168
Electrochemical Supercapacitors for Energy Storage and Delivery
Another benefit of crystalline metal oxides is that their high conductivities
allow charge propagation along the lattice structures of thin surface layers.
However, conductivity is lower in the commonly used hydrous metal oxides
that are amorphous. It is possible to increase conductivity through crystallization but this significantly reduces active surface area via the removal of
water and elimination of pore spacing within the material structure [79].
Ruthenium dioxide (RuO 2) is widely used because it is highly reversible,
exhibits very high capacitance, and presents good cycle life. The fast redox
reaction of RuO 2 follows the mechanism in the following reaction and contains three oxidation states (0 ≤ b ≤ 2) within a 1.2 V potential window:
RuO x (OH) v + bH + + be – 㲗RUO x–b (OH) y+b
(4.4)
The ability of the oxidizing materials to transition from state to state allows
overlapping of charge windows and can provide a constant current over
operating potential. RuO 2 -based devices show strong performance of 350
F.g –1 at 100 mV.s –1 scan rates [80]. Zheng et al. [81] illustrated that anhydrous
salt forms (RuO 2 -xH 2 O) of the metal oxide could achieve optimum performance as high as 750 F.g –1 when annealed at temperatures just below the
crystallization point of the material. Chen et al. [82] showed results reaching
1500 F.g –1 using a porous carbon support.
However, ruthenium is a rare earth mineral that is highly toxic and its
high cost prevents its market use. Due to the high cost of RuO 2 , deposition
techniques must be optimized to deposit small quantities and enhance material utilization within a device. To further optimize use, composites with
high-surface-area activated carbon supports can be used. Despite the strong
performance characteristics of RuO 2 , inevitable supply and demand issues
push the market toward other pseudocapacitive materials.
A study of alternative transition oxides by Cottineau et al. yields important information about the performances of oxides of vanadium (V 2 O 5 ), iron
(Fe 3 O 4 ), and manganese (MnO 2 ) in aqueous electrolytes [83]. The synthesis
of powders showed MnO 2 and V 2 O 5 had amorphous structures, while iron
had a propensity to crystallize and form magnetite crystals. Composite electrodes were made on AC and tested with cyclic voltammetry (Figure 4.20).
The potential windows show that the oxide materials coupled with the
neutral electrolyte kinetically limit water decomposition (unrestricted water
decomposition occurs at V < –0.57 for hydrogen and V > 0.67 for oxygen
versus Ag–AgCl reference at neutral pH) [83]. Fe 3 O 4 pushes the negative
potential limit while MnO 2 and V 2 O 5 show stability up to +1 V. AC materials
show the onset of oxygen evolution beyond 0.6 V, but the kinetic limitation of hydrogen evolution on AC in K 2 SO 4 allows operation down to –1 V
(Figure 4.21) [83]. This highlights the potential for an asymmetric device with
increased potential window of 2 V using positive MnO 2 and negative AC–
Fe 3 O 4 electrodes. The curve shapes of the oxides can be seen in Figure 4.22.
Electrochemical Supercapacitors for Energy Storage and Delivery
Another benefit of crystalline metal oxides is that their high conductivities
allow charge propagation along the lattice structures of thin surface layers.
However, conductivity is lower in the commonly used hydrous metal oxides
that are amorphous. It is possible to increase conductivity through crystallization but this significantly reduces active surface area via the removal of
water and elimination of pore spacing within the material structure [79].
Ruthenium dioxide (RuO 2) is widely used because it is highly reversible,
exhibits very high capacitance, and presents good cycle life. The fast redox
reaction of RuO 2 follows the mechanism in the following reaction and contains three oxidation states (0 ≤ b ≤ 2) within a 1.2 V potential window:
RuO x (OH) v + bH + + be – 㲗RUO x–b (OH) y+b
(4.4)
The ability of the oxidizing materials to transition from state to state allows
overlapping of charge windows and can provide a constant current over
operating potential. RuO 2 -based devices show strong performance of 350
F.g –1 at 100 mV.s –1 scan rates [80]. Zheng et al. [81] illustrated that anhydrous
salt forms (RuO 2 -xH 2 O) of the metal oxide could achieve optimum performance as high as 750 F.g –1 when annealed at temperatures just below the
crystallization point of the material. Chen et al. [82] showed results reaching
1500 F.g –1 using a porous carbon support.
However, ruthenium is a rare earth mineral that is highly toxic and its
high cost prevents its market use. Due to the high cost of RuO 2 , deposition
techniques must be optimized to deposit small quantities and enhance material utilization within a device. To further optimize use, composites with
high-surface-area activated carbon supports can be used. Despite the strong
performance characteristics of RuO 2 , inevitable supply and demand issues
push the market toward other pseudocapacitive materials.
A study of alternative transition oxides by Cottineau et al. yields important information about the performances of oxides of vanadium (V 2 O 5 ), iron
(Fe 3 O 4 ), and manganese (MnO 2 ) in aqueous electrolytes [83]. The synthesis
of powders showed MnO 2 and V 2 O 5 had amorphous structures, while iron
had a propensity to crystallize and form magnetite crystals. Composite electrodes were made on AC and tested with cyclic voltammetry (Figure 4.20).
The potential windows show that the oxide materials coupled with the
neutral electrolyte kinetically limit water decomposition (unrestricted water
decomposition occurs at V < –0.57 for hydrogen and V > 0.67 for oxygen
versus Ag–AgCl reference at neutral pH) [83]. Fe 3 O 4 pushes the negative
potential limit while MnO 2 and V 2 O 5 show stability up to +1 V. AC materials
show the onset of oxygen evolution beyond 0.6 V, but the kinetic limitation of hydrogen evolution on AC in K 2 SO 4 allows operation down to –1 V
(Figure 4.21) [83]. This highlights the potential for an asymmetric device with
increased potential window of 2 V using positive MnO 2 and negative AC–
Fe 3 O 4 electrodes. The curve shapes of the oxides can be seen in Figure 4.22.