Urgent exploitation of various renewable and sustainable energy sources, such as wind and solar energy, has been prompted by the environmental concerns related to the continuous consumption of nonrenewable resources and the increasing complexity of power distribution systems. Efficient usage of these new energy sources is crucial concerning their non-stable power generation. Hence, a popular strategy is to develop advanced energy storage devices for delivering energy on demand in a cost-effective manner. Currently, energy storage systems are available for various large?scale applications and are classified into four main types: mechanical, chemical, electrical, and electrochemical.

Urgent exploitation of various renewable and sustainable energy sources, such as wind and solar energy, has been prompted by the environmental concerns related to the continuous consumption of nonrenewable resources and the increasing complexity of power distribution systems. Efficient usage of these new energy sources is crucial concerning their non-stable power generation. Hence, a popular strategy is to develop advanced energy storage devices for delivering energy on demand in a cost-effective manner. Currently, energy storage systems are available for various large?scale applications and are classified into four main types: mechanical, chemical, electrical, and electrochemical.

Mechanical energy storage via pumped hydroelectricity is currently the dominant energy storage method. However, electrochemical energy storage (EES) systems in terms of electrochemical capacitors (ECs) and batteries have demonstrated great potential in powering portable electronics and the electrification of the transportation sector due to the advantageous features of high round?trip efficiency, long cycle life, and potential to be implemented with various chemistries based on cheap, sustainable and recyclable materials, and low maintenance cost.

Generally, electric energy is stored in EES in two ways: directly via a non?faradaic process or indirectly via a faradaic process. The non?faradaic technologies store electricity directly in an electrostatic way. Typically, electric double?layer capacitors (EDLCs) are efficient (≈100%) and suitable for power management (e.g., frequency regulation), but deliver a low energy density with limited discharge time. Alternatively, electrical energy can be stored by converting it to available chemical energy, requiring faradaic oxidization and reduction of the electrochemically active reagents, and reversibly release the energy on demand. Typical examples of faradaic systems include pseudocapacitors and various batteries. Ragone compares the power and energy relationship of various EES systems. Pike Research forecasted that the grid?scale stationary EES system revenues will grow from $1.5 billion in 2010 to $25.3 billion over the following ten years, with the most significant growth in EES technologies

Principle of Energy Storage in ECs

EC devices have attracted considerable interest over recent decades due to their fast charge–discharge rate and long life span. Compared to other energy storage devices, for example, batteries, ECs have higher power densities and can charge and discharge in a few seconds. Since General Electric released the first patent related to ECs in 1957. These devices have been applied in many fields, including power capture and supply, power quality applications, and backup power.

• Basics of Double Capacitance and Pseudocapacitance

ECs are classified into two types based on their energy storage mechanisms: EDLCs and pseudocapacitors. In EDLCs, energy is stored via electrostatic accumulation of charges at the electrode–electrolyte interface. In the case of pseudocapacitors, energy is stored by the electrosorption and/or reversible redox reactions at or near the surface of the electrode material, usually a conducting polymer or transition metal oxide. In general, both these mechanisms exist in a supercapacitor device.

• Charge Storage Mechanism in EDLCs

The energy storage of EDLCs is via charge adsorption at the surface of the electrode without any faradaic reactions. During the charge/discharge processes, the arrangement of the charges in the Helmholtz double layer results in a displacement current. Since the materials can respond quickly to the change of potential and the physical reaction in nature, EDLCs can deliver energy quickly. However, due to the confinement of the electrode surface, the amount of stored energy is limited and much lower than that of pseudocapacitors and batteries.

The full potential of nanostructured capacitive materials, especially extrinsic pseudocapacitive materials, and hybrid electrodes has not yet been realized. The performance, in terms of the capacitance, rate capability, and cycle stability, needs to be further improved and a proper balance needs to be considered. However, some fundamental criteria for identifying potential high?performance pseudocapacitive electrode materials have been proposed, along with strategies for hybrid electrode design. Intrinsic and extrinsic pseudocapacitive materials have been identified from both thermodynamic and kinetic point of view. Advanced approaches, aiming at introducing more electrochemically active sites and shortening the transport path for electrons and diffusion length for ions, have been discussed. This is achieved through the selection of an appropriate pseudocapacitive material and the careful design of the hybrid electrode architecture. Furthermore, the ability to quantitatively differentiate between the capacitive and diffusion?controlled processes assists in tailoring the hybrid electrode for different applications.

Ternary hybrid structures have been explored in order to take advantage of the different merits of the components (conductive additives, pseudocapacitive metal oxides, and/or conducting polymers). One typical ternary electrode composed of MnO2, CNTs, and PEDOT?PSS demonstrated significant improvement in the electrochemical performance.Each component in the MnO2/CNTs/PEDOT?PSS hybrid structure contributed to the improved electrochemical properties. The MnO2 nanospheres provided high specific capacitance, the CNTs offered high surface area for the deposition of MnO2 and provided good electrical conductivity and mechanical stability, and the PEDOT?PSS acted as an effective dispersant for the MnO2/CNTs composite, and a good conductive binder for ensuring good electric contact between the MnO2 nanoparticles and CNTs.

There are several important points to consider regarding the topic of pseudocapacitive materials and hybrid electrodes:

• The same electrode material, depending on its structure, morphology, particle size, and guest ion, may display pseudocapacitive or battery?like behavior. The identification of the types of such electrode materials through the quantitative differentiation between capacitive and diffusion?controlled processes is critically important.
• The development of ternary hybrid structures is a promising direction for optimal electrode design as the positive properties of all components can be combined. A good understanding of surface chemical interactions between the components is essential to boost synergistic effects to enhance charge transfer and storage. This knowledge is extremely lacking for ternary hybrids, although the charge storage mechanism in binary structures has been well explained.
• Going beyond hybrid electrodes, hybrid energy storage devices consisting of a Faradaic battery?type electrode and a Faradaic pseudocapacitive or a non?Faradaic double layer electrode, or consisting of hybrid battery?capacitor electrodes, could be promising alternatives to break the energy density limitation of traditional electrochemical capacitors and the kinetic limitation of batteries. The key challenge lies in a thorough understanding of the basic electrochemistry of the double hybridization approach. In addition, the selection of appropriate electrode materials and the design of unique hybrid electrodes are key factors in realizing the full potential of hybrid electrode materials and hybrid energy storage devices.