Local Impact of Humidification on Degradation in Polymer Electrolyte Fuel Cells

Water management represents one of the main challenges in the design and operation of Polymer Electrolyte Fuel Cells (PEFCs). Besides performance, the water level also affects the durability of the cell. Understanding the degradation processes is of vital importance for extending durability of PEFCs by suitable mitigation strategies. In this work, the degradation processes related to operation with fully- and non-humidified gas streams were locally studied. The differences were analyzed using in-situ diagnostic tools, such as segmented cell for local current density measurements, during a 300 h test operating under constant conditions, in combination with local post-test analysis, i.e. SEM/EDX and XPS. The results showed the deep impact of the RH on homogeneity during the degradation process due to the fact that different water distribution influences the chemical environment. Under non-humidified gas streams, the cathode inlet region exhibited increased degradation, whereas with fully humidified gases the bottom of the cell had the higher performance losses. The degradation and the degree of reversibility produced by Pt dissolution, PTFE defluorination, and contaminants such as silicon (Si) and nickel (Ni) were locally evaluate

Local Impact of Pt Nanodeposits on Ionomer Decomposition in Polymer Electrolyte Membranes

Based on recent theoretical studies, we designed a multistep experimental protocol to understand the impact of environmental conditions around Pt nanodeposits on Membrane chemical degradation. The first experiment probes the local potential at a Pt microelectrode for different rates of permeation of hydrogen and oxygen gases from anode and cathode side. The subsequent degradation experiment utilizes the local conditions taken from the first experiment to analyze local rates of ionomer degradation. The rate of ionomer decomposition is significantly enhanced in the anodic H2-rich membrane region, which can be explained with the markedly increased amount of H2O2 formation at Pt nanodeposits in this region

Enveloping of Catalyst Powder by Ionomer for Dry Spray Coating in Polymer Electrolyte Membrane Fuel Cells

This study presents innovative concepts for improving performance of membrane electrode assemblies (MEAs) prepared by the dry-spraying method introduced by the German Aerospace center (DLR). Dry-spraying is a time and cost effective method that involves solvent-free spraying of catalyst powder on polymer electrolyte membrane. The issue which is resolved in this work is the large ionomer particle size in the conventional method. With mechanical grinding, particle size of the ionomer less than 100 nm were not been achieved. However, here the reactive interface of dry-sprayed MEA is optimized by improving ionic conductivity. Our approach is to modify a carbon support by partially enveloping with Nafion® ionomer followed by incorporating Pt black with it. Additionally, commercial catalyst powder was also modified by two-step preparation process with Nafion® dispersion. In this research, both of these modified powders are sprayed over membrane; hot-pressed; characterized, and have shown improved ionic network and distribution, which corresponds to their higher performances. The improvement in the performance does not correlate with electrode surface area but with the ionomer resistance of the catalytic layer. Therefore, with this study we demonstrate a pathway and methodology to further improve performance by optimizing ionomer structure and networks in the catalytic layer

Einfluss von Platin-Abscheidungen auf die Membrandegradation in Polymerelektrolytbrennstoffzellen

Bei der Polymerelektrolytbrennstoffzelle (proton exchange membrane fuel cell, PEMFC) kann sich im Betrieb in der Elektrode Pt auflösen und in der Elektro-lytmembran wieder abscheiden. Da sich zugleich die Diffusion der Reaktanten der Zelle, H2 und O2, durch die Membran nicht vermeiden lässt, reagieren an den katalytisch aktiven Pt-Abscheidungen beide Stoffe, wobei u. a. schädliche re-aktive Spezies entstehen können. Dies führt in den meisten Fällen zu einem dras-tischen Anstieg der normalerweise geringen Rate der Membranzersetzung. Aller-dings wurde in einigen Experimenten auch das Gegenteil berichtet, nämlich eine Verringerung der Zersetzungsrate. Zur Erklärung des ambivalenten Effekts der Pt-Abscheidungen auf die Membrandegradation gibt es in der Literatur bisher zwei Theorien. Zum einen wird der örtlichen Verteilung der Abscheidungen ein entscheidender Einfluss zugesprochen, zum anderen den elektrochemischen Bedingungen an den Pt-Abscheidungen. Die Identifikation des tatsächlichen Einflussfaktors bzw. der Faktoren ist notwendig, um ein besseres Verständnis von der chemischen Degradation aufzubauen und Vermeidungsstrategien entwickeln zu können. Um die Aufklärung voranzutreiben, werden in dieser Arbeit die beiden vor-geschlagenen Theorien experimentell überprüft. Dies erfolgt anhand von in-situ Degradationstests mit Brennstoffzellen, in deren Membran zuvor Pt abge-schieden wurde. In verschiedenen Experimenten wurden die Einflussfaktoren elektrochemische Bedingungen und Partikelverteilung systematisch variiert und die Wirkung auf die Membranzersetzung untersucht. Die größten Heraus-forderungen bei diesen Versuchen waren zum einen geeignete Experimente zu entwickeln und zum anderen die weitere Abscheidung von Pt während des Degradationstests zu verhindern. Zur Lösung dieses Problems wurde die Zellkonfiguration experimentabhängig angepasst und gewechselt, indem Elektroden mit entweder stabilem oder instabilem Pt verwendet wurden. Hierfür wurden im Vorhinein geeignete Elektroden in einem Vergleichstest identifiziert. In den Experimenten zeigte sich eine starke Abhängigkeit der Membrandegrada-tion von der Verteilung der Pt-Abscheidungen, vor allem von der Partikeldichte. Dagegen konnte der Einfluss der elektrochemischen Bedingungen auf die Memb-randegradation letztlich nicht validiert werden. Offenbar war im dazu durch-geführten Experiment der Effekt des Potentialeinflusses zu gering gewesen, wie sich in der anschließenden simulativen Untersuchung herausgestellt hatte. Somit konnte nur die Theorie über den Zusammenhang von Pt-Verteilung und Ausmaß der chemischen Degradation bestätigt werden. Der degradationshemmende Effekt von Pt konnte allerdings in keiner der Messungen beobachtet werden. Hierfür war die Partikeldichte vermutlich zu gering. Zusammengefasst kann gesagt werden, dass die chemische Membrandegradation durch Pt-Abscheidungen ein praktisch kaum vermeidbarer und beeinflussbarer Prozess ist. Eine Verhinderung bzw. Abschwächung erfordert in erster Linie andere oder stabilere Materialien im Katalysator und in der Elektrolytmembran. Jedoch besteht durch die Wahl geeigneter Betriebsbedingungen ein gewisses Potential zu Milderung der Degradation durch Pt-Abscheidungen

Influence of Platinum Precipitation on Properties and Degradation of Nafion® Membranes

In this study, the role of Pt concentration and distribution in the membrane on ionomer degradation was investigated. Nafion® membranes were impregnated with different mass fraction of Pt via ion-exchange and operated as membrane electrode assemblies (MEAs) at open circuit voltage (OCV). Various MEAs with known Pt concentration were characterized by electrochemical in-situ measurements regarding both, membrane properties and ionomer degradation. Additionally, Pt particle distribution in the membrane cross sections was analyzed. The presence of Pt precipitations was found to influence ionic conductivity and the sensitivity of cell performance for humidification. Ionomer decomposition increased with concentration and surface area of Pt deposits as well as distance between particles. The comparison between Pt precipitated artificially and during operation showed a considerable difference in particle distribution and extent of internal membrane humidification due to water formation at Pt sites

Hydrophobicity Patterning of Gas Diffusion Media and Performance in Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells with their high gravimetric energy density face a water balance problem especially under variable loads, e.g. automotive operation: The excess product water needs to be removed from the fuel cell while maintaining a humidified membrane. The gas diffusion layer (GDL), which also provides contact to the electrochemically active components, is responsible for a passive water management of the cell. The adjustment of the hydrophobicity of the GDL is crucial for stable operation. In polymer electrolyte fuel cells they typically consist of conductive medium, either a carbon based powder in the microporous layer or carbon felt/fibres/cloth in the macroporous backing, and a hydrophobicity impregnation agent like polytetrafluoroethylene (PTFE). The ratio determines the hydrophobicity and thus the performance. Modifications of the hydrophobicity were applied with the goal to form a structured, non-uniform wettability of the gas diffusion media, thus introducing more hydrophilic evasion pathways for water while maintaining more hydrophobic areas. It is shown, that these modifications can result in considerable performance improvement and more homogeneous current density

The Role of the Platinum Band for Membrane Degradation in PEMFC – An Investigation using an Artificial Platinum Band

The impact of the Pt band on ionomer degradation is controversial. Many studies found increased degradation in the presence of Pt deposits. However, in a recent publication, it was reported that the Pt band suppressed ionomer degradation.3 In their theoretical work, Gummalla et al. explained this discrepancy by the distribution of Pt particles: at low particle density, radicals are free to attack the ionomer, at high density, they are scavenged.4 Experimental results based on membranes in which Pt was deposited chemically support this theory.5,6 However, in those membranes, Pt particles were dispersed homogeneously, not locally as a thin band. In order to examine the influence of the Pt band more accurately, it is necessary to prepare membranes containing Pt particles which are distributed similarly to the Pt band. In this work, we investigated the role of Pt concentration in the Pt band on membrane degradation. For this purpose, we conducted a degradation experiment with membrane electrode assemblies (MEAs) containing an artificial Pt band with different amount of Pt. The MEAs contained two Nafion® membranes between which Pt was deposited mechanically. In the degradation test, they were operated with H2/O2 for ca. 100 h at open circuit voltage. Degradation was investigated based on electrochemical characterization and fluoride emission. In addition, Pt particle distribution at the membrane cross sections was analyzed with scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX). 1. A. B. LaConti et al., in Handbook of Fuel Cells: Fundamentals, Technology and Applications, Vol. 3, 647–663, Chicester, England (2003). 2. J. Aragane, J. Appl. Electrochem., 26, 147–152 (1996). 3. N. Macauley et al., ECS Electrochem. Lett., 2, F33–F35 (2013). 4. M. Gummalla et al., J. Electrochem. Soc., 157, B1542–1548 (2010). 5. M. P. Rodgers et al., J. Electrochem. Soc., 160, F1123–F1128 (2013). 6. S. Helmly et al., ECS Trans., 58, 969–990 (2013)

Electrocatalyst Stability under Dynamic and Stationary Operation of Fuel Cells

Durability and cost are two barriers recognized for the extensive application of proton exchange membrane (PEM) fuel cell as a promising clean energy technology. To date, considerable effort has been made to study the performance and component degradation via diverse methods and for accelerated degradation, routines have been developed. For instance, operation at open circuit voltage (OCV) is one of the most frequently employed high potential stressors for a PEM fuel cells leading to Pt electrocatalyst dissolution and membrane decomposition. Single cells as well as short stacks were aged under different operation conditions which are discussed in the literature to accelerate degradation of electrocatalyst and membranes. An example of the aging of a commercial MEA under OCV for 1600 h leads to significant performance loss and membrane thinning. The loss of electrochemical surface area is evident from the impedance and the CVs. Also a decrease of membrane resistance is observed. For further characterization of aging inner interfaces and cross section of MEAs were investigated with material sensitive PeakForce Quantitative Nanomechanical Property Mapping (QNM™). Platinum particles could be unequivocally identified at the inner interfaces and cross sections. With scanning electron microscopy (SEM) and energy dispersive element analysis (EDX) of cross sections platinum bands at the cathode, but in some cases also at the anode side, were found as well as a significant platinum content within the whole membrane. The influence of platinum inside the membrane on the membrane degradation is discussed. Also the different aging mechanisms associated to the operation conditions are elaborated. A general conclusion from these measurement is that Pt dissolution plays an important – perhaps even the crucial – role for the loss of electrochemical surface area and membrane decomposition. To demonstrate this aspect, membranes were first impregnated with Pt ions which were reduced and the fluoride emission rate was measured