Fabrication and structural characterization of self-supporting electrolyte membranes for a micro solid-oxide fuel cell

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Joshua L. Hertz and Harry L. Tuller Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Srikar T. Vengallatorec) and S. Mark Spearing Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Martin A. Schmidt Microsystems Technology Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 23 March 2004; accepted 14 May 2004)

Micromachined fuel cells are among a class of microscale devices being explored for portable power generation. In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells. Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition. Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 ␮m for YSZ and 525 ␮m for GDC, with high processing yields for YSZ. Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to −155 MPa) in as-deposited evaporated and sputtered films, respectively. Tensile evaporated films fail via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio. Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films. Sputtered 1025-␮m membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K).

I. INTRODUCTION

Miniaturized fuel cells are potential high efficiency, high energy density replacements for batteries in the mW–W power generation market for portable consumer and military electronic devices. Small-scale fuel cells utilizing low temperature proton exchange membranes (PEMFC) have been the primary focus of development because they do not require complex heat integration strategies for efficient operation. However, to avoid significant cost, complexity, and technological difficul-

a)

Present address: School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 b) Address all correspondence to this author. e-mail: [email protected] c) Present address: Department of Mechanical Engineering, McGill University, Quebec, Canada H3A 2K6 DOI: 10.1557/JMR.2004.0350 2604

http://journals.cambridge.org

J. Mater. Res., Vol. 19, No. 9, Sep 2004 Downloaded: 14 Mar 2015

ties, most systems require hydrogen storage methods to circumvent challenging fuel reforming and H2 purification processes.1–3 Direct methanol fuel cells (DMFC) benefit from direct fuel utilization, and micro-scale versions have been developed4,5; however, concentrated methanol solutions are required to realize beneficial energy densities for portable power applications, and under these condit

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