Synergistic CoFe₂O₄ Spinel–Cobalt Phthalocyanine–Carbon Nanotube Hybrid Catalyst for Highly Selective Electrochemical Nitrate-to-Ammonia Conversion

Abstract

In this study, we developed an optimized catalyst, CoFe2O4/CoPc/CNT, synthesized using 1 mmole (Co(NO3)2.6H2O and 0.125 mmole FeSO4.7H2O in DMF, along with 0.1 mmole H2Pc and 40 mg acid-functionalized CNT, at 110 °C for 6 h followed by washing and heat treatment at 500 °C for 90 minute. Powder X-ray Diffraction (PXRD) and Fourier Transform Infrared Spectroscopy (FTIR) analyses confirm the formation of the CoFe₂O₄/CoPc/CNT composite. Field Emission Scanning Electron Microscopy (FESEM) combined with Energy Dispersive X-ray Spectroscopy (EDS) shows the uniform distribution of CoFe₂O₄ over the CNT surface, while High Resolution Transmission Electron Microscopy (HRTEM) reveals the primarily deposition of very small CoFe₂O₄ particles with sizes of approximately 5 nm. HRTEM analysis further reveals the formation of CoPc with sheet-type morphology. Scanning Transmission Electron Microscopy EDS mapping reveals the CoPc nanostructures are wrapped or grown over CNT. The uniform dispersion and smaller particle size in the optimized sample are expected to provide a higher density of active sites, enhancing its performance toward electrochemical nitrate reduction (eNO₃RR). Electrochemical Impedance Spectroscopy (EIS) measurements show that the optimized sample exhibits the lowest charge transfer resistance among the various composite samples synthesized using different ratios of Co(NO₃)₂·6H₂O (1 mmole) and FeSO₄·7H₂O (1–0.125 mmole), while keeping all other reaction conditions constant. The presence of oxygen vacancies in CoFe2O4 and a substantial amount of CoPc in the optimized sample, as revealed by X-ray Photoelectron Spectroscopy (XPS) and PXRD analyses, could enhance the extent of NO3- adsorption and thereby enable efficient NH₃ production (~51.47 mg h⁻¹ cm⁻² or 102.94 mg h⁻¹ mg⁻¹ cat) at –0.9 V vs. RHE. The optimized catalyst also shows higher selectivity at different NO₃⁻ concentrations and applied potentials, demonstrating its superior efficiency toward eNO₃RR.