Highly Active and Stable ORR Electrocatalysts with Ultra-Low Content of Platinum Derived from Metal-Organic Framework

Proton exchange membrane fuel cells (PEMFCs) have emerged as promising candidates for the next generation of energy sources due to their pollution-free power generation, high energy density, and abundant hydrogen fuel supply. However, the sluggish kinetics of the oxygen reduction reaction (ORR) necessitates the use of platinum (Pt) catalysts, significantly increase the overall cost of PEMFC stacks. To enhance the efficiency and cost competitiveness of PEMFCs, it is crucial to develop highly active and durable ORR electrocatalysts containing ultra-low amount of Pt.<br/>In this study, ultra-low Pt-cobalt (Co) alloy catalysts were synthesized using core-shell Zeolitic Imidazolate Frameworks (ZIFs). The internal Zn-ZIF (ZIF-8) facilitated the formation of a hollow structure during carbonization, significantly enhancing the porosity of the catalyst. The Co derived from the external Co-ZIF (ZIF-67) was alloyed with 5 wt% Pt, resulting in the high activity of the catalyst. Additionally, the catalyst exhibited a graphitic structure due to the catalyst-induced graphitization of Co during the carbonization and alloying process, thereby increasing its durability.<br/>In a half-cell system, the catalyst achieved a half-wave potential of 0.916 V and a mass activity of 0.191 A/mg_pt, with the half-wave potential decreasing by only 12 mV after 100,000 cycles. Remarkably, in a single-cell system, the MEA employing this catalyst with an ultralow Pt loading of 0.04 mg/cm² demonstrated a high peak power density of 1.380 W/cm² and a mass activity of 0.197 A/mg_pt. Furthermore, the MEA retained 56% of its initial mass activity, with a peak power density of 0.623 W/cm² after 30,000 square wave cycles based on the protocol of the Department of Energy. Therefore, we believe that the ultra-low loaded Pt-Co alloy catalyst derived from core-shell ZIF-8@ZIF-67 promotes the interaction with oxygen molecules and lowers the activation energy barrier for the ORR, making it a promising solution for more efficient and cost-effective PEMFCs.