Hydrogen Spillover Synergistically Enhances Electrocatalytic Nitrate Reduction to Ammonia on SnO₂‐CuO Heterointerfaces

ABSTRACT

The electrocatalytic nitrate reduction reaction (NO₃RR) offers an ideal solution to addressing nitrogen cycle imbalances and energy challenges. However, the competing hydrogen evolution reaction and limited efficient catalysts result in unsatisfactory NO₃RR efficiency. Herein, SnO₂‐CuO hollow nanofibers with hierarchical heterointerfaces were prepared via an electrospinning and calcination strategy. Experimental and theoretical calculations demonstrate that Sn sites in the SnO₂‐CuO hierarchical heterojunction can effectively promote water splitting to provide active hydrogen (^*H), while Cu sites facilitate the adsorption and reduction of NO₃ ⁻, which accelerates the hydrogenation reaction of ^*NO_x intermediates. Furthermore, charge redistribution at the SnO₂‐CuO interface generates a built‐in electric field, significantly reducing the diffusion energy barrier of ^*H and promoting the migration kinetics of electrons/^*H, thereby achieving efficient collaboration between hydrogen spillover and electron transfer at the interface. The as‐prepared SnO₂‐CuO composite achieved superior Faradaic efficiency (94.89%, – 0.9 V vs RHE) and NH₃ yield (628.14 µmol h⁻¹ cm⁻²). A rechargeable Zn‐NO₃ ^– battery fabricated using this catalyst continuously produces NH₃ while discharging electrical energy, highlighting the feasibility of an integrated platform for pollutant resource recovery and energy storage. This study provides effective insights into the advanced structural and functional design of highly efficient NO₃RR catalysts.

Nitrate adsorbs and activates on Cu sites, while Sn sites promote water dissociation to generate reactive surface hydrogen that transfers across the SnO₂‐CuO interface to hydrogenate ^*NO_x intermediates, enabling selective nitrate reduction to NH₃ with suppressed HER.

1. Introduction

Ammonia(NH₃) has emerged as an attractive carbon‐free energy carrier due to its high volumetric energy density (4.32 kWh L⁻¹) and exceptional hydrogen storage capacity (17.6 wt.%) [1, 2]. Electrocatalytic nitrate reduction (NO₃RR), which uses NO₃ ⁻ as the N source and H₂O as the H source, enables simultaneous nitrate remediation and sustainable NH₃ production, contributes to establishing a positive cycle between energy and the environment, and has thus garnered extensive attention from researchers [3, 4]. However, the NO₃RR process is complex, involving an overall transfer of eight electrons and nine protons, with proton generation and electron transfer exerting significant influence on the reaction kinetics [5, 6] This complexity also makes the process prone to generating various by‐products such as NO₂ and N₂H₄ [7]. Moreover, the intrinsic kinetic barriers necessitate an additional overpotential, which readily exacerbates the competing hydrogen evolution reaction (HER), thereby suppressing both the NH₃ yield and the Faradaic efficiency (FE) [8, 9]. The NO₃RR also provides a promising pathway for constructing high‐value electrochemical energy conversion systems. Zn‐NO₃ ⁻ batteries, in which NO₃ ⁻ serves as the cathodic oxidant, have recently attracted extensive attention. This system not only enables electrochemical denitrification, but also couples nitrate removal with energy output by utilizing a Zn anode as the electron donor, thereby achieving synergistic environmental remediation and energy conversion [10, 11]. Consequently, the rational design of highly active and selective NO₃RR electrocatalysts is pivotal for advancing the industrialization of electrocatalytic NH₃ production.

Cu‐based catalysts exhibit a strong binding affinity toward ^*NO_x intermediates during NO₃RR, thereby lowering the energy barrier of the nitrate‐to‐nitrite determining step [2, 12, 13]. Their intrinsically high selectivity toward NH₃‐N can, in turn, effectively suppress the competing HER [14, 15]. Consequently, such catalysts, characterized by high catalytic activity and selectivity, have attracted extensive research attention, with CuO being a typical example. However, recent studies indicate that CuO binds protons relatively weakly, which is unfavorable for generating ^*H required in NO₃RR and thus impedes the subsequent hydrogenation of ^*NO_x species [16, 17]. To address this limitation, intensive efforts have been devoted to structural and compositional modulation of CuO to optimize its NO₃RR performance [18, 19] For instance, Chen et al. synthesized a series of copper‐oxide nanorods with tunable Cu valence states, the uniform rod‐like morphology and mixed‐valence copper oxides promoted adsorption of key ^*NO_x species, delivering outstanding NO₃RR activity with an NH₃ Faradaic efficiency (FE) of 93% [20]. Likewise, Liu et al. anchored Pd single atoms at interlayer unsaturated bonds of CuO and exploited shear stress to facilitate ^*H formation, achieving $F{E}_{N{H}_3}=$90% at − 0.5 V vs. RHE [21]. Despite some progress, the catalysts obtained through the above strategies still exhibit limited generation of ^*H during the reaction, which is insufficient to meet the demands of the NO₃RR process. Moreover, the core mechanisms governing the generation, transfer, and utilization pathways of ^*H during the reaction require deeper investigation and elucidation.

Research has shown that heterojunction engineering can effectively tailor the local electronic environment of electrocatalysts by inducing interfacial charge redistribution within the material, thereby establishing a built‐in electric field (BIEF) that accelerates electron/ion diffusion and transport [22]. Notably, heterojunctions can also integrate the complementary physicochemical attributes of distinct components to offset their individual limitations, offering pronounced advantages for NO₃RR [23]. Ye et al. fabricated a MOF‐derived Ni₃Se₄/CoSe₂ heterostructure. Benefiting from heterojunction optimization of the reaction pathway and synergistic catalysis between the two components, the catalyst delivered an NH₃ yield of 1.44 mmol cm⁻² h⁻¹ at a current density of 350 mA cm⁻² [24]. Recent studies confirm that introducing an efficient ^*H‐generating component into materials with weak proton‐adsorption affinity can effectively address insufficient ^*H supply during the NO₃RR process by leveraging the hydrogen spillover effect [25, 26]. Mechanistically, hydrogen spillover in the NO₃RR involves the initial formation of ^*H at hydrogen‐generating sites, followed by their subsequent interfacial migration to adjacent catalytic sites, where they participate in stepwise hydrogenation processes. Given that the NO₃RR requires a continuous and adequate ^*H supply to drive the sequential reduction of NO₃ ⁻ to NH₃, conventional single‐component catalysts often suffer from sluggish water dissociation kinetics and severe competition between hydrogen adsorption and nitrate adsorption for limited active sites [25, 27]. These limitations typically lead to insufficient local ^*H coverage, thereby forcing the reaction to proceed via energetically less favorable concerted electron‐proton transfer pathways [28]. The hydrogen spillover effect enables a spatially decoupled “H‐producing site/H‐consuming site” cooperative configuration, which not only compensates for the inadequate ^*H‐binding capability of weak hydrogen‐adsorbing sites but also effectively suppresses competing HER, thereby significantly enhancing the selectivity toward the NO₃RR. Hu et al. developed a Co@Ga core‐shell catalyst, in which the hydrogen spillover effect enables spontaneous migration of ^*H generated on Ga sites to adjacent Co sites, thereby effectively enhancing the efficiency of the ^*NO_x hydrogenation reaction. Therefore, when a highly active ^*H generation component is introduced into copper oxide to form a heterointerface, the generated hydrogen atoms can dynamically migrate to the adjacent CuO surface due to interfacial electronic coupling and lattice continuity, which will effectively address the inherent drawback of insufficient ^*H production during the NO₃RR process on CuO, thereby achieving unexpected performance. However, during NO₃RR, the interfacial transfer mechanisms of electrons and protons across the heterointerface remain insufficiently understood. In particular, how this heterointerface kinetically orchestrates the “H producing site/ H consuming site” coupling, and thereby enables synchronized hydrogen generation and utilization, which still requires further elucidation.

Herein, we fabricated a SnO₂‐CuO composite hollow nanofiber catalyst featuring a hierarchical heterointerface. Experimental results and theoretical calculations demonstrate that ^*H generated on the SnO₂ surface can transfer to adjacent CuO sites, forming a hydrogen spillover effect, which liberates CuO from reliance on its intrinsic activity for surface ^*H provision. CuO gains access to abundant external ^*H supply, enabling it to focus on its advantageous function of adsorbing and hydrogenating ^*NO_x intermediates. The BIEF formed at the SnO₂‐CuO heterojunction interface promotes charge/proton transfer, reduces the energy barrier for ^*H migration across the interface, and optimizes the reaction kinetics during ^*NO_x hydrogenation. The SnO₂‐CuO catalyst achieves a coordinated division of labor between functional components at the interface, delivering superior NO₃RR performance. This work explores the reaction mechanism of NO₃RR at heterointerfaces and the practical application potential of the catalyst.

2. Results and Discussion

The preparation of the SnO₂‐CuO hollow nanofibers with hierarchical heterointerface composites is shown in Figure 1a. First, a polyvinylpyrrolidone (PVP) solution containing Sn²⁺ and Cu²⁺ is subjected to electrospinning to obtain polymer fiber precursors (Sn/Cu‐PVP), which were subsequently subjected to calcination to yield hollow nanofiber composites (SnO₂‐CuO). For comparison, single oxide counterparts were prepared under identical conditions by introducing only a copper salt or a tin salt into the spinning solution, yielding CuO and SnO₂, respectively. As shown in Figure S1, the as‐prepared SnO₂‐CuO composite exhibits a well‐defined fibrous morphology with an average diameter of approximately 300 nm. Pure SnO₂ also retains a uniform fiber‐like morphology, whereas CuO appears predominantly as bulky aggregates, which is attributed to the hygroscopic nature of CuCl₂·2H₂O that disrupts fiber formation during electrospinning. The hollow architecture of the SnO₂‐CuO catalyst is clearly evident in Figure 1b, and the fiber surface is decorated with protruding nanorods, which can expose abundant accessible active sites [29]. High‐resolution transmission electron microscopy reveals distinct lattice fringes associated with the two oxide phases. The nanorods on the fiber surface display a lattice spacing of 0.342 nm, assignable to the (110) planes of SnO₂ (Figure 1c and Figure S2a), whereas the fiber wall predominantly shows lattice fringes of 0.268 nm, corresponding to the (110) planes of CuO (Figure 1d and Figure S2b). These observations indicate that CuO is mainly enriched at the inner wall of the hollow nanotube, while the surface nanorods are primarily composed of SnO₂. Speculating on the formation mechanism of this structure: During the initial stage of calcination, the evaporation of ethanol drives SnCl₂·2H₂O and CuCl₂·2H₂O to migrate toward the outer layer of the fibers, thereby forming a precursor‐rich shell. Subsequently, the thermal decomposition of PVP generates a large amount of gas, resulting in a hollow structure. At the same time, due to the greater oxygen affinity and faster oxidation kinetics of Sn species, the Sn‐containing precursors preferentially diffuse outward and crystallize on the outer surface, forming SnO₂ nanorod structures. In contrast, Cu species exhibit stronger coordination interactions with the carbonyl groups of PVP and have relatively lower diffusion mobility; therefore, they are primarily confined within the fiber and subsequently transform into CuO. Energy dispersive x‐ray spectroscopy analysis further confirms the coexistence of Cu, Sn, and O throughout the SnO₂‐CuO composite nanofiber catalyst (Figure 1e), indicating that large‐scale SnO₂/CuO heterointerfaces are constructed on the hollow nanofibers of SnO₂‐CuO composite. The hollow fiber structure and the hierarchical heterointerface featuring a BIEF facilitate the exposure of more active sites, ensure intimate contact between the catalyst and the electrolyte solution, and accelerate electron/^*H transfer, thereby effectively enhancing the reaction kinetics of the NO₃RR process.

FIGURE 1.

(a) Schematic illustration of the synthesis of SnO₂‐CuO. (b) TEM image. (c,d) HRTEM images with the corresponding inverse fast Fourier transform images. (e) HAADF STEM image and elemental mapping of SnO₂‐CuO.

The x‐ray diffraction (XRD) peaks of the SnO₂‐CuO sample can be well indexed to tetragonal SnO₂ (JCPDS 41–1445) and monoclinic CuO (JCPDS 48–1548) (Figure 2a), indicating the successful formation of SnO₂/CuO heterostructure [30]. In addition, the single‐component SnO₂ and CuO reference samples were also obtained under identical conditions. Raman spectroscopy further corroborates the phase composition. Raman spectra of SnO₂, CuO, and SnO₂‐CuO show the characteristic vibrational modes of both oxides, confirming the coexistence of SnO₂ and CuO in the composite (Figure 2b). A broad band centered at around 270 cm⁻¹ is observed for SnO₂, which is attributed to defect‐induced Raman activation and phonon confinement in nanostructured SnO₂, originating from oxygen vacancy‐related symmetry breaking and surface disorder [31]. Compared with the single‐phase samples, the composite exhibits an intensified and broadened band around 600 to 650 cm⁻¹, indicating strengthened interfacial interaction and an altered local lattice environment. Electron paramagnetic resonance (EPR) measurements (Figure 2c) reveal that the SnO₂‐CuO composite exhibits the strongest EPR signal at g = 2.003, which is typically associated with oxygen vacancy‐related paramagnetic centers [32]. This suggests that the introduction of SnO₂ effectively promotes the formation of oxygen vacancies, modulates the local electronic structure, and enhances the charge transfer efficiency [33]. The increased oxygen vacancy concentration not only modifies the local electronic structure but also facilitates the partial reduction and stabilization of Cu species through electron donation and charge compensation.

FIGURE 2.

(a) XRD patterns, (b) Raman spectra, and (c) EPR spectra of CuO, SnO₂, and SnO₂‐CuO. (d) Cu 2p XPS spectra of CuO and SnO₂‐CuO. (e) Sn 3d XPS spectra of SnO₂ and SnO₂‐CuO. (f) O 1s XPS spectra. (g) Charge density difference plot at the SnO₂‐CuO interface and (h) electron gain/loss of Sn and Cu atoms calculated by Bader charge analysis.

X‐ray photoelectron spectroscopy (XPS) was employed to determine the surface composition of the catalysts and the oxidation states of C, O, Sn, and Cu. The XPS survey spectra of SnO₂‐CuO, SnO₂, and CuO are presented in Figure S3. In the high‐resolution Cu 2p spectrum of CuO (Figure 2d), five peaks are observed, including the characteristic Cu 2p_3/2 peak at 934.2 eV, the Cu 2p_1/2 peak at 954.0 eV, and three shake‐up satellite features at 941.7, 944.2, and 962.3 eV [34]. By contrast, SnO₂‐CuO exhibits two additional components at 932.6 and 952.4 eV, which can be assigned to Cu⁺ species. This emergence of Cu⁺ is attributed to the interfacial interaction between Cu and Sn species in the composite, which induces lattice strain and promotes the formation of oxygen vacancies at the heterointerface. Consistently, Cu Auger analysis of SnO₂‐CuO yields a Cu⁺ to Cu²⁺ ratio of 28.45 to 71.55 (Figure S4). The Sn 3d spectra further support electronic modulation in the composite. As shown in Figure 2e, pristine SnO₂ displays a single Sn 3d doublet at 489.0 and 495.4 eV, corresponding to Sn⁴⁺ 3d_5/2 and Sn⁴⁺ 3d_3/2, respectively. In SnO₂‐CuO, the Sn 3d peaks shift slightly toward lower binding energy, appearing at 486.6 and 495.0 eV, which can be rationalized by interfacial charge transfer driven by the different Fermi levels of CuO and SnO₂, namely, electron transfer from SnO₂ to CuO [35]. The O 1s spectrum (Figure 2f) can be deconvoluted into three components assigned to lattice oxygen, defect‐related oxygen, and surface hydroxyl species. Notably, SnO₂‐CuO shows the highest fraction of defect oxygen, in agreement with the EPR results, indicating an increased oxygen vacancy density [35]. During the NO₃RR process, electron‐rich oxygen vacancies can serve as catalytically active sites, thereby enhancing the adsorption of NO₃ ⁻ species. In addition, oxygen vacancies facilitate N‐O bond activation and the subsequent formation of key reaction intermediates. More importantly, by modulating the local electronic structure, oxygen vacancies promote interfacial charge transfer kinetics, thereby accelerating the overall reaction pathway.

Density functional theory (DFT) calculations were employed to conduct an in‐depth investigation into the charge characteristics of the SnO₂‐CuO composite interface (Figure 2g). The computational results demonstrate that a pronounced interfacial coupling effect is established at the SnO₂‐CuO heterointerface, inducing an inhomogeneous distribution of charge within the interfacial region. Obvious charge rearrangement features are observed at the interface, blue regions indicate decreased charge density, while yellow regions represent increased charge density. Quantitative analysis based on the Bader charge decomposition method further reveals that approximately 3.77 electrons undergo net transfer from the SnO₂ phase to the CuO phase (Figure 2h). Combined analyses of charge density difference, Bader charge transfer, and the experimentally observed XPS binding energy shifts collectively indicate a pronounced electron redistribution from SnO₂ to CuO. This interfacial charge transfer intrinsically gives rise to a built‐in electric field at the heterojunction, which serves as a driving force for the directional migration of charge carriers and ^*H.

The electrocatalytic NO₃RR performance of SnO₂‐CuO was evaluated in an H‐type cell using a standard three‐electrode configuration. Under ambient conditions, the concentrations of the major NO₃RR products, including NH₃ and NO₂ ⁻, were quantified by ion chromatography in an electrolyte. All potentials reported in this work are referenced to the reversible hydrogen electrode. Figure 3a compares the linear sweep voltammetry curves of SnO₂‐CuO recorded in 0.5 m K₂SO₄ with and without 0.1 m KNO₃. In the presence of nitrate, SnO₂‐CuO delivers a markedly higher current density, providing initial evidence that nitrate reduction is kinetically favored over HER on this catalyst. To benchmark the overall NO₃RR activity, the single oxide counterparts CuO and SnO₂ were tested under identical conditions. As shown in Figure S5, the three catalysts exhibit pronounced differences in current response, with SnO₂‐CuO displaying a more positive onset potential and a higher current density, indicative of superior electrocatalytic activity toward nitrate reduction.

FIGURE 3.

(a) LSV curves of SnO₂‐CuO in 0.5 M K₂SO₄ with /without 0.1 M KNO₃. (b) NH₃ yield rate and Faradaic efficiency of SnO₂‐CuO at different potentials. (c) Comparison of NH₃ yield rate and Faradaic efficiency for CuO, SnO₂, and SnO₂‐CuO at − 0.9 V vs RHE. (d) ¹H NMR spectra of electrolytes after NO₃RR using ¹⁴NO₃ ^– or ¹⁵NO₃ ^– as the nitrogen source. (e) NMR calibration curve for quantifying NH₄ ⁺ concentration. (f) Comparison of NH₃ yield rate and Faradaic efficiency determined by ion chromatography and NMR at − 0.9 V vs RHE. (g) Performance comparison of SnO₂‐CuO with reported catalysts. (h) Cycling stability for SnO₂‐CuO over 11 consecutive cycles at − 0.9 V vs. RHE.

Subsequently, potentiostatic electrolysis was conducted at a series of applied potentials ranging from − 0.6 to −1.0 V vs RHE for 1 h, and the resulting current time curves are shown in Figure S6. The concentration of NH₄ ⁺ in the diluted electrolyte was quantified by ion chromatography using an external calibration curve (Figure S7). As the applied potential became more cathodic, the NH₃ production rate of SnO₂‐CuO increased monotonically. In contrast, the Faradaic efficiency exhibited a volcano‐type dependence, which can be attributed to the progressively intensified competition from the hydrogen evolution reaction at more negative potentials (Figure 3b). Notably, at − 0.9 V vs. RHE, SnO₂‐CuO delivered a maximum Faradaic efficiency of 94.89% together with an NH₃ yield rate of 628.14 µmol h⁻¹ cm⁻². Even at − 1.0 V vs RHE, the catalyst maintained a Faradaic efficiency above 90.00%. The SnO₂‐CuO composite catalyst significantly outperforms the single metal oxides at the optimal potential, indicating a synergistic catalytic effect at the bimetallic oxide interface (Figure 3c). Analysis of the main by‐products (see Figures S8 and S9) indicates that the SnO₂‐CuO composite exhibits lower NO₂ ⁻ yield and FE compared to its single‐oxide counterparts. This performance enhancement originates from the SnO₂‐CuO heterointerface, where interfacial charge redistribution establishes a BIEF that promotes electron and ^*H migration and reinforces the division‐of‐labor synergy between CuO and SnO₂. To unambiguously verify that the nitrogen in the produced NH₃ originated from nitrate in the electrolyte rather than external contamination, isotope labeling experiments were performed by replacing ¹⁴NO₃ ⁻ with ¹⁵NO₃ ⁻, followed by ¹H NMR (500 MHz) analysis after electrolysis. As shown in Figure 3d, the electrolyte containing ¹⁴NO₃ ⁻ exhibits the characteristic triplet signal, whereas the ¹⁵NO₃ ⁻ electrolyte shows the diagnostic doublet associated with ¹⁵N, confirming that NH₃ is generated from the electroreduction of NO₃ ⁻. A quantitative calibration curve was further established by NMR (Figure 3e and Figure S10). For SnO₂‐CuO electrolyzed at − 0.9 V vs RHE for 1 h, the NH₃ amount determined by NMR agrees well with that obtained by ion chromatography (Figure 3f), validating the reliability of NH₃ quantification throughout this work. Furthermore, benchmarking against representative reported catalysts under neutral conditions shows that SnO₂‐CuO delivers highly competitive NO₃RR activity at − 0.9 V vs. RHE, featuring both higher Faradaic efficiency and a higher NH₃ yield rate (Figure 3g, details summarized in Table S1).

To assess the durability of SnO₂‐CuO, consecutive electrolysis was performed at − 0.9 V vs. RHE for 11 cycles. As shown in Figure 3h, both the NH₃ yield rate and the Faradaic efficiency exhibit only slight variations throughout the cycling test, indicating excellent electrochemical stability. Scanning electron microscopy images show that SnO₂‐CuO largely retains its fibrous morphology after prolonged operation (Figure S11a). Consistently, XRD patterns of the SnO₂‐CuO sample after reaction confirm that the diffraction peaks can still be indexed to CuO and SnO₂, indicating favorable lattice stability of the catalyst during the reaction process (Figure S11b). Collectively, these results demonstrate that the hollow nanofiber architecture with hierarchical heterointerfaces functional architecture of SnO₂‐CuO endows it with highly efficient and stable NH₃ productivity during the NO₃RR process.

Cyclic voltammetry (CV) curves were recorded for the three catalysts within a non‐Faradaic potential window at scan rates from 30 to 90 mV s⁻¹ (Figure S12). Based on the CV curve exhibiting good center symmetry, the calculated bilayer capacitance of SnO₂‐CuO (0.359 mF cm⁻²) was higher than that of the single metal oxides CuO (0.257 mF cm⁻²) and SnO₂ (0.242 mF cm⁻²). This indicates that the composite catalyst increased the electrochemically active specific surface area, exposing more active sites (Figure 4a) [26, 36]. These results indicate that the heterostructure offers a higher electrochemically active surface area and exposes more accessible active sites. The increased density of active sites can strengthen the adsorption and activation of ^*NO_x, thereby improving NO₃RR kinetics and overall catalytic efficiency [37].

FIGURE 4.

(a) Estimation of the Cdl. Operando Bode phase plots of SnO₂‐CuO at different potentials in electrolyte (b) without and (c) with 0.1 M NO₃ ^–. (d) Structural models of CuO, SnO_2, and SnO₂‐CuO, (e) their projected density of states, and (f) total density of states.

Electrochemical impedance spectroscopy was employed to probe the NO₃RR kinetics of the catalysts. An equivalent circuit model was established as shown in Figure S13, where R_s, R_ct, and C_dl denote the solution resistance, charge transfer resistance, and constant phase element, respectively [38, 39]. According to the Nyquist responses, the overall impedance associated with NO₃RR decreases as the applied potential becomes more cathodic (Figure S14) [18, 40]. During the NO₃RR process, the R_ct values of SnO₂‐CuO, CuO, and SnO₂ decrease as the potential increases, which can be attributed to the accumulation of ionic species in the electrolyte, such as NH₄ ⁺ and NO₂ ⁻, thereby improving electrolyte conductivity and facilitating charge transport (Figure S15) [41]. Operando Bode phase analysis of SnO₂‐CuO, CuO, and SnO₂ was performed to elucidate the competition between NO₃RR and the hydrogen evolution reaction. Under HER conditions (Figure 4b and Figure S16), the SnO₂‐CuO heterostructure exhibits a more pronounced phase response due to the inhomogeneous charge distribution at the interface induced by the hydrogen evolution reaction [42], indicating that the heterostructure facilitates the formation of ^*H. In contrast, under NO₃RR conditions (Figure 4c and Figure S17), the evolution of phase characteristics in SnO₂‐CuO occurs at a lower phase angle and frequency, suggesting accelerated interfacial kinetics and a reaction pathway more inclined toward nitrate reduction [17]. Collectively, these results demonstrate that the SnO₂‐CuO composite effectively promotes NO₃RR in neutral media while concurrently mitigating the competing HER [43]. In addition, Tafel analysis was performed for SnO₂‐CuO, CuO, and SnO₂ under NO₃RR conditions. The SnO₂‐CuO catalyst displays the lowest Tafel slope, further confirming its most favorable reaction kinetics (Figure S18) [39].

To gain deeper insight into the origin of the improved NO₃RR kinetics, density functional theory calculations were performed for SnO₂‐CuO, CuO, and SnO₂, and the optimized structural model is shown in Figure 4d. CuO and SnO₂ exhibit a relatively low density of states at the Fermi level, whereas the SnO₂‐CuO heterostructure displays a pronounced increase in DOS near the Fermi level, arising from the combined contributions of Cu d, Sn p, and O p states, indicating strengthened interfacial electronic coupling and orbital mixing (Figure 4e) [44]. In addition, SnO₂‐CuO presents a higher total DOS in the vicinity of the Fermi level, and the enriched electron density facilitates rapid charge transfer (Figure 4f), suggesting enhanced electronic conductivity and more favorable interfacial charge transport [45]. This electronic enrichment provides a more sufficient electron supply for nitrate adsorption and the subsequent reduction steps. Additionally, the enhanced reaction kinetics of SnO₂‐CuO can be specifically attributed to its hollow nanofiber architecture with abundant hierarchical heterointerfaces. First, the strong electronic interaction at the SnO₂‐CuO heterointerface generates a BIEF, which optimizes the proton/electron transfer environment. Electrons transfer from SnO₂ to CuO, facilitating the electron‐gaining reduction of NO₃ ⁻ at copper sites. Simultaneously, ^*H formed by water dissociation on SnO₂ spontaneously migrates toward CuO under the driving force of the BIEF, thereby promoting the hydrogenation process [46]. Second, the hollow and porous architecture introduces a confinement effect that enriches the local NO₃ ⁻ concentration, enhances mass transport, and accelerates the nitrate reduction process [16, 47].

To elucidate the mechanism by which SnO₂‐CuO promotes NO₃RR, 5,5 dimethyl 1 pyrroline N oxide (DMPO) was employed as an ^*H spin trap, and operando EPR was used to monitor the formation and consumption of ^*H during HER and NO₃RR [16, 17]. As shown in Figure 5a, in a nitrate‐free electrolyte, SnO₂‐CuO displays nine characteristic DMPO ^*H signals with an intensity ratio of 1:1:2:1:2:1:2:1:1 [48, 49] and the overall signal is stronger than that of CuO. This result indicates that the Sn and Cu dual metal sites facilitate water dissociation and thereby promote ^*H generation. Because NO₃RR requires ^*H for sequential hydrogenation, a moderate ^*H supply from water splitting is beneficial for driving nitrate reduction. Upon introducing NO₃ ⁻, the DMPO ^*H signal intensity decreases markedly, suggesting rapid consumption of ^*H via hydrogenation steps during NO₃RR (Figure 5b). Notably, under nitrate‐containing conditions, SnO₂‐CuO exhibits a lower DMPO ^*H signal than CuO, implying that the SnO₂‐CuO catalyst not only enhances ^*H generation but also more efficiently channels ^*H toward hydrogenation of adsorbed NO₂ intermediates rather than the competing HER, thereby favoring NO₃RR. CV measurements (Figure S19a) reveal that the SnO₂‐CuO composite exhibits a markedly enhanced current response within the ^*H‐associated potential region relative to pristine SnO₂ and CuO, indicating more efficient generation of ^*H. Moreover, as shown in Figure S19b, the introduction of NO₃ ⁻ results in a pronounced suppression of the ^*H‐related current, suggesting that the generated ^*H is preferentially consumed during the NO₃RR process rather than accumulating on the catalyst surface. To further elucidate the role of ^*H, quenching experiments using tert‐butanol (TBA) were conducted. With increasing TBA concentration, the current density decreases systematically, providing compelling evidence that ^*H plays a decisive role in governing the NO₃RR process (Figure S20).

FIGURE 5.

(a,b) DMPO assisted EPR spectra of CuO and SnO₂‐CuO in electrolyte without and with 0.1 M KNO₃. (c) HER pathways on Cu and Sn sites of SnO₂‐CuO. (d) NO₃ ⁻ adsorption energies on Cu and Sn sites of SnO₂‐CuO. (e) Operando FTIR spectra of NO₃RR on SnO₂‐CuO recorded at different potentials from − 0.1 to −1.0 V vs RHE. (f) Gibbs free energy changes of key intermediates along the NO₃RR pathway on SnO₂‐CuO.

Furthermore, DFT calculations were performed to gain molecular‐level insight into the origin of the high activity of the SnO₂‐CuO catalyst. As shown in Figure 5c, the adsorption energy of H₂O, the energy barrier for H₂O dissociation, and the adsorption energy of ^*H were evaluated on Sn and Cu sites. These energetics are substantially higher on Cu sites than on Sn sites, indicating that Sn sites are more favorable for water activation and ^*H generation, consistent with a higher intrinsic tendency toward HER [50, 51] As shown in Figure 5d, the lower adsorption energy of NO₃ ⁻ at Cu sites suggests stronger binding affinity, supporting the conclusion that Cu serves as the active site for the NO₃RR [52, 53]. This difference in adsorption preference reflects a competitive adsorption behavior between H₂O and NO₃ ⁻ on distinct sites, leading to a spatial separation of ^*H generation and NO₃ ⁻ activation. From an energetic perspective, Cu sites promote NO₃ ⁻ adsorption and subsequent activation while simultaneously suppressing the competing HER, whereas Sn sites provide a controlled supply of reactive ^*H through moderate water splitting, which is essential for the hydrogenation of ^*NO_x intermediates. This migration process is not purely driven by concentration gradients, but is further facilitated by the built‐in electric field at the heterointerface. The BIEF provides a directional driving force that promotes ^*H transfer from SnO₂ to CuO and lowers the interfacial diffusion barrier, thereby accelerating the hydrogenation of ^*NO_x intermediates. This division of roles and the intersite synergy between Cu and Sn collectively enhance both the catalytic activity and product selectivity of NO₃RR on SnO₂‐CuO.

Operando FTIR was performed to monitor reaction intermediates over a potential window from − 0.1 to −1.0 V vs RHE. As shown in Figure 5e and Figure S21, the band at 3340 cm⁻¹ can be assigned to O─H stretching vibrations of interfacial water associated with SO₄ ²⁻ in the electrolyte, whereas the feature at 1640 cm⁻¹ corresponds to water dissociation [54]. Notably, pronounced bands emerge at approximately 1220, 1150, and 1110 cm⁻¹, which are attributable to key NO₃RR intermediates including ^*NO₂, ^*NH₂, and ^*NH₂OH, respectively, and their intensities gradually increase as the potential becomes more cathodic [16, 55]. The band at 1440 cm⁻¹ can be assigned to NH₄ ⁺ related vibrations, indicating the formation of NH₃ during NO₃RR. In addition, a relatively weak band near 3720 cm⁻¹ is associated with an N─H vibrational mode of NH₃ [56]. Its low intensity suggests that the generated NH₃ desorbs rapidly from the catalyst surface, which is beneficial for sustaining continuous catalysis [57]. Operando FTIR results support a stepwise nitrate reduction pathway in the sequence NO₃ ⁻→ ^*HNO₃→ ^*NO₂→ ^*NO₂H→ ^*NO→ ^*NOH→ ^*NHOH→ ^*NH₂→ NH₃. This pathway highlights the crucial role of electron transfer and ^*H derived from water dissociation in driving the stepwise hydrogenation steps during NO₃RR. Based on the operando FTIR evidence, the Gibbs free energy profile (ΔG) for each elementary step along the NO₃RR pathway was constructed (Figure 5f). On the SnO₂‐CuO surface, NO₃ ⁻ is first adsorbed on Cu sites to form ^*NO₃, followed by hydrogenation to ^*HNO₃. This initial hydrogenation step from ^*NO₃ to ^*HNO₃ represents the rate‐determining step, with an energy barrier of only 0.58 eV, indicating favorable activation of adsorbed nitrate at the heterointerface. The free NO₃ ⁻ in the electrolyte preferentially adsorbs onto copper ion sites, and electron transfer from SnO₂ to CuO across the heterointerface promotes the reduction of adsorbed NO₃ ⁻ at copper sites, accelerating N─O bond cleavage to generate ^*NO₂ and ^*NO intermediates. Meanwhile, Sn sites produce abundant ^*H, which rapidly migrates to Cu sites driven by the built‐in electric field at the heterointerface, accelerating subsequent hydrogenation reactions for efficient ammonia production. Overall, the catalytic enhancement can be attributed to a spatially separated dual‐site cooperative mechanism. Sn sites preferentially facilitate water dissociation to generate reactive ^*H [58]. Cu sites serve as the primary centers for nitrate adsorption and reduction. The proximity between these two sites enables efficient interfacial hydrogen transfer. This process helps overcome the kinetic limitations of sequential proton‐coupled electron transfer steps.

A Zn‐NO₃ ⁻ battery was assembled using the SnO₂‐CuO composite as the cathode with 0.5 m K₂SO₄ and 0.1 m KNO₃ as the catholyte and a Zn foil as the anode with 1 m NaOH as the anolyte, enabling concurrent NH₃ synthesis and electrical energy output (Figure 6a). Figure 6b shows that the battery delivers a stable open‐circuit voltage (OCV) of 1.55 V. From the discharge linear sweep voltammetry curve (Figure 6c), the power density increases with current density and then decreases, reaching a peak value of 6.33 mW cm⁻². Galvanostatic discharge tests conducted at current densities from 0 to 20 mA cm⁻² show a gradual decrease in output voltage with increasing current (Figure 6d). After discharge, the cell voltage recovers well, and stable discharge plateaus are maintained across all tested currents, indicating robust long‐term electrochemical stability during energy delivery [59]. Reversibility arises from the Zn dissolution and deposition process at the anode and the reversible cathode reactions involving nitrate reduction during discharge and oxygen evolution from water during charge, rendering the Zn‐NO₃ ⁻ battery rechargeable [60]. At a constant current density of 8 mA cm⁻², the cell sustains 50 charge and discharge cycles with good electrochemical reversibility and operational reliability (Figure 6e). Continuous discharge also enables effective NH₃ production, achieving an NH₃ yield of 58.14 µmol h⁻¹ cm⁻² and FE of 83.10% at 15 mA (Figure 6f and Figure S22). To further evaluate the practical performance of the SnO₂‐CuO‐based Zn‐NO₃ ⁻ battery, its NH₃ yield rate and peak power density were compared with recently reported Zn‐NO₃ ⁻ battery systems. As shown in Figure S23 and Table S2, the SnO₂‐CuO battery delivers an NH₃ yield rate of 8.14 µmol h⁻¹ cm⁻² and a peak power density of 6.33 mW cm⁻², outperforming several representative systems. These results demonstrate that SnO₂‐CuO delivers highly efficient electrocatalytic ammonia synthesis from nitrate while also offering practical promise for applications in energy storage and nitrate‐contaminated wastewater remediation.

FIGURE 6.

(a) Schematic illustration for Zn‐NO₃ ⁻. (b) Open circuit voltage profile of the assembled Zn‐NO₃ ⁻ battery and a photograph of the device (inset). (c) Discharge polarization curve and the corresponding power density curve of the Zn‐NO₃ ⁻ battery. (d) Galvanostatic discharge curves. (e) Charge and discharge cycling profiles of the Zn‐NO₃ ⁻ battery at a constant current density. (f) NH₃ production test.

3. Conclusions

In this work, a SnO₂‐CuO hollow nanofiber architecture with hierarchical heterointerfaces, featuring SnO₂ nanorods embedded within CuO nanotube frameworks, was designed and constructed for efficient NO₃RR. The heterostructured hollow nanofiber architecture cooperatively induces interfacial charge reorganization, establishing a heterojunction‐spanning BIEF that enables more efficient electron transport and ^*H spillover, which imparts additional kinetic advantages for NO₃RR. Theoretical calculations indicate that Sn sites favor H₂O dissociation to generate ^*H, whereas Cu sites preferentially promote NO₃ ⁻ reduction to ^*NO₂. The in situ generated ^*H is transferred from Sn sites to Cu sites through a hydrogen spillover process, which accelerates the subsequent hydrogenation steps in NO₃RR and suppresses the competing hydrogen evolution reaction, enabling efficient NH₃ synthesis through a synergistic division of functions. The catalyst delivers outstanding performance, achieving a Faradaic efficiency of 94.89% and an NH₃ yield rate of 628.14 µmol h⁻¹ cm⁻² at an optimal potential of − 0.9 V vs. RHE, while maintaining stable activity over eleven consecutive cycles. Practical utility was further demonstrated by constructing a rechargeable Zn‐NO₃ ⁻ battery employing SnO₂‐CuO as the cathode. During discharge, this device continuously produces NH₃ while delivering a stable output voltage of 1.55 V and a peak power density of 6.33 mW cm⁻². This study provides an effective strategy that integrates environmental remediation with sustainable chemical production.

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll74149‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.