In the development of efficient electrocatalysts for proton exchange membrane fuel cells (PEMFCs), catalyst stability is too often overshadowed by the pursuit of higher and higher activities. With evolving complexity of three-dimensional, nanoporous catalytic nanomaterials which exhibit unique and unusual catalytic properties, it is important to find an optimal balance between activity and stability. In addition to the standard mechanisms of electrocatalyst degradation including Pt dissolution/Ostwald ripening and agglomeration, nanoporous materials can lose electrochemically active surface area (ECSA) through surface smoothening driven by coarsening, the tendency to reduce the overall surface free energy, leading to the evolution of particle morphology from a porous to a nonporous structure. Here we have studied the morphological and compositional evolution, through a combination of in-situ and ex-situ accelerated degradation testing (ADT), of a unique nanoscale catalyst composed of nanoporous PtNi nanoparticles formed by the dealloying of PtNi<sub>3</sub> precursor nanoparticles. With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we have developed strategies to mitigate coarsening and improve catalyst stability through slowing electrochemically enhanced surface diffusion and limiting step edge movement. Taking advantage of the free volume within the nanoporous nanoparticles, we have created a composite catalyst architecture through the incorporation of a hydrophobic, protic ionic liquid (IL), specifically [MTBD][beti]. In addition to the improved oxygen reduction reaction (ORR) kinetics, the interfacial IL structure minimize anion/metal interaction and limit the potential dependent metal/ion complex formation, impairing electrochemically enhanced surface diffusion. Another approach limits morphology evolution by impeding step edge movement through the use of foreign metal and/or metal-oxide adsorbates on the surface. For example, with a lower intrinsic rate of surface diffusion, Ir was used as the foreign impurity species to decorate step edges resulting in decreased morphological evolution and transition metal loss. The ORR activity, ECSA and residual transition metal content of these complex nanoporous electrocatalysts was tracked as a function of cycle number during ADT providing insight into the mechanism of catalyst degradation and the effect of the different approaches on catalyst stability.

Morphological and compositional instability in nanoporous nanoparticle electrocatalysts for oxygen reduction reaction

253rd National Meeting (2017)

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