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

Journal name:
Nature
Volume:
516,
Pages:
78–81
Date published:
DOI:
doi:10.1038/nature13970
Received
Accepted
Published online

Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors—in particular, pseudocapacitors1, 2. Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium3. The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre4, 5; hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films6. Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the ‘MXene’ family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a ‘MAX’ phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre7, 8. Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres 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 is almost twice that of our previous report8, and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.

At a glance

Figures

  1. Schematic of MXene clay synthesis and electrode preparation.
    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 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 coating (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 the letter M (~1 cm) and dried, yielding a conductive solid (labelled with the experimental conductivity of ‘clay’ rolled to 5 µm thickness). The etched material is referred to as Ti3C2Tx, where the T denotes surface terminations, such as OH, O and F.

  2. Structural characterization of MXene.
    Figure 2: Structural characterization of MXene.

    a, XRD patterns of samples produced by etching in LiF + HCl solution. The pink trace is for multilayer Ti3C2Tx, showing a sharp, intense peak (0002) and higher-order (000l) peaks, corresponding to a c lattice parameter of 28 Å and high order in the c direction. The blue trace is for the same sample after rolling into an approximately 40-µm-thick film; c-direction peaks are preserved, but the prominent (110) peak is no longer observed, showing substantial reduction of 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 angle 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 overall 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-µm-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 film (~30 µm), showing poorer overall alignment of flakes in the interior of the film.

  3. Electrochemical performance of rolled, free-standing electrodes.
    Figure 3: Electrochemical performance of rolled, free-standing electrodes.

    a, Cyclic voltammetry profiles at different scan rates for a 5-µm-thick electrode in 1 M H2SO4. b, Comparison of rate performances reported in this work and previously for HF-produced MXene8. c, Capacitance retention test of a 5-µm-thick rolled electrode in 1 M H2SO4. Inset shows galvanostatic cycling data collected at 10 A g−1. d, Cyclic voltammetry profiles collected at 2 mV s−1 and 20 mV s−1 with hatched portions of the contributions of the processes not limited by diffusion, that is, capacitive (‘C-’); vertical lines limit the cyclic voltammetry area used in calculations. e, f, Rate performance (e) and electrochemical impedance spectroscopy data (f) of 5-µm-thick (red stars), 30-µm-thick (black circles) and 75-µm-thick (olive triangles) rolled electrodes. The inset in f shows the magnified high-frequency region.

  4. Processing of MXene clay.
    Extended Data Fig. 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.

  5. SEM images.
    Extended Data Fig. 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 60° angle peak in the XRD pattern.

  6. Contact angle.
    Extended Data Fig. 3: Contact angle.

    Digital image showing contact angle of a water droplet on rolled MXene film, indicating its hydrophilic surface.

  7. TEM characterization of dispersed Ti3C2Tx flakes.
    Extended Data Fig. 4: TEM characterization of dispersed Ti3C2Tx flakes.

    a, Representative TEM image showing the morphology and size of a large single-layer Ti3C2Tx flake. Note folding on all sides of this large flake. b, The lateral size distribution of the dispersed Ti3C2Tx flakes. ce, Representative TEM 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 overestimated owing to inevitable restacking and edge folding of single-layer flakes during preparation of samples for TEM analysis. Edge folding is clearly seen in a. An example of restacking is shown in Extended Data Fig. 5.

  8. TEM image showing the restacking of single- or double-layer MXene flakes into few-layer MXene.
    Extended Data Fig. 5: TEM image showing the restacking of single- or double-layer MXene flakes into few-layer MXene.
  9. Gravimetrically normalized capacitance.
    Extended Data Fig. 6: Gravimetrically normalized capacitance.

    Cyclic voltammetry profiles at different scan rates for 5-µm-thick (a), 30-µm-thick (b) and 75-µm-thick (c) electrodes in 1 M H2SO4. d, Gravimetric rate performances of rolled electrodes, 5 µm thick (black squares), 30 µm thick (red circles) and 75 µm thick (blue triangles).

Tables

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

References

  1. Simon, P. & Gogotsi, Y. Materials for electrochemical capacitors. Nature Mater. 7, 845854 (2008)
  2. Augustyn, V. et al. High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance. Nature Mater. 12, 518522 (2013)
  3. Gogotsi, Y. & Simon, P. True performance metrics in electrochemical energy storage. Science 334, 917918 (2011)
  4. Murali, S. et al. Volumetric capacitance of compressed activated microwave-expanded graphite oxide (a-MEGO) electrodes. Nano Energy 2, 764768 (2013)
  5. Yang, X., Cheng, C., Wang, Y., Qiu, L. & Li, D. Liquid-mediated dense integration of graphene materials for compact capacitive energy storage. Science 341, 534537 (2013)
  6. Zheng, J. P., Cygan, P. J. & Jow, T. R. Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J. Electrochem. Soc. 142, 26992703 (1995)
  7. Naguib, M. et al. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv. Mater. 23, 42484253 (2011)
  8. Lukatskaya, M. R. et al. Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide. Science 341, 15021505 (2013)
  9. Nicolosi, V., Chhowalla, M., Kanatzidis, M. G., Strano, M. S. & Coleman, J. N. Liquid exfoliation of layered materials. Science 340, 6139 (2013)
  10. Ghaffari, M. et al. High-volumetric performance aligned nano-porous microwave exfoliated graphite oxide-based electrochemical capacitors. Adv. Mater. 25, 48794885 (2013)
  11. Tao, Y. et al. Towards ultrahigh volumetric capacitance: graphene derived highly dense but porous carbons for supercapacitors. Sci. Rep. 3, 2975 (2013)
  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, 42834287 (2008)
  13. Barsoum, M. W. MAX Phases: Properties of Machinable Ternary Carbides and Nitrides (John Wiley & Sons, 2013)
  14. Naguib, M., Mochalin, V. N., Barsoum, M. W. & Gogotsi, Y. MXenes: a new family of two-dimensional materials. Adv. Mater. 26, 982 (2014)
  15. Xie, X. et al. Surface Al leached Ti3AlC2 substituting carbon for catalyst support served in a harsh corrosive electrochemical system. Nanoscale 6, 1103511040 (2014)
  16. Peng, Q. et al. Unique lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide. J. Am. Chem. Soc. 136, 41134116 (2014)
  17. Tang, Q., Zhou, Z. & Shen, P. Are MXenes promising anode materials for Li ion batteries? computational studies on electronic properties and Li storage capability of Ti3C2 and Ti3C2X2 (X = F, OH) monolayer. J. Am. Chem. Soc. 134, 1690916916 (2012)
  18. Mashtalir, O. et al. Intercalation and delamination of layered carbides and carbonitrides. Nature Commun. 4, 1716 (2013)
  19. Halim, J. et al. Transparent conductive two-dimensional titanium carbide epitaxial thin films. Chem. Mater. 26, 23742381 (2014)
  20. Chang, F., Li, C., Yang, J., Tang, H. & Xue, M. Synthesis of a new graphene-like transition metal carbide by de-intercalating Ti3AlC2. Mater. Lett. 109, 295298 (2013)
  21. Enyashin, A. N. & Ivanovskii, A. L. Two-dimensional titanium carbonitrides and their hydroxylated derivatives: structural, electronic properties and stability of MXenes Ti3C2−xNx(OH)2 from DFTB calculations. J. Solid State Chem. 207, 4248 (2013)
  22. Madsen, F. T. & Müller-Vonmoos, M. The swelling behaviour of clays. Appl. Clay Sci. 4, 143156 (1989)
  23. Hensen, E. J. & Smit, B. Why clays swell. J. Phys. Chem. B 106, 1266412667 (2002)
  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, 11381142 (2014)
  25. Mashtalir, O., Naguib, M., Dyatkin, B., Gogotsi, Y. & Barsoum, M. W. Kinetics of aluminum extraction from Ti3AlC2 in hydrofluoric acid. Mater. Chem. Phys. 139, 147152 (2013)
  26. Conway, B. Electrochemical capacitors based on pseudocapacitance. In Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (Kluwer Academic/Plenum, 1999)
  27. Dmowski, W., Egami, T., Swider-Lyons, K. E., Love, C. T. & Rolison, D. R. Local atomic structure and conduction mechanism of nanocrystalline hydrous RuO2 from X-ray scattering. J. Phys. Chem. B 106, 1267712683 (2002)
  28. Wang, J., Polleux, J., Lim, J. & Dunn, B. Pseudocapacitive contributions to electrochemical energy storage in TiO2 (anatase) nanoparticles. J. Phys. Chem. C 111, 1492514931 (2007)
  29. Levi, M. D. et al. Solving the capacitive paradox of 2D MXene by electrochemical quartz-crystal admittance and in situ electronic conductance measurements. Adv. Energy Mater. http://dx.doi.org/10.1002/aenm.201400815 (2014)
  30. Simon, P., Gogotsi, Y. & Dunn, B. Where do batteries end and supercapacitors begin? Science 343, 12101211 (2014)

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Author information

  1. These authors contributed equally to this work.

    • Michael Ghidiu &
    • Maria R. Lukatskaya

Affiliations

  1. Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA

    • Michael Ghidiu,
    • Maria R. Lukatskaya,
    • Meng-Qiang Zhao,
    • Yury Gogotsi &
    • Michel W. Barsoum

Contributions

M.G. conducted material synthesis and XRD analysis. M.R.L. performed electrochemical measurements and SEM analysis. M.-Q.Z. performed TEM analysis. M.W.B. and Y.G. planned and supervised the research. M.R.L., M.G., M.W.B. and Y.G. wrote the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding authors

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Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Processing of MXene clay. (117 KB)

    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.

  2. Extended Data Figure 2: SEM images. (204 KB)

    a, Multilayer MXene particle. b, Cross-section of rolled Ti3C2 film, showing shearing that is most probably responsible for the loss of the 60° angle peak in the XRD pattern.

  3. Extended Data Figure 3: Contact angle. (97 KB)

    Digital image showing contact angle of a water droplet on rolled MXene film, indicating its hydrophilic surface.

  4. Extended Data Figure 4: TEM characterization of dispersed Ti3C2Tx flakes. (334 KB)

    a, Representative TEM image showing the morphology and size of a large single-layer Ti3C2Tx flake. Note folding on all sides of this large flake. b, The lateral size distribution of the dispersed Ti3C2Tx flakes. ce, Representative TEM 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 overestimated owing to inevitable restacking and edge folding of single-layer flakes during preparation of samples for TEM analysis. Edge folding is clearly seen in a. An example of restacking is shown in Extended Data Fig. 5.

  5. Extended Data Figure 5: TEM image showing the restacking of single- or double-layer MXene flakes into few-layer MXene. (417 KB)
  6. Extended Data Figure 6: Gravimetrically normalized capacitance. (458 KB)

    Cyclic voltammetry profiles at different scan rates for 5-µm-thick (a), 30-µm-thick (b) and 75-µm-thick (c) electrodes in 1 M H2SO4. d, Gravimetric rate performances of rolled electrodes, 5 µm thick (black squares), 30 µm thick (red circles) and 75 µm thick (blue triangles).

Extended Data Tables

  1. Extended Data Table 1: Effect of film thickness and scan rate on mass- and volume-normalized capacitance values (149 KB)

Additional data