The present invention relates to an electrochemical device comprising an anode, a cathode and an electrolyte membrane disposed between said anode and said cathode, wherein said electrolyte membrane comprises a material of formula La1-xSRxGa1-yMGyO3-o.5(x+y) wherein x and y are independently a value of form 0.1 to 0.3 extremes included, said material having a relative density of at least 90% and comprising LaSrGaO4 in a percentage of from 0.05 vol%, extremes included. The invention also relates to a method for producing energy and to a method for separating oxygen from a gas mixture.
