Scientists at Lawrence Livermore National Laboratory and UC Santa Cruz have successfully 3D-printed periodic graphene composite aerogel microlattices for supercapacitor applications, using a technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing.
The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. Supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g−1) and power densities (>4 kW·kg−1) that equal or exceed those of reported devices made with electrodes 10−100 times thinner. A paper on their work is published in the ACS journal Nano Letters.
The researchers suggested that their ultra-lightweight graphene aerogel supercapacitors open the door to novel, unconstrained designs of highly efficient energy storage systems for smartphones, wearables, implantable devices, electric cars and wireless sensors.
… graphene-based materials hold a large advantage in supercapacitor applications due to their large specific surface area, superior elasticity, chemical stability, and excellent electrical conductivity. However, graphene’s propensity toward aggregation and restacking can significantly affect capacitive performance by reducing the ion-accessible surfaces and limiting ion and electron transport due to narrower channels. Therefore, a variety of three-dimensional (3D) graphene-based materials (i.e., hydrogels, aerogels, sponges, and porous films ) have been extensively explored to overcome these limitations by providing a network of interconnected pores. Their unique properties and porous morphology not only provide additional ion-accessible surface for charge storage but also facilitate the ion diffusion within the structure. A key challenge for 3D graphene lies in controllable large-scale assembly into desired architectures while maintaining inherent properties of the graphene (e.g., large surface area, high electrical conductivity, exceptional mechanical properties, etc.).
… the use of 3D printing techniques to produce free-standing reduced GO [graphene oxide] (rGO) assemblies has been reported. However, these attempts have several drawbacks, such as the inability to synthesize samples with large surface area and the need for polymer matrices. Recently, we utilized an extrusion-based 3D printing technique, known as direct-ink writing (DIW), to fabricate highly compressible graphene aerogel microlattices. These 3D-printed graphene aerogels showed even better mechanical strength than most bulk graphene assemblies while maintaining the large surface area of single graphene sheets.
… Here, we demonstrate a fabrication strategy for 3D-printed graphene composite aerogels (3D-GCAs) with designed architecture for microsupercapacitor applications. Our approach is based on a printable ink consisting of both GO and graphene nanoplatelets (GNP). By adding GNP into the GO solution, the electrical conductivity of the GO-GNP composite is greatly improved without a detrimental loss of surface area. Furthermore, by building the electrodes with engineered macroporosity to facilitate mass transport, extremely thick electrodes (ca. 1 mm) were produced with capacitance retention (ca. 90% from 0.5 to 10 A·g−1) and power densities (>4 kW·kg−1) that equal or exceed those of reported devices made with electrodes 10−100 times thinner.—Zhu et al.
The DIW technique uses a three-axis motion stage to assemble 3D structures by robotically extruding a continuous “ink” filament through a micronozzle at room temperature in a layer-by-layer scheme. The key to this method is designing gel-based viscoelastic ink materials possessing shear thinning behavior to facilitate extrusion flow under pressure and a rapid pseudoplastic-to-dilatant recovery resulting in shape retention after deposition.
The inks’ physical and electrochemical properties can be significantly improved to even realize multifunctionality by the addition of functional fillers, such as conductive nanoparticles, nanotube/wires, as well as nanofibers.
In their ink, the researchers increased the GO concentration to 40 mg·cm−3, and added hydrophilic fumed silica to serve as a viscosifier to meet the rheology requirements of reliable flow through a fine nozzle under shear force and shape retention after deposition.
The silica imparted both shear-thinning behavior and a shear yield stress to the GO suspension to further enhance the printability of the GO-based inks.
GNPs were added along with the reactants used to induce gelation postprinting via organic sol−gel chemistry.
This breaks through the limitations of what 2D manufacturing can do. We can fabricate a large range of 3D architectures. In a phone [for instance] you would only need to leave a small area for energy storage. The geometry can be very complex.—Cheng Zhu, the paper’s lead author
Supercapacitors also can charge incredibly fast, Zhu said, in theory requiring just a few minutes or seconds to reach full capacity. Zhu and his fellow researchers believe that in the future, newly designed 3D-printed supercapacitors will be used to create unique electronics that are currently difficult or even impossible to make using other synthetic methods, including fully customized smartphones and paper-based or foldable devices, while at the same time achieving unprecedented levels of performance.
We’re pioneering the marriage of 3D-printing and porous materials. Think of a supercapacitor as a portable energy device, so anything that needs electricity would benefit from such a supercapacitor. If we can replace the standard [technology] with our lightweight, compact and high-performance supercapacitor, that would be a radical change.—Fang Qian, co-author
Graphene-based inks, the researchers said, have a distinct advantage over carbon-based materials due to their ultrahigh surface area, lightweight properties, elasticity and superior electrical conductivity. The graphene composite aerogel supercapacitors also are extremely stable, the researchers reported, capable of nearly fully retaining their energy capacity after 10,000 consecutive charging and discharging cycles.
The Lab researchers worked closely with UC Santa Cruz professor Yat Li and grad student Tianyu Liu, who performed the electrochemical characterizations and optimized the materials used in the process.
Over the next year, the researchers intend to expand the technology by developing new 3D designs, using different inks and improving the performance of existing materials.
Funding for the research came from the internal Laboratory Directed Research & Development (LDRD).
Cheng Zhu, Tianyu Liu, Fang Qian, T. Yong-Jin Han, Eric B. Duoss, Joshua D. Kuntz, Christopher M. Spadaccini, Marcus A. Worsley, and Yat Li (2016) “Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores” Nano Letters doi: 10.1021/acs.nanolett.5b04965