Review—Wetting Phenomena in Catalyst Layers of PEM Fuel Cells: Novel Approaches for Modeling and Materials Research

Abstract

The development of high-performance polymer electrolyte fuel cells increasingly relies on modeling to optimally tune cathode catalyst layers (CCL) to desired properties. This includes models to rationalize the role of water as a promoter and asphyxiant to the oxygen reduction reaction. Existing models are able to reproduce or predict, using assumed parameters, the performance of the cell. However, consideration of the wetting properties of the composite has remained elusive. Experiments to characterize these properties are difficult to perform. There is thus a gap in theory for relating material choices with wetting properties. Here we elaborate on this gap and presents a novel conceptual approach to close it. Fundamental modeling approaches, molecular dynamics studies, and experimental works have shown that the interaction of ionomer with the Pt/C surface exerts a major impact on wetting behavior and water sorption properties of the porous CCL composite. In our approach, the state of molecular alignment of ionomer sidechains and backbones is linked to the structural characteristics of the Pt/C catalyst. From this rationalization, wetting properties of the CCL can be deduced. An analysis of these correlations supports a crucial hypothesis: lowering the platinum loading leaves the CCL more prone to flooding.

Ultrahigh Mass Activity Pt Entities Consisting of Pt Single atoms, Clusters, and Nanoparticles for Improved Hydrogen Evolution Reaction

“…It is worth noting that great attention must be paid to the interpretation of the absolute data obtained as hydrophilicity of Pt based carbon electrocatalysts in catalyst layers depends on multiple factors, such as morphology, surface composition, Pt loading and ionomer composition. [ 74 ] Thus, solely the trend of the hydrophilicity in the order of Pt/CNF > Pt/MPNC ≈ Pt/C is discussed in regard to electrochemical HER performance in an aqueous electrolyte.…”

Impacts of Pt/Carbon Black Catalyst Surface Hydrophilicity on Ionomer Distribution and Durability during Water-Generating Load Cycling of Polymer Electrolyte Fuel Cells

“…We focused on the catalyst surface hydrophilicity to evaluate the balance between functional groups and the Pt oxidation state. According to previous reports, the hydrophilicity of a material’s surface can be evaluated by the specific surface area detected by comparing two adsorption isotherms for adsorbents that can wet all surfaces (e.g., N 2 , argon) versus those that can wet selected surfaces (H 2 O). − Other systematic summaries, such as those using contact angles to assess the adsorption state of ionomer, also exist . We calculated the catalyst surface hydrophilicity using N 2 BET and H 2 O BET-specific surface areas obtained from the N 2 -pore volume (PV) and water vapor adsorption measurements, respectively.…”

Modification of Nafion's nanostructure for the water management of PEM fuel cells

“…The liquid water saturation level, , in the GDM and CL reduces the actual gas porosity thereby hindering the gas transport. The surface with the higher hydrophobicity can accelerate the capillary‐pressure‐driven liquid water drainage process and reduce the gaseous oxygen transport resistance in the gas pore direction 7 …”

“…The surface with the higher hydrophobicity can accelerate the capillary-pressure-driven liquid water drainage process and reduce the gaseous oxygen transport resistance in the gas pore direction. 7 The oxygen transport in the second direction, the normal direction of the ionomer layer surface, is affected by the accumulated water film covering the ionomer surface (i.e., gaseous oxygen has to dissolve and diffuse in the water film) and the oxygen solubility/diffusivity of the ionomer in the catalyst agglomerates. As shown in Figure 1B, the hydrophobic Nafion ionomer interface may create the water-film-free oxygen transport pathway from the gas pores to the active sites.…”