Porous BN with vacancy defects for selective removal of CO from H2 feed gas in hydrogen fuel cells: a DFT study

Abstract

Developing highly efficient sorbent materials for separation and capture of CO from H₂ feed gas is of great importance in fuel cell technology in terms of the efficient use of the Pt anode catalyst and the cycle lifetime of the system. Using density functional theory (DFT) calculations, we showed that porous BN with introduced monovacancy defects can be an effective sorbent for CO capture. The adsorption energies were calculated to be 0.075 and −0.92 eV for H₂ and CO absorbed on pristine porous BN, respectively. However, with introduction of vacancy defects, the adsorption energies were estimated to be 5.56 and 3.79 eV for CO absorbed on V_B and V_N defects, respectively. Meanwhile, the calculated adsorption energies of H₂ on the V_B and V_N defects decrease to −0.71 and −0.30 eV, respectively. Our result indicates that the capture of CO on defective porous BN is highly preferred over H₂ molecules. Moreover, the energy difference between E_ads(CO) and E_ads(H₂) is 2 to 4 times higher than that of the reported metal/graphene system, suggesting that p-BN with vacancy defects is a more efficient sorbent. The electronic structure calculations suggest that introduction of vacancy defects into porous BN induces defect electronic states in the energy-gap region with a strong impact on the adsorption performance of the material. To evaluate energetic stability, the defect formation energies were also calculated. The formation energy of vacancy defects strongly depends on the chemical environment, and the selection of different reactants (B or N sources) would achieve the goal of reducing the formation energy of the vacancy defects. These findings should provide useful guidance for the design and fabrication of porous BN sorbents useful for removal of CO from H₂ feed gas feed in hydrogen fuel cells.