Oxygen reduction reaction (ORR) electrocatalysts with high activity, extended durability and lower cost are needed for proton exchange membrane fuel cells (PEMFCs). Carbon-supported catalysts result in significant catalyst degradation at high potentials, and therefore carbon-free structures are of interest to provide improved durability. We have recently developed bimetallic two-dimensional (2D) nanoframes as an approach to translate 2D materials into self-supported, carbon-free electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts.¹ Bimetallic Pt-M (M=Ni, Co) 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated M(OH)₂ nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-M alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The Pt lattice distances within the Pt-M alloy were significantly smaller compared to those of Pt nanoparticles. Based on the unique local and extended structure, the ORR specific activity of Pt-Ni 2D nanoframes (5.8 mA cm_Pt^-2) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 V_RHE showed that metallic Pt-Ni 2D nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Different temperature/atmosphere treatments were determined to significantly alter the atomic-level structure and influence the electrochemical activity. Additional efforts are aimed at understanding the effect of altering the transition metal and treatment conditions on the structure and ORR activity and stability. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of carbon-free nanoarchitectures that function as high activity and stability oxygen reduction electrocatalysts for PEM fuel cells and other applications.

References

1. Godínez-Salomón, F.; Mendoza-Cruz, R; Arellano-Jimenez, M.J., Jose-Yacaman, M.; Rhodes, C.P; Metallic Two-dimensional Nanoframes: Design of Carbon-free Hierarchical Nickel-Platinum Alloy Electrocatalyst Nanoarchitecture with Enhanced Oxygen Reduction Activity and Stability, ACS Applied Materials & Interfaces, 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043