Understanding of Regeneration Processes of Ruthenium-Poisoned ORR Catalysts

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PEM fuel cells, that are operated with a reformate gas (H2, CO2, CO), are of great interest in maritime applications. Despite its outstanding activity in the hydrogen oxidation reaction (HOR), platinum can be quickly poisoned by only very small amounts of CO. Therefore, high CO tolerant anode catalyst materials such as platinum (Pt)-ruthenium (Ru) alloy nanoparticles (NPs) supported on carbon (Pt-Ru/C) are used for PEMFCs supplied with reformate gas. (1) However, these Pt-Ru/C catalysts strongly suffer from the Ru dissolution followed by the crossover through the membrane to the cathode. At the cathode, the soluble Ruz+ species quickly re-deposit at the Pt/C catalyst, resulting in a dramatic loss of performance for the oxygen reduction reaction (ORR). For instance, the kinetics of the ORR decreases by a factor of 8, when the Ru coverage on the surface of Pt nanoparticles is in the range of 20 at.%. (2, 3) Very recently, we have reported different electrochemical regeneration strategies of Ru-poisoned cathode catalyst using rotating disc electrode (RDE) technique. (4) Although the poisoned catalyst could be mostly re-activated, the atomic processes during the regeneration are poorly understood to date. Therefore, operando and in-situ spectroscopic and microscopic techniques are needed.In this work, we combined the electrochemical regeneration protocols with in-situ XAS technique to better understand the re-dissolution processes of the RuOx species from the Pt-based catalyst particle surface.Our regeneration strategies can be categorized into dynamic and steady-state conditions. Each protocol involved an initial pre-activation process by holding the potential at 1.40 VRHE for 5 minutes in 0.1 M HClO4. The dynamic regeneration protocol was performed by potential pulsing between 1.40 VRHE and 1.60 VRHE in 10-second intervals using chronoamperometric method. Additionally, single pulse measurements at 1.60 VRHE with a duration time of 100 s and 300 s were applied for the steady-state regeneration protocol. The regeneration procedure plays an important role in the recovery of the Ru-poisoned ORR activity. The best results of a recovery procedure for PtRu2 and PtRu catalysts are obtained under steady-state condition at 1.60 VRHE for 100 s.The next step was to correlate the electrochemical regeneration experiments with in-situ XAS investigations on commercially available PtRu2 and PtRu catalysts. The Ru K edge and Pt L2/3 edges XAS measurements were carried out in a home-made electrochemical flow cell. Based on the in-situ XAS data, we were able to monitor the changes in the electronic structure and atomic environment of both elements in the alloy nanoparticles as a function of the recovery parameters (potential, holding time). In addition, ex-situ scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDX) investigations were carried out on Ru-poisoned catalysts after the regeneration protocol to collaborate with the results from XAS.In summary, XAS is a powerful technique to elucidate the electronic structure and atomic arrangement of Ru and Pt atoms and link them with the electrochemical parameters of the regeneration protocol.