Electrostatic Potential Regulation in Composite-Functionalized Graphene for Highly Selective CO2/H2 Separation

Selective elimination of CO2 from H2-containing streams requires surface sites that preferentially stabilize CO2 while limiting the H2 uptake. This work explores a structure-oriented modification strategy for monolayer graphene by introducing phosphorus-centered groups with different phenyl/hydroxyl environments at a fixed nominal grafting site density. GCMC simulations were used to quantify pure-gas and binary CO2/H2 adsorption, while DFT-derived electrostatic potential, van der Waals potential, and charge-density distributions were analyzed to connect surface polarity with adsorption behavior. The results show that phosphoryl functionality strengthens the CO2 affinity, while phenyl substitution modifies the spatial distribution of favorable adsorption regions without promoting H2 adsorption. Noncovalent interaction and adsorption energy analyses further indicate a stronger local binding preference of G-DPPA toward CO2 than H2, consistent with the preferential CO2 adsorption observed in the GCMC simulations. Among the investigated models, diphenylphosphinic acid-functionalized graphene (G-DPPA) exhibits the most favorable separation behavior. At 273 K and 1 bar, its CO2 uptake is 44.5% higher than that of pristine graphene, and the CO2/H2 selectivity for an equimolar mixture increases by 70.66%, giving an adsorbed-phase CO2 fraction of ∼96%.