Polymer electrolyte membrane lifetime

Figures for the time required for a smooth operation of polymer electrolyte membrane fuel cells (and other fuel cells used in the same applications) are given variously as 2000-3000 h for the power plants in portable devices, as up to 3000 h over a period of 5-6 years for the power plants in electric cars, and as 5-10 years for stationary power plants. Much time will, of course, be required to collect statistical data for the potential lifetime of different kinds of fuel cells. Research efforts, therefore, concentrate on finding the reasons for the gradual decline of performance indicators and for premature failure of fuel cells. In recent years, many studies have been conducted in this area.

The influence of a variety of contaminants in reactants and in the fuel cells themselves on the polymer electrolyte membrane fuel cell lifetime, as well as the mechanisms of this influence, have been examined in a review by Cheng et al. (2007). 

The scientific community has made great progress in increasing the durability of polymer electrolyte membrane fuel cell (PEMFC) systems, but durability must further increase before we can consider fuel cells economically viable [1]. As durability increases, new modes of fuel cell contamination and failure are exposed. We expect state of the art PEMFC systems to run for thousands of hours. This means that each sulfonate group in typical per-fluorosulfonic acid (PFSA) membranes used in today s PEMFC systems will associate with several million protons over the lifetime of the systems. Even if other cations replace only a small fraction of the protons entering the electrolyte membrane, these contaminant cations can build up in the system and degrade the fuel cell system performance over time. 

The construction of ISEs used in clinical measurements is of the membrane electrode type, i.e., the ion-sensitive membrane separates the sample from an internal reference electrolyte, which is the site of the internal reference element, usually a silver wire covered by silver chloride. The membrane can be shaped to different forms such as flat, convex, tubular, etc. Sodium sensitive membranes are made from special composition glass, the other ion-sensitive membranes from a polymer matrix such as plasticized polyvinylchloride (PVC) or silicon rubber. The particular selectivity of polymer membranes is first of all due to a small percentage of active material, e.g., valinomycin, dissolved in the polymer. Important secondary effects have been attributed to the type and permittivity of the polymer. The useful lifetime of the sensors also depends on the polymer. The time response [13] may again depend on membrane composition. 

Figure I.6a also reveals the timeline of milestones in fuel cell design. The leftmost curve is the performance curve of the first practical H2/O2 fuel cell, built by Mond and Langer in 1889 (Mond and Langer, 1889). The electrodes consisted of thin porous leafs of Pt covered with Pt black particles with sizes of 0.1 lam. The electrol)de was a porous ceramic material, earthenware, that was soaked in sulfuric acid. The Pt loading was 2 mg cm and the current density achieved was about 0.02 A cm at a fuel cell voltage of 0.6 V. The next curve in Figure I.6a marks the birth of the PEFC, conceived by Grubb and Niedrach (Grubb and Niedrach, 1960). In this cell, a sulfonated cross-linked polystyrene membrane served as gas separator and proton conductor. However, the proton conductivity of the polystyrene PEM was too low and the membrane lifetime was too short for a wider use of this cell. It needed the invention of a new class of polymer electrolytes in the form of Nafion PFSA-type PEMs to overcome these limitations.

Open-air bending actuators can also be obtained as pseudo-trilayer devices where the conducting polymer electrodes are interpenetrated in the both faces of a solid polymer electrolyte (SPE) membrane. Since the electrodes are entangled in the SPE, the resulting material avoids any delamination issue during actuation. If an ionic liquid is used as electrolyte, very high lifetime can be obtained (Vidal et al. 2004). The typical procedure requires (1) to synthesize the SPE membrane first and then (2) to incorporate the conducting polymer electrodes. 

Solid Polymer Electrolyte The most common solid polymer electrolytes consist of a hydrophobic and inert polymer backbone which is sulfonated with hydrophilic acid clusters to provide adequate conductivity as discussed in Chapter 5. In order to ensure adequate performance, some membrane hydration is required. However, excess water in the electrodes can result in electrode flooding, so that a precarious balance must be achieved. Modern perflourosulfonated ionomer electrolytes for H2 PEFCs are 18-25 xm thick with a practical operating temperature limit of 120°C, although PEFC operation is rarely greater than 90°C due to excessive humidity requirements and operational low lifetimes. 

The preparation complexity of perfluorosulfonated membrane and the high cost have restricted PEMFC from commercialization. Many researchers are dedicated to the development of nonflnorinated PEM. The American company Dais has developed styrene/ethylene-bntylene/styrene triblock polymer [51]. This membrane is especially snitable for small power PEMFC working at room temperature. The lifetime of the membrane is up to 4000 h. Baglio did some experiments to test the performance comparison of portable direct methanol fuel cell mini-stacks between a low-cost nonfluorinated polymer electrolyte and Nafion membrane. He found that at room temperature, a single-cell nonfluorinated membrane can achieve maximum power density of about 18 mW/cm. As a comparison, the value was 31 mW/cm for Nafion 117 membrane. Despite the lower performance, the nonfluorinated membrane showed good characteristics for application in portable DMFCs especially regarded to the perspectives of significant cost reduction [52]. 

For the continuous process, a special divided cell (Pb02/steel anode, steel cathode, Nafion as cation exchange membrane) based on the principle of a tubular reactor was developed. The final product can be removed in gaseous form, so that the electrolyte can be recycled in a simple manner. The membrane and electrodes are supposed to have lifetimes of at least one year 63). Hexafluoropropylene is a useful monomer for fluorine-containing polymers, e.g., fluorinated polyethers. 

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