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

Carbon-Supported PtNi Nanocrystals for Alkaline Oxygen Reduction and Evolution Reactions: Electrochemical Activity and Durability upon Accelerated Stress Tests

International audiencePtNi is amongst the most active electrocatalyst for the oxygen reduction reaction, but its stability in operation is uncertain. Intuitively, alkaline environments lead to milder degradations than acidic ones, although carbon-supported Pt-group metal nanoparticles are particularly degraded even in dilute alkaline electrolytes. To date, PtNi catalysts durability has not been characterized for alkaline oxygen reduction and evolution reactions (ORR and OER). Herein, carbon-supported shape controlled PtNi catalysts were compared in terms of activity and durability during alkaline ORR and OER. The PtNi catalysts are shape-controlled Pt-rich alloy, Ni-rich alloy, and Pt core/Ni shell (Pt@Ni) synthesized on Vulcan XC72R carbon. Their morphology and composition were evaluated by identical-location transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction pre and post accelerated stress test. Compared to Pt/C and Ni/C benchmark catalysts, the core-shell and Ni-rich alloy catalysts gave high and stable OER activities. After accelerated stress test, the catalysts show two features which are believed to play a major role in the durability: a Ni-enrichment at the nanoparticles' surface and an improved attachment of the catalyst to the carbon support

Size dependent oxygen reduction and methanol oxidation reactions: catalytic activities of PtCu octahedral nanocrystals

The synthetic control through colloidal synthesis led to a remarkable increase in platinum mass activity in octahedral nanocrystals with a Pt-rich surface. In this manuscript, we demonstrate that the ratio of surfactant can tune the size of Pt surface enriched PtCu nano-octahedra from 8 to 18 nm with homogeneous size and shape on the carbon support. For the nano-octahedra, the Pt-rich surface has been determined by high-angle annular dark field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. The Pt-rich surface exhibits an increasing compressive strain with increasing surface of the {111} facets. With increasing surface, the PtCu nano-octahedra display higher oxygen reduction reaction (ORR) activity, which however leads to higher onset over-potentials in the methanol oxidation reaction (MOR) and CO-stripping. This observed trend for a series of size-selected nano-octahedra demonstrates the benefits of controlling the strained {111} Pt surface for the ORR and MOR activity

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