As electronics shrink, they require smaller components—including for energy storage. Batteries store a lot of energy but deliver power more slowly, whereas supercapacitors can rapidly charge and discharge but don’t hold much energy. This is the case not only for larger bulk batteries and supercapacitors, but also at much smaller scales, with both microbatteries and microsupercapacitors having the same limitations as their bigger counterparts.

Now researchers from University College London (UCL) have developed a hybrid energy-storage option that offers a better balance between storage capacity and discharge rates. The component, called a zinc-ion micro-capacitor (ZIMC), could one day find its way into compact devices, such as wearables, medical implants, and IoT devices.

“It wasn’t our aim to outperform microbatteries and microsupercapacitors in every way,” says Buddha Deka Boruah, a lecturer in energy storage at UCL, “but to create a middle-ground device that balances energy and power in a small footprint.” The researchers published their results in March in ACS Nano.

The device combines aspects of microcapacitors and microbatteries in a small, on-chip format. The researchers developed porous, three-dimensional interdigitated electrodes (IDEs) out of gold as current collectors using dynamic bubbling electrodeposition, which creates a porous structure with a relativelylarge surface area.

Current collectors are thin metallic layers that transport the electrons between the electrodes and the external circuit (both ways to and from the electrode). Reducing the thickness of current collectors is one of the key ways of reducing the size of energy storage devices.

A microplotter fabrication technique loaded the porous current collectors with different materials to create the two electrodes. The researchers added zinc ions to the IDE to form a battery-like anode and activated carbon coated with a conductive polymer called PEDOT to create a capacitor-like hybrid cathode.

The porous nature of the IDEs allows more active material―the zinc ions and activated carbon-PEDOT material―to be loaded into the electrodes. This improves the device’s energy capacity, as well as how easily the current flows, because the contact area between the current collectors and electrodes is relatively large.

The porous nature of the IDEs also improves the ion movement in and out of the electrodes to increase the amount of charge that can be stored. In the device, the zinc anode stores energy like a battery through the plating and stripping of zinc ions. However, the cathode rapidly stores and releases energy using both double-layer capacitance and fast redox reactions.

The zinc-ion microcapacitor’s design allows it to store more energy than a supercapacitor but release power more quickly The advantageous structure of the 3D Au Zn//AC-PEDOT SIMC.Yujia Fan, Nibagani Naresh et al./ACS Nano

The development offers more energy and power in a smaller package than microsupercapacitors, but it doesn’t store as much energy as microbatteries. It instead adopts a middle ground between the two conventional architectures: Fast charging and discharging rates, a long cycle life of thousands of charge-discharge cycles, a lower risk of overheating, and a device area of just 0.4 square centimeters.

“Devices of just a few hundred micrometers wide are possible,” says Boruah. “They could even be made smaller, and we are intensively working on it.”

The ZIMCs developed in this study store around 1.2 microwatt-hours of energy per square centimeter, which is less than the 0.37 milliwatt-hours per square centimeter for microbatteries—however the ZIMCs charge faster and last for more charge cycles. Compared to microsupercapacitors, the ZIMCs have a much higher power areal density (640 microwatts per square centimeter compared to 0.0056 μW/cm²), which means that a ZIMC can rapidly discharge more energy than a microsupercapacitor.

The ZIMCs also perform well against other hybrid energy-storage devices that have been developed in the past. Lithium-based and sodium-based hybrid energy-storage devices store more energy than these ZIMCs, but the ZIMCs are potentially safer and longer-lasting. The ZIMCs can also be manufactured directly onto chips using simple processing methods.

“Rather than replacing existing microbatteries or microsupercapacitors, our device brings together the strengths of both, offering a compact and efficient solution for next-generation on-chip electronics” says Boruah.

Despite the developments, Boruah noted using gold for the current collectors could potentially be too expensive for commercial use despite the performance of the device. Additionally, while the material choices and architecture are flexible and can bend and stretch more than other, more rigid energy storage options, the researchers have not yet explicitly tested how the device performs under bending and mechanical stress.

The researchers plan to build on this research by improving the scalability, flexibility, and integration of these ZIMCs into real-world devices―including integrating the ZIMCs alongside microsensors for system on a chip( SoC) microsystems. They also plan to explore alternative current-collector materials to replace gold―but with the same level of performance and at a more commercially feasible cost―and optimize the electrode design to further improve the energy density.