Influence of processing conditions on the microstructure of NiO-YSZ supporting anode for solid oxide fuel cellsCeramics International

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Authors
Michele Casarin, Vincenzo M. Sglavo
Year
2015
DOI
10.1016/j.ceramint.2014.10.168
Subject
Process Chemistry and Technology / Ceramics and Composites / Electronic, Optical and Magnetic Materials / Materials Chemistry / Surfaces, Coatings and Films

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CERAMICS

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Ceramics International 41 (20 o s

Vi ity o rm 6 N aim miz reduction in their viscosity, shear stress and yield stress; concurrently, solvent evaporation and hysteresis in flow behaviour are minimized, thus improving the suspension stability. The corresponding sintered compacts produced from the optimized slurry demonstrated uniform high efficiency and wide range in power generation applications strongly reduced at lower temperature with adverse effects in the cell performance [5]. Nonetheless, SOFC with high performance zirconia (YSZ) and strontium-doped lanthanum manganite diminishes with porosity but anodes composed by particles of smaller size demonstrate much higher strength at lower porosity [9]. Therefore, a fine and uniform microstructure with no agglomerates and limited porosity is essential in anodes with good mechanical properties and reliability [10]. http://dx.doi.org/10.1016/j.ceramint.2014.10.168 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. nCorresponding author. Tel.: þ39 0461 281922; fax: þ39 0461 281945.

E-mail address: nirasac@gmail.com (M. Casarin).[1]. Much effort in the development of SOFC concerns cell reliability and cost-effective fabrication processes. For devices operating at temperature of 800 1C or below, the improvement of long term stability and reliability as well as less demanding choice in expensive materials for interconnectors make the commercialization of SOFC increasingly affordable [2,3]. Moreover, lower operating temperature limits the problem of rapid thermal cycling as it is required for SOFC in mobile applications [4]. On the other hand, the kinetic of electrochemical reactions at electrodes and ionic conductivity in the solid oxide electrolyte are (LSM), already at 800 1C [6–8].

In anode-supported SOFC, the anode must resist to the mechanical stresses arising during cell fabrication and service to guarantee structural integrity and reliability of the cell. The mechanical properties in NiO-YSZ anodes such as elastic moduli, strength and fracture toughness are significantly controlled by the microstructure and in particular by the porosity [9]. For example, the Young‘s modulus of NiOYSZ decreases from 210 GPa to 110 GPa when porosity increases from 0 to 23% [10]; the fracture resistance alsoby controlling the conditions of drying, burn-out and sintering process; the lower limit allows effective gas permeation during anode reduction owing to an interconnected pore structure obtained already at 12–14% porosity. This indicates a stable pore structure derived from the burn-out of pore former even for sintering temperature as high as 1400 1C. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Anode-supported SOFC; Colloidal process control; NiO-YSZ composite; Pore former 1. Introduction

Among energy conversion devices, solid oxide fuel cells (SOFC) have attracted considerable interests as a result of their can be fabricated in the anode-supported configuration where a thin (10 mm) electrolyte layer can be easily produced and power densities in excess of 1 W/cm2 are achieved with the state of art materials, that is, nickel oxide (NiO), yttria-stabilizedmicrostructure with specific porosity distribution. It is shown that the residual porosity can be tailored in the range of 12–23% exclusivelyInfluence of processing conditions supporting anode for

Michele Casarinn,

Department of Industrial Engineering, Univers

Received 16 July 2014; received in revised fo

Available online

Abstract

In the present work, the control of a ceramic colloidal process is porosity for anode-supported solid oxide fuel cell applications. The optiINTERNATIONAL 15) 2543–2557 n the microstructure of NiO-YSZ olid oxide fuel cells ncenzo M. Sglavo f Trento, via Mesiano 77, 38123 Trento, Italy 13 October 2014; accepted 30 October 2014 ovember 2014 ed to fabricate NiO-YSZ composite with specific microstructure and ation of dispersant and solid concentration in anode slurries provides a www.elsevier.com/locate/ceramint

The anode suspensions were composed by NiO and 8 mol%

Y2O3-ZrO2 (58:42 weight ratio); graphite (5 wt% of the total solid loading) was added as pore former. The optimal concentration of dispersant was determined at the minimum of viscosity at 0.85 s1 in screening suspensions (70 wt% solid loading) and corresponds to 1.04% (dry weight of powder basis - dwb). This dispersant concentration was then

Fig. 1. SEM micrographs of the commercial powders after 2 h milling; (a) nickel oxide, (b) ytrria-stabilized zirconia and (c) graphite (pore former). s InOn the other hand, the anode microstructure also plays an important role in the electrochemical performance of SOFC [11–14]; the anode must possess suitable pore structure to allow gas permeation and transport to minimize the concentration polarization [13]. Large pores guarantee extensive gas permeability but the electrochemical reaction sites (triple phase boundaries) are reduced and the corresponding higher activation polarization weakens the electrochemical performance.

Therefore, a trade-off between gas permeation and electrochemical performance in conjunction with mechanical properties is achieved by producing an optimal porosity [15,16].

The porosity in the NiO-YSZ anodes is commonly controlled by the use of pore formers, which provide, after densification, an interconnected residual porosity. Such pore network suitable for gas permeation is commonly achieved at 30–40% porosity [16,17]. For NiO-YSZ anodes, a porosity increase of about 23% arises also from the reduction of NiO into Ni during the first operation cycle; therefore, an initial porosity between 10% and 15% is therefore expected to produce a porosity of 35% after complete reduction, adequate for gas permeation while maintaining consistent electrochemical and mechanical properties. The final specific microstructure in terms of interconnected porosity and NiOYSZ composite architecture is strictly correlated to the raw materials and processing conditions [11,18,19].