1 Introduction

Artificial nitrogen fixation to ammonia (NH3) plays a critical role in fabricating agricultural fertilizers and maintaining the earth’s ecosystems [1,5]. In recent years, new strategies, such as biological [6], photocatalytic [7] and electrocatalytic [8,9,10] approaches, have been reported for ammonia synthesis. In particular, electrocatalytic nitrogen reduction reaction (N2RR) can use water as hydrogen source and proceed in ambient conditions, suggesting an attractive feature of clean ammonia production with low carbon footprint [11]. Nevertheless, the development of N2RR has been largely limited by its low current densities, limited Faradaic efficiency (FE) values, and slow NH3 production rates, which are ascribed to the large reaction energy barriers during NH3 adsorption and activation processes [12]. It is critical to design robust electrocatalysts that can efficiently adsorb, activate and convert N2 into NH3.

Two-dimensional (2D) materials, such as graphene [13], metal–organic frameworks [14], black phosphorus [15], have been drawing great attention of researchers for N2RR, owning to their unique 2D structures and unconventional chemical properties [16]. MXenes, one of the novel 2D materials synthesized by selective etching of the aluminum layers from the precursor MAX phases [17], have been demonstrated with applications in supercapacitors [18], batteries [19], and electrochemical N2RR [20,21,22]. For instance, Luo et al. [23] reported that Ti3C2Tx MXene on stainless steel mesh functioned as efficient N2RR electrocatalysts with a FE of 5.78%. On the other hand, the terminal groups (Tx), mainly oxygen (O)-containing or fluorine (F)-terminations, can be tuned to affect the electrocatalytic performances of Ti3C2Tx MXene [24]. Previously, density functional theory (DFT) calculations suggested that Ti3C2 MXene with O-containing terminal groups can combine N2 more strongly than that with F-terminal groups [1c). The color of few-layer Ti3C2Tx MXene appeared as a blackish green color instead of pure black (Additional file 1: Fig. S2a), and the Tyndall scattering effect was also observed (Additional file 1: Fig. S2b). These two empirical phenomena indicated the formation of few-layer, sheet-like structure of Ti3C2Tx MXene.

Fig. 1
figure 1

a Schematic of the synthesis procedure of the Ti3C2Tx MXene. b SEM images of Ti3C2Tx MXene. c EDX elemental map** profiles of Ti3C2Tx MXene with Ti (red), F (purple), C (green), and O (blue) distributions. d XRD spectra of Ti3C2Tx MXene loaded on carbon paper (CP)

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The X-ray powder diffraction (XRD) pattern exhibited the characteristic peaks of Ti3C2Tx MXene, which differed greatly from the Ti3AlC2 MAX precursor (Fig. 1d). After etching, the peak (2θ ≈ 9.28°) on Ti3AlC2 MAX phase exhibited a negative shift to a low angle (2θ ≈ 5.89°), corresponding to the characteristic lattice (002) plane of Ti3C2Tx MXene [30]. No additional peaks were observed for Ti3C2Tx MXene except for the peaks of carbon paper. The differences of the XRD patterns between Ti3C2Tx MXene and Ti3AlC2 MAX indicated the successful etching of Al layers from Ti3AlC2 MAX phases.

Scanning electron microscopy (SEM) images showed that all the synthesized Ti3C2Tx MXene were 2D sheet-like structures (Additional file 1: Fig. S3). The ratio of F-terminal groups to all the surface groups of Ti3C2Tx MXene was measured by EDS profiles (Additional file 1: Fig. S4), which was calculated as 82% (designated as Ti3C2Tx-high F, indicating that it was etched by a high-concentration fluorinated acid) and 48% (designated as Ti3C2Tx-medium F, indicating that it was etched by a medium-concentration fluorinated acid). To further decrease the ratio of fluorine groups on the surface of MXene, an alkalization treatment was conducted to Ti3C2Tx-medium F MXene (Experimental section), and the fluorine proportion was reduced to 24% (designated as Ti3C2Tx-low F).

To investigate the structures of Ti3C2Tx MXene with different amounts of fluoride, Raman and X-ray photoelectron spectroscopy (XPS) were conducted to qualitatively investigate the fluorine ratio in all the terminal groups. As shown in the Raman spectra (Fig. 2a), the relative intensity of vibrational modes for Ti3C2Tx indicated the densities of terminal groups on MXene. The Raman shifts at 205 and 366 cm−1 were respectively associated to the A1g and Eg vibration modes of the C–Ti–O structure. The Raman peak at 614 cm−1 corresponded to the C–Ti–OH structure [23], and the Raman peak at 706 cm−1 represented the A1g vibrations of carbon. The relatively large intensity of the characteristic Raman peaks at 205, 614, and 366 cm−1 corresponded to the O-containing terminal groups, and suggested that the densities of O-terminal groups were in the order of Ti3C2Tx-low F > Ti3C2Tx-medium F > Ti3C2Tx-high F. This trend also confirmed that the densities of F-terminal groups followed the order as: Ti3C2Tx-high F > Ti3C2Tx-medium F > Ti3C2Tx-low F.

Fig. 2
figure 2

a Raman spectra and b XPS spectra of Ti3C2Tx-low F, Ti3C2Tx-medium F and Ti3C2Tx-high F. c F 1 s spectra, and d Ti 2p XPS spectra of Ti3C2Tx-low F, Ti3C2Tx-medium F and Ti3C2Tx-high F MXene samples

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The survey XPS spectra of all the Ti3C2Tx MXene samples with different densities of fluorine terminal groups exhibited same element peaks (Fig. 2b), which assigned to F 1 s, O 1 s, Ti 2p and C 1 s, respectively. Among these samples, Ti3C2Tx MXene-high F showed the highest intensity of F 1 s and the lowest intensity of O 1 s, indicating the highest ratio of fluorine terminal groups among these samples, in good accord with the EDX result. The F 1 s XPS spectra presented two main peaks at 684.5 and 685.9 eV (Fig. 2c), corresponding to the F–Ti and F–C bonds, respectively. For the O 1 s spectra (Additional file 1: Fig. S5), the peaks at 529.3, 531.2 and 533.4 eV were ascribed to C–Ti–O, C–Ti–OH species, and the adsorbed H2O on the MXene surface. With the increased concentrations of alkali solutions, the density of C–Ti–O species on the MXene surface increased and the fluorine terminal groups decreased. For the Ti 2p XPS spectrum (Fig. 2d), four doublets were fitted to indicate the valance and bond structures of Ti. The peaks centered at 454.4 and 460.6 eV referred to Ti3+; the peaks at 455.0 and 461.7 eV were assigned to Ti2+; the peaks at 456.2 and 462.9 eV were associated with the Ti−C bond; the peaks at 458.4 and 464.3 eV were ascribed to the Ti−O bond; and the peak at 459.5 eV was attributed to the Ti−F bond. The relative intensities of Ti−O and Ti−F bonds also indicated that the fluorine terminal group density on the MXene surface followed the order of Ti3C2Tx-high F > Ti3C2Tx-medium F > Ti3C2Tx-low F.

The capability of different F-terminating surface functionalizations of Ti3C2Tx MXene for enhancing the N2RR catalytic activity was then investigated. All the electrochemical tests were conducted in N2-saturated 0.01 M Na2SO4 electrolyte, and all the potentials presented in this work were converted as values versus reversible hydrogen electrode (RHE). The linear sweep voltammetry (LSV) curves of Ti3C2Tx MXene with different proportions of fluorine terminal groups were recorded (Fig. 3a). The experimental overpotentials of Ti3C2Tx MXene displayed the following order of Ti3C2Tx-high F > Ti3C2Tx-medium F > Ti3C2Tx-low F at ‒10 mA·cm−2 current density, which was associated with the hydrogenation step during electrochemical reduction. Owning to the different densities of F-termination on the MXene surface, the hydrogenation step could be inhibited [31]. The Ti3C2Tx-high F sample had the highest density of fluorine terminal groups, resulting in the inhibition of H+ adsorption [32] and eventually the decline of N2RR activity. In contrast, Ti3C2Tx-low F had a low density of fluorine terminal groups and abundant O-containing termination, and presented the highest hydrogen evolution reaction (HER) performance but low N2 adsorption and activation capabilities. In comparison, Ti3C2Tx-medium F MXene had a medium density of fluorine terminal groups, and presented the highest N2RR electrocatalytic activity.

Fig. 3
figure 3

a Linear sweep voltametric curves of Ti3C2Tx-low F, Ti3C2Tx-medium F and Ti3C2Tx-high F in N2-saturated 0.01 M Na2SO4 electrolyte with a scan rate of 5 mV·s−1. b LSV curves of Ti3C2Tx-medium F in N2-saturated (red curve) and Ar-saturated (black curve) 0.01 M Na2SO4 electrolyte with a scan rate of 5 mV·s−1. c UV–Vis absorption spectra of N2RR products of Ti3C2Tx-medium F at potentials between ‒0.4 and ‒0.8 V over Ti3C2Tx -48% F. d NH3 yield rate (left y-axis) and Faradaic efficiencies (right y-axis) of Ti3C2Tx-medium F MXene at the corresponding potentials

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The Ti3C2Tx-medium F MXene catalyst was further tested in both Ar-saturated and N2-saturated electrolytes (Fig. 3b). The current density in N2-saturated electrolyte (red curve) exceeded that in Ar-saturated electrolyte (black curve) in the voltage range between − 0.4 and − 0.8 V, indicating the occurrence of electrochemical N2RR on the catalyst surface. Both the chronoamperometric tests and the salicylic acid indicator method were adopted to determine the amount of produced NH3. All the yields of ammonia were calculated from the standard curves (Additional file 1: Fig. S6). In addition, each experiment was also conducted in Ar-saturated electrolyte to serve as the background. The corrected rate of NH3 yield (YRcorrected) was calculated from the following equation: YRcorrected = YRN2 – YRAr. The NH3 yield rate from Ti3C2Tx-medium F MXene was calculated based on the corresponding UV–Vis absorption spectra at the potential range between ‒0.4 and ‒0.8 V (Fig. 3c). The value of average background (YRAr) was calculated as (2.03 ± 0.2) × 10–5 μmol·s−1·cm−2. As shown in Fig. 3d, the maximum FE for NH3 production by Ti3C2Tx-medium F MXene was 42.7% at ‒0.5 V, while the highest NH3 partial current density after background correction was 18.3 μA·cm−2 at ‒0.7 V, corresponding to an FE of 7.4% and the NH3 production rate of 2.81 × 10–5 μmol·s−1·cm−2.

The chronoamperometry curves and the corresponding UV–Vis absorption spectra over Ti3C2Tx MXene with different densities of F-terminal groups at all applied potentials were displayed (Additional file 1: Fig. S7). Compared to Ti3C2Tx MXene counterparts with higher or lower surface densities of fluorine terminal groups (i.e., Ti3C2Tx-high F and Ti3C2Tx-low F), the Ti3C2Tx-medium F MXene catalyst covered with medium fluorine terminal group density exhibited the highest N2RR catalytic performance (Fig. 4a, b). The corrected NH3 yield rate (YRcorrected) with Ti3C2Tx-medium F catalyst (2.81 × 10–5 μmol·s−1·cm−2) was 1.6 and 1.7 times higher than that of Ti3C2Tx-high F (1.75 × 10–5 μmol·s−1·cm−2) and Ti3C2Tx-low F (1.67 × 10–5 μmol·s−1·cm−2) at ‒0.7 V.

Fig. 4
figure 4

a The ammonia production rates and b partial current densities of NH3 production of Ti3C2Tx MXene samples with different surface densities of fluorine terminal groups. c Chronoamperometry curve of Ti3C2Tx-medium F MXene for 18 h under ‒0.7 V. d The NH3 yield rate (left y-axis) and Faradaic efficiency (right y-axis) of Ti3C2Tx-medium F MXene at ‒0.7 V for 6 times

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The electrochemical stability of the Ti3C2Tx-medium F MXene catalyst was further interrogated. As shown in Fig. 4c, the total electrolysis current density in N2-saturated electrolyte was maintained relatively stable over 18 h. Moreover, cycling test of six continuous times was conducted, and the corresponding chronoamperometric measurements and UV–Vis absorption spectra were examined after each cycle (Additional file 1: Fig. S7e, f). Both of NH3 production rate (2.67 ± 0.16 × 10–5 μmol·s−1·cm−2) and the FE values (7.5% ± 0.5%) were within the error range of 7.4% after the continuous chronoamperometric measurement for 6 times, with 1 h of measurement each time (Fig. 4d), which suggesting the excellent durability of Ti3C2Tx-medium F MXene. Furthermore, the XRD patterns of Ti3C2Tx MXene before and after electrochemical nitrogen reduction reaction were also displayed (Additional file 1: Fig. S8), the unvaried peak position of XRD patterns also testified the stable crystal phases and structure of Ti3C2Tx MXene. Thus, Ti3C2Tx-medium F MXene with a medium density of surface fluorine terminal groups was demonstrated as an optimal catalyst for N2RR.

4 Conclusions

In summary, we demonstrated the surface functionalization of fluorine terminal groups on MXene to tune the N2RR catalytic activity at ambient conditions, in which different densities of surface fluorine terminal groups allowed to affect the capability of N2 adsorption and activation. The Ti3C2Tx MXene catalyst with a medium F-termination proportion (Ti3C2Tx-medium F) showed the optimal N2RR activity, with the highest NH3 yield rate of 2.81 × 10–5 μmol·s−1·cm−2 at ‒0.7 V, substantially exceeding that of Ti3C2Tx-high F and Ti3C2Tx-low F. Further study and development of surface functionalization toward N2 adsorption and activation can serve as a powerful toolkit for improving artificial N2 fixation.