The practical potential of MXenes (two-dimensional, a few atoms thick layers of transition-metal carbides, nitrides, and carbonitrides) as supercapacitor electrodes has become more promising with the successful vertical alignment of MXene nanosheets on substrates. These electrodes exhibit ultrafast charging capabilities. This leap in energy storage, published recently in Nature (doi:10.1038/s41586-018-0109-z), was achieved by the collaboration between Shu Yang’s group at the University of Pennsylvania and Yury Gogotsi’s group at Drexel University.

The conventional configuration is composed of horizontally stacked MXene sheets that is undesirable for fast-rate charging as ion diffusion through the sheets is severely impeded by the compact-film configuration. The sluggish ion movement leads to deterioration of energy-storage capacity at elevated charging rates. This problem is exacerbated when the film thickness approaches or exceeds 10 µm, far less than the industrial thickness standard of 100 µm for active materials used in supercapacitors. To resolve this issue, developing electrodes with straight ion-movement channels extending from the electrode surface to the substrate is critical.

(a) The horizontally stacked configuration induces slow ion transport. (b) The vertical alignment architecture provides straight ion-movement channels that accelerate ion transport. (c) A top-view scanning electron microscope image showing the morphology of the vertically aligned Ti₃C₂T_x nanosheets. Credit: Nature.

Inspired by the anisotropy and self-assembly of liquid-crystalline materials, Yang and Gogotsi’s groups converted Ti₃C₂T_x (T_x: surface functional groups) nanosheets, the most widely studied MXene, into liquid-crystal phases. They hoped to vertically align Ti₃C₂T_x in the specific liquid-crystal phase. However, this was not a trivial task, according to Yu Xia, first author of the article: “The biggest challenge is that very little is known in the field of two-dimensional materials-based liquid crystals.... Everything had to start from scratch.”

A solution came soon. The researchers attached a surfactant, hexaethylene glycol monododecyl ether (C₁₂E₆), onto the surface of Ti₃C₂T_x through hydrogen bonds, which turned the Ti₃C₂T_x nanosheets into high-order liquid crystals, coined lamellar nematics. Upon applying a shear force, these surface-modified Ti₃C₂T_x sheets “stand” straight on the substrate, forming orderly distributed arrays, as suggested by liquid-crystal theory. These vertical arrays were maintained after the C₁₂E₆ surfactants were removed. This structure contains abundant inter-sheet slits that serve as ion-movement “expressways” to allow quick ion diffusion.

Electrochemical testing revealed that the vertically aligned Ti₃C₂T_x nanosheets are capable of being charged rapidly. Cyclic voltammetry tests showed that the 200-µm-thick film electrode lost little of its charge-storage capacity at a fast charging rate of 1000 mV s^–1. This behavior was observed in electrodes that were 40–320 µm thick. The control sample, the same Ti₃C₂T_x nanosheets but stacked parallel to the substrate, abruptly lost its charge-storage capability starting at an intermediate rate of 10 mV s^–1, even though the thickness was only 35 µm. The results unambiguously highlight the advantage of the vertical arrangement in retaining the fast charge-storage characteristic of MXene-based supercapacitor electrodes.

“It is amazing that simply making MXenes ‘stand up’ leads to such a big difference,” says Yat Li of the University of California, Santa Cruz. “This work shows an innovative method of controlling the alignment of electrochemically active materials to achieve ultrafast directional ion diffusion in thick electrodes,” he says. Li was not involved in this study.

This method may encourage a plethora of future work. Gogotsi says, “Alignment of mechanically strong and electrically conductive MXene layers may lead to manufacturing of MXene fibers, membranes, coatings, and other forms with unique and anisotropic properties for various applications beyond energy storage.”