B-Doped MnN₄-G Nanosheets as Bifunctional Electrocatalysts for Both Oxygen Reduction and Oxygen Evolution Reactions

Non-platinum-group (NPG) single-metal-atom electrocatalysts with bifunctional catalytic capabilities for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly sought for producing sustainable and renewable energy sources. Doping nonmetal atoms next to the active single-metal-atom can be an effective way to further improve the overall catalytic activities. Herein, we report a nonmetal B-doping strategy for enhancing the catalytic activity of the graphene-based single-metal-atom Mn-pyridine-N₄-graphene catalysts [named MnN₄B_n-G (n = 0-8)] toward ORR and OER. The underlying chemical mechanism for this design is that the B dopant can lower the charge density corresponding to the d-orbital of the Mn atom, thereby weakening the adsorption strength of oxygen intermediates (O/OH/OOH) on the active Mn site, leading to higher ORR electrocatalytic activity. Not only can this doping strategy enhance ORR activity, but it can also retain high OER activity. Our results show that the MnN₄B₅-G catalyst entails both relatively high onset potential for ORR (U^onset = 0.78 V) and relatively low overpotential for OER (η^over = 0.43 V), rendering it being a compelling oxygen redox bifunctional electrocatalyst. The newly designed single-metal-atom Mn-pyridine-N₄-graphene catalysts offer alternative NPG metals and bifunctional catalysts for renewable energy applications.