Zinc-Derived Microporous Structure in Non-Precious Metal Catalysts for Polymer Electrolyte Fuel Cell Cathodes
Authors: Lin Ling , Chung Hoon T , Yin Xi , Martinez Ulises , Zelenay Piotr 
Published: February 2020

The kinetically sluggish oxygen reduction reaction (ORR) at the cathode of polymer electrolyte membrane fuel cells (PEFCs) requires higher content of platinum at the cathode than the facile hydrogen oxidation at the anode. Due to the scarcity and high cost of platinum, the development of inexpensive platinum group metal-free (PGM-free) ORR catalyst is one of the most urgent needs in the PEFC field. Transition metal-nitrogen-carbon (M-N-C) catalysts obtained by heat-treating transition metals, nitrogen, and carbon precursors have been viewed as the promising PGM-free catalysts for PEFC cathodes though not yet meeting all the performance characteristics required. High ORR activity, excellent mass-transport properties and practical long-term durability must be achieved simultaneously for PGM-free catalyst to replace platinum-based catalysts in the PEFC system.
Specific catalyst precursors and heat-treatment conditions are critical to obtaining PGM-free catalysts with the desired structure. Iron- and cobalt-based zeolitic imidazolate frameworks (ZIFs), with uniformly distributed transition metals coordinated by N-containing ligands, have been viewed as highly suitable precursors for ORR catalysts. However, catalysts derived from these ZIFs often have low surface area and porosity, which are not conducive to efficient mass transport. On the contrary, ZIF-8, a zinc based ZIF, yields carbon with high surface area and porosity. Dodelet et al. used ZIF-8 as a microporous host for Fe and N precursors [1]. Following a heat-treatment at a relatively high temperature of 1050 ºC, they obtained an ORR catalyst with enhanced fuel cell performance. Unlike Fe and Co in Fe- and Co-based ZIFs, zinc in ZIF-8 is volatile during high-temperature treatment, likely acting as pore-forming agent. Base on that assumption, we used in this work a zinc salt instead of zinc ZIF as a precursor responsible for the formation of micropores in the catalyst. By using high heat-treatment temperature we also achieved more corrosion-resistant catalysts, while increasing the porosity and specific surface area from 315 m2 g-1 to 910 m2 g-1due to Zn evaporation.
Fe-CM-PANI(Zn) catalyst was synthesized using the two nitrogen-precursor (cyanamide and PANI) approach, developed previously by LANL[2]. ZnCl2 was mixed with nitrogen precursors and heat-treated at 1000 ºC to ensure complete removal of Zn. The resulting CM-PANI-Fe(Zn) catalyst had much higher surface area than the catalyst obtained without Zn under the same conditions. The half-wave potential (E½
) for CM-PANI-Fe(Zn) in RDE testing in acidic electrolyte, 0.79 V vs. RHE, was similar to that obtained with Zn-free CM-PANI-Fe heat-treated at 900 ºC (optimal heat-treatment temperature for the Zn-free system), but by 0.16 V higher than E½
measured with the Zn-free catalyst heat-treated at 1000 ºC (Figure 1). While no immediate increase in the activity can be demonstrated with CM-PANI-Fe(Zn), this catalyst promises to have much improved stability thanks to being treated at a much higher temperature. Electrochemical and fuel cell testing of the Zn-derived catalyst, focusing in particular on the durability, is underway, as is the catalyst structure optimization. The results will be present at the meeting.

References

[1] E. Proietti, F. Jaouen1, M. Lefèvre, N. Larouche1, J. Tian, J. Herranz J.-P. Dodelet. Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells. Nat. Commun. 2011, 2, 416.
[2] P. Zelenay. Non-Precious Metal Fuel Cell Cathodes: Catalyst Development and Electrode Structure Design. 2014 Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation Meeting (2014) available online at: http://www.hydrogen.energy.gov/pdfs/review14/fc107_zelenay_2014_o.pdf

Figure 1