Effects of conjoint chemical and mechanical stress on PFSA membranes

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Degradation mechanisms of membrane-electrode assemblies (MEA)

Over the past decades, a lot of studies considered aging and reliability of the MEA and provided some understandings on their degradation mechanisms, with the overall objectives to improve their durability and reduce their cost [49,153–155].
This section summarizes the main degradation mechanisms of MEA components, with an emphasis on the membrane in § 3.3. Although interest of this thesis work is focused on PFSA membrane, the two following sections aim at providing an overview of the gas diffusion media and catalyst layers degradation, which are components as much important as membrane in the efficient operation of PEMFC. Nevertheless, these sections are not indispensable for the understanding of subsequent chapters of the manuscript. On another note, it is important to keep in mind that the other cell components (i.e. bipolar plates and gaskets) are also likely to deteriorate during fuel cell operation, although their degradation mechanism will not be fully described in this memoire. In short, gaskets made from silicon are prone to degrade and pollute the membrane and the electrodes [156] while metallic bipolar plates can release cationic impurities into the MEA [157].

Gas diffusion media

Gas diffusion layer (GDL) and microporous layer (MPL) aim at facilitating reactant gases transport as well as to evacuate the water produced by the electrochemical reaction. Some studies have demonstrated that these materials degrade during fuel cell operation, leading to a hydrophobicity loss and a mass reduction [158–160]. Moreover, MPL are found to be vulnerable to chemical oxidation by water, entailing the formation of surface carbon oxides (CO and CO2) [161,162]. This carbon loss by chemical oxidation diminishes MPL hydrophobicity with, as a result, the accumulation of more water in the pores, thus inducing a restricted access of gases to the actives sites. Several investigations [163–167] have highlighted the relation between microstructure changes and mass transport properties, showing that a decrease of GDL and MPL hydrophobicity and a modification of pore size can significantly impact the mass transport.
The catalyst layers are composed of three elements: carbon-based support, Pt or Pt-alloy catalyst and ionomer binder. Each of them can be the subject of various degradation processes during fuel cell operation leading to a loss of electrochemical active surface area (ECSA) and thus to the decrease of fuel cell performances.
Mechanisms of Pt catalyst degradation during fuel cell operation have been intensively investigated during the past decades. It is well established that the decrease of ECSA is far more severe at the cathode side than at the anode side due to higher local potentials in regular operation as well as during transients [168,169].
First of all, Pt particles are sensitive to their environment and may be contaminated by impurities coming from the gases (hydrogen or air), the fuel cell or its auxiliaries (metallic components or other chemicals) [170]. The presence of contaminants in the fuel cell system can generate reversible or irreversible performance losses [171,172]. Indeed, Pt particles are sensitive to ionic impurities, such as Cl-, F- or HSO4- [154,170]. Schmidt et al. [173] especially highlighted a reduction of ORR activity due to the presence of ClO4-, HSO4- and Cl-. Contamination of the fuel cell by carbon oxide (CO), hydrogen sulfide (H2S) or ammonia (NH3) species have also been extensively examined in the literature [170,174,175]. Indeed, they can be absorbed at the Pt surface blocking the access of the catalytic sites to H2 or O2. The ECSA losses can originate from multiple catalyst degradation mechanisms [154,176–178]. They are generally divided into four categories:
• Particles dissolution and redeposition.
• Electrochemical Ostwald ripening.
• Particles migration and coalescence/agglomeration.
• Particles detachment.
A summary of these degradation mechanisms is illustrated in the Figure 1.19.

Degradation mechanisms of the carbon-based support

The degradation of the carbon-based support is another important issue which limits the catalyst layers durability [161,196–198]. The carbon-based support is thermodynamically unstable in the operating conditions of the PEMFC and is particularly prone to oxidation during FC startup and shutdown, or other fuel starvation events [199,200]. The reaction of carbon with water entails the formation of CO and CO2:
+ 2 → +2 ++2 − 0 = 0.207 Eq. 1.17.
+ 2 →+ 4 + + 4 − 0 = 0.518 Eq. 1.18.
CO2 formation reaction prevails on CO formation, following a two-step process involving surface oxides and their oxidation by O-abstraction from water:
+ 2 2 →+ 2 + + 2 − Eq. 1.19.
+→+ 4 + + 4 − Eq. 1.20.
And one can easily understand from these equations that a high relative humidity of gases as well as water accumulation within the cell will increase the carbon corrosion rate [196,201]. In addition, high temperatures also increase the kinetic of carbon corrosion.
In addition to the temperature and the humidity level, other factors greatly affect the carbon-based support degradation during the fuel cell operation such as the potential and consequently the operating conditions of the PEMFC (OCV or repeated startup and shutdown conditions, for example), the nature of the catalyst as well as the chemical state of carbon. In the list below is detailed the impact of these various factors on the carbon corrosion rate.

.Formation of hydrogen peroxide and reactive oxygen species (ROS)

The formation of hydrogen peroxide during fuel cell operation have been observed into the membrane [240,241] as well as in exhaust water from anode and cathode sides [242,243]. More especially, Liu et al. [240] estimated H2O2 concentration between 0.1 and 1.1 mmol.L-1, depending on the membrane thickness and operating conditions. Two distinct mechanisms of H2O2 formation have been proposed in the literature: the first has been suggested by Pozio et al. [157] and consists in the incomplete reduction of oxygen on the platinum surface at the cathode side: + 2 + + 2 − = 0 = 0.67 Eq. 1.21.
This parasitic 2-electron reduction reaction (Eq. 1.21) is encouraged when cell voltage is below to 0.67 V at the expense of the ORR (Eq. 1.2). Furthermore, PFSA membrane not being perfectly impermeable to gases, oxygen crossover can diffuse towards the anode side and promote the formation of H2O2 since anode cell voltage is lower than cathode one [238,243]. LaConti et al. [238] proposed another mechanism based on the gas crossover through the membrane and their combination on the platinum surface to form H2O2 according to the following three-step process: 2 → 2 • (via Pt catalyst) • + 2 (crossover) → • • + • → 2 2 (which can diffuse into the membrane). The gas crossover and the presence of platinum catalyst are prerequisite conditions to entail H2O2 formation during fuel cell operation, which confirm that H2O2 can be observed both to the anode and the cathode sides or even within the membrane. However, H2O2 itself is not harmful for the polymer structure but its decomposition into free radicals expose the membrane to chemical degradation [244].
Formation of ROS occurs through the homolysis of O–O bond in H2O2 which is catalyzed by metal cations like Fe2+, Cu2+ or Ti3+ [185,238,239]:
2 2+ + → ( +1)++ •+ − Eq. 1.25.
22+ •→ •+ 2 Eq. 1.26.
Metal ions in MEA can originated from the corrosion of stack and/or cell components as well as impurities in air steams, coolants or humidifiers [157]. Pozio et al. [157] studied the influence of end plate composition on the membrane degradation and highlighted a correlation between fluoride release and metal ion contamination. On another note, sulfonic acid groups have a higher affinity for metal ionsthan protons, except for Li+, which leads to detrimental loss of conductivity, less water-uptake and thus reduced cell performances [214,245]. Moreover, it has also been suggested that contaminant ions accelerate the membrane degradation by catalyzing H2O2 decomposition and consequently ROS formation [238].

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Chemical changes observed after in-situ experiments

Analysis of PEMFC collecting water during fuel cell operation permitted to highlight the membrane chemical decomposition thanks to the detection of various degradation products. The best-known degradation product is fluoride ion, resulting from the production of hydrofluoric (HF) during PFSA decomposition [185,209,212,229,243,247]. In addition to fluoride ions, sulfate ions have been revealed thanks to ion chromatography analysis [185,212] while the use of 19F NMR spectroscopy permits to identify several perfluorinated carboxylic acids [213,229]. The main degradation products identified in the literature are summarized in Table 1.1.
Fluoride ion emission rate (FER) measured in fuel cell water exhaust or solution extracts of aged MEA provides an accurate indication of chemical degradation level of the PFSA membrane during or after fuel cell operation. It has been highlighted that FER depends upon chemical nature and relative humidity of reactant gases [209,243,250], the current density of the cell [250], temperature [251] as well as the membrane thickness [247,252]. Indeed, Zhao et al. [252] recently demonstrated the importance of membrane thickness on fluoride emissions and thus polymer chemical decomposition which are directly correlated to hydrogen permeation (Figure 1.28). On another note, the counter-ion nature of sulfonic groups also plays a crucial role on membrane degradation. It has been shown that FER was higher in the case of membranes in proton form in comparison to membranes which are polluted with alkaline or alkaline earth metal ions [247,253]. However, Kinumoto et al. [253] also demonstrated that the presence of Fe2+ or Cu2+ ions strongly increases the polymer decomposition.

PFSA membrane degradation through ex-situ aging protocols

Membrane exposure to hydrogen peroxide (H2O2) solution are commonly-used as ex-situ aging tests in order to investigate the impact of chemical stress without considering the other stresses that could affect the membrane durability during in-situ aging tests. This approach enables to mimic the aggressive chemical environment, and especially the formation of free radicals encountered during fuel cell operation, while accelerating the overall membrane degradation. As previously described, H2O2 decomposition into free radicals can be catalyzed by metal ions (eq. 1.25) and more particularly the reaction between H2O2 and ferrous ions Fe2+, known as Fenton’s reaction, constitutes the most widely-used accelerated ex-situ aging test. However, ROS generation does not necessarily require the presence of metal ions. Few authors have indeed initiated the formation of highly reactive radicals thanks to UV irradiation [264], gamma-ray irradiation [265] or electron-beam irradiation [266], but these approaches remain very minority at this time. Alternatively, aging method based on gas-phase H2O2 exposure have been developed in order to better replicate fuel cell operating conditions [216,254,257,267– 269]. Hommura et al. [267] indeed suggested that this accelerated aging method is more suitable for membrane degradation studying since no contamination is endured by the membrane during the test. It has been nevertheless suggested that gas-phase H2O2 exposure induces more severe degradation than Fenton’s reagents exposure [268] but also that degradation mechanism were distinct from that occurring in aqueous conditions [269].
Investigations on membrane exposure to H2O2 or Fenton’s reagents, regardless of the aging method used, highlighted the release of several degradation products such as fluoride ions, sulfate or hydrogen sulfate ions as well as various fluorinated carboxylic acids (Table 1.1). FTIR and/or NMR analyses of H2O2 or Fenton’s solution after the chemical degradation process permitted to detect several decomposed fragments coming from PFSA side chains. The appearance of -C=O, -S=O and -CF stretching bands on IR spectra [270,271] as well as the detection of -CFx (x = 0–3) and -C=O resonance peaks by 13C-NMR spectroscopy [270] reveals the presence of -SO3-, -COOH and -CF- groups in solution. More particularly, Healy et al. [229] investigated the degradation of Nafion™ membranes through in-situ (fuel cell operation) and ex-situ (Fenton’s reaction) experiments and demonstrated by 19F-NMR and mass spectroscopies that a similar fluorocarboned molecule directly deriving from the PFSA side-chain were released in both cases: the perfluoro(4-methyl-3-oxa)pentane-1-sulfonic-5-carboxylic diacid, HOOC–CF(CF3)–O–CF2–CF2–SO3H.

Table of contents :

Chapter I – State of the art of proton-exchange membrane fuel cell (PEMFC) systems
1. Overview of PEMFC
1.1. Operating principle of PEMFC
1.2. Fuel cell components
2. Generalities on PFSA membranes
2.1. Chemical structure and morphology
2.2. Sorption and transport of water and protons
2.3. Mechanical properties
3. Degradation mechanisms of membrane-electrode assemblies (MEA)
3.1. Gas diffusion media
3.2. Catalyst layers
3.3. PFSA membranes
Conjoint chemical and mechanical degradations
4. Objectives of the thesis work
Chapter II – Experimental techniques
1. Chemical and electrochemical characterizations
1.1. Fluoride emissions measurement via ion-selective electrode (ISE)
1.2. ATR-FTIR spectroscopy
1.3. NMR spectroscopy
2. Characterization of membrane functional properties
2.1. Liquid-state 1H-NMR
2.2. Water uptake measurements
Chapter III – Accelerated chemical degradation of PFSA membranes: Fenton’s reaction protocol
1. State of the art: chemical degradation of PFSA membranes induced by Fenton’s reaction
2. Description of the aging and cleaning protocols
2.1. Sample pretreatment
2.2. Aging protocol based on Fenton’s reaction and operating conditions
2.3. Cleaning of aged samples
3. Effect of Fenton’s reagent concentrations on the chemical degradation of PFSA membranes
3.1. Macroscopic morphology evolution of aged membranes
3.2. Fenton solutions analysis: quantification of the chemical degradation
4. Conclusions
Chapter IV – Time-resolved monitoring of PFSA membranes degradation induced by Fenton’s reaction
1. Introduction
2. Establishment of the time-resolved monitoring of ex-situ chemical degradation
3. Chemical structure evolution after exposure to Fenton’s reagents
3.1. ATR-FTIR spectroscopy
3.2. Solid-state 19F-NMR spectroscopy
4. Quantification of the chemical degradation
4.1. Weight loss and fluoride emissions
4.2. Liquid-state 19F NMR spectroscopy
4.3. Correlation between weight loss and emissions of degradation products
5. Impact of the degradation on PFSA membranes functional properties
5.1. Water sorption capacity in aged membranes
5.2. Water self-diffusion after chemical degradation
6. Discussions
6.1. Comparison of PFSA membrane degradation with literature
6.2. Contribution of reinforcement layer and radical scavengers against chemical degradation
7. Conclusions
Chapter V – Effects of conjoint chemical and mechanical stress on PFSA membranes
1. Introduction
2. Experimental device and protocols
2.1. Description of the aging device
2.2. Ex-situ coupled mechanical and chemical stress tests
2.3. Electrochemical tests in single cell
3. Characterization of membrane degradation
3.1. Preliminary tests
3.2. Cyclic compression stress
3.3. Influence of the mechanical strength
3.4. Impact of aging test duration
3.5. Impact of the presence of GDL
4. Impact of conjoint chemical and mechanical stress on the membrane structure and functional properties
4.1. Chemical structure evolution of membranes after conjoint chemical and mechanical stress
4.2. Evolution of water sorption and transport properties in aged membranes
4.3. Cell performances after conjoint mechanical and chemical stresses
5. Contribution of the mechanical stress on membrane properties: comparison with pure ex-situ chemical stress tests
6. Conclusions
Conclusions and Perspectives
Appendix A – Optimization of the experimental protocols
Appendix B – Impact of a static compressive stress on the functional properties of PFSA membranes

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