Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu2O Nanocubes: Active Species and Reaction Mechanisms

Abstract

The electrochemical reduction of nitrate (NO₃^-) and nitrite (NO₂^-) enables sustainable, carbon-neutral, and decentralized routes to produce ammonia (NH₃). Copper-based materials are promising electrocatalysts for NO_x^- conversion to NH₃. However, the underlying reaction mechanisms and the role of different Cu species during the catalytic process are still poorly understood. Herein, by combining quasi in situ X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption spectroscopy (XAS), we unveiled that Cu is mostly in metallic form during the highly selective reduction of NO₃^-/NO₂^- to NH₃. On the contrary, Cu(I) species are predominant in a potential region where the two-electron reduction of NO₃^- to NO₂^- is the major reaction. Electrokinetic analysis and in situ Raman spectroscopy was also used to propose possible steps and intermediates leading to NO₂^- and NH₃, respectively. This work establishes a correlation between the catalytic performance and the dynamic changes of the chemical state of Cu, and provides crucial mechanistic insights into the pathways for NO₃^-/NO₂^- electrocatalytic reduction.

Figure 1

Characterization and electrocatalytic NO₃RR and NO₂RR performances of Cu₂O NCs. (a) TEM image of the as prepared Cu₂O NCs. (b) Linear scan voltammograms (LSV) of Cu₂O NCs without adding NO₃^–/NO₂^– sources (black) and in the presence of NaNO₃ (red) and NaNO₂ (blue), respectively, using a scan rate of 50 mV/s. (c,d) Potential-dependent FEs for NH₃/NO₂^– production during 2 h chronoamperometric NO₃RR (c) and NO₂RR (d). The error bars represent the standard deviation from three independent experiments. Experimental conditions: 0.1 M Na₂SO₄ electrolyte (pH 12) + 8 mM NaNO₃ or NaNO₂.

Figure 2

Quasi in situ and postreaction ex situ characterization of Cu₂O NCs after NO₃RR. (a) Quasi in situ XPS Cu LMM AES of Cu₂O NCs after NO₃RR (1 h) at different potentials. (b,c) Ex situ GI-PXRD of Cu₂O NCs before and after NO₃RR at 0.10 V_RHE (b) and −0.30 V_RHE (c). TEM of Cu₂O NCs after NO₃RR at 0.10 V_RHE (d) and −0.30 V_RHE (e). Electrolyte: 0.1 M NaSO₄ + 8 mM NaNO₃, pH 12.

Figure 3

Operando XAS data for Cu₂O NCs during NO₃RR. (a) Normalized Cu K-edge XANES spectra collected at OCP, 0.10 V_RHE and −0.30 V_RHE. (b) Linear combination analysis (LCA) results of the XANES spectra, showing the variation of the Cu oxidation state. The full squares indicate the time-dependent FE(NO₂^–) (red) and FE(NH₃) (blue) obtained by NO₃RR electrolysis experiments. The first data point is measured at 0.10 V_RHE for 2 h, while the other data points are measured at −0.30 V_RHE with the corresponding reaction times. The error bars are calculated based on three independent experiments. (c) Fourier-transformed (FT) Cu K-edge EXAFS spectra of Cu₂O NCs at OCP, 0.10 V_RHE, and −0.30 V_RHE. (d) EXAFS fitting results showing the evolution of Cu–Cu and Cu–O coordination numbers. The error bars indicate the fitting uncertainty. Electrolyte: pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃.

Figure 4

Electrokinetic analysis of NO₃RR and NO₂RR in alkaline conditions. (a) Tafel plots of Cu₂O NCs at different pH (11.5–12.9) derived from staircase voltammetry. Electrolyte: 0.1 M Na₂SO₄ with 8 mM NaNO₃ at different pH values. (b) Scan rate-dependent CVs of Cu₂O NCs at pH 12.0. Typical reductive peaks are labeled. (c) Rotating rate dependent LSVs of Cu₂O NCs on rotating disk electrodes at pH 12.0. (d) Tafel plots of Cu₂O NCs at different pH (11.5–12.9) derived from staircase voltammetry. Electrolyte: 0.1 M Na₂SO₄ with 8 mM NaNO₂ with different pH values.

Figure 5

In situ Raman spectroscopy data of Cu₂O NCs. (a) Enlarged in situ Raman spectra during NO₃RR in the range of 950–1650 cm^–1 Raman shift, with the applied potentials ranging from the 0.1 to −0.3 V_RHE. Full spectra are shown in Figure S49a, Supporting Information. Electrolyte: pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃. The NO₃^–, NO₂^–, NH₂OH and NH₃ related peaks are labeled with red, violet, orange, and blue dotted lines, respectively. The asterisk denoted the atoms adsorbed on the catalyst surface. Peaks at 983 and 1065 cm^–1 correspond to SO₄^2– and CO₃^2– in solution (labeled with a black dotted line),^, respectively, as they always appeared in the electrolyte containing Na₂SO₄ (pH 12). (b) In situ Raman spectra in the range of 1100–1250 cm^–1 during NO₃RR (left, pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃), NO₂RR (middle, pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₂), NH₂OH reduction (right, pH 12, 0.1 M Na₂SO₄ + 8 mM NH₂OH). The NH₂OH related peaks are labeled with orange dashed lines. The intensity of NO₃RR Raman spectra is manually increased five times for clarity.

Figure 6

Active species and possible reaction mechanisms. (a) Schematics of the structural and oxidation state change of the catalyst under NO₃RR/NO₂RR conditions. (b) Proposed mechanisms for NO₃RR/NO₂RR to ammonia over Cu₂O NCs. The species that been determined based on experimental evidence (either detected by in situ Raman or deduced from electrokinetic analysis) are in bold. The left side (yellow background) correspond to reaction conditions where Cu(I)/Cu(0) species are present, while on the right-hand side (red background) the catalyst is mostly metallic.