Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes

Abstract

Transport phenomena are key in controlling performance of electrochemical energy-conversion technologies and can be highly complex involving multiple length-scales and materials/phases.Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell (PEFC) electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions.We examine electrode overall gas-transport resistance and its components as a function of ionomer content and chemistry. Low equivalent-weight ionomers allow better dissolved-gas and proton transport due to greater water uptake and low crystallinity, but also cause significant interfacial resistance due to high density of sulfonic-acid groups. These effects of equivalent weight are also observed via in-situ ionic conductivity and CO displacement measurements. Of critical importance, the results are supported by ex-situ ellipsometry and x-ray scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Structural and resultant performance changes in the electrode are shown to occur above a threshold sulfonic-group loading highlighting the significance of ink-based interactions. Our findings and methodologies are applicable to a variety of solid-state energy-conversion devices and material designs.

Fully Hydrocarbon Membrane Electrode Assemblies for Proton Exchange Membrane Fuel Cells and Electrolyzers: An Engineering Perspective

“…The novel ionomer should have higher gas permeability and decent proton conductivity at low humidity but simultaneously should not have a too high interfacial resistance to the catalyst, which can be accounted to, for instance, the high anion concentration. [ 9 ] Requirements and targets for fuel cells and electrolyzers are discussed in the following sections.…”

“…To achieve these very ambitious goals, some "early stage" materials were combined to estimate a polarization curve (dashed green line) that theoretically can exceed the 2030 target value (solid green line). Those materials include increased catalytic active sites and intrinsic activity [5,6] and catalyst layers with mesoporous Pt/C from GM R&D. [7] The optimal electrode ionomer should provide high proton conductivity at high temperatures (>100 °C) and low humidity (<50% relative humidity (RH)), thus high ion exchange capacity (IEC) [8] without poisoning the catalyst [9,10] and finally a high oxygen permeability. [11,12] The gas diffusion layers should also be improved to significantly reduce the gas transport losses.…”

Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers

“…In sub-micrometer-thick films of conventional ionomers (such as Nafion), the phase segregation and ionic domain connectivity are sacrificed due to dimensional and interfacial constraints. ,− These cause a decrease in the size of ion-conducting domains to ∼1.5–2 nm in sub-micrometer-thick, hydrated Nafion films from ∼4 nm in corresponding bulk membranes . Based on the current understanding of ionomers, a common school of thought is that ion channels with a narrow diameter (≤2 nm) cause nanoconfinement effects on flow and negatively impact the proton conductivity. , However, the trends in proton conduction through narrow (1–2 nm) versus very narrow (sub-nanometer) ion channels are very thought-provoking .…”

Nonconformal Particles of Hyperbranched Sulfonated Phenylated Poly(phenylene) Ionomers as Proton-Conducting Pathways in Proton Exchange Membrane Fuel Cell Catalyst Layers

“…6,9,12,13 However, it has been reported that the ultrathin coating of the ionomer introduces an O 2 permeation barrier to catalyst particles within the primary pores and on the surface of the secondary pore structure, a primary factor in the added mass transport resistances that cannot be explained by the agglomerate model alone. 14 As a result, there has been intense research effort in understanding the O 2 permeation through thin ionomer films, 15−17 the effect of the chemical nature of the ionomer, 14,18 the impact of porosity of the support, 19 and the influence of catalyst ink composition on mass transport limitations within the CL. 20,21 Despite intense research surrounding PFSA ionomers, there are growing concerns over the use of potentially hazardous chemical feedstocks, and the complex synthesis results in only a handful of chemical manufacturing companies having the capability to produce them, adding to the expense of PFSAs.…”

“…These are critical in facilitating the diffusion of gases within and the removal of water from the CL, as described by agglomerate models. , Polyelectrolytes, such as the typically used perfluorosolfonic acid (PFSA) ionomer Nafion, are also included in the ink dispersion to encapsulate and bind the Pt/C agglomerates, providing proton conductive pathways throughout the entire CL. ,,, However, it has been reported that the ultrathin coating of the ionomer introduces an O 2 permeation barrier to catalyst particles within the primary pores and on the surface of the secondary pore structure, a primary factor in the added mass transport resistances that cannot be explained by the agglomerate model alone . As a result, there has been intense research effort in understanding the O 2 permeation through thin ionomer films, − the effect of the chemical nature of the ionomer, , the impact of porosity of the support, and the influence of catalyst ink composition on mass transport limitations within the CL. , …”