Abstract
R2R processing is used to manufacture a wide range of products for various applications which span many industrial business sectors. The overall R2R methodology has been in use for decades and this continuous technique traditionally involves deposition of material(s) onto moving webs, carriers or other continuous belt-fed or conveyor-based processes that enable successive steps to build a final version which serves to support the deposited materials. Established methods that typify R2R processing include tape casting, silk-screen printing, reel-to-reel vacuum deposition/coating, and R2R lithography. Products supported by R2R manufacturing include micro-electronics, electro-chromic window films, PVs, fuel cells for energy conversion, battery electrodes for energy storage, and barrier and membrane materials. Due to innovation in materials and process equipment, high-quality yet very low-cost multilayer technologies have the potential to be manufactured on a very cost-competitive basis. To move energy-related products from high-cost niche applications to the commercial sector, the means must be available to enable manufacture of these products in a cost-competitive manner that is affordable. 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. However, more early-stage research is needed to solve the challenge of linking the materials (particles, polymers, solvents, additives) used in ink and slurry formulations and the coating and drying processes to the ultimate performance of the final R2R product, especially for a process that uses multiple layers of deposition to achieve the end product.
To solve the problems associated with these challenges, the R2R Collaboration is executing a research program with outcomes that will ultimately link modeling, processing, metrology and defect detection tools, thereby directly relating the properties of constituent particles and processing conditions to the performance of final devices. The Collaboration team and their research efforts with industry involvement are illustrated schematically in Figure 1. This collaborative approach was designed to foster identification and development of materials and processes related to R2R for clean-energy materials development. Using computational and experimental capabilities by acknowledged subject matter experts within the supported National Laboratory system, this project leverages the capabilities and expertise at each of five National Laboratories to further the development of multilayer technologies that will enable high-volume, cost-competitive platforms.
Figure 1. The R2R AMM DOE Laboratory Collaboration team and major research efforts. Source: ORNL
A typical R2R process has three steps: (1) mixing of particles and various constituents in a slurry, (2) coating of the ink/slurry mixture on a substrate, and (3) drying/curing and processing of the coating. Final performance of devices made via R2R processes is dependent on the active materials (e.g., electrochemical particles in battery or fuel cell electrodes) and the device structure that stems from the governing component interactions within the various steps. However, a fundamental understanding of the underlying mechanisms and phenomena is still lacking, which is why industrial-scale R2R process development and manufacturing is still largely empirical in nature.
The FY 2019 through FY 2021 program addresses aspects of the following two targets from the AMO Multi-Year Program Plan:
• Target 8.1 Develop technologies to reduce the cost per manufactured throughput of continuous R2R manufacturing processes.
o Increasing throughput of R2R processes by 5 times for batteries (to 50 square feet per minute (50 ft2/min)) and capacitors and 10 times for printed electronics and the manufacture of other substrates and MEs used in support of these products.
o Developing resolution capabilities to enable registration and alignment that will detect, align, and co-deposit multiple layers of coatings and print < 1-micron (1 µm) features using continuous process scalable for commercial production.
o Developing scalable and reliable R2R processes for solution deposition of ultra-thin (<10 nm) films for active and passive materials.
o Develop in-line multilayer coating technology on thin films with yields greater than 95%.
• Target 8.2 Develop in-line instrumentation tools that will evaluate the quality of single and multilayer materials in-process.
o Developing in-line QC technologies and methodologies for real-time identification of defects and expected product properties “in-use/application” during continuous processing at all size-scales with a focus on the “micro” and “nano” scale traces, lines, and devices
o Developing technologies to increase the measurement frequency of surface rheology without significant cost increases with a goal of a 10-nanometer in-line profilometry at a production rate of 100,000 square millimeters per minute (100,000 mm2/min).
The category of electrochemical conversion devices that apply to fuel cells was selected to begin the FY 2019 efforts; however, research was kept general enough that other applications could include solid-state batteries (SSBs), low-temperature water electrolysis, CO2 separation/reduction and water filtration/purification. Of specific interest was ES technology. ES is a technology platform that is capable of fabricating advanced nanofiber-based materials across a wide range of applications and industrial domains. This process offers a high surface-to-volume ratio which makes nanofibers the ideal candidate for various applications where high porosity and high surface areas are desirable. There are currently a few barriers to commercialization of ES. It is limited to filtration applications produced in single-step, batch processes; advanced materials used in electronics, fuel cells and batteries that require continuous, post-ES processing; complex multi-step physiochemical processes that create challenges transferring technology from the bench to commercial production; and time-consuming material transfer process steps. However, integration of R2R processing can enable the adoption of electrospun advanced materials into a wider range of applications. In FY 2020, the R2R Collaboration successfully completed all tasks toward development of an ES method using a R2R manufacturing process. Additionally, efforts continued for continuum-scale modeling, simulation, processing, and manufacturing techniques and metrology that demonstrate the feasibility and potential for scale-up.
ANL conducted in-situ annealing studies at the Advanced Photon Source on LLZO nanofibers with results that showed the fibers were slightly richer in lithium as compared to the stoichiometric composition. These studies suggested that the optimum thermal treatment temperature of the as-spun LLZO nanofibers is around 700 °C. Electrospun nanofiber filter media of ~35 µm thickness and 97% filtration efficiency was developed that can tolerate common hospital disinfection processes, such as autoclaving and alcohol soak, which enables reuse of N95 respirators and masks as part of a multi-lab COVID-related project sponsored by the DOE Basic Energy Sciences Office. Disinfection evaluations demonstrated no drop in filtration efficiency, showing great promise in terms of reusability. The ES recipe was improved to increase production rate by six-fold which reduces production cost and facilitates commercialization of the electrospun filter media technology.
ORNL conducted annealing studies that showed no significant change in the LLZO fiber morphology for the three different annealing times; however, the fibers must be annealed in argon to remove the carbonates and increase their conductivity. Baseline data for particles sizes from different mixing experiments were provided to LBNL to support modeling efforts. The thickness and Pt loading of a coating prepared by different mixing techniques was determined to have a larger variability than is typical for slot die coating. A gas diffusion catalyst layer loading of 0.1 mgPt/cm2 was achieved across multiple electrodes with 8 wt.% Pt/C slurries processed with a R2R slot-die. Wetting experiments at ORNL using 1-propanol and ethanol solutions with water showed that a lower alcohol content (higher water content) results in higher contact angles because of the hydrophobic microporous layer surface. Addition of Nafion ionomer did not decrease the contact angle; however, the high viscosity of the ink due to the presence of Pt/C decreased the apparent contact angle. Coatings trials were performed with coating gaps smaller and larger than the wet coating thickness. Larger gaps gave smaller Pt loading variations (4-7%) than the smaller gaps (Pt loading variation is typically 10-40%). Composite Al-LLZO/PAN nanofibers were synthesized that had diameters from approximately 800 nm to 1.4 µm. Spectroscopic analyses showed that the Al-LLZO is uniformly distributed on carbon fiber structures. Deposition of an ionomer overlayer on a PEMFC cathode at the same time as the cathode electrocatalyst layer was achieved and trials were completed that eliminated the need for it altogether. Coin cells based on composite cathodes were assembled and tested. The composite cathodes exhibit more noticeable power law contributions, in the low frequency regime, which is indicative of a more pronounced ion diffusion mechanism. . At higher C-rates, a marked improvement in the specific capacity is evident for the cells based on composite cathodes and especially for the cathode filled with porous Al-LLZO fibers.
NREL received a three-layer slide die and multiple high-resolution cameras were identified for use in flow imaging and bracketry was obtained to mount the cameras in different configurations. The slide die was installed in a hood for initial flow testing and the newly machined flow inlets were installed to make the die operational. NREL provided SNL a detailed listing of target multilayer coating structures to clearly define priority modeling scenarios as the 2-layer and 3-layer slide die models are being developed. In situ testing was performed using R2R slot-die-coated GDEs. Electrochemical performance evaluation determined that catalyst layers without ionomer overlayers (spray or dual slot-die coated) performed better than catalyst layers with ionomer overlayers. None of the overlayer GDEs performed better than the single-layer GDE; however, the two-layer GDE with the least amount (thickness) of overlayer provided the best performance of the two-layer GDEs. NREL fabricated MEAs at different line speeds and different Nafion overlayer feed rates. Electrochemical impedance spectroscopy measurements were performed for two MEAs, and the results were largely consistent with the polarization data. A high level of non-uniformity in the loading as well as the visual appearance of the electrodes was observed. Multilayer and ES structures were identified that will be produced in sheet or roll form, along with known or expected metrology needs. NREL developed spectroscopy methods to be used for heterogeneous particle-polymer inks focused on continuing to improve FTIR-ATR measurement methodology by exploring scattering and index-of-refraction-matching (Christiansen effect) the spectroscopic behaviors that may be affecting measurements. Indices of refraction data for Nafion were determined to overlap near the previously measured absorption troughs indicating that the Christiansen effect may be prevalent. Mid- to far-IR measurements of common porous carbon substrates were made to understand the transmission (penetration of dissimilar layers) characteristics of these materials and elucidate possible modes of measurement for observing active layers adjacent to and “through” the porous substrate.
LBNL designed a coating and drying observation table that will be used to simulate the fabrication conditions for materials being investigated at the other Labs, but at a scale that is small enough to be easily combined with analytical tools such as the Advanced Light Source beamlines. An automated mixing experimental setup comprised of custom electronics and computer-controlled disperser, pump and motorized lab jack was constructed. The setup also includes a flow-through particle size analyzer operating at a 5 Hz sampling rate. Preliminary experiments were performed on carbon black dispersed in mixtures of 70% alcohol in water and 10% alcohol in water. Results showed significant effects of solvent composition on particle size distribution. The chiller for the mixing experimental setup was upgraded and additional code was added to the equipment control library to control and query the chiller which will improve compatibility with the dispersion preparation techniques used by the other R2R teams. Custom temperature bath sample holders were designed that are compatible with the motorized sample stages to load samples consistently and keep them at a controlled temperature while preventing the sample container exteriors from being contaminated with coolant. A portable custom cart with an adjustable platform was designed to isolate the motorized components of the mixing experiment setup (mixer, stage, pump, and chiller) from vibration-sensitive instruments (particle size analyzer and soon, confocal microscope) and to position the mixer and pump in a consistently reproducible position relative to different instruments. The LBNL computer-controlled mixing setup was transformed into a portable mixing station, allowing reliable recreation of samples in the vicinities of instruments in different locations. An update was released to LBNL’s highly flexible open-source simulation package that will be used to implement upcoming models.
SNL completed the deposition models of single- and two-layer slot-die coating flow along with a working user interface to improve usability. Rheological data for a representative single- and two-layer-slot coating stack were fit with standard constitutive equations and implemented in the models. The initial polymer (PMA) and solvent (toluene) single- and two-layer drying model was completed with comparable values found in the literature. The single-layer model operating space was validated using process inputs from ORNL. SNL models predicted the coating window (11 in H₂0 for 60 µm gap and 14 in H₂0 for 60 µm vacuum at 3 ft/min coating speed) for ORNL single-layer slot die coating trials with a complex 8 wt % Pt/C catalyst ink that guided the experiments aimed at increasing the coating gap (slot-to-web) from 60 µm to 90 µm for better coating quality at a 30 µm coating wet thickness. A new mesh/solid-model design for the slot die model was developed to ease in gap-based parameter studies. A significant finding that needs to be experimentally verified is that the models predict that the low-vacuum limit disappears at all web speeds for smaller gaps and concentrated Pt/C inks. Stability analysis on the slide-only multilayer models has revealed potential for interlayer defects. Development continued for the first-of-a-kind, 3-layer slide die model. Configurations were planned for the slide die coating trials. A special slide-only model is being used to help NREL redesign the lip geometry. SNL and UNM continued to advance a machine-learning model to connect coating defects of particulate conductive films (pinholes, striations) to process parameters for gravure coating/printing. Data was captured with high-end optical microscopy coupled with open computer vision image recognition tools.
Technology transfer for these and other technologies applicable to R2R manufacturing was initiated through collaboration with industry partners. Three CRADA projects with industry continued and two of those projects completed in FY 2020.