Fabrication, Performance, and Durability of Roll-to-Roll Coated Iridium-Based Anodes

Abstract

To reduce hydrogen production costs for low temperature water electrolysers to meet the Hydrogen Shot goal of $1/kg manufacturing methods need to be translated from batch to continuous processes. For production of IrO2 anode layers, roll-to-roll (R2R) methods are well suited due to their potential for high throughput and uniformity. There are a variety of coating methods for R2R, each with their own operating limitations such as coating fluid viscosity and liquid film thickness. For any coating method there will be a region of operating conditions, known as the coating window, where stable coatings can be obtained. Related to this is the ink formulation and its solids (catalyst and ionomer) concentration which influences its viscosity and determines the required liquid film thickness to achieve a specified target loading. Increasing solids concentration is desirable to reduce solvent content enabling reduced dryer loads and/or increased line speeds. However, this reduces the liquid film thickness, potentially to values outside the coating window.To better understand the relationship between coating method and formulation we conducted a study exploring the range of Ir loadings attainable with different formulations and coating methods. Catalyst inks were formulated with 10, 20, and 30 wt% IrO2 with a fixed I:Cat ratio of 0.2. Slot die and gravure coating were used to coat the catalyst layers onto a decal substrate at a variety of loadings ranging from 0.06 mgIr/cm2 to 0.65 mgIr/cm2. Slot coating produced uniform coatings with 20 and 30 wt% IrO2 inks but the low viscosity of the 10 wt% IrO2 ink resulted in poor control of coating width and uniformity. In contrast gravure coating was able to produce uniform coatings with all formulations due to it being better suited for low viscosity fluids. For coatings within the coating window the catalyst layers had high uniformity with loading variations below 10%. However, optical and scanning electron microscopy revealed microscale heterogeneity of the catalyst layers with low loaded catalyst layers (< 0.2 mgIr/cm2) appearing to have voids in the coating.MEAs were fabricated from these R2R-coated catalyst layers for comparison to spray-coated catalyst layers. These MEAs were tested for both performance and durability. R2R-catalyst layers with 0.4 mgIr/cm2 had identical initial performance compared to spray-coated catalyst layers. In contrast, R2R-coated CLs with 0.2 mgIr/cm2 performed significantly worse than spray-coated catalyst layers, likely due to the heterogeneities. These results illuminate the challenges in moving towards very low catalyst loadings. Results will also be presented on efforts to improve the homogeneity catalyst layers at 0.2 mgIr/cm2 through changes in ink formulation and processing.This work was authored in part 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 U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Hydrogen and Fuel Cells Technology 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.