The modulating effect of N coordination on single-atom catalysts researched by Pt-Nx-C model through both experimental study and DFT simulation

Abstract

N-doped carbon-based single-atom catalysts (NC-SACs) are widely researched in various electrochemical reactions due to high metal atom utilization and catalytic activity. The catalytic activity of NC-SACs originates from the coordinating structure between single metal site (M) and the doped nitrogen (N) in carbon matrix by forming M-Nx-C structure (1≤ x≤ 4). The M-N4-C structure is widely considered to be the most stable and effective catalytic site. However, there is no in-depth research for the “x” modulation in Pt-Nx-C structure and the corresponding catalytic properties. Herein, atomically dispersed Pt on N-doped carbon (Pt-NC) with Pt-Nx-C structure (1≤ x≤ 4), as a research model, is fabricated by a ZIF-8 template and applied to catalytic oxygen reduction. Different carbonization temperatures are used to control N loss, and then modulate the N coordination of Pt-Nx-C structure. The Pt-NC has the predictable low half-wave potential (E1/2) of 0.72 V vs RHE compared to the Pt/C 20% of 0.81V due to low Pt content. Remarkably, the Pt-NC shows a high onset potential (1.10 V vs RHE, determined for j = -0.1 mA cm2) and a high current density of 5.2 mA cm−2, more positive and higher than that of Pt/C 20% (0.96 V) and 4.9 mA cm−2, respectively. As the structural characterization and DFT simulation confirmed, the reducing Pt-N coordination number induces low valence of Pt atoms and low free energy of oxygen reduction, which is responsible for the improved catalytic activity. Furthermore, the Pt-NC shows high mass activity (172 times higher than that of Pt/C 20%), better stability and methanol crossover resistance.