Electrochemical N2-to-NH3 with high faradaic efficiency on ZrO2/carbon quantum dots catalysts in neutral electrolyte

Abstract

Electrochemical nitrogen reduction (NRR) to synthesize NH3 is limited by low Faraday efficiency (FE). Developing electrocatalysts that can efficiently adsorb/activate N2 and suppress hydrogen evolution reaction (HER) to enhance FE is needed. In this work, x-ZrO2/CQDs-T series catalysts were successfully prepared. The results demonstrate that: (I) The appropriate introduction of carbon quantum dots (CQDs) effectively enhances the surface adsorption capacity of ZrO2/CQDs for N2. (II) By high-temperature calcination eliminates the alkaline functional group (-NH2) of CQDs, which reduces the basicity of ZrO2/CQDs catalyst and inhibits the adsorption of protons on the catalyst surface. (III) Due to the semiconductor properties of ZrO2, the charge transfer efficiency of ZrO2/CQDs catalyst is significantly reduced, thereby weakening the binding of electrons to the adsorbed protons. As a result, 1-ZrO2/CQDs-750 catalyst appropriately and effectually restricts the accessibility of protons/electrons and promotes more N2 molecules to occupy active sites, thus achieving up to 66.62 % FE, which is superior to most reported NRR catalysts so far. Meanwhile, electrochemical in situ ATR-SEIRAS technique reveals that 1-ZrO2/CQDs-750 catalyst surface occurs in associative alternating reactions. The introduction of CQDs not only effectively affects the adsorption of N2 and N-related intermediates on ZrO2/CQDs catalyst and limits the proton/electron accessibility to improve FE of NRR, but also promotes the exposure of active sites to increase NH3 yield.