Article pubs.acs.org/cm
Optimized Sol−Gel Routes to Synthesize Yttria-Stabilized Zirconia Thin Films as Solid Electrolytes for Solid Oxide Fuel Cells Emilie Courtin,*,† Philippe Boy,† Clément Rouhet,† Luc Bianchi,‡ Eric Bruneton,§ Nathalie Poirot,∥ Christel Laberty-Robert,⊥ and Clément Sanchez⊥ †
Laboratoire Sol−Gel et Simulation, ‡Laboratoire Projection THermique, and §Laboratoire Microstructure et Comportement, CEA, DAM Le Ripault, F-37260, Monts, France ∥ Groupe de Recherche en Matériaux microélectronique Acoustique Nanotechnologie, UMR 7347, IUT de Blois, F-41029 Blois Cedex, France ⊥ Laboratoire de Chimie de la Matière Condensée de Paris, Université Paris 6, UMR-7574, Collège de France, F-75231 Paris Cedex 05, France S Supporting Information *
ABSTRACT: Sol−gel strategies are carefully evaluated and compared to synthesize electrolyte materials for Intermediate Temperature SOFCs (Solid Oxide Fuel Cells). Robust 10−20 μm thick cubic Yttria-Stabilized Zirconia (YSZ), (Y2O3)0.08(ZrO2)0.92, films have been prepared by using nanocomposite stable sols composed with “sol−gel made” YSZ binders and YSZ powder. Two different strategies have been investigated for the synthesis of 8YSZ binders: a mixed alkoxide-nitrate route in alcoholic media, and an environmentfriendly hydrothermal route, in water media. Ionic conductivities of ∼2.5 × 10−2 S cm−1 are obtained at 800 °C for powders made by the hydrothermal and the alkoxide routes. Films synthesized from composite sols that contain commercial YSZ powder and the organic binder or the water-based binder, have been deposited on YSZ-NiO cermets and integrated in an entire single fuel cell. The two optimized routes yield to gastight YSZ electrolytes. In particular with the organic-based binder, a maximum power density of 250 mW cm−2 at 850 °C has been achieved. The proposed route can be extended to other materials for different applications such as ferroelectric and dielectric materials. KEYWORDS: solid oxide fuel cells, sol−gel, thin films, 8 percent Y2O3−ZrO2, electrolyte, fuel cell performance
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INTRODUCTION Solid Oxide Fuel Cells (SOFC) are of particular interest because of their high energy conversion efficiency, low pollution emission, and ability to work with various fuels.1 Yttria-Stabilized Zirconia (YSZ), (Y2O3)0.08(ZrO2)0.92, is the most widely used material for electrolyte because of its high ionic conductivity and good chemical and mechanical stability.2 However, SOFCs originally worked at high temperatures, above 850 °C, which could lead to a rapid degradation of the stack, and expensive interconnect materials are generally needed.1,3 To decrease this operating temperature to 800 °C and even lower, new electrolyte materials have to be designed and processed. The most common challenges targeted are the increase of the ionic conductivity and the decrease of the electrolyte thickness to reduce the ohmic loss.4,5 Sol−gel routes are highly versatile therefore providing interesting strategies to process thin films or powder that can be used as electrolytes in SOFCs. However, one step processed sol−gel films are usually very thin (≤1 μm) and therefore their use as electronic ceramics or solid electrolytes need multiple depositions and numerous adequate thermal treatments that generate additional cost and difficulties (porosity decohesion, cracks and failures © 2012 American Chemical Society
associated to thermo-mechanical strains) to obtain coatings with optimized properties.6 To synthesize thicker films (10−20 μm) in one deposition step by dip-coating or spin-coating, strategies using composite sols, as the one proposed by Barrow et al. for piezoelectric materials, have been developed.7 It consists in mixing with a limiting amount of organic matter, a powder (commercially available or “sol−gel made”) and a colloidal sol (called the binder) made with the same composition.8 The YSZ binder synthesis is a key issue in this process. This binder must be stable with time, and its stoichiometry perfectly controlled. The grain size should be small (
