Tech Briefs

Transparent and stretchable supercapacitors are used as portable energy sources for flexible electronics in biomedical, energy, and wearable systems.

Stretchable electronic devices, including solar cells, light-emitting diodes, batteries, and textile supercapacitors, have been developed to retain their functions even when under large strains (up to 40%). Some transparent solar cells, batteries, and supercapacitors have also been developed. However, most of the currently developed electrodes and the associated supercapacitor cells cannot be both transparent and stretchable.

Schematic representation of the procedures for producing wrinkled graphene sheets for the fabrication of transparent and stretchable supercapacitors.

It is highly desirable to integrate the stretchable and transparent characteristics into a single device for aesthetically pleasing wearable electronics, and integrated energy conversion and storage systems. However, it is still a challenge to construct both stretchable and transparent electronic devices because most of the existing electrodes are neither stretchable nor transparent.

Highly transparent (up to 60% at 550 nm) and stretchable multilayer graphene sheets with a wrinkled structure were synthesized, and after being transferred onto a polydimethylsiloxane (PDMS) substrate, were used as both the current collector and active electrodes for the development of high-performance transparent (57%) and stretchable (up to 40% strain) all-solid supercapacitors with excellent stability, even over hundreds of stretching cycles.

In spite of its excellent electrical, optical, and mechanical properties, graphene has rarely been discussed for applications as stretchable electrodes since stretching often reduces its electrical conductivity dramatically. In addition, the process to transfer a large-area graphene film from the growth substrate to a pre-strained elastic substrate (e.g., PDMS) often causes serious cracking or breakage of the graphene sheet. As such, very limited effort has been made to develop transparent and stretchable graphene electrodes, which is very difficult, if not impossible.

Owing to its high conductivity and excellent transparency (transmittance up to 95% for 2 nm thick film), the oneatom-thick graphene sheet provides an ideal electrode material for high-performance stretchable and transparent optoelectronics. The first wrinkled graphene sheet of a large area was synthesized by chemical vapor deposition (CVD) of methane with the carrier gas of argon and hydrogen under 1000 °C. The wrinkled graphene sheet was then transferred with its structural integrity onto a PDMS substrate to exhibit high transparency and stretchability. The resistance of the newly synthesized wrinkled graphene sheet composited with polyvinyl alcohol (PVA) to be used as a protecting layer and/or electrolyte matrix increased by only 170%, even when it was stretched up to 40% strain.

As shown in the figure, a tweezer with an appropriate wrinkled structure was used to produce a wrinkled copper (Cu) foil by sliding it over the Cu foil. The resultant wrinkled Cu foil was then used as the substrate for the graphene growth by CVD of methane as the carbon source under the mixture carrier gas of argon and hydrogen at 1000 °C in a tube furnace, followed by coating a thin layer of PDMS onto the top surface of the asprepared graphene sheet, and thermally solidified at 75 °C for 1 hour. By removing the Cu substrate in an aqueous solution, a large piece of stretchable wrinkled graphene sheet on PDMS was obtained. Finally, the transparent and stretchable all-solid-state supercapacitors were assembled by pressing two of the PDMS-supported graphene electrodes together with a transparent layer of polymer electrolyte between both the electrolyte and separator.

Depending on the graphene growth durations, the PDMS-supported wrinkled graphene sheets exhibited an optical transmittance in the range of 50 to 60%, which are comparable to the multi-layered planar graphene sheet prepared under the same condition. The wrinkled graphene sheets showed transparency slightly lower than that of the planar graphene sheets synthesized at the same condition, which can be attributed to the light diffuse reflectance and light-scattering effects associated with the wavy surface.

Indeed, the stretchability of both the planar and wrinkled graphene sheets was improved significantly by PVA coating, which was used as both the protecting layer and electrolyte matrix. Compared with the PVA-coated planar graphene sheet, the PVA-coated wrinkled graphene sheet exhibited an even better stretchability.

There should be a delicate balance between the stretchability and transparency for the graphene sheets to be used in the high-performance stretchable and transparent supercapacitors being developed. The high transmittance of the resultant supercapacitors is evident, showing optical transmittances in the range of 48 to 57%, depending on the growth time (i.e., the layer number) of the graphene sheets.

For supercapacitors based on both the planar and wrinkled graphene sheets, their CVs and charging/discharging performance, as well as their specific capacitance, was almost unchanged when they were stretched up to 40% strain. Furthermore, these transparent and stretchable supercapacitors also showed an outstanding stability as their CVs and capacitances did not vary over hundreds of cycles of stretching up to 40% strain. These results clearly indicate that the newly developed transparent supercapacitors are highly stretchable and stable.

This work was done by Ajit K. Roy of the Air Force Research Laboratory; and Tao Chen, Yuhua Xue, and Liming Dai of Case Western Reserve University. AFRL-0235