Conductive Two-Dimensional Titanium Carbide 'Clay' with High Volumetric

LETTER doi:10.1038/nature13970

Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance

Michael Ghidiu1*, Maria R. Lukatskaya1*, Meng-Qiang Zhao1, Yury Gogotsi1 & Michel W. Barsoum1

19 Safe and powerful energy storage devices are becoming increasingly thin Ti3AlC2 films with ammonium bifluoride . The change in MXene important. Charging times of seconds to minutes, with power den- properties upon intercalation and the compositional variability of fluo- sities exceeding those of batteries, can in principle be provided by ride salts suggested the possibility of a one-step procedure for the syn- electrochemical capacitors—in particular, pseudocapacitors1,2. Recent thesis of many MXenes, with tunable structures and properties. research has focused mainly on improving the gravimetric perform- The MXenes reported in this study were prepared by dissolving LiF ance of the electrodes of such systems, but for portable electronics in 6 M HCl, followed by the slow addition of Ti3AlC2 powders and heat- and vehicles volume is at a premium3. The best volumetric capaci- ing of the mixture at 40u C for 45 h.After etching,the resulting sediments tances of carbon-based electrodes are around 300 farads per cubic were washed to remove the reaction products and raise the pH (several centimetre4,5; hydrated ruthenium oxide can reach capacitances of cycles of water addition, centrifugation and decanting). The resulting 1,000 to 1,500 farads per cubic centimetre with great cyclability, but sediment formed a clay-like paste that could be rolled, when wet (Fig. 1a), only in thin films6. Recently, electrodes made of two-dimensional between water-permeable membranes in a roller mill to produce flex- titanium carbide (Ti3C2, a member of the ‘MXene’ family), produced ible, free-standing films (Fig. 1c) in a matter of minutes, in contrast to by etching aluminium from titanium aluminium carbide (Ti3AlC2,a those previously produced by the laborious technique of intercalation, ‘MAX’ phase) in concentrated hydrofluoric acid, have been shown to delamination, and filtration18. have volumetric capacitances of over 300 farads per cubic centimetre7,8. A graphical depiction of the processing is provided in Extended Data Here we report a method of producing this material using a solution Fig. 1. Further, scaling was not limited to the size of the filtration appa- of lithium fluoride and hydrochloric acid. The resulting hydrophilic ratus; films of any dimensions could readily be produced. Additionally, material swells in volume when hydrated, and can be shaped like clay when wet, the ‘clay’ could be moulded and dried to yield various shapes and dried into a highly conductive solid or rolled into films tens of that were highly conductive (Fig. 1d). Diluted, it could also be used as an micrometres thick. Additive-free films of this titanium carbide ‘clay’ ink to deposit (print) MXene on various substrates. Like clay, the mate- have volumetric capacitances of up to 900 farads per cubic centimetre, rial could be rehydrated, swelling in volume, and shrinking when dried with excellent cyclability and rate performances. This capacitance is (Fig. 1b). almost twice that of our previous report8, and our synthetic method Energy-dispersive spectroscopy confirmed that aluminium (Al) was also offers a much faster route to film production as well as the avoid- removed, and X-ray diffraction (XRD) revealed the disappearance of ance of handling hazardous concentrated hydrofluoric acid. Ti3AlC2 peaks (traces can be seen in the case of incomplete transforma- In the search for new electrode materials, two-dimensional solids are tion). Multilayer particles did not show the accordion-like morphology of particular interest owing to their large electrochemically active surfaces9. seen in HF-etched MXenes reported to date14,20; rather, particles appeared For example, activated graphene electrodes have capacitances of 200– tightly stacked, presumably as a result of water and/or cationic interca- 350 F cm23 compared to 60–100 F cm23 for activated porous carbons10,11. lation (see Extended Data Fig. 2a). Fluorine and oxygen were observed Yet graphene is limited to the chemistry of carbon, does not tap into in energy-dispersive spectroscopy; this, coupled with X-ray photoelec- metal redox reactions as in ruthenium oxide (RuO2) (ref. 6), and its tron spectroscopy showing evidence of Ti–F and Ti–O bonding, sug- conductivity is substantially decreased by the addition of redox-active gests O- and F-containing surface terminations, as has been discussed 12 14,21 functional groups . MXenes (of the formula Mn 1 1XnTx, where M is a at length for HF-produced MXenes . The yield of MXene after etch- 14 transition metal, X is C and/or N, and Tx denotes surface functionali- ing, calculated as described previously , is around 100%, which is com- zation) are a relatively young class of two-dimensional solids, produced parable with the HF-etching method. Our new method thus does not by the selective etching of the A-group (generally group IIIA and IVA lead to material losses, although an accurate yield determination is dif- elements) layers fromthe MAX phases,which comprise a .70-member ficult owing to the variability of surface groups and amount of inter- family of layered, hexagonal early-transition-metal carbides and nitrides13. calated water. To date, all MXenes have been produced by etching MAX phases in XRD patterns of the etched material, in its air-dried multilayered concentrated hydrofluoric acid (HF)14–16. state, showed a remarkable increase in the intensity and sharpness of the MXenes have already proved to be promising candidates for elec- (000l) peaks (Fig. 2a, pink); in some cases the full width at half maxi- trodes in lithium (Li)-ion batteries17,18 and supercapacitors8, exhibiting mum (FWHM) was as small as 0.188u, as opposed to the broad peaks volumetric capacitances that exceedmostpreviously reportedmaterials. typical of HF-etched MXene7, and more typical of intercalated MXenes18. However, the path to electrode manufacturing required the handling of Further, compared to a lattice parameter of c < 20 A˚ for HF-produced concentrated HF and a laborious multi-step procedure. Here we sought Ti3C2Tx, the corresponding value in this work was 27–28 A˚ . XRD pat- a safer route by exploiting the reaction between common, inexpensive terns of still-hydrated sediment showed shifts to even higher spacings: hydrochloric acid (HCl) and fluoride salts, leading to dissolution of alu- lattice parameters as high as c < 40 A˚ have been measured. These large minium and the extraction of two-dimensional carbide layers. Further- shifts are suggestive of the presence of water, and possibly cations, between more, given the ability of MXenes to preferentially intercalate cations the hydrophilic and negatively charged MXene sheets. From these sub- (post-synthesis)8, a related question was whether etching and intercala- stantial increases in c and the clay-like properties (see below), it is reason- tion might be achieved in a single step, as was observed for etching of able to assume that—as in clays22,23—the swelling is due to the intercalation

1Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA. *These authors contributed equally to this work.

a Etching Washing Rolling Shaping Figure 1 | Schematic of MXene clay synthesis and electrode preparation. a, MAX phase is etched in a solution of acid and fluoride salt (step 1), then washed with water to remove reaction products and raise the pH towards neutral (step 2). The resulting sediment behaves like a clay; it can Painting be rolled to produce flexible, freestanding films (step 3), moulded and dried to yield conducting objects of desired shape (step 4), or diluted and painted onto a substrate to yield a conductive Ti3AlC2 Ti3C2Tx ‘Clay’ Electrode coating (step 5). b, When dried samples (left, showing cross-section and top view) are hydrated b c d (right) they swell; upon drying, they shrink. 1,500 S cm–1 c, Image of a rolled film. d, ‘Clay’ shaped into the letter M (,1 cm) and dried, yielding a conductive solid (labelled with the experimental conductivity of ‘clay’ rolled to 5 mm thickness). The etched material is referred to as Ti3C2Tx, where the T denotes surface terminations, such as OH, O and F. of multiple layers of water and possibly cations between the MXene The c parameter expansion also resulted in the weakening of inter- sheets. Interfacial water has a more structured hydrogen-bonding net- actions between the MXene layers, as evidenced by the easy delamina- work than bulk H2O (ref. 24). The MXene surface, holding a negative tion of multilayered particles by sonication, as is done for van der Waals electric charge, may act to align the dipoles of water molecules between solids9. In our previous work, typical sonication times for delamination MXene layers. (after post-synthesis intercalation withdimethyl sulphoxide) were of the When the‘clay’ was rolled into freestandingfilms, XRD patterns again order of 4 h (ref. 18). Here, sonication times of the order of 30–60 min showed strong ordering in the c direction (Fig. 2a, blue). Films, ranging resulted in stable suspensions with concentrations as high as 2 g per litre, in thicknesses from submicrometre to about 100 mm, were readily pro- higher than observed previously. Remarkably, the yield from multilayer duced by this method. The most compelling evidence for particle shearing to dispersed flakes was about 45% by mass. Freestanding films were also is the marked intensity decrease of the (110) peak around 61u, indi- readily fabricated by filtering these suspensions, as reported previously8. cating a reduction of ordering in the non-basal directions while order The fact that the LiF 1 HCl etchant was much milder than HF resulted in the c direction was maintained (see blue XRD pattern in Fig. 2a and in flakes with larger lateral dimensions (Fig. 2b) that did not contain the scanning electron microscopy (SEM) image in Extended Data Fig. 2b). nanometre-size defects frequently observed in HF-etched samples25. Morphologically, the thinner films showed more overall shearing of the Transmission electron microscopy (TEM) analysis showed that, of 321 multilayer particles when viewed in cross-section (Fig. 2e, f) and exhib- flakes analysed, over 70% had dimensions of 0.5–1.5 mm (Extended ited substantial flexibility, even when allowed to dry thoroughly (inset Data Fig. 4a, b). Single layers about 10 A˚ thick were imaged using TEM to Fig. 2e). The contact angle of water ontherolled MXene film was mea- (Fig. 2c, d), confirming that the material is indeed two-dimensional. sured as 21.5u, confirming its hydrophilic nature (Extended Data Fig. 3). Analysis of 332 flakes suggested that roughly 70% of the flakes were 1–2 Attempts to hydrate and roll HF-produced MXene were unsuccessful; layersthick (Extended Data Fig. 4c–f). We note that, since the restacking we propose that the intercalated water acts as a lubricant that allows or folding of flakes can lead to higher apparent thicknesses (Extended facile shearing. Data Fig. 5), the 70% estimate is conservative. Thus, using this method,

a b Figure 2 | Structural characterization of MXene.

Ti3AlC2 a, XRD patterns of samples produced by etching × 3 (0 0 2) 8.0 10 in LiF 1 HCl solution. The pink trace is for * Ti3C2 (HF) 6.0 × 103 multilayer Ti3C2Tx, showing a sharp, intense peak (0002) and higher-order (000l) peaks, ˚ × 3 corresponding to a c lattice parameter of 28 A 1.5 10 * 2 nm–1 and high order in the c direction. The blue trace (0 0 4) 1.0 × 103 (0 0 6) is for the same sample after rolling into an (0 0 8) (0 0 10) (0 0 12) (1 1 0) Offset intensity approximately 40-mm-thick film; c-direction peaks 5.0 × 102 Rolled clay are preserved, but the prominent (110) peak is Clay no longer observed, showing substantial reduction 0.0 of order in non-basal directions. In both cases, 10º 20º 30º 40º 50º 60º 50 nm 2Θ traces of Ti3AlC2 are still present (red diamonds). The MXene (0002) peak is at a much lower angle e f c than that typical of MXene produced by HF (green star). b, TEM image of several flakes, showing lateral sizes up to a few hundred nanometres. Few defective areas are present. The inset shows the 9–10 Å 10 nm overall selected area electron diffraction pattern. c, d, TEM images of single- and double-layer flakes, d respectively. Insets show sketches of these layers. d e, SEM image of a fracture surface of a ,4-mm- thick film produced by rolling, showing shearing of layers; the flexibility of the film is demonstrated in the inset. f, Fracture surface of a thicker rolled μ μ 1 m 2 m 10 nm film (,30 mm), showing poorer overall alignment of flakes in the interior of the film.

4 DECEMBER 2014 | VOL 516 | NATURE | 79 ©2014 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER large fractions of single-layered MXene flakes with high yields, large this analysis—summarized in Fig. 3d—show that, at scan rates below lateral sizes, and good quality can be readily produced. The flake lateral 20 mV s21, there is a noticeable, yet not prevailing, contribution of sizes reported here are larger than those reported for HF-etched Ti3AlC2 diffusion-limited processes to the total capacitance. At scan rates of (ref. 18); the milder delamination conditions may be partially respon- 20 mV s21 and higher, the response is not diffusion-controlled but is sible for this. rather due to surface capacitive effects, whether electrostatic or pseu- Previously we have shown that MXene ‘paper’—made by filtration of docapacitive. This observation is in agreement with the conjecture of 29 solutions containing delaminated Ti3C2Tx flakes—exhibited volumetric Levi et al. about the presence of shallow- and deep-trap sites in MXene capacitances of ,350 F cm23 at 20 mV s21 (and 450 F cm23 at 2 mV s21) structures. Further, if there are also redox contributions from changes in potassium hydroxide (KOH) electrolyte8. For comparison, we charac- in the oxidation states of surface Ti atoms28, the redox processes are not 30 terized the electrochemical performances of rolled, freestanding Ti3C2Tx diffusion-limited, and thus represent ‘intrinsic’ capacitive behaviour . films in 1 M sulphuric acid (H2SO4). The advantages of acidic electro- When the electrochemical responses of three rolled clay electrodes lytes include not only their excellent conductivities but also that protons, (5 mm thick, 30 mm thick and 75 mm thick) were compared (Fig. 3e, f), being the smallestcations, are known to allow forsurface redoxreactions not surprisingly, the volumetric capacitances decreased with increased in transition-metal oxide electrodes, such as RuO2, MnO2 and some thickness. These thickness-dependent differences can be partially traced others, and may contribute to the capacitance via fast surface redox1,26. to the electrode morphologies. As noted above, electrodes thinner than At a scan rate of 2 mV s21, capacitance values reached 900 F cm23 10 mm showed good flake alignment (Fig. 2e) with typical densities of (Fig. 3a) and a good rate handling ability was observed (Fig. 3b). The 3.6–3.8 g cm23. At 2.2–2.8 g cm23, the densities of the thicker (15 mm results—summarized and compared with previous work8 in Fig. 3b— and larger) rolled electrodes were lower, which is a reflection of the fact clearly showthat rolled Ti3C2Tx clay electrodes show outstanding capac- that their core seemed to be more open (Fig. 2f). And while the lower itive performance, not only volumetrically but gravimetrically as well, densities led to lower volumetric capacitances, their more open struc- achieving 245 F g21 at 2 mV s21. This can be ascribed to the smaller size ture ensured accessibility to ions and thus similar rate performances as of H1 compared to other intercalating cations, surface redox processes, their thinner counterparts (Fig. 3e, f). The lower densities also ensured and improved accessibility of interlayer spacing in LiF 1 HCl-etched that the drop in gravimetric capacitances with thickness (see Extended MXene owing to pre-intercalated water, compared to the previously Data Fig. 6) was not substantial. A summary of key mass- and volume- studied HF-etched samples. It is worth noting that a similar positive normalized capacitance values as a function of electrode thickness is pro- role of structural water for capacitive performance in acidic electrolytes vided in Extended Data Table 1. Although the voltage window used for was observed for hydrated ruthenium oxide27. The electrodes showed testing is relatively narrow, it can be expanded by conducting tests in other no measurable capacitance losses even after 10,000 cycles (Fig. 3c). Cou- types of electrolytes, such as neutral aqueous and organic electrolytes, or lombic efficiency is close to 100% (inset in Fig. 3c), confirming that the using MXenes as negative electrodes in asymmetric cell configurations. outstanding performance is not due to parasitic reactions. The good capacitive rate performance of the 75-mm-thick electrodes To quantify the capacitive and diffusion limited contributions to (Fig. 3e) is noteworthy, however, and demonstrates scalability and huge the total capacitances, we used the approach of ref. 28. The results of promise of MXenes for application as negative electrodes of hybrid

a 1,500 b c

1,000 Ti3C2Tx clay in 1M H2SO4 100 1,000 (this work) 0.3 ) 100 mV s–1 80 ) –3 800 500 –1 0.2 50 mV s –3 20 mV s–1 0 60 0.1 (F cm 10 mV s–1 600 0.0 5 mV s–1 Ti3C2Tx paper in –500 2 mV s–1 1M KOH (ref. 8) 40 –0.1 400 –1,000 –0.2 20 –0.3

Capacitance 200 Ti C T in various electrolytes (ref. 8) –1,500 3 2 x Capacitance (F cm 0102030405060 Potential, V versus Ag/AgCl

Capacitance retention (%) 0 Time (s) –2,000 0 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 1 2510 2050100 0 2,500 5,000 7,500 10,000 Potential, V versus Ag/AgCl Scan rate (mV s–1) Cycle number

d e f 0.4 μ 1,000 5 m 4 0.2 5 μm 70 μ μ 5 m 0.0 30 m μ ) 60 μ 3 30 m –0.2 –3 800 75 m C-2 mV s–1 ) –0.4 50 Ω (

–1 ) 2 mV s 30 μm ) 2

–0.6 600 Ω ( 40 )

3.0 Z μ –Im( Z 400 75 m 30 1 75 μm Current (mA) 1.5 –Im( 0.0 20 0 –1.5 200 Capacitance (F cm 01234 –1 10 –3.0 C-20 mV s Re(Z) (Ω) 20 mV s–1 –4.5 0 0 –0.3 –0.2 –0.1 0.0 0.1 0.2 1 2510 2050100 0102030405060708090100 Potential, V versus Ag/AgCl Scan rate (mV s–1) Re(Z) (Ω) Figure 3 | Electrochemical performance of rolled, free-standing electrodes. and 20 mV s21 with hatched portions of the contributions of the processes not a, Cyclic voltammetry profiles at different scan rates for a 5-mm-thick electrode limited by diffusion, that is, capacitive (‘C-’); vertical lines limit the cyclic in 1 M H2SO4. b, Comparison of rate performances reported in this work voltammetry area used in calculations. e, f, Rate performance (e) and and previously for HF-produced MXene8. c, Capacitance retention test of a electrochemical impedance spectroscopy data (f)of5-mm-thick (red stars), 5-mm-thick rolled electrode in 1 M H2SO4. Inset shows galvanostatic cycling 30-mm-thick (black circles) and 75-mm-thick (olive triangles) rolled electrodes. data collected at 10 A g21. d, Cyclic voltammetry profiles collected at 2 mV s21 The inset in f shows the magnified high-frequency region.

80 | NATURE | VOL 516 | 4 DECEMBER 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH large-scale energy storage devices. Electrodes of that thickness cannot 10. Ghaffari, M. et al. High-volumetric performance aligned nano-porous microwave exfoliated graphite oxide-based electrochemical capacitors. Adv. Mater. 25, be produced by filtration and the MXene clay-like characteristics add 4879–4885 (2013). important versatility to electrode manufacturing, allowing films of the 11. Tao, Y. et al. Towards ultrahigh volumetric capacitance: graphene derived highly required thicknesses to be rolled. Note that the capacitance values reported dense but porous carbons for supercapacitors. Sci. Rep. 3, 2975 (2013). herein are still preliminary. As better understanding of how the films’ 12. Jung, I., Dikin, D. A., Piner, R. D. & Ruoff, R. S. Tunable electrical conductivity of individual graphene oxide sheets reduced at ‘‘low’’ temperatures. Nano Lett. 8, morphologies affect their capacitances is gained, enhancements in the 4283–4287 (2008). latter should ensue. 13. Barsoum, M. W. MAX Phases: Properties of Machinable Ternary Carbides and Nitrides In terms of versatility, the LiF 1 HCl solution was also capable of (John Wiley & Sons, 2013). 14. Naguib, M., Mochalin, V. N., Barsoum, M. W. & Gogotsi, Y. MXenes: a new family of etching other MAX phases, for example, Nb2AlC and Ti2AlC. In the case two-dimensional materials. Adv. Mater. 26, 982 (2014). of Ti2AlC, we delaminated the multilayer powders in a similar fashion 15. Xie, X. et al. Surface Al leached Ti3AlC2 substituting carbon for catalyst support to Ti3C2Tx to produce suspensions of Ti2CTx flakes, as well as Ti2CTx served in a harsh corrosive electrochemical system. Nanoscale 6, 11035–11040 ‘paper’, which had not been previously reported. These considerations (2014). 16. Peng, Q. et al. Unique lead adsorption behavior of activated hydroxyl group in hint at the potential of this new etching method for the synthesis ofother two-dimensional titanium carbide. J. Am. Chem. Soc. 136, 4113–4116 (2014). MXenes, which will be explored in future studies. 17. Tang, Q., Zhou, Z. & Shen, P. Are MXenes promising anode materials for Li ion This method of MXene production was successful to varying degrees batteries? computational studies on electronic properties and Li storage capability of Ti3C2 and Ti3C2X2 (X 5 F, OH) monolayer. J.Am.Chem.Soc.134, 16909–16916 for other fluoride salts, such as NaF, KF, CsF, tetrabutylammonium fluo- (2012). ride, and CaF2 in HCl, all of which showed similar etching behaviour. 18. Mashtalir, O. et al. Intercalation and delamination of layered carbides and When H2SO4 was used instead of HCl, MXenes were still obtained. We carbonitrides. Nature Commun. 4, 1716 (2013). note here that these systems are options and merit further study; the 19. Halim, J. et al. Transparent conductive two-dimensional titanium carbide epitaxial thin films. Chem. Mater. 26, 2374–2381 (2014). ability to fine tune the reaction based on reagents used will indubitably 20. Chang, F., Li, C., Yang, J., Tang, H. & Xue, M. Synthesis of a new graphene-like lead to potentially useful variations in compositions and properties, espe- transition metal carbide by de-intercalating Ti3AlC2. Mater. Lett. 109, 295–298 cially since itis reasonableto assume that different acids and salts should (2013). modify the surface chemistries and pre-intercalate different ions. 21. Enyashin, A. N. & Ivanovskii, A. L. Two-dimensional titanium carbonitrides and their hydroxylated derivatives: structural, electronic properties and stability of In summary, a new high-yield method for MXene synthesis that is MXenes Ti3C22xNx(OH)2 from DFTB calculations. J. Solid State Chem. 207, 42–48 safer, easier, and provides a faster route to delaminated flakes has been (2013). detailed. This method yields a clay-like material (for a discussion of the 22. Madsen, F. T. & Mu¨ ller-Vonmoos, M. The swelling behaviour of clays. Appl. Clay Sci. 4, 143–156 (1989). effect of experimental conditions on properties, see Methods), which 23. Hensen, E. J. & Smit, B. Why clays swell. J. Phys. Chem. B 106, 12664–12667 can be shaped to give conductive solids of desired forms, or rolled into (2002). thin sheets, for a host of applications. When the rolled films were used 24. Lis, D., Backus, E. H. G., Hunger, J., Parekh, S. H. & Bonn, M. Liquid flow along a solid surface reversibly alters interfacial chemistry. Science 344, 1138–1142 (2014). as supercapacitor electrodes in a H2SO4 electrolyte, the performances 23 25. Mashtalir, O., Naguib, M., Dyatkin, B., Gogotsi, Y. & Barsoum, M. W. Kinetics of were extraordinary, with volumetric capacitances up to 900 F cm or aluminum extraction from Ti AlC in hydrofluoric acid. Mater. Chem. Phys. 139, 21 3 2 245 F g . When it is further appreciated that these numbers are ‘first- 147–152 (2013). generation’ numbers that will no doubtincrease aswe better understand 26. Conway, B. Electrochemical capacitors based on pseudocapacitance. In Electrochemical Supercapacitors: Scientific Fundamentals and Technological the underlying processes and modify the material structure and chem- Applications (Kluwer Academic/Plenum, 1999). istry, the potential of these non-oxide two-dimensional materials to 27. Dmowski, W., Egami, T., Swider-Lyons, K. E., Love, C. T. & Rolison, D. R. Local atomic push electrochemical energy storage to new heights is clear. structure and conduction mechanism of nanocrystalline hydrous RuO2 from X-ray scattering. J. Phys. Chem. B 106, 12677–12683 (2002). Online Content Methods, along with any additional Extended Data display items 28. Wang, J., Polleux, J., Lim, J. & Dunn, B. Pseudocapacitive contributions to and Source Data, are available in the online version of the paper; references unique electrochemical energy storage in TiO2 (anatase) nanoparticles. J. Phys. Chem. C to these sections appear only in the online paper. 111, 14925–14931 (2007). 29. Levi, M. D. et al. Solving the capacitive paradox of 2D MXene by electrochemical Received 8 August; accepted 13 October 2014. quartz-crystal admittance and in situ electronic conductance measurements. Adv. Energy Mater. http://dx.doi.org/10.1002/aenm.201400815 (2014). Published online 26 November 2014. 30. Simon, P., Gogotsi, Y. & Dunn, B. Where do batteries end and supercapacitors begin? Science 343, 1210–1211 (2014). 1. Simon, P. & Gogotsi, Y. Materials for electrochemical capacitors. Nature Mater. 7, 845–854 (2008). 1 Acknowledgements We thank O. Mashtalir and Z. Ling for help with material 2. Augustyn, V. et al. High-rate electrochemical energy storage through Li characterization. This work was supported by the US National Science Foundation intercalation pseudocapacitance. Nature Mater. 12, 518–522 (2013). under grant number DMR-1310245. Electrochemical research was supported by the 3. Gogotsi, Y. & Simon, P. True performance metrics in electrochemical energy Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier storage. Science 334, 917–918 (2011). Research Center funded by the US Department of Energy, Office of Science, and Office 4. Murali, S. et al. Volumetric capacitance of compressed activated microwave- of Basic Energy Sciences. XRD, X-ray photoelectron spectroscopy, SEM and TEM expanded graphite oxide (a-MEGO) electrodes. Nano Energy 2, 764–768 (2013). investigations were performed at the Centralized Research Facilities at Drexel 5. Yang, X., Cheng, C., Wang, Y., Qiu, L. & Li, D. Liquid-mediated dense integration of University. graphene materials for compact capacitive energy storage. Science 341, 534–537 (2013). Author Contributions M.G. conducted material synthesis and XRD analysis. M.R.L. 6. Zheng, J. P., Cygan, P. J. & Jow, T. R. Hydrous ruthenium oxide as an electrode performed electrochemical measurements and SEM analysis. M.-Q.Z. performed TEM material for electrochemical capacitors. J. Electrochem. Soc. 142, 2699–2703 analysis. M.W.B. and Y.G. planned and supervised the research. M.R.L., M.G., M.W.B. and (1995). Y.G. wrote the manuscript. 7. Naguib, M. et al. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv. Mater. 23, 4248–4253 (2011). Author Information Reprints and permissions information is available at 8. Lukatskaya, M. R. et al. Cation intercalation and high volumetric capacitance of www.nature.com/reprints. The authors declare no competing financial interests. two-dimensional titanium carbide. Science 341, 1502–1505 (2013). Readers are welcome to comment on the online version of the paper. Correspondence 9. Nicolosi, V., Chhowalla, M., Kanatzidis, M. G., Strano, M. S. & Coleman, J. N. Liquid and requests for materials should be addressed to M.B. ([email protected]) or exfoliation of layered materials. Science 340, 6139 (2013). Y.G. ([email protected]).

21 21 METHODS from 1 mV s to 100 mV s . Electrochemical impedance spectroscopy was performed at open circuit potential, with a 10-mV amplitude, and frequencies that Synthesis of Ti3AlC2. The MAX phase used as precursor for MXene synthesis ranged from 10 mHz to 200 kHz. Galvanostatic cycling was performed at 1 A g21 herein—Ti3AlC2—was prepared by mixing commercial Ti2AlC powders (Kanthal, and 10 A g21 between the potential limits of 20.3 V to 0.25 V versus Ag/AgCl. Capac- Sweden) with TiC in a 1:1 molar ratio (after adjusting for the ,12 wt% Ti3AlC2 already present in the commercial powder), followed by ball milling for 18 h. The itance data reported in the article were calculated from the slope of the discharge mixture was then heated at 5 uC min21, under flowing argon (Ar) in a tube furnace curve. for 2 h at 1,350 uC. The resulting lightly sintered brick was ground with a TiN-coated Characterization of structure and properties. XRD patterns were recorded with ˚ milling bit and sieved through a 400 mesh sieve producing powder with particle a powder diffractometer (Rigaku SmartLab) using Cu Ka radiation (l 5 1.54 A)with size less than 38 mm. 0.2u 2h steps and 0.5 s dwelling time. Scanning electron microscopy was performed on a Zeiss Supra 50VP (Carl Zeiss Synthesis of Ti3C2Tx MXene. Concentrated HCl (Fisher, technical grade), was added to distilled water to prepare a 6 M solution (30 ml total). 1.98 g (5 molar equiv- SMT AG, Oberkochen, Germany) equipped with an energy-dispersive spectroscope alents) of LiF (Alfa Aesar, 981%) was added to this solution. The mixture was stirred (Oxford EDS, with INCA software). Most energy-dispersive spectroscope scans for 5 min with a magnetic Teflon stir bar to dissolve the salt. were obtained at low magnification (1003 to 2003) at random points of pow- dered samples. Three grams of Ti3AlC2 powders were carefully added over the course of 10 min to avoid initial overheating of the solution as a result of the exothermic nature of Transmission electron microscopy of the MXene flakes was performed on a the reactions. The reaction mixture was then held at 40 uC for 45 h, after which the JEM-2100 (JEOL, Japan) using an accelerating voltage of 200 kV. The TEM sam- mixture was washed through ,5 cycles of distilled water addition, centrifugation ples were prepared by dropping two drops of diluted colloidal solution of MXene (3,500 r.p.m. 3 5 min for each cycle), and decanting, until the supernatant reached flakes onto a copper grid and drying in air. The flake size and number of layers per a pH of approximately 6. The final product, with a small amount of water, was filtered flake distributions were obtained through statistical analysis of more than 300 MXene on cellulose nitrate (0.22 mm pore size). At this stage, the filtrate exhibited clay-like flakes in the TEM images. properties and could be directly processed into films by rolling. Resistivity measurements were performed with a 4-point probe (ResTest v1, Preparation of Ti3C2Tx ‘paper’. The Ti3C2Tx flakes were dispersed in distilled Jandel Engineering, UK). Measured resistivity was automatically multiplied by the water (2 g MXene per 0.5 litre of water), deaerated with Ar, followed by sonication proper thickness correction factor given by the Jandel software. for 1 h. The mixture was then centrifuged for 1 h at 3,500 r.p.m., and the superna- Temperatureand timein theMXene synthesis. We found that reaction conditions tant,which was dark green in colour, was collected. This dispersion was filtered using of 35 uC for 24 h rather than 40 uC for 45 h produced a material with persistent a membrane (3501 Coated PP, Celgard, USA) to yield flexible, freestanding Ti3C2Tx MAX-phase peaks in the XRD patterns, and higher Al content revealed by energy- paper. The weight percentage of MXene delaminated into stable suspension in this dispersive spectroscopy, but that gave reliable high yields of delaminated flakes case was around 45 wt%. upon sonication. The Ti3AlC2 etched at higher temperatures showed lower Al con- Ti3C2Tx clay electrodes. Preparation of the clay electrodes is depicted step-by-step tent but did not always readily delaminate and disperse by sonication. in Extended Data Fig. 1. The dried and crushed Ti3C2Tx powder is hydrated to the Capacitance calculations. The volumetric capacitance determined from the cyclic consistency of a thick paste, roughly two parts powder to one part water (Extended voltammetry data is given by: ð Data Fig. 1a–c), which turns it into a plastic, clay-like state, that can be formed and 1 jdV moulded. The ‘clay’ is then rolled using a roller mill with water-permeable Celgard C~ ð1Þ DV s sheets (Extended Data Fig. 1d) on either side, resulting in the formation of a freestanding film (Extended Data Fig. 1e), which was readily lifted off the membrane and the volumetric capacitance determined from the galvanostatic charge/dis- upon drying (Extended Data Fig. 1f). charge data is given by: Activated carbon electrodes. The activated carbon electrodes were prepared by C~ðÞjt =V ð2Þ mechanical processing of a pre-mixed slurry, containing ethanol (190 proof, Decon 2 Laboratories), YP-50 activated carbon powder (Kuraray, Japan), and polytetrafluo- where C is the normalized capacitance (in units of F cm 3), j is the current density 23 21 roethynene (PTFE) binder (60 wt% in H2O, Sigma Aldrich). The resulting compo- (in A cm ), s is the rate (in V s ), V is the voltage (in V), DV is the voltage window 2 sition of the activated carbon electrodes was 95 wt% activated carbon and 5 wt% (in V) and t is time (in s). Calculations of the gravimetric capacitance (in F g 1)were 2 PTFE. They had thicknesses that varied between 100 mm and 150 mm; the mass den- performed using current density per electrode weight (in A g 1). sities per unit area were in the 10–25 mg cm22 range. Analysis of the limiting processes for charge storage. To quantify the diffusion- Electrochemical setup. All electrochemical measurements were performed in three- limited contribution to capacitance, the relationship between the current i(V) (at a electrode Swagelok cells, in which MXene served as the working electrode, over- given voltage V, in units of mA) and scan rate u(in units of V s21), was assumed to be27: capacitive activated carbon films were used as the counter electrode, and Ag/AgCl iVðÞ~k uzk u0:5 in 1 M KCl was the reference electrode. Two layers of the Celgard membranes were 1 2 used as separators. The electrolyte was 1 M H2SO4 (Alfa Aesar, American Chemical where k1 and k2 are constants. For the cyclic voltammetry experiments, at scan rates Society grade). from 1 mV s21 to 20 mV s21, current values were extracted, and i/u0.5 versus u0.5 0.5 0.5 Electrochemical measurements. Cyclic voltammetry, electrochemical impedance was plotted at each voltage and linear fitting was performed: i(V)/u 5 k1u 1 k2. spectroscopy, and galvanostatic cycling were performed using a VMP3 potentiostat The slope k1, for each voltage, describes the contributions of the non-diffusion con- (Biologic, France). Cyclic voltammetry was performed using scan rates that ranged trolled processes to the overall process.

Extended Data Figure 1 | Processing of MXene clay. a, Dried and crushed powder. b, c, Hydrated clay is plastic and can be readily formed and moulded. d, Demonstration of films produced in the roller mill. e, f, Rolled freestanding film being lifted off Celgard membranes.

Extended Data Figure 2 | SEM images. a, Multilayer MXene particle. b, Cross-section of rolled Ti3C2 film, showing shearing that is most probably responsible for the loss of the 60u angle peak in the XRD pattern.

Extended Data Figure 3 | Contact angle. Digital image showing contact angle of a water droplet on rolled MXene film, indicating its hydrophilic surface.

Extended Data Figure 4 | TEM characterization of dispersed Ti3C2Tx flakes. flakes. f, Statistical analysis of layer number distribution of dispersed Ti3C2Tx a, Representative TEM image showing the morphology and size of a large flakes. Note that the fractions of double- and few-layer flakes are overestimated single-layer Ti3C2Tx flake. Note folding on all sides of this large flake. b,The owing to inevitable restacking and edge folding of single-layer flakes during lateral size distribution of the dispersed Ti3C2Tx flakes. c–e, Representative preparation of samples for TEM analysis. Edge folding is clearly seen in a. TEM images showing single-layer (c), double-layer (d) and triple-layer (e) An example of restacking is shown in Extended Data Fig. 5.

Extended Data Figure 5 | TEM image showing the restacking of single- or double-layer MXene flakes into few-layer MXene.

Extended Data Figure 6 | Gravimetrically normalized capacitance. rate performances of rolled electrodes, 5 mm thick (black squares), 30 mm thick Cyclic voltammetry profiles at different scan rates for 5-mm-thick (a), (red circles) and 75 mm thick (blue triangles). 30-mm-thick (b) and 75-mm-thick (c) electrodes in 1 M H2SO4. d, Gravimetric

Extended Data Table 1 | Effect of film thickness and scan rate on mass- and volume-normalized capacitance values