Capillary penetration method for measuring wetting properties of carbon ionomer films for proton exchange membrane fuel cell (PEMFC) applications

In this work, capillary rise experiments were performed to assess the wetting properties of carbon-ionomer (CI) films. The samples were attached to a micro-balance and then immersed into liquid water to (i) measure the mass gain from the liquid uptake and (ii) estimate the (external) contact angle to water (typical value around 140°). The results showed that drying the CI films under low vacuum significantly impacted the CI film wettability. The influence of the ionomer content on the CI films’ wettability was investigated with various ionomer to carbon (I/C) ratios: 0.8, 1.0, 1.2 and 1.4. No significant variation of the contact angle to water extracted from the capillary rise experiment was measured. However, water uptake increased with the I/C ratio suggesting a more hydrophilic behavior. This observation was in good agreement with the measurement from the sessile drop method showing a slight decrease of the contact angle to water: from 155° for an I/C of 0.8 to 135° for I/C = 1.4

Local transient phenomena in PEM fuel cell with dead-ended anode

Cette thèse concerne les phénomènes locaux qui se produisent dans une pile à combustible à membrane (PEMFC) fonctionnant en mode bouché. Ce mode de fonctionnement consiste à alimenter l’anode en hydrogène sec tout en maintenant sa sortie fermée ce qui favorise l’accumulation d’eau et d’azote (issus du compartiment cathodique) près de la sortie anodique. Certaines régions sont donc convenablement alimentées en gaz tandis que d’autres ne le sont plus. Ces déséquilibres s’accompagnent de hausses localisées de potentiel (à l'anode et à la cathode) qui accélèrent la dégradation du catalyseur et de son support carboné à la cathode. Afin d’étudier ces dégradations à une échelle locale, une cellule segmentée novatrice permettant la mesure simultanée des densités de courant et des potentiels locaux a été développée. Des protocoles de vieillissement accélérés reposant sur un fonctionnement prolongé en mode bouché montrent que les pics de potentiel ont pour conséquence, après quelques heures, une distribution non-uniforme de la surface active (ECSA) à la cathode et des courants le long de la cellule : les dommages sont plus prononcés dans les zones les plus touchées par le déficit en hydrogène. Des études paramétriques et un modèle numérique permettent de comprendre que le déficit en hydrogène résulte principalement de l’accumulation d’eau liquide dans les canaux de l’anode, bien que l'azote joue également un rôle. Par conséquent, la gestion de l’eau impacte fortement les variations de potentiel à la cathode et donc leurs conséquences en termes de dégradation ; basées sur ces constatations, des solutions sont proposées pour améliorer les durée de vie des pilesThis work investigates the local transient phenomena occurring in proton exchange membrane fuel cells (PEMFC) operated with a dead-ended anode. The dead-end mode consists in closing the anode outlet, which leads eventually to local hydrogen starvation due to the excessive accumulation of liquid water and nitrogen (because of membrane crossover) in the anode compartment. Such fuel-starvation events may remain undetected but can entail a significant rise of the anode (and thus cathode) potentials and accelerate carbon corrosion and catalyst degradation. To access local information, we developed an innovative segmented linear cell with reference electrodes along the gas channels. By simultaneously monitoring the local potentials and current densities during operation, we assessed the impact of fuel starvation and observed strong local cathode potential excursions close to the anode outlet. Aging protocols based on fuel cell operation with a dead-ended anode (longer than in real use condition) showed non-uniform cathode ElectroChemical Surface Area (ECSA) losses and performance degradation along the cell area: the damage was more severe in the regions suffering the longest from fuel starvation. Parametric studies completed by numerical simulations showed that the fuel starvation is mainly governed by liquid water accumulation in the anode channels, as well as nitrogen crossover through the membrane. As a consequence, water management impacts significantly the cathode potential variations and thus the resulting electrode degradation. Starting from this founding, we propose strategies to improve fuel cell lifetim

Phénomènes locaux instationnaires dans les piles à combustible à membrane (PEMFC) fonctionnant en mode bouché (dead-end)

This work investigates the local transient phenomena occurring in proton exchange membrane fuel cells (PEMFC) operated with a dead-ended anode. The dead-end mode consists in closing the anode outlet, which leads eventually to local hydrogen starvation due to the excessive accumulation of liquid water and nitrogen (because of membrane crossover) in the anode compartment. Such fuel-starvation events may remain undetected but can entail a significant rise of the anode (and thus cathode) potentials and accelerate carbon corrosion and catalyst degradation. To access local information, we developed an innovative segmented linear cell with reference electrodes along the gas channels. By simultaneously monitoring the local potentials and current densities during operation, we assessed the impact of fuel starvation and observed strong local cathode potential excursions close to the anode outlet. Aging protocols based on fuel cell operation with a dead-ended anode (longer than in real use condition) showed non-uniform cathode ElectroChemical Surface Area (ECSA) losses and performance degradation along the cell area: the damage was more severe in the regions suffering the longest from fuel starvation. Parametric studies completed by numerical simulations showed that the fuel starvation is mainly governed by liquid water accumulation in the anode channels, as well as nitrogen crossover through the membrane. As a consequence, water management impacts significantly the cathode potential variations and thus the resulting electrode degradation. Starting from this founding, we propose strategies to improve fuel cell lifetime.Cette thèse concerne les phénomènes locaux qui se produisent dans une pile à combustible à membrane (PEMFC) fonctionnant en mode bouché. Ce mode de fonctionnement consiste à alimenter l’anode en hydrogène sec tout en maintenant sa sortie fermée ce qui favorise l’accumulation d’eau et d’azote (issus du compartiment cathodique) près de la sortie anodique. Certaines régions sont donc convenablement alimentées en gaz tandis que d’autres ne le sont plus. Ces déséquilibres s’accompagnent de hausses localisées de potentiel (à l'anode et à la cathode) qui accélèrent la dégradation du catalyseur et de son support carboné à la cathode. Afin d’étudier ces dégradations à une échelle locale, une cellule segmentée novatrice permettant la mesure simultanée des densités de courant et des potentiels locaux a été développée. Des protocoles de vieillissement accélérés reposant sur un fonctionnement prolongé en mode bouché montrent que les pics de potentiel ont pour conséquence, après quelques heures, une distribution non-uniforme de la surface active (ECSA) à la cathode et des courants le long de la cellule : les dommages sont plus prononcés dans les zones les plus touchées par le déficit en hydrogène. Des études paramétriques et un modèle numérique permettent de comprendre que le déficit en hydrogène résulte principalement de l’accumulation d’eau liquide dans les canaux de l’anode, bien que l'azote joue également un rôle. Par conséquent, la gestion de l’eau impacte fortement les variations de potentiel à la cathode et donc leurs conséquences en termes de dégradation ; basées sur ces constatations, des solutions sont proposées pour améliorer les durée de vie des piles

High-temperature PEM Fuel Cell Characterization: an Experimental Study Focused on Potential Degradation due to the Polarization Curve

High-Temperature Proton Exchange Membrane Fuel Cell constant current ageing tests highlighted that the characterizations used to monitor the state of health of single cells could be potentially degrading. An experimental campaign to analyze potential degradation due to polarization curves was carried out. More exactly, four methodologies to generate a polarization curve including Electrochemical Impedance Spectroscopies (EIS) were cycled 30 times. The tested single cells were based on a commercial PBI Membrane Electrodes Assembly (MEA) with an active surface of 45 cm2 (BASF Celtec®-P 1100 type). Before the first cycling test and after the last cycling one, complete characterizations, composed by a voltammetry and a polarization curve including EIS, were performed. The results show that one of the MEA has a voltage which increased for one of the four methods to obtain the polarization curve. This growth is linked to a decrease of ohmic losses: in an unexpected way, it could be considered as a way to improve the break-in period. Similarly, the monitoring of CO2 emission (as corrosion has been suspected to be involved at high voltage, i.e. low current density) confirms the potential degradation of the electrodes during the measurement of the polarization curve

Tracking Changes in Surface Chemistry and Structure of Supported IrO x Nanoparticles in Acidic Oxygen Evolution Reaction Conditions

International audienc

Voltage Readjustment Methodology According to Pressure and Temperature Applied to a High Temperature PEM Fuel Cell

The operating conditions can have uncontrolled effects on the voltage of a High-Temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC). For instance, the HT-PEMFC can be used at ambient pressure, i.e., without having a back pressure regulator. In this case, the variation in the atmospheric pressure directly affects pressures inside the fuel cell, which induces voltage variation. Moreover, in transient phases, several coupled phenomena can have an uncontrolled effect on the voltage. For example, following a change in the current operating point, thermal conditions in the fuel cell can vary, and the temperature stabilization then leads to a voltage variation. This article introduces a readjustment method for the fuel cell voltage to compensate for the effects of the pressure and temperature variations that are undergone and to decouple their effects. This methodology is based on the realization of a design of experiments to characterize the voltage sensitivity to pressure ([1; 1.5 bar]) and temperature ([120; 180 °C]) between 0.2 and 1 A/cm2 of an Advent PBI MEA (formerly BASF Celtec®-P 1100 W). The data obtained allowed identifying an empirical model that takes into account the aging caused by the experiment. Finally, the methodology is criticized before proposing an alternative method

Tracking Changes in Surface Chemistry and Structure of Supported IrO x Nanoparticles in Acidic Oxygen Evolution Reaction Conditions

International audienc

Probing Surface Oxide Formation and Dissolution on/of Ir Single Crystals via X-ray Photoelectron Spectroscopy and Inductively Coupled Plasma Mass Spectrometry

International audienc

About fuel starvation in PEMFC: characterization of the mechanism of degradation and mitigation strategies

International audienc

Oxygen Evolution Reaction Investigation on Model Catalysts in Acidic Medium: electro-oxidation and catalytic activity of Ir(111), Ir(210) and nanostructured Ir(210)

International audienceProton exchange membrane water electrolysers (PEMWEs) are perceived as one of the most promising technology for a clean hydrogen production but the sluggish oxygen evolution reaction (OER, anodic reaction) kinetics and the poor stability of the anodic material still limit their widespread development [1]. Iridium oxides (IrO2) are ones of the most active electrocatalysts for the OER [2], able to maintain high OER kinetics over the long term in the harsh operating conditions of a PEMWE anode [3]. Considering the low abundance of iridium on the Earth's crust and its high cost, designing and synthesizing tailored OER nanocatalysts is an essential step. Switching from bulk surfaces to nanocatalysts requires investigations at atomically smooth model surfaces such as single crystals [4]. In this study, the effect of different crystal facets of Ir electrodes (Ir(111), Ir(210) and nanostructured Ir(210)) on the initial catalytic performance for the OER were compared. To go further, an ageing test consisting of applying 1.7 V vs. RHE for 2h was conducted to evaluate the stability of these surfaces. The amount of iridium dissolved was quantified by Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) (Figure 1B). Atomic Force Microscopy (AFM) was used to study the surface morphologies and the chemical compositions were obtained by X-ray Photoelectron Spectroscopy (XPS). Although the surface structure (low-coordinated atoms and surface defects) and composition play a role on the initial catalytic performances, these differences vanish upon ageing (Figure 1A). By establishing the structure-activity-stability relationships, this fundamental study demonstrates the impact of low-coordinated atoms on the iridium dissolution and the correlation between the oxide composition and the OER activity. This study will help to improve the understanding of iridium electrocatalytic behavior upon OER and to design efficient catalysts for PEMWEs