Direct Solar Power‐Driven Durable Ammonia Electrosynthesis From Nitrate Under Fluctuating Voltage Conditions

ABSTRACT

Nitrate electroreduction (NO₃RR) powered by solar energy offers a sustainable platform for ammonia (NH₃) synthesis. However, the variability of solar energy, leads to fluctuating voltages and frequent start‐stop of the system, presents significant challenges for catalysts in maintaining high efficiency and stability. In this work, the symmetry of charge distribution across distinct Cu facets was manipulated, achieving high NH₃ faradaic efficiencies (FEs) above 90% across a wide potential window of 800 mV in an H‐cell condition. Moreover, the membrane electrode assembly directly powered by a commercial silicon‐based solar panel operated for 8 days at a current density of 300 mA cm⁻² under fluctuating natural solar conditions (1.6 V–2.9 V), maintaining high NH₃ FE between 81%–94%. Mechanistic studies reveal that incorporating isolated Sn on the Cu (111) facet triggers a highly asymmetric charge distribution, which not only induces a weak field–dipole interaction between NO₃ ⁻ and SnCu111 surface but also promotes water dissociation kinetics and key intermediate hydrogenation, maximizing the free energy differentiation from the competing HER. This work provides a blueprint for practical catalyst design, offering a proof of concept for direct solar power‐driven NH₃ electrosynthesis.

Isolated Sn sites on Cu(111) induce a highly asymmetric charge distribution, which promotes water dissociation, accelerates key intermediate hydrogenation, and suppresses competing hydrogen evolution. As a result, SnCu111 delivers NH3 Faradaic efficiencies above 90% over an 800 mV potential window and enables stable direct solar power‐driven NH₃ synthesis under fluctuating operating conditions.

1. Introduction

Ammonia (NH₃) is not only one of the most important nitrogen‐containing compounds in modern agriculture but also a key intermediate in the chemical industry, energy storage, and carbon‐neutral energy systems [1, 2, 3]. The conventional Haber–Bosch process synthesizes NH₃ from nitrogen (N₂) and hydrogen (H₂) under high temperatures (400 °C–500 °C) and high pressures (10–30 MPa). Although this process enables large‐scale industrial production, it relies heavily on fossil fuels, resulting in extremely high energy consumption and substantial carbon emissions [4, 5]. In contrast, the electrochemical nitrate (NO₃ ⁻) reduction reaction (NO₃RR) to NH₃ presents a sustainable alternative that not only facilitates green NH₃ synthesis under ambient conditions but also enables the simultaneous remediation of NO₃ ⁻‐contaminated wastewater [6, 7, 8, 9, 10]. Therefore, developing low‐cost and energy‐efficient electrocatalytic NO₃RR systems for NH₃ synthesis is essential for enabling a green NH₃ economy.

The source of the supply power has great impact on the efficiency and sustainability of electrocatalytic NH₃ synthesis system. The grid‐powered operation condition (Figure 1a) provides stable voltage supply, while the predominance of fossil fuel‐based power generation results in a high carbon footprint and substantial cost [11, 12]. Switching to renewable energy sources (e.g., solar or wind) represents a necessary and promising alternative (Figure 1b) [13, 14, 15, 16]. However, the inherent intermittency of renewables poses a fundamental challenge: most electrocatalysts operate efficiently only within a narrow voltage window, and power fluctuations lead to declining conversion efficiency and unwanted byproducts (e.g. nitrite (NO₂ ⁻), H₂) [17, 18, 19]. To stabilize voltage fluctuations, energy storage devices or voltage regulators are required, which inevitably increase system complexity and overall cost [20]. According to recent benchmark analyses by the National Renewable Energy Laboratory (NREL) (Figure S1), incorporating energy storage systems can increase the levelized cost of electricity of commercial photovoltaic systems by nearly 70%, highlighting the significant economic burden associated with voltage stabilization infrastructure. Therefore, a more ideal scenario is to address the compatibility issue between fluctuating renewable power input and NO₃RR electrocatalysis system through rational catalyst design, requiring catalysts that can sustain high NH₃ Faradaic efficiency (FE) over wide voltage ranges and maintain long‐term stability under dynamic operating conditions (Figure 1c).

FIGURE 1.

Schematic illustration of power supply strategies for electrochemical nitrate reduction (NO₃RR): (a) stable voltage supply via grid‐powered operation; (b) stable voltage supply via renewable energy with intermediate energy storage; (c) fluctuating voltage supply via direct renewable energy input.

As is known, NO₃RR is a complex process involving multiple proton‐coupled electron transfer (PCET) steps. Moreover, it is highly sensitive to voltage fluctuations, which can disrupt the reaction equilibrium, leading to the desorption of key intermediates such as NO₂ ⁻ and promoting the competing hydrogen evolution reaction (HER) [21, 22, 23, 24, 25]. Although numerous pioneering studies have achieved substantial improvements in NO₃RR efficiency and selectivity, most have been conducted under idealized steady‐state laboratory conditions [5, 26, 27, 28, 29, 30, 31, 32, 33]. In contrast, investigations under realistic, dynamically fluctuating voltage inputs remain scarce. Therefore, there is an urgent need to develop robust electrocatalysts that can be systematically integrated into renewable‐energy‐driven systems to enable stable and efficient NH₃ synthesis under realistically harsh operating conditions.

In this work, we developed a facet‐engineered Sn‐doped Cu catalyst with predominantly exposed (111) facets (SnCu111) to achieve durable and efficient NH₃ synthesis under fluctuating voltage conditions. The SnCu111 catalyst exhibited exceptional performance in an H‐cell, achieving >90% FE for NH₃ over an 800 mV potential window from −0.17 to −0.97 V vs. RHE. When applied to a membrane electrode assembly (MEA), the catalyst still achieved more than 90% FE for NH₃ within a 700‐mV cell‐voltage range. Furthermore, when directly coupled with commercial silicon‐based solar cells under naturally fluctuating light conditions, the system operated for eight days at an industrial‐level current density of 300 mA cm⁻², maintaining an NH₃ FE of 81%–94%, demonstrating excellent tolerance to nature solar power fluctuations. Together with in situ Fourier‐transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations, these results demonstrate that isolated Sn incorporation on the Cu (111) facet triggers an asymmetric charge distribution, inducing a localized polarization field. This field disrupts the interfacial hydrogen‐bond network, thereby promoting water dissociation. Moreover, it effectively stabilizes multiple N‐containing intermediates and facilitates their subsequent hydrogenation. As a result, a significant free‐energy differentiation between NO₃RR and the competing HER is established, realizing an efficient and continuous production of NH₃ under unstable voltage supplied by solar cells.

2. Results and Discussion

2.1. Synthesis and Characterization of Facet Engineered SnCu Catalyst

Cu, the most common catalyst for NO₃RR, was selected as our model system. To broaden its efficient voltage window for fluctuating voltage applications, it is essential to suppress both the HER and NO₂ ⁻ accumulation, which are the two main competing pathways on Cu‐based catalysts [34, 35]. Incorporating Sn has been widely recognized to mitigate HER [26, 36, 37], whereas the surface orientation of Cu also plays a crucial role in determining NO₃RR selectivity [38, 39, 40]. To investigate the effect of the two factors, Cu crystals with distinct crystal orientation were selected. The octahedral and cubic shapes predominantly expose the (111) and (100) facets, respectively (Figure 2a). Sn atoms were then introduced onto Cu111 and Cu100 to construct four atomic models (Figure S2). DFT calculations were then used to probe the Gibbs free energy changes (ΔG) for HER and the critical *NO₂ → *NO₂H hydrogenation step on these four models. As shown in Figure 2b, Sn incorporation increased the Gibbs free energy required for the HER on both facets, with SnCu111 exhibiting the highest value (0.86 eV). Moreover, SnCu111 was the only surface where the *NO₂ → *NO₂H step was thermodynamically spontaneous (ΔG < 0). These results indicate a powerful synergy between the (111) facet and Sn doping, which concurrently impedes H₂ evolution and facilitates the hydrogenation of *NO₂, thereby expected to steer the reaction selectively toward NH₃.

FIGURE 2.

a) Atomic models of Cu111 and Cu100. b) Gibbs free energy changes of HER and *NO₂ → *NO₂H step on SnCu111, SnCu100, Cu111, and Cu100 models. c) Projected density of states (PDOS) of Cu d orbitals on Cu100, SnCu100, Cu111, and SnCu111. d) Bader charge distribution of surface atoms on SnCu100 and SnCu111. e) Adsorption configurations of NO₃ ⁻ on SnCu100 and SnCu111. f) Adsorption free energies of NO₃ ⁻ (ΔG_NO₃) and NH₃ (ΔG_NH₃) on Cu100, SnCu100, Cu111, and SnCu111. g) Schematic illustration of the electroreduction transformation from Sn/Cu₂O precursors to SnCu111 and SnCu100. h) XRD patterns of Sn/Cu₂O111 after different electroreduction times. i) High angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) images of SnCu111 (inset figure: transmission electron microscopy (TEM) image). j) Intensity profile extracted from a selected region in the HAADF image.

To gain deeper insight into the electronic structure variations induced by Sn incorporation, total density of states (TDOS) analysis was conducted (Figure S3). Compared with pristine Cu111 and Cu100, both SnCu111 and SnCu100 exhibit significantly higher TDOS near the Fermi level, suggesting enhanced electronic conductivity and facilitated interfacial electron transfer [41]. Such electronic characteristics are essential for sustaining the multi‐electron transfer process involved in the eight‐electron NO₃RR pathway. Further projected density of states (PDOS) analysis of the Cu d orbitals reveals that Sn incorporation induces an overall downshift of the Cu d band center relative to pristine Cu surfaces. Specifically, the d band center shifts from −1.9647 eV on Cu100 to −2.2797 eV on SnCu100, and from −2.1114 eV on Cu111 to −2.2165 eV on SnCu111, respectively (Figure 2c). Notably, SnCu111 exhibits a medium d band center among these surfaces, indicating a moderate tuning of intermediate adsorption strength [42, 43].

To further elucidate the potential influence of interfacial charge distribution, Bader charge analysis was performed (Figure 2d). The results reveal that although electron transfer from Sn to Cu occurs on both facets, their spatial charge distribution characteristics differ fundamentally. On the SnCu100 surface, the redistributed charges remain highly symmetric, with the Sn atom carrying a positive charge of +0.28 e and the surrounding Cu atoms uniformly gaining approximately −0.03 e, resulting in a relatively uniform distribution. In contrast, SnCu111 exhibits pronounced charge asymmetry, with the charges on neighboring Cu atoms fluctuating significantly in the range of −0.02 e to −0.06 e. This anisotropic charge redistribution constructs a nonuniform local polarization field on the SnCu111 surface [44, 45]. Upon NO₃ ⁻ adsorption, this field acts on the originally planar‐symmetric NO₃ ⁻ and induces geometric distortion, with the out‐of‐plane tilt angle of the N–O bond decreasing from 180° to 176° (Figure 2e). This symmetry breaking enhances the dipole moment of the adsorbed NO₃ ⁻. Consequently, the polarized NO₃ ⁻ further couples strongly with the local polarization field through field–dipole interaction [46, 47], providing an additional electrostatic stabilization for the adsorption of NO₃ ⁻.

Further adsorption energy analysis reveals that Sn incorporation leads to a concurrent weakening of NO₃ ⁻ and NH₃ adsorption on the Cu (100) facet (Figure 2f). In contrast, on the Cu (111) facet, although Sn doping similarly weakens NH₃ adsorption, the adsorption of NO₃ ⁻ is significantly enhanced due to the additional electrostatic stabilization provided by the local polarization field. Consequently, SnCu111 achieves differential regulation of the adsorption energies for both NO₃ ⁻ and NH₃, thereby breaking the linear scaling relationship, which is expected to enhance the overall NO₃RR reaction kinetics.

To verify the theoretical predictions, Sn‐Cu catalysts were synthesized through sequential chemical and electrochemical reduction. Initially, Sn‐doped Cu₂O samples with distinct crystal orientations were prepared by chemical reduction. As shown in Figure S4, the x‐ray diffraction (XRD) patterns of Cu₂O111, Cu₂O100, Sn/Cu₂O111, and Sn/Cu₂O100 all display characteristic diffraction peaks at approximately 36.5° and 42.4°, corresponding to the (111) and (200) planes of Cu₂O, respectively [48, 49]. No reflections of Sn or SnO _x are observed, indicating that Sn atoms were incorporated into the Cu₂O lattice rather than forming separate phases. During subsequent in situ electrochemical reduction (Figure 2g), these oxides were gradually converted into metallic SnCu alloys. The XRD evolution of Sn/Cu₂O111 at different reduction times (Figure 2h) shows the emergence of metallic Cu peaks (43.3° and 50.4°) concurrent with the decay of the Cu₂O signals. After 2 h, the complete disappearance of Cu₂O peaks confirms the full reduction to a metallic SnCu111 alloy with a predominant (111) facet. Consistently, the XRD patterns of the other three electroreduced final catalysts (Figure S5) confirm full conversion to the corresponding metallic phases and retention of their facet preference, yielding Cu111, Cu100, and SnCu100.

The electronic structure evolution of the catalysts was further investigated by x‐ray photoelectron spectroscopy (XPS). As shown in Figure S6a, the Sn 3d peak of the Sn/Cu₂O111 precursor shifts to lower binding energies after electrochemical reduction, indicating the reduction of Sn oxide species [37]. A similar shift toward lower binding energy is also observed for SnCu100 (Figure S6b), further confirming the reduction of Sn species during the electrochemical reduction process. In addition, the Cu 2p spectra before and after reduction of SnCu111 are presented in Figure S7a. No characteristic shake‐up satellite peaks associated with Cu²⁺ species are observed, indicating the absence of Cu²⁺ in both samples. Because Cu⁰ and Cu⁺ possess very similar Cu 2p binding energies, Cu LMM Auger spectroscopy was further employed (Figure S7b). The results reveal that the Cu⁺ species in the Sn/Cu₂O111 precursor are converted into predominantly metallic Cu⁰ after reduction. Furthermore, the Cu LMM spectra of SnCu100, Cu111, and Cu100 (Figure S8) are also dominated by the characteristic Cu⁰ feature. These results collectively demonstrate the successful formation of Sn‐modified metallic Cu catalysts through the electrochemical reduction process. Inductively coupled plasma optical emission spectrometry (ICP‐OES) confirms the successful incorporation of Sn into the Cu matrix, with Sn content around 3 wt% (Table S1) This comparable Sn content thereby attributes the performance differences between SnCu111 and SnCu100 primarily to their facet orientation rather than composition.

The morphologies of the catalysts were examined by scanning electron microscopy (SEM). Figure S9 shows that SnCu111 and Cu111, together with their oxide precursors, exhibit well‐defined octahedral morphologies without noticeable structural changes after electrochemical reduction or Sn incorporation. Similarly, SnCu100 and Cu100, along with their Cu₂O precursors, retain cubic morphologies (Figure S10). To further elucidate the structure of SnCu111 at the atomic scale, high‐resolution transmission electron microscopy (HRTEM) and aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) were conducted. The HRTEM image shows lattice fringes with an interplanar spacing of approximately 0.208 nm, corresponding to the Cu111 planes (Figures 2i and S11) [50]. No separate crystalline Sn domains are detected, suggesting the formation of a homogeneous alloy structure. In the HAADF‐STEM image, isolated bright spots can be resolved on the Cu lattice, and the corresponding intensity profile verifies atomic‐level dispersion of Sn within the Cu framework (Figure 2j). Energy‐dispersive x‐ray spectroscopy (EDX) mapping further confirms a uniform Sn distribution across the alloy particles with no significant aggregation (Figure S12). For comparison, similar structural characteristics were also observed for SnCu100. HRTEM analysis confirmed the preservation of the Cu(100) lattice after Sn incorporation, while atomic‐resolution HAADF‐STEM images revealed isolated bright contrast features attributable to individual Sn atoms (Figures S13 and S14). Furthermore, EDX mapping demonstrated a homogeneous distribution of Sn without detectable Sn‐rich domains or aggregation (Figure S15). Collectively, these results provide strong evidence that Sn is highly dispersed and predominantly exists as isolated atomic sites in both SnCu111 and SnCu100 catalysts.

2.2. The NH₃ Electrosynthesis Performance of Catalysts in H‐Type Cell

The electrochemical NO₃RR performance was first evaluated in 1 M KOH + 0.1 M KNO₃ using a three‐electrode H‐type cell (Figure S16). As a preliminary assessment, linear sweep voltammetry (LSV) was recorded in electrolytes with and without 0.1 M NO₃ ⁻. As shown in Figures 3a and S17, all catalysts display a pronounced increase in cathodic current upon the addition of NO₃ ⁻, implying their catalytic activity toward NO₃RR. Notably, SnCu111 exhibits the largest current enhancement after NO₃ ⁻ introduction, together with the most positive onset potential (≈0.20 V vs. RHE), indicating superior intrinsic NO₃RR activity and a lower barrier for NO₃ ⁻ activation. Then, chronoamperometry (CA) was carried out in 1 M KOH with 0.1 M KNO₃ at applied potentials from −0.17 to −0.97 V vs, RHE to further evaluate NO₃RR performance. NH₃ was quantified using the indophenol blue method (Figure S18), while the by‐products NO₂ ⁻ and H₂ were analyzed using the Griess method and gas chromatography, respectively (Figures S19 and S20) [26]. In addition, no detectable N₂ was observed for any catalyst under the investigated operating conditions according to gas chromatography analysis, suggesting that N₂ was not a major by‐product in the present NO₃RR system. As shown in Figures 3b and S21, FEs for NH₃ production were compared across the four catalysts over a range of applied potentials. SnCu111 exhibits outstanding selectivity, maintaining FE values above 90% across an 800 mV potential window from −0.17 to −0.97 V vs. RHE. Particularly, at −0.47 V vs. RHE, the FE remains nearly 100%, indicating highly efficient electron utilization toward NH₃ formation. In contrast, Cu111 maintains NH₃ FEs above 90% only within a narrow potential window from −0.37 to −0.57 V vs. RHE, beyond which its FE decreases sharply, reaching only 47% at −0.97 V vs. RHE. As indicated in Figure S22, this decline results from a markedly HER. This trend highlights that Sn incorporation effectively suppresses HER. A similar behavior is observed on Cu100, where severe HER at high potentials limits the maximum NH₃ FE to below 90%. Upon introducing Sn, HER suppression is achieved on SnCu100 as well, enabling it to maintain NH₃ FE above 90% between −0.37 and −0.67 V vs. RHE. Notably, Sn incorporation suppresses HER most effectively on the Cu (111) facet, allowing SnCu111 to retain high NH₃ FE even at more negative potentials. Furthermore, Figure S23 shows that SnCu111 exhibits the lowest FE toward NO₂ ⁻, demonstrating that the further hydrogenation of NO₂ ⁻ is most favorable on the Sn‐doped Cu (111) facet, thereby effectively suppressing its accumulation. As a result, the improved suppression of HER together with the enhanced NO₂ ⁻ hydrogenation enables SnCu111 to deliver the highest NH₃ yield across the entire potential range, reaching up to 19.8 mg h⁻¹ cm⁻² at −0.87 V vs. RHE (Figure S24). In addition, the half‐cell energy efficiency (EE) of the four catalysts was evaluated to assess their energy utilization during NO₃RR. As shown in Figure S25, SnCu111 consistently exhibits the highest EE over the entire investigated potential range, reaching a maximum value of 36.5% at −0.17 V vs. RHE. In comparison, SnCu100, Cu111, and Cu100 show lower energy efficiencies under identical conditions. These results further demonstrate that the synergistic effect of Sn incorporation and the Cu(111) facet not only enhances NH₃ production and selectivity but also improves the overall energy utilization efficiency of the NO₃RR process.

FIGURE 3.

a) Linear sweep voltammetry (LSV) polarization curves of Cu111, SnCu100, and SnCu111 with/without NO₃ ⁻ in 1 M KOH. b) NH₃ FE over different catalysts at various potentials. c) NH₃ yield rate and FE of SnCu111 at –0.47 V vs RHE at different NO₃ ⁻ concentrations. d) Comparison of FE and potential window (FE > 90%) of SnCu111 with reported Cu‐based catalysts. e) Recycling stability test of SnCu111 at –0.47 V vs RHE over 15 cycles.

To exclude the possible contribution of surface area effects, the electrochemically active surface areas (ECSAs) of Cu111, SnCu111, Cu100, and SnCu100 were evaluated by double‐layer capacitance (Cdl) measurements (Figures S26 and S27a). The catalysts exhibit comparable Cdl values, indicating that Sn incorporation does not substantially change the electrochemically active surface area. Furthermore, the NH₃ yields were normalized by ECSA (Figure S27b). SnCu111 remains the most active catalyst after ECSA normalization, indicating that its superior NO₃RR performance originates from enhanced intrinsic catalytic activity rather than differences in electrochemically active surface area. Collectively, these results confirm that the combined effect of Sn incorporation and the Cu (111) facet creates an optimized surface environment that simultaneously suppresses HER and facilitates NO₂ ⁻ conversion, thereby effectively broadening the high‐FE potential window for NH₃ production, in agreement with the theoretical predictions.

To further investigate the influence of Sn loading on NO₃RR performance, additional SnCu111 catalysts with nominal Sn contents of 1% and 5% were synthesized. ICP‐OES measurements confirmed actual Sn contents of 1.06% and 4.67%, respectively (Table S1). SEM and XRD analyses revealed no significant changes in morphology or crystal structure with varying Sn loading (Figures S28 and S29). As shown in Figure S30, all SnCu111 catalysts exhibit highly similar LSV curves, NH₃ FEs, and NH₃ yields over the investigated potential range. These results indicate that varying the Sn loading has only a minor influence on NH₃ selectivity, suggesting that the enhanced NH₃ selectivity primarily originates from the incorporation of isolated Sn species rather than a specific Sn loading. To further verify the reliability of NH₃ quantification, ¹H NMR spectroscopy was employed to measure NH₃ yield and FE (Figure S31) [51]. The results obtained by NMR are consistent with those from the indophenol blue method, confirming the accuracy of the measurements. In addition, a series of control experiments were conducted to exclude possible NH₃ contamination from atmospheric N₂, reagents, and non‐electrochemical processes (Figure S32 and Table S4). Similar NH₃ yields were obtained under ambient and Ar‐purged conditions, while negligible NH₃ was detected in NO₃ ⁻‐free electrolyte and under open‐circuit conditions. Moreover, no NH₃ were found in the raw materials used for electrolyte preparation. These results collectively demonstrate that the detected NH₃ originates from the electrochemical reduction of NO₃ ⁻.

Given the wide variation of NO₃ ⁻ concentrations in wastewater, the performance of SnCu111 was further evaluated at different NO₃ ⁻ levels (Figure 3c). At −0.47 V vs. RHE, SnCu111 maintains NH₃ FE above 90% across 25–500 mM NO₃ ⁻, demonstrating robust selectivity over a broad concentration range and suitability for streams with diverse NO₃ ⁻ loadings. To benchmark the catalytic performance, Figure 3d and Table S2 compare the maximum NH₃ FE and the potential window sustaining FE > 90% among various Cu‐based catalysts. Most reported Cu‐based catalysts can only achieve their high FE within a narrow potential window. In contrast, SnCu111 delivers an almost 100% maximum FE and maintains FE above 90% across an exceptionally wide 800 mV potential window, highlighting its superior selectivity. Finally, a CA cycling test was employed to assess the durability of SnCu111. As shown in Figure 3e, the NH₃ FE and yield remain essentially unchanged over 15 consecutive cycles at −0.47 V vs. RHE, indicating excellent cycling stability.

2.3. Full Cell Performance of SnCu111

To further evaluate the practical applicability of SnCu111, a MEA configuration was constructed, as illustrated in Figure 4a. The MEA employed SnCu111 as the cathode, a proton exchange membrane (PEM) as the separator, and a stainless‐steel mesh anode for the oxygen evolution reaction (OER). A catholyte of 1 M KOH + 0.1 M KNO₃ and an anolyte of 1 M KOH were used. To benchmark the device‐level performance, Cu111, Cu100, and SnCu100 were also evaluated under identical MEA conditions. As shown in Figure S33, LSV curves of the catalysts were recorded in the MEA configuration. Among them, SnCu111 delivers the highest current density across the entire voltage range, indicating superior NO₃RR activity under device‐level operation. Chronopotentiometric tests were subsequently performed at different cell voltages to evaluate NH₃ production performance (Figures 4b and S34). Consistent with the results obtained in the H‐type cell, SnCu111 maintains NH₃ selectivity above 90% across a wide cell‐voltage window of up to 700 mV, demonstrating its strong capability to suppress competing reactions even under device‐level operation. In contrast, SnCu100 shows NH₃ FEs above 90% over a narrower voltage window of approximately 300 mV. In addition, Cu111 and Cu100 exhibit much lower NH₃ FEs at the same voltage range. Notably, at a cell voltage of 3.0 V, SnCu111 delivers a high current density of 330 mA cm⁻², meeting the benchmark for industrially relevant performance [52, 53, 54]. These results demonstrate that the combination of Sn incorporation and the Cu(111) facet remain highly effective in suppressing competing reactions and preserving NH₃ selectivity under practical device conditions.

FIGURE 4.

a) Schematic of the membrane–electrode assembly (MEA) structure. b) NH₃ FE and current density at different cell voltages. c) NH₃ FE under simulated voltage fluctuations. d) Voltage and NH₃ FE under real sunlight fluctuations in one day from 9 am to 3 pm. e) Current density and NH₃ FE of the direct solar‐driven NO₃RR system over 8 days from 9 am to 3 pm.

To investigate catalyst robustness under realistic intermittent‐power conditions, voltage fluctuation tests were conducted using all four catalysts. As shown in Figure S35, all catalysts exhibit rapid and reversible current responses following the applied voltage profile. However, markedly different NH₃ selectivities are observed among the catalysts during voltage fluctuations (Figures 4c and S36). SnCu111 consistently maintains NH₃ FEs above 90% throughout the entire test, whereas SnCu100 exhibits NH₃ FEs of approximately 80%. In comparison, Cu111 and Cu100 only maintain NH₃ FEs of approximately 60%–70%. The superior NH₃ selectivity retained by SnCu111 under continuously changing operating voltages further confirms its strong resistance to HER and its ability to sustain efficient NO₃ ⁻‐to‐ NH₃ conversion under fluctuating power inputs.

We then constructed a demonstration system to evaluate the feasibility of direct solar‐driven NO₃RR by coupling the SnCu111‐MEA device with a commercial silicon photovoltaic panel. Under standard AM 1.5G illumination (100 mW cm⁻²), the current density‐voltage (J–V) characteristics of the solar cell exhibit an open‐circuit voltage (Voc) of 3.71 V, a short‐circuit current (Isc) of 354 mA, and a maximum power point (MPP) at approximately 3.0 V and 330 mA (Figure S37a). Under simulated 1 sun operation, the photovoltaic panel was directly connected to the SnCu111‐MEA device without any external bias or energy‐storage unit. The coupled system maintained a stable operating current of approximately 328 mA while preserving an average NH₃ FE of 93.15%, corresponding to a solar‐to‐ammonia (STA) efficiency of 1.83% (Figure S37b).

To further evaluate the performance under practical operating conditions, outdoor solar‐driven NO₃RR experiments were subsequently conducted using the same photovoltaic‐powered system (Figure S38). The product was collected every two hours for NH₃ quantification, and the corresponding FE values represent the average performance over each two‐hour interval. As shown in Figure 4d, throughout a single operating day (9:00 am–3:00 pm), the system voltage fluctuated continuously in response to changes in solar irradiance. Despite these variations, the MEA consistently maintained a high NH₃ FE (>85%), demonstrating strong tolerance toward real‐time solar‐driven voltage dynamics. To further evaluate durability under practical outdoor conditions, the device was operated under natural sunlight for eight days. As shown in Figures 4e and S39, both the cell voltage and current density varied significantly with weather conditions. The voltage fluctuated between 1.6 and 2.9 V, and the current density reached up to 300 mA cm⁻² at peak sunlight. Remarkably, the NH₃ FE remained consistently high, exceeding 90% on sunny days and staying above 80% even on cloudy days. Occasional decreases in FE may be associated with periods of rapidly changing solar irradiance, where voltage fluctuations temporarily shifted the operating conditions away from the optimal NO₃RR window. Despite experiencing pronounced voltage and current fluctuations, as well as repeated daily start–stop cycles, the system showed no noticeable decline in current density or NH₃ FE on the eighth day compared with the first day, demonstrating the excellent stability of the SnCu111‐based MEA under real outdoor operating conditions. Post‐reaction XRD, XPS, ICP‐OES, electron microscopy, and elemental mapping analyses further confirmed that the crystal structure, surface chemical state, and highly dispersed Sn sites were preserved after long‐term operation (Figures S40–S43 and Table S1). Together, these results confirm that SnCu111 enables a robust and durable direct solar‐driven NO₃RR system.

2.4. The Electracatalytic Mechanism Analysis

To elucidate the roles of Sn incorporation and crystal facet in governing the NO₃RR process, a series of mechanistic investigations were conducted. Electrochemical impedance spectroscopy (EIS) was first performed to evaluate the interfacial charge‐transfer behavior of the catalysts. As shown in Figure 5a, all Nyquist plots display typical semicircles that can be fitted using an equivalent circuit consisting of solution resistance (Rs), charge‐transfer resistance (R _ct), and a constant phase element (CPE). Among the four catalysts, SnCu111 exhibits the smallest semicircle diameter, corresponding to the lowest R _ct, followed by SnCu100, whereas Cu111 and Cu100 display significantly larger charge‐transfer resistances. These results indicate that both Sn incorporation and the Cu(111) facet facilitate interfacial electron transfer, with the strongest effect observed for SnCu111.

FIGURE 5.

a) Electrochemical impedance spectroscopy (EIS) Nyquist plots of SnCu111, SnCu100, Cu100 and Cu111. b) In situ Fourier‐transform infrared (FTIR) spectroscopy of SnCu111 at various potentials. c) In situ FTIR spectroscopy of Cu111 at various potentials. d) Deconvolution of the O–H stretching region for SnCu111 at different potentials. e) Potential‐dependent proportion of free H₂O on SnCu111, SnCu100, Cu111, and Cu100 derived from FTIR peak fitting. f) NH₃ yield of SnCu111 in the absence and presence of tert‐butyl alcohol (TBA). g) Kinetic isotope effect (KIE) values of SnCu111, SnCu100, and Cu111 at –0.47 V vs. RHE. h) Calculated free energy for water dissociation on SnCu111, SnCu100, Cu100, and Cu111. i) Free energy diagrams for the NO₃RR to NH₃ on SnCu111.

Cyclic voltammetry (CV) measurements were further conducted to probe the intrinsic redox properties of the catalysts. As shown in Figure S44a, all catalysts exhibit a distinct reduction peak at approximately 0.35–0.37 V vs. RHE, attributed to the reduction of Cu₂O (Cu⁺ → Cu⁰), together with an oxidation peak at approximately 0.59–0.62 V vs. RHE corresponding to Cu oxidation (Cu⁰ → Cu⁺). A closer examination of the reduction peak (Figure S25b) reveals that SnCu111 exhibits the most positively shifted reduction potential (∼370 mV), compared with SnCu100 (∼356 mV), Cu111 (∼355 mV), and Cu100 (∼354 mV). Conversely, the oxidation peak of SnCu111 appears at ∼593 mV, negatively shifted relative to SnCu100 (∼614 mV), Cu111 (∼616 mV), and Cu100 (∼617 mV) (Figure S44c). Consequently, SnCu111 exhibits the smallest redox potential gap among all catalysts. Such positively shifted reduction and negatively shifted oxidation peaks indicate more facile redox conversion and faster interfacial electron‐transfer kinetics [55, 56]. These observations are consistent with the EIS results, further demonstrating that the combination of isolated Sn sites and the Cu(111) facet synergistically lower the charge‐transfer resistance and promotes NO₃RR kinetics.

To directly probe the surface intermediates and reaction pathway, in situ FTIR spectroscopy was employed to monitor the NO₃RR process. The potential‐dependent spectra of the four catalysts are shown in Figures 5b,c and S45. In the low‐wavenumber region (1200–1700 cm⁻¹), they show similar peaks at 1097 cm, 1104 cm, 1240 cm, 1300 cm, 1460 cm, 1638 cm and 1654 cm⁻¹, assigned to NO₃ ⁻, NH₂OH, NO₂ ⁻, NH₂, NH₃, H₂O δ(H–O–H) bending, NO, respectively [57, 58, 59, 60]. The appearance of these common intermediates indicates that all catalysts follow a similar associative NO₃RR pathway. Notably, the NO₂ ⁻ signal on SnCu111 is significantly weaker than that on Cu111, particularly at low overpotentials (Figure 5b,c). A similar trend is also observed for SnCu100 relative to Cu100 (Figure S45), indicating that Sn incorporation facilitates the further conversion of NO₂ ⁻ intermediates on both Cu facets and reduces their surface accumulation [61]. In contrast, the NH₃ signal follows the opposite trend. Among all catalysts, SnCu111 exhibits the strongest NH₃ band intensity, while SnCu100 also displays a stronger NH₃ signal than Cu100. These results suggest that Sn incorporation promotes the hydrogenation of NO_x intermediates toward NH₃ formation.

Further insights into interfacial water activation were obtained from the O–H stretching region (3000–3700 cm⁻¹). As shown in Figures 5d and S46, the broad water band can be deconvoluted into strongly hydrogen‐bonded water (4‐HB H₂O), weakly hydrogen‐bonded water (2‐HB H₂O), and free H₂O components [62, 63]. Quantitative analyses of the H₂O populations are provided in Figures 5e and S47. Notably, the proportion of free H₂O increases with increasing cathodic polarization for all catalysts (Figure 5e). For both Cu facets, Sn incorporation leads to a substantially higher free‐H₂O content, with SnCu111 and SnCu100 consistently exhibiting larger free‐H₂O populations than their corresponding Cu counterparts. In addition to the Sn effect, the crystal facet also influences the interfacial water structure. The Cu(111)‐derived catalysts consistently exhibit higher free‐H₂O contents than their Cu(100)‐derived counterparts, with SnCu111 displaying the highest free‐H₂O proportion among all catalysts. Since free H₂O molecules are less restricted by hydrogen‐bond interactions and can more readily participate in interfacial dissociation processes [64], the elevated free‐H₂O population on SnCu111 indicates the most favorable environment for water activation and surface‐active hydrogen (*H) generation. This enhanced interfacial water activation is expected to facilitate the hydrogenation of NO _x intermediates, consistent with the FTIR observations and the superior NH₃ production performance of SnCu111.

To further probe the role of *H species during NO₃RR, the *H quenching test was conducted with the addition of tert‐butyl alcohol (TBA) as a scavenger [42, 65]. As shown in Figures 5f and S48, the NH₃ yields of SnCu111 and the other three catalysts decreases markedly after the addition of TBA, indicating that quenching *H species significantly suppresses NH₃ formation. This result confirms that hydrogenation mediated by surface *H is a crucial step in the NO₃RR process. Moreover, the pronounced performance loss after TBA addition highlights the important role of interfacial water activation in supplying *H species required for the hydrogenation of NO_x intermediates.

To further verify the role of interfacial water activation, kinetic isotope effect (KIE) experiments were performed by replacing H₂O with D₂O [33, 62, 66]. As shown in Figure S49, all catalysts exhibit lower NH₃ yields in D₂O than in H₂O, confirming the involvement of water‐derived hydrogen species in NO₃RR. The calculated KIE values are summarized in Figure 5g. Compared with the corresponding pristine Cu catalysts, Sn incorporation decreases the KIE value from 1.22 to 1.11 on Cu111 and from 1.37 to 1.20 on Cu100, indicating that Sn facilitates interfacial water activation on both facets. Notably, SnCu111 exhibits the lowest KIE value among all catalysts, suggesting that the promoting effect of Sn on water dissociation is more pronounced on the Cu(111) facet. Along with the FT‐IR results, this behavior is likely associated with the asymmetric charge distribution on SnCu111, which induces a localized polarization field capable of restructuring the interfacial hydrogen‐bond network. The reconstructed water network facilitates water dissociation and increases the availability of *H species for the hydrogenation of N‐containing intermediates during NO₃RR [67, 68].

To gain atomic‐level insights into the reaction mechanism, DFT calculations were performed. First, the thermodynamics of water dissociation were evaluated to probe interfacial water activation. As shown in Figure 5h, the free‐energy barrier for water dissociation decreases in the order Cu100 (0.93 eV) > Cu111 (0.81 eV) > SnCu100 (0.73 eV) > SnCu111 (0.61 eV). These results indicate that both Sn incorporation and the Cu(111) facet facilitate water activation, with SnCu111 providing the most favorable environment for water dissociation. Inspection of the optimized adsorption configurations (Figure S50) reveals that water dissociation on the Sn‐containing catalysts occurs at an interfacial Sn–Cu site. Specifically, H₂O initially adsorbs on the isolated Sn atom, followed by O─H bond cleavage, with the OH fragment remaining bound to Sn while the dissociated H is transferred to a neighboring Cu site. This cooperative adsorption mode highlights the role of the oxophilic Sn site in water activation and contributes to the reduced water dissociation barrier [69, 70]. The substantially lower barrier on SnCu111 compared with SnCu100 further suggests that the Cu(111) facet provides a more favorable geometric and electronic environment for water activation. These results are in excellent agreement with the in situ FTIR and KIE measurements, consistently demonstrating enhanced interfacial water activation on SnCu111.

The free‐energy profiles for NO₃ ⁻ reduction to NH₃ on SnCu111 and the other three catalysts are shown in Figures 5i and S51–S53, while the corresponding adsorption configurations are presented in Figure S54–S57. For all catalysts, NO₃RR proceeds through a common associative pathway involving sequential hydrogenation of *NO₃ → *NO₂ → *NO₂H → *NO → *NHO → *NHOH → *NH₂OH → *NH₂ →*NH₃. Although the overall reaction pathway remains unchanged, both Sn incorporation and crystal facet markedly influence the reaction energetics. Specifically, SnCu111 and Cu111 share the same PDS of NH₃ desorption, with ΔG values of 0.22 eV and 0.37 eV, respectively, indicating that Sn incorporation on the (111) facet significantly facilitates NH₃ release. For SnCu100, the PDS shifts to the hydrogenation of *NO to *NHO (ΔG = 0.34 eV), substantially higher than the corresponding step on SnCu111 (0.18 eV), demonstrating that the Cu(111) facet provides a more favorable surface environment for *NO hydrogenation under Sn incorporation. For Cu100, the PDS corresponds to the hydrogenation of *NO₂ to *NO₂H (ΔG = 0.39 eV), the highest among all four catalysts. These contrasts collectively highlight the synergistic role of Sn incorporation and the (111) facet in lowering the energy barriers associated with key NO _x hydrogenation steps. As a result, SnCu111 provides the most favorable thermodynamics for the conversion of N‐containing intermediates. This difference may be attributed to the localized polarization field induced by isolated Sn atoms on the Cu(111) facet, which may stabilize key reaction intermediates and reduce the free‐energy requirements of the corresponding hydrogenation steps. Overall, the incorporation of isolated Sn atoms on the Cu (111) facet induces an asymmetric charge distribution, giving rise to a localized polarization field. This field not only facilitates interfacial water activation for *H supply but also stabilizes multiple N‐containing intermediates and facilitates their hydrogenation, thereby achieving a maximized free‐energy differentiation between NO₃RR and the competing HER.

3. Conclusion

In summary, by combining facet engineering with atomic metal doping, SnCu111 catalyst was developed that exhibits excellent voltage‐fluctuation tolerance and operational stability in a direct solar‐powered NO₃RR system. Mechanistic analysis reveals that atomic Sn on the Cu (111) facet creates an asymmetric charge distribution that not only enhances interfacial *H generation and promotes the hydrogenation of key N‐containing intermediates but also effectively suppresses the competing HER, thereby imparting strong tolerance to voltage fluctuation. This study provides a design blueprint for directly coupling renewable energy with electrocatalytic processes without relying on energy‐storage components, and the fluctuation‐tolerant catalytic concept established here is expected to be extendable to a broad range of multi‐electron conversion reactions.

Author Contributions

X.W. and Q.W. conceived and designed the project. J.C. and Q.W. supervised the project. X.W. prepared the samples and performed the electrocatalytic measurements. X.W., J.L., L.W., and S.T. performed the characterizations, including XRD, SEM, TEM, HAADF‐STEM, FTIR, XPS, in situ FTIR and ICP‐OES, etc. X.W. and Q.W. analyzed the electrocatalytic mechanism. J.L. performed the DFT calculations. X.W. wrote the manuscript. Q.W., C.L. and J.C. helped to revise the manuscript. X.W. and J.L. contributed equally to this work. All the authors discussed the results and commented on the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie74173‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.