3.3 Gas separation performance
In order to identify the most suitable gas separation system, the single gas permeation experiment was first performed on the tubular MXene/SS membrane. As shown in Figure 9A, different gases were tested and the ideal selectivity of each gas was calculated. According to the experimental data, the permeance of H2, CO2, N2, CH4, and C2H4 were 1450 GPU, 22 GPU , 61 GPU, 45 GPU, and 8 GPU, respectively, while the corresponding ideal selectivity of S(H2/CO2), S(H2/N2), S(H2/CH4) and S(H2/C2H4) were 65, 24, 33, and 186, which were all larger than the Knudsen selectivity. Obviously, the MXene/SS membrane showed a cut-off between H2 and CO2 molecules. Moreover, the effect of different electrophoretic deposition time on the gas separation performance of the tubular MXene/SS membranes was also investigated. As shown in Figure 9B, for better comparison, the H2 gas permeance of the bare stainless steel substrate and the substrate after modification were tested to be 5323 GPU and 4908 GPU with similar H2/CO2 selectivity of ~5, respectively. The comparable results indicated the use of filamentous CNTs as surface fillers could only modify the substrate surface without reducing the gas permeance of the original substrate, which was a promising candidate as a substrate modification. With the prolongation of electrophoresis time, the MXene membrane grew thicker, resulting in fewer defects, so the hydrogen permeance of the tubular MXene/SS membrane became lower, while the gas selectivity increased initially and then stayed almost unchanged. When the electrophoresis time was 2.5 min, the H2/CO2 selectivity of the membrane reached the maximum of 55 with comparable hydrogen permeance of 1290 GPU, thus the tubular MXene/SS2.5 membrane was the optimal one in this study.