Application of hard carbon synthesized from leaves and cardboard for the cathodes of
potassium-air batteries
Shuo Liu
Upper Arlington High School
February, 2015
Application of hard carbon synthesized from leaves and cardboard for the anodes of
potassium-air batteries
Shuo Liu, 2817 Welsford Road, Upper Arlington, Ohio 43221
Upper Arlington High School, Upper Arlington, Ohio 43221
Mentor: Yiying Wu
The reliability and efficiency of energy and energy storage is becoming ever more important as
supplies of fossil fuel dwindle. Alternative sources of energy and new battery technologies have
become focal points of major research. The purpose of this study was to determine if hard
carbon could be successfully synthesized from different carbon sources, specifically leaves and
cardboard, and then utilized in the cathode of a potassium air battery. A hard carbon powder
was synthesized through pyrolysis and then mixed into a viscous slurry and applied to the
anode. The battery coin cell was tested and results showed a high level of function. The voltage
was consistent with other potassium air batteries and also exhibited stable long term charging
and discharging. This could lead to further improvements in efficiency of the potassium-air
battery and opens up the possibility of even using organic or waste materials to do so. This
study has important implications for the future of battery technology and energy storage.
Acknowledgements
First and foremost I would like to thank Mrs. Laura Brennan, the Science Research
coordinator at Upper Arlington High School for making this all possible and helping me through
my research project. I would also like to thank my mentor, Dr. Yiying Wu, a professor of
chemistry at Ohio State University who allowed me to conduct research in his labs and guided
me through my project. My thanks also goes out to Ren Xiaodi who supervised me and helped
me out with my experiments in the lab. Last but not least, I would like to thank my parents for
supporting and encouraging me all the way through my project.
Table of Contents
Introduction………………………………………………………………………………....5
Materials and Methods………...……………………………………………....………….6
Results……………………………………………………………………..………………..8
Discussion and Conclusion…………………………………………………………..……9
References………………………………………………………..………………….……..11
Appendices……………………………………………………..……………………..…….12
Introduction
The basic theory behind a battery is storing up chemical energy which can then be converted
into electrical energy for use. The reaction that occurs is a flow of ions through the cell and out,
and thus a flow of electrons, which provides the electrical energy. There are also usually
electrolytes within the cell that help facilitate the movement of electrons and ions. Battery
efficiency mainly depends on how well ions can flow within the battery and how restorable the
anode and cathode materials are, which allows the battery to maintain longevity. As the world’s
energy needs constantly ramp up, the need for efficient energy is also made ever more
apparent. Currently, lithium ion batteries are an effective way to power important items, such as
phones, laptops, or parts of electric vehicles. Hard carbon anodes have been incorporated into
cells since they increase the irreversibility of the cell. The lithium ions fit into stacked graphene
sheets which preserves the anode material for longer use (Buiel and Dahn, 1998).
My approach was to use the hard carbon anode concept and apply it to potassium air batteries
that the group I am working with is developing. The concept of the battery is that the cathode is
a porous carbon material which is permeable to oxygen, such that the oxygen is able to diffuse
through the cell to the potassium at the anode and oxidize it. The electrolyte solution is
specifically KPF6, which is also permeable to oxygen (Ren and Wu, 2013). The problem with
the current set up is that the oxygen comes into contact with the potassium too much, and forms
a layer of potassium oxide above the anode. This reduces conductivity of the cell and causes
the cell to lose efficiency. The problem with a direct application of the same graphene sheets is
that potassium ions are much larger, meaning that they won’t be able to fit. Thus, we had to
synthesize hard carbon, which has a different molecular structure comprised of hopefully
vacuous spaces for the larger potassium ions to fit into (Ponrouch, Goni and Palacin, 2013).
This paper details the synthesis and application of hard carbon to potassium cells and
compares their performances to control cells.
Materials and Methods
Hard Carbon Synthesis
Two dried oak leaves and a sheet of cardboard were collected. They first cut into smaller
pieces, about 0.5 cm by 0.5 cm squares and then underwent a retting process. They were
cleaned in separate beakers with 400 mL of deionized water. They were boiled off on hot plates
at 100 degrees Celsius and with a stir bar at 400 rpm. They were heated until five minutes after
they started to boil. The beakers were drained and the pieces of cardboard and leaves were
collected with filter paper then dried off in a convection oven overnight. Afterwards, the dried
leaves and cardboard were then separately pyrolysed in the tube furnace at 700 degrees
Celsius for two hours under a nitrogen atmosphere. The calcinated carbon was then massed
and ground using mortar and pestle into hard carbon powder.
Cell Construction
A viscous slurry was created by mixing the hard carbon powder with Super P, a commercial
conductive carbon black, PVDF, a commercial thermoplastic fluoropolymer, and NMP, an
organic solvent. The slurry was mixed in a 85% ratio hard carbon, 5% Super P, and 10% PVDF.
This resulted in a 52.50 mg hard carbon, 3.088 mg Super P, 6.176 mg PVDF, and 500
microliters NMP mix for the cardboard and a 33.20 mg hard carbon, 1.953 mg Super P, 3.906
mg PVDF, and 300 microliters NMP mix for the leaf. The slurries were mixed by alternating two
minutes on the Vortex Genie and then 10 minutes in the sonicator for 3 to 4 cycles.
At first, a Ni mesh was used as an electrode. 8 0.5 in diameter circles were punched out of a
sheet of Ni mesh and were cleaned with deionized water and acetone. 60 microliters of slurry
were piped onto the mesh circles, four for each the leaf and cardboard. The slurry was too thin
to properly integrate into the mesh, so a different substrate had to be used.
1 cm by 1 cm copper sheets were cut out. They were massed and then 60 microliters of the
slurry mixture was dripped on using a micropipette. The coated sheets were heated in a vacuum
oven at 60 degrees Celsius overnight. Afterwards the cells were constructed in a glovebox on a
Kimwipe. The electrode was placed onto the cell bottom with the copper side facedown. Then a
separator and 100 microliters of NaPF6 PC/EC electrolyte solution was added. On top of that a
stainless steel current collector and a piece of potassium was placed. The argon atmosphere in
the glove box prevented the potassium from reacting with oxygen. Then another separator, a
wave collector, and the cell top was put on. The cell was placed and centered on a compression
machine. The pressure was increased to about 1000 PSI and then it was all released and the
cell was retrieved. The side vent was vacuumed out and then the cell was taken out to ensure
no oxygen would enter the glovebox environment. The same procedure was repeated for all the
coin cells.
Analysis
A bit of the hard carbon powder for each sample was saved to use for analysis. A Raman
microprobe analysis was run on Super P and both hard carbons to compare the order and
vibrations of molecules to determine if it was truly a hard carbon with a more porous
morphology. Infrared spectroscopy was also run to determine if hard carbon exhibited a similar
structure to the normal carbon. X-ray diffraction was the last analysis run on the samples, which
was to ensure the molecular structure of the hard carbon differed from graphite. SEM
microscopy was the final analysis run to get a visual representation of the hard carbon.
The battery coin cells were taken and attached with clamps to a current measuring device. A
computer program would charge and discharge the cell and take measurements based on a set
of given parameters. The cell’s efficiency was measured by recording voltage and examining
the continuation of charging and discharging cycles. The results were controlled against the
standard potassium air battery coin cell.
Results
The hard carbon synthesis was successful for both the leaf and the cardboard. The samples
exhibited significant deviation from graphite in the analyses. Figures 1-3 display the difference in
results for the analyses. Figure 4 clearly shows a different morphology of the hard carbon
samples compared to graphite under SEM. The hard carbon has a more chaotic morphology
with vacuous spaces in between while the graphite is a clearly layered and more rigid structure.
The hard carbon from both the leaf and the cardboard was then applied in potassium air battery
coin cells for testing. The samples were compared against a standard potassium air battery coin
cell to ensure proper function and hopefully efficient application. All the coin cells were tested for
voltage and capacity and length of charge/discharge cycles.
Figure 5 shows the different voltage-capacity profiles for the cells. Although the standard coin
cell was slightly better in terms of efficiency, the different hard carbon coin cells showed
promising results. They had slightly less voltage, but seemed to still be able to function perfectly
fine, even using waste and organic materials like leaves and cardboard. In Figure 6 the charge
and discharge cycles are being compared between the cells. A major issue with all oxide
batteries is the degradation of the electrolyte and the formation of an insulating layer on the
metal electrode. The constant flow of oxygen and side products through the electrolyte causes
degradation, and at the electrode the side products react to form an insulating potassium layer
on top of the electrode. The layer prevents continued efficient diffusing of potassium in the cell,
thus severely limiting the life cycle of the cell. The hard carbon actually improved the life cycle of
the cell and kept up more consistent charge and discharge cycles over the control cell.
Discussion/Conclusion
Battery technology is becoming ever more important as a field of research. With the dwindling
supplies of fossil fuels, new sources of energy need to be utilized and more efficient ways of
using and storing that energy need to be developed. Different battery types are being refined
and experimented on. This study focuses specifically on the potassium air battery, a potentially
useful battery in the future due to its high specific energy and stability from low overpotentials.
This study addresses on the longevity problem of the potassium air battery by applying hard
carbon synthesized from leaves and cardboard, which are easily obtainable organic and waste
products.
The hard carbon was effectively synthesized and had a much different morphology from
graphite. Through the different analyses, it could be seen that the hard carbon synthesized from
both leaves and cardboard was much more chaotic and had more vacuous spaces in its
morphology while graphite were clearly layered and orderly. The utilization of the hard carbon
still allowed the potassium air battery coin cells to function correctly, although at a slightly lower
voltage level. This could have happened because the cell was not able to maximize the first
charge cycle since the flow of electrons through the cell might be disrupted slightly by the hard
carbon morphology. However, the hard carbon cells actually ended up having more consistent
charge and discharge cycles as time progressed, which meant they provided better cell life and
longevity. That was one of the fundamental issues of a potassium air battery because of the
degradation of the electrolyte from diffusion of oxygen and side products, as well as the
formation of an insulating layer on the electrode from the side products reacting with potassium.
However, in this study, the hard carbon actually helped the issue, since the flow of potassium
ions is much improved. With graphite, potassium ions are unable to integrate smoothly in the
cathode region since the layered sheets are not accommodating enough for the size of the
potassium ions. However, because the hard carbon has a much more variable morphology,
there are many spaces for potassium ions to fit into and reduces the formation of the insulating
layer, thus allowing for more long term function and consistent charge and discharge cycles.
This study is also subject to some limitations. Most of the results found were due to the
application of hard carbon. There were not many significant results drawn between hard carbon
synthesized from leaves or hard carbon synthesized from cardboard, so it’s difficult to tell what
difference the source of carbon makes. Also, the study was mainly focused on whether hard
carbon would be applicable in the battery, and not so much the efficiency of the battery. There
could be more analyses conducted about different variables of battery function between the
hard carbon cell and the control. This could be even further extended to comparing it against
commercial batteries to determine what the next step to improving viability of the potassium air
battery is. However, this study did yield promising results for the application of hard carbon and
opens up many possibilities for future research.
References
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