Effect of Ionomer Equivalent Weight and Side Chain Length on Catalyst Layer Transport Resistances

Abstract

Current state of the art Proton Exchange Membrane fuel cell cathode catalyst layer can be described as a complex, heterogeneous porous system with multiple components across various length scales at which both the electrochemical reaction and mass transport of reactants/products occur. The functional components in the catalyst layer consists of the catalyst nanoparticles (few nm in diameter) for electrochemical reaction, carbon support (few tens of nm to µm) for electron conduction, ionomer chains (few nm to µm) for proton transport and pore structure (few nm to few hundred nm) for gas transport. [1] There is a strong impetus to decrease the catalyst loading to minimize Pt material cost and ease the supply-demand scenario. While extensive research into the development of highly active Pt-alloy nanoparticle catalyst systems has led to tremendous improvements in the low current density kinetic performance region, the current state of the art cathode electrodes suffer from a poor high current density (HCD) performance at low Pt loadings which leads to an increase in stack area and cost. This brings into focus the need to fundamentally understand the functional role and microstructure-property relationships of the various components in the cathode catalyst layer towards mitigating the HCD mass transport losses at low precious metal loadings. [2] Recent studies have pointed to the presence of a local transport resistance in the cathode catalyst layer that affects high current density performance via i) decrease in O2 permeability through the ionomer thin film covering the Pt/C agglomerates, and/or ii) O2 transport within the Pt/C agglomerates. [2, 3] In this context, the chemical composition of the PFSA ionomer and its aggregate structure plays a key role in not only enabling the bulk proton transport in the catalyst layer but also in directly affecting the local O2 transport resistance through the ionomer thin film and at the catalyst-ionomer interface. Current state-of-the-art perfluorosulfonic acid (PFSA) ionomers that are composed of both hydrophobic fluoroethylene backbone and sulfonic acid end groups are understood to form a non-homogeneous thin film with varying thickness of few nanometers around the catalyst particles. The phase-segregated structure of these ionomers plays a key role enabling a high proton conductivity and O2 permeability through the film. There are two characteristic factors of PFSA ionomers that control ionomer phase segregation. These are i) side chain length/chemistry and ii) equivalent weight (EW). These two factors are related by the equation EW = 100m + MWside-chain, where m is the fluorocarbon backbone length (number of TFE units) and MWside-chain is the molecular weight of the side chain. For a given EW, ionomers with shorter side chain length (m) yield a higher backbone fraction. Similarly, for a given side-chain length, increasing EW indicates a higher backbone fraction. Hence, both the side-chain length/chemistry and the EW could be tuned to affect the backbone length of the ionomer and hence the phase separation behavior. [4] In this study, we have systematically examined the ex situ and in situ characteristics of a few select PFSA ionomers of varying side-chain length and EW. Ex situ analysis of the ionomer include x-ray scattering, size exclusion chromatography, ionic conductivity, and water uptake measurements of the thin films. In situ measurements include H2/air polarization curves under differential cell conditions and electrochemical diagnostics to quantify proton and local-O­2 transport resistances. Figure 1 shows a snapshot of H2/air polarization curves and local O2 transport resistances measured using PFSA ionomers of different chain lengths (short – SSC, medium – MSC and long – LSC) and EW. The two lower EW ionomers were found to significantly decrease the transport resistance and increase the high current density performance. The measured differential cell data will be used in a 1-D model to quantify the performance loss terms. The effects of ionomer EW and side-chain length on the catalyst layer transport resistance and the corresponding impact on the high current density performance will be discussed. References S. Holdcroft, Fuel Cell Catalyst Layers: A Polymer Science Perspective, Chem. Mater. 2014, 26 (1), pp 381-393A. Kongkanand, M. F. Mathias, The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells, J. Phys. Chem. Lett., 2016, 7 (7), pp 1127–1137S. Jomori et al., An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC, J. Electrochem. Soc. 2013, 160 (9), pp F1067-F1073A. Kusoglu, A. Weber, New Insights into Perfluorinated Sulfonic-Acid Ionomers, Chem. Rev., 2017, 117 (3), 987-1104 Figure 1