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.