Influence of Ionomer Composition and Distribution on PEM Fuel Cell Performance of PGM-Free Catalyst

Abstract

One of the effective ways to reduce the cost of polymer electrolyte membrane fuel cells (PEMFCs) is to reduce the amount of platinum-group metal (PGM) catalysts used for the oxygen reduction reaction (ORR). Recently, highly active platinum group metal-free (PGM-free) catalysts were developed and applied for the ORR reaction [1-3]. Significant developments have been made on the chemistry of PGM-free catalysts and their kinetic performance [4-6]. However, despite all the progress, some challenges remain before they are commercially viable. Mass-transport within the very thick catalyst layer (CL) displays a key challenge for further improvement of the fuel cell performance of PGM-free catalysts. Understanding of the structure of CL including the distribution of active sites and the ionomer inside the CL play an important role in the development of stable PGM-free cathodes capable of high power density. In the present work, our objective has been to demonstrate the effects of the ionomer content, solvents, and equivalent weight (EW) on electrode performance. The structure of the ionomer (i.e., short-side-chain vs long-side chain ionomer) as well as the effect of the relative humidity (RH) have been considered. To increase the proton conductivity of the cathode and improve the catalysts’ effectiveness across its thickness, it is important to optimize the ionomer type and its EW. In this work, we used the nanoscale X-ray computed tomography (nano-CT) method to characterize the morphology and transport properties of PGM-free cathodes. Our results show the sensitivity of the cathode’s performance to the hydrophilicity of the ionomer films. We found that electrodes with a lower ionomer EW have a tendency to retain more water than electrodes with a higher EW. Thus, at high current density conditions where significant water is being generated, the lower EW ionomers showed reduced performances due to electrode flooding when using the same processing and ionomer loading. In general, higher MEA performance at 100% RH was achieved using more hydrophobic ionomers (higher EW) with a greater proportion of hydrophobic fluoropolymer backbone or by using ink formulations that enhanced the hydrophobicity of a particular ionomer.AcknowledgmentThis material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008076.