Pyrolysis in Molten Salts Converts Plastics into a Mesoporous Electrocatalyst with a High Density of Atomic Fe–N–C Active Site

Abstract

Upcycling plastic waste into value-added products is a promising strategy for both economic and environmental sustainability. However, achieving control over structure–property correlations during the upcycling process remains challenging. Here, we investigate the modulation of active site formation during pyrolytic conversion of plastics to carbon by composition-controlled molten salts. Using melamine formaldehyde (MF) foams as a precursor, we demonstrate that their pyrolysis in eutectic chloride molten salts with Fe2+, Na+, and K+ ions directs the carbonization process toward the formation of mesoporous functional carbon enriched with atomically dispersed Fe–N–C sites, which are well-known for their catalytic activity in oxygen reduction reaction (ORR). Compared to pyrolysis in the FeCl2-only salt, the incorporation of alkali metal ions (Na+, K+) in the eutectic mixture facilitates nitrogen retention in the carbon matrix and modulates iron speciation. This synergistic environment promotes a higher density and more uniform dispersion of Fe–N–C moieties within the carbon matrix. Consequently, the resulting catalyst exhibits a high surface area, hierarchical porosity, and improved electrochemical properties. When tested as an ORR catalyst, the Fe–N–C-enriched catalyst delivers a high half-wave potential (E1/2 = 0.875 V vs RHE) with nearly exclusive 4e– pathway. These properties are comparable to those of commercial Pt/C catalysts, along with improved stability.