Low-Cost Biomass-Derived Lignosulfonate with Tin Plating/Stripping for Acidic Aqueous Redox Flow Storage

Biomass-derived redox couples that sustain fast, reversible electrochemistry at practical current densities are of growing interest for low-cost (bio)polymer redox flow batteries. Sodium lignosulfonate (NaLS), a lignin-derived biopolymer, exhibits markedly enhanced proton-coupled redox kinetics and reversibility in strongly acidic chloride media, with 2 M HCl outperforming H2SO4, neutral NaCl, and alkaline KOH electrolytes. Leveraging this acid compatibility, we develop an acidic Sn||NaLS hybrid aqueous redox flow battery that couples NaLS redox on porous carbon felt with Sn2+/Sn0 plating–stripping from SnCl2 in 2 M HCl. Negative-substrate screening identifies Cu as the most reversible Sn plating surface, enabling stable cycling at 10 mA cm–2. At 10 mM NaLS (250 g L–1), the optimized cell delivers 3.69 Ah L–1 (14.55 mAh gNaLS–1) with an average discharge voltage of ∼0.73 V and ∼47% average energy efficiency over 80 cycles. Benchmarking against state-of-the-art aqueous lignin-based RFBs, this work achieves one of the highest volumetric capacities and energy densities (2.69 Wh L–1) among lignin-based systems. Post-cycling electrochemical and spectroscopic characterization reveals that capacity fading is primarily governed by the chemical degradation of NaLS. These findings identify critical optimization targets such as understanding electrolyte-lignin compatibility and enhancing Sn plating/stripping reversibility, to increase Coulombic efficiency and ensure long-term cycling stability.