“…Over the past forty years, the proton-conducting solid oxides went beyond perovskites family ( Figure 1 ). After variously doped barium cerates and zirconates [ 8 , 9 , 10 ], classes of proton-conducting materials like hexagonal [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ], oxygen-deficient [ 18 , 19 , 20 ], block-layered [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ] perovskites, brownmillerites [ 32 , 33 , 34 ], pyrochlores [ 35 , 36 , 37 , 38 ], apatites [ 39 ], monazites [ 40 , 41 , 42 ], gallium-based oxides [ 43 , 44 , 45 ], materials with fluorite- [ 46 , 47 , 48 ], fergusonite- [ 49 , 50 , 51 ], and weberite-types [ 52 , 53 ] of structures were described.…”