Precoordination Mediated Active Center Evolution for Constructing Cluster-Single Atom Fe-Nx Sites from Sludge toward Efficient Oxygen Reduction

The sluggish kinetics of the oxygen reduction reaction (ORR) remains a critical limitation for green energy conversion technologies. Although Fe-N-C single-atom catalysts offer high atomic utilization, their symmetric coordination environments limit electronic flexibility and catalytic activity. Herein, a precoordination-mediated active center evolution strategy is developed to construct dual scale cluster-single atom Fe-N x active centers using municipal sludge as a low-cost precursor. During the citric acid-assisted hydrothermal process, protein hydrolysis induces the release and redistribution of nitrogen-containing species, forming a precoordinated environment that enables the formation of subnanometer clusters and isolated single-atom sites. Subsequent activation removes mineral species and constructs a hierarchical porous carbon framework with high specific surface area and enhanced mass transport. Density functional theory calculations reveal that electronic coupling between clusters and single atoms induces long-range electronic modulation, shifting the d-band center and optimizing the adsorption energetics of oxygen intermediates. As a result, the catalyst exhibits excellent ORR activity across a wide pH range, delivering a half-wave potential of 0.86 V in alkaline media and outperforming Pt/C in acidic and neutral electrolytes. A Zn-air battery achieves a peak power density of 195 mW cm -2 , while microbial fuel cells deliver 1589 mW m -2 with 54% pollutant degradation. Life-cycle assessment indicates reduced carbon emissions compared with sludge incineration. This work provides a controllable strategy for constructing dual-scale active centers and highlights their promise in green energy conversion and nano-enabled electrocatalysis.