Solid oxide fuel cells (SOFCs) are viewed as a highly attractive technology for energy conversion with high efficiency, fuel flexibility and low emissions. However, it still faces many technical obstacles, particularly the durability bottle-necked by material degradation in the laminated cell components due to the challenging high-temperature operation. The delamination between rigid-natured cathode and electrolyte is one of the most typical failures due to their thermo-mechanical incompatibility.
Regrettably, most highly active cathode materials are represented by cobalt-based oxides (e.g. SrNb0.1Co0.9O3−δ), of which the thermal expansion coefficient (TEC) is significantly higher than that of ceramic electrolyte (e.g. Sm0.2Ce0.8O1.9). Such a mismatch in thermal expansion behaviour creates large stress interfacially and thus the delamination during operation or thermal cycling. In the past 15 years, we have devoted significant efforts to reducing the TEC of the cathodes, such as lattice doping with transition metals containing d0 orbitals, compositing the perovskite electrode material with electrolyte material, introducing A-site deficiency into the perovskite, or pursuing in situ formation of a thermal expansion inhibiting phase. However, most of these strategies can reduce the TEC value of cobalt-based electrodes to a very modest extent, not able to fully match that of the electrolyte, and may cause negative side-effects on oxygen reduction reaction (ORR) activity. Thus, it necessitates the development of cathode with both high catalytic activity and matched TECs with state-of-art electrolyte.
We present an effective approach to achieving full thermo-mechanical compatibility between cathode and electrolyte by introducing a concept of thermal-expansion offset, which has never been reported. The Y2W3O12 (YWO), a negative thermal expansion material, is composited with SrNb0.1Co0.9O3−δ (SNC) perovskite of a high positive TEC, to form the cathode (denoted as c-SYNC). (Fig. 1) The apparent TEC of c-SYNC is offset to be only 12.9×10–6 K–1, which perfectly matches that of Sm0.2Ce0.8O1.9 electrolyte (12.3×10–6 K–1) and leads to the enhanced cathode-electrolyte compatibility and thus the excellent stability of the cell (stable performance at 600 °C for 200 h and under 40 cycles of thermal shock). Moreover, a new interphase SrWO4 is formed due to limited reaction between the two materials in the composite during the calcination process that creates as well A-site deficiencies in the perovskite. As a result, the composite shows both high activity and excellent stability.
Fig. 1. a, STEM-HAADF of c-SYNC with each phase marked by dashed lines. Corresponding element mapping of Sr, W and Co for STEM and corresponding FFT images of the SWO phase. b, Thermal expansion curves from 100 to 800 °C in air of dense c-SYNC and SNC bar specimens. c, The ASR (Rp) response of SNC and c-SYNC based symmetric cell electrodes during 40 thermal cycles between 600 °C and 300 °C at a heating rate of 30 °C min–1 and passive cooling at ~7.5 °C min–1, 90 h total cumulative testing.
We do not only demonstrate a completely new TEC compatible SOFC cathode with high performance and enhanced durability by compositing YWO, but also propose a new concept of “offsetting thermal expansion to develop fully compatible thermo-mechanical electrodes for solid oxide fuel cells”, that is facile, effective and universal for developing durable and high-performance SOFCs. In addition, our work serves for inspiring interests in the exploration of new couples of negative thermal materials and ORR-active perovskites in a broader scope for the potential application of this TEC offset approach, aiming for the development of high performance SOFC cathode with good thermo-mechanical compatibility.
For more information please check out our article “A Thermal-Expansion Offset Approach for High Performance Fuel Cell Cathodes” published in Nature. DOI: 10.1038/s41586-021-03264-1