MEMS-based thin-film solid-oxide fuel cells
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ntroduction Solid-oxide fuel cells (SOFCs) are efficient electrochemical devices that convert the chemical energy of a fuel into electrical energy and typically employ an extrinsically doped ceramic oxide electrolyte membrane to transport ions through its crystal lattice, while the electrons traverse the external circuit providing useful electrical work. Thin-film SOFCs based on processing techniques used for microelectromechanical systems (MEMS) offer an attractive platform to study contributions from rate-controlling processes (i.e., the slowest step in the rate processes that governs the overall fuel cell performance). Naturally, as one slow step is enhanced, the next slow step becomes the dominant factor in determining the overall cell performance. In particular, thin-film electrolyte membranes can help reduce ohmic losses to such an extent that cathode activation losses become dominant. As the rate of the hydrogen oxidation reaction at the anode is considerably faster than the cathodic reaction, this article focuses on expanding the understanding of the oxygen reduction reaction (ORR)
at the cathode followed by oxide ion incorporation into the electrolyte (see the Introductory article in this issue). In principle, ORR involves the reduction of molecular oxygen to oxide ions, which are then incorporated into vacant sites (i.e., oxide ion vacancies) in the oxygen sublattice of the ceramic electrolyte membrane. Using advanced analytical methods with high spatial and energy resolution that allowed us to trace the path of oxygen during ORR, we were able to elucidate the role of surface grain boundaries in this important reaction. This article draws heavily from major results and findings reported in recent articles by us. First, we briefly review MEMS-based SOFC fabrication techniques followed by fabrication of thin-film electrolyte membranes. As these cells typically operate at temperatures lower than 500°C, they feature an ionically conducting ceramic electrolyte membrane with porous platinum electrodes at the anode and the cathode. Next, we present two engineering strategies to enhance the cathode reaction, which is the ratedetermining step in MEMS-based SOFC: engineering of
Jihwan An, Department of Mechanical Engineering, Stanford University, USA, and Seoul National University of Science and Technology, Korea; [email protected] Joon Hyung Shim, Department of Mechanical Engineering, Korea University, South Korea; [email protected] Young-Beom Kim, Department of Mechanical Engineering, Hanyang University, South Korea; [email protected] Joong Sun Park, Argonne National Laboratory, USA; [email protected] Wonyoung Lee, School of Mechanical Engineering, Sungkyunkwan University, South Korea; [email protected] Turgut M. Gür, Department of Materials Science and Engineering, Stanford University, USA; [email protected] Fritz B. Prinz, Departments of Mechanical Engineering and Materials Science and Engineering, Stanford University, USA; [email protected] DOI: 10.1557/mrs.2014.171
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MRS BULLETIN • VOLUME 39 • SEPTEMBER
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