3D Printing Graphene Oxide Hydrogels with Direct Ink Writing for Microsupercapacitors

In the recently published ‘Direct 3D printing of a graphene oxide hydrogel for fabrication of a high areal specific capacitance microsupercapacitor,’ researchers consider the capability for a graphene oxide (GO) hydrogel to be used as a bioink referred to as GO ink.

As 3D printing grows in popularity with users around the world, the development of smaller electronics and miniaturized platforms grows. This includes:

• Micro-electromechanical systems
• Biomedical sensors
• Wireless sensors
• Actuator drives

As the demand for devices increases, so does that of energy storage like microsupercapacitors (MSCs)—micro-scale platforms offering high-power density, a long cycle life, and more. Previously, methods for creating microsupercapacitors have resulted in electrodes with limited thickness, unable to store sufficient energy. For improving capacity, active materials must be more powerfully loaded with increased amounts for each electrode.

3D printing can be used here as it has shown not only versatility in such applications but also the ability to create thicker electrodes simply by creating more layers.

“As one typical type of the 3D printing technique, direct ink writing (DIW) is most commonly used because of its simple printing mechanism, low-budget fabrication process and wide adaptability to different types of materials,” state the researchers. “For the fabrication of MSCs through DIW, it is of crucial importance to find a suitable material with optimized rheological properties for use as the ink.”

Graphene and derivatives exhibit great potential for 3D aerogels due to:

• High surface area
• Extraordinary mechanical properties
• Super electrical/thermal conductivity
• Honeycomb framework

GO nanosheets may exhibit hydrophilicity in oxygenated regions showing defects:

“Therefore, GO sheets obtained from oxidation and exfoliation of graphite can be well dispersed in aqueous media,” stated the researchers. “Depending on the factors such as degree of oxidation of the sheets, concentration, pH and salt concentration, the GO suspensions can exist as viscoelastic liquid, soft solid, liquid crystal, gel and glass.”

Direct ink writing has been popular for fabrication of 3D structures, the researchers explain that dilute GO suspension usually ‘fails as a candidate for DIW because of its liquid behavior and low solid containing.’ Most methods suggest adding polymers and inorganic nanoparticles; however, other methods have been suggested too such as freeze-casting. In most cases, the problem is still an overall lack of comprehension regarding the rheological properties of GO inks and how they affect the direct-writing process.

In this research, the authors experimented with 3D printing an MSC electrode, using a hydrogel 3D printer (Cell Assembler II), developed at the Department of Mechanical Engineering, Tsinghua University. They found that GO suspensions with different suspensions yielded different results; for instance, concentrated suspensions are suitable for direct ink writing because of the gel-liquid transition and shear-thinning behavior. Parameters were fixed based on rheological tests, extrusion experiments and printing experiments.

(a) Schematic illustration of the fabrication process of 3DHG-MSCs, (b) structural decomposition diagram of 3DHG-MSCs, (c) optical image of the 3D interdigitated architecture composed of three-pair fingers with 5 printed layers, (d) optical microscopic image of 3DHG electrode.

Reduced graphene oxide (RGO) sheets were used to form the 3DHG electrodes. The inks demonstrated ‘significant’ shear thinning and rapid viscosity recovery—all very conducive for DIW.

“The optimal concentration range of the GO suspensions and proper needle size were determined from the 3D printing, based on the comprehensive study of the feasibility for 3D printing, structure precision, and optimizing electrode function,” concluded the researchers. “Through the investigation on the smoothness of the extrusion, controllability, and stability of the actual printing process, the appropriate scanning speed and extrusion speed region were determined to construct the precise 3D structure.

“The fabricated all-solid-state 3D MSCs achieved a high areal specific capacitance of 101 mF cm⁻² at a current density of 0.5 mA cm⁻², and the capacitance of 111 mF cm⁻² at a scan rate of 10 mV s⁻¹, which are higher than most of the all-solid-state MSCs using carbon-based materials reported until now. The desirable rheological properties of the highly concentrated GO suspension and optimized processing parameters of 3D printing are critically important to contribute to the feasible fabrication and excellent performance of the energy storage devices.”

(a) SEM image of the GO sheets, (b) AFM image of the GO sheets, (c) Raman spectra of GO and RGO used for the 3DHG electrode, (d) XPS survey spectrum, (e) C1s XPS spectra, (f) XRD curves of GO, RGO and graphite.

As bioprinting continues to become further relied on for tissue engineering, researchers work toward finding a cure for cancer, creating hardware for military installations, and even tissue engineering at the International Space Station. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

Extrusion experiment results of the GO suspensions with the different concentrations (17.5, 21.5, 25.5, 31.5, 40.0, 50.0 mg mL−1) for three different nozzle sizes (27G, 30G, 32G).