Nitrogen-doped magnetic onion-like carbon as support for Pt particles in a hybrid cathode catalyst for fuelcells

Abstract

Pt and non-precious metal catalysts were combined to build a hybrid cathode for fuel cell application, with the aim of dramatically reducing the amount of Pt and increasing the overall catalytic performance. An active nitrogen-doped magnetic onion-like graphitic carbon material (N-Me-C) was synthesized by pyrolyzing a hexamethylene diamine-Me (Me: Co and Fe) complex. The N-Me-C materials proved capable of effectively catalyzing the oxygen reduction reaction (ORR), as evidenced by rotating disk/ring electrode (RDE/RRDE) data showing significant positive shifts of onset and half-wave (E_½) potentials and a drop of H₂O₂ yield, when compared to traditional carbon supporting materials. In the hybrid cathode catalyst, ultra-low loading Pt nanoparticles (2 wt%) were subsequently anchored to the N-Me-C support through a chemical reduction method. The configuration using ultra-low Pt loading is advantageous for mitigating particle agglomeration and improving Pt utilization due to isolated particle distributions and smaller particle sizes. Electrochemical and fuel cell data confirmed that the use of the N-Me-C support leads to a significant enhancement of ORR catalytic activity. It is quite significant that the 2% Pt/N-Me-C cathode with an ultra-low Pt loading of 0.04 mg_-Pt cm⁻² is effective in generating a current density of ca. 0.14 and 0.59 A cm⁻² at cell voltages of 0.80 and 0.65 V operated in a H₂-air cell, respectively. The corresponding mass activity (A mg_-Pt⁻¹) was increased by factors of 1.4 and 3.5 at 0.65 V, when compared with 2% Pt/C and commercial E-TEK 20% Pt/C cathodes. Extensive physical and electrochemical characterization revealed that the significant improvement in mass activity is mainly attributable to the non-precious ORR active sites on M-Me-C, and also partially to the beneficial support effect of nitrogen doping associated with stronger support-metal interactions and smaller particle sizes.