Controlling and optimizing activity/stability balance in morphologically complex electrocatalytic materials

Yawei Li

doi:10.17918/00000101

The development of efficient and stable catalysts for the cathodic oxygen reduction reaction (ORR) is one of the greatest challenges facing commercialization of polymer electrolyte membrane (PEM) fuel cells. To date, Pt-based electrocatalysts exhibit the best combination of activity and stability. Considerable efforts have been devoted to improving Pt-based catalysts through the use of transition metal alloying components and the design of unique three-dimensional nanomaterials to improve Pt mass-based utilization. Using nanoporous NiPt alloy nanoparticles (np-NiPt) as a representative three-dimensional electrocatalyst morphology, we have studied the unique mechanisms of degradation under relevant PEMFC operational conditions and proposed degradation mitigating strategies, affording a better balance between activity and stability. Using a combination of in-situ and ex-situ experimental techniques, we tracked sources instability of np-NiPt including surface oxide induced solubilization, anodic corrosion, and coarsening. In contrast to solid nanoparticles, where the modes of active area loss include anodic corrosion and agglomeration, morphologically complex three-dimensional, porous nanostructures can also degrade through a process termed coarsening. As a consequence of the high radius of curvature features and the presence of both positive and negative curvature, chemical potential gradients drive diffusion of material from areas of positive curvature to areas of negative curvature in order to reduce total surface free energy. With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we conclude that the dominant degradation mechanism for three-dimensional, porous electrocatalysts is coarsening. However, in the electrochemical context, coarsening is not purely driven by surface diffusion driven, rather it is governed by a dissolution/redeposition process. With this new insight, we have developed two strategies to mitigate coarsening and improve catalyst stability: (1) surface decoration with slow moving species to limit step-edge motion and diffusional coarsening, and (2) hydrophobic ionic liquid (IL) thin films to exclude water from the surface and lower the degree of Pt oxidation/dissolution. With these strategies we will show how more detailed insight into the atomic processes that govern electrocatalytic material instability can begin to break the inverse correlation between activity and durability. For the composite catalysts incorporating IL thin films, we have also assessed the impact of the hydrophobic IL on ORR catalytic activity and reaction mechanism. We demonstrate that the IL acts to disrupt the interfacial water structure and change the degree of solvation/stabilization of adsorbed intermediates, potentially limiting the scaling behavior associated with different oxygenate adsorbates. Additionally, the impact of IL to limit the deleterious effects of Nafion, a common catalyst layer ionomer in PEMFCs, including specific adsorption of charged groups, is also investigated. We have applied the insight derived from the study of platinum group metal (PGM) materials to the assessment and optimization of the activity/stability balance in non-PGM systems. Through this work we have demonstrated the inadequacy of singular activity and durability descriptor metrics. Here we present a systemic assessment of the compositional dependent HER activity and stability for Co-based mixed chalcogen, CoS_xSe_[2-x], pyrite transition metal dichalcogenides (TMDs). Through this study, we find that the hydrogen adsorption free energy ([delta]GHad), as a singular activity descriptor, is insufficient to fully characterize the activity of non-PGM materials. This observed compositional trend in HER activity can be explained by the unique combination of compositional dependent [delta]GHad and bulk resistivity/conductivity of the pyrite TMDs. Through further stability tests under constant potential HER electrolysis, Se-rich Co-based pyrite TMDs are found to be more durable than S-rich samples. Therefore, with an HER activity matching that of CoS₂, but with a dramatic improvement in stability, CoSe₂ breaks away from the traditional inverse activity/stability relationship and represents a promising material for non-PGM HER electrocatalysis in acidic based PEM electrolyzers. The thesis outlines a fundamental investigation into the underlying physical and chemical processes that define the mechanisms of morphological and compositional evolution in electrocatalytic nanostructures and provides insight and useful strategies for controlling and optimizing the activity/stability balance in electrocatalytic materials.

Controlling and optimizing activity/stability balance in morphologically complex electrocatalytic materials

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Controlling and optimizing activity/stability balance in morphologically complex electrocatalytic materials

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