Vat Photopolymerization of High-Tannin-Loading Suspensions for the Fabrication of High-Mechanical-Strength Hard Carbon

Hard carbon derived from bio-based sources such as tannins is gaining attention for applications in energy storage, catalysis, and lightweight structures, due to its sustainability and high mechanical properties. However, the fabrication of 3D hard carbon objects via vat photopolymerization (VP) remains challenging because of rheological instability, polymerization inhibition by tannins, and shrinkage during pyrolysis. In this study, we demonstrate that tannin can be processed using professional VP with an optimized photocurable formulation composed of highly reactive, viscous dipentaerythritol hexaacrylate (DPHA), 1,6-hexanediol diacrylate (HDDA) as a fluidifying agent, and a photoinitiator, without added dispersants. By systematically increasing the tannin content (30–50 wt%) and adjusting the DPHA/HDDA ratio, we optimized viscosity, suspension stability, curing behavior, and dimensional fidelity during pyrolysis. Unlike previous approaches that mainly focus on porous carbon architectures, our method maximizes tannin loading (up to 45 wt%) to achieve nearly fully dense and high mechanical performance hard carbon with controlled shrinkage deformations. The optimized formulation, featuring a DPHA/HDDA ratio of 80/20, delivers the best compromise between rheology, curing performance, and structural integrity. Significantly improved directional mechanical properties were obtained, with 39 MPa of compressive strength, which is between porous hard carbon (<5 MPa) and dense graphite (∼150 MPa). This work establishes a robust pathway for VP-based fabrication of complex, defect-free 3D hard carbon objects from bio-derived precursors, offering high mechanical performance and pyrolysis-based tunable electro-thermal properties ranging from an insulating to a conductive state. This represents potential applications in high-strength, high-temperature materials or 3D electronic architectures with tunable properties.