Thin-Film SOFC Fabrication Using Flash Light Annealing

Abstract

Solid oxide fuel cell (SOFC) is a promising energy conversion technology that achieves high conversion efficiency while remaining environmentally friendly. As a result, many researchers have intensively studied SOFCs in recent years, and considerable progress has been made. Typically, SOFCs were operated at high temperatures above 800 oC [1, 2]. Because SOFCs exist in the solid state, including the electrolyte, high temperature operation is unavoidable for achieving satisfactory ionic transport. The relatively low oxygen reduction reaction (ORR) rate is another critical issue that necessitates a high operating temperature. However, due to the high thermal stability and chemical compatibility required for proper operation of high-temperature SOFCs, the component materials are quite costly. Lowering the operating temperature would reduce the maintenance cost and improve the startup time of SOFCs considerably, and has therefore been the focus of several investigations [3-5]. In recent years, through various and concerted efforts, it has been achieved reasonable fuel cell performances in the low operation temperature regime (below 500 oC). Because the ionic conductivity and ORR rate both fall off with decreasing operating temperature, minimizing these losses in low-temperature SOFCs (LT-SOFCs) is a significant challenge. One way to address this problem is to reduce the thickness of the electrolyte. Many studies have reported electrolyte thicknesses on the scale of a few micrometers, and a few even reported thicknesses of just tens of nanometers. By cutting the distance of ionic transport paths to the nanometer scale, Ohmic losses can be decreased proportionally. Another is to reduce the electrode interface resistance at the cathode side. Compared with the hydrogen oxidation reaction (HOR) rate at the anode, the ORR at the cathode is quite slow, and is usually considered as the rate-determining step. Therefore, introducing an additional functional layer between the cathode and electrolyte is one way to facilitate surface oxygen kinetics that can significantly improve the power output. Moreover, increasing the electrochemical active area by engineering the surface structure, thereby increasing the output power proportionally has been also carried out upon the functional layer studies. Recently, solution based deposition method for thin film MEA fabrication has been widely used, which has the wide array of advantages such as high purity and controllability of stoichiometry even though the cost of this process is simple and inexpensive. However, behind these advantages, additional heat treatments must be followed subsequently due to the nature of the wet chemical solution process, and these procedures could be inconvenient and inefficient. For these reasons, instead of conventional sintering equipment, a flash light sintering process is introduced. In this work, cathode and electrolyte thin films were deposited by the metallo-organic chemical solution deposition (MOCSD) method, and flash light sintering process was utilized as a subsequence instead of conventional sintering. Microstructures, crystalline phases and electrical properties were investigated to examine the effectiveness of flash light. Fig. 1 SEM images of (a) and (b) (cross-view), (c) and (d) (top view) References [1] S.C. Singhal, Solid oxide fuel cells for stationary, mobile, and military applications, Solid State Ionics 152–153 (2002) 405-410. [2] M.J.L. Østergård, C. Clausen, C. Bagger, M. Mogensen, Manganite-zirconia composite cathodes for SOFC: Influence of structure and composition, Electrochimica Acta 40(12) (1995) 1971-1981. [3] T. Matsui, M. Inaba, A. Mineshige, Z. Ogumi, Electrochemical properties of ceria-based oxides for use in intermediate-temperature SOFCs, Solid State Ionics 176(7–8) (2005) 647-654. [4] T. Matsui, T. Kosaka, M. Inaba, A. Mineshige, Z. Ogumi, Effects of mixed conduction on the open-circuit voltage of intermediate-temperature SOFCs based on Sm-doped ceria electrolytes, Solid State Ionics 176(7–8) (2005) 663-668. [5] D. Hirabayashi, A. Tomita, S. Teranishi, T. Hibino, M. Sano, Improvement of a reduction-resistant Ce0.8Sm0.2O1.9 electrolyte by optimizing a thin BaCe1−xSmxO3−α layer for intermediate-temperature SOFCs, Solid State Ionics 176(9–10) (2005) 881-887.