Polymer electrolyte membrane fuel cells (PEMFCs) are energy conversion devices that change chemical energy into electric energy. PEMFCs have high power density and efficiency wiht no pollution due to hydrogen as a fuel [1]. However, their high cost and low stability hindered PEMFCs from full-commercialization. This issue is originated from large amount of noble platinum catalyst in the cathode electrode to overcome the sluggish oxygen reduction reaction (ORR) [2]. To increase ORR activity with small amount of Pt, Pt-M alloy (M: Fe, Ni, Co, etc) catalysts have been studied over the past years. Particularly, Pt-M intermetallic catalysts showed significant enhancement of ORR activity and stability because of their abundant active sites and strong interactions between Pt and metal [3].

In this report, Pt₃Mn intermetallic nanoparticles on carbon support (Pt₃Mn/C) for ORR in PEMFCs was produced by using a facile synthesis. By using commercial Pt/C and Mn precursor, Pt₃Mn/C catalyst could be easily synthesized. Total synthesis can be separated into 2 steps; first, Pt/C was sonicated in hexane, and Mn(acac)₂, HDD, diphenyl ether was mixed and heated for 30 min. at 250 ^oC. Secondly, As-synthesized Pt₃Mn/C was heat-treated under 700 ^oC for 4 hours in H₂/Ar atmosphere to form Pt₃Mn intermetallic nanoparticles.

Pt₃Mn intermetallic catalyst demonstrated a high mass activity compared to commercial Pt/C. While initial mass activity of Pt/C was 0.125 A/mg_Pt, Pt₃Mn/C showed 0.386 A/mg_Pt, which was more than 3 times higher activity. Furthermore, Pt₃Mn/C not only exhibited a high initial ORR activity, but also showed a great stability in a voltage-cycling test. After accelerated degradation test for 10k cycles, mass activity of Pt/C was reduced into 0.028 A/mg_Pt (77.6 % decrease), when Pt₃Mn/C showed 0.276 A/mg_Pt (28.5% decrease).

Additionally, Pt₃Mn intermetallic catalyst showed a fast kinetic in alcohol oxidation reactions compared to commercial Pt/C. In methanol oxidation reaction, mass activity of Pt/C was about 0.10 A/mg_Pt, when Pt_­3Mn/C showed 0.15 A/mg_Pt, which was 1.5 times higher. In ethanol oxidation reaction, Pt/C showed 0.035A/mg_Pt, when Pt₃Mn/C showed 0.070 A/mg_Pt, which was 2 times higher than Pt/C.

Further details of Pt₃Mn intermetallic catalysts was characterized by adopting XAS analysis and DFT calculations to verify their high catalytic activity and stability. Through XAFS data, heat-treated Pt₃Mn intermetallic nanoparticle showed higher coordination number, which implies its higher ordering than as-synthesized Pt₃Mn. From XANES analysis, stability of catalysts can be compared. DFT calculation was also performed to compare ORR dissociative mechanism between Pt₃Mn(111) and Pt(111). While ORR spontaneously occurs Pt₃Mn(111) under potential of 0.87 V, Pt(111) needs potential of 0.53 V, which indicates high ORR activity of Pt₃Mn.

Finally, we demonstrated opportunity of utilizing in actual applications by the single-cell test. These results verified that intermetallic Pt₃Mn/C catalyst can be used as an active and stable ORR catalyst for PEMFCs. Catalyst loading was 0.15 mg_Pt/cm² for both anode and cathode. Initial performance of MEA with Pt₃Mn/C cathode was 551 mA/cm², when Pt/C showed 420 mA/cm² @ 0.7 V. After long term durability test of each MEA, single cell performance of Pt₃Mn catalyst decreased into 486 mA/cm², which was 88% of initial performance. However, Pt/C catalyst decreased into 308 mA/cm², which was 73% of initial performance, larger degradation than Pt₃Mn/C.

References

[1] S. K. Oh, T. H. Cho, M. J. Kim, J. H. Lim, K. S. Eom, D. H. Kim, E. A. Cho, H. S. Kwon, International Journal of Hydrogen Energy (2017)

[2] M. J. Kim, C. R. Kwon, K. S. Eom, J. H. Kim, E. A. Cho, Scientific Reports, 7, 44411 (2017)

[3] C. W. Jung, C. S. Lee, K. H. Bang, J. H. Lim, H. I. Lee, H. J. Rye, E. A. Cho, H. M. Lee, ACS Appl. Mater. Interfaces, 9 (37), 31806-31815 (2017)