Hydrophilic catalyst layer

To overcome these disadvantages, a thin-film CL technique was invented, which remains the most commonly used method in PEM fuel cells. Thin-film catalyst layers were initially used in the early 1990s by Los Alamos National Laboratory [6], Ballard, and Johnson-Matthey [7,8]. A thin-film catalyst layer is prepared from catalyst ink, consisting of uniformly distributed ionomer and catalyst. In these thin-film catalyst layers, the binding material is not PTFE but rather hydrophilic Nafion ionomer, which also provides proton conductive paths for the electrochemical reactions. It has been found that the presence of hydrophobic PTFE in thin catalyst layers was not beneficial to fuel cell performance [9]. 

Contact angle measurements on the CL may also be useful in the characterization of catalyst layer degradation in a fuel cell. Yu et al. [19] found that the contact angle of a degraded CL became smaller compared to that of an unused catalyst layer, indicating more hydrophilic behavior accompan3dng degradation. 

Thin-film catalyst layers are usually hydrophilic, with no hydrophobic ingredients added inside the CL. Although PTFE is generally unnecessary for thin-film catalyst layers, sometimes hydrophobicity maybe required for better transport in the CL. Zhang et al. [11] designed a dual-bound composite CL that contained... 

Zhang and Shi [36] found that the dual-bound composite catalyst layer exhibited higher performance than either a PTFE-bound CL or a thin-film CL, as shown in Figure 2.9. Optimization of the dual-bound CL showed that impregnation of Nation between the two layers could lead to decreased cell performance [37]. Thus, the optimal structure for a dual-bound CL was a separate hydrophilic layer on top of a hydrophobic layer. 

Yu, H. M., Ziegler, C., Oszcipok, M., Zobel, M., and Hebling, C. Hydrophilicity and hydrophobicity study of catalyst layers in proton exchange membrane fuel cells. Electrochimica Acta 2006 51 1199-1207. 

Jung, U. H., Park, K. T., Park, E. H., and Kim, S. H. Improvement of low-humidity performance of PEMFC by addition of hydrophilic SiOj particles to catalyst layer. Journal of Power Sources 2006 159 529-532. 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

Davis et al. described a new type of effective chiral catalysts, the so called "supported aqueous-phase catalysts". This hydrophilic complex is supported on a hydrophilic solid to create a large interface between the catalytic species and the organic reactants. The hydrophilicity of the ligands and the support creates an interaction sufficient to maintain immobilization of the sulfonized BINAP ligand (Scheme 7.12.) in a layer on the carrier. 

As a major conclusion, primary pores inside agglomerates and ultrathin catalyst layers should be hydrophilic (maximum wetting). Under such conditions effectiveness of catalyst utilization can approach 100%. Moreover, the microscopic mechanism of the electrochemical reaction, represented by the transfer coefficient a, is essential for the effectiveness of catalyst utilization. 

In some of the cases listed in Table 3.1 catalytic membranes are used. For instance the catalytic membranes prepared by Peters et al. (2007) have a catalyst layer deposited on a composite hydrophilic membrane. The dual-layer structure allows the independent optimization of the selective and the catalytic properties and partially satisfies contradicting material properties , such as the necessity to achieve a low permeability of the reactants together with a high availability of the same reactants on the catalyst. The value of 5 can be varied by changing the number of dip-coat... 

The preparation of a GDL involves the use of a substrate, carbon cloth or paper [6-8], which are in general commercially available. They are usually treated to have hydrophobic/hydrophilic properties, typically using polytetrafluoroethylene (PTFE) [9]. A microporous carbon layer, made with carbon and PTFE with controlled porosity is applied to the substrate in the catalyst layer side or to both sides [10]. This improves the gas and water transport properties. 

The use of porous catalyst layer (CL) is based on the consideration that pores are needed for transporting gaseous reactants and water because a material diffuses much faster through empty space than through liquid or solid materials, and both hydrophobic and hydrophilic pores are needed for transporting gases and water, respectively. Numerous work has been done in order to create and maintain both kinds of pores within the CL, but an ideal situation is not yet achieved. A major problem encountered with such a CL with a thickness around 10 -20 pm is that flooding appears to be unavoidable. In... 

The reaction reduces the proton concentration and thus the proton conductivity of the ionomer in both the PEM and the catalyst layers. Metal ions will do similar harm, and the impact of highly charged cations such as Fe + is most harmful. It not only replaces H+ but may also restructure the hydrophilic domains of the ionomer (e.g., crosslinking the side chains by ionically interacting with the -SO3 groups). Obviously, the impact caused by cations is cumulative. 

The MEA is composed of three main parts, e.g., polymer electrolyte membrane (PEM), gas diffusion medium, and catalyst layer (CL). The membrane, with hydrophilic proton-conducting channels embedded in a hydrophobic structural matrix, plays a key role in the operation of PEFCs. The PEMs for PEFCs commonly use perfluorosulfonic acid (PFSA) electrolytes such as Nation , with the chemical structure shown in Fig. 2, because of its high proton conductivity as well as chemical and thermal stability [1]. The gas diffusion medium (GDM), including both the microporous layer (MPL) and the gas diffusion layer (GDL), which typically is based on carbon fibers, is also an important component. The GDM is designed with three distinct... 

The electrode performance in any electrochemical system depends on the complex interaction between intrinsic kinetics and various transport processes involving reactants, products, and the electrolyte. In the particular case of direct fuel cells, the catalyst support (when employed), the hydrophobic-hydrophilic properties of the diffusion substrate, die ionomer load in the catalyst layer, and the electrode design, including the current collector, all have a great impact on die power output. The goal in the present section is to give an overview of the experimental advances in the area of catalyst layer engineering and anode structures. 

This coarse-grained molecular dynamics model helped consolidate the main features of microstructure formation in CLs of PEFCs. These showed that the final microstructure depends on carbon particle choices and ionomer-carbon interactions. While ionomer sidechains are buried inside hydrophilic domains with a weak contact to carbon domains, the ionomer backbones are attached to the surface of carbon agglomerates. The evolving structural characteristics of the catalyst layers (CL) are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2) and water as well as the distribution of electrocatalytic activity at Pt/water interfaces. In principle, such meso-scale simulation studies allow relating of these properties to the selection of solvent, carbon (particle sizes and wettability), catalyst loading, and level of membrane hydration in the catalyst layer. There is still a lack of explicit experimental data with which these results could be compared. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

As discussed above, it is vital to extend the eontact area between the catalyst and the protonic ionomer in order to improve catalyst utilization. To meet this requirement, ionomer-bonded hydrophilie eatalyst layers have been developed. The hydrophilic ink, prepared by mixing the catalyst and ionomer directly, ensures sufficient contact between the eatalyst particles and the ionomer. It was found that an ionomer-bonded hydrophilic catalyst layer could improve Pt utilization by up to 45.4% [25],... 

Membrane-based Hydrophilic Catalyst Layer. Wilson and Gottesfeld [8, 21-23] suggested an ionomer-bonded hydrophilic catalyst layer prepared with the decal transfer method. The so-called decal transfer process includes two key steps (1) coating catalyst ink onto a blank substrate (e.g., PTFE film) then (2) transferring the coat onto the membrane (as shown in Figure 19.6). A typical preparation procedure is as follows ... 

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