Exploring the Active Site and Catalytic Activity of N-Coordinated Ni2 Dual-Atom Catalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is an indispensable electrochemical reaction in fuel cells. However, the performance of fuel cells has been affected by the slack kinetics of the ORR. Hence, the development of efficient and affordable electrocatalysts for the reduction of O₂ is necessary for the large-scale commercialization of fuel cells. Here, we present a Ni₂ dual-atom anchored on a N-doped carbon nanotube (Ni₂_N₃_CNT and Ni₂_N₄_CNT) and a Ni single-atom anchored on N-doped carbon nanotube (Ni₁_N₃_CNT and Ni₁_N₄_CNT) catalysts with two possible active sites, namely, Ni-site and N-site, as efficient catalysts toward the ORR. We have analyzed the energetically favorable active site for O₂ reduction on the surface of the Ni₁_N₃_CNT, Ni₁_N₄_CNT, Ni₂_N₃_CNT, and Ni₂_N₄_CNT catalysts by employing the density functional theory method with van der Waals (vdW) dispersion corrections (in short the DFT-D3) method. Among all possible configurations, Ni₂_N₃_CNT is a more favorable configuration with the Ni catalytic active site toward the ORR. Then, we have studied the structural, electronic, and catalytic activity of Ni₂_N₃_CNT by using the same DFT-D3 method. The analysis of the ORR intermediate species reveals that the associative reaction pathway is a more favorable path for reducing the O₂ into H₂O at the Ni catalytic site of Ni₂_N₃_CNT than the dissociative reaction pathway. In the free energy profile, all of the ORR reaction intermediate steps are downhill, indicating the good catalytic activity of Ni₂_N₃_CNT toward the ORR. Moreover, we have also studied the structural and electronic properties of all of the reaction intermediate steps by employing the same DFT-D3 method. These findings point out that the Ni₂ dual-atom catalysts provide an efficient electrocatalytic activity toward the ORR, and it holds great promise as a replacement for Pt-based catalysts in future proton-exchange membrane fuel cells. This work highlights the potential and importance of the subject material as a durable electrocatalyst for the ORR, offering insights into Ni₂ dual-atom electrochemistry and the design of advanced catalysts, which may be a promising candidate to substitute for Pt electrodes, and it is an excellent material for fuel-cell components.