Reduced-Temperature Solid Oxide Fuel Cells Research Articles

Heterointerface Engineered Oxygen Electrode for High Performance Reversible Solid Oxide Fuel Cells (R-SOFCs)

Solid oxide fuel cells (SOFCs) are promising alternatives to combustion technology for electric energy production due to no emission and high efficiency in chemical to electric energy conversion. Despite such advantages, the commercialization of SOFCs is hindered by the high operating temperature, resulting in a few problems, such as expensive system cost and short lifetime. Thus, while reduced-temperature SOFCs have been a long-lasting interest in the SOFC community, the sluggish oxygen reduction reaction (ORR) occurring at the oxygen electrode at intermediate temperature is the most critical challenge for the enhanced overall performance of SOFCs. As a promising strategy to achieve higher catalytic activity of oxygen electrode, the infiltration method has been employed due to not only extended active site for ORR by electrode surface decoration with active nanoparticle catalysts, but enriched oxygen vacancies additionally formed on the surface of infiltrates. Furthermore, heterointerface engineering in the infiltrated oxygen electrode has, more recently, emerged as a promising approach to optimize oxygen electrode performance. The lattice strain, induced by lattice parameter mismatch in heterostructured materials, can significantly influence the ORR/OER (oxygen evolution reaction) kinetics by changing oxygen defect formation energy and migration barrier. We report the enhancement in the electrochemical performance of the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) based heterostructured oxygen electrode, when infiltrated with aliovalent cation doped La2-xAxNi1-yByO4 (Ruddlesden-Popper, RP), leading to different intrinsic catalytic ability according to dopant; Sr and Co for A and B site, respectively, to generate oxygen defect (e.g., oxygen vacancy or interstitial O2-). This study demonstrates the impact of the oxygen defect in the infiltrates on the heterointerface, dominating ORR & OER kinetics in the heterostructured oxygen electrode, and investigate the primary oxygen defect for RP structured infiltrate to realize high ORR & OER activity. To characterize the morphology of infiltrates anchored on the oxygen electrode surface and the heterointerface synthesized between infiltrated layers and backbones, scanning electron microscope (SEM) and transmission electron microscopy (TEM) are performed. The electrochemical performances of the infiltrated SOFCs were examined by electrochemical impedance spectroscopy and analyzed in terms of the distribution of relaxation time (DRT) from impedance data. Thermogravimetric analysis (TGA) and iodometric titration are conducted to understand the correlation between oxygen defect produced by aliovalent doping and electrochemical ORR activity.

Synergy Effect of Mixed Precursor Infiltrated Cathode on Solid Oxide Fuel Cell Performance

The Solid oxide fuel cells (SOFCs) are promising alternative to combustion technology for electric energy production due to no emission and high efficiency in chemical to electric energy conversion. However, the commercialization of SOFCs is hindered by the high operating temperature, resulting in few of main problems, such as expensive system cost and short lifetime. Thus, while reduced-temperature SOFCs has been a long lasting interest in SOFC community, the sluggish oxygen reduction reaction (ORR) occurring at cathode at intermediate temperature is the most critical challenge for enhanced overall performance of SOFCs. In recent years, the surface modification of cathode materials by infiltration process shows outstanding performance. In this process, it is important to select the proper infiltrate materials since which play important role in enlarging active sites for ORR. However, the existing infiltration processes have limiting effect on performance of SOFC due to coarsening of particles at high temperature. Therefore, the selection of appropriate infiltrate materials along with the suppression of particle growth is the key requirement for enhanced ORR, dominating overall SOFC performance. In this paper, we attempt to systemically investigate the various metal oxide as infiltrate material and extend active site for ORR by controlling the size of infiltrated oxide. The nitrate precursor of corresponding oxide was infiltrated into the cathode, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), in symmetric cell to test the effect of each infiltrated oxide on ORR. Among the tested samples, Pr infiltrated LSCF show lower polarization resistance (Rp) at 500 °C; 40 % lower Rp for Pr infiltrated LSCF than that of bare LSCF. Furthermore, mixed precursor of two different oxides, chemically inactive each other, were infiltrated. This process can effectively suppress particle growth at high temperature, resulting in more active site for ORR due to increased surface area. To confirm the morphology of infiltrated oxides anchored on the cathode surface, scanning electron microscope (SEM) and transmission electron microscopy (TEM) are performed.Also, electrochemical impedance spectroscopy (EIS), thermogravimetric analysis (TGA) and iodometric titration are conducted to understand correlation between produced oxygen vacancy by infiltrated oxide and ORR. In conclusion, this study shows that mixed precursor-infiltrated cathode has significant influence on ORR in SOFCs.

Structural, optical and magnetic investigation of Gd implanted CeO2 nanocrystals

Gadolinium implanted cerium oxide (Gd-CeO2) nanocomposites is an important candidate which have unique hexagonal structure and high K- dielectric constant. Gd-CeO2 nanoparticles were synthesized using hydrothermal method. X-ray diffraction (XRD) results showed that the peaks are consistent with pure phase cubic structure the XRD pattern also confirmed crystallinity and phase purity of the sample. Nanocrystals sizes were found to be up to 25nm as revealed by XRD and SEM. It is suggested that Gd gives an affirmative effect on the ion influence behavior of Gd-CeO2. XRD patterns showed formation of new phases and SEM micrographs revealed hexagonal structure. Photoluminescence measurement (PL) reveals the systematic shift of the emission band towards lower wavelength thereby ascertaining the quantum confinement effect (QCE). The PL spectrum has wider broad peak ranging from 390nm to 770nm and a sharp one centered on at 451.30nm which is in tune with Gd ions. In the Raman spectra showed intense band observed between 460cm−1 and 470cm−1 which is attributed to oxygen ions into CeO2. Room temperature ferromagnetism was observed in un-doped and Gd implanted and annealed CeO2 nanocrystals. In the recent studies, ceria based materials have been considered as one of the most promising electrolytes for reduced temperature SOFC (solid oxide fuel cell) system due to their high ionic conductivities allowing its use in stainless steel supported fuel cells. CeO2 having an optical bandgap 3.3eV and n-type carrier density which make it a promising candidate for various technological application such as buffer layer on silicon on insulator devices.

BaCo0.7Fe0.22Y0.08O3−δ as an Active Oxygen Reduction Electrocatalyst for Low-Temperature Solid Oxide Fuel Cells below 600 °C

Solid oxide fuel cells (SOFCs) offer great promise as sustainable energy conversion devices due to their high chemical-to-electrical conversion efficiency, flexible fuel sources, and low pollutions. In recent years, much effort has been devoted to developing intermediate temperature SOFCs. Central to the devices is the availability of a highly effective electrocatalyst for oxygen reduction reaction with reduced temperature operation, especially below 600 °C. Here we present a novel B-site Y-doped perovskite-type oxide BaCo0.7Fe0.22Y0.08O3−δ (BCFY) with extremely low polarization resistances (e.g., 0.10 Ω cm2 at 550 °C), which is ascribed to high cubic symmetry structure and fast oxygen kinetics. The superior electrocatalytic activity and stability enable BCFY to be a promising cathode candidate toward the application of reduced temperature SOFCs.

Shape-Dependent Activity of Ceria for Hydrogen Electro-Oxidation in Reduced-Temperature Solid Oxide Fuel Cells.

Single crystalline ceria nanooctahedra, nanocubes, and nanorods are hydrothermally synthesized, colloidally impregnated into the porous La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) scaffolds, and electrochemically evaluated as the anode catalysts for reduced temperature solid oxide fuel cells (SOFCs). Well-defined surface terminations are confirmed by the high-resolution transmission electron microscopy--(111) for nanooctahedra, (100) for nanocubes, and both (110) and (100) for nanorods. Temperature-programmed reduction in H2 shows the highest reducibility for nanorods, followed sequentially by nanocubes and nanooctahedra. Measurements of the anode polarization resistances and the fuel cell power densities reveal different orders of activity of ceria nanocrystals at high and low temperatures for hydrogen electro-oxidation, i.e., nanorods > nanocubes > nanooctahedra at T ≤ 450 °C and nanooctahedra > nanorods > nanocubes at T ≥ 500 °C. Such shape-dependent activities of these ceria nanocrystals have been correlated to their difference in the local structure distortions and thus in the reducibility. These findings will open up a new strategy for design of advanced catalysts for reduced-temperature SOFCs by elaborately engineering the shape of nanocrystals and thus selectively exposing the crystal facets.

Towards a Fundamental Understanding of the Oxygen Reduction Mechanism

The kinetics of oxygen reduction are thermally activated, therefore as solid oxide fuel cell (SOFC) operating temperature is lowered, the rates of chemical reactions drop exponentially, and activation polarization at the cathode dominates SOFC performance. Hence, cathode polarization is widely regarded to be the key issue hindering reduced temperature SOFC development. In order to understand the various mechanistic contributions to cathode polarization and apply this knowledge to development of lower-polarization/lower-temperature SOFC cathodes, we have embarked on a multi-faceted, multi-disciplinary approach to deconvolute the various contributions to SOFC cathode polarization. This approach includes FIB/SEM to quantify the cathode microstructure and heterogeneous catalysis techniques and O-isotope exchange to evaluate the O2 reduction mechanism at the gas-solid reaction interface. These results are then combined (and contrasted) with more conventional electrochemical polarization techniques to try and elucidate each of the mechanisms as a function of composition and microstructure. The progress to date is summarized.

Electronic nature of the enhanced conductivity in YSZ-STO multilayers deposited by PLD

The search for new strategies to enhance the oxide ionic conductivity in oxide materials is a very active field of research. These materials are needed for application in a new generation of more efficient and durable solid state electrochemical devices such as reduced-temperature Solid Oxide Fuel Cells (SOFC's). Recently published results have claimed that sputtered yttria stabilised zirconia (YSZ)/SrTiO 3 (STO) heterostructures show enhanced oxygen ion conductivity, by about eight orders of magnitude, with respect to that expected for YSZ and STO bulk values. The YSZ/STO heterostructures obtained in this work from Pulsed Laser Deposited (PLD) films, also show enhanced conductivity of a similar order. However, the fact that our structures show different relative orientations indicates that the conductivity enhancement may not be associated to a particular crystallographic arrangement at the interface. The combined characterisation of the conductivity dependence on oxygen partial pressure and direct oxygen diffusion by means of tracer experiments clearly demonstrate that the enhancement observed is related to the electronic rather than ionic conductivity.

Heat resistant alloys as interconnect materials of reduced temperature SOFCs

Heat-resistant alloys, Haynes 230 and SS310, were exposed to air and humidified H 2 at 750 °C for up to 1000 h, respectively, simulating the environments in reduced temperature solid oxide fuel cells (SOFCs). The oxidized samples were characterized by using SEM, EDS and X-ray diffraction to obtain the morphology, thickness, composition and crystal structure of the oxide scales. A mechanism for the formation of metallic Ni-rich nodules on top of the oxide scale in Haynes 230 sample oxidized in humidified H 2 was established. Thermodynamic analysis confirmed that MnCr 2O 4 is the favored spinel phase, together with Cr 2O 3, in the oxide scales.

Oxidation of Haynes 230 alloy in reduced temperature solid oxide fuel cell environments

Haynes 230 alloy was exposed to reducing and oxidizing environments at 750 °C for 1000 h, simulating the conditions in a reduced temperature solid oxide fuel cell (SOFC). The oxidized specimens were characterized in terms of the oxide morphology, composition and crystal structure. The oxide scale in each environment was identified as Cr 2O 3 with the existence of Cr 2MnO 4. Ni remained metallic in the reducing atmosphere, and NiO was detected in the sample exposed to air. The oxide scale is around 1 μm thick after 1000 h of oxidation in both situations. The area specific resistance (ASR) contributed by the oxide scale is expected less than 0.1 Ω cm 2 after 40,000 h of exposure when a parabolic oxide growth rate is assumed, demonstrating the suitability of the interconnect application of this alloy in the reduced temperature SOFCs.

Electrochemical properties of reduced-temperature SOFCs with mixed ionic–electronic conductors in electrodes and/or interlayers

We have investigated the electrochemical properties of two types of reduced-temperature solid oxide fuel cells (SOFCs) in which the mixed ionic–electronic conductors are used to improve their performances. Electrolyte-supported cells, in which doped LaGaO 3 strengthened by Al 2O 3 dispersion is used as the electrolyte, were prepared and tested. Samaria-doped ceria (SDC) interlayers of 0.3-μm thickness were fired onto both surfaces of the electrolyte of 0.2-mm thickness at 1523 K, before firing the Ni–Sm 0.1Ce 0.9O 1.95–(CeO 2) 0.1[(Y 2O 3) 0.08(ZrO 2) 0.92] 0.9 (Ni–SDC–CeYSZ) (10 mol% ceria-doped yttria-stabilized zirconia [YSZ]) cermet anode at 1723 K and La 0.7Sr 0.3Co 0.2Fe 0.8O 3–Sm 0.2Ce 0.8O 1.9 (LSCF–SDC) composite cathode at 1373 K. The cells have a nominal size of 60×60 mm 2 with an effective electrode area of 4 cm 2. The single cell thus prepared showed a high power density of 0.67 W cm −2 at 1073 K and long-term stability during the operation time of 1000 h. Anode-supported cells with a thin YSZ electrolyte film approximately 30 μm thick were also prepared by co-sintering of screen-printed YSZ paste on a compacted anode substrate. The cells have a nominal size of 50×50 mm 2 with an effective electrode area of 4 cm 2. The single cell with the LSCF–SDC composite cathode having SDC interlayer showed the maximum power density of 0.648 W cm −2 at 1023 K. The bilayer cathode also showed high resistance against degradation by Cr-poisoning.

Ni/Ceria Cermet as Anode of Reduced-Temperature Solid Oxide Fuel Cells

Anode characteristics of a cermet on the electrolyte were studied by a three-probe ac impedance method combined with dc polarization, instead of the conventional current-voltage curve measurement. The experimentally obtained Cole-Cole plots of ac impedance showed a shift with added dc voltage. Both bulk resistance and electrode reaction resistance increased with increasing anodic dc current. The shift of bulk resistance is explained by the electronic conduction of the electrolyte. The increase of electrode reaction resistance is thought to be a result of discrepancy between the ionic and the external current, differing from Ni/yttria-stabilized zirconia anode. An interface resistance lower than 0.1 Ω/cm2 was obtained below 700°C by using a coprecipitation method to prepare the anode precursor and cofiring it with the electrolyte to improve the contact. © 2002 The Electrochemical Society. All rights reserved.

Pt-cermet cathode for reduced temperature SOFCs

Polarization characteristics of Pt–(Sc 0.10Ce 0.01)Zr 0.89O 2 (SSZ) cermet cathodes were studied in order to develop new cathodes for reduced temperature solid oxide fuel cells (SOFCs). Several Pt–SSZ cermet electrodes were prepared by mixing Pt and SSZ powders using a high-energy ball mill. A Pt–SSZ cermet cathode on yttria-stabilized zirconia (YSZ) electrolyte achieved high cathodic activity; the cathode interfacial conductivity reached as high as 6.7 S/cm 2 at 973 K and 0.8 S/cm 2 at 873 K. These results suggest that this Pt–SSZ cermet cathode has enough capability at 973 K, but is still not satisfactory at 873 K. This high activity can be obtained by ball milling SSZ/Pt mixture of proper ratios in vacuum and by controlling electrode thickness, sintering temperature, and Ca impurity. These electrodes showed two optima of interfacial conductivity vs. SSZ/Pt ratio and remarkable dependence on cathode thickness. It was considered that the first optimum is based on two-dimensional reaction sites on YSZ electrolyte surface and the second one originates from three-dimensional expansion of reaction sites into the cermet cathode.

Reduced-Temperature Solid Oxide Fuel Cells Fabricated by Screen Printing

Electrolyte films of samaria-doped ceria (SDC, are fabricated onto porous NiO-SDC substrates by a screen printing technique. A cathode layer, consisting of and 10 wt % SDC, is subsequently screen printed on the electrolyte to form a single cell, which is tested at temperatures from 400 to 600°C. When humidified (3% hydrogen or methane is used as fuel and stationary air as oxidant, the maximum power densities are 188 (or 78) and 397 (or 304) mW/cm2 at 500 and 600°C, respectively. Impedance analysis indicates that the performances of the solid oxide fuel cells (SOFCs) below 550°C are determined primarily by the interfacial resistance, implying that the development of catalytically active electrode materials is critical to the successful development of high-performance SOFCs to be operated at temperatures below 600°C. © 2001 The Electrochemical Society. All rights reserved.

Development of Reduced-Temperature Solid Oxide Fuel Cell Power Systems

Development of Reduced-Temperature Solid Oxide Fuel Cell Power Systems

Soft chemical route and new solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are a promising technology for electric power generation in the 21st century. Recently, research are focusing on reduced temperature SOFCs. The fabrication of thin film electrolytes and electrode membranes for reduced temperatures by a soft chemical route is discussed.