Comparison of Drop Deposition and Layer-by-layer Approach to Form Thin Carbon-nanotube-membranes as Catalyst Layer in Fuel Cells

In this study two different approaches to make thin multifunctional membranes of carbon nanotubes will be compared. The membranes can be used to form a catalytic layer in a proton exchange membrane fuel cell. The most straight forward method is to directly depose drops of previously dispensed nanotubes on a Si-substrate. For easy release of the membrane the substrate is coated with an additional hydrophilic sacrificial layer, in this case a sputter deposited thin Al-film. The advantages of this method are the easy handling of the process and short fabrication times. But, there is a high probability of pin-hole generation within the membrane if it undergoes a thickness of approximately 6 mu m. In contrast, the layer-by-layer deposition technique follows a bottom-up approach, making subsequently bilayers of functionalized nanotubes attached by electrostatic forces. This method provides the possibility to control the layer thickness very precisely and to achieve a minimal defect density. Like every step-by-step method, the processing time depends on the final thickness which can be disadvantageous when targeting thicker membranes.