Bis(2,2′-biphenoxy)borates for electrochemical double-layer capacitor electrolytes

Article abstract of DOI:10.1002/chem.201003449

Fluorine makes the difference! Bis(2,2′-biphenoxy)borates decorated with fluorine substituents have been synthesized and studied in supercapacitor test cells (see scheme). A clear trend towards higher electrochemical stability with the increase of the fluorine content has been observed. For a maximum performance, only two fluorine substituents per benzene moiety are required.

Full text of DOI:10.1002/chem.201003449

DOI: 10.1002/chem.201003449
Bis(2,2’-biphenoxy)borates for Electrochemical Double-Layer Capacitor
Electrolytes
Robert Francke,[a] Dario Cericola,[b] Rꢀdiger Kçtz,[b] Gregor Schnakenburg,[c] and
Siegfried R. Waldvogel*[a]
Research on supercapacitors as energy storage devices
has recently attracted significant attention because this tech-
nique can combine high power output with high energy den-
sity applications.[1] Electrochemical supercapacitors can
store electrical energy on the basis of double-layer charging
or partial electrochemical conversion at the capacitor elec-
trode interface.[2] Such systems can be discharged at tremen-
dously higher rates than conventional batteries with an
almost unlimited number of cycles. Compared with conven-
tional capacitors, supercapacitors have a much higher specif-
ic capacity and energy, depending on the electrode surface
and pore structure.[3] The most prominent and powerful
electrode material for double-layer capacitors is activated
carbon because of its high porosity and high surface area,
good electronic conductivity, and electrochemical inert-
ness.[4] Several variations of carbon were intensively studied,
thus including mesoporous and microporous activated
carbon,[5] xerogel and aerogel carbon,[6] carbon nanostruc-
tures,[7] and glassy carbon.[8] Modern systems use an electro-
lyte containing tetraalkylammonium salts, preferably tetra-
fluoroborate, in an aprotic solvent such as acetonitrile.[9] The
electrochemical window allows operations at significantly
higher voltages compared with aqueous electrolytes. There-
fore, such supercapacitors can accommodate higher charge
densities and provide larger specific energy storage because
the storable energy increases with the square of the voltage
attainable on charge.[2]
inhibitors in such devices.[11] Despite some lack of stability
in the anodic region, we found that bis(2,2’-biphenoxy)bo-
rates can be used for supercapacitor cells.[12] A survey of the
known borates is given in context of secondary battery ap-
plications.[13]
Herein we report biphenoxyborates with a suitable stabili-
ty as supporting electrolytes for capacitive energy storage.
In the course of selective anodic coupling of simple phe-
nols to 2,2’-biphenols,[14,15] we developed a novel borate-tem-
plated strategy which allows this reaction to proceed on
large scale.[15] In this electroorganic conversion bis(2,2’biphe-
noxy)borates are formed as intermediates which turned out
to be extraordinarily stable. This prompted us to test them
in the supercapacitor cell.[12] Because of the limits detected
for the anodic range, we envisioned to improve the stability
towards oxidation by the installation of fluorine substituents.
The fluorination causes a considerable electronic perturba-
tion of the aromatic systems. This results in inverted electro-
static potential surfaces compared to the nonfluorinated spe-
cies.[16] Not surprisingly, such highly fluorinated compounds
are of interest for energy storage applications.[17]
A direct oxidative coupling to the corresponding fluori-
nated 2,2’-biphenols by standard methods[18] fails because of
the deactivating properties of the fluoro substituents.[15,19]
Only when boron-doped diamond anodes are employed, less
fluorinated 2,2’-biphenols can be obtained in moderate
yields.[20] However, we developed a reliable and general se-
quence to prepare the highly fluorinated 2,2’-biphenols 3
(Scheme 1).[21,22] The synthesis starts with fluorinated phe-
nols which are converted into the iodo anisoles 2 by a tele-
scoped iodination/methylation sequence.[21] Subsequent Ull-
mann-type coupling and deprotection provide the desired
fluorinated 2,2’-biphenols in good yields.[22]
The limited thermal stability of tetrafluoroborates and the
liberation upon decomposition incidents of highly toxic hy-
drogen fluoride create the need for alternative materials.
Recently, borate salts like lithium bisoxalatoborate were
identified as very powerful electrolyte systems for superca-
pacitors.[10] Beneficially, these borates can act as corrosion
The sodium bis(2,2’-biphenoxy)borates 4 can be obtained
in good to excellent yields by conversion of the correspond-
[a] R. Francke, Prof. Dr. S. R. Waldvogel
Institute for Organic Chemistry, Mainz University
Duesbergweg 10–14, 55128 Mainz (Germany)
Fax : (+49)6131-3926069
E-mail: waldvogel@uni-mainz.de
[b] D. Cericola, Dr. R. Kçtz
Paul-Scherrer-Institute, 5232 Villigen PSI (Switzerland)
[c] Dr. G. Schnakenburg
X-ray Analysis Department
Institute for Inorganic Chemistry, Bonn University
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003449.
Scheme 1. Synthesis of the biphenoxy ligands. DMF=N,N-dimethyl-
AHCTUNGTRENNGfUN ormamide.
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COMMUNICATION
ing 2,2’-biphenol 3 with sodium borohydride (Scheme 2 and
Table 1). In contrast to the tetrafluoroborate salts, the
sodium salts 4 can be handled at ambient conditions and are
stable in alcoholic solutions. With decomposition tempera-
tures higher than 3208C, these species exhibit an excellent
thermal stability.
trode (Figure 2). A comparison of the oxidation potentials
of 4 (Table 1, Figure 2) reveals a clear trend towards higher
anodic stability with the increase of the fluorine content in
Scheme 2. Synthesis of the bis(2,2’-biphenoxy)borates.
Figure 2. Cyclic voltammograms of bis(2,2’-biphenoxy)borates 4a–4e re-
corded at diluted concentrations; conditions: working electrode=Pt,
counter electrode=glassy carbon, reference electron=Ag/AgCl, c=
5·10À3 m, supporting electrolyte: NBu4ClO4 (0.1m) in CH3CN; n=
50 mVsÀ1; the whole potential range is shown exemplarily for 4e (inset).
Table 1. Yields of isolated products and oxidation potentials of sodium
bis(2,2’-biphenoxy)borates with different substitution patterns.
[a]
Compound
R1
R2
R3
Yield [%]
Eox vs. Ag/AgCl
4a
4b
4c
4d
4e
Me
F
Me
F
H
H
H
H
F
Me
Me
F
F
F
85
82
93
92
89
1.03
1.38
1.38
1.69
1.70
the ligand sphere. The replacement of a single methyl
moiety on a benzene ring of 4a by fluorine atoms in o- or p-
positions results in an anodic shift of the peak potential of
about 350 mV (4b and 4c, respectively). A further increase
of the peak potential of 310 mV is achieved by substitution
of the second methyl group in o- and p-position of the ben-
zene moiety by fluorine (4d). The introduction of additional
fluorine atoms in the m-position does not result in any sig-
nificant shift of the potential. Therefore, the installation of
four fluorine substituents on a 2,2’-biphenol results in a
maximum electrochemical stability and represents a com-
promise in synthetic effort and fluorine content. In the re-
ductive regime the potential window is limited by the de-
composition of the solvent (see inset in Figure 2).
F
[a] Peak potential; for conditions see Figure 2.
The structures of sodium bis(2,2’-bipenoxy)borates 4c, 4d,
and 5b were confirmed by X-ray analyses of suitable single
crystals (Figure 1, see the Supporting Information). The
borate anions have a disc-like shape with boron in a slightly
deformed tetrahedral coordination. The phenyl rings of the
biphenol ligands are tilted along their biaryl axis for about
40–438.
Since in supercapacitor cells a high concentration of the
conducting salt is required, the solubility of the salts in polar
aprotic solvents such as acetonitrile was optimized by cation
exchange (Scheme 3). The nonsymmetrical N-butyl-N-meth-
ylpyrrolidinium salts 5 turned out to have the highest solu-
bility in such solvents. Furthermore, ammonium ions are
known to be electrochemically stable within a wide potential
range.[23]
The capacitive performance of the different salts in a ca-
pacitor cell were tested with standard activated carbon elec-
trodes (YP17) in a limited potential window around the
open circuit potential of the carbon (see Figure 3 and
Table 2). For negative polarization, the resulting capacitan-
ces compare well to those obtained in the standard electro-
lyte based on Et4NBF4. For positive polarization, however,
(+0.5 V) the achieved capacitance is smaller than for the
same cell utilizing standard electrolyte. The current response
Figure 1. Molecular structures obtained by X-ray analysis of single crys-
tals of 4d (left) and 4c (right).
To study the influence of the substitution pattern on the
anodic stability of the bis(2,2’-biphenoxy)borate anions, we
performed cyclic voltammetry experiments in a standard
three electrode cell. To this aim, we used an acetonitrile-
based supporting electrolyte and platinum as a working elec-
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S. R. Waldvogel et al.
Scheme 3. Synthesis of the bis(2,2’-biphenoxy)borates 5 by cation ex-
change.
Figure 4. Cyclic voltammograms of 5c–5e in EDLC cells at elevated vol-
tages; conditions: see Figure 3.
rents are observed. The total available potential window
could be estimated to more than 3.0 V.
Conductivities of borate salts 5c–5e versus the concentra-
tion are plotted in Figure 5. For 5c a maximum appears at
about 0.4m, which can be explained by the increasing viscos-
Figure 3. Cyclic voltammograms of N-butyl-N-methylpyrrolidinium salts
5c–5e (saturated solutions) and NEt4BF4 (1m; dotted line) recorded
using EDLC test cells. Conditions: asymmetric three electrode cell, sol-
vent: CH3CN, n=1 mVsÀ1, c=0.9m (5c), c=0.25m (5d and 5e), Work-
ing, counter, and reference electrodes=activated carbon (YP17).
Table 2. Specific capacitances (C) and decomposition voltages (Eonset) on
activated carbon of the N-butyl-N-methylpyrrolidinium salts of bis(2,2’-
biphenoxy)borates 5c–5e.
Compound
C at À0.5 V [FgÀ1
]
C at 0.5 V [FgÀ1
]
Eonset [V]
5c
5d
5e
71
108
93
69
80
75
0.75
1.18
1.12
at the vertex potentials is rather slow for the tested salts
compared to Et4NBF4 in acetonitrile indicating higher resist-
ance of the bulk electrolyte and in the pores of the activated
carbon.
Figure 5. Conductivities of bis(2,2’-biphenoxy)borates 5c–5e plotted
versus their concentration in CH3CN.
The cyclic voltammograms for a much wider potential
range are reproduced in Figure 4 for salts 5c, 5d, and 5e.
Very nice capacitive behavior is observed for the negative
charging of the carbon down to 2.5 V with an average capac-
itance of 100 F/g. In the case of positive polarization, the po-
tential window is significantly smaller with limiting poten-
tials between +0.75 V for 5c and +1.18 V for salt 5d.
Beyond these limiting potential, significant oxidation cur-
ity of the electrolyte system. The saturated solutions of the
bis(2,2’-biphenoxy)borates are comparable to other acetoni-
trile-based electrolytes. However, the conductivity of the in-
vestigated systems does not yet reach the value of the cur-
rent standard electrolyte for electrical double-layer capaci-
tors.[23]
In conclusion, fluorinated bis(2,2’-biphenoxy)borates are
appealing supporting electrolytes for electrochemical
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Supporting Electrolytes for Electrochemical Capacitors
COMMUNICATION
double-layer capacitors. The stepwise introduction of fluoro
substituents enlarges the electrochemical window for these
bis(2,2’-biphenoxy)borates more than 600 mV. The maxi-
mum effect is already reached when two fluorine atoms are
installed per benzo moiety. The thermal and chemical stabil-
ity of these fluorinated bis(2,2’-biphenoxy)borates is by far
superior than the standard electrolyte systems. The found
electrochemical characteristics are close to the standard
electrolyte. The salts show good capacitive performance in
combination with the activated carbon YP17, especially for
negative charging. A total cell voltage of above 3.0 V can be
expected for the capacitor device. The performance ob-
served for positive charging can possibly be improved by tai-
loring the pore-size distribution of the activated carbon elec-
trode.
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Experimental Section
General procedure for the synthesis of the sodium bis(2,2’-biphenoxy)bo-
rates: Under inert conditions 200 mg (5.287 mmol) of sodium borohy-
dride and 10.574 mmol of the corresponding 2,2’-biphenol were placed in
a 50 mL Schlenk flask and 20 mL of dry THF were added slowly (atten-
tion: vigorous hydrogen evolution!). The reaction mixture was stirred at
room temperature until hydrogen evolution ceased. After slow elevation
of the temperature it was stirred for 6 h to reflux. The reaction mixture
was concentrated under reduced pressure to about a quarter of the
volume and then 15 mL of diethylether were added slowly. The precipi-
tated product was filtered off, washed two times with diethylether
(10 mL) and dried for 10 h at 1208C (5·10À3 mbar).
General procedure for the cation exchange reaction: A solution of
5 mmol of the corresponding sodium bis(2,2’-biphenoxy)borate in 30 mL
acetone was added to 0.889 g (5 mmol) of N-butyl-N-methyl pyrrolidini-
um chloride which was weighed in before in a Schlenk flask under inert
conditions due to its highly hygroscopic nature. The mixture was stirred
for three hours at reflux conditions, brought to room temperature and fil-
tered. The filtrate was concentrated under reduced pressure and recrys-
tallized from boiling ethanol. The product was dried for 10 h at 1008C
(5·10À3 mbar).
Cyclic Voltammetry in supercapacitor test cells: Cyclic voltammetry was
performed in a three electrode arrangement using high surface area acti-
vated carbon (~1600 m2 gÀ1) as working, reference and counter electrode
active material. The counter electrode was largely oversized with respect
to the working electrode. Cyclic voltammetries were measured with the
scan rate of 1 mvsÀ1
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Acknowledgements
Financial support by the Bundesministerium fꢁr Bildung und Forschung
(HE-Lion, 03X4612 J) is highly appreciated.
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Keywords: borates · energy storage · fluorine · phenols ·
Received: November 29, 2010
super capacitors
Published online: February 15, 2011
Chem. Eur. J. 2011, 17, 3082 – 3085
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