–
Understanding mutual relationships between electrolyte
and electrode functional groups on redox reactions of interest for advanced
supercapacitors
*
*
+
Süheda Isikli , Jesús Palma, Marc A. Anderson, Raül Díaz
Electrochemical Processes Unit, IMDEA Energy, c/Tulipán, s/n, E-28933 Móstoles (Madrid), Spain
*email: [email protected]; [email protected]
2.3. Steric effects
Introduction
CH3
The main issue to be overcomed for the implementation of renewable
energies is the development of suitable energy storage devices.
Electrochemical devices are the most promising alternative, but while
batteries lack power density supercapacitors lack energy density. The
use of supercapacitors with hybrid carbon materials having
pseudocapacitive redox properties may fill this gap, and the presence
of quinonic moieties on the surface of carbon electrodes has long
been known to give rise to highly reversible pseudocapacitances [1].
Phosphate buf.pH7
Phosphate buf.pH8.75
0,3
H3C
0,2
H3C
I/mA
-0,1
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
0,2
1,2
2
Fig. 2. Cyclic Voltammetries of p-BQ in phosphate buffer solutions
at pH 7 and 8.75.Scan rate:50mV/s
-0,6
-1,6
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
Ewe/V
0,4
Sodium borate buf.pH8.75
Phosphate buf.pH8.75
Figure 5. Cyclic voltammetry of DMBQ and DTBBQ in acetonitrile
with TBAPF6. Potentials vs Ag/Ag/Cl, current densities.
0,2
Table 3. Peak positions extracted from Figure 5.
I/mA
0,0
-0,2
DTBBQ
-0,4
-1,0
-0,5
0,0
0,5
1,0
Fig. 3. Cyclic Voltammetries of p-BQ in sodium borate and
phosphate buffer at pH8.75. Scan rate 50mV/s
Table 1. Peak positions extracted from Figure 3.
First
Oxidation
peak (Vvs
Ag/AgCl)
0.149
0.055
-0.107
-0.068
O
-
-
O
O
O
Q
Q.-
Q2-
2.2. Electron density
2e-, 2H+
[H+] ›[Q]
2,5-dimethyl1,4-benzoquinone
(DMBQ)
OH
(H 2 O)n
O
O
CH3
H3C
+ 2e-
DMBQ:50mV/s
p-BQ:50mV/s
0,4
(H 2 O)n
1,4-benzoquinone
(p-BQ)
O
O
[H+]‹ [Q]
-1,053
-0,594
-0,696
-1,252
0,199
0,102
O
O
CH3
CH3
C
OH
Eox1Ered2
2-Hydroxymethyl-6methoxy1,4-benzoquinone
(HMMBQ)
0.256
0.123
2. Non-aqueous electrolytes: acetonitrile
O
Second
Eox2reduction Ered1
peak (Vvs
Ag/AgCl)
O
2.1 Electrochemical reaction mechanism in aprotic media
O
First
reduction
peak (Vvs
Ag/AgCl)
OH
Thus, care must be taken when analyzing results of redox reactions of quinones in
different media, as in [2]. We hereby state that, while phosphate and bis-tris
buffers, the two buffers typically used due to their close relationship with biologic
media, give similar results at neutral pH, phosphate buffer at basic pH gives rise
to a different behaviour, in contrast to the use of borate buffer. The different
characteristic of phosphate buffer in alkaline solution arises from the formation of
transition state with PO43- ions during the redox process.
-
Second
oxidation
peak (Vvs
Ag/AgCl)
2.4. Hydrogen bonding
Second
Reduction Eox1Oxidation peak (Vvs Ered1
Peak(Vvs Ag/AgCl)
Ag/AgCl)
Phosphate -0.031
Borate
First
oxidation
peak (Vvs
Ag/AgCl)
DMBQ
-0,87
-0,493
-0,614
-0,974
0,121
0,104
When comparing DTBBQ and DMBQ, while keeping the effects related to the
electron donating strength of the substituents, the main additional difference is the
strong additional displacements of the first oxidation and the second reduction
peaks to more negative potentials. The close structural relationship between
DTBBQ and DMBQ allows to attribute this displacement to the steric effects due
to the bulky tert-butyl groups of DTBBQ. It si interesting to note that this would
mean that the first radical anion will be preferentially reduced at the quinonic
group closer to the substituents.
-0,6
1. Aqueous electrolytes
O
0,0
-0,2
-0,4
1.1. Effect of pH in buffered and unbuffered solutions
It has been discussed that there is an influence of the pH and of the
use of buffered or unbuffered solutions. Two different mechanisms
were proposed [2]:
O
J(mA/cm )
Ewe/V
Results and discussion
O
DTBBQ:50mV/s
DMBQ:50mV/s
0,4
-0,5
Experimental
O
DMBQ
-0,3
Ewe/V
Unbuffered
Electrolyte
O
-0,2
As a first step in the development of controlled quinonic compounds
leading to advanced supercapacitors, we hereby present a study of
several quinonic compounds in aqueous and non aqueous media.
Interestingly, in aqueous medium different reaction mechanisms
appear to occur at differing conditions of pH, and strong effects of
interaction with the electrolyte used are also present, difficulting the
study of neutral unbuffered electrolytes typically used in aqueous
supercapacitors. When using acetonitrile with tetrabutylammonium
salts as electrolyte, it is demonstrated that the electrochemistry of a
particular quinonic compound is strongly dependent on its particular
electron density and structure –viz, steric effects; hydrogen bonding.
O
CH3
2,6-Di-tert-butyl1,4-benzoquinone O
(DTBBQ)
0,0
Optimal device operation requires optimization of surface functional
groups for each one of the two electrodes in a particular electrolyte.
While the lack of control of organic moieties present in carbons has
complicated this optimization, the study of controlled quinonic
compounds in supercapacitor devices has been scarce, and the study
of redox reactions of quinonic compounds in solution is mainly
focused on their biological applications.
Buffered
Electrolyte
O
CH3
CH3
CH3
-0,4
O
CH3
0,1
While aqueous supercapacitors are more environmentally friendly,
non aqueous electrolytes offer a wider potential window which results
in better performances. Quinonic compounds offer the oportunity to
study both kinds of device.
Materials. 1,4-Benzoquinone (BQ, >99.5%), 2-hydroxymethyl-6-methoxy1,4-benzoquinone (HMMBQ, 97%), 2,5-dimethyl-1,4-benzoquinone
(DMBQ,>99.5%), 2,6-ditert-butyl-1,4-benzoquinone (DTBBQ, 98%),
tetrabutylammonium hexafluorophosphate (TBAPF6, 99%), acetonitrile
(99.5+%) were purchased from Sigma Aldrich. TBAPF6 was purified by
recrystallization in ethanol, and the other chemicals were used as received.
Phosphate (disodium hydrogen phosphate, >99.0%,Fluka+HCl, p.a.,37% /
NaOH, p.a.,99%,Scharlau), sodium borate (Boric Acid, p.a., Sigma Aldrich
+NaOH) and BIS-TRIS(2,2-bis(hydroxymethyl)-2,2¨,2”nitrilotriethanol,99.0%,Sigma Aldrich+ HCl) buffers with various pH range
were prepared for the experiments in aqueous media.
Methods. Cyclic voltammetry experiments were done using a conventional
three electrode configuration, with glassy carbon (0.5mm vitrous carbon
foil,Goodfellow) as working electrode, a platinum grid (99.9% metal basis,
Sigma Aldrich) counter electrode, and an Ag/AgCl reference electrode.
0.1M TBAPF6 was used in all experiments as supporting electrolyte. A
VMP3 BioLogic potentiostat was used to record the data. Quinone
concentrations used were, unless otherwise stated, 2 mM. Extended Ar
bubbling was performed when working in air to remove oxygen.
O
H3C
O
O
DMBQ
Table 4. Peak positions extracted from cyclic voltammetries.
First
oxidation
peak
(Vvs
Ag/AgCl)
HMMBQ -1,023
20mV/s
DMBQ
-0,87
20mV/s
HMMBQ -1,052
50mV/s
Second
oxidation
peak (Vvs
Ag/AgCl)
DMBQ
50mV/s
-0,88
Second
Eox2reduction Ered1
peak (Vvs
Ag/AgCl)
Eox1Ered2
-0,502
First
reduction
peak
(Vvs
Ag/AgCl)
-0,594
-1,090
0,092
0,067
-0,493
-0,614
-0,974
0,121
0,104
-0,495
-0,603
-1,122
0,108
0,070
-0,466
-0,665
-1,029
0,1988
0,149
HMMBQ -1,056
-0,495
-0,602
-1,112
0,107
0,056
100mV/s
DMBQ
-0,827
-0,442
-0,651
-1,023
0,2093
0,196
100mV/s
When comparing HMMBQ and DMBQ, the main feature is the outstanding kinetic
and thermodynamic stability of HMMBQ. We attribute this effect to the
stabilization provided by hydrogen bonding through the –OH group of the
methanol substituent of HMMBQ.Our reasoning follows those of similar protoncoupled electron transfer pathways [3].
Conclusions
0,2
O
O
2
J(mA/cm )
Based on the results presented in the next section, we propose an
additional mechanism, present in phosphate buffer at basic pH:
PO 4
-0,4
+ 2e-
Basic
Phosphate
Buffer
-0,6
-1,6
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
Ewe/V
O
O
Figure 4. Cyclic voltammetry of p-BQ and DMBQ in acetonitrile
with TBAPF6.
PO 4
1.2. Influence of the electrolyte
Table 2. Peak positions extracted from Figure 4.
First
oxidation
peak (Vvs
Ag/AgCl)
Second
oxidation
peak (Vvs
Ag/AgCl)
First
reduction
peak (Vvs
Ag/AgCl)
Second
Eox2reduction Ered1
peak (Vvs
Ag/AgCl)
Eox1Ered2
BQ
-0,832
-0,346
-0,467
-0,937
0,121
0,105
DMBQ
-0,87
-0,493
-0,614
-0,974
0,121
0,104
Bis-Tris buf.pH 7
Phospahe buf.pH7
0,4
0,2
I/mA
0,0
-0,2
0,0
-0,2
-0,4
-0,6
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
Ewe/V
Figure 1. Cyclic Voltammetry of p-BQ in Phosphate and Bis-Tris
buffer solutions at pH7.Scan rate:50mV/s
When comparing BQ and DMBQ, it is clear that the introduction of electron
donating groups in the benzene ring does not influence the reversibility of the
redox processes and shifts to more negative potentials all the peaks, but specially
the second oxidation peak and the first reduction peak. Thus, electron donating
groups destabilize the first radical anion.
We verified that the mechanism of electron transfer was highly affected by
both the nature of the medium and the quinonic compound. For example, in
aqueous electrolytes it would appear that different reaction mechanisms occur
at differing conditions of pH, and a strong effect of the electrolyte used is
detected.
In acetonitrile, electron donating groups destabilize the first radical anion
formed after oxidation but do not affect greatly the initial product or the
second radical anion formed, while steric effects must also be taken into
account. Interestingly, the presence near the quinonic groups of substituents
able to form hydrogen bonds stabilizes all the quinonic redox processes,
making them more reversible and kinetically faster.
These results will help to synthesize quinonic compounds with tailored
properties adapted to supercapacitor devices covering different requirements.
References
[1] H.A. Andreas, B.E. Conway, Electrochimica Acta (2006), 51, 6510.
[2] M. Quan et al., J. Amer. Chem. Soc. (2007), 129, 12847.
[3] C. Costentin et al., Acc. Chem. Res. (2010), ASAP.
Acknowledgements
The authors wish to thank “Comunidad de Madrid” and “European Social Fund”
for its financial support to the SOLGEMAC project through the Programme of
Activities between Research Groups (S2009/ENE-1617).
R.D. thanks MICINN for the “Ramon y Cajal” contract.