146
Electrochemical Supercapacitors for Energy Storage and Delivery
be overcome by the driving potential of the capacitor during charging. This
discovery illustrates potential for improvement with subnanometer- and
nanometer-sized pore micropore structures to increase the capacitances
of electrode materials. It also highlights the complex tunable pore design
required to optimize electrode structures for EDLC application. At the same
time, complex pore structure and material choices for electrolytes can significantly alter performance characteristics, advantages, and drawbacks.
Other important properties to consider in EDLCs are pore regularity and
distance. In classical electrochemistry, ion transport time has a quadratic
link to transport length [17]. Regular pore structures provide reliable short
diffusion pathways between storage sites, but pore defects disrupt pore
regularity, which increases the pore interspacing and causes ion scattering. Irregular pore spacing and ion scattering detract from ion transport
speed and throughput, resulting in fewer accessible ions and lower power.
Scattering can also be caused by poor interfacial wetting of the electrode.
Depending on the character of the electrolyte solvent, hydrophobic or hydrophilic dopants or functional groups can be applied to the material.
4.2.6 Functionalization Effects on EDLCs
Small functional groups on the electrode layer materials, such as carbon particles, improve wetting with the electrolyte and allow increased solvent penetration within small pores of the electrode materials. For example, nitrogen
dopants and oxygen functional groups can improve wettability in aqueous systems by altering surface effects [18]. Further, active heteroatoms, such as oxygen
and nitrogen, give carbon materials a very stable covalently bound acidic (oxygen, electron acceptor) or basic character (nitrogen, electron donor) that introduces a pseudocapacitive component on top of the EDLC capacitance [19].
Pseudocapacitive metal oxide and metal nitride coatings deposited and
adsorbed on carbon supports are also effective in increasing the maximum
energy density possible for an ES and are discussed in detail later in this
chapter. Oxygen dopants such as carboxyl, carbonyl, and hydroxyl groups
can improve wetting and open surface area for increased capacitance and
higher power [20]. Activated carbons can incorporate oxygen groups through
steam activation during carbonization or through a secondary KOH activation reaction. An overabundance of oxygen functionalities disrupts the graphitic π-bonding network of the material and reduces conductivity of the
material too much. In active carbons, this can occur when activation temperatures and activation reagent concentrations are too high.
Graphene oxide (GO), the precursor material of graphene, contains a very
large number of oxygen groups introduced by a harsh oxidation reaction
from bulk graphite. The precursor material is insulating until it is reduced
with a strong reduction technique (sodium borohydride, high temperature,
and hydrazine) that aims to remove all the oxygen functionalities and restore
the graphitic plane to create conductive few-layer graphene for use in ESs.
Electrochemical Supercapacitors for Energy Storage and Delivery
be overcome by the driving potential of the capacitor during charging. This
discovery illustrates potential for improvement with subnanometer- and
nanometer-sized pore micropore structures to increase the capacitances
of electrode materials. It also highlights the complex tunable pore design
required to optimize electrode structures for EDLC application. At the same
time, complex pore structure and material choices for electrolytes can significantly alter performance characteristics, advantages, and drawbacks.
Other important properties to consider in EDLCs are pore regularity and
distance. In classical electrochemistry, ion transport time has a quadratic
link to transport length [17]. Regular pore structures provide reliable short
diffusion pathways between storage sites, but pore defects disrupt pore
regularity, which increases the pore interspacing and causes ion scattering. Irregular pore spacing and ion scattering detract from ion transport
speed and throughput, resulting in fewer accessible ions and lower power.
Scattering can also be caused by poor interfacial wetting of the electrode.
Depending on the character of the electrolyte solvent, hydrophobic or hydrophilic dopants or functional groups can be applied to the material.
4.2.6 Functionalization Effects on EDLCs
Small functional groups on the electrode layer materials, such as carbon particles, improve wetting with the electrolyte and allow increased solvent penetration within small pores of the electrode materials. For example, nitrogen
dopants and oxygen functional groups can improve wettability in aqueous systems by altering surface effects [18]. Further, active heteroatoms, such as oxygen
and nitrogen, give carbon materials a very stable covalently bound acidic (oxygen, electron acceptor) or basic character (nitrogen, electron donor) that introduces a pseudocapacitive component on top of the EDLC capacitance [19].
Pseudocapacitive metal oxide and metal nitride coatings deposited and
adsorbed on carbon supports are also effective in increasing the maximum
energy density possible for an ES and are discussed in detail later in this
chapter. Oxygen dopants such as carboxyl, carbonyl, and hydroxyl groups
can improve wetting and open surface area for increased capacitance and
higher power [20]. Activated carbons can incorporate oxygen groups through
steam activation during carbonization or through a secondary KOH activation reaction. An overabundance of oxygen functionalities disrupts the graphitic π-bonding network of the material and reduces conductivity of the
material too much. In active carbons, this can occur when activation temperatures and activation reagent concentrations are too high.
Graphene oxide (GO), the precursor material of graphene, contains a very
large number of oxygen groups introduced by a harsh oxidation reaction
from bulk graphite. The precursor material is insulating until it is reduced
with a strong reduction technique (sodium borohydride, high temperature,
and hydrazine) that aims to remove all the oxygen functionalities and restore
the graphitic plane to create conductive few-layer graphene for use in ESs.