Designing Regenerable CO2 Sorbents: Ni/Ga Codoped Mg-MOF-74 with Controlled Adsorption Thermodynamics

Metal-organic frameworks with exposed metal sites are promising platforms for postcombustion CO2 capture, however balancing strong uptake with low recovery time remains a challenge. In this study, we use spin-polarized density functional theory (DFT) calculations to understand the stability, electronic structure, and CO2 adsorption thermodynamics in Ni/Ga codoped Mg-MOF-74. Thermodynamic screening and 300 K AIMD find six viable compositions such that Ni2+-only substitutions are stable, but Ga3+-only substitutions are unstable without charge compensation. Therefore, placement of an -OH moiety maintains charge neutrality while preserving the stability of the sample. Electronic analyses reveal a dopant-controlled PDOS behavior where Ni-only doped MOFs are metallic and all Ni-Ga codoped systems are half-metallic, where a single conducting spin channel arises from the Ni-3d states controlled by the Ga-OH electrostatics. Single-molecule adsorption shows consistent CO2 selectivity over N2 and H2O across all stable codoped compositions; however, strong binding with CO2 makes desorption difficult, resulting in a long recovery time. Two codoped compositions, i.e., 5% Ni-5%Ga and 10% Ni-5% Ga, deliver a moderate, reversible CO2 binding window. Exposure to multiple CO2 molecules on these optimized codoped compositions reaches a practical limit of eight CO2 per supercell, leading to a gravimetric capacity of 15.55% in the 5% Ni-5% Ga-doped sample, with average adsorption energy weakening as coverage increases due to site saturation and CO2-CO2 interactions. These results define a codoping strategy that stabilizes Mg-MOF-74, introduces half-metallicity, and tunes CO2 binding into a regenerable regime.