Rational design of Fe-Nx self-doped carbon nanosheets from oily sludge for advanced energy storage

Large amounts of oily sludge generated during petroleum production pose dual challenges, including severe environmental risks and the inefficient utilization of intrinsic functional elements. Herein, hierarchical porous carbon nanosheets derived from acid-pretreated oily sludge (denoted as HOS) are prepared through a three-step process involving acid pretreatment, controlled pyrolysis, and post-activation, without introducing any external iron or nitrogen precursors. Benefiting from precursor-level regulation of intrinsic iron and nitrogen species, the optimized sample (HOS-8) exhibits a two-dimensional nanosheet architecture, hierarchical porosity, a high specific surface area, and abundant Fe-N_x active sites. These structural and chemical features collectively facilitate rapid ion/electron transport and efficient charge storage. As a result, HOS-8 delivers a high specific capacitance of 333 F g^-1, excellent rate capability with 79.4% capacitance retention at 10 A g^-1, and outstanding cycling stability with 95.5% retention after 10,000 cycles. A symmetric supercapacitor device assembled using HOS-8 achieves an energy density of 13.6 Wh kg^-1, outperforming many reported heteroatom-doped carbon electrodes. This work demonstrates a feasible strategy for converting iron- and nitrogen-containing oily sludge into functional carbon materials and provides insights into the rational design of waste-derived electrodes for energy storage applications.