MXene Nanofibers as Highly Active Catalysts for Hydrogen Evolution

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 8976−8982

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*,§ Science and Technology on Thermostructural Composite Materials Laboratory and ‡State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China § Lab of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Northwest University, Xi’an 710069, People’s Republic of China ACS Sustainable Chem. Eng. 2018.6:8976-8982. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/23/19. For personal use only.

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S Supporting Information *

ABSTRACT: Nonprecious metal catalysts for hydrogen evolution reaction (HER) have recently received growing attention. Herein, we designed a highly active MXene nanofiber catalyst with a 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 a 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 exposed active sites are responsible for the enhanced activity. This work provides a novel synthesis method for MXene nanofibers, and MXene nanofibers are also promising for applications in batteries, supercapacitors, and catalytic fields. KEYWORDS: MXene nanofibers, Hydrogen evolution reaction, Specific surface area, Active sites

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the active sites of Mo−S−C, and highly dispersed Mo2C.16 Besides the hybrid structure, designing highly porous structures has proved to be another approach to create active sites. The two-dimensional (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, and recently attracted extensive interest.17,18 Various nanostructures of TMCs, from onedimensional (1D) to three-dimensional (3D) architectures, have been explored to improve electrocatalytic performance.19,20 One-dimensional nanostructure, including nanowires, nanofibers, nanorods, etc., recently achieved growing attention owing to abundant active sites, large specific surface areas, and fast charge-transfer process.18,21,22 However, it has remained 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 first 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.

INTRODUCTION Hydrogen, as a kind of green energy source, which has the potential to overcome the global energy and environmental crisis, has attracted growing attention.1,2 The electrocatalytic hydrogen evolution reaction (HER) is considered to be a highly active approach to produce hydrogen via the splitting of water.3 Noble metals and noble metal oxides, such as Pt, RuO2, etc., are the most efficient catalysts.4 However, high cost and low reserves limit their applications.5 Hence, many efforts have been devoted to develop highly efficient catalysts with low cost, mainly including metal carbides, metal sulfides,6 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 from the requirements for industrial applications.11 Therefore, various approaches, including doping, hybrid structure, 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 © 2018 American Chemical Society

Received: March 25, 2018 Revised: May 30, 2018 Published: June 12, 2018 8976

DOI: 10.1021/acssuschemeng.8b01348 ACS Sustainable Chem. Eng. 2018, 6, 8976−8982

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic diagram for 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 inset is the SAED image. (f) XRD spectra of Ti3C2 NFs and Ti3C2 flakes. (g) Raman spectra of Ti3C2 NFs and Ti3C2 flakes. synthesized via the selective etching of Al from Ti3AlC2 NFs. Typically, 1 g of Ti3AlC2 (purchased from FORSMAN Co., China) was added into 20 mL of 10 wt % HF solution. The solution was stirred for 2 h at 40 °C. Then, the material was centrifuged and washed in deionized water until pH ∼7. After drying at 30 °C for 24 h under N2 atmosphere, Ti3C2 NFs were obtained. The tranditional Ti3C2 flakes were synthesized via the direct HF etching of bulk Ti3AlC2 which has been reported in our previous work.27 Typically, 0.5 g of Ti3AlC2 (purchased from FORSMAN Co., China) was added into 10 mL of 40 wt % HF solution. The solution was stirred for 72 h at 40 °C. Then, the material was centrifuged and washed in deionized water until pH ∼7. After drying at 30 °C for 24 h under N2 atmosphere, Ti3C2 flakes were obtained. Characterization. X-ray diffraction (XRD) patterns were obtained from the sample powders directly with a Bruker D8 ADVANCE X-ray diffractometer equipped with Cu Kα radiation. The microstructure was observed by a scanning electron microscope (SEM; S4700, Hitachi, Japan) equipped with an energy dispersive Xray spectrometer (EDS). A transmission electron microscope (TEM; F30G, Tecnai, Netherlands) 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 pore size distribution were tested by a N2 adsorption instrument (Micromeritics ASPA 2460,USA) using the Brunauer−Emmett−Teller (BET) method at 77 K. Raman spectra were obtained from a Renishaw Ramascope (confocal Raman microscope, Renishaw, Gloucestershire, 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,

synthesized a hybrid catalyst of 2D Mo2C and graphene, and showed a current density of 10 mA·cm−2 at a low overpotential of 87 mV.26 Ti-based MXenes delivered performances inferior to those of Mo-based MXenes. To create more active sites for MXenes and enhance their HER activity, we designed an MXene nanofiber (NF) structure with a high specific surface area (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 selective 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 significantly enhanced. To the best of our knowledge, to date, this 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 insights for designing novel structures for water splitting.

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EXPERIMENTAL SECTION

Materials. Ti3AlC2 (200 mesh, 98 wt %) was purchased from FORSMAN Co., China. KOH (AR) was purchased from Aladdin Co., Shanghai, China. HF (40 wt %) was purchased from Aladdin Co., Shanghai, China. All reagents were used without further purification. Synthesis of Ti3C2 NFs and Ti3C2 Flakes. Ti3AlC2 NFs were first prepared via the hydrolyzation process of Ti3AlC2. Typically, 2 g of Ti3AlC2 was added into 20 mL of 6 M 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 °C for 24 h, Ti3AlC2 NFs were obtained. Ti3C2 NFs were 8977

DOI: 10.1021/acssuschemeng.8b01348 ACS Sustainable Chem. Eng. 2018, 6, 8976−8982

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. XPS survey (a), Ti 2p (b), C 1s (c), and O 1s (d) spectra of Ti3C2 NFs.

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. NFs with an 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 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 Ti3C2 MXene NFs with abundant functional groups could be obtained. The HF etching method has been proved as an efficient approach to obtain Ti3C2 MXenes from bulk Ti3AlC2 powders via eq 4:31

Shanghai, China). A glassy carbon working electrode with a diameter of 3 mm was fabricated by drop-casting the catalyst ink. Typically, 5 mg of catalyst and 100 μL of 5 wt % Nafion solution were dispersed in 900 μL of 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 glassy carbon electrode (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 eq 1: E RHE = EAG/AgC1 + 0.21 + 0.059pH

(1)

Linear sweep voltammetry (LSV) was carried out in 0.5 M H2SO4 at a scan rate of 5 mV·s−1. Tafel plots were fitted to the Tafel equation:

η = b log j + a

(2)

Ti3AlC2 + 3HF → Ti3C2 + AlF3 + 3/2H 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 100 kHz−0.1 Hz under the potential of 100 mV. The electrochemically active surface area (ECSA) was measured by double layer capacitance (Cdl) in a nonfaradaic potential range of 0.21−0.31 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

(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,c. The NF morphology can be well-preserved during the HF etching process. A 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 widths 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 (inset 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 nonhydrolyzed Ti3C2 nano-

(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. First, 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 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 8978

DOI: 10.1021/acssuschemeng.8b01348 ACS Sustainable Chem. Eng. 2018, 6, 8976−8982

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. N2 absorption−desorption isotherm (a) and pore size distributions (b) of Ti3C2 flakes and Ti3C2 NFs.

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

MXene flakes and previously 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 signals similar to those of 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 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 successful selective etching of Al. In the high-resolution Ti 2p XPS spectrum, two peaks of the Ti− C bond at 454.3 eV and Ti 2p1/2 at 460.3 eV, which is attributed by Ti3C2, can be clearly observed.37 The peak at 464.6 eV which is assigned to the Ti−O bond, suggests the existence of oxygen-containing functional groups (−OH,

sheets in the obtained Ti3C2 NF sample still can be observed (Figure S3). Besides, the effects of KOH concentration on the morphologies of Ti3C2 NFs were also studied, and the results (Figure S4) show that the KOH concentration does not affect the morphologies including the width of NFs, when the concentration is