Graphene has generated substantial interest as a filler due to its exceptional strength, flexibility, and conductivity but faces obstacles in supply and implementation. A renewable, plant-based graphene nanoparticle (pGNP) presents a more accessible and sustainable filler with the same properties as mineral graphenes. In this study, the mechanisms of graphene reinforcement in carbon fiber reinforced plastic (CFRP) were examined, along with the resulting improvements to mechanical strength, resistance to crack propagation, electrical and thermal conductivity at elevated temperatures. pGNP, produced from renewable biomass, was shown to have a graphitic structure with flakes 3-10 layers thick and a median lateral size of 240 nm with epoxide and carboxyl functional groups. pGNP was sprayed on carbon fiber/epoxy prepreg at loadings from 1.1 g/m2 to 4.2 g/m2 . An even particle dispersion was achieved using a spray application in a water/alcohol suspension with the addition of surfactants and dispersion aides. Results show interlaminar pGNP improved Mode I fracture toughness at crack initiation by 146% at 20°C and 126% at 90°C, with fracture toughness improved by 53% and 52% during propagation at 20°C and 90°C, respectively. Mode II fracture toughness was not changed at 20°C and improved 55% at 90°C. pGNP addition increases flexural modulus by 15%, flexural strength by 17%, and interlaminar shear strength by 17%, as well as electrical conductivity by 397% (κ₂₂) and thermal conductivity by 27% (k₁₁), with these improvements observed at 1.1- 2.3 g/m2 spray loadings. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and dynamic mechanical analysis (DMA) show polymer crosslinking with graphene surface groups and the resulting restriction of side chain movement. These restrictions improve composite performance at ambient and elevated temperatures, extending the damage process zone and increasing fracture toughness, as well as improve particle/matrix interaction, leading to improved conductivity
