Flexible photo-charging power sources for wearable electronics

Critical importance of current collector property to the performance of flexible electrochemical power sources

With the emergence of disposable electronics, compatible safe, flexible, and recyclable power sources have become a critical challenge. Here, an ultra‐thin paper‐based iontronic power source is enabled by highly efficient translational Li+ transport within 2D nanofluidic channels of graphene oxide under a salinity gradient and the fine‐tuned interfacial redox reactions. The paper‐based source can generate volumetric power and energy densities of 438.02 mW cm−3 and 30.02 mWh cm−3, respectively. Its areal power density is 1095.05 mW cm−2, surpassing most flexible batteries. It maintains a working state when bent or even cut and can be simply recycled by incineration. By filling 2D nanofluidic inks in different pens, the power source can be drawn on paper when needed, which not only overcomes the inherent defect of self‐discharge for most batteries but also enables writing directly on any insulating substrates. Furthermore, all‐in‐one disposable electronics comprised of an energy management system (paper‐based triboelectric nanogenerator and iontronic power source) and wireless sensing system (temperature sensor with NFC circuits) are integrated onto one piece of paper by duplex printing, demonstrating the huge potential of such integratable iontronic power sources for soft, wireless, and conformable disposable electronics.

The future wearable/portable electronics need flexible power sources with higher storage capability. Lithium-sulfur (Li-S) battery is very promising for the development of next-generation high-energy battery due to its ultra-high theoretical capacity. However, lithium-sulfur battery manufacturing still encounters several roadblocks, including low sulfur utilization, severe capacity fading, poor cycling life and bad Coulombic efficiency, which can be ascribed to the insulating nature of sulfur (~5×10-30 S cm-1 at 25 ℃), the “shuttle effect” of soluble reaction intermediates (polysulfides), as well as the uncontrolled side reaction at lithium anode. To date, various strategies have been explored to push up the potential of Li-S systems, which indeed help reaching an encouraging practical capacity (~550 mAh g-1), more than twice of the state of the art lithium-ion batteries (~180 mAh g-1) [1,2]. Obviously, Li-S batteries hold great promise for the development of next-generation flexible power sources because of their superior energy density. However, the development of flexible Li-S batteries are still bothered by its fast capacity decay and lack of suitable flexible substrates. Our previous studies have demonstrated that the activated cotton textile (ACT) converted from cotton textile could be an excellent platform for constructing flexible energy systems (e.g. flexible supercapacitors and flexible lithium-ion batteries) due to their excellent conductivity and eminent flexibility [3-5]. However, to the best of our knowledge, ACTs have not been explored for flexible Li-S batteries. Herein, a conductive activated cotton textile (ACT) with porous tubular structure was first derived from natural cotton textile to load sulfur, which was further wrapped with partially reduced graphene oxide (ACT/S-rGO) to immobilize lithium polysulfides. Meanwhile, the partially reduced graphene oxide nanosheets could be served as a conductive coating, which further mitigated the poor conductivity of sulfur and enabled fast electron transportation along ACT fibers. Furthermore, a KOH-activated ACT with micropore size distribution was inserted between cathode and separator to mitigate the “shuttle effect” of polysulfides. Finally, the assembled ACT/S-rGO cathode with porous ACT interlayer exhibited an exceptional rate capability and durable cyclic performance (with a well-retained capacity of ~1016 mAh g−1even after 200 cycles). A flexible Li-S cell with ACT/S-rGO as a cathode was also assembled to demonstrate its superior potential as flexible power sources for future wearable electronic devices.

Over the past decade, researchers have devoted considerable attention to the integration of living organisms with electronic elements to yield bioelectronic devices. Not only is the integration of DNA, enzymes, or whole cells with electronics of scientific interest, but it has many versatile potential applications. Researchers are using these ideas to fabricate biosensors for analytical applications and to assemble biofuel cells (BFCs) and biomolecule-based devices. Other research efforts include the development of biocomputing systems for information processing. In this Account, we focus on our recent progress in engineering at the bioelectrochemical interface (BECI) for the rational design and construction of important bioelectronic devices, ranging from electrochemical (EC-) biosensors to BFCs, and self-powered logic biosensors. Hydrogels and sol-gels provide attractive materials for the immobilization of enzymes because they make EC-enzyme biosensors stable and even functional in extreme environments. We use a layer-by-layer (LBL) self-assembly technique to fabricate multicomponent thin films on the BECI at the nanometer scale. Additionally, we demonstrate how carbon nanomaterials have paved the way for new and improved EC-enzyme biosensors. In addition to the widely reported BECI-based electrochemical impedance spectroscopy (EIS)-type aptasensors, we integrate the LBL technique with our previously developed "solid-state probe" technique for redox probes immobilization on electrode surfaces to design and fabricate BECI-based differential pulse voltammetry (DPV)-type aptasensors. BFCs can directly harvest energy from ambient biofuels as green energy sources, which could lead to their application as simple, flexible, and portable power sources. Porous materials provide favorable microenvironments for enzyme immobilization, which can enhance BFC power output. Furthermore, by introducing aptamer-based logic systems to BFCs, such systems could be applied as self-powered and intelligent aptasensors for the logic detection. We have developed biocomputing keypad lock security systems which can be also used for intelligent medical diagnostics. BECI engineering provides a simple but effective approach toward the design and fabrication of EC-biosensors, BFCs, and self-powered logic biosensors, which will make essential contributions in the development of creative and practical bioelectronic devices. The exploration of novel interface engineering applications and the creation of new fabrication concepts or methods merit further attention.

Integration of patterned electrolyte film and sacrificial substrate serpentine electrode of low curvature for high stretch supercapacitor, physiological signal detection

A simple inorganic hybrids strategy for graphene fibers fabrication with excellent electrochemical performance

Sodium-ion batteries offer an attractive option for potential low cost and large scale energy storage due to the earth abundance of sodium. Red phosphorus is considered as a high capacity anode for sodium-ion batteries with a theoretical capacity of 2596 mAh/g. However, similar to silicon in lithium-ion batteries, several limitations, such as large volume expansion upon sodiation/desodiation and low electronic conductance, have severely limited the performance of red phosphorus anodes. In order to address the above challenges, we have developed a method to deposit red phosphorus nanodots densely and uniformly onto reduced graphene oxide sheets (P@RGO) to minimize the sodium ion diffusion length and the sodiation/desodiation stresses, and the RGO network also serves as electron pathway and creates free space to accommodate the volume variation of phosphorus particles. The resulted P@RGO flexible anode achieved 1165.4, 510.6, and 135.3 mAh/g specific charge capacity at 159.4, 31878.9, and 47818.3 mA/g charge/discharge current density in rate capability test, and a 914 mAh/g capacity after 300 deep cycles in cycling stability test at 1593.9 mA/g current density, which marks a significant performance improvement for red phosphorus anodes for sodium-ion chemistry and flexible power sources for wearable electronics.

Micro-level energy sources from the ambient environment can be a viable alternative to increase the life of the sensor nodes from a WSN network. Harvesting energy from the environment - particularly solar energy, is a technique that can support the operation of the sensor nodes. The power extracted from the luminous radiation is extremely variable, depending on day/night alternation, atmospheric conditions, temperature, geographical location, indoor or outdoor environment - which makes the energy harvested insufficient for use as an exclusive power source to supply sensor nodes. A special case is the indoor environment, in which the solar radiation is strongly reduced, compared to the outside environment, and artificial lighting levels are usually below 1000 Lux. We made experiments on an indoor solar cell using illumination from three different sources: fluorescent, incandescent, LED. Detailed measurements, performance comparisons, and conclusions are presented.

In the new power system with renewable energy as the main body, wind power and photovoltaic have the characteristics of volatility, uncertainty, and poor predictability. To improve the penetration rate of renewable energy, sufficient flexible and adjustable power is vital to absorb large-scale renewable energy sources, especially wind power. Therefore, it is necessary to consider a reasonable proportion of wind power, flexible and adjustable power sources and ordinary conventional coal-fired power sources in the power grid coordination planning. In the paper, the K-Means clustering method is proposed to sample the load and wind speed values at different times of a typical day from the actual data. A mathematical model for coordinated planning of dynamic power grids with coal-fired power generation is then built up. The goal of this model is to make optimal investment in candidate generating units and candidate lines under the premise of satisfying the constraints of safe operation of the power grid, so as to ensure that the total present value of investment cost and operating cost is the smallest during the planning period. At the same time, the simplified unit combination model is used to simulate the dispatching operation of various types of units in the typical intraday power system, so as to illustrate the flexible adjustment effect of gas-fired generating units on wind power and load fluctuations. The established mathematical model is a mixed integer linear programming model, and the model is solved by using GAMS software to realize programming. The IEEE-30 node case verifies the validity of the proposed model.

Electric eel is an excellent example to harness ion-concentration gradients for sustainable power generation. However, current strategies to create electric-eel-inspired power sources commonly involve manual stacking of multiple salinity-gradient power source units, resulting in low efficiency, unstable contact, and poor flexibility. Here we propose a consecutive multimaterial printing strategy to efficiently fabricate biomimetic ionic hydrogel power sources with a maximum stretchability of 137%. The consecutively-printed ionic hydrogel power source filaments showed seamless bonding interface and can maintain stable voltage outputs for 1000 stretching cycles at 100% strain. With arrayed multi-channel printhead, power sources with a maximum voltage of 208 V can be automatically printed and assembled in parallel within 30 min. The as-printed flexible power source filaments can be woven into a wristband to power a digital wristwatch. The presented strategy provides a tool to efficiently produce electric-eel-inspired ionic hydrogel power sources with great stretchability for various flexible power source applications.

Flexible power sources in FHE applications may be subjected to loads of flexing, twisting, folding and stretching. Lithium-ion batteries less than 1-mm in thickness are capable of powering thin flexible wearable electronics. While the reliability and performance of thick block lithiumion batteries has been studied in great detail - relatively little is known about thin flexible power sources under stresses of daily-motion. In this study, degradation of thin-flexible power source reliability has been studied under twist stresses of magnitude to replicate stresses of daily motion of human body using motion-control setups in a lab-environment. The samples are subjected to 150 charging-discharging cycles and the 60 twist-motion per every discharge cycle. The twist motion and the charging-discharging of the battery have been controlled via micro-controller and the LabView, respectively. Additionally, FEM model have been developed to predict the life of the battery and analyze acceleration factors between test conditions and use conditions.

Photovoltaic cells generally suffer a loss of output power due to ubiquitous shadows, which can greatly limit their practical use, especially in metropolitan regions. Very recently, a shadow-effect energy generator (SEG), a new platform that can scavenge energy under shadow conditions, has been demonstrated. In contrast to previously reported SEG based on silicon system, in this study, for the first time, methylammonium lead iodide (MAPbI3) perovskite-based flexible SEG with high-performance and long-term stability is achieved through interface engineering strategy, involving inclusion of atomic-layer-deposited (ALD) Al2O3 and poly{2,5-bis(2-dodecylhexadecyl)-3,6-di(thiophen-2-yl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-(E)-1,2-bis(3-cyanothiophen-2-yl)ethene} (DPP-CNTVT) interfacial layers between the electrode and perovskite layer. The incorporation of the ALD Al2O3 film effectively suppresses the mobile iodide ion-caused electrode corrosion problem and ensures the formation of a Schottky potential barrier. On the other hand, the inclusion of DPP-CNTVT layer can passivate the defects within the perovskite layer and facilitate charge generation and extraction. With all these advantages, the resulting flexible SEG devices exhibit remarkable power density up to 18.22 μW cm–2 under half-in-shadow conditions, which can light up 7 light-emitting diodes and a calculator. To the best of our knowledge, this is the first successful demonstration of perovskite SEG devices, and the power density achieved herein even outperforms that of state-of-the-art silicon-based SEG measured under the same condition. Additionally, through the use of UV-cured epoxy/fluorinated polymer poly(perfluoro(1-butenyl vinyl ether) (CYTOP) bilayer film as the encapsulation layer, long-term stable SEG devices can be realized. Importantly, our strategy has shown great promise for extending the applicability of SEG to flexible semitransparent power sources and self-powered photodetectors. This work provides important guidelines for the exploitation of high-performance and stable perovskite SEG through interface engineering, which sets a new milestone in this field and presents a significant step toward the practical applications of this emerging technology.

Transparent, flexible and high efficient power sources are important components of organic electronic and optoelectronic devices. In this work, based on the principle of the previously demonstrated triboelectric generator, we demonstrate a new high-output, flexible and transparent nanogenerator by using transparent polymer materials. We have fabricated three types of regular and uniform polymer patterned arrays (line, cube, and pyramid) to improve the efficiency of the nanogenerator. The power generation of the pyramid-featured device far surpassed that exhibited by the unstructured films and gave an output voltage of up to 18 V at a current density of ∼0.13 μA/cm(2). Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg, ∼3.6 Pa in contact pressure) and a falling feather (20 mg, ∼0.4 Pa in contact pressure) with a low-end detection limit of ∼13 mPa.

Microsized supercapacitors (mSCs) with small volume, rapid charge-discharge rate, and ultralong cyclic lifetime are urgently needed to meet the demand of miniaturized portable electronic devices. A versatile self-shrinkage assembling (SSA) strategy to directly construct the compact mSCs (CmSCs) from hydrogels of reduced graphene oxide is reported. A single CmSC is only 0.0023 cm3 in volume, which is significantly smaller than most reported mSCs in fiber/yarn and planar interdigital forms. It exhibits a high capacitance of up to 68.3 F cm-3 and a superior cycling stability with 98% capacitance retention after 25 000 cycles. Most importantly, the SSA technique enables the CmSC as the building block to realize arbitrary, programmable, and multi-dimensional integration for adaptable and complicated power systems. By design on mortise and tenon joint connection, autologous integrated 3D interdigital CmSCs are fabricated in a self-holding-on manner, which thus dramatically reduces the whole device volume to achieve the high-performance capacitive behavior. Consequently, the SSA technique offers a universal and versatile approach for large-scale on-demand integration of mSCs as flexible and transformable power sources.

Natural and artificial muscle elements as robot actuators