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The design of catalysts for Polymer Electrolyte Membrane Fuel Cells is guided by two important principles, improving specific catalytic activity and increasing lifetime [1]. Over the past 3 decades, there was a tremendous effort to understand at the atomic and molecular levels the key parameters that control the catalytic activity of the interface for the oxygen reduction reaction (ORR), the limiting reaction for high performance devices. Understanding the functional links between structure-activity relationships from Pt single crystalline surfaces to PtNi alloy single crystals established the importance of controlling structure, composition in both bulk and particularly at the surface, leading to real catalytic reactivity enhancement of almost 2 orders of magnitude [2]. These studies paved the way to synthesis of advanced nanostructures with well-defined parameters such as particle size, distribution, and alloy composition, culminating with PtNi nanoframes, displaying one of the highest specific activities measured for the ORR [3]. While there are room for improvement with increasing precision in nanostructure synthesis, the corresponding understanding of durability of Pt-based is far less developed. One of the limitations is that measuring durability has always been linked to a particular experimental protocol, leading to the development of accelerated stress tests (AST). These protocols measure the changes in surface area, as well as catalytic activity as a function of cycle number, but provide very little information about the mechanism of degradation. Transmission Electron Microscopy helped shed light on particle size evolution as a function of cycle number, where small particles (ca. 3nm) often coalesce and grow to large sizes, ranging from 5 to 7nm [4]. Carbon corrosion can also contribute to Pt degradation especially under start-stop conditions. However, only recently after the development of an in situ that is sensitive enough to measure rates of Pt dissolution as a function of electrode potential we were able to begin establishing structure-stability relationships that goes beyond simple changes to surface area. In this talk, we will present how the stability trends observed from well-defined single crystal Pt surfaces can help understand durability of nanoparticle systems. First, we reveal how the mechanism of Pt-oxide induced dissolution is highly sensitive on surface structure, where dissolution from (110) surface is almost 10 times higher than on (111) surfaces [5]. The effect anions and other species present at the interface is also relevant for stability of Pt surface atoms. Beyond Pt dissolution, we discuss the effects of redeposition processes that can occur after fast changes in electrode potential, leading to a clear pathway to understand the kinetics of nanoparticle evolution. Further comparison between the rates of Pt dissolution measured from nanoparticles to extended surfaces (single crystal and thin films), helped elucidate the importance of controlling particle size, distribution and support loading. Overall, establishing the functional link between structure-stability can provide a deeper understanding on how to control the durability of fuel cell catalysts, together with advances in catalytic activity that can bring fuel cell technology one step closer to a cost–effective widespread commercialization. [1] Stamekovic, V.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M., Nature Materials, 16, 57-69, 2017 [2] Stamenkovic, V. et al., Science 315, 493-497, 2007 [3] Chen, C. et al., Science 343, 1339-1343, 2014 [4] Borup, R. et al., Chemical Reviews, 107, 3904-3951, 2007 [5] Lopes, P. P. et al., ACS Catalysis, 6, 2536-2544, 2016 |
