Hairy carbon is a new material developed by growing submicron carbon filaments on conventional carbon substrates. Typical substrate materials include carbon black, graphite powder, carbon fibers, and glassy carbon. A catalyst is used to initiate hair growth with carbonaceous gases serving as the carbon source. To study the electrochemical behavior of hairy carbons, cyclic voltammetry (CV) and discharge testing were conducted. In both cases, hairy carbon results surpassed those of the substrate material alone

Chung, Deborah D. L.

Frysz, Christine A.

Shui, Xiaoping

English

NASA Technical Reports Server

N94"23367
HAIRY CARBON ELECTRODES
STUDIED BY CYCLIC VOLTAMMETRY AND BATTERY DISCHARGE TESTING
Deborah D. L. Chung and Xiaoping Shui
Composite Materials Research Laboratory, State University of New York at Buffalo
Buffalo, NY 14260-4400
Christine A. Frysz
Technology Division, Wilson Greatbatch, Ltd.
10,000 Wehfle Drive,. Clarence, NY 14031
ABSTRACt
Hairy carbon is a new material developed by growing submicron carbon filaments on
conventional carbon substrates. Typical substrate materials include carbon black, graphite powder,
carbon fibers and glassy carbon. A catalyst is used to initiate hair growth with carbonaceous gases
serving as the carbon source. To study the electrochemical behavior of hairy carbons, cyclic
voltammetry (CV) and discharge testing were conducted. In both cases, hairy carbon results
surpassed those of the substrate material alone.
INTRODUCTION
Because carbon and graphite exhibit an advantageous blend of chemical, electrical,
mechanical and thermal properties, they are very often considered for electrochemical applications.
Chemically, carbon and graphite offer good corrosion resistance, are available in high purity and
readily form intercalation products. Not only are they good electrical conductors, but they also
exhibit high thermal conductivity and low thermal expansion. Carbon and graphite are lightweight,
easily handled, and are also available in a variety of physical structures including carbon fibers,
submicron carbon filaments, carbon blacks and graphite powders. The battery electrode is among
the electrochemical applications requiring the characteristics just described.
Military, space and commercial applications continue to become more and more demanding.
Today's technological strides require that batteries be compact but capable of providing higher
energy and power density while still maintaining long shelf life and good discharge characteristics.
Continued advancement in carbon and graphite technology can contribute toward achieving these
goals by providing improvements to the characteristics that most influence performance.
Hairy carbon, a new material comprising tiny carbon filaments grown on conventional
carbon substrates, have been developed. The hairs on the carbon are submicron filaments with
diameters ranging between 0.05 Ixm and 0.2 Inn. Figures la and b are scanning electron
photomicrographs of hairy carbon grown on carbon black. Two different magnifications of the same
sample are displayed. At the lower magnification, Figure la, a small portion of a hairy carbon black
cluster is shown. The submicron nature of the hairs can very easily be seen at the higher
magnification presented in Figure lb.
Hairy carbons are not to be confused with conventional carbon fibers or carbon filaments as
developed by Tibbetts and coworkers at General Motors. 1-3 Whereas hairy carbon is catalytically
grown from carbonaceous gases, carbon fibers are typically prepared either from a pitch material or
from a polymer precursor such as polyacrylonitrile. In contrast to hairy carbon, carbon fibers have
diameters of the order of 10 Ima. Carbon filaments, like hairy carbon, are submicron in nature
(typical diameters of 0.2 _tm) and are also catalytically grown from carbonaceous source gases, but
carbon filaments are very long and tend to cling to each other as does cotton wool. Consequently,
dispersion and mixing are very difficult. Since hairy carbons can be produced on particulate carbon
substrates, the particulate nature of the finished product is preserved. Dispersion and mixing,
therefore, remain very easy. An added advantage is that the length of the 'hair' can be easily
controlled gearing the finished product toward the intended application. In fact, the hairs can be so
short that they resemble nodules.
251
https://ntrs.nasa.gov/search.jsp?R=19940018894 2020-06-16T16:05:10+00:00Z
Carbon filaments, like hairy carbon, can be grown using carbon monoxide or many different
hydrocarbons as the source. 4-t0 Methane and other aliphatics, olefins, and aromatics have all been
shown to produce carbon filaments when exposed to appropriate catalysts. Thepresence of gases
which can gasify carbon (hydrogen, water, carbon dioxide) reduces the driving force and lowers the
rate of filament growth, ultimately stopping it completely. Carbon filament formation takes place
under conditions where, in the absence of the 'catalyst,' no carbon deposition would take place.
Hairy carbon can easily satisfy two battery;electrode applications: (1) a reduction electrode
when used independent of other cathode mixes; and, (2) a conductive additive to otherwise
nonconductive or low conductivity cathode mixes such as manganese dioxide and carbon
monofluoride. For the former application, the submicron filamentous nature of the carbon can
provide a novel means for controlling connectivity, porosity and surface area. These attributes, along
with the improved physical properties, offer advantages over state-of-the-art carbon electrodes.
Regarding the latter application, hairy carbon, being more graphitic and exhibiting high aspect ratio,
in addition to improving electrical conductivity, could result in the addition of a smaller volume
fraction for conductivity enhancement allowing for addition of more active cathode material or
reduction in overall cathode size. The end result is high power and high energy density composite
cathodes with more energy output from the same package design or equal energy output from more
compact designs. This could prove beneficial to implantable device manufacturers, space exploration
programs and original equipment manufacturers requiring size and weight constraints or increased
energy from current design packages. This paper focuses on the former application, that is, the
carbon reduction electrode. Discharge testing was conducted in BCX electrolyte. BCX is a
codepolarizer of the interhalogen, bromine chloride, in thionyl chloride. 11
EXPERIMENTAL
The electrochemical behavior of the hairy carbons was studied using cyclic voltammetry
(CV). A Bioanalytical Systems CV cell, Figure 2, ¢vas used in conjunction with the Headstart
Electrochemistry program (developed by EG&G Princeton Applied Research), a potentiostat and an
IBM personal computer. A saturated calomel electrode served as reference with a platinum wire used
as the auxiliary electrode. The working electrode was a purchased holder typically used for carbon
paste electrodes. The hairy carbon filled the cavity as does carbon paste, however, was used without
the conventional paraffin oil binder. Testing was conducted in 6 mM potassium ferrocyanide as the
electroactive species in 1 M potassium nitrate in water as the supporting electrolyte. This solution
allowed study of the Fe2+/Fe 3+ redox couple.
To assess the discharge capability of hairy carbon, constant current plate to plate discharge
testing was conducted using the apparatus shown in Figure 3. A cut-down AA battery case was used
as the positive electrode current collector. The hairy carbon was placed inside the case and separated
from the lithium by a nonconductive glass fiber paper. Insulative polymer disks were placed on top
of the assembly and held in place with a metal clip. The assembly was immersed in the BCX
electrolyte. A Keithley power source was used to apply current. Data was collected on a 'randy
computer.
RESULTS AND DISCUSSION
Compared in Figures 4a - f are the voltammetric responses for hairy versus plain graphite
powder, hairy versus plain carbon black and hairy versus plain carbon fibers. The addition of hair
dramatically improves electrochemical performance. Note the increase in electron transfer rate and
enhanced reversibility of the redox reactions. The improvements are attributed to increased surface
area and particulate connectivity which translates into improved conductivity. Figures 5a and b
compare a cyclic voltammogram of conventional carbon paste using paraffin oil with dry hairy
graphite powder. The graphite powder used to grow hairs for this experiment was the same powder
used to manufacture the carbon paste commercially available for analytical electrodes. The results
indicate that the electrochemical response of the dry hairy graphite powder surpasses that of current
carbon paste technology.
252
Discharge performance using hairy versus plain graphite powder is shown in Figure 6. An
improvement in cell capacity per unit area of greater than 100% was achieved using hairy graphite
powder. The substantial increase in capacity is attributed to increased electrode surface area as well as
conductivity. The significance in this result is the fact that the graphite powder tested would never be
considered for lithium battery use primarily because of its low surface area. Given these results and
that many varieties of carbon forms can be made hairy, a new group of materials become available
for research by battery manufacturers.
In addition to surface area, pore size and distribution are of importance for carbon reduction
electrodes. Carbon blacks traditionally exhibit high surface areas. Electrode preparation methods
are developed to achieve optimum electrode pore size and distribution. To maintain good electrode
pore size and distribution, yet assess the effect of increased electrode conductivity, discharge testing
of carbon black electrodes employed blends of plain carbon black with hairy carbon black. The
plain carbon black provided retention of good pore size and distribution while the hairy carbon black
contributed to electrode conductivity. Figure 7 compares discharge performance of plain carbon
black with a blend of 50 weight percent hairy carbon black and 50 weight per cent plain carbon
black. An improvement of approximately 40 mA,h per unit density was achieved when discharged
to2 volts.
CONCLUSIONS
Hairy carbon is electrochemically superior to plain carbon whether in the form of carbon
black, graphite powder or carbon fibers. Hairy carbon can improve electrode performance as
observed by plate to plate discharge testing. Compared to plato graphite powder deemed unusable
for battery applications, hairy graphite powder used in a lithium soluble cathode system, BCX, as a
reduction electrode shows increased capacity by greater than 100%. The capacity of electrodes made
with a blend of hairy carbon black and plain carbon black was also improved. Capacity
improvements due to hairy carbon, however, are larger for graphite powder than for carbon black.
The difference in improvement is attributed to increased electrode surface area as well as conductivity
on graphite powder whereas conductivity alone is increased for hairy carbon black. Hairy carbon is a
new material. Its electrochemical behavior shows it to be attractive for electrode use. In addition to
offering a variety of new materials for research by battery manufacturers, the improved
electrochemical responses make hairy carbon desirable candidates for analytical electrodes, be they
chemical or biomedical. Other potential applications include electronic thermal conductors and
conductive composites.
ACKNOWLEDGMENTS
The authors would like to thank New York State Energy Research and Development
Authority for funding this work and Professor Carl R. F. Lund of SUNY/Buffalo for technical
assistance in hair growth.
REFERENCES
1. Tibbetts, G. G., Cryst, J., Growth, 66, 632 (1984).
2. Tibbetts, G. G., DeVour, M. G., and Rodda, E. J., Carbon, 25, 367 (1987).
3. Bradley, J. R., and Tibbetts, G. G., Carbon,23, 423 (1985).
4. Keep, C. W., Baker, R. T. K., and France, J. A., Journal of Catalysis,47, 232 (1977).
5. Baker, R. T. K., Barber, M. A., Harris, P. S., Feates, E S., and Waite, R. J., Journal of Catalysis, 26,
51 (1972).
6. Rostrup-Nielsen, J. R., Journal of Catalysis, 27, 343 (1972).
253
7. Obedin, A., Endo, M., and Koyama, T., Journal of Crystal Growth, 32, 335 (1976).
8. Jackson, S. D., Thompson, S. J., and Webb, G.,Journal of Catalysis,70, 249 (1981).
9. Audier, M., and Coulon, M., Carbon, 23, 317 (1985).
10. Lobo, L. S., and Tdmm, D. L.,Journal of Catalysis,29, 15 (1973).
11. C. C. Liang, P. Krehl, D. A. Danner, Journal of Applied Electrochemistry, 11, (1981) 563-571.
254
Ca)
2 tm
Figure 1 - Scanning electron photomicrographs of hairy carbon black.
A portion of a cluster of hairy carbon black is shown in (a). The
submicron nature of the hairs is readily observed in the higher
magnification view in (b).
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Figure 2 - The cyclic voltammetry cell used
to study the Fe2+/Fe 3+ redox couple.
Figure 3 - Hairy carbon discharge test cell.
Capacity comparisons were made by.
conducting plate to plate discharge testing.
255
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Figure 4 Cyclic voltammograms of plain versus hairy carbon substrates. Plain versus hairy
graphite powder is compared in (a) and (b) respectively, plain versus hairy carbon black in
(c) and (d) respectively, and plain versus hairy carbon fibers in (e) and (f) respectively.
256
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Figure 5 - Cyclic voltammetry comparison between conventional carbon
paste using paraffin oil binder (a) and dry hairy graphite powder (b). The
graphite powder for both (a) and Co) is the same except that the powder
in (b) is hairy.
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Figure 6 - Plate to plate d!scharge test
results for (a) hairy and (b) plain graphite
powder. The plain graphite powder is not
used by battery manufacturers because of
its low surface area.
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CAPACITY PER UNIT DENSITY (mA,h/g.cc" l)
Figure 7 - Plate to plate discharge test results
for (a) hairy and (b) plain carbon black.
The hairy carbon black electrode is a 50:50
blend by weight of hairy carbon black with
plain carbon black.
257
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This document contains the proceedings of NASA's fourth Space Electrochemical Research and Technology
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Hairy carbon electrodes studied by cyclic voltammetry and battery discharge testing

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