First-principles study on the hydrogenation of the O2 on TiN4 embedded graphene

As a kind of clean and high efficient energy conversion devices, the proton exchange membrane fuel cell (PEMFC) is a promising technology for clean and sustainable power generation. Metal-coordinated nitrogen-doped graphene is attractive since its use as a cathode material for the PEMFC. The mechanism of O2 activation and hydrogenation on TiN4 embedded graphene has been investigated in terms of the dispersion-corrected density functional theory (DFT-D) method. It is found that: 1) O2 prefers to stay on top of the Ti atom with the side-on configuration, forming the O-Ti-O three-member ring with an adsorption energy of 4.96 eV. 2) According to the Mulliken atomic charges analysis, the absorbed O2 molecule are negatively charged by 0.60 e in the side-on configuration. 3) Upon the chemisorption of the O2 on TiN4-graphene, there are two possible pathways during the activation of the O2 molecule: dissociation and hydrogenation. In the dissociation pathway, the adsorbed O2 molecule is first dissociated into two O atoms, with a fairly big reaction barrier of 0.95 eV and an endothermic reaction energy of 0.20 eV. Subsequently, the two O atoms are hydrogenated into O+OH with a reaction barrier of 0.40 eV and an exothermic reaction energy of 2.46 eV. In the hydrogenation pathway, the reaction barrier of the hydrogenation of the adsorbed O2 is 0.52 eV. The OOH formed subsequently is dissociated into O+OH with a small reaction barrier of 0.04 eV and an exothermic reaction of 2.14 eV. The hydrogenation pathways of the adsorbed O2 is more preferable, and the corresponding rate-limiting step of this pathway is the hydrogenation of the O2 with a reaction barrier of 0.52 eV and an exothermic reaction energy of 0.64 eV.#br#In summary, the preferable path of the hydrogenation reactions of O2 on TiN4-Graphene is O2(ads)+H(ads) → OOH(ads)→O+OH(ads). Current results may be benefitial to the design of new electrocatalyst materials based on graphene.