Stretchable Supercapacitors as Emergent Energy Storage Units for Health Monitoring Bioelectronics

Emerging health monitoring bioelectronics require energy storage units with improved stretchability, biocompatibility, and self‐charging capability. Stretchable supercapacitors hold great potential for such systems due to their superior specific capacitances, power densities, and tissue‐conforming properties, as compared to both batteries and conventional capacitors. Despite the rapid progress that has been made in supercapacitor research, practical applications in health monitoring bioelectronics have yet to be achieved, requiring innovations in materials, device configurations, and fabrications tailored for such applications. In this review, the progress in stretchable supercapacitor‐powered health monitoring bioelectronics is summarized and the required specifications of supercapacitors for different types of application settings with varying demands on biocompatibility are discussed, including nontouching wearables, skin‐touching wearables, skin‐conforming wearables, and implantables. The perspective of this review is then broadened to focus on integration of stretchable supercapacitors in bioelectronics and aspects of energy harvesting and sensing. Finally further insights on the existing challenges in this developing field and potential solutions are provided.     

Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities

In recent years, tremendous research effort has been aimed at increasing the energy density of supercapacitors without sacrificing high power capability so that they reach the levels achieved in batteries and at lowering fabrication costs. For this purpose, two important problems have to be solved: first, it is critical to develop ways to design high performance electrode materials for supercapacitors; second, it is necessary to achieve controllably assembled supercapacitor types (such as symmetric capacitors including double‐layer and pseudo‐capacitors, asymmetric capacitors, and Li‐ion capacitors). The explosive growth of research in this field makes this review timely. Recent progress in the research and development of high performance electrode materials and high‐energy supercapacitors is summarized. Several key issues for improving the energy densities of supercapacitors and some mutual relationships among various effecting parameters are reviewed, and challenges and perspectives in this exciting field are also discussed. This provides fundamental insight into supercapacitors and offers an important guideline for future design of advanced next‐generation supercapacitors for industrial and consumer applications.     

A New View of Supercapacitors: Integrated Supercapacitors

Charging times ranging from seconds to minutes with high power densities can be achieved by electrochemical capacitors in principle. Over the past few decades, the performance of supercapacitors has been greatly improved by the utilization of new materials, preparation of unique nanostructures, investigation of electrolytes, and so on. However, the discovery of the related basic theory is very limited. Herein, a new view of a supercapacitor called the “integrated supercapacitor” is proposed. The electrode of the integrated supercapacitor consists of certain positive and negative materials. With this design, a single integrated electrode can work in both the positive and negative potential windows simultaneously. Additionally, the integrated full supercapacitor device shows a much higher capacitance and wider potential window than traditional single symmetric and asymmetric supercapacitors, which results from its multiple mechanisms, including the traditional positive//positive symmetric, positive//negative asymmetric, and negative//negative symmetric full supercapacitor mechanisms.     

Highly Stable Supercapacitors with Conducting Polymer Core‐Shell Electrodes for Energy Storage Applications

Conducting polymers such as polyaniline (PAni) show a great potential as pseudocapacitor materials for electrochemical energy storage applications. Yet, the cycling instability of PAni resulting from structural alteration is a major hurdle to its commercial application. Here, the development of nanostructured PAni–RuO₂ core–shell arrays as electrodes for highly stable pseudocapacitors with excellent energy storage performance is reported. A thin layer of RuO₂ grown by atomic layer deposition (ALD) on PAni nanofibers plays a crucial role in stabilizing the PAni pseudocapacitors and improving their energy density. The pseudocapacitors, which are based on optimized PAni–RuO₂ core–shell nanostructured electrodes, exhibit very high specific capacitance (710 F g^?1 at 5 mV s^?1) and power density (42.2 kW kg^?1) at an energy density of 10 Wh kg^?1. Furthermore, they exhibit remarkable capacitance retention of ≈88% after 10 000 cycles at very high current density of 20 A g^?1, superior to that of pristine PAni‐based pseudocapacitors. This prominently enhanced electrochemical stability successfully demonstrates the buffering effect of ALD coating on PAni, which provides a new approach for the preparation of metal‐oxide/conducting polymer hybrid electrodes with excellent electrochemical performance.