Regulation of the coordination number of Zn single atoms to boost electrochemical sensing of H2O2†

Compared with transition metals with partially occupied 3d orbitals, Zn has a filled 3d¹⁰ configuration, which severely restricts electron mobility and hence usually renders Zn²⁺ intrinsically inactive for electrochemical sensing. Metal single-atom catalysts are a new kind of sensing material. Owing to their unique coordination structure and high atomic utilization rate, metal single-atom catalysts show unique properties, which makes them promising for use in the field of electrochemical sensing. However, whether Zn single atoms are active sites remains to be elucidated. In this study, we prepared nitrogen-doped carbon (NC) materials by pyrolyzing ZIF-8 at high temperatures and reported that when the pyrolysis temperature was 800 °C, many Zn single atoms with Zn-N₄ coordination structures remained in the NC material. Even when the pyrolysis temperature is increased to 1000 °C, a small number of Zn single atoms remain, and the coordination structure changes from Zn-N₄ to Zn-N₃. Furthermore, unexpectedly, both residual Zn single atoms showed electrocatalytic activity for H₂O₂ reduction. In particular, the electrocatalytic activity was significantly enhanced after the coordination structure was changed from Zn-N₄ to Zn-N₃. Density functional theory (DFT) calculations indicate that the coordination structure of Zn-N₃ optimizes the adsorption and desorption strength of oxygen-containing species in the electrocatalytic reaction process, which lowers the energy barrier of the rate-determining step and increases the detection sensitivity of H₂O₂ nearly 4.1 times. This study revealed new properties of Zn single atoms for the electrocatalytic reduction of H₂O₂ and developed a strategy to increase the electrocatalytic activity of metal single-atom catalysts through coordination number regulation, which lays the foundation for the use of Zn single atoms in the field of electrochemical sensing and provides ideas for the design of new highly active sensing materials.