Microstructured catalysts mass transport

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

Electrodes consisting of supported metal catalysts are used in electrosynthesis and electrochemical energy conversion devices (e.g., fuel cells). Nanometer-sized metal catalyst particles are typically impregnated into the porous structure of an sp -bonded carbon-support material. Typical carbon supports include chemically or physically activated carbon, carbon black, and graphitized carbons [186]. The primary role of the support is to provide a high surface area over which small metallic particles can be dispersed and stabilized. The porous support should also allow facile mass transport of reactants and products to and from the active sites [187]. Several properties of the support are critical porosity, pore size distribution, crush strength, surface chemistry, and microstructural and morphological stability [186]. 

The result of these degradation mechanisms is that that the GDL and the MPL both lose their hydrophobic character [133, 153, 154], and that the pore structure of the materials changes. The relation between microstructure and surface properties and mass transport properties has been the subject of several recent experimental studies [155,156], which indicate that indeed mass transport can be seriously affected by the hydrophobicity of the GDL and MPL as well as by the pore size. This will contribute to the gradual decay of the performance, though it is hard to distinguish the effects of changes in the GDL/MPL to those of changes in the catalyst layer. 

Regarding the solvent used to prepare the catalyst ink, its properties in catalyst ink should be mentioned as it also plays an important role in determining the microstructure and cataljAic activity of the CL. When ionomer such as Nafion solution is mixed with solvent, the mixture may become a solution, a colloid, or a precipitate due to the different dielectric constants of the solvent. When the dielectric constant is more than 10, a solution is formed between three and 10, a colloidal solution is formed and less than 3, precipitation occurs.If the mixture is a solution (i.e., the solution method ), excessive ionomer may cover the carbon surface, resulting in decreased Pt utilization. However, when the mixture is a colloid (the colloidal method ), ionomer colloids adsorb on the catalyst powder and the size of the catalyst powder agglomerates increases, leading to an increased porosity of the CL. In this case, the mass transfer resistance could be diminished because of the continuous network of ionomers throughout the CL, which then improves the proton transport from the catalyst to the membrane. ... 

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