Confined growth of pyridinic N–Mo₂C sites on MXenes for hydrogen evolution

Abstract

Developing low-cost and high-performance hydrogen evolution reaction (HER) electrocatalysts is a key research area for scalable hydrogen production from water electrolysis. Here, a hybrid of nitrogen-doped carbon encapsulated Mo₂C nanodots on Ti₃C₂T_x MXene (Mo₂C/Ti₃C₂T_x@NC) is developed through in situ polymerization of dopamine and a Mo precursor on the Ti₃C₂T_x MXene surface. During the annealing treatment, the polydopamine plays multiple roles in forming N-doped carbon, confining MoO₄²⁻ ions into ultrasmall Mo₂C nanodots, and stabilizing the MXene flakes against spontaneous oxidation. The as-synthesized hybrid exhibits excellent HER activity in acidic electrolyte with an overpotential of 53 mV at 10 mA cm⁻² and excellent stability over 30 hours. The combination of experiments and simulations demonstrates that pyridinic N-doped carbon coated Mo₂C nanodots serve as the active sites and Ti₃C₂T_x MXene facilitates the charge transfer, synergistically contributing to the superior HER performance.

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Owing to the high energy density and zero carbon emission, hydrogen is considered as an optimal solution to address the energy crisis and alleviate the environmental pollution.^1,2 Although industrial hydrogen production still relies on fossil fuels, water electrolysis shows promise as a clean and sustainable route for hydrogen generation.^3,4 Thus, developing efficient hydrogen evolution reaction (HER) electrocatalysts is of great importance to realize scalable electrocatalytic hydrogen production. Until now, platinum (Pt) and its derivatives remain the best HER electrocatalysts with low overpotentials and fast reaction kinetics, but their low abundance and high cost severely hinder their large-scale use.⁵ Recently, intensive research efforts have been made to explore low-cost and earth-abundant alternatives with comparable HER performance.^6–15 Among them, molybdenum carbide (Mo₂C) holds great promise due to its Pt-like d-band electronic structure.¹⁶ In general, the HER activity of Mo₂C mainly relies on the accessibility of active sites and conductivity. Mo₂C nanoparticles, even ultrasmall nanodots, are preferable but tend to agglomerate when processed into electrodes. In this context, introducing a highly conductive substrate that could simultaneously optimize the dispersion of ultrasmall Mo₂C would be of importance.^17,18

Recently, an emerging family of two-dimensional (2D) transition metal carbides and nitrides, called MXenes, have attracted much attention due to their unique characteristics of metallic conductivity (up to 10 000 S cm⁻¹), hydrophilicity and diversity stemming from various surface terminations.^19,20 Typically, the most studied Ti₃C₂T_x MXene (T_x denotes the surface –OH, –O and –F groups) shows great promise in various applications including supercapacitors,²¹ sensors,²² water purification,²³ electromagnetic interference shielding²⁴ and catalysis.^25–29 It would be natural to assume that coupling Mo₂C and MXenes may deliver satisfactory HER performance, while it remains challenging due to the following factors. First, the synthesis of Mo₂C always requires high annealing temperature. Proverbially, MXenes with high surface energy are prone to oxidation at evaluated temperatures, although protected by an inert atmosphere, leading to inferior conductivity.³⁰ In addition, the high synthesis temperature would accelerate the agglomeration of Mo₂C nanoparticles and thus result in poor accessibility of active sites.³¹ Accordingly, protecting MXenes from surface oxidization and Mo₂C nanoparticles from aggregation during the calcination process is of great importance to realize MXene–Mo₂C hybrid HER electrocatalysts.

Here, we report a 2D coupled nanohybrid of Mo₂C nanodots and Ti₃C₂T_x MXene encapsulated by nitrogen-doped carbon layers (denoted as Mo₂C/Ti₃C₂T_x@NC) synthesized using molybdate ions chelated in polydopamine (Mo-PDA) as the precursor (Fig. 1). Notably, Ti₃C₂T_x flakes are fully capped with Mo-PDA by in situ polymerization, which could inhibit the surface oxidation of Ti₃C₂T_x during annealing treatment and thus retain the excellent conductivity.³² Moreover, due to the confinement effect of the Mo-PDA structure, the ultrasmall Mo₂C nanodots are uniformly dispersed in the carbon matrix without agglomeration. Demonstrated by theoretical and experimental analyses, the highly accessible pyridinic N–Mo₂C sites coupled with the metallic Ti₃C₂T_x support of Mo₂C/Ti₃C₂T_x@NC can deliver remarkable HER activities with an overpotential of 53 mV at 10 mA cm⁻², a Tafel slope of 40 mV dec⁻¹ and excellent stability over 30 hours in acidic electrolyte, which are superior to those of reported noble metal-free electrocatalysts.

Fig. 1 Synthetic illustration of the preparation of Mo₂C/Ti₃C₂T_x@NC.

As demonstrated previously, PDA may have a positive confinement effect on the dispersion of Mo₂C nanodots, in which a hierarchical microflower carbon structure was formed (Fig. S1†). Even though Mo₂C nanodots may be well dispersed, such a microflower structure would show inferior HER performance in terms of low conductivity. We assume that 2D Ti₃C₂T_x MXene with various terminations may be a good candidate as a metallically conductive substrate to form a flat PDA layer, realizing both well dispersed Mo₂C nanodots and fast electron transport. To verify this idea, we first check the morphology of polydopamine on the MXene by in situ polymerization. The ultrathin Ti₃C₂T_x with a lateral size of several microns exhibits single crystallinity (Fig. S2†). By in situ polymerization of dopamine on the surface of Ti₃C₂T_x MXene, the polydopamine/Ti₃C₂T_x (denoted as PDA/Ti₃C₂T_x) shows a similar morphology to the pristine MXene (Fig. 2a). After annealing at 750 °C, the pristine MXene is severely oxidized with many TiO₂ nanoparticles/nanorods generated on the surface (Fig. S3a†), while the N-doped carbon coated Ti₃C₂T_x MXene (denoted as Ti₃C₂T_x@NC) derived from PDA/Ti₃C₂T_x retains the flat surface without the presence of TiO₂ particles (Fig. 2b), which is confirmed from the enlarged TEM image (Fig. S3b†). The high-resolution TEM (HRTEM) image (Fig. 2c) clearly shows that the Ti₃C₂T_x MXene layers are fully covered by carbon.

Fig. 2 TEM images of (a) PDA/Ti₃C₂T_x, (b) Ti₃C₂T_x@NC and (d) Mo₂C/Ti₃C₂T_x@NC. (c) HRTEM image of a Ti₃C₂T_x@NC edge showing the carbon layer covered Ti₃C₂T_x MXene structure. (e) Enlarged TEM image of Mo₂C/Ti₃C₂T_x@NC showing the uniform dispersion of Mo₂C nanodots on the surface of Ti₃C₂T_x MXene. (f) Size distribution of Mo₂C nanodots in Mo₂C/Ti₃C₂T_x@NC counted in (e). HRTEM image (g) and HAADF-STEM image (h) of Mo₂C/Ti₃C₂T_x@NC. (i) EDX elemental mapping from the marked region in (h).

The structures of these samples were further investigated by X-ray diffraction (XRD) and Raman spectroscopy. The pristine MXene exhibits the typical peak at ∼7° corresponding to the (002) plane of Ti₃C₂T_x MXene (Fig. S4a†).²³ The (002) peak of PDA/Ti₃C₂T_x shifts to ∼6° due to the expanded interlayer spacing of Ti₃C₂T_x layers. The annealed pristine MXene does not show characteristic Ti₃C₂T_x MXene peaks but shows typical peaks of rutile TiO₂ (JCPDS no. 21-1276).³⁴ Notably, no TiO₂ peaks are observed in the Ti₃C₂T_x@NC XRD pattern. In addition, the (002) peak of Ti₃C₂T_x MXene disappeared, which should be attributed to the suppressed restacking of MXene flakes by carbon layers on the surface.³⁵ The Raman spectra (Fig. S4b†) show that all the pristine Ti₃C₂T_x, PDA/Ti₃C₂T_x and Ti₃C₂T_x@NC samples exhibit typical peaks corresponding to Ti₃C₂T_x MXene while annealed Ti₃C₂T_x shows the characteristic peaks at 237.7, 446.9 and 609.8 cm⁻¹ of rutile TiO₂.³⁶ In addition, X-ray photoelectron spectroscopy (XPS) analyses (Fig. S5†) confirm the oxidation of Ti₃C₂T_x MXene into TiO₂ during the annealing process. Based on the above results, it can be concluded that dopamine self-polymerized on the surface of the MXene could protect it from oxidation during the annealing treatment by in situ forming N-doped carbon layers.

Previous studies have reported that molybdate ions can react with dopamine and then polymerize into the Mo-PDA complex, which can be used to fabricate nanostructured Mo compound/carbon composites for various applications.^33,37,38 Based on this inspiration, here we prepared Mo-PDA/Ti₃C₂T_x through a facile in situ polymerization method, as depicted in Fig. 1. The SEM and TEM images (Fig. S6†) of Mo-PDA/Ti₃C₂T_x show a similar ultrathin morphology to PDA/Ti₃C₂T_x, confirming that Ti₃C₂T_x flakes are tightly covered by Mo-PDA layers. Mo₂C/Ti₃C₂T_x@NC was obtained by annealing Mo-PDA/Ti₃C₂T_x under Ar at 750 °C. As shown in Fig. 2d and Fig. S7,† the morphology of Mo₂C/Ti₃C₂T_x@NC is similar to that of Ti₃C₂T_x@NC (Fig. 2b). The enlarged TEM image (Fig. 2e) evidences that high-density ultrasmall Mo₂C nanodots with an average diameter of 1.1 nm (Fig. 2f) are uniformly anchored on the surface of Ti₃C₂T_x flakes. The high-resolution TEM image (Fig. 2g) exhibits the lattice fringes of ∼0.23 nm on the nanodots, corresponding to the d-spacing of the β-Mo₂C (101) plane.¹⁷ The background lattices belong to Ti₃C₂T_x, confirming its hexagonal structure. The high-angle annular dark field scanning TEM (HAADF-STEM) image (Fig. 2h) reveals numerous bright dots decorated on the MXene sheets, further demonstrating the uniform dispersion of Mo₂C nanodots. The corresponding energy dispersive X-ray spectroscopy (EDX) elemental mapping images (Fig. 2i) confirm the distribution of Ti, Mo, C and N elements on the Mo₂C/Ti₃C₂T_x@NC surface.

Fig. 3a displays the XRD patterns of the samples. The characteristic peaks of both Mo-PDA and the MXene are observed in the XRD pattern of Mo-PDA/Ti₃C₂T_x. Similar to PDA/Ti₃C₂T_x, the (002) peaks of Ti₃C₂T_x negatively shift due to the expanded interlayer spacing. The absence of the (002) peak in Mo₂C/Ti₃C₂T_x@NC results from the separation of MXene layers by the Mo₂C nanodots and carbon layers. Through investigation using N₂ adsorption–desorption isotherms, as shown in Fig. 3b, the Brunauer–Emmett–Teller (BET) surface areas of Mo₂C/Ti₃C₂T_x@NC and Mo₂C@NC were calculated to be 241.4 and 86.4 m² g⁻¹, respectively. Both samples exhibit type IV isotherms of mesoporous structures, with pores in the range from 2 to 50 nm. The larger surface area and mesoporous structure of Mo₂C/Ti₃C₂T_x@NC could facilitate the accessibility of active sites and charge transfer during the HER process.

Fig. 3 (a) XRD patterns of pristine Ti₃C₂T_x MXene, Mo-PDA, Mo-PDA/Ti₃C₂T_x and Mo₂C/Ti₃C₂T_x@NC. (b) N₂ adsorption–desorption isotherms and the corresponding pore size distributions of Mo₂C@NC and Mo₂C/Ti₃C₂T_x@NC. High-resolution XPS spectra of (c) Ti 2p, (d) Mo 3d and (e) N 1 s of Mo₂C/Ti₃C₂T_x@NC. The Ti 2p survey of pristine Ti₃C₂T_x MXene is presented in (c) for comparison. The inset in (e) is the calculated contents of pyridinic N, pyrrolic N and graphitic N in Mo₂C/Ti₃C₂T_x@NC.

The valence states and compositions of Mo₂C/Ti₃C₂T_x@NC were elucidated by XPS analyses. The XPS full scan (Fig. S8a†) evidences the existence of Ti, Mo, C, N, and O in Mo₂C/Ti₃C₂T_x@NC. The Ti 2p core level can be fitted with four doublets for Ti 2p_1/2 and Ti 2p_3/2, as shown in Fig. 3c. Typically, the Ti 2p_3/2 components at 454.3, 455, 456.1, and 457.8 eV are assigned to Ti–C (Ti⁺), Ti–X (Ti²⁺), Ti_xO_y (Ti³⁺), and TiO₂ (Ti⁴⁺), respectively.²² Compared to the pristine Ti₃C₂T_x MXene, the binding energy of Ti–C positively shifts by about 0.3 eV, indicating the electron transfer from Ti₃C₂T_x to Mo₂C nanodots in the hybrid.²⁶ The deconvoluted Mo 3d XPS spectrum (Fig. 3d) reveals that the peaks at 228.8 eV and 231.9 eV are attributed to Mo₂C. Meanwhile, due to inevitable surface oxidation, the peaks at 229.6, 232.5, 233, and 235.8 eV stem from Mo⁴⁺ 3d_5/2, Mo⁶⁺ 3d_5/2, Mo⁴⁺ 3d_3/2 and Mo⁶⁺ 3d_3/2, respectively.³³ As shown in Fig. 3e, the deconvolution of the N1s XPS spectrum reveals the peaks at 398.6, 400.5 and 401.8 eV, which could be assigned to pyridinic, pyrrolic and graphitic N, respectively. Accordingly, the content is determined to be 71.64%, 18.43% and 9.93%, respectively. The C 1 s spectrum (Fig. S8b†) was fitted with five components at 282, 284.6, 285.5, 287.1, and 289.1 eV, stemming from Ti–C, C–C, C–N, C–O, and –COO, respectively. Upon comparison, the XPS spectra of Mo₂C@NC are similar to those of Mo₂C/Ti₃C₂T_x@NC (Fig. S9†). The above XPS analyses imply that Mo₂C nanocrystals are intensely coupled on Ti₃C₂T_x MXene and encapsulated in the pyridinic N-rich carbon layer.

The HER performance of Mo₂C/Ti₃C₂T_x@NC was evaluated by investigating the polarization curves in 0.5 M H₂SO₄ with all the potentials converted to the reversible hydrogen electrode (RHE). For comparison, Mo₂C@NC, Ti₃C₂T_x@NC and commercial 20 wt% Pt/C catalyst were also measured. As shown in Fig. 4a, the commercial 20 wt% Pt/C exhibits the best activity with an onset overpotential of zero. Mo₂C/Ti₃C₂T_x@NC shows a low onset overpotential of 6 mV vs. RHE (Fig. S10a†), which is very comparable to that of Pt/C but much lower than that of Mo₂C@NC (53 mV) and Ti₃C₂T_x@NC (432 mV). Impressively, Mo₂C/Ti₃C₂T_x@NC shows better performance than Pt/C at higher overpotentials (>106 mV). To elucidate the HER mechanism, Tafel plots of these catalysts were calculated based on the Tafel equation η = a + b log|j|, where j represents the current density and b is the Tafel slope.³⁹ As shown in Fig. 4b, the Tafel slope of Mo₂C/Ti₃C₂T_x@NC is determined to be 40 mV dec⁻¹, close to that of Pt/C (31 mV dec⁻¹), and lower than those of Mo₂C@NC (67 mV dec⁻¹) and Ti₃C₂T_x@NC (113 mV dec⁻¹). The value implies that the HER operates through the Volmer–Heyrovsky mechanism on Mo₂C/Ti₃C₂T_x@NC.⁴⁰ The exchange current density (j₀) for these catalysts was determined by fitting the linear portion of the Tafel plots at low current density (Fig. S10b†).⁴¹ As displayed in Fig. 4c, the j₀ value of Mo₂C/Ti₃C₂T_x@NC is 0.52 mA cm⁻², smaller than that of Pt/C (0.841 mA cm⁻²) but much higher than those of Mo₂C@NC (0.127 mA cm⁻²) and Ti₃C₂T_x@NC (2.6 × 10⁻⁵ mA cm⁻²), indicating the superior intrinsic activity of Mo₂C/Ti₃C₂T_x@NC. To achieve a current density of 10 mA cm⁻², the overpotentials of 27, 53, 121, and 631 mV are required for Pt/C, Mo₂C/Ti₃C₂T_x@NC, Mo₂C@NC and Ti₃C₂T_x@NC, respectively. As listed in Table S1,† such a low overpotential indicates that the Mo₂C/Ti₃C₂T_x@NC exhibits one of the best HER performances among those of previously reported non-noble-metal electrocatalysts, such as MoC–Mo₂C/PNCDs,⁹ MoS₂/Ti₃C₂T_x,²⁶ Co-SAC,¹⁰ and 1T′-ReSSe QDs.⁴² Moreover, Mo₂C/Ti₃C₂T_x@NC also demonstrated excellent HER performance in basic (1 M KOH) and neutral (1 M PBS) electrolytes with an overpotential of 75 and 114 mV at 10 mA cm⁻² and a Tafel slope of 59.2 and 80.3 mV dec⁻¹, respectively, comparable to that of state-of-the-art non-noble-metal electrocatalysts (Fig. S11†).^29,43–45

Fig. 4 (a) Polarization curves and (b) corresponding Tafel plots of Mo₂C/Ti₃C₂T_x@NC, Mo₂C@NC, Ti₃C₂T_x@NC and commercial 20% Pt/C in 0.5 M H₂SO₄. (c) Comparison of overpotentials at 10 mA cm⁻² (η₁₀) and exchange current densities (j₀) of the four catalysts. (d) Comparison of ΔG_H* of p-N-C, g-N-C, Mo₂C, Ti₃C₂T_x as well as their heterostructures. (e) Time-dependent current density curve of Mo₂C/Ti₃C₂T_x@NC at a constant overpotential of 100 mV. The inset gives the polarization curves of Mo₂C/Ti₃C₂T_x@NC initially and after 1000 CV cycles.

The above results imply that Ti₃C₂T_x itself shows negligible HER activity but coupling of Mo₂C with conductive Ti₃C₂T_x can modulate the morphological structure to optimize the HER activity. The Nyquist plots of the electrochemical impedance spectroscopy (EIS) responses (Fig. S12†) show a much smaller charge transfer resistance for Mo₂C/Ti₃C₂T_x@NC than that for Mo₂C@NC during the HER process, evidencing the faster HER kinetics due to the introduction of the conductive Ti₃C₂T_x support.⁴⁶ To further understand the intrinsic origin, the electrochemical surface area (ECSA) and per-site turnover frequency (TOF) of Mo₂C/Ti₃C₂T_x@NC and Mo₂C@NC were typically compared. The ECSA was estimated from the electrochemical double-layer capacitance (C_dl) by performing cyclic voltammetry in the region from 0.2 to 0.3 V at rates varying from 10 to 100 mV s⁻¹ (Fig. S13†). The ECSA of Mo₂C/Ti₃C₂T_x@NC (603.75 cm²) is larger than that of Mo₂C@NC (539 cm²), indicating that the former has a higher number of active sites. As the Ti₃C₂T_x@NC has very poor HER activity, the active sites should arise from Mo₂C. Here, we assume that all the Mo atoms in the samples are active sites, which is determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Mo₂C/Ti₃C₂T_x@NC has a TOF of 0.246 s⁻¹ at an overpotential of 100 mV (Fig. S14†), much higher than that of Mo₂C@NC (0.014 s⁻¹), which is comparable to those of reported non-noble-metal HER electrocatalysts, such as 3DHP-Mo₂C (0.045 s⁻¹)⁴⁷ and CoN_x/C (0.39 s⁻¹).⁴⁸

The electrochemical results reveal that Mo₂C/Ti₃C₂T_x@NC exhibits superior HER activity to that of its counterparts. To investigate the underlying mechanism, we performed density functional theory (DFT) calculations (Fig. S15–S17†). Here we considered a simple HER pathway containing three states: the initial state of H⁺ + e⁻, the intermediate state of adsorbed H (H*), and the final state of H₂ molecule.³⁹ The Gibbs free energy of the H* adsorption (ΔG_H*) was calculated on the considered systems. Generally, a high-performance HER catalyst has a ΔG_H* value close to zero.⁴¹Fig. 4d shows the calculated free energy diagrams for the HER. Pristine Mo₂C and Ti₃C₂T_x MXene have a ΔG_H* of −0.78 and −0.43 eV, respectively, suggesting their strong interactions with H and consequently poor HER performance. A previous study demonstrated that pyridinic N and graphitic N in carbon contribute to the HER activity of composite catalysts.⁴⁹ The modeled pyridinic N doped graphene (p-N-C) and graphitic N doped graphene (g-N-C) possess a ΔG_H* of 0.49 and 0.69 eV, respectively, which implies their unfavorable interactions with H and poor HER activities. Coupling N-doped carbon with Mo₂C could lead to enhanced HER performance. Typically, the ΔG_H* values of Mo₂C@p-N-C and Mo₂C@g-N-C were 0.22 and 0.44 eV, respectively, which were lower than those without N-doped carbon. Meanwhile, N-doped carbon coupled with Ti₃C₂T_x MXene shows high ΔG_H* with unfavorable HER activity. The DFT results are in good agreement with the above experimental results, revealing that Mo₂C@p-N-C serves as the active sites while the MXene support facilitates the charge transfer. Notably, Chen et al.⁵⁰ reported an efficient HER catalyst of Mo single atoms anchored on N-doped carbon synthesized by an appropriate annealing method, which could be formed in the as-prepared Mo₂C/Ti₃C₂T_x@NC. Accordingly, the Mo–N–C active site, determined as Mo₁N₁C₂, is also considered. As shown in Fig. S18 and S19,† both Mo₁N₁C₂ and Mo₁N₁C₂/Ti₃C₂T_x exhibit extremely low ΔG_H* values, suggesting that they are highly active toward the HER. The density of state near the Femi level for Mo₁N₁C₂/Ti₃C₂T_x is higher than that of Mo₁N₁C₂, indicating that the Ti₃C₂T_x MXenes facilitate the charge transfer during the HER process.⁵¹

The stability of HER electrocatalysts is also a critical indicator for practical applications. As shown in Fig. 4e, the time-dependent current density curve of Mo₂C/Ti₃C₂T_x@NC at a constant overpotential of 100 mV shows a slight decrease over 30 h, which is much better than that of commercial 20 wt% Pt/C, demonstrating excellent durability. In addition, the stability of Mo₂C/Ti₃C₂T_x@NC was studied using CV cycles in the range of 0.1 to −0.2 V vs. RHE. The polarization curve of Mo₂C/Ti₃C₂T_x@NC after 1000 cycles (inset of Fig. 4e) shows a negligible decrease in current density compared to the initial curve, implying its long-term stability. Detailed characterization of Mo₂C/Ti₃C₂T_x@NC after the stability test (Fig. S21†) further confirms the outstanding durability of its morphology and structure, which could be attributed to the protection of the covered carbon layers.

In controlled experiments, the effect of carbonization temperature (650 °C and 850 °C) on HER performance was investigated. The samples are named M-650 and M-850. TEM images (Fig. 5a, b and S22†) reveal that MoO_x nanodots in M-650 (not completely carbonized) have an average size of 1 nm while the size of Mo₂C nanodots in M-850 is 2.6 nm. It is in good agreement with a previous report which showed that the particle size would increase with the carbonization temperature.⁵² The XRD patterns of the two samples are similar to that of Mo₂C/Ti₃C₂T_x@NC (here defined as M-750) except for the presence of TiO₂ signals in M-850 (Fig. S23†). The XPS results (Fig. 5c) suggest that M-650 contains MoO_x rather than Mo₂C, which could arise from the insufficient temperature. Meanwhile, as shown in Fig. 5d, M-850 has a similar Mo 3d XPS spectrum to M-750, suggesting the formation of Mo₂C nanodots. As shown in Fig. 5e, the overpotentials of M-650 and M-850 for achieving 10 mA cm⁻² are 248 and 83 mV, respectively. And the corresponding Tafel slopes are 116.3 and 58.3 mV dec⁻¹, respectively (Fig. 5f). Obviously, M-750 exhibits the best HER activity among the three samples, which could be attributed to the following two factors: the MoO_x generated at a lower temperature (650 °C) has poorer intrinsic activity than Mo₂C, and the higher temperature (850 °C) induces aggregation of Mo₂C nanodots and partial oxidation of the MXene which would reduce the density of active sites and electron conductivity.

Fig. 5 TEM images of (a) M-650 and (b) M-850. The inset gives the size distributions of nanoparticles in the samples. High-resolution XPS Mo 3d spectra of (c) M-650 and (d) M-850. Polarization curves (e) and Tafel plots (f) of M-650, M-750, and M-850.

Based on the aforementioned considerations, the extraordinary HER performance of Mo₂C/Ti₃C₂T_x@NC is postulated to arise from the following synergistic aspects: (1) in situ polymerization of dopamine retains the structural stability of Ti₃C₂T_x MXene at high carbonization temperature, facilitating the charge transfer; (2) polydopamine chelated MoO₄²⁻ delivers highly disperse ultrasmall Mo₂C nanodots on the surface of the MXene, favoring the high accessibility of active sites for the HER; (3) the pyridinic N dopants in carbon coupled with Mo₂C induce activated active sites for the HER; (4) the intensely coupled Mo₂C on Ti₃C₂T_x provides a resistance-less path favorable for fast electron transfer; (5) the geometric confinement of Mo₂C and the MXene within the carbon layer guarantees the excellent stability of the HER performance during long-term operation.

Conclusions

In summary, we developed an in situ polymerization strategy to stabilize MXenes against oxidation during high-temperature treatment, which was extended to the fabrication of a hierarchical Mo₂C/Ti₃C₂T_x@NC composite. Serving as a HER catalyst, Mo₂C/Ti₃C₂T_x@NC exhibits excellent activity in the entire pH range and robust stability. Theoretical simulations demonstrated that the coupled Mo₂C and pyridinic N doped carbon serve as the most active sites for H adsorption, while the strongly incorporated metallic Ti₃C₂T_x MXene provides a fast charge transfer pathway, which synergistically lead to much superior HER performance. This strategy not only provides a highly active HER catalyst, but also opens a new avenue for designing MXene-based nanocomposites that require high synthesis temperature, which could be applied in various energy-related fields.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the AcRF Tier 1 grant (RG105/19) from the Ministry of Education in Singapore and the National Natural Science Foundation of China (U1401248, 11874044). The DFT calculations were supported by TianHe-1(A) at the NSCC in Tianjin. The authors also thank Prof. Yury Gogotsi from Drexel University for helpful comments on the manuscript.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta01697g

‡ These authors contributed equally to this work.

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