Electrocatalysts with Tailored Properties
Authors: Kang Yijin , Snyder Joshua David , Li Dongguo , Markovic Nenad M , Stamenkovic Vojislav R 
Published: February 2020

Hindering the global commercialization of polymer electrolyte membrane fuel cells are the limitations of the current technology to provide long operational lifetimes at a minimized cost. These issues can be tied, to a large degree, to the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) where significant quantities of precious metal based catalyst are required to produce optimal power. Ability to control structure at the atomic level can effectively tailor catalytic properties of materials, enabling enhancements in activity and durability. Intense research efforts are focused on development of efficient electrocatalysts with minimal amount of precious metal and low cost. Alloying Pt with non-noble metals was found to be effective approach in reducing the Pt content in electrocatalysts by increasing their intrinsic activity. It was demonstrated that the formation of a nano-segregated Pt(111)-Skin structure over a bulk single-crystal alloys could enhance the ORR activity (vs. Pt/C) by two orders of magnitude through altered electronic structure of Pt surface atoms. In spite of large improvements, these materials cannot be part of electrochemical devices but their outstanding catalytic performance can serve to guide the research aimed to development of more active nanoscale materials that offer high surface area utilization. The effect of nanoparticle size, surface mophology, composition and near-surface composition, shape, architecture will be thorougly discussed.

The main emphasis will be on the caged, hollow nanoframes that offer a new direction in the catalyst development and great promise to meeting the performance goals. The hollow interior greatly diminishes buried non-functional precious metal atoms, and their uncommon geometry provides a pathway for tailoring physical and chemical properties. The open structure of the Pt3Ni nanoframes address some of the major design criteria for advanced nanoscale electrocatalysts, namely, high surface-to-volume ratio, 3D surface molecular accessibility, and optimal precious metal utilization. The approach presented here for the structural evolution of a bimetallic nanostructure from solid polyhedra to hollow highly crystalline nanoframes with controlled size, structure and composition can be readily applied to other multimetallic electrocatalysts.

In adition, we further push the evolution of nanoscale ORR electrocatalyst design where the use of targeted deposition, for both well-defined thin-films with layered structures and monodisperse nanocrystals with well-controlled core-shell structures, of strategically selected alloying components yields a multilayered electrocatalyst with enhanced activity and durability as well as optimized precious metals utilization. Through careful Au doping of Pt alloy catalysts we have determined: 1) sublayer Au atoms can effectively stabilize Pt-alloy ORR electrocatalysts without sacrificing activity by adjusting the thermodynamics of subsurface oxygen induced place-exchange, whereas surface Au atoms, while also enhancing durability, deactivate Pt-alloy electrocatalysts through a blocking of active sites and promotion of 2-electron rather than 4-electron ORR; 2) transition metal core with a Pt-alloy shell effectively reduces the overall amount of electrochemically inactive precious metal content while maintaining high ORR activity.

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