Challenges and opportunities for supercapacitors

In recent times of energy scarcity, energy harvesting from renewable energy sources has been the prime goal of the research community. Adjacently, researchers are also engaged to devise methods for storing this energy in the form of electricity. Supercapacitors are one of the most efficient energy storage devices. Supercapacitors form a bridge between conventional capacitors and secondary ion batteries.^1–7 They have many advantages, such as high power density, high energy density, long cycle life, fast charge and discharge, instantaneous high current discharge, low cost, easy maintenance and no pollution to the environment (Table I).^8–12 Supercapacitors are continuously developing energy devices that are designed for high power supply compared to secondary ion batteries. The last few decades have seen a gradual expansion in scientific interest for use of various nanostructured engineered materials for application in supercapacitors. Generally, these nanostructured materials reveal better capacitance behavior than their bulk counterparts. The nanostructured materials are gifted with better chemical kinetics, chemical activity, shorter ionic diffusion path lengths, high surface area, and surplus of active sites for electrochemical reactions. It involves materials, energy, chemistry, electronic devices, and other disciplines and has become one of the hotspots of interdisciplinary research.^13,14 As a new type of energy storage device with environment benign nature and excellent performance, great application values, and market potential, it can be excavated in many fields such as industrial control, power, transportation, intelligent instruments, consumer electronic products, national defense, communications, medical equipment like defibrillators, pulsed lasers, new energy vehicles, and so on.^5,14–18 Although having unlimited potentials and opportunities, the supercapacitor faces enormous challenges.^19–23 

In this paper, the opportunities, challenges, and development trends of supercapacitors are summarized based on the current research situation.

A supercapacitor is an electrochemical device, which was developed in 1970s and 1980s to store energy by polarized electrolyte. It is different from the traditional chemical power supply. It is a kind of device with performance between traditional capacitors and batteries.²⁴ According to charge storage mechanism, supercapacitors are broadly classified into two classes, viz. electric double layer capacitors (EDLCs) and pseudocapacitors. The former mainly relies on the nonfaradaic double layer charge stored at the electrode-electrolyte interface and the latter on the redox pseudocapacitance due to the redox reactions at the interfaces to store electric energy.^25–28 Research and commercialization on supercapacitors started early in the United States, Japan, Russia, Switzerland, South Korea, France, and other European and American countries. Maxwell of the United States, Japan’s NEC, Panasonic, Tokin and Russian Econd; these companies occupy most of the global market. As early as 1879, Helmholtz discovered the properties of double-layer capacitance and proposed the concept of double-layer, but it is only in recent decades that double-layer is used for energy storage.²⁴ In 1957, Bcker first proposed that smaller capacitors could be used as energy storage devices, which had a specific energy close to that of batteries. In 1968, Standard Oil Company Sohio first proposed a patent for making double-layer capacitors from carbon materials with high specific surface area. The patent technology was transferred to NEC, which began to produce supercapacitors for starting systems of electric vehicles in 1979. Almost at the same time, Panasonic studied supercapacitors with activated carbon as the electrode material and organic solution as the electrolyte. Since then, supercapacitors have begun to be industrialized on a large scale, and various kinds of supercapacitors have been introduced.

The industrialization of supercapacitors began in 1980’s-Generation-1980 NEC/Tokin and 1987 Panasonic and Mitsubishi Products. In the 1990s, Econd and ELIT launched the electrochemical capacitors for high power start-up power applications. Nowadays, companies such as Panasonic, NEC, EPCOS, Maxwell, and NESS are very active in the research of supercapacitors (Fig. 1).

At present, the products of the United States, Japan, and Russia almost occupy the whole supercapacitor market. Each country’s supercapacitor products have its own characteristics and advantages in power, capacity, price, and so on. Supercapacitors have attracted worldwide attention since they have been introduced in the market. The rapid expansion of global demand has become a new bright spot in the field of chemical power. According to Bosch’s ⟨2007–2022 Research Report on the Current Situation and Investment Prospects of China’s Supercapacitor Market⟩, the global supercapacitor market has reached $16 × 10⁹ US dollars in 2015, and analysts expect the supercapacitor market to exceed $92.3 × 10⁹ by 2020, with a compound annual growth rate of 39% (Fig. 2). From this point of view, the development market of supercapacitors in the world is growing. With the breakthroughs in the key technologies of materials and processes, the quality and performance of supercapacitors has been progressively enhanced, as a result the new generation supercapacitors have begun to be industrialized on a large scale. In addition, as a product, supercapacitors have become well established, and their application scope has been continuously expanding. It has been widely used in industry, consumer electronics, communications, medical equipment, national defense, military equipment, transportation, and other fields.^29,30

In the last few decades, supercapacitors have evolved as special energy storage devices with small capacity to large-scale power storage, from separate energy storage to hybrid energy storage with batteries or fuel cells, supercapacitors have shown many unique advantages. In conclusion, supercapacitors have exceptional performance advantages, which lead to their extensive research, application, and huge potential in the consumer electronic industry and market. Therefore, we believe that supercapacitors will have unlimited opportunities in the coming future.^31,32

Supercapacitors are widely used in transportation, industry, military, consumer electronics, and other fields because of their excellent characteristics. However, these devices have some shortcomings. The existing problems that need to be solved are mainly described in the following four aspects (Fig. 3).

Energy densities of supercapacitors are not very high. At present, there is still a certain gap between supercapacitors (<20 Wh kg⁻¹) and batteries (30–200 Wh kg⁻¹) in terms of energy densities, how to improve the energy density is still the research focus and difficulty in the field of supercapacitors.^5,33,34 Improvement of manufacturing process and technology is an effective way to improve the storage capacity of supercapacitors, but in the long run, it is essential and difficult to find new electrolyte and electrode active materials with higher corresponding electrochemical performance. Low energy density supercapacitors result in bulkier devices and hence they are not compact. Energy densities of supercapacitors can be enhanced by increasing the effective surface area of electrode materials in double layer capacitors or increasing the operation voltage window or both. More and more research is going on to develop novel materials with high surface area and using suitable organic electrolytes which can endure a larger voltage window. If these expanses are properly addressed energy densities of supercapacitors can become comparable to batteries.

In some areas, the supercapacitor model can be equivalent to the ideal model, but in military applications, especially in power supply applications of satellites and spacecraft, some nonideal parameters may bring potential risks, which cannot be unheeded. Resonance caused by ordinary signal, filter, and energy storage capacitor has a mature solution because of its limited energy. Supercapacitors have the ability of instantaneous throughput and huge energy because of its high energy. Therefore, it is very important to have a reliable design to study the impact on load nature, load fluctuation or external environment, and accidental disturbance on the system stability.³⁵ 

The rated voltage of a supercapacitor is very low (less than 2.7 V), which requires a lot of series connection for practical applications. Because of the need for high current charging and discharging in applications, and overcharging has a serious impact on the life of capacitors, it is very important whether the voltages on individual capacitors in series are consistent or not.

Supercapacitors have a short development time and a fast speed. Enterprises engaged in the supercapacitor industry have different levels. As a new energy storage device, the healthy development of supercapacitors cannot be separated from the industry and market supervision, which aims to formulate practical industry standards, national standards, and even international standards. A set of technical standard system such as terms, the classification model naming method, the electrical performance test method, safety technical requirements, general specification, electrode material specification, electrolyte specification, charger specification series, production technical requirements, transportation requirements, recovery, and destruction requirements should be established for supercapacitors. For instance, general requirements and storage and management requirements of supercapacitor monomers and modules in the disposal of scrap, including dismantling of scrap monomer and super capacitor modules and handle recycling, electrolyte capacitor shell processing, plate processing, the processing of the diaphragm and other aspects, aims to guide and standardize the super capacitor industry achieve the goal of low cost, green recycling disposal. It is a necessary means to promote the healthy development of the industry.

With the rapid advancement of portable electronic products and the concept of wearable electronics, flexible energy storage devices have become popular with researchers. It is of great implication to develop energy storage devices which are flexible and small but endowed with high electrochemical properties.^36–38 However, the traditional supercapacitors, due to the unbending nature of the electrode, are greatly restricted to the shape of the device, and in the preparation of the electrode, the metal collector and the bonding agent, which also reduce the electrochemical performance of the supercapacitor.³⁹ So a flexible supercapacitor matched with a portable electronic product will be the development direction of the next generation of flexible storage devices. Flexible supercapacitors with different microstructures and macromorphologies have been widely reported. Unlike traditional nonflexible supercapacitors, in flexible supercapacitors, positive and negative electrodes, diaphragms, electrolytes, fluid collectors, and packaging shells are flexible, which gives flexible supercapacitors assembled in a thin, light and smart designs of any shape and size, thus increasing their potential for application in flexible and wearable filed.^40–42 From the point of view of practical research, the fabrication of flexible supercapacitors depends on obtaining suitable flexible electrodes. Therefore, researchers in this field also focus on the fabrication of high-performance flexible electrodes. Up to now, the research on flexible electrodes and flexible supercapacitors has formed a huge and complex system. The prepared flexible electrodes and flexible supercapacitors show rich and colorful physical morphology and functional characteristics. Flexible electrodes can be prepared by ex situ or in situ growth of the active material on a flexible substrate like carbon cloth, conducting polyethylene terephthalate (PET), etc. In case of in situ growth, the direct deposition of active material on the flexible substrate requires no use of polymeric binders and hence besides being flexible the electrodes also show superior electrochemical performance. In terms of device structures to design, it is necessary to maximize the utilization rate of electrode materials, ensure good compatibility between gel electrolyte and electrode materials, and avoid electrolyte leakage and encapsulation process simplification. With the continuous exploration by researchers, flexible supercapacitors can be divided into one-dimensional fiberlike and two-dimensional planar (Paper structure, textile structure) flexible supercapacitors in device structures.^43–48 The flexible electrode is the foundation of flexible device. At present, there are many methods for the preparation of flexible electrodes. A flexible electrode with simple process, good electrochemical performance and flexibility, which integrates active substances, flexible substrate and electrolyte, will be the main research direction in the future. In order to make the electrode more miniaturized without losing energy density, it is crucial to design nanolevel electrode materials to provide more electrochemical reactive sites or to compound the materials. This can be achieved by special designing of nano heterostructures which provides a surplus of redox reaction sites and enhanced surface area. For instance, Zhao et al.⁴⁹ assembled a flexible all-solid-state supercapacitor using nitrogen, phosphorus, and oxygen co-doped graphene. The assembled paper structure flexible all-solid-state symmetric supercapacitor exhibits a mass energy density of 25.3 Wh kg⁻¹ and a volume energy density of 25.2 Wh l⁻¹. Chen et al.⁵⁰ reported a stretchable one-dimensional linear SC, which is continuously wrapped on a prestretched elastic filament using carbon nanotube film electrodes. The CV curves of the assembled devices under different bending states do not change significantly. The flexibility of the linear device is further demonstrated by extending from 0% to 370%, while only slight capacitance loss occurs in the process of extending beyond 250%. Textile electronic products can be used in high-tech sportswear, work clothes, portable energy systems, health monitoring systems and military camouflage and other fields. Therefore, electronic textiles, intelligent textiles and wearable electronic products will have a greater development prospect and market in the future. Compared with paper-based substrate, fabric-based substrate has the advantages of high area and mass load of active material, which improves the area power density and energy density.⁴² Low-cost and efficient textile-based SCs have also been applied in wearable electronic devices.⁵¹ For example, Bao and Li⁵² assembled an MnO₂/ACT(flexible activated carbon textiles)//ACT asymmetric supercapacitor using cotton t-shirts as a substrate for the active electrode material. The maximum energy and power density of the Scs is reached 66.7 Wh kg⁻¹ and 4.97 kW kg⁻¹, respectively. Dong et al.^53,54 has also synthesized high electrochemical performance textiles electrode by chose activated carbon fiber cloth (ACFC) as flexible substrates. Their work is expected to promote the development of flexible electrodes and renewable energy storage devices. Meanwhile, the development of flexible all-solid SC is also conducive to its miniaturization.^55–58 Microsupercapacitor possess desirable merits of ultrahigh power delivery, outstanding rate capability, and high-frequency response.^59,60 For example, the researchers have obtained fluorine doped graphene microelectrodes by means of mask filter assistance, and successfully fabricated high specific energy all solid state supercapacitors by using high voltage ionic liquid gel as electrolyte.⁶¹ (Fig. 4) It is noteworthy that the fabricated all-solid-state planar SC exhibits a highly stable pseudocapacitance behavior when its volume capacitance is ∼582 F cm⁻³ at 10 mV s⁻¹ and 8.1 F cm⁻³ even at the ultrahigh rate of 2000 V s⁻¹, its ultrafast frequency response has a short-term constant of 0.26 ms, and its ultrahigh power density is ∼1191 W cm⁻³. Therefore, we believe that flexible and micro capacitors can expand the application of capacitors to wearable, microelectronic products, and in other fields.

Constructing hybrid battery-supercapacitors (battery-supercapacitor are the systems that one electrode stores charge by a battery-type Faradaic process while the other stores charge based on a capacitive mechanism) is an effective way to solve the problem of low energy density of supercapacitors. Hybrid battery-supercapacitor devices such as lithium/sodium/potassium/magnesium ion hybrid battery-supercapacitors inherit the high power (∼0.1–30 kW kg⁻¹) of supercapacitors and the high energy density (∼5–200 Wh kg⁻¹) of secondary batteries. In addition, these devices have the advantages of stable long cycle performance and low cost.^5,14,62 For instance, Wang et al.⁶³ using amorphous carbon (DC) and mesoporous graphene (MG) as negative and positive electrodes assembled a sodium ion hybrid battery-supercapacitor, which had an energy density of 168 Wh kg⁻¹ at 501 W kg⁻¹ and a maximum power density of 2432 W kg⁻¹ (98 Wh kg⁻¹), with a retention rate of 85% after 1200 cycles. Lu et al.⁶⁴ designed a nonaqueous mixed device of potassium ion battery and capacitor by combining the new potassium ion electrolyte with soft carbon as the negative electrode and commercial activated carbon as the positive electrode. The cycle life of the device can reach 1500 times, and the energy density is as high as 120 Wh kg⁻¹, showing a good application prospect. In recent years, batteries based on the storage theories of polyvalent ions (Zn²⁺, Mg²⁺, Ca²⁺, Al³⁺, etc.) have shown different dynamic and thermodynamic characteristics and electrochemical properties from lithium ion batteries and have gradually become a research hotspot in the field of electrochemical energy storage. At the same time, as a new type of electrochemical energy storage system, the polyvalent ionic hybrid capacitor came into being.⁶⁵ Relatively, the research of zinc-ion hybrid capacitor is getting more and more.⁶⁶ Fortunately, the research shows the advantages of high safety and long cycle life. For example, Dong et al.⁶⁷ developed a new type of extremely safe, high-rate, and ultralong-life hybrid supercapacitor based on the +2-valence zinc ion as the active carrier. This zinc-ion hybrid supercapacitor has excellent electrochemical performance: at 14.9 kW kg⁻¹ power density, the energy density is higher than 84 Wh kg⁻¹; After 10 000 cycles, the volume retention rate was higher than 91%. This research result has a good application prospect in the field of renewable and clean energy. However, for hybrid battery-supercapacitors, some problems need to be solved at the same time. One is that due to the different charge storage kinetics of positive and negative electrode materials, electrolyte ions embedded in battery materials tend to move more slowly than the interfacial adsorption and desorption of double-layer electrodes, resulting in the lack of high energy density at a small current and serious attenuation at a large current density. Recent studies have shown that some battery electrode materials exhibit a certain degree of pseudocapacitance at a certain nanoscale or with a specific nanostructure, which is called extrinsic pseudocapacitance. The charge transfer in the electrochemical process of these materials will become more complex, in which the charge-discharge curve platform is not obvious, and the peak potential in the CV curve does not change significantly with the scanning rate, and the rate performance of these materials has been improved.^68,69 Studies have shown that most battery materials exhibit capacitance characteristics of varying degrees under specific conditions by analyzing the energy storage behavior of battery materials. For example, the delithiation potential of LiCoO₂ is 3.9 V, but when the particle size of LiCoO₂ is controlled at 17 nm and steadily decreased, the voltage platform tilted gradually. When the particle size decreased to 6 nm, the voltage platform almost disappeared and became an approximate linear discharge curve.⁷⁰ The results show that the electrolyte ion transfer retardation can be effectively alleviated by two-dimensional or nanometer method of the battery-type electrode materials. Therefore, how to properly match the anode and cathode materials has a significant impact on the performance of hybrid supercapacitors, which is still a great challenge. In addition to the matching of anode and cathode materials, the voltage window and the choice of electrolyte also have an important impact on the hybrid battery-supercapacitors. With further research, hybrid supercapacitors which possess both high power density and high energy density is expected to be an ideal power source in a wider range of applications. Under such a scenario, supercapacitors can compete well with secondary batteries as energy storage devices and may even emerge as the better substitute.

With the rapid development of intelligent electronic devices, people are in urgent need of intelligent and controllable multifunctional electrochemical energy storage devices. The intelligence of electronic devices allows manufacturers and users to program them to perform different functions for different requirements in real life. Through the development of new materials and design of new structures combined with supercomputer simulation and artificial intelligence, we can expect to develop customizable devices to create user-friendly and personalized future interactions between wearable and biointegrated electronics.⁷¹ For example, Yin et al.⁷² reported a water-actived primary battery assembled from biodegradable, environmentally friendly, and biocompatible materials. Interestingly, Wang et al.⁷³ proposed the concept of edible supercapacitor and proved that a single edible supercapacitor the size of a capsule has enough energy and power to function as an independent device. More interestingly, Dong et al.⁷⁴ used air-floating paper as a flexible structural carrier to prepare breathable and wearable paper electrode of supercapacitor by carbon deposition using CNTs and MnO₂ as electrochemical active materials. They also used the electrode material to assemble flexible, breathable, asymmetric ultracapacitors with an electrolyte of PVA/KOH that displayed an energy density of 6.9 μWh cm⁻² at current of 1 mA cm⁻² and good performance of 0°–180° reciprocating bending capacity retention. Transparency is also an attractive feature of future optoelectronic devices. Although transparent devices have not been reported, transparent flexible multifunctional energy storage electrodes and devices have been receiving increasing attention. For instance, Gao et al. prepared MXene-rGO composite aerogel with excellent mechanical properties. The composite aerogel combined with the large specific surface area of rGO and the high conductivity of MXene effectively prevented the self-stacking of layered structures. The 3D MXene-rGO aerogel based microsupercapacitor provides an areal capacitance of up to 34.6 mF cm⁻² and retains an electrical capacity of up to 91% after 15 000 cycles. By further wrapping the polyurethane shell with self-healing properties, the device showed excellent self-healing ability (81.7% capacitance retained after the fifth healing).⁷⁵ With the development of fully transparent concept electronic devices, transparent energy storage devices will become the key to portable devices and have broad prospects in the future electronic product market.^76,77 Xu et al. used self-supporting ultrathin micro-nano-metal grid/nano-manganese oxide composite electrode materials, and they used aqueous system PVA/LiCl as solid electrolyte to build ultrathin self-supporting flexible transparent all-solid supercapacitor devices. The device has ultrathin thickness (<20 μm), exceptional electrochemical energy storage properties, high transparency (>80%), and outstanding resistance to bending properties, after many folds and fully kneads into a ball. The device capacitance is nearly constant, not obvious attenuation in performance, and in return to the initial flat form performance goes back to the initial state, reflecting very good resistance to bending properties. At the same time, this flexible transparent capacitor is so soft that it can withstand any deformation. Therefore, it provides the possibility for the birth of the concept of fully transparent electronic products⁷⁸ (Fig. 5). This work offers an opinion to design and manufacture the next generation of long-life, multifunctional electronic devices in order to further meet the needs of sustainable development. In the future, the integration of supercapacitors with electrochromic, shape memory, and even self-repair functions will be very attractive.

To meet social needs and promote industrial development, the application aspects of supercapacitors are of prime importance. With the rapid development of the electronic industry, the demand for high capacity portable power supply becomes more and more crucial. On the one hand, people all over the world are paying more and more attention to energy consumption and environmental protection. They are eager for more and more clean energy to be used; mankind is actively seeking solutions. On the other hand, with the development of the electronics industry, there is an urgent need to provide high-capacity, portable backup power for all kinds of electronic devices, which drives the development of supercapacitors. These social demands promote the rapid development of the supercapacitor industry to a certain extent, and the market prospect is very broad.

Beginning in 1992, the U.S. department of energy and USABC organized national laboratories and industries (e.g., Maxwell, GE, etc.) to jointly develop dual-layer supercapacitors using carbon materials. The initial goal is to increase the energy density of a supercapacitor to 5 Wh kg⁻¹, while maintaining a power density of 1 kW kg⁻¹. This target has been basically achieved. In 1996, the European Community formulated the development plan of electric vehicle supercapacitors, led by the SAFT company. The target is to achieve specific energy of 6 Wh kg⁻¹, specific power of 1500 W kg⁻¹, cycle life of more than 100 000 times, meeting the requirements of electrochemical cell and fuel cell electric vehicles. According to the relevant data, if the specific energy of supercapacitor reaches 20 Wh kg⁻¹, it will be ideal for hybrid cars. Therefore, the current research level needs to be improved to meet the requirements of social development. In addition, the new energy passenger cars, rail transit, smart meters, wind turbines, power grid equipment, port heavy machinery, and national defense military industry and other fields are widely used, and with the strengthening of communication and cooperation between upstream and downstream enterprises, the application field of supercapacitors will continue to expand.

Improving product performance and reducing production cost is the first and most significant criterion to be considered for any industry to endure. To improve the technology of supercapacitor itself, in addition to improving the manufacturing process and technology, to find stable and effective electrode and electrolyte materials to improve the performance, and at the same time to reduce the cost is also the research focus in this field. Full Power Technologies, an American firm, is developing low-cost ultracapacitors. From the way of cost reduction analysis, one has to (1) find new low-cost raw materials, such as natural mineral resources; (2) seek the combination of low price raw materials and high price raw materials, so as to realize the purpose of complementary performance and low overall price; (3) improve the production process (such as simplifying the process) and the production equipment, at the same time achieve low cost. Therefore, this will also be the main direction and strategic goal of the product’s future development; (4) pay attention to the industrialization prospects of the materials and the cost issues on application of new electrode materials, such as carbon fiber graphene; (5) matching research of existing electrode materials, such as matching of existing electrode materials with the electrolyte; (6) the research of group module should pay more attention to the overall service life characteristics and capacitors and the management system to enhance reliability and security.

In brief, the development of supercapacitors is inseparable from the progress of science and technology and the demand of application (Fig. 6). With the popularity of new energy vehicles and smart wearable devices, we believe that the development of supercapacitors will be more rapid and far reaching.

As a new type of green and efficient energy storage device, supercapacitors have shown great potential in many industries and fields. The huge potential market will also bring infinite opportunities for the development of supercapacitors. However, there are still problems with these virtuous energy storage devices. With the popularity of new energy vehicles and smart wearable devices, it is an important goal to expand the application field of supercapacitors, reduce costs, and improve energy density, while electrode materials that restrict the performance and cost of supercapacitors will remain the focus of future research.

We thank the financial support from the National Natural Science Foundation of China (Grant No. 51774251), the Hebei Natural Science Foundation for Distinguished Young Scholars (Grant No. B2017203313), the Hundred Excellent Innovative Talents Support Program in Hebei Province (Grant No. SLRC2017057), the Talent Engineering Training Funds of Hebei Province (Grant No. A201802001), and the opening project of the state key laboratory of Advanced Chemical Power Sources (Grant No. SKL-ACPS-C-11).