Catalyst binder polytetrafluoroethylene

In preparing the electrodes for low-temperature fuel cells that use membrane electrolytes, a polymer binder is essential to bind discrete catalyst particles to form a porous catalyst layer that simultaneously facilitates the transfer of ions, electrons, and reactants/products. Firstly, polytetrafluoroethylene (PTFE) was used... 

The catalyst ink usually includes catalyst, carbon powder, binder, and solvent. Sometimes, other additives are added to improve the dispersion of the components and stabilize the catalyst ink. The catalyst either covers the surface of the GDL or directly coats the surface of the membrane (catalyst coated membrane, CCM). The CL usually consists of (1) an ionic conductor such as perfluorosulfonate acid (PFSA) ionomer to provide a passage for protons to be transported in or out, (2) metal catalysts supported on a conducting matrix like carbon, to provide a means for electron conduction, and (3) a water-repelling agent such as polytetrafluoroethylene (PTFE) to provide sufficient porosity for the gaseous reactants to be transferred to catalyzed sites [5, 6]. Every individual factor must be optimized to provide the best overall performance of a CL. 

Air cathodes are typically constructed in layers. The uppermost layer is exposed to the air and must allow oxygen gas to enter, so this material is a porous structure often made of PTFE (polytetrafluoroethylene). The catalysts are then found in the next layer. This layer is a porous carbon layer that contains the catalysts. This carbon layer can function on its own for reaction with oxygen gas, but the presence of catalysts can improve the performance of the cell. Catalysts are added to this layer as the pure catalyst itself or as a catalyst coated on carbon powder. This can be done with a wet or dry process. Depending on the wet process to manufacture the air cathode, the catalyst must be able to not decompose in the solvent such as water or alcohol. For the dry process the catalyst must readily mix with the carbon power and binders. Many air cathode manufacturing processes involve heat, some up to 300 °C, which requires thermal stability of the catalyst. The next layer down is a metal mesh current collector that is used to conduct the electrons and provide mechanical support. This air cathode represented here is typical of an air cathode manufactured by Yardney Technical Products Inc. and it has a second layer for catalysts. This second layer improves performance of the air cathode. 

Typically, composite electrodes are employed. The electrodes are composed of high-surface-area carbon powders mixed with a catalyst and Teflon (DuPont s registered trademark for polytetrafluoroethylene) and laminated with a current collector made of expanded metal mesh. The use of the Teflon binder in the air... 

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