**Toward High-Voltage/Energy Symmetric Supercapacitors via Interface Engineering Supercapacitors via Interface Engineering**

**Toward High-Voltage/Energy Symmetric** 

Yaqun Wang and Guoxin Zhang Yaqun Wang and Guoxin Zhang Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73131

#### **Abstract**

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This chapter includes elaborately selected recent literatures on electrochemical energy storing in symmetric supercapacitors (SSCs) with high operating voltages (voltage >1.6 V) and high specific energy. SSCs are a typical sort of electrochemical capacitors with larger energy density than conventional capacitors; by involving electrode materials with stable interfaces (for instance, nitrogen-doped carbon materials) and electrolytes with wide safe potential window (for instance, ionic liquids), they can supply competitive energy relative to batteries. Fundamentals of SSCs are first introduced, aiming at clarifying some critical interfacial phenomena that are critical to enhance overall capacitive performance. State-of-the-art SSCs are included as demonstrations from the aspects of both enhanced capacitances and expanded voltages. We also provide a few feasible strategies for the design high-voltage/energy SSCs such as using inactive electrode materials.

DOI: 10.5772/intechopen.73131

**Keywords:** symmetric supercapacitor, electrode materials, electrochemical interface, high voltage, high energy

## **1. Introduction**

The growing concerns over fossil fuels, in terms of global warming, pollution, and resource depletion, call for clean and renewable energy such as sunlight, wind, and hydrogen energy [1]. Consequently, great necessity has been urged regarding the development of rapid and efficient energy storage upon the generation of huge amount of the aforementioned types of energy. Supercapacitors (SCs), storing energy in or on the interfaces of electrode materials, are capable of fully charging/discharging within seconds or minutes, making them excellent candidates for the fast accumulation of these transient types of energy [2–4]. Also, taking advantage of the interfacial energy storage mechanism, unlike the deep-conversion mechanism in batteries, SCs

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

have excellent recyclability, typically >5000 times [5]. The past decade has witnessed significant progresses in SCs researches, many types of SCs have come to existence, including but not limited to electrochemical double-layer capacitors (EDLCs) and asymmetric supercapacitors (ASSCs), and they can be further sorted by applicable electrolytes (mainly three types electrolytes: aqueous, organic, and ionic liquid electrolytes) [2]. Still, they are not satisfactory enough from the perspectives of energy stored, which is mainly due to low capacitances or narrow potential windows especially in aqueous electrolytes [6, 7]. Coupling SCs with pseudo-capacitive electrode materials such as transition metal-based materials or electronically conductive polymers is feasible to enlarge the specific energy; however, the lifetime of pseudo-capacitors normally goes down quickly in a few hundred cycles [8, 9]. Currently, it is still very hard to simultaneously obtain large capacitance, high-operating voltage, and high-cycling stability.

Symmetric supercapacitors (SSCs), mainly including carbon-based EDLCs and a few SSCs with identical metallic component- or conductive polymer-based electrodes, supply much higher specific power and cycling stability than pseudo-capacitors, due to the interfacial charging/ discharging mechanism [2, 10]. Their energy are given by the equation (1), in which E is the energy stored in capacitor cell, CT is the total capacitance, and V is the operating voltage. E is proportional to total capacitance and square voltage, which means that specific energy E can be improved via two ways: increasing specific capacitance and expanding operating voltage [11]. Both of these aforementioned two aspects are highly related to the interfacial chemistry and phenomenon [5]. According to the electrochemical double-layer phenomenon established by Helmholtz, the electrochemical interface consists of electrode surface and thin layered electrolyte (containing ions or cations) adjacent to electrode surface. In the first place, this thin layer of electrolyte plays the fundamental roles of conducting ions and facilitating charge compensation on electrode interface; additionally, they can be decomposed or transformed to supply non-Faradaic current once charge transfer occurs when the electrons arrive in or depart from the conduction band of electrode [6]. Before that, electrolyte ions and molecules are forced to strongly absorb onto electrode surface to form a tightly packed stern layer [6, 7]. According to previous literatures, the efficiency and strength of absorption are highly depend on the surface properties of electrode materials, doping, defect, and functionality and can significantly alter the interactions between electrolyte and electrode interface [11–13]. Therefore, in order to achieve high operating voltage as well as high energy, it is critical to address the interface issues regarding both the surface properties of electrode materials and the applicable electrolytes.

$$\mathbf{E} = 1/2\mathbf{C}\_{\mathbf{r}}\mathbf{V}\_2 \tag{1}$$

SSCs with large operating voltage and specific energy, and last but not the least, feasibility of safely expanding operating voltage for the achievement of high-energy SSCs. In addition, the importance of carbonaceous electrode materials that are inactive for water splitting is highlighted, in which their specific energy can be significantly improved due to greatly expanded operating voltages. The prospects of SSCs developments are speculated based on the interface engineering on carbonaceous materials, highlighting the practical feasibilities of high-energy SSCs for the progressing smooth swing to renewable energy from traditional fossil fuels.

Toward High-Voltage/Energy Symmetric Supercapacitors via Interface Engineering

http://dx.doi.org/10.5772/intechopen.73131

Symmetric supercapacitors (SSCs) are mainly built on electrochemical double-layer configured identical positive and negative electrodes; most applicable electrode materials are carbon-based due to their high chemical stability of carbon materials [1, 14]. The electrochemical doublelayer model, first established by Helmholtz, reveals that two oppositely charged ionic layers are formed at electrode-electrolyte interfaces under electrochemical forces driven. Afterward, Stern recognized that there are two regions of ion distribution at the electrode-electrolyte interfaces: one inner layer and one outer layer, as schematically depicted in **Scheme 1**. The inner region, where ions are strongly absorbed onto the electrode surface, is called the compact layer (or Stern layer); and the outer layer consists of a continuous distribution of ions in solution [10]. The capacitance at electrode-electrolyte interface (CEDL) with electrochemical double-layer configuration can be divided into capacitance from the inner compact layer (CH) and capacitance

*CEDL*

fied due to conductivity and cost issues, which we will summarize later in Section 3.

Besides the electrode materials selection, it is also important to choose a proper electrolyte and solvent to form a robust electrode-electrolyte interface since energy stored in a SSCs

= \_\_\_1 *CH* + \_\_\_\_ <sup>1</sup> *CDiff*

There are several critical factors that give significant impact on CEDL, mainly including the conductivity of electrode material, the surface area of electrode materials, the accessibility to the inner electrochemical surface, the electric field across the electrode, and the electrolyte/solvent properties [15]. For instance, SCs with high-surface area porous carbon electrode materials (such as activated carbon) can store much more capacitances by several orders of magnitude. There are also a few other ways that are feasible to enlarge the capacitances including doping heteroatom elements and compositing stable metal oxides or conductive polymers [11, 16]. Heteroatom doping is able to break down the high symmetry of graphitic carbon, creating a large amount of defects, leading to the easy formation of compact inner absorption layer [16]. The advantages of using stable metal oxides or conductive polymers as electrochemical interfaces instead of carbon materials are obvious; they are capable of supplying much more capacitance through pseudo-capacitive absorption besides the EDL capacitance [17]. However, the greatly enlarged capacitances are often obtained on the basis of compromising the efficiency and cycling stability, and only a minor few metal oxides and conductive polymers are quali-

(2)

119

**2. Basics of symmetric supercapacitors (SSCs)**

from the diffuse layer (CDiff), as described in equation (2)

\_\_\_\_ <sup>1</sup>

In the past few decades, many review articles have discussed the investigations on materials selections and device fabrications for developing high-performance SCs, but few accounted for the interface designing and engineering. Also, research progresses from different angles (material synthesis, electrolyte selections, and device fabrications) have come to the point calling for a generic summary for improving the integrated performance of SCs on the clear understanding of electrode interfacial phenomenon. This chapter aims to present and discuss a number of relevant issues, including fundamentals of interfacial (mainly electric double layer (EDL)) capacitance, nanoscale charge transfer, discussions on a few benchmarked SSCs with large operating voltage and specific energy, and last but not the least, feasibility of safely expanding operating voltage for the achievement of high-energy SSCs. In addition, the importance of carbonaceous electrode materials that are inactive for water splitting is highlighted, in which their specific energy can be significantly improved due to greatly expanded operating voltages. The prospects of SSCs developments are speculated based on the interface engineering on carbonaceous materials, highlighting the practical feasibilities of high-energy SSCs for the progressing smooth swing to renewable energy from traditional fossil fuels.
