Fenton’s test

Special test One drop of 25% iron (II) sulfate, 2 or 3 drops of hydrogen peroxide produces deep violet-blue color (Fenton s test)... 

Fenton s test Add 1 drop of a 25 per cent solution of iron(II) sulphate to about 5 ml of the neutral or acid solution of the tartrate, and then 2-3 drops of hydrogen peroxide solution (10 volume). A deep-violet or blue colouration is developed on adding excess sodium hydroxide solution. The colour becomes more intense upon the addition of a drop of iron(III) chloride solution. 

Both, ex-situ Fenton s test and in situ fuel cell tests, suggest similar kind of degradation mechanism, as similar polymer fragments were detected under both conditions. The nature of the fragmeuts indicates stepwise degradation mechanism beginning at unstable polymer end groups. 

In order to solve this chemical stability problem, a new proprietary PFSA ionomer synthesis procedure has been developed at DuPont that results in a reduction of the reactive end groups. This approach has been referred as chemical stabilization (CS) technology. Fluoride emission from chemically stabilized polymer in a Fenton s test was found to be eight times lower than a nonchemically stabilized polymer. 

The chemical degradation was studied both in situ (during fuel cell operation) and ex situ (by Fenton s reagent test) [68]. The structure of the examined polymer is given in Fig. 1. Except for fluoride release, the same product shown below was identified using NMR and mass spectroscopy in both the Fenton s test water and a residue extracted from MEAs that were heavily degraded during fuel cell operation. 

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... 

Unlike Fe " ions, the mitigating ions should not reduce H VH202 as this would result in OH formation, as is the basis of Fenton s test [87]. 

Table 4.2 Characterization results of the blend membranes (and of pure PBIs, where applicable) before and after Fenton s Test (FT)...

Also, blends of FBI with these aromatic polyethers resist Fenton s test [13], while FBI and its blends with sulfonated aromatic polysulfones did not withstand such a treatment... 

Chen et al. (2007) used FTIR to study the chemical structure of Nafion membranes before and after Fenton s test. After Fenton s test, a very weak absorption at 1434 cm is evident and assigned to a... 

Meng s group reported that a new approach to the preparation of ionomers from polyCphthalazinone ether ketone)s that have been recently synthesized by a N-C coupling reaction [43,44], These new polymers are claimed to exhibit improved oxidative resistance by the Fenton s test when compared with other sulfonated polymers. The structures of these polymers are shown in Fig. 4.11. 

Results from Fenton s tests conducted at different temperatures indicate that higher temperature leads to more severe membrane degradation (Chen et al. 2007a). The FRR increases 2 times when the temperature increases by 10°C. 

To detect the radicals in Fenton s test, 5,5 -dimethyl-l-pyrroline-A-oxide (DMPO) was used in a Fenton-type test where a membrane was submerged in 0.3 wt% HjOj at room temperature (Aoki et al. 2006c). 4-Hydroxy-2,2,6,6-teramethylpiperidine-A-oxyl was used for spin calibration. The amount of hydroxyl radical is quantified lOmin after H O addition. It was found that the number of... 

Nonsupported metal black catalysts were then used to eliminate the strong ESR signal from the carbon support. No radicals could be detected for perfluorobutane sulfonate nor for the Nafion ionomer dispersion presumably owing to the short lifetime of the radicals (Vogel et al. 2(X)7). With a DMPO spin trap, HO adduct can be easily observed in Fenton s test, while a small amount of HO, can also be detected. 

The oxygen content of membranes after Fenton s test increases significantly (Chen et al. 2007a). It is believed that peroxide or carbonyl groups are formed during the degradation process. 

F NMR spectra reveal that degradation species extracted from fuel cell degraded membrane are similar to the compounds found in Fenton s test bath water (Healy et al. 2005). Various peaks have been assigned to different atoms of the side chain (Fig. 16). A similar study was also described in a more recent report (Kinumoto et al. 2006). 

As discussed earlier, stabilization of PFS A end groups successfully reduced manbrane degradation in Fenton s test (Curtin et al. 2004). Unfortunately, such stabilizalion does not help improve membrane lifetime in actual fuel cell tests. 

As shown in Table 4, hydrocarbon compounds react with HO significantly faster than the self-quenching reaction H0 +H202 —> H O + HO. Although the recombination reaction of HO is also very fast, it should not be considered as a competitive reaction owing to the extremely low concentration of the radicals. Thus, Fenton s test can be considered as a stabihty test against HO attack. 

As discussed in previous sections, the hydrocarbon membranes are generally believed to be intrinsically less stable than perfluorinated membranes owing to lower bond strength of C-H. Results from Fenton s test prove the instability of hydrocarbon membranes to hydroxyl radicals. In contrast, several hydrocarbon membranes show excellent real fuel cell durability. 

Fig. 2 Examples of membrane screening tests, (a) Typical Fenton s test results, (b) Relative humidity cycle profile, (c) Typical gas crossover data resulting from the relative humidity (RH) test...

Tartaric add. (d) Make a saturated aqueous solution of ferrous sulphate (do not heat). Add one drop of it to the neutral solution of the compound, followed by two drops of 10 volume hydrogen peroxide and an excess of sodium hydroxide solution. An intense violet colour due to the ferric salt of dihydroxyfumaric acid, is a positive test (Fenton s test). 

Ex situ accelerated methods were applied to study the degradation of PFS A membranes. Ex situ accelerated chemical degradation experimentation of PFSA most commonly employs Fenton s testing. Fenton s reagents include hydrogen peroxide with Fe ions in order to produce hydroxyl radicals as follows ... 

It should be noted that there are no strict standard operation conditions for Fenton s test and different testing conditions may be employed by different researchers. 

---