Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using Bimetallic Covalent Organic Frameworks with Tandem Active Sites

Abstract

Electrochemical nitrate reduction reaction (NO₃RR) is a promising approach to simultaneously realize pollutant removal and ammonia generation. However, this process involves the transfer of eight electrons and nine protons along with multiple by-products, resulting in a significant challenge for achieving high ammonia yield and selectivity. Herein, we introduced bimetallic covalent organic frameworks catalysts with Cu and Co active sites to achieve a two-step tandem reaction, avoiding excessive nitrite accumulation and enabling efficient NO₃RR. For the initial two-electron process, the Cu sites in the bimetallic catalyst exhibit a strong binding affinity with nitrate, promoting their conversion to nitrite. The Co sites enhance the supply and adsorption of active hydrogen and stabilize the subsequent six-electron process, thereby improving the overall catalytic efficiency. Compared to monometallic Cu and Co catalysts, the CuCo bimetallic catalyst demonstrates superior ammonia yield and Faradaic efficiency (NH₃ yield rate = 20.8 mg·h^-1·cm^-2, FE = 92.16% in 0.3 M nitrate). Such coordinated two-step process advances the efficiency and applicability of NO₃RR through optimizing a cascade catalytic reaction, thereby establishing an innovative path for the engineering of NO₃RR electrocatalysts.

Keywords: Ammonia synthesis; Cascade electrocatalysis; Covalent organic frameworks; Nitrate reduction; Tandem active sites.

Figure 1

Synthesis process and characterizations of catalysts. a) The schematic diagram of preparing TTA‐TPH and TTA‐TPH‐CuCo. b) PXRD patterns and AA stacking simulation of TTA‐TPH. c) Top and side views of the refined AA model for TTA‐TPH. d) Nitrogen adsorption‐desorption isotherms and pore size distributions (inset) of TTA‐TPH and TTA‐TPH‐CuCo. (e, f) HR‐TEM images and EDS mapping images of TTA‐TPH and TTA‐TPH‐CuCo, inset is the distance of (001) lattice plane.

Figure 2

Structural characterizations of catalysts. a) XPS N1s spectra of TPH monomer, TTA‐TPH and TTA‐TPH‐CuCo. b) XPS Cu 2p spectra and c) XPS Co 2p spectra of TTA‐TPH‐CuCo. d) Cu K‐edge XANES of TTA‐TPH‐CuCo and its valence state fitting calibration curve (inset). e) FT‐EXAFS spectra of Cu for TTA‐TPH‐CuCo, CuO, Cu₂O, Cu phthalocyanine (Pc) and Cu foil. f) WT‐EXAFS plots of Cu for CuO, CuPc, and TTA‐TPH‐CuCo. g) Co K‐edge XANES of TTA‐TPH‐CuCo and its valence state fitting calibration curve (inset). h) FT‐EXAFS spectra of Co for TTA‐TPH‐CuCo, CoO, Co₂O₃, CoPc, and Co foil. i) WT‐EXAFS plots of Co for CoO, CoPc and TTA‐TPH‐CuCo.

Figure 3

NO₃RR performance. a) LSV curves in 0.5 M K₂SO₄ with and without 0.1 M NO₃ ⁻. b) The C _dl comparison of all samples. c) NH₃ yield rate of TTA‐TPH‐CuCo with different CuCo proportions. d) NH₃ yield and e) FE of TTA‐TPH‐CuCo under different potentials and NO₃ ⁻ concentrations. f) The NH₃ yield and FE of TTA‐TPH‐CuCo at −0.75 V versus RHE during fifty cycling tests. g) ¹H NMR spectra of TTA‐TPH‐CuCo in ¹⁵NO₃ ⁻ and ¹⁴NO₃ ⁻ electrolytes before and after NO₃RR. h) Comparison of NH₃ yield and FE of TTA‐TPH‐CuCo with recently reported NO₃RR electrocatalysts (see Supporting Information for detailed references). (Error bars in 3c–e correspond to the standard deviations of three measurements, the center value of error bars is the average of three measurements).

Figure 4

In situ characterizations and reaction mechanism. a) In situ DEMS patterns of TTA‐TPH‐CuCo. b) In situ ATR‐IRAS measurements under different potentials. c) Gaussian fitting on O‐H stretching bands of TTA‐TPH‐CuCo and the corresponding structures for 4‐HB‐H₂O, 2‐HB‐H₂O and K⁺‐H₂O. d) The relative proportions of three types of water as a function of potential. e) In situ ATR‐IRAS measurements during 30‐min test (at −0.75 V vs. RHE). f) EPR spectra with and without 0.1 M NO₃ ⁻. g) Energy barriers during the processes of H₂O adsorption, dissociation and *H adsorption. h) The adsorption and dissociation process of water over the Co site.

Figure 5

Theoretical calculations and reaction pathway. PDOS spectra of TTA‐TPH‐CuCo (a) before and (b) after NO₃ ⁻ adsorption. c) Differential charge density of TTA‐TPH‐CuCo before and after NO₃ ⁻ adsorption. d) Energy barriers for the formation of *NO₃H and *NHO/*NOH intermediate over TTA‐TPH‐CuCo active sites. e) Free energies of the NO₃RR reaction pathway at Cu site (NO₃ ⁻ to NO₂ ⁻) and Co site (NO₂ ⁻ to NH₃). f) The schematic illustration of the tandem mechanism on TTA‐TPH‐CuCo.