Effects of Porosity and Ionomer Composition on Fuel Cell Performance of PGM-Free ORR Catalysts
Authors: Yin Xi , Lin Ling , Chung Hoon T , Komini Babu Siddharth , Martinez Ulises , Purdy Geraldine M , Zelenay Piotr 
Published: February 2020

Recently, the carbon-based platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts have received intensive research interest as a low-cost alternative to Pt-based ORR catalysts.1-3 However, due to their relatively low specific activity the application of carbon-based PGM-free catalysts requires higher loadings than Pt-based materials, resulting in thick catalyst layers on the order of 100 µm.4,5 Such thick electrodes lead to the mass transport loss and strong dependence of the electrode performance on the ionomer content and catalytic layer porosity.4,6,7

In this work, we systematically studied the effect of electrode porosity, equivalent weight (EW) and content of proton-conductive ionomers on the fuel cell performance of PGM-free catalysts, utilizing our previously reported highly porous CM-PANI-Fe-C(Zn) ORR catalyst.8 Thanks to the high intrinsic porosity and high surface area of > 1000 m2/g due to the use of the pore forming effect of cyanamide and ZnCl2, this catalyst showed high activity in electrochemical cell test, with the half-wave potential of 0.81 V vs. RHE. In order to maximize the fuel cell performance, the electrode porosity and the ionomer content were further optimized in this work. The porosity of electrode was controlled through the hot-pressing conditions and gasket thickness during membrane electrode assembly (MEA) fabrication and fuel cell assembly. Electrodes with fully optimized porosity showed significantly improved fuel cell performance due to facilitated mass transport. Such electrodes were then used to study the effect of EW and ionomer content (25 wt%, 35 wt%, 45 wt%, and 55 wt%). The studied perfluorosulfonic acid ionomers were Nafion® (EW of 1100 g/mol SO3H), Aquivon® D83 (EW of 830 g/mol SO3H), and Aquivon® D72 (EW of 700 g/mol SO3H). It was found that found an increase in the ionomer content from 25% to 55% resulted in improved performance in the kinetic region of the fuel cell polarization plot, while negatively affecting performance in the mass transport region. Among the tested ionomers, Aquivon® D83 (EW of 830 g/mol) provided the most “balanced” performance of the MEA. The results are summarized in the form of 2D matrices to guide the design of PGM-free catalyst electrode further. Cathodes with optimized porosity, ionomer content, and E.W. allowed for reaching current densities of > 115 mA/cm2 and > 550 mA/cm2 at 0.80 V and 0.65 V, respectively, as well as the maximum power density of 530 mW/cm2. All fuel cell testing was performed at 1.0 bar pressure of H2 and air at 80°C. These results demonstrate the critical effect of electrode design on the performance fuel cell with PGM-free catalysts.

Acknowledgements

Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. The author would like to thank Dr. Natalia Macauley and Dr. Tommy Rockward at LANL for helpful discussion. X. Y. and L. L. contributed equally to this work.

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