Magnetic Purification of PGM-Free Catalysts
Authors: Yin Xi , Martinez Ulises , Chung Hoon T , Lin Ling , Zelenay Piotr 
Published: February 2020

To date, Pt-based catalysts have remained the sole, though expensive, solution in electrocatalysis of the sluggish oxygen reduction reaction (ORR) at the hydrogen fuel cell cathode.1 Recently, platinum group metal-free (PGM-free) materials have drawn attention as promising low cost replacement for Pt-based ORR catalysts.2 A prevailing approach in the synthesis of highly active PGM-free catalysts is based on a two-stage heat treatment of precursors containing C, N and transition metals.2,3,4 Studies suggest that Fe-N moieties formed during the heat treatment may play critical role as active sites.5 However, this state-of-the-art synthesis approach involves the acid-leaching of excess non-active iron-rich phases, which inevitably damages some of the active sites. A prolonged second heat treatment at high temperature (700°C-1000°C) is essential to recover the lost active sites. This procedure not only increases the chemical demands and cost to produce ORR PGM-free catalysts, but, more importantly, may not lead to full recovery of all the active sites initially formed. More economical and efficient methods are in demand to purify the PGM-free catalysts.

In this presentation, we demonstrate an approach to magnetically purify active PGM-free catalysts while preserving the initial density of active sites. During the process of magnetic purification, the pyrolysis products are subjected to shear mixing in inert solvent to break down the agglomerations. The aggregates containing ferromagnetic phases, such as iron and iron carbides, are extracted efficiently in the magnetic field of a permanent magnet. The recovered active carbon-rich phase is collected and further characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy to understand the chemical and physical properties of the active phase. Our results show that the ferromagnetic metal-rich phases are efficiently removed, leaving behind carbon-based phases. Electrochemical characterization and fuel cell testing show that the magnetically purified catalyst outperforms catalysts prepared with the conventional two-stage heat treatment. The nature of active sites in the purified carbon phases are also studied with X-ray photoelectron spectroscopy and discussed. This innovative approach enables the green and economical purification of active non-precious metal ORR catalysts, without compromising their catalytic performance.

Acknowledgement

Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

References

Spendelow, J.; Marcinkoski, J., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2014, (2014).

Wu, G.; Zelenay, P., Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res.
46, 1878-1889 (2013).

Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science
332, 443-447 (2011).

Zhao, D.; Shui, J.-L.; Grabstanowicz, L. R.; Chen, C.; Commet, S. M.; Xu, T.; Lu, J.; Liu, D.-J., Highly Efficient Non-Precious Metal Electrocatalysts Prepared from One-Pot Synthesized Zeolitic Imidazolate Frameworks. Adv. Mater.
26, 1093-1097 (2014).

Holby, E. F.; Wu, G.; Zelenay, P.; Taylor, C. D., Structure of Fe–Nx–C Defects in Oxygen Reduction Reaction Catalysts from First-Principles Modeling. J. Phys. Chem. C
118, 14388-14393 (2014).