Concentrated Solar Combined With Hydrothermal Treatment to Unlock Lignin Graphitization Mechanisms

Biocarbons are carbon-rich materials produced from biomass and are increasingly used as sustainable alternatives to fossil-based materials for energy and environmental applications. Their production for commercial usages requires high temperature (above 2000°C), which calls for particular attention on energy consumption and environmental impact. Hydrothermal carbonization, selected as prestructuration step, produces small graphitic units with 6 stacked layers of 2.34 nm diameter. Further pyrolysis at 800°C increases layer length and the number of small graphitic units, giving an ideal graphitization precursor. Conventional and concentrated solar carbonization are compared in the graphitization step. At 1800°C, conventional carbonization leads to a turbostratic structure with a quality of graphitic domains characterized by low interlayer spacing (d₀₀₂ = 0.357 nm) and long defective graphene layers (L_a(XRD) = 5.81 nm, L_a(Raman) = 3.17 ± 0.46 nm). Solar carbonization at 1800°C also yields long defective graphene layers, but the structure is heterogeneous with both a turbostratic (L_c = 2.41 nm, d₀₀₂ = 0.368 nm) and a graphitic (L_c = 5.58 nm, d₀₀₂ = 0.345 nm) phase. Even though solar carbonization relies on an intermittent energy source, it enables higher graphene layer stacking biocarbon as compared to conventional carbonization route.