Durable Hybrid Electrocatalysts for Proton Exchange Membrane Fuel Cells
Authors: Xiao Fei , Xu Gui-Liang , Sun Cheng-Jun , Hwang Inhui , Xu Mingjie , WU Hsiwen , Wei Zidong , Pan Xiaoqing , Amine Khalil , Shao Minhua 
Published: December 2020

Developing transition metal and nitrogen co-doped carbon (Me-N-C) catalysts with comparable ORR activity is a feasible approach to addressing the material cost issue of proton exchange membrane fuel cells (PEMFCs).1-3 However, the low durability of Me-N-C hinders their practical application. It was found that the half-wave potential of Fe-N-C catalysts decreased significantly (16-30 mV) only after 10000 cycles between 0.6 and 1.0 V in acidic electrolytes,4-5 and above 50% current density loss only after 20 h at 0.4 V in sing cell tests.6-7

Herein, we rationally designed a hybrid Pt-Fe-N-C electrocatalyst consisting of abundant Pt and Fe single atoms homogeneously dispersed on the nitrogen-doped carbon support and a small amount of Pt-Fe alloy nanoparticles. The distributions of single atoms and morphologies of carbon framework were obtained with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The oxidation state and chemical bond information was characterized by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The optimized Pt-Fe-N-C consisting of 0.21wt.% Pt and 2.4 wt.% Fe demonstrated unprecedented durability upon potential cycling with no change in the half-wave potential after 70000 cycles between 0.6 and 1.0 V (Figure 1a). The higher durability was also confirmed in the fuel cell testing. As shown in Figure 1b, after keeping the voltage at 0.4 V for 85 h, the current density of the fuel cell using Pt-Fe-N-C as the cathode material only dropped by 20%. For comparison, the current density dropped by more than 50% after only 23 h for the fuel cell using Fe-N-C as the cathode material.

This work demonstrates the feasibility of improving the durability of Fe-N-C material via ultra-low Pt doping. In addition, it highlights the importance of the synergy effect between Pt and Fe single atoms and the strong interaction between active sites and carbon support, which may shed light on improving the durability of non-precious metal catalysts via a hybrid structure.

Figure 1. (a) Steady-state polarization curves of oxygen reduction reaction activity Pt-Fe-N-C after 70000 cycles in 0.1 M HClO4 electrolyte. The catalyst loading is 0.51 mg cm-2. (b) Long-term durability test of Pt-Fe-N-C and Fe-N-C catalyst at 0.4 V (1 bar H2/1 bar air; anode Pt loading: 0.1 mg cm-2; cathode catalyst loading: 3.2 mg cm-2; cell temperature 80 °C; 100% RH).

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Figure 1