The electrocatalytic NO
3–RR performance of CoSe
2/CC was evaluated in both NO
3–-free and NO
3–-containing 0.1 mol/L alkaline electrolyte. The possible NO
3–RR products, like NH
3, N
2H
4, and NO
2–, were quantified using distinct colorimetric methods, with their respective UV–vis spectra and calibration curves detailed in Figs. S9-S11 (Supporting information). Initial assessment of CoSe
2/CC's NO
3–RR electrocatalytic efficiency employed linear sweep voltammetry (LSV) curves. As depicted in
Fig. 2a, the current density (
j) of CoSe
2/CC significantly increased upon the introduction of 0.1 mol/L NO
3– into the 0.1 mol/L NaOH electrolyte. Comparative LSV tests of pure CC, the precursor Co(OH)F/CC, and CoSe
2 powder/CC under identical conditions (Figs. S12 and S13 in Supporting information) revealed lower
j for both pure CC (–71.2 mA/cm
2), Co(OH)F/CC (–97.6 mA/cm
2), and CoSe
2 powder/CC (–128.0 mA/cm
2), as compared to CoSe
2/CC (–207.6 mA/cm
2) at –1.0 V. We further compared the three catalysts (CoSe
2/CC, CoSe
2/CC-500, and CoSe
2/CC-10) generated under different preparation conditions for NO
3–RR. Preliminarily, it was found that the
j of these three catalysts did not differ much within the alkaline electrolyte with and without NO
3– (Fig. S14 in Supporting information). Chronoamperometry (CA) tests, conducted for 1 h at various potentials ranging from –0.3 V to –0.9 V (
Fig. 2b), further probed CoSe
2/CC's NO
3–RR capabilities. The UV–vis spectra post-CA tests (Fig. S15 in Supporting information) exhibited progressively intensifying absorption peak intensities with increased negative potentials, indicating a preference for NH
3 synthesis at higher negative potentials.
Fig. 2c illustrates that the NH
3 yield of CoSe
2/CC increased consistently with more negative operating potentials, reaching a peak yield of 818.0 µmol h
–1 cm
–2 at –0.9 V. Additionally, the FE curve displayed a volcano-shaped profile, achieving a maximum FE of 97.6% at –0.6 V, correlating with an NH
3 yield of 517.7 µmol h
–1 cm
–2, thereby confirming the superior NO
3–RR performance for NH
3 synthesis. The electrochemical NO
3– reduction to NH
3 capabilities of Co(OH)F/CC, CC, and CoSe
2 powder/CC were compared under identical conditions at –0.6 V (
Fig. 2d and Fig. S16 in Supporting information). The results showed that the NH
3 yields and FEs for Co(OH)F/CC (292.6 µmol h
–1 cm
–2, 65.3%), CC (19.7 µmol h
–1 cm
–2, 23.6%), and CoSe
2 powder/CC (150.1 µmol h
–1 cm
–2, 71.9%) were significantly lower than those of CoSe
2/CC (517.7 µmol h
–1 cm
–2, 97.6%). Also, the NH
3 yields and FEs of CoSe
2–500 (498.0 µmol h
–1 cm
–2, 90.0%) and CoSe
2/CC-10 (438.2 µmol h
–1 cm
–2, 83.4%), prepared under different conditions, were only slightly lower than that of CoSe
2/CC (Fig. S17 in Supporting information). Remarkably, the highest NH
3 yield and FE of CoSe
2/CC surpass those of some recently reported NO
3–RR electrocatalysts (
Fig. 2e and Table S1 in Supporting information). This exceptional NO
3–RR performance of CoSe
2/CC substantiates the role of CoSe
2 as the active phase for efficient and highly selective electrocatalytic conversion of NO
3– to NH
3.