Ti₃C₂T_x MXenes and Mn₃O₄ nanoparticles synergistically promote the electrochemical synthesis of ammonia under ambient conditions

Abstract

Ammonia serves as a hydrogen energy carrier and a renewable, zero-carbon fuel alternative that is safely transportable. The electrochemical catalytic reduction of N₂ to NH₃ in aqueous electrolytes at ambient temperature and pressure (eNRR) using electricity generated from renewable energy sources such as solar and wind power can provide an environmentally friendly approach. To effectively suppress the occurrence of hydrogen evolution side reactions, it is necessary to design and synthesize catalysts with high selectivity for N₂ adsorption. Owing to the ability of transition metals with unoccupied d orbitals to significantly promote the adsorption of N₂ molecules and the activation of inert bonds, researchers have explored manganese-oxide catalysts through both experimental and theoretical studies. However, manganese oxides are semiconductor materials with poor conductivity. To solve this problem, the Ti₃C₂T_x MXene material can be introduced as a carrier for manganese oxide particles. In this study, the Ti₃C₂@Mn₃O₄ composite was used as an electrocatalyst for ammonia synthesis under ambient conditions using a simple method. Benefiting from the synergistic catalytic effect of MXene and Mn₃O₄, the composite exhibits excellent catalytic performance for ammonia synthesis, with an NH₃ yield rate of 53.7 μg h⁻¹ mg_cat.⁻¹ and satisfactory FE of 10.4% at −0.6 V (vs. RHE) under ambient conditions. The composite catalyst exhibits excellent stability, durability, and selectivity, with outstanding synergistic effects, surpassing most reported NRR electrocatalysts. This simple and versatile strategy may offer researchers inspiration for rationally designing highly efficient NRR electrocatalysts.

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1. Introduction

Ammonia is a hydrogen energy carrier that can be easily and safely transported, as well as a renewable and zero-carbon fuel. However, the Haber–Bosch process for producing ammonia is a high-temperature, high-pressure process with high energy consumption in the chemical industry, consuming about 2% of the world's energy annually. At the same time, the H₂ required for the reaction is mainly produced through the steam reforming of natural gas, with the required energy mainly derived from fossil fuels, resulting in annual CO₂ emissions of approximately 320 million tons.¹ Therefore, finding a green, environmentally friendly, and energy-efficient method for ammonia synthesis is of great significance for the sustainable development of the economy and ecology.

The electrochemical catalytic reduction of N₂ in aqueous electrolytes under ambient conditions to synthesize ammonia (eNRR), utilizing electricity generated from renewable energy sources such as solar and wind power, can provide an alternative and environmentally benign method. This method not only overcomes the limitations of thermodynamic equilibrium for the thermodynamically unfavorable NH₃ synthesis reaction with the driving force of electricity but also eliminates or reduces the constraints of thermodynamic equilibrium, thereby avoiding the reliance on temperature and high pressure conditions and reducing CO₂ emissions, becoming a highly researched field.^2–4 In acidic aqueous electrolytes, the reaction occurring at the cathode is⁵

N₂ (g) + 6H⁺ (aq.) + 6e⁻ → 2NH₃ (g) (E⁰ = 0.0577 V vs. SHE) (1)

At the same time, a side reaction occurs:

2H⁺ (aq.) + 2e⁻ → H₂ (g) (E⁰ = 0 V vs. SHE) (2)

During the eNRR process, N₂ molecules need to be adsorbed and activated on the catalyst surface, followed by hydrogenation reactions. N₂ molecules are highly inert, so active H⁺ ions are more easily adsorbed onto the catalyst surface, leading to strong competitive hydrogen evolution reactions (HER), with low faradaic efficiency (FE) for eNRR and slow ammonia synthesis rates.^6,7 Therefore, suppressing the hydrogen evolution reaction is key to improving the performance of eNRR.

To effectively suppress the occurrence of hydrogen evolution side reactions, it is necessary to design and prepare a catalyst with high selectivity for N₂ adsorption. In light of the fact that transition metals with unoccupied d orbitals can significantly promote the adsorption of N₂ molecules and the activation of inert bonds, researchers have explored manganese-based catalysts through experimental and theoretical studies.^8–11 Kong et al. used MnO₂ as an NRR electrocatalyst and achieved an NH₃ yield rate of 34.12 μg h⁻¹ mg_cat.⁻¹ in 0.1 M HCl;¹² Wu et al. synthesized Mn₃O₄ nanocubes as an NRR electrocatalyst with an NH₃ yield rate of 11.6 μg h⁻¹ mg_cat.⁻¹ and FE of 3.0% in 0.1 M Na₂SO₄;¹³ Zheng et al. reported a MnO-based composite, which displayed a low FE of 1.5% in 0.1 M Na₂SO₄.¹⁴

As a transition metal oxide, manganese oxides are semiconductor materials with poor conductivity. In order to improve its conductivity, the Ti₃C₂T_x MXene material can be introduced as a carrier for manganese oxide particles. MXenes (M_n+1X_n, n = 1, 2, and 3)are 2D layered transition metal carbides and nitrides that exhibit excellent conductivity, good hydrophilicity, and a variety of surface terminal groups (T_x, such as =O, –OH, and –F).^15–17 The synthesized manganese oxide nanoparticles can be firmly anchored on the surface and interlayers of Ti₃C₂T_x MXene materials due to the electronegativity of the surface terminals (T_x). In addition, recent research suggests that Ti₃C₂T_x exhibits good NRR activity.^18–21

In this study, a Ti₃C₂@Mn₃O₄ composite was used as an electrocatalyst for ammonia synthesis under ambient conditions in a simple method. Benefiting from the synergistic catalytic effect of MXene and Mn₃O₄, the composite exhibits excellent catalytic performance for ammonia synthesis with an NH₃ yield rate of 53.7 μg h⁻¹ mg_cat.⁻¹ and satisfactory FE of 10.4% at −0.6 V (vs. RHE) under ambient conditions. The composite catalyst exhibits excellent stability, durability, and selectivity, with outstanding synergistic effects, surpassing most reported NRR electrocatalysts. This simple and versatile strategy may offer researchers inspiration for rationally designing highly efficient NRR electrocatalysts.

2. Experimental section

2.1 Material preparation

First, an appropriate amount of Ti₃AlC₂ MAX material was weighed and etched with hydrofluoric acid (HF) to remove the Al layer. The mixture was then filtered, washed, and vacuum dried to obtain the layered Ti₃C₂T_x MXene material.²² Subsequently, 0.1 g of Ti₃C₂T_x MXenes was ultrasonically dispersed in 50 mL of deionized water until a uniform dispersion was formed, followed by the addition of 15 mL of a MnSO₄ solution (0.05 g mL⁻¹) into the dispersion with continuous stirring. Due to the abundant terminal groups (T_x, such as O, –F, and –OH) of Ti₃C₂T_x MXene and electronegativity, positively charged manganese ions (Mn²⁺) will be electrostatically attracted to intercalate between the Ti₃C₂T_x MXene layers and firmly adsorb on its surface; meanwhile, the intercalation of manganese ions will effectively increase the interlayer spacing of Ti₃C₂T_x MXenes.²³

Then, 10 mL of KOH solution with a concentration of 0.2 g mL⁻¹ was added dropwise to the dispersion under continuous stirring for 30 min. The added hydroxide ions (OH⁻) reacted with Mn²⁺ to form manganese hydroxide (Mn(OH)₂), which was immediately oxidized to manganese tetroxide (Mn₃O₄) in the air atmosphere. The resulting Mn₃O₄ was firmly anchored to the surface of the Ti₃C₂T_x MXene material to form the Ti₃C₂@Mn₃O₄ composite materials, thereby preventing further growth of Mn₃O₄ nanoparticles. The resulting dispersion was vacuum-filtered and washed until the pH was 7–8, then dried under vacuum at 100 °C for 12 h. After cooling to room temperature, the material was used for electrode preparation. At the same time, pure Mn₃O₄ particles were synthesized using the same method for comparative experiments.

2.2 Material characterization

The phase structure of the prepared materials was characterized using an X-ray diffractometer (XRD, DX-600X, Cu-Kα1). The surface elemental composition and the corresponding chemical structure of the prepared materials were analyzed using an X-ray photoelectron spectrometer (XPS, Thermo Scientific NEXSA, Al-Kα). The micro-morphology of the materials was observed using a field emission scanning electron microscope (SEM, FEI Inspect F50), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM, JEM2100). The manganese (Mn) content in the Ti₃C₂@Mn₃O₄ composite material was analyzed using an inductively coupled plasma emission spectrometer (ICP-OES, AGILENT 730).

2.3 Electrochemical analysis

The electrochemical tests were conducted in a two-compartment electrolytic cell, separated by a Nafion membrane. A platinum electrode and saturated Hg/Hg₂SO₄ electrode were applied as the counter electrode and reference electrode, respectively. For the working electrode, an appropriate amount of catalyst and Nafion solution (5 wt%) were ultrasonically dispersed in a mixed solution of pure water and anhydrous ethanol to form a uniform dispersion. Then, a small amount of the dispersion was loaded onto a carbon cloth with an area of 1 × 1 cm². The final catalyst loading is about 0.5 mg cm⁻². The electrolysis cell (H₂SO₄, pH = 2) was continuously purged with high-purity N₂ for 30 min under sealed conditions to exclude air while ensuring that the N₂ was saturated. The corresponding electrochemical tests were performed at room temperature and atmospheric pressure using an electrochemical workstation. All electrode potentials were converted to those corresponding to the reversible hydrogen electrode (E (vs. RHE) = E (vs. Hg/Hg₂SO₄) + 0.616 + 0.059 × pH).

The linear sweep voltammetry (LSV) curves were determined at a scan rate of 5 mV s⁻¹ and the corresponding chronoamperometry tests were carried out at a series of negative potentials. Throughout the experiment, a continuous flow of high-purity nitrogen gas was maintained to the cathodic compartment. Cyclic voltammetry (CV) curves were conducted in the range of 0.35 and 0.45 V with the scan rates of 20, 40, 60, 80, and 100 mV s⁻¹.

2.4 Determination of NH₃

The yield of ammonia (NH₃) produced by the electrocatalytic reduction of N₂ was determined using the indophenol blue method.^24,25 4 mL of the electrolyte after the reaction was mixed with 500 μL of sodium salicylate (C₇H₅NaO₃) solution (0.4 M C₇H₅NaO₃, 0.32 M NaOH), 50 μL of sodium hypochlorite (NaClO) solution (0.05 M NaClO, 0.75 M NaOH), and 50 μL of sodium nitroprusside (C₅FeN₆Na₂O) solution (1 wt%). The mixed solution was then kept in the dark for 2 h. Then, the absorbance of the mixed solution was measured using a UV-visible spectrophotometer. The concentration–absorbance curve was plotted using a standard NH₄Cl solution of 0∼2 μg mL⁻¹. The yield of ammonia was calculated using eqn (3):

r_NH3 = (c_NH3V)/(tm) (3)

where c_NH3 is the measured concentration of ammonia (μg mL⁻¹), V is the volume of the electrolyte (mL), t is the reaction time (h), and m is the amount of catalyst used (mg).

2.5 Determination of N₂H₄

The Watt–Chrisp method was used for hydrazine detection.^24,25 First, p-dimethylaminobenzaldehyde (0.998 g) was dissolved in ethanol (50 mL) to act as the colorimetric reagent. Then, the electrolyte (2 mL) was thoroughly mixed with the colorimetric reagent (2 mL) for 20 min, and the absorbance was detected using a UV-visible spectrophotometer (λ = 455 nm). The absorbance curve was plotted using standard hydrazine solutions (0–1 μg mL⁻¹).

2.6 FE of NH₃ production

The FE for ammonia synthesis is the ratio of the charge transferred for ammonia production to the total consumed charge, which is given by eqn (4):

FE = 3Fn_NH3/Q (4)

F is the Faraday constant (96 485 C mol⁻¹), n_NH3 is the molar amount of produced ammonia (mol), and Q is the total consumed charge (C).

3. Results and discussions

3.1 Morphology and structure

Fig. 1a shows the XRD patterns of the samples. When Mn₃O₄ nanoparticles were in situ grown on the surface of Ti₃C₂T_x MXene layers to form the Ti₃C₂@Mn₃O₄ composites, the original XRD characteristic peak at 8.85° shifted to a lower angle at 6.9°, and the interlayer spacing of Ti₃C₂T_x MXene increased from 0.998 to 1.280 nm, indicating manganese ions were effectively intercalated into the Ti₃C₂T_x MXene layers; the diffraction peaks at 18.0°, 2.9°, 32.3°, 36.1°, 36.4°, 44.4°, 59.8° matched with the standard card of Mn₃O₄ (PDF# 24-0734), corresponding to the diffraction peaks of (1 0 1), (1 1 2), (1 0 3), (2 1 1), (2 02), (2 2 0), and (2 2 4), respectively. The ICP-OES analysis revealed that the manganese content in Ti₃C₂@Mn₃O₄ was 0.456 g, and thus the calculated content of Mn₃O₄ in the Ti₃C₂@Mn₃O₄ composite was 63.3%. Fig. 1b–f show the XPS spectra of the Ti₃C₂@Mn₃O₄ material. The Ti2p spectrum (Fig. 1c) exhibits three sets of resolved peaks at 455.3 and 461.0, 456.8 and 462.4, 458.6 and 464.2 eV, corresponding to Ti–C, Ti³⁺, and Ti⁴⁺(Ti–O),²³ respectively. The C 1 s spectrum (Fig. 1d) can be deconvoluted into four Gaussian peaks, with the strongest peak at 284.8 eV corresponding to the C–C bonds, while the other peaks at 285.2, 286.4 and 288.3 eV correspond to the C–Ti, C–O and C–F bonds, respectively.²³ As for Mn 3s (Fig. 1e), the peaks at 83.2 and 88.9 eV correspond to Mn 3s_3/2 and Mn 3s_1/2, respectively, and the binding energy difference (ΔE) between these two peaks is 5.7 eV, which is lower than that of Mn²⁺ (ΔE = 6 eV) and higher than that of Mn³⁺ (ΔE = 5.3 eV), indicating that the valence state of Mn in the Ti₃C₂@Mn₃O₄ composites is a mixed valence state of Mn²⁺ and Mn³⁺.²⁶ The O 1s spectrum (Fig. 1f) can be deconvoluted into three peaks at 529.7, 530.1, and 531.4 eV, corresponding to O–C, Mn–O–Mn (Ti–O), and O–H,²⁶ respectively. All the above analysis indicates that the prepared Ti₃C₂T_x MXene materials indeed have rich surface termination structures, such as =O, –OH, and –F, thereby endowing Ti₃C₂T_x MXenes with electronegativity.

Fig. 1 XRD patterns of (a) Ti₃C₂T_x MXenes, Mn₃O₄ and Ti₃C₂@Mn₃O₄; (b) overall survey, (c) Ti 2p, (d) C 1s, (e) Mn 3s, and (f) O 1s XPS spectra of Ti₃C₂@Mn₃O₄.

The acid treatment (Fig. 2a and b) results in the etching of Al, yielding Ti₃C₂T_x MXenes with a smooth accordion-like surface and a significantly increased layer spacing. The prepared Ti₃C₂@Mn₃O₄ composites were observed and analyzed using TEM and HRTEM. As can be seen Fig. 2c and d, due to the strong electrostatic attraction between the surface terminals T_x (=O, –F, and –OH) of Ti₃C₂T_x MXenes and Mn²⁺, Mn²⁺ will be inserted into the interlayer of Ti₃C₂T_x MXenes and adsorbed on its surface, thus anchoring the generated Mn₃O₄ particles firmly in the interlayer and on the surface of Ti₃C₂T_x MXenes, inhibiting their growth or aggregation into secondary large particles. Meanwhile, the presence of interlayer Mn₃O₄ nanoparticles can effectively expand the interlayer spacing of Ti₃C₂T_x MXenes, which increased from 0.998 to 1.280 nm, as revealed by the aforementioned XRD (Fig. 1a). The planar spacings of the generated nanoparticles are 0.313, 0.294, and 0.254 nm, corresponding to the (1 1 2), (2 0 0), and (2 1 1) planes of Mn₃O₄, respectively, analyzed by HRTEM (Fig. 2d). Meanwhile, the laminar structure of Ti₃C₂T_x MXenes can be clearly seen. The distributions of Mn, O, Ti, and C in the Ti₃C₂@Mn₃O₄ composites are shown in Fig. 2e, indicating these elements are uniformly dispersed in the material. This result demonstrates that Mn₃O₄ nanoparticles are evenly grown on Ti₃C₂T_x MXenes.

Fig. 2 (a) and (b) SEM images of Ti₃C₂T_x MXenes; TEM (c) and HRTEM (d) images of Ti₃C₂@Mn₃O₄; (e) corresponding elemental mappings of Mn, O, Ti, and C.

3.2 Electrocatalytic performance

Firstly, the linear sweep voltammetry (LSV) curves were determined to evaluate the NRR activity of Ti₃C₂@Mn₃O₄ in N₂- and Ar-saturated H₂SO₄ solutions (pH = 2) (tests confirm that neither Ar nor N₂ contained any other nitrogen species (Table S1†)). As shown in Fig. 3a, the two curves of the catalyst under N₂ and Ar conditions exhibit similar trends but different current densities. Apparently, at the same voltage, the current density is higher in N₂ atmosphere, indicating a more active NRR reaction. In general, the more negative the voltage, the more pronounced the reduction reaction. However, based on the competitive hydrogen evolution side reaction of the NRR reaction, the hydrogen evolution becomes more intense as the voltage increases, leading to a gradual decrease in FE. To further evaluate the HER activity of Ti₃C₂@Mn₃O₄, the LSV curve of the carbon cloth without the electrocatalyst was determined under an Ar atmosphere as a control experiment. The current density with the catalyst is higher than that of carbon cloth at the same potential, indicating that the catalyst possesses HER catalytic activity. The corresponding Tafel analysis is presented in Fig. 3b.

Fig. 3 (a) LSV curves in N₂- and Ar-saturated solutions. (b) Tafel slope with carbon cloth alone and in the presence of a catalyst. (c) Chronoamperometry results at the corresponding potentials. (d) UV-vis absorption spectra of electrolytes stained with indophenol indicator for 2 h. (e) NH₃ yield rate and FEs for Ti₃C₂@Mn₃O₄ at the corresponding potentials. (f) ¹H-NMR spectra of the electrolyte after electroreduction under ¹⁴N₂ (6 h) and ¹⁵N₂ (2 h) feed gases.

In acidic electrolytes, the HER process primarily involves three steps: electrochemical hydrogen adsorption (Volmer step), electrochemical desorption (Heyrovsky step), and chemical desorption (Tafel step), as shown in the following equations:²⁷

Volmer: H₃O⁺ + e⁻→ H_ads + H₂O (120 mV dec⁻¹) (5)

Heyrovsky: H_ads + H₃O⁺ + e⁻ → H₂ + H₂O (40 mV dec⁻¹) (6)

Tafel: 2H_ads → H₂ (30 mV dec⁻¹) (7)

H₃O⁺ combines with electrons on the catalyst surface to readily form H_ads, which constitutes the Volmer step, with a Tafel slope of 120 mV dec⁻¹. Subsequently, the generated H_ads may follow two distinct pathways: The adsorbed H_ads combines with another H₃O⁺ and gains an electron to form H₂, which then desorbs from the active site (Heyrovsky step), with a Tafel slope of 40 mV dec⁻¹. Alternatively, two adjacent H_ads species directly combine to produce H₂, followed by desorption (Tafel step), with a Tafel slope of 30 mV dec⁻¹. Tafel slope serves as a crucial indicator of the kinetics in electrochemical reactions, particularly in HER. Notably, under an argon atmosphere, the Tafel slope recorded with just a carbon cloth or in the presence of the catalyst is 188 and 153 mV dec⁻¹, respectively. Both values significantly exceed the 120 mV dec⁻¹ threshold, strongly suggesting that the Volmer step serves as the rate-determining step in the HER mechanism.

In order to determine the optimal working voltage for the NRR reaction, a series of NRR experiments were carried out using the comparative potentials of −0.4 to −0.80 V, and the resulting time-dependent current density curves are shown in Fig. 3c. The UV-vis absorption spectrum of the electrolytes after 2 h of electrocatalysis (Fig. 3d) shows an obvious absorption, which proves the NRR catalytic performance of Ti₃C₂@Mn₃O₄. With the increase in voltage, the absorption peak intensity shows a trend of first increasing and then decreasing, with the strongest peak at −0.6 V. The yield of ammonia tested by the indophenol method (Fig. S1†) also shows a trend of first increasing and then decreasing with the rise in voltage, with the highest yield (53.7 μg h⁻¹ mg_cat.⁻¹) at −0.6 V, while FE shows a gradually declining trend (Fig. 3e). Although a high FE is observed at −0.4 V, the NH₃ yield rate is too low; therefore, −0.6 V was chosen as the optimal operating voltage, at which the highest ammonia production rate was obtained and the FE was also high (10.4%). Hydrazine (N₂H₂) is a potential secondary product in the electrocatalytic NRR process, which was barely detected by the Watt and Crisp methods (Fig. S2 and S3†).

Subsequently, isotopic labeling measurements with ¹⁴N₂ and ¹⁵N₂ gas feeds were conducted to confirm the true source of NH₃. ¹⁴N₂ exhibits a reaction duration of 6 hours (h), whereas ¹⁵N₂ completes the same process within 2 h. The results (Fig. 3f) of the ¹H-NMR spectrum display the characteristic triplet and doublet peaks of ¹⁴NH₄⁺ and ¹⁵NH₄⁺, corresponding to the used ¹⁴N₂ and ¹⁵N₂ gas feeds, respectively.^28–30 This indicates that the nitrogen in the ammonia indeed comes from the nitrogen gas used. As evidenced by the ¹H-NMR spectrum analysis, the nitrogen isotopic composition exhibits a distinct temporal pattern with the ¹⁴N₂ peak intensity (21 a.u.) demonstrating a three-fold enhancement compared to the ¹⁵N₂ counterpart (7 a.u.). This proportional relationship aligns with the temporal progression of the reaction, providing quantitative evidence for the time-dependent accumulation of ammonia products.

In order to verify that N₂ is the main nitrogen source for the ammonia synthesis reaction, the following comparative experiments were conducted. (i) The Ti₃C₂@Mn₃O₄ catalyst was electrolyzed at the open-circuit voltage in a N₂-saturated electrolyte for 2 h. (ii) The black CP was electrolyzed at −0.6 V in a N₂-saturated electrolyte for 2 h. (iii) The Ti₃C₂@Mn₃O₄ catalyst was electrolyzed at −0.6 V in an Ar-saturated electrolyte for 2 h. The UV-vis absorption spectra of the electrolytes obtained from the above three comparisons, treated with indophenol indicator, are shown in Fig. 4a, which shows that only a slight amount of NH₃ was detected in cases (i), (ii), and (iii) compared to Ti₃C₂@Mn₃O₄, indicating that electrocatalytic NRR occurs only under a N₂-saturated atmosphere and a specific applied voltage. The durability and stability of catalysts are also important indicators for evaluating the performance of electrocatalysts. Fig. 4b shows the NH₃ yield rates and corresponding FEs of Ti₃C₂@Mn₃O₄ with alternating 2 h-cycles in N₂-saturated electrolytes (Fig. S4†). As can be seen, during the 8 cycles of intermittent operation, the NH₃ yield rates remained largely unchanged despite minor fluctuations and the corresponding FEs also remained nearly stable. The NH₃ yield rates and corresponding FEs of the Ti₃C₂@Mn₃O₄ composite material during alternating 2 hour cycles in N₂- and Ar-saturated electrolytes, respectively, are shown in Fig. 4c and S5.† In the N₂ atmosphere, the NH₃ yield rate remained stable at approximately 55 μg h⁻¹ mg_cat.⁻¹, with the FE maintained at around 7%. In contrast, under an Ar atmosphere, the detected NH₃ yield was negligible (∼2 μg h⁻¹ mg_cat.⁻¹), and the FE was essentially 0% due to no appreciable NH₃ production. These results demonstrate that the catalyst shows effective catalytic performance for eNRR under ambient conditions. After a long cycle of 22 h (Fig. 4d), there was no significant difference in the current density of catalysis at an applied potential of −0.6 V. Moreover, the XRD patterns of the electrode before and after cycling are identical (Fig. S6†). The excellent catalytic performance of the Ti₃C₂@Mn₃O₄ material is attributed to the synergistic catalytic effect of Mn₃O₄ and MXenes, as evident from Fig. 4e and S7.† The amount of ammonia produced at the Mn₃O₄, Ti₃C₂ and Ti₃C₂@Mn₃O₄ electrodes after electrolysis for 2 h under ambient conditions at −0.6 V is 22.5, 15.4 and 53.7 μg h⁻¹ mg_cat.⁻¹, respectively.

Fig. 4 (a) UV-vis absorption spectra of the electrolyte after charging at −0.6 V for 2 h under different electrochemical conditions. (b) NH₃ yield rate and the corresponding FEs of Ti₃C₂@Mn₃O₄ with alternating 2 h cycles in the N₂-saturated electrolytes. (c) NH₃ yield rate and the corresponding FEs of Ti₃C₂@Mn₃O₄ with alternating 2 h cycles in N₂- and Ar-saturated electrolytes. (d) Time-dependent current density curve for the Ti₃C₂@Mn₃O₄ catalyst at −0.6 V for 22 h. (e) NH₃ yield of Mn₃O₄, Ti₃C₂T_x and Ti₃C₂@Mn₃O₄.

As summarized in Table 1, the present work demonstrates superior NH₃ yield and FE compared with state-of-the-art electrocatalysts reported in the literature. Although ref. 38 demonstrates a high FE of 73.3%, it employs the noble metal gold as the catalyst and achieves only a modest NH₃ yield of 23 μg h⁻¹ mg_cat.⁻¹. Similarly, ref. 39 reports an FE of 56.5%, but with a considerably lower NH₃ yield of merely 7.4 μg h⁻¹ mg_cat.⁻¹. In comparison, our work achieves a superior NH₃ yield of 53.7 μg h⁻¹ mg_cat.⁻¹ while maintaining competitive FE performance.

Table 1 Comparison with other literature

Materials	Optimal operating voltage (V vs. RHE)	NH3 yield rate (ug h^−1 mgcat.^−1)	FE (%)	References
	−0.6	53.7	10.4	This work
SnS2	−0.5	3.3	10.8	8
MnO2-Ti3C2Tx	−0.55	34.1	11.3	12
Mn3O4 nanocube	−0.8	11.6	3	13
MnO-carbon nanofibers	−1.25	35.9	1.5	14
Ti3C2Tx MXene nanosheets	−0.4	20.4	9.3	20
Fluorine-free Ti3C2Tx nanosheets	−0.3	36.9	9.1	21
Mn3O4 nanoparticles	−0.5	25.9	5.5	31
CuAg/Ti3C2	−0.5	4.1	9.7	32
Molybdenum carbide	−0.5	7.7	1.8	33
Pd films grown on Ni foam	−0.1	18.2	10.3	34
SnO2	−0.6	25.2	11.4	35
BiOCl-modified 2D titanium carbide MXene	−0.1	4.0	11.9	36
1T/2H mixed-phase MoSSe	−0.45	32.3	12.6	37
Gold nanoparticles	−0.3	23.0	73.3	38
FeSA–N–C	0	7.4	56.5	39

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To understand the intrinsic advantages of Ti₃C₂@Mn₃O₄ for NRR catalytic activity, the electrochemical active surface area was estimated by the double-layer capacitance (C_dl) from CV experiments in the range of 0.35–0.45 V (Fig. 5a–c). The C_dl value of Ti₃C₂T_x is calculated to be 0.59 mF cm⁻² (Fig. 5d), which is larger than that of Mn₃O₄ (0.19 mF cm⁻²), and the value of Ti₃C₂@Mn₃O₄ is calculated to be 0.23 mF cm⁻². As revealed from this data, the high specific surface area of MXene and its interaction with Mn₃O₄ can indeed effectively increase the electrochemical surface area of Mn₃O₄, thereby improving its catalytic activity. In order to investigate the intrinsic reason for the high electrocatalytic performance of Ti₃C₂@Mn₃O₄, N₂ temperature-programmed desorption (N₂-TPD) experiments were carried out, and the results are displayed in Fig. 5e. The chemical desorption peaks of Mn₃O₄ and Ti₃C₂ were located at 90 °C and 164 °C, respectively, in the N₂-TPD spectra. As for Ti₃C₂@Mn₃O₄, there were two characteristic peaks at around 90 °C and 169 °C, which are associated with the peak for the chemical adsorption of nitrogen for Mn₃O₄ and Ti₃C₂, respectively. This additionally confirms that the synergistic effect of Mn₃O₄ and Ti₃C₂ promoted the NRR catalytic performance of the Ti₃C₂@Mn₃O₄ material.

Fig. 5 (a) and (b), and (c) CV curves of different catalysts at various scan rates (20–100 mV s⁻¹) in the region of 0.35 to 0.45 V vs. Hg/Hg₂SO₄; (d) capacitive current densities at 0.40 V vs. Hg/Hg₂SO₄ as a function of scan rate for different catalysts. Slopes (2C_dl) were used to calculate the ECSA. (e) N₂-TPD profiles for Mn₃O₄, Ti₃C₂T_x and Ti₃C₂@Mn₃O₄.

4. Conclusion

The Ti₃C₂@Mn₃O₄ composite was used as an electrocatalyst for ammonia synthesis under ambient conditions via a very simple method. Benefiting from the synergistic catalytic effect of MXenes and Mn₃O₄, the composite exhibits excellent catalytic performance for ammonia synthesis with an NH₃ yield rate of 53.7 μg h⁻¹ mg_cat.⁻¹ and satisfactory FE of 10.4% at −0.6 V under ambient conditions. The composite catalyst exhibits excellent stability, durability, and selectivity, with outstanding synergistic effects, surpassing most reported NRR electrocatalysts. This simple and versatile strategy may offer researchers inspiration for rationally designing highly efficient NRR electrocatalysts.

Data availability

All data included in this study are available from the corresponding author upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research was supported by the Sichuan Science and Technology Program (2024NSFSC0236).

References

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra01799h

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