FIGURE 9 A) Series of gas permeability through the tubular MXene/SS2.5 membrane. B) Effect of electrophoresis time on the H2/CO2 separation performance of the membranes. Tests condition: the feeding mixed gas (50 ml: 50 ml of H2/CO2) at 1 atm and 25oC.
In addition, the effect of the operation parameters on the gas permeance, such as temperature and feed gas humidity, had also been investigated. The temperature-dependent H2/CO2 separation performance of the tubular MXene/SS2.5 membrane was depicted in Figure 10A, where three temperature cycles included heating and cooling processes. When the temperature was raised from 25 oC to 125oC, the hydrogen permeance gradually was raised from 1283 GPU to 1880 GPU, while the H2/CO2selectivity decreased from 55 to 19. During the cooling stage, the hydrogen permeance fell from 1880 GPU to 1634 GPU, and the selectivity was raised from 19 to 50. Furthermore, the performance of the MXene/SS2.5 membrane could still recover well after three heating and cooling cycles, with H2/CO2 selectivity of 51 and hydrogen permeance of 1568 GPU , demonstrating the tubular MXene/SS2.5 membrane had good thermal resistance. The apparent activation energy of the system was illustrated in Figure 10B via Arrhenius equation. The apparent activation energies Eact (H2) and Eact (CO2) of the two gases were 1.61 kJ/mol and 11.04 kJ/mol, respectively. It could be found that the apparent activation energies were all positive, which indicated that gas diffusion rather than adsorption was primarily responsible for controlling the gas separation process, which was consistent with the literatures.66 Moreover, the synthesis gas produced by the methane steam reforming process always contains water vapor (usually < 3 vol%), so a water vapor-containing test on the separation system was required. Different saturated salt solutions were used to produce water vapor with different relative humidity to explore gas permeability.4 As shown in Figure 10C, as the relative humidity increased from 0% to 92%, the hydrogen permeance decreased from 1244 to 1168 GPU, while the CO2 permeance slightly increased from 21 to 22 GPU, thus the corresponding H2/CO2 selectivity decreased from 57 to 51. Obviously, as the relative humidity of the feed gas increased, the condensable water molecules in the feed gas condensed in the pores or interlayer spacing of the membrane, obstructing the transmission of non-condensable small molecular gas of H2. Because CO2 is more soluble in condensed water than H2, it diffused through the membrane more quickly, reducing the selectivity of H2/CO2, which was consistent with the phenomenon of layered graphene at various humidity conditions.4