Adjusting OH tolerance of Ni4 clusters supported on ultra-small carbon nanotube with lattice vacancies for hydrogen oxidation catalysts

Abstract

The hydroxide (OH) binding on Ni4 clusters supported on the (5, 5) ultra-small carbon nanotubes (CNTs) with lattice vacancies is theoretically investigated by applying the density functional theory (DFT) method. All Ni4 clusters are strongly adsorbed on the (5, 5) ultra-small CNTs with lattice vacancies and formed very stable Ni4/Vn structures. The results show that the OH binding energy (OHBE) at a Ni site nearly increases linearly with increasing Ni d band center. On the other side, this OHBE also increases as the number of vacancies increases. These findings could be attributed to interactions with carbon dangling bonds, which alter the electronic structure of the CNT-supported Ni4 clusters. Furthermore, an unusual behavior is observed for a Ni4 cluster supported on the (5, 5) ultra-small without a vacancy, where its OHBE is noticeably stronger than the others. These assertions are also supported by the projected crystal orbital overlap population (pCOOP), the projected crystal overlap Hamiltonian population (pCOHP), and the Mulliken–Löwdin analyses, which confirm the presence of bonding, anti-bonding, and non-bonding of OH states on Ni4/Vn. As a result, we can conclude that OH strongly binds to all Ni4/Vn. Adjusting OHBE tolerance to the Ni4 clusters is regarded as evidence of the CNT support effect, resulting in an increase in the activity of the hydrogen oxidation reaction (HOR) catalyst in an alkaline environment. It is necessary to make the high performance of the Ni cluster catalysts.