as follows:

$$\text{Anode:}\text{H}{}_2\text{→2H}{}^{\mathrm{+}}\text{+2e}{}^{\mathrm{-}}$$

$$\text{Cathode:}\text{1/2O}{}_2\text{+2H}{}^{\mathrm{+}}\text{+2e}{}^{\mathrm{-}}\text{→H}{}_2\text{O}$$

$$\text{Overall:}\text{H}{}_2\text{+1/2O}{}_2\text{→H}{}_2\text{O+Electrical}\text{Energy+Heat}\text{Energy}$$

In recent years, PEFCs have been identified as promising power sources for vehicular transportation and for other applications requiring clean, quiet, and portable power. Hydrogen-powered fuel cells in general have a high power density and are relatively efficient in their conversion of chemical energy to electrical energy. Exhaust from hydrogen-powered fuel cells is free of environmentally undesirable gases such as nitrogen oxides, carbon monoxide, and residual hydrocarbons that are commonly produced by internal combustion engines. Carbon dioxide, a greenhouse gas, is also absent from the exhaust of hydrogen-powered fuel cells. Thus, transportation uses, especially fuel cell electric vehicles (FCEV), are on attractive and effective application because of not only clean exhaust emissions and high-energy efficiencies but also effective solution to the coming petroleum shortage. While FCEV might provide the greatest societal benefits, its total impact would be small if only a few FCEVs are sold due to lack of fueling infrastructure or due to high vehicle cost. The major obstacles for the commercial use of FCEV are expensive materials and low performances at high temperatures (over 100°C) and low humidities.

In general, proton-conducting polymers are usually based on polymer electrolytes, which have negatively charged groups attached to the polymer backbone. These polymer electrolytes tend to be rather rigid and are poor proton conductors unless water is absorbed. The proton conductivity of hydrated polymer electrolytes dramatically increases with water content and reaches values of 10⁻²–10⁻¹ S cm⁻¹.

The first PEFC used in an operational system was the GE-built 1 kW Gemini power plant [1]. This system was used as the primary power source for the Gemini spacecraft during the mid-1960s. The performances and lifetimes of the Gemini PEFCs were limited due to the degradation of poly(styrene sulfonic acid) membrane employed at that time. The degradation mechanism determined by GE was generally accepted until the present time. It was postulated that HO₂ radicals attack the polymer electrolyte membrane. The second GE PEFC unit was a 350 W module that powered the Biosatellite spacecraft in 1969. An improved Nafion^® membrane manufactured by DuPont was used as the electrolyte. Fig. 2 shows the chemical structures of Nafion^® and other perfluorinated electrolyte membranes. The performance and lifetime of PEFCs have significantly improved since Nafion^® was developed in 1968. Lifetimes of over 50,000 h have been achieved with commercial Nafion^®120.

Nafion^® 117 and 115 have equivalent repeat unit molecular weights of 1100 and thicknesses in the dry state of 175 and 125 μm, respectively. Nafion^® 120 has an equivalent weight of 1200 and a dry state thickness of 260 μm. Ballard Technologies Corporation showed the possibility of the application of PEFC for electric vehicles by using experimental perfluorinated membranes developed by Dow Chemical. Development of PEFC has been accelerated year by year after the report of Ballard Technologies Corporation. The Dow membrane has an equivalent weight of approximately 800 and a thickness in the wet state of 125 μm. In addition, Flemion^® R, S, T, which have equivalent repeat unit molecular weights of 1000 and dry state thicknesses of 50, 80, 120 μm, respectively, were also developed by Asahi Glass Company [2]. Asahi Chemical Industry manufactured a series of Aciplex^®-S membranes, which have equivalent repeat unit molecular weights of 1000–1200 and dry state thicknesses of 25–100 μm.

These perfluorinated ion exchange membranes including Neosepta-F^® (Tokuyama) and Gore-Select^® (W. L. Gore and Associates, Inc.) have been developed for chlor-alkali electrolysis. The water uptake and proton transport properties of this type of membrane have significant effects on the performance of PEFCs. These membranes have water uptakes of above 15 H₂O/–SO₃H, and maximizing membrane water uptake also maximizes the proton conductivity. In general, conductivities can reach values of 10⁻²–10⁻¹ S cm⁻¹. All of these membranes possess good thermal, chemical, and mechanical properties due to their perfluorinated polymer backbones.

A limiting factor in PEFCs is the membrane that serves as a structural framework to support the electrodes and transport protons from the anode to the cathode. The limitations to large-scale commercial use include poor ionic conductivities at low humidities and/or elevated temperatures, a susceptibility to chemical degradation at elevated temperatures and finally, membrane cost. These factors can adversely affect fuel cell performance and tend to limit the conditions under which a fuel cell may be operated. For example, the conductivity of Nafion^® reaches up to 10⁻² S cm⁻¹ in its fully hydrated state but dramatically decreases with temperature above the boiling temperature of water because of the loss of absorbed water in the membranes. Consequently, the development of new solid polymer electrolytes, which are cheap materials and possess sufficient electrochemical properties, have become one of the most important areas for research in PEFC and FCEV.

Proton-conducting polymer electrolyte membranes for high performance PEFCs have to meet the following requirements, especially for electric vehicle applications [3]:

• low cost materials;

• high proton conductivities over 100°C and under 0°C;

• good water uptakes above 100°C;

• durability for 10 years.

The challenge is to produce a cheaper material that can satisfy the requirements noted above. Some sacrifice in material lifetime and mechanical properties may be acceptable, providing cost factors are commercially realistic. Good electrochemical properties over a wide temperature range may help the early marketing of PEFCs. Presently, one of the most promising routes to high-performance proton-conducting polymer electrolyte membranes is the use of hydrocarbon polymers for polymer backbones. The use of hydrocarbon polymers as polymer electrolytes was abandoned in the initial stage of fuel cell development due to the low thermal and chemical stability of these materials. However, relatively cheap hydrocarbon polymers can be used for polymer electrolytes, since the lifetime of electrolytes required in FC vehicles are shorter when compared to use in space vehicles. Also, catalyst and FC assembly technologies have improved and brought advantages to the lifetimes of PEFCs and related materials.

There are many advantages of hydrocarbon polymers that have made them particularly attractive:

• Hydrocarbon polymers are cheaper than perfluorinated ionomers, and many kinds of materials are commercially available.

• Hydrocarbon polymers containing polar groups have high water uptakes over a wide temperature range, and the absorbed water is restricted to the polar groups of polymer chains.

• Decomposition of hydrocarbon polymers can be depressed to some extent by proper molecular design.

• Hydrocarbon polymers are easily recycled by conventional methods.

Based on numerous works by other authors and our own research group, we review the syntheses of new proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. The characteristics of these new materials which determine their potential applications, are discussed in detail. A review of electrochemical properties, water uptake, and thermal stability makes possible a comprehensive understanding of the proton conduction mechanism and physical state of absorbed water in these systems.