7.2 Flexible supercapacitors
3 min read•august 15, 2024
are revolutionizing energy storage for wearable electronics. These devices store energy through , allowing for rapid charging and discharging. Their bendable and stretchable materials make them ideal for integration into clothing and accessories.
Compared to traditional batteries, flexible supercapacitors offer higher and longer cycle life. They can charge in seconds to minutes and function over a wider range of conditions. These advantages make them perfect for powering fitness trackers, , and other wearable tech.
Working principles of flexible supercapacitors
Energy storage mechanism and flexibility
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- Flexible supercapacitors store energy through electrical double layer formation at electrode-electrolyte interface enabling rapid charge and discharge cycles
- Achieve flexibility using bendable and stretchable materials for electrodes and electrolytes maintaining performance under mechanical deformation
- Operate over wide temperature range enhancing reliability in wearable applications
- Design with thin, lightweight structures minimizing bulk and weight of wearable electronic systems
- Conform to various shapes withstanding repeated bending or stretching making them ideal for integration into clothing and accessories (smart textiles)
Advantages for wearable electronics
- Offer high power density allowing quick energy delivery for sudden power needs (fitness trackers)
- Provide long cycle life exceeding 100,000 charge-discharge cycles outperforming typical 500-1000 cycles of lithium-ion batteries
- Enable fast charging capabilities with full charge achieved in seconds to minutes rather than hours
- Demonstrate lower compared to traditional batteries improving long-term energy storage
- Maintain functionality over wider range of operating conditions relevant to wearable applications (outdoor sports monitors)
Materials for flexible supercapacitors
Electrode materials
- Utilize carbon-based materials (, , ) due to high surface area and conductivity
- Incorporate (, ) enhancing flexibility and providing pseudocapacitive effects
- Explore novel approaches like and creating complex electrode structures improving device performance
- Integrate () with conductive materials using pre-stretching and patterning techniques maintaining electrical connectivity during deformation
Electrolytes and fabrication methods
- Employ solid or (, ) ensuring flexibility and preventing leakage in wearable applications
- Implement , , and for large-scale production of flexible supercapacitor components
- Fabricate micro-supercapacitors using photolithography and etching techniques creating interdigitated electrode patterns on flexible substrates
- Develop hybrid systems combining flexible supercapacitors with batteries leveraging high power density of supercapacitors with high of batteries
Flexible supercapacitors vs other energy storage devices
Performance comparison
- Demonstrate lower energy density compared to lithium-ion batteries but offer significantly higher power density allowing rapid energy delivery
- Achieve charging and discharging rates much faster than traditional batteries with full charge in seconds to minutes versus hours
- Exhibit linear discharge profile differing from relatively constant voltage output of batteries necessitating different power management strategies in
- Maintain superior temperature performance compared to batteries functioning over wider range of operating conditions relevant to wearable applications (extreme weather conditions)
Energy storage characteristics
- Store energy through electrical double layer formation contrasting with chemical reactions in batteries
- Provide cycle life exceeding 100,000 charge-discharge cycles vastly outperforming typical 500-1000 cycles of lithium-ion batteries
- Show higher self-discharge rates than batteries but lower than conventional capacitors affecting long-term energy storage capabilities
- Offer potential for integration with energy harvesting systems in wearables storing energy from sources like body heat or motion for later use (self-powered health monitors)
Applications of flexible supercapacitors in wearables
Potential uses and advantages
- Power smart textiles, health monitoring devices, and in wearable electronics
- Serve as energy storage components in self-powered wearable systems harvesting energy from body heat or motion
- Function as power buffers in handling peak power demands and extending life of primary batteries in wearable devices
- Enable rapid charging capabilities for on-the-go use minimizing downtime for wearable devices (smartwatches)
Limitations and challenges
- Face restrictions in applications requiring long-term, high-capacity energy storage due to lower energy density compared to batteries
- Encounter higher costs of materials and fabrication processes compared to conventional energy storage devices limiting widespread adoption
- Address challenges in scaling up production while maintaining consistent performance and flexibility across large areas for mass-market wearable applications
- Overcome engineering hurdles in integrating flexible supercapacitors with other wearable components and ensuring user safety especially in close contact with skin
- Develop strategies to mitigate potential electromagnetic interference with other electronic components in compact wearable designs
Key Terms to Review (37)
3D printing: 3D printing, also known as additive manufacturing, is a process that creates three-dimensional objects from a digital file by layering materials. This technology is revolutionizing various fields by enabling the rapid prototyping and production of complex shapes, making it particularly valuable in the development of wearable and flexible electronics.
Activated carbon: Activated carbon is a form of carbon that has been processed to make it extremely porous, increasing its surface area and adsorption capacity. This unique structure allows activated carbon to effectively capture and store various substances, making it a key material in applications like filtration and energy storage.
Biodegradable materials: Biodegradable materials are substances that can be broken down by natural processes, typically through the action of microorganisms, into simpler, non-toxic components. These materials are designed to decompose in the environment, reducing pollution and waste. The growing use of biodegradable materials is crucial in developing sustainable technologies, particularly in applications that require temporary use and disposal, such as energy storage devices and medical implants.
Capacitance: Capacitance is the ability of a system to store an electrical charge per unit voltage. It plays a crucial role in energy storage devices, like capacitors and supercapacitors, determining how much energy can be stored in relation to the voltage applied. Understanding capacitance is essential for developing efficient flexible electronics, particularly those that require lightweight and space-efficient energy storage solutions.
Carbon Nanotubes: Carbon nanotubes (CNTs) are cylindrical nanostructures made up of carbon atoms arranged in a hexagonal lattice, known for their remarkable mechanical, electrical, and thermal properties. Their unique structure makes them ideal for enhancing the performance of wearable and flexible electronics, enabling innovative applications in sensing, energy storage, and smart textiles.
Conductive Polymers: Conductive polymers are organic polymers that can conduct electricity due to the presence of conjugated double bonds in their structure. These materials have gained significant attention in various applications, particularly in wearable and flexible electronics, where their unique properties can be utilized to create lightweight, flexible devices with electrical functionalities.
Cycle Stability: Cycle stability refers to the ability of a flexible supercapacitor to maintain its performance over multiple charge and discharge cycles. It is a crucial characteristic that indicates how well the device can withstand repeated use without significant degradation in capacitance, energy density, or power delivery. High cycle stability means that the supercapacitor can endure numerous charging cycles while retaining its effectiveness, which is essential for applications in wearable and flexible electronics.
Elastomers: Elastomers are a class of polymers known for their elastic properties, meaning they can stretch significantly and return to their original shape after deformation. This unique characteristic makes them ideal for applications that require flexibility, durability, and resilience, such as in stretchable and self-healing materials, interconnects and circuits, as well as in flexible supercapacitors. Their ability to withstand mechanical stress while maintaining performance is crucial in the development of advanced electronic devices.
Electrical double layer formation: Electrical double layer formation refers to the structure that occurs at the interface between an electrode and an electrolyte, where two layers of charge develop. The inner layer, or Stern layer, consists of ions that are closely associated with the surface of the electrode, while the outer layer, or diffuse layer, contains ions that are more loosely associated and extend into the electrolyte. This phenomenon is crucial for the operation of flexible supercapacitors as it influences their capacitance and energy storage capabilities.
Energy Density: Energy density is a measure of how much energy is stored in a given volume or mass of a material. It is crucial for evaluating the performance and efficiency of energy storage devices and systems, determining how much energy can be delivered in a specific context, such as size, weight, or operational environment. High energy density is desirable for applications requiring compact and lightweight power sources, making it a key consideration in developing flexible batteries, supercapacitors, and energy harvesting technologies.
Esr - equivalent series resistance: Equivalent series resistance (ESR) is a measure of the resistive losses in a capacitor or supercapacitor when it is subjected to an AC signal. It reflects the internal resistance that causes energy loss, affecting the efficiency and performance of flexible supercapacitors in applications. A lower ESR is desirable as it indicates better energy efficiency and faster charging/discharging capabilities, which are crucial for the effectiveness of energy storage devices.
Fiber supercapacitors: Fiber supercapacitors are energy storage devices that integrate the properties of traditional supercapacitors into flexible and lightweight fiber formats. This innovative design allows for the seamless incorporation of energy storage capabilities into wearable technologies, textiles, and other flexible electronics, enhancing their functionality without compromising comfort or aesthetics.
Flexible displays: Flexible displays are thin, lightweight electronic screens that can bend, fold, and stretch without losing functionality. These displays enable innovative applications in wearable and flexible electronics, integrating seamlessly into devices while offering new form factors and user experiences.
Flexible supercapacitors: Flexible supercapacitors are energy storage devices that combine the high power density of conventional supercapacitors with a flexible design, allowing them to be integrated into a variety of surfaces and materials. They are characterized by their ability to store and release energy quickly while maintaining mechanical flexibility, making them ideal for use in wearable electronics and other applications where space and weight are critical.
Gel electrolytes: Gel electrolytes are a type of electrolyte that has a gel-like consistency, offering a balance between solid and liquid states. This unique form provides enhanced ionic conductivity while maintaining flexibility and mechanical stability, making them particularly useful in applications like flexible supercapacitors. By incorporating polymeric materials, gel electrolytes can also improve the safety and stability of electrochemical devices.
Graphene: Graphene is a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice, known for its exceptional electrical, thermal, and mechanical properties. This unique structure allows graphene to be a leading material for wearable and flexible electronics, enabling the development of lightweight, flexible devices with high performance and efficiency.
Hybrid energy systems: Hybrid energy systems are integrated energy systems that combine two or more different types of energy generation technologies to provide a more efficient and reliable power supply. These systems can include combinations of renewable sources, like solar and wind, alongside traditional energy sources, such as fossil fuels or battery storage, to optimize energy production and consumption. The aim is to enhance system reliability, reduce emissions, and improve energy efficiency.
Inkjet printing: Inkjet printing is a digital printing technology that creates images by propelling tiny droplets of ink onto various substrates. This method allows for high-resolution prints and is particularly valuable in creating flexible and wearable electronics, as it can be easily applied to a range of materials without significant heat or pressure.
Ionic liquids: Ionic liquids are salts that exist in a liquid state at relatively low temperatures, typically below 100°C. They are composed entirely of ions and have unique properties such as low volatility, high thermal stability, and the ability to dissolve a variety of materials. These characteristics make ionic liquids particularly valuable in applications like flexible supercapacitors, where they can serve as efficient electrolytes to enhance energy storage and performance.
Laser Scribing: Laser scribing is a precise technique that uses focused laser beams to create fine lines or patterns on a substrate, often employed in the fabrication of flexible and wearable electronics. This method allows for selective removal or modification of material layers, which is crucial in creating efficient and lightweight components, such as those found in flexible supercapacitors. The high precision and speed of laser scribing make it ideal for developing intricate designs that maintain the functionality of electronic devices while adhering to the demands of flexibility.
Planar supercapacitors: Planar supercapacitors are energy storage devices designed with a flat, two-dimensional structure that allows for efficient charge and discharge cycles. These devices leverage the principles of electrochemical capacitance to store energy, making them ideal for applications in flexible and wearable electronics due to their compact design and lightweight nature.
Polyaniline: Polyaniline is a conductive polymer known for its unique electrical properties and ability to be processed in various forms, such as films, fibers, and powders. This versatility makes it a key material in the development of stretchable interconnects and circuits, as well as in flexible supercapacitors. Its electrical conductivity can be tuned through chemical doping, making it suitable for applications that require flexibility and stretchability without sacrificing performance.
Polymer Electrolytes: Polymer electrolytes are solid or gel-like materials that conduct ions and are typically made from polymers combined with ionic salts. These electrolytes play a crucial role in energy storage devices, particularly in flexible supercapacitors, as they enable ionic conduction while maintaining mechanical flexibility and structural integrity.
Polypyrrole: Polypyrrole is a conducting polymer formed by the polymerization of pyrrole monomers, known for its excellent electrical conductivity and environmental stability. This material is particularly important in energy storage applications, as its unique properties make it suitable for flexible supercapacitors, enabling them to store and release energy efficiently while maintaining a lightweight and flexible form factor.
Power Density: Power density refers to the amount of power (energy per unit time) produced or consumed per unit volume or mass of a device or system. This metric is crucial in evaluating the efficiency and performance of energy storage and conversion technologies, as it indicates how much energy can be harnessed in a given space. High power density is particularly desirable in applications where size and weight are limited, such as portable and wearable electronics.
Recyclability: Recyclability refers to the ability of a material to be collected, processed, and transformed into new products after its initial use. This concept is increasingly vital in the context of electronics, as it addresses the environmental impact and sustainability of materials used in various applications. By focusing on recyclability, industries aim to reduce waste, conserve resources, and minimize the ecological footprint of their products, especially in sectors like flexible electronics, energy storage devices, and packaging solutions.
Roll-to-roll processing: Roll-to-roll processing is a manufacturing technique that allows the production of flexible electronic devices on large rolls of substrate material. This method enables high-speed, cost-effective fabrication by utilizing continuous production processes, making it particularly valuable for industries focused on flexible and stretchable electronics, lighting, energy storage, and packaging.
Screen printing: Screen printing is a versatile and widely used technique for applying inks onto various substrates using a mesh screen to transfer the ink in desired patterns. This method is crucial in producing electronic components as it allows for the precise deposition of conductive materials on flexible substrates, enhancing their functionality in wearable and flexible electronics.
Self-discharge rates: Self-discharge rates refer to the rate at which a capacitor loses its stored energy when not in use, due to internal leakage currents. In the context of flexible supercapacitors, low self-discharge rates are crucial because they determine the efficiency and performance of energy storage systems, particularly in wearable and portable applications. A lower self-discharge rate means that the energy can be retained for a longer time, making these devices more effective for practical use in flexible electronics.
Self-healing supercapacitors: Self-healing supercapacitors are advanced energy storage devices designed to automatically repair damage, thereby maintaining functionality and performance over time. This capability is particularly beneficial in flexible electronics, where mechanical stress and damage can occur frequently, allowing these supercapacitors to extend their lifespan and reliability in various applications.
Smart textiles: Smart textiles are fabrics that have been enhanced with technology to provide added value and functionality beyond traditional textiles. They can sense environmental changes, respond to stimuli, or interact with electronic devices, making them integral to the advancement of wearable and flexible electronics.
Solid Electrolytes: Solid electrolytes are materials that conduct ions while remaining in a solid state, crucial for the functioning of various electrochemical devices. These materials facilitate ion transport without the need for a liquid medium, allowing for lightweight and flexible designs in energy storage systems like supercapacitors. Their unique properties enable enhanced performance, safety, and stability in various applications.
Specific Capacitance: Specific capacitance is a measure of a capacitor's ability to store electrical energy per unit mass or volume, expressed in farads per gram (F/g) or farads per cubic centimeter (F/cm³). This term is crucial in the development of flexible supercapacitors, as it indicates how effectively these devices can store energy while maintaining lightweight and compact designs. A high specific capacitance is desirable in flexible electronics to enhance performance without compromising flexibility or weight.
Stretchable substrates: Stretchable substrates are flexible materials designed to maintain their structural integrity and functionality while undergoing deformation, such as stretching or bending. These substrates are essential in applications requiring flexibility, such as wearable devices and flexible supercapacitors, as they allow for seamless integration into dynamic environments and can conform to the surface of the human body or other surfaces.
Transparent supercapacitors: Transparent supercapacitors are advanced energy storage devices that combine high capacitance with transparency, allowing them to be integrated into various applications where visibility is essential. These devices leverage materials like conductive polymers and transparent electrodes to maintain both performance and aesthetic appeal, making them ideal for use in wearable electronics and flexible displays.
Voltage Window: The voltage window refers to the range of voltage levels within which a supercapacitor can operate safely and effectively without undergoing degradation or failure. This range is crucial for ensuring the longevity and performance of flexible supercapacitors, as exceeding this window can lead to irreversible damage to the materials or the device itself. Understanding the voltage window helps in optimizing the design and application of supercapacitors in wearable and flexible electronic devices.
Wearable devices: Wearable devices are smart electronic gadgets that can be worn on the body, often incorporating sensors and software to collect data and provide functionality in real-time. These devices can enhance daily activities by monitoring health metrics, providing notifications, or offering interactive features. Their integration with flexible display technologies and energy storage solutions allows for innovative designs that adapt to user needs while maintaining comfort and usability.