Fenton test

The evolution of fluoride ions in actual fuel cell effluent and during laboratory accelerated life studies has been reported. One common example of radical generation from peroxide decomposition is in the Fenton test, where peroxyl or hydroxyl radicals can be formed through the reaction of hydrogen peroxide with Fe(II) (Scheme 3.2). 

Fenton test (3% H2O2 solution containing 2 ppm FeS04) results for sul-fonated aromatic polymers have also been reported. The stability of sulfonated aromatic polymers depends on backbone structure, sulfonation degree, and testing temperature. For polyimides, the time before the polymer is totally dissolved ranges from several minutes to 9 hours at or as... 

Cross-linked, sulfonic-acid-substituted, polyphosphazene-based PEMs have primarily been examined for potential use in DMFC applications due to their low MeOH crossover with reported values 2.5 times lower than that of Nafion. These materials have also been shown to display good thermomechanical and chemical stability (in a Fenton test). Sulfonamide-substituted polyphosphazenes have exhibited very high power densities that are comparable with Nation and may be suitable for use in PEMFC applications. ... 

Control After Fenton Test Control After Fenton Test... 

A study on the feasibility of the enhanced EK-Fenton process for the remediation of hexachlorobenzene (HCB) in a low-permeability soil was conducted by Oonnittan, Shrestha, and Sillanpaa (2008). In that work, kaolin was spiked with HCB and treated by the EK and EK-Fenton processes. )3-cyclodextrin was used to enhance the solubility of HCB in pore fluid. Two EK experiments were conducted to observe the suitability of j8-cyclodextrin as a flushing solution for these processes. Another test conducted was the EK-Fenton test using )3-cyclodextrin as an enhancing agent. Results showed that the type of flushing solution, system pH, electric current, and EO flow are of significance for HCB removal. 

Two different colorimetric tests are used in the pharmacopoeia for identifying tartrates, the Fenton test and the Pesez test. 

As for the chemical stability, the Fenton s reagent prepared from H2O2 aqueous solution and FeS04 is often used. But it is well known that chemical degradation caused by H2O2 is most vigorous at the state of open circuit in dry conditions. Fenton tests are conducted in aqueous solution. A new... 

Fenton s test, which consists in the immersion of the membrane in a H2O2 solution (30 w/w%) containing 20 ppm of Fe, monitoring the rate of fluoride ions release into the solution, has become a common ex situ accelerated test for membrane durability [91, 92]. Fenton test usually yields lifetimes much greater... 

The interesting feature of this new polymer blend is that, in contrast to pure FBI and PBI/SPSF, the miscible PBI/PPy(50) coPSF polymer blends are chemically stable under the highly oxidative Fenton test conditions. In this respect it can be considered as chemically stable under the aggressive oxidative conditions (peroxide species formation at the cathode) under fuel cell operation. 

Figure 12. H NMR of Copolymer V , (Mn = 38000) with the assignment of the respective peaks in CDCI3 (a) before and (b) after the Fenton test. Inlet size exclusion chromatography for Copolymer V using CDCI3 as eluent before (solid line) and after the Fenton test. ...

L. Ghassemzadeh, K.D. Kreuer, J. Maier, K. Mueller, Evaluating chemical degradation of proton conducting perfluorosulfonic acid ionomers in a Fenton test by sohd-state F NMR spectroscopy, J. Power Sources 196 (2011) 2490—2497. 

The question of how critical is the oxidative stability of the materials used as membranes in fuel cells has been contradictory for long time. As it is known, FBI and FBI-related materials show a moderate stability in Fenton test conditions [14] while especially the doped FBI membranes lose their integrity within the Fenton test solution even after a few hours [15],... 

To compare different polymers with respect to their chemical stability, the Fenton test is widely applied in fuel cell membrane research. In these tests, membrane samples are immersed in hydrogen peroxide solution containing a small amount of Fe " ", e.g., iron(lI)sulfate. In the presence of the metal ion, the decomposition of hydrogen peroxide is accelerated. The ongoing reactions are very complex, and several reactive intermediates are formed. Just as an example and demonstrating the catalytic nature, the following partial reactions of the so-called Haber-Weiss mechanism are highlighted, as shown in (6.28)-(6.31) [49, 50]. 

Obvious shortcomings of Fenton tests are their strong dependence on not standardized test protocols and the often qualitative nature of the monitored parameter, e.g., time until the membrane breaks or when floccules start to precipitate (see Table 6.3). Even within one paper, several test conditions may be used in parallel. Furthermore, due to the thermal instability of peroxide solutions, the solutions must be refreshed regularly. Sometimes, samples are immediately re-immersed, in other cases, samples are washed... 

Table 6.3 Selected literature examples for Fenton tests...

In brief, the Fenton test for pristine PBI membranes is informative as an aging tool for quaUtative comparison of the chemical stability of different materials. However, the test is apparently an overdoing method. The materials that withstand the test are surely durable in the real fuel cells, but those that cannot survive the test might still be sufficiently durable in fuel cells. 

The presence of phosphoric acid makes the chemistry in the Fenton test far more complex [69]. Firstly, the presence of phosphoric acid was shown to suppress the formation of radicals due to the lowered pH. Secondly, it was suggested that the presence of phosphoric acid further suppressed the degradation rate, possibly due to the formation of iron phosphates or by the ionization of the polymer. It was concluded that the presence of ferrous ion in the membrane-electrode interface had a negative impact on the fuel cell durability. However, this effect was mainly connected to the deterioration of the catalyst activity [69]. 

It should be mentioned that the relevance of the Fenton test to predict lifetime of a fuel ceU has been frequently debated. It is a common view that it is a very tough test and that fast degradation cannot be interpreted as a disqualification of the membrane material for fuel cell use. However, it is an easy tool for comparative accelerated testing of polymer materials and improved resistance to the Fenton test is an indication of improved oxidative stability. 

Table 22.3 summarizes the literature data of Fenton test results for mPBI, its structurally... 

Table 22.3 Polybenzimidazole degradation during the Fenton test translated into the rate of the polymer weight loss (in percent)...

Fenton test is almost the most widely used method in polymer electrolyte membrane degradation studies because it saves time, is well controlled, and most importantly makes it easy to obtain an undamaged sample without interference from the Pt CL during analysis (Schumb et al., 1955 Guo et al., 1999 ... 

By treating Nafion with fluorine gas for 50 h, Curtin et al. (2004) found that the number of hydrogen-containing end groups can be reduced, and this improves the chemical stahihty against the hydroxyl and hydroperoxy radicals (a 56% decrease in released fluoride ions in the Fenton test). 

Owing to the extremely short lifetime of radicals, low temperatnres and/or spin trapping have to be used to detect the radicals in ESR experiments (Kadirov et al. 2005). In a UV-assisted Fenton test, it is proposed that side chain C-S bond scission, promoted by the presence of ferric ions, leads to a high membrane degradation rate -OCFjCF SOj +Fe " -> -0CF2CF2S03 +Fe ". The UV irradiation may also play an important role in this mechanism. The radical quintet detected in these tests suggests that the fluorine atom attached to the side-chain-linked carbon atoms can be attacked and leads to possible chain scission (Fig. 14). 

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