Production and Performance of Copper-based Anode-supported SOFCs

The production of tubular SOFCs supported by a cermet anode formulated with copper was successfully achieved. The optimization and the experience developed in the process of extrusion allowed to produce the composite anodes quickly and with good reliability. The low melting temperature of copper and its two oxides imposed a limit on the maximum temperature reachable during co-sintering of the cell. The study of the effect of different sintering aids on the densification of the GDC10 permitted to select Li₂O as the most effective, being able to enhance the sinterability of the material and to decrease of more than 600°C its sintering temperature. Although its effect was remarkable, the densification of the thin electrolytic layer when bonded to the supporting anode could not be accomplished without analyzing and solving the issues related to its constrained sintering. A complete cell with a 10 um perfectly dense and crack free Li₂O-GDC10 electrolyte, Li₂O-GDC10:Cu₂O 75:25 vol% anode and GDC10:LSF20 50:50 vol% cathode was obtained after one step sintering for 3h at 1020°C in argon atmosphere. The co-sintering permitted perfect adhesion of the three layers and no sign of debonding. One of the main fabrication problems related to the employment of Cu₂O in the cermet anode was its low coefficient of thermal expansion that caused the onset of tensile stress in the electrolyte during cooling and the opening of brittle fractures on its surface. When the Cu₂O was increased to 35vol% to insure the electrical conductivity of the anode, the sintering temperature had to be decreased to 995°C to minimize the cracks formation, although this affected the densification of the electrolyte layer. Cells formulated with Cu₂O suffered from incomplete densification, cracks and oxidation of the cuprite during reduction. These issues made impossible to perform electrochemical tests.

Employment of CuO was less problematic, thanks to its resistance to oxidation and its higher CET. SOFCs were fabricated with Li₂O-GDC10 electrolyte, Li₂O-GDC10:CuO 60:40 vol% anode and GDC10:LSF20 50:50 vol% cathode. Sintering at 1020°C resulted in a fully dense electrolyte with a fine grain structure, although some cracks due to differential shrinkage could not be completely avoided. Despite this, the cell had a maximum OCV of 0.54 V and the highest measured power density was 8.98 mW cm^-2 with a peak power density projected to be 27.5 mW cm^-2 when tested at 600°C in dry H₂. More test at lower temperatures are currently in progress.

These results do show that SOFCs can be produced with a copper cermet anode using similar methods used with NiO-based anodes by a one-step firing procedure at relatively low temperatures, thus having cost and high energy savings. The use of copper can allow for the use of light hydrocarbons in the cell without external reforming equipment or need of high steam to carbon ratio, and without the risk of carbon deposition in the cell