Durable Mn-Based PGM-Free Electrodes for Polymer Electrolyte Membrane Fuel Cells
Authors: Yang Fan , Spinetta Magali , Zhong Sichen , Chen Mengjie , Wu Gang , Xu Hui 
Published: February 2020

The development of low cost and high performance cathode for the oxygen reduction reaction (ORR) remains a grand challenge for transportation applications of polymer electrolyte membrane fuel cells (PEMFCs). Fe and Co-based platinum group metal (PGM)-free catalysts have been previously studied, however facing poor long-term durability and may potentially accelerate membrane degradation caused by oxygen radicals1. To overcome the above weakness, Li et al.2 has developed Mn based PGM-free catalyst which can mitigate membrane degradation, meanwhile showing a competitive catalyst activity compared to Fe based catalyst. The activity of the Mn-N-C catalysts measured using rotating disk electrodes (RDEs) in acidic electrolytes approaches state-of-the-art Fe-N-C catalysts3. More importantly, the Mn-N-C catalysts have demonstrated enhanced stability using potential cycling (0.6-1.0 V) in O2-saturated acidic electrolytes.
In addition to the Mn-based PGM-free catalyst development, the electrode design is also critical for the MEA performance. The oxygen reduction reaction (ORR) may occur at different interface in the catalyst layers for PGM and PGM-free catalysts. For PGM catalysts, the ORR takes place on the Pt/ionomer interface, while Pt is on the surface of the catalyst support. However, since the Mn active sites are embedded inside the metal organic framework (MOF), it is more difficult to establish the catalyst/ionomer interface PGM-free catalyst. Hereby, the ionomer with lower equivalent weight (EW=830) has been proposed to replace the conventional Nafion ionomer (EW=1100). The shorter chain of the low EW ionomer can penetrate into the micro-pores in the catalyst layer easier, which may promote their interaction with the catalyst active sites. In addition to the low EW ionomer, the ionomer to carbon (I/C) ratio also has a great impact on the proton, gas and water transports that can further affect the MEA performance. The particle size of the catalyst also affects the MEA performance. In the RDE studies, the Mn catalyst with a smaller average particle size (50 nm) shows a better ORR activity than the one with a larger average particle size (80nm). However, in the MEA studies, catalysts with larger average particle size seems to demonstrate better performance, likely due to optimal pore structures of electrodes.
This work will provide a systematical understanding and guidance of the MEA design not only for Mn-based catalyst but also for other PGM-free catalysts.

Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0008075.
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