(Nanowerk Spotlight) MXenes are a promising class of 2D materials with unique intrinsic physical and chemical properties, including excellent conductivity, hydrophilicity and high density when compared to graphene.
Formulating 3D-printable MXene inks and integrating them into customized 3D device architectures would provide a high degree of architectural control, scalability, and cost-effectiveness. However, this would require that inks with single to few layer MXene can achieve the rheological properties required for 3D printing.
3D-printed architectures composed of MXenes are particularly attractive for energy storage applications such as rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries and supercapacitors. They could also be used in other applications such as energy conversion, photocatalytic fuel production or electromagnetic shielding.
Researchers at the University of Manchester have demonstrated for the first time the possibility to print three-dimensional freestanding MXene objects. They developed 3D-printing inks that are composed solely of large few-layer MXene flakes and water as the solvent – without the need for additives to control the ink's rheological properties.
The capability to print customized MXene architectures in three dimensions opens new opportunities to realize high-performance multiscale and multidimensional devices, as required for many different energy, catalysis, and transportation applications.
"We formulated our aqueous inks with 2D Ti3C2Tx – the most studied MXene material – with ideal viscoelastic properties for extrusion-based 3D printing of freestanding, high specific surface area architectures of different sizes and shapes," Dr Suelen Barg, a Lecturer in Structural Materials at the Department of Materials, tells Nanowerk. "The inks we developed provide a good translation of properties from the 2D material into 3D, resulting in architectures with significantly improved specific surface area compared to other approaches."
3D-printed multiscale architectures after freeze-drying. a,b) SEM and optical photographs (inset) of freestanding Ti3C2Tx microlattice (a) and hollow rectangular prism (b) printed through 330 and 250 µm nozzles, respectively. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
In this first study of the 3D-printability of MXene, the team demonstrates that by formulating inks composed of large size (8 µm average lateral size) and few-layers thin (1–3 nm thick) MXene flakes and water, the rheological properties can be tuned to allow their extrusion through micrometer-sized nozzles. Furthermore, the extruded filaments can retain their shape for more than 20 printed layers.
After printing, the team freeze-dried the wet 3D structures to protect the internal integrity and the external shape of the structures with low shrinkage. The results are freestanding MXene architectures of well-defined
shapes without the necessity of any further thermal or chemical treatments.
"In addition to various 3D architectures such as microlattices and hollow rectangular prisms, we further applied the inks to directly print interdigitated symmetric supercapacitors with a solid electrolyte using Ti3C2Tx as both the electrode material and current collector," says Barg. "This eliminates the need to employ noble metals or other metallic materials used as current collector in 3D-printed electrode designs."
The electrodes, with an active material loading of about 8.5 mg cm-2, exhibit an areal capacitance of 2.1 F cm-2, high power and energy densities, and capacitance retention above 90% for 10,000 cycles.
Illustration of 3D-printed MXene architectures with 25 printed layers each. Left: 3D-printed microlattice. Right: Rectangular hollow prism. Printed through 330 µm and 250 µm nozzles, respectively. Scale bar: 3 mm. (Reprinted with permission by Wiley-VCH Verlag)
"These results are among the highest in the literature and confirm that our synthesis and processing route allows an excellent translation of properties from the 2D material to the assembled 3D device," Wenji Yang,
currently a PhD student in the Barg Group and the first author of the paper, points out.
As a next step, the group will optimize their MXene ink formulations and additive manufacturing protocol to fabricate bespoke energy storage devices in asymmetric configurations, incorporating multi-materials printing and alternative electrolytes to overcome the voltage window limitations and further enhance energy density in energy storage devices.
They are also exploring their 3D printable inks in other applications that can benefit from MXene tunable 3D architectures such as electromagnetic shielding and catalysis.
"The 3D-printability of MXenes encourages us to explore the development of inks incorporating other functional materials," Barg concludes. "Particular challenges and potential research directions include the control of MXene oxidation inside aqueous inks and the development of a better control and understanding of the role of water intercalation in the formation and properties of MXene inks and their relation to printability."