171
Components and Materials for Electrochemical Supercapacitors
Mn(II) that results in dissolution of negative electrode material at a cell voltage of 0.5 V (corresponding to approximately –0.25 V on the negative MnO 2
electrode) during the charging cycle [85]. Due to slight variations in electrode
mechanisms, the positive Mn electrode experiences a set of reduction reactions from Mn(III) to Mn(IV) that are stable and reversible [83,86]. The combination indicates the practical use of MnO 2 is in hybrid systems that employ
pseudocapacitive positive electrodes with different negative electrodes.
Layer shape and thickness are also important parameters in developing
MnO 2 electrodes. Pure films of MnO 2 exhibit lower pseudocapacitance (150
F.g –1 ) than RuO 2 (600 F.g –1 ) in aqueous electrolyte [1]. The capacitance of MnO 2
is restricted by poor electronic conductivity. Further, evaluation of the oxidation stress on thicker films shows that only the top few layers of material are
available for use because of limited ion accessibility [87]. This means that the
high capacitance of MnO 2 is not easily achieved at practical mass loading or
higher power.
The use of conducting metal or carbon substrates with controlled thin film
deposition (<100 nm) of MnO 2 demonstrates that a much higher maximum
performance of 1200 F.g –1 was possible for low mass loading [1]. Belanger
et al. showed that MnO 2 capacitance could reach 900 to 1380 F.g –1 when
mass loading was low (5 to 30 μg.cm –2 ) [88]. Zhitomirsky et al. studied the
improved performance retention possible by doping MnO 2 with conductive
additives such as silver [89] and CNTs [90] at material loading as high as
150 to 300 μg.cm –2 . Using CNTs to generate an open surface network created
higher power electrodes containing amorphous MnO 2 with capacitance up
to 568 F.g –1 [91].
Controlling hydrothermal synthesis of MnO 2 could create thin layers of
crystalline material with a variety of nanoarchitectures such as nanowires,
nanobelts, nanorods, and hollow nanospheres and urchins. Morphology and
crystal type (α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 , and ξ-MnO 2 ) are controlled
by temperature, pH, mole ratios, and reaction time. Yang et al. [92] demonstrated thin 17 nm layers of δ-MnO 2 nanoflowers that exhibited a maximum
capacitance of 260 F.g –1 at low current of 70 mA.g –1 . Zhang et al. [93] showed
α-MnO 2 nanourchins (basic pH) and belts (acidic pH) with maximum capacitances of 161 F.g –1 and 262 F.g –1 , respectively, at 250 mA.g –1 . However, the performance of these materials (Figure 4.22) was only 199 F.g –1 and 121 F.g –1 for
current discharge at 1 A.g –1 in aqueous electrolyte compared to EDLC electrodes [93]. Despite the interesting morphology, performance of amorphous
MnO 2 remains superior.
4.2.9.3 Transition Metal Nitrides
Metal nitrides are transition metal materials (titanium, vanadium, and
molybdenum) that are receiving attention for pseudocapacitive study
because they exhibit high electronic conductivities compared to transition
oxides. Liu et al. [94] studied molybdenum nitride (Mo x N) and showed that
Components and Materials for Electrochemical Supercapacitors
Mn(II) that results in dissolution of negative electrode material at a cell voltage of 0.5 V (corresponding to approximately –0.25 V on the negative MnO 2
electrode) during the charging cycle [85]. Due to slight variations in electrode
mechanisms, the positive Mn electrode experiences a set of reduction reactions from Mn(III) to Mn(IV) that are stable and reversible [83,86]. The combination indicates the practical use of MnO 2 is in hybrid systems that employ
pseudocapacitive positive electrodes with different negative electrodes.
Layer shape and thickness are also important parameters in developing
MnO 2 electrodes. Pure films of MnO 2 exhibit lower pseudocapacitance (150
F.g –1 ) than RuO 2 (600 F.g –1 ) in aqueous electrolyte [1]. The capacitance of MnO 2
is restricted by poor electronic conductivity. Further, evaluation of the oxidation stress on thicker films shows that only the top few layers of material are
available for use because of limited ion accessibility [87]. This means that the
high capacitance of MnO 2 is not easily achieved at practical mass loading or
higher power.
The use of conducting metal or carbon substrates with controlled thin film
deposition (<100 nm) of MnO 2 demonstrates that a much higher maximum
performance of 1200 F.g –1 was possible for low mass loading [1]. Belanger
et al. showed that MnO 2 capacitance could reach 900 to 1380 F.g –1 when
mass loading was low (5 to 30 μg.cm –2 ) [88]. Zhitomirsky et al. studied the
improved performance retention possible by doping MnO 2 with conductive
additives such as silver [89] and CNTs [90] at material loading as high as
150 to 300 μg.cm –2 . Using CNTs to generate an open surface network created
higher power electrodes containing amorphous MnO 2 with capacitance up
to 568 F.g –1 [91].
Controlling hydrothermal synthesis of MnO 2 could create thin layers of
crystalline material with a variety of nanoarchitectures such as nanowires,
nanobelts, nanorods, and hollow nanospheres and urchins. Morphology and
crystal type (α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 , and ξ-MnO 2 ) are controlled
by temperature, pH, mole ratios, and reaction time. Yang et al. [92] demonstrated thin 17 nm layers of δ-MnO 2 nanoflowers that exhibited a maximum
capacitance of 260 F.g –1 at low current of 70 mA.g –1 . Zhang et al. [93] showed
α-MnO 2 nanourchins (basic pH) and belts (acidic pH) with maximum capacitances of 161 F.g –1 and 262 F.g –1 , respectively, at 250 mA.g –1 . However, the performance of these materials (Figure 4.22) was only 199 F.g –1 and 121 F.g –1 for
current discharge at 1 A.g –1 in aqueous electrolyte compared to EDLC electrodes [93]. Despite the interesting morphology, performance of amorphous
MnO 2 remains superior.
4.2.9.3 Transition Metal Nitrides
Metal nitrides are transition metal materials (titanium, vanadium, and
molybdenum) that are receiving attention for pseudocapacitive study
because they exhibit high electronic conductivities compared to transition
oxides. Liu et al. [94] studied molybdenum nitride (Mo x N) and showed that