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.

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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.