Electrocatalyst for oxygen reduction reaction (ORR) has been an area of intensive research for decades because of both its fundamental complexity and practical importance for many applications, including polymer electrolyte membrane fuel cells (PEMFCs) and metal-air batteries. The ORR activity and electrocatalyst durability are two confronted difficulties in the research. Pt remains the most effective element for ORR in acidic electrolyte for both reaction kinetics and durability considerations. Because of a high Pt price and its insufficient activity, extensive studies have been devoted to decreasing the Pt usage by searching new Pt nanostructures with improved ORR property. The discovery that single crystalline Pt₃Ni (111) can exhibit exceptionally high ORR activity (18 mA/cm² Pt at 0.9 V), which is about 90 times the activity of commercial Pt/C, points to an ideal strategy for solving the problem by using octahedral Pt alloy nanoparticles, which are enclosed by the (111) planes and have large specific active area. Several research groups have made good progresses by synthesizing these shaped particles and reporting high ORR activity. However there are two challenges remained with the octahedral Pt alloy nanocatalyst strategy: (1) The developed wet synthetic methods have low scaling-up capacity and surface contamination issue and are thus not suitable for mass production of the shaped Pt alloy nanoparticles, and (2) Currently researched octahedral Pt alloy nanoparticles have insufficient durability.

We realize scalable and surfactant-free production of octahedral Pt alloy nanoparticle catalysts by developing a facile solid-state chemistry method. The prepared octahedral Pt_1.5Ni/C exhibited high ORR activities of 3.99 mA/cm² Pt and 1.96 A/mg Pt at 0.90 V vs. RHE, which are about 20 and 10 times the values for commercial Pt/C specifically.^[1] The prepared octahedral Pt₂CuNi/C uniform alloy nanoparticle catalyst exhibited both excellent ORR mass activity of 2.35 A/mg Pt at 0.90 V vs. RHE and promising stability, with 81.3% and 68.1% retained activity after 4,000 and 10,000 cycles of durability test.^[2] The values make the octahedral Pt₂CuNi/C a significantly more stable catalyst than the octahedral Pt-Ni/C, which retained only 52.9% and 35.0% of the initial mass activity after the two tests. The much improved durability was attributed to a uniform alloy composition of the ternary nanoparticles, which slowed down the leaching process of Ni and Cu and helped to preserve the octahedral morphology. The finding provides insights on factors determining the particle stability and offers guidance on development of highly active and durable ORR catalyst for real applications.

References

[1] C. L. Zhang, S. Y. Hwang, A. Trout, Z. M. Peng, Solid-state chemistry-enabled scalable production of octahedral Pt-Ni alloy electrocatalyst for oxygen reduction reaction, Journal of the American Chemical Society 2014, 136, 7805-7808.

[2] C. L. Zhang, W. Sandorf, Z. M. Peng, Octahedral Pt2CuNi Uniform Alloy Nanoparticle Catalyst with High Activity and Promising Stability for Oxygen Reduction Reaction, ACS Catalysis, 2015, 5, 2296-2300.