Thin Film Solid Oxide Fuel Cells Research Articles

Surface Modification of NiFe Anode-Support for Thin-Film Solid-Oxide Fuel Cell

Surface Modification of NiFe Anode-Support for Thin-Film Solid-Oxide Fuel Cell

Gadolinia Doped Ceria with Engineered Grain Boundaries as Potential Electrolyte Material for Thin-Film Solid Oxide Fuel Cells

Thin-film solid oxide fuel cells are receiving increasing attention within the scientific community thanks to their high energy density and high efficiency in different temperature ranges. To be utilized as power supply systems for portable electronic devices, thin-film solid oxide fuel cells must demonstrate high power supply and low-temperature operation capabilities. Both capabilities are highly dependent on the function of the electrolyte layer, commonly comprised of ceramic mixed-ionic-electronic conductors [1]. The state-of-the-art thin-film solid oxide fuel cells utilize yttrium stabilized zirconia as an electrolyte material, despite its relatively low ionic conductivity within the typical working-temperature range of solid oxide fuel cells (up to 1000 °C). Moreover, the ionic conductivity of yttrium stabilized zirconia dramatically reduces with decreased temperature, limiting its performance as an electrolyte material for low-temperature (below 600 °C) solid oxide fuel cells [2].Gadolinia doped ceria is an alternative promising electrolyte material for low temperature solid oxide fuel cells. Its relatively high ionic conductivity below 600 °C makes it attractive as an electrolyte material although its partial electronic conductivity is a significant drawback [3]. This drawback is more pronounced in ultra-thin layers (sub-micron scale) solid oxide fuel cells devices. As a result, gadolinia doped ceria have been utilized only to a limited extent for thin-film solid oxide fuel cells.The issue of partial electronic conductivity might be resolved thanks to a non-conventional deposition technique of polycrystalline gadolinia doped ceria thin films. The proposed sputtering technique includes metallic alloy target and results in a granular morphology, as opposed to the columnar morphology obtained using the conventional sputtering method. As electrical conductivity of polycrystalline thin films is higher along the grain boundaries [4], gadolinia doped ceria layers with random crystallographic orientation are expected to be less electronically conductive. Here, we investigated the integration and the performance of granular gadolinia doped ceria as an electrolyte material for thin-film solid oxide fuel cells. The electronic-conduction properties along the grain boundaries of gadolinia doped ceria were studied, and a thin-film solid oxide fuel cell prototype was fabricated. Such devices with engineered grain boundaries hold the potential for low-temperature operation, high power output, and silicone-compatibility for novel integration opportunities and bulk-fabrication.

Thin film Si solar cell and solid oxide fuel cell technologies for a low cost, environmentally friendly and sustainable source of energy

Thin-film solar cell technology based on hydrogenated amorphous silicon has matured over the last two decades and is capable to deliver commercial modules with almost 10 % stabilized efficiency. Moreover attention on hydrogen fuel cells is increasing as the range of potential commercial applications expands, with more than 3800 fuel cell systems being installed and operated worldwide. These systems attain high theoretical efficiencies in the range of 43.7-70.4 %, with an additional 20% as heat recovery, and operating at carbon-free conditions between 50 °C and 900 °C. In this paper the status of thin-film silicon solar cell technology is reviewed by comparing it to other major solar cell technologies. A discussion is also made upon the benefits obtained from the few potential applications, as long-term potential actions for sustainable development, and the future of Solid Oxide Fuel Cells (SOFCs). Applications of thin-film silicon modules and SOFCs are presented. EPIA (European Photovoltaic Industry Association) estimates that half of the growth in the next two years will come from thin-film technology.

Nanocrystal Engineering of Thin-Film Yttria-Stabilized Zirconia Electrolytes for Low-Temperature Solid-Oxide Fuel Cells.

To overcome significantly sluggish oxygen-ion conduction in the electrolytes of low-temperature solid-oxide fuel cells (SOFCs), numerous researchers have devoted considerable effort to fabricating the electrolytes as thin as possible. However, thickness is not the only factor that affects the electrolyte performance; roughness, grain size, and internal film stress also play a role. In this study, yttria-stabilized zirconia (YSZ) was deposited via a reactive sputtering process to fabricate high-performance thin-film electrolytes. Various sputtering chamber pressures (5, 10, and 15 mTorr) were investigated to improve the electrolytes. As a result, high surface area, large grain size, and residual tensile stress were successfully obtained by increasing the sputtering pressure. To clarify the correlation between the microstructure and electrolyte performance, a YSZ thin-film electrolyte was applied to anodized aluminum oxide-supported SOFCs composed of conventional electrode materials which are Ni and Pt as the anode and the cathode, respectively. The thin-film SOFC with YSZ deposited at 15 mTorr exhibited the lowest ohmic resistance and, consequently, the highest maximum power density (493 mW/cm2) at 500 °C whose performance is more than five times higher than that of the cell with YSZ deposited at 5 mTorr (94.1 mW/cm2). Despite the basic electrode materials, exceptionally high performance at low operating temperature was achieved via controlling the single fabrication condition for the electrolyte.

Development of robust YSZ thin-film electrolyte by RF sputtering and anode support design for stable IT-SOFC

Thin-film solid oxide fuel cells (TF-SOFCs) have attracted attention as a strategy for lowering the operating temperature of SOFCs. However, the porous and rough surface of the SOFC support and the shadowing effect have hindered the construction of pinhole-free thin-film electrolytes during sputtering. In this study, we report the deposition of a gas-tight YSZ thin-film electrolyte (approximately 0.8 μm) by RF sputtering on a Ni-YSZ anode support. To utilize a sputtered thin film as an electrolyte, we examined the relationship between the deposition pressure and thin-film properties and determined the optimal deposition conditions. By designing the anode functional layer (AFL) and annealing, a thin-film electrolyte is successfully deposited on the porous Ni-YSZ support without cracks and pinholes. The resulting thin-film electrolyte cell achieves a high open-circuit voltage (OCV) of approximately 1.09 V and a maximum power density of 0.446 W/cm2 at 600 °C, which implies high ionic conductivity and gas tightness. Moreover, the single cell exhibited exceptional long-term stability (140 h) despite the extremely thin and vacuum-deposited electrolyte components. This study provides guidelines for the practical application of thin films as electrolytes to lower the operating temperature of SOFCs.

A Comprehensive Investigation on the Direct-Ammonia Thin-Film Solid Oxide Fuel Cells (TF-SOFCs) and the Effect of Porosity-Control of the Anode Support Layer

Ammonia is highly regarded as a hydrogen storage and transportation medium due to its various advantages. Solid oxide fuel cells (SOFCs) can achieve high synergy by using ammonia directly as fuel without the ammonia decomposition reactor and filtration devices. We confirmed the distribution of hydrogen produced by the decomposition of ammonia through 2D multi-physics numerical modeling and predicted the decomposition rate and maximum current density according to the tortuosity. Based on the results, we adjusted the pore volume of the anode-support layer using PMMA to reduce tortuosity and facilitate hydrogen supply and produced anode support layer-anode functional layer substrate. We then manufactured a Thin-film SOFC and tested using hydrogen and ammonia fuel, and achieved high performance of 1897 and 1357 mW/cm2 at 650 °C.

Fabrication of Low-Temperature Solid Oxide Fuel Cells via Development of Magnetron Sputtering Process

We herein report the development of the sputtering process for the fabrication of low-temperature thin film solid oxide fuel cells (TF-SOFCs) supported by anodized aluminum oxide (AAO). The process development has been conducted for each component, anode, electrolyte, electrolyte/cathode interlayer, and cathode to scale up the cell from 1cm x 1cm to 15 cm x 15 cm. In order to fabricate a uniform thin film regardless of the size of AAO, sputtering conditions such as power, pressure, angle, target-to-substrate distance (TSD), and time have been developed. The recipe for the anode and cathode has aimed at a porous structure with nano-grains for high reactivity. In contrast, the recipe for the electrolyte has been optimized for thickness uniformity and high density. With these sputtering recipes TF-SOFCs with the required microstructure and thickness uniformity have been successfully fabricated on 15 cm x 15 cm AAO substrates.

A Self-Crystallized Nanofibrous Ni-GDC Anode by Magnetron Sputtering for Low-Temperature Solid Oxide Fuel Cells.

The optimum composition ratio of the anode cermet (Ni-GDC) for solid oxide fuel cells (SOFCs) varies because the electron-collecting mechanism is different depending on its applications. A Co-sputtering method facilitates ratio control with sputtering power adjustment. However, there is a practical issue with fabricating anode cermet with various ratios attributed to the large sputtering yield gap of the metal target, Ni, and the ceramic target, gadolinia-doped ceria (GDC). Therefore, in this study, a Gd-Ce metal alloy was applied instead of GDC to match the sputtering rate with that of Ni, which enables a wide ratio range achievement. A thin film of Gd-Ce oxidized after deposition and successfully transformed to crystallized GDC under a SOFC operation environment. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) confirmed its crystallinity, and the film deposited with various power ratios was sputtered on the ScSZ electrolyte pellet to clarify the optimum Ni-GDC ratio for thin-film SOFCs. Last, the Ni-GDC was applied to anodized aluminum oxide (AAO)-supported SOFCs to maximize the performance. The performance change according to the thickness of Ni-GDC was identified, and the best performance among them was 638 mW/cm2 at 500 °C.

Fabrication of AAO-Based Four-Stack Thin Film Solid Oxide Fuel Cells for the Low-Temperature Operation Using Sputtering Method

Anodic Aluminium Oxide (AAO) based four thin-film solid oxide fuel cell (TF-SOFC) stack was fabricated. The fuel cell stack was tested and analyzed at the low-temperature window. Since the low-temperature operation has many advantages, such as lowering the system cost and widening the material selection, there has been much effort to reduce conductive resistance, mainly at the electrolyte, which is the most problematic issue in low-temperature operation. One of the solutions is TF-SOFC deposited on the various substrates or supporting materials. AAO, a non-conductive and nanoporous substrate, has been widely used in fabricating TF-SOFC as a substrate because of its uniformly arranged pores and highly developed manufacturing process.However, AAO is a poor template for stacking TF-SOFCs because it has lower mechanical strength than metal, has no electrical conductivity, and has a different coefficient of thermal expansion from metal. In this study, therefore, planarly stacked TF-SOFC were fabricated to expand its limits of use as a template for the stacked fuel cell. Nickel-based anode, yttria-stabilized zirconia (YSZ) electrolyte with functional layers, and platinum cathode and current collector were sequentially deposited using sputtering and atomic layer deposition methods. All thin-film components of four cells were deposited on the AAO at the same time, making good connections between each electrode. By effectively setting the deposition process, unnecessary pores of AAO were blocked, and current collecting layers were stably formed in between the structure of the cells with a step difference.Then, the stacked TF-SOFC were electrochemically analyzed, including the current density-voltage-power density curve (i-V-P curve) and electrochemical impedance spectroscopy (EIS) being compared to a single cell. The microstructure of the fuel cell and its electrically connected structures were observed by scanning electron microscopy (SEM) and focused ion beam (FIB) cross-sectional SEM analyses. The cell’s overall voltage was comparable to four times that of a single cell, and the power density was obtained. We revealed the correlation between the electrochemical performance and the microstructure of individual cells and the whole stack. The full stack achieved electrochemical and mechanical stability by forming a complete fuel cell structure and enhanced the applicability of the planar cell stack on the AAO substrate.

(Invited) Effect of Anode Thickness on Thin-Film Solid Oxide Fuel Cells Deposited on Nanoporous Substrates

In this study, low-temperature solid oxide fuel cells having different thin-film anode thicknesses were fabricated, and their characteristics were studied. Since the low-temperature solid oxide fuel cells (LT-SOFCs) have the advantages of a low-cost fuel cell system and a wide selection of components’ materials, many researchers have made efforts to lower the operating temperature range. Naturally, thin-film electrolyte technology has been improved to reduce the exponentially increasing ohmic resistance of oxygen conduction when the working temperature is lowered. Various substrates or even no substrate were used to fabricate thin-film solid oxide fuel cells (TF-SOFCs). However, free-standing TF-SOFCs have poor mechanical strength, and the active area was limited to tens of square micrometers. A nanoporous substrate such as Anodic aluminum oxide (AAO) has been a great platform on which TF-SOFCs were stably deposited and operated. AAO has well-arranged and straight pores from its bottom to the top but no electrical conductivity. Due to the resistance characteristics of TF-SOFCs based on AAO, the anode’s thickness and the nanostructure significantly influence the overall performance of the fuel cell. According to previous studies, the performance tends to increase as the thickness increases with well-maintained porosity in the thin-film anode. To confirm this tendency and further improve cell performance, AAO substrates with 250nm-size pores and relatively rough surfaces were used. On the AAO substrate, nickel gadolinium-doped-ceria (Ni-GDC) mixed ionic and electronic conductor (MIEC) was introduced as the anode material, which increased hydrogen oxidation reaction (HOR) reactivity at the anode. In addition, the glancing angle deposition (GLAD) of the sputtering method was applied to maintain porosity better than the conventional method. With this method, thickening the thin-film anode improved the current collecting performance while the pores were well connected to the anode-electrolyte interface. Following that, the yttria-stabilized zirconia (YSZ) electrolyte was densely deposited, and the porous deposited platinum cathode completed the entire cell. The fabricated cells were tested at 500C and electrochemically analyzed by i-V-P curves and the EIS method. Also, the anode’s nanostructures were revealed by various analyzing methods such as surface FESEM, FIB cross-sectional SEM, TEM-EDS, and 4-probe conductivity measurement. Each cell’s stable structure and the uniformity of some fuel cell components such as the electrolyte and the cathode were confirmed. We found that the electrochemical performance increases as the anode thickness increases while its porosity is well-maintained. About 550mW/cm2 was measured as the highest maximum power density, which is more than a 20% increased result than the other fabricated cells. Those investigations also enabled the anode nanostructure to be identified, such as the anode thin-film porosity, conductivity, and three-dimensional morphology. The correlation between the anode nanostructure and the fuel cell performance was discussed, and the optimized cell structure was found. This study enlarged the understanding of nanoporous substrate-based TF-SOFC manufacturing and their expandability.

Manipulation of Anode Nanostructure and Composition By Glancing Angle Deposition for Thin-Film Solid Oxide Fuel Cells

Co-sputtering is a simple yet effective technique for fabricating a thin film of composite materials and different compositions by adjusting the electrical power of each sputtering target. However, practical issues arise when depositing cermet materials such as Ni-YSZ and Ni-GDC because of the contrasting characteristics of metal and ceramic sputtering targets. For example, the surface binding energy of GDC is significantly higher than that of Ni, resulting in a considerably lower sputtering yield of GDC. The difference in sputtering yield is further exacerbated by the low power density limit of the GDC target, which is set by its unfavorable material properties such as brittleness and low thermal conductivity. In contrast, due to its magnetic property, Ni requires sufficiently high sputtering power during magnetron sputtering.This work combines co-sputtering with advanced sputtering techniques such as glancing angle deposition (GLAD) and oblique angle deposition (OAD) to fabricate otherwise unobtainable Ni-GDC anode microstructure with various compositions. Reduced flux capture area of Ni caused by the high incident angle of GLAD allowed control of thin-film compositions, and optimal composition was determined based on fuel cell performance and electrochemical impedance spectroscopy (EIS) analysis. A sufficient amount of GDC was necessary to expand the triple-phase boundary (TPB) and suppress the thermal agglomeration of highly mobile Ni.The effect of electrode nanostructure on fuel cell performance was also investigated. The ballistic shadowing effect during OAD was utilized to produce thin-film electrodes with various porosities. With the increase of the deposition angle, the porosity increased, whereas the in-plane electrical conductivity decreased due to the reduced density. The columnar structure was also manipulated by managing the substrate position through azimuthal rotation. As a result, various columnar structures such as vertical posts, slanted posts, and zig-zag were fabricated, and the optimal microstructure was determined via electrochemical examinations.

Reactive Sputtered NiO-YSZ Anode Functional Layer for Thin Film Low-Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is one of the most promising next-generation electrochemical energy conversion devices because of its wide choice of fuels, cleanliness and, high efficiency of up to 70-85%( with heat utilization). Recently, low-temperature SOFC (< 600 ℃) has been studied to overcome the limitations of conventional SOFC, e.g., thermal stability, cost, and limited application. However, lowering operated temperature is challenging due to slow charge transfer and ion transport rates. Therefore, SOFC design with thin-film stacks, namely thin-film SOFC (TF-SOFC) fabricated on porous anode support, have been usually adopted for sufficient power density even at this temperature regime.A ceramic-metal composite (cermet) of Ni and YSZ (yttria-stabilized zirconia) is the most common anode or anode functional layer (AFL) material of TF-SOFC, because Ni-YSZ cermet has reasonable cost, chemical stability in a reducing atmosphere at high temperature, and similar thermal expansion coefficient to that of YSZ electrolyte. Various thin-film techniques have been reported for AFL fabrication, e.g., pulsed laser deposition (PLD), RF sputtering, etc. Among those, reactive sputtering method, which uses metal target in controlled gas conditions (e.g., oxygen condition for oxide deposition), is a versatile method to deposit various types of thin films with fast deposition speed (up to > 1um/hr), which is beneficial in manufacturability regarding the usual thickness range of AFL (a few microns).In this study, therefore, we studied physical and chemical properties of nanocomposite Ni-YSZ films fabricated by using the reactive sputtering deposition technique, focusing on the effect of sputter parameters. In particular, Oxygen partial pressure significantly affects deposition speed, surface morphology, composition, electrochemical performance, and thermal durability at 450 ℃. It is shown that excessive O2/Ar partial pressure ratio (> 0.2) significantly reduces the deposition speed by approximately one order of magnitude compared to the lower ratio case. In morphological analysis, O2/Ar partial pressure ratio affect nano-structured of anode, after annealing process at 1200℃ with air. Also, the Ni-YSZ anode sputtered at the environment of O2/Ar partial pressure ratio of 0.1 sample shows the lowest activation resistance when applied to LT-SOFC’s anode. The O2/Ar 0.5 sample shows surface Ni agglomeration due to high NiO-to-metallic Ni ratio.

A comprehensive investigation of direct ammonia-fueled thin-film solid-oxide fuel cells: Performance, limitation, and prospects

SummaryAmmonia is a promising carbon-free hydrogen carrier. Owing to their nickel-rich anodes and high operating temperatures, solid oxide fuel cells (SOFCs) can directly utilize NH3 fuel—direct-ammonia SOFCs (DA-SOFCs). Lowering the operating temperature can diversify application areas of DA-SOFCs. We tested direct-ammonia operation using two types of thin-film SOFCs (TF-SOFCs) under 500 to 650°C and compared these with a conventional SOFC. The TF-SOFC with a nickel oxide gadolinium-doped ceria anode achieved a peak power density of 1330 mW cm−2 (NH3 fuel under 650°C), which is the best performance reported to date. However, the performance difference between the NH3 and H2 operations was significant. Electrochemical impedance analyses, ammonia conversion quantification, and two-dimensional multi-physics modeling suggested that reduced ammonia conversion at low temperatures is the main cause of the performance gap. A comparative study with previously reported DA-SOFCs clarified that incorporating a more active ammonia decomposition catalyst will further improve low-temperature DA-SOFCs.

Metal Supported Thin Film SOFC Fabrication Via RF Sputtering

Thin film solid oxide fuel cell (TF-SOFC) has been studied for past few decades to lower its operating temperature to intermediate temperature range (400℃-700℃). However, it is a challenge to deposit a pinhole free electrolyte thin film on a conventional porous ceramic support. In the case of the Ni-Fe based metal support, even if NiO-Fe2O3 is manufactured as a dense structure, it is reduced to Ni-Fe in a hydrogen atmosphere and forms pores. In this study, Ni-Fe based metal support has been introduced as TF-SOFC support to deposit pinhole free electrolyte. Metal support was fabricated as 290㎛ thick porous bulk(34% porosity) with 10㎛ thick dense layer(<1% porosity) by using tape casting process. Ni-Fe ratio (9:1. 7:3, 5:5, 3:7) and sintering condition (1300℃-1500℃) of metal support were optimized to minimize a mismatch of TEC with thin film and improve microstructure stability of support. After that, NiO-YSZ anode functional layer (AFL) and YSZ electrolyte were continuously deposited on metal support through RF sputtering. By tailoring sputtering condition (substrate temperature, operating pressure, reactive gas flow), 39vol%NiO-YSZ composition AFL and 8.6mol%YSZ pinhole free electrolyte were deposited. Annealing was performed at a temperature of 1100℃ or less to improve the crystallinity of the thin film and suppress Ni agglomeration of Ni-YSZ AFL while minimizing diffusion of Fe in the AFL. In cell operating condition (600℃-H2 atmosphere), the dense layer of metal support and NiO-YSZ AFL formed 28% and 24% of pores, respectively. By using LSM-YSZ cathode through the screen printing process, Metal support (300㎛) / Ni-YSZ (1.3㎛) / YSZ (0.8㎛) / LSM-YSZ (20㎛) button cell was fabricated, and electrochemical analysis was performed. Key wards: Sputtering, Thin film, Metal Support, Ni-Fe, YSZ, Ni-YSZ

Tailoring 3D structured nanofibrous nickel/gadolinium-doped ceria anodes for high-performance thin-film solid oxide fuel cells

Solid Oxide Fuel cells (SOFCs) operating at low temperature (450–600 °C) have been investigated in order to achieve rapid on/off, improved long-term stability and a wide selection of low cost materials, but decrease in ionic conductivity and slowed electrode kinetics still remain as challenges. Anodized aluminum oxide (AAO)-supported thin-film SOFCs (TF-SOFCs) fabricated by thin film deposition techniques exhibit exceptional performances at low temperature owing to extremely reduced electrolyte thickness and nanostructured electrodes. However, limited electrical conduction in anodes on AAO coerces metallic anodes and studies on cermet anodes are still insufficient. In this work, nickel-gadolinium doped ceria (Ni-GDC) thin-film anodes with different thickness are co-sputtered on AAOs with subsequently sputter-deposited yttria-stabilized zirconia electrolyte and platinum cathode. The TF-SOFCs represent maximum power densities of 223, 281, 353 and 177 mW/cm2 at 500 °C for Ni-GDC thickness of 300, 500, 800, 1000 nm, respectively. 800 nm thickness shows the best performance since porosity is maximized by columnar growth behavior during sputtering and lateral electrical conduction is consistently improved as the anode becomes thicker, which is confirmed by electrochemical impedance spectroscopy, field-emission scanning electron microscopy, and focused ion beam. Transmission electron microscopy/energy-dispersive X-ray spectroscopy are utilized to get structural and chemical information.

Solid Oxide Fuel Cells with a Thin Film Electrolyte: A Review on Manufacturing Technologies and Electrochemical Characteristicses

Solid oxide fuel cells (SOFCs) are electrochemical systems converting the energy released during fuel oxidation into electrical energy. SOFCs are considered as a promising clean energy technology due to the high efficiency of fuel-to-power conversion and environmental friendliness. The potential applications of SOFCs extend from stationary power generation units for industrial and household facilities to auxiliary power units in vehicles and portable power sources. One of the main elements of SOFCs is a solid oxide electrolyte possessing ionic conductivity at high temperatures (above 700 °C). The main challenge in the SOFC commercialization is related to their high operating temperature, which entails materials degradation, short life-time, long start-up and shut-down times, and high cost. One of the most effective ways to reduce the SOFC operating temperature is to minimize the electrolyte thickness. In this regard, fabrication of SOFCs with a thin film electrolyte has been attracting high research activity over the past few decades. Different fabrication techniques were reported to be applicable for manufacturing thin film SOFCs, and the fuel cell performance was found to be highly dependent on the appropriate selection of materials and processing technologies. The present review is focused on state-of-the-art fabrication technologies of the thin film SOFCs. A brief survey of configurations and geometries of the thin film SOFCs and methods of deposition of solid-oxide films is given. Special attention is focused on the electrical generation performance of the thin film SOFCs.

Effect of Oxygen Partial Pressure on Reactive-Sputtered Ni-YSZ Anode Properties for Low-Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is a promising next-generation energy conversion device because of its high efficiency, cleanliness, and fuel flexibility. Recently, SOFC operating at low- to intermediate-temperature (LT to IT) range (< 600 ℃) has been studied to overcome the limitations of conventional SOFC, e.g., thermal stability, cost, and limited application. In lowered temperature range, however, charge transfer and transport rates slow down; therefore, SOFCs design with thin film stacks, namely thin film SOFC (TF-SOFC) fabricated on porous anode support have been usually adopted for sufficient power density even at this temperature regime.TF-SOFC usually uses Ni-YSZ (nickel-yttria-stabilized zirconia) cermet for anode functional layer (AFL) to accommodate the deposition of dense electrolyte layers on anode support with relatively large pores because of its reasonable cost, catalytic activity, and electrical conductivity. While various thin film techniques have been reported for AFL fabrication, e.g., pulsed layer deposition (PLD), RF sputtering, etc. Among such techniques, reactive sputtering method, which uses metal target in controlled gas environment (e.g., oxygen environment for oxide deposition), is a versatile method to deposit various types of thin films with fast speed (up to > 1um/hr), which is beneficial in manufacturabilily regarding the usual thickness range of AFL (a few microns). Furthermore, the reactive co-sputtering method can control anode properties such as composition ratio, deposition speed, and morphology in wide ranges.In this study, we fabricate reactive sputtered Ni-YSZ cermet anodes, as prototypes for AFLs of TF-SOFCs, by using a single metal target with various oxygen partial pressure conditions, and characterize their physical and chemical properties. Oxygen partial pressure significantly affects deposition speed, surface morphology, composition, electrochemical performance and its thermal durability at 450 ℃. The Ni-YSZ anode sputtered at the environment of O2/Ar partial pressure ratio of 0.1 sample shows the lowest activation resistance when applied to LT-SOFC’s anode. It is also shown that excessive O2/Ar partial pressure ratio (> 0.2) significantly reduces the deposition speed by approximately one order of magnitude compare to the lower ratio case.

Three-Dimensionally Architected Solid Oxide Fuel Cells for Low-Temperature Operation

Recently, while eco-friendly energy storage/conversion technologies have been attracting attention, researches have been reported to improve performance and provide multiple functions by applying three-dimensional nano-/micro-structures to energy devices. In general energy devices, the electrolyte and electrodes located on both sides have been designed to have a flat interface, but in the end, developed structures in consideration of three-dimensional efficiency should be required. Among them, in solid oxide fuel cells (SOFCs), which have advantages such as high efficiency and fuel flexibility, studies on the use of advanced cell structures were conducted to lower the operating temperature. Although micro-SOFC using etching or laser processing has been reported, there are limitations in practical operation due to poor material/structural stability and low pattern resolution. In this paper, we developed a novel ceramic micropatterning process by imprinting a pyramid-shaped polymer mold made by soft lithography on the anode support. Furthermore, we have implemented a three-dimensional SOFC with record-high cell performance through the combination of thin film process and large-area cell manufacturing technology. For microscale patterning of the anode, polymer-ceramic anode films with low elastic modulus was prepared by the tape casting process, and the anode support with pyramidal patterns was fabricated through the subsequent imprinting process. The imprinted pattern was maintained after the sintering process. Then, through RF sputtering and pulsed laser deposition (PLD) process, the nanoscale anode functional layer (nAFL), Y2O3-stabilized ZrO2 (YSZ), Gd0.1Ce0.9O1.95 (CGO) and La0.6Sr0.4CoO3- δ (LSC) layer were deposited to fabricate a full cell. To analyze the electrochemical effect of three-dimensional electrode/electrolyte interface, 3DA-cell with ceramic micropatterning process and Planar-cell with flat interface were compared. Interestingly, 3DA-cell showed higher performance than Planar-cell at all operating temperatures, and resulted in 55 % enhanced maximum power density (PDmax) and 19 %, 52 % decreased polarization, ohmic resistance at 500 °C, respectively. Notably, the CGO-based 3DA-cell using CGO as the main electrolyte material recorded a high PDmax of 0.931 W/cm2 at 500 °C. Therefore, to find the factors that improve the electrochemical performance, the microstructures of two different cells were analyzed by scanning electron microscope (SEM), transmission electron microscopy (TEM) and 3D reconstruction method. From the 3D reconstruction result, the nAFL/YSZ interface area, CGO/LSC interface area, and active TPB length of AFL of 3DA-cell increased by about 33 %. 23 %, and 64 % compared to Planar-cell, whereas pore volume ratio of AFL did not change. Therefore, it was confirmed that the increase of the electrode/electrolyte interface area by ceramic micropatterning had a direct effect on the improvement of cell performance, and the internal structure of AFL was not affected by the imprinting process. Furthermore, to demonstrate the versatility of ceramic micropatterning process, a 5 cm x 5 cm large-area three-dimensional thin film SOFC was fabricated and its performance was evaluated. As a result, we set a new milestone in SOFC technology by reporting a power of ~ 13 W at 500 °C, and confirmed the consistency of our process through a cell reproducibility test. Additionally, the stability of the three-dimensional structure was verified through the operation without notable degradation for 500 hours under the condition of 5 A and 8 A current load. This work exhibits an outstanding breakthrough to accomplish high-performance, high-stable large-area SOFCs, and can accelerate commercialization to the portable and transportation devices.

Thin-Film SOFC Enabling Low-Temperature and Low-Hydrogen-Concentration Operation

Solid oxide fuel cells (SOFCs), which have the highest power-generation efficiency among generators, are key in achieving a low-carbon society. To reduce the cost of an SOFC system including mounting cost, thereby, expand its application, we have been developing a thin-film SOFC suitable for low-temperature operations.Since the anode, solid-electrolyte, and cathode layers are formed by sputtered thin films, the internal resistance of the cell decreases; thus, the operating temperature can be lowered [1, 2]. Figure 1 shows a cross-sectional micrograph of the developed SOFC. In this study, nanoporous anodic aluminum oxide (AAO) was used as a support substrate [3, 4, 5]. The pore diameter was 55 nm and thickness was 100 μm. On the AAO substrate, a stack of dense (70 nm) and porous-platinum (80 nm) layers was deposited for the anode. Pores were formed in the dense platinum layer because the surface of the underlying AAO support substrate has 55-nm pores. A dense yttria-stabilized zirconia (YSZ) was used for the solid electrolyte layer (290 nm). The density of the YSZ layer is significant for inhibiting both gas and electron/hole leakage between the anode and cathode sides. Reactive sputtering by using a metal Zr84-Y16 target under a DC bias condition was key to deposit the dense YSZ layer on the porous-platinum anode layer. The interfacial layer (40 nm) of gadolinia-doped ceria (GDC) was deposited on the YSZ layer. The cathode layer (20 nm) of GDC-platinum composite material was then deposited on the interfacial layer. The characteristics of our SOFC was measured at 453°C, as shown in Fig. 2. The air was flowed to the cathode, and wet 3%-hydrogen of nitrogen base was flowed to the anode. An open circuit voltage of 1.049 V and output power density of 203 mW/cm2 were achieved using 3% hydrogen. At the cathode side, the polarization resistance was reduced by forming a gadolinium-doped ceria layer at the interface [5]. In addition, the GDC-platinum cathode was found to reduce the polarization resistance compared to the porous platinum cathode. At the anode side, the hydrogen diffused through the pores of the AAO substrate and reached the anode layer of the SOFC. The triple-phase boundaries formed among gas-, porous-anode-, and solid-electrolyte phases arguably enhanced the performance of the anode. The water vapor produced at the anode side diffused through the pores of the AAO and joined the exhaust gas. The power density at 453°C is expected to increase by using high-concentration hydrogen [6], which we plan to demonstrate for future study. Apart from operation in high-concentration hydrogen, our SOFC has another application. The experimental results indicate that our SOFC can generate high power by using the low-concentration hydrogen from hydrous bioethanol even at low temperatures.

Sputtered Thin-Film Solid Oxide Fuel Cells

An all-sputter process has been developed and optimized to fabricate high-performance thin-film solid oxide fuel cells (TF-SOFCs) using conventional materials. The process involves sequential sputtering of cell components onto a porous substrate. An example structure comprises of an anode such as nickel-yttria stabilized zirconia (Ni-YSZ), an electrolyte such as YSZ, an interlayer such as gadolinium doped ceria (GDC), and then a cathode such as lanthanum strontium cobalt perovskite (LSC)-GDC. These are sputtered onto a porous substrate such as anodized aluminum oxide (AAO). Process conditions and sputtering procedures have been tailored and engineered to produce components with desired structural characteristics, i.e., a fully dense electrolyte and interlayer and a porous anode and cathode. The porous electrodes show a columnar structure with branch-like nano-fibers. Sputtered TF-SOFCs exhibit superior performance at reduced temperatures; for example, the peak power density was about 3.0 W/cm2 at 650oC (with H2 for the cell Ni-YSZ/YSZ/GDC/LSC-GDC) and the peak power density was about 2.6 W/cm2 at 700oC (with CH4 for the cell Ni-YSZ/YSZ/GDC/LSC-GDC).