From this analysis we determined that particle aggregation is a failure mode for streaking defects. Particle aggregation is caused by a combination of inter-particle attraction that binds particles and coating flow that creates the opportunity for particles to collide. Our work utilizes a combination of pilot line coating, catalyst ink formulation and rheology, and visualizing catalyst ink under flow to probe the origin of catalyst particle aggregation. Inter-particle attraction is studied by varying the surface-active properties with different catalyst carbon supports. Flow conditions that collide particles are studied with fluid rheology and slot-die lip operation to probe where in the slot-die coater particles may be concentrating. From coating trials, particles are seen to punctuate the end of each streak defect, indicating that aggregates are forming and clogging the slot-die coater until they are flushed out. Inter-particle attraction that creates aggregates can be tuned with the catalyst carbon support. Catalyst inks known to cause streaking use a high surface area carbon support(HSC) and reducing that attraction with lower surface area carbon support(LSC) stopped streak formation. Visualizing the catalyst inks under flow also confirmed that HSC ink had visible aggregates while the LSC ink did not. Roll-to-roll coating also confirmed that HSC inks would produce streaks while LSC ink would not.
The die cavity and the die lips are two regions for the slot-die coater that can concentrate particles and encourage aggregation during coating. In the die cavity there is a transition in flow from a narrow channel to a wide channel and back to a narrow channel. This contraction and expansion can cause flow recirculation and provide a region of low shear stress and be an opportunity for particle accumulation, increasing collisions that can lead to aggregation. If the fluid also has a yield stress, then flow can cease at low shear stress and promote aggregation. We characterize the yield stress of HSC and LSC catalyst inks and compared against comparable flow conditions expected in the slot-die coater to confirm that catalyst ink yield stress is not related to streaking. The die lips are another transition region as flow narrows from the die cavity and out of the slot-die coater. This contraction provides an opportunity for HSC ink aggregates to span the slot-die gap—restricting flow, encouraging aggregation, and inducing clogging. By widening the slot-die gap during roll-to-roll coating and reducing the probability for particle bridging, streaks were no longer formed by HSC ink. In this work, we address the coupling between inter-particle attraction and coating flow to explore aggregate formation that leads to clogging and streaking defects. From this analysis we offer formulation and operational solutions to prevent streak defects in slot die coating of Platinum on carbon catalyst inks.
This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the Hydrogen and Fuel Cell Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.