High-temperature Catalyst Layers - Components and Structure

4 High-temperature Catalyst Layers - Components and Structure 

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. 

Phosphoric acid-doped polybenzimidazole (PA-PBl) membrane is the most common type for HT-PEMFCs. It was also introduced in the catalyst layer as an ionomer to improve proton conductivity [8, 80-83]. Similar to die apphcation of Nafion ionomer in the catalyst layer, polybenzimidazole (PBl) solution was incorporated into the catalyst layer by an impregnation method (die electrode was soaked in a PBI solution), by a gluing method (the PBl membrane was coated by PBI solution), or by a brushing method (the PBl solution was brushed onto the electrode) [80]. 

Unfortunately, while catalyst components and structures in low-temperature MEAs have attracted considerable attention, optimization of high-temperature catalyst layer structures and components seems little studied. Lobato et al. [83] investigated the effect of the catalytic ink preparation method on the performance of HT-PEMFCs. They employed two methods for catalyst layer preparation the solution method and the colloid method. In the solution method, catalyst ink was prepared by mixing the catalyst (20% Pt/C) and PBl solution (5% PBl in dimethylacetamide). In the colloid method, acetone was added to the mixture of catalyst and PBI solution, which made the PBI form a colloid suspended in the solvent. They found that electrodes prepared by the solution method showed better performances at 150 °C and 175 °C, and that the electrodes prepared by die colloid method gave a better performance at 125 °C. This is probably due to differences in catalyst layer structure (see Section 18.2.7). 

5 Strategies for HT Catalyst/Catalyst Layer Performance Improvement and Mitigation 

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