Antioxidants in non-perfluorinated fuel cell membranes: prospects and limitations

Abstract

Phenol type antioxidants, covalently attached to the polymer trunk to prevent leaching out, provide effective stabilization of the electrolyte membrane against chemical degradation in the fuel cell. Implications of the use of phenolic compounds as antioxidants in non-perfluorinated fuel cell membranes are discussed.

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The electrochemical components of the polymer electrolyte fuel cell (PEFC) are exposed to considerable oxidative stress during operation, owing to the presence of reactive oxygen species (ROS). These are formed in the catalyst layer and membrane as intermediates and can attack the polymer of the proton conducting membrane electrolyte.¹ For perfluorosulfonic acid (PFSA) type membranes, which are the most widely used class of ionomers in fuel cells, it has been established that the presence of H₂, O₂ and Pt catalyst is required for membrane degradation to occur.² HO˙ and HOO˙ are the ROS held responsible for creating oxidative stress. These radicals attack the perfluorinated ionomer, leading to chain scission, unzipping, and loss of functional groups.³ The loss of ionomer constituents entails gradual thinning of the membrane and, eventually, pinhole formation and/or shorting of the cell.⁴ It has been discovered that the incorporation of manganese oxide or ceria, or the corresponding transition metal ions, into the membrane leads to a substantial improvement of the chemical stability of PFSA membranes, measured as a reduction of fluoride emission, in some cases lower by an order of magnitude.^5–7 The presence of a multivalent transition metal ion, which can cycle between two oxidation states, such as Ce^3+/4+ or Mn^2+/3+, is a key feature of this stabilization mechanism, since it allows for a regenerative radical scavenging: the oxidized transition metal ion is effectively reduced, mainly by H₂O₂ but also by HOO˙, which can both be found in the membrane electrode assembly^8,9 to restore the HO˙ scavenging capacity.^5,10

Alternative membranes based on hydrocarbon polymers or partially fluorinated polymers are being developed with the prospect of reducing cost as well as addressing some of the main shortcomings of PFSA type membranes, such as the strong coupling between chemical and mechanical degradation.¹¹ Radiation grafted membranes based on a semi-crystalline fluoropolymer have shown some promise in this respect with encouraging performance and durability attributes.¹² The main challenge regarding radiation grafted membranes, in analogy to other partially fluorinated membranes, is the higher susceptibility of the polymer to oxidative degradation. HO˙ radicals react readily with aromatic compounds, typically with almost diffusion limited rates.¹³ Therefore, the lifetime of HO˙ in membranes with abundant aromatic units is much lower, i.e., on the order of nanoseconds, compared to PFSA membranes, where HO˙ lifetimes are on the order of microseconds.¹⁴ Thus, in the case of PFSA ionomers, there is sufficient time for Ce³⁺ and Mn²⁺ to react with HO˙, providing effective stabilization against ionomer attack. In alternative membranes containing sulfonated aromatic moieties, cerium (or manganese) cannot provide notable stabilization.^10,15,16 In addition, the oxidized form or the transition metal ion may have a sufficiently high redox potential to attack weak points in the polymer, such as the α-position in poly(styrenesulfonic acid) (PSSA).¹⁴

It is therefore necessary to look for other, more suitable stabilizers. Phenolic compounds suggest themselves, as this type of antioxidant reacts very rapidly with HO˙. In fact, phenol derivatives are widely used as antioxidants in plastics.¹⁷ Furthermore, using non-ionic additives as antioxidants is not associated with a loss of proton exchange sites, which in the case of cerium has shown to lead to performance loss⁵ and adverse effects of ion migration.¹⁸ In order to prevent washing out of the antioxidant, we have reported recently the incorporation of a polymer-bound, phenolic type antioxidant (tyramine) into radiation grafted membranes.¹⁹ In an accelerated fuel cell test under open circuit voltage (OCV) conditions, the tyraminated membranes had substantially improved chemical stability compared to non-stabilized membranes. The OCV hold time in these tests was merely 4 h to allow a comparison with styrene-only grafted membranes. In this article, we report accelerated stress tests over 140 h to investigate the stability of the radiation grafted membranes with antioxidants under more meaningful test conditions.

The proton conducting membranes were prepared by pre-irradiation induced grafting (“radiation grafting”) using 25 μm ETFE (Tefzel® 100LZ, DuPont) as base film. For the baseline membrane, styrene was used as grafting monomer. Sulfonation of the grafted film introduced proton exchange sites. For the membranes with polymer-bound antioxidants, glycidyl methacrylate (GMA) was co-grafted with styrene. GMA acted as a “linker” monomer, which allowed subsequent attaching of tyramine via a polymer-analogous nucleophilic substitution reaction. The tyramine represents the phenol type antioxidant. Finally, the graft copolymer was sulfonated (Scheme 1). Details of the synthesis procedure and compositional analysis can be found elsewhere.¹⁹

Scheme 1 Chemical structure of ETFE (25 μm) based radiation grafted membranes used in this study, with “PSSA” baseline (left) and styrene/glycidylmethacrylate (GMA) co-grafted membrane functionalized with tyramine (“GMA(Tyr)”, right), a phenolic compound serving as antioxidant.

The grafted membranes with bound tyramine, designated GMA(Tyr), had a graft level of 41%, with a molar fraction of styrene and GMA in the grafts of 73 and 27%, respectively. The conversion of the tyramination reaction was around 50%. Thus, around 13% of the monomer units in the grafted chain contained a tyramine unit. The sulfonation reaction, on the other hand, is typically next to complete, yielding a degree of sulfonation of close to 100%. The ion exchange capacity (IEC) of the tyraminated membranes was around 1.7 mmol g⁻¹. For comparison, a pure styrene grafted membrane (“PSSA”) with a graft level of 21% and an IEC of 1.4 mmol g⁻¹ was used.

The membranes were assembled with carbon paper type gas diffusion electrodes (ELE0162, Johnson Mathey Fuel Cells, UK) into single cells of 16 cm² active area and operated on H₂ and O₂ at a temperature of 80 °C. After conditioning overnight at a current density of 0.5 A cm⁻² polarization curves were recorded and the hydrogen crossover was determined electrochemically.²⁰ The cell was subsequently switched to OCV for a predefined duration, in case of the tyraminated membranes 144 h and in case of the non-stabilized PSSA membranes 4 and 12 h, respectively. After switching back to a current density of 0.5 A cm⁻² for conditioning, another polarization curve was recorded and the H₂ crossover was determined. Two GMA(Tyr) membranes were tested in this manner. The first one, designated GMA(Tyr) “fresh”, was assembled into the single cell and tested immediately after synthesis of the membrane. The second membrane, intended for a reproducibility test, was prepared in the same batch as the first membrane, yet stored for about one week (GMA(Tyr) “stored”) in an Ar purged box in a fridge (4 °C) before it was assembled into the single cell and tested. Polarization curves of the “fresh” GMA(Tyr) membrane, the reference styrene-only grafted membrane and Nafion 212 are shown in Fig. 1. The tyraminated membrane showed performance slightly superior to that of the PSSA membrane and close to that of Nafion 212. The performance of the “stored” membrane was identical to that of the “fresh” membrane (not shown). Fig. 1 also shows the polarization curve after 144 h at OCV for the GMA(Tyr) “fresh” type membrane and after 4 h for the styrene-only grafted membrane. It is evident that the tyraminated membrane showed a dramatically improved stability, as the performance of the styrene-only membrane was markedly decreased after the short time at OCV compared to the tyraminated membrane after 6 days at OCV. The more pronounced degradation of the PSSA membrane is also indicated by the increased high frequency resistance (HFR), indicative of a loss of grafts and thus conductivity.²¹

Fig. 1 Polarization curves and high frequency (HF) resistance (1 kHz) of H₂/O₂ (fully humidified) single cells at 80 °C and 2.5 bar_a backpressure for MEAs at the beginning of test (BOT), and after a period of accelerated stress testing at open circuit voltage (OCV). For comparison, the performance of a Nafion 212 based MEA is shown.

The evolution of cell voltage and HFR, which is a measure for the membrane resistance and thus its state of health, during the period of OCV hold is shown in Fig. 2. The “fresh” GMA(Tyr) type membrane showed a slightly lower OCV decay rate than the “stored” GMA(Tyr) type membrane, which suggests a lower rate of membrane degradation for the “fresh” membrane. With a sharp increase in HFR from 69 mΩ cm² to 454 mΩ cm² within merely 12 h, the cell with the pure styrene grafted membrane showed the most pronounced degradation. In contrast, both tyraminated membranes, which share a similar initial HFR, exhibit a much slower rate of HFR increase. However, the HFR of the “fresh” membrane increased only slightly from 75 to 77 mΩ cm² over 144 h at OCV, whereas the “stored” membrane showed an accelerated rate of HFR increase after 50 h, which led to a higher HFR (206 mΩ cm²) at the end of test (EOT). The results suggest that the “stored” membrane is less stabilized by the tyramine than the “fresh” membrane, perhaps because of the loss of the antioxidant functionality during storage, although the stabilizing effect is still impressive in comparison to the non-stabilized membrane.

Fig. 2 History of cell voltage and HF resistance during the accelerated stress test at OCV (conditions cf. Fig. 1). The membranes with polymer-bound antioxidant, GMA(Tyr), were tested immediately after membrane preparation (“fresh”) and after storing for 7 days (“stored”), respectively.

After discontinuation of the fuel cell tests, the cells were disassembled and the membrane electrode assembly (MEA) removed. The electrodes were delaminated and the catalyst layers removed by immersing the MEA in a 1/1 v/v mixture of water and ethanol. The post-test IEC of the retrieved membrane was determined by titration. A comparison of membrane properties before and after the OCV hold test is given in Table 1, highlighting changes in the IEC of the membrane, ohmic resistance (HFR) of the cell, and hydrogen crossover. The “fresh” GMA(Tyr) type membrane exhibited the highest IEC retention with two thirds of the initial IEC remaining after the 144 h OCV test, which is in contrast to the “stored” GMA(Tyr) type membrane, which lost approximately two thirds of its initial IEC. The styrene-only grafted membrane lost the graft component completely after an OCV hold time of merely 12 h, indicating the poor chemical stability of this non-stabilized membrane. The resistance of the cell with the “fresh” GMA(Tyr) membrane showed almost no increase in HFR, whereas the “stored” membrane showed a considerable increase from 75 to 206 mΩ cm². The increase in HFR is, as expected, most pronounced in case of the styrene-only grafted membrane. It is also interesting to compare the H₂ crossover data before and after the accelerated aging protocol. The value decreases for the “fresh” tyraminated membrane and increases for the other two membranes. A decrease upon accelerated stress testing was observed before for tyraminated membranes.¹⁹ The hydrogen crossover of a membrane usually increases during an OCV hold test as a result of fragmentation of the graft component, loss of chain constituents and concomitant increase of the porosity of the membrane. The decrease in H₂ crossover in case of the “fresh” tyraminated membrane could be a result of crosslinking through recombination of phenoxyl radicals. For butylated phenol, a widely used antioxidant in technical plastics products,²² recombination of the phenoxyl radicals is well known.^23,24

Table 1 Key membrane and MEA properties before and after the accelerated stress test. BOT/EOT = beginning/end of test, i_x = hydrogen crossover (measured electrochemically)

Membrane	GMA(Tyr) “fresh”	GMA(Tyr) “stored”	PSSA	Unit
a Determined by FTIR.
Time at OCV	144	144	12	h
IEC pristine	1.79 ± 0.02	1.69 ± 0.04	1.79 ± 0.08	mmol g^−1
IEC post-test	1.14 ± 0.05	0.57 ± 0.11	(0)^a	mmol g^−1
IEC retention	65 ± 3	34 ± 6	0	%
HFR BOT	75	75	69	mΩ cm^2
HFR EOT	77	206	454	mΩ cm^2
ix(H2) BOT	0.89 ± 0.04	0.97 ± 0.03	1.14 ± 0.09	mA cm^−2
ix(H2) EOT	0.76 ± 0.01	1.26 ± 0.07	2.58 ± 0.03	mA cm^−2

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A comparison of the change in HFR, indicating membrane aging in situ, and IEC loss for the three tested membranes is shown in Fig. 3. The post-test analysis correlates well with the in situ behavior. Yet it is somewhat surprising that virtually no increase in HFR is observed for a membrane that lost 35% of its IEC in case of the “fresh” GMA(Tyr) membrane. Also, the non-stabilized membrane still shows “measurable” HFR at the end of test, while according to the IEC measurement, all of its grafts have been lost. This may be related to the fact that during the cell test, cleaved fragments of the grafted chain remain in the membrane, as they are not actively washed out in the absence of current and electroosmotic drag. The fragments, although detached from the polymer backbone, may still contribute to the conductivity of the membrane. This is a well-known phenomenon and has been described before.¹⁹ After disassembly of the MEA and the membrane work-up procedure, these fragments are likely to be washed out, tantamount to a loss of IEC.

Fig. 3 Membrane degradation during the accelerated stress test at OCV, expressed as increase in HFR (measured in situ) and loss of IEC (measured by titration of the membrane after disassembly and comparison with pristine membrane).

The difference in the activity of the “fresh” and “stored” tyraminated membrane suggests that the antioxidant functionality is somehow impaired during the storage period of one week, despite the fact that the membrane was kept in a fridge under Ar in the dark. The mechanism of deactivation of the antioxidant is currently unknown. Possibly, a slow acid catalyzed reaction consumes the phenol, resulting in a lower capacity to scavenge aggressive intermediates. The abrupt increase in HFR of the GMA(Tyr) “stored” membrane after around 50 h on test may indicate that the antioxidant is depleted at that point. It is therefore likely that the antioxidant functionality in the “fresh” membrane will also be consumed at some point. It has to be furthermore considered that the ester group of GMA can slowly undergo acid-catalysed hydrolysis under the operating conditions of the fuel cell. Therefore, we performed post-test FTIR spectroscopic analysis of the tested membranes (cf. ESI, Fig. S1†). In case of hydrolysis and formation of the carboxylic acid, a peak shift in the C=O stretch vibration at 1730 cm⁻¹ or appearance of another peak would be observed,²⁵ which is not the case here. This finding is in agreement with previously reported studies of GMA based membranes tested in fuel cells.²⁶ Hydrolysis is slow in the case of a methacrylate but is nevertheless expected to take place over long periods of operation.

The results presented here demonstrate that phenol type antioxidants are promising and effective in stabilizing non-perfluorinated fuel cell membranes against oxidative attack. Increasing the concentration of antioxidant in the membrane can be a means to prolong the stabilizing effect. Yet current limitations were also highlighted in this study. Therefore, future work needs to be directed to tackle these issues. The chemistry of the antioxidant group itself needs to be adapted to improve its shelf life and prevent adverse side reactions. In the plastics industry, “hindered” phenols are used as antioxidants, such as compounds based on 2,6-di-tert-butylphenol.¹⁷ In addition, it may be advisable to use dihydroxy compounds, e.g., derivatives of hydroquinone or catechol, which can undergo reversible oxidation/reduction reactions.²⁷ Hence, instead of tyramine, dopamine could be used during membrane preparation. Furthermore, other functional groups to deactivate reactive intermediates may be explored, such as lactones or amines.²² An entirely different strategy would be to target the “parent compound” H₂O₂ rather than the radicals derived from it. Since H₂O₂ is much longer lived than the radicals,¹⁴ deactivation may be easier to achieve.

Since the ionomer in the fuel cell is exposed to a continuous bombardment with radicals, sustained stabilization over thousands of hours in all likelihood requires a regenerative antioxidant functionality. Regeneration by H₂O₂, as in case of cerium and manganese, is unlikely to work for thermodynamic reasons.‡ It is conceivable to increase the redox potential of the phenol by introducing electron-withdrawing substituents, such as nitro-groups. Yet even if the electrode potential can be increased above that of the HOO˙/H₂O₂ couple, the resulting reaction is expected to be very slow.³⁰ In a recent study, Yao et al. claim that vitamin E, also a phenol type antioxidant, is regenerated by H₂ in the fuel cell, yet solid proof is not given.³¹ In fact, the regeneration through H₂ is thermodynamically or kinetically unfavorable. We attributed the drop in H₂ crossover seen for membranes with antioxidants exposed to periods of oxidative stress to follow-up reactions of the antioxidant, resulting in crosslinking of the polymer.¹⁹ Other chemistries may be more amenable to a regenerative mechanism. In the plastics industry, hindered amine light stabilizers (HALS), typically piperidine derivatives, are used to protect the polymer from light-induced degradation.³² In particular, HALS can undergo regenerative antioxidant action in a process referred to as Denisov cycle, a cyclic mechanism of chain termination, similar to the “ping-pong” mechanism used by superoxide dismutase in living cells.³³ Whether this mechanism can also work in the context of a partially fluorinated fuel cell membrane is not known. Clearly, the conditions are rather specific, such as the low pH, under which an (acidified) hindered amine may not be effective.³⁴

Conclusions

Fuel cell membranes based on hydrocarbon or partially fluorinated polymers containing aromatic sulfonic acid moieties as protogenic groups require a different antioxidant strategy than perfluorosulfonic acid (PFSA) membranes, where cerium and manganese ions were found to be effective and regenerative radical scavengers. Polymer-bound antioxidants of the phenol type have been introduced into radiation grafted membranes and shown to be very effective and promising in stabilizing the membrane against chemical degradation. Membranes containing the antioxidant showed a much lower rate of degradation compared to a non-stabilized membrane. However, a significant difference in the stabilization effect was observed between a membrane tested immediately after synthesis and a membrane stored for one week. It therefore appears that antioxidant functionality is gradually lost. To exploit the potential of this antioxidant concept, future work ought to be directed towards finding more stable functional groups and designing those to operate in regenerative manner to sustain stabilization over thousands of hours in the fuel cell.

Acknowledgements

Y.B. and Z.Z. acknowledge financial support by the Swiss National Science Foundation (project no. 2000_132382) and Swiss Federal Office of Energy (contract no. 102245), respectively. The authors wish to thank Prof. Dr W.H. Koppenol, L. Bonorand and Prof. Dr T.J. Schmidt for fruitful discussions.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09792k

‡ E°(HOO˙,H⁺/H₂O₂) = 1.46 V at pH 0.²⁸ E°(PhO˙,H⁺/PhOH) = 1.31 V at pH 0.²⁷ E°(Ce⁴⁺/Ce⁴⁺) = 1.44.²⁹

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