Electrode Degradation in Polymer Electrolyte Fuel Cells

To mitigate the climate crisis, and reduce carbon emissions from e.g. thetransport and energy sectors, hydrogen has been proposed to be used as anenvironmentally friendly alternative energy carrier. Proton exchange membranefuel cells (PEMFCs) use hydrogen as a fuel to create electricity with the onlybyproducts being water and heat, and are well suited as a power source for e.g.vehicles. However, for successful commercialisation of PEMFCs, some hurdlesneed to be overcome. In particular, lifetime is a limiting factor for PEMFCdue to harsh operational conditions. To improve lifetime, the mechanisms bywhich the materials in PEMFCs degrade must first be better understood.In this thesis, I present a study on the behaviour of Pt, which is currentlythe sate-of-the-art catalyst for PEMFC, during electrochemical procedures inliquid electrolytes, studied using electrochemical quartz crystal micro-balance(EQCM). Mass response and dissolution rates for Pt thin films were studiedin acid and alkaline environments. The Pt dissolution rate was found to besimilar in alkaline and acidic electrolyte when normalised to electrochemicalsurface area. Furthermore, I present identical location (IL) microscopy implemented ina real 5 cm2 single-cell fuel cell, to follow the degradation of Pt catalyst oncarbon support under realistic operation conditions. With both IL scanningelectron microscopy (IL-SEM) and IL transmission electron microscopy (ILTEM),I show that the degradation processes can be followed during differenttypes of ageing processes. IL-SEM show that the carbon support material isstable during normal fuel cell operation conditions, while the Pt particles grow.IL-TEM show similar result for the normal condition operation as seen withthe IL-SEM. However, during start-up/shutdown conditions, IL-TEM showthat the carbon support lose volume, and collapse on weak points, which bringsPt particles together, and promotes Pt particle growth. The developed ILtechniques presented in this thesis helps distinguish the degradation effects ofdifferent operation conditions and opens up for further testing of degradationprocesses under real fuel cell conditions

Impact of Accelerated Stress Tests on the Cathodic Catalytic Layer in a Proton Exchange Membrane (PEM) Fuel Cell Studied by Identical Location Scanning Electron Microscopy

Platinum is the most used electrocatalyst in proton exchange membrane fuel cells (PEMFCs). Nonetheless, it suffers from various types of degradation. Identical location electron microscopy has previously been used to observe local catalyst changes under accelerated stress tests (ASTs), giving insight into how individual catalyst particles degrade. However, it is important that such studies are carried out under relevant reaction conditions, as these can differ substantially between liquid half-cells and real PEMFC conditions. In this work, a single cell PEMFC was used to study the degradation of a commercial Pt-catalyzed membrane electrode assembly by performing square wave voltage ASTs in a potential range of 0.6 to 1.0 V. Identical location scanning electron microscopy (IL-SEM) was used to follow the degradation of the cathodic catalytic layer (CL) throughout 14,000 AST cycles. From the IL-SEM, we can conclude that the Pt nanoparticles degrade via Ostwald ripening, crystal migration, and coalescence. Small Pt nanoparticles agglomerate to larger particles or dissolve and redeposit to more stable particles, increasing the average particle size during the ASTs. In addition, cross-sectional TEM images show thinning of the ionomer layer during the AST procedure. The IL-SEM technique facilitates observation of local degradation of the CL in real PEMFCs, which will help to understand different degradation mechanisms, allowing for better solutions to be designed

Fuel cell electrode degradation followed by identical location transmission electron microscopy

Identical location transmission electron microscopy (IL-TEM) is a powerful technique that has previously been used to study degradation of catalyst materials for proton exchange membrane fuel cells (PEMFCs) in half-cell environments. Here, we demonstrate that IL-TEM can be used to follow degradation at the top of the catalytic Pt/C layer in a real PEMFC on the atomic scale under operation. We find that during an accelerated stress test (AST), mimicking normal operation, Pt nanoparticles grow mainly by Ostwald ripening, while the carbon support is stable. Under AST mimicking start-up/shutdown conditions, the carbon support degrades mainly by loss of volume and collapse, which forces the Pt nanoparticles closer, promoting additional particle growth. The observed degradation correlates with the measured decrease in electrochemical performance for the respective AST. The results show the feasibility of performing IL-TEM imaging in PEMFCs under real-operating conditions, opening up the possibility for similar studies in other fully operational systems

Comparison of Oxygen Adsorption and Platinum Dissolution in Acid and Alkaline Solutions Using Electrochemical Quartz Crystal Microbalance

Platinum (Pt) is a widely used electrocatalyst material in fuel cells and electrolysers. Proton exchange membrane (PEM) fuel cells and electrolysis operate under highly acidic conditions whereas the more recently developed anion exchange membrane (AEM) processes take place under alkaline conditions. Pt dissolution and Pt oxidation during operation and varying potentials has been studied mainly for the acidic PEM and less for the alkaline AEM. This study presents a comparison of Pt dissolution and Pt oxidation in 0.5 M H2SO4 and 1 M KOH using electrochemical quartz crystal microbalance (EQCM) on Pt thin films. Physical characterisation using electron microscopy and atomic force microscopy (AFM) revealed small, yet significant differences in the Pt film surface structure, which is related to differences in measured electrochemical surface area (ECSA). The mass increase from adsorption of oxygenated species and Pt oxidation is higher in alkaline conditions compared to in acid while dissolution of Pt is similar

Fundamental insight into enhanced activity of Pd/CeO2 thin films in hydrogen oxidation reaction in alkaline media

Palladium supported on ceria (Pd/CeO2) has recently raised strong interest as an alternative catalyst to platinum on the anode electrode in anion exchange membrane fuel cells. Herein, we provide new insight into the enhanced activity of Pd/CeO2 in hydrogen oxidation reaction (HOR) in alkaline media. Using well-defined model thin films, we show that Pd/CeO2 thin films lead to enhanced activity in HOR compared to pure Pd thin films. In situ characterization using electrochemical quartz crystal microbalance provide in-depth understanding of the role of CeO2. CeO2 leads to fundamental differences in adsorption and absorption of key reaction intermediates during HOR. In combination with characterization and theoretical calculations, Pd atoms embedded in CeO2 are shown to be present on the prepared thin films and active for hydrogen activation but are not able to bind CO during CO-stripping characterization. Finally, an estimation of the source of hydroxyl intermediates provided by CeO2 - which could be directly participating in the reaction - is presented herein. Fundamental understanding of the Pd-CeO2 interface in HOR opens new ways to reduce the amount of noble metals in alkaline fuel cells