Nanoscale Compositionally Graded Thin‐Film Electrolyte Membranes for Low‐Temperature Solid Oxide Fuel Cells

Controllable fabrication of compositionally graded Gd_0.1Ce_0.9O_2‐δ and Y_0.16Zr_0.84O_2‐δ electrolytes using co‐sputtering is demonstrated. Self‐supported membranes were lithographically fabricated to employ the new electrolytes into thin film solid oxide fuel cells. Devices integrating such electrolytes demonstrate performance of over 1175 mW cm^?2 and 665 mW cm^?2 at 520 °C using hydrogen and methane as fuel, respectively. The results present a general strategy to fabricate nanoscale functionally graded materials with selective interfacial functionality for energy conversion.     

Recent Progress in the Development of Anode Materials for Solid Oxide Fuel Cells

The field of research into solid oxide fuel cell (SOFC) anode materials has been rapidly moving forward. In the four years since the last in‐depth review significant advancements have been made in the reduction of the operating temperature and improvement of the performance of SOFCs. This progress report examines the developments in the field and looks to draw conclusions and inspiration from this research. A brief introduction is given to the field, followed by an overview of the principal previous materials. A detailed analysis of the developments of the last 4 years is given using a selection of the available literature, concentrating on metal‐fluorite cermets and perovskite‐based materials. This is followed by a consideration of alternate fuels for use in SOFCs and their associated problems and a short discussion on the effect of synthesis method on anode performance. The concluding remarks compile the significant developments in the field along with a consideration of the promise of future research. The recent progress in the development of anode materials for SOFCs based on oxygen ion conducting electrolytes is reviewed.     

High‐Performance Silver Cathode Surface Treated with Scandia‐Stabilized Zirconia Nanoparticles for Intermediate Temperature Solid Oxide Fuel Cells

This work introduces a novel silver composite cathode with a surface coating of scandia‐stabilized zirconia (ScSZ) nanoparticles for application in intermediate temperature solid oxide fuel cells (IT‐SOFCs). The ScSZ coating is expected to maximize the triple boundary area of the Ag electrode, ScSZ electrolyte, and oxygen gas, where the oxygen reduction reaction occurs. The coating also protects the porous Ag against thermal agglomeration during fuel cell operation. The ScSZ nanoparticles are prepared by sputtering scandium‐zirconium alloy followed by thermal oxidation on Ag mesh. The performance of the solid oxide fuel cells with a gadolinia‐doped ceria electrolyte support is evaluated. At temperatures <500 °C, our optimized Ag‐ScSZ cathode outperforms the bare Ag cathode and even the platinum cathode, which has been believed to be the best material for this purpose. The highest cell peak power with the Ag‐ScSZ cathode is close to 60 mW cm^?2 at 450 °C, while bare Ag and optimized Pt cathodes produce 38.3 and 49.4 mW cm^?2, respectively. Long‐term current measurement also confirms that the Ag‐ScSZ cathode is thermally stable, with less degradation than bare Ag or Pt.     

Anion Doping: A New Strategy for Developing High‐Performance Perovskite‐Type Cathode Materials of Solid Oxide Fuel Cells

Overcoming the sluggish activity of cathode materials is critical to realizing the wide‐spread application of intermediate‐temperature solid oxide fuel cells. Herein, a new way is reported to tune the performance of perovskite‐type materials as oxygen reduction electrodes by embedding anions (F^?) in oxygen sites. The obtained perovskite oxyfluorides SrFeO_{3?σ ?δ} F_σ and SrFe_0.9Ti_0.1O_{3?σ ?δ} F_σ (σ = 0.05 and 0.10) show improved electrocatalytic activity compared to their parent oxides, achieving area specific resistance values of 0.875, 0.393, and 0.491 Ω cm² for SrFeO_3?δ , SrFeO_2.95?δ F_0.05, and SrFeO_2.90?δ F_0.10, respectively, at 600 °C in air. Such improved performance is a result of the improved bulk diffusion and surface exchange properties due to anion doping. Moreover, favorable stability in performance is also demonstrated for the F^? anion‐doped perovskites as oxygen reduction electrodes at 650 °C for a test period of ≈200 h. A combination of anion doping and cation doping may provide a highly attractive strategy for the future development of cathode materials.     

Chemically Stable Yttrium and Tin Co‐Doped Barium Zirconate Electrolyte for Next Generation High Performance Proton‐Conducting Solid Oxide Fuel Cells

BaZr_0.7Sn_0.1Y_0.2O_3–δ (BZSY) is developed as a novel chemically stable proton conductor for solid oxide fuel cells (SOFCs). BZSY possesses the same cubic symmetry of space group Pm‐3m with BaZr_0.8Y_0.2O_3‐δ (BZY). Thermogravimetric analysis (TGA) and X‐ray photoelectron spectra (XPS) results reveal that BZSY exhibits remarkably enhanced hydration ability compared to BZY. Correspondingly, BZSY shows significantly improved electrical conductivity. The chemical stability test shows that BZSY is quite stable under atmospheres containing CO₂ or H₂O. Fully dense BZSY electrolyte films are successfully fabricated on NiO–BZSY anode substrates followed by co‐firing at 1400 °C for 5 h and the film exhibits excellent electrical conductivity under fuel cell conditions. The single cell with a 12‐μm‐thick BZSY electrolyte film outputs by far the best performance for acceptor‐doped BaZrO₃‐based SOFCs. With wet hydrogen (3% H₂O) as the fuel and static air as the oxidant, the peak power density of the cell achieves as high as 360 mWcm^?2 at 700 °C, an increase of 42% compared to the reported highest performance of BaZrO₃‐based cells. The encouraging results demonstrate that BZSY is a good candidate as the electrolyte material for next generation high performance proton‐conducting SOFCs.     

Advances in Cathode Materials for Solid Oxide Fuel Cells: Complex Oxides without Alkaline Earth Metal Elements

Solid oxide fuel cells (SOFCs) represent one of the cleanest and most efficient options for the direct conversion of a wide variety of fuels to electricity. For example, SOFCs powered by natural gas are ideally suited for distributed power generation. However, the commercialization of SOFC technologies hinges on breakthroughs in materials development to dramatically reduce the cost while enhancing performance and durability. One of the critical obstacles to achieving high‐performance SOFC systems is the cathodes for oxygen reduction reaction (ORR), which perform poorly at low temperatures and degrade over time under operating conditions. Here a comprehensive review of the latest advances in the development of SOFC cathodes is presented: complex oxides without alkaline earth metal elements (because these elements could be vulnerable to phase segregation and contaminant poisoning). Various strategies are discussed for enhancing ORR activity while minimizing the effect of contaminant on electrode durability. Furthermore, some of the critical challenges are briefly highlighted and the prospects for future‐generation SOFC cathodes are discussed. A good understanding of the latest advances and remaining challenges in searching for highly active SOFC cathodes with robust tolerance to contaminants may provide useful guidance for the rational design of new materials and structures for commercially viable SOFC technologies.     

Low‐Temperature Micro‐Solid Oxide Fuel Cells with Partially Amorphous La0.6Sr0.4CoO3‐δ Cathodes

Partially amorphous La_0.6Sr_0.4CoO_3‐δ (LSC) thin‐film cathodes are fabricated using pulsed laser deposition and are integrated in free‐standing micro‐solid oxide fuel cells (micro‐SOFC) with a 3YSZ electrolyte and a Pt anode. A low degree of crystallinity of the LSC layers is achieved by taking advantage of the miniaturization of the cells, which permits low‐temperature operation (300–450 °C). Thermomechanically stable micro‐SOFC are obtained with strongly buckled electrolyte membranes. The nanoporous columnar microstructure of the LSC layers provides a large surface area for oxygen incorporation and is also believed to reduce the amount of stress at the cathode/electrolyte interface. With a high rate of failure‐free micro‐SOFC membranes, it is possible to avoid gas cross‐over and open‐circuit voltages of 1.06 V are attained. First power densities as high as 200–262 mW cm^?2 at 400–450 °C are achieved. The area‐specific resistance of the oxygen reduction reaction is lower than 0.3 Ω cm² at 400 °C around the peak power density. These outstanding findings demonstrate that partially amorphous oxides are promising electrode candidates for the next‐generation of solid oxide fuel cells working at low‐temperatures.     

Solid Oxide Fuel Cell Anode Materials for Direct Hydrocarbon Utilization

Solid oxide fuel cells (SOFC) are highly efficient energy conversion devices with the advantage of directly utilizing hydrocarbon fuels. Starting with a short introduction about the fuel challenges and early achievements in this field, this review paper focuses on advances in oxygen‐ion conducting electrolyte‐based SOFC during the last 15 years. Robust anodes immune to carbon deposition are a prerequisite for direct hydrocarbon SOFC. In this paper, direct hydrocarbon SOFC anode materials are classified into three general categories: Ni‐cermet, Cu‐cermet, and oxide‐based anodes. Oxide anodes are further classified in terms of their crystalline structures, namely fluorite, rutile, tungsten bronze, pyrochlore, perovskite, and double perovskite. Achievements and recent advances on these SOFC anodes are reviewed and discussed. The concluding remarks summarize the pros and cons of direct hydrocarbon SOFC anode materials along with the perspective of future research trends.     

Exceptionally Enhanced Electrode Activity of (Pr,Ce)O2−δ‐Based Cathodes for Thin‐Film Solid Oxide Fuel Cells

It is shown that an electrochemically‐driven oxide overcoating substantially improves the performance of metal electrodes in high‐temperature electrochemical applications. As a case study, Pt thin films are overcoated with (Pr,Ce)O_2?δ (PCO) by means of a cathodic electrochemical deposition process that produces nanostructured oxide layers with a high specific surface area and uniform metal coverage and then the coated films are examined as an O₂‐electrode for thin‐film‐based solid oxide fuel cells. The combination of excellent conductivity, reactivity, and durability of PCO dramatically improves the oxygen reduction reaction rate while maintaining the nanoscale architecture of PCO layers and thus the performance of the PCO‐coated Pt thin‐film electrodes at high temperatures. As a result, with an oxide coating step lasting only 5 min, the electrode resistance is successfully reduced by more than 1000 times at 500 °C in air. These observations provide a new direction for the design of high‐performance electrodes for high‐temperature electrochemical cells.     

Microstructure of Nanoscaled La0.6Sr0.4CoO3‐δ Cathodes for Intermediate‐Temperature Solid Oxide Fuel Cells

Nanocrystalline La_1‐xSr_xCoO_3‐δ (LSC) thin films with a nominal Sr‐content of x = 0.4 were deposited on Ce_0.9Gd_0.1O_1.95 electrolyte substrates using a low temperature sol‐gel process. The structural and chemical properties of the LSC thin films were studied after thermal treatment, which included a calcination step and a variable, extended annealing time at 700 °C or 800 °C. Transmission electron microscopy combined with selected‐area electron diffraction, energy‐dispersive X‐ray spectrometry, and scanning transmission electron microscopy tomography was applied for the investigation of grain size, porosity, microstructure, and analysis of the local chemical composition and element distribution on the nanoscale. The area specific resistance (ASR) values of the thin film LSC cathodes, which include the lowest ASR value reported so far (ASR_chem = 0.023 Ωcm² at 600 °C) can be interpreted on the basis of the structural and chemical characterization.