Work was conducted that could lead to a high energy density electrochemical capacitor, completely free of liquid electrolyte. A three-dimensional RuO sub x-ionomer composite structure has been successfully formed and appears to provide an ionomer ionic linkage throughout the composite structure. Capacitance values of approximately 0.6 F/sq cm were obtained compared with 1 F/sq cm when a liquid electrolyte is used with the same configuration

Forchione, J.

Griffith, A.

Laconti, A. B.

Lessner, P.

Sarangapani, S.

English

NASA Technical Reports Server

N90-20464
ADVANCED DOUBLE LAYER CAPACITORS
S. Sarangapani, P. Lessner, J. Forchione,
A. Griffith and A.B. LaConti
Giner, Inc.
Waltham, Massachusetts 02254
Work has been conducted that could lead to a high energy density electro-
chemical capacitor, completely free of liquid electrolyte. A three-dlmensional
RuOx-ionomer composite structure has been successfully formed and appears to
provide an ionomer ionic linkage throughout the composite structure. Capaci-
tance values of approximately 0.6 F/cm z have been obtained compared I F/cm 2 when
a liquid electrolyte is used with the same configuration.
INTRODUCTION
Electrochemical capacitors store energy by utilizing double layer and sur-
face redox type processes. Table I compares energy density, power density, and
cycle life figures for batteries, electrochemical capacitors, and conventional
capacitors. The projected energy density for electrochemical capacitors are two
orders of magnitude lower than that of batteries, but power densities (on the
millisecond to second) time scale are three orders of magnitude higher. Energy
density is much better than for conventional capacitors, but the nature of the
electrochemical processes make the electrochemical capacitors suitable for rela-
tively long pulses (milliseconds) and low to intermediate power applications.
Table I
Device Wh/L" W/L Cycle Life
Batteries 50-250 150 <104
Electrochemical Capacitors 5 >105 >105
Conventional Capacitors 0.05 >108 >106
Several applications of devices with the characteristics of electrochemical
capacitors can be envisioned: lightweight electronic fuses, burst power for
battery-powered vehicles and backup power for computer memory. The last appli-
cation takes advantage of the high cycle life of electrochemical capacitors,
instead of the high power density. Commercial devices, based on high surface
area carbons and sulfuric acid and marketed by NEC under the SUPERCAP brand
label, are already being sold for this application. Preliminary projections
83
https://ntrs.nasa.gov/search.jsp?R=19900011148 2020-03-19T22:44:18+00:00Z
indicate that the advanced electrochemical capacitors that are being developed
will have an order of magnitude higher specific energy and energy density than
these carbon-based capacitors.
A simplified equivalent circuit of a single electrochemical capacitor cell
is shown in Figure I. A device consists of two electrodes separated by an elec-
trolyte. Capacitances for metals and carbons in contact with aqueous solutions
are 10-40 mlcrofarads/real cm 2. Certain noble metal oxides in contact with
aqueous solutions have capacitances in the range of approximately 150 micro-
farads/cm 2 (Kleljn and Lyklema, 1987). Materials in contact with organic
solvent-electrolytes may also have high capacities. In addition, pseudocapacity
due to fast surface redox reactions can contribute to energy storage which is
accessible on a similar time scale as double layer energy storage. High surface
area porous electrodes can be used to give capacitances up to several farads per
geometric cm 2 .
The equivalent circuit also illustrates several resistances present in the
system. The resistance in parallel with the capacltance represents a leakage_
path. It should be as high as possible. The series resistance between the
electrodes ultimately limits the power that can be delivered by the system.
The major contributor to this is the resistance Or the electrolyte between the
electrodes. In addition, for practical systems using porous electrodes, the
distributed electrolyte resistance in the electrode contributes to the series
resistance.
Because of the decomposition of water, aqueous-based devices are limited to
about I V per cell; organlc-based devices may have higher cell voltages because
organic solvents have higher decomposition voltages. To build up the voltage,
several cells need to be stacked in series.
.... These physical principles provide an explanation for some of the figures of
merit listed in Table I. Energy density of electrochemical capacitors is lower
than that of batteries because the capacitors use surface processes to store
energy instead of bulk processes. However, these surface processes are accessi-
ble on a much shorter time scale than bulk processes giving the electrochemical
capacitor a higher power density. Cycle life is greater than that of batteries
because the surface processes result in no or small morphological changes in the
material. In a sense, the electrochemical capacitor can be thought of as a
specialized type of battery.
Some Work_has been conducted in the development of solvent-electrolyte-
based electrochemical capacitors (Holleck,et al., 1988; Lee, et al., 1988).
A problem with these systems is the possibility of electrolyte leakage. This
can occur because of external pressure changes or gas evolution due to cell
overcharge. The typical sulfuric acid electrolyte (3 to 4M H2SO4) is very cor-
rosive and leakage could cause the module and ancillary equipment to be damaged
or destroyed. This has led us to investigate devices where the liquid electro-
lyte is replaced by a solid ionomer electrolyte. Perfluorosulfonated ionomer
electrolytes, such as Nafion (R_, consist of a perfluoro polymer backbone to
which sulfonlc acid sites are permanently anchored. The only liquid or vapor
present in these devices would be pure water.
(R) E.I. DuPont Trademark
84
i
R
=
z
J
!
z
i
The device concept is shown in Figure 2. The capacitor electrodes (high
surface area metal oxides) are separated by thin sheets of ionomer membrane.
Current collection is accomplished via lightweight foils or screens. The con-
figuration allows the individual cells to be stacked into a module. The repeat-
ing element consists of the thin bipolar metal collector in intimate contact
with a unitized particulate-solid ionomer composite structure.
The major challenge to be met when replacing the liquid electrolyte with
the solid ionomer electrolyte is maintaining a high surface area electrode-
electrolyte contact. A liquid electrolyte is sorbed into the macropores of the
electrode and micropores down to about i00 A by capillary action. An illustra-
tion of the type of composite that needs to be formed with the solid electrode-
solid ionomer electrolyte structure is shown in Figure 3. A film of ionomer is
needed from the face of the electrode in contact with the ionomer membrane
separator extending back into the electrode toward the current collector. The
electrode particulates must make intimate electronic contact with each other and
with the current collector. Ideally, there is ionomer ionic linkage throughout
the thin particulate-solid ionomer composite structure and extends from metal
collector to metal collector.
There are several requirements for the electrode material to be used in the
electrochemical capacitor. Most importantly, it should have a high double layer
capacity and pseudocapacity on a real area basis. In order to have a high
capacitance on a geometric area basis, the material should have a high surface
area. The material should be able to be fabricated into a porous electrode
forming a high surface area interface with the solid ionomer electrolyte and
bonded to the ionomer membrane separator.
RuO 2 was selected as the electrode material to be studied initially because
it meets all of these requirements. The oxide has a high double layer capacity
of about 150 microfarads/real cm 2 (Kleijn and Lyklema, 1987). There is also a
substantial pseudocapacity due to the surface reaction which can be written
approximately as:
2RuO 2 + 2H+ + 2e- --> Ru203 + H20 [I]
This type of surface reaction in combination with double layer processes has
been shown to be capable of sustaining high current densities on the millisecond
scale (Holleck, et al., 1988; Lee, et al., 1988). RuO 2 can be made into a high
surface area coating (Trasatti and Lodi, 1980) or as high surface area particu-
lates (Balko, et al., 1980). Bonding of particulates to an ionomer membrane has
been demonstrated for RuO 2 anodes for use in chlor-alkali cells (Coker, et al._
1980) where membrane and electrode assemblies with active areas of up to 35 ft _
have been prepared.
EXPERIMENTAL METHODS
RuO 2 was prepared by thermal decomposition of RuCI 3 in a NaNO 3 flux (Balko,
et al., 1980). The resulting powder was characterized by scanning electron
microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction
(XRD) and nitrogen surface area (BET method).
85
Two approaches were used to form the RuO2-ionomer composite electrodes:
i) impregnation of a PTFE-bonded RuO 2 electrode with ionomer solution, and
2) mixing of ionomer solution with RuO 2 powder, followed by f0rmation into an
electrode. In all cases, the ionomer solution was 5% Naflon 117 in an alcohol-
water mixture (Aldrich Chemicals).
The RuO2-Nafion composite electrodes were hot-pressed to a Naflon 117
membrane. A diagram of the test cell is shown in Figure 4. The electrode
arrangement for testing consisted of two electrodes pressed on opposite sides
of the membrane. The membrane and electrode (M and E) assembly waS_1_9_gen
two acrylic pi@tes which contained a water reservoir to keep the M and E wet.
A tail of the Nafion 117 membrane-dips into a beaker Of H2SO4-which contain_ a_
Hg/Hg2SO 4 reference electrode. Some of the M and E assemblies had an integrally
bonded Pt/alr reference eiectrode.
The effectiveness of forming the extended RuO2-ionomer interfac e and the
maximum electrochemical limits of operation were assessed by using cyclic
voltammetry. The electrode was cycled between voltage limlts where only double
layer and reversible surface redox processes occur. For double layer or surface
processes, the current is related to sweep rate by:
dV
i - C -- [2]
dt
where C is the capacitance of the electrode.
RESULTS AND DISCUSSION
Powder x-ray diffraction showed the material to be substantially RuO 2.
There was some undecomposed _uCl 3 present. Nitrogen adsorption measurements
gave surface areas near 50 m=/g. The SEM ph0tograph in Figure 5_shovs the
particles to be pyramid-shaped and on the order of 0.i to 5 microns i_slze.
Clearly these surface areas cannot be explained on the basis of impermeable
particles of this size. TEM examination of a 50 m2/g powder (Figure 6) shows
that the particles have considerable internal porosity with crystallites on the
order of 200 _. Assuming spherical^particles of this diameter, the calculated
specific surface area is about 40 mZ/g, which is in good agreement with the
measured BET surfac_ area. This powder was heat treated which reduced its
surface area to 3 m=/g. Figure 7 shows that this surface area loss was due to
the loss of the small size particles by sintering. This type _f morphology in
the oxide is not unexpected. The density of RuCI 3 is 3.1 g/cm , while RuO 2 has
a density of 7 g/cmJ. When the decomposition reaction is accompanied by a large
decrease in specific volume (as is the case here) the final products tend to
have the external shape and dimensions of the precursor, but with many small
voids (Volpe and Boudart, 1985). This appears to be the case here.
Voltammograms of a RuO 2 electrode bonded to a Nafion 117 membrane are shown
in Figure 8. At potentials more negative than -0.4 V vs. Hg/HgSO 4, RuO 2 is
irreversibly reduced to lower oxides (shown as the beginning of a reduction wave
in Figure 8). Above about 0.5 V, oxygen evolution is possible. This gives a
single cell a usable voltage of 0.9 V. In the intermediate region, the voltam-
mogram is fairly flat with peaks due to reversible surface conversion of oxides
86
as in Reaction i. Trassati and Lodl (1980) have stated that the oxide cycles
between the +3 and +4 states in the region of -0.3 to 0.3 V. The current in
the potential region near 0.15 V was plotted against sweep rate to obtain
capacitance.
The capacitances of 50 m2/g powder bon_ed to the Nafion 117 membrane with
no ionomer impregnation are about 0.15 F/cm =. This capacitance is independent
of RuO 2 loading. This is approximately one order of magnitude lower than the
capacitance if the electrode was completely flooded with sulfuric acid.
Our most successful attempts to extend the area of electrode-electrolyte
contact has been via the technique of suspending RuO 2 in a Naflon solution and
spreading it onto the Naflon 117 membrane, followed by evaporation of the
solvent. Figure 9 shows the results obtained by using this technique with a low
surface area powder and 5 wt% Nafion in the final electrode. Although the abso-
lute values of capacitances are low, a clear correlation is seen between RuO 2
loading and capacitance. Figure i0 shows that with a higher surface area
powder, similar results can be achieved. Another technique is to form the elec-
trode on a separate thin sheet, followed by transfer to the membrane. Figure 10
shows that this method gives similar results; it _s also more amenable for
making larger electrodes. Electrodes up to 25 cm _ have been made with this
technique.
SUMMARY
The all-solid electrochemical capacitor concept offers the advantage of
greater reliability and safety. If the electrode-ionomer structure can be
optimized further and sufficiently thin ionomer membrane separators can be
developed, then the energy and power density for the all-solld capacitor should
be comparable to other advanced electrochemical capacitors being developed (Lee,
et al., 1988 and Table I). The techniques of sealing/water management, thermal
management and power transfer/control are expected to be similar to scaled-up
all-solid-state proton exchange membrane bipolar electrolysis devices (C_ker, et
al., 1982). These devices containing up to I00 bipolar elements (0.3 ftL) have
been marketed as reliable devices for over 5 years.
ACKNOWLEDGEMENT
This work is being supported by the Office of Naval Research under Contract
No. N00014-88-C-0391.
REFERENCES
Balko, E.N.C.R. Davidson and A.B. LaConti. 1980. Solid Solutions of RuO 2 and
IrO 2. J. Inorg. Nucl. Chem. 42:1778.
Coker, T.G., R.M. Dempsey and A.B. LaConti. 1980. Production of Halogens in an
Electrolysis Cell with Catalytic Electrodes Binded to an Ion Transporting
Membrane and an Oxygen Depolarized Cathode. U.S. Patent No. 4,191,618.
87
Coker, T.G., A.B. LaConti and L.J. Nuttall. 1982. Industrial and Government
Applications of SPECR)Fuel Cell and Electrolyzers. Paper published in
Proceedings of the Electrochemical Society Symposium on Membranes and Ionic and
Electronic Conductlng Polymers, Case Western Reserve, Cleveland Ohio.
Holleck, G.L., B.H. Jackman and R.D. Rauh. May 1988. High Rate Electrodes for
Pulse Power, Iridium Oxide and Ruthenium Oxide. Abstract No. 18, The 173rd
Electrochemical Society Meeting, Atlanta, Georgia.
Kleijn, J.M. and J. Lyklema. 1987. The Electrical Double Layer on Oxides:
Specific Adsorption of Chloride and Methylviologen on Ruthenium Dioxide.
Journal of Colloid and Interface Science. 120:511.
Lee, Ho-Lun, G. Mason and K. Kern. 1988. High Power Pulse Energy System
Concept. Fina_Report for ONR Contract No. N00014-87-C-0705.
Trasatti, S. and C. Lodl. 1980. Properties of Conductive Transition Metal
Oxides with Rutile-Type Structure. In Electrodes of Conductive Metallic Oxides
FartA, Chapter 7, S. Trasatti (Ed.), Elsevier, Amsterdam.
Volpe, L. and M. Boudart. 1985. Topotactic Preparation of Powders with High
Specific Surface area. Catal. Rev. Sci. Eng. 27:515.
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88
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Figure 2. Unitized Electrochemical Capacitor Module.
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Figure 3. Structure of lonomer-Hetal Oxide Composite Electrode.
89
ORIGINAL PAGE iS
OF POOR QUALITY
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Figure 5. SEM Photograph of RuO 2 Particulates. =
90
OR_GIN._L PI_GE G_
OF POOR QUALITY
Figure 6. TEM Photograph of 50 m2/g RuO 2 Particulates.
Figure 7. TEM Photograph of 3 m2/g RuO 2 Particulates.
91
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Advanced double layer capacitors

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