In situ evolution of electrocatalysts for enhanced electrochemical nitrate reduction under realistic conditions

Abstract

Electrochemical nitrate reduction to ammonia (ENRA) is gaining attention for its potential in water remediation and sustainable ammonia production, offering a greener alternative to the energy-intensive Haber-Bosch process. Current research on ENRA is dedicated to enhancing ammonia selectively and productivity with sophisticated catalysts. However, the performance of ENRA and the change of catalytic activity in more complicated solutions (i.e., nitrate-polluted groundwater) are poorly understood. Here we first explored the influence of Ca²⁺ and bicarbonate on ENRA using commercial cathodes. We found that the catalytic activity of used Ni or Cu foam cathodes significantly outperforms their pristine ones due to the in situ evolution of new catalytic species on used cathodes during ENRA. In contrast, the nitrate conversion performance with nonactive Ti or Sn cathode is less affected by Ca²⁺ or bicarbonate because of their original poor activity. In addition, the coexistence of Ca²⁺ and bicarbonate inhibits nitrate conversion by forming scales (CaCO₃) on the in situ-formed active sites. Likewise, ENRA is prone to fast performance deterioration in treating actual groundwater over continuous flow operation due to the presence of hardness ions and possible organic substances that quickly block the active sites toward nitrate reduction. Our work suggests that more work is required to ensure the long-term stability of ENRA in treating natural nitrate-polluted water bodies and to leverage the environmental relevance of ENRA in more realistic conditions.

Keywords: Ammonium; Cathodic corrosion; Groundwater; Hardness ions; In situ activation.

Graphical abstract

Fig. 1

Electrochemical nitrate reduction with Ni foam cathode. a, Influence of Ca²⁺ and bicarbonate on nitrate removal over ten-cycle recycling. b–c, XRD patterns (b) and Raman spectrum (c) of fresh and used Ni foam under different ion compositions. d, Evolution of NH₄⁺, NO₂⁻, NO₃⁻, and other nitrogen species in nitrate-only conditions. e–f, LSV curves (e) and Nyquist plots (f) of fresh and used Ni foam under different test conditions, both LSV and EIS were recorded with electrolytes containing 50 mM Na₂SO₄ and 4 mM NaNO₃. g–h, SEM images of fresh (g) and used (h) Ni foam in NO₃⁻ condition. i–j, O 1s and Ni 2p XPS spectra of fresh (i) and used (j) Ni foam in nitrate-only condition. CPE, constant phase element, R_ct, charge transfer resistance, R_s, resistance of bulk solution.

Fig. 2

Influence of ion composition. a–c, The evolution of NH₄⁺, NO₂⁻, NO₃⁻, and other nitrogen species in NO₃⁻ + Ca²⁺ (a), NO₃⁻ + HCO₃⁻ (b), and NO₃⁻ + Ca²⁺ + HCO₃⁻ (c) condition. d–f, SEM images of used Ni foam in NO₃⁻ + Ca²⁺ (d), NO₃⁻ + HCO₃⁻ (e), and NO₃⁻ + Ca²⁺ + HCO₃⁻ (f) condition. g, O 1s and Ni 2p XPS spectra of Ni foam in NO₃⁻ + Ca²⁺, NO₃⁻ + HCO₃⁻, and NO₃⁻ + Ca²⁺ + HCO₃⁻ condition.

Fig. 3

Influence of cathode material. a, Electrochemical nitrate removal with Cu foam, Ti plate, and Sn plate electrodes in nitrate-only solution over a ten-cycle test. b, LSV curves of Cu foam, Ti plate, and Sn plate in the absence or presence of 4 mM NO₃⁻; 50 mM Na₂SO₄ were added as supporting electrolytes. c–e, The determination of double layer capacitance of Cu foam (c), Ti plate (d), and Sn plate (e) under fresh and NO₃⁻ conditions. f–h, XRD patterns of Cu foam (f), Ti plate (g), and Sn plate (h) in nitrate-only condition.

Fig. 4

Joint effects of coexisting ions and cathode material. a–c, Electrochemical nitrate reduction with Cu foam (a), Ti plate (b), and Sn plate (c) in the presence of different ions over a ten-cycle test. d–f, LSV curves of Cu foam (d), Ti plate (e), and Sn plate (f) under different ion compositions. g–i, The Nyquist plots for the EIS spectra of Cu foam (g), Ti plate (h), and Sn plate (i) under different ion compositions. LSV and EIS were collected with electrolytes containing 50 mM Na₂SO₄ and 4 mM NaNO₃. CPE, constant phase element, R_ct, charge transfer resistance, R_s, resistance of bulk solution.

Fig. 5

Long-term performance. a, Nitrate removal efficiency over continuous flow operation mode in treating simulated water, nitrate spiked tap water, and actual nitrate-polluted groundwater. b–c, XRD patterns of used Cu foam in simulated water (b) and groundwater (c). d, Change of Ca²⁺ and Mg²⁺ concentration over tests with nitrate-polluted groundwater. The same Cu foam cathode was used for the tests with synthetic nitrate-containing water. For the tests with nitrate-spiked tap water and actual nitrate-polluted groundwater, new Cu foams were introduced.