Proton Exchange Membrane fuel cells or Polymer Electrolyte Membrane (PEM fuel cells) operate by transforming the energy released from hydrogen and oxygen reactions into electrical energy. The PEM fuel cells utilize a membrane, which performs the important function of transmitting protons while hampering the flow of electrons. Platinum acts as a catalyst in hydrogen fuel cells by splitting hydrogen molecules. Platinum mixed with carbon black and water is layered onto the fuel cell membrane.
Let us look deeper into the construction of these fuel cells and how this material is layered onto the membrane. Platinum plays a central role in the operation of a PEM fuel cell; it is responsible for the oxidation of hydrogen and reduction of oxygen hence it needs to cover the maximum surface area of the membrane in order to be exposed to the most injected gas. It is also important to maximize the surface area of the platinum catalyst particles, by using the smallest sized particles possible. Smaller the size, the more surface area that is exposed to the injected gas. Lastly, it is important that the platinum particles be layered consistently and in such a way as to avoid clumping or agglomeration.
The next objective that needs to be observed in the construction of the PEM fuel cell is the even thickness of the platinum catalyst coating. It is imperative to create a leveled or uniform film thickness thickness so that there is an equal amount of hydrogen oxidation across the film. If there should be an inconsistent layer of platinum, the less dense section would achieve a lower rate of oxidation while a thicker than normal layer can cause other problems. Hence, determining the correct density of the platinum film thickness is a critical issue so it is important that the platinum carbon black coating on the fuel cell's membrane be highly uniform for optimum hydrogen conversion. The platinum density acts as an important factor in determining the amount of gas that reaches the membrane of the hydrogen fuel cell. The platinum carbon black combination should be such as to allow a permissible degree of contact between the gas and the membrane. A dense layer would obviously provide a measure of resistance leading to a reduced rate of contact between the platinum and the gas, and subsequently a lesser rate of chemical reaction. A less dense layer would lead to hot spots and other problems. Thus uniformity, both in the layering of platinum and the carbon web is a necessity.
Pressing, knife-edge and printing methods are found to create these non-uniform coating thicknesses and/or hot spots on fuel cell membranes. Ultrasonic spray technology is an ideal solution for applying a coating of platinum catalyst to the polymer membrane. First, platinum is of course a costly material and ultrasonic spray nozzles due to their soft or low forward velocity, minimize bounce or over spray. Secondly, coating applications should be extremely precise to facilitate optimal results, which can be achieved with ultrasonics. Lastly, the coating process should not damage what it is you are trying to coat. By press methods, the internal apparatus can be harmed by prolonged exposure to the environment. Whereas the rolling processes, also subjected to prolonged exposure to the environment, can cause platinum particles to agglomerate. The hydraulic spraying process is far more suitable but it has its many pitfalls as well. Dispersion via hydraulic spraying is often inconsistent, which can lead to hot spots. There is also unnecessary waste of platinum due to the high-pressure velocity of the fluid resulting in fluid "bounce."
tag : film thickness
