The primary PEMFC catalyst layer architecture used since the early 1990’s is a porous dispersed Pt-supported carbon catalyst with recast solid ionomer component forming a continuous (percolating) network through the electrode for proton transport; the carbon facilitating the electrical conduction; the active electrochemical reactions conducted on the metallic Pt sites [1,2]. The thin catalyst layer consists of microscale carbon particles each supporting nanoscale platinum catalyst particles all loosely embedded in a matrix of ionomer creating a porous structure.

The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, provides both electrical and proton conductivity pathways. This has been traditionally referred to as the 3-phase boundary of an electrolyte, an electrode, and a gaseous fuel. In addition to the gas-reactant transport to the active site, the result product water must also be managed both in terms of product removal and optimal hydration of the membrane/ionomer providing the proton conductivity.

Various methods are used to produce these thin-layer-electrode films including decal transfer (in both H+ and Na+ forms), direct membrane application, catalyst-coated-membranes and catalyst-coated-gas-diffusion-media. However the resulting structures all must result in providing the 3-phase boundary regions for good performance.

As ubiquitous as the thin layer electrode structure has been used, it is still an active field of research to properly understand the structure and improve the reactant transport and catalyst utilization. A demonstration of the 3-phase boundary is shown in Figure 1 by high resolution TEM [3]. Platinum nanoparticles are observed attached to the carbon support with a thin layer of surrounding ionomer. However, this observed structure is not consistent through-out the electrode; regions where agglomerates of ionomer occur, including Pt nanoparticles separated from the carbon support. One consortia working to better understand and improve the catalyst layer structure is FC-PAD (Fuel Cell-Performance and Durability) consortia, comprised of five U.S. National Labs: Argonne, Berkeley, Los Alamos, Oak Ridge and National Renewable Energy Lab.

Recent results from the FC-PAD consortia include elemental mapping of the electrode layers (see Figure 2). This type of elemental mapping suggests an inhomogeneous distribution in the electrode layer. Such an inhomogeneous distribution suggests that catalyst utilization is not fully optimized. Additionally, the appropriate pore structure in the catalyst layer is also necessary for providing transport paths for reactants to reach reaction sites as well as allow water mobility throughout the catalyst layer. Various methods have also been utilized to map the electrode layer pore-structure including digitizing of microscopy data, MIP and BET analysis.

Because performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain 100% ionomer contact with the platinum nanoparticles to maximize catalyst utilization. In addition, the thickness of ionomer film over the catalyst nanoparticles should be optimal to facilitate gas diffusion and water balance without sacrificing its protonic conductivity. Partial ionomer contact renders a portion of the catalyst to be electrochemically inactive and can also limit proton conduction. However, increasing ionomer content (to increase contact with the catalyst) leads to a thicker ionomer film, which results in an increased gas and water diffusional barrier.

Some novel MEA fabrications various developers are exploring include:

• NSTF (Nano Structure Thin Film) catalyst and electrodes
• Catalyst support functionalization to control the catalyst/ionomer interface
• Stratified catalyst layers
• Ionomer “nanofiber” inclusion in the catalyst layer
• Ionomer-coated MWCNTs
• High temperature membrane electrode assemblies

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.

References

1. S. Wilson and S. Gottesfeld, Thin- film catalyst layers for polymer electrolyte fuel cell electrodes, J. Appl. Electrochem., 22, 1 (1992)
2. MS Wilson, S Gottesfeld, High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells, Journal of the Electrochemical Society, Electrochem. Soc. 1992.
3. Moore, R Borup, KS Reeves, ECS Trans. 3(1), 717-733 (2006)