Technological Advances In Flexible Energy Storage Devices And Flexible Electronics

Flexible electronics has emerged as one of the most potential technologies that could revolutionize the modern society, including military defense, medical diagnosis and treatment, wellness preventive care, recreation sports, and education.

With the vigorous process of nanofabrication and nanotechnology, an array of flexible electronics devices have been demonstrated, which, however, still largely rely on rigid and bulky power sources. Substantial efforts have been focused on investigating energy devices that are flexible and portable. Among them, supercapacitors received considerable attention due to their outstanding performances in high power density, ultra-long cycle life, facileness, and low-cost in fabrication.

In a recent paper published in Advanced Functional Materials, researchers from the University of Texas at Austin reported the creation of an innovative type of all-solid-state flexible supercapacitors by employing unique three-dimensional (3D) graphite foams with tunable hierarchical porosities as electrode support. After optimization, the 3-D graphite foams with 1.9 mm micropores show ultralow mass density (~0.15 mg/cm²) and approximately two-time enhancement in the volumetric surface area compared to conventional graphite foams (0.7 to 1.5 mg/cm²) without interfering with the electric conductivity (~10 S/cm).

Moreover, the strategically engineered corrugations created on the 3D hierarchical graphite foams not only provide substantially greater loading efficiency of pseudocapacitive materials and higher electric conductivity for fast charging and discharging, but also accommodate mechanical strains much more effectively. When loaded with manganese oxide (Mn₃O₄) nanoparticles, the composites achieve one of the best performances in energy storage, i.e., offering specific capacitances of 538 F/g (1 mV/s) and 260 F/g (1 mV/s) based on the mass of pure Mn₃O₄ and entire electrode composite, respectively.

Furthermore, they can be integrated as all-solid-state flexible symmetric supercapacitors with a full cell-specific capacitance as high as 53 F/g based on the entire electrode mass, which is even comparable to some of those obtained based on the net mass of active materials. Furthermore, they retain 80% of peak capacitance after 1000 continuous mechanical bending cycles.

It is potent to integrate flexible energy devices with flexible electronics for practical applications. Here, the authors seamlessly integrated the flexible all-solid-state supercapacitors with wearable strain sensors into self-powered strain sensors. Leveraging the low capacitance variance of the supercapacitor under mechanical deformation and ultra-sensitivity of the strain sensor, the integrated flexible device powers itself conforms to human skins, and readily detects both coarse and fine motions.

For instance, when attached to a finger, the device shows periodic electric signals with bending of the finger. When attached to skins near the carotid artery of a volunteer, electric signals show a heart beating rate of 96 per minute. The design, fabrication, and applications reported in this work are expected to inspire new approaches to achieve fully integrated flexible energy storage devices with outstanding performances for wearable electronics.

The work is led by Prof. Donglei “Emma” Fan and her Ph.D. student Weigu Li at the University of Texas at Austin. The study, Ultralight and Binder-Free All-Solid-State Flexible Supercapacitors for Powering Wearable Strain Sensors was recently published in the journal Advanced Functional Materials.