Structural and transport properties of ultrathin perfluorosulfonic acid ionomer film in proton exchange membrane fuel cell catalyst layer: A review

To explore the effects of solvent–ionomer interactions in catalyst inks on the structure and performance of Cu catalyst layers (CLs) for CO2 electrolysis, we used a “like for like” rationale to select acetone and methanol as dispersion solvents with a distinct affinity for the ionomer backbone or sulfonated ionic heads, respectively, of the perfluorinated sulfonic acid (PFSA) ionomer Aquivion. First, we characterized the morphology and wettability of Aquivion films drop-cast from acetone- and methanol-based inks on flat Cu foils and glassy carbons. On a flat surface, the ionomer films cast from the Aquivion and acetone mixture were more continuous and hydrophobic than films cast from methanol-based inks. Our study’s second stage compared the performance of Cu nanoparticle CLs prepared with acetone and methanol on gas diffusion electrodes (GDEs) in a flow cell electrolyzer. The effects of the ionomer–solvent interaction led to a more uniform and flooding-tolerant GDE when acetone was the dispersion solvent (acetone-CL) than when we used methanol (methanol-CL). As a result, acetone-CL yielded a higher selectivity for CO2 electrolysis to C2+ products at high current density, up to 25% greater than methanol-CL at 500 mA cm–2. Ethylene was the primary product for both CLs, with a Faradaic efficiency for ethylene of 47.4 ± 4.0% on the acetone-CL and that of 37.6 ± 5.5% on the methanol-CL at a current density of 300 mA cm–2. We attribute the enhanced C2+ selectivity of the acetone-CL to this electrode’s better resistance to electrolyte flooding, with zero seepage observed at tested current densities. Our findings reveal the critical role of solvent–ionomer interaction in determining the film structure and hydrophobicity, providing new insights into the CL design for enhanced multicarbon production in high current densities in CO2 electrolysis processes.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Effect of Dispersing Solvents for an Ionomer on the Performance of Copper Catalyst Layers for CO₂ Electrolysis to Multicarbon Products

Idros,

Wu,

Duignan

et al. 2023

“…However, ionomer intrusion into the small nanopore <4 nm would not be natural because the ionomer reportedly formed rod-like micelles with respective lengths and diameters of 20–100 and 3–5 nm in solution. , Notably, the σ of the ionomer referred to the bulk value of the membrane . Recently, ionomer thin films with thicknesses of less than several tens of nanometers exhibited lower σ values than that of the bulk ionomer. − Consequently, the σ of an ionomer coating thinner than the size of a typical Pt/C particle (several tens of nanometers) should naturally be lower than that of the bulk. As ionomer intrusion and bulklike conductivity may be unreasonable in a catalyst layer, a novel model, with a reduced σ and without nanopore-intruding ionomer, is necessary.…”

Section: Introductionmentioning

confidence: 99%

Equation Elucidating the Catalyst-Layer Proton Conductivity in a Polymer Electrolyte Fuel Cell Based on the Ionomer Distribution Determined Using Small-Angle Neutron Scattering

Harada,

Kadoura,

Takata

et al. 2023

The performance of a polymer electrolyte fuel cell can be enhanced by improving the proton conductivity of the catalyst layer, where the oxygen reduction reaction generates electrochemical power. Protons are conducted through the ionomer coatings on catalyst-supporting carbon particles, which form porous structures that facilitate oxygen diffusion during the reaction within the catalyst layer. Therefore, while a higher ionomer content in the catalyst layer is favorable, the proton conductivity is additionally governed by the type of carbon support. As the influence of the ionomer distribution is not fully understood, we introduce a novel proton conductivity model for use in simulating catalyst layers with various amounts of ionomers and different carbon types. This proton conductivity model considers that several ionomers occur as thin films with drastically suppressed proton conductivities. Although evaluating the thin-film ionomer fraction is challenging, proton-conducting ion clusters in thick-film ionomers have been detected by characterizing the catalyst layers via small-angle neutron scattering. Our model reveals that reducing the fraction of the thin-film ionomer or avoiding factors that suppress its proton conduction improves the performance of the catalyst layer.

“…It was found that the short-side-chain (SSC) PFSA, such as Solvay-D790, enabled higher mass transport and proton conduction networks because it tends to be tiled on the surface of the catalyst. The higher proton conductivity and oxygen permeability lead to higher catalyst utilization compared with long-side-chain (LSC) ionomers. − Several research groups succeeded in decreasing the local O 2 transport resistance using highly oxygen-permeable ionomers. − For example, a recent study by Toyota Central R&D Labs reported performance enhancement via incorporation, in the cathode catalyst layers, of a ring-structured backbone matrix into ionomer . Although this method reduced O 2 transport resistance in the ionomer film effectively, its proton conductivity and stability have not been fully studied.…”

Section: Introductionmentioning

confidence: 99%

Micromodification of the Catalyst Layer by CO to Increase Pt Utilization for Proton-Exchange Membrane Fuel Cells

Wen

Banham

et al. 2022

Improving the utilization of platinum in proton-exchange membrane (PEM) fuel cells is critical to reducing their cost. In the past decade, numerous Pt-based oxygen reduction reaction catalysts with high specific and mass activities have been developed. However, the high activities are mostly achieved in rotating disk electrode (RDE) measurement and have rarely been accomplished at the membrane electrode assembly (MEA) level. The failure of these direct translations from RDE to MEA has been well documented with several key reasons having been previously identified. One of them is the resistance caused by complex mass transport pathways in the MEA. Herein, we improve the proton and oxygen transportations in the MEA by building a thin and uniform distribution of ionomer on the catalyst surface. As a result, a PEM fuel cell design is capable of showing a current density improvement of 38% at the same voltage (0.6 V) under the H 2 /air operation.