ABSTRACT
A fuel cell, which directly converts chemical energy into electricity, is known to be an important energy conversion device in the emerging hydrogen economy. Among the most researched fuel cell types, solid oxide fuel cells (SOFCs) have been recognized to have efficient energy conversion and can readily utilize hydrocarbon fuels. The main drawback that limits the application of conventional SOFCs is the extremely high operating temperature of more than 800°C, which is necessary to activate electrochemical charge transfer and ionic charge transportation. The high operating temperature also imposes difficulty on mobile applications and requires special materials for sealing and heat management and therefore causes design challenges and high cost.
Thin-film solid oxide fuel cells (TF-SOFCs), or micro-SOFCs, are a subcategory developed from conventional SOFCs with the purpose of decreasing the operating temperature by minimizing the ohmic resistance of ionic transportation across the electrolyte. Typical SOFC electrolyte thickness ranges from hundreds of micrometers up to millimeters, while the thickness of TF-SOFCs ranges from tens of micrometers all the way down to nanometers. In addition to the linear decrease of ohmic loss, interesting phenomena associated with improved cathode electrochemical reactions were also observed when a working fuel cell having nanogranular thin-film electrolyte was made possible.
As the thickness of solid oxide electrolyte decreases, its mechanical strength decreases, and the electrolyte is no longer self-supporting, and therefore a suitable supporting substrate is required. Especially for the sub-micrometer thickness electrolytes, despite the impressive performance of TF-SOFCs reported, scaling up of the electrolyte films requires a suitable and robust supporting substrate to make a working fuel cell. As such, exploring unconventional substrate supports for thin-film electrolytes has been one of the main topics for TF-SOFCs. Currently, supporting substrates for TF-SOFCs mainly fall into two configurations: (1) micromachined silicon wafers and (2) porous substrates with surface nanopores.
The main discussion in this chapter will focus on TF-SOFCs with electrolyte thickness under micrometer scale. Current development of TF-SOFC architectures in terms of a supporting substrate and state-of-the-art methodology to improve stability of TF-SOFCs against mechanical and thermal stress will be reviewed.
