Significant effort has been devoted to reduce the cathode platinum loading for proton exchange membrane fuel cell (PEMFC). Apart from advanced electrocatalysts, the role of ionomer properties, Pt nanoparticle dispersion and catalyst layer microstructures are of equal significance to achieve low Pt loading [1-3]. The polarization loss due to oxygen transport limitations on reduced Pt loading electrodes was reported to account for ~50% of the total loss at high current density operation [1]. Since ionomer (Nafion ®) is usually incorporated with the Pt/C catalyst, such loss has been attributed to the oxygen transport resistance in the interface between the ionomer and Pt [3,4]. Therefore, it is imperative to have a comprehensive understanding of the polarization behavior for the low-Pt-loading electrodes and reduce the polarization loss due to oxygen transport through optimizing the ionomer-to-carbon (I/C) ratio.

In this study, a systematic breakdown of six types of polarization sources [5] is presented to elaborate the effect of I/C ratios and carbon support on the cathode polarization behavior in air-breathing fuel cells. The sources of polarization is summarized in Table I. A novel electrode fabrication method, reactive spray deposition technology (RSDT), was employed to deposit low Pt loading (anode, 0.05 mg cm^-2; cathode, 0.1 mg cm^-2) catalyst-coated membranes (CCMs) using high-surface-area carbon black (KB) and multi-walled carbon nanotube (MWNT) as catalyst supports. This technique is unique in combining the catalyst synthesis and deposition into one step and allows for independent control of the catalyst, support and ionomer compositions in the electrode.

The analysis of the H₂/Air polarization distinguished three Tafel regions with different controlling processes for RSDT-derived cathode electrodes. The non-electrode concentration overpotential dominated in the region where the current densities were above 1000 mA cm^-2, while the cathode electrode concentration overpotential dominated the current densities from 100 to 1000 mA cm^-2. Below 100 mA cm^-2, the ORR overpotential following Tafel kinetics is dominant. Non-electrode concentration overpotential (η_corr3) is controlled by the molecular diffusion of oxygen in the gas diffusion layer (GDL) and mesopores in the catalyst layer. It was reduced with higher back pressure or higher pore volume in the pore range of 10 nm-100 nm. The cathode electrode concentration overpotential (η_corr4) is related to the oxygen transport in the micropores (Knudsen diffusion) and through ionomer thin film. The I/C ratio for optimal fuel cell performance is related to the lowest η_corr4. As Pt/KB cathodes exhibited higher microporosity than that of Pt/MWNT cathodes, the η_corr4 for Pt/KB was also higher. The ORR overpotential (η_corr5  and η_corr6) decreased with increasing I/C ratios for both Pt/KB and Pt/MWNT CCMs, suggesting an improved triple-phase boundary between Pt, carbon and ionomer with enhanced ionomer coverage on the carbon. As a result, the ORR activities at 0.9V_iR-freeincreased with I/C ratio.

Table I. List of fuel cell polarization sources and the technique of separation [5].

References

[1] A. Kongkanand, M.F. Mathias, J.Phys.Chem.Lett. 7 (2016) 1127-1137.

[2] S. Holdcroft, Chem. Mater. 26 (2014) 381-393.

[3] A.Z. Weber, A. Kusoglu, J.Mater.Chem.A. 2 (2014) 17207-17211.

[4] N. Nonoyama, S. Okazaki, A.Z. Weber, Y. Ikogi, T. Yoshida, Journal of The Electrochemical Society. 158 (2011) B416-B423.

[5] M.V. Williams, H.R. Kunz, J.M. Fenton, Journal of The Electrochemical Society. 152 (2005) A635-A644.