Scalable production of graphene inks via wet-jet milling exfoliation for screen-printed micro-supercapacitors

Abstract: The miniaturization of energy storage units is pivotal for the development of next-generation portable electronic devices. Micro-supercapacitors (MSCs) hold a great potential to work as on-chip micro-power sources and energy storage units complementing batteries and energy harvester systems. The scalable production of supercapacitor materials with cost-effective and high-throughput processing methods is crucial for the widespread application of MSCs. Here, we report wet-jet milling exfoliation of graphite to scale-up the production of graphene as supercapacitor material. The formulation of aqueous/alcohol-based graphene inks allows metal-free, flexible MSCs to be screen-printed. These MSCs exhibit areal capacitance (Careal) values up to 1.324 mF cm-2 (5.296 mF cm-2 for a single electrode), corresponding to an outstanding volumetric capacitance (Cvol) of 0.490 F cm-3 (1.961 F cm-3 for a single electrode). The screen-printed MSCs can operate up to power density above 20 mW cm-2 at energy density of 0.064 uWh cm-2. The devices exhibit excellent cycling stability over charge-discharge cycling (10000 cycles), bending cycling (100 cycles at bending radius of 1 cm) and folding (up to angles of 180{\deg}). Moreover, ethylene vinyl acetate-encapsulated MSCs retain their electrochemical properties after a home-laundry cycle, providing waterproof and washable properties for prospective application in wearable electronics.

Scalable Production of Graphene Inks via Wet-Jet Milling Exfoliation for Screen-Printed Micro-Supercapacitors

The paper "Scalable production of graphene inks via wet-jet milling exfoliation for screen-printed micro-supercapacitors" addresses the critical need for scalable and cost-effective production of materials for micro-supercapacitors (MSCs), facilitating their integration into miniaturized electronic devices. The authors present a significant advancement in the production method for graphene through a wet-jet milling (WJM) exfoliation process. This innovative approach optimizes the production of defect-free few-layer graphene flakes, enabling large-scale fabrication of graphene inks.

Wet-Jet Milling Technique

Graphene's potential as an active material for supercapacitors is well-recognized due to its high specific surface area and excellent electrical conductivity. The paper highlights the limitations of conventional methods such as ultrasonication and ball milling, which are non-scalable and often result in defective materials. The wet-jet milling method offers a notable improvement, achieving high concentration (10 g L^-1) dispersions in NMP, with a considerable reduction in processing time (<3 minutes per gram) and high exfoliation yield (~100%).

Characterization and Screen Printing of Graphene Inks

The paper provides detailed morphological and structural analyses of the produced graphene using TEM and Raman spectroscopy, confirming the production of high-quality single-/few-layer graphene flakes. The resulting graphene inks, with controlled viscosity and surface tension, are screen-printable onto flexible polyethylene terephthalate (PET) substrates. It explores the role of Single-Walled Carbon Nanotubes (SWCNTs) as spacers to avoid restacking of graphene flakes, enhancing the surface area and porosity of the electrodes.

Performance of Micro-Supercapacitors

The MSCs created exhibit promising electrochemical performance with substantial areal capacitance values (up to 1.324 mF cm^-2), corresponding to a volumetric capacitance of 0.490 F cm^-3. The devices display robust high-rate capabilities (power density above 20 mW cm^-2), substantial cycling stability over 10,000 cycles, and resilience to mechanical deformation. Furthermore, the encapsulation with ethylene vinyl acetate ensures the MSCs remain functional after exposure to home-laundry cycles, enhancing their practicality for wearable electronics.

Implications and Future Developments

The results pave the way for industrial-scale production of graphene-based MSCs, potentially transforming the landscape of portable and wearable electronics. The WJM process represents a valuable advancement in graphene production, facilitating the integration of MSCs as efficient on-chip micro-power sources. Future research could explore optimization of the solvent-exchange process and further industrial applications of graphene inks.

Overall, this paper provides a comprehensive exploration and validation of wet-jet milling as a scalable method for producing high-quality graphene inks, enabling the fabrication of high-performance MSCs on flexible substrates. The documented advancements hold potential for significant impact on the development of next-generation electronic devices, where efficient energy storage systems are crucial.