Ultrathin Ni-Co nanosheets with disparate-CO2-affinity nanodomains in membranes to improve gas separation

One of the biggest challenges of Pebax-based membranes is how to simultaneously achieve high CO2 permeability and selectivity. Hence, mixed matrix membranes (MMMs) were designed and fabricated by incorporating the ultrathin Ni-Co nanosheet (Ni-Co-NS) fillers into Pebax® MH 1657 (Pebax) matrix to improve CO2 separation performance. The ultrathin Ni-Co-NS fillers had disparate-CO2-affinity nanodomains and were endowed with three main functions in the Pebax/Ni-Co-NS MMMs: i) First, Ni and Co metals constituted CO2-philic and non-CO2-philic nanodomains, respectively, because the two metals had disparate CO2 adsorption enthalpy. CO2-philic nanodomains rendered a high affinity capacity, whereas non-CO2-philic nanodomains rendered low-friction diffusion. Thus, a combination of the disparate-CO2-affinity nanodomains significantly facilitated CO2 transport. ii) Second, the pore size of the nanosheets decreased from 0.79 nm to 0.49 nm by introducing the second Co metal into Ni-NS, and the decreased pores size contributed to strengthen CO2/CH4 selectivity. iii) Third, the uniform nanopores of Ni-Co-NS that paralleled to the gas concentration gradient constituted high-speed CO2 transport channels, which was conducive to improving CO2 permeability of MMMs. Therefore, CO2 separation performance of MMMs was remarkably improved by Ni-Co-NS fillers. Pebax/Ni-Co-NS MMMs with the content of 5 wt% Ni-Co-NS fillers showed the highest CO2 permeability of 483 ± 11 Barrer and CO2/CH4 selectivity of 39 ± 1.1. It implies that the introduction of nanosheets with two different metals is an effective method to fabricate MMMs with an excellent CO2 separation performance.