Heterogeneous 2D/2D MnO₂/MXene catalyst for nitrate-to-ammonia electrochemical reduction and Zn-nitrate battery behavior

Summary

Ambient temperature electrochemical ammonia synthesis via nitrate reduction reaction (NO₃RR) is a promising alternative to the energy-intensive Haber-Bosch process but lacks effective electrocatalysts for practical applications. In this work, MnO₂ 2D-nanoflakes are anchored onto 2D-MXene Ti₃C₂T_x, forming a heterogeneous catalyst for NO₃RR with an ammonia yield of 14.06 ± 0.48 mg h⁻¹.mg_cat⁻¹ at −1.2 V versus reversible hydrogen electrode (vs. RHE) and a Faradaic efficiency (FE) of 85.23 ± 1.62% at −1.0 vs. RHE, along with good stability over six consecutive cycles, with NH₃ FE exceeding 75% at −1.0 V vs. RHE in 0.5 M K₂SO₄ and 0.1 M KNO₃ electrolyte. As a cathode in a Zn-NO₃⁻ battery with a polished Zn plate from spent Zn-C batteries as anode, it delivers a power density of 0.323 mW cm⁻² and NH₃ FE of ∼79.9%. This study highlights a highly effective electrocatalyst for NO₃RR and its potential in self-powered NH₃ production.

Graphical abstract

Highlights

• •2D/2D MnO₂/MXene hybrids are used for electrochemical nitrate-to-ammonia conversion
• •The optimized MnO₂/MXene exhibits NH₃ Faradaic efficiency over 85%
• •The optimized MnO₂/MXene shows promising Zn-NO₃⁻ battery behavior

Chemistry; Energy engineering; Materials science

Introduction

Ammonia (NH₃) is an indispensable precursor for nitrogen-related industrial products and serves as a promising hydrogen carrier (17.6 wt % of hydrogen).^1,2 At present, the practical-scale NH₃ production depends predominantly on the conventional energy-intensive Haber-Bosch process, which operates under extreme conditions (300°C–500°C, >200 atm).^3,4 Despite its efficiency for a century, its substantial environmental impact remains a significant concern, as it constitutes ∼1%–2% of the total global energy consumption per annum and contributes ∼1.3% of energy-related CO₂ emissions.⁵ Achieving carbon neutrality necessitates the urgent development of sustainable and efficient technologies for synthesizing ammonia and nitrogen-based products that can partially or entirely replace the Haber-Bosch process.^6,7

The nitrate-to-ammonia electrochemical reduction (NO₃RR) has emerged as an appealing replacement to the century-old Haber-Bosch process because of its ability to utilize renewable energy sources and operate under mild reaction conditions.^8,9,10 In comparison to the nitrogen-to-ammonia reduction reaction (NRR), NO₃RR exhibits greater thermodynamic favorability owing to the substantially lower bond dissociation energy of the N=O bond (204 kJ mol⁻¹) in contrast to the considerably higher dissociation energy of the N≡N bond (941 kJ mol⁻¹).¹¹ Additionally, the substantial potential gap between NO₃RR and the competitive hydrogen evolution reaction (HER) (0.69 V) is markedly larger than that between NRR and HER (0.09 V), indicating that NO₃RR is less susceptible to HER interference.¹² However, NO₃RR proceeds through complex multi-step reaction pathways, requiring the transfer of eight electrons and nine protons in aqueous media, which results in sluggish reaction kinetics and the generation of multiple byproducts.¹³ Moreover, parasitic HER may still occur at highly negative potentials, competing with NO₃RR and limiting NH₃ production efficiency, thereby hindering its practical application.^14,15,16 Consequently, developing highly efficient and cost-effective electrocatalysts for NO₃RR to NH₃ remains an urgent yet formidable challenge.

Possessing well-defined structures and tunable electronic properties, two-dimensional (2D) materials with massive active sites and a high surface area are suitable candidates for electrocatalysts.^17,18 Currently, titanium carbide MXene (Ti₃C₂T_x) etched from MAX phase (Ti₃AlC₂) exhibits outstanding electrical conductivity, hydrophilicity, and an easily modifiable surface area, which can be attributed to its distinctive structural characteristics.^19,20 Besides that, owing to the dense functional groups, Ti₃C₂T_x MXene serves as an effective platform for anchoring and dispersing various electrocatalysts.^21,22 The functional surface groups (e.g., -F, -O, and -OH), formed via the etching and delamination process, enhance the dispersibility of Ti₃C₂T_x MXene in aqueous solutions and facilitate its integration with other 2D materials via solution-based assembly techniques.²³ Meanwhile, manganese dioxide (MnO₂) has attracted considerable interest for its remarkable electrochemical properties, which can be tailored through synthetic modifications to optimize its redox behavior.^24,25,26 However, MnO₂ exhibits limited electrical conductivity, which can be significantly improved through hybridization with MXene.^27,28

Inspired by the above discussion, we have developed heterogeneous 2D/2D MnO₂/MXene hybrids, wherein MnO₂ nanoflakes are grown on MXene nanosheets to facilitate NO₃RR performance. This hybrid structure is expected to provide an increased number of active sites and mass transport channels, thereby improving NO₃RR performance. The proportion of MXene to MnO₂ can be precisely regulated by modifying the concentration of KMnO₄ precursor, with the most catalytically active configuration exhibiting a dense flower-like morphology at the nanoscale. The variations in compositional, structural, and electrochemical properties of the synthesized catalysts were thoroughly examined utilizing advanced spectroscopic and electrochemical techniques. The as-prepared dense MnO₂/MXene hybrid exhibited an NH₃ yield rate of 14.06 ± 0.48 mg h⁻¹.mg_cat⁻¹ at −1.2 V vs. RHE and a maximum NH₃ FE of 85.23 ± 1.62% at −1.0 V vs. RHE. Furthermore, the catalyst maintained NH₃ FE above 75% over six consecutive cycles with a long-term operational stability of 6 h. Finally, a fabricated zinc-nitrate (Zn-NO₃⁻) battery system, utilizing a polished Zn plate recovered from spent Zn-carbon batteries, demonstrated the potential for multifunctional applications, including material recycling, high-power-density electricity generation, and ammonia production.

Results and discussion

Materials synthesis

As shown in Figure 1A, a schematic diagram illustrates the synthesis process of MnO₂/MXene catalysts through a three-step process (detailed in the Experimental Section). In the first step, multi-layer MXene was thermally etched by concentrated HF at 50°C from MAX phase according to a method reported elsewhere with some modifications.¹⁹ Subsequently, the collected multi-layer MXene was dissolved in DMSO for intercalation, followed by sonication treatment for peeling it off into few-layer flakes with small particle size, which increases the available surface area. In the final stage, the precipitated few-layer MXene was dispersed in ethanol to generate a homogeneous solution. Then, by slowly adding KMnO₄ precursor to the above solution, MnO₂ nanoflakes were vertically grown on the MXene surface while preserving MXene morphology.

Figure 1.

Materials synthesis and characterization

(A) Schematic illustration of the preparation of as-prepared MnO₂/MXene catalysts. Characterization of as-prepared catalysts: (B) XRD patterns, (C) Raman, (D) BET surface area measured from nitrogen adsorption-desorption isotherms and, (E) the corresponding pore-size distribution.

Phase composition and crystallographic analysis

The phase composition and crystallographic properties of MnO₂/MXene, along with single-component MnO₂ and MXene, were initially examined through X-ray diffraction (XRD) analysis. As depicted in Figure 1B, there are several characteristic diffraction peaks observed in the MXene component, where the diffraction peaks at 2θ = 8.61°, 18.25°, 22.27°, 28.04°, and 60.63° are related to the (002), (004), (006), (008), and (110) planes of MXene Ti₃C₂T_x, respectively, which aligns with findings reported in previous literature.²⁷ In the XRD pattern of pure MnO₂, the peaks located at 15.60°, 22.16°, and 41.52° represent the (001), (002), and (200) planes, respectively, which are assigned to δ-MnO₂ (JCPDS80-1098).²⁹ Following the addition of the KMnO₄ solution, the MnO₂/MXene hybrids displayed well-defined diffraction peaks attributable to both MnO₂ and MXene, validating the effective incorporation of these two phases within the composite structure.

Vibrational characteristics and interfacial interaction revealed by Raman spectroscopy

The Raman spectra of the synthesized catalysts, obtained under excitation with a 532 nm green laser, are presented in Figure 1C. The Raman spectrum of MXene exhibited a distinctive peak at 150 cm⁻¹, attributed to the in-plane vibrational mode (A_1g) of the Ti-C bond, aligning with previously reported findings.³⁰ One major peak was observed in the MnO₂ Raman spectrum related to the symmetrical stretching vibrations (E_g) around 600–650 cm⁻¹ of Mn-O, which are associated with the MnO₆ octahedral.³¹ The Raman spectra of the MnO₂/MXene hybrids showed the characteristic peaks of both MnO₂ and MXene.³² From the sparse MnO₂/MXene to MnO₂ urchin/MXene with the increase in MnO₂ loading, the intensities of Mn-O symmetric stretching vibration increased and shifted slightly toward a higher frequency, which can be associated with the increased force constant of stretching vibrations of Mn–O bonds.³³ Due to the interaction of MXene component, electrons tend to transfer from Mn³⁺ sites in MnO₂ to Ti⁴⁺ sites in MXene, further strengthening the Mn–O bonds, which can be explained further by X-ray photoelectron spectroscopy (XPS) spectra in the following section.

Mesoporous structure formation in MnO₂/MXene hybrids

The characterization of pore architecture was conducted via Brunauer-Emmett-Teller (BET) testing, which exhibited a type IV profile accompanied by H3 hysteresis loops, indicating the presence of a mesoporous framework predominantly consisting of lamellar, slit-like voids (Figures 1D and S1, supplemental information).^34,35 Growing MnO₂ nanoflakes presents a distinctive approach to augmenting the effective surface area while mitigating the undesirable restacking of MXene layers, leading to the enhanced surface area of MnO₂/MXene hybrids compared to single component MXene, as shown in Table S1 (supplemental information). Besides that, except MXene, the MnO₂ and MnO₂/MXene hybrids present the pore size in 2–20 nm, confirming the mesoporous structures (Figures 1E and S2, supplemental information), where V and D denoted as pore volume and pore diameter, respectively. This demonstrates that incorporating MnO₂ into MXene nanosheets can form the porous structure, favoring the mass/ions transport.

Morphological and crystallographic characterization of MnO₂/MXene hybrids

As depicted in Figure S3 (supplemental information), the field emission scanning electron microscopy (FE-SEM) mapping of MnO₂ revealed a sea urchin-like structure with burrs and uniform particle size, with diameter around 100–150 nm, while MXene specimen exhibited a stack of smooth nanolayers. After functionalization of MXene with precursor KMnO₄ solution with different concentrations in 0.001 mol L⁻¹ and 0.005 mol L⁻¹, the formed MnO₂ nanoflakes are uniformly decorated on the MXene surface, resulting in the fabrication of sparse MnO₂/MXene and dense MnO₂/MXene heterostructures which possess the massive mass transport channel and expose active sites (Figures 2A and 2B). Apparently, once increasing the precursor KMnO₄ solution to 0.01 mol L⁻¹, the component appears both MnO₂ nanoflakes and massive urchin-like particles, which may lead to congestive mass transport channels and hinder the mass transportation due to the inherent low conductivity of extensive MnO₂ amount (Figure 2C). As illustrated in Figure 2D, the distinct few-layered structure of MXene is clearly observed via high-resolution transmission electron microscopy (HR-TEM) image, confirming the successful fabrication of few-layer MXene. Besides that, MnO₂ parts are distributed on the MXene skeleton which are identified through lattice fringes 0.35 nm of MnO₂ (002) planes (Figures 2E and 2F).^36,37

Figure 2.

Morphological and crystallographic characterization

FE-SEM micrographs of as-prepared catalysts: (A) sparse MnO₂/MXene (scale bar, 250 nm), (B) dense MnO₂/MXene (scale bar, 250 nm), and (C) MnO₂ urchin/MXene (scale bar, 250 nm).

(D–F) HRTEM images (scale bar = 10 nm), (E and F) corresponding lattice space (scale bar, 2 nm), (G) The SAED pattern (scale bar, 5 nm⁻¹), and (H) EDS mapping (scale bar , 1 μm) of dense MnO₂/MXene.

To analyze the crystallographic characteristics of the dense MnO₂/MXene composites, selected area electron diffraction (SAED) was employed. As depicted in Figure 2G, three distinct concentric diffraction rings are discernible, each corresponding to specific lattice planes of different crystalline phases. Notably, the innermost ring is associated with the (002) plane of MnO₂, which formed during the MnO₂ growth process. Meanwhile, the yellow rings, exhibiting progressively larger diameters, correspond to the (102) plane of MXene, whereas the last ring with the largest diameter represents the (110) plane of MXene.³⁸ These results confirm the successful formation of the dense MnO₂/MXene hybrid structure, wherein MnO₂ nanoflakes are uniformly anchored onto the MXene framework, and both MnO₂ and MXene maintain their crystalline nature. Furthermore, the surface element distribution of the dense MnO₂/MXene was depicted in Figure 2H, where Mn, O, Al, Ti elements were detected and distributed on the catalyst surface.

Surface elemental composition and electronic coupling in MnO₂/MXene hybrids

The surface component information of all as-prepared catalysts was revealed by XPS. In the full-scale XPS survey (Figure S4, supplemental information), an apparent Ti 2p signal is observed for the MXene and MXene/MnO₂ hybrids. For comparison, the peaks assigned to C–Ti (281.26 eV), C-C (284.26 eV), C-O (287.26 eV), and C=O (288.86 eV) are present in the high-resolution C 1s XPS spectrum of MXene catalyst (Figure 3A), while the C 1s XPS spectrum of MnO₂ shows with the concomitant disappearance of the C–Ti peak and three peaks at C-C (284.52 eV), C-O (286.12 eV), and C=O (288.52 eV).³⁹ Four distinct peaks, representing the four distinct oxygen-containing functional groups, can be distinguished in the O 1s spectrum of MXene which are Ti-O bond at 528.16 eV, C-Ti-O_x bond at 529.36 eV, Ti-O-Ti bond at 531.56 eV, and C-Ti-(OH)_x bond at 534.16 eV (Figure 3B).⁴⁰ Besides that, the O 1s spectrum of MnO₂ sample includes two distinct peaks at 529.52 eV and 531.92 eV, which are indicated to Mn-O-Mn and Mn-O-H, respectively. In MnO₂/MXene hybrids, the fluctuation in density of deconvoluted O 1s peaks demonstrates the electronic interaction between MnO₂ and MXene.

Figure 3.

Evaluation surface elemental composition

X-ray photoelectron spectroscopy spectra of all as-prepared catalysts: (A) C 1s, (B) O 1s, (C) Ti 2p, and (D) Mn 2p.

As shown in Figure 3C, in the Ti 2p spectrum of MXene, four major peaks were detected at 454.66, 456.06, 458.26, and 460.66 eV, which can be indicated to Ti-C 2p_3/2, Ti²⁺ 2p_3/2, Ti³⁺ 2p_3/2, and Ti⁴⁺ 2p_3/2, respectively.²¹ After compositing with MnO₂, the difference in the positions of Ti 2p_3/2, between MXene and MnO₂/MXene hybrids is due to the decrease in Ti⁴⁺ intensity and the redshift in energy binding of Ti⁴⁺, which may be attributed to an electron transfer from MnO₂ to MXene during the composite process. Besides that, the electronic structure of the magnesium in the MnO₂ and MXene/MnO₂ composites was further examined (Figure 3D).

In the MnO₂ sample, the Mn 2p XPS spectrum exhibited a broad peak spanning from 638 to 651 eV, corresponding to the spin-orbit state of Mn 2p_3/2. Deconvolution of this peak revealed two distinct components which are the Mn(IV) species at approximately 644.92 eV and the Mn(III) species at around 642.12 eV, indicating the coexistence of Mn(IV) and Mn(III) within the MnO₂ structure. Upon integration of MnO₂ with the MXene framework, the binding energy of Mn³⁺ 2p_3/2 underwent a positive shift, implying the electron transfer from Mn³⁺ sites in MnO₂ to Ti⁴⁺ sites in MXene, further strengthening the Mn–O bonds, which is consistent with Raman data. The modifications in the electronic structure of MnO₂/MXene hybrids suggest a strong electronic interaction between MnO₂ and MXene, which facilitates enhanced charge transfer between Ti₃C₂T_x and MnO₂. This interaction is expected to improve the electrochemical performance of the material by promoting more efficient electron transport within the composite system.^32,41

Nitrate-to-ammonia electrochemical reduction performance

The synergistic effect between the MnO₂ and MXene components was systematically investigated by comparing the electrochemical nitrate reduction reaction (NO₃RR) performance of the heterogeneous 2D/2D MnO₂/MXene hybrids with those of the single-component MnO₂ and MXene catalysts. The NO₃RR activity was evaluated using a standard 3-electrode configuration H-type electrolysis cell (Figure S5, supplemental information) and preliminary catalytic activity was assessed through linear sweep voltammetry (LSV) in 0.5 M K₂SO₄ and 0.1 M KNO₃. As depicted in Figures 4A and S6a (supplemental information), all as-prepared catalysts exhibited a markedly higher current density in the NO₃⁻-containing electrolyte compared to bare K₂SO₄ solution, indicating that the enhanced current density primarily originates from the electrocatalytic reduction of NO₃⁻ ions. The dense MnO₂/MXene hybrid demonstrated superior current density from the onset potential, achieving 91.6 mA cm⁻² at −1.0 V vs. RHE, significantly outperforming single-component MnO₂ and MXene catalysts. Moreover, the MnO₂/MXene hybrids exhibited enhanced electron transfer kinetics, as evidenced by their lower Tafel slopes: 0.353 mV.dec⁻¹ for dense MnO₂/MXene, 0.373 mV.dec⁻¹ for sparse MnO₂/MXene, and 0.402 mV.dec⁻¹ for MnO₂ urchin/MXene—compared to MXene (0.434 mV.dec⁻¹) and MnO₂ (0.878 mV.dec⁻¹) (Figures 4B and S6b, supplemental information). These results suggest that the dense MnO₂/MXene hybrid exhibits the lowest energy barrier for NO₃⁻ adsorption and subsequent conversion.²⁷

Figure 4.

Electrochemical evaluation of as-prepared catalysts for NO₃RR

(A) Linear sweep voltammetry (LSV) at 5.0 mV s⁻¹ in 0.5 M K₂SO₄ with and without 0.1 M KNO₃, In 0.5 M K₂SO₄ and 0.1 M KNO₃ electrolyte: (B) corresponding Tafel plot of nitrate reduction kinetics; (C) Electrochemical impedance spectroscopy (EIS) with 5 mV alternating current amplitude at an applied potential of −0.6 V vs. RHE; (D) Double-layer capacitance (C_dl) measurement for determining electrochemically active surface area; (E) Faradaic efficiency (FE) and (F) yield rate of NH₃ as a function of overpotential for different catalysts. Error bars represent the standard deviation in yield rate and FE calculated after three tests for repeatability.

To gain deeper insights into the charge resistance characteristics of the synthesized catalysts, electrochemical impedance spectroscopy (EIS) was performed, and the associated Nyquist plots were analyzed (Figure S7a, supplemental information) reveal that MnO₂/MXene hybrids exhibit smaller semicircle diameters than single-component MnO₂ and MXene catalysts, indicative of enhanced charge transfer due to the interfacial synergy of the single-component 2D MnO₂ and 2D MXene. Equivalent circuit modeling (Figure 4C) demonstrates that for MnO₂/MXene hybrids, an uncompensated solution resistance (R_S) is connected in series to two parallel constant phase elements (CPE1 and CPE2) and resistors (R₁ and R_CT).¹⁹ Conversely, for single-component MnO₂ and MXene catalysts, R_S is connected to a single parallel combination of a singular parallel network comprising a constant phase element (CPE) and a charge transfer resistance (R_CT) (Figure S7b, supplemental information).⁴² The charge transfer resistance (R_CT) at low frequencies, representing electron transfer across the electrode-electrolyte interface, was significantly lower for dense MnO₂/MXene (5.001 Ω), confirming its superior charge transfer capability and efficient electron transport kinetics during NO₃RR. Additionally, the presence of R₁ in MnO₂/MXene hybrids at high frequency is associated with interfacial resistance between MnO₂ and MXene.⁴³ Notably, the dense MnO₂/MXene catalyst, characterized by abundant MnO₂ nanoflakes, facilitates extensive electron/ion transport channels, thereby minimizing R₁ (0.353 Ω) (Table S2, supplemental information). In contrast, the sparse MnO₂/MXene and MnO₂ urchin/MXene hybrids exhibit either insufficient or congested mass/ions transport pathways, leading to sluggish NO₃RR kinetics. As a result, these findings demonstrate that the dense MnO₂/MXene catalyst with optimal vertical-grown MnO₂ nanoflakes can greatly facilitate fast electron/ion transfer, leading to promoted NO₃RR activity.

The superior catalytic performance of the dense MnO₂/MXene hybrid can be attributed to its increased electrochemically active surface area (ECSA), which facilitates extensive exposure of active sites. This was confirmed by determining the double-layer capacitance (C_dl) serving as a quantitative proxy for ECSA, calculated from cyclic voltammetry (CV) measurements recorded at various scan rates (Figure S8, supplemental information).⁴⁴ As illustrated in Figures 4D and S9 (supplemental information), MnO₂/MXene hybrids exhibit significantly higher C_dl values: 4.49 mF cm⁻² for dense MnO₂/MXene, 3.58 mF cm⁻² for sparse MnO₂/MXene, and 2.74 mF cm⁻² for MnO₂ urchin/MXene, compared to single-component MXene (2.74 mF cm⁻²) and MnO₂ (0.581 mF cm⁻²), demonstrating the dense MnO₂/MXene catalyst possesses the largest ECSA. The partial current density, which is proportional to NH₃ yield rate, clearly demonstrates the enhanced activity and selectivity of the MnO₂/MXene hybrids compared to single-component MnO₂ and MXene catalysts (Figure S10, supplemental information). In particular, the dense MnO₂/MXene exhibited the highest NH₃ partial current density, with values 15.14- and 3.18-fold higher than those of MnO₂ and MXene, respectively, at −1.0 V vs. RHE. Therefore, the improved NO₃RR activity of MnO₂/MXene hybrids suggests the presence of a synergistic effect between single component 2D MnO₂ and 2D MXene catalysts.

Ammonia production efficiency

To further assess catalytic efficiency, electrolysis experiments were conducted at controlled potentials ranging from −0.7 to −1.2 V vs. RHE for 1 h in 0.5 M K₂SO₄ and 0.1 M KNO₃ electrolyte, using all as-prepared catalysts as cathodes. Following chronoamperometry, the electrolyte was analyzed via ultraviolet-visible spectrophotometry to quantify NH₃ and NO₂⁻ concentrations based on calibration curves (Figure S11, supplemental information). As shown in Figures 4E and S12a (supplemental information), NH₃ yield rates progressively increased with more negative applied potentials, while FE of NH₃ exhibited a volcano-shaped trend (Figures 4F and S12b, supplemental information), slightly decreasing at more negative potentials due to the competition of parasitic HER.⁴⁵

Remarkably, the dense MnO₂/MXene catalyst exhibited exceptional NO₃RR activity, attaining the highest NH₃ production rate of 14.06 ± 0.48 mg h⁻¹.mg_cat⁻¹ at −1.2 V vs. RHE, along with a maximum NH₃ FE of 85.23 ± 1.62% at −1.0 V vs. RHE. In conclusion, the superior NO₃RR performance of the dense MnO₂/MXene catalyst can be attributed to two key factors: (1) the synergistic interaction between 2D MnO₂ and 2D MXene, which enhances charge transfer and boosts intrinsic electrocatalytic activity, and (2) the dense MnO₂/MXene catalyst with the optimized MnO₂ nanoflake density, which maximizes active sites and facilitates improved NO₃RR kinetics.

Time-dependent nitrate reduction and ammonia formation kinetics

The relationship between the reaction time of the dense MnO₂/MXene catalyst and the fluctuation of NO₃⁻, NO₂⁻, and NH₃ concentrations during the reduction process was recorded at −1.0 V vs. RHE over 8 h in 0.5 M K₂SO₄ and 0.1 M KNO₃ (6200 ppm NO₃⁻) (Figure 5A). The NO₃⁻ concentration gradually decreases, while the NH₃ concentration increases, demonstrating the linear conversion of nitrate to ammonia and indicating that the nitrogen in NH₃ comes from NO₃⁻. Meanwhile, the low concentration of produced NO₂⁻ ions implies high NH₃ selectivity in the initial process and then gradually declines with reaction time, because of the subsequent reduction of the generated NO₂⁻ intermediate into several N-intermediates.^46,47 However, due to significant NO₃⁻ consumption, the corresponding yield rate and FE of NH₃ notably decrease after 8 h (Figure 5B). Furthermore, the negligible yield rate and FE of NO₂⁻ follow a trend similar to NH₃, indicating that the change in NO₃⁻ concentration affects NO₂⁻ production during NO₃RR (Figure S13). In addition, the time-concentration relationship of consumed NO₃⁻ was examined using a pseudo-first-order kinetic model (Figure 5C). The NO₃⁻ consumption rate increases rapidly in the first hour and then gradually slows until reaching equilibrium. The pseudo-first-order kinetic model estimates a maximum NO₃⁻ consumption of around 3979.43 ppm and a rate constant of 0.291 h⁻¹.⁴⁸

Figure 5.

NO₃RR performance evaluation of dense MnO₂/MXene

In 0.5 M K₂SO₄ and 0.1 M KNO₃ at −1.0 V vs. RHE: (A) concentration-time curves of the NO₃⁻, NO₂⁻, and NH₃ in NO₃RR for 8 h; and (B) the corresponding and yield rate and FE of NH₃ in NO₃RR for 8 h; (C) the logarithmic relationship between time and consumed NO₃⁻ concentration during NO₃RR for 8 h. FE and yield rate of NH₃ in (D) 0.5 M K₂SO₄ and 0.1 M KNO₃ solution with different pH; and in (E) 0.5 M K₂SO₄ solution with different concentrations of NO₃⁻; (F) cyclic stability test with refreshing the electrolyte on FE and yield rate of NH₃ in 0.5 M K₂SO₄ and 0.1 M KNO₃, (1 cycle/1 h); and (G) performance comparison of different reported Mn-based and MXene-based catalysts for NO₃RR to NH₃ production in neutral electrolyte (See detailed information in Table S3, supplemental information).

Effect of electrolyte pH and nitrate concentration on NO3RR performance

To investigate the competition between NO₃RR and the hydrogen evolution reaction (HER), NO₃RR was conducted at 0.5 M K₂SO₄ and KNO₃ 0.1 M under different pH conditions. In acidic electrolytes, HER dominates due to its favorable kinetics, leading to low values of NH₃ yield rate and FE (Figure 5D). In contrast, NO₃⁻-to-NH₃ performance increases and reaches a maximum NH₃ FE of 88.2% ± 3.21% at pH 10, indicating that NO₃RR is more favorable in neutral and alkaline electrolytes. This result suggests that OH⁻ concentration enhances H_ads (adsorbed hydrogen) generation, which plays a crucial role in the hydrogenation step of NO₃RR.^49,50 Consequently, the yield rate of NO₂⁻, nitrite is regarded as a vital nitrogen-intermediate in the NO₃RR process, resulting from the initial 2e-reduction of NO₃⁻-to-NO₂⁻, increases with pH due to enhanced hydrogenation in high-pH environments (Figure S14). However, an excessive amount of OH⁻ ions leads to the competitive adsorption of OH⁻ ions on the catalyst surface, reducing NO₃RR performance.⁵¹

Verification of ammonia origin and catalyst durability

To investigate the impact of NO₃⁻ concentration, the NO₃RR performance of the dense MnO₂/MXene hybrid was evaluated in 0.5 M K₂SO₄ with varying nitrate concentrations. As shown in Figure 5E, at low NO₃⁻ concentration (0.01 M), NH₃ yield rate remains low but significantly increases at higher NO₃⁻ concentrations, suggesting that NO₃RR strongly depends on NO₃⁻ concentration.⁵² Higher NO₃⁻ concentrations provide more effective reactive substrates for NO₃RR, indicating that elevated NO₃⁻ concentrations favor catalytic activity. Besides that, N-containing intermediates react with H_ads, and a high NO₃⁻ concentration maintains an effective supply of nitrogen-containing intermediates, suppressing water dissociation and ensuring NH₃ production.⁵³ Additionally, the yield rate and FE of NO₂⁻ at 0.02 M NO₃⁻ electrolyte are significantly higher than those at lower NO₃⁻ concentrations (0.01 M), further confirming that high NO₃⁻ concentrations enhance NH₃ efficiency by promoting intermediate formation (Figure S15, supplemental information).

To quantitatively prove that the ammonia originates from the nitrate, the electrolysis process in the electrolyte with and without NO₃⁻ is performed (Figure S16, supplemental information). As expected, negligible NH₃ was detected in the post-electrolysis 0.5 M K₂SO₄ solution, effectively ruling out potential interference from nitrogen-containing impurities to the experiment conclusion. Additionally, to exclude the possible influence of carbon cloth substrate in the NO₃RR performance, only a minimal amount of NH₃ was observed following electrolysis with the blank carbon cloth, indicating its inherently low NO₃RR activity. This finding confirms that the catalytic performance primarily originates from the intrinsic properties of the synthesized catalysts.

To assess the durability of the dense MnO₂/MXene catalyst, a cycling test was conducted over 6 h by replenishing the solution every hour under −1.0 V vs. RHE (Figure 5F). The NH₃ FE values remain above 75% after 6 cycles, demonstrating the acceptable durability of the dense MnO₂/MXene catalyst. To compare the NH₃ performance of this work with previous reports, a comparative chart was constructed based on NH₃ yield and FE in neutral electrolyte. In conclusion, the dense MnO₂/MXene catalyst exhibits acceptance performance compared to reported Mn-based and MXene-based catalysts in terms of NH₃ yield and FE in neutral environments (Figure 5G and Table S3, supplemental information).

Zinc-nitrate battery behavior

Owing to the high energy density of NH₃ (4.32 kWh.L⁻¹) and its capability for simultaneous electricity generation, Zn-NO₃⁻ batteries have emerged as promising energy storage devices with both high efficiency and environmentally friendly sustainability. This can be attributed to their dual functionality, enabling both power generation and the electrochemical conversion of NO₃⁻ from wastewater into value-added NH₃. Inspired by the acceptance NO₃RR activity of dense MnO₂/MXene catalyst, a Zn-NO₃⁻ battery was constructed using dense MnO₂/MXene as the cathode in an electrolyte containing 0.5 M K₂SO₄ and 0.01 M Zn(CH₃COO)₂, while a polished Zn plate, sourced from a spent Zn-carbon battery, served as the anode in 0.5 K₂SO₄ and 0.5 M KNO₃ (Figure 6A). As depicted in Figure 6B, the Zn-NO₃⁻ battery exhibited an open-circuit potential of 1.04 V, which remained stable over a period exceeding 10 h with negligible fluctuation. During the discharge process, the electrochemical reduction of NO₃⁻ to NH₃ was facilitated by Zn dissolution. The electrode reactions during discharge are delineated as follows:

Cathode: NO₃⁻ + 6H⁺ + 8e⁻ → NH₃ + 3OH⁻

Anode: 4Zn + 8OH⁻ → 4ZnO + 4H₂O + 8e⁻

Overall reaction: 4Zn + NO₃⁻ + 2H₂O → 4ZnO + NH₃ + OH⁻

Figure 6.

Zinc-nitrate battery behavior

Electrochemical evaluation of the dense MnO₂/MXene-based Zn-NO₃^- battery in 0.5 M K₂SO₄ and 0.5 M KNO₃: (A) schematic diagram; (B) open-circuit potential; (C) polarization discharge curves and power density; (D) FE and yield rate of NH₃ in Zn-NO₃^- battery operated at different current densities; (E) discharging potentials at different current densities.

A critical performance metric for battery evaluation is power density. As revealed by the discharge polarization curve with 5 mV s⁻¹ at scan rate, the battery achieved a maximum power density of 0.323 mW cm⁻² (Figure 6C). Furthermore, Figure 6D illustrates the NH₃ yield rate and the corresponding NH₃ FE under varying discharge current densities. Notably, at a discharge current density of 2.5 mA cm⁻², the Zn-NO₃⁻ battery exhibited NH₃ FE of 79.63% ± 4.51% alongside an NH₃ yield rate of 157.72 ± 9.84 μg h⁻¹.mg_cat⁻¹, demonstrating its capacity for both effective NH₃ synthesis and energy output. Moreover, the long-term electrochemical stability of the battery was confirmed through trapezoidal discharge profiles at incremental current densities ranging from 0.2 mA cm⁻² to 2.5 mA cm⁻², as presented in Figure 6E. The highly stable discharge voltage plateaus across varying current densities underscores the excellent rate performance of the Zn-NO₃⁻ battery employing the dense MnO₂/MXene cathode. In conclusion, the previous results highlight the considerable potential of Zn-NO₃⁻ batteries with the dense MnO₂/MXene cathode for the on-site generation of value-added ammonia from nitrate while concurrently serving as a reliable energy source.

Conclusion

In summary, 2D MXene skeleton functionalized with 2D nanoflakes MnO₂ can enhance the NO₃RR catalytic activity by alternating the electronic structure and enhanced mass transport channels. Using different KMnO₄ concentrations leads to the formation of sparse MnO₂/MXene, dense MnO₂/MXene, and MnO₂ urchin/MXene catalysts. The changes in morphology and electronic properties of as-prepared catalysts are elucidated through characterization tests. The optimal density of MnO₂ nanoflakes distributing onto MXene substrate in the dense MnO₂/MXene hybrid exhibited an acceptance performance, achieving an NH₃ FE of 85.23% at −1.0 V vs. RHE in a neutral environment. Furthermore, the catalytic stability is retained over six consecutive cycles, with NH₃ FE consistently exceeding 75%. This exceptional catalytic activity is attributed to the finely optimized electronic properties and geometric configuration, enhancing the mass transfer capabilities and catalytic active sites. Moreover, the Zn–NO₃⁻ battery including the dense MnO₂/MXene cathode exhibits a remarkable NH₃ FE of 79.63% ± 4.51%, and an NH₃ production rate of 157.72 ± 9.84 μg h⁻¹.cm⁻² at a discharge current density of 2.5 mA cm⁻². This study not only introduces a high-effective heterogeneous 2D/2D catalyst for NO₃RR but also presents a promising strategy for simultaneous energy storage and ammonia synthesis through a Zn–NO₃^- battery system.

Limitations of the study

Although significant efforts were made to mitigate the limitations of this study, some factors may still influence the results and their interpretation, including: (1) the lack of confirmation of the nitrogen source origin using nuclear magnetic resonance and the detection of other by-products such as N₂, H₂, NO_x, N₂H₄, etc., (2) the absence of post-treatment characterization of the NO₃RR process using techniques such as XPS, TEM, SEM, and XRD to assess changes in the morphology and electronic properties of the as-prepared catalysts, and (3) the lack of theoretical calculations to elucidate the NO₃RR reaction pathway.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Phi Long Nguyen (long.np2@vinuni.edu.vn).

Materials availability

This study did not generate new unique reagents.

Data and code availability

• •Adjective data reported in this paper will be shared by the lead contact upon request.
• •This study did not generate original code.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Acknowledgments

This work was financially supported by the grant no. VUNI.2324.SG05 and grant no. VINUNI.CEI.FS_0007 from VinUniversity, Vietnam.

Author contributions

T.K.C.P: writing – original draft, writing – review and editing, data curation, conceptualization; formal analysis, investigation, methodology; T.N.P: writing – review and editing; A.-G.N: visualization, writing – review and editing; T.N.T: writing – review and editing; T.M.-A.T: investigation, methodology; N.N.L: conceptualization, writing – review and editing, resources, and supervision; P.L.N: conceptualization, writing – review and editing, and supervision; T.V.B.P: conceptualization, writing – review and editing, funding acquisition, and supervision.

Declaration of interests

The authors declare no conflict of interest.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the authors used ChatGPT in order to improve readability and language of the work. After using this tool/service, the author reviewed and edited the content as needed and took full responsibility for the content of the publication.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Other
MAX phase (Ti3AlC2)	Luoyang Tongrun Nano Technology Co. Ltd., China	https://www.alibaba.com/product-detail/Max-Phase-Titanium-Aluminum-Carbide-Powder_1600216351996.html?spm=a2700.shop_index.89.1.30061495fwVBMS
Hydrofluoric acid (HF)	Sigma-Aldrich	Cat#7664393
Dimethyl sulfoxide (DMSO)	Sigma-Aldrich	Cat#67685
Potassium sulfate (K2SO4)	Sigma-Aldrich	Cat#7778805
Sulfamic acid (H3NSO3)	Sigma-Aldrich	Cat#5329146
p-aminobenzensulfonamide (C6H8N2O2S)	Sigma-Aldrich	Cat#9863741
Zinc acetate (Zn(CH3COO)2	Sigma-Aldrich	Cat#5970456
Potassium nitrate (KNO3)	Sigma-Aldrich	Cat#7757791
Potassium nitrite (KNO2)	Sigma-Aldrich	Cat#7758090
Phosphoric acid (H3PO4)	Sigma-Aldrich	Cat#7664382
Sodium potassium tartrate (C4H4KNaO6)	Sigma-Aldrich	Cat#6381595
Nessler’s reagent	Sigma-Aldrich	Cat#7783337
Potassium hydroxide (KOH), 85%	Thermal Fisher	Cat#1310583
N-(1-Naphthyl)-ethylenediamine dihydrochloride, (C12H16Cl2N2) >98%	Thermal Fisher	Cat#1465254
Hydrogen peroxide (H2O2) 35% w/w aq. soln., stab.	Thermal Fisher	Cat#7722841
Potassium permanganate (KMnO4)	Weng Jiang Reagent	Cat#7722647
Ethanol (C2H5OH)	Duc Giang Chemicals Group, Vietnam	https://ducgiangchem.vn/en/category/san-pham/pure-chemicals/
Isopropanol (C3H7OH)	VietChem, Vietnam	https://vietchem.com.vn/san-pham/isopropyl-alcohol--ipa-99--c3h8o-singapore-163kg-phuy.html
Sulfuric acid (H2SO4), 97%	Samchun Chemical Co., Ltd., South Korea	Cat#005S1430
Hydrochloric acid (HCl), 35.0–37.0%	Samchun Chemical Co., Ltd., South Korea	Cat#005H0256
Carbon cloth	Suzhou Sinero Technology Co., Ltd, China	W0S1011

Method details

Catalyst preparation

Synthesis of 2D Ti₃C₂T_x nanosheets from MAX phase Ti₃AlC₂

The synthesis of 2D few-layer Ti₃C₂T_x MXene nanosheets was carried out by etching Ti₃AlC₂ powders in concentrated HF, following a previously reported method. Specifically, 200 mg of Ti₃AlC₂ was gradually introduced into 5 mL of concentrated HF solution (48%), with continuous magnetic stirring at 50°C for approximately 36 h. Subsequently, 30 mL of DIW was slowly added, and stirring was continued for 20 min at room temperature. The resulting multilayer Ti₃C₂T_x MXene nanosheets were separated by centrifugation at 5000 rpm, and repeatedly washed with DIW until the pH of the supernatant reached approximately 6. To obtain fewer-layer Ti₃C₂T_x MXene nanosheets, 10 mL of DMSO was added to the supernatant, followed by simultaneous sonication and stirring at 300 rpm for 36 h. Finally, the fewer-layer Ti₃C₂T_x MXene was washed by centrifugation at 5000 rpm using DIW multiple times until a neutral pH was achieved, then diluted to 10 mL for further use, with an estimated concentration of 20 mg mL⁻¹.

Synthesis of 2D/2D-MnO₂/MXene

A series of KMnO4 solutions with varying concentrations (0.001 M, 0.005 M, and 0.01 M) were prepared in a volume of 100 mL, designated as solution A. Concurrently, 2 mL of a few-layer Ti₃C₂T_x MXene nanosheet dispersion (20 mg mL⁻¹) was ultrasonically treated in 100 mL of ethanol for 15 min to obtain solution B. Solution A was then gradually added dropwise to solution B under continuous magnetic stirring at 700 rpm for 2 h. The resulting precipitate was collected by centrifugation, washed five times with DIW, and subsequently dried in a vacuum oven at 35°C for 48 h. The final products - sparse MnO₂/MXene, dense MnO₂/MXene, and MnO₂ urchin/MXene - corresponded to the used KMnO₄ concentrations of 0.001 M, 0.005 M, and 0.01 M, respectively. For comparison, MnO₂ catalyst was synthesized following the same procedure as the MnO₂/MXene catalysts but without the addition of MXene.

Characterization

The morphology of the synthesized catalysts was analyzed using field emission scanning electron microscopy (FE-SEM, SU-8010, Hitachi) and high-resolution transmission electron microscopy (HR-TEM, JEM2100), along with energy-dispersive X-ray spectrometry (EDS) mapping. Phase identification of the prepared samples was conducted via X-ray diffraction (XRD) within the 2θ range of 10°–80° at a scan rate of 5° min⁻¹ using an X-ray diffractometer (D8 ADVANCE, Bruker) equipped with Cu-Kα radiation (λ = 0.154 nm). The chemical state of the central element was examined through X-ray photoelectron spectroscopy (XPS) using a standard monochromatic Al-Kα source (JPS-9030 Photoelectron Spectrometer), with all spectra calibrated to the main C1s peak at 284.6 eV. The surface area and pore size distribution were determined via nitrogen adsorption-desorption isotherms, employing the multipoint BET (Brunauer-Emmett-Teller) method and BJH (Barrett-Joyner-Halenda) methods at 77.3 K using a Quantachrome Instrument. Raman spectra were acquired with a laser confocal Raman microscope (XploRA ONE, Horiba) using a 432 nm excitation laser. Ultraviolet-visible (UV-Vis) spectroscopy measurements were performed with a NOVEL-102S Double Beam Spectrophotometer.

Electrochemical nitrate reduction test

All electrochemical measurements were carried out using an electrochemical workstation (Corrtest C@310M). A catalyst ink was prepared by dispersing 10 mg of the synthesized catalyst in a 1:1 volume ratio of isopropanol and ethanol (0.95 mL total) under ultrasonic agitation for 1 h. Subsequently, 0.05 mL of Nafion solution (5 wt. %) was added, followed by sonicating for 15 to ensure homogeneity. The obtained catalyst slurry (0.1 mL) was drop-cast onto a 1 × 1 cm² carbon cloth substrate, yielding an estimated catalyst loading of 1 mg cm⁻². Prior to electrode preparation, the carbon cloth was ultrasonically cleaned in ethanol and deionized water (DIW) for 15 min in each solvent to remove surface impurities. The dried catalyst-coated carbon cloth was then employed as the working electrode, in conjunction with either an Ag/AgCl reference electrode for acidic and neutral electrolytes or a Hg/HgO reference electrode for alkaline electrolytes.

Nitrate electrochemical reduction experiments were conducted in a three-electrode system within an H-type electrochemical cell, where the cathode and anode compartments were separated by a Nafion 115 membrane (Thinkru Hydrogen Ltd, China). The membrane was pretreated by sequential immersion in 5% H₂O₂ at 80°C for 1 h, DIW at 80°C for 2 h, and 5% H₂SO₄ at 80°C for 1 h, followed by multiple rinses with DIW. The electrolyte solution comprised 0.5 M K₂SO₄ and 0.1 M KNO₃ (pH 6.7), with pH adjustments made using either saturated H₂SO₄ or saturated KOH solutions. All measured potentials were referenced to the reversible hydrogen electrode (RHE) using the following equations: E(RHE) = E(Ag/AgCl) + 0.059 $\times$ pH + 0.209 or E(RHE) = E(Hg/HgO) + 0.059 $\times$ pH + 0.140.

Linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV s⁻¹ from 0.00 V to −1.40 V (vs. RHE) until repeatable results were obtained. Electrochemical impedance spectroscopy (EIS) was carried out over a frequency range of 100 kHz to 0.1 Hz, with an alternating current perturbation amplitude of 10 mV at −0.7 V vs. RHE. Cyclic voltammetry (CV) was conducted within a non-Faradic region at six different scan rates (100, 80, 60, 40, 20, and 10 mV s⁻¹). The double-layer capacitance (C_dl) was determined by plotting the CV curve area against the scan rate, with the slope of the linear fit corresponding to the capacitance value. Chronoamperometric (CA) measurements were performed at varying applied potentials for 1 h. The yield rate, Faradaic efficiency, and corresponding error bars were obtained from three independent samples subjected to identical testing conditions. The partial current density for NH₃ production was calculated using the equation: j_NH3 = j $\times$ FE_NH3. Where j is the current density normalized to the area of the working electrode.

The electrochemical performance of the hybrid liquid Zn-NO₃^- battery was evaluated in a two-electrode configuration within an H-type cell separated by a Nafion 115 membrane. A polished Zn plate (1 × 1 cm², 0.5 mm thickness) recovered from a spent zinc-carbon battery was utilized as the anode, while the cathode consisted of a carbon cloth loaded with dense MnO₂/MXene (1 × 1 cm², 1 mg loading). Before use, the Zn electrode was ultrasonically cleaned in a sequence of soap solution, DIW, and diluted H₂SO₄ to remove surface impurities. The anolyte contained 60 mL of 0.5 M K₂SO₄ and 0.01 M Zn(CH₃COO)₂, while the catholyte contained 60 mL of 0.5 M K₂SO₄ and 0.5 M KNO₃.

Product quantification

The ultraviolet-visible (UV-Vis) spectrophotometer was employed to quantify ion concentrations in the post-experiment electrolytes. Prior to analysis, the samples were appropriately diluted to ensure their absorbance values fell within the linear range of the calibration curves. The specific detection procedures are detailed as follows.

NO₃⁻ determination

A specific volume of electrolyte was withdrawn from the electrolysis cell and diluted to 5 mL to ensure compatibility with the detectable concentration range. Subsequently, 0.1 mL of 1 M HCl and 0.1 mL of a 0.8 wt % sulfamic acid solution were introduced into the prepared sample. The absorption spectrum was acquired via UV-Vis spectrophotometry, with absorbance recorded at 220 nm and 274 nm. The final absorbance value was calculated using the equation: A = A_220nm – 2A_274nm. A calibration curve was generated based on a series of potassium nitrate standard solutions (Figures S11A and S11B, supplemental information).

NO₂⁻ determination

The colorimetric reagent was formulated by dissolving 0.4 g of p-aminobenzenesulfonamide and 0.01 g of N-(1-naphthyl) ethylenediamine dihydrochloride in 9 mL of deionized water (DIW), followed by the addition of 1 mL of phosphoric acid (ρ = 1.71 g/mL). A predetermined volume of electrolyte was extracted from the electrolysis cell, diluted to 5 mL, and thoroughly homogenized. The reaction mixture was allowed to develop for 20 min before measuring the absorbance at 540 nm. The nitrite-N concentration was determined through a calibration curve established using potassium nitrite standard solutions (Figures S11C and S11D, supplemental information).

NH₃ determination

The quantification of NH₃ was conducted utilizing the Nessler’s reagent assay. A predefined aliquot of electrolyte was extracted from the electrochemical system and subsequently diluted to 5 mL to ensure it fell within the measurable concentration range. Thereafter, 0.1 mL of a 0.1 M potassium sodium tartrate solution was introduced and homogenized, followed by the addition of 0.1 mL of Nessler’s reagent. The mixture was then left undisturbed for 20 min to allow for complete reaction, after which the absorbance at 424 nm was measured. A calibration curve was constructed using a series of ammonia standard solutions (Figures S11E and S11F, supplemental information).

Calculation of yield rate (r) and Faradaic Efficiency (FE)

For nitrate-to-ammonia electrochemical reduction, the yield rate (r) (μg h-1 m_cat⁻¹) and FE of NH₃ or NO₂⁻ were defined according to the following equations:

$${\mathrm{r}}_{\text{NH}3}=\left(\left[{\text{NH}}_3\right]\times \mathrm{V}\times 3600\right)/\left(\mathrm{t}\times {\mathrm{m}}_{\text{cat}}\right)$$

$${\text{FE}}_{\text{NH}3}=\left(8\times 96500\times \left[{\text{NH}}_3\right]\times \mathrm{V}\right)/\left(1000000\times 17\times \int idt\right)\times 100\%$$

$${\mathrm{r}}_{{\text{NO}2}^{-}}=\left(\left[{\text{NO}}_{2^{-}}\right]\times \mathrm{V}\times 3600\right)/\left(\mathrm{t}\times {\mathrm{m}}_{\text{cat}}\right)$$

$${\text{FE}}_{{\text{NO}2}^{-}}=\left(2\times 96500\times \left[{\text{NO}}_{2^{-}}\right]\times \mathrm{V}\right)/\left(1000000\times 46\times \int idt\right)\times 100\%$$

where [NH₃] and [NO₂⁻] are the ammonia and nitrite concentration (μg mL⁻¹), respectively, m_cat. is the mass catalyst on the working electrode (1 mg), V is the volume (mL) of the electrolyte at anode part or cathode part in H-cell (60 mL), i is the current (A), t is the reaction time (h) of chronoamperometry tests, $\int idt$ is the total consumed charge (C).

The reaction rate (k with units of h⁻¹) was calculated by assuming first-order dependence on consumed nitrate concentration (ΔC_NO3-):

$$\left[\Delta {\mathrm{C}}_{{\text{NO}3}^{-}}\left(\mathrm{t}\right)\right]=\left[\Delta {\mathrm{C}}_{{\text{NO}3}^{-}}\left(\mathrm{e}\right)\right]\ast \left(1-{\mathrm{e}}^{\text{kt}}\right)$$

Where

• [ΔC_NO3- (t)] is the consumed NO₃⁻ concentration at time t.

• [ΔC_NO3- (e)] is the consumed NO₃⁻ concentration at the reaction equilibrium.

• k is the pseudo-first-order rate constant.

• t is reaction time (h)

Quantification and statistical analysis

There are no quantification or statistical analyses to include in this study.

Published: May 22, 2025