Stereolithography-Derived Three-Dimensional Pyrolytic Carbon/Mn₃O₄ Nanostructures for Free-Standing Hybrid Supercapacitor Electrodes

The development of permeable three dimensional (3D) macroporous carbon architectures loaded with active pseudocapacitive nanomaterials offers hybrid supercapacitor materials with higher energy density, shortened diffusion length for ions and higher charge/discharge rate capability, and thereby is highly relevant for electrochemical energy storage (EES). Herein, structurally complex and tailorable 3D pyrolytic carbon/Mn₃O₄ hybrid supercapacitor electrode materials are synthesized through self-assembly of MnO₂ nanoflakes and nanoflowers onto the surface of stereolithography (SLA) 3D printed architectures via a facile wet chemical deposition route, followed by a single thermal treatment. The thermal annealing of the MnO₂ nanostructures concurrent with carbonization of the polymer precursor leads to formation of a 3D hybrid supercapacitor electrode material with unique structural integrity and uniformity. The microstructural and chemical characterization of the hybrid electrode reveals the predominant formation of crystalline hausmannite-Mn₃O₄ after the pyrolysis/annealing process, which is a favourable pseudocapacitive material for EES. With the combination of the 3D free-standing carbon architecture with self-assembled binder-free Mn₃O₄ nanostructures, electrochemical capacitive charge storage with very good rate capability, gravimetric and areal capacitances (186 Fg^-1 and 968 mFcm^-2, respectively) and long lifespan (˃92% after 5000 cycles) is demonstrated. It is worth noting that the gravimetric capacitance value is obtained by considering the full mass of the electrode including the carbon current collector. When only the mass of the pseudocapacitive nanomaterial is considered, a capacitance value of 457 Fg^-1 is achieved, which is comparable to state-of-the-art Mn₃O₄-based supercapacitor electrode materials.