Fundamental Understanding of Electronic Structure in FeN4 Site on Electrocatalytic Activity via dz2-Orbital-Driven Charge Tuning for Acidic Oxygen Reduction

The structural diversity of active sites resulting from traditional pyrolysis hinder our understanding of the local coordination environment (LCE) around the active site, and its effects on performance in the oxygen reduction reaction (ORR). We created a series of FeN4 active-site configurations via a pyrolysis-free approach where LCEs are defined by covalent organic polymers (COPs). Our results suggest a more positive charge on iron atoms in the vicinity of an electron-withdrawing side-chain; that is, a high-valence configuration (FeH+N4) that is achieved with a COPBTC@Cl-CNTs catalyst subject to dz2 ${{d}_{{z}<^>{2}}}$ -orbital tuning. A new descriptor xi, defined as the band center of iron atoms projected on the 3dz2 ${{3d}_{{z}<^>{2}}}$ -orbital, was introduced to quantitively explain a volcano-like regulation mechanism. When xi is distributed between -1.887 and -1.862 eV, the catalytic performance of the COPBTC@Cl-CNTs electrocatalyst is optimized.