Thanks to rapid technological innovation over the last decade, a wide range of wearable devices have emerged, playing an inspiring role in healthcare, diagnostics, and sports monitoring. This review summarizes recent progress in the development and application of electrochemical (EC) sweat sensors and self-powered systems for wearable applications. Presented here are the most common types of EC sweat sensors, namely potentiometric ion-selective electrodes, amperometric enzymatic and non-enzymatic sensors, as well as differential pulse voltammetric sensors. The sensing principle and novel functional nanomaterials that are used to enhance the performance of wearable sweat sensors and their applications are summarized. The recent advancement of battery-free wearable devices is presented, including self-powered biofuel cells (BFC), energy harvesting from novel materials and its strategy, are also discussed. The challenges in, and opportunities for, the development of sweat sensor-based wearable systems moving forward are reviewed, with the aim of shedding some light on future directions.

Field-effect transistors have attracted significant attention in chemical sensing and clinical diagnosis, due to their high sensitivity and label-free operation. Through a scalable photolithographic process in this study, we fabricated graphene-based ion-sensitive field-effect transistor (ISFET) arrays that can continuously monitor sodium ions in real-time. As the sodium ion concentration increased, the current–gate voltage characteristic curves shifted towards the negative direction, showing that sodium ions were captured and could be detected over a wide concentration range, from 10⁻⁸ to 10⁻¹ M, with a sensitivity of 152.4 mV/dec. Time-dependent measurements and interfering experiments were conducted to validate the real-time measurements and the highly specific detection capability of our sensor. Our graphene ISFETs (G-ISFET) not only showed a fast response, but also exhibited remarkable selectivity against interference ions, including Ca²⁺, K⁺, Mg²⁺ and NH₄⁺. The scalability, high sensitivity and selectivity synergistically make our G-ISFET a promising platform for sodium sensing in health monitoring.

Technological demands in HEVs, renewable systems, and electrical storage systems coupled with its mature industrial process, recyclability and low cost has furthered the interests in Lead acid (LAB) systems. The LAB Positive active materials, due to low utilization and life cycle, severely limits the competitiveness of the traditional battery. Reported attempts to optimize the lead acid system using nano-size materials including graphene and carbon nanotubes, has yielded increase in some performance metrics, but little is known about the responsible mechanisms ^{1,2 3}. Based on the gel-crystal theory, Pavlov et al. ^4–6has established that the lead acid active mass particles comprises interfaces made of intermediate amorphous molecules, within which the electrochemical reactions take place. Little or nothing is known about the effects of additives of micro- and sub-micro sizes on the redox activity in the gel zone, and how the chemistry of the active mass is improved to result in global increase in capacity and cycle life of the positive active mass. This research demonstrates highly improved discharge capacity and cycle life obtained from careful combination of cathode materials with tailored graphene based additives: Graphene Oxide (GO), chemically converted graphene (CCG) and pristine graphene (GX). The chemical and electrochemical phenomena responsible for the high activity of the positive active particles in the cathode were established. PbO-GO had the best performance with highest utilization of 41.8%, followed by PbO-CCG (37.7%), PbO-control (29.7%), and PbO-GX (28.7%) at 2.5C rate. PbO-CCG seemed to have the best performance: better on charging, lower internal resistance, but poorer cycle life compared to PbO-GO. GO & CCG optimized samples had better discharge capacity and cyclic performance. Cycled at 2.5C rate, all samples but the control, had Increasing capacity till after ~50 cycles. This is due to electro-functionalization of the graphene sheets which tailored the growth of PbO₂crystals in the gel zone. Also, there is a difference in the response of graphene composites to the chemistry of the reacting species on charging and discharging. The extra-ordinary mobility of ions was facilitated by electrochemical functionalization⁷and controlled nano-recrystallization of cathode particles along graphene sheets embedded with the gel-crystal interface. Porosity or surface area increase had no effects. REFERENCES 1. Xiang, J. et al. Beneficial effects of activated carbon additives on the performance of negative lead-acid battery electrode for high-rate partial-state-of-charge operation. J. Power Sources 241,150–158 (2013). 2. Wang, H., Yu, J., Zhao, Y. & Guo, Q. A facile route for PbO@C nanocomposites: An electrode candidate for lead-acid batteries with enhanced capacitance. J. Power Sources 224,125–131 (2013). 3. Yolshina, L. A., Yolshina, V. A., Yolshin, A. N. & Plaksin, S. V. Novel lead-graphene and lead-graphite metallic composite materials for possible applications as positive electrode grid in lead-acid battery. J. Power Sources 278,87–97 (2015). 4. Pavlov, D. & Balkanov, I. Hydration and Amorphization of Active Mass Pb02 Particles and Their Influence on the Electrical Properties of the Lead-Acid Battery Positive Plate. 136,(2003). 5. Pavlov, D. & Petkova, G. Phenomena That Limit the Capacity of the Positive Lead Acid Battery Plates. J. Electrochem. Soc. 149,A654 (2002). 6. Pavlov, D. The Lead-Acid Battery Lead Dioxide Active Mass: A Gel-Crystal System with Proton and Electron Conductivity. J. Electrochem. Soc. 139,3075 (1992). 7. Dada, O. In-Situ Electrochemical Functionalization of Reduced Graphene Oxide: Positive Lead Acid Electrode Case. in 227th ECS Meeting (May 24-28, 2015). Controlled Nano-Crystallization for Optimizing of Lead Acid Positive Active Material Using Graphene Nano-Composites Technological demands in HEVs, renewable systems, and electrical storage systems coupled with its mature industrial process, recyclability and low cost has furthered the interests in Lead acid (LAB) systems. The LAB Positive active materials, due to low utilization and life cycle, severely limits the competitiveness of the traditional battery. Reported attempts to optimize the lead acid system using nano-size materials including graphene and carbon nanotubes, has yielded increase in some performance metrics, but little is known about the responsible mechanisms ^{1,2 3}. Based on the gel-crystal theory, Pavlov et al. ^4–6 has established that the lead acid active mass particles comprises interfaces made of intermediate amorphous molecules, within which the electrochemical reactions take place. Little or nothing is known about the effects of additives of micro- and sub-micro sizes on the redox activity in the gel zone, and how the chemistry of the active mass is improved to result in global increase in capacity and cycle life of the positive active mass. This research demonstrates highly improved discharge capacity and cycle life obtained from careful combination of cathode materials with tailored graphene based additives: Graphene Oxide (GO), chemically converted graphene (CCG) and pristine graphene (GX). The chemical and electrochemical phenomena responsible for the high activity of the positive active particles in the cathode were established. PbO-GO had the best performance with highest utilization of 41.8%, followed by PbO-CCG (37.7%), PbO-control (29.7%), and PbO-GX (28.7%) at 2.5C rate. PbO-CCG seemed to have the best performance: better on charging, lower internal resistance, but poorer cycle life compared to PbO-GO. GO & CCG optimized samples had better discharge capacity and cyclic performance. Cycled at 2.5C rate, all samples but the control, had Increasing capacity till after ~50 cycles. This is due to electro-functionalization of the graphene sheets which tailored the growth of PbO₂ crystals in the gel zone. Also, there is a difference in the response of graphene composites to the chemistry of the reacting species on charging and discharging. The extra-ordinary mobility of ions was facilitated by electrochemical functionalization⁷ and controlled nano-recrystallization of cathode particles along graphene sheets embedded with the gel-crystal interface. Porosity or surface area increase had no effects. REFERENCES 1. Xiang, J. et al. Beneficial effects of activated carbon additives on the performance of negative lead-acid battery electrode for high-rate partial-state-of-charge operation. J. Power Sources 241, 150–158 (2013). 2. Wang, H., Yu, J., Zhao, Y. & Guo, Q. A facile route for PbO@C nanocomposites: An electrode candidate for lead-acid batteries with enhanced capacitance. J. Power Sources 224, 125–131 (2013). 3. Yolshina, L. A., Yolshina, V. A., Yolshin, A. N. & Plaksin, S. V. Novel lead-graphene and lead-graphite metallic composite materials for possible applications as positive electrode grid in lead-acid battery. J. Power Sources 278, 87–97 (2015). 4. Pavlov, D. & Balkanov, I. Hydration and Amorphization of Active Mass Pb02 Particles and Their Influence on the Electrical Properties of the Lead-Acid Battery Positive Plate. 136, (2003). 5. Pavlov, D. & Petkova, G. Phenomena That Limit the Capacity of the Positive Lead Acid Battery Plates. J. Electrochem. Soc. 149, A654 (2002). 6. Pavlov, D. The Lead-Acid Battery Lead Dioxide Active Mass: A Gel-Crystal System with Proton and Electron Conductivity. J. Electrochem. Soc. 139, 3075 (1992). 7. Dada, O. In-Situ Electrochemical Functionalization of Reduced Graphene Oxide: Positive Lead Acid Electrode Case. in 227th ECS Meeting (May 24-28, 2015).

Human sweat, a biomarker-rich physiological fluid, is an ideal candidate for non-invasive and real-time health monitoring. State-of-the-art electrochemical sweat sensors, that detect concentrations of analytes, such as electrolytes, through ion-selective membranes (ISM) in ion-selective electrodes (ISEs), show great potential for monitoring bodily conditions with high selectivity and affordability. Ion detection can be accomplished by utilizing the electrochemical potential gradient across an ISM within the solid-contact ISE. The solid-contact interface, located between the sensing membrane and electron-conducting substrate, plays a crucial role in ion-to-electron transduction. This critical layer serves to minimize the formation of the water layer, enabling stable potential responses and maintaining long-term stability. This work focuses on the use of chemical vapor deposition-grown, hydrophobic, gradient porous graphene (3D-G) material as the solid-contact layer in the ISE, offering a wider detection range, enhanced sensitivity, and improved long-term stability. The highly porous structure promotes ion recognition, diffusion, and adsorption onto the enhanced electrode surface, resulting in long-term stability, rapid analyte diffusion, and enhanced ion-to-electron transduction for sweat electrolyte sensing. A three-dimensionally interconnected porous electrode design can significantly enlarge the sensing areas in comparison to a 2D planar electrode, thereby improving the overall sensing performance of the sweat sensor. Owing to their large interface area, coupled with large double-layer capacitance in all-solid-state ISEs, hydrophobic nanostructured materials have been demonstrated to facilitate the ion-to-electron transduction process. Employing the 3D-G ISEs for health assessment has been exemplified through an on-body sweat sensing experiment, wherein the obtained results exhibit notable similarity with those generated by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Figure 1