PBI membranes with highly symmetric sponge structures were prepared via water vapor induced phase inversion method [13]. The as-prepared polymer was dissolved in DMAC to form a 15 wt% solution. After degassing, the solutions were cast on a glass plate and quickly transferred to a box with constant temperature (50 °C) and relative humidity (100%) under normal atmospheric pressure. After 10 min, the membranes were peeled off and immersed in water.