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.

Hierarchically Engineered Nanocarbon Florets as Bifunctional Electrode Materials for Adsorptive and Intercalative Energy Storage.

Three-dimensional dendritic nanostructured carbon florets (NCFs) with tailored porosity are demonstrated as electrochemically versatile electrodes for both adsorptive and intercalative energy storage pathways. Achieved through a single-step template-driven approach, the NCFs exhibit turbostratic graphitic lamellae in a floral assembly leading to high specific surface area and multi-modal pore distribution (920 m2/g). The synergism in structural and chemical frameworks, along with open-ended morphology, enables bifunctionality of hard carbon NCFs as symmetric adsorptive electrodes for supercapacitors (SCs) and intercalation anodes for hybrid potassium-ion capacitors (KICs). Flexible, all-solid-state SCs through facile integration of NCF with the ionic-liquid-imbibed porous polymeric matrix achieve high-energy density (20 W h/kg) and power density (32.7 kW/kg) without compromising on mechanical flexibility and cyclability (94% after 20k cycles). Furthermore, NCF as an anode in a full-cell hybrid KIC (activated carbon as cathode) delivers excellent electrochemical performance with maximum energy and power densities of 57 W h/kg and 12.5 kW/kg, respectively, when cycled in a potential window of 1.0-4.0 V. The exceptional bifunctional performance of NCF highlights the possibility of utilizing such engineered nanocarbons for high-performance energy storage devices.

Interwoven Carbon Nanotube Wires for High-Performing, Mechanically Robust, Washable, and Wearable Supercapacitors.

An energy storage system with large storage capacity, rapid power release, and simultaneous tolerance to harsh mechanical stresses is a major bottleneck for realizing self-sustaining, wearable electronics. Addressing this, we demonstrate carbon nanotube wire (CNT-wire) interwoven solid-state supercapacitive energy storage devices (sewcaps) exhibiting superior storage capacity (30 Wh/kg, compared to electrochemical capacitors at ∼10 Wh/kg) and 14-fold higher power density (3511 W/kg) compared to Li-ion batteries (∼250 W/kg). While the high specific surface area and electrical conductivity of CNT-wires and high ionic conductivity of the electrolyte enable high energy density, the device design enables the combination of planar and radial diffusive pathways for ultralow interface resistance (∼0.2 mΩ/sewcap) and rapid charging-discharging ability (τ = 1.16 ms). Thus, this versatile approach of interweaving to form functional devices provides tunable power delivery across six orders of magnitude (2 µW to 2 W) through reconfiguration of the interweaving pattern and density. Importantly, such textile-integrated sewcaps exhibit unaltered performance (>95% retention across 4000 charge-discharge cycles) under extreme mechanical punishments such as repeated laundering, flexing (∼68°), rolling (360°), and crushing (∼21.8 kPa), implying direct interfacing with wearable platforms.

Real-Time, Wearable, Biomechanical Movement Capture of Both Humans and Robots with Metal-Free Electrodes.

We demonstrate an all-carbon-based, flexible, conformal movement-capturing device capable of precisely monitoring biomechanical movements of both humans and robots. Mechanically robust, metal-free electrodes form a unique component of the device responsible for qualitatively and quantitatively transducing biomechanical movements without any signal artifacts. Importantly, the device withstands and operates in a wide dynamic range for both stretching (25% strain) and bending (140°) actions with minimal cycling hysteresis (2.0), high repeatability (>100 cycles), low creep, and humidity-independent rapid response (∼200 ms). Furthermore, the device qualitatively distinguishes movements such as bending of finger, knuckle, and wrist and also provides quantitative information on the extent of such movements. We establish that single-wall carbon nanotubes (CNTs) embedded in ultralow concentration (0.016 wt %) within an elastomeric matrix undergo three-dimensional conformational changes during biomechanical movements that are subsequently transduced as signals. In addition, such CNT-elastomer strips exhibit enhanced stretchability (>100%) and elasticity (∼77%) in comparison to those of pure elastomers, leading to a wider dynamic working range of the device. Furthermore, seamless integration of a versatile gesture tracker on ubiquitous platforms, such as human skin, kinesiologic tapes, gloves, and robotic arms, is achieved, thereby catering to applications ranging from healthcare monitoring and physiotherapy to robotics and wearable technologies.