Co(OH)2/Cu NWs with interface modification of hypophosphite ions for sustainable ammonia synthesis from nitrate at ~2.8 A·cm-2

doi:10.1016/S1872-2067(26)65241-2

Abstract

Abstract:

Ammonia production via nitrate electroreduction (NO₃RR) offers an environmentally benign alternative, yet it is still extremely difficult to simultaneously attain superior Faradaic efficiency and long-lasting durability when operating at the high current densities. Herein, we report a self-supporting electrode comprising Cu nanowires uniformly coated with cobalt hydroxide incorporating surface-coordinated hypophosphite ions (H₂PO₂^-) (denoted as P-Co(OH)₂/Cu NWs), directly fabricated on Cu foam. The P-Co(OH)₂/Cu NWs catalyst exhibited remarkable NO₃RR performance in 0.1 mol·L^-1 NaNO₃/1 mol·L^-1 NaOH, producing NH₃ at a rate of 204.3 mg·h^-1·cm^-2 (-1.0 V) with a peak Faradaic efficiency of 96.8% (-0.7 V), both potentials referenced to RHE. Notably, the electrode maintains stable electroactivity and NH₃ selectivity over 20 consecutive test cycles (1 h per cycle) under approximately 2.8 A·cm^-2. In-situ attenuated total reflection Fourier transform infrared spectroscopy reveals that the P-Co(OH)₂/Cu NWs electrode promotes enhanced deoxygenation of *NO₃ and deep hydrogenation of *NO_x intermediates. Cyclic voltammetry test and In-situ Raman spectroscopy demonstrate that the incorporated H₂PO₂^- induce and stabilize the in-situ formation of metallic Co⁰ phase under operative electrolysis conditions. Density functional theory calculations further elucidate that the metallic Co⁰ phase, generated in situ during NO₃RR, optimizes the adsorption energetics of *NOH, thereby substantially facilitating the energy-demanding step (*NO → *NOH) relative to Co(OH)₂, while concurrently strengthening *H adsorption. This work presents a viable electrode design strategy for securing efficient and stable electrocatalytic NO₃^--to-NH₃ conversion under high-rate operation.

Key words: Nitrate reduction, Industrial current density, Hypophosphite ion, Ammonia, Interface modification

Ruihan Zhou, Tixuan Xia, Yingchen Yang, Dongbo Xu, Weidong Shi. Co(OH)₂/Cu NWs with interface modification of hypophosphite ions for sustainable ammonia synthesis from nitrate at ~2.8 A·cm^-2[J]. Chinese Journal of Catalysis, 2027, 92: 365-375.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65241-2

https://www.cjcatal.com/EN/Y2027/V92/I1/365

Figures/Tables 5

Fig. 1. (a) Schematic diagram depicting the preparation of P-Co(OH)2. SEM image of Cu(OH)2 NWs (b), Cu NWs (c), and P-Co(OH)2/Cu NWs (d). EDS mapping images (e), TEM images (f), and HRTEM images (g) of P-Co(OH)2/Cu NWs.

Fig. 2. XRD pattern (a), CV curves (b), Co 2p (c), O 1s (d) XPS spectra, and FT-IR spectra (e) of P-Co(OH)2/Cu NWs and Co(OH)2/Cu NWs.

Fig. 3. The potential-dependent FE of ammonia (a), The ammonia yield rate (b), and the selectivity of NO2- (c) for Cu NWs, Co(OH)2/Cu NWs, P-Co(OH)2/Cu NWs and P-Co(OH)2/CC in 0.1 mol·L-1 NaNO3 and 1 mol·L-1 NaOH. (d) NO3- removal of catalysts measured in a 1 mol·L-1 NaOH with 0.1 mol·L-1 NO3- electrolyte at -0.7 V vs. RHE. (e) The NH3 yield rate and FE of P-Co(OH)2/Cu NWs at -1 V vs. RHE in cycling tests conducted continuously for 20 h. (f) 1H NMR spectra using 15NO3- and 14NO3- over P-Co(OH)2/Cu NWs. (g) comparison of NO3RR performance of P-Co(OH)2/Cu NWs with reported state-of-the-art electrocatalysts in 0.1 mol·L-1 NO3-. (h) comparison of NH3 yield rate for recently reported state-of-the-art electrocatalysts with different concentrations of NO3-.

Fig. 4. (a,b) CV curves in 1 mol·L-1 NaOH. (c) CV curves in 1 mol·L-1 NaOH + 0.1 mol·L-1 NaNO3. In-situ Raman spectra (d,e), in-situ ATR-IR spectra (f,g), and EPR spectra (h,i) in 1 mol·L-1 NaOH with and without 0.1 mol·L-1 NaNO3 of P-Co(OH)2/Cu NWs, Co(OH)2/Cu NWs and Cu NWs.

Fig. 5. (a) Gibbs free energy diagram of NO3- to NO2- generated during NO3RR. (b) Adsorption energy of NO3- and NO2-. (c) The hydrogen evolution reaction pathways on Cu (111), Co(OH)2 (101), and Co(101). (d) Gibbs free energy diagram of NO2- to NH3 generated during NO3RR. (e) PDOS along with d-band center for Co(OH)2 and Co.

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Co(OH)₂/Cu NWs with interface modification of hypophosphite ions for sustainable ammonia synthesis from nitrate at ~2.8 A·cm^-2

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