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

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 NO3-RR are advantageous in terms of their customizable Cu-coordinative micro-environment and facile preparation, however, they are easy to agglomerate due to their large π-conjugating system, which severely lowers the active site density and increases internal resistance. This study demonstrates that Ti3C2Tx MXene is a promising support to disperse and anchor the molecular catalyst (copper phthalocyanine, CuPc) to construct the 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 remained high ammonia selectivity (~80%) after seven recycling tests. The combination of online differential electrochemical mass spectroscopy (DEMS) and density functional theory (DFT) calculations have 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 MXene support is applicable to many metal phthalocyanines (MPcs, M=Fe, Co, Ni), demonstrating its potential in other electrochemical catalysis applications.