Comparative study of electrochemical properties of mixed conducting Ln2NiO4 + δ (Ln = La, Pr and Nd) and La0.6Sr0.4Fe0.8Co0.2O3 − δ as SOFC cathodes associated to Ce0.9Gd0.1O2 − δ, La0.8Sr0.2Ga0.8Mg0.2O3 − δ and La9Sr1Si6O26.5 electrolytes