Fuel cells offer the possibility of zero-emissions electricity generation and increased energy security. We review here the current status of solid oxide (SOFC) and polymer electrolyte membrane (PEMFC) fuel cells. Such solid electrolyte systems obviate the need to contain corrosive liquids and are thus preferred by many developers over alkali, phosphoric acid or molten carbonate fuel cells. Dramatic improvements in power densities have been achieved in both SOFC and PEMFC systems through reduction of the electrolyte thickness and architectural control of the composite electrodes. Current efforts are aimed at reducing SOFC costs by lowering operating temperatures to 500–800 °C, and reducing PEMFC system complexity be developing ‘water-free’ membranes which can also be operated at temperatures slightly above 100 °C.

Fuel cell materials and components

Bismuth oxide systems exhibit high oxide ion conductivity and have been proposed as good electrolyte materials for applications such as solid oxide fuel cells and oxygen sensors. However, due to their instability under conditions of low oxygen partial pressures there has been difficulty in developing these materials as alternative electrolyte materials compared to the state-of-the-art cubic stabilised zirconia electrolyte. Bismuth oxide and doped bismuth oxide systems exhibit a complex array of structures and properties depending upon the dopant concentration, temperature and atmosphere. In this paper we comprehensively review the structures, thermal expansion, phase transitions, electrical conductivity and stability of bismuth oxide and doped bismuth oxide systems. ©

Bismuth based oxide electrolytes— structure and ionic conductivity

A review of numerical modeling of solid oxide fuel cells

We report a correlation between the microstructure of the anode electrode of a solid oxide fuel cell (SOFC) and its electrochemical performance for a tubular design. It was shown that the electrochemical performance of the cell was extensively improved when the size of constituent particles was reduced so as to yield a highly porous microstructure. The SOFC had a power density of greater than 1 watt per square centimeter at an operating temperature as low as 600°C with a conventional zirconia-based electrolyte, a nickel cermet anode, and a lanthanum ferrite perovskite cathode material. The effect of the hydrogen fuel flow rate (linear velocity) was also examined for the optimization of operating conditions. Higher linear fuel velocity led to better cell performance for the cell with higher anode porosity. A zirconia-based cell could be used for a low-temperature SOFC system under 600°C just by optimizing the microstructure of the anode electrode and operating conditions.

http://science.sciencemag.org/content/sci/325/5942/852.full.pdf

Impact of Anode Microstructure on Solid Oxide Fuel Cells

Solid oxide fuel cell: Decade of progress, future perspectives and challenges

Design and fabrication of a 100 W anode supported micro-tubular SOFC stack

Porous ceramic microstructures play an important role in enhancing the performance of solid oxide fuel cells (SOFCs). The development of SOFCs that can generate high power densities requires improvements in the electrodes. The engineering of the highly porous electrodes is required to maximize the number of electrochemical reaction sites and to facilitate the gas diffusion. Various sacrificial template methods are capable of generating appropriate porous microstructures and can give a strict control over the porosity amount, pore sizes, and morphology. The present review provides a broad overview on SOFCs from the perspectives of creating porous electrodes using sacrificial template methods as a state of the art strategy. In addition, the manufacturing processes for the production of SOFCs are overviewed and described with focus on the advantages and limitations of each method.

Review on fabrication techniques for porous electrodes of solid oxide fuel cells by sacrificial template methods

Fabrication and properties of anode-supported tubular solid oxide fuel cells

Processes to fabricate tubular solid oxide fuel cells (SOFCs) using various electrolyte and electrode materials have been investigated. Single-phase perovskite strontium- and magnesium-doped lanthanum gallate (LSGM) was synthesized. Mechanical and thermal properties, and the cell performance of LSGM electrolyte fuel cells were studied and compared with yttria-stabilized zirconia (YSZ) electrolyte systems. Paste extrusion was identified as a suitable method for high-speed manufacture of the tubular support components. Procedures were developed to extrude dense, straight, round (OD: 2-4 ±0.1 mm), thin-walled (0.2 ± 0.02 mm) tubular LSGM and YSZ electrolytes, and porous anode or cathode support (5-20 mm OD, 0.5-2 mm wall thickness). The research systematically investigated developing a fabrication process, designing the extrusion die, the effects of formulation and process additives, and how to handle, dry, and sinter the extrudate. Dip coating, brush painting, vacuum infiltration, electrophoretic coating, and plasma spray were examined for applying thin films on the tubular support. The average three-point bending strengths of extruded and sintered LSGM were determined to be 287, 195, 184, and 147 MPa at room temperature, 600°C, 800°C, and 1000°C, respectively. Room-temperature burst strengths (average) of the tubular electrolytes made from YSZ and LSGM were 127 and 40 MPa, respectively. Room-temperature bending strength and burst strength (average) of the as-sintered NiO-YSZ anode were 300 and 100 MPa, respectively. The average thermal expansion coefficients of YSZ and LSGM were 10.18 x 10 -6 /°C and 11.01 x 10 -6 /°C, respectively, between room temperature and 800°C. The maximum power density at 800°C for a 220-μm-thick LSGM electrolyte was 460-482 mW/cm 2 , which is more than double the power density for YSZ cells (200-220mW/cm 2 ) at 850°C. Reducing LSGM thickness from 550 to 220 μm increased the maximum power densities by more than 30%. Repeatable cell power outputs per cell of 2.5 W at 800°C and 2.8 W at 850°C were obtained. The cell durability was evaluated during a 500-h test. This comprehensive study intended to better understand the relationship between the materials, processing, manipulation of the microstructure, properties, and tubular SOFC performance.

Fabrication and characterization of tubular solid oxide fuel cells

Yttria-stabilized zirconia (YSZ) is the most common electrolyte material for solid oxide fuel cells. Herein, we conducted a comparative study on the densification behavior of three different kinds of commercial 8 mol% YSZ powders: (i) TZ-8Y (Tosoh, Japan), (ii) MELox 8Y (MEL Chemicals, UK), and (iii) YSZ-HT (Huatsing Power, China). The comparison was made on both the self-supporting pellets and thin-film electrolytes coated onto a NiO–YSZ anode support. For the pellets, MELox 8Y showed the highest densification at lower sintering temperatures with 93% and 96% of the theoretical density at 1250 and 1300 °C, respectively. Although YSZ-HT showed a higher sintering rate than TZ-8Y, a sintering temperature of 1350 °C was required for both the powders to reach 95% of the theoretical density. For the thin-film electrolytes, on the other hand, YSZ-HT showed the highest sintering rate with a dense microstructure at a co-sintering temperature of 1250 °C. Our results indicate that besides the average particle size, other factors such as particle size distribution and post-processing play a significant role in determining the sintering rate and densification behavior of the YSZ powders. Additionally, a close match in the sintering shrinkage of the electrolyte and anode support is important for facilitating the densification of the thin-film electrolytes.

/pdf/densification-behavior-of-yttria-stabilized-zirconia-powders-173oprjnfq.pdf

Densification behavior of yttria-stabilized zirconia powders for solid oxide fuel cell electrolytes

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