Rational Design of Nitrogen‐Doped Low Pt-Loading Electrocatalysts for Fuel Cells: Binary to High-Entropy Alloys and Intermetallics
Authors: Sasaki Kotaro , Zhao Xueru 
Published: December 2024

PtM alloy electrocatalysts (M = Fe, Co, Ni, Cu) have been the subject of many investigations aimed at increasing their attractive properties, in particular, their oxygen reduction reaction (ORR) activity. Despite some success, these catalysts still have relatively high Pt content, unsatisfactory ORR activity, and lack the necessary durability as M metals leach out from the alloys during potential cycling. To alleviate these drawbacks, the BNL team has employed the approach to dope nitrogen (N) in PtM alloy catalysts for the last decade.1-4 The PtMN/C catalysts synthesized by nitriding the core metals through annealing in a flowing NH3 gas exhibited much higher ORR activity and stability compared to those of PtM alloy counterparts. On the other hand, PtM nanoparticles with ordered intermetallic structures are generally more stable against chemical oxidation/dissolution than solid-solution structures. We synthesized a N-doped L10-ordered intermetallic PtNiN core−shell catalyst (Int-PtNiN/C),5 and the result demonstrated that the strategy of combining N-doping and structure ordering can effectively improve the ORR performance of the PtM system in operating fuel cells.6 The Int-PtNiN/C catalyst has been evaluated for heavy-duty vehicle (HDV) applications in the Million Mile Fuel Cell Truck (M2FCT) consortium formed by DOE HFTO. During the development of Int-PtNiN/C in the M2FCT, we found that a new synthesis method using high-pressure nitriding (HPN) could further improve the durability of the catalyst (HPN-Int-PtNiN/C), which met the target under the M2FCT 90K AST protocols for HDV applications.
Recently, we extended our strategy to multi-elemental systems; under the DOE L’Innovator project, we applied the N-doping and structural ordering to a quaternary PtCoNiCr system. The resultant Int-PtCoNiCrN/C catalyst showed much improved ORR activity and durability in MEA testing operating at 160ºC compared to those of commercial Pt catalysts, demonstrating that the N-doped intermetallic quaternary catalyst is promising for high-temperature (HT)-FC applications including aviation FC use. We also synthesized a N-doped high-entropy alloy (HEA) catalyst, which comprises a Pt-rich shell and a N-doped PtCoFeNiCu core on a carbon support (donated N-Pt/HEA/C).7 The N-Pt/HEA/C showed high mass activity and excellent stability in RDE and MEA testing, which could be rationalized by the formation of stable multiple M-N bonds coupled with the high-entropy effect. In the paper, we will address the recent development of HPN-Int-PtNiN/C, Int-PtCoNiCrN/C, and N-Pt/HEA/C catalysts and discuss the origins of enhanced electrocatalytic performance based on the results of operando X-ray absorption spectroscopy and scattering measurements, S/TEM/EDS analysis, and theoretical calculations.
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