The current disclosure teaches one to achieve PBI membranes with high ionic conductivity and low mechanical creep for the first time. This is in contrast to previous teachings of PBI membrane fabrication methods, which yield PBIs with either high ionic conductivity and high mechanical creep or low ionic conductivity and low mechanical creep. The membranes produced according to the disclosed process provide doped membranes for applications in fuel cells and electrolysis devices such as electrochemical separation devices.