High-performance thermochemical energy storage from FeMn co-modified steel slag

CaO-based thermochemical energy storage (TCES) materials have attracted wide attention due to their high energy density, favorable reaction reversibility, and potential integration with CO 2 capture. Steel slag, as a Ca-rich solid waste naturally containing Fe and Mn, offers intrinsic advantages for developing structurally stable and solar-absorptive heat storage materials. However, current steel slag-derived TCES materials still suffer from limited understanding of the functional role of inherent transition-metal elements during repeated high-temperature cycling. In this work, Fe/Mn modified steel slag-derived CaO-based composites were rationally designed to clarify the individual and synergistic effects of Fe and Mn during high-temperature cycling. Fe/Mn containing phases formed a stable framework among CaO particles, effectively suppressing grain sintering and pore collapse. The Fe Mn co-modified sample exhibited superior overall performance. After 20 cycles, Fe Mn co-modified composite maintained a heat storage density of 1263 kJ/kg, corresponding to 97% retention of its initial capacity. The Fe Mn co-modified composite achieved a solar absorptance of 82% and a crushing strength of 1.26 N, which were 1.2 and 4.1 times higher than those of the unmodified sample, respectively. Moreover, the coexistence of Fe and Mn promoted electron transfer and oxygen vacancy formation through double-exchange interactions, thereby facilitating the carbonation reaction. This study demonstrates a targeted Fe Mn co-modification strategy based on the intrinsic Fe-rich feature of steel slag and provides new insights into the high-value utilization of steel slag for sustainable and durable TCES materials.