Why Ni is absent from the surface of La2NiO4+delta?

La2NiO4+delta (LNO214) is a potentiaL intermediate temperature saki oxide fueL ceLL (IT-SOFC) cathode materiaL which beLongs to the RuddLesden-Popper (RP) structure series A(n+1)B(4)O3(n+1). There is interest in this materiaL as it offers a way to avoid Sr segregation and associated degradation, as LNO214 can take in oxygen interstitiaLs and become cataLyticay active without A-site doping. WhiLe the buLk ionic conduction mechanisms are weLL studied, its surface structure and chemistry are stiLL a matter of debate. Recent experimentaL studies (both with and without dopants) reveaL that it has a La-terminated surface and a highLy Ni deficient surface Layer. These resuLts disagree with previous computer simuLations, and undermine the conventionaL expLanation for the oxygen reduction process at the surface. In this work we evaLuate the thermodynamic stabiLity of La2NiO4+delta at IT-SOFC operation temperatures. We find that the decomposition of La2NiO4+delta to produce La2O3 and higher order RP phases is indeed thermodynamicay favourabLe. A hypothesis for the formation mechanism of the La-terminated and Ni deficient surface based on partiaL decomposition and surface passivation is proposed and evaLuated.