Manufacturing of Direct Methanol Fuel Cell Electrodes by Spraying

Spraying is a well-established coating process used to fabricate electrodes for both polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cells (DMFCs), and also for the fabrication of gas-diffusion media (GDM) used in fuel cells. Despite its popularity as a process there is little basic research on how spray parameters and nozzle characteristics affect the droplet sizes of catalyst inks, and how the droplet sizes affect the electrode structure, and eventually the overall membrane-electrode assembly (MEA) performance. We present results from an experimental study to quantify key process parameters in the electrode spraying process, characterizing the spray nozzle, measuring droplet diameters, and investigating the microstructural effects on electrode performance. For this purpose, a spraying apparatus was developed and calibrated, and MEA's were fabricated with fixed electrode loadings but with different droplet sizes. Droplet sizes were controlled by characterizing the spray nozzle and measuring the spray optically by utilizing high-speed photography. It is shown that increasing spraying pressures generally reduces the mean droplet size of the spray, which affects the microstructure of the electrode produced and results in higher MEA performance. Of the costs involved in hydrogen-based PEMFCs and DMFCs, the material cost of the catalyst and the ion-exchange membrane are key contributors to the overall system cost, but even if much less expensive materials were available, the labor-intensive nature of the membrane electrode assembly (MEA) processing contributes a major portion of the manufacturing costs, particularly at low production volumes. Performance issues, which range from overall system energy density, to long-term performance and stability, are likewise tied directly to the materials and the manufacturing process choices. The porous electrode in the MEA is formed by depositing catalyst ink onto a substrate, which, upon drying, forms a three-dimensional porous structure, which allows for the transport of the fuel and oxidant to the electrode and for the removal of reaction products from the electro catalysts. The MEA electrode must be electronically conductive to allow for the movement of the electrons to and from the external circuit, and must maintain an ionic pathway for the protons to move through the ionomer to the other electrode to complete the overall reaction. This process is shown schematically in Figure 1. Limiting the mobility and transport of any of these species reduces the catalyst utilization and the overall performance that can be extracted from a given amount of catalyst.