A boundary layer could be observed near the substrate where the gas velocity was significantly reduced. A two-dimensional (2D) plot of the gas velocity, 1 mm above the substrate, on a 6 × 16 cm area at the center of the substrate is shown in Figure . The influence of the reactor geometry is apparent with varying gas velocity in different parts of the reactor. The gas flow was highest in the center and decreased at the edges of the reactor. The average gas velocity near graphene was 0.06 ± 0.04 cm s -1 (𝜐 ! ! ,#$%& ), indicating an uneven flow. A high standard deviation in velocity is not desired for obtaining uniformly porous graphene in scaled-up samples. To address this, a quartz semi-cylindrical block (12 cm in diameter) was placed in the reactor, occupying and blocking the bottom half of the unnecessary space for the ozone reaction. Figure shows the tubular reactor hosting the semi-cylindrical block and a 55 cm-long graphene substrate plate. Attributing to the reduction in the cross-sectional area of the flow by the block, the gas develops a laminar flow at a short distance after the inlet (Figure ). A 2D plot of the gas velocity near graphene reveals a significantly uniform flow profile (Figure ). The velocity increased three-fold to 0.17 ± 0.02 cm s -1 (𝜐 ! ! ,'(#) ). This led to a significant improvement in the porosity of graphene, reflected by a drastically improved CO2 permeance. An average CO2 permeance from the 1-cm-scale membranes of 13105 GPU and CO2/N2 selectivity of 15.1 could be achieved. Furthermore, the 50 cm 2 PG membrane in the 5 × 10 cm 2 cross-flow module yielded an attractive CO2 permeance of 11799 and 8792 GPU, and CO2/N2 selectivity of 15.9 and 17.6, respectively (Figure and Table ). This shows that a carefully designed scaled-up reactor and transfer strategy allows one to achieve attractive performance from large-area graphene membranes.