Highly compressible 3D periodic graphene aerogel microlattices

Abstract

Graphene is a two-dimensional material that offers a unique combination of low density, exceptional mechanical properties, large surface area and excellent electrical conductivity. Recent progress has produced bulk 3D assemblies of graphene, such as graphene aerogels, but they possess purely stochastic porous networks, which limit their performance compared with the potential of an engineered architecture. Here we report the fabrication of periodic graphene aerogel microlattices, possessing an engineered architecture via a 3D printing technique known as direct ink writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit supercompressibility (up to 90% compressive strain). Moreover, the Young's moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. Adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications.

Figure 1. Fabrication strategy and GO ink's rheological properties.

Log–log plots of (a) apparent viscosity as a function of shear rate and (b) storage and loss modulus as a function of shear stress of GO inks with and without silica fillers. (c) Schematic of the fabrication process. Following the arrows: fumed silica powders and catalyst (that is, (NH₄)₂CO₃ or R–F solution) were added into as-prepared aqueous GO suspensions. After mixing, a homogeneous GO ink with designed rheological properties was obtained. The GO ink was extruded through a micronozzle immersed in isooctane to prevent drying during printing. The printed microlattice structure was supercritically dried to remove the liquid. Then, the structure was heated to 1,050 °C under N₂ for carbonization. Finally, the silica filler was etched using HF acid. The in-plane centre-to-centre rod spacing is defined as L, and the filament diameter is defined as d.

Figure 2. Morphology and structure of graphene aerogels.

(a) Optical image of a 3D printed graphene aerogel microlattice. SEM images of (b) a 3D printed graphene aerogel microlattice, (c) graphene aerogel without R–F after etching and (d) graphene aerogel with 4 wt% R–F after etching. Optical image of (e) 3D printed graphene aerogel microlattices with varying thickness and (f) a 3D printed graphene aerogel honeycomb. Scale bars, 5 mm (a), 200 μm (b), 100 nm (c,d), 1 cm (f).

Figure 3. Raman and XRD spectra of graphene aerogels.

(a) Raman and (b) XRD spectra of 3D printed graphene aerogel microlattices made with various ink formulations. Spectra of highly oriented pyrolytic graphite (HOPG) and graphene oxide (GO) powder are included for reference.

Figure 4. Compressive properties of graphene aerogels.

Stress–strain curves during loading–unloading cycles in sequence of increasing strain amplitude for (a) bulk graphene aerogel (31 mg cm⁻³) and (b) 3D printed graphene aerogel microlattice (16 mg cm⁻³) using the GO ink without R–F, (c) bulk graphene aerogel (123 mg cm⁻³) and (d) 3D printed graphene aerogel microlattice (53 mg cm⁻³) using the GO ink with R–F.

Figure 5. Physical properties of graphene aerogels.

(a) Compressive stress–strain curves of 10 cycles of loading–unloading. (b) Maximum stress and energy loss coefficient during 10 cycles. (c) Electrical resistance change when repeatedly compressed up to 50% of strain for 10 cycles. The graphene aerogel microlattice used for cyclic compression and conductivity measurements (a–c) has a geometric density of 53 mg cm⁻³. (d) The relationships between Young's modulus and density of bulk and printed graphene aerogels.