Catalyst layer ionomer structure

Another approach has been developed to fabricate electrodes with loading as low as 0.1 mg Pt/cm (32). The electrode structure was improved by increasing the contact area between the electrolyte and the platinum clusters. The advantages of this approach are that a thinner catalyst layer of 2 to 3 microns and a uniform mix of catalyst and ionomer are produced. For example, a cell with a Pt loading of 0.17 to 0.13 mg/cm has been fabricated. The cell generated 3 A/cm at > 0.4V on pressurized O2 and 0.65 V at 1 A/cm on pressurized air (32,... 

Ionomer in Catalyst Layers Structure Formation and Performance... 

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Antolini et al. [6] have provided empirical equations to calculate the optimal Nafion loading in the catalyst layer as a function of electrode structure. In the case of a catalyst layer containing Pt/C and Nafion ionomer, the optimal Nafion load (in mg/cm2) is expressed as... 

MEA performance is mainly limited by ORR kinetics, as well as oxygen transport to the cathode catalyst. Another major loss is due to proton conduction, in both the membrane and the cathode catalyst layer (CL). Characterization of the ionic resistance of fuel cell electrodes helps provide important information on electrode structure optimization, and quantification of the ionomer degradation in the electrodes [23],... 

Most recently, an alternative low-platinum-loading catalyst layer structure has been developed by Wilson at LANL. In this structure, recast ionomer is used instead of PTFE to bind the catalyst layer structure together, and the low-loading catalyst layer is applied to the membrane, rather than to the gas-diffusion structure (mode A3)... 

The theory was also used to explore novel design ideas. It was predicted that functionally graded layers with enhanced ionomer content near the membrane interface and correspondingly reduced ionomer content near the GDL side would result in better performances compared to standard CCLs with uniform composition [122]. This prediction was recently verified in experiment [123]. Correspondingly, fabricated catalyst layers with a three-sublayer structure result in enhancements of fuel cell voltage by 5-10%. 

The thin-film catalyst layers are typically cast from inks consisting of the supported Pt catalyst and solubilized ionomer [10]. Because the ionomer must bind the thin film structure together, special treatments of the recast films are necessary during fabrication to impart robustness to... 

Figure 2.1. Structure and composition of catalyst layers at three different scales At the nanoparticle level, anode and cathode processes are depicted, including possible anode poisoning by CO. At the agglomerate level, ionomer functions as binder and proton-conducting medium are indicated, and points with distinct electrochemical environments are shown (double- and triple-phase boundary). At the macroscopic scale, the interpenetrating percolating phases of ionomer, gas pores, and solid Pt/Carbon are shown, and the bimodal porous structure is indicated.

In Refs. [18,19], the macrohomogeneous theory was extended to include concepts of percolation theory. The resulting structure-based model correlates the performance of the CCL with the volumetric amounts of Pt, C, ionomer, and pores. A detailed review of macroscopic catalyst layer theory can be found in Ref. [17]. A further extension of this theory in Ref. [25] explores the key role of the CCL for the fuel cell water balance. This function is closely linked to the pore size distribution. Major principles of these models will be reproduced here. The details can be found in the literature cited. 

The structures of PFSA membranes have been analyzed and discussed by many researchers, and the cluster-network model for hydrated membranes proposed by Gierke [22] has been a basic model symbolic of the PFSA characteristics up to now. As for the structure of the diluted aqueous solution of PFSA, it is important to understand the structure of ionomer dispersion and catalyst ink, comprising catalyst particles, ionomer, and solvent, for the preparation of cast membrane and catalyst layer, respectively. Aldebert et al. 

Delamination of the MPL from the GDL substrate has not been widely reported but may occur during freeze-thaw cycles, as occurs with catalyst-layer delamination from the membrane [131, 132]. A different situation occurs in the GDL/MPL, where the pore diameters are on the order of a micron or larger and the water is not hydrating the sulfraiic acid of the ionomer. The volume expansion caused by ice formation can produce large isotropic stresses that can damage the structure of the catalyst layer, the MPL, or the GDL. 

Generally, the presence of an anionic binder in the catalyst layer improves the cell performance, and the optimum ionomer content goes through a maximum. However, as film-like structures are likely to form when the ionomer is incorporated into the catalyst layer, the active sites may be covered by ionomer films, leading to a decrease in the active surface area and, as a cmisequence, in the cell performance. 

---