Balancing Hydrogen Evolution and Hydrogenation Reaction via Facet Engineering for Efficient Conversion of Nitrate to Ammonia in Actual Wastewater

Abstract

Due to the competitive relationship between nitrate reduction reaction (NO3-RR) and hydrogen evolution reaction (HER), the conventional approach to improve Faradaic efficiency is to select a catalyst without HER activity. Nevertheless, such strategy not only limits the application of HER catalysts in NO3-RR, but also causes insufficient hydrogen source, thereby sacrificing ammonia yield rate. We believe that HER catalysts should not be excluded from hydrogenation reduction, and HER-involvement in NO3-RR can achieve both high Faradaic efficiency and ammonia yield rate. Herein, taking traditional water electrolysis material Co3O4 as model system, we employ facet engineering and obtain a Faradaic efficiency of 97.51% and an ammonia yield rate of 4.33 mg-N·cm-2·h-1 . Excellent performance is attributed to the oxygen vacancy on crystal facet, which greatly promote water dissociation and capture HER intermediate for NO3-RR, successfully shifting the reaction pathway from hydrogen evolution to nitrate hydrogenation. Beyond material development, we construct a hybrid reactor to address and achieve an ammonia recovery rate of 1216.8 g-N·m-2·d-1 in nuclear industry wastewater with ultra-high nitrate concentration. This study breaks through the limitation of HER catalyst in NO3-RR, which provides a significant insight into the catalyst designing and hydrogenation mechanism.