Fundamental Understanding of Electronic Structure in FeN₄ Site on Electrocatalytic Activity via <i>dz</i>^<i>2</i>‐Orbital‐Driven Charge Tuning for Acidic Oxygen Reduction

Abstract

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 (Fe^H+ N₄ ) that is achieved with a COP_BTC @Cl-CNTs catalyst subject to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>d</mml:mi> <mml:msup><mml:mrow><mml:mi>z</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msup> </mml:msub> <mml:annotation>${{d}_{{z}^{2}}}$</mml:annotation> </mml:semantics> </mml:math> -orbital tuning. A new descriptor ξ, defined as the band center of iron atoms projected on the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mrow><mml:mn>3</mml:mn> <mml:mi>d</mml:mi></mml:mrow> <mml:msup><mml:mrow><mml:mi>z</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msup> </mml:msub> <mml:annotation>${{3d}_{{z}^{2}}}$</mml:annotation> </mml:semantics> </mml:math> -orbital, was introduced to quantitively explain a volcano-like regulation mechanism. When ξ is distributed between -1.887 and -1.862 eV, the catalytic performance of the COP_BTC @Cl-CNTs electrocatalyst is optimized.