A New Strategy to Overcome Repulsion of Cathodic Electric-Double-Layer to Cr2o72-: Establishing Electron Shuttling in Microchannels

Abstract

The intrinsic limitation of Cr2O72- electroreduction is the electric repulsion of the cathodic electric-double-layer to anions. To overcome this limitation, we propose a new strategy: to shuttle electrons from cathode surface to bulk solution via NO3-/ NO2- redox cycle while to shorten the diffusion distance of anions to cathodic surface down to 10 μm by using cathodic microchannels. After Cr2O72- solution involving NO3- flowed through the cathodic microchannels, the reduction rate of Cr2O72--Cr achieved 18.9 mg·L-1·h-1, and the volumetric mass transfer efficiency value was 0.35. Which is 7.1 times and 8.8 times as many as that for flat electrodes without electron shuttles, respectively. The contribution of microchannels and electron shuttles to Cr2O72- reduction was 32% and 68%, respectively. This electron shuttling in microchannels was applicable in a wide pH range and worked stably during 70 hours of continuous Cr2O72- reduction. The produced Cr3+ will be converted into Cr(OH)3 precipitation by raising pH value of the cathodic chamber via electrochemical water splitting. These results demonstrated a novel method for Cr2O72- removal and Cr(OH)3 recovery.

Keywords: Cr2O72- reduction, Electron shuttle, Microchannel cathode, Electrostatic repulsion