Functionally graded composite La2NiO4+δ and La4Ni3O10−δ solid oxide fuel cell cathodes

•Novel functionally graded Ruddlesden–Popper cathodes have been developed.•These contain a thin compact La2NiO4+δ layer adjacent to the electrolyte.•This layer improves the overall electrolyte/electrode connectivity.•The bulk of the electrode is comprised of a La2NiO4+δ + La4Ni3O10−δ composite.•Graded cathodes give better performance than traditional single-layer porous cathodes.

Functionally graded electrodes have been fabricated from members of the (LaNiO3)nLaO Ruddlesden–Popper family. These consist of a thin compact layer of La2NiO4 + δ deposited on the electrolyte, topped by a thicker porous La2NiO4 + δ + La4Ni310 − δ composite layer, completed by a thin porous La4Ni3O10 − δ layer acting as a current collector. The microstructure and electrochemical role of these layers are discussed in detail. It was found that using a 50:50 wt.% mix in the composite layer gave the best performance; ASRs at 500, 550, 600, 650, and 700 °C were 15.6, 5.53, 2.29, 1.06 and 0.53 Ω cm2 respectively. This represents high performance for this class of material, particularly notable at the lower-end of the temperature range. As such this grading process is an attractive proposition for further research towards lowering the operating temperature of SOFCs.