An Empirical Model for the Flooding Behavior of Ultra-thin PEM Fuel Cell Electrodes

Abstract

A quantitative empirical model is applied to explain the onset of cathode flooding behavior of PEM fuel cell membrane electrode assemblies having the ultra-thin 3M NSTF (nanostructured thin film) electrocatalysts and first generation Toray carbon paper gas diffusion layers (GDL). An empirical method is used to define the current density at the onset of cathode flooding, and calculated vapor phase water removal rates at specific water balance conditions are found to give excellent quantitative agreement with the measured rates of water generation for three different first generation GDL's. Effective water vapor diffusion coefficients porosites and saturation levels for the GDL's can be extracted from the water balance data at the in-situ operating conditions. Application of the method to later generation GDL's for which the flooding occus at much lower temperatures is also discussed, with the conclusion that liquid phase transport plays a greater role for these GDL's.

Engineering Catalyst Layers for Next‐Generation Polymer Electrolyte Fuel Cells: A Review of Design, Materials, and Methods

“…[ 209,255 ] NSTF electrodes have clearly shown that high current densities can be achieved using only water as the proton conductor. [ 50–52 ] However, since O 2 diffusion through water is slower than through air, excess localized liquid water can limit O 2 diffusion. Muzaffar et al.…”

“…[ 58,59 ] Thin, low‐Pt loaded electrodes have been reported to be more prone to water management problems. [ 50,51,239 ] Hence, it is suggested the O 2 transport limitations are caused by the extended water diffusion pathways to the surface of the catalyst. This hypothesis suggests that thin CLs are more prone to flooding, and as such, water management is a high priority for low Pt loading electrodes.…”

“…[ 199 ] The thin structure of NSTFs limits water vaporization and makes them susceptible to flooding at high current densities. [ 50,51 ] Due to their thin CL and dependence on high catalyst surface areas for high performance, NSTFs are typically highly engineered such as those shown as Figure 33 ; however, this does not guarantee high Pt utilization. For example, Deng et al.…”

“…The electrons are transported to the CL via electrical contact between support particles, while the protons are typically transported via the PFSA ionomer or localized liquid water, produced by high humidity operating conditions or as the byproduct of the ORR reaction. [ 50–52 ] The combination of these three reactants at what is known as the triple phase boundary (TPB) is required for the reaction to occur, as shown in Figure 2b. If a catalyst particle is not connected to any one of these three then the catalyst will not be utilized in the reaction.…”

Tipping water balance and the Pt loading effect in polymer electrolyte fuel cells: a model-based analysis

“…2 The 3M approach to MEA fabrication has been the most impressive demonstration to date that a drastic reduction of Pt loading, m pt , is achievable without sacrificing performance, as these MEAs indeed exhibited the highest power performance reported to date 2 . However liquid water removal clearly transpired as the Achilles heel of catalyst layer design 6 . Moreover, the 3M design demonstrated an important principle of catalyst layer operation: liquid water is sufficient as the medium for proton transport in the catalyst layer.…”