Nafion membranes lifetime

Run/Nin heteronuclear complexes such as (653), in which a photosensitizer [Ru(bpy)3]2+ or [Ru(phen)3]2+ is covalently attached to the Ni1 cyclam complex, have been synthesized in order to improve the efficiency of electron transfer from the photoexcited photosensitizer to the catalytic site.1 44-1646 However, these complexes did not perform particularly well, either due to unfavorable configuration of the Nin-cyclam subunit and the resulting steric hindrance or due to short lifetime of the excited states of the Ru photosensitizer moieties. A stable catalytic system has been prepared by immobilizing macrocyclic Ni11 complexes and [Ru(bipy)3]2+ in a nafion membrane.164... 

In a fiirther series of experiments, phosphotungstic acid (PWA) was impregnated onto silica particles and the resulting material was loaded in the recast Nafion. As is well known, heteropolyacids (HPAs) have demonstrated suitable characteristics to be used as proton conductive materials in fuel cells [4-6]. Due to dissolution phenomena in water, however, previous experiments using solid heteropolyacid did not result in stable fuel cell performance [5]. To overcome this problem, resulting in a short lifetime of the fuel cell, experiments of blocking the HPA in a host material were carried out [7-9]. Thus, in this work the phosphotungstic acid-modified membrane was compared, in terms of performance, with the bare silica-recast Nafion membrane in direct methanol fuel cell at 145°C. 

In order to increase the proton conductivity, a lower EW ionomer can be made. However, when the EW is too low (e.g., less than 700), the mechanical strength of the membrane becomes unacceptable, especially after the membrane is fully hydrated, which will in turn shorten the membrane lifetime. The proton conductivity and the mechanical strength of a membrane also depend on the side chain length and structure. The PFSAs made by Dow and 3M have shorter side chains than DuPont s Nafion, as shown in Pigure 1.3. 

Repeated attempts have been made to use materials less highly fluorinated than Nafion, to reduce the costs. However, such membranes have a markedly lower chemical stability, which inevitably would cut the expected membrane lifetime in fuel cells. The search continues for suitable membrane materials having a completely different chemistry. 

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.

The Teflon-like molecular backbone gives these materials excellent long-term stability in both oxidative and reductive environments. A lifetime of over 60,000 h under fuel cell conditions has been achieved with commercial Nafion membranes. These membranes exhibit proton conductivity as high as 0.1 S cm" under fully... 

While the objective is an application in fuel cells, most of the studies performed on new materials do not include a fuel cell evaluation. Moreover, when some fuel cell tests are presented, they are restricted to a polarization curve to estimate the fuel cell performance in comparison with Nafion and only scarce works concern the long-term stability. The specifications for automotive application are 5000 h of operation at 80°C over 5-10 years and more than 10,000 start-stop cycles (typically 3 cycles per day over 10 years). The latter constraint is probably the most difficult to achieve since Nafion membranes are able to operate for more than 10,000 h under stationary load and temperature conditions, but the fifetime is reduced to a few hundred hours when operating under cycling conditions [ 145]. The membrane lifetime is defined as the duration of fuel cell operation until a total or partial rupture induces gas mixing. It is well known that the membrane stability can be significantly enhanced by increasing the membrane thickness or decreasing the ion content, but this stability would be obtained at the expense of the fuel cell performance, which is not acceptable. 

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]. 

In situ FC tests have shown that these membranes are capable of performances at least comparable to Nafion under fully humidified, 60°C operation conditions. Lifetimes were typically short however, with the introduction of DVB as a cross-linker during the graft copolymerization with styrene, lifetimes could be improved to around 1,000 hours. This could be further improved to over 6,(300 hours using a combination of DVB and triallyl cyanuarate as cross-linkers. ... 

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. 

Evaluate FC lifetime and performance of benchmark Nafion and candidate membranes (high temperature membrane)... 

Initially, Nafion-type membranes had been developed for the needs of the chlorine industry. Chlorine electrolyzers with such membranes were widely introduced. Apart from chlorine, they yield alkali that is very pure as the second major electrolysis product. Soon after the advent of these membranes, work on fuel cells that would include them started. Fuel cells with such membranes had lifetimes two to three orders of magnitude longer. 

The sensitivity to hydrolysis is a key issue in many applications. The ester bond in 4GT-PTMO copolymers is sensitive to hydrolysis however, it is fairly protected since most of the ester is contained in a crystalline structure. The addition of a small amount (1-2%) of a hindered aromatic polycarbodiimide substantially increases the lifetime of this material in the presence of hot water or steam [160]. Polyurethanes are susceptible to hydrolytic attack, especially those with polyester soft segments. However, polyester soft segment polyurethanes are generally more resistant to oils, organic solvents, and thermal degradation. lonomers will swell when exposed to water in fact, a commercial hydrated perfluorosulfonic ionomer (Nafion) is used as a membrane separator in chlor-alkali cells. Styrene-diene copolymers and polyolefin TPEs are insensitive to water. 

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