In situ ammonia synthesis and energy generation via aqueous Zn–NO₃⁻ batteries

Abstract

The sustainable generation of electricity and ammonia (NH₃) is essential for modern industrial advancement. The electrochemical nitrate reduction reaction (NO₃RR) provides a sustainable pathway for NH₃ production, complemented by pollution mitigation. Nevertheless, this process is constrained by limited nitrate adsorption, multiple competing reactions, and sluggish kinetics involving coupled proton–electron transfer steps, thereby reducing NH₃ selectivity. Herein, we have utilised FeCu(1:2)S_x as a cathode catalyst in Zn–NO₃⁻ battery to produce NH₃ via the NO₃RR, simultaneously generating electricity. The assembled Zn–NO₃⁻ battery demonstrated a remarkable faradaic efficiency (F.E.) of 98.96% for NH₃ and a power density of 5.2 mW cm⁻². Real time NH₃ production examined by in situ electrochemical Raman and attenuated total reflectance-Fourier transform infrared (ATR FT-IR) spectroscopy revealed that Cu promotes NO₃⁻ adsorption and reduction to NO₂⁻, while Fe facilitates hydrogenation resulting in an NH₃ yield of 3.69 mg h⁻¹ cm⁻² at −0.9 V vs. RHE. As a proof of concept, two Zn–NO₃⁻ batteries connected in series powered 62 LEDs for over 70 h.

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Introduction

The rising global energy demands are increasingly outpacing the limited availability of fossil fuel resources, thereby intensifying the need for advancements and innovations in energy storage technologies.^1,2 Among emerging systems, the zinc-based battery has garnered substantial interest as a promising next-generation energy storage system, owing to its high theoretical energy density and exceptional current output capabilities.^3,4 The economic feasibility of these batteries is further enhanced by the widespread availability and low cost of zinc (Zn), making them an appealing option for sustainable energy solutions.^5,6 However, in such batteries, the electrocatalytic oxygen evolution and reduction reactions (OER and ORR) occur at the cathode during charging and discharging, respectively and are significantly impeded by the intricate gas–solid–liquid triple-phase interface and sluggish kinetics; moreover, these processes yield no commercially valuable product.⁷ Thus, replacing air with nitrate (NO₃⁻) in these batteries could potentially provide higher output voltage than O₂-based batteries.^6,7 Moreover, the Zn–NO₃⁻ battery is considered environmentally advantageous due to its ability to generate electricity while converting NO₃⁻ from wastewater into value-added NH₃.^5,6,8 Thus compared to the well-established Zn–air (oxygen) battery, the Zn–NO₃⁻ battery offers a ‘one stone-three birds’ strategy: pollutant remediation with NH₃ production, energy storage & conversion.⁷ Moreover, the produced NH₃ is the second most widely synthesised chemical in the world, with an annual demand exceeding 200 million metric tonnes, with its crucial role as a necessary component in the production of fertilisers and other advantageous chemical compounds.^8–10 Despite these advantages, the Zn–NO₃⁻ battery faces significant challenges in meeting the performance criteria required to power modern electrical appliances. The primary limitation in the NO₃RR arises from its complex eight-electron transfer mechanism, which is hindered by slow kinetics and the substantial energy barrier associated with breaking N=O bonds (204 kJ mol⁻¹).^11–14 Furthermore, the reaction often leads to the generation of undesirable by-products, including NO₂⁻, N₂H₄, and NH₂OH, thereby diminishing both selectivity and overall efficiency.^12,15

Another significant challenge lies in the co-existence of desorbed and non-desorbed intermediates, which complicates the favourable adsorption of specific intermediates, such as NO₂⁻, due to the limitations imposed by scaling relationships.^16–18 These factors collectively underscore the difficulty and necessity of designing an efficient electrocatalyst capable of promoting selective adsorption and conversion of certain intermediates such as NO₂⁻.

Transition metal-based catalysts, owing to their earth abundance and cost-effectiveness, have become a feasible choice for the NO₃RR to NH_3, particularly due to their superior adsorption strength to NO₃⁻ through N and O atoms.^19–21 Among these, copper-based catalysts have gained immense attention due to the similar energies of Cu d-orbital and π* (LUMO) molecular orbital of NO₃⁻.^22–25 While this compatibility facilitates strong NO₃⁻ adsorption and promotes its initial conversion, however, it concurrently results in the accumulation of NO₂⁻ by-products. The limited ability of Cu to further cleave the N–O bond in NO₂⁻ stems from its weak adsorption of NO₂⁻ intermediates. Moreover, the electroreduction of NO₃⁻, governed by complex proton-coupled electron transfer (PCET) reactions, is further constrained by Cu's poor catalytic activity for hydrogen atom evolution, thereby impeding efficient PCET progression. As a result, monometallic Cu systems often fail to achieve high NH₄⁺ selectivity, likely due to suboptimal hydrogenation steps. Inspired by the catalytic activity of iron (Fe) in nitrate reductase enzymes in plants, Fe has been employed in electrochemical NH₃ synthesis.^26–28 Its favourable binding energies with N-containing intermediates make it particularly effective.^12,29–31 Additionally, Fe exhibits sufficient catalytic properties for hydrogen evolution, thereby enhancing the stepwise PCET reactions involved in the NO₃RR.

Thus, a bimetallic Fe and Cu-based catalyst is predicted to enhance electrochemical NO₃⁻ reduction activity towards NH₃ production. Inspired by the aforementioned consideration in mind, we synthesised FeCu(a:b)S_x as a catalyst for the NO₃RR. The optimised catalyst FeCu(1:2)S_x gave a high NH₃ F.E. of 96.5% at a potential of −0.6 V vs. RHE. Furthermore, in situ Raman spectra and ATR FT-IR analyses showed formation of NO₂⁻ and NH₃ during the NO₃RR. To evaluate the influence of rational catalyst design on catalytic activity, we assessed the performance of monometallic control catalysts. The FeCu(1:2)S_x catalyst was further utilised as a cathode material in the Zn–NO₃⁻ battery with a Zn plate as the anode (Scheme 1). The Zn–NO₃⁻ battery assembled with FeCu(1:2)S_x as the cathode demonstrated a power density of 5.2 mW cm⁻² and high F.E. of 98.96% for NH₃ production. This research offers profound insights into the design of efficient electrocatalysts for the NO₃RR and Zn–NO₃⁻ battery.

Scheme 1 Schematic representation of the assembled Zn–NO₃⁻ battery with catalyst design and ammonia application.

Results and discussion

Electrochemical nitrate reduction (NO₃RR)

The potential applicability of the designed FeCu(1:2)S_x catalyst towards the NO₃RR was assessed in 0.5 M K₂SO₄ electrolyte using 0.1 M KNO₃ as the nitrate source. Initially, linear sweep voltammetry (LSV) experiments were performed in Ar-saturated 0.5 M K₂SO₄ electrolyte containing 0.1 M KNO₃. As seen from Fig. 1a, a rapid increase in the reduction current density was observed attributed to the NO₃RR. In the absence of 0.1 M KNO_3, a similar behaviour was observed, but at a more reductive potential. These observations indicate the electrocatalytic activity of the designed catalyst towards the NO₃RR (inset, Fig. 1a and S1).

Fig. 1 (a) LSV curves of CuS_x, FeS₂, and FeCu(1:2)S_x catalysts in 0.5 M K₂SO₄ containing 0.1 M KNO₃ (inset shows LSV for FeCu(1:2)S_x in the absence and presence of NO₃⁻) (b) F.E. of NH₃ and NO₂⁻ for various catalysts in Ar saturated 0.5 M K₂SO₄ in the presence of 0.1 M KNO₃. In situ potential-dependent ATR FT-IR spectra of (c) FeCu(1:2)S_x and (d) CuS_x catalysts. (e) In situ potential-dependent electrochemical Raman spectra of the FeCu(1:2)S_x catalyst.

To understand the role of each element of the designed catalyst towards NO₃RR, control experiments were performed using the monometallic catalyst viz. CuS_x and FeS₂. The comparative LSV curves in Fig. 1a displayed an onset potential of −0.47 V vs. RHE (@10 mA cm⁻²) for FeCu(1:2)S_x with a maximum current density (cd) of 224 mA cm⁻² @ −1.0 V vs. RHE. Conversely, CuS_x and FeS₂, exhibited a cd of 90 mA cm⁻² and 112 mA cm⁻², respectively, at @ −1.0 V vs. RHE and with the onset potentials of −0.52 V and −0.51 vs. RHE (@10 mA cm⁻²), signifying a superior activity of the FeCu(1:2)S_x catalyst towards the NO₃RR. To gain a deeper understanding on the product formation during the NO₃RR, chronoamperometric (CA) measurements were conducted at a fixed potential of −0.6 V vs. RHE for 1 h, and the obtained products were quantified using colorimetric methods. Notably, FeCu(1:2)S_x demonstrated a significantly enhanced NH₃ yield rate of 1542 µg h⁻¹ cm⁻², surpassing those of monometallic FeS₂ (746 µg h⁻¹ cm⁻²) and CuS_x (566 µg h⁻¹ cm⁻²) (Fig. 1b and S1–S4). In contrast, CuS_x achieved the highest NO₂⁻ yield rate of 1149 µg h⁻¹ cm⁻², with a F.E. of 13.08%, which was superior to that of FeS₂ (686 µg h⁻¹ cm⁻², 6.83%) and FeCu(1:2)S_x (518 µg h⁻¹ cm⁻², 2.99%) (Fig. 1b and S2).

To provide more compelling evidence for the synthesis of NH₃ from NO₃⁻ and the role of each metal, potential dependent in situ electrochemical ATR-FTIR spectroscopic analysis was performed for CuS_x and FeCu(1:2)S_x catalysts. The FTIR spectra of FeCu(1:2)S_x displayed peaks at ∼3851, ∼3752 and ∼1106 cm⁻¹ corresponding to intermediates NH, NH₂, and NH_3, respectively (Fig. 1c and d).³² As the sample was polarized to a more negative potential the intensity of these bands increases with the increase in the potential, which highlights the progression of NH₃ production. Amid the finding, a pair of characteristic peaks at ∼1548 cm⁻¹ and 1360 cm⁻¹ represent the asymmetric and symmetric vibration modes of the absorbed NO₂⁻ intermediates.³³ These observations unequivocally underscore the remarkable potential of FeCu(1:2)S_x catalysts in facilitating the hydrogenation of NO₃⁻ to NH₃ via the NO₂⁻ intermediate. Conversely, for CuS_x ATR-FT-IR measurements exhibit peaks at ∼1106 cm⁻¹ and ∼1360 cm⁻¹, corresponding to NH₃ and adsorbed NO₂⁻. However, the minimal presence of hydrogenation intermediates (*NH₂ and *NH) indicates the limited efficacy of CuS_x for hydrogenation to NH₃. In the case of FeS₂, ATR-FT-IR measurements display a pronounced peak at ∼1106 cm⁻¹ attributed to NH₃, accompanied by a negligible hump at ∼1360 cm⁻¹ for NO₂⁻ (Fig. S5). This observation clearly indicates that FeS₂ facilitates more complete hydrogenation and suppresses intermediate accumulation compared to CuS_x. Additional peaks at ∼1646 cm⁻¹ and ∼3371 cm⁻¹ are assigned to water.

To gain deeper insight into the NO₃RR, Raman spectra were recorded over a range of potentials (−0.4 V to −0.9 V vs. RHE) and at open-circuit potential (OCP) (Fig. 1e). As anticipated, at OCP, two distinct peaks were observed at ∼1043 cm⁻¹ and ∼979 cm⁻¹ corresponding to the presence of NO₃⁻ and SO₄²⁻, respectively.^34,35 The emergence of a peak at ∼1439 cm⁻¹, attributed to the N=O stretching vibration of the bridged nitro group, confirmed the formation of NO₂⁻ during the NO₃RR.³⁶ Additional peaks at ∼1315 cm⁻¹ and ∼1586 cm⁻¹ were associated with the symmetric and antisymmetric bending vibrations of HNH of NH_3, respectively, and exhibited increasing intensity with a more negative applied potential.^34,37 The pronounced NH₃ peak intensity, accompanied by a diminished NO₂⁻ hump, signifies the catalyst's selective facilitation of NH₃ formation on its surface. Based on in situ Raman spectroscopic analysis, it was conclusively established that NO₃RR intermediates, including NO₂⁻, and NHN, predominantly contribute to the electrochemical production of NO₂⁻ and NH₃ on the catalyst surface.

The electrochemical activity of FeCu(1:2)S_x was further tested at various potentials to quantify NO₃RR products (Fig. S6). Then the product was evaluated by UV-Vis spectral analysis via an Indophenol blue method (Fig. S6). With an increase in the negative potential during the NO₃RR, an increasing trend in NH₃ yield rate was observed, indicating higher NH₃ synthesis at larger negative potential,⁶ and the highest NH₃ yield rate of 3.69 mg h⁻¹ cm⁻² was attained at −0.9 V vs. RHE (Fig. S6). Furthermore, the F.E. of NH₃ was studied at various potentials, revealing a F.E. of 96.5% and NH₃ yield rate of 1.54 mg h⁻¹ cm⁻² (Fig. 1d) at −0.6 V vs. RHE with a turn over frequency (TOF) of 0.154 h⁻¹, suggesting its efficient performance in carrying out the NO₃RR in a neutral medium. This prompted further experiments with other variants to discern the influence of the Cu and Fe ratio in FeCu(a:b)S_x catalysts. The activity was found to be in the order FeCu(1:2)S_x > FeCu(1:1)S_x > FeCu(2:1)S_x (Fig. S7). Furthermore, to compare the NH₃ production rates and efficiency of other bimetallic variants with CuFe(1:2)S_x, the CA experiments were performed at a potential of −0.6 V vs. RHE under similar experimental conditions (Fig. S5b). Amongst these, FeCu(1:2)S_x showed the highest NH₃ yield rate and F.E., indicating its superior NO₃RR activity (Fig. S7d). In order to substantiate the synthesis of NH₃ from NO₃⁻, time-dependent in situ ATR FTIR analyses were performed on FeCu(1:2)S_x (Fig. 2a). These exhibited peaks indicating intermediates NH, NH₂, and NH₃ for ammonia (Fig. 2a).³² The progressive increase in the intensity of these bands over time underscores the advancement of NH₃ production. Among these, a pair of characteristic peaks comprises vibration modes of the absorbed NO₂⁻ intermediates.³³ Further analysis identifies peaks at ∼1646 cm⁻¹ and ∼3371 cm⁻¹, which originate from the O–H stretching and bending modes of water, signifying its role as a proton source for subsequent hydrogenation reactions.^32,33 These observations clearly highlight the significant potential of FeCu(1:2)S_x catalysts in promoting the hydrogenation of NO₃⁻ to NH₃ via the NO₂⁻ intermediate.

Fig. 2 (a) In situ time-dependent ATR FT-IR spectra of the FeCu(1:2)S_x catalyst with the zoomed parts in i and ii. (b) Schematic illustration for the reaction mechanism.

In light of our comprehensive in situ analytical observations, coupled with corroborative electrochemical data and pertinent literature, it can be conclusively inferred that Cu is highly effective in promoting NO₃⁻ adsorption and its reduction to NO₂⁻, whereas Fe plays a pivotal role in facilitating the hydrogenation to NH₃.^11,38 The electrochemical reduction of NO₃⁻ to NH₃, governed by the reaction NO₃⁻ + 6H₂O + 8e⁻ → NH₃ + 9OH⁻, proceeds through a well-defined mechanistic pathway (Fig. 2b).^11,38 This pathway is initially characterized by a sequence of deoxygenation reactions (*NO₃⁻ → *NO₂ → *NO → *N), followed by a series of hydrogenation steps (*N → *NH → *NH₂ → *NH₃). Such mechanistic elucidation highlights the intricate interplay of deoxygenation and hydrogenation processes pivotal to the NO₃⁻ reduction pathway.

The structural features of the synthesised variants of FeCu(a:b)S_x along with its monometallic variant FeS₂ and CuS_x were analysed via powder X-ray diffraction (P-XRD). Fig. 3a demonstrates the P-XRD pattern of FeCu(1:2)S_x, FeS₂ and CuS_x. The P-XRD pattern of the synthesised FeS₂ displays diffraction peaks in agreement with JCPDS No. 42-1340. Additionally, the P-XRD pattern of CuS_x shows a combination of peaks attributed to CuS (JCPDS No. 06-0464) and Cu₂S (JCPDS No. 53-0522), with the most prominent peak at 31.785° assigned to the (103) plane of CuS, and a major peak at 46.105° corresponding to the (220) plane of Cu₂S. The P-XRD pattern of the synthesised FeCu(a:b)S_x catalysts indicates the coexistence of FeS₂, CuS, Cu₂S and CuFeS₂ in all the variants, consistent with their respective JCPDS standards (Fig. 3a and S8).

Fig. 3 (a) P-XRD pattern for FeS₂, CuS_x, and FeCu(1:2)S_x catalysts. FE-SEM images at (b) 1 µm and (c) 100 nm magnification for the FeCu(1:2)S_x catalyst. Deconvoluted XPS spectra of (d) Fe 2p and (e) Cu 2p for the FeCu(1:2)S_x catalyst.

The field emission scanning electron microscopy (FE-SEM) images of FeCu(1:2)S_x reveal microflower-shaped morphology (Fig. 3b and c). Energy dispersive X-ray spectroscopy (EDS) elemental dot mapping also confirms the coexistence and uniform distribution of Cu, Fe, and S within the microflowers (Fig. S8). The high-resolution transmission electron microscopy (HR-TEM) images of the FeCu(1:2)S_x catalyst show distinct lattice fringes with measured interplanar spacings of 0.303 nm, 0.189 nm, and 0.163 nm, which can be indexed to the (112) plane of CuFeS₂, the (110) plane of CuS, and the (311) plane of FeS₂, respectively (Fig. S9a and b). The corresponding selected area electron diffraction (SAED) patterns further corroborate these assignments, displaying diffraction spots consistent with the planes of CuFeS₂, CuS and FeS₂, thereby confirming the structural coherence between HR-TEM and SAED observations (Fig. S9c).

The survey XPS (X-ray photoelectron spectroscopy) spectra of FeCu(1:2)S_x indicate the presence of Fe, Cu, and S (Fig. S10a). Fe 2p XPS spectra unveil binding energies of Fe 2p_3/2 (710.9 eV) and Fe 2p_1/2 (724.5 eV), attributed to Fe³⁺ in CuFeS₂.³⁹ Additionally, two peaks at 707.7 eV and 720.7 eV correspond to the Fe 2p_3/2 and Fe 2p_1/2 binding energies of Fe²⁺ in FeS₂, respectively (Fig. 3d).^39,40 The deconvoluted Cu 2p XPS spectra shown in Fig. 3e revealed BE peaks at 932.0 eV and 951.9 eV corresponding to 2p_3/2 and 2p_1/2, respectively, corresponding to Cu⁺. Furthermore, peaks at 932.9 eV and 954.5 eV corresponded to 2p_3/2 and 2p_1/2 of Cu²⁺.^41,42 Thus, we can conclude that Fe is present in the +2 and +3 oxidation state and Cu in both the Cu(I) and Cu(II) oxidation state in the FeCu(1:2)S_x catalyst. Moreover, the deconvoluted S 2p XPS spectrum shows peaks at 161.0 eV and 162.0 eV, corresponding to S 2p_3/2 and S 2p_1/2 bonded with metals. Additionally, peaks at 163.5 eV and 168.5 eV indicate the presence of divalent sulfides and sulfates, respectively (Fig. S10b).⁴³ Furthermore, the XPS spectra of FeCu(1:2)S_x exhibits a decrease of ∼0.2 eV in the Cu 2p binding energy and a concomitant positive shift of ∼0.3 eV in the Fe 2p binding energy relative to their respective monometallic counterparts. These shifts provide strong evidence for effective Cu–Fe electronic coupling within FeCu(1:2)S_x (Fig. S11a and b). Comparative XPS analysis thus highlights a pronounced chemical interaction between Cu and Fe, consistent with electron transfer from Fe to Cu atoms within FeCu(1:2)S_x.^6,44,45 To further clarify the oxidation state of Cu, Auger emission (AE) spectra reveal a distinct Auger Cu LMM peak corresponding to Cu²⁺ and Cu¹⁺, located at 568.5 eV and 569.8 eV, respectively (Fig. S11c).^46,47 Notably, no signal is observed at 568 eV, confirming the absence of Cu⁰.⁴⁷ The Raman spectrum exhibits a distinct peak at ∼470 cm⁻¹, which can be attributed to the Cu–S vibrational modes characteristic of the chalcopyrite CuFeS₂ phase (Fig. S12).^48,49 The powder X-ray diffraction (P-XRD) patterns of FeCu(a:b)S_x catalysts with varying Fe : Cu ratios indeed show noteable differences. Specifically, when the Fe : Cu ratio is increased from 1 : 1 to 1 : 2, the diffraction peaks corresponding to CuS at 2θ = 32.85° and 47.94° display enhanced intensity, indicating enrichment of copper-containing phases. Conversely, when the Fe : Cu ratio is adjusted from 1 : 1 to 2 : 1, the characteristic CuS peak at 2θ = 47.94° diminishes, reflecting a relative decline in copper content. Thus the structural features are governed by the dominant elemental composition (Fig. S13). Fig. S14 depicts the FE-SEM images and EDS dot mapping for the FeCu(1:1)S_x and FeCu(2:1)S_x catalysts.

The superior activity of FeCu(1:2)S_x could be due to the faster kinetics at the electrode–electrolyte interface, which was evidenced by the lower Tafel slopes compared to the other catalyst variants (Fig. S15 and Table S1). This was further supported by electrochemical impedance spectroscopy (EIS) analysis, where in Fig. S16, a lower charge transfer resistance (R_ct) was observed for the FeCu(1:2)S_x catalyst (Table S2), signifying fast charge transfer kinetics at the electrode–electrolyte interface.⁸ The FeCu(1:2)S_x catalyst shows a higher conductivity of 5.32 mS cm⁻¹ compared to its monometallic variants (Table S2). The facile kinetics could also be attributed to the increased number of accessible electrochemically active sites on the catalyst surface which was further evaluated using double-layer capacitance in the non-faradaic region. The electrochemical surface area (ECSA) of the FeCu(1:2)S_x catalyst was found to be 329 cm² which is superior to that of other variants (Fig. S17 and Table S1). Thus, larger ECSA of FeCu(1:2)S_x justified its superior activity towards the NO₃RR. The energy efficiency was found to be 28.47% at a potential of −0.4 V vs. RHE (details in the SI).

Furthermore, the validation of NH₃ generated and reliability of the data for the same have emerged as critical and significant criteria. As a result, a strict methodology described in the literature was followed, including cleaning of cell components (Scheme S1).⁵⁰ Also, the control experiments were performed by conducting CA measurement in Ar-saturated 0.5 M K₂SO₄ (in the absence of NO₃⁻) at a potential of −0.6 V vs. RHE, on a bare electrode in the presence of NO₃⁻. The UV-Vis spectra obtained for the electrolyte for NH₃ production, indicated negligible NH₃ production, thereby endorsing that the source of N is exclusively from the added NO₃⁻ and no existence of potential interferences (Fig. 4a). Furthermore, to confirm that NH₃ generated in our experiment comes from the NO₃RR only, isotope labelling experiments were performed using ¹⁴NO₃⁻ and ¹⁵NO₃⁻ as the N source. For the quantitative analysis of ¹⁵NH₄⁺/¹⁴NH₄⁺ acquired from the isotope labelling studies, the calibration curves were extracted for different known concentrations of ¹⁵NH₄⁺/¹⁴NH₄⁺ using ¹H NMR spectroscopy (Fig. S18–S20). After electrolysis, ¹H NMR spectra of ¹⁴NO₃⁻ and ¹⁵NO₃⁻ solutions showed a triplet and doublet with coupling constants of 52 Hz and ∼72 Hz, respectively, and no doublet or triplet was observed in the absence of NO₃⁻, indicating the reliability of produced data (Fig. 4b and S20). Fig. 4c demonstrates the NH₃ yields for both ¹⁵NH₄⁺ and ¹⁴NH₄⁺ obtained using the Indophenol blue method and ¹H NMR spectra showing negligible change in the NH₃ yield. NH₃ yield was further quantified spectrophotometrically using Nessler's reagent, yielding 1.53 mg h⁻¹ cm⁻² at −0.6 V vs. RHE, validating the results from the Indophenol blue method (Fig. S21). Apart from NH₃, other possible reduction products produced during the NO₃RR such as NO₂⁻, H₂, N₂H₄, and NH₂OH, were quantified, after the CA test at −0.6 V vs. RHE for 1 h. The by-products NO₂⁻, N₂H₄ and NH₂OH were tested and quantified by UV-Vis spectroscopy as illustrated in Fig. S22 and S23 (detailed in the SI). It is worth noting that N₂H₄ and NH₂OH were not detected (Fig. S16 and S17); however, NO₂⁻ was detected with a yield rate of 1.29 mg h⁻¹ cm⁻² and F.E. of 2.99%. Thus, the relatively high selectivity for NH₃ synthesis by the FeCu(1:2)S_x catalyst with a total F.E. for the NO₃RR at −0.6 V vs. RHE was found to be 99.49%, with F.E. for NH₃, and NO₂⁻ to be 96.5% and 2.99% respectively (Fig. S24).

Fig. 4 (a) Bar diagram representing NH₃ yield for the FeCu(1:2)S_x catalyst with and without NO₃⁻ and at the bare electrode. (b) ¹H NMR spectra obtained after the NO₃RR for FeCu(1:2)S_x in Ar saturated ¹⁴NO₃⁻, and ¹⁵NO₃⁻ containing electrolyte solution. (c) Bar diagram representing the comparison of the NH₃ yield by the Indophenol method and NMR for ¹⁴NH₄⁺ and ¹⁵NH₄⁺ produced during the NO₃RR.

Along with the electrochemical activity, stability is an important parameter for evaluating the performance of the catalyst. The electrochemical stability of FeCu(1:2)S_x towards the NO₃RR was then investigated using consecutive electrolysis for 10 cycles at a potential of −0.6 V vs. RHE (Fig. S25). The CA curves revealed a negligible change in the current response and its corresponding UV-Vis spectra for 10 consecutive cycles. A steady F.E. and NH₃ yield was maintained throughout the duration of 10 h of the stability test pointing towards reproducibility of the catalyst's performance (Fig. S25c). The changes in the catalyst after the stability test were also investigated by P-XRD, FE-SEM, EDS and XPS techniques. The XRD pattern showed that crystalline structures are well preserved (Fig. S26a) and, post XPS analysis (Fig. S26b–d) showed no change in the chemical composition of elements present in FeCu(1:2)S_x, endorsing the high stability of the catalyst for the NO₃RR. Furthermore, the FE-SEM image (Fig. S26e), showed that morphology is well maintained. Also, EDS spectra and dot mapping images showed the presence of all the elements in the catalyst even after the stability test (Fig. S26f–i). Table S3 shows the atomic % of the elements present in FeCu(1:2)S_x, demonstrating almost negligible change in the composition. The aforementioned results show appreciable activity towards the NO₃RR in neutral media in comparison to reported literature (Table S4).

Zn–NO₃⁻ battery

Motivated by the superior performance of FeCu(1:2)S_x towards the NO₃RR, a Zn–NO₃⁻ battery was assembled to demonstrate that the battery could be used for producing electric current and NH₃.

The assembled Zn–NO₃⁻ battery comprise FeCu(1:2)S_x coated over Ni foam as the cathode in Ar saturated 0.5 M K₂SO₄ solution containing 0.1 M KNO₃ and a polished Zn plate was employed as the anode in 1 M KOH containing 0.02 M Zn(CH₃COO)₂. During the discharge process, the anodic Zn gets oxidised, followed by spontaneous production of zincate anions which upon reaching saturation convert to ZnO (eqn (2)). At the cathode, NO₃⁻ reduction takes place assisted by electrons released by Zn metal oxidation (eqn (3))^6,51

Anode:

4Zn + 8OH⁻ → 4ZnO + 4H₂O + 8e⁻ (1)

Cathode:

NO₃⁻ + 7H₂O + 8e⁻ → NH₄OH + 9OH⁻ (2)

Overall reaction

4Zn + NO₃⁻ + 3H₂O → 4ZnO + NH₄OH + OH⁻ (3)

The assembled battery showed an open circuit voltage (OCV) of 1.32 V vs. Zn/Zn²⁺ (calculations detailed in the SI) and it remained stable for more than 20 h (Fig. 5a). Furthermore, as shown in the discharge polarisation curve (Fig. 5b), a maximum current density of 30 mA cm⁻² was obtained with a maximum power density of 5.2 mW cm⁻² @ a current density of 16 mA cm⁻² in the assembled battery, which is comparable to that in reported literature in neutral media (Table S5). Moreover, the sequential chronopotentiometric measurements at various discharge current densities ranging from −0.1 mA cm⁻² to −2.5 mA cm⁻² (Fig. 5c) were recorded and a stable response was obtained with negligible change in the potential over a time scale of 1 h indicating fast mass transfer and conductivity.⁶ Furthermore, the production and quantification of NH₃ during battery discharging was analysed by recording chronopotentiometry measurements at different current densities ranging from −2 to −10 mA cm⁻² (Fig. S27a) and then quantified by UV-Vis spectroscopy (Fig. S27b). Interestingly, the highest F.E. of 98.96% was obtained at a current density of −4 mA cm⁻² with an NH₃ yield of 158 µg h⁻¹ cm⁻² and a maximum NH₃ yield of 257 µg h⁻¹ cm⁻² at a current density of −10 mA cm⁻² (Fig. 5d). Moreover, upon discharging at −1 mA cm⁻², the battery exhibited a remarkable energy density of 620.12 Wh kg⁻¹ w.r.t. the weight of Zn consumed. Again, after mechanical polishing of the Zn anode and replacing the anodic electrolyte, an energy density of 614.74 Wh kg⁻¹ was achieved, indicating only a slight decrease and stable response of FeCu(1:2)S_x as the cathode (Fig. S28). To illustrate the practical utility of the Zn–NO₃⁻ battery system, two home-made Zn–NO₃⁻ batteries were connected in series, forming an energy unit capable of powering an array of 62 blue light-emitting diodes (LEDs). These LEDs were configured in a parallel arrangement to ensure uniform voltage distribution and stable operation. Remarkably, the system sustained uninterrupted illumination of the LEDs for a prolonged duration exceeding 70 h, as depicted in Fig. 5e–h. This compelling demonstration highlights the battery's potential for real-world energy storage and delivery applications, emphasising its robustness and prolonged operational efficacy in powering small-scale electronic devices. These results prove that FeCu(1:2)S_x could be an efficient and cost-effective cathode material for NH₃ production and electricity generation simultaneously.

Fig. 5 (a) OCV recorded for the assembled Zn–NO₃⁻ battery, (b) discharge curve with the corresponding power density at a scan rate of 5 mV s⁻¹. (c) Discharge curves at different current densities. (d) Bar diagram representing the NH₃ yield rate and F.E. for the Zn–NO₃⁻ battery at different discharge current densities with the FeCu(1:2)S_x catalyst as the cathode. (e–h) Photographs showing 62 blue LEDs powered by two Zn–NO₃⁻ batteries connected in series using the FeCu(1:2)S_x catalyst as a cathode for more than 70 h.

Conclusions

In conclusion, we have designed FeCu(1:2)S_x as an effective catalyst for the NO₃RR showing the highest NH₃ yield of 3.69 mg h⁻¹ cm⁻² at a potential of −0.9 V vs. RHE and a high NH₃ F.E. of 96.5% at −0.6 V vs. RHE. In situ studies and electrochemical analyses validate the distinct roles of Fe and Cu in the NO₃RR. Cu effectively promotes NO₃⁻ adsorption and reduction to NO₂⁻, while Fe is essential for hydrogenation to NH₃. Benefitting from superior electrochemical performance of FeCu(1:2)S_x a Zn–NO₃⁻ battery was assembled utilising the FeCu(1:2)S_x catalyst as the cathode. It exhibited a high OCV of 1.32 V with a power density of 5.2 mW cm⁻² and a maximum NH₃ F.E. of 98.96%. The two home-made Zn–NO₃⁻ batteries connected in series could power 62 blue LEDs for more than 70 h. This work enriches the use of Zn-based batteries for electrocatalysis and widens the scope of bimetallic catalysts for electrochemical NH₃ synthesis.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental procedures, additional characterization data, and figures that validate the findings presented in the main text. See DOI: https://doi.org/10.1039/d5ta08241b.

Acknowledgements

The authors thank the Central Research Facility (CRF), IIT Ropar, for FE-SEM, XPS, and Raman spectroscopy facilities. T. C. Nagaiah thanks the Anusandhan National Research Foundation (ANRF/PAIR/2025/000006/PAIR-A) for financial support. A. Chaturvedi thanks I. I. T Ropar for a fellowship and research facilities. S. Mehta thanks CSIR (09/1005(0045)/2020-EMR-I) for a fellowship.

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