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Components and Materials for Electrochemical Supercapacitors
Activated carbon (AC) materials made from coal pitch precursors are the
industrial standards for ESs, although newer materials and process variation
can alter the characteristics of AC materials.
Improvements to AC are under investigation in attempts to improve pore
structures and increase surface areas through templating AC carbon, functionalization, and generating AC using different precursor materials. More
advanced carbons, such as carbon nanotubes (CNTs), carbon nano-onions,
and graphene, may offer a variety of improved properties because of their
graphitic plane structures. These advanced carbons also exhibit very small
size scales in at least one dimension, enabling the possibility of large area
and improved power density through stronger electrolyte access.
Pseudocapacitive materials are used to increase energy density and
undergo redox reactions to store more than one charge per reactive surface
site and also allow storage deeper than the surface layer. Pseudocapacitance
has been investigated in transition metal oxide materials that can shift
between oxidation states quickly and in electrically conductive polymers
that store charges along reactive groups in their polymer backbones.
Commercialized electrolyte materials in ES devices are dominated by
organic electrolytes containing quaternary salts (tetraethylammonium tetrafluoroborate, TEA + BF 4
– ) because of their moderate ionic conductivities and
voltage windows [4]. The major solvents include acetonitrile and polypropylene carbonate. Aqueous electrolytes have very high ion conductivity rates
and are easier to handle than organic electrolytes, but they are unstable at
high potential (potential window limited to approximately 1 V). Common
aqueous electrolyte solvents used in the development of electrode materials include sulfuric acid (H 2 SO 4 ), potassium hydroxide (KOH), potassium
chloride (KCl), and sulfates (K 2 SO 4 , Na 2 SO 4 ). Attempts to design new and
improved electrolyte performance have led to ionic liquids that are capable
of high voltage windows (3 to 5 V). Also, attempts to reduce packaging complexity and minimize corrosion and safety problems have led to investigations of gel and solid-state polymers that combine both the separator and the
electrolyte into a single component within an ES.
4.2 Anode and Cathode Structures and Materials
4.2.1 Overview of Battery Functions and Materials
Batteries are operated by converting the energy stored in chemical bonds
to electrical energy. A constant potential is produced when a stable resistive load is applied and is equivalent to the difference between the half-cell
potentials of the electrode materials. Figure 4.1 illustrates an example of the
ion double-layer and nominal voltage generated by the half-cell reactions of