Tailoring Ionomer Chemistry for Improved Oxygen Transport in the Cathode Catalyst Layer of Proton Exchange Membrane Fuel Cells

Abstract

Reducing the local oxygen transport resistance in the cathode catalyst layer of the proton exchange membrane fuel cell (PEMFC) is crucial for the improvement of cell performance, particularly for low platinum loading. In this work, we report that the performance of PEMFC can be significantly improved by replacing Nafion with a highly oxygen-permeable perfluoronated ionomer containing dioxole segments on the backbone as both proton conductor and catalyst binder in the cathode catalyst layer, as evidenced by the increased peak power density of 100 mW cm −2 . Molecular dynamics simulation results reveal that the synthesized ionomer has higher diffusion coefficient and higher oxygen solubility compared with Nafion. Benefiting from the high oxygen permeability, both the specific activity and mass specific activity of a Pt/C catalyst using the designed ionomer as catalyst binder for the oxygen reduction reaction are three times higher than those of a catalyst using Nafion as catalyst binder. The results demonstrate that the rational design of highly oxygen-permeable ionomers applied in the catalyst layer could be an effective strategy for a highperformance PEMFC with low platinum loading.

Effects of Humidity and Produced Water on Specific Adsorption of High Oxygen Permeability Ionomers Composed Entirely of Cyclic Monomers on Cathode Performance for Polymer Electrolyte Fuel Cells

“…The unchanged R DM for both is due to the GDL and MPL, which significantly impact molecular diffusion, being common to both. Consistent with other HOPI studies, the HOPI showed a marked decrease in R other , approximately half that of Nafion. − For the first time, we separated R other into R CL,gas and R CL,ion in the HOPI, finding that both resistances decreased to approximately half of those in Nafion. The decrease in R CL,ion can be attributed to the ionomer’s bulky molecular structure increasing gas permeability and reducing the formation of high-density layers on the Pt surface, as reported in other HOPI studies.…”

“…They also proposed the concept that it would be possible to decrease the amount of Pt used while maintaining the same level of cell performance. , Shimizu et al demonstrated that the application of HOPI not only improves power generation performance but also affects the oxygen diffusion in the CL and the durability of the catalyst, depending on the IEC, providing guidelines for the design of CLs using HOPIs . Recent research has proposed several specific monomer examples that increase the oxygen permeability of the ionomer, and they are effective for improving cell performance by copolymerization of monomers that can introduce a cyclic structure into the main chain of the polymer, such as 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (MDO), perfluoro­(2-methylene-4-methyl-1,3-dioxolane) (MMD), and perfluoro­(2,2-dimethyl-1,3-dioxole) (PDD). − This is thought to be due to the inhibition of crystallization that can occur with the packing of the main chain, as seen in Nafion, and the increase in oxygen solubility due to the decrease in polymer density and increase in free volume, caused by the steric hindrance around the main chain by the cyclic structure. , …”

A Numerical Assessment of Mitigation Strategies to Reduce Local Oxygen and Proton Transport Resistances in Polymer Electrolyte Fuel Cells

“…Nevertheless, it is worth noting that achieving a remarkable high performance may only be possible if enhanced diffusion is accompanied by a proper CL design in terms of high porosity and ionomer morphology. The increase in oxygen diffusivity shown in previous work is typical of order unity, so a disruptive increase in oxygen diffusivity may not be possible by solely altering the chemical ionomer structure [54,[70][71][72][73].…”

Preparation of an Integrated Membrane Electrode Assembly for Proton Exchange Membrane Fuel Cells Using a Novel Wet-Bonding Strategy

“…As shown in Figure d and Table , the EIS curves of MEA-CCM and MEA-NR-Ca presented almost identical leftmost intersections with X -axis at the high-frequency zone, meaning similar R Ω values for these MEAs. , This indicates that direct depositions of both the ionomer on the CL and catalyst slurry on the PEM could produce an excellent CLs/PEM interfacial connection. However, the R ct of the MEA-CCM is 8.4% higher than that of the MEA-NR-Ca, suggesting better charge conduction in the MEA-NR-Ca, , which could be related to the slight permeation of ionomers into the CL, which facilitates proton conduction and maximizes the PEM/CL interface . The membrane layer, in its dispersed form, can more fully combine with the CL, resulting in sufficient adhesion that enhances the charge transport and catalytic activity in CL.…”