Multivariate covalent organic frameworks with tailored electrostatic potential promote nitrate electroreduction to ammonia in acid

Abstract

The direct synthesis of ammonia from nitrate (NO₃^-) reduction in acid is a promising approach for industrialization. However, the difficulty arises from the intense competition with the inevitable hydrogen evolution reaction, which is favoured due to the overwhelming protons (H⁺). Here, we systematically explore and rationally optimize the microenvironment using multivariate covalent organic frameworks (COFs) as catalyst adlayers to promote the nitrate-to-ammonia conversion in acid. With the application of tailored positive electrostatic potential generated over the multivariate COFs, both the mass transfer of NO₃^- and H⁺ are regulated via appropriate electrostatic interactions, thus realizing the priority of NO₃RR with respect to HER or NO₃^--to-NO₂^-. As a result, an NH₃ yield rate of 11.01 mmol h^-1 mg^-1 and a corresponding Faradaic efficiency of 91.0% are attained, and solid NH₄Cl with a high purity of 96.2% is directly collected in acid; therefore, this method provides a practical approach for economically valorising wastewater into valuable ammonia.

Fig. 1. Rational design and theoretical analysis of multivariate COFs.

a Synthetic scheme of multivariate COFs. Aromatic rings are coloured to assist in the differentiation of the origin of the different moieties (orange, 2,4,6-triformylphloroglucinol, TP; green, benzidine, BD; azure, ethidium bromide, EB). b Electrostatic potential (ESP) maps and (c) corresponding distribution ranges of TPBD₃, TPBD₂EB₁, TPBD₁EB₂, and TPEB₃. d Electrostatic interactions of H⁺ and NO₃^– with BCOF, BECOF, and ECOF.

Fig. 2. Physical characterisation of multivariate COFs.

a, b Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF STEM) images and (c) corresponding elemental mappings of the synthesised BECOF. Scale bars, a 5 nm; b 1 nm; c 50 nm. d X-ray diffraction (XRD) patterns, e Fourier transform infrared (FTIR) spectra, f ¹³C solid-state nuclear magnetic resonance (SSNMR) spectra, g X-ray photoelectron spectroscopy (XPS) survey spectra, and (h) high-resolution N 1s XPS spectra of BCOF, BECOF, and ECOF.

Fig. 3. Experimental verification of the regulated mass transfer of NO₃^– and H⁺.

a Configuration of the tailor-made cell and schematic illustration of the detection range for in situ FTIR characterisation without an applied potential. Blue spheres, N; red spheres, O; white spheres, H. b In situ FTIR spectra of the Au substrate loaded with or without BECOF in 0.1 M nitrate solution. c In situ Raman spectra of the blank, BCOF-coated, BECOF-coated, and ECOF-coated substrates without an applied potential in 0.1 M nitrate solution. d Schematic of the H⁺ diffusion test. e pH changes versus time when glass microfibre (GF), BCOF/GF, BECOF/GF, and ECOF/GF were used as the membranes.

Fig. 4. A multivariate COF adlayer promotes the NO₃RR in acid, and the corresponding schematic depiction is shown.

a Linear sweep voltammetry (LSV) curves of different electrodes in Ar-saturated 0.05 M H₂SO₄ without (dashed line) and with (solid line) 0.1 M KNO₃ (mass loading was 0.2 mg cm^–2 and reaction temperature was 25 °C) at a scan rate of 5 mV s^–1. b NH₃ yield rates, c corresponding Faradaic efficiencies, d H₂ Faradaic efficiencies, and (e) NO₂^– yield rates of PdCu, BCOF/PdCu, BECOF/PdCu, and ECOF/PdCu at each given potential. The electrochemical cell resistance was 4.4 ± 0.3 Ω, and all the potentials were not iR-corrected. The error bars correspond to the standard deviations of measurements of three separately prepared samples under the same conditions. f Schematic illustration for improving NO₃RR performance by using multivariate COFs with tailored ESPs. COFs are coloured to assist in the differentiation of the origins of the different moieties (orange vertices, TPs; green linkers, BDs; blue linkers, EBs). Blue spheres, N; red spheres, O; white spheres, H.

Fig. 5. Analysis of the NO₃RR electrolysis and practical ammonia product synthesis.

Operando Raman spectra of (a) BECOF/PdCu and (b) PdCu collected at −1.3 V versus RHE as a function of time. Operando FTIR spectra of (c) BECOF/PdCu and (d) PdCu collected at −1.3 V versus RHE as a function of time. e ¹H NMR spectra of standard samples and the NO₃RR products using ¹⁴NO₃^– and ¹⁵NO₃^– as the isotopic nitrate source in 0.05 M H₂SO₄ electrolyte. f Comparison of the calculated NH₃ yield rates and Faradaic efficiencies at −1.3 V versus RHE using different quantification methods. The potential was not iR-corrected. g Schematic of the ammonia product synthesis process from nitrate-containing influent to NH₄Cl(s). h XRD analysis results of generated and commercial NH₄Cl solid products.