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This is the third quarterly report covering the period from July to September 2022 and includes a review of “The Big Challenge of Carbon in the Additive Manufacturing of Solid Oxide Electrolyzers (SOEC) and Solid Oxide Fuel Cells (SOFC)”, prepared and submitted for publication in a peer-reviewed journal of ceramics. Below is the first part of a review paper covering the additive manufacturing literature for SOFCs and TECs. The second part focuses on the opportunities and challenges of additive manufacturing technology and will be published in January 2023. #nasf #additive manufacturing
The NASF-AESF Research Committee selected the Electrodeposition Project to develop low-cost and scalable manufacturing processes for hydrogen fuel cells and electrolyzers for clean transportation and distributed energy applications. This report is the third quarter report covering the period from July to September 2022. Since the start of the project in January 2022, project leader Majid Minari Jolandan, Ph.D., has served at Arizona State University (ASU) as an adjunct professor of mechanical and aerospace engineering at the Ira A. Fulton College of Engineering. , Transport and energy.
During this quarter from July to September, Dr. Minari decided to move to the University of Texas at Dallas (UTD). Therefore, this report covers the enrichment period from July 1 to August 8, 2022.
As is standard practice in academic contracts, grants will follow the nominated professor (junior Ph.D.). It is logical that there is a shift in the plan, and from August 8 to October 1, 2022, work is not being carried out. On October 1, UTD initiated the disbursement of funds and this work will continue for the next three years and one quarter, with each additional year pending Board approval.
During the interim period, a review was prepared and submitted to the peer-reviewed journal Journal of Ceramics for publication. Below is the first part of the review file. The second part will be released in January 2023. Click here for a printable PDF version of the first part.
The Big Challenge of Additively Manufacturing Solid Oxide Electrolyzers (SOEC) and Solid Oxide Fuel Cells (SOFC) to Power Electrolyzed Hydrogen Economics on the Road to Global Decarbonization – Part 1
Majid Minari Jolandan Faculty of Mechanical Engineering, University of Texas at Dallas Richardson, Texas, USA
Solid oxide electrolyzers (SOEC) and solid oxide fuel cells (SOFC) are the main high-temperature devices for realizing the global “hydrogen economy”. These devices are multimaterial in nature (ceramics and cermets). They have multilayer configurations of different scales (from micrometers to hundreds of micrometers) and different morphological requirements (porosity and compaction) for each layer. Adjacent layers must be chemically and thermally compatible and mechanically resistant to high temperatures. In addition, many elements must be combined with each other to obtain acceptable power. The most significant obstacles to the widespread global adoption of these devices are their high cost, as well as problems with reliability and durability. Given its complex structure and stringent requirements, additive manufacturing (AM) has been proposed as a possible technological route to low-cost production of durable devices to achieve economies of scale. However, there is currently no single AM ​​technology capable of 3D printing these devices at a full battery charge, or more difficult at the stack level. This article describes the challenges that need to be addressed if additive manufacturing is to become a viable way to make SOECs and SOFCs. A list of suggestions is provided to facilitate such efforts.
Key words: SOFC, SOFC, hydrogen economy, renewable energy, decarburization, additive manufacturing, market competitiveness, scaling and large-scale production.
As an important chemical feedstock in the global economy, hydrogen is in growing demand in transportation, steel production, power generation and load balancing for network services. Recently, significant global investments have been made in the “hydrogen economy”, which, in turn, will stimulate the production and processing of clean hydrogen technologies. For example, in the United States, the Department of Energy’s “Hydrogen Shot” mission is to bring the price of hydrogen to $1 per kg within 1 decade (“1 1 1″). Industries such as long-haul transportation of heavy and medium-sized vehicles, high-temperature thermal power generation, energy storage and synthetic fuels for aviation and marine transport are among the most energy-intensive and difficult to decarburize. Hydrogen has been proposed as the main energy source for the decarburization of these sectors (Fig. 1).
Hydrogen is the simplest element on Earth, but it is not usually found alone in nature. It must be obtained as a result of a chemical reaction of the compounds that contain it. Currently, most (~95%) of the world’s hydrogen is produced by steam reforming of methane (SMR), which releases the greenhouse gas CO2. Electrolytic hydrogen (without any contamination) is more expensive than hydrogen produced using the SMR process. 1 The current market for hydrogen is approximately 10 million tons per year (MTPA) in the US and 65-100 million tons per year worldwide. However, only about 2% of the world’s hydrogen production is produced by electrolysis. The market for electrolytic hydrogen is likely to grow significantly, to at least 100 million tonnes per year by 2050, to meet potential future demand and help hard-to-decarbonize industries. To meet this market size, U.S. electrolyzer capacity is likely to increase from 0.17 gigawatts (GW) today to 1,000 GW by 2050, or a compound annual growth rate of 20 percent between 2021 and 2050, with annual production demand exceeding 100 GW/year. .2 In addition, the decarburization scenario would require more than 50 GW of internal fuel cell capacity, with annual production requirements in excess of 3 GW/yr. Investments in manufacturing and process development, as well as scaling and industrialization of production, will reduce the cost of hydrogen electrolysis.
SOFC and SOEC are considered indispensable electrochemical energy storage and conversion devices for the global advancement of the hydrogen economy. SOEC is an energy storage device that produces stored hydrogen from electricity and water (water electrolysis), CO and oxygen from CO2 electrolysis, or even co-electrolysis of water and CO2 to produce syngas (CO + H2) and oxygen. 3 High temperature steam electrolysers use electricity (preferably renewable energy) and heat (preferably waste heat or low cost heat generators such as nuclear reactors) because they are powered by steam. SOFC converts the chemical energy stored in the fuel (H2, CO, CH4, etc.) directly into electrical energy through an electrochemical reaction (by oxidizing the fuel). SOFCs typically consist of about 40-60 individual cells, each producing about 25 watts, interconnected to form a module. four
The main barriers to using existing technologies are production time and cost, quality assurance and quality control, and packaging durability. Despite their high efficiency, the introduction of these devices to the global market currently lacks economies of scale. The benefits of the hydrogen economy will be best realized in large deployments and across multiple applications. However, the high cost of these devices compared to alternative energy systems is perhaps the most important factor hindering their widespread adoption.
SOFCs, SOFCs and their packages are geometrically complex, inherently multimaterial and multilayer devices. Batteries are made from thin active elements (~10-50 microns of electrolyte and ~50-300 microns of anode and cathode) of various composition and microstructure (porous anode and cathode and dense electrolyte). A complete traditional manufacturing process can include more than a hundred steps, including injection molding, screen printing, slip casting, slurry spraying, spray pyrolysis, dip coating, thin film deposition, chemical infiltration and catalyst ex-solution, and laser cutting. , punching, laminating, stacking and firing/sintering of production tapes. Figure 5-7 Numerous steps, most of which require manual entry and multiple connections and seals, result in poor reliability, durability and reproducibility, high cost, and long time to market (Figure 2). To accommodate these devices on a global scale, manufacturing techniques are needed to reduce the number of battery components in the battery pack, reduce processing temperature, reduce the number of processing steps, and reduce overall processing time. These improvements can lead to higher yields and lower costs for large scale production. 2
Figure 2. Problems with current fabrication technologies used to manufacture SOFCs and SOECs and attributes of ideal fabrication technologies for these devices.
The development or application of suitable additive manufacturing (AM) methods can reduce manufacturing costs, reduce the waste of expensive raw materials, provide more environmentally friendly materials and processing methods, and use fewer solvents. Additive manufacturing techniques can reduce the number of steps and lead to stronger and more reliable devices. Another benefit of additive manufacturing technology could be increased space for designing more efficient devices, such as the realization of complex geometries beyond flat and tubular, or increased surface area and increased power density of electrochemical reaction centers. 8 Thermomechanical Simulation of SOFCs and 3D Electrodes The rational design of 3D fabricated composite electrodes demonstrates the performance benefits of 3D printing when certain design criteria are met. 9.10
Given the largely nascent stage of the SOEC and SOFC industries, data on supply chain requirements and constraints is limited. 1 High-volume production of these energy devices requires the creation of multidisciplinary supply chains to support components, materials, and equipment. 2 Some cells may experience supply chain issues for materials and components such as interconnects because interconnects are more prone to degradation (cracking, delamination and pinholes in the coating). AM allows for distributed production, which can raise some issues in the supply chain.
The purpose of this article is to provide a brief overview of the issues that need to be addressed if additive manufacturing is to become a viable way to manufacture these energy devices. The aim is to help identify current bottlenecks and the required R&D strategies that will result in maturation of these technologies and at-scale production of these devices. The aim is to help identify current bottlenecks and the required R&D strategies that will result in maturation of these technologies and at-scale production of these devices. The goal is to help identify current bottlenecks and the necessary research and development strategies that will lead to the improvement of these technologies and the mass production of these devices. The goal is to help identify current bottlenecks and the necessary R&D strategies that will lead to the improvement of these technologies and the mass production of these devices. A list of suggestions is provided to facilitate such efforts. This article does not discuss in detail the various additive manufacturing processes and how they work. We encourage readers to refer to more accurate reviews of the various processes. 5:11-16
At a basic level, these electrochemical devices consist of an electrolyte and two electrodes (anode and cathode). Complete cells and stacks also require connecting and sealing materials. The electrolyte and electrodes must be of appropriate thickness to reduce resistance and diffusion resistance. The microstructure and to some extent the thickness of the functional materials in these devices primarily determine the performance of the device. 17 The electrolyte is pure ceramic, while the anode and cathode are cermet composites (cermets). A dense thin electrolyte is needed to separate the oxidizing gas from the combustible gas. When the battery is supported by electrodes, the thickness of the electrolyte can be greatly reduced (down to a few microns), which greatly reduces the overall ohmic resistance of the battery. However, thinner electrolytes limit the number of applicable 3D printing techniques. The cathode and anode are a mixture of electrolyte and electrode materials, which is the first choice for reducing polarization and expanding the three-phase boundary (TPB).
ZrO2 doped with yttrium (Y) or scandium (Sc) is a conductor of oxygen ions above 800°C. At present, yttria stabilized zirconium oxide (YSZ) is the newest electrolyte material for SOFC and CHP. YSZ can usually be sintered in the range of 1300-1500°C. 18 Sc-stabilized zirconia (ScSZ) and gadolinium-doped cerium (GDC) have also been used as electrolytes. 19 The electrolyte must be sufficiently dense to avoid fuel leakage/oxidizer gas on the electrodes and to reduce the resistance to diffusion of oxygen ions in the electrolyte. The electronic conductivity of the electrolyte must be low to prevent losses due to leakage currents. The density of an electrolyte, related to porosity, plays an important role in its conductivity. Defects, pinholes, and other electrolyte defects can significantly degrade the electrochemical performance of a battery. Therefore, the sintering step of the electrolytic ceramic is of decisive importance.
Nickel-YSZ (Ni-YSZ) cermets are used as anodes in SOFCs and as cathodes in SOFCs (considered as fuel electrode in both devices). The YSZ ceramic in this cermet provides ionic conductivity and structural support, while Ni acts as a catalyst and electron conductor. 20 The cathode in SOFC and the anode (or oxygen electrode) in SOEC can be made of mixed conductors such as lanthanum strontium cobalt ferrite (LSCF) or lanthanum strontium cobaltate (LSC). LSCF is a mixed ionic-electronic conductor capable of rapidly conducting oxygen ions and electrons. Promotes the oxygen reduction reaction as a highly active catalyst. Highly alloyed strontium (Sr) LaMnO3 (LSM) in YSZ cermets can be used for less demanding applications. LSM has good compatibility and low reactivity with YSZ, as well as a coefficient of thermal expansion (CTE) similar to YSZ. In this case, the LSMs provide the electronic conductivity and catalytic function, while the YSZs are structural components and provide the ionic conductivity. Some designs use a gadolinium-doped cerium oxide (GDC) buffer layer between the electrolyte and the LSCF cathode. To prevent the oxygen electrode material from reacting with YSZ, a thin layer (0.1 to 5 microns) of GDC can also be used. 3
Nickel and lanthanum elements are considered environmentally burdensome materials in terms of SOFC and SOEC recovery and circular economy. This burden can be addressed by redesigning and considering circular economy approaches (estimated at about 70%). twenty one
The cathode and anode are porous, conductive, and must have high catalytic activity for fuel oxidation and oxygen reduction, which requires a high density of electrochemical reaction centers or a three-phase boundary TPB. Electrochemical reactions take place in the TPB where electrons, ions and reactants meet. Porosity is necessary to provide mass transport channels, i.e. diffusion of gaseous fuels and by-products. The polarization in each electrode includes ohmic polarization, activation polarization, and concentration polarization and must be optimized to minimize overall cellular polarization. 22 Ohmic polarization, activation polarization, and concentration polarization are related to conductivity, three-phase boundary, and porosity, respectively. The volume percentage (vol.%) of pores is an important factor. In addition, factors such as the correct connection (on/off) of the pores, the size and distribution of the pores by size, and the tortuosity of the pores play a dominant role in influencing the polarization properties.
In addition to the pores formed during the reduction of NiO to Ni, porosity is usually provided by pore formers (such as graphitic carbon, short carbon fibers, polymer beads, flour, rice, starch, etc.). 22 In general, larger blowing agents (~20 µm) are more effective than smaller ones (several microns). 22 A certain volume percentage of pore former is required to form an open permeable network of pores, typically ~30 volume percent. It has also been suggested that composite blowing agents consisting of two or more blowing agents of different size ranges can be used to improve pore network connectivity and tune shrinkage kinetics. 22 Other methods such as cryocasting can also be used to create pores. In freeze casting, the sublimation of ice in an aqueous suspension creates porosity. twenty three
Figure 3 – Schematic side view (left) of a layered structure in SOEC/SOFC. Relevant material, morphology and other properties of each layer (right).
An interconnect is a layer that sits between each individual cell and connects them in series. Interconnects are exposed to the oxidation and reduction sides of the cell at high temperatures and therefore place the highest demands on the materials of other battery components in terms of stability. Two types of interconnects are commonly used in these devices: metal oxide and ceramic oxide. 24 Ceramics are more stable (especially long-term) in oxidizing environments, but less electrically conductive and more expensive than metals. Metal interconnects are cheaper and more conductive, but they are not as stable at high temperatures as ceramics. One way to improve the stability of metallic interconnects is to coat them with protective ceramic layers, including oxides, perovskites, and spinels. The most common ceramics used in interconnects include lanthanum and yttrium chromites (YCrO3 and LaCrO3) and p-type perovskite semiconductors. 24 The additive manufacturing processes for this specialty ceramic are very limited. The main problem with these materials is that it is difficult to sinter chromium-containing oxides, since the evaporation of Cr-O particles complicates the sintering process.
Ferritic stainless steels (FSS) are good metal candidates because they are cheap, have good CTE, are easy to fabricate, and form highly conductive oxides on their surface. However, evaporation of chromium (Cr) at high operating temperatures has been a major limiting factor. The formation of native chromium oxide increases ohmic resistance and chromium poisoning of SOFC cathodes, which are the two main degradation mechanisms in these devices. 24 Ceramic-metal composites (cermets) are also being considered for interconnects and good electrical conductivity due to their thermal stability at high temperatures.
Sealant is another important component of these devices, and additive manufacturing has not yet been reported. Typically, the maximum operating temperature of these devices is determined by the glass transition temperature of the sealant. Airtight (airtight) sealants provide electrical insulation (prevent short circuits) and prevent mixing of fuel and oxidizers. Glass-ceramic sealants are inexpensive, have acceptable characteristics and stability (in reducing and oxidizing environments). 25 The thermal properties of a sealant, including CTE, glass transition temperature, crystallization temperature, and melting point, are critical parameters in selecting a sealant. Glass-ceramic sealants form a chemical bond with adjacent components, so no external load is required during operation. These sealants have low cost and sufficient stability, as well as flexible design through compositional changes. Partial crystallization can be achieved by sintering at a temperature above the operating temperature of the device, resulting in a hermetic seal. Currently, glass ceramics are mainly produced by rolling, casting, pressing, centrifugal casting and other methods. Both the sealant and the interconnect can be made from ceramic materials. Therefore, additive manufacturing processes based on all-ceramics and cermets can be developed.
Several AM processes have been used to 3D print these devices, though mostly for some devices. 13,16 These methods include inkjet printing (IJP),11,26-41 stereolithography (SL)8,42 and digital light processing (DLP)18,43. Nowadays, inkjet printing is a popular method. However, with the possible exception of printed corrugated surfaces, the printed cells and functional layers have so far been flat and advanced 3D configurations with the potential to achieve higher power densities have not been reported. We also note that most of the reports refer to SOFC. However, given that these devices are very similar in structure and operation, the process can be applied to printed SOECs.
Inkjet printing of SOFC components is widely described in the literature. The first report on fuel cell inkjet printing dates back to 2008, when the authors printed a NiO-YSZ interlayer and a YSZ electrolyte layer (both about 6 µm thick) on a commercial NiO-YSZ anode substrate. 44 Since then, various SOFC components have been printed using inkjet printing, including electrolytes29,40,41,44 anode micropillars,45 oxide cathodes and composite cathodes,28,32-34 cathode intermediate layers,35 cathode, intermediate layer, and electrolyte. , 30 anode and electrolyte, 36 even the whole SOFC. 26 It has been demonstrated that inkjet printing can be used to produce complete SOFCs with electrochemical properties consistent with conventional processing methods. 30 Electrolyte layers have been reported in thicknesses ranging from submicron26 to several microns. Most inkjet batteries support anodes. 26,44,45 Additional layers are often added using traditional manufacturing processes such as screen printing or brushing. In addition to printed structures, inkjet printing has also been used to introduce or infiltrate other chemicals such as yttrium-doped barium zirconate into porous electrodes. 31
Flanders et al. Using optimized ink formulations to print arrays of micropillars and squares in a sintered structure, a minimum feature size of 35 µm was achieved. 29 Khan et al. The entire submicron YSZ anode SOFC was printed using a commercial low cost office printer (HP inkjet printer). 26 For ink synthesis, the authors used a granulometric composition of 0.15–0.19 µm, which is smaller than the printer nozzle diameter. 26 The printed SOFC maintained a high open-circuit voltage and a strong and uniform microstructure during electrochemical testing, and achieved an output power of 730 mW/cm2 and a low degradation rate of 0.2 mV/h at 650°C during endurance testing.
In 2022, Jang and Kelsall reported using inkjet printing to print 3D NiO-YSZ structures to improve SOFC performance. 45 In particular, bars 50 µm in diameter and 100 µm apart were printed using custom NiO-YSZ inks. A column height of about 28 µm was obtained with 90-layer printing. Initially, the authors prepared porous particles of the NiO-YSZ carrier by mixing the powder with a graphitized soot blowing agent, pressing the particles, and heating to 800°C. Then, the NiO-YSZ columnar structure layer was printed on the substrate with an inkjet printer, and then the YSZ electrolyte was dipped onto the column surface. The small particle size compared to the substrate is used to prevent clogging of the nozzle and no blowing agents are used. The YSZ electrolyte was sintered at 1450° for 5 hours. The cells were made by applying LSM-YSZ ink to a sintered YSZ surface followed by heat treatment at 1000°C for 2 hours. 45
The authors argue that in the structure of the NiO-YSZ column, the increase in power density is associated not only with a larger area of ​​the electrode/electrolyte interface, but also with an increased length of TPB in the Ni-YSZ column. Because the inkjet-printed NiO-YSZ bollards do not have a blowing agent, the porosity in the bollards is only due to the reduction in volume associated with reduction of NiO to Ni, which is less than the porosity in the substrate coming from the blowing agent. Less porosity in the column reduces gas permeability, especially for taller columns with smaller diameters. Therefore, it is necessary to determine the optimal height of the column. 45
Huang et al reported printing microtubular SOFCs using inkjet printing. 46 Anode (NiO-YSZ), electrolyte (YSZ) and cathode layers were inkjet printed on a cylindrical ceramic substrate. According to the cross-sectional SEM images, the thickness of the cathode and anode is less than 30 µm. The 3D printed battery lasted over 4,000 hours at 18.5A DC and completed over 1,000 fast thermal cycles without battery failure. 46
Inkjet printing is compatible with metallic, resin, ceramic and composite inks. This requires relatively inexpensive equipment, and traditional office printers can be modified for this purpose. The most important aspects of the manufacturing process include active material ink formulation, inkjet deposition, print optimization, and characterization of inkjet printed films. These parameters together affect the electrochemical characteristics of printed batteries. Inkjet printing requires “printing” inks that require certain rheological properties. For “stabilized” ink, a suitable dispersant should be used to prevent settling and agglomeration of particles that can clog the nozzle. 37 The particle size must also be much smaller than the nozzle diameter. This may require custom ink synthesis. 47
Inkjet-printed SOFC and SOEC devices can achieve lower operating temperatures because the printed electrodes and electrolytes can be thin films (a few micrometers to sub-micrometers), which reduces ion transport energy loss. In principle, inkjet printing can be extended to large area production because, in addition to nozzle movement, the substrate can also move under the nozzle. For example, with the right design, inkjet printers can be integrated into roll-to-roll processes.
If inkjet printing is used to print several functional layers, considering different sintering temperatures, not all layers can be sintered at one time, multi-stage sintering is usually used. For example, in one study, the anode/anode/electrolyte sandwich was fired together at 1400°C for 2 hours, and after the sandwich cathode and cathode were printed, the cell was fired again at 1200°C for 1 hour. 30 In the same vein, new designs for monolithic fuel cell stacks that require only one heat treatment during fabrication are promising. 48
Ink jet ink has a low viscosity and therefore a low solids content. It is believed that thermal inkjet printing (as opposed to the more traditional piezoelectric inkjet printing) allows the use of ink with a higher solids content, which improves printing efficiency. 38 In inkjet printing, the required electrode porosity can be controlled by adjusting the print density using “greyscale” in the digital print file. 32 To produce LSCF cathodes with controlled microstructure, porosity and thickness, Han et al. Adjust the grayscale, brightness, or “brightness” value of the black and white picture in the software from 0 (black) to 255 (white). 32 A similar approach can be used in multi-cartridge printers to fabricate composition-controlled composite cathodes.33 In particular, for printing LSCF/GDC composites, the content and porosity of the LSCF and GDC layers were controlled by controlling the image and print cycle times for tuning. 33 The authors concluded that the optimal dose of HDC in the composite cathode improves the rate of oxygen reduction. Similarly, inkjet printing of composite cathodes (LSCF-GDC) has been reported. The composition and microstructure of the composite cathode are controlled by adjusting the ratio of raw materials in the ink and changing the printing parameters. 33
However, for SOEC and SOFC printing, inkjet printing has inherent limitations. This process is limited to thin films (hence the flat designs) and requires special design to produce non-flat surfaces. It should be noted that micropillar-type geometry was obtained using this process. 29,45 For example, in 90-layer printing, the column height is about 28 µm. 45 Obtaining thicker samples as substrates requires many layers of printing and is therefore time consuming. Substrate wetting and film curing become important for multi-layer printing and must be considered in process design. Some inks use organic solvents, which may not be ideal. 11 Although there are numerous reports of the use of aqueous inkjet inks. 29,40,41
Aerosol inkjet printing (AJP) has also been used to print SOFC components. This is a more complex and expensive device than an inkjet printer. Sukeshini et al. Deposition of YSZ electrolytes and functionally graded anode interlayers by AJP was reported using NiO and YSZ ink slurries with different compositions. 27 The system’s dual spray configuration allows materials to be mixed on demand for composite sandwich anode deposition. For the composite anode layer, the authors prepared two separate paints with a solids content of about 35 wt.% using YSZ and NiO powders, solvents, dispersants, binders and plasticizers. The NiO-YSZ composite with a composition gradient was deposited on a YSZ substrate and sintered at a temperature of 1400°C. A hand glued LSM sintered at 1200°C was used as the cathode layer to complete the cell. 27 Prior to electrochemical characterization, the anode side was reduced for several hours in 5% hydrogen in argon. By sorting the anode in such a way that the area adjacent to the electrolyte has a larger volume fraction of YSZ compared to Ni, a decrease in ohmic resistance is expected and better electrochemical characteristics will be achieved, which, according to the authors, can be achieved through further optimization. execute. 27
The advantages of DLP and SL are good surface quality and high dimensional accuracy. DLP printers typically have a resolution of about 50 µm in a plane (XY plane) with 18 layers in thicknesses ranging from 25 µm18 to 50 µm. 43 It is here that the main problem in SOEC and SOFC printing is the production of thin electrolyte layers (~5–10 µm) using lithography-based printers, since several layers are often required to obtain structures with acceptable mechanical properties. Therefore, these processes (DLP and SL) are not suitable if it is necessary to obtain a dilute electrolyte (and thus reduce the loss of ions). Thus, all current reports of lithographic printing of these devices are supported by the electrolyte due to the thick electrolyte layer. 8,18,42,43 The thickness of the printed electrolytes in these reports ranged from 200 µm to 500 µm. 8, 18, 42, 43
If only one battery component (usually electrolyte) is printed using DLP and SL processes, the anode and cathode are added by conventional methods, including brushing, sputtering, etc. followed by heat treatment (or annealing), the temperature of NiO-YSZ is usually higher, than LSM-YSZ. 8,18,42,43 For example, NiO-8YSZ slurry and LSM slurry were applied to the surface of the 8YSZ sintered electrolyte layer with a brush. NiO-8YSZ and LSM slurries were prepared using the appropriate commercial powders. 18 Wei et al. Sputtered cermets composed of Ag and GDC have been used as anode and cathode materials on printed electrolytes. 43 After application, anneal the anode and cathode materials. In another report, commercial NiO-YSZ and LSM-YSZ slurries were deposited on 3D printed YSZ electrolytes as fuel and oxygen electrodes, respectively, and then cooled at 1400°C (3 hours) and 1200°C (1 hour) with heating. treatment) respectively. 8 In this work, a 250 µm electrolyte supported 8YSZ SOFC with a conventional high aspect ratio flat and corrugated electrolyte was printed using the SL process. 8 Performance of cells with corrugated layers increased by 57% in fuel cell and co-electrolysis (CO2 and steam) modes in the temperature range of 800-900°C. This improvement is due to the larger area (~60%) compared to cells with flat layers. The degradation rate of the printed battery was 0.035 mV/h. 8 In another electrolyte-supported design, the authors printed a honeycomb cell consisting of 260 µm thick hexagonal cells forming a network connected by beams 530 µm thick and 220 µm wide. Simulation studies confirm that a honeycomb structure improves battery performance compared to a flat structure. The authors suggest that this is due to the use of thinner membranes and, in part, to the use of an increased area associated with the beam. 42
To achieve desired densification and prevent warpage, crack formation and delamination during shrinkage in debinding and sintering in lithography-based printing, high solids loading (>30-60%), and stable and uniform photocurable slurries are required.18,43 Successful sintering and debinding additionally requires optimization of thermogravimetric properties of the binder, which is nontrivial for multi-layers and multi-materials in SOECs and SOFCs. To achieve the desired densification and prevent warpage, crack formation and delamination during shrinkage in debinding and sintering in lithography-based printing, high solids loading (>30-60%), and stable and uniform photocurable slurries are required.18,43 Successful sintering and debinding additionally requires optimization of thermogravimetric properties of the binder, which is nontrivial for multi-layers and multi-materials in SOECs and SOFCs. Для достижения желаемого уплотнения и предотвращения коробления, образования трещин и расслоения во время усадки при удалении связующего и спекании в литографической печати требуется высокое содержание твердых частиц (> 30-60%), а также стабильные и однородные фотоотверждаемые суспензии.18,43 Успешное спекание и удаление связующего дополнительно требует оптимизации термогравиметрических свойств связующего, что нетривиально для многослойных и многокомпонентных материалов в ТОТЭ и ТОТЭ. To achieve the desired densification and to prevent warping, cracking, and delamination during shrinkage during debinding and sintering, lithographic printing requires a high solids content (>30-60%) as well as stable and uniform photocurable slurries.18,43 Successful sintering and removal of the binder additionally requires optimization of the thermogravimetric properties of the binder, which is not trivial for multilayer and multicomponent materials in SOFCs and SOFCs.为了实现所需的致密化并防止在基于光刻的印刷中脱脂和烧结收缩过程中发生翘曲、裂纹形成和分层，需要高固体含量(>30-60%) 和稳定均匀的光固化浆料。为了 实现 所 需 的 致密化 防止 在 基于 光刻 的 中 脱脂 和 烧结 收缩 过程 中 发生 、 裂纹 形成 和 ， 需要 高固体 含量 含量 (> 30-60%) 和 稳定 均匀 光固化浆 光固化浆 的 的 的 的料。 Для достижения желаемого уплотнения и предотвращения коробления, образования трещин и расслоения при удалении связующего и усадки при спекании в литографической печати требуется материал с высоким содержанием сухих веществ (>30-60%) и стабильная гомогенная фотоотверждаемая паста. To achieve the desired densification and to prevent warping, debinding cracking and delamination and sintering shrinkage, lithographic printing requires a material with a high solids content (>30-60%) and a stable homogeneous photocurable paste. 18,43 Successful sintering and removal of the binder also requires optimization of the thermogravimetric properties of the binder, which is a challenging task for multilayer and multicomponent materials in SOFCs and SOFCs. Generally, a viscosity < 5-20 Pa at a shear rate of 30 s-1 is recommended for a photocurable resin,49 which makes using highly-loaded resins challenging. Generally, a viscosity < 5-20 Pa at a shear rate of 30 s-1 is recommended for a photocurable resin,49 which makes using highly-loaded resins challenging. Как правило, для фотоотверждаемой смолы рекомендуется вязкость < 5-20 Па при скорости сдвига 30 с-1,49 что затрудняет использование смол с высокой нагрузкой. Generally, a viscosity of < 5-20 Pa at a shear rate of 30 s-1.49 is recommended for a photocurable resin, making it difficult to use high load resins.通常，对于光固化树脂，建议在30 s-1 的剪切速率下粘度< 5-20 Pa，49 这使得使用高负载树脂具有挑战性。通常，对于光固化树脂，建议在30 s-1 的剪切速率下粘度< 5-20 Pa，49 Как правило, для светоотверждаемых смол рекомендуется вязкость <5-20 Па при скорости сдвига 30 с-149, что затрудняет использование смол с высокой нагрузкой. Generally, a viscosity of <5-20 Pa at a shear rate of 30 s-149 is recommended for light-cured resins, making it difficult to use high load resins. Bath heating during printing can be used to reduce viscosity. The incorporation of various particle sizes into the slurry helps to achieve a high solids content while maintaining a somewhat low viscosity. 43
Sharp interfaces printed layer by layer can compromise the mechanical and electrical properties of the printed ceramic (and cermet) layers and affect the electrochemical performance of the battery. Xing et al. Compared to cells with similar electrolyte thickness, the DLP-printed electrolyte provides lower power density, which the authors attribute mainly to layer boundaries between 50 µm thick DLP-printed layers and separation between the cathode layer and electrolyte. Separation.18 Although the cell obtained an OCV of ~1.1, this is more indicative of the sealing of the printed electrolyte.
It is unclear whether the DLP or SL processes are capable of printing with porous electrodes. One of the possible ways to obtain porous parts is to add pore formers to the photocurable resin. However, the addition of blowing agents can result in poor light diffraction and geometric tolerances, and even partial cure. Porosity can also be obtained by partial sintering, which is undesirable due to the deterioration of mechanical properties. Reduction of NiO to Ni is associated with 40% volume reduction.22 Therefore, depending on the amount of NiO, small pores (either open or closed) (<1-3 μm) can be obtained by NiO to Ni reduction. Reduction of NiO to Ni is associated with 40% volume reduction.22 Therefore, depending on the amount of NiO, small pores (either open or closed) (<1-3 μm) can be obtained by NiO to Ni reduction. Восстановление NiO до Ni связано с уменьшением объема на 40%.22 Следовательно, в зависимости от количества NiO, небольшие поры (открытые или закрытые) (<1-3 мкм) могут быть получены восстановлением NiO до Ni. The reduction of NiO to Ni is associated with a volume reduction of 40%.22 Therefore, depending on the amount of NiO, small pores (open or closed) (<1-3 µm) can be obtained by reduction of NiO to Ni. The reduction of NiO to Ni was associated with a 40% reduction in volume. 22 因此，根据NiO 的量，可以通过NiO 还原为Ni 获得小孔（开放或闭合）（<1-3 μm）。 22 因此，根据NiO 的量，可以通过NiO 还原为Ni 获得小孔（开放或闭合）（<1-3 μm）。 22 Таким образом, в зависимости от количества NiO мелкие поры (открытые или закрытые) (<1–3 мкм) можно получить восстановлением NiO до Ni. 22 Thus, depending on the amount of NiO, small pores (open or closed) (<1–3 µm) can be obtained by reduction of NiO to Ni.
It should be noted that the electrodes (cathode and anode) can also be first printed and then impregnated. 17 In this case, the ceramic phase of the cermet is 3D printed (for example, the YSZ phase in NiO-YSZ) and then impregnated (impregnated) with the corresponding metal phase. Generally, there are three methods of impregnation, including metal salt solution impregnation with various additives, nanoparticle suspension impregnation, and molten salt impregnation. 17 Indeed, impregnation has certain advantages, since the catalytic phase does not sinter the sintered ceramic phase at the high temperatures required for synthesis. They can simply be fired and dried at lower temperatures. This lower processing temperature and small catalyst particle size can potentially prevent nickel migration and coarsening as well as complex microstructure evolution.
The Robocasting (or direct ink writing) process is basically compatible with any material and blowing agent. 5 However, the resolution of this process is relatively low. In addition, obtaining thin electrolytes in the range of several micrometers using robotic casting is not a trivial task. Therefore, this method must be combined with other hybrid process methods to print a complete battery. Anelli et al. reported a symmetrical cell with LSM-YSZ/YSZ/LSM-YSZ composition using a hybrid technique of mechanical casting and inkjet printing followed by a co-sintering step. 50 LSM-YSZ electrodes were printed by adding pore formers by mechanical casting, and water-based YSZ inks were printed using inkjet printing. After all layers were printed, the fully printed cells were co-sintered in air at 1200°C for 1 hour. The thickness of the final sintered electrolyte is about 2.8 µm. For this cell, the electrochemical characterization resulted in an area resistivity (ASR) value of ~2.1 Ωcm2 at 750°C.
There are other AM processes that could potentially help fabricate these electrochemical devices. For example, the layered nature of these devices is compatible with Laminated Object Manufacturing (LOM) processes,51 but not currently at scale. It may be helpful to develop a process such as LOM, which has the potential to produce thinner laminates. Laser processing of ceramic materials may be suitable for these devices, more for patterning or surface modification for subtractive processes such as drilling and machining. 52
Table 1 compares the two main AM processes for printing SOEC and SOFC. The first column lists the benefits of each printing process. The second column lists the limitations of each SOFC and SOEC printing process. The third column adds additional considerations to consider when printing these devices with each process. Table 2 provides a summary of current published work.
This work was supported by the AESF Foundation and the National Science Foundation (CMMI Award No. 2152732) through the AESF Research Program.