Layered PGM-Free Electrode for Improved Mass Transport

Abstract

Increased interest in platinum group metal-free (PGM-free) catalysts as a low-cost replacement for PGM catalysts for the oxygen reduction reaction (ORR) has resulted in significant progress in activity and durability over the past decade (1-4). However, PGM-free catalysts still require high catalyst loadings to generate an acceptable level of performance due to their low active site density and turnover frequency. The thick catalyst layers suffer from both high ionic resistance and gas-transport limitations. Consequently, improvements to the morphology of PGM-free electrodes is as important to enhancing the ORR performance as is catalytic activity. Previously, Yin et al. were able to improve PGM-free electrode performance by developing a new fabrication technique for PGM-free membrane electrode assemblies (MEAs) and by optimizing ionomer loading and equivalent weight (5). Previous studies by Komini Babu et al. suggested that more substantial advancements in fuel cell performance could be accomplished by improving water management in the conventional PGM-free electrode (6, 7). Conventionally thick PGM-free electrodes suffer from mass transport resistance and challenging water management, limiting high catalyst utilization to only near the polymer membrane (6). Therefore, in order to increase catalyst utilization improved water management can be achieved by decreasing hydrophilicity in the catalyst layer (CL) near the microporous layer (MPL), which can be achieved by reducing the ionomer content or by replacing the low equivalent weight (EW) ionomer with a high-EW. In this work, we introduce PGM-free cathodes with improved high-current performance (mass transport-controlled range of current densities) by depositing two catalyst layers with varied ionomer loading (IL) or equivalent weight (EW) in order to decrease hydrophobicity near the MPL. For example, the EW effect was studied by preparing a MEA with two cathode layers consisting of 55 wt.% IL with EW of 1100 near the MPL and EW of 830 near the membrane. A different MEA with varying IL layers was prepared using 830 EW ionomer with 55wt. % IL near the membrane and 35 wt.% IL near the MPL. Our preliminary results (Figure 1) show that the MEA with the layered PGM-free cathode demonstrated improved performance when compared to the conventional single layer MEA, with the highest performance observed from the IL layered MEA. Acknowledgments This research was supported by DOE-EERE’s Fuel Cell Technologies Office (FCTO) through Electrocatalysis Consortium (ElectroCat). References G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 332, 443 (2011).H. T. Chung, D. A. Cullen, D. Higgins, B. T. Sneed, E. F. Holby, K. L. More and P. Zelenay, 484, 479 (2017).U. Martinez, S. Komini Babu, E. F. Holby, H. T. Chung, X. Yin and P. Zelenay, Advanced Materials, 1806545 (2019).U. Martinez, S. K. Babu, E. F. Holby and P. Zelenay, Current Opinion in Electrochemistry (2018).X. Yin, L. Lin, H. T. Chung, S. Komini Babu, U. Martinez, G. M. Purdy and P. Zelenay, ECS Transactions, 77, 1273 (2017).S. K. Babu, H. T. Chung, P. Zelenay and S. Litster, Journal of The Electrochemical Society, 164, F1037 (2017).S. Komini Babu, H. T. Chung, P. Zelenay and S. Litster, ACS Applied Materials & Interfaces, 8, acsami.6b08844 (2016). Figure 1