Nanostructured materials for supercapacitor applications

Sanket Bhoyate, Pawan K. Kahol and Ram K. Gupta

DOI: 10.1039/9781788013871-00001

Insightful growth in nanostructure materials has flourished the field of nano-energy research, especially, supercapacitors, as an answer to worldwide energy issues. In this chapter, application of nanostructured materials for supercapacitors focusing on latest exploration and development in nano-energy materials and their effect over charge storage capacities has been described. Several factors such as safety, environment friendliness, durable life cycle, higher coulombic efficiency, energy, and power densities, play a critical role in defining supercapacitors as a device of interest. A major part of this chapter focuses on the development of nanostructured materials for electrochemical double layer capacitors (EDLCs) and pseudocapacitors. However, brief background over charge storage mechanism has also been considered.

1 Introduction

In 2003, Nobel Laureate Richard E. Smalley delineated that energy is the topmost problem faced by human society. Increasing global population has caused raising demands for technological gears, especially in portable devices and automobiles. This has led to the gradual depletion of fossil fuels and caused increasing environmental issues. According to the report of International Renewable Energy Agency (IRENA), International Energy Agency (IEA) and the Renewable Energy Policy Network for the 21st century (REN21), about 29% of total final energy is consumed by transportion. Considering this, several automotive companies are focusing to develop electric vehicles and hybrid electric vehicles for which efficient energy storage devices are needed. Batteries and fuel cells are widely used energy storage devices providing high energy density; however, they lack the ability to provide higher power, safer operations, longer life cycle, and faster charge-discharge cycles.

Recently, serious safety issues related to Li-ion batteries caused due to fires or explosions in devices such as cell phones, laptops, electric vehicles, and airplanes have sparked worldwide attention. Few examples include fires or explosions in Samsung Note 7, Tesla electric car battery fire, and the Boeing 787 Dreamliner battery issues, remind us about the most urgent issues to be resolved before being proposed in the practical application. Moreover, charging time required for batteries especially in electric vehicles is another big challenge. A recent article in The New York Times addresses this issue as "Charging Time Trauma" which could be a limiting factor for the people to switch over from gas vehicles to electric vehicles. Such issues can be addressed through nanomaterial research and solving the current drawbacks of traditional devices. Researchers have provided significant attention over developing supercapacitors type batteries to address previous issues and create a more reliable and safe source of energy. Supercapacitors provide high power density, longer cycle life, ultra-fast charge–discharge, and safer operation, leading to appreciable attention as a device of interest. Use of nanostructured materials have shown promising capabilities for fabricating supercapacitors with higher efficiency to solve growing energy problems. Till date, thousands of reports and patents covering aspects of the electrical double layer (EDL), reversible redox reaction, and hybrid mechanisms have been developed. This technology has flourished into a multi-billion dollar industry covering a wide range of applications such as telecommunication devices, smart electronic devices, industrial actuators, automobiles, aerospace appliances, power tools, and many more.

Currently, supercapacitor research focuses on developing nanostructured material arrays providing flexibility, transparency, smaller-size, higher energy and power densities, and cost-effectiveness. This could be governed by considering basic electrode materials of capacitor devices summarized in later sections. However, before debating on this topic, it is essential to understand the general charge-storage mechanism of the supercapacitors.

The first generation capacitor was called a "Leyden Jar" that stored the charge electrostatically over the surface. Later, the concept was properly defined as EDL by Helmholtz. Based on this concept the first patent was filed for an electrochemical capacitor device by General Electric. After a few years, a new patent was granted on an electrochemical capacitor where a new concept of reversible redox reactions over the interface of electrodes was observed providing higher capacitance compared to the previous devices. The concept seems to combine behavior of high surface area carbon along with reversible redox materials of batteries. Although the device performance was high, a concept study of such mechanisms was not clearly defined based on its capacitance type. In 1971, an electrochemical capacitor based on RuO2 was discovered storing the charge based on faradic reactions following the nature as a capacitor. The concept was termed as pseudocapacitance and the device was termed as a pseudocapacitor. EDL capacitors provided high power density while pseudocapacitors delivered higher energy density. Hence, today's capacitor research tends to combine the concepts of EDLCs and pseudocapacitors to obtain a hybrid device with high energy and power densities. Thus, the capacitor's charge storage mechanism can be categorized as an electric double layer, pseudocapacitance, and hybrid mechanism and capacitor device is termed as an electric double layer capacitor, pseudocapacitor, and hybrid capacitor, respectively.

2 Charge storage mechanism

The charge storage...