Scalable thin film micro-solid oxide fuel cells with enhanced mechanical stability for low temperature operation
Baek, Jong Dae
Date of Issue2016-03
School of Mechanical and Aerospace Engineering
A fuel cell, which directly converts chemical energy into electricity has been a promising candidate as a next-generation renewable energy source. Especially, solid oxide fuel cells (SOFCs) have been recognized as high energy conversion efficient and fuel flexible devices. However, conventional SOFCs are typically operated above 700°C to activate ionic transport across an ionic conducting electrolyte. Such high operating temperatures limit to expand the scope of their applications and require special materials for sealing and heat management which is expensive and cause design challenges. Therefore, lowering operating temperature is a primary concern to expand the field of feasible application. Various efforts have been made to reduce operating temperature either by thinner electrolyte or by better catalytic materials. In particular, thin film electrolytes significantly reduce ohmic resistance of SOFCs and accordingly, provides good fuel cell performances at a low operating temperature regime (300 to 500°C). Such thin film can be fabricated as a free-standing membrane by the help of silicon micromachining technology. However, in order to minimize the internal resistance of the cell, the membrane is usually made to be only a few tens of nanometers in thickness, and subsequently its membrane stability is deteriorated. In addition, the lateral dimension of a fuel cell determines the total power output of this device, and enlarging the electrolyte to obtain higher power output without membrane failures becomes nearly impossible. In this dissertation, new fabrication methods to realize thin film SOFCs with enhanced membrane mechanical stability are presented. Different from conventional square thin films having geometrical discontinuities such as sharp corners, circular thin films have uniform stress distribution and no stress concentration points along the boundary due to axisymmetric geometry. By taking mechanical advantages of a circular thin film, we successfully fabricated millimeter-scale and 100 nm-thick circular yttria-stabilized zirconia (YSZ) and yttria-stabilized barium zirconate (BYZ) electrolyte SOFCs with a high yield rate above 80% by combination of silicon wet etching and plasma etching. Circular electrolytes were sustained by a silicon-edge support which acts as a stress absorber. Compared with the clamped square membranes, the circular membrane reinforced with a silicon-edge support showed 40% reduction in the maximum principal stress. Moreover, electrochemical performances of the circular electrolyte membranes showed that high open circuit voltage (OCV) above 1.07 V and maximum power densities of 437 mW/cm2 and 76 mW/cm2 at 400°C of the 1.5 mm diametral YSZ cell and 1.6 mm diametral BYZ cell, respectively. To further enlarge electrochemically active area with maintaining mechanical stability, a fabrication method of circular array SOFCs applied with silicon-edge reinforcement is demonstrated. In this work, we fabricated a new thin film SOFC architecture that can provide upward membrane scalability to obtain higher device power, and at the same time enhance membrane mechanical stability for stable fuel cell operation. This SOFC array achieved high OCV of 1.1 V and provided 1.38 mW of total power output at 400 °C with the array of 3.6 mm in lateral dimension. Through long-term OCV test and thermal cycling test, functional and thermal stability of the circular array SOFCs with edge-reinforcement were confirmed. With further design and process optimization, higher total power output can be achieved at low temperature by this μ-SOFC technology.