Preparation of a catalyst ink for manufacturing a catalyst coated membrane (CCM)
Applicant: TEMASEK POLYTECHINC [SG] / WO 2023014300 A1
This patent application from Temasek Polytechnic looks to overcome problems with the existing methods for preparation of a catalyst coated membrane (CCM). Transfer of electrocatalyst layers via a flexible substrate to the membrane surface is the first of these methods (roll-to-roll or decal). Roll-to-roll has the downside of requiring multiple complex stages and high cost. The decal method can lead to poor dispersion of catalyst particles, incomplete transfer of catalyst onto the membrane and catalyst degradation during the hot-pressing process. The direct spray deposition of catalyst onto the membrane surface might relieve some of the complexity however, this is time consuming and the need for several different inks can result in high catalyst wastage, increased likelihood of defects and structural issues, as well as agglomeration and nozzle blockage during the spraying process. This patent application refers to work to improve the direct spray method incorporating milling steps to reduce the problem of nozzle blockage.
Based on a desired ionomer weight percentage of 40% (c.f. catalyst powder), a method is used whereby 6.25 g of catalyst powder (Tanaka TEC10V40E, 40 wt.% Pt/C), is dispersed in 15 g of deionized water and ground with zirconia balls in an enclosed grinding bowl with addition of 50 g of Nation solution (79.61 g Chemours D520, 5% Nation by weight). Then ball milled in a planetary ball mill PM 100 at room temp. with a rotation speed of 300 rpm for 5 min (total time of 10 min inclusive of a 1 min interval). The obtained ink mixture can then be filtered through a 400-mesh filter. The filtered ink is then mixed with 30 g of isopropanol (IPA) and sonicated at 25 kHz, room temp. for 15 min using an Elma Multi-frequency ultrasonic unit (Model: TI-H-10) to produce a homogenous catalyst ink with the desired viscosity. Then primed in an ultrasonic syringe holder for at least 30 mins before spray coating. The spray coating is carried out by spraying multiple coats on each side of the membrane using a Sono-tek XYZ motion ultrasonic coating systems (Model: ExactaCoat). The number of coats may be calculated based on the loading of catalyst required [which differs for the cathode size (e.g., 0.4 mg/cm2) and anode side (e.g. 0.1 mg/cm2)]. A single spray cycle of 3 mL ink on a 20×20 cm membrane achieves 0.1 mg/cm2 catalyst loading. The catalysts were incorporated into an air-cooled open-cathode PEMFC stack consist of 5 cells in series with an active area of 8 cm2 for testing.
The above system is referred to as the “three component ink formulation”. In another example, the isopropanol was added prior to ball milling and referred to as the “four component ink formulation”. The three-component system performed best in electrochemical testing, likely due to the mixture for the ball mill being too dilute for complete homogenisation.
The work is important because it improves and simplifies the production process for CCM. When the market is at full scale, it will be important for higher throughput and lower downtime in a fuel cell production plant, and this work would improve those factors.
