Accelerated Cr₂O₇^2- electroreduction with NO₃^-/NO₂^- electron shuttle and in cathode microchannels

The primary limitation of Cr₂O₇^2- electroreduction arises from the electrostatic repulsion between the cathodic electric double layer and anionic species. To overcome this limitation, we developed a strategy that involved two steps: transfering electrons from the cathode surface to the bulk solution via the NO₃^-/NO₂^- redox cycle, and shortening the diffusion distance of anions to the cathodic surface by means of cathodic microchannels whose average radius is only 10 μm. When Cr₂O₇^2- solution containing NO₃^- flowed through the cathodic microchannels, the Cr₂O₇^2- removal rate reached 18.9 mg·L^-1·h^-1, and the volumetric mass transfer efficiency was 0.35. The value was 7.1 and 8.8 times higher, respectively, than that achieved using flat electrodes without electron shuttles. The contribution of microchannels and electron shuttles to Cr₂O₇^2- removal was 32 % and 68 %, respectively. This electron shuttling in microchannels was effective across a wide pH range and remained stable during 70 h of continuous operation. The resulting Cr³⁺ was subsequently converted into Cr(OH)₃ precipitate by reacting with OH- generated from water electrolysis over the cathode. These results demonstrate a novel approach for Cr₂O₇^2- removal and Cr(OH)₃ recovery. SYNOPSIS: This study presents a new strategy using electron shuttles for the efficient treatment and resource recovery of the challenging Cr₂O₇^2-.