Design Principles for Manipulating Electrochemical Interfaces in Solid-State Supercapacitors for Wearable Applications

Storage and delivery of electrical energy form the heart of the rapidly expanding domain of wearable electronics, with applications ranging from point-of-care medical diagnostics to Internet-of-Things (IoT). Solid-state, electrochemical, double-layer-based supercapacitive energy storage devices, with high power density, ability to interface with intermittent energy harvesters, long lifetime, and cyclability, offer attractive possibilities for self-sustaining power sources in such portable applications. This mini-review highlights the need for a multipronged approach involving (a) development of materials for electrodes and electrolyte and (b) utilizing the right kind of design principles, processing techniques, and fabrication approaches to (c) achieve seamless all-solid electrode-electrolyte interfaces providing (d) facile integration onto wearable platforms. Importantly, a comprehensive figure-of-merit (FOM) accounting for both the electrochemical performance and the mechanical robustness of flexible supercapacitors is proposed. This is expected to facilitate uniform comparison of performance across devices differing in their design approaches and materials. Finally, new operando and in situ techniques for probing and understanding such all-solid interfaces are presented. The iterative cycle of scientific understanding, furthering technological advancements, seeks to provide future directions for achieving mechanically robust supercapacitors with enhanced energy density and power density for wearable and portable applications.