Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices. By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors. This review provides comprehensive knowledge to this field. We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area. Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories. We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors.

The demand for flexible/wearable electronic devices that have aesthetic appeal and multi-functionality has stimulated the rapid development of flexible supercapacitors with enhanced electrochemical performance and mechanical flexibility. After a brief introduction to flexible supercapacitors, we summarize current progress made with graphene-based electrodes. Two recently proposed prototypes for flexible supercapacitors, known as micro-supercapacitors and fiber-type supercapacitors, are then discussed. We also present our perspective on the development of graphene-based electrodes for flexible supercapacitors.

Origami-inspired self-folding graphene papers show remote control grasping, manipulation, and walking behaviors.

Anion exchange membrane (AEM) electrolysis is a promising technology to produce hydrogen through the splitting of pure water. In contrast to proton-exchange-membrane (PEM) technology, which requires precious-metal oxide anodes, AEM systems allow for the use of earth-abundant anode catalysts. Here we report a study of first-row transition-metal (oxy)­hydroxide/oxide catalyst powders for application in AEM devices and compare physical properties and performance to benchmark IrO x catalysts as well as typical catalysts for alkaline electrolyzers. We show that the catalysts’ oxygen-evolution activity measured in alkaline electrolyte using a typical three-electrode cell is a poor indicator of performance in the AEM system. The best oxygen-evolution-reaction (OER) catalysts in alkaline electrolyte, NiFeO x H y oxyhydroxides, are the worst in AEM electrolysis devices where a solid alkaline electrolyte is used along with a pure water feed. NiCoO x -based catalysts show the best performance in AEM electrolysis. The performance can be further improved by adding Fe species to the particle surface. We attribute the differences in performance in part to differences in the electrical conductivity of the catalyst phases, which are also measured and reported.

A facile and passive multiply flexible thin-film sensor is demonstrated based on thermoelectric effects in graphene. The sensor is highly conductive, free-standing, flexible, and elastic. It senses heat and cold, and measures heated/cooled areas; it also discerns human touch from other pressures, locates human touch, and measures pressure levels. All of these sensing abilities are demonstrated without any internal/external power supply.

The ability to achieve simultaneous intrinsic deformation with fast response in commercially available materials that can safely contact skin continues to be an unresolved challenge for artificial actuating materials. Rather than using a microporous structure, here we show an ambient-driven actuator that takes advantage of inherent nanoscale molecular channels within a commercial perfluorosulfonic acid ionomer (PFSA) film, fabricated by simple solution processing to realize a rapid response, self-adaptive, and exceptionally stable actuation. Selective patterning of PFSA films on an inert soft substrate (polyethylene terephthalate film) facilitates the formation of a range of different geometries, including a 2D (two-dimensional) roll or 3D (three-dimensional) helical structure in response to vapor stimuli. Chemical modification of the surface allowed the development of a kirigami-inspired single-layer actuator for personal humidity and heat management through macroscale geometric design features, to afford a bilayer stimuli-responsive actuator with multicolor switching capability.

Quasi-solid-state micro-supercapacitors with cellular graphene film as the active material and polyvinyl alcohol/H3PO4as the gel electrolyte have been fabricated. The 3D porous graphene films not only serve as high performance supercapacitor electrodes, but also provide an abundant ion reservoir for the gel electrolyte.

Using a 3D stretching method, a highly elastic reduced graphene oxide (rGO)/polyacrylic ester hierarchically wrinkled elastic transparent conductor (HWETC) is fabricated. Periodic hierarchical N-rGO layer wrinkling allows the HWETC to show high conductivity (100-457 Ω ◻ ) and transmittance (67-85%) under substantial stretching (>400%) and bending deformation (≈180°), which enables electrothermal actuation and strain sensing.