Walnut shell-derived biochar as a sustainable carbon electrode for aqueous sodium-ion supercapacitors

The development of sustainable electrode materials for aqueous sodium-ion supercapacitors is critical for cost-effective and environmentally friendly energy storage. In this work, we report the use of walnut shell-derived biochar as a low-cost and renewable electrode material operating in neutral sodium aqueous electrolytes. A key strength of this study is the systematic evaluation of different sodium-based electrolytes on the same biochar electrode, providing clear insight into electrolyte–electrode interactions, which are often overlooked despite their importance in device performance. Comprehensive electrochemical characterization in both three-electrode and symmetric two-electrode configurations reveals typical electrical double-layer capacitive behavior, low internal resistance, and efficient ion transport within the porous carbon structure. Importantly, the results show a clear electrolyte-dependent response: NaNO₃ delivers higher initial capacitance values (up to 204 F g⁻¹ at 0.2 A g⁻¹) yet suffers from limited electrochemical stability causes rapid capacitance fading. In contrast, Na₂SO₄ demonstrates superior electrochemical robustness, maintaining ∼85.3% capacitance after 6000 cycles and supporting higher current operation. Electrochemical impedance analysis further confirms favorable interfacial kinetics and near-ideal capacitive response, while also highlighting the role of electrolyte identity in governing charge transfer resistance and ion transport dynamics. Overall, this study reveals that the selection of sodium-ion electrolyte is a decisive factor in defining the performance and stability of walnut shell-derived biochar electrodes. Beyond identifying efficient, low-cost, and sustainable electrode material, the work provides important mechanistic insight into electrolyte-dependent behavior, establishing meaningful basis for rational electrolyte selection in future aqueous supercapacitor systems.