Comparative Study on the Catalytic Activity of the TM–N2 Active Sites (TM = Mn, Fe, Co, Ni) in the Oxygen Reduction Reaction: Density Functional Theory Study

We investigate the interaction of transition metal–nitrogen (TM–N2; TM = Mn, Fe, Co, and Ni) active sites with oxygen reduction reaction (ORR) related molecules using the density functional theory (DFT) calculations. Generally, the trend of molecular adsorption energy in TM–N2 systems is Mn–N2 > Fe–N2 > Co–N2 > NiN2. In the case of O2 adsorption, the O2 molecule is adsorbed with a symmetric side-on configuration on the TM–N2 systems, regardless the type of TM atom. We also find that when the reaction of the adsorbed HO2 molecule and an H atom takes place, instead of forming H2O2 molecule the reaction produces two OH radicals. From the evaluation of the potential energy surface profiles of the oxygen reduction reaction (ORR), we find that a direct four-electron reduction pathway could be facilitated on the TM–N2 active sites. However, all of the TM–N2 systems share the same main rate-limiting reaction, which is the OH reduction step. Generally, the edge-like TM–N2 active site has stronger molecular adsorption energy as compared to the TM–pyrrole-like active site. However, the strong molecular adsorption energy in the edge-like TM–N2 active site results in uphill energy profiles for some of ORR steps, even at a low electrode potential. This suggests that edge-like TM–N2 active site will have a relatively slower ORR rate as compared to the TM–pyrrole-like active site.