Adding a New Member to the MXene Family: Synthesis, Structure, and

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Adding a new member to the MXene family: Synthesis, structure and electrocatalytic activity for the Hydrogen Evolution Reaction of V4C3Tx Minh Hai Tran, Timo Schäfer, Ali Shahraei, Michael Dürrschnabel, Leopoldo Molina-Luna, Ulrike I. Kramm, and Christina S. Birkel ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00652 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Applied Energy Materials

Adding a New Member to the MXene Family: Synthesis, Structure and Electrocatalytic Activity for the Hydrogen Evolution Reaction of V4C3Tx Minh H. Tran,† Timo Schäfer, † Ali Shahraei,‡ Michael Dürrschnabel,§,# Leopoldo Molina-Luna,§ Ulrike I. Kramm, †,‡ and Christina S. Birkel*,† †

Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, 64287 Darmstadt, Germany.

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Institut für Materialwissenschaft, Technische Universität Darmstadt, 64287 Darmstadt, Germany

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Department of Material- and Earth Sciences, Electron Microscopy Center Darmstadt (EMC-DA), Technische Universität Darmstadt, 64287 Darmstadt, Germany. Key words: MAX phase, MXene, V4AlC3, V4C3Tx, hydrogen evolution reaction, electrocatalysis, carbides ABSTRACT: Two-dimensional transition metal-based carbides (or nitrides), so-called MXenes, that can be derived from the three-dimensional MAX phases, have attracted considerable attention throughout the last couple of years. The particular structure together with their hydrophilic and metallic nature make them promising candidates for a plethora of applications, such as sensors, electrodes and catalysts. Obviously, the respective chemical and physical properties are highly dependent on the chemical composition, stoichiometry and surface structure of the MXene. Here, we introduce a new member of the MXene family, V4C3Tx (T representing the surface groups), based on the chemical exfoliation of the 413 MAX phase V4AlC3 by treatment with aqueous hydrofluoric acid. X-ray powder diffraction data together with scalebridging electron microscopy studies proof the successful removal of aluminum from the MAX phase structure. The electrocatalytic activity for the hydrogen evolution reaction of this new MXene is tested in acidic solution over the course of 100 cycles. Interestingly, we find a significant improvement of the catalytic performance over time (i.e. the overpotential required to achieve a current density of 10 mA cm-2 decreases by almost 200 mV) that we assign to the removal of an oxide species on the surface of the MXene, as shown by XPS measurements. Our study provides crucial experimental data of the electrocatalytic activity of MXenes together with the evolution of its surface structure that is also relevant for other transition-metal based MXenes in the context of further potential applications.

INTRODUCTION Transition metal-based carbides that belong to the family of so-called MXenes, have been identified as a new class of two-dimensional materials.1–3 They can be obtained by the removal of the A element – mostly Al – from ternary MAX phases (transition metal-based carbides and nitrides). MAX phases possess a lamellar structure with alternating layers of M6X octahedra and the A element along the c-axis.4 Due to the different bonding strength between M and X (carbon or nitrogen) and M and A, the weaker bound A element can selectively be removed. Synthetically, this exfoliation is mainly realized by treatment/etching with aqueous hydrofluoric acid leaving –F, OH and –O groups (commonly summarized and denoted as Tx) on the surface of the two-dimensional MXenes.5 Members of the MXene family that have already been prepared include Ti2CTx, Nb2CTx, Mo2CTx V2CTx, as well as

the solid solutions (Ti/V)2CTx and (Ti/Nb)2CTx, Ti3C2Tx, Zr3C2Tx together with several solid solutions (e.g. between Ti/V, Mo/Ti, Cr/V and Cr/Ti).6 For the 4:3 stoichiometry, only Ti4N3Tx, Nb4C3Tx and Ta4C3Tx as well as the solid solutions (Ti/Nb)4C3Tx and (Nb/Zr)4C3Tx have been synthesized. Yet, a significantly higher number of MAX phases are known many of which were synthesized and structurally characterized by Jeitschko in the 1960s.7,8 As a result of their two-dimensionality, MXenes with varying chemical composition are already discussed in the context of a plethora of different potential applications.6 Very prominently, their application as energy storage and energy conversion materials has been described.6,9–11 However, the latter, i.e. their possible use as catalysts for the hydrogen evolution reaction, has mostly been investigated theoretically.12 Here, many first-principles studies suggest that MXenes can be excellent candidates as cata-

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lysts for water splitting.12–14 Obviously, the surface structure/chemistry plays a major role for the catalytic activity and usually fully oxidized MXene surfaces (MXO2) are considered. Besides, transition-metal modification of the surface of the two-dimensional MXene was shown to greatly benefit the performance of the catalyst.15,16 Experimental studies targeting the use of MXenes as catalysts for the hydrogen evolution reaction (HER) are scarce. Seh et al. investigated different transition metalbased MXenes theoretically but also presented experimental data of the catalytic activity of Ti2CTx and Mo2CTx.17 Their work shows the higher stability and HER activity of the Mo- in contrast to the Ti-based system. Theoretical investigations also show the potential of Vcontaining MXenes14,15 and it is pointed out that the layer thickness will have a large influence on the respective catalytic activity.14 It becomes clear that further experimental investigations – not only on exfoliated 211 but also “higher” MAX phases, such as 413 MAX phases – are greatly needed in order to drive this exciting research area further. As for many other MAX phases, the first report on V2AlC dates back to 1963 when Jeitschko et al. synthesized and structurally characterized this then called H-phase.7 A phase diagram of V-Al-C was published in 1980, yet no ternary phase beside V2AlC was mentioned.18 Later, a high-temperature ternary phase was synthesized – first in the form of carbon-deficient single crystals V4AlC3-x (x≈0.31)19 and then as a minor side phase (~18 wt%) in polycrystalline VC.20 With these findings, another socalled 413 MAX phase had been added to the family of MAX phases. Crystallographically, they show a different packing sequence along the c-axis in comparison to the 211 and 312 MAX phases, i.e. four layers of transition metal are found between the layers of A element (instead of two and three for the 211 and 312 phases, respectively). Quite recently, our group has successfully synthesized polycrystalline V4AlC3 using microwave heating.21 Here, we present the exfoliation of V4AlC3 resulting in the new MXene V4C3Tx. The product is structurally characterized by X-ray powder diffraction and further analyzed in depth by electron microscopy techniques. Additionally, we investigated its electrocatalytic activity for the hydrogen evolution reaction (HER) as well as its stability under these acidic conditions. MATERIALS AND METHODS For the initial V4AlC3 synthesis, powders of vanadium (>99.5%, –325mesh, Sigma-Aldrich), aluminum (>99.97, – 325mesh, ChemPUR) and carbon (99,9999%, 2–15 micrometers, Carbone of America) were used in a 4:5.2:3 molar ratio. Briefly, the powders were thoroughly ground, pressed and sealed in an evacuated quartz ampoule. The ampoule was embedded in 7 g granular graphite (Grüssing, acting as a susceptor) and reacted using a microwave oven – in contrast to the originally published

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work,21 a laboratory grade microwave oven was used (CEM, see Figure 1). The temperature profile of the susceptor material (Figure 1) was recorded using an IR pyrometer (Optris GmbH) that is attached to the top of the microwave oven. The microwave power level was adjusted to 1600 W and the reaction was run for 60 min. Within 23 min, temperatures > 1000 °C are reached and a bright glowing of the graphite susceptor is observed (see Figure 1). The temperature further increases to around 1360 °C that is maintained for another 45 min. It is important to note that temperatures exceeding 1300 °C are essential in order to obtain the V4AlC3 phase. The sample is allowed to cool down slowly to room temperature within the insulation housing. The 413 MAX phase was then thoroughly ground using an agate mortar and pestel. 200 mg of V4AlC3 were carefully added to ~15 ml 40% HF in a PTFE beaker and stirred for 165 hours. Please note that working with hydrofluoric acid (HF) requires particular care and equipment (calcium gluconate gel needs to be accessible close by in case of an accident). During the exfoliation process, the temperature increases and the reaction mixture was therefore cooled using an ice/water bath. Small gas bubbles were released according to the following chemical equation: V4AlC3 + 3 HF V4C3 + AlF3 + 1.5 H2

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The resulting black product was washed six times with water using PTFE centrifuge tubes in a centrifuge (Heraeus, 5000 rpm) until neutral and then dried at 120 °C in a drying chamber over night. To investigate the crystallographic structure of the samples, X-ray powder diffraction measurements were performed. MAX phase V4AlC3 powders were prepared on a flat sample holder while MXene V4C3Tx powders were loaded into a 0.05 mm quartz capillary. Diffraction data were acquired using a powder diffractometer system STOE STADI P with monochromatized Cu radiation in transmission geometry. Data of V4AlC3 were refined using TOPAS Academic.22 The sample was finely ground and dry sprayed onto the Cu grid for electron microscopy analyses by a home-made instrument. Scanning transmission electron microscopy (STEM) in combination with electron energy-loss spectroscopy (EELS) was carried out in a JEOL ARM 200F equipped with a Gatan Enfina spectrometer. The acceleration voltage was 200 kV. High-angle annular dark- field images (HAADF) as well as EEL spectra were acquired using a camera length of 6 cm. The energy-resolution in EELS, as measured by the full-width half-maximum of the zero-loss peak and was estimated to be about 1 eV. Conventional diffraction contrast imaging, selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDX) were done in a JEOL 2100F equipped with an Oxford X-Max80 provided with an 80 mm2 Silicon Drift Detector (SDD) detector. The microscope was operated at 200 kV.


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ACS Applied Energy Materials perform the electrochemical conditioning for 100 cycles. Afterwards, the electrode was rinsed with water and the GC-rod again trans-

Figure 1: Temperature profile of the susceptor material during the microwave-based (1600 W) solid state synthesis of MAX phase V4AlC3 (blue line) showing the high heating rate and final susceptor temperature of 1367 °C. The laboratory grade microwave oven (MARS) is also shown together with glowing susceptor material where an evacuated quartz ampoule holding the precursor mixture is embedded.

For the evaluation of HER activity we applied similar conditions as given by Seh et al.17 that is to the best of our knowledge the first experimental study on HER activity of MXene. A three-electrode setup with a working electrode (glassy carbon disc, A = 0.1963 cm2), carbon rod as counter electrode and Ag/AgCl reference electrode was used. For the catalyst ink, 5 mg of catalyst powder were sonicated with 5 µl of a Nafion in 500 µl ethanol for 30 min (N/C ratio = 0.25). Then 2 µl of this suspension were drop casted on the working electrode and left to dry to yield a catalyst loading of 0.1 mg cm-2. Measurements were performed in 0.5M H2SO4 at rpm 1600. The given voltammograms are iR corrected using the iR correction as provided by the potentiostat software (Nordic Electrochemistry). Measurements were performed in a potential range of 0.1 V to -1.0 V with a sweep rate of 10 mV s-1 to evaluate the HER activity at Beginning of Life (B.o.L.). In addition to this, afterwards the electrode was continuously cycled in the same potential range for 100 cycles. Then again, the HER activity was evaluated and assigned as End of Life (E.o.L.) activity. For investigation of possible structural changes induced by this cycling, the composition of the electrode was checked in the as-prepared state and after the E.o.L. measurement was made. X-ray-induced Photoelectron Spectroscopy (XPS) was measured using a SPECS PHOIBOS 150 hemispherical analyzer and a SPECS XR50M Al Kα X-ray source (E = 1486.7 eV). The system operates at a pressure of p < 10-9 mbar. For the XPS measurements, the catalyst ink was drop-casted on the top-surface of a glassy carbon rod (A = 0.1963 cm²) and mounted on a specially designed XPS holder. After XPS measurement, the GC-rod equipped with the catalyst layer was mounted on a RDE shaft to

Figure 2: X-ray powder diffraction data of (a) MAX phase V4AlC3 and (b) MXene V4C3Tx. Upon exfoliation, a clear shift of the (002) peak to smaller angles can be observed (red arrow) resulting from a larger unit cell parameter c. Besides, peak intensities corresponding to the MAX phase decrease significantly while the high background shows the presence of an amorphous component. Crystal structures of the 413 MAX phase and respective MXene are shown on the right (orange = V, dark blue = Al, dark grey = C).

ferred to XPS to investigate structural changes on the surface. For the survey scans the step size was 1 eV and two scans were performed. Fine scans were performed with an energy step size of 0.05 eV for the following regions: V 2p (20 scans), O 1s (20 scans), C 1s (15 scans), whereas the number in parenthesis gives the number of overlayed scans. The software CasaXPS was used in order to analyze the data. A Shirley background was used and fits were made allowing for both Gaussian and Lorentzian (GL30) contributions to the peaks. RESULTS AND DISCUSSION Both samples, MAX phase V4AlC3 and MXene V4C3Tx, are structurally characterized by X-ray powder diffraction (Figure 2). For V4AlC3, the structural model with space goup P63/mmc (as reported by Hu et al.20) was fitted (orange line in Figure 2 (a)) to the obtained powder diffraction data (dark blue line in Figure 2 (a)). The resulting refined unit cell parameters of a = 2.924(16) Å and c = 22.643(17) Å match those reported in the literature of a = 2.931 Å and c = 22.7192 Å well. Side phases are not included in the fit in Figure 2 (a) but can be identified as 211 MAX phase V2AlC and VC and account for a combined ~15 % of the final product. The Xray powder diffraction data of the product after exfoliation with HF has changed quite significantly (Figure 2 (b)). One striking feature is the broadening and shift to smaller diffraction angles of the (002) reflection (indicated by a red arrow in Figure 2). This corresponds to an increase in the unit cell parameter c (~ 29.63 Å) due to the removal of Al and insertion of water molecules between



the V4C3 sheets. Accordingly, all (00l) peaks broaden and shift to lower angles which is not visible here as a result of

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using the crystal structures (on the right in Figure 2, orange = V, dark blue = Al, dark grey = C). Successful exfoliation of the MAX phase is also proven by electron microscopy studies (Figure 3). Scanning electron

Figure 4: (a) High-angle annular dark-field (HAADF) image of a freestanding MXen V4C3Tx layered structure. (b) HAADF image of the analyzed area. (c) StripeSTEM-like electron energy-loss (EEL) map created from the 2D EEL data of the analyzed area. (d) EEL spectra extracted from position (1) and (2).

the high amorphous background at angles below 30°. This phenomenon Figure 3: Electron micrographs of (a) MAX phase V4AlC3 and (b) MXene V4C3Tx including overview images and EDX analyses (insets), the white boxes indicating where the EDX analyses were performed. Both, the images as well as the EDX data show the successful removal of Al from the initial MAX phase yielding two-dimensional V4C3Tx. The two-dimensional V4C3Tx was analyzed by STEM-HAADF as shown in (c) and (d), respectively.

is routinely reported for two-dimensional MXenes.2,3 For the 4:3 stoichiometry, increased unit cell parameters of 30.47 Å and 30.34 Å are reported for Nb4C3Tx23 and Ta4C3Tx,2 respectively. Contrarily, the (110) peak maintains most of its intensity since it is not affected by the increased distance between the V4C3 sheets after the Al removal. At the same time, the intensity of V4AlC3 reflections between 35 and 45° decreases significantly. All of those observed features in the X-ray diffraction data show that the exfoliation process of V4AlC3 and removal of Al atoms was successful yielding 2D V4C3Tx, as shown

micrographs of V4AlC3 show the layered morphology typical of the MAX phase family (Figure 3 (a)).4 EDX measurements (inset in Figure 3 (a)) further show the presence of V, Al and C alongside Si that most probably stems from the quartz ampoule used as the reaction container. After the treatment with hydrofluoric acid, the morphology clearly changes with structures that are fanned out and shifted (Figure 3 (b)). The V4C3 layers have separated as a result of the removal of the Al (upper inset in Figure 3 (b)) that is also shown by the EDX analysis where no Al signal is detected (lower inset in Figure 3 (b)). Based on the SEM/EDX measurements, it is very likely that some minor amount of Al has remained in the structure since weak Al signals are found in some areas of the sample. On a local scale, transmission electron micrographs (Figure 3 (c + d)) further demonstrate the exfoliation of the initial MAX phase. The light dots show the location of the heaviest present element vanadium while the dark regions correspond to amorphous areas. The 4:3 stoichiometry of the V4C3 layers is clearly visible in the lower right image (i.e. four “lines” of light elements separated by dark amorphous layers) as also indicated by the overlaid crystal structure image of V4C3Tx. The individual V4C3 layers are roughly 1 nm thick and not delaminated since no ions or small molecules have been inserted in between these sheets. A freestanding MXene V4C3Tx layered structure was also further investigated by STEM-EELS (Figure 4). Figure 4 (a) shows a STEM-HAADF image of a MXene V4C3Tx layered structure. From the part indicated by the white rectangle (shown in Figure 4 (b)) a 2D STEM-EELS map was acquired in the low-loss range. In order to clearly reveal


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the layered structure in the spectral data, the 2D EELS data cube was evaluated by generating a stripeSTEM-like image in false colors (Figure 4 (c)). The layered structure can also be observed in the spectral image. In order to reveal a difference

regular reflection. Figure 5 (c) shows an EDX point spectrum of a representative area of the particle. Only V and C are present as expected. The Cu peak is due to stray radiation from the copper grid. Figure 5 (d-f) show the same analysis for a second particle, respectively. Superstructure reflections also are present here, this indicates a possible ordering of

Figure 6: Survey scan (a) as well as fine scans of O1s (b) and C1s (c) of the as-prepared electrode (black line) and after 100 cycles of electrochemical conditioning (green line).

Figure 5: (a) Bright-field TEM image of particle 1. (b) Selected area electron diffraction pattern of the marked area in (a). The zone axis orientation is [111] and the pattern indexation carried out according to the V4C3 structure (https://materials.springer.com/isp/crystallographic/docs/sd_112920 5). Superstructure reflections are present (see arrows). (c) EDX spectrum of particle 1. (d) to (f) Same as in (a) to (c) but for a second particle in oriented in [101] zone axis orientation, respectively.

between the MXene layer (1) and the amorphous interspacing (2), two representative EELS spectra were extracted from the data cube (Figure 4 (d)). We observed a peak around 10 eV, which is attributed to hydrogen containing surface groups.24 Besides, a V-M2,3 low-loss edge was present in both spectra. However, an Al-L2,3 low-loss edge was not observed in both spectra demonstrating again the removal of aluminum. The sample thickness was estimated to be between one and two mean free paths (mfp). In addition to the EELS measurements, we conducted combined electron diffraction and EDX experiments (Figure 5). Figure 5 (a) shows a TEM bright-field image of a freestanding MXene V4C3Tx layers. The white circle indicates the area from which the selected area electron diffraction pattern shown in Figure 5 (b) was acquired. Superstructure reflections are visible at 1/3 and 2/3 of each

local point defects on the C sublattice in the substoichiometric samples.19,25 For the initial catalyst (as deposited with Nafion on a glassy carbon disc) and after electrochemical conditioning, the elemental composition was determined from X-ray induced photoelectron spectroscopy (XPS). The survey scans as well as O 1s and C1 s fine scan regions are presented in Figure 6. In the as-prepared electrode the presence of vanadium, carbon, nitrogen, oxygen and fluorine (the last named components from Nafion) are depicted, whereas there is no signal that stems from aluminum. This indicates a complete removal of aluminum from the surface-near region of the MXene. It is important to note that the O 1s signature of the as-prepared electrode clearly indicates the presence of Me-O due to the oxidation of the MXene surface,26 as discussed in the introduction part. In contrast, after electrochemical conditioning the related peak completely vanishes and only adsorbed water and C-O remains. The removal of oxidized states from the surface is also visible in Figure 7 (c) that shows the V 2p finescan region. The peaks at 516.9 and 513.4 eV, respectively, can be attributed to the V4+ and V-C with a valence state of zero to one, respectively, based on the literature.27,28 More importantly, the loss of almost 90 % of vanadium (as of the oxidized state) has no negative effect on HER activity. Instead, the overpotential required to achieve a current density of 10 mA cm-2 decreases by almost 200 mV, as visible in Figure 7 (a). Two important conclusions can be made from this result: i) The vanadium is only HER active as V4C3Tx and ii) oxidized surface



layers that are formed during or subsequent to the exfoliation treatment can be removed by electrochemical cycling but are not required for HER activity of this MXene. To get further insights, the Tafel slope was determined for the initial and conditioned catalyst. The hydrogen evolution reaction either proceeds via Volmer-Heyrovsky or Volmer-Tafel mechanism.

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ever, we might conclude that the removal of the surface oxide layer is required to enable a non-limited HER on MXenes. Besides it is important to note that the electrocatalytic properties of any given MXene is not only dependent on the M-element but also on a complex interplay of additional variables, such as the surface groups, possible defects and adatoms and interlayer distance and interaction. For example, Handoko et al. studied the HER performance of Ti3C2Tx with different amounts of F-surface groups and found the highest activity for the material with the lowest fluorine coverage.30 Therefore, in order to fully explain all of the observed phenomena, more systematic studies of MXenes based on different transition metals are needed. Particularly, experimental investigations are necessary that can be compared with each other and discussed in the context of existing theoretical studies. CONCLUSION

Figure 7: (a) Voltammograms and related Tafel plots of (b) V4C3Tx at B.o.L. (black line) and after 100 cycles in 0.5M H2SO4 (green line, rpm 1600, 0.1 mgcat cm-2). In (c) the V 2p fine scan region of the initial electrode (prior to electrochemical tests, black line) and after the 100 cycles (green line) are given.

Especially in older literature, it is discussed that depending on the rate determining step (RDS) different Tafel slopes are observed: Step 1 (Volmer): H+ + e- Hads (120 mV dec-1) (2) Step 2a (Heyrovsky): H+ + Hads + e- H2 ↑ (40 mV dec-1) (3) Step 2b (Tafel): 2 Hads H2 ↑ (30 mV dec-1) (4) The Tafel slopes, as-given in parentheses, are related to a transfer coefficient of α = 0.5. However, as described recently by Shinagawa et al.,29 the Tafel slope might also change due to variations in the surface termination of the catalyst. Anyhow, for the initial material it becomes clear that with a Tafel slope of almost 240 mV dec-1 neither of the above mentioned steps is rate determining. Hence, most probably on V4C3Tx a “reaction” occurs in advance to the initial electron transfer step and seems rate limiting. This could be the removal of oxidized surface states or the diffusion of protons to free V0 states. In contrast, the conditioned catalyst reaches values at low and high overpotentials that are typically assigned to the Volmer and Heyrovsky step, respectively, as rate limiting steps. According to Shinagawa et al. instead of a change of the RDS a change of the surface termination with intermediates formed earlier in the reaction could also cause a variation of the Tafel slope.29 In order to distinguish both effects (change in RDS vs. surface termination) a more detailed correlation between HER activity measurements and related changes in XPS would be nice in the future and best to be done in-situ. On the basis of our results; how-

Based on our previous successful preparation of bulk V4AlC3, we performed its chemical exfoliation using aqueous hydrofluoric acid. With that, we were able to add the new member V4C3Tx to the growing MXene family. X-ray powder diffraction data reflect the more amorphous nature as well as the increased unit cell parameter c in contrast to the parent 413 MAX phase, as is commonly found in the MXene literature. Electron micrographs, that show the V4C3-sheets detaching from one another, further proof the successful exfoliation. On the microscale, most aluminum has been removed as supported by EDX measurements. On the atomic scale, stacks of multiple V4C3 layers are observed with amorphous areas in between them where the aluminum has been selectively etched. Here, EELS and EDX measurements on single grains also demonstrate no signs of remaining aluminum. Electron diffraction patterns can be indexed according to the V4C3 structure and superstructure peaks are observed that might stem from ordering of point defects on the C sublattice. Among many other potential applications, MXenes are discussed in the context of their electrocatalytic activity – although not much is known experimentally about their HER (hydrogen evolution reaction) performance. We have therefore used the new MXene V4C3Tx as a catalyst and studied its electrochemical behavior in acidic solution. Interestingly, we find a significant enhancement of the catalytic activity over the course of 100 cycles, i.e. the overpotential required to achieve a current density of 10 mA cm-2 decreases by almost 200 mV. XPS studies before and after catalytic cycling clearly show the removal of oxide species on the surface of the MXene. This is also observed in a similar study on a molybdenum-based MXene.17 However, their initial catalytic activity is much higher than what we find while their performance decreases over time. It becomes clear that further systematic


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experimental studies are greatly needed to fully understand the observed processes. Our results on the catalytic activity of a new MXene are therefore very valuable to enhance this understanding and ultimately to develop new HER catalysts. Furthermore, the surface structure and its modification upon electrochemical treatments is also a crucial factor for other potential applications, such as the use of MXenes as supercapacitors or electrodes in battery systems.

AUTHOR INFORMATION

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Corresponding Author * [email protected] (12)

Present Addresses # Present address: Karlsruhe Institute of Technology (KIT) Institute for Applied Materials, Applied Materials Physics Department: Metallic Materials, Hermann-von-HelmholtzPlatz 1, D-76344 Eggenstein-Leopoldshafen

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Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. /

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Funding Sources

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CSB and MHT would like to thank the DFG for financial support (BI 1775/2-1). MD and L.M-L. acknowledge financial support from the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE RESPONSE. L.M-L acknowledges financial support from DFG Grant MO3010/3-1. AS and UIK acknowledge financial support by the graduate school of excellence energy science and engineering (GSC1070).

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ACKNOWLEDGMENT

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AS and UIK acknowledge the use of the XPS setup DAISYMAT of the group of Wolfram Jaegermann at TU Darmstadt. (20)

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Temperature profile of the susceptor material during the microwave-based (1600 W) solid state synthesis of MAX phase V4AlC3 (blue line) showing the high heating rate and final susceptor temperature of 1367 °C. The laboratory grade microwave oven (MARS) is also shown to-gether with glowing susceptor material where an evacuated quartz ampoule holding the precursor mixture is embedded. 58x54mm (300 x 300 DPI)


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X-ray powder diffraction data of (a) MAX phase V4AlC3 and (b) MXene V4C3Tx. Upon exfoliation, a clear shift of the (002) peak to smaller angles can be observed (red arrow) resulting from a larger unit cell parameter c. Besides, peak intensities corresponding to the MAX phase decrease significantly while the high background shows the presence of an amorphous component. Crystal structures of the 413 MAX phase and respective MXene are shown on the right (orange = V, dark blue = Al, dark grey = C). 93x68mm (300 x 300 DPI)



Electron micrographs of (a) MAX phase V4AlC3 and (b) MXene V4C3Tx including overview images and EDX analyses (insets), the white boxes indicating where the EDX analyses were performed. Both, the images as well as the EDX data show the successful removal of Al from the initial MAX phase yielding two-dimensional V4C3Tx. The two-dimensional V4C3Tx was analyzed by STEM-HAADF as shown in (c) and (d), respectively.


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(a) High-angle annular dark-field (HAADF) image of a freestanding MXen V4C3Tx layered structure. (b) HAADF image of the analyzed area. (c) StripeSTEM-like electron energy-loss (EEL) map created from the 2D EEL data of the analyzed area. (d) EEL spectra extracted from position (1) and (2). 160x113mm (300 x 300 DPI)



(a) Bright-field TEM image of particle 1. (b) Selected area electron diffraction pattern of the marked area in (a). The zone axis orientation is [111] and the pattern indexation carried out according to the V4C3 structure (https://materials.springer.com/isp/crystallographic/docs/sd_1129205). Superstructure reflections are present (see arrows). (c) EDX spec-trum of particle 1. (d) to (f) Same as in (a) to (c) but for a second particle in oriented in [101] zone axis orientation, respectively. 160x216mm (300 x 300 DPI)


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Survey scan (a) as well as fine scans of O1s (b) and C1s (c) of the as-prepared electrode (black line) and after 100 cycles of electrochemical conditioning (green line). 138x103mm (150 x 150 DPI)



(a) Voltammograms and related Tafel plots of (b) V4C3Tx at B.o.L. (black line) and after 100 cycles in 0.5M H2SO4 (green line, rpm 1600, 0.1 mgcat cm-2). In (c) the V 2p fine scan region of the initial electrode (prior to electrochemical tests, black line) and after the 100 cycles (green line) are given. 161x120mm (150 x 150 DPI)


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Adding a New Member to the MXene Family: Synthesis, Structure, and

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