(Nanowerk Spotlight) Graphene has a unique combination of properties that is ideal for next-generation electronics, including mechanical flexibility, high electrical conductivity, and chemical stability. The burgeoning field of wearable electronics – ‘smart’ fabrics with invisibly integrated energy harvesting, energy storage, electronics and sensor systems – benefits from graphene in numerous ways. Graphene materials, be they pristine or composites, will lead to smaller high-capacity and fast-charging supercapacitors, completely flexible and even rollable electronics and energy-storage devices, and transparent batteries.
To realize the commercial potential of graphene, it is necessary to develop reliable, cost-effective and facile processes for the industry-scale fabrication of graphene-based devices.
One possible route is inkjet printing, already extensively demonstrated with conductive metal nanoparticle inks. Although liquid-phase graphene dispersions have been demonstrated, researchers are still struggling with sophisticated inkjet printing technologies that allow efficient and reliable mass production of high-quality graphene patterns for practical applications.
As a proof-of-concept, the researchers fabricated a stretchable micro-supercapacitor (S-MSC) demonstrating the highest capacitance reported for a graphene-based highly stretchable MSC to date. This also is the first example of using laser-induced graphene in the form for a powder preparation of graphene-based inks and subsequently for use in screen-printing of S-MSC.
Illustration of the processing steps for screen-printed flexible supercapacitor fabricated from laser-induced graphene ink. (Image: Laboratory for Nanobioelectronics, UC San Diego) (click on image to enlarge)
Back in 2014, researchers at Rice University created flexible, patterned sheets of multilayer graphene from a cheap polymer by burning it with a computer-controlled laser, a technique they called laser-induced graphene (LIG). This high-yield and low-cost graphene synthesis process works in air at room temperature and eliminates the need for hot furnaces and controlled environments, and it makes graphene that is suitable for electronics or energy storage.
“LIG can be prepared from a few polymeric substances, such as Kapton polyimide and polyetherimide, as well as various sustainable biomasses, including wood, lignin, cloth, paper, or hydrothermal carbons,” Farshad Tehrani, the paper’s first author, tells Nanowerk. “On the other hand, LIG has considerably enhanced dispersion in typical solvent and binders due to its inherently abundant defects and surface functional groups.”
He points out that the team’s novel method, while maintaining the distinct advantages of the direct-written LIG, unlocks untapped potentials of the LIG material in several areas:
Mechanical stretchability: In this study, the inherently brittle and mechanically fragile LIG electrodes are turned into a mechanically robust, highly stretchable electrodes, with the new ink attractive for diverse wearable electronic devices.
Enhanced electrochemical performance: The areal capacitance of the team’s S-MSC has far surpassed that of direct-written laser LIG and has produced the highest areal capacitance reported for highly stretchable supercapacitors.
Customized composite formulations: The basic ink formulation is compatible with a wide range of compositions using the LIG as an attractive conductive filler.
Substrate versatility: Unlike direct-laser writing, which is limited to polymeric substrates and several biomasses, the LIG ink can be printed on almost any stretchable and non stretchable substrate, such as polymeric substrates, fabrics, or textiles.
“During the development of our new supercapacitor, we discovered a specific synergic effect between polymeric binders poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) mixed with Polyurethane (PU), PEDOT:PSS-PU and graphene sheets in producing exceptional electromechanical performances,” adds Fernando Soto, a co-author of the paper. “We realized that when both sides of the graphene sheets are thoroughly covered with the conductive/elastic PEDOT:PSS/PU polymer, it results in a robust composite that withstands severe shear stresses during stretching.”
“Not only that, but it also maintains above 85% of its electrochemical performance such as its charge storing capacitance properties, composite conductivity and electrochemical stability at high charge-discharge cycles,” he adds.
A scotch tape experiment showing the flexibility and stretchability of the S-MSC. (Video: Laboratory for Nanobioelectronics, UC San Diego)
In developing wearable electronic devices, researchers need to deal with a range of issues where stretchability and mechanical performance of the device is as important as its electronic properties such as conductivity, charge storage properties and, generally, its high electrochemical performance.
Rather than focusing on one of these specific problems, the team’s work addresses a series of challenges that include high mechanical and electrochemical performance while keeping the costs at their lowest possible point for realistic commercialization scenarios.
“From the design to the implementation stages of our study, the primary focus has been devoted to scalability, versatility and cost efficiency of a high performance platform that can potentially spark further innovations using nanocomposite materials in the field of wearable electronics,” notes Tehrani.
The next stages of the team’s work in this area of wearable applications will see the integration of these high-performance S-MSCs with batteries and energy harvesting systems such as biofuel cells, triboelectrics, and piezoelectrics.
