Free-standing membrane incorporating single-atom catalysts for ultrafast electroreduction of low-concentration nitrate

Abstract

The release of wastewaters containing relatively low levels of nitrate (NO₃^-) results in sufficient contamination to induce harmful algal blooms and to elevate drinking water NO₃^- concentrations to potentially hazardous levels. In particular, the facile triggering of algal blooms by ultra-low concentrations of NO₃^- necessitates the development of efficient methods for NO₃^- destruction. However, promising electrochemical methods suffer from weak mass transport under low reactant concentrations, resulting in long treatment times (on the order of hours) for complete NO₃^- destruction. In this study, we present flow-through electrofiltration via an electrified membrane incorporating nonprecious metal single-atom catalysts for NO₃^- reduction activity enhancement and selectivity modification, achieving near-complete removal of ultra-low concentration NO₃^- (10 mg-N L^-1) with a residence time of only a few seconds (10 s). By anchoring Cu single atoms supported on N-doped carbon in a carbon nanotube interwoven framework, we fabricate a free-standing carbonaceous membrane featuring high conductivity, permeability, and flexibility. The membrane achieves over 97% NO₃^- removal with high N₂ selectivity of 86% in a single-pass electrofiltration, which is a significant improvement over flow-by operation (30% NO₃^- removal with 7% N₂ selectivity). This high NO₃^- reduction performance is attributed to the greater adsorption and transport of nitric oxide under high molecular collision frequency coupled with a balanced supply of atomic hydrogen through H₂ dissociation during electrofiltration. Overall, our findings provide a paradigm of applying a flow-through electrified membrane incorporating single-atom catalysts to improve the rate and selectivity of NO₃^- reduction for efficient water purification.

Keywords: activity and selectivity improvement; carbonaceous interwoven structure; free-standing electrified membrane; low-concentration nitrate reduction; single-atom catalyst.

Fig. 1.

Characterization of copper single-atom anchored on N-doped carbon (Cu₁/NC) catalysts and free-standing electrified CNT membrane incorporating Cu₁/NC (Cu₁/NC@CNT-FEM). (A) HAADF-STEM image of Cu₁/NC. (B) Normalized Cu K-edge XANES and (C) FT-EXAFS spectra of the Cu₁/NC, copper foil (Cu-Cu reference), and copper phthalocyanine (CuPc, Cu-N reference). (D) Fitting of the Cu₁/NC FT-EXAFS spectrum. Inset is the corresponding K-space spectrum. (E) Schematic illustrating the fabrication procedure of the Cu₁/NC@CNT-FEM. (F) Photographs of the Cu₁/NC@CNT-FEM. The Inset shows the folded membrane. (G) SEM image of the Top view of the Cu₁/NC@CNT-FEM. (H) EDS mapping images (overlapped with C, N, and Cu elements) of the Cu₁/NC@CNT-FEM. White circles indicate the areas where the Cu₁/NC are visible in G. SEM images of the cross-sectional view of the Cu₁/NC@CNT-FEM at (I) low and (J) high magnifications. (K) SEM image depicting the Cu₁/NC bound in the CNT interwoven structure. (L) Water contact angles (Top) and water flux (Bottom) of the Cu₁/NC@CNT-FEM and a free-standing electrified CNT membrane (CNT-FEM) without incorporating Cu₁/NC. (M) EIS spectra of the Cu₁/NC@CNT-FEM, the CNT-FEM, and a Cu₁/NC functionalized ceramic membrane (Cu₁/NC-CM) over a frequency range of 1 to 10⁶ Hz in 10 mM Na₂SO₄ solution.

Fig. 2.

Electrochemical nitrate reduction performance of the Cu₁/NC@CNT-FEM. (A) Schematic illustration of the cross-flow membrane electrofiltration system. The membrane filtration cell contains a feed chamber (orange) with a RuO₂-IrO₂/Ti mesh electrode and a permeate chamber (blue). The Cu₁/NC@CNT-FEM (effective area of 12.6 cm²) and the RuO₂-IrO₂/Ti mesh serve as the cathode and anode, respectively. Experiments were conducted at a cross-flow rate of 200 mL min⁻¹ and a permeate flow rate of 1 mL min⁻¹, resulting in a residence time of 10 s in the membrane. (B) CV curves of the Cu₁/NC and N-doped carbon in different electrolytes (100 mM Na₂SO₄ or 100 mM Na₂SO₄ + 100 mg-N L⁻¹ NaNO₃) using glassy carbon as the support at a scan rate of 20 mV s⁻¹. The dashed line indicates the potential where NO₃⁻ reduction occurs. (C) Effect of current density (0.5 to 2.5 mA cm⁻²) on the distribution of nitrogen species in the permeate (left axis) and N₂ selectivity (right axis) using the Cu₁/NC@CNT-FEM in the electrofiltration system. The Right panel shows the NO₃⁻ reduction results of the membrane in a batch system at a current density of 2.5 mA cm⁻². Both filtration and batch experiments were performed by treating 60 mL feed solution with 10 mg-N L⁻¹ NaNO₃ and 10 mM Na₂SO₄ for 1 h. Error bars represent SDs from triplicate measurements. (D) NO₃⁻ removal efficiency (Top) and loss of Cu (Bottom) of the Cu₁/NC@CNT-FEM as a function of filtration cycles. After operating for 2 h, the membrane was rinsed and dried and then used for the next cycle.

Fig. 3.

Mechanism investigation for electrochemical nitrate reduction using the Cu₁/NC@CNT-FEM. (A) Effect of modifying the relative contributions of various hydrogen species, including hydrogen ion (H⁺), hydrogen gas (H₂), and atomic hydrogen (H*), on nitrogen speciation (left axis) and N₂ selectivity (right axis) using the Cu₁/NC@CNT-FEM. Investigation of the H⁺, H₂, and H* species was performed by lowering the feed solution pH to 3.0, applying surface flushing to remove the electrogenerated hydrogen bubbles, and injecting t-BuOH (200 mM) into the feed for radical quenching, respectively. The Left (green shading) and Right (gray shading) panels show the nitrogen reduction results obtained using the Cu₁/NC@CNT-FEM or a CNT-FEM incorporating N-doped carbon without Cu₁ doping under electrofiltration conditions. Experiments were performed in the filtration system using a feed solution with 10 mg-N L⁻¹ NaNO₃ and 10 mM Na₂SO₄ at a current density of 2.5 mA cm⁻². Error bars represent SDs from triplicate measurements. (B) EPR spectra of the electrofiltration system at different voltages using DMPO (50 mM) as the spin-trapping agent. −3.5 V and −1.5 V (vs. Ag/AgCl) represent the voltages with and without significant contributions from the hydrogen evolution reaction, respectively, during the NO₃RR. Asterisks (*) indicate the characteristic peaks of H*. Mass density distributions of NO under different collision frequency distributions, estimated through molecular dynamics simulation, when the molecules (C₁) transport to the surface and (C₂) penetrate through the surface, where the surface possesses a Cu−N₄ moiety-doped carbon (Cu−N₄/C) structure. Low (representing flow-by mode) and high (representing flow-through mode) collision frequency distributions were determined considering collision probability mean values of 0.4 and 0.8, respectively.

Fig. 4.

Proposed mechanisms for highly efficient and selective electrochemical nitrate reduction using the Cu₁/NC@CNT-FEM. (A) Schematic describing the structural characteristics and properties of the free-standing electrified membrane (EM) during single-pass electroreduction of NO₃⁻, including the immobilization of catalysts in the CNT interwoven structure and advection-enhanced mass transport. (B) Schematic illustrating the mechanism for achieving highly efficient and selective NO₃⁻ reduction to N₂ under flow-through electrofiltration, including enhancing the adsorption of NO and matching the supply of H* through H₂ dissociation.