Thermomechanical and conductivity studies of doped niobium titanates as possible current collector material in the SOFC anode

In the reducing atmosphere of the SOFC anode at operating temperatures of 800 degreesC and. above Nb2TiO7 is reduced to Nb1.33Ti0.67O4. This material displays very high electronic conductivity of >100 Scm(-1), suitable for use in such applications as a current collector. It has a low thermal expansion coefficient of 3 x 10(-6) K-1, however, which may cause problems due to mismatch with other SOFC components, e.g. YSZ. Doping with Fe2O3 successfully increased the thermal expansion to a maximum of 6 x 10(-6) K-1. A conductivity of 140 Scm(-1) at 900 degreesC in dry 5% H-2/Ar, with an activation energy of 0.18 eV, was achieved for the Nb1.344Ti0.642Fe0.014O4, making it suitable for the use as a current collector. Conductivity runs in wet 5%H-2/Ar showed lower conductivities of 15-18 Scm(-1) and lower activation energies of 0.08 - 0.09 eV. Single cell tests of Nb1.33Ti0.67O4 showed power outputs of 5.5 - 7.2 mW.cm(-2) at 850 degreesC, lower than for Ni with 150 - 200 mW.cm(-2) at 850 degreesC, however, this material displayed much better stability at high temperatures than Ni.