Energy Recovery from Hexavalent Chromium Reduction for In Situ Electrocatalytic Hydrogen Peroxide Production

Recovering chemical energy embedded in pollutants is significant in achieving carbon-neutral industrial wastewater treatment. Considering that industrial wastewater is usually treated in a decentralized manner, in situ utilization of chemical energy to achieve waste-to-treasure should be given priority. Herein, the chemical energy released by the electroreduction of Cr(VI) was used to enhance on-site H₂O₂ generation in a stacked flow-through electrochemical system. The driving force of water flow efficiently coupled O₂ evolution with 2-e O₂ reduction to facilitate H₂O₂ generation by transporting anode-produced O₂ to the cathode. Meanwhile, the chemical energy released by Cr(VI) promoted O₂ evolution and impeded H₂ evolution by regulating the electrode potentials, accounting for the enhanced H₂O₂ generation. The system could completely reduce 10-100 ppm of Cr(VI), reaching the maximum H₂O₂ concentration of 2.41 mM. In particular, the H₂O₂ concentrations in the Cr(VI)-containing electrolyte were 10.6-88.1% higher than those in the Cr(VI) free electrolyte at 1.8-2.5 V. A 24-day continuous experiment demonstrated the high efficiency and stability of the system, achieving a 100% reduction efficiency for 100 ppm of Cr(VI) and producing ∼1.5 mM H₂O₂ at 1.8 V. This study presents a feasible strategy for Cr(VI) detoxification and synchronous on-site H₂O₂ generation, providing a new perspective for innovative Cr(VI) wastewater treatment toward resource utilization.