Design and Testing of Autonomous Chargeable and Wearable Sweat/Ionic Liquid-Based Supercapacitors

A Protic Ionic Liquid Promoted Gel Polymer Electrolyte for Solid-State Electrochemical Energy Storage

This study presents the synthesis of a transparent, flexible gel polymer electrolyte (GPE) based on the protic ionic liquid BMImHSO4 and on polyvinyl alcohol (PVA) through solution casting and electrochemical evaluation in a 2.5 V symmetrical C/C electrical double-layer solid-state capacitor (EDLC). The freestanding GPE film exhibits high thermal stability (>300 °C), wide electrochemical windows (>2.7 V), and good ionic conductivity (2.43 × 10-2 S cm-1 at 20 °C). EDLC, using this novel GPE film, shows high specific capacitance (81 F g-1) as well as good retention above 90% of the initial capacitance after 4500 cycles. The engineered protic ionic liquid GPE is, hopefully, applicable to high-performance solid-state electrochemical energy storage.

Cellulose-based Conductive Materials for Bioelectronics

The growing demand for electronic devices has led to excessive stress on Earth's resources, necessitating effective waste management and the search for renewable materials with minimal environmental impact. Bioelectronics, designed to interface with the human body, have traditionally been made from inorganic materials, such as metals, which, while having suitable electrical conductivity, differ significantly in chemical and mechanical properties from biological tissues. This can cause issues such as unreliable signal collection and inflammatory responses. Recently, natural biopolymers such as cellulose, chitosan, and silk have been explored for flexible devices, given their chemical uniqueness, shape flexibility, ease of processing, mechanical strength, and biodegradability. Cellulose is the most abundant natural biopolymer, has been widely used across industries, and can be transformed into electronically conductive carbon materials. This review focuses on the advancements in cellulose-based conductive materials for bioelectronics, detailing their chemical properties, methods to enhance conductivity, and forms used in bioelectronic applications. It highlights the compatibility of cellulose with biological tissues, emphasizing its potential in developing wearable sensors, supercapacitors, and other healthcare-related devices. The review also addresses current challenges in this field and suggests future research directions to overcome these obstacles and fully realize the potential of cellulose-based bioelectronics.

Simultaneous detection of tyrosine and uric acid in sweat using CoWO4@CNT with a hydrogel modified electrochemical biosensor

The levels of uric acid (UA) and tyrosine (Tyr) in sweat reflect a person's overall health. However, simultaneously identifying several components in sweat remains challenging. Here, we achieve simultaneous detection of UA and Tyr by synthesizing CoWO4@CNT in a single step using a hydrothermal method. CoWO4's high catalytic efficacy and large CNT reaction area allow the detection of 1-1000 μM UA (LOD = 0.14 μM) and 5-1000 μM Tyr (LOD = 4.2 μM). To increase sweat collection, we developed a polydopamine-polyacrylamide (PDA-PAM) hydrogel with a sweat absorption rate of up to 226%. Finally, by monitoring sweat at various times of day, our sensors can discriminate between UA and Tyr in real sweat, and the results are consistent with the individuals' activity levels. Overall, the effective electrocatalytically active materials and PDA-PAM hydrogel improve the detection of UA and Tyr. The remarkable performance of CoWO4@CNT in real samples shows that it has the potential to improve health detection and real-time sweat analysis.

Direct ink writing 3D printing of low-dimensional nanomaterials for micro-supercapacitors

Micro-supercapacitors (MSCs) have attracted significant attention for potential applications in miniaturized electronics due to their high power density, rapid charge/discharge rates, and extended lifespan. Despite the unique properties of low-dimensional nanomaterials, which hold tremendous potential for revolutionary applications, effectively integrating these attributes into MSCs presents several challenges. 3D printing is rapidly emerging as a key player in the fabrication of advanced energy storage devices. Its ability to design, prototype, and produce functional devices incorporating low-dimensional nanomaterials positions it as an influential technology. In this review, we delve into recent advancements and innovations in micro-supercapacitor manufacturing, with a specific focus on the incorporation of low-dimensional nanomaterials using direct ink writing (DIW) 3D printing techniques. We highlight the distinct advantages offered by low-dimensional nanomaterials, from quantum effects in 0D nanoparticles that result in high capacitance values to rapid electron and ion transport in 1D nanowires, as well as the extensive surface area and mechanical flexibility of 2D nanosheets. Additionally, we address the challenges encountered during the fabrication process, such as material viscosity, printing resolution, and seamless integration of active materials with current collectors. This review highlights the remarkable progress in the energy storage sector, demonstrating how the synergistic use of low-dimensional nanomaterials and 3D printing technologies not only overcomes existing limitations but also opens new avenues for the development and production of advanced micro-supercapacitors. The convergence of low-dimensional nanomaterials and DIW 3D printing heralds the advent of the next generation of energy storage devices, making a significant contribution to the field and laying the groundwork for future innovations.

Built-In Piezoelectric Nanogenerators Promote Sustainable and Flexible Supercapacitors: A Review

Energy storage devices such as supercapacitors (SCs), if equipped with built-in energy harvesters such as piezoelectric nanogenerators, will continuously power wearable electronics and become important enablers of the future Internet of Things. As wearable gadgets become flexible, energy items that can be fabricated with greater compliance will be crucial, and designing them with sustainable and flexible strategies for future use will be important. In this review, flexible supercapacitors designed with built-in nanogenerators, mainly piezoelectric nanogenerators, are discussed in terms of their operational principles, device configuration, and material selection, with a focus on their application in flexible wearable electronics. While the structural design and materials selection are highlighted, the current shortcomings and challenges in the emerging field of nanogenerators that can be integrated into flexible supercapacitors are also discussed to make wearable devices more comfortable and sustainable. We hope this work may provide references, future directions, and new perspectives for the development of electrochemical power sources that can charge themselves by harvesting mechanical energy from the ambient environment.

Molecular dynamics study of fluorosulfonyl ionic liquids as electrolyte for electrical double layer capacitors

The development of high-performance supercapacitors is an important goal in the field of energy storage. Ionic liquids (ILs) are promising electrolyte materials for efficient energy storage in supercapacitors, because of the high stability, low volatility, and wider electrochemical stability window than traditional electrolytes. However, ILs-based supercapacitors usually show a relatively lower power density owing to the inherent viscosity-induced low electrical conductivity. Fluorosulfonyl ILs have aroused much attention in energy storage devices due to its low toxicity and excellent stability. Here, we propose that structural modification is an effective way to improve the energy storage performance of fluorosulfonyl ILs through the classical molecular dynamics (MD) method. Four fluorosulfonyl ILs with different sizes and symmetries were considered. Series of properties including conductivity, interface structure, and double-layer capacitance curves were systematically investigated. The results show that smaller size and more asymmetric structure can enhance self-diffusion coefficient and conductivity, and improve the electrochemical performance. Appropriate modification of the electrodes can further enhance the capacitive performance. Our work provides an opportunity to further understand and develop the fluorosulfonyl ILs electrolyte in supercapacitors.

Self-chargeable supercapacitor made with MXene-bacterial cellulose nanofiber composite for wearable devices

The development of wearable electronics is restricted by the developments of supporting energy storage devices, especially flexible supercapacitors. Nowadays, miniaturized supercapacitors based on MXenes due to their obvious advantages in the specific capacity have received extensive attention. The energy existing in the surrounding environment has been used to directly charge energy storage devices. However, the hybrid wearable electronics integrated supercapacitors are mechanically connected through metal wires leading to non-compact devices. Thus, it is urgent to develop a general and universal method to fabricate high-performance robust MXene-based flexible electrodes with high electrical conductivity and apply them to self-chargeable supercapacitors and compact wearable devices. Herein, the bacterial cellulose (BC) nanofibers are used as a crosslinking agent to connect two-dimensional MXene nanosheets through the hydrogen bond, which greatly improves the mechanical strength of MXene-bacterial cellulose (MXene-BC) composite films (Young's modulus reaching 6.8 GPa). The supercapacitors made with the electrodes of MXene-BC composite films (BC content is 10%) present high capacitance behavior (areal capacitance up to 346 mF cm^-2) because the introduction of BC nanofibers increases the interlayer spacing of MXene nanosheets, providing more storage space for the ions in the electrolyte. Then, a self-chargeable supercapacitor is proposed based on the combination of a zinc-air (Zn-air) battery and a supercapacitor. The self-chargeable supercapacitor can realize self-charging after dropping a drop of electrolyte solution into the Zn-air battery. The charging voltage of a single self-chargeable supercapacitor can reach 0.6 V after adding artificial sweat as the electrolyte. Finally, a smart wristband with the function of self-charging is proposed, which can absorb the sweat generated by the human for self-chargeable supercapacitors to drive the pedometer integrated within the smart wristband to work. The proposed self-chargeable supercapacitors are simple and effective, not restricted by the use environment, providing a promising way for self-powered wearable electronics.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.