Flexible Supercaps Enable Body-Sensing Harvesting-Powered Apps

Supercapacitors are widely used for energy storage and often associated with harvesting applications. However, their physical rigidity makes them a less-than-desired option for many such situations, especially those that use a body’s motion as the energy source. Among the limitations of conventional micro-supercapacitors, with their “sandwich-like” stacked geometry, are poor flexibility, long ion-diffusion distances, and a complex integration process when combined with wearable electronics.

Now, a research team based at Pennsylvania State University (Penn State), along with members from Minjiang University and Nanjing University (both in China), has devised and tested planar micro-supercapacitor arrays (MSCAs) (Fig. 1). These are based on hybrid electrodes using ultra-thin zinc-phosphor (Zn₃P₂) nanosheets anchored on 3D laser-induced graphene foams (ZnP@LIG), which are arranged in an island-bridge device architecture. The MSCAs can complement or replace lithium-ion batteries in wearable devices due their small footprint, high power density, and the ability to charge and discharge quickly.

1. These micro-supercapacitor arrays (MSCAs) can provide energy storage along with flexibility and stretchability. (Source: Pennsylvania State University)

Led by Prof. Huanyu “Larry” Cheng, the team explored alternative device architectures and integration processes to advance the use of micro-supercapacitors in wearable devices. They found that arranging micro-supercapacitor cells in a serpentine, island-bridge layout enables the configuration to stretch and bend at the bridges, while reducing deformation of the micro-supercapacitors (the islands). Hence the designation as micro-supercapacitors arrays.

Prof. Cheng noted, “By using an island-bridge design when connecting cells, the micro-supercapacitor arrays displayed increased stretchability and allowed for adjustable voltage outputs. This allows the system to be reversibly stretched up to 100%.”

By using non-layered, ultra-thin zinc-phosphorus nanosheets and 3D laser-induced graphene foam—a highly porous, self-heating nanomaterial—to construct the island-bridge design of the cells, Cheng and his team saw significant improvements in electric conductivity and the number of absorbed charged ions.

The MSCA numbers tell the story: The hybrid electrodes with a large specific surface area demonstrate excellent ionic and electrical conductivities, with gravimetric and areal capacitances of 1425 F/gram and 7.125 F/cm² at 1 A/gram, respectively. In addition, they achieved high energy density of 245 mW-hr/cm² along with power density of 12.50 mW/kg at 145 mW-hr/cm². They adjusted and controlled voltage- and current-output ratings as needed by varying serial and parallel connections of MSCA cells within the island-bridge design.

The basic MSCA numbers alone are only part of the story. The team integrated stretchable MSCAs with a “crumpled” Au-based triboelectric nanogenerator (TENG) plus a stretchable, crumpled graphene-based strain sensor to demonstrate a self-powered, energy-harvesting stretchable system (Fig. 2).

2. The MSCAs (center) are coupled with a triboelectric nanogenerator (left) and a graphene-based strain sensor (right). Key performance attributes are shown below each segment.

These MSCAs with adjustable voltage/current outputs can be reversibly stretched to 100% and integrated with a TENG and sensors to create a viable solution for a self-powered stretchable system. The details are in their paper published in Nano Energy with the formidable title, “High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems.”