B-site high-entropy tailoring K2NiF4 oxide as an effective cathode for proton-conducting solid oxide fuel cells

•A K2NiF4-type oxide is firstly modified via B-site high-entropy tailoring.•the B-site transition metals in LSMFCNC evolve into various compound states.•LSMFCNC's crystal structure expansion and a decrease in the bond strength between cations and oxygen ions.•B-site high-entropy tailoring boosts the bulk oxygen/proton diffusion coefficient by 3 to 6 times.•The durable LSMFCNC cathode achieves 1126 mW/cm2 at 600  °C.