The mechanisms of calcium-catalyzed graphenization of cellulose and lignin biochars uncovered

A recent study has shown that highly crystalline graphene-based materials can be obtained from poorly organized carbon precursors using calcium as a non-conventional catalyst. XRD and TEM analyses of calcium-impregnated cellulose and lignin biochars showed the formation of well-ordered graphenic structures (L c > 7 nm, d 002 < 0.345 nm) above 1200 °C, far below the standard graphenization temperatures (T > 2000 °C). Herein, we propose new insights on the mechanism controlling the formation of highly graphenic biochars using Ca as a catalyst. We postulate that the calcium-catalyzed graphenization occurs through the formation of a metastable calcium carbide by reaction between CaO particles and amorphous carbon between 1000 and 1200 °C. CaC 2 decomposes into calcium vapor and a graphenic shell covering the CaC 2 particles as confirmed by TEM analysis. The thickness and planarity of the graphenic shell increase with the CaC 2 initial particle size (between 20 and 200 nm), and its growth is controlled by the diffusion of the calcium vapor through the graphene layer. A much effective graphenization was obtained for the lignin biochars compared to cellulose, with L c > 10 nm and d 002 < 0.340 nm, attributed to the insertion of sulfur in the graphenic shells, which favors their ruptures and the decomposition of CaC 2 into graphene. We believe that these findings would enable the reduction of costs and environmental impact of graphene-based materials synthesis using cheap and abundant renewable feedstocks and catalysts as well.