To efficiently and extensively utilize hydrogen for transportation and stationary power generation, the development of low-cost and efficient proton exchange membrane fuel cells (PEMFC) is essential.¹ Platinum (Pt) is used as catalyst in PEMFC due to its high performance, but its high cost has been one of the major barriers to the extensive use of PEMFC systems for transportation. In particular, high loadings of Pt are required at the PEMFC cathode due to the sluggish kinetics of the oxygen reduction reaction (ORR).² One of the strategies adopted to overcome this barrier, is the development of low-cost platinum group metal (PGM)-free catalysts.³ Both the ORR activity and durability of PGM-free catalysts has improved considerably in recent years,⁴ but the mass activity of these materials remains much lower compared to Pt-based catalysts, requiring the use of higher catalyst loadings on the electrode. As a direct consequence, typical PGM-free catalyst layers (CL) are about 1 order of magnitude thicker than Pt-based ones (~100 µm vs. ~10 µm), creating more challenging conditions for transport of O₂ and H⁺ to the active sites and removal of liquid water within the CL.⁵

A series of in-situ electrochemical diagnostics methods to measure the mass transport resistance in PGM-fee CLs based on H₂ and O₂ limiting currents have been developed within the DOE-sponsored ElectroCat consortium.^4,6 We will show the application of these methods, in conjunction to other well-established in-situ and ex-situ characterizations (cyclic voltammetry, impedance spectroscopy, SEM, X-ray tomography), to explain the performance trend observed in different PGM-free CLs. We examined the impact of different CL fabrication variables like the ionomer-to-catalyst (I/C) ratio, the ink solvent composition, and the ionomer equivalent weight (EW), evidencing the ones providing harsher conditions for mass transport. The results show the importance achieving optimal transport conditions by selecting a proper combination of these fabrication parameters.^7,8

With the aim of improving mass transport and ionic conductivity and expanding the CL operational robustness over a broader range of operating conditions, we developed an innovative electrode architecture having differentiated and ordered domains.⁹ In particular, we designed a CL divided into alternated catalyst and void domains (grooves). We investigated the fabrication of the groovy CL using different methods and tested the performance under different relative humidity conditions. The results show how the groovy CL structure provides performance enhancements compared to a traditional planar CL in conditions more challenging for mass transport, e.g., at high relative humidity and for electrodes prepared with high I/C and low EW ionomer. In addition, we demonstrated that filling the grooves with a material more hydrophobic than the main catalyst domain (e.g., catalyst mixed with ionomer with high EW and low I/C, or carbon mixed with PTFE) we can largely expand the operational robustness in oversaturated conditions.

References

1. D. A. Cullen et al., Nat. Energy (2021).
2. D. Banham et al., Sci. Adv., 4, 1–7 (2018).
3. L. Osmieri et al., Curr. Opin. Electrochem., 25, 100627 (2020).
4. P. Zelenay and D. J. Myers, DOE Annual Merit Review - ElectroCat 2.0 (Electrocatalysis Consortium) (2021).
5. L. Osmieri and Q. Meyer, Curr. Opin. Electrochem., 31, 100847 (2021).
6. A. G. Star, G. Wang, S. Medina, S. Pylypenko, and K. C. Neyerlin, J. Power Sources, 450, 227655 (2020).
7. L. Osmieri et al., Nano Energy, 75, 104943 (2020).
8. G. Wang, L. Osmieri, A. G. Star, J. Pfeilsticker, and K. C. Neyerlin, J. Electrochem. Soc., 167, 044519 (2020).
9. J. S. Spendelow, DOE Annual Merit Review - Accessible PGM-free Catalysts and Electrodes (2021).