Proton exchange membrane stability

ESR spectroscopy, used in the direct detection or spin trapping modes, is a sensitive method for the detection of polymer fragments and for determining the degradation mechanism. Recent applications for the study of stability in ionomer membranes used as proton exchange membranes in fuel cells demonstrate the capability of ESR to detect details that cannot be obtained by other methods. 

One of the earliest proton exchange membranes was based on sulfonated polystyrene where divinylbenzene was used as a cross-linking unit for extra stability. Developed by General Electric, this membrane (21) was cheap and easy to manufacture, and it was used for fuel cells in the Gemini space pro-gram.i However, due to the sensitivity of the benzylic hydrogen to radical attack, lifetimes for these membranes under FC operating conditions were quite low. Thus, little work has been carried out on these systems since their inception. 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

Figure 29. Conductivity of some intermediate-temperature proton conductors, compared to the conductivity of Nafion and the oxide ion conductivity of YSZ (yttria-stabilized zirconia), the standard electrolyte materials for low- and high-temperature fuel cells, proton exchange membrane fuel cells (PEMFCs), and solid oxide fuel cells (SOFCs).

Haugen, G.M. et al.. Increased stability of PFSA proton exchange membranes under fuel cell operation by the decomposition of peroxide catalyzed by heteropoly acids, ECS Trans., 3, 551, 2006. 

B. Gupta, G.G. Scherer, and J.G. Highfield. Thermal stability of proton exchange membranes prepared by grafting of styrene into pre-irradiated FEP films and the effect of crossUnking. Angewandte Makromolekulare Chemie 256, 81-84 1998. 

Fluorene-containing sulfonated PODAs have been considered for the application as proton exchange membranes for polymer electrolyte membrane fuel cells. The polymers are highly stable thermally and show improved oxidation stability. However, the proton conductivity is around 10 S cm, which is by a factor of 50 lower than that of Nafion membranes. 

In the PEFC, the membrane, together with the electrodes, forms the basic electrochemical unit, the membrane electrode assembly (MEA). The first and foremost function of the electrolyte membrane is the transport of protons from anode to cathode. On one hand, the electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and, on the other hand, they provide pathways for reactant supply to the catalyst and removal of products from the catalyst. The components of the MEA need to be chemically stable for several thousands of hours in the fuel ceU under the prevailing operating and transient conditions. PEFC electrodes are wet-proofed fibrous carbon sheet materials of a few 100 ttm thickness. The functionality of the proton exchange membrane (PEM) extends to requirements of mechanical stability to also ensure effective separation of anode and... 

From 2,5-bis(4-lluorophenyl)-1,3,4-oxadiazole and 2,2-bis(4-hydroxypheny)propane bisphenol A and 3, 3-disulfonate-4 4-dichloro diphenyl sulfone, a poly(aryl ether sulfone) containing 1,3,4-oxadiazole moieties was obtained [65]. From this polymer, proton exchange membranes have been fabricated. The sulfonated polyoxadiazole membranes exhibit excellent thermal, dimensional and oxidative stability, and a low methanol diffusion coefficient. Thus, these sulfonated polyoxadiazole membranes may be alternative materials for proton exchange membranes at medium to high temperature operations. 

Sohn Y J, Kim M and Lee W Y (2011), The computer-aided analysis for the driving stability of a plug-in fuel cell vehicle using a proton exchange membrane fuel cell. International Journal of Hydrogen Energy,21,1-12. 

Sasaki K, Shao MH, Adzic RR (2009) Dissolution and stabilization of platinum in oxygen cathodes. In Buchi EN, Inaba M, Schmidt T (eds) Proton exchange membrane fuel cell durability. Springer, New York, pp 7-28... 

Chhina H, Campbell S, Kesler O (2008) High surface area synthesis, electrochemical activity, and stability of tungsten carbide supported Pt during oxygen reduction in proton exchange membrane fuel cells. J Power Sources 179(l) 50-59... 

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