First-principles study of adsorption and migration of alkali and alkaline earth metals (Rb, Cs, and Ba) on graphene

The thermionic energy converters (TECs), with graphene as the coating of the anode, greatly improves the electron emission capability, thereby increasing the output power of the devices. Various types of defects are generated inside the graphene since the TECs used in the miniature space thermionic reactor is irradiated with high-energy neutrons and ions. Defective sites tend to be the most stable adsorption sites for adatoms, but greatly reduce the mobility of adatoms on the surface of graphene. The migration barriers of Rb, Cs, and Ba on the pristine graphene surface are 0.054 eV, 0.048 eV, and 0.105 eV, respectively, and the migration barriers on the oxygen-doped graphene surface are increased obviously. The inhibition of metal migration ability by defect sites is detrimental to the formation of electric dipoles, which may be the main reason for the increase in the work function of defective graphene. Simultaneously, oxygen-containing defects are ubiquitous in graphene due to their low formation energies, and the lowest work function of defective graphene is 2.74 eV, which is reached after cesium adsorption at defect sites, whereas this is still expected to lead to a drastic drop in device output performance. ____________________________________________________________________ • The alkali and alkaline earth metals (Rb, Cs, and Ba) are selected as the adatoms on the surface of graphene, which can address the challenges of developing a novel coating of electrode materials for thermionic energy converters. • Considering that the cohesive energy is affected by the bonding valence, it can be predicted that the cohesive characteristics of the metal ions adsorbed on the graphene surface will change after the ionization, and the metal ions tend to aggregate together. • The Stone-Wales defect, oxygen substitutional defect, boron substitutional defects, and single carbon vacancy are introduced in monolayer graphene, which allows us to study the effect of defects created by high-energy neutrons and protons through cascading collisions on the performance of electrode materials. • Oxygen-containing defects are ubiquitous in graphene due to their low formation energies, and the lowest work function of defective graphene is 2.74 eV, which is reached after Cs adsorption at 2O C (two C sites substituted with O) defect sites, whereas this is still expected to have detrimental effects on the electron emission capability of graphene-based anode materials. The adsorption and migration characteristics of Rb, Cs, and Ba on pristine and defective graphene have been systematically investigated by utilizing the density functional theory (DFT) method, which could help address the challenges of developing novel anode materials of thermionic energy converters (TECs). In this work, we found that the work function of the adsorption system is lower as Cs is adsorbed on graphene compared with Rb and Ba, which indicates that the electron emission capability has been greatly improved. Computational results of defective graphene demonstrate that defect sites act as traps for metals. The diffusion near the Stone-Wales defect and vacancy is severely hindered and the migration barrier on the surface of B/O doped graphene is also slightly increased. Particularly, the work functions of all defective graphenes are significantly increased, which is attributed to the decrease in the probability of electric dipole formation and the increase of the absolute cohesive energy. O-containing defects are ubiquitous in graphene due to their low formation energies, and the lowest work function of defective graphene is 2.74 eV, which is reached after Cs adsorption at 2O C defect sites, whereas this is still expected to have detrimental effects on graphene-based anode materials.