Levitation-guided disorder engineering unlocks efficient nitrite-to-ammonia electroconversion

Abstract

Electrocatalytic conversion of nitrite (NO₂⁻) to ammonia (NH₃) via the NO₂⁻ reduction reaction (eNO₂RR) presents a promising approach. Prussian blue analog (PBA)–based electrocatalysts are potential candidates for eNO₂RR owing to their good activity and selectivity. Herein, to the best of our knowledge, for the first time, we report a facile synthesis of a flower-like copper (Cu)–cobalt (Co) PBA sulfide (CuCoPBA-S) using pulsed laser irradiation in liquid and investigate its formation mechanism using acoustic levitation coupled with in situ Raman spectroscopy. This approach enables contaminant-free, rapid, and cost-effective synthesis of electrocatalysts. The sulfurization process is shown to be time-dependent in the formation of ordered/disordered flower-like CuCoPBA-S structures. CuCoPBA-S considerably influences the eNO₂RR, achieving a NH₃ faradaic efficiency (FE) of 80.91% and an NH₃ yield rate of 3394.1 μg h⁻¹ cm⁻² at a fixed potential of −0.5 V vs. the reversible hydrogen electrode (RHE). Moreover, density functional theory analysis validates the eNO₂RR pathway facilitated by CuCoPBA-S during the electrocatalytic conversion of NO₂⁻ to NH₃, and the rate-determining step in the pathway is the hydrogenation of *NH₂O to *NH₂OH.

Keywords: Prussian blue analog; Acoustic levitation; Nitrite reduction; Electrocatalysis; Ammonia synthesis.

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1 Introduction

Nitrate and nitrite (NO₂⁻) are water contaminants that pose a serious threat to daily life owing to their adverse effect of blocking oxygen transport in blood vessels.^1,2 Hence, their removal from water bodies is essential, though the process typically demands substantial energy. A sustainable approach that removes nitrate/NO₂⁻ through a catalytic process while simultaneously generating energy offers a versatile solution aligned with Sustainable Development Goal 6: Clean Water and Sanitation and Sustainable Development Goal 7: Affordable and Clean Energy, as set by the United Nations. The ultimate objective of wastewater treatment is the complete reduction of nitrate/NO₂⁻ to value-added products such as nitrogen (N₂) or ammonia (NH₃). Photocatalytic and electrocatalytic approaches have been widely explored for nitrate and NO₂⁻ reduction. Among these, conversion of nitrate to NH₃ via the electrocatalytic nitrate reduction reaction and NO₂⁻ to NH₃ via the electrocatalytic NO₂⁻ reduction reaction (eNO₂RR) have become viable options for electrochemists, enabling the production of NH₃ under ambient conditions.^3,4 Although nitrate is more abundant in natural and industrial environments, NO₂⁻ plays a critical role as a key intermediate in the nitrate reduction pathway and is frequently present in wastewater streams originating from agricultural runoff, biological nitrification–denitrification processes, and industrial effluents. Under incomplete denitrification conditions, NO₂⁻ can accumulate and persist at appreciable concentrations, making its selective removal both environmentally important and necessary to prevent secondary pollution.

NH₃ is an essential feedstock for the manufacture of fertilizers, pharmaceuticals, and other industrial chemicals.^5–7 Currently, large-scale NH₃ production relies predominantly on the Haber–Bosch process, which is associated with high energy consumption.⁸ Although the catalytic nitrogen reduction reaction provides an alternative route to NH₃ synthesis, it is limited by the low solubility of N₂ in aqueous electrolytes and the high dissociation energy of the N≡N bond (∼914 kJ mol⁻¹). In contrast, the electrocatalytic nitrate reduction reaction and eNO₂RR offer more feasible pathways for NH₃ production. Notably, NO₂⁻ is a more readily reducible nitrogen source than dinitrogen, enabling lower overpotentials and higher reaction efficiencies. Our focus on NO₂⁻ reduction is motivated by both mechanistic and practical considerations. From a fundamental perspective, eNO₂RR involves fewer proton-coupled electron transfer steps than nitrate reduction, enabling clearer identification of intrinsic catalytic activity and reaction pathways. This makes NO₂⁻ an ideal model substrate for probing catalytically active sites and reaction kinetics, which can inform the rational design of catalysts for the more complex nitrate reduction process. From an application standpoint, although large-scale NH₃ production directly from NO₂⁻ may be limited by feedstock availability, NO₂⁻-containing streams are commonly encountered in decentralized and intermediate stages of nitrogen treatment systems, where localized NH₃ production could be feasible. Furthermore, mechanistic insights gained from eNO₂RR are directly transferable to nitrate reduction systems, as NO₂⁻ is an essential intermediate in the overall nitrate-to-NH₃ conversion pathway. Thus, eNO₂RR provides a dual benefit: effective removal of toxic nitrogen-containing contaminants from water and simultaneous NH₃ generation, making it a key focus in sustainability research. In this context, eNO₂RR presents a feasible and low-cost process for NH₃ production.⁹ This approach integrates wastewater remediation with value-added chemical production and supports the development of a circular nitrogen economy. Despite these advantages, eNO₂RR still faces limitations, including low yield, limited selectivity, and the high cost of catalysts.¹⁰ Therefore, the rational design of efficient and economical electrocatalysts is critical for improving eNO₂RR performance.¹¹

Consequently, nanostructured catalysts with high electrical conductivity, catalytic activity, stability, and selectivity are essential prerequisites for efficient eNO₂RR.¹² Noble metal catalysts, such as platinum (Pt),¹³ gold (Au),¹⁴ silver (Ag),¹⁵ ruthenium (Ru),¹⁶ palladium (Pd),¹ and iridium (Ir),¹⁷ are suitable candidates with high eNO₂RR selectivity and NH₃ product yield. However, owing to the high cost of noble metals, developing low-cost catalysts for the eNO₂RR process is imperative. Tremendous efforts have been made for developing non-noble metal catalysts, such as copper (Cu), cobalt (Co), nickel (Ni), and iron (Fe). Among these, Cu and Co have received much attention owing to their excellent selectivity toward eNO₂RR, strong activity, and high faradaic efficiency (FE). The hydrogen evolution reaction (HER) is a competing process that adversely affects the eNO₂RR activity of the electrocatalyst. Cu/Co-based electrocatalysts exhibit low or negligible HER activity and are therefore highly favored for eNO₂RR.¹⁸ Natural abundance, high NO₂⁻ selectivity, and fast reaction kinetics further enhance the appeal of Cu/Co-based catalysts.¹⁹ In this context, the synthesis of Cu/Co-based electrocatalysts with nanostructured morphologies offers a cost-effective and viable approach for synthesizing catalysts. Recent studies highlight the effectiveness of nanostructured Cu/Co-based catalysts. For example, isolated Cu atoms anchored on MnO₂ achieved an NH₃ yield rate (YR (NH₃)) of 439.8 μmol h⁻¹ cm⁻² with a FE (NH₃) of 93.3%.²⁰ Wang et al. reported an indium (In)–Cu single-atom alloy (In₁Cu) catalyst that delivered an YR (NH₃) of 1042.1 μmol h⁻¹ cm⁻² and a FE (NH₃) of 94.7% at 354 mA cm⁻².²¹ Cobalt metallopeptide catalysts have also demonstrated eNO₂RR performance in aqueous buffers near neutral pH, achieving an FE (NH₃) of 90 ± 3%,¹⁸ while cobalt sulfide (CoS₂) nanorods supported on nickel foam reached an FE (NH₃) of 97.9% and selectivity of 98.7%.²² A Pd/CuO electrocatalyst in 0.1 M K₂SO₄ containing 0.01 M KNO₂ achieved a YR (NH₃) of 906.4 μg h⁻¹ cm⁻² at −0.84 V vs. RHE.²³ The Co@JDC composite in 0.1 M NaOH with 0.1 M NO₂⁻ electrolyte delivered a high YR (NH₃) of 2.8 ± 0.1 mol h⁻¹ g_Co⁻¹ at −1.0 vs. RHE.²⁴ Cu@TiO₂/TP in 0.1 M Na₂SO₄ containing 0.1 M NO₂⁻ achieved a YR (NH₃) of 760.5 μmol h⁻¹ cm⁻² at −0.8 V vs. RHE.²⁵ Other representative catalysts include Cu₃P in 0.1 M PBS with 0.1 M NaNO₂, which delivered a YR (NH₃) of 1626.6 ± 36.1 μg h⁻¹ cm⁻² at −0.5 V vs. RHE,²⁶ and CoB in 0.1 M Na₂SO₄ with 400 ppm NO₂⁻, which achieved a YR (NH₃) of 233.1 μmol h⁻¹ cm⁻² at −0.7 V vs. RHE.²⁷ Additionally, a CoP electrocatalyst demonstrated a YR (NH₃) of 2260.7 ± 51.5 μg h⁻¹ cm⁻² at −0.2 V vs. RHE in 0.1 M PBS containing 500 ppm NO₂⁻ electrolyte.²⁸

Prussian blue and Prussian blue analogs (PBAs), members of the metal–organic framework family, have received great attention in electrocatalysis owing to their peculiar characteristics such as good electronic conductivity, excellent electrocatalytic activity, strong electrochemical stability, and low cost.^29,30 The structural tunability of PBAs is a fascinating feature that enables their application in eNO₂RR. For instance, PBA-derived high-entropy oxides have been reported as efficient eNO₂RR electrocatalysts, achieving a YR (NH₃) of 894.3 μg h⁻¹ cm⁻² and a FE (NH₃) of 88.9% at −1.0 V vs. Ag/AgCl.³¹ While Cu/Co-based catalysts have been extensively studied for NO₃RR, their application in eNO₂RR remains comparatively underexplored. Xin et al. demonstrated a bimetallic Cu@Co-MOF-74 catalyst for NO₃RR with a YR (NH₃) of 307.98 mmol h⁻¹ g_cat⁻¹ and a FE (NH₃) of 84.01% at −0.88 V vs. RHE.³² He et al. reported a Cu–Co binary sulfide catalyst enabling high-rate tandem electroreduction to NH₃, achieving a YR (NH₃) of 1.17 mmol cm⁻² h⁻¹ in 0.1 M NO₃⁻ at −0.175 V vs. RHE.³³ Similarly, a CoCuO_x@CuO_x/Cu foam catalyst delivered an YR (NH₃) of 519.1 μg h⁻¹ cm⁻² with an FE (NH₃) of 99.83% in 1 mM NO₃⁻ electrolyte at −0.2 V vs. RHE.³⁴

The synthesis methods of catalysts for the electroconversion of NO₂⁻ to NH₃ include high-temperature pyrolysis,³⁵ hydrothermal/solvothermal method,^36,37 and pulsed laser deposition.³⁸ Pulsed laser irradiation in liquid (PLIL) has emerged as a green route for synthesizing catalyst materials in a short processing time in an environmentally friendly manner.^39–41 PLIL is a simple process for producing high-yield materials.^42,43 Electrocatalysts prepared via PLIL have shown remarkable versatility and efficiency for converting nitrogen-containing compounds to NH₃. Acoustic levitation coupled with PLIL is a versatile method that enables free-standing material synthesis through the formation of single droplets.^44,45 A key advantage of this method is the elimination of the possible interactions of chemical agents with solid surfaces or boundary effects.^46,47 Electrocatalysts can easily be synthesized in an ultraclean and confined environment that allows in situ analysis using characterization tools such as ultraviolet-visible (UV-vis) or Raman spectroscopy.⁴⁸ The simultaneous synthesis and characterization is time-saving and has attracted considerable research interest, especially in electrocatalysis.

The electrocatalytic conversion of NO₃⁻ to NH₃ proceeds through an eight-electron transfer pathway, whereas eNO₂RR involves a six-electron transfer process and remains comparatively less explored than NO₃RR.⁴⁹ This knowledge gap has motivated us to investigate the catalytic activity and reaction mechanisms of eNO₂RR using hierarchical Cu₃[Co(CN)₆]₂ (denoted as CuCoPBA) cubes and flower-like CuCoPBA sulfide (CuCoPBA-S) catalysts. Herein, we report the synthesis of CuCoPBA-S via a two-step process involving co-precipitation, followed by PLIL acoustic levitation coupled with PLIL. During the formation of CuCoPBA-S, a time-dependent sulfurization was observed and its mechanism was studied. CuCoPBA cubes transformed into a flower-like ordered/disordered CuCoPBA-S heterostructure during sulfurization. The sulfurization mechanism in CuCoPBA was investigated through in situ Raman spectroscopy. Furthermore, the electrocatalytic performance of CuCoPBA-S toward eNO₂RR was evaluated in alkaline electrolyte (1.0 M KOH) both in the absence and presence of 0.1 M KNO₂. The alkaline medium was selected because it enhances reaction kinetics, stabilizes key nitrogen-containing intermediates, suppresses competing side reactions such as HER, and provides high ionic conductivity. The YR (NH₃) during the eNO₂RR was quantified using UV-vis spectroscopy using the Indophenol blue method and validated via a ¹⁴NH₄⁺ isotope–labeling study. Density functional theory (DFT) analysis was used to evaluate the effect of structurally-oriented active sites in ordered/disordered CuCoPBA-S on the eNO₂RR activity. This study highlights the novel use of acoustic levitation coupled with PLIL to elucidate the formation mechanism of the ordered/disordered CuCoPBA-S heterostructure using in situ Raman spectroscopy.

2 Results and discussion

2.1 Microstructure and surface morphology

The coprecipitation method is a versatile method for synthesizing metal nanostructures with desired morphologies and sizes. We synthesized CuCoPBA using a facile coprecipitation method, which was further converted to CuCoPBA-S through photoreduction via the PLIL method. Fig. 1a shows a schematic diagram of synthesizing CuCoPBA-S using CuCoPBA by PLIL method. Laser irradiation induces the sulfurization of CuCoPBA, resulting in the formation of flower-like CuCoPBA-S. The growth of the flower-like CuCoPBA-S differs from CuCoPBA, which exhibits a nanocube morphology. Fig. 1b shows the Fourier transform infrared (FTIR) spectra of CuCoPBA and CuCoPBA-S. In the spectrum of CuCoPBA-S, the prominent peaks centered at ∼3428, 3312, and 3202 cm⁻¹ correspond to cobaltpentlandite (Co₉S₈). The peak positioned at ∼2139 cm⁻¹ is assigned to copper(I)thiocyanate (CuSCN). The presence of Co₉S₈ and Cu₂S is further confirmed by peaks positioned at ∼1617 and 1401 cm⁻¹, and ∼1096 and 700 cm⁻¹, respectively. Fig. 1c shows the Raman spectra of CuCoPBA and CuCoPBA-S. In the spectrum of CuCoPBA, the prominent peaks centered at 187.5 and 264.3 cm⁻¹ are attributed to Cu–CN–Co deformation vibrations, whereas those at 489.3 and 524.2 cm⁻¹ correspond to Cu–C and Co–C stretching vibrations.⁵⁰ The Raman spectrum of CuCoPBA-S exhibits peaks corresponding to the vibrations associated with Cu–S (177.8 cm⁻¹), Cu–C (489.3 cm⁻¹), Co–S (711.6 cm⁻¹), and CuSCN (2161.6 cm⁻¹). Although with low intensity, peaks positioned at ∼1399 and 1418 cm⁻¹ are assigned to the stretching vibrations of Cu–N and Co–N, respectively. The Raman spectrum of CuCoPBA-S also exhibits characteristic peaks of pure sulfur powder in the range of 100–400 cm⁻¹, associated with the vibration of the S–S bond in the S8 species.⁵¹ This shows that a small amount of pure sulfur is formed during the photoreduction process, which is further supported by X-ray diffraction (XRD) analysis. Fig. S1 shows the XRD pattern of CuCoPBA and CuCoPBA-S. The XRD pattern of CuCoPBA exhibits major peaks positioned at 2θ values of 17.6°, 25.0°, 35.7°, 40.1°, 44.1°, 51.3°, 54.7°, and 57.9°, corresponding to (200), (220), (400), (420), (422), (440), (600), and (620), respectively (PDF#: 02-0169). After the formation of CuCoPBA-S, these peaks exhibit slight shifts. The XRD pattern of CuCoPBA-S appears complex owing to its ordered/disordered crystal structure, in which sulfurization results in the formation of individual sulfide phases on the PBA network, which is driven by the differences in their formation energies. Although the individual sulfide phases are structurally ordered, the entire CuCoPBA-S crystal structure is considered disordered. To quantitatively resolve the phase composition and structural characteristics of CuCoPBA-S, detailed Rietveld refinement analysis was performed. The refined XRD pattern of CuCoPBA-S is shown in Fig. S2. In addition to the parent CuCoPBA phase (PDF# 02-0169), the refinement confirms the formation of multiple sulfide-related phases, including CuSCN (PDF# 22-0253), Cu₂S (PDF# 12-0174), Co₉S₈ (PDF# 02-0169), and elemental sulfur (PDF# 08-0247). The presence of these phases substantiates the heterogeneous sulfurization process and supports the formation of an ordered/disordered CuCoPBA-S heterostructure. The refined lattice parameters, phase fractions, and reliability factors obtained from the Rietveld analysis are summarized in Table S1, providing quantitative validation of the phase evolution in CuCoPBA-S. The photoreduction of thiourea yields a small amount of elemental sulfur, which is unavoidable during the processing. The XRD analysis reveals that the sulfurization process forms various sulfide compounds, such as CuSCN, Co₉S₈, and Cu₂S, within a PBA network. Fig. 1d shows the field-emission scanning electron microscopy (FESEM) images of CuCoPBA nanocubes at two different magnifications, along with SEM–energy-dispersive X-ray spectroscopy (EDS) elemental mapping images for N, C, Cu, and Co. Pulsed laser irradiation envisages the formation of CuCoPBA-S, with thiourea serving as the sulfur source. Fig. 1e shows the FESEM image of flower-like CuCoPBA-S and the corresponding FESEM–EDS elemental mapping images for S, N, C, Cu, and Co. The FESEM–EDS elemental mapping confirmed the even distribution of sulfur throughout the material. The flower-like morphology of CuCoPBA-S, observed in the FESEM image, results from the sulfurization of CuCoPBA, aided by thiourea as the sulfur precursor. Fig. 1f presents high-resolution transmission electron microscopy (HRTEM) images at different magnifications, verifying the flower-like morphology of CuCoPBA-S. High-magnification images reveal lattice fringes corresponding to Cu₂S and Co₉S₈, indicating that CuCoPBA-S exhibits a crystalline nature (ordered) within the PBA network structure (disordered). Lattice spacings of 0.314 nm (Cu₂S (111)) and 0.29 nm (Co₉S₈ (311)) are observed in the HRTEM images (selected region is given in a yellow box) of CuCoPBA-S, as depicted in Fig. 1f.

Fig. 1 (a) Schematic diagram showing the synthesis of CuCoPBA-S via pulsed laser irradiation in liquid (PLIL) method; (b) FTIR spectra of CuCoPBA and CuCoPBA-S; (c) Raman spectra of CuCoPBA and CuCoPBA-S; (d) FESEM images of CuCoPBA with corresponding EDS elemental mapping images; (e) FESEM image of CuCoPBA-S with corresponding EDS elemental mapping images; (f) HRTEM images of CuCoPBA-S at different magnifications, and high-resolution images showing lattice fringes corresponding to Cu₂S and Co₉S₈.

Surface chemical analysis of CuCoPBA and CuCoPBA-S was performed using X-ray photoelectron spectroscopy (XPS). The XPS survey spectra obtained for CuCoPBA and CuCoPBA-S are depicted in Fig. 2a, confirming the presence of various elements in the samples. A small oxygen peak is observed, attributed to the inevitable formation of native oxide in the open environment. Fig. 2b shows the XPS C 1s deconvolution spectra for CuCoPBA and CuCoPBA-S. In the CuCoPBA spectrum, the prominent peak positioned at 285.7 eV corresponds to the C–N bond, which diminishes in intensity in CuCoPBA-S owing to the ordered/disordered sulfurization process. Another peak positioned at 284.6 eV in the CuCoPBA spectrum, corresponding to the C–C bond, remains unaltered in CuCoPBA-S, though its intensity varies, indicating the structural integrity of the PBA network. Oxygen-containing surface functional groups are observed in both CuCoPBA and CuCoPBA-S, with peaks positioned at 287.3 and 288.8 eV corresponding to C=O and O–C=O, respectively. Fig. 2c shows the XPS N 1s deconvolution spectra for CuCoPBA and CuCoPBA-S. In CuCoPBA, peaks at 397.60, 398.26, 399.11, and 399.92 eV are assigned to pyridinic-N, cyanide-N, pyrrolic-N, and C₃-N species, respectively. Upon sulfurization, the cyanide-N, pyridinic-N, and pyrrolic-N peaks shift slightly toward higher binding energies (398.34, 397.68, and 399.16 eV, respectively), indicating electronic redistribution around the nitrogen environment. Notably, the intensity of the cyanide-N peak decreases after sulfurization, while the pyridinic-N and pyrrolic-N contributions increase, suggesting partial disruption of the cyanide framework and the formation of defect-rich nitrogen environments. Meanwhile, the C₃-N peak exhibits increased intensity and shifts slightly toward lower binding energy (399.90 eV), reflecting rehybridization and structural rearrangement of nitrogen species. A peak corresponding to N–C₃ shifts from 400.0 eV in CuCoPBA to 400.8 eV in CuCoPBA-S, further evidencing changes in local nitrogen coordination. The Co 2p deconvoluted XPS spectra of CuCoPBA and CuCoPBA-S are shown in Fig. 2d. For CuCoPBA, the Co 2p_3/2 and Co 2p_1/2 peaks corresponding to mixed Co²⁺/Co³⁺ states appear at 782.64/781.23 eV and 797.66/796.64 eV, respectively. After sulfurization, the Co³⁺-related peaks in CuCoPBA-S shift to lower binding energies, appearing at 780.31 eV (Co 2p_3/2) and 795.12 eV (Co 2p_1/2), corresponding to an approximate negative shift of ∼1.1 eV. This shift indicates increased electron density around Co centers due to the formation of Co–S bonds and altered coordination environments. A similar trend is observed in the Cu 2p spectra (Fig. 2e). In CuCoPBA, Cu 2p_3/2 peaks at 931.44 and 932.95 eV correspond to Cu⁺ and Cu²⁺ species, respectively, while Cu 2p_1/2 peaks at 951.69 and 952.87 eV are also associated with Cu⁺ and Cu²⁺ states. In CuCoPBA-S, the Cu⁺-related peaks shift to lower binding energies, appearing at 930.17 eV (Cu 2p_3/2) and 950.19 eV (Cu 2p_1/2), representing a negative shift of ~1.5 eV. This behavior suggests partial reduction of Cu species and enhanced electron donation upon sulfurization. Fig. 2f shows the S 2p spectra for CuCoPBA and CuCoPBA-S. No sulfur peaks are observed in CuCoPBA, as it lacks sulfur content, but in the spectra of CuCoPBA-S, peaks located at 163.5, 162.9, and 162.1 eV correspond to C–S–C, Co–S, and Cu–S, respectively.

Fig. 2 (a) XPS survey spectra of CuCoPBA and CuCoPBA-S. XPS deconvolution spectra of CuCoPBA and CuCoPBA-S for (b) C 1s, (c) N 1s, (d) Co 2p, (e) Cu 2p, and (f) S 2p.

2.2 In situ Raman analysis of CuCoPBA-S via acoustic levitation

In situ Raman spectroscopic analysis, coupled with acoustic levitation, investigated the ordered/disordered structural formation of CuCoPBA-S. The left panel of Fig. 3a shows a digital photograph of the water droplet (reactant) ready for acoustic levitation (labelled as levitated droplet (LD)), irradiated with a 532-nm laser at a power of 100 mJ. Upon laser irradiation, CuCoPBA and thiourea reacted to form the ordered/disordered flower-like CuCoPBA-S structure. The right panel of Fig. 3a depicts the digital photograph of the laser-irradiated CuCoPBA-S, showing a decrease in the size of the reactant droplet. During the laser irradiation, in situ Raman spectroscopy recorded structural changes in CuCoPBA-S at 1 min intervals for continuous 12 min, with the resulting contour plots depicted in Fig. 3b. A detailed investigation of the formation of the ordered/disordered flower-like CuCoPBA-S was performed via in situ Raman spectroscopy, with data collected at 1 min intervals for 12 min. From Fig. 3c, the formation of Cu(SCN) is observed during the first 1 min 20 s, indicated by the increased intensity peaks at 2161.6 cm⁻¹. Immediately after 1 min 20 s, the formation of Cu₂S became dominant, as indicated by the increase in its peak intensity at 177.8 cm⁻¹, which continued to increase, reaching saturation by 3 min 45 s. Finally, the formation of Co₉S₈ became prominent, as evidenced by the increase in peak intensity at 711.6 cm⁻¹. The in situ Raman spectroscopic analysis, coupled with acoustic levitation, reveals that the formation of the ordered/disordered CuCoPBA-S structure is time-dependent. The enthalpy of formation for each sulfur-based compound differs as it is associated with the thermochemical properties of the compounds.⁵² The flower-like morphology of CuCoPBA-S results from the different enthalpy of formation, determining the sulfurization reaction with the metal atoms, leading to the sequential formation of Cu(SCN), Cu₂S, and Co₉S₈. This provides insights into the bottom-up synthesis of CuCoPBA-S in an ordered/disordered manner through a time-dependent sulfurization reaction.

Fig. 3 (a) Digital photograph of the acoustic levitated droplet (LD) under laser irradiation (left panel) and the water droplet after the formation of CuCoPBA-S (right panel); (b) in situ Raman contour plots of CuCoPBA-S over a 12 minute reaction period, and (c) in situ Raman spectra of CuCoPBA-S.

2.3 Electrocatalytic NO₂RR (eNO₂RR) performance evaluation

The electrocatalytic performance of CuCoPBA and CuCoPBA-S was evaluated using a standard H-cell in a three-electrode configuration, as shown schematically in Fig. 4a. The electrocatalyst material (CuCoPBA and CuCoPBA-S) coated carbon cloth (CC) served as the working electrode, a graphite rod as the counter electrode, Hg/HgO as the reference electrode, a Nafion membrane as the electrolyte separator, 1 M KOH as the anolyte, and 1.0 M KOH + 0.1 M KNO₂ as the catholyte. Fig. 4b shows the linear sweep voltammetry (LSV) curves of CuCoPBA and CuCoPBA-S obtained in a 1.0 M KOH electrolyte, with and without the addition of 0.1 M KNO₂. The LSV curves reveal that both catalysts exhibit high current densities within a reduction potential range of 0.0 to −0.6 V vs. reversible hydrogen electrode (RHE) when NO₂⁻ is present in the electrolyte, suggesting their potential for the eNO₂RR. A noticeable variation in the LSV curve for CuCoPBA-S shows a more positive onset potential and higher current density compared to CuCoPBA. Notably, the eNO₂RR performance of CuCoPBA-S is much higher than that of CuCoPBA, as evidenced by the LSV curves. However, at highly negative potentials, the current density for CuCoPBA-S exceeds that of CuCoPBA owing to the slower reaction kinetics of CuCoPBA, as reflected in the Tafel slopes (Fig. 4c) and Nyquist plots (Fig. 4d). In addition, the Nyquist plot for CuCoPBA and CuCoPBA-S obtained in the 1.0 M KOH electrolyte, with and without 0.1 M KNO₂, displays lower ohmic resistance for CuCoPBA-S compared to CuCoPBA, attributed to the higher ionic conductivity of CuCoPBA-S. The charge transfer resistance (R_ct) for CuCoPBA-S is lower than that for CuCoPBA, confirming enhanced conductivity of CuCoPBA-S after sulfurization, likely owing to the creation of additional ionic channels that facilitate the easy penetration of electrolyte ions into the inner pores of the catalyst electrode, thereby enhancing the catalytic activity of CuCoPBA-S. FESEM analysis further reveals that CuCoPBA-S has a larger active surface area compared to CuCoPBA. This increase in surface area contributes to the enhanced catalytic activity of CuCoPBA-S. Moreover, the flower-like morphology of CuCoPBA-S aids in the easy penetration of electrolyte ions, as the pores of the electrode nanostructure are more favorable for ion diffusion into the inner pores.

Fig. 4 Electrocatalytic performance evaluation of CuCoPBA and CuCoPBA-S for eNO₂RR in 1 M KOH electrolyte with and without 0.1 M KNO₂. (a) Schematic diagram of the H-cell setup for eNO₂RR testing; (b) LSV profiles recorded with and without 0.1 M KNO₂; (c) Tafel plots; (d) Nyquist plots; (e) comparison of the FE (NH₃) for CuCoPBA and CuCoPBA-S at various reduction potentials; (f) comparison of YR (NH₃) for CuCoPBA and CuCoPBA-S at various reduction potentials; (g) ¹H NMR spectrum of the catholyte after 60 min of the eNO₂RR at −0.5 V vs. RHE; (h) variation in the YR (NH₃) and FE (NH₃) over 10 consecutive cycles at −0.5 V vs. RHE.

The cyclic voltammetry (CV) curves of CuCoPBA and CuCoPBA-S, measured in the range of −0.1 to 1.1 V vs. RHE at 50 mV s⁻¹ in 1 M KOH electrolyte (Fig. S3), show a large area under the curve for CuCoPBA-S. The active sites were calculated as 1.85 × 10⁻⁷ and 2.31 × 10⁻⁶ mol cm⁻² for CuCoPBA and CuCoPBA-S, respectively. This indicates that CuCoPBA-S achieves a superior eNO₂RR activity compared with CuCoPBA, which can be further verified by turnover frequency (TOF), as depicted in Fig. S4. TOF, which reflects the efficiency of the catalyst for eNO₂RR, was 0.004, 0.006, and 0.042 s⁻¹ for CuCoPBA at different reduction potentials of −0.4, −0.5, and −0.6 V, respectively. By contrast, CuCoPBA-S showed a rapid increase in TOF, with 0.042, 0.083, and 0.232 s⁻¹ at −0.4, −0.5, and −0.6 V, respectively. These higher TOFs suggest that CuCoPBA-S exhibits superior efficiency toward eNO₂RR. The CV curves in the non-Faradaic potential region (0.65–0.70 V vs. RHE) at various scan rates in the 1 M KOH electrolyte, as shown in Fig. S5, reveal that CuCoPBA-S has a larger area under the curve than CuCoPBA, indicating a higher electrochemical activity. The double-layer capacitance (C_dl) for CuCoPBA and CuCoPBA-S, calculated from Fig. S6, is 58 and 354 mF cm⁻², respectively, further confirms the superior electrochemical activity of CuCoPBA-S.

After the eNO₂RR, quantitative analysis of the reaction products—including nitrogen (N₂), hydrogen (H₂), and NH₃—was carried out. The YR (NH₃) and FE (NH₃) of both CuCoPBA and CuCoPBA-S were evaluated at different applied reduction potentials. The YR (NH₃) was determined using the indophenol blue method in conjunction with UV-vis absorption spectroscopy. The corresponding UV-vis spectra (Fig. S7a) and calibration curves (Fig. S7b) for NH₄⁺ were derived from the absorbance of NH₄Cl standard solutions with known concentrations. The eNO₂RR catalysis was conducted for 1 h, and the concentration of NH₄⁺ was determined every 15 min. To verify that the detected NH₄⁺ originated exclusively from NO₂⁻ reduction rather than from extraneous nitrogen sources, a control experiment was performed in 1.0 M KOH electrolyte without the addition of KNO₂, under otherwise identical electrolysis conditions. The current profiles recorded at different applied potentials are shown in Fig. S8a. Importantly, the corresponding UV-vis absorption spectra (Fig. S8b) exhibit no detectable changes across the tested potentials, confirming that NH₄⁺ formation occurs solely from the electrochemical reduction of NO₂⁻. Fig. 4e shows the variation in FE (NH₃) with respect to the reduction potential for CuCoPBA and CuCoPBA-S. For CuCoPBA, FE (NH₃) values of 27.91%, 41.28%, 45.55%, 39.13%, and 29.41% are obtained at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. In contrast, CuCoPBA-S exhibits significantly higher FE (NH₃) values of 31.15%, 60.24%, 80.91%, 57.98%, and 44.71% at the corresponding potentials. In both cases, FE (NH₃) values increase with increasing cathodic potential from −0.3 to −0.5 V vs. RHE, followed by a decline at more negative potentials. This decrease is attributed to the enhanced contribution of competing side reactions, such as the HER, at higher overpotentials. The maximum FE (NH₃) value for CuCoPBA is 45.55% at −0.5 V versus RHE, whereas CuCoPBA-S achieves a significantly higher FE (NH₃) value of 80.91% under the same conditions. Accordingly, −0.5 V vs. RHE was selected as the optimal operating potential for subsequent electrochemical measurements. The electrocatalytic performance of CuCoPBA-S, evaluated in terms of YR (NH₃), is compared with that of representative catalysts reported in the literature in Table S2. This benchmarking analysis demonstrates that CuCoPBA-S outperforms many previously reported systems, highlighting its strong potential and competitiveness for eNO₂RR. The NH₃ quantification was further confirmed using NMR spectroscopy at different reduction potentials. To determine the NH₄⁺ concentration during the conversion of NO₂⁻ to NH₄⁺, ¹H NMR analysis was performed using ¹⁴NH₄⁺ as the feedstock. The NMR spectra and calibration curves are depicted in Fig. S9. Electrolysis was evaluated at −0.5 V in a 0.5 M ¹⁴NO₂⁻ electrolyte, and the collected electrolyte showed standard triplet peaks for ¹⁴NH₄⁺ in the spectra (Fig. 4g). ¹H NMR spectra obtained for CuCoPBA and CuCoPBA-S at different reduction potentials for NH₃ quantification are shown in Fig. S10. A comparison of the FE (NH₃) for CuCoPBA and CuCoPBA-S, calculated from UV-vis absorption spectroscopy and ¹H NMR spectroscopy at different reduction potentials, is presented in Fig. S11. The FE (NH₃) for CuCoPBA from ¹H NMR analysis was found to be 30.71%, 42.83%, 47.65%, 41.30%, and 30. 22% at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. Similarly, the FE (NH₃) for CuCoPBA-S from ¹H NMR analysis was found to be 31.03%, 62.22%, 83.03%, 58.03%, and 44.54% at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. These data show no significant difference in the FE (NH₃) values obtained for CuCoPBA and CuCoPBA-S when calculated from UV-vis and ¹H NMR spectra. The YR (NH₃) calculated from UV-vis spectroscopic analysis at different reduction potentials for both electrocatalysts is depicted in Fig. 4f. In both cases, the YR (NH₃) increases with highly negative reduction potentials; however, CuCoPBA-S exhibits a sudden increase compared with CuCoPBA. CuCoPBA-S exhibits a higher YR (NH₃) (3394.1 μg h⁻¹ cm⁻²) than CuCoPBA (756.7 μg h⁻¹ cm⁻²) at −0.5 V vs. RHE. Furthermore, the YR (NH₃) steadily increased at highly negative potentials, reaching values of 577.5 μg h⁻¹ cm⁻² for CuCoPBA-S and 2570.9 μg h⁻¹ cm⁻² for CuCoPBA at −0.7 V vs. RHE. The YR (NH₃) obtained for CuCoPBA and CuCoPBA-S at different reduction potentials, expressed in μmol h⁻¹ cm⁻², is shown in Fig. S12. Fig. S13 shows the gravimetric YR (NH₃) of CuCoPBA and CuCoPBA-S at different reduction potentials, expressed in μmol h⁻¹ mg_cat⁻¹. The YR (NH₃) obtained for CuCoPBA and CuCoPBA-S is further compared with data from ¹H NMR analysis. The area-specific YR (NH₃) obtained from UV-vis and NMR analyses for CuCoPBA and CuCoPBA-S at different reduction potentials, expressed in μg h⁻¹ cm⁻² and μmol h⁻¹ cm⁻², are shown in Fig. S14 and S15, respectively. Fig. S16 compares the gravimetric YR (NH₃) expressed in μmol h⁻¹ mg_cat⁻¹, calculated from UV-vis and ¹H NMR spectra for CuCoPBA and CuCoPBA-S at different reduction potentials. These data show no significant difference in the YR (NH₃) values obtained for CuCoPBA and CuCoPBA-S when calculated from UV–vis and ¹H NMR spectra. Further, N₂ and H₂ were also quantified by gas chromatography. During the analysis, no detectable N₂ was found at any reduction potential. However, H₂ was systematically quantified for both catalysts, CuCoPBA and CuCoPBA-S, at different reduction potentials. Fig. S17 shows the FE (H₂) and YR (H₂) calculated for CuCoPBA and CuCoPBA-S at different reduction potentials. Fig. S17a shows the variation in FE (H₂) and YR (H₂) with reduction potential for CuCoPBA. FE (H₂) values of 2.38%, 2.64%, 6.14%, 22.52%, and 36.94% were achieved at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. Similarly, YR (H₂) values of 6.82, 8.31, 6.29, 52.01, and 141.38 μmol h⁻¹ cm⁻² were achieved at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. For CuCoPBA-S (Fig. S17b), the FE (H₂) was found to be 3.05%, 3.56%, 3.61%, 14.61%, and 24.33% at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. Similarly, YR (H₂) values of 6.79, 8.32, 12.21, 42.01, and 115.38 μmol h⁻¹ cm⁻² were achieved at −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE, respectively. Overall, a lower YR (H₂) was observed for CuCoPBA-S compared to CuCoPBA. While the present study focuses on intrinsic catalytic activity in a conventional three-electrode configuration, evaluating NO₂⁻ reduction in a flow-cell system represents an important step toward practical implementation. Flow cells can significantly enhance mass transport, enabling operation at higher current densities and improved reactant utilization under continuous conditions. Given the porous architecture and high surface area of the developed catalyst, it is expected to perform favorably in flow-cell configurations, where efficient electrolyte permeation and reactant accessibility are critical. Future studies will therefore focus on integrating the catalyst into gas-diffusion or flow-through electrodes to assess performance under industrially relevant conditions, including high current density operation, long-term stability, and NH₃ production rates.

The enhanced eNO₂RR performance of CuCoPBA-S is likely due to the greater availability of accessible active sites for interaction with the electrolyte compared to CuCoPBA, which facilitates more efficient catalytic processes. In general, the surface area of a catalyst is a key factor governing its activity, as materials with larger surface areas typically provide more active sites and thus exhibit improved catalytic performance relative to those with smaller surface areas. To investigate the role of surface area in influencing eNO₂RR activity, Brunauer–Emmett–Teller (BET) surface area measurements were conducted for both CuCoPBA and CuCoPBA-S. Fig. S18 shows the N₂-sorption isotherms of CuCoPBA and CuCoPBA-S. From these adsorption–desorption data, the BET surface areas for CuCoPBA and CuCoPBA-S were calculated to be5.38 and 3.78 m² g⁻¹, respectively. This result indicates that the enhanced catalytic activity of CuCoPBA-S is not primarily due to its surface area, as CuCoPBA actually exhibits a larger BET surface area. Instead, the origin of the activity is attributed to the active sites of CuCoPBA-S, which will be further elucidated with the help of DFT predictions, in situ Raman, and ex situ FTIR analyses in forthcoming sections. To investigate the long-term stability of CuCoPBA-S, electrolysis was performed for 10 h, with YR (NH₃) and FE (NH₃) calculated each hour. The electrochemical stability of CuCoPBA-S for eNO₂RR was recorded over 10 consecutive cycles at −0.5 V vs. RHE (each cycle lasting 1 h with electrolyte replacement after each cycle). No profound variation in UV-vis absorbance was observed after 10 cycles, indicating excellent cyclic stability of CuCoPBA-S for eNO₂RR. The variation in the YR (NH₃) and FE (NH₃) (Fig. 4h) indicates no substantial variation after 10 h of cyclic stability testing. To elucidate possible structural and morphological changes of the catalyst electrode (CuCoPBA-S coated on carbon cloth) after cycling, post-stability characterizations were carried out using FESEM imaging, SEM-EDS elemental mapping, ex situ XRD, and ex situ XPS analyses. Fig. S19 compares FESEM images and SEM–EDS elemental mapping images of the CuCoPBA-S catalyst electrode before and after the stability test. No discernible morphological changes are observed after cycling, and the characteristic flower-like morphology is well preserved. Moreover, SEM–EDS elemental mapping analysis confirms a uniform distribution of the constituent elements both before and after cycling, demonstrating that the catalyst remains compositionally homogeneous during prolonged operation. Fig. S20 shows the XRD patterns of the CuCoPBA-S catalyst electrode before and after the stability test. A broad diffraction peak at 25.82° is observed in both patterns and is attributed to the C(002) plane of the carbon cloth substrate. No additional phase formation or peak shift is detected after cycling, indicating preserved crystallographic integrity. XPS analyses were further performed to probe surface chemical stability of the catalyst electrode. The deconvoluted C 1s spectra before and after cycling (Fig. S21) display peaks at 284.01, 284.83, 285.85, and 287.81 eV, corresponding to C–C, C–N, C=O, and O–C=O species, respectively, with no appreciable shifts after the stability test. Similarly, the deconvoluted N 1s spectra (Fig. S22) show peaks at 397.97, 398.84, 399.47, and 400.17 eV, assigned to pyridinic-N, cyanide-N, pyrrolic-N, and C₃-N species, respectively, which also remain unchanged after cycling. The deconvoluted Co 2p spectra (Fig. S23) exhibit Co 2p_3/2 peaks at 780.66 and 782.07 eV corresponding to Co³⁺ and Co²⁺, and Co 2p_1/2 peaks at 796.21 and 797.08 eV corresponding to Co³⁺ and Co²⁺, respectively. No binding energy shifts are observed after the stability test. Likewise, the deconvoluted Cu 2p spectra before and after cycling (Fig. S24) show Cu 2p_3/2 peaks at 931.35 and 932.62 eV (Cu⁺ and Cu²⁺) and Cu 2p_1/2 peaks at 950.96 and 952.22 eV (Cu⁺ and Cu²⁺), with no significant changes after 10 hours of operation. The deconvoluted S 2p spectra (Fig. S25) display peaks at 162.45, 163.46, and 164.31 eV corresponding to Cu–S, Co–S, and C–S–C species, respectively. After cycling, these peaks shift slightly to lower binding energies (162.01, 163.28, and 164.29 eV), indicating minor electronic redistribution at sulfur sites. In addition, weak peaks at 168.91 and 170.29 eV are observed and assigned to C–SO₃⁵³ and Cu–S–O species,⁵⁴ respectively, which are attributed to partial reduction or decomposition of Nafion on the catalyst electrode during prolonged electrolysis.

2.4 In situ and ex situ analysis of CuCoPBA-S for eNO₂RR

To elucidate the origin of catalytic activity and identify reaction intermediates on CuCoPBA-S during the eNO₂RR, in situ Raman spectroscopy and ex situ FTIR analyses were performed. A schematic illustration of the in situ Raman setup is shown in Fig. 5a, with a digital photograph of the experimental configuration provided in Fig. S26. The in situ Raman spectra of CuCoPBA-S collected at OCP and at applied potentials ranging from −0.2 to −0.7 V vs. RHE, are presented in Fig. 5b. Two broad bands centered at ~1348 and ~1610 cm⁻¹, corresponding to the D and G bands of the carbon cloth substrate, are consistently observed across all spectra. A characteristic peak at ∼1250 cm⁻¹, assigned to NO₂⁻ species, appears at all tested potentials, confirming the presence and electrochemical consumption of NO₂⁻ during reduction. Upon applying a cathodic potential of −0.2 V vs. RHE, new Raman features emerge at ~940 and ~1500 cm⁻¹, which are attributed to the *NH₂OH intermediate and NH₃ product vibrations, respectively.^55,56 In addition, a shoulder near ∼1220 cm⁻¹ develops under cathodic bias and is assigned to surface-adsorbed *NO₂ species. This feature initially intensifies and then diminishes at more negative potentials, indicating rapid conversion of *NO₂ intermediates and fast eNO₂RR kinetics, as further evidenced in Fig. S27. To probe solution-phase intermediates and products, ex situ ATR–FTIR spectroscopy was conducted on catholyte samples collected after 1 h of eNO₂RR at OCP and at applied potentials between −0.2 and −0.7 V vs. RHE. The potential-dependent ATR–FTIR spectra (Fig. 5c) and corresponding contour plots (Fig. 5d and e) reveal several bands that increase in intensity with increasing cathodic bias. Distinct absorption features at ∼1095 and 1236 cm⁻¹ are assigned to *NH₂OH and *NH₂ vibrations, respectively, while the band at 1640 cm⁻¹ and the broad feature centered at 3368 cm⁻¹ are attributed to the *NO intermediate and the NH₃ product. Collectively, the in situ Raman and ex situ ATR–FTIR results support a stepwise eNO₂RR pathway on CuCoPBA-S involving *NO, *NH₂OH, and *NH₂ as key intermediates en route to NH₃ formation.⁵⁷ To demonstrate practical conversion of the produced NH₃ into a value-added product, an acid-trap method was employed to convert NH₃ into ammonium chloride (NH₄Cl). A schematic illustration of the NH₃-to-NH₄Cl conversion setup is shown in Fig. 5f. After eNO₂RR catalysis, the catholyte was collected and heated at 70 °C under argon purging to release dissolved NH₃ gas. The evolved NH₃ was subsequently passed through an acid trap containing 2 M HCl, where it was quantitatively converted into NH₄Cl. The successful formation of NH₄Cl was confirmed by XRD analysis (Fig. 5g), with diffraction peaks in good agreement with the standard reference pattern (PDF# 04-0836). A digital photograph of the NH₄Cl product obtained after the acid-trap process is shown as an inset image in Fig. 5g. These results highlight the mechanistic understanding and practical relevance of CuCoPBA-S for eNO₂RR and underscore its potential for future electrocatalytic NH₃ manufacturing and nitrogen upcycling applications.

Fig. 5 (a) Schematic depiction of the in situ Raman experimental setup; (b) in situ Raman spectra recorded during the eNO₂RR using CuCoPBA-S at different reduction potentials; (c) ex situ FTIR spectra and (d and e) contour plots of the catholyte collected after 1 h eNO₂RR using CuCoPBA-S at different reduction potentials; (f) schematic of the acid-trap experiment for real-time production of NH₄Cl after the eNO₂RR using CuCoPBA-S; (g) XRD spectra of NH₄Cl after the eNO₂RR using CuCoPBA-S (inset: digital image of NH₄Cl collected in a weighing paper after acid-trap).

2.5 Density functional theory calculations

Theoretical calculations using density functional theory (DFT) analysis were performed to understand the reaction mechanisms and pathways of CuCoPBA and CuCoPBA-S during eNO₂RR. All first-principles DFT calculations are carried out using the Quantum ESPRESSO (QE) package with a plane-wave pseudopotential framework. The studied systems included CuCoPBA, Cu₂S, Co₉S₈, and CuCoPBA-S. The CuCoPBA-S heterostructure is constructed using a 2 × 2 × 1, 4 × 4 × 1, and 3 × 3 × 1 supercell of CuCoPBA, Cu₂S, and Co₉S₈, respectively. The coupling between Co–S and Cu–S is considered in the construction of CuCoPBA-S. The surfaces of CuCoPBA, Cu₂S, and Co₉S₈ were arranged in a stacked configuration along the x-axis with respect to the (001) plane, which was expected to enhance the visibility of additional active sites for eNO₂RR. However, there is little variation in the typical lattice structures of CuCoPBA, Cu₂S, and Co₉S₈ along all axes. Consequently, a 1% strain is applied to both Cu₂S and Co₉S₈. The exchange-correlation energy is described using the Perdew–Burke–Ernzerhof (PBE) functional in the generalized gradient approximation (GGA), and core-electron interactions are represented by ultrasoft pseudopotentials. The DFT-optimized side-view and top-view of the crystal structures for Cu₂S, Co₉S₈, and CuCoPBA are shown in Fig. S28–S30, respectively. The side and top views of the CuCoPBA-S crystal structure are shown in Fig. S31, 6a and b, respectively. Charge distribution profiles for Cu₂S, Co₉S₈, CuCoPBA, and CuCoPBA-S are shown in Fig. S32–S35, respectively, with CuCoPBA-S displaying a higher charge distribution, facilitating fast electron transfer during eNO₂RR. DFT-optimized electron localized function (ELF) profiles (Fig. S36) of Cu₂S, Co₉S₈, CuCoPBA, and CuCoPBA-S showed that CuCoPBA-S has more spatially distributed valence electrons than CuCoPBA, thereby enhancing electron transport. This peculiar electronic structure benefits charge transfer during catalysis, resulting in enhanced conductivity and efficiency. The total density of states (DOS) diagrams for Cu₂S, Co₉S₈, CuCoPBA, and CuCoPBA-S are shown in Fig. S37, with CuCoPBA-S exhibiting a higher DOS than other materials, indicating greater carrier concentration and more favorable electron energy distributions in the material. The partial DOS (PDOS) for Cu₂S and Co₉S₈ given in Fig. S38a and b, respectively, show continuous and more broader bands for Co₉S₈ compared with Cu₂S, whereas the PDOS for CuCoPBA (Fig. 6c) and CuCoPBA-S (Fig. 6d) shows that CuCoPBA-S has broader electronic states than CuCoPBA. The energy-band diagrams of Cu₂S (Fig. S39a), Co₉S₈ (Fig. S39b), CuCoPBA (Fig. 6e), and CuCoPBA-S (Fig. 6f) further confirm that CuCoPBA-S possesses stronger conduction and valence bands crossing near the Fermi level, representing superior electronic conductivity. Two possible pathways for the reduction of NO₂⁻ to NH₃ are outlined. The incorporation of sulfur into the structure of CuCoPBA during the sulfurization procedure modulates the electronic structure of the Co site of CuCoPBA in CuCoPBA-S, thereby boosting eNO₂RR by effectively capturing the NO₃⁻ on the CuCoPBA-S surface. The DFT-optimized crystal structure of CuCoPBA/Cu₂S/Co9S8 is shown in Fig. 6g and the adsorption energy for NO₂⁻ at various sites of CuCoPBA/Cu₂S/Co₉S₈ is calculated. The adsorption configurations of NO₂⁻ at different interfacial metal sites in the CuCoPBA/Cu₂S/Co₉S₈ heterostructure were systematically investigated by DFT calculations. Specifically, NO₂⁻ adsorption at (i) the Cu site of Cu₂S at the CuCoPBA/Cu₂S/Co₉S₈ interface, (ii) the Co site of Co₉S₈ at the same interface, (iii) the bimetallic Co–Cu interfacial site of CuCoPBA/Cu₂S/Co₉S₈, (iv) the Co-site of CuCoPBA at the interface, and (v) the Cu site of CuCoPBA at the interface is shown in Fig. 6h–l, respectively. The corresponding adsorption energies (E_ads) are −1.49, −1.57, −1.96, −1.24, and −1.24 eV, respectively. Among these configurations, NO₂⁻ adsorption at the Cu site of Cu₂S and the Co site of Co₉S₈ at the CuCoPBA/Cu₂S/Co₉S₈ interface exhibits the most negative adsorption energy, indicating the most favorable adsorption for initiating eNO₂RR. Notably, bidentate adsorption of NO₂⁻ across interfacial Co–Cu dual sites is thermodynamically more favorable than monodentate adsorption on isolated Cu or Co sites. This enhanced stability arises from stronger interfacial bonding, improved electron transfer, and more optimal intermediate energetics, all of which collectively enhance the kinetics and selectivity of NO₂⁻ reduction to NH₃. Hence, DFT calculations further reveal that the superior eNO₂RR activity of the Co–Cu interfacial sites originates from energetically favorable bidentate NO₂⁻ adsorption compared with single-metal adsorption behavior. Charge density difference analysis confirms pronounced charge redistribution between the dual-metal sites and the adsorbed NO₂⁻ species, indicating efficient electron donation into the antibonding orbitals of NO₂⁻. This electron transfer weakens and elongates the N–O bonds, thereby facilitating bond activation and cleavage.⁵⁸ Consequently, the energy barriers for key elementary steps—particularly N–O bond dissociation and subsequent proton-coupled hydrogenation of *NO_x intermediates—are significantly reduced on the Co–Cu dual sites. Moreover, the alloyed interfacial environment provides moderated binding energies for critical reaction intermediates (*NO₂, *NO, and *NH_x), approaching the Sabatier optimum. This balance prevents both overly weak adsorption (typical of isolated Cu sites) and excessive overbinding (often observed on Co sites), leading to a lower limiting potential and more favorable reaction energetics. These results confirm that the enhanced catalytic performance of the CoCu sites originates from intrinsic synergy and interfacial dual-metal effects rather than from isolated single-metal adsorption. Based on these insights, two DFT-optimized reaction pathways (pathway-1 and pathway-2) for eNO₂RR on CuCoPBA/Cu₂S/Co₉S₈ were identified and are schematically illustrated in Fig. 7a. The Gibbs free-energy profiles indicate that the pathway: *NO₂ → *NO₂H → *NO → *NHO → *NH₂O → *NH₂OH → *NH₂ → *NH₃ (pathway-1) is energetically more favorable than the alternative *NO₂ → *NO₂H → *NO → *NOH → *N → *NH → *NH₂ → *NH₃ route (pathway-2). The adsorption configurations of reaction intermediates at the Cu site of Cu₂S and the Co site of Co₉S₈ at the CuCoPBA/Cu₂S/Co₉S₈ interface leading to NH₃ formation are shown in Fig. 7b. The calculated Gibbs-free energy profiles for the Co-site of CuCoPBA in CuCoPBA-S, the Cu-site of CuCoPBA in CuCoPBA-S, the Cu-site of Cu₂S, and the Co-site of Co₉S₈ are depicted in Fig. S40. Similarly, the calculated Gibbs-free energy profiles for the Co-site of CuCoPBA in CuCoPBA-S, the Cu-site of CuCoPBA in CuCoPBA-S, the Cu-site of Cu₂S in CuCoPBA-S, and the Co-site of Co₉S₈ in CuCoPBA-S are depicted in Fig. S41. In both cases, the potential determining step (PDS) is the hydrogenation of *NH₂O to *NH₂OH. Among all Gibbs-free energy diagrams, the Co-site of CuCoPBA in CuCoPBA-S requires less energy for hydrogenation and subsequent electrochemical reduction to NH₃, highlighting its superior electrocatalytic activity for converting NO₂⁻ to NH₃ compared with the other sites.

Fig. 6 (a) Top-view and (b) side-view of the DFT-optimized geometry of CuCoPBA-S; partial density of states (PDOS) for (c) CuCoPBA and (d) CuCoPBA-S; band structure for (e) CuCoPBA and (f) CuCoPBA-S; schematic representation of NO₂⁻ adsorption at various sites of CuCoPBA/Cu₂S/Co₉S₈: (g) DFT-optimized geometry of CuCoPBA/Cu₂S/Co₉S₈, (h) NO₂⁻ adsorption at the Cu-site of Cu₂S at the interface of CuCoPBA/Cu₂S/Co₉S₈, (i) NO₂⁻ adsorption at the Co-site of Co₉S₈ at the interface of CuCoPBA/Cu₂S/Co₉S₈, (j) NO₂⁻ adsorption at the Cu-site of Cu₂S and the Co-site of Co₉S₈ at the interface of CuCoPBA/Cu₂S/Co₉S₈, (k) NO₂⁻ adsorption at the Co-site of CuCoPBA at the interface of CuCoPBA/Cu₂S/Co₉S₈, and (l) NO₂⁻ adsorption at the Cu-site of CuCoPBA at the interface of CuCoPBA/Cu₂S/Co₉S₈.

Fig. 7 (a) Gibbs free energy diagram for the two DFT-optimized reaction pathways during the eNO₂RR and (b) schematic representation of the reduction of NO₂⁻ to NH₃⁺ with various reaction pathways (green arrow: pathway-1; red arrow: pathway-2) over the surface of the CuCoPBA-S interface.

3 Conclusions

In this study, we successfully synthesized CuCoPBA-S with a well-defined flower-like morphology via a facile PLIL strategy using CuCoPBA as the precursor. The formation mechanism of CuCoPBA-S was investigated via acoustic levitation coupled with the PLIL method. The contaminant-free synthesis of CuCoPBA-S via acoustic levitation coupled with PLIL showed that the sulfurization process is time-dependent, leading to the evolution of an ordered/disordered flower-like CuCoPBA-S heterostructure. Detailed FESEM and HRTEM analyses confirmed the preservation of the hierarchical flower-like architecture after laser processing. The as-synthesized CuCoPBA-S was then used as a catalyst for the electroreduction of NO₂⁻ to NH₃ via eNO₂RR. CuCoPBA-S exhibited excellent catalytic activity toward the electroreduction of NO₂⁻ to NH₃, achieving a maximum FE (NH₃) of 80.91% and an YR (NH₃) of 3394.1 μg h⁻¹ cm⁻² at −0.5 V vs. RHE. Moreover, the catalyst demonstrated excellent operational stability, maintaining consistent electrochemical performance over 10 h of continuous eNO₂RR electrolysis at −0.5 V vs. RHE with hourly catholyte replacement. Post-reaction characterizations using XRD, FESEM, and SEM–EDX confirmed that the bulk structure and morphology of CuCoPBA-S were well preserved after prolonged operation. Mechanistic insights obtained from in situ Raman spectroscopy revealed the formation of key reaction intermediates including *NO, *NO₂, *NH₂, and *NH₂OH during the eNO₂RR pathway, which were further corroborated by ex situ FTIR analysis. DFT calculations demonstrated that NO₂⁻ adsorption at the Cu site of Cu₂S and the Co site of Co₉S₈ at the CuCoPBA/Cu₂S/Co₉S₈ interface is energetically most favorable for initiating eNO₂RR. In particular, bidentate NO₂⁻ adsorption across interfacial Co–Cu dual sites is thermodynamically superior to monodentate adsorption on isolated Cu or Co sites, owing to enhanced electronic coupling, stronger stabilization of intermediates, and more favorable reaction energetics. This dual-site synergy significantly accelerates NO₂⁻ activation and improves both the kinetics and selectivity of NH₃ formation. Overall, this study introduces a novel strategy for elucidating the formation mechanisms of PLIL-derived catalysts by integrating in situ Raman spectroscopy with acoustic levitation.

4 Experimental section

4.1 Materials

Potassium hexacyanocobaltate(III) (K₃[Co(CN)₆]), copper nitrate (Cu(NO₃)₂), and sodium citrate (Na₃C₆H₅O₇) were purchased from Sigma-Aldrich. Deionized water was used in all experiments.

4.2 Synthesis of CuCoPBA and CuCoPBA-S

Initially, CuCoPBA nanocubes were synthesized via a modified coprecipitation method reported in the literature.⁵⁹ In a typical procedure, 2 mM K₃[Co(CN)₆], 2 mM Cu(NO₃)₂, and 2.25 mM Na₃C₆H₅O₇ were dissolved in 20 mL of deionized water under magnetic stirring and let undisturbed for 24 h at room temperature. The resulting precipitate was collected via centrifugation at 8000 rpm for 5 min, washed several times with water, and dried in a vacuum oven at 60 °C for 6 h to obtain CuCoPBA powder. The as-prepared CuCoPBA was then used to synthesize CuCoPBA-S via acoustic levitation coupled with the PLIL method. In a typical procedure, 5 mg of CuCoPBA was dispersed in 10 mL of deionized water in a 20 mL glass vial, followed by the addition of 5 mg of thiourea and thorough stirring. The dispersion was then irradiated with a Ng:YAG laser (10 Hz, 7 ns) at 532 nm with a laser power of 200 mJ per pulse for 30 min. The resulting powder was filtered and dried at 60 °C to obtain CuCoPBA-S.

Details of material characterization, electrochemical measurements, NH₃ quantification using the calorimetric indophenol blue method and ¹H nuclear magnetic resonance (NMR), calculation of the NH₃ yield rate (YR (NH₃)) and NH₃ faradaic efficiency (FE (NH₃)), N₂ and H₂ quantification by gas chromatography (GC), H₂ yield rate (YR (H₂)) and H₂ faradaic efficiency (FE (H₂)), in situ Raman spectroscopy measurements, acoustic levitation measurements, and DFT computations are provided in the SI.

Author contributions

Juhyeon Park: conceptualization, data curation, investigation, writing – original manuscript. Jayesh Cherusseri: methodology, investigation, formal analysis, writing – review & editing. Jayaraman Theerthagiri: methodology, investigation, formal analysis, writing – review & editing. Ahreum Min: investigation, resources, writing – review & editing. Anuj Kumar: investigation, resources, writing – review & editing. Gyeong-Ah Kim: formal analysis, investigation. Nivarthi Rajiv Bharadwaj: data curation, formal analysis, Sanggyeong Lee: investigation, formal analysis, Rimal Isaac R. S.: investigation, formal analysis. Myong Yong Choi: funding acquisition, project administration, resources, supervision, writing – review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6im00053c.

Acknowledgements

This research was supported by the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (no. 2019R1A6C1010042, RS-2024-00434932, RS-2025-12872968, and 2021R1A6C102A526), and by the Korea government (MSIT) (PG2025039-02). The authors acknowledge financial support from the National Research Foundation of Korea (NRF) (RS-2025-02634080, 2022R1A2C2010686, RS-2024-00405324, and RS-2024-00443792). This work was supported by the Research Resurgence under the Glocal University 30 Project at Gyeongsang National University in 2025.

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Footnote

† These authors contributed equally to this work.

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