Energy-Positive Thiosulfate Production Enabled by Coupling Waste Sulfide Oxidation with Oxygen Reduction

Abstract

Sulfide-rich waste streams are commonly treated as among the most common environmental liabilities, yet they also represent an abundant sulfur-derived chemical resource if energy-efficient upcycling processes can be developed. Electrochemical sulfide oxidation offers a promising route for sulfur recovery, but conventional systems are typically energy-intensive and produce low-value products. Here we develop a galvanic electrochemical-chemical cascade strategy that couples the sulfide oxidation reaction (SOR) with the two-electron oxygen reduction reaction (ORR), enabling sulfide valorization with net positive electrical energy output. Specifically, a Ce-modified Cu2S anode is designed to enhance both SOR activity and selectivity. Ce incorporation electronically modulates the Cu–S framework, providing a favorable intrinsic pathway for deeper sulfide oxidation and sulfur-chain propagation, while the resulting polysulfide distribution is further governed by the electrochemical reaction conditions. The optimized Ce–Cu2S/NF electrode achieves 100 mA cm–2 at 0.28 V versus the reversible hydrogen electrode and maintains stable SOR operation for over 110 h. At the cathode, graphitized carbon nanotubes selectively generate H2O2. Through membrane engineering and a relay-type cyclic process, polysulfide intermediates and H2O2 are coupled in a dosage-controlled manner to produce Na2S2O3·5H2O with a sulfide-to-thiosulfate conversion yield exceeding 60% and a purity of approximately 80%. Importantly, the integrated flow cell simultaneously generates electricity with a peak power density of approximately 4 mW cm–2. Preliminary techno-economic analysis further indicates that ORR coupling reverses the electricity term from a cost penalty to an energy credit. This work demonstrates an energy-positive approach for sulfide waste valorization to value-added thiosulfate.