Sustainable photo-assisted electrocatalysis of struvite fertilizer via synchronous redox catalysis on bifunctional yolk-shell Cu2O@NiFe2O4 Z-scheme nanoreactor

doi:10.1016/S1872-2067(25)64755-3

Abstract

Abstract:

In this study, we developed a tandem photo-assisted electrochemical (PA-EC) chemical strategy for both energy-saving ammonia/fertilizer synthesis and comprehensive nitrogen- and phosphorus-rich wastewater treatment, in which synchronous hypophosphite ion (H₂PO₂^-) oxidation to phosphate ion (PO₄^3-) (POR) and nitrate reduction (NO₃RR) to ammonia (NH₃) occur, followed by cascade chemical precipitation to generate struvite. Herein, a bifunctional Cu₂O@NiFe₂O₄ Z-scheme heterojunction with a yolk/shell structure and oxygen vacancies (O_Vs) was designed and developed to optimize the NO₃RR/POR. Serving as a key component, the established PA-EC system consisted of a Janus Cu₂O@NiFe₂O₄/NF self-supporting integrated photocathode and a Cu₂O@NiFe₂O₄/NF photocathode with efficient struvite PA-EC synthesis performance under a low cell voltage of 1.6 V vs NHE. Specifically, Janus Cu₂O@NiFe₂O₄/NF photocathode exhibits superior performance with a high NH₃ yield of 38.06 mmol L^-1 and a faradaic efficiency (FE) of 92.31% at 1.6 V vs. NHE and enables ammonia FE over 60% in a broad NO₃^- concentration window of 0.005-0.5 mol L^-1. The photoassisted electrochemical catalytic mechanism and reaction pathway for struvite synthesis on Cu₂O@NiFe₂O₄ were investigated through a series of experiments and theoretical calculations. The results demonstrated the critical roles of the interfacial electric field, void confinement, and oxygen vacancies in promoting the overall catalytic efficiency. These encouraging results warrant further studies on combined P and N recovery for efficient production of valuable fertilizers.

Key words: Nitrate reduction, Hypophosphite oxidation, Struvite photo-assisted electrocatalysis, Nutrient recovery, Wastewater treatment

Yan Sun, Lei Zhang. Sustainable photo-assisted electrocatalysis of struvite fertilizer via synchronous redox catalysis on bifunctional yolk-shell Cu₂O@NiFe₂O₄ Z-scheme nanoreactor[J]. Chinese Journal of Catalysis, 2025, 76: 230-241.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64755-3

https://www.cjcatal.com/EN/Y2025/V76/I9/230

Figures/Tables 10

Scheme 1. Schematic illustration of struvite fertilizer synthesis and utilization.

Scheme 2. Schematic diagram of the synthesis process for the self-supporting Cu2O@NiFe2O4/NF integrated photoelectrode.

Fig. 1. SEM images of Cu2O/NF (a1), Cu2O@NiFe2O4/NF (b1), and NiFe2O4/NF (c1). SEM images of Cu2O (a2), Cu2O@NiFe2O4 (b2) and NiFe2O4 (c2). TEM images of Cu2O (a3), Cu2O@NiFe2O4 (b3-b5) and NiFe2O4 (c3). (b5) EDS mapping of Cu2O@NiFe2O4.

Fig. 2. (a) XRD patterns of catalysts. High-resolution XPS spectra of Ni 2p (b), Fe 2p (c), Cu 2p (d), and O 1s (e). (f) Tauc’s bandgap plots. (g) Mott-Schottky plots. (h) Schematic illustration of space-charge region at the contact zone of Cu2O and NiFe2O4 heterojunction.

Fig. 3. (a) LSVs of catalysts in 50 mmol L-1 NaNO3 and 0.5 mol L-1 Na2SO4. (b) PL spectra. (c) Transient photocurrent responses of catalysts. (d) NH3 yield. (e) EIS Nyquist plots. (f) Catalysts with different hollow void; the OCP change of Cu2O@NiFe2O4 with different hollow void in 0.5 mol L-1 Na2SO4 and 0.5 mol L-1 Na2SO4 + 50 mmol L-1 NaNO3 solution. (g) Yield of NH3 of photocathode with different sprayed times. (h) CA of photocathode. (i) NH3 yield and FE of NO3RR for Janus Cu2O@NiFe2O4/NF photocathode at different potentials.

Fig. 4. (a) Time-dependent concentration changes of NO3--N, NH3-N, and NO2--N by Janus Cu2O@NiFe2O4/NF photocathode (b) YieldNH3 and FENH3 of Janus Cu2O@NiFe2O4/NF photocathode with concentrations of NO3- ranging from 5 to 0.5 mol L-1. (c) Stability test of Janus Cu2O@NiFe2O4/NF photocathode for 10-h continuous NO3RR. (d) Comparison of the NO3RR performance of the Janus Cu2O@NiFe2O4/NF with other catalysts reported in the literature.

Fig. 5. (a) The logarithm of jNH3 at the potential of -0.8 V vs. NHE as a function of the logarithm cNO3- on the Cu2O, NiFe2O4, Cu2O@NiFe2O4. Finite element simulation result: Surface NO3- density distribution (b); Electric field distribution (c); Surface potential distribution (d). Around the yolk-shell model (left); the nanocube model (right).

Fig. 6. (a) Double layer capacitance for the Cu2O, NiFe2O4 and Cu2O@NiFe2O4 electrodes with/without 50 mmol L-1 NaNO3 in 0.5 mol L-1 Na2SO4 electrolyte based on the CVs at various scan rates. (b) Zeta potentials of Cu2O, NiFe2O4 and Cu2O@NiFe2O4. (c) FTIR of Cu2O@NiFe2O4 before and after illumination. (d) EPR spectra of the solutions obtained after 10 min of photocatalysis by Cu2O@NiFe2O4 at light or dark using DMPO as the *H-trapping reagent. (e) The evolutions of NO2- along with the reaction time during NO3RR conducted by different electrodes (left), yieldNH3 of NO2RR for Janus Cu2O@NiFe2O4/NF photocathode (right). (f) Koutecky-Levich plots of Cu2O@NiFe2O4.

Scheme 3. Schematic illustration of the role of IEF and confinement effect for NO3RR.

Fig. 7. (a) LSVs of Cu2O, NiFe2O4 and Cu2O@NiFe2O4 for POR. (b) Conversion rate of H2PO2− at various voltages. (c) i-t curve and conversion rate of H2PO2− over Cu2O@NiFe2O4 electrode for consecutive cycles at 2.2V vs. NHE. (d) Schematic illustration of the PEC system for NO3RR and the POR. (e) LSV curves of the Cu2O@NiFe2O4 electrode as the cathode and anode in NO3RR||POR and NO3RR||OER. (f) NH3 yield rate, NH3 FE and H2PO2− conversion rate of the Cu2O@NiFe2O4 electrode in NO3RR||POR at various voltages. (g) XRD of the N-P precipitates. (h) Effect of MgNH4PO4 on soybean growth.

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Sustainable photo-assisted electrocatalysis of struvite fertilizer via synchronous redox catalysis on bifunctional yolk-shell Cu₂O@NiFe₂O₄ Z-scheme nanoreactor

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