N, S, P-Codoped Carbon Quantum Dots as Corrosion Inhibitors for Mg–Air Batteries

In Mg–air batteries, electrolyte-induced self-corrosion and parasitic H2 evolution on Mg anodes lead to low anode utilization and poor discharge stability. Developing eco-friendly, highly efficient interface-regulating additives is therefore critical. Carbon quantum dots (CQDs) offer chemical stability and abundant surface groups, demonstrating strong potential for corrosion inhibition and electrochemical control. In this study, nitrogen, sulfur, and phosphorus ternary co-doped CQDs (NSP-CQDs) were fabricated via ultrasonic oxidation, wherein the orange peel acted as a carbon precursor, and thiourea and sodium tripolyphosphate acted as dopants. The as-prepared NSP-CQDs were added to a sodium chloride electrolyte to explore their effects on Mg anode corrosion and the discharge capability of Mg–air cells. Structural characterization showed that NSP-CQDs have a low-crystallinity carbon structure and exhibit localized graphite-ordering characteristics, with the surface rich in functional groups containing N, S, and P. EIS fitting results revealed that NSP-CQDs suppress Mg alloy self-corrosion, yielding an Rct of 319.5 Ω cm2 and an Rf of 915.1 Ω cm2. At 300 mg/L NSP-CQDs and 5 mA·cm–2, galvanostatic discharge produced a stable 1.24 V and 2005.4 Wh kg–1 specific energy. Furthermore, under intermittent operation at 10 mA·cm–2, the discharge voltage remained stable at 1.11∼1.13 V, with a cumulative operating time of up to 160 h.