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