Upcycling Bio and Battery Waste Into Multivalent Calcium–Manganese Oxide Nanocomposites

Transforming chemically complex waste streams into high‐performance calcium–manganese oxide multivalent nanocomposites (MNC) provides a promising route toward circular and sustainable materials engineering. Here, we report the synthesis of compositionally tunable Ca–Mn MNC derived entirely from waste eggshells and spent alkaline batteries through hydrothermal processing. In situ X‐ray diffraction (XRD) revealed strongly composition‐dependent thermal phase evolution, with Ca:Mn = 25:75 sample stabilizing mixed‐valence Mn oxide phases with cubic CaMn 7 O 12 quadruple perovskite phases while Ca:Mn = 75:25 sample was dominated by highly crystalline CaO phases. STEM‐EELS and XPS analyses demonstrated substantial heterogeneity of Mn valence between the 25CaMn and 50CaMn catalysts, confirming the coexistence of Mn 2+ , Mn 3+ , and Mn 4+ ‐type environments through spatial variation in the Mn L 3 / L 2 ratio associated with 3 d electronic transitions. Among the investigated compositions, 25CaMn exhibited a CO 2 space–time yield (STY) of and achieved ∼85% CO conversion near 300 °C, whereas 50CaMn delivered the highest overall CO 2 productivity with a maximum STY of between 350 and 400 °C. Excessive Ca incorporation in 75CaMn suppressed Mn lattice participation and lowered catalytic activity at intermediate temperatures. This work demonstrates how waste‐derived precursor engineering can generate multivalent electronic structure and catalytic functionality.