MXene Nanofibers as Highly Active Catalysts for Hydrogen Evolution

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MXene Nanofibers as Highly Active Catalysts for Hydrogen Evolution Reaction Wenyu Yuan, Laifei Cheng, Yurong An, Heng Wu, Na Yao, Xiaoli Fan, and Xiaohui Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01348 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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ACS Sustainable Chemistry & Engineering

MXene Nanofibers as Highly Active Catalysts for Hydrogen Evolution Reaction Wenyu Yuan,a Laifei Cheng,a Yurong An,b Heng Wu,a Na Yao,c Xiaoli Fan,b Xiaohui Guo*c a

Science and Technology on Thermostructural Composite Materials Laboratory,

Northwestern Polytechnical University, Xi’ an 710072, P. R. China. b

State Key Laboratory of Solidification Processing, Northwestern Polytechnical

University, 710072, Xi'an, P. R. China c

Lab of Synthetic and Natural Functional Molecule Chemistry of Ministry of

Education, Northwest University, Xi’ an 710069, P. R. China. *Email: [email protected]

ACS Paragon Plus Environment

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Abstract: None-precious metal catalysts for hydrogen evolution reaction (HER) recently received growing attention. Herein, we designed a highly active MXene nanofiber catalyst with high specific surface area (SSA) via the hydrolyzation of bulk MAX ceramics, and a subsequent HF etching process. Compared with traditional MXene flakes, the MXene nanofibers delivered much higher SSA, and exposed more active sites in the cross section. As a result, the MXene nanofiber delivered an enhanced HER activity with a low overpotential of 169 mV at a current density of 10 mA cm-2, a depressed Tafel slope of 97 mV dec-1 and low electrochemical resistance. The improved SSA and exposing active sites are responsible for the enhanced activity. This work provide a novel synthesis method for MXene nanofibers, and the MXene nanofibers are also promising to be applied in batteries, supercapacitors, and catalytic fields. Keywords: MXene nanofibers; Hydrogen evolution reaction; Specific surface area; Active sites.


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Introduction Hydrogen, as a kind of green energy source, which is potential to overcome the global energy & environmental crisis, has attracted growing attention.1-2 Electrocatalytic hydrogen evolution reaction (HER) is considered as a highly active approach to produce hydrogen via the splitting of water.3 Nobel metals and nobel oxides, such as Pt, RuO2, etc, are most effecient catalysts.4 However, the high cost and low reserves limit their applications.5 Hence, many efforts have been devoted to develop high effecient catalysts with low cost, mainly including metal carbides, metal sulfides6 and metal nitrides7,8 to replace noble metals.9,10 Among these non-noble metal catalysts, transition metal carbides (TMCs) were considered as the candidates for hydrogen production owing to their low cost, high electrical conductivity and high catalytic activities. However, the real catalytic performance of these TMCs is still far away from the requirements for industrial applications. 11 Therefore, various approaches, including doping, hybrid structure, and architecture design, etc., were carried out and achieved remarkable progress.12-14 To create more active sites for HER, Gao et al reported a P-Mo2C@carbon nanowire structure for HER, in which P-doping can weaken the strength of Mo–H and carbon can accelerate the charge transfer, leading to a remarkable catalytic performance.15 Bao et al reported a hybrid-structure of vertically aligned Mo2C-MoS2 nanosheets on carbon paper and exhibited high catalytic activity owing to the active sites of Mo−S−C, and highly dispersed Mo2C.16 Besides of the hybrid structure, designing highly porous structure has been proved to be another approach to create active sites.



The 2D porous TMCs reported by Huang et al demonstrated a low onset overpotential of 25 mV and a small Tafel slope of 40 mV/dec. Furthermore, the structure can directly affect the catalytic process, active sites and charge transfer, recently attracted extensive interests.17-18 Various nanostructures of TMCs, from 1D to 3D architecture, have been explored to improve the electrocatalytic performance.19-20 1D nanostructure, including nanowires, nanofibers, nanorods, etc., recently achieved growing attention owing to the abundant active sites, large specific surface area and fast charge-transfer process.18,

21-22

However, it has remained to be challenging to fabricate 1D

nanostructures with high catalytic activity via a simple, low cost and stable approach. Very recently, MXenes, a new kind of 2D TMCs, have gained widespread attention as potential catalysts for HER.23-24 Gogotsi’s group firstly reported the HER performance of MXenes in 2016.25 They found that Mo2C exhibited higher HER activity than Ti2C. The overpotential of Mo2C was up to 0.283 V, limiting the applications of MXenes. Loh et al synthesized a hybrid catalyst of 2D Mo2C and graphene, and shows a current density of 10 mA·cm-2 at a low overpotential of 87 mV.26

Ti-based MXenes delivered inferior performance than Mo-based MXenes. To

create more active sites for MXenes and enhance their HER activity, we designed an MXene nanofiber (NF) structure with high SSA, which can expose more active sites, and enhance the HER activity. Bulk MAX ceramics can be directly hydrolyzed into uniform nanofibers under alkaline condition. After the selectively etching of “A” phases, the MXene NF structure can be simply obtained. Owing to the improved SSA and abundant active sites on the surface, the HER activity of MXene NFs has been


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significantly enhanced. To the best of our knowledge, to date, it is the first time to report the HER performance of MXene NFs. The synthesis method can further applied to prepare other MXene NFs owing to the large MXene family, and this work would open new sights for designing novel structure for water splitting.

Experimental Section Materials Ti3AlC2 (200 mesh, 98 wt.%) was purchased from FORSMAN Company, China. KOH (AR) was purchased from Aladdin Company, Shanghai, China. HF (40 wt.%) was purchased from Aladdin Company, Shanghai, China. All reagents were used without further purification. Synthesis of Ti3C2 NFs and Ti3C2 flakes Ti3AlC2 NFs was firstly prepared via the hydrolyzation process of Ti3AlC2. Typically, 2 g Ti3AlC2 was added into 20 mL 6M KOH solution. The solution was stirred for 96 h at room temperature. After that, the material was centrifuged and washed in deionized water until pH~7. After drying at 40 oC for 24h, The Ti3AlC2 NFs was obtained. Ti3C2 NFs was synthesized via the selective etching of Al from Ti3AlC2 NFs. Typically, 1 g Ti3AlC2 (purchased from FORSMAN Company, China) was added into 20 mL 10 wt.% HF solution. The solution was stirred for 2 h at 40 oC. Then, the material was centrifuged and washed in deionized water until pH~7. After drying at 30 oC for 24h under N2 atmosphere, The Ti3C2 NFs was obtained. The tranditional Ti3C2 flakes was synthesized via the directly HF etching of bulk Ti3AlC2 which has



been reported in our previous work.27 Typically, 0.5 g Ti3AlC2 (purchased from FORSMAN Company, China) was added into 10 mL 40 wt.% HF solution. The solution was stirred for 72 h at 40 oC. Then, the material was centrifuged and washed in deionized water until pH~7. After drying at 30 oC for 24h under N2 atmosphere, The Ti3C2 flakes was obtained. Characterization X-ray diffraction (XRD) patterns were obtained from the sample powders directly with Bruker D8 ADVANCE X-ray diffractometer equipped with Cu Kɑ radiation. The microstructure was observed by a scanning electron microscopy (SEM, S4700, Hitachi, Japan) equipped with an energy dispersive X-ray spectrometer (EDS). Transmission electron microscope (TEM, F30G, Tecnai, Nederland) was used to investigate the structure of Ti3C2 flakes and NFs. X-ray photoelectron spectroscopy (XPS) (PHI 5400, PE, USA) was used to measure the elemental composition and chemical bonds of Ti3C2 NFs. The SSA and the distribution of pore size were tested by N2 adsorption instrument (Micromeritics ASPA 2460, the United States) using Brunauer–Emmett–Teller (BET) method at 77 K. Raman spectra were obtained from Renishaw Ramascope (Confocal Raman Microscope, Renishaw, Gloucester-shire, U.K.) equipped with a He–Ne laser (λ = 532 nm). Electrochemical Measurements The HER activity was studied by a typical three-electrode system consisting of a working electrode, a graphite rod counter electrode, and a Ag/AgCl (3.0 M KCl) reference electrode on a CHI 660E electrochemical workstation (Chenhua, Shanghai,


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China). A glass carbon working electrode with a diameter of 3 mm was fabricated by the drop-casting the catalyst ink. Typically, 5 mg of catalyst and 100 µL of 5 wt% Nafion solution were dispersed in 900 µL ethanol by 20 min sonication to form a homogeneous ink. After that, 4.3 µL of the above obtained ink (containing ~21.5 µg of catalyst) was coated on the glass carbon electrode (the loading mass: ~ 0.3 mg/cm2). After drying at room temperature, the working electrode was obtained. The Ag/AgCl reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE) by the equation (1): ERHE=EAg/AgCl+0.21 + 0.059pH.

(1)

Linear sweep voltammetry (LSV) were carried out in 0.5 M H2SO4 at a scan rate of 5 mV s-1. Tafel plots are fitted to the Tafel equation: η=b log j + a

(2)

where η is the overpotential, b is the Tafel slope, j is the current density and a is the Tafel intercept relative to the exchange current density j0. Electrochemical impedance spectroscopy (EIS) analysis was tested in the frequency range of 100 KHz to 0.1 Hz under the potential of 100 mV. The electrochemically active surface area (ECSA) was measured by the double layer capacitance (Cdl) in a non‐faradaic potential range of 0.21 to 0.31 V at different scan rates of 20, 40, 60, 80, 100, and 120 mV·s-1. The ECSA was calculated by the following equation: ECSA = Cdl/Cs

(3)

Where Cs is the specific capacitance. The durability test was carried out at a static overpotential of 200 mV for 12 h in 0.5 M H2SO4.



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Results and discussion The synthetic procedures for the Ti3C2 NFs are illustrated in Figure 1a. Firstly, Ti3AlC2 MAX NFs were synthesized via the hydrolyzation of Ti3AlC2 MAX phases in 6 M KOH solution with strong string. The bulk Ti3AlC2 before hydrolyzation process can be clearly observed from the Figure S1. The thickness of bulk Ti3AlC2 is ~2-5 µm and the particle size is ~5-20 µm. Previous works demonstrated that 2D materials are susceptible to hydrolysis.28 The Ti-C bonds near defect sites were easily affected by the OH- ions, and the OH- would work like a scissor creating a crack in the 2D Ti3AlC2 framework. The shear force caused by string would cause the propagation of the cracks until reaching the edges. Therefore, Ti3AlC2 NFs were obtained and the morphology of Ti3AlC2 NFs is shown in Figure S2. The NFs with average width of 50 nm can be obtained. The scissor role of OH- to tailor the 2D frameworks into 1D NFs recently was also widely to be used to synthesize various 1D nitrides and carbides.29-30 After the etching in hydrofluoric acid (HF) solution, the Al was selectively etched out and the Ti3C2 MXene NFs with abundant functional groups can be obtained. The HF etching method has been proved as an efficient approach to obtain the Ti3C2 MXenes from bulk Ti3AlC2 powders via the following equation (4):31

Ti3 AlC 2 + 3HF → Ti3C 2 + AlF3 + 3 / 2H 2 ↑

(4)

Compared with the bulk Ti3AlC2, the above reaction for Ti3AlC2 NFs is more efficient owing to the improved SSA and shortened ion transfer paths, leading to an enhanced etching efficiency. The morphologies of Ti3C2 NFs are shown in Figure 1b, 1c. The NF morphology can be well preserved during HF etching process. A


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high-magnification SEM image in Figure 1c disclosed the thickness of the Ti3C2 NFs ranging from 40 to 60 nm. The TEM image in Figure 1d clearly displayed the NF structure of Ti3C2 MXene. The width of NFs are ~50 nm. The d-spacing of Ti3C2 from the high-resolution TEM (HRTEM) image was measured to be 0.97 nm in Figure 1e, which is consistent with the reported Ti3C2 MXene material, suggesting that the Al was etching away and the d-spacing was enlarged owing to the functional groups (-OH, etc.) on the surface of Ti3C2.32 Although Al atom layers in Ti3AlC2 were selectively removed, the high crystallinity was well preserved, as shown in the selected area electron diffraction (SAED) image (the insert of Figure 1e). However, owing to the large particle size of bulk Ti3AlC2, the hydrolyzation cannot fully transform the Ti3AlC2 into NF structure, especially for the inner Ti3AlC2. Therefore, some non-hydrolyzed Ti3C2 nanosheets in obtained Ti3C2 NF sample still can be observed (Figure S3). Besides, the effects of KOH concentration on the morphologies of Ti3C2 NFs were also been studied, and the results (Figure S4) show that the KOH concentration does not affect the morphologies including the width of NFs, when the concentration < 8 M KOH.



Figure 1. The schematic diagram for the synthesis of Ti3C2 NFs. b) SEM image of Ti3C2 NFs. c) high magnification SEM image of Ti3C2 NFs. d) TEM image of Ti3C2 NFs. e) HRTEM image of Ti3C2 NFs. The insert is the SAED image. f) XRD spectra of Ti3C2 NFs. g) Raman spectra of Ti3C2 NFs.

To further identify that the Ti3C2 structure has been well preserved, the Ti3C2 flakes synthesized via one-step HF etching of bulk Ti3AlC2 was used as the reference sample, and the comparison of XRD and Raman spectra were carried out. The morphology of Ti3C2 flake is shown in Figure S5. The XRD spectra of Ti3AlC2, hydrolyzed Ti3AlC2 and Ti3C2 NFs are shown in Figure S6. The (002) peak of Ti3AlC2 at 9.5o is shifted to lower 2θ value of 9.0o and is broaden in Ti3C2 NFs, indicating that the Al was selectively etched and Ti3C2 MXene NFs was successfully synthesized.33-34


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We further compared the XRD spectra of Ti3C2 flakes and obtained Ti3C2 NFs (Figure S7) and the enlarged XRD spectra in 5-25 o are shown in Figure 1f. All XRD peaks agreed well with the Ti3C2 MXene flakes and previous reported Ti3C2 MXenes synthesized via the common HF etching method from bulk Ti3AlC2.35 Raman spectroscopy (Figure 1g) was then carried out to identify the structure of Ti3C2 NFs. Compared to Ti3C2, the Ti3C2 NFs displayed similar signals with the Ti3C2 flakes and the intensity of Ti3C2 NFs has been enhanced, suggesting the high crystallinity. Three broad Raman peaks centered nearly around 256, 417, and 612 cm-1, which can be attributed to the vibrations from Ti3C2.36

Figure 2. XPS survey (a), Ti2p (b), C1s (c), O1s (d) spectra of Ti3C2 NFs.

In order to analyze the chemical bonds of Ti3C2 NFs, XPS measurements of Ti3C2 NFs were carried out, as shown in Figure 2. Figure 2a shows that elements of O, Ti and C are the main elements in obtained Ti3C2 NFs. No Al element was



observed indicating the successfully selective etching of Al. In high-resolution Ti2p XPS spectrum, two peaks of Ti-C bond at 454.3 eV and Ti2p1/2 460.3 eV, which is attributed by Ti3C2, can be clearly observed.37 The peak at 464.6 eV which is assigned to be Ti-O bond, suggests the existence of oxygen-contained functional groups (-OH, etc.). 36 An obvious peak at 281.8 eV in high-resolution C1s XPS spectrum is assigned to be C-Ti bond. The C-O and O-C=O bond at 286.4 eV and 288.6 eV is caused by that many functional groups were introduced in the hydrolyzation process and the following HF etching process.36 To further identify the functional groups on the surface of Ti3C2 NFs, we further analyzed the high-resolution O1s XPS spectrum and found that the oxygen-contained functional groups not only existed at the surface of Ti layers, also anchored on the surface of carbon atom layers, and the main specie among these functional groups is hydroxyl (-OH). The XPS spectra proved that we have successfully synthesized Ti3C2 NFs. Besides, owing to instability caused by the high SSA and etching process, many –OH groups exists in the obtained Ti3C2 NFs. The SSA and pore size distribution of Ti3C2 flakes and Ti3C2 NFs were characterized via by N2 adsorption instrument using BET method at 77 K, and the results are shown in Figure 3. The Ti3C2 flakes suffer from low SSA (8.5 m2·g-1), meanwhile the SSA of Ti3C2 NFs reached up 58.5 m2·g-1. The pore size distribution in Figure 3b suggests that Ti3C2 NFs have more abundant pore structure than Ti3C2 flakes, which is responsible for the enhanced electrochemical performance.


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Figure 3. The N2 absorption-desorption isotherm (a) and the pore size distribution (b) of Ti3C2 flakes and Ti3C2 NFs

The catalytic activities of the Ti3C2 flakes and Ti3C2 NFs samples for the HER were measured using a standard three-electrode electrochemical system in 0.5 M H2SO4, as shown in Figure 4. As a reference, commercial Pt catalyst (20 wt% Pt/C) was also tested. Figure 4a shows HER polarization curves of different samples. Ti3C2 flakes delivered very poor electrocatalytic activity for HER, and the overpotential is 385 mV at 10 mA·cm-2, which is much lower than that of Ti3C2 NFs samples (169 mV) and Pt/C catalyst. The low overpotential of Ti3C2 NFs indicates that Ti3C2 NFs process higher activity than Ti3C2 flakes, owing to the high SSA and abundant active sites, which is consistent with that we discussed above. In order to understand the kinetics of HER processes of Ti3C2 NFs catalysts in HER, Tafel slopes, which reveal the rate-determining step of HER, were used to show the involved possible steps, as shown in Figure 4b. The rate-determining step will be identified as the Tafel, Heyrovsky, or Volmer steps, respectively, if Tafel slope is 30, 40, or 120 mV/decade.38 Ti3C2 NFs resulted in a Tafel slope of 97 mV/dec, meanwhile



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the Tafel slopes of commercial Pt/C and Ti3C2 flakes are 40 and 188 mV/dec, respectively,

suggesting

that

the

splitting

of

water

is controlled

by

a

Volmer–Heyrovsky Mechanism that include a fast discharge Volmer Reaction (H3O++e-1——Hads+H2O) and a relative slow desorption Heyrovsky reaction (Hads+H3O++e-1——H2+H2O). The –OH group termination is considered as the active site for HER based on the previous reported works.39-40 The mechanism can be concluded as following: 1) the H3O+ ion was adsorbed on the –OH group active site, following by combining with an electron to produce an H atom; 2) the generated H atom combined with an electron and a H3O+ ion, and generate a H2 molecule. The desorption of hydrogen (step 2) is the rate-determining step. The exchange current density (j0) is also an important parameter to describe the catalytic efficiency. The obtained j0 of Ti3C2 NFs is 0.36 mA cm−2, higher than those of Ti3C2 NFs (0.11 mA cm−2) and most of non-noble metal catalysts.41-43 The high j0 of Ti3C2 NFs suggests a high catalytic efficiency, further confirming a favorable HER kinetics at the Ti3C2 NFs electrolyte interface. Table S1 compares the HER performances of the synthesized Ti3C2 NFs and reported MXene HER catalysts, and the Ti3C2 NFs in this work is superior to the reported MXene catalysts and most of non-noble metal electrocatalysts.25, 41-46 To better understand the intrinsic activity of Ti3C2 NFs catalyst, the ECSA (Figure S8) was estimated via testing the Cdl in the non-faradaic potential region.47 The Ti3C2 NFs has a much larger Cdl (21 mF·cm-2), which is about 6 times that of Ti3C2 flakes (Figure S8b, d), indicating that Ti3C2 NFs possess abundant catalytic


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active sites for the HER. Combining the ECSA and SSA results, we can draw a conclusion that the enhanced catalytic activity mainly comes from the improved SSA of Ti3C2 NFs. Moreover, to reveal the electron-transfer kinetics in HER, EIS was carried out and the Nyquist plots display in Figure 4c. The charge transfer resistance (Rct) can be obtained by fitting the impedance spectra to the referencing equivalent circuit. It was found that the Rct of Ti3C2 NFs (224.3 Ω) is much lower than that of Ti3C2 flakes (475.6 Ω), suggesting that the NF structure can shorten the diffusion pathway for electrons, promote the charge transfer and therefore enhance HER performance.

Figure 4. The LSV (a) and Tafel slopes (b) of Ti3C2 NFs, Ti3C2 flakes and Pt/C electrodes. (c) The EIS plots of Ti3C2 NFs, and Ti3C2 flakes. (d) The LSV of Ti3C2 NFs before and after cyclic test. The insert is the plots of current density vs. times for Ti3C2 NFs under a potential of 200 mV.



The stability is another important factor to assess the performance of catalysts. To evaluate the cyclic stability of Ti3C2 NFs, the time dependence of current density at the overpotential of 0.2 V, and the polarization curves before and after cyclic test were tested and the results are shown in Figure 4d. The polarization curves of Ti3C2 NFs before and after 12 h cyclic test shows that the potential difference of Ti3C2 NFs is very little. In addition, the curve under the overpotential of 0.2 V (inset in Figure 4d) is very stable and after 12 h continuous electrolysis, the current density remained almost constant, indicating that the Ti3C2 NFs catalyst is extraordinary stable, suggesting remarkably cycling stability and long-term viability. The morphology of NF can also be clearly observed in the SEM image (Figure S9) of Ti3C2 NFs after cyclic test, indicating the high structural stability of Ti3C2 NFs.

Conclusion In summary, we designed a Ti3C2 NF structure with enhanced HER activity via the hydrolyzation in KOH solution and a subsequent selectively etching. Compared with common Ti3C2 flakes, the SSA has been greatly improved, and more active sites for catalysis has been exposed, leading to an enhanced HER activity. As a result, the Ti3C2 NFs delivered a low overpotential of 169 mV at a current density of 10 mA cm-2, a depressed Tafel slope of 97 mV dec-1 and low electrochemical resistance. Besides, the Ti3C2 NFs shows high cycling stability and long-term viability. This synthesis method can further applied to prepare other MXene NFs owing to the large MXene family, and the MXene NFs are promising to be applied in batteries, supercapacitors, catalytic and other-related fields.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at XXXXXXX. The SEM images of bulk Ti3AlC2; The morphology of hydrolyzed Ti3AlC2; TEM image of Ti3C2 NFs with partial non-hydrolyzed nanosheets; The SEM images of Ti3C2 NFs treated in 10 M KOH, 8 M KOH, and 6 M KOH, respectively; SEM image of common Ti3C2 flakes; XRD of Ti3AlC2, hydrolyzed Ti3AlC2 and Ti3C2 NFs; the XRD comparison of Ti3C2 flakes and Ti3C2 NFs; Table S1; The CV for Ti3C2 flakes and Ti3C2 NFs with different scan rate from 20 to 120 mVs-1; The plots of capacitive current at 0.25 V vs. scan rate of Ti3C2 flakes and Ti3C2 NFs; SEM image of Ti3C2 NFs after cyclic tests (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (No. 2017YFB1103500), and the National Natural Science Foundation of China (No. 51302220, 51672218 and 51632007).



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TOC GRAPHIC MXene nanofiber, which is synthesized via the hydrolyzation of bulk MAX ceramics and a subsequent HF etching process, is a potential catalyst for hydrogen evolution reaction.


MXene Nanofibers as Highly Active Catalysts for Hydrogen Evolution

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