Highly efficient and selective nitrate electroreduction to ammonia catalyzed by molecular copper catalyst@Ti3C2Tx MXene

Abstract

The efficient electroreduction reaction of nitrate (NO₃⁻RR) at low concentrations to ammonia is energetically favorable for ammonia production and environmentally essential to treat water contamination. Copper-based conjugated molecule electrocatalysts for the NO₃⁻RR are advantageous in terms of their customizable Cu-coordinative micro-environment and facile preparation; however, they easily agglomerate due to their large π-conjugating system, which severely lowers the active site density and increases the internal resistance. This study demonstrates that Ti₃C₂T_x MXenes are promising supports to disperse and anchor a molecular catalyst (copper phthalocyanine, CuPc) to construct a CuPc@MXene electrocatalyst in order to significantly promote NO₃⁻RR performance. 10% CuPc@MXene achieved higher ammonia selectivity (94.0%) and higher nitrate conversion (90.5%) compared to those of unsupported CuPc, and retained high ammonia selectivity (∼80%) after seven recycling tests. The combination of online differential electrochemical mass spectroscopy (DEMS) and density functional theory (DFT) calculations has showed that CuPc molecules are able to inhibit the hydrogen evolution reaction and effectively convert nitrate into ammonia through the ONH reaction pathway. The results showed that the MXene support is applicable to many metal phthalocyanines (MPcs, M = Fe, Co, Ni), demonstrating its potential in other electrochemical catalysis applications.