In this study, we present the fabrication of fibers with tunable morphology and porosity from an intrinsically microporous polymer, PIM-1, using a simple one-step electrospinning technique. The method involves the dissolution of PIM-1 in a good solvent tetrahydrofuran (THF) and cosolvents composed of THF and a nonsolvent, either dimethylformamide (DMF) or toluene, for electrospinning without the requirement of post-treatments. Electrospun PIM-1 fibers obtained from THF and different cosolvents reveal unique variations in fiber morphology, starting from changes in fiber surface texture to cross-sectional shape and internal/external pores. The change in the cross-sectional shape from corrugated circles to ribbon-like dumbbells can be attributed primarily to a buckling instability, while phase separation induced by the nonsolvent can be used to explain the formation of a highly porous structure both on the fiber surface and in the fiber interior at different solvent/nonsolvent ratios. With an increase in toluene fraction, the surface pores become more developed, whereas the interior pores demonstrate a decrease in size and density due to the competing effects between phase separation and fiber solidification. The hierarchical porosity of the fiber mats that comes from the macro (>50 nm)/meso pores (2-50 nm) created from nonsolvent induced phase separation (NIPS) during electrospinning and the intrinsic microporosity (<2 nm) from the PIM-1 rigid backbone contribute to an increase in the surface area of the fiber mats. These results are confirmed by nitrogen physisorption analysis and an improvement of CO2 adsorptive capacity. CO2 adsorption kinetics under different temperatures and adsorption/desorption cyclic stability investigated using a gravimetric approach show PIM-1 fiber mats to hold strong promise in applications for gas capture.