Chemistry Reference
In-Depth Information
feature increased capacitance that can be significantly higher than the
general capacitance of EDLC. The effective capacitance of pseudocapacitors
can be expressed in an additive manner as
C
¼
C
E
þ
C
P
(9.6)
d
n
3
r
4
n
g
|
4
where C
E
and C
P
indicate non-faradaic and faradaic contribution to the
capacitance, respectively.
Metal oxides and conductive polymers are widely used as electroactive
materials for pseudocapacitor electrodes. The pseudocapacitive metal oxides
should be not only reversibly electroactive for redox reactions but also
electrically conductive to deliver electrons that are essential for continuing
the reactions. Ruthenium oxide (RuO
2
) and manganese oxide (MnO
2
) are the
most representative metal oxides for pseudocapacitors. Other oxides such as
vanadium oxide (V
2
O
5
),
16
nickel oxide (NiO),
17
cobalt oxide (Co
3
O
4
),
18
and
tin oxide (SnO
2
)
19
can be potentially used as economic materials for
pseudocapacitor electrodes.
Ruthenium oxide has been widely investigated due to its superior per-
formance compared with other metal oxides. Ruthenium oxide has three
distinct redox reactions that are active within 1.2 V:
RuO
2
þ
xH
1
þ
xe
2
RuO
2
x
(OH)
x
(9.7)
where 0
o
x
o
2.
19
The specific capacitance of ruthenium oxide could reach
up to 720 F g
1
.
20
Ruthenium oxide also has high thermal stability, a rela-
tively long lifetime, and good electrical and proton conductivities. In con-
trast, the use of ruthenium oxide is restricted because of its high price and
toxicity.
Manganese oxide (MnO
2
) is a promising material alternative to ruthenium
oxide especially in terms of cost. From 2013, the price (per gram) of
elemental manganese is almost 800 times lower than that of ruthenium.
Moreover, manganese oxide is environmentally benign. The relevant redox
reactions of manganese oxide can be expressed as:
.
MnO
2
þ
E
1
þ
e
2
MnO
2
E
(9.8)
where E
1
indicates the cation of the electrolyte.
19
Notwithstanding the listed promising attributes of manganese oxide, it
has a very poor electrical conductivity (10
5
-10
6
Scm
1
).
21
When in-
dependently used as an electrode, therefore, manganese oxide suffers a
high ESR and poor power output. In addition, the ion intercalation-
deintercalation can cause mechanical instability under cyclic charging-
discharging loads. Moreover, chemical corrosion of the metal oxide, espe-
cially in aqueous electrolytes, can shorten the lifetime of the electrode. Re-
cently, conductive polymer wrapping has been suggested to compensate for
the low conductivity and protect MnO
2
from corrosion.
22,23
Conductive polymers such as polypyrrole (PPy), polyaniline (PANI), and
polythiophene (PT) are also promising candidates for pseudocapacitors.
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