Thin Film Solid Oxide Fuel Cells Research Articles

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).

Scalable fabrication process of thin-film solid oxide fuel cells with an anode functional layer design and a sputtered electrolyte

The fabrication process for anode-supported thin-film solid oxide fuel cells (SOFCs) was investigated by using scalable and cost-effective methods. The anode functional layer (AFL) was introduced on the surface of the substrate to stably deposit the thin-film electrolyte. In previous studies, the AFL has been generally designed to increase the catalytic activity; however, in this study, additional design parameters including the roughness and density were controlled to achieve a pinhole-free thin-film electrolyte and structural stability. Through the developed process, button and large-sized cells were fabricated, and the electrochemical performance evaluation showed potential power density and impedance values at relatively low operating temperature. Microstructural analyses showed that each layer of the AFL, electrolyte, and cathode was uniformly coated on the substrate. The thin-film electrolyte was densely deposited without cracks or pinholes. The electrochemical performance and microstructure confirmed that the developed thin-film SOFCs are reliable and reproducible without costly processes or materials.

Atomic layer deposition of GDC cathodic functional thin films for oxide ion incorporation enhancement

AbstractIn this paper, we report successful fabrication of a gadolinia‐doped ceria (GDC) thin film using atomic layer deposition (ALD) for improving the performance of solid oxide fuel cells (SOFCs). By varying the deposition conditions and adjusting the configuration of the ALD supercycle, the doping ratio of ALD GDC was controlled. The morphology, crystallinity, and chemical composition of ALD GDC thin films were analyzed. ALD GDC showed different surface chemistry, including oxidation states, at different doping ratios. The application of ALD GDC in a SOFC led to an output power density enhancement greater than 2.5 times. With an anodic aluminum oxide (AAO) porous support structure, an ALD GDC thin film SOFC (TF‐SOFC) showed a high power density of 288.24 mW/cm2 at an operating temperature of 450°C.

Nano-Ceramic Cathodes via Co-sputtering of Gd–Ce Alloy and Lanthanum Strontium Cobaltite for Low-Temperature Thin-Film Solid Oxide Fuel Cells

We report the electrochemical performance and structural characteristics of porous nanostructured ceramic cathodes for thin-film solid oxide fuel cells (TF-SOFCs) based on yttria-stabilized zirconi...

Sintering-resistant platinum electrode achieved through atomic layer deposition for thin-film solid oxide fuel cells

Abstract Thin-film solid oxide fuel cells (TF-SOFCs) using a dense and thin electrolyte have attracted much attention as a promising portable power generator because they can lower the operating temperature of devices, which is a key issue related to conventional SOFCs, to below 500 °C and are compatible with several microfabrication processes. Highly porous interconnected Pt thin films are now widely used as oxygen electrodes, but their poor thermal stability seriously hampers the sustainable operation of TF-SOFCs. Here, we demonstrate how Al2O3 layers coated through atomic layer deposition effectively suppress the degradation of nanoporous Pt thin-film electrodes. Although Al2O3 is an electrical insulator, the selection of an appropriate overcoat thickness ensures stable electrochemical reaction sites of the Pt electrode at high temperatures even without serious current collection or gas flow issues. As a result, a 3.6-nm-thick Al2O3 layer maintains the high specific surface area morphology of Pt thin films at 450 °C and improves the electrode activity by more than twofold compared to an uncoated sample. These results suggest that a simple and scalable coating strategy enables the implementation of TF-SOFCs with ideal performance and durability outcomes.

Investigation of Reducing In-Plane Resistance of Nickel Oxide-Samaria-Doped Ceria Anode in Thin-Film Solid Oxide Fuel Cells

Metal/NiO-Smarium-doped ceria (SDC) nano-composite thin film anodes were deposited on anodic aluminum oxide by co-sputtering to enhance the in-plane current-collecting ability and investigated by varying the composition of metal materials (Pt and Au). Full fuel cells with these nano-composites were fabricated and tested at 500 °C. Columnar anodes with a sponge structure were fabricated by varying the DC sputtering source power and they were thermally stable at the operating temperature. By adding metal material, the ohmic resistance, including the current collecting resistance, was drastically reduced and the polarization resistance also decreased. The nano-composite electrode with a Pt content of 61 at% showed the highest performance, which is a maximum power density of 212.5 mW/cm2 at 500 °C. In addition, Au was considered to reduce the current collecting resistance and the corresponding power density was 3 times higher than that with the NiO-SDC anode.

All-Sputtered, Superior Power Density Thin-Film Solid Oxide Fuel Cells with a Novel Nanofibrous Ceramic Cathode.

Thin film solid oxide fuel cells (TF-SOFCs) are attracting attention due to their ability to operate at comparatively lower temperatures (400-650 °C) that are unattainable for conventional anode-supported SOFCs (650-800 °C). However, limited cathode performance and cell scalability remain persistent issues. Here, we report a new approach of fabricating yttria-stabilized zirconia (YSZ)-based TF-SOFCs via a scalable magnetron sputtering process. Notable is the development and deposition of a porous La0.6Sr0.4Co0.2Fe0.8O2.95(LSCF)-based cathode with a unique fibrous nanostructure. This all-sputtered cell shows an open-circuit voltage of ∼1.0 V and peak power densities of ∼1.7 and ∼2.5 W/cm2 at 600 and 650 °C, respectively, under hydrogen fuel and air along with showing stable performance in short-term testing. The power densities obtained in this work are the highest among YSZ-based SOFCs at these low temperatures, which demonstrate the feasibility of fabricating exceptionally high-performance TF-SOFC cells with distinctive dense or porous nanostructures for each layer, as desired, by a sputtering process. This work illustrates a new, potentially low-cost, and scalable platform for the fabrication of next-generation TF-SOFCs with excellent power output and stability.

Fabrication and Characterization of Direct Methane Fueled Thin Film SOFCs Supported by Microchannel-Structured Microtubular Substrates

Fuel flexibility is an attractive advantage of solid oxide fuel cell (SOFC) technology; however, carbon deposition issue with widely used Ni-cermet anodes in hydrocarbon fuels seriously limits its ...

고체 산화물 연료전지를 위한 SrCo0.8Nb0.1Ta0.1O3-δ 공기극 펄스레이저 증착

Recently, new perovskite cathode material, SrCoSUB0.8/SUBNbSUB0.1/SUBTa0.1OSUB3-δ/SUB (SCNT) was reported, showing high oxygen reduction reaction (ORR) activity. This study demonstrates thin film deposition of SCNT by pulsed laser deposition technique applied to anodic aluminum oxide (AAO) based thin-film solid oxide fuel cells (TF-SOFCs) to assess the possibility of SCNT application to TF-SOFCs. The SCNT powder and the target were prepared by the solid state reactive sintering method (SSRS). This target was then mounted to the pulsed laser depositing machine and deposited on the Si wafer, and the nano-porous substrate, AAO. The physical structure and the chemical phase were investigated by the field emission scanning electron microscope, focused ion beam scanning electron microscope, and X-ray diffraction. On the top of the AAO, thin Pt film and yttria stabilized zirconia (YSZ) were first deposited by sputtering and the SCNT was deposited on the top of it afterward. The open circuit voltage of AAO cell was tested at 500°C, and successful polarization activity of SCNT was observed.

(Invited) High-Performance Thin-Film Solid Oxide Fuel Cells

A process based on sputtering has been developed for fabricating thin-film solid oxide fuel cells (TF-SOFCs) that have a total thickness of ~3 µm and are based on the conventional configuration using common SOFC materials. Single cells (with dense electrolytes, dense interlayers, and porous electrode films) on various substrates were effectively fabricated through sputtering/co-sputtering of metal and/or ceramic targets and the electrodes exhibited a fine nanostructure consisting of very small columns of roughly 10 nm in diameter. Fabricated cells exhibit open circuit voltages of ~1.0 V and exceptional power densities at reduced temperatures (500o-650oC) on different fuels. Single cells (of the composition lanthanum strontium cobaltite-gadolinium doped ceria (LSC-GDC) cathode/GDC interlayer/yttria stabilized zirconia (YSZ) electrolyte/Ni-GDC anode) show record power densities of ~3.5 W/cm2 with hydrogen/air and ~1.5 W/cm2 with dry methane/air at 650oC.

Fabrication and Characterization of Thin Film SrCo0.8Nb0.1Ta0.1O3-Δ By Pulsed Laser Deposition

High working temperature(≥800 °C) of solid oxide fuel cell(SOFC) causes high cost of balance of plant, long start up time, and low durability. To overcome these barriers, lowering the working temperature of SOFC is one of the most attractive ways and such strategy has been researched through the fabrication of thin electrolyte and developing new perovskite cathode material. Recently, mixed ionic electronic conducting(MIEC) cathode material, SrCo0.8Nb0.1Ta0.1O3-δ (SCNT), has been developed by Li et al. and the SOFC with the cathode showed high performance(1.22W/cm2) at low working temperature (500 °C) with 14µm thick gadolinia doped ceria(GDC) electrolyte[1]. To apply SCNT to SOFC with thin electrolyte(≤1µm), the cathode must be fabricated on the electrolyte without high temperature sintering process(≥1000 °C) because of the weak mechanical strength of thin electrolyte. In this work, pulsed laser deposition(PLD) method was employed for the fabrication of thin film SCNT. PLD has been used widely for perovskite material deposition because of its high preservability of stoichiometric ratio of its original target material. However, the deposition temperature of PLD has a great influence on both physical and chemical structure of the film which means oxygen reaction activity(ORR) of the film also varies as temperature changes. Therefore, thin film SCNT was deposited by PLD at various temperature condition in this work to see the diversity of thin film characteristic for understanding the feasibility of application of SCNT to thin film SOFC (TF-SOFC). Powder and PLD target of SCNT were both prepared by the method Li et al. used, solid state reactive sintering(SSRS). The film was firstly deposited on Si wafer at various temperature (room temp, 500, 600, 700 °C) by PLD. Field emission scanning electron microscope (FE-SEM) and focused ion beam-SEM (FIB-SEM) were used to see the nano-structure of the film and deposition rate. 75mTorr of oxygen atmosphere was enough to make the structure of thin film deposited at room temperature very porous but dense film was observed at the temperature higher than 500 °C. The atomic ratio and crystallinity of each film were identified by X-ray spectroscopy(XPS) and X-ray diffraction(XRD) respectively. Through the understanding the physical and chemical variation of PLD deposited SCNT at different temperature condition, the best PLD deposition condition of SCNT for TF-SOFC was derived. Acknowledgment This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2012M3A6A7054855) References [1] M. Li et al., “A niobium and tantalum co-doped perovskite cathode for solid oxide fuel cells operating below 500 °c,” Nat. Commun., vol. 8, no. 5, pp. 1–9, 2017.

Development of Thin-Film Solid Oxide Fuel Cells Supported on Anode/Metal Substrates

Lowering the operating temperature of solid oxide fuel cell (SOFC) has been studied at worldwide institutes over the past decades. In the process development, nano-structuring electrode catalysts for better reaction kinetics and thin-film electrolyte for reducing the ionic conduction resistance have been explored so far. Thin-film electrolyte enables low-temperature operation of SOFCs using conventional materials. Therefore, if the cell fabrication using the thin film electrolyte can be implemented with relatively low-cost and scalable technology, it will be possible to overcome the many difficulties that researchers suffered due to the high temperature. In this study, we developed a process for fabricating a button-size and large-area SOFC unit cells by introducing a thin-film electrolyte, supported on anode/metal substrates. Morphology analysis confirmed that the pinhole-free thin-film electrolyte was uniformly deposited. Electrochemical performance evaluation showed sufficient power density and area-specific resistance without using new materials.

Effect of nano-pinholes within ceramic electrolytes of thin-film solid oxide fuel cells

The effect of nano-pinholes in the ceramic electrolyte (yttria-stabilized zirconia) on the performance of a thin-film solid oxide fuel cell (SOFC) is investigated. As a result, the open-circuit voltage of the thin-film SOFC was not severely affected by the existence and the size of a pinhole. However, the pinhole induced a critical electrical shortage between the anode and cathode if the cathode was deposited onto the pinhole. Thus, a nano-sized pinhole does not significantly induce fuel crossover or critically reduce the performance. It could, however, reduce the performance significantly during the common fabrication step, i.e. deposition of thin-film cathode on an electrolyte, of a thin-film SOFC, especially for larger pinholes.

Effects of Nanoscale PEALD YSZ Interlayer for AAO Based Thin Film Solid Oxide Fuel Cells

Performance of thin film solid oxide fuel cells (TF-SOFCs) were improved by inserting plasma enhanced atomic layer deposition (PEALD) of yttira-stabilized zirconia (YSZ) interlayers. By controlling the ratio between Y2O3 and ZrO2 in YSZ deposition supercycles, Y2O3 mol.% in YSZ films were changed. High Y2O3 contained PEALD YSZ interlayered cell showed higher maximum power density (123.6 mW/cm2 at 500 °C, 286.1 mW/cm2 at 550 °C) and smaller polarization resistance compared with reference Y2O3 concentrated PEALD YSZ interlayer cell. (108.5 mW/cm2 at 500 °C, 181.1 mW/cm2 at 550 °C) Exchange current densities of TF-SOFCs at 500 °C also improved ~ 59.3% at high Y2O3 concentrated PEALD YSZ interlayer cells. These phenomena caused by high density of oxygen vacancies in high Y2O3 concentrated PEALD YSZ interlayer, which helps oxygen incorporation reactions at cathode-interlayer interface. Therefore, cathodic polarization loss related with oxygen reduction reactions was decreased, and then, performance and exchange current density were improved. Results of this study imply that insertion of simple PEALD YSZ interlayer at cathode-electrolyte interface efficiently improves performance and electrochemical characteristics of TF-SOFCs.

Au-Coated Lanthanum Strontium Cobalt Ferrite Cathode for Lowering Sheet Resistance of a Solid Oxide Fuel Cell

In this paper, we fabricate a thin film solid oxide fuel cell (SOFC) with a lanthanum strontium cobalt ferrite (LSCF) cathode which is coated with an Au layer by sputtering. The fabricated thin film SOFC has a 370-nm-thick Ni anode/880-nm-thick YSZ electrolyte/440-nm-thick LSCF cathode/130-nm-thick Au layer. The electrochemical performances of the thin film SOFC with the LSCF cathode coated with a 130-nm-thick Au layer and another SOFC without an Au layer are measured at the operating temperature of 600 °C. Since the LSCF cathode coated with a 130-nm-thick Au layer significantly lowers the sheet resistance (in-plane direction resistance), the peak power densities of the thin film SOFC with the LSCF cathode coated with a 130-nm-thick Au layer and the thin film SOFC with the LSCF cathode without a 130-nm-thick Au layer are 500 mW/cm2 and 49 mW/cm2, respectively. Due to the LSCF cathode coated with a 130-nm-thick Au layer, the peak power density of the thin film SOFC with the Au-coated LSCF cathode is 10 times higher than that of the LSCF cathode without Au layer.

Optimization of Y2O3 dopant concentration of yttria stabilized zirconia thin film electrolyte prepared by plasma enhanced atomic layer deposition for high performance thin film solid oxide fuel cells

In this study, the Y2O3 doping concentration of yttria stabilized zirconia (YSZ) thin film electrolyte deposited by plasma enhanced atomic layer deposition (PEALD) is optimized to maximize the performance of thin film solid oxide fuel cells (TF-SOFCs). The PEALD YSZ thin films are highly crystalline, and the Y2O3 concentration is controlled by changing the ratio between ZrO2 and Y2O3 ALD cycles. Electrochemical performances of TF-SOFCs are strongly dependent on the Y2O3 doping concentration in electrolytes. The cell with 10.7 mol% doped YSZ achieves the best performance (180 mW/cm2) at 450 °C due to decreased polarization loss because of its higher density of oxygen vacancies. These results demonstrate the effectiveness of PEALD process to deposit crystalline YSZ thin film electrolytes with optimal doping for high performance TF-SOFCs.

Effect of plasma-enhanced atomic layer deposited YSZ inter-layer on cathode interface of GDC electrolyte in thin film solid oxide fuel cells

We have fabricated thin film-solid oxide fuel cells (TF-SOFCs) with tri-layer electrolytes, which are composed of YSZ deposited by sputtering, YSZ deposited by plasma enhanced atomic layer deposition (PEALD), and GDC deposited by sputtering. Then, we have investigated the effects of the PEALD YSZ on the GDC layer and the whole cell. Open circuit voltages (OCVs) and peak power densities of the cells kept being enhanced as the thickness of the PEALD YSZ layer increases. The 1000 cycle of PEALD YSZ deposited TF-SOFC exhibited 214 mW/cm2 of peak power density, while TF-SOFC without PEALD YSZ inter-layer showed 124 mW/cm2, almost half of that of the 1000 cycle sample. FE-SEM images and AFM analysis of the top surface of GDC electrolytes sustain this phenomenon. As the PEALD YSZ inter-layer thickness increases, the top surface of GDC electrolyte become rough, with pin-holes disappeared, and the grain size of GDC electrolyte grew smaller. As a result, the improved quality of the electrolytes enhanced OCV and smaller grain size of the electrolytes caused better catalytic activity on the electrolyte-cathode interface, leading to lower faradaic impedances in electrochemical impedance spectroscopy (EIS).

A nanoporous substrate-based low temperature solid oxide fuel cell using a thin film Ni anode

A thin film solid oxide fuel cell (TF-SOFC) with a Ni anode on a nanoporous anodic aluminum oxide substrate was demonstrated. In our experiments, Ni was used as an anode material, while a very small amount of Pt was used as a cathode material, which reduced the thickness of the thin film cathode to 65 nm. In addition, a plasma-enhanced atomic layer deposition method was used to prepare a yttria-stabilized zirconia electrolyte. The open circuit voltage and the peak power density of the as-fabricated fuel cells, both of which were measured at 500 °C, were 1.04 V and 150 mW/cm2, respectively.

Pulsed Laser Deposition of SrCo0.8Nb0.1Ta0.1O3-Δ As a Cathode for Thin Film Solid Oxide Fuel Cells

Recently, Li et al. reported co-doped perovskite SrCo0.8Nb0.1Ta0.1O3-δ (SCNT), which shows high oxygen reduction reaction (ORR) activity [1]. This research demonstrates thin film deposition of SCNT by pulsed laser deposition technique and successfully applied to thin-film solid oxide fuel cells (SOFCs). For decades, there have been many researches on SOFCs operating at intermediate or low temperature range (400-600 °C). The operating temperature-lowering strategy can be divided into two: material development and nano-thin architecturing. The material development approach is to develop brand-new materials and the aforementioned SCNT is one of such approaches. According to [1], it shows area-specific charge transfer resistance as low as 0.16 ~ 0.68 Ω·cm2, which is one of the state-of-the-art ORR kinetics in literature, and the SCNT was successfully applied to Gd-doped CeO2 electrolyte-based SOFC as a cathode. The SOFC with SCNT exhibited comparable electrochemical performance in spite of the operating temperature range below 500 °C. Another strategy is to make the ceramic electrolyte as thin as possible because most of the performance comes from the ohmic resistance of the electrolyte inside SOFCs. There have been also many reports on such thin film SOFCs operating below 500 °C, but in this case even with the most well-known yttria-stabilized zirconia (YSZ) did the SOFCs exhibit the comparable performance to normal SOFCs operating at high temperature (>700 °C). However, the previous researches mostly employed Pt as a cathode, which is not suitable for industrial applications due to expensive price. Therefore, it is believed that champion thin film SOFCs without novel metals will be feasible if both materials development and thin-film architecturing are combined together. Otherwise, such thin film SCNT can be used as an interfacial (functional) layer between electrolyte and electrode since thin film could have the fine nano-morphology favorable to electrochemical reaction. The SCNT target was prepared by solid state reactive sintering method and described in detail elsewhere [1]. This target was then mounted to pulsed laser depositing machine and deposited onto the Si wafer. Here, two depositing conditions are selected as the morphology-influencing parameters: internal pressure and substrate temperature. By changing these parameters, we seek the conditions with which the deposited thin film SCNT has columnar structure. Such thin film SCNT was subsequently applied to thin film SOFCs. Here, the thin film SOFCs were fabricated on nano-porous substrates. On top of the nano-porous substrate, Thin Ni film was firstly deposited by sputtering. After that, the YSZ was also deposited by sputtering. The as-fabricated thin film SOFCs with SCNT cathodes were operated at 450 and 500 °C. Polarization characteristics and corresponding electrochemical impedance spectroscopy were employed to deep understandings into the electrochemical kinetics of the fuel cells. Reference 1. M. Li, M. Zhao, F. Li, W. Zhou, V. K. Peterson, X. Xu, Z. Shao, I. Gentle, Z. Zhu, Nat. Commun., 8, 13990(2017)