When you’re exercising with a smartwatch, do you often worry about the battery suddenly running out? And as IoT sensors proliferate in homes and cities, how can we ensure these tiny devices have a long‑lasting, reliable power supply? With the explosive growth of portable and wearable electronics and IoT technologies, the limitations of conventional energy‑storage devices—such as their inability to achieve miniaturization, high integration, and flexibility—are becoming increasingly apparent.

Recently, research teams from institutions including the Guangdong–Israel Institute of Technology and the Southern University of Science and Technology published a groundbreaking achievement in ACS Applied Energy Materials: using aerosol-jet 3D printing, they successfully fabricated an array of graphene‑based hollow‑pillar electrode (HPE) microsupercapacitors (MSCs). These devices not only deliver a high output voltage of 10 V but also boast an exceptionally high integration density of 102 cm⁻², while exhibiting excellent bending stability, thus paving a new path for the development of miniature energy‑storage devices.

The Urgent Demand and Bottlenecks of Micro‑Energy Storage

The widespread adoption of the Internet of Things (IoT) has made “everything interconnected” a reality. From smart wristbands and flexible displays to implantable medical devices and miniature sensors, these devices are typically compact and highly adaptable, placing stringent demands on their accompanying energy‑storage systems: they must be small enough to integrate with microscale components, offer high capacity and rapid charging to support prolonged use, and exhibit excellent mechanical flexibility to accommodate complex application scenarios.

Micro-supercapacitors (MSCs), with their high power density, rapid charge–discharge capabilities, and exceptionally long cycle life, have emerged as an ideal energy-storage solution for such miniature devices. Meanwhile, graphene, as a “star player” among electrode materials, boasts ultra‑high electrical conductivity, an enormous specific surface area, and outstanding mechanical flexibility, naturally making it a central focus of research.

However, the development of conventional micro-supercapacitors faces two major core bottlenecks:

• First, there are limitations inherent in manufacturing processes. Conventional 3D printing techniques, such as direct ink writing, impose extremely stringent requirements on ink viscosity, leading to issues like nozzle clogging and structural instability. Moreover, the resulting interdigitated electrode (IDE) structures tend to be bulky, making it difficult to achieve high‑density integration.
• Second, defects in the electrode structure—such as the non-uniform electric field distribution and limited effective surface area inherent in conventional planar or solid electrodes—make it difficult to simultaneously optimize capacitance performance and voltage output. Moreover, these electrodes often lack sufficient mechanical flexibility, leading to failure under bending.

How can we overcome the dual constraints of fabrication processes and structural design to develop miniature supercapacitors that are “small in size, high in capacitance, high in voltage, and highly flexible”? The research team turned to aerosol jet printing technology and an innovative hollow‑pillar electrode architecture.

Graphene ink + 3D printing create a “hollow energy column”

The core innovation of this study lies in the synergistic breakthroughs achieved in both “material formulation” and “manufacturing process.” By customizing graphene ink and employing precisely controlled aerosol jet printing, the researchers successfully fabricated hollow‑pillar electrodes with outstanding performance.

1. Customized: Stable, Easy-to-Print Graphene Ink

The performance of electrode inks directly determines the success or failure of 3D printing and the resulting device characteristics. The research team used solution-exfoliated graphene nanosheets as the core material and employed poly(propylene carbonate) (PPC) as a stabilizer to develop a graphene ink suitable for aerosol-jet printing.

The incorporation of PPC plays a crucial role: on the one hand, it adsorbs onto the graphene surface via CH–π interactions, creating steric hindrance that effectively prevents the restacking of graphene nanosheets and ensures stable dispersion of the ink; on the other hand, PPC enhances the mechanical strength of the printed structure, providing structural support for the fabrication of three-dimensional architectures.

To optimize printing performance, the team systematically adjusted the mass ratio of graphene to PPC and found that a 5:5 ratio strikes the optimal balance between ink viscosity and flowability—enabling uniform micro‑pillar structures with vertical stacking while preventing collapse or deformation. This ink formulation not only addresses the poor printability of conventional graphene inks but also lays the groundwork for subsequent 3D structural fabrication.

2. Precision Intelligent Manufacturing: The “Micro-Sculpting Art” of Aerosol Jet Printing

If customized inks represent “raw-material assurance,” then aerosol jet printing (AJP) technology is the “manufacturing powerhouse.” Compared with conventional 3D‑printing techniques, aerosol jet printing offers distinct advantages: it uses ultrasonic atomization to convert ink into tiny droplets, which are then precisely deposited onto a substrate via pneumatic focusing provided by a carrier gas and a coaxial sheath gas. This approach not only accommodates a broader range of ink viscosities but also enables high‑precision patterning.

The research team precisely controlled the printing parameters—nozzle diameter of 300 μm, carrier gas flow rate of 30 sccm, sheath gas flow rate of 90 sccm, and substrate temperature of 80°C—while simultaneously rotating the substrate in a circular motion with a 250-μm diameter. This arrangement enabled graphene nanosheets to stack along the circumference, ultimately yielding a hollow‑column electrode structure with a wall thickness of approximately 50 μm. More importantly, the electrode height could be accurately tuned by adjusting the number of printing passes, thereby enabling customizable structural design.

This hollow‑column architecture stands in stark contrast to conventional interdigitated electrodes: the latter are solid, elongated strips, with the electric field concentrated at the electrode tips and distributed unevenly, whereas the hollow‑column electrode is a hollow cylinder, allowing the electric field to be uniformly distributed across both its inner and outer surfaces. This not only effectively increases the reactive surface area but also reduces resistance to ion diffusion.

3. Device Integration: Solid-State Electrolyte + Series-Parallel Design

To enable the practical application of the device, the research team also employed an ionic gel as a solid-state electrolyte, which was directly deposited onto the hollow‑pillar electrodes via aerosol jet printing, thereby fabricating a fully printed solid‑state microsupercapacitor. This solid-state electrolyte not only eliminates the leakage issues associated with liquid electrolytes but also establishes excellent interfacial contact with the electrodes, enhancing the device’s stability.

High Performance: High Capacitance, Bend-Resistant, and High-Density All-in-One

Superior capacitor performance: At the same electrode thickness of 220 μm, hollow‑pillar electrodes (HPEs) achieve a areal capacitance of 2.5 mF cm⁻² and a volumetric capacitance of 114 mF cm⁻³, significantly outperforming conventional interdigitated‑electrode (IDE) devices. This improvement stems from the hollow‑pillar architecture, which enables uniform electric‑field distribution and a larger effective surface area. COMSOL simulations reveal that, in IDEs, the electric field is concentrated at the finger tips, whereas in HPEs, the field extends not only over the outer surface but also into the interior, allowing more graphene active sites to participate in charge storage.

Mechanical Flexibility and Ultra‑Durability: For wearable devices, mechanical flexibility is an essential property. The research team subjected the devices to rigorous bending tests: at a small bending radius of 2.5 mm, after 1,000 bending–recovery cycles, the hollow‑pillar electrode device retained 97% of its initial capacitance, with virtually no distortion in the CV curves; by contrast, the conventional interdigitated electrode device experienced a sharp drop in capacitance to just 43% of its initial value, accompanied by obvious cracking of the electrodes.

This difference stems from the mechanical properties of the two electrode structures: the hollow‑pillar electrode, with its circular cross‑section, forms point contacts with the substrate, resulting in a uniform stress distribution during bending and minimizing stress concentrations; in contrast, the interdigitated electrode’s elongated, strip‑like architecture exhibits greater stiffness, making it prone to localized overloading and fracture under bending. This exceptional bending stability enables the device to seamlessly integrate into complex applications such as flexible wearables and curved‑surface electronics.

Record-breaking integration density: High integration density is key to realizing on-chip energy storage in miniature devices. This hollow‑pillar electrode device has a footprint of only 0.0098 cm² per unit, and an array of ten devices achieves an integration density of 102 cm⁻², with a surface voltage output of 102 V cm⁻²—far surpassing previously reported 3D‑printed microsupercapacitor arrays.

This means that on a substrate no larger than a fingernail, hundreds of such miniature supercapacitors can be integrated, providing ample power to densely packed IoT nodes and microsensor arrays, thereby enabling a truly unified “microdevice–microenergy-storage” design.

Furthermore, the device exhibits excellent cycling stability, with a capacitance retention exceeding 98% after 10,000 charge–discharge cycles—comparable to that of conventional graphene‑based microsupercapacitors—thus meeting the requirements for long-term operation.

Image and Text Guide

Figure 1. Schematic illustration of the AJP process for 3D graphene MSCs with HPEs. (a) Graphene exfoliation using PPC as a stabilizer. (b) Ink formulation containing exfoliated graphene and PPC. (c) Ink setup for 3D AJP of graphene ink. (d) AJP of three-dimensional graphene HPEs. (e) AJP of an ionogel electrolyte onto the printed electrodes to complete the fabrication of the MSC.

Figure 2. Exfoliated graphene and its ink. (a) Scanning electron microscope image of exfoliated graphene nanosheets. (b) Transmission electron microscope image of graphene nanosheets, along with (c) the selected-area electron diffraction pattern. (d) Raman spectra of exfoliated graphene and graphite, highlighting the characteristic D, G, D′, and 2D bands. (e) Photograph of graphene ink prepared using exfoliated graphene nanosheets and poly(p‑phenylene vinylene) (PPC). (f) Viscosity of the graphene ink as a function of shear rate at 20 °C, for a graphene-to-PPC weight ratio of 5:5.

Figure 3. Printed graphene–polymer electrodes (HPEs). (a) Top-view and (b) side-view scanning electron microscope images of HPEs fabricated via the AJP method. (c) Thickness of the HPEs prepared at different numbers of printing passes. (d) Top-view and (e) side-view optical microscope images of HPEs printed on a polyimide substrate. (f) Photograph of an HPE placed on a fingertip.

Figure 4. Electrochemical performance of a single device. Cyclic voltammograms of MSCs at scan rates ranging from 5 to 100 mV s−1: (a) using IDEs, and (b) using HPEs. Both IDEs and HPEs were printed with the same 220 μm thickness. (c) Area capacitance of MSCs as a function of scan rate, obtained from cyclic voltammetry measurements. Horizontal cross-sectional views of the electric field distribution in MSCs, simulated using COMSOL, for (d) IDEs and (e) HPEs. (f) Nyquist plots of the device, with insets showing magnified views of the high‑frequency and mid‑frequency regions. Galvanostatic charge–discharge (GCD) curves of MSCs fabricated with (g) IDEs and (h) HPEs, measured at current densities ranging from 10 to 100 μA cm−2. (i) Area capacitance of the device as a function of current density, derived from GCD measurements.

Figure 5. Cycling and bending stability. (a) Capacitance retention of MSCs after 10,000 charge–discharge cycles using IDEs and HPEs. Photographs of MSCs on a bent substrate with a bending radius of 2.5 mm, obtained using (b) IDEs and (c) HPEs. (d) CV curves of the HPE device during the 1st and 10,000th charge–discharge cycles. CV curves of (e) IDEs and (f) HPEs before and after 1,000 bending cycles. All CV curves were recorded at a scan rate of 100 mV s−1 with the devices lying flat after the bending cycles. (g) Schematic illustration of the HPE and (h) IDE before and after bending.

Figure 6. (a) CV curves of the array and a single HPE device at a scan rate of 100 mV s⁻¹; (b) GCD curves of the array and a single device at a current of 0.1 μA; (c) Schematic and photograph of ten HPE devices connected in series; (d) CV curves of the series‑connected array and a single HPE device over scan rates from 100 to 500 mV s⁻¹; (e) GCD curves of the series‑connected array and a single device at currents ranging from 0.1 to 0.5 μA; (f) GCD curves of the series‑connected array and a single device at currents ranging from 0.1 to 0.5 μA; (g) CV curve of the series‑connected array over scan rates from 100 to 500 mV s⁻¹; (h) GCD curve of the series‑connected array at currents ranging from 0.1 to 0.5 μA; (i) CV curve of the series‑connected array over scan rates from 100 to 500 mV s⁻¹; (j) GCD curve of the series‑connected array at currents ranging from 0.1 to 0.5 μA; (k) Comparison of the integrated density (1/Ssingle) and the voltage output per unit area (Voutput/single device) of the previously reported 3D‑printed series‑connected MSC array under high‑voltage operation, where Ssingle and Voutput denote, respectively, the area of a single device and the output voltage of each device.

From traditional interdigitated electrodes to innovative hollow‑pillar electrodes, and from conventional complex 3D printing to precision aerosol‑jet printing, this research has, through the synergistic innovation of materials, structures, and processes, successfully overcome multiple limitations in microsupercapacitors—namely, integration density, voltage output, and mechanical flexibility—thereby setting a new benchmark for the advancement of micro‑energy‑storage technologies.

As the Internet of Things, wearable devices, and flexible electronics continue to advance, demand for miniature energy‑storage devices is set to grow steadily. This 3D‑printed microsupercapacitor, built on graphene hollow‑pillar electrodes, not only addresses key challenges in existing technologies but also underscores the powerful potential of “materials innovation plus process breakthroughs.” Looking ahead, with further technological refinements—such as higher energy density, lower manufacturing costs, and expanded material platforms—these high‑performance microsupercapacitors are poised for large‑scale commercial deployment, providing a stable and reliable “miniature energy core” for the era of the Internet of Everything.

The “density revolution” in micro‑energy storage has already begun, and we may soon experience the enhanced battery life this technology brings to the smart devices around us.