With the emerging demands for clean energy and an economy with net-zero greenhouse gas emissions, electrocatalysis areas have attracted tremendous interest in recent years. The electrochemical devices that use electrocatalysis, such as fuel cells, electrolyzers, and flow batteries, consist of hierarchical structures, requiring comprehension and rational designs across scales from millimeter and micrometer all the way down to atomic scale. In past decades, electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been extensively utilized for imaging different scales of these devices in both two and three dimensions. However, electron-based techniques for high-resolution imaging require uninterrupted maintenance of a high-vacuum environment, leading to difficulties of sample preparation and lack of integrated observation without intrusion/ disassembly. To overcome these disadvantages, more and more efforts have been dedicated to the development of X-ray imaging techniques recently, specifically absorption-based two-dimensional (2D) transmission X-ray microscopy and three-dimensional (3D) X-ray tomography, due to much better transmission behaviors of X-rays than electrons. X-ray tomography imaging mostly focuses on answering questions related to morphology and morphological changes at the microscale or near 1 μm resolution and nanoscale of 30 nm resolution. The method is nondestructive and it allows for the visualization of operando electrochemical devices, such as fuel cells, electrolyzers, and redox flow batteries. Operando X-ray microscopic tomography typically focuses on catalyst layers and morphology changes during degradation, as well as mass transport. Nanoscale tomography still predominantly is used for ex situ studies, as multiple challenges exist for operando studies implementation, including X-ray beam damage, sample holder design, and beamline availability. Both microscale and nanoscale tomography beamlines now couple various spectroscopic techniques, enabling electrocatalysis studies for both morphology and chemical transformations. This viewpoint highlights the recent advances in X-ray tomography for electrocatalysis, compares it to other tomographic techniques, and outlines key complementary techniques that can provide additional information during imaging. Lastly, it provides a perspective of what to anticipate in coming years regarding the method use for electrocatalysis studies.

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.

As the United States energy infrastructure moves towards the integration of hydrogen energy, the reliable production of hydrogen through water electrolyzers is imperative. In proton exchange membrane water electrolyzes (PEMWE’s), the porous transport layer (PTL) plays an important role. When choosing PTL material, one should consider corrosion resistance, electrical conductivity, and ability to support the structure of the electrochemical cell. Due to these requirements, titanium is the current state-of-the-art anode PTL material. However, titanium quickly forms a layer of titanium oxide which significantly decreases conductivity of the PTL and respectively decreases the overall efficiency of the PEMWE system. Coatings are commonly applied to the PTL to combat this complication, but the use of noble metals leads to cost concerns. Additionally, the degradation of coated PTLs is not yet well known. Advanced physicochemical characterization of PTLs at various stages of fabrication and testing is vital to understand their properties and the impact on electrochemical performance in order to improve durability and meet cost targets. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is currently used to cross section areas of the PTL for subsequent analysis with Scanning Transmission Electron MicroscopyEnergy-dispersive X-ray Spectroscopy (STEM-EDS) analysis to visualize the elemental information of the PTL materials and PTL coatings, specifically looking at detrimental oxide layer formation. Unfortunately, this approach is time-consuming and difficult, motivating the development of alternative approaches that allow the characterization of wide sample sets more efficiently. Additionally, STEM-EDS analysis only provides elemental information, so if several oxide layers preside, it can be difficult to differentiate them. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiling is a very powerful technique that has been commonly used to characterize thin films and buried interfaces. Unlike the FIB-SEM and STEM-EDS combination, ToF-SIMS can be performed relatively quickly in several areas of the PTL, provides chemical information, and is sensitive to trace elements. An added advantage of ToF-SIMS is ability to do 2D and 3D chemical analysis on different scales. This enables visualization of elemental and chemical distribution on both larger scale (70x70 um) areas, and smaller areas (20x10 um) which provides more detailed surface and interface information. This allows for detailed tracking of all major constituents of the PTL and coating. It also allows to follow changes in oxide layers upon fabrication and after electrochemical testing, as well as track homogeneity of the surface and oxide layer interfaces throughout different parts of the PTL. This presentation will demonstrate capabilities of this technique for characterization of PTLs, along with progress and potential challenges. ToF-SIMs results will be compared to TEM analysis of cross-sections obtained with FIB-SEM. This study will highlight similarities and differences between the techniques, technique optimization for these morphologically challenging samples, and paths for future investigation moving forward.