Electrolyte development for a SOFC operating at low temperature

RWTH Publications (RWTH Aachen)

Abstract

Solid oxide fuel cells (SOFCs) operating at low temperature ( 500 °C) enable applications, such as auxiliary power units (APUs) or portable devices. However, the state-of-the-art electrolyte material (yttria-stabilized zirconia (YSZ)) used in intermediate-temperature SOFCs does not provide a sufficiently high ionic conductivity. Two approaches can therefore be taken to deal with that. First, an alternative electrolyte material to YSZ with a higher conductivity. However, when looking for alternatives, the conductivity values for each material found in widely-cited literature can be confusing, as the reported values are sometimes in conflict with each other. Second, an electrolyte film with thinner thickness. While spin coating is reported to be able to fabricate a YSZ electrolyte with thickness as thin as 1 μm, further thickness decrease by spin coating is a big challenge. Moreover, spin coating is very time consuming, needing multiple steps of coating, drying and heat-treatment, which could take several days. Therefore, in this study we present a systematic comparison of the conductivity of the three most popular electrolyte materials, i.e., YSZ, scandium-stabilized zirconia (ScSZ), and gadolinium-doped ceria (GDC). Using electrochemical impedance spectroscopy (EIS) to characterize the ionic conductivity, we find that at 500 °C, GDC has a higher ionic conductivity (5.8e-3 S cm-1) than ScSZ (2.5e-3 S cm-1) and YSZ (1.1e-3 S cm-1). The properties of the starting powders, powder processing and the microstructure after sintering are all taken into account. Following up on this, a GDC electrolyte is developed on an industrial scale anode, with a dimension of 5 cmx5 cm, by screen printing. After sintering at 1400 °C for 5 h, a thin and dense GDC electrolyte, with thickness of 3.5 μm and air leakage rate of 3.54e-6 hPa dm2 s-1 cm-2, is achieved. The single cell test shows the cell has a high cell performance, a measured voltage of 0.84 V at a current density of 2Acm-2 and 750 °C (air,H2 with 10%H2O). According to the EIS, a quite low ohmic resistance is achieved, 125.2 milli ohm cm2 at 500 °C. This comparison of conductivity can be used as a guide when deciding on electrolyte materials for different SOFC applications, especially when the fabrication of the electrolyte layer of different thickness has to be considered, and rectify misleading information in the literature. The development of the thin and dense GDC electrolyte provides a strategy of using GDC as electrolyte for SOFC and confirms the advantage of using GDC while not YSZ for low temperature SOFC, which is the fabrication challenge can be decreased significantly.

Authors 1

1. Jun Zhang corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen