Pushing the limits of screen printing towards low iridium loadings of catalyst layers for PEM water electrolysis

In this study, the unique international collaboration of Fraunhofer ISE with the National Renewable Energy Laboratory (NREL), University of Connecticut (UCONN) and Colorado School of Mines (Mines) within the OREO project ("Overall Research on Electrode Coating Processes") allows a comprehensive fabrication, testing and in-situ and ex-situ analysis of catalyst coated membranes (CCMs) for PEM electrolyzers. Additionally, the project partners from industry are providing the materials like catalyst powders from Heraeus Deutschland GmbH & Co. KG as well as ionomers and membrane from Fumatech, ensuring the comparability of measurements. From these materials, catalyst layers (CLs) are produced with slot die coating at NREL and screen printing at ISE. Moreover, STEM/EDS analysis of the cross sections are conducted at UCONN to visualize and quantify catalyst and ionomer distribution, thickness of the layers, porosity, and particle size distribution. At Mines, the CLs are measured using SEM/EDS for surface analysis of the produced layers. The performance of the CCMs is determined with the 4 cm2 test cell from ISE using polarization curves and impedance spectroscopy. Within this work, the focus lies on the ink and screen printing process development to enable production towards low Ir-loading CLs for industrially relevant production technologies. Screen printing is a robust and high-throughput coating technology, which can be used to produce CLs for proton exchange membrane (PEM) electrolyzers. This printing method can produce CLs with complex designs, that are not possible with coating methods such as slot-die. The aim of this work is to reach the lowest possible iridium loading that can be printed with commercially available meshes. Two catalyst pastes are prepared with unsupported iridium oxide powder (Heraeus), Ionomer dispersion (SSC, Fumatech) and solvents, resulting in a paste solid content of 16.67 wt% and 18 wt%. The printing is done with a semiautomatic flatbed screen printer (EKRA XH STS, ASYS Group) and three different screens (KOENEN GmbH): 230/30, 400/18 and 520/11 (Asada Mesh) (mesh count per inch/ mesh diameter in μm). At least 100 CLs are produced with each screen-variation, so that an analysis of the printing process is carried out in view of a larger production scale. The results of the study show that the DOE target of 0.4 mgIr/cm2 in 2026 [1] in Ir-loading reduction can be successfully reached with a 520/11 screen at 0.34 mgIr/cm2. The electrochemical operation of the CCMs reveals the good result of an iR-free corrected voltage of 1.586 V at 1 A/cm2.