The insufficient long-term stability of Solid Oxide Cells (SOCs) is one of the major drawbacks, limiting commercial usage of these electrochemical devices. The main reason for possible damage or degradation comes from the thermal expansion coefficient (TEC) mismatch between the electrode and electrolyte, which causes unwanted strain, cracks, and layer delamination. The problem may be mitigated by using negative thermal expansion coefficient materials (NTEs) that have been known for decades for their ability to decrease their spatial dimensions with increasing temperature. This interesting property along with the high electronic conductivity was recently reported for perovskite-based compounds like NdMnO₃ and Sm_1-xA_xMnO_3-δ (A = Zn, Cu; x ≤ 0.15) [1,2,3]. The addition of such materials to form composite-type oxygen electrodes for SOCs significantly lowers the TEC, adjusting it to the chosen electrolyte material, and diminishing possible degradation mechanisms. Therefore, the appropriately designed and produced composite electrode should possess suitably low polarization resistance, as well as is expected to deliver enhanced long-term stability, which in effect would greatly increase not only the performance but also the practical applicability of the SOC-based devices.

Double perovskite oxides from REBa_0.5Sr_0.5CoCuO_5+δ (RE: Pr, Nd, Sm, Gd) group are known for their high mixed ionic-electronic conductivity, as well as suitable electrocatalytic activity, and are reported as attractive oxygen electrode material for SOCs. In this work, those materials were prepared by the typical sol-gel method or the electrospinning method. Among initially studied compositions, SmBa_0.5Sr_0.5CoCuO_5+δ (SBSCCO) was found to be the best candidate, exhibiting demanded physicochemical and electrocatalytic properties. Also, this single-phase material is chemically stable and compatible with LSGM electrolyte. The total conductivity of SBSCCO is around 100 S·cm^-1 between 300°C and 900°C, while if used as the oxygen electrode, the measured polarization resistance (R_p) in SBSCCO|LSGM|SBSCCO symmetrical cell was only 0.17 Ω·cm² at 800°C. The long-term stability tests showed that the relative increase of R_p is around 20% after 5 days at 700°C.

To further enhance the characteristics of the SBSCCO electrode, selected NTEs: Sm_1-xZn_xMnO_3-δ (SZMx) and Sm_1-xCu_xMnO_3-δ (SCMx) were synthesized through the solid state reaction, with final annealing performed at 1300°C for 10 h in air. The conducted dilatometry measurements confirmed the negative TEC for those materials, ranging from -0.1·10^-6 1/K to -11·10^-6 1/K between in the 200-900°C range. Also, the total conductivity of SZM15 was found to be around 20 S·cm^-1 at 900°C. Then, the previously obtained SBSCCO and chosen NTEs were used to prepare several composite oxygen electrodes, by adding different amounts of the respective powders. The preliminary results show that the R_p of the composite electrode with 10 wt.% addition of SZM15, as measured in the SBSCCO:SZM15|LSGM|SBSCCO:SZM15 symmetrical cell in the 700-900°C range, was significantly lower when compared with the unmodified SBSCCO electrode. For instance, R_p at 800°C was 0.12 Ω·cm² giving a 30% relative decrease. Additionally, the I-V characteristics were measured for commercial anode-supported fuel cells comprising Ni-YSZ|YSZ support, GDC10 buffer layer, and the SBSCCO:SZM15 composite electrode. The maximum peak power density recorded at 800°C was 850 mW·cm^-2, which is around 60% higher when compared to the same cell prepared with the SBSCCO electrode without NTE addition.

All the obtained results clearly show that the state-of-the-art innovative application of NTEs in SOCs indeed allows for decreasing the TEC of the composite and adjusting it to the solid electrolyte, but also enables a significant decrease of the polarization resistance. Further experiments are ongoing, regarding the long-term performance of the composite electrodes, SEM and TEM measurements involving the electrolyte/electrode interface.