d<i>z</i>²-Orbital Modulation of CoN₄ Sites via Charge Environment Reconstruction in Covalent Organic Polymers for Proton-Exchange Membrane Fuel Cells.

Abstract

Precise modulation of the electronic structure of transition metal sites in oxygen reduction reaction (ORR) catalysts is key to enhancing their intrinsic activity. However, the structural diversity of active sites created by conventional pyrolysis hinders the establishment of clear structure-activity relationships between the local coordination environment and ORR performance. Here, via a pyrolysis-free kinetically irreversible polymerization strategy, we successfully constructed a covalent organic polymer (COP) featuring well-defined Co-N₄ centers and proposed an approach to modulate the d<i>z</i>² orbital occupancy of cobalt sites by reconstructing their surrounding charge environment. Theoretical calculations reveal that the introduction of an electron-rich nitrogen matrix effectively elevates the valence state of the cobalt center, induces energy-level splitting of its d<i>z</i>² orbital, and leads to a high-spin state. This electronic configuration not only optimizes the adsorption strength of oxygenated intermediates but also significantly lowers the energy barrier for O₂ activation. Significantly, the proton exchange membrane fuel cells (PEMFCs) fabricated with electron-rich N-modulated COP as the cathode catalyst demonstrated 1.5-fold and 3.5-fold increases in peak power density compared to the S- and C-doped COP analogues, respectively. Unraveling the modulation mechanism of the charge environment on catalytic performance at the atomic orbital level provides a paradigm for designing high-performance molecular electrocatalysts for PEMFC cathodes.