New Insights into Pt Dissolution Mechanisms from SFC-ICP-MS Measurements for Well-Defined Surfaces

Abstract

In the last years, polymer electrolyte membrane fuel cell (PEMFC) technology has progressed notably, allowing its extended implementation in the transport sector. For example, the fuel cell cars currently being commercialized use PEMFCs with Pt nanoparticles for the oxygen reduction reaction (ORR) at the cathode, primarily because of their better long-term stability in comparison to other electrocatalysts. However, even pure Pt catalysts degrade under real-life conditions, since a PEMFC is expected to resist hundreds of thousands of load cycles and tens of thousands of start-up/shut-down cycles during its lifetime, leading to a high number of platinum oxidation/reduction cycles and resulting in its extensive degradation.1 This degradation of Pt catalysts is mainly linked to electro-oxidation and dissolution processes, which have been investigated for a long time in polycrystalline and supported nanoparticle catalysts. More recently, investigations with Pt single crystal electrodes have been carried out, which offer the possibility of a more detailed understanding of these processes at the atomic level. These works include studies using in-situ surface X-ray diffraction (SXRD) and on-line inductively coupled mass spectrometry (ICP-MS), which have for example shown differences in the onset potential for anodic dissolution on Pt(100) and Pt(111) that have their origin in the different atomic structures of the initial oxide.2 The present work focuses on the dissolution behaviour of the well-defined surfaces of the three Pt basal planes investigated by scanning flow cell inductively coupled mass spectrometry (SFC-ICP-MS). Further investigations providing new information about the dissolution mechanisms are carried out by varying a wide variety of parameters: upper potential limit, scan rate, potential holding times, electrolyte composition, pH, purging gas and cooling atmosphere. Some of the results will be presented in combination with SXRD measurements in order to explain different aspects of the restructuring and dissolution mechanisms for these surfaces. For example, the dissolved amounts for Pt(111), Pt(100) and Pt(110) in 0.1 M HClO4 and 0.1 M H2SO4 after cyclic voltammetry at a scan rate of 0.05 V s-1 for three different upper potential limits, 1.20 V, 1.40 V and 1.60 V vs. RHE, were obtained. In all cases dissolution follows the order Pt(110) > Pt(100) > Pt(111), which is in agreement with the previous works performed in 0.1 M HClO4.3 Higher dissolution is always observed in the case of H2SO4 electrolyte. Potentiostatic hold experiments for Pt(100) have been carried out, allowing the separation of the anodic and cathodic dissolution peaks. It can be observed that the cathodic dissolution increases proportionally with the increase in the oxidation potential. The combination of this data with SXRD measurements suggests that there are specific species in the oxide structure at high potentials that are the responsible for the majority of the cathodic dissolution in the Pt(100) surface.References M. K. Debe, Nature, 486 (2012) 43-51.Fuchs, T.; Drnec, J.; Calle-Vallejo, F.; Stubb, N.; Sandbeck, D.; Ruge, M.; Cherevko. S.; Harrington, D. A.; Magnussen, O. M. Nature Catalysis 3 (2020) 754-761.Sandbeck, D. J. S.; Brummel, O.; Mayrhofer, K. J. J.; Stubb, N.; Libuda, J.; Katsounaros, I.; Cherevko. S. ChemPhysChem 20 (2019) 2997-3003