Redox Additive Electrolytes in Pseudocapacitors

Supercapacitors (SCs) have attracted widespread attention due to their high power density,
fast charging/discharging capability, long cycle life, and high safety. The SCs are generally
classified into two classes to be electric double layer capacitors (EDLCs) and pseudocapacitors
based on their different charge mechanisms. In which, the pseudocapacitors proposes a higher
capacity than the former class. Thus, it receives a significant interest in both industrial and
academic researches. However, in overall, it should be noted that both SCs classes still exhibit
unsatisfactory and much lower energy density than rechargeable batteries, and thus extensive
research efforts are devoted to increasing the energy density of SCs.
The energy density of a SC is determined by both the electrode materials and the
electrolyte. Along with the development and modification of electrode materials to improve
capacity, exploration of advanced electrolytes is considered as an effective approach to enhance
the SC performances. Recently, redox additive electrolytes obtained a growing interest due to an
additional redox activity from electrolytes, which offers an increased charge storage capacity in
SCs. It also proposes potential solutions for how to effectively increase the energy density of the
SCs while maintaining their high power and long life. This work reports several aspects of redox
additives electrolytes used in pseudocapacitors including types, mechanisms, performances, and
drawbacks of redox additive electrolytes.

The type of redox additive electrolytes
Various types of redox additives have been explored and used in SCs. The redox species
in electrolytes can be catalogued into inorganic additives and organic ones. The inorganic additives
are halides (KI, NaI, KBr, NaBr, FeBr3), K3Fe(CN)6, K4Fe(CN)6, vanadium complexes, copper
salts, sodium disulfide, and silver nitrate. The organic additives such as benzyl viologen (BV),
ethyl viologen (EV), heptyl viologen (HV), methyl viologen (MV), methylene blue, and quinones
are also investigated. The use of redox additives depends on the redox potential, supporting
electrolyte (basic, neutral, or acid), and the charge storage mechanism of electrode materials.
The standard redox potentials of redox couples from the redox additives can determine which
redox couples can be used in the SCs, as shown in Figure 1.
1

Figure 1. Reduction potentials of the couples considered relative to the thermodynamic stability
window of water at neutral pH (white region). The lines coloured in red, green and blue are for
couples stable in acidic (1M acid), neutral and basic (1M base) conditions, respectively.

For pseudocapacitor, KI, and K3[Fe(CN)6] are the two most widely used. The redox reaction from
the redox additives during charging/discharging processes are shown as below
1. For KI:

2. For K3[Fe(CN)6]:

2


Mechanisms
Figure 2 schematically shows the simple charge storage mechanism of a redox active
electrolyte with an electrode material of Co-Al LDH.
The ions from redox additives are adsorbed on the electrode surface from the bulk electrolyte, and
these adsorbed ions are than directly involved in the redox reaction at electrode–electrolyte
interface. These electrochemical redox reactions are reversible. K 3Fe(CN)6 can act as an
electrolyte for charge storage and also as redox mediator because Fe(CN)63- ions are involved in
the redox reaction and then converted into Fe(CN)64-. The high reversibility of the
Fe(CN)63−/Fe(CN)64− ion pair at the electrode/electrolyte interface coupled with the redox
transition of Co(II)/Co(III) in the Co−Al LDH electrode is believed to be the key factor to improve
the electrochemical performance. The additional redox reaction thus enhances the capacitance.

Figure 2. Illustration of the role of K3Fe(CN)6 in the courses of (a) charge and (b) discharge for
the Co−Al LDH electrode. (Phys. Chem. Chem. Phys. 2009, 11, 2195− 2202).


Performance
In the inorganic redox additives used in pseudocapacitors, K3Fe(CN)6 and K4Fe(CN)6, are
the most widely used redox additives in aqueous electrolyte and have attracted considerable
attention in recent years. The first example of K3Fe(CN)6 as a redox additive was proposed in
2009. By adding 0.1 M K3Fe(CN)6 into 1 M KOH as electrolyte and using Co−Al-layered double
hydroxide (Co−Al LDH), the capacitance increased from 226 F g−1 to 712 F g−1. The redox additive
also exhibits a great enhancement electrochemical performances of Co 3O4 and CuFe2O4
pseudocapacitive materials as shown in Table 1. Furthermore, constructing an asymmetric
3


supercapacitor (ASCs) is regarded as an effective method to improve the energy density of the
device. Thus the redox additive was also applied in the ASCs to enhance the energy density. The
assembled an CuFe2O4||CNT/carbon black ASC in the 2 M KOH + 0.02 M K3Fe(CN)6 electrolyte
exhibited a high energy density of 128 W h kg−1 at a power density of 900 W kg−1 and 95% capacity
retention after 10000 cycles. Meanwhile, 3 M KOH added 0.03 M K4[Fe(CN)6] electrolyte showed
a great enhancement in energy density of CoTe||AC ASC from 23.5 to 67 W h kg−1 at a power
density of 793.5 W kg−1. The results successfully demonstrate the practical applicability of a
K3Fe(CN)6 and K4[Fe(CN)6] redox additives.
It is known that KI species exhibit well reversible redox reactions at the
electrode−electrolyte interface and are applied in various SCs. The KI was introduced as a
promising redox additive to evaluate and improve the electrochemical performance in both three
and two electrode systems. Senthilkumar et. al developed a new ASC using flower like Bi2O3 as
negative and bio-waste derived activated carbon as positive electrodes with Li2SO4/KI as redox
additive electrolyte. A nearly threefold improved specific capacitance and energy density of 99.5
F g−1 and 35.4 Wh kg−1, respectively was achieved by adding of KI into Li2SO4 electrolyte. An
addition of 0.0075 M KI onto 1 M Li2SO4 electrolyte, MWCNTs/ZrO2||MWCNTs/WO3 ASC also
showed an increment of energy density from ~65 Wh kg-1@950 W kg-1 to ~133 Wh kg-1@950 W
kg-1. The system still remained 85.3% capacitance after 3000 charge/discharge cycles. The
enhancement in energy density and capacitance could be explained by the addition of redox

reactions from the redox additives as shown above. The performances are summarized in Table 1
and Table 2.

Table 1. Electrochemical performance of pseudocapacitive materials in redox electrolytes (three
electrodes system).
Materials

CuFe2O4

Redox

Supporting

additives

electrolyte

-

2 M KOH

Capacitance

Rate capability

Cycling

Reference

stability

160 mAh g− 1 at 1 A g− 1

85% at 10 A g− 1

91%

after

10000 cycles
0.02

M

2 M KOH

450 mAh g− 1 at 1 A g− 1

83.7% at 10 A g− 1

K3[Fe(CN)6]
Co3O4

-

93.5% after
10000 cycles

3 M KOH

1560 F g-1 at 1 A g− 1


69.7% at 15 A g− 1

102%

after

10000 cycles

4

10.1016/j.cej.2
020.127779


0.01

M

3 M KOH

2670 F g-1 at 1 A g− 1

55.1% at 15 A g− 1

76%

K3[Fe(CN)6]
0.02


M

3 M KOH

6580 F g-1 at 1 A g− 1

31.1% at 15 A g− 1

K3[Fe(CN)6]

after

10.1016/j.ener

10000 cycles

gy.2020.11943

69%

6

after

10000 cycles

Table 2. Electrochemical performance of ASCs in redox electrolytes.
SCs

Redox


Supporting

additives

electrolyte

Capacitance

Rate

Energy density

Cycling

capabili

Reference

stability

ty
CuFe2O4||CNT

-

2 M KOH

/carbon black


63 mAh g

− 1

at 1 A g

−1

67% at
10 A g

−1

~47

kg-

Wh

1

-1

@~750 W kg

91%

after

10000


10.1016/j.cej.2
020.127779

cycles
0.02

M

2 M KOH

142 mAh g

−1

at 1 A g− 1

K3[Fe(CN)6]

78% at

~128 Wh kg

10 A g− 1

1

-

@~900 W kg-1


95%

after

10000
cycles

Co3O4||carbon

0.01

nano-onion

K3[Fe(CN)6]

Bi2O3||AC

M

-

2 M KOH

−1

120 F g

at


1 A g− 1

1 M Li2SO4

− 1

29 F g

at

1.5 mA cm− 2

25% at

~42.5 Wh kg

15 A g− 1

1

@~425 W kg-1

~25% at

35.4

6

1


mA

-

Wh

kg

-

75%

after

10000

gy.2020.11943

cycles

6

-

10.1016/j.elect

@822.6 W kg-1

acta.2013.10.1


cm− 2
0.06 M KI

1 M Li2SO4

−1

99.5 F g

at

1.5 mA cm− 2

10.1016/j.ener

99

45% at

10.2

6

1

mA

Wh

kg


-

72%

after

@822.6 W kg-1

1000 cycles

kg-

89.9% after

10.1038/srep2

3000 cycles

5793

−2

cm
MWCNTs/Zr

-

1 M Li2SO4


100 F g

0.0075 M KI

1 M Li2SO4

−1

196 F g

-

3 M KOH

at

60% at

~65

10 A g− 1

1

58% at

~133 Wh kg

10 A g


67.3 F g− 1 at

92.5%

23.5

at 10 A

1

1Ag

−1

−1

Wh

@950 W kg-1

−1

1Ag
CoTe//AC

at

1 A g− 1

O2||MWCNTs/

WO3

−1

1

-

-1

@950 W kg
Wh

85.3% after
3000 cycles

kg-

92.3% after

10.1039/c7ra1

-1

3000 cycles

2919j

kg-


80.7% after

-1

3000 cycles

@793.5 W kg

g− 1
0.03

M

K4[Fe(CN)6]

192.1 F g−
at 1 A g

−1

Drawbacks
5

1

85% at
5Ag

−1


67
1

Wh

@793.5 W kg


1. Numerous electrode materials are available but only a few materials are studied with redox
additive/active electrolyte.
2. Redox additives are mostly used in aqueous electrolytes and still not tested with organic
electrolytes etc.
3. There are many redox active materials or compounds available but they are yet to be utilized as
redox additives in electrolytes for SC applications.
4. The low rate capability of SCs having redox additives in pseudocapacitor should be enhanced.
5. Capacitance retention of devices also need to concern. Controlling the adding concentration of
redox additives may work.

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