A Sustainable and Low‐Cost Zn‐Lignosulfonate Redox Flow Battery

Aqueous organic redox flow batteries offer great promise for long‐duration stationary energy storage but are often hindered by high costs associated with synthetic redox‐active materials and expensive ion‐exchange membranes. Polymer‐based redox species allow the use of cheaper microporous membranes, yet synthetic redox polymers remain costly to produce. In this study, a cost‐effective zinc/lignosulfonate hybrid redox flow battery (RFB) is presented, employing commercial sodium lignosulfonate (NaLS) as a biopolymer catholyte, Zn foil as a low‐cost and abundant anode, and a cost‐effective size‐exclusion cellulose membrane as the separator. The sluggish electrochemical behavior of the NaLS is enhanced by using carbon‐impregnated felt electrodes. Using 10 mM NaLS (20 kDa), the system achieves an average discharge voltage of 0.98 V and an initial capacity of 1.41 Ah L −1 (9.4 mAh g −1 ), outperforming previous lignin‐based RFBs. Increasing NaLS concentration to 30 mM boosts capacity to 3.52 Ah L −1 , although the cycling stability in 1 M ZnSO 4 remain moderate. However, increasing the ZnSO 4 concentration to 3 M further improves cyclability, with capacity retention rising to 72% over 28 days, compared to 66% over 17 days for 1 M ZnSO 4 . This work represents the first demonstration of commercial lignosulfonate as a catholyte in Zn‐hybrid RFBs, showcasing its potential as a sustainable active material with a remarkably low electrolyte cost of 26 € kWh −1 .