The synergistic effect of the adjacent dual NiNx sites to boost the oxygen reduction reaction activity for proton exchange membrane fuel cell

The investigation on the application of dual metallic sites in electro-catalysis for oxygen reduction reaction (ORR) has been given a lot of consideration. In this study, the reaction mechanisms and corresponding thermodynamic energy evolution in the dual Ni saturated coordinate with N atoms embedded in graphene have been investigated using first-principles density functional theory (DFT) calculations. The results show that the dual NiNx active sites are more stable than that of the NiN4 in view of the comparison of their formation energies. The reduction reaction of O2∗ on NiN4/G obviously promotes the two-electron reduction process due to its loosely bonded with O2. While the oxygen reduction reaction process on these dual NiNx active sites is thermally downhill, all of which promote a single site with four electron ORR process. The dual Ni atoms directly bonded catalysts (Ni2N6-ST2/G and Ni2N6-ST3/G) are in favor of the ORR mechanism through the reaction pathway of O2→O2∗→OOH∗→O∗→OH∗→H2O. However, the dual NiNx catalysts of others (Ni2N6-ST1/G and Ni2N7-ST4/G) prefer to undergo an unconventional 2OH∗ mechanism (O2→O2∗→OOH∗→2HO∗→OH∗→H2O). Moreover, it was found that the first reduction step (O2∗→OOH∗) is the thermodynamic rate-determining step (RDS) for the ORR on these catalysts. Both of Ni2N6-ST2/G and Ni2N6-ST3/G exhibit outstanding ORR activity with the on-set potentials of 0.65 V and 0.68 V, respectively. In particular, the two catalysts are able to adsorb the H2O2 and further reduce it into H2O through a two-electron pathway. Furthermore, it was found that the most stable intermediates in the oxygen reduction pathway depend on their lowest free energy at a given set of pH and applied potential. The free energy for these O-contained intermediates is linearly decreasing with the increment of applied potential. The critical potential for these O-contained intermediates is also linearly decreased with the increment of pH value.