Bio-Derived Tannin-Modified Phenolic Carbon Cryogels with Optimized Microstructure for Supercapacitors Application

Bio-derived carbon cryogels have garnered significant interest as promising electrode materials for supercapacitors due to their high specific surface area (SSA), hierarchical porosity, and eco-friendly synthesis methods. In this study, a tannin-modified phenolic hydrogel was synthesized using a sustainable tannin–phenol precursor system and subsequently subjected to three distinct drying methods-freeze-drying (FD), supercritical drying (SCD), and ambient pressure drying (APD)-to systematically evaluate their influence on structural integrity, porosity, and electrochemical behavior. Among these, the sample obtained via freeze-drying (TPUF-FD) maintained the most intact porous network, minimizing structural collapse during sublimation of ice under vacuum. This preservation of hierarchical micro- and mesopores facilitated enhanced ion diffusion, leading to the highest SSA and favorable nitrogen/oxygen functionalities that contribute to both electric double-layer capacitance and pseudocapacitance. The TPUF-FD electrode exhibited a high specific capacitance of 127.6 F g−1 at 0.5 A g−1, maintaining 107.0 F g−1 at 10 A g−1, which corresponds to a rate retention of 83.9%. When assembled into a symmetric device, the supercapacitor achieved an energy density of 8.47 Wh kg−1 at a power density of 562.5 W kg−1. Notably, the device retained 100% of its initial capacitance after 9000 charge–discharge cycles at 10 A g−1 with excellent coulombic efficiency (108.3%). These results underscore the crucial role of freeze-drying in preserving both the microstructural features and surface chemistry of biomass-derived carbon cryogels, which enhances ion accessibility and contributes to the stable, high-performance supercapacitor applications.