Conductive two-dimensional titanium carbide ‘clay’with high volumetric capacitanceMichael Ghidiu1*, Maria R. Lukatskaya1*, Meng-Qiang Zhao1, Yury Gogotsi1 & Michel W. Barsoum1
Safe and powerful energy storage devices are becoming increasinglyimportant. Charging times of seconds to minutes, with power den-sities exceeding those of batteries, can in principle be provided byelectrochemical capacitors—in particular, pseudocapacitors1,2. Recentresearch has focused mainly on improving the gravimetric perform-ance of the electrodes of such systems, but for portable electronicsand vehicles volume is at a premium3. The best volumetric capaci-tances of carbon-based electrodes are around 300 farads per cubiccentimetre4,5; hydrated ruthenium oxide can reach capacitances of1,000 to 1,500 farads per cubic centimetre with great cyclability, butonly in thin films6. Recently, electrodes made of two-dimensionaltitanium carbide (Ti3C2, a member of the ‘MXene’ family), producedby etching aluminium from titanium aluminium carbide (Ti3AlC2, a‘MAX’ phase) in concentrated hydrofluoric acid, have been shown tohave volumetric capacitances of over 300 farads per cubic centimetre7,8.Here we report a method of producing this material using a solutionof lithium fluoride and hydrochloric acid. The resulting hydrophilicmaterial swells in volume when hydrated, and can be shaped like clayand dried into a highly conductive solid or rolled into films tens ofmicrometres thick. Additive-free films of this titanium carbide ‘clay’have volumetric capacitances of up to 900 farads per cubic centimetre,with excellent cyclability and rate performances. This capacitance isalmost twice that of our previous report8, and our synthetic methodalso offers a much faster route to film production as well as the avoid-ance of handling hazardous concentrated hydrofluoric acid.
In the search for new electrode materials, two-dimensional solids areof particular interest owing to their large electrochemically active surfaces9.For example, activated graphene electrodes have capacitances of 200–350 F cm23 compared to 60–100 F cm23 for activated porous carbons10,11.Yet graphene is limited to the chemistry of carbon, does not tap intometal redox reactions as in ruthenium oxide (RuO2) (ref. 6), and itsconductivity is substantially decreased by the addition of redox-activefunctional groups12. MXenes (of the formula Mn 1 1XnTx, where M is atransition metal, X is C and/or N, and Tx denotes surface functionali-zation) are a relatively young class of two-dimensional solids, producedby the selective etching of the A-group (generally group IIIA and IVAelements) layers from the MAX phases, which comprise a .70-memberfamily of layered, hexagonal early-transition-metal carbides and nitrides13.To date, all MXenes have been produced by etching MAX phases inconcentrated hydrofluoric acid (HF)14–16.
MXenes have already proved to be promising candidates for elec-trodes in lithium (Li)-ion batteries17,18 and supercapacitors8, exhibitingvolumetric capacitances that exceed most previously reported materials.However, the path to electrode manufacturing required the handling ofconcentrated HF and a laborious multi-step procedure. Here we soughta safer route by exploiting the reaction between common, inexpensivehydrochloric acid (HCl) and fluoride salts, leading to dissolution of alu-minium and the extraction of two-dimensional carbide layers. Further-more, given the ability of MXenes to preferentially intercalate cations(post-synthesis)8, a related question was whether etching and intercala-tion might be achieved in a single step, as was observed for etching of
thin Ti3AlC2 films with ammonium bifluoride19. The change in MXeneproperties upon intercalation and the compositional variability of fluo-ride salts suggested the possibility of a one-step procedure for the syn-thesis of many MXenes, with tunable structures and properties.
The MXenes reported in this study were prepared by dissolving LiFin 6 M HCl, followed by the slow addition of Ti3AlC2 powders and heat-ing of the mixture at 40uC for 45 h. After etching, the resulting sedimentswere washed to remove the reaction products and raise the pH (severalcycles of water addition, centrifugation and decanting). The resultingsediment formed a clay-like paste that could be rolled, when wet (Fig. 1a),between water-permeable membranes in a roller mill to produce flex-ible, free-standing films (Fig. 1c) in a matter of minutes, in contrast tothose previously produced by the laborious technique of intercalation,delamination, and filtration18.
A graphical depiction of the processing is provided in Extended DataFig. 1. Further, scaling was not limited to the size of the filtration appa-ratus; films of any dimensions could readily be produced. Additionally,when wet, the ‘clay’ could be moulded and dried to yield various shapesthat were highly conductive (Fig. 1d). Diluted, it could also be used as anink to deposit (print) MXene on various substrates. Like clay, the mate-rial could be rehydrated, swelling in volume, and shrinking when dried(Fig. 1b).
Energy-dispersive spectroscopy confirmed that aluminium (Al) wasremoved, and X-ray diffraction (XRD) revealed the disappearance ofTi3AlC2 peaks (traces can be seen in the case of incomplete transforma-tion). Multilayer particles did not show the accordion-like morphologyseen in HF-etched MXenes reported to date14,20; rather, particles appearedtightly stacked, presumably as a result of water and/or cationic interca-lation (see Extended Data Fig. 2a). Fluorine and oxygen were observedin energy-dispersive spectroscopy; this, coupled with X-ray photoelec-tron spectroscopy showing evidence of Ti–F and Ti–O bonding, sug-gests O- and F-containing surface terminations, as has been discussedat length for HF-produced MXenes14,21. The yield of MXene after etch-ing, calculated as described previously14, is around 100%, which is com-parable with the HF-etching method. Our new method thus does notlead to material losses, although an accurate yield determination is dif-ficult owing to the variability of surface groups and amount of inter-calated water.
XRD patterns of the etched material, in its air-dried multilayeredstate, showed a remarkable increase in the intensity and sharpness of the(000l) peaks (Fig. 2a, pink); in some cases the full width at half maxi-mum (FWHM) was as small as 0.188u, as opposed to the broad peakstypical of HF-etched MXene7, and more typical of intercalated MXenes18.Further, compared to a lattice parameter of c < 20 A for HF-producedTi3C2Tx, the corresponding value in this work was 27–28 A. XRD pat-terns of still-hydrated sediment showed shifts to even higher spacings:lattice parameters as high as c < 40 A have been measured. These largeshifts are suggestive of the presence of water, and possibly cations, betweenthe hydrophilic and negatively charged MXene sheets. From these sub-stantial increases in c and the clay-like properties (see below), it is reason-able to assume that—as in clays22,23—the swelling is due to the intercalation
*These authors contributed equally to this work.
1Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA.
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of multiple layers of water and possibly cations between the MXenesheets. Interfacial water has a more structured hydrogen-bonding net-work than bulk H2O (ref. 24). The MXene surface, holding a negativeelectric charge, may act to align the dipoles of water molecules betweenMXene layers.
When the ‘clay’ was rolled into freestanding films, XRD patterns againshowed strong ordering in the c direction (Fig. 2a, blue). Films, rangingin thicknesses from submicrometre to about 100mm, were readily pro-duced by this method. The most compelling evidence for particle shearingis the marked intensity decrease of the (110) peak around 61u, indi-cating a reduction of ordering in the non-basal directions while orderin the c direction was maintained (see blue XRD pattern in Fig. 2a andscanning electron microscopy (SEM) image in Extended Data Fig. 2b).Morphologically, the thinner films showed more overall shearing of themultilayer particles when viewed in cross-section (Fig. 2e, f) and exhib-ited substantial flexibility, even when allowed to dry thoroughly (insetto Fig. 2e). The contact angle of water on the rolled MXene film was mea-sured as 21.5u, confirming its hydrophilic nature (Extended Data Fig. 3).Attempts to hydrate and roll HF-produced MXene were unsuccessful;we propose that the intercalated water acts as a lubricant that allowsfacile shearing.
The c parameter expansion also resulted in the weakening of inter-actions between the MXene layers, as evidenced by the easy delamina-tion of multilayered particles by sonication, as is done for van der Waalssolids9. In our previous work, typical sonication times for delamination(after post-synthesis intercalation with dimethyl sulphoxide) were of theorder of 4 h (ref. 18). Here, sonication times of the order of 30–60 minresulted in stable suspensions with concentrations as high as 2 g per litre,higher than observed previously. Remarkably, the yield from multilayerto dispersed flakes was about 45% by mass. Freestanding films were alsoreadily fabricated by filtering these suspensions, as reported previously8.
The fact that the LiF 1 HCl etchant was much milder than HF resultedin flakes with larger lateral dimensions (Fig. 2b) that did not contain thenanometre-size defects frequently observed in HF-etched samples25.Transmission electron microscopy (TEM) analysis showed that, of 321flakes analysed, over 70% had dimensions of 0.5–1.5 mm (ExtendedData Fig. 4a, b). Single layers about 10 A thick were imaged using TEM(Fig. 2c, d), confirming that the material is indeed two-dimensional.Analysis of 332 flakes suggested that roughly 70% of the flakes were 1–2layers thick (Extended Data Fig. 4c–f). We note that, since the restackingor folding of flakes can lead to higher apparent thicknesses (ExtendedData Fig. 5), the 70% estimate is conservative. Thus, using this method,
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Ti3AlC2 Ti3C2Tx ‘Clay’ Electrode
Etching Washing Rolling Shapinga
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Figure 1 | Schematic of MXene clay synthesisand electrode preparation. a, MAX phase isetched in a solution of acid and fluoride salt(step 1), then washed with water to remove reactionproducts and raise the pH towards neutral (step 2).The resulting sediment behaves like a clay; it canbe rolled to produce flexible, freestanding films(step 3), moulded and dried to yield conductingobjects of desired shape (step 4), or diluted andpainted onto a substrate to yield a conductivecoating (step 5). b, When dried samples (left,showing cross-section and top view) are hydrated(right) they swell; upon drying, they shrink.c, Image of a rolled film. d, ‘Clay’ shaped into theletter M (,1 cm) and dried, yielding a conductivesolid (labelled with the experimental conductivityof ‘clay’ rolled to 5 mm thickness). The etchedmaterial is referred to as Ti3C2Tx, where the Tdenotes surface terminations, such as OH, O and F.
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Figure 2 | Structural characterization of MXene.a, XRD patterns of samples produced by etchingin LiF 1 HCl solution. The pink trace is formultilayer Ti3C2Tx, showing a sharp, intensepeak (0002) and higher-order (000l) peaks,corresponding to a c lattice parameter of 28 Aand high order in the c direction. The blue traceis for the same sample after rolling into anapproximately 40-mm-thick film; c-direction peaksare preserved, but the prominent (110) peak isno longer observed, showing substantial reductionof order in non-basal directions. In both cases,traces of Ti3AlC2 are still present (red diamonds).The MXene (0002) peak is at a much lower anglethan that typical of MXene produced by HF (greenstar). b, TEM image of several flakes, showinglateral sizes up to a few hundred nanometres. Fewdefective areas are present. The inset shows theoverall selected area electron diffraction pattern.c, d, TEM images of single- and double-layer flakes,respectively. Insets show sketches of these layers.e, SEM image of a fracture surface of a ,4-mm-thick film produced by rolling, showing shearing oflayers; the flexibility of the film is demonstrated inthe inset. f, Fracture surface of a thicker rolledfilm (,30mm), showing poorer overall alignmentof flakes in the interior of the film.
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large fractions of single-layered MXene flakes with high yields, largelateral sizes, and good quality can be readily produced. The flake lateralsizes reported here are larger than those reported for HF-etched Ti3AlC2
(ref. 18); the milder delamination conditions may be partially respon-sible for this.
Previously we have shown that MXene ‘paper’—made by filtration ofsolutions containing delaminated Ti3C2Tx flakes—exhibited volumetriccapacitances of ,350 F cm23 at 20 mV s21 (and 450 F cm23 at 2 mV s21)in potassium hydroxide (KOH) electrolyte8. For comparison, we charac-terized the electrochemical performances of rolled, freestanding Ti3C2Tx
films in 1 M sulphuric acid (H2SO4). The advantages of acidic electro-lytes include not only their excellent conductivities but also that protons,being the smallest cations, are known to allow for surface redox reactionsin transition-metal oxide electrodes, such as RuO2, MnO2 and someothers, and may contribute to the capacitance via fast surface redox1,26.
At a scan rate of 2 mV s21, capacitance values reached 900 F cm23
(Fig. 3a) and a good rate handling ability was observed (Fig. 3b). Theresults—summarized and compared with previous work8 in Fig. 3b—clearly show that rolled Ti3C2Tx clay electrodes show outstanding capac-itive performance, not only volumetrically but gravimetrically as well,achieving 245 F g21 at 2 mV s21. This can be ascribed to the smaller sizeof H1 compared to other intercalating cations, surface redox processes,and improved accessibility of interlayer spacing in LiF 1 HCl-etchedMXene owing to pre-intercalated water, compared to the previouslystudied HF-etched samples. It is worth noting that a similar positiverole of structural water for capacitive performance in acidic electrolyteswas observed for hydrated ruthenium oxide27. The electrodes showedno measurable capacitance losses even after 10,000 cycles (Fig. 3c). Cou-lombic efficiency is close to 100% (inset in Fig. 3c), confirming that theoutstanding performance is not due to parasitic reactions.
To quantify the capacitive and diffusion limited contributions tothe total capacitances, we used the approach of ref. 28. The results of
this analysis—summarized in Fig. 3d—show that, at scan rates below20 mV s21, there is a noticeable, yet not prevailing, contribution ofdiffusion-limited processes to the total capacitance. At scan rates of20 mV s21 and higher, the response is not diffusion-controlled but israther due to surface capacitive effects, whether electrostatic or pseu-docapacitive. This observation is in agreement with the conjecture ofLevi et al.29 about the presence of shallow- and deep-trap sites in MXenestructures. Further, if there are also redox contributions from changesin the oxidation states of surface Ti atoms28, the redox processes are notdiffusion-limited, and thus represent ‘intrinsic’ capacitive behaviour30.
When the electrochemical responses of three rolled clay electrodes(5mm thick, 30mm thick and 75mm thick) were compared (Fig. 3e, f),not surprisingly, the volumetric capacitances decreased with increasedthickness. These thickness-dependent differences can be partially tracedto the electrode morphologies. As noted above, electrodes thinner than10mm showed good flake alignment (Fig. 2e) with typical densities of3.6–3.8 g cm23. At 2.2–2.8 g cm23, the densities of the thicker (15mmand larger) rolled electrodes were lower, which is a reflection of the factthat their core seemed to be more open (Fig. 2f). And while the lowerdensities led to lower volumetric capacitances, their more open struc-ture ensured accessibility to ions and thus similar rate performances astheir thinner counterparts (Fig. 3e, f). The lower densities also ensuredthat the drop in gravimetric capacitances with thickness (see ExtendedData Fig. 6) was not substantial. A summary of key mass- and volume-normalized capacitance values as a function of electrode thickness is pro-vided in Extended Data Table 1. Although the voltage window used fortesting is relatively narrow, it can be expanded by conducting tests in othertypes of electrolytes, such as neutral aqueous and organic electrolytes, orusing MXenes as negative electrodes in asymmetric cell configurations.
The good capacitive rate performance of the 75-mm-thick electrodes(Fig. 3e) is noteworthy, however, and demonstrates scalability and hugepromise of MXenes for application as negative electrodes of hybrid
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Figure 3 | Electrochemical performance of rolled, free-standing electrodes.a, Cyclic voltammetry profiles at different scan rates for a 5-mm-thick electrodein 1 M H2SO4. b, Comparison of rate performances reported in this workand previously for HF-produced MXene8. c, Capacitance retention test of a5-mm-thick rolled electrode in 1 M H2SO4. Inset shows galvanostatic cyclingdata collected at 10 A g21. d, Cyclic voltammetry profiles collected at 2 mV s21
and 20 mV s21 with hatched portions of the contributions of the processes notlimited by diffusion, that is, capacitive (‘C-’); vertical lines limit the cyclicvoltammetry area used in calculations. e, f, Rate performance (e) andelectrochemical impedance spectroscopy data (f) of 5-mm-thick (red stars),30-mm-thick (black circles) and 75-mm-thick (olive triangles) rolled electrodes.The inset in f shows the magnified high-frequency region.
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large-scale energy storage devices. Electrodes of that thickness cannotbe produced by filtration and the MXene clay-like characteristics addimportant versatility to electrode manufacturing, allowing films of therequired thicknesses to be rolled. Note that the capacitance values reportedherein are still preliminary. As better understanding of how the films’morphologies affect their capacitances is gained, enhancements in thelatter should ensue.
In terms of versatility, the LiF 1 HCl solution was also capable ofetching other MAX phases, for example, Nb2AlC and Ti2AlC. In the caseof Ti2AlC, we delaminated the multilayer powders in a similar fashionto Ti3C2Tx to produce suspensions of Ti2CTx flakes, as well as Ti2CTx
‘paper’, which had not been previously reported. These considerationshint at the potential of this new etching method for the synthesis of otherMXenes, which will be explored in future studies.
This method of MXene production was successful to varying degreesfor other fluoride salts, such as NaF, KF, CsF, tetrabutylammonium fluo-ride, and CaF2 in HCl, all of which showed similar etching behaviour.When H2SO4 was used instead of HCl, MXenes were still obtained. Wenote here that these systems are options and merit further study; theability to fine tune the reaction based on reagents used will indubitablylead to potentially useful variations in compositions and properties, espe-cially since it is reasonable to assume that different acids and salts shouldmodify the surface chemistries and pre-intercalate different ions.
In summary, a new high-yield method for MXene synthesis that issafer, easier, and provides a faster route to delaminated flakes has beendetailed. This method yields a clay-like material (for a discussion of theeffect of experimental conditions on properties, see Methods), whichcan be shaped to give conductive solids of desired forms, or rolled intothin sheets, for a host of applications. When the rolled films were usedas supercapacitor electrodes in a H2SO4 electrolyte, the performanceswere extraordinary, with volumetric capacitances up to 900 F cm23 or245 F g21. When it is further appreciated that these numbers are ‘first-generation’ numbers that will no doubt increase as we better understandthe underlying processes and modify the material structure and chem-istry, the potential of these non-oxide two-dimensional materials topush electrochemical energy storage to new heights is clear.
Online Content Methods, along with any additional Extended Data display itemsandSourceData, are available in the online version of the paper; references uniqueto these sections appear only in the online paper.
Received 8 August; accepted 13 October 2014.
Published online 26 November 2014.
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Acknowledgements We thank O. Mashtalir and Z. Ling for help with materialcharacterization. This work was supported by the US National Science Foundationunder grant number DMR-1310245. Electrochemical research was supported by theFluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy FrontierResearch Center funded by the US Department of Energy, Office of Science, and Officeof Basic Energy Sciences. XRD, X-ray photoelectron spectroscopy, SEM and TEMinvestigations were performed at the Centralized Research Facilities at DrexelUniversity.
Author Contributions M.G. conducted material synthesis and XRD analysis. M.R.L.performed electrochemical measurements and SEM analysis. M.-Q.Z. performed TEManalysis. M.W.B. and Y.G. plannedandsupervised the research. M.R.L., M.G.,M.W.B. andY.G. wrote the manuscript.
Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to comment on the online version of the paper. Correspondenceand requests for materials should be addressed to M.B. ([email protected]) orY.G. ([email protected]).
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METHODSSynthesis of Ti3AlC2. The MAX phase used as precursor for MXene synthesisherein—Ti3AlC2—was prepared by mixing commercial Ti2AlC powders (Kanthal,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. Themixture was then heated at 5 uC min21, under flowing argon (Ar) in a tube furnacefor 2 h at 1,350 uC. The resulting lightly sintered brick was ground with a TiN-coatedmilling bit and sieved through a 400 mesh sieve producing powder with particlesize less than 38mm.Synthesis of Ti3C2Tx MXene. Concentrated HCl (Fisher, technical grade), wasadded to distilled water to prepare a 6 M solution (30 ml total). 1.98 g (5 molar equiv-alents) of LiF (Alfa Aesar, 981%) was added to this solution. The mixture was stirredfor 5 min with a magnetic Teflon stir bar to dissolve the salt.
Three grams of Ti3AlC2 powders were carefully added over the course of 10 minto avoid initial overheating of the solution as a result of the exothermic nature ofthe reactions. The reaction mixture was then held at 40 uC for 45 h, after which themixture was washed through ,5 cycles of distilled water addition, centrifugation(3,500 r.p.m. 3 5 min for each cycle), and decanting, until the supernatant reacheda pH of approximately 6. The final product, with a small amount of water, was filteredon cellulose nitrate (0.22mm pore size). At this stage, the filtrate exhibited clay-likeproperties and could be directly processed into films by rolling.Preparation of Ti3C2Tx ‘paper’. The Ti3C2Tx flakes were dispersed in distilledwater (2 g MXene per 0.5 litre of water), deaerated with Ar, followed by sonicationfor 1 h. The mixture was then centrifuged for 1 h at 3,500 r.p.m., and the superna-tant, which was dark green in colour, was collected. This dispersion was filtered usinga membrane (3501 Coated PP, Celgard, USA) to yield flexible, freestanding Ti3C2Tx
paper. The weight percentage of MXene delaminated into stable suspension in thiscase was around 45 wt%.Ti3C2Tx clay electrodes. Preparation of the clay electrodes is depicted step-by-stepin Extended Data Fig. 1. The dried and crushed Ti3C2Tx powder is hydrated to theconsistency of a thick paste, roughly two parts powder to one part water (ExtendedData Fig. 1a–c), which turns it into a plastic, clay-like state, that can be formed andmoulded. The ‘clay’ is then rolled using a roller mill with water-permeable Celgardsheets (Extended Data Fig. 1d) on either side, resulting in the formation of a free-standing film (Extended Data Fig. 1e), which was readily lifted off the membraneupon drying (Extended Data Fig. 1f).Activated carbon electrodes. The activated carbon electrodes were prepared bymechanical processing of a pre-mixed slurry, containing ethanol (190 proof, DeconLaboratories), YP-50 activated carbon powder (Kuraray, Japan), and polytetrafluo-roethynene (PTFE) binder (60 wt% in H2O, Sigma Aldrich). The resulting compo-sition of the activated carbon electrodes was 95 wt% activated carbon and 5 wt%PTFE. They had thicknesses that varied between 100mm and 150mm; the mass den-sities per unit area were in the 10–25 mg cm22 range.Electrochemical setup. All electrochemical measurements were performed in three-electrode Swagelok cells, in which MXene served as the working electrode, over-capacitive activated carbon films were used as the counter electrode, and Ag/AgClin 1 M KCl was the reference electrode. Two layers of the Celgard membranes wereused as separators. The electrolyte was 1 M H2SO4 (Alfa Aesar, American ChemicalSociety grade).Electrochemical measurements. Cyclic voltammetry, electrochemical impedancespectroscopy, and galvanostatic cycling were performed using a VMP3 potentiostat(Biologic, France). Cyclic voltammetry was performed using scan rates that ranged
from 1 mV s21 to 100 mV s21. Electrochemical impedance spectroscopy was per-formed at open circuit potential, with a 10-mV amplitude, and frequencies thatranged from 10 mHz to 200 kHz. Galvanostatic cycling was performed at 1 A g21
and 10 A g21 between the potential limits of 20.3 V to 0.25 V versus Ag/AgCl. Capac-itance data reported in the article were calculated from the slope of the dischargecurve.Characterization of structure and properties. XRD patterns were recorded witha powder diffractometer (Rigaku SmartLab) using Cu Ka radiation (l 5 1.54 A) with0.2u 2h steps and 0.5 s dwelling time.
Scanning electron microscopy was performed on a Zeiss Supra 50VP (Carl ZeissSMT AG, Oberkochen, Germany) equipped with an energy-dispersive spectroscope(Oxford EDS, with INCA software). Most energy-dispersive spectroscope scanswere obtained at low magnification (1003 to 2003) at random points of pow-dered samples.
Transmission electron microscopy of the MXene flakes was performed on aJEM-2100 (JEOL, Japan) using an accelerating voltage of 200 kV. The TEM sam-ples were prepared by dropping two drops of diluted colloidal solution of MXeneflakes onto a copper grid and drying in air. The flake size and number of layers perflake distributions were obtained through statistical analysis of more than 300 MXeneflakes in the TEM images.
Resistivity measurements were performed with a 4-point probe (ResTest v1,Jandel Engineering, UK). Measured resistivity was automatically multiplied by theproper thickness correction factor given by the Jandel software.Temperature and time in the MXene synthesis. We found that reaction conditionsof 35 uC for 24 h rather than 40 uC for 45 h produced a material with persistentMAX-phase peaks in the XRD patterns, and higher Al content revealed by energy-dispersive spectroscopy, but that gave reliable high yields of delaminated flakesupon sonication. The Ti3AlC2 etched at higher temperatures showed lower Al con-tent but did not always readily delaminate and disperse by sonication.Capacitance calculations. The volumetric capacitance determined from the cyclicvoltammetry data is given by:
C~1DV
ðjdV
sð1Þ
and the volumetric capacitance determined from the galvanostatic charge/dis-charge data is given by:
C~ jtð Þ=V ð2Þwhere C is the normalized capacitance (in units of F cm23), j is the current density(in A cm23), s is the rate (in V s21), V is the voltage (in V),DV is the voltage window(in V) and t is time (in s). Calculations of the gravimetric capacitance (in F g21) wereperformed using current density per electrode weight (in A g21).Analysis of the limiting processes for charge storage. To quantify the diffusion-limited contribution to capacitance, the relationship between the current i(V) (at agiven voltage V, in units of mA) and scan rate u (in units of V s21), was assumed to be27:
i Vð Þ~k1uzk2u0:5
where k1 and k2 are constants. For the cyclic voltammetry experiments, at scan ratesfrom 1 mV s21 to 20 mV s21, current values were extracted, and i/u0.5 versus u0.5
was plotted at each voltage and linear fitting was performed: i(V)/u0.5 5 k1u0.5 1 k2.The slope k1, for each voltage, describes the contributions of the non-diffusion con-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 forthe loss of the 60u angle peak in the XRD pattern.
Extended Data Figure 3 | Contact angle. Digital image showing contact angleof a water droplet on rolled MXene film, indicating its hydrophilic surface.
Extended Data Figure 4 | TEM characterization of dispersed Ti3C2Tx flakes.a, Representative TEM image showing the morphology and size of a largesingle-layer Ti3C2Tx flake. Note folding on all sides of this large flake. b, Thelateral size distribution of the dispersed Ti3C2Tx flakes. c–e, RepresentativeTEM images showing single-layer (c), double-layer (d) and triple-layer (e)
flakes. f, Statistical analysis of layer number distribution of dispersed Ti3C2Tx
flakes. Note that the fractions of double- and few-layer flakes are overestimatedowing to inevitable restacking and edge folding of single-layer flakes duringpreparation of samples for TEM analysis. Edge folding is clearly seen in a.An example of restacking is shown in Extended Data Fig. 5.
Extended Data Figure 6 | Gravimetrically normalized capacitance.Cyclic voltammetry profiles at different scan rates for 5-mm-thick (a),30-mm-thick (b) and 75-mm-thick (c) electrodes in 1 M H2SO4. d, Gravimetric
rate performances of rolled electrodes, 5 mm thick (black squares), 30mm thick(red circles) and 75mm thick (blue triangles).
