This work investigates the use of roll-to-roll coating methods for the production of iridium oxide catalyst layers for proton exchange membrane water electrolyzers. Catalyst layers were produced using two coating methods: slot die and gravure. By varying the solids content of the catalyst ink and coating process variables loadings between 0.08 and 0.64 mgIr cm-2 were prepared with relatively high spatial uniformity. However, at loadings below 0.2 mgIr cm-2 microscopy reveals voids in the catalyst layer due to similar length scales of catalyst agglomerates and overall layer thickness. Electrochemical testing shows that these voids do not impact initial membrane electrode assembly performance but lead to increased performance losses after potential cycling compared to spray coated catalyst layers.

The proton exchange membrane integrity can be compromised during hot-press fabrication of membrane electrode assemblies (MEAs) causing premature cell failures during operation. In this work, infrared (IR) thermography was used as a diagnostic tool to spatially visualize hydrogen (H2) crossover and identify process-induced- membrane irregularities (PIMs). These irregularities were identified as seed locations for MEA failures. Fine tuning of hot-press conditions was used to mitigate premature cell failures informed by accelerated stress testing (AST). The impact of PIMs on the initial performance, high-frequency resistances, open-circuit voltage, and H2 crossover are reported. Nafion XL and 212 membranes, hot-pressed with a force of 16 kg/cm2 and temperature of 120 degrees C, were found to be consistently irregularity-free. Irregularity-free MEAs using Nafion 211, 212, and XL membranes demonstrated AST lifetime improvements of 58, 64 and 400%, respectively, compared to those fabricated with non-optimized conditions. This work highlights the importance of fabrication parameters on premature cell failures.

Iridium oxide (IrO2) is recognized as a state-of-art catalyst for anodes of low-temperature polymer-electrolyte membrane water electrolyzers (PEMWE), one of the promising clean energy technologies to produce hydrogen, a critical energy carrier for decarbonization. However, typical IrO2 ink formulations are challenging to process in liquid-film coating processes because of their poor stability against gravitational settling and low viscosities. Here we report on time evolution of the microstructure of concentrated IrO2 inks in a water-rich dispersion medium, probed using a combination of rheology and X-ray scattering for up to four days. The inks progressively evolve from a predominantly liquid-like to a gel-like material with increasing aging time that can be leveraged as a formulation strategy to enhance their stability against sedimentation, and processability during electrode fabrication. We also elucidate the aging behavior by investigating the effects of ink formulation composition - ionomer concentration and solvent composition - and using the extended-DLVO theory. The implications of aging on electrode fabrication, including via direct coating onto membranes and porous transport layers, and membrane-electrode-assembly performance has also been examined. Our findings offer not only a facile but also an environmentally benign formulation strategy to enhance ink processibility, expand practical fabrication approaches, and advance PEMWE manufacturing.

Cracks in catalyst layers (CLs) are a potential source of long-term failure in a fuel cell membrane electrode as-sembly (MEA). While modifications to the CL ink formulation can affect the degree of cracking, these changes may lead to lower initial performance than their cracked analogues due to the established link between formulation and performance. In this work, we explored the use of polymeric additives to mitigate CL cracks. Small quantities of poly (acrylic acid), poly (ethylene oxide), poly (methyl methacrylate), or poly (vinyl alcohol) - 5 wt% relative to ionomer mass - were added to the ink prior to its final mixing. Poly (vinyl alcohol) resulted in crack-free CLs, whereas the other polymers resulted in CLs with similar crack percentages as the control CL. Through a combination of transmission electron microscopy, X-ray computed tomography, and infrared spectroscopy, we ascribed the crack-mitigating mechanism of poly (vinyl alcohol) to its ability to hydro-gen-bond with Nafion, the ion conducting polymer binder in the catalyst ink. Initial performance of this non --cracked electrode exhibited nearly identical electrochemical behavior to its cracked counterpart, demonstrating that PVA additives successfully reduce cracks while maintaining cell initial performance.

A major factor driving fuel cell costs is the quantity of precious metal required. Therefore, it is important to understand a timeframe where inks can be reused. In this work, we explore differences between a freshly prepared catalyst ink and one that has been stored for over a year - comparing ink properties, cathode catalyst layer microstructure, and their respective fuel cell performance. Ink studies revealed smaller agglomerate sizes and a decrease in shear viscosity for the aged ink. Longer storage time also results in fewer cracks and a more uniform ionomer distribution, as evidenced by microscopy characterization of rod-coated electrodes. Lastly, polarization curves show improved performance at higher current densities for the electrode prepared from the aged ink. We rationalize such effect in terms of enhanced ionomer adsorption onto the catalyst over time.

This work demonstrates the application of high-resolution, 3D imaging to characterize micro- and nano-scale features of iridium oxide (IrO2) anode catalyst layers (CLs) in proton exchange membrane water electrolysis (PEMWE). Scanning electron microscopy (SEM) and nanoscale X-ray computed tomography (nano-CT) reveal differences in micro-scale features between spray-coated and blade-coated CLs, and in blade-coated CLs along the fabrication and testing timeline. The electrode thickness distribution of the tested blade-coated CL suggests increased thinning of regions compressed by titanium fibers of the porous transport layer (PTL). Ultra-high-resolution imaging of Xe-plasma focused ion beam with SEM (pFIB-SEM) with the development of a segmentation method is used to investigate nano-scale features between these regions of the tested blade-coated CL, showing variations in porosity and pore sizes. The findings provide insights and modeling inputs for the morphology of PEMWE anode CLs towards a better understanding of the relationship between fabrication, operation, and transport behavior.

We investigate the effect of dispersion media composition,water-to-isopropanol(IPA) ratio, on the structure and rheology of perfluorinated sulfonicacid (PFSA) ionomers using short-side-chain PFSA (EW & SIM; 725g/mol) as a model ionomer. In the semidilute unentangled regime, theirviscosity scaling remains unaffected by the IPA% in the solvent mixture.The dispersions display a similar polyelectrolyte-like scaling & eta;(0) & SIM; c (0.5) for all IPA%, indicatingthe role of electrostatic interactions, particularly intrachain electrostaticrepulsions, resulting in an extended conformation at dilute concentrationslike in salt-free polyelectrolytes. This is supported by the small-angleX-ray scattering results revealing a rod-like structure at low concentrationsfor all IPA%. In contrast, in the semidilute entangled and concentratedregimes, as the IPA% is increased, their viscosity scaling becomesincreasingly stronger, increasingly deviating from salt-free polyelectrolytesolutions. Such behavior is accompanied by strong viscoelastic nonlinearities,particularly shear-thickening and strain-stiffening nonlinearitieswith maximum intensities at intermediate IPA%. These trends suggestIPA is inducing ionomer association at higher concentrations via ion-dipolarinteractions due to its lower polarity. These findings showing a dramaticeffect of solvent composition on the aggregated structure and rheologyof ionomer dispersions, particularly at higher concentrations, havesignificant implications for both optimization of their morphologyand processing during membrane casting.

Image processing is a powerful tool that allows for rapid and automated data parsing in settings that occupy large variable spaces and require large data sets. Feature detection on difficultly discerned backgrounds is a subset of image processing that facilitates the extraction of quantitative metrics from otherwise subjective data. Crack detection and quantification is an important capability in polymer electrolyte membrane fuel cell quality control, failure analysis, and optimization. This work presents a technique to perform crack detection and quantification which overcomes challenges faced by commonly used image segmentation techniques. We demonstrate the use of a geometrically filtered noise-level detection technique to select a binary threshold value from which we then quantify how cracked a sample is. We demonstrate the accuracy of our technique using programmatically generated test images of known crack amounts and their performance on real-world fuel cell catalyst layer samples.

We present a study of mid-infrared (mid-IR) transmission through typical proton-exchange fuel cell and water electrolyzer catalyst-coated membrane materials, and the determination of membrane layer thickness, and catalyst layer loading, from the power of the transmitted beam. Measured membrane thicknesses lay in the range of 25-125 mu m, and catalyst areal loadings range from 0.05 to 0.35 mg/cm2. We show that the transmission spectrum correlated to membrane thickness and catalyst loading values separately, and also used a mathematical model to calculate both quantities simultaneously, from a single measurement of transmission. Measurements were carried out using a Fourier-transform infrared (FTIR) spectrometer, in specular transmission mode, in the wavelength range of 3-13 mu m (3333-770 cm-1). We discuss the potential application of this method to the development of roll-to-roll manufacturing full-area-scanning quality inspection of catalyst-coated membrane (CCM) materials.

This work investigates the influence of ballmilling (sometimes also referred to as jar roller milling) time on cathode catalyst layer (CL) inks and electrode properties using formulations and coating methods relevant for industrial manufacturing. Four CL inks with the same composition were milled for 24, 48, 72, or 96 h. Rheological investigation of these inks showed a reduction of elastic moduli and steady-shear viscosity with continuous ink milling, which is correlated to a decrease in particle-particle interactions as well as formation of smaller agglomerates. Optical microscopy (OM) analysis of the fabricated electrodes revealed a trend in surface crack formation; formulations milled for 24 h contained the lowest average surface crack area percentages of 0.370% at heavy-duty loadings of similar to 0.300 mgPt cm(-2), compared to 2.418% for the ink milled for 96 h. Further characterization of the CL through transmission electron microscopy (TEM) imaging showed a decrease in the mean agglomerate and pore size with milling time. These smaller electrode features were consistent with reduced fracture resistance and, hence, development of larger stresses during drying. Our results highlight the need to consider ink processing as an important component in defect-free CL manufacturing.

We investigate the effect of alcohol fraction (isopropanol, IPA) in a binary water-alcohol solvent mixture on the shear and extensional rheological properties, as well as the role of viscoelasticity on fiber formation of poly(acrylic acid) (PAA) in electrospinning. Comparison of the scaling of both specific viscosities eta(sp) and extensional relaxation times lambda(E) of PAA in water-IPA mixtures, showed stronger scaling compared to salt-free aqueous polyelectrolyte solutions, except for the eta(sp) in the unentangled regime displaying a polyelectrolyte-like scaling eta(sp) similar to c(0.5) for all IPA%. Such deviation suggested IPA induces association/aggregation of PAA. However, the trends between eta(sp) and lambda(E) magnitudes as a function of IPA% differ for concentrations compared in the entangled regime. The eta(sp) as well as their elastic moduli exhibit a maximum, whereas lambda(E) increases monotonically with IPA%, suggesting a complex interplay of various interactions are dictating their structure in water-IPA mixtures, affecting their shear and extensional response differently. Electrospinning experiments showed increasing IPA% reduces the onset of both beaded and uniform fibers. Analysis using dimensionless numbers indicated the enhancement of their elasticity by IPA, and the consequent stabilizing effect on their jets/filaments against break-up during electrospinning, plays a role in the improvement of their fiber formation.

This work reports the characterization of catalyst layer (CL) structure and composition for a set of gas-diffusion electrodes (GDEs) fabricated by different coating methods with the goal of developing a better understanding of CL processing-structure-property-performance relationships. The CL coating techniques used in this study were ultrasonic spray coating, and two roll-to-roll (R2R) methods – gravure and slot die. Scanning transmission electron microscopy (STEM) coupled with X-ray energy dispersive spectroscopy (EDS) analysis of GDE cross-sections provided bulk information, while scanning electron microscopy (SEM) with EDS and X-ray photoelectron spectroscopy (XPS) were used to investigate the near surface and surface composition of the CLs that will be in direct contact with the membrane once the membrane electrode assembly (MEA) is constructed. This study demonstrates that common quantification parameters, namely the F:Pt ratio extracted from cross-sectional STEM-EDS maps and top-down CL measurements with SEM and XPS, along with roughness parameters quantified from SEM micrographs are useful parameters in explaining and potentially predicting performance trends. Additionally, it highlights the importance of understanding key CL surface properties for advancing the manufacturability of high-performance components for polymer electrolyte membrane (PEM) technologies.

Low-temperature polymer electrolyte membrane water electrolyzers (PEMWE) are an attractive clean energy technology to produce hydrogen (H2), which is an energy carrier for several applications such as transportation and grid-scale energy storage and distribution (as supported by the US Department of Energy’s H2@Scale initiative). The catalyst layers -- composed of catalyst particles and ionomer, which acts as a binder for the catalyst and a proton conducting medium -- are key components of the PEMWE membrane electrode assembly (MEA). The catalyst layers are commonly fabricated by solution-processing an ink, which is a mixture of catalyst and ionomer often dispersed in a water-alcohol solvent mixture. Tuning the rheological properties of the anode inks (typically composed of iridium oxide catalyst, IrOx), particularly increasing their viscosity without significantly increasing the solids loading, to suit various scalable coating methods, is generally a challenge due to relatively low porosity and high density of the IrOx catalysts compared to the carbon-supported cathode catalysts. The typically low viscosities of the anode inks combined with high particle densities often cause stability/settling issues and challenges obtaining unform coatings, leading to inhomogeneous distribution of the catalyst that may have a negative effect on electrode performance. In this presentation we report on a dramatic enhancement in the viscoelasticity of the anode inks with aging, where the ink transitions from a liquid-like to a weak gel-like structure. The steady-shear and oscillatory shear rheology characterizations of the inks as a function of aging/time, the impact of formulation conditions (ionomer-to-catalyst ratio and dispersion media composition) on the viscoelastic enhancement with aging, and possible mechanisms for the observed behavior will be discussed. In addition to the rheological measurements, X-ray scattering characterization of the ink structure will be presented. The implications of the rheological changes on ink stability and processing will also be discussed. Additionally the impact of ink age on MEA performance will be presented.

Proton exchange membrane fuel cells (PEMFCs) are a progressive technology capable of providing on-demand energy without contributing to greenhouse gases. To meet the needs of transitioning from lab-scale fabrication to large-scale manufacturing, further analysis of catalyst layers is necessary. A common issue within the cathode catalyst layer (CL) is non-uniform distribution of the ionomer within the catalyst layer. Additionally, the interface between catalyst and ionomer is not well understood. X-ray photoelectron spectroscopy (XPS) is a highly surface-sensitive technique primed to provide new insights into the challenging interfacial interactions and chemistries among the catalyst, support, and ionomer. Although Nafion is inherently susceptible to X-ray degradation, XPS can be used effectively through a modified acquisition strategy developed previously in our group. In this work, XPS has been used to probe CLs with focus on the catalyst-ionomer interface and interactions using a series of electrodes prepared by a Mayer rod coating method. The catalyst-ionomer interface was investigated as a function of support material, Pt loading on the support, and ionomer loading. Surface information was acquired using ex situ and in situ XPS to emphasize the evolution of this technique’s capabilities at probing ionomer interactions. Complementary characterization with scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDS) mapping of catalysts and electrode cross-sections were utilized to visualize distribution of catalyst, carbon, and ionomer in the CL to assist with interpretation of XPS data. Results from this dataset emphasize the potential of this technique to study complex interfaces in PEM catalysts layers motivating further work expanding to other catalysts and ionomers.

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.

The current use of expensive and geographically-limited precious metal catalysts, typically iridium, in proton exchange membrane water electrolyzer (PEMWE) anodes is necessary to accelerate the sluggish oxygen evolution reaction (OER). However, high loadings of iridium catalyst drastically increase the capital costs of PEM electrolysis, while low loadings of iridium lead to a significant decline in the efficiency of hydrogen production. Thus, PEMWE anode fabrication methods must be optimized to achieve the highest performance with lower iridium loadings. Using high resolution characterization methods, we show nanoscale differences in anode morphology between different IrO2 anode fabrication methods, such as spray coated versus blade coated anodes. We also explore the evolution of the electrode morphology throughout its fabrication and testing lifetime. The characterization methods used in this work include high resolution nanoscale X-ray computed tomography (nano-CT) as well as plasma-focused ion beam cross-sectioning and scanning electron microscopy (pFIB-SEM). Additionally, we present quantitative data such as solid particle and pore size distributions as well as pore network extractions derived from our image-based characterizations to elucidate the morphological differences between IrO2 anodes. This conference presentation was developed based upon funding from the Alliance for Sustainable Energy, LLC, Managing and Operating Contractor for the National Renewable Energy Laboratory for the U.S. Department of Energy.

by R2R manufacturing include micro-electronics, electro-chromic window films, PVs, fuel cells for energy conversion, battery electrodes for energy storage, and barrier materials. Due to innovation in materials and process equipment, high-quality yet very low-cost multi-layer technologies can be manufactured on a very cost-competitive basis. To move energy-related products from high-cost niche applications to the commercial sector, a means must be available to enable manufacture of these products in a cost-competitive manner that is affordable by the general consumer. Fortunately, products such as fuel cells, thin- and mid-film PVs, batteries, electrochromic and piezoelectric films, water separation membranes, and other energy saving technologies readily lend themselves to manufacture using R2R approaches.Within the DOE EERE AMO, it was recognized that establishment of a program supported at the DOE National Laboratories, along with their immense design of materials and equipment modelling capabilities enabled with use of high-performance computing, could take advantage of available R2R infrastructure to manufacture new technologies. In FY 2016, a R2R Consortium was established and provided with initial "seed" funding to take an approach that was envisioned to be supportable by advanced manufacturing R2R processes. This collaborative approach was designed to foster identification and development of materials and processes related to R2R for clean-energy product development. Using computational and experimental capabilities by acknowledged subject matter experts within the supported National Laboratory system, this collaborative project would leverage the capabilities and expertise at each of four laboratories to further the development of an enabling high-volume cost-competitive platform technology. The collaboration team that is comprised of ORNL, ANL, NREL and LBNL, coordinating with EPB and other selected industry partners, was formed in April 2016 to initially address enhancing battery electrode performance and R2R manufacturing challenges. The research efforts were to predict and measure changes and results in electrode morphology and performance based on process condition changes; to evaluate mixed, active, particle size deposition and drying for novel electrode materials; to model various process condition changes and the resulting morphology and electrode performance; and to develop and validate NDE techniques for in-line measurement of battery electrode material properties. These efforts carried through FY 2017 and completed at the end of FY 2018.The approach was to look at compositions of materials with different particle sizes to make electrode samples using a R2R manufacturing process. The shape, size, and morphology of the materials, the chemistry of the formulation, the nature of slurries, their coating rate, the rate of drying all play a role in determining the final coating architecture, quality, and performance. A commercial cathode material was selected to make a series of cathodes and anodes by single pass, dual pass and slot die methods. Analysis of all the compiled results for this battery electrode development were that the best performing cathodes included: dual-pass electrode with large particles near the foil, mixed small and large particles, and small particles only. Whereas, the best performing anodes were with a mix of small and large graphite particles. An additional core project was added in FY 2017 to conduct studies of fuel cell materials that can be produced using R2R processes. The goal of this project is to explore, understand and optimize material and process parameters to support increased throughput, increased quality, and reduced cost for high volume production of gas-diffusion electrodes (GDEs) for PEMFCs. Project work in FY 2018 were to use a R2R process to fabricate electrodes without ionomer overlayer that can produce the equivalent mass activity as spray-coated electrodes with an ionomer overlayer. Oxygen-limiting current measurements were utilized to optimize oxygen mass transport. Alcohol-rich solvents were investigated, and results were that the alcohol and water ratio can be tuned to control ionomer distribution. Water-rich solvents produce a more dispersed ink which results in better high-current density performance.

As the production of polymer electrolyte fuel cells expands, novel quality control methods must be invented or adapted in order to support expected rates of production. Ensuring the quality of deposited catalyst layers is an essential step in the fuel cell manufacturing process, as the efficiency of a fuel cell is reliant on the catalyst layer being uniform at both the target platinum loading and the target ionomer content. Implementing a quality control method that is sensitive to these aspects is imperative, as wasting precious metals and other catalyst materials is expensive, and represents a potential barrier to entry into the field for manufacturers experimenting with novel deposition processes. In this work, we analyzed catalyst inks to determine if their ionomer content could be quantized spectroscopically. Attenuated total reflection (ATR) Fourier transform infrared spectroscopic technique was investigated producing a signal proportional to the ionomer content. ATR spectroscopy was able to quantitatively differentiate samples in which the ionomer to carbon mass ratio (I/C) varied between 0.9 and 3.0. The I/C ratio was correlated to the measured ATR signal near the CF2 vibrational bands located between 1100 cm-1 and 1400 cm-1. The experimental results obtained constitute a step toward the development of novel quality control methodologies for catalyst inks utilized by the fuel cell industry.