Harnessing Nickel Phthalocyanine-Based Electrochemical CNT Sponges for Ammonia Synthesis from Nitrate in Contaminated Water

Abstract

Electrochemical reduction of nitrate to ammonia is of great interest in water treatment with regard to the conversion of contaminants to value-added products, which requires the development of advanced electrodes to achieve high selectivity, stability, and Faradaic efficiency (FE). Herein, nickel phthalocyanine was homogeneously doped into the fiber of a carbon nanotube (CNT) sponge, enabling the production of an electrode with high electrochemical double-layer capacitance (C DL) and a large electrochemically active surface area (ECSA). The as-prepared NiPc-CNT sponge could achieve 97.6% nitrate removal, 88.4% ammonia selectivity, and 86.8% FE at a nitrate concentration of 50 mg-N L–1 under an optimized potential of −1.2 V (vs Ag/AgCl). Meanwhile, the ammonia selectivity could be further improved at the high nitrate concentration. Density functional theory calculations showed that the exposure of Ni–N4 active sites could effectively suppress the hydrogen evolution reaction and dinitrogen generation, enhancing the ammonia selectivity and Faradaic efficiency. Overall, this work sheds light on the conversion of nitrate to ammonia on the metal phthalocyanine-based electrode, offering a novel strategy for managing nitrate in wastewater.

Molecular inspired electrocatalyst materials for environmental remediation

“…Cobalt and copper phthalocyanines (CoPc, CuPc) anchored on carbon nanotubes electrochemically convert nitrate to ammonia with higher activity and selectivity than aggregated Pc samples. [77][78][79] The CNT-supported CuPc material is 30% more active than the carbon black-supported CuPc. 77 CuPc requires a lower NO 3 − reduction overpotential (η) compared to the cobalt analog to reach a j of 10 mA cm −2 and a faradaic efficiency (FE) over 98%.…”

“…NiPc-CNT sponges achieve 98% nitrate removal after 4 h of electrolysis due to the high electrochemically active surface area (ECSA) layer of the CNT sponge. 78 CoPc has the overall highest performance for nitrate reduction, with an ammonia conversion rate >97%. 79 75 At acidic pH values, hydroxylamine is the main product at 100% selectivity with the Co-PP system.…”

“…Carbonaceous supports such as glassy carbon, 58,71–74 graphite, 75,76 and carbon nanotubes 77–79 have been utilized to enhance the electrochemical performance and stability of metal-based catalysts. Cobalt and copper phthalocyanines (CoPc, CuPc) anchored on carbon nanotubes electrochemically convert nitrate to ammonia with higher activity and selectivity than aggregated Pc samples.…”

“…NiPc-CNT sponges achieve 98% nitrate removal after 4 h of electrolysis due to the high electrochemically active surface area (ECSA) layer of the CNT sponge. 78 CoPc has the overall highest performance for nitrate reduction, with an ammonia conversion rate >97%. 79 A family of Co(dmgH 2 )Cl 2 (dmg = dimethylglyoximato) immobilized on multiwalled CNTs (MWCNT) show a 5-fold increase in current density compared to bare MWCNTs in the nitrite to ammonia electrochemical conversion with FE of 86–98% and preference to ammonia.…”

Controlling the Metal–Ligand Coordination Environment of Manganese Phthalocyanine in 1D–2D Heterostructure for Enhancing Nitrate Reduction to Ammonia

“…Nitrogen in nitrate ions has an oxidation state of +5, which can form a number of nitrogenous products in oxidation states from +3 to −3, including nitrite (+3, NO 2 – ), nitric oxide (+2, NO), nitrous oxide (+1, N 2 O), nitrogen (0, N 2 ), hydroxylamine (−1, NH 2 OH), hydrazine (−2, N 2 H 4 ), and ammonia (−3, NH 3 ). ,− Development of numerous electrocatalytic systems using a variety of heterogeneous catalysts including Cu, Ag, Au, Rh, Ru, Ir, Pd, Pt, etc. often converts NO 3 – to N 2 via a five-electron transfer process. − Several research groups have also shown that electrochemically NO 3 – can be converted to hydroxylamine, nitrite, and hydrazine. − Electrocatalytic nitrate to ammonia conversion [NO 3 – + 6H 2 O + 8e – → NH 3 + 9OH – ] via an eight-electron transfer process using metal, nonmetal, and transition-metal-based electrocatalysts would be an alternative option for next-generation ammonia production. − The selectivity toward ammonia synthesis was unsatisfactory, and generally a broad range of products is obtained. This is due to the complexity of the process, strong competition from HER, and the production of various byproducts, which reduce the faradaic efficiency (FE) and selectivity of ammonia .…”