Solid oxide fuel cells (SOFC) are an energy efficient, low pollution technology for generating power and have the potential to revolutionize energy generation. Tubular and planar concepts are the most common SOFC systems. The primary objective of this study was to develop a cost-effective process for fabricating micro-tubular SOFCs and to investigate how to improve fuel cell performance.
Two technical approaches were investigated for increasing the cell performance and reducing SOFC costs: (a) reducing electrolyte resistance by minimizing the electrolyte thickness, which consequently increases oxygen flux to the anode; and (b) using new electrolyte materials thereby making it possible for the fuel cell to operate at lower temperatures.
The new material primarily investigated for the SOFC was strontium- and magnesium-doped lanthanum gallate (LSGM), in which the single-phase perovskite LSGM region was determined. Yttria-stabilized zirconia (YSZ) and gadolinium doped ceria (CGO) electrolyte systems were also studied.
Paste extrusion was identified as a suitable method for high-speed manufacture of the tubular support components of SOFCs. Procedures were developed to extrude dense, straight, round (OD: 4.0±0. l mm), thin walled (0.2±0.02 mm) tubular LSGM and YSZ electrolytes. 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.
The three-point bending strength of extruded and sintered LSGM was determined to be 287 MPa, 195 MPa, 184 MPa and 147 MPa at room temperature, 600°C, 800°C and 1000°C respectively. Room temperature burst strengths of the tubular electrolytes made from YSZ, LSGM and CGO were 127 MPa, 40 MPa and 63 MPa respectively. The average thermal expansion coefficients between room temperature and 800°C were 10.18xl0-6 /°C, 11.0lxl0-6 /°C and 12.04x10-6/°C, for YSZ, LSGM, and CGO respectively.
The maximum power density at 800°C for a 220-μm thick LSGM electrolyte was 460-482 mW/cm2, which is more than double the power density for YSZ cells (200-220 mW/cm2) at 850°C. Reducing LSGM thickness from 550 μm 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. Cell performance degradation was measured at 12% during the 500-hour test.
A novel tubular stack design concept was developed to significantly increase stack volumetric power packing density by 84% for a three-cell module or 116% for a five-cell module, compared to a single-tube cell design. This design can also increase fuel efficiency.
Recommendations for future work are given. It is suggested that (1) an anodesupported nanosize LSGM electrolyte system be used for the next generation of intermediate-temperature SOFCs; (2) the LSGM layer be applied by plasma spraying; and (3) nickel-free anodes for LSGM electrolyte systems are developed.
This comprehensive study discusses the relationship between the materials, processing, manipulation of the microstructure, properties and tubular SOFC performance. The unique process developed for producing tubular support components has been scaled to commercial production and represents a significant scientific, engineering and commercial perspective contribution to fuel cell technology.
