3D carbon sponge-derived from red onion skin for solid-state supercapacitor

Hierarchical pore development has been widely explored with various biomass precursors using one or more surface activating agents and porogens to prepare three-dimensional (3D) carbon materials such as activated carbons (ACs) with high specific surface areas (SSA) for the fabrication of electrical double layer (EDLC) supercapacitor (SC) for efficient charge storage. However, purity, quality and performance of biomass derived ACs are usually concerned as toxic gases are produced from activating agents and porogens. In further connection with an effective pore structure control in such ACs, selection of the activating agent and the carbonization conditions is highly crucial. We noted that zinc chloride (ZnCl₂) activation has not been attempted with red onion (Allium cepa. L) skins for energy storage applications. This motivated us to have a detailed investigation of the ZnCl₂ effect on red onion skins at different temperatures. In this study to see if we can correlate the activation process to be investigated with the pore structure management in the ACs derived, mainly to see if we can deduce some meaningful relationship with the energy storage performance of the resulting 3D carbon structures. We found that 3D carbon sponges can be derived from red onion skins at 900 °C for 3 h under inert atmosphere due to the inherent assembly of quercetin molecules and anthocyanins via hydrogen bonding and π–π stacking interactions assisted surface activation, carbonization, and aromatization processes. Surface porosity measurements using BET method revealed that the SSA (∼2398 m² g⁻¹) of 3D porous carbon sponges is comparable or higher than the most other biomass derived ACs. High resolution transmission electron microscopic (HRTEM) results confirmed that around each micropore and mesopore, five to ten graphitic nanolayers were created, which further interacted to form conducting networks on the 3D sponge surface. Such conducting networks stabilized the hierarchical pores and circulated the electrolyte in and around the micro/-nano cavity via controlled diffusion process which promoted an efficient charge storage at the electrochemical interface. As a result, the 3D carbon material provided a specific capacitance (C_sp) value of 265 F g⁻¹ at a current density (CD) of 1.0 Ag⁻¹, with two-fold higher than that provided by commercial AC materials. The all-solid-state SC fabricated with 3D carbon sponge provided a high energy density (ED) of 19.9 Wh kg⁻¹ at a power density (PD) of 12.5 KW kg⁻¹ with minimum IR drop (∼0.05 V), which is comparable to the ED and PD values for biomass-derived ACs reported in the literature. This work provides new insights into the preparation of 3D nanostructured ACs with sponge-like texture from a biomass precursor with good control over 3D structure, graphitic networks, and porosity development for improved energy storage applications.