Phase Diagram-Guided Molten Salt Engineering of Biocarbon Pores at Low Temperatures

Pore structure engineering of biocarbon is crucial for enhancing its performance. Herein, a phase diagram-guided molten salt activation method is systematically developed to prepare biocarbon with a tunable pore structure at low temperature (500 °C). This strategy incorporates key activation variables on the phase diagram of binary salts, including salt type, ratio, phase partition, and operating temperature. Among various salts, ZnCl₂ with a low melting point (≈290 °C) is selected as the "pioneering" primary salt to swell biomass precursor and create porous channels conducive to the ion diffusion of secondary salts. Based on the liquid-solid equilibrium curve, specific amounts of secondary salts can be dissolved into molten ZnCl₂, facilitating the delivery of secondary salt ions to the inner active sites. The carefully selected secondary salts with varied solubility, reactivity, and ionic sizes further enhance pore creation (reaching a specific surface area of 2491 m² g^-1) and regulate pore sizes by catalytic reactions and pore filling. The understanding of the underlying ion diffusion and pore evolution mechanism (formation, growth, and stabilization) in binary molten salt systems provides a unique and versatile guide for producing biocarbon with tunable pore structure at low temperatures, supporting applications including energy storage and carbon capture.