Efficient Electrocatalysts for Hydrogen Evolution Reaction Using Heteroatom-Doped MXene Nanosheet

develop a low-cost hybrid electrocatalysts for hydrogen production. Due to their layered structure and strong electrical conductivity, MXene-based materials have been lately used more and more in energy storage devices. Herein, heteroatom-(boron and sulfur-) doped MXene (B, S-Ti 3 C 2 T x ) nanosheets are developed as e ﬃ cient electrocatalysts for the hydrogen evolution reaction (HER). The synthesized B, S-Ti 3 C 2 T x has a large surface area and exhibits excellent electrocatalytic activity in acidic media. The prepared B, S-2-Ti 3 C 2 T x catalyst exhibits a low overpotential of − 110 mV vs. reversible hydrogen electrode for the HER and a low Tafel slope of ∼ 54 mV dec − 1 . Furthermore, B, S-2-Ti 3 C 2 T x shows a double-layer capacitance of 1.05 mF/cm 2 and maintains a steady catalytic activity for the HER for over 1000 cycles.

Researchers have recently become interested in MXenes, a family of two-dimensional TMCs, notably Ti 3 C 2 T x (where the T x represents the surface functionalization, such as -O, -OH, and -F).Due to the fact that MXenes have a high surface area, great electrical conductivity, and strong chemical and mechanical stabilities [35][36][37], MXenes have been extensively investigated for use in electrochemical applications such as lithium and sodium ion batteries [38], flexible electrodes [39,40] as well as supercapacitors [41][42][43].Several studies have also been conducted on MXenes as efficient catalysts for HER [44][45][46][47][48][49][50][51].However, the intrinsic catalytic activity of MXenes is not nearly as strong as that of Pt and TMDs.To further enhance catalytic performance, MXenes have been nanocomposited with various materials, such as carbonaceous materials, oxides, and metal-organic frameworks [52,53].
Heteroatom doping of MXene is a promising method for developing efficient electrocatalysts for HER.Recently, Le et al. synthesized nitrogen-doped Ti 3 C 2 T x , with enhanced electrocatalytic activity toward HER, via heat treatment in an ammonia atmosphere [54].With an overpotential of 198 mV and a Tafel slope of 92 mV dec -1 , the catalyst in its as-obtained state demonstrated strong catalytic activity.Ding et al. studied the HER activities of Ti 2 C and Mo 2 C doped with nonmetallic heteroatoms such as N, B, P, and S by making use of DFT simulations in their research [55].All the samples showed a lower Gibbs-free energy than pristine Ti 2 C and Mo 2 C, indicating their significant potential as HER active catalysts.N-Ti 2 CO 2 showed a very small ΔG H of 0.087 eV and abundant active sites.
Herein, we report Ti 3 C 2 T x codoped with boron and sulfur (B, S-Ti 3 C 2 T x ) as an efficient catalyst for electrocatalytic HER.We investigated the catalytic performance of the asprepared samples at different amounts of the boron precursor.The introduction of boron and sulfur modified the conductivity and increased the active surface area.Specifically, the B, S-2-Ti 3 C 2 T x sample exhibited the highest HER performance with a low overpotential of 110 mV compared to the other samples (B, S-1-Ti 3 C 2 T x and B, S-3-Ti 3 C 2 T x ).This improved performance was attributed to the synergistic contributions of boron-and sulfur-coordinated species.

Synthesis of Ti 3 C 2 T x .
To obtain Ti 3 C 2 T x MXene, the starting material Ti 3 AlC 2 (MAX) was subjected to an etching process where the Al layer was removed using a LiF/HCl solution.A solution for etching was prepared by adding 0.8 g of LiF to a 30 mL HCl solution, followed by stirring.1 g of Ti 3 AlC 2 (MAX) powder was then added to the etching solution at 40 °C and reacted for 24 hours until the Al layer was completely removed.The bulk Ti 3 C 2 T x MXene was collected by centrifugation, washed with deionized (DI) water to achieve a final pH of about 6, and then dried under vacuum overnight at 60 °C.

Synthesis of B-Doped Ti 3 C 2 T x .
A simple hydrothermal procedure was used to fabricate B-doped Ti 3 C 2 T x specimens.100 mg Ti 3 C 2 T x and various concentrations of boric acid (0.75, 1.5, and 3 g) were typically added to 60 mL of DI water and mixed for 1 hour at room temperature.The solution was then placed into a Teflon-lined autoclave reactor and hydrothermally heated for 24 hours at 180 °C.The centrifuged suspension was rinsed with DI water and dried at 80 °C for 12 hours.The as-prepared B-doped specimens were labeled B-1-Ti 3 C 2 T x , B-2-Ti 3 C 2 T x , and B-3-Ti 3 C 2 T x , corresponding to 0.75, 1.5, and 3 g of boric acid precursor, respectively.
2.4.Synthesis of S-Doped Ti 3 C 2 T x .S-doped Ti 3 C 2 T x was synthesized using thiourea as the sulfur source.100 mg of Ti 3 C 2 T x powder and 1.2 g of thiourea were added to 60 mL of DI water and mixed at room temperature for one hour.The solution was then placed into the Teflon-lined autoclave reactor and hydrothermally heated for 24 hours at 180 °C.S-Ti 3 C 2 T x was collected by centrifugation, rinsed with DI water, and dried in a vacuum oven at 80 °C for 12 hours.
2.5.Synthesis of B, S-Ti 3 C 2 T x .B, S-Ti 3 C 2 T x was successfully synthesized through a single-step reaction.Ti 3 C 2 T x (100 mg), thiourea (1.2 g), and different amounts of boric acid (0.75, 1.5, or 3 g) were dissolved in 60 mL of DI water with continuous magnetic stirring for 1 h at room temperature.The solution was then placed into an autoclave reactor coated with Teflon and hydrothermally heated at 180 °C for 24 hours.The resulting specimens (designated as B, S-1-Ti 3 C 2 T x , B, S-2-Ti 3 C 2 T x , and B, S-3-Ti 3 C 2 T x ) were obtained via centrifugation and drying at 80 °C for 12 h.2.6.Materials Characterization.The crystal structure of the synthesized specimens was studied via X-ray diffraction (XRD; Bruker New D8-Advance, Seoul, Korea) using Cu Kα radiation (λ = 0:154 nm).Raman spectra were obtained using a Raman spectrometer (LabRAM-HR Evolution).Field-emission scanning electron microscopy (FE-SEM; Zeiss 300 VP, Seoul, Korea) images were obtained at an acceleration voltage of 10 kV to study the morphology of the as-prepared samples.Transmission electron microscopy (TEM; JEOL-2100F) was conducted to further confirm the morphology.The X-ray photoelectron spectroscopy (XPS) technique was applied to verify the elemental build of the catalysts as well as the oxidation states of the constituent elements.

Electrochemical Measurements.
All electrochemical measurements were carried out at room temperature using an Ivium potentiostat V55630 and a typical three-electrode electrolytic setup.As the reference, counter, and working electrodes, respectively, the saturated calomel electrode (SCE), graphite rod, and B, S-Ti 3 C 2 T x on a glassy electrode were used.At a scan rate of 5 mV s1, linear sweep voltammetry (LSV) was used to measure the HER activity.Each and every measurement was carried out in 0.5 M H 2 SO 4 while iR compensation was used.To determine the level of stability, continuous cyclic voltammograms were obtained at a scan rate of 50 mV s -1 for a total of 1000 cycles.In addition, electrochemical impedance spectroscopy, known as EIS, was carried out at a potential of -230 mV vs. RHE and at frequencies ranging from 100 kHz to 0.1 Hz.Using the following equation, each potential was correlated to the RHE to ensure accuracy: 2 International Journal of Energy Research   To verify the morphology and interlayer distance as well as the corresponding plane, TEM analysis was carried out.The high-resolution TEM image of B, S-2-Ti 3 C 2 T x is shown in Figure S1(a-c We used XPS to verify the successful synthesis of Ti 3 C 2 T x as well as the existence of constituent atoms in B, S-2-Ti 3 C 2 T x and B-2-Ti 3 C 2 T x (Figures S2 and S3).The fact that the survey scan revealed the presence of titanium, carbon, oxygen, and boron is evidence that the Ti 3 C 2 T x and B, S-2-Ti 3 C 2 T x MXenes were successfully prepared.This is illustrated in Figure 3(a).On the other hand, S is not easily discernible in the survey scan, which leads one to believe that the amount of S that was doped into the sample was quite low.As a result of this, high-resolution XPS scans were carried out in order to obtain additional confirmation that B and S were present in the sample.High-resolution Ti 2p spectra of Ti 3 C 2 T x can be divided into four peaks at 455.N-dopedTi 3 C 2 [41] MTC@NF [55] P@MNTC [55] Ti 3 C 2 @MoO 3 [57] Ru@B-Ti 3 C 2 [54] (c) The electrochemical analyses were conducted using a three-electrode system to evaluate the HER catalytic properties of the Ti 3 C 2 Tx, B-Ti 3 C 2 T x , and B, S-Ti 3 C 2 T x electrocatalysts in 0.5 M of H 2 SO 4 .The commercial Pt/C catalyst was employed as a reference, and it demonstrates the highest HER activity, with practically zero onset potential and low overpotential (at10 mA cm -2 ), in contrast to the pristine Ti 3 C 2 T x catalyst, which demonstrates a significantly lower level of catalytic activity [66].Initially, the B-doped Ti 3 C 2 T x catalysts were examined for their properties (Table S1).The B-2-Ti 3 C 2 T x catalyst displayed a level of activity that was noticeably higher than that of the other catalysts.Specifically, B-2-Ti 3 C 2 Tx shows an overpotential of 170 mV (at 10 mA cm −2 ), whereas B-1-Ti 3 C 2 T x and B-3-Ti We also investigated the S-doped Ti 3 C 2 T x catalyst (Table S2).S-Ti 3 C 2 T x exhibits significantly higher activity than pristine Ti 3 C 2 T x (Figure S9(a)).Specifically, it shows an overpotential of 230 mV (at 10 mA cm −2 ), whereas Ti 3 C 2 T x exhibits an overpotential of 300 mV (Figure S9(b)).The corresponding Tafel slopes are calculated to be 86.6 mV dec −1 (Figure S9(c)).Our goal was to achieve maximum HER activity by adjusting the concentration of the boron precursor on the surface of the MXene.It may come as a surprise, but the overpotential of B-2-Ti 3 C 2 T x at 10 mA cm -2 is just 170 mV.This value is significantly lower than that of B-1-Ti 3 C 2 T x .If the catalytic activity of B-doped electrocatalysts was exclusively dependent on the B-O-Ti composition, then B-3-Ti 3 C 2 T x with a greater B-doping content would demonstrate higher performance than B-2-Ti 3 C 2 T x .This finding, on the other hand, demonstrates that additional interactions, such as O-Ti-C, also contribute to the improvement in catalytic activity.This also suggests that the optimal quantity of the B precursor is critical for achieving the highest possible level of HER activity from the catalyst.

Results and Discussion
To further enhance the HER kinetics of B-doped catalysts, S was added to the as-prepared catalyst, and the cata-   CV curves at scan rates of 20-150 mV s −1 were used to determine the Cdl (Figure S11).The Cdl of B, S-2-Ti 3 C 2 T x is 1.05 mF cm −2 (Figure 4(e)).The enhanced catalytic performance of B, S-Ti 3 C 2 T x can be attributed to the high conductivity of B, S-Ti 3 C 2 T x , which enables fast charge transfer and increases the number of active sites, allowing the faster reduction of adsorbed hydrogen ions.
Long-time stability is an important parameter for the catalysts.Therefore, long-term cycling tests of B, S-2-Ti 3 C 2 T x were conducted for 1000 cycles in an acidic medium.As shown in Figure 4(f), the sample does not show significant differences before and after cycling, indicating that the B, S-2-Ti 3 C 2 T x catalyst has excellent HER activity with long-term stability.Additionally, the electrochemical surface area of the prepared samples was estimated using double-layer capacitance (C dl ) to investigate whether the number of active sites has increased after the incorporation of B and S.
Based on the obtained HER performance of B, S-Ti 3 C 2 T x , an electrocatalytic mechanism has been proposed, as shown in Figure 5.This process occurs at the surface.Boron, sulfur, and Ti 3 C 2 , with their unique advantages, play specific roles in different elementary reactions to synergistically improve the HER kinetics.

Conclusions
B, S-Ti 3 C 2 T x MXenes were fabricated via a facile one-step hydrothermal process.B, S-Ti 3 C 2 T x showed excellent electrocatalytic HER performance compared to pristine Ti 3 C 2 T x .Specifically, the B, S-2-Ti 3 C 2 T x catalyst exhibited high catalytic activity with a low overpotential of 110 mV at 10 mA cm −2 for HER and excellent stability over 1000 continuous CV cycles.Therefore, this study demonstrates a promising strategy for the metal-free codoping of MXene nanosheets.

Figure 1 (
Figure1(a) shows the procedure for preparing B, S-Ti 3 C 2 T x .Ti 3 AlC 2 was used as the precursor in the preparation of Ti 3 C 2 Tx.The B, S-Ti 3 C 2 T x hybrid was fabricated through a simple hydrothermal method.The crystal structure of the bare Ti 3 AlC 2 , Ti 3 C 2 T x MXene, and B, S-Ti 3 C 2 T x was analyzed using XRD (Figure1(b)).The peaks of the bare Ti 3 C 2 T x corresponding to (002), (006), (008), and (001)

Figure 1 :
Figure 1: (a) Schematic illustration of the synthesis procedure of B, S-Ti 3 C 2 T x .Ti 3 AlC 2 was used as the precursor for the preparation of Ti 3 C 2 T x , of which was then autoclaved to result in B, S-Ti 3 C 2 T x .(b) X-ray diffraction (XRD) data showing the crystal structures.(c, d) Raman spectra showing the chemical compositions as well as the origin of disordered carbon of as-synthesized Ti 3 C 2 T x and B, S-Ti 3 C 2 T x samples.

Figure 2 (
g) shows a high-resolution TEM image of B, S-2-Ti 3 C 2 T x at 200 nm magnification, with the dashed frames indicating the aggregated B and S doped on the Ti 3 C 2 T x nanosheet.
) (supporting information), with a magnification range of 50 to 200 nm.Crystal defects were observed in B, S-2-Ti 3 C 2 T x enclosed by a yellow dashed line, which can be explained by the substitution of B and S for the C sites.To confirm the successful preparation of B, S-2-Ti 3 C 2 T x , EDX elemental mapping was taken of a specific area (Figure 2(h)).The elements B, S, Ti, C, and O were uniformly dispersed.
3, 456.1, 457.3, and 458.8 eV, which are attributed to Ti−C, Ti 2+ , Ti 3+ , and Ti−O, respectively (Figure 3(a)), as previously reported [59].Compared with the Ti 3 C 2 T x sample, a smaller-intensity peak of Ti−C at 454.9 eV and a peak shift of 0.4 eV are observed, whereas a stronger peak of Ti−O appears at 458.7 eV in the B-Ti 3 C 2 T x MXene sample (Figure.S4(a)).Based on previous reports, the presence of Ti 3+ in B-Ti 3 C 2 T x , which is formed through the reduction of Ti 4+ , leads to electron transfer from Ti to B and produces a B-Ti-O substitutional site

Figure 4 :
Figure 4: (a) LSV curves and (b) Tafel plots of the as-prepared catalysts.(c) Comparison of the overpotential required to achieve a current density of 10 mA/cm 2 .(d) EIS spectra of B, S-Ti 3 C 2 T x at −0.3 V vs. RHE in 0.5 M H 2 SO 4 (inset: equivalent circuit).(e) double-layer capacitance (c dl ) of Ti 3 C 2 T x and B, S-Ti 3 C 2 T x electrocatalysts at 0.15 V vs. RHE as a function of the scan rate.(f) polarization curves of B, S-Ti 3 C 2 T x catalysts before and after 1000 CV cycles (at a scan rate of 50 mV s −1 ) in 0.5 M H 2 SO 4 .
3 C 2 T x exhibit overpotentials of 250 and 220 mV, respectively (Figure.S7(a)).Their electrocatalytic performances are compared in Figure.S7(b) based on the potential obtained at a constant current density of 10 mA cm −2 .We believe that the synergistic effects of active sites on Ti-C, B-O-B, and B-O-Ti species are responsible for the improvement in HER reactivity that we observed following B-doping.The corresponding Tafel slopes of the as-prepared catalysts, which can provide information about the mechanism of HER electrocatalysis, were calculated to be 112.2,98, 64, and 80 mV dec1 for pristine Ti 3 C 2 T x , B-1-Ti 3 C 2 T x , B-2-Ti 3 C 2 T x , and B-3-Ti 3 C 2 T x , respectively; B-2-Ti 3 C 2 T x has the lowest Tafel slope (Figure.S8(a)).
lytic activity was investigated.The introduction of S significantly improved the catalytic performances of the catalysts.B-2-Ti 3 C 2 T x shows an overpotential of 170 mV at a current density of 10 mA cm −2 ; at the same time, B, S-2-Ti 3 C 2 T x catalyst exhibits a significantly lower overpotential (110 mV) than both B-Ti 3 C 2 T x and S-Ti 3 C 2 T x , demonstrating the synergistic effect of the dual-heteroatom doping of 2D Ti 3 C 2 T x nanosheets (Figure4(a)).Similarly, B, S-2-Ti 3 C 2 T x and B, S-3-Ti 3 C 2 T x exhibit overpotentials (at 10 mA cm −2 ) of 180 and 200 mV, respectively, which are lower than that of B-Ti 3 C 2 T x .In their work, Le et al. investigated the kinetics and electronic behavior of doped MXene [54].DFT calculations were carried out, and the electronic band structure and charge difference for hydrogen adsorption of the Ru@B-Mxene are illustrated.The calculated values of ΔG H * for atomic H adsorption on the Ru@B-Mxene compared to the nondoped MXene are low, which plays a crucial role in reducing the charge-transfer resistance of the 2D MXene nanosheet for the HER.The Tafel slopes of B, S-1-Ti 3 C 2 T x , B, S-2-Ti 3 C 2 T x , and B, S-3-Ti 3 C 2 T x are 78, 54, and 77 mV dec −1 , respectively, which are lower than those of the respective B-doped Ti 3 C 2 T x specimens, signifying faster HER kinetics (Figure 4(b)).The figure (Figure 4(c)) shows a comparison of overpotential at 10 mA/cm 2 with other reported MXene electrocatalysts [54, 67-70].In addition, the electrocatalytic performance based on the potential obtained at a constant current density of 10 mA cm −2 of the prepared catalyst is shown in Figure.S10.EIS measurements were conducted to further study the kinetics of the catalyst for HER.The EIS plots in Figure 4(d) show that the charge-transfer resistance (R ct ) of an electrode, represented by the semicircle, significantly decreases after B and S doping of Ti 3 C 2 T x .Consequently, the Rct of B, S-2-Ti 3 C 2 T x is significantly smaller than that of B-2-Ti 3 C 2 T x .This confirms the higher rate of charge transport in the B and S codoped MXenes.

Figure 5 :
Figure 5: Schematic illustration of the HER catalysis mechanism for B, S-Ti 3 C 2 T x in an acidic medium.