From Starch to Carbon Materials: Insight into the Cross-Linking Reaction and Its Influence on the Carbonization Process

As a renewable biomass product, starch is a fantastic source for preparing various advanced carbon materials. But starch shows poor thermal stability. Its original spherical morphology tends to be disrupted by direct pyrolysis, and the carbon yield is low. Thus, prestabilization by chemical cross-linking is an effective approach to address the above issue. Herein, corn starch was cross-linked by (NH4)2HPO4, followed by carbonization to obtain uniform carbon microspheres. The chemical evolution from starch to carbon was studied systematically using thermogravimetric-mass spectrometry, in situ Fourier transform infrared, X-ray photoelectron spectroscopy, nuclear magnetic resonance, and in situ X-ray diffraction technique. The mechanism of the cross-linking reaction and its influence on the carbonization process were proposed. The introduction of (NH4)2HPO4 promoted the dehydrogenation reaction of starch and further improved its carbonization behaviors. With the increased temperature, more stable heterocyclic aromatic moieties, such as amines, pyridine, pyrrole, and quaternary type N sites, formed in the carbon skeleton, which boosted the growth in cyclization and the size of the polyaromatic units. The further formation of C–O–PO3, C–P–O3, and C2–P–O2 played a critical role in cross-linking of polyaromatic unit fragments into graphitic crystalline, which facilitated the preservation of the natural microspheres morphology. The insights into the thermochemical conversion of starch paved a way for the controllable transformation from organic biomass to inorganic carbon materials, with desired structure and properties.