Catalyst layers hydrophobic component

Besides the catalyst gradient in the catalyst layer, other components such as the hydrophobic agent (PTFE) and proton conductive polymer (Nafion) may also need to be adjusted in order to optimize gas/water transportation and electron/proton transfer. It can be expected that the catalyst layer adjacent to the gas diffusion layer side should be more hydrophobic to ensure much more of the reactants penetrates the inside of the electrode. While near the membrane side, more proton conductive polymer is needed to ensure a continuous network for proton conduction. Therefore, a non-uniform catalyst layer with a decreasing PTFE loading and an increasing Nafion content along the through-plane direction from GDL to membrane should be more efficient. 

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

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 GDL is usually made of a carbon-based porous substrate, such as carbon paper or carbon cloth, with a thickness of about 0.2 to 0.5 mm and a dual-layer structure. A schematic of the GDL between the flow field and the catalyst layer is presented in Figure 1.13. The first layer of the GDL, in contact with the flow field and the inlet gas in the flow channels, is a macro-porous carbon substrate, serving as a current collector, a physical support for the catalyst layer, and an elastic component of the MEA. The elastic component is necessary for the fuel cell to handle the compression needed to establish an intimate contact. The second layer of the GDL, in contact with the catalyst layer, is a thiimer microporous layer consisting of carbon black powder and some hydrophobic agent, which provides proper surface pore size and hydrophobicity to avoid flooding and to enhance intimate electronic contact at its interface with the catalyst layer [33]. 

The catalyst layer structures and components in HT-PEMFCs should be different from diose used in low-temperature PEMFCs. For example, water management in an HT-PEMFC is not a problem and thus the required hydrophobicity of the catalyst layer might not be a factor for HT-MEAs. Obviously, Nafion, the commonly used ionomer for low-temperature MEAs, is not suitable for HT applications. Unfortunately, die design and evaluation of HT catalyst layers have not yet attracted attention. This is most likely due to the lack of suitable materials for HT-MEA fabrication. For example, although a significant number of publications look at high-temperature membrane and ionomer development, these ionomers have seldom been used in a catalyst layer to replace Nafion, possibly due to the technical difficulties of doing so. 

Regarding the inherent problems with proton/gas transport for the hydrophobic or hydrophilic types of catalyst layers, a dual-bonded composite CL was suggested first by Zhang et al. [50, 51] in order to alleviate the drawbacks and promote the merits of the individual CL. Zhang and Shi [52, 53] also investigated such dual-bonded composite CLs by optimizing various components used. This dual-bonded CL cathode has two layers. The first is the hydrophobic layer with PTFE as a binder material, which is fabricated directly on the surface of the GDL. The second is a hydrophilic layer with Nafion as a binder material, which is fabricated on top of the hydrophobic layer surface. The typical preparation process can be summarized as follows ... 

The effect of impurities on fuel cells, often referred to as fuel cell contamination, has been identified as one of the most important issues in fuel cell operation and applications. Studies have shown that the component most affected by contamination is the MEA [3]. Three major effects of contamination on the MEA have been identified [3,4] (1) the kinetic effect, which involves poisoning of the catalysts or a decrease in catalytic activity (2) the conductivity effect, reflected in an increase in the solid electrolyte resistance and (3) the mass transfer effect, caused by changes in catalyst layer structure, interface properties, and hydrophobicity, hindering the mass transfer of hydrogen and/or oxygen. 

Diffusion Media The diffusion media (DM) consists of a carbon fiber or woven cloth macroporous layer and possibly a highly hydrophobic microporous layer (MPL) that we will treat separately in this text. The flexible DM is a critical component in the PEFC and was originally developed to enable better electrical contact between the catalyst layer and lands but really serves four primary functions ... 

Various studies and some patents have been published on the use of membrane catalysts for the direct synthesis of H202 [73-81]. The redox treatment of the membrane influences the properties both in the synthesis and decomposition of H202. Formation of a hydrophobic layer improves the selectivity, because it limits the consecutive decomposition of hydrogen peroxide, limiting the chemisorption of H2 and re-adsorption of H202 [73]. Either polymeric or ceramic-type membranes could be used, but the latter are preferable to allow more robust operations. The mono- or bi-metallic Pd-based active component could be deposited either in the form of dispersed particles (e.g., by precipitation-deposition) or of a thin film (e.g., by... 

The topmost molecular layer of solid materials normally determines their properties to the environment, e.g. to body fluids and their reactivity to chemical systems such as of catalysts. Therefore, surface modification of solid polymers with ultrathin layers of reactive, hydrophilic, hydrophobic and amphiphilic components or polymers, solely or in combination, is chosen to improve surface and interface properties to other systems. 

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