Nitrogen-Doped Hard Carbon from Glucose and Sewage-Sludge Hydrolysate via Cohydrothermal Carbonization for Sodium-Ion...

Municipal sewage-sludge hydrolysate contains soluble nitrogen-bearing organic matter but is compositionally variable and does not, by itself, provide a controlled carbon framework for hard-carbon synthesis. Here, glucose was used as a reproducible carbon-framework precursor, while sewage-sludge hydrolysate was introduced as a nitrogen-containing functional cofeedstock and structural regulator. A series of precursors containing 5 g glucose and 0-70 mL hydrolysate within a fixed total liquid volume of 70 mL were hydrothermally treated at 240 °C and subsequently carbonized at 1300 °C. The sample prepared with 70 mL hydrolysate (10AQGL) exhibited an expanded interlayer spacing of 0.386 nm, moderate defect density, 0.8 wt % surface nitrogen, and a low specific surface area of 8.91 m²·g^-1. These coupled structural and chemical characteristics limited excessive interfacial side reactions and supported an initial Coulombic efficiency of 84.2%. The 10AQGL electrode delivered a reversible capacity of 307.7 mAh·g^-1 and retained approximately 240 mAh·g^-1 after 600 cycles at 1.0 A·g^-1. The improvement cannot be assigned to nitrogen doping alone because nitrogen content, interlayer spacing, defect density, and pore structure changed concurrently. This study therefore demonstrates the use of sewage-sludge hydrolysate as a functional cofeedstock in glucose-derived hard carbon, while industrial scalability and net environmental benefits remain to be established through continuous processing, techno-economic analysis, and life-cycle assessment.