Sustainable Composites from Almond and Hazelnut Shells for Green Laser-Induced Graphene

Recently, the scientific community has been focusing on processing and further use of biomass in various fields. In particular, the use of biowaste for green electronics applications has an impressive number of advantages, such as low cost, low density, non-toxicity, biocompatibility, and biodegradability. Within the framework of this study, almond shell powder (ASP) and hazelnut shell powder (HSP) were used as the main component for creating biowaste-based composites. The high lignin content of the shells increases the thermal stability of the composites, making them an ideal precursor for Laser-Induced Graphene (LIG). <br/>LIG is a three-dimensional porous and conductive carbon structure obtained via localized pyrolysis by irradiation with a CO₂ IR laser. LIG synthesis and patterning is a cost-effective one-step process that allows to avoid the use of chemicals and the need for long-term use of high temperatures. <br/>Depending on the precursor composition, the quality and properties of LIG can vary. Among biomasses, chitosan is of great interest for the development of sustainable polymers, mainly due to its low cost and complete biodegradability, thus it was adopted as a matrix for ASP and HSP composites. These materials were prepared by mixing shell powders, glycerol and chitosan dissolved in acetic acid, with specific ratios. Optimization of the shell concentration has been carried out to obtain the best trade-off between good mechanical (i.e., flexibility and stiffness) and electrical properties. <br/>We obtained flexible and strong composites with a high content of ASP and HSP (Young's modulus = 25.4 MPa), which served as good precursors for the creation of LIG with a low sheet resistance (≈ 10-100 Ω/sq, depending on the specific composition and laser settings used). An investigation of the obtained materials’ structure and composition has been carried out by means of SEM and Raman spectroscopy, which showed the typical graphene features and thus confirmed the successful conversion of the composite biowaste precursors into conductive LIG. <br/>These results are a step towards green LIG for electronics applications starting from ASP and HSP, two low-value waste products which are produced in huge quantities in the agri-food industry and open a path for future studies on the topic.