Synthesis and Testing of BCZY/LNZ Mixed Proton–electron Conducting Composites… Related Interaction of SrTi0.65Fe0.35O3-δ with LaNi0.6Fe0.4O3-δ, La0.6Sr0.4Co0.2Fe0.8O3-δ and Ce0.8Gd0.2O2-δ Electrocatalytic Gas Sensor with Reference Layer