PROJECT PROGRESS REPORT Electrocatalytic Energy Conversion at the Interfaces of Hybrid Carbon-Bismuth Nanoparticle Assemblies Submitted To The 2014 Summer NSF CEAS REU Program Part of NSF Type 1 STEP Grant Sponsored By The National Science Foundation Grant ID No.: DUE-0756921 EEC: 1004623 College of Engineering and Applied Science University of Cincinnati Cincinnati, Ohio Prepared By Trevor Yates, Chemical Engineering, University of Cincinnati Adam McNeeley, Chemical Engineering, University of Cincinnati Will Barrett, Chemical Engineering, University of Cincinnati Report Reviewed By:
Transcript
PROJECT PROGRESS REPORT
Electrocatalytic Energy Conversion at the Interfaces of Hybrid Carbon-Bismuth Nanoparticle Assemblies
Submitted ToThe 2014 Summer NSF CEAS REU Program
Part ofNSF Type 1 STEP Grant
Sponsored ByThe National Science Foundation
Grant ID No.: DUE-0756921 EEC: 1004623
College of Engineering and Applied Science University of Cincinnati
Cincinnati, Ohio
Prepared By
Trevor Yates, Chemical Engineering, University of CincinnatiAdam McNeeley, Chemical Engineering, University of Cincinnati
Will Barrett, Chemical Engineering, University of Cincinnati
Report Reviewed By:
Dr. Anastasios Angelopoulos
REU Faculty Co-MentorAssociate Professor
Department of Biomedical, Chemical and Environmental EngineeringUniversity of Cincinnati
June 29, 2014
1. Abstract.
Bismuth has potential as an electrocatalyst because of its conductive properties and low cost.
Former studies have electrodeposited Bismuth nanoparticles on Graphite Felt electrodes. They
have found that Bismuth improves the energy conversion efficiency of Vanadium Redox Flow
Batteries (VRFBs). [2,5] In this project, Bismuth nanoparticles will be further investigated as an
electrocatalyst by implementing Standard Layer by Layer (sLBL) assemblies with Carbon and a
cationic polymer on a Glassy Carbon electrode.
2. Introduction.
Making solar energy more economical is one of the 14 National Academy of Engineering grand
challenges presented to all engineers because of the necessity to increase sustainability. Solar
energy’s major flaw is its intermittence. When it is cloudy or dark, there is no sun available, thus
no electricity can be generated to support a power grid. Having an effective way to store large
amounts of energy would be a major step towards making solar energy viable. One method of
storing energy that shows promise in this application is the VRFB. The VRFB is a redox flow
battery that stores charged Vanadium in electrolyte tanks and carries out redox reactions in the
cell stacks, just like a voltaic cell. A VRFB includes storage tanks, pumps, electrodes, and a
membrane that is permeable to cation diffusion. The VRFB has tremendous potential for energy
storage because the electrolyte solution is inert to degradation during the charging and
discharging cycles.2 The capacity and power output of the VRFB is also very customizable
depending on the volume of electrolyte and number of cell stacks. If more energy is needed,
then increase the size of the tanks or the number of cell stacks. The major issues with the VRFB
include poor energy conversion, rate capability, and power density.2 These issues keep the VRFB
from becoming a viable solution to this grand challenge.
There needs to be a larger effort to transition away from fossil fuels. Renewable energy sources
including wind, hydro, solar, wind, biofuels, and other biomass made up only 7.8% of the total
energy consumption in the United States in 2013. Wind and solar, the primary applications for
the VRFB, make up only 29% of that renewable energy.6 The United States Department of
Energy claimed in 2010 that it will take 104 years for the world to deplete all of its fossil fuel
reserves. This is assuming that the consumption rate is constant throughout. Since projections
show a doubling in energy demand every 14 years, due to population increase, it appears likely
that, unless major change takes place, the young generation will face a major energy crisis.8
The VRFB is expensive and must be improved in order to make intermittent sources more
economical options for replacing fossil fuels. A VRFB unit is estimated to have a production cost
of $217/kWh and an electricity storage cost of $0.10/kWh.8
A brief cost analysis will show the price of a 1 kWh unit and show how the overall cost
compares with fossil fuel energy. A VRFB consists of many components which will be looked at
individually.
The Vanadium electrolyte solution stores the potential energy at 25 Wh/kg and costs $1.60/kg
when purchased in bulk quantities. This means that $64 must be spent on the electrolyte solution.
In addition to paying for the electrolyte solution, the storage tanks have to be considered. Since
there is 153 L of solution total then two 100 liter tanks will be suitable. Tanks vary in cost
depending on the material, ranging from $432 (PCO Poly) to $637 (Stainless Steel). It is safe to
assume that the tanks will cost around $500. Next will be the pumps which cycle the electrolyte
solutions from the cell stacks to the storage tanks. Each pump must be able to move 153 L of
electrolyte at a rate of 0.0866 L/min.
100 L tank is expected to be at a height of 0.5 m. The energy required from a pump is so small
for this unit that it can be assumed to be one horsepower at around $110. The size of the
electrode is also important to consider. Since 800 A of current will be required with a nominal
voltage of 1.26 V, the surface area must be 2.5 m2. This same surface area will be assumed for
the proton exchange membrane with Nafion 117. The electrodes will cost around $40. The
ultimate price to pay will be the membrane. At around $100/ft2 the membrane will cost $2,634 in
a 1 kWh unit. The total for every component comes to $3,238. It cannot be assumed that the unit
cost will scale linearly with energy increase due to the many factors involved; however, as a
rough estimate, a 1 MWh unit should cost roughly 3.2 million dollars. Making this system
cheaper and more cost effective is the ultimate result that this project hopes to accomplish
through the development of a good electrocatalyst.
The first goal of this project is to investigate Bismuth as an electrocatalyst in hybrid electrodes to
improve energy conversion and rate capability. Bismuth has great promise because it is cheap
and serves as an excellent conductor. However, Bismuth as an electrocatalyst has not yet been
extensively documented, but on the other hand, previous research found that when Bismuth is
electrodeposited on graphite felt electrodes in VRFBs, there is an increase of energy efficiency
by 11% at high current density (150 mA/cm^2).2
The next goal of this project is to test the performance of a hybrid Carbon-Bismuth electrode
using Layer by Layer assembly. Carbon itself is a stable electrocatalyst but does not produce an
adequate amount of current density to be effective. The hope is that Bismuth will increase the
current density without losing the stability characteristic of Carbon.
3. Methods.
Particle and Polymer Preparation
The preparation of Bismuth nanoparticles and the cationic polymer have been important to the
testing because they were the two main compounds used to coat the electrode in Layer by Layer
applications. The nanoparticles proved to be particularly difficult to deal with. The solution of
dark black nanoparticles had a tendency to coagulate and precipitate out of solution. The
solution had to be thoroughly agitated before use in order to ensure that it was well mixed. The
Bi-Sn nanoparticles stability was dependent on the solution pH, which was found to be ideal at 3.
If the pH was higher than 3, the Bi-Sn structure would not hold together and be stable. If the pH
was lower than 3, then the Bismuth would oxidize.
The first Bi-Sn nanoparticles used were old and leftover from a previous experiment. These
particles had a very thick consistency, and the solution oxidized and turned white. Fresh
particles were prepared and required three days to set up. The new particles had a much thinner
consistency and were easier to work with. After the old particles had been oxidized, extra care
was taken to prevent the new ones from being exposed to air. Parafilm was wrapped around the
lid of the vial containing the particles when it was no longer being used and nitrogen was
pumped into the vial to provide protection from oxygen.
The polymer was prepared prior to any testing. The concentration of the polymer was 0.12 g/L
and was a clear solution. The same polymer solution was used for all of the tests. After initial
test results showed the polymer having a poor effect on the Bi-Sn nanoparticles, a portion of the
existing polymer was used to create a polymer with a slightly lower pH. The original polymer
had a measured pH of 4.4 and was reduced to 2.67 by adding 2 M H2SO4 drop wise. This
modified polymer was used for only one experiment just to see if it had a major effect. Every
other test used the same cationic polymer with a pH of 4.4.
The stability of the Bismuth nanoparticles had become a major concern in the experiment. As
mentioned earlier, after a few days of exposure to air the Bismuth nanoparticles were observed to
have a yellow white film that separated on top of the black solution. After an extended amount
of time, usually a few days later, the solution would turn completely white. Along with this
noted change in physical appearance, there was a consistent decline in electroactivity with the
aging of the particles. This decline was noticed because tests conducted with the older particles
consistently produced smaller Bismuth peaks when scanned. It was hypothesized that the
Bismuth nanoparticles were being oxidized in air over time. To prevent this from happening,
measures were taken to keep the nanoparticles from being exposed to Oxygen. When the
particles were used for experimentation, a pure Nitrogen air stream was blown into the vial after
uncapping until Bismuth was extracted using an automatic pipette. This Nitrogen stream was
usually applied for about five seconds. After the Bismuth was extracted, the Nitrogen stream
was reapplied for 30 seconds before recapping the vial. This procedure was repeated every time
the nanoparticles needed to be applied to the electrode. Once the nanoparticles were no longer
needed for the experiment, Nitrogen was blown into the vial for two minutes before being
capped and sealed with Parafilm.
4. V3+ Preparation
To prepare the V3+ electrolyte solution, it needed to be electrochemically reduced from V4+. To
do this two equal volumes of 0.1M V4+ solution were put into two halves of a cell. A potential
was then applied to the cell to charge it. Once the cell was charged, the negative side of the cell
contained V3+ and the positive side contained V5+. These two solutions were then transferred to
two sealable containers. Before they were put away for storage, Nitrogen was pumped into the
container to remove any Oxygen. This was done because these two solutions oxidized easily in
air. Once the Nitrogen was put into the containers, they were then sealed, with Parafilm wrapped
around the lids, making them completely air-tight.
Cleaning Process
The Glassy Carbon Electrode (GCE) used for the assembly of the electrodes being tested,
underwent a vigorous cleaning after each test trial. To make sure accurate data was being
collected, the GCE needed to be free of any impurities before being used. The GCE was tested
for impurities by connecting it to the cyclic voltammeter. The voltammeter ran at a preset sweep
rate of 250 mV/s. This was later changed to 50 mV/s to be used as a baseline for data analysis.
The resulting graph was then analyzed for current peaks that would signify if any electroactive
impurities were present. The GCE was scanned for 25 cycles which provided an opportunity for
any minor impurities to be removed electrolytically. When no current peaks were observed the
GCE would be ready for experimentation.
A standard cleaning procedure was first used to clean the GCE. This involved first scrubbing the
electrode on a buffer pad using Alumina paste and deionized water. The GCE was scrubbed
with its face flat to the buffer pad applying light pressure in a figure eight motion. The GCE was
scrubbed for a short length of time and was then rinsed with deionized water. The GCE was then
placed in a 150 mL beaker half-filled with deionized water. This beaker was then placed in an
ultrasonicator turned on to the max setting. The beaker was allowed to sit in the ultrasonicator
for at least five minutes to allow for any particulates to vibrate off of the GCE. The GCE was
then removed from the 150 mL beaker and rinsed with deionized water. It was then connected to
the cyclic voltammeter to perform cleaning sweeps. After confirming the GCE was free of
electroactive impurities from the cleaning scan, the GCE was removed from the cyclic
voltammeter. The GCE was then rinsed with deionized water and placed on a stand to dry.
Once dry, the GCE would be ready for experiments. It was found that the Bi-Sn nanoparticles
were very difficult to remove from the GCE. Bismuth oxidation peaks were consistently
observed in the cleaning scans. This increased cleaning time because all of the cleaning steps
needed to be repeated until the cleaning scan showed the absence of a Bismuth peak or any other
impurity.
To make sure that the electrode was clean extra steps were added to the process. This step was
done after removing the GCE from the ultrasonicator. Aqua regia was prepared in a vial by
mixing 18M HCl with 18M HNO3 in a 3:1 ratio. The Aqua regia was stored in a capped vial
inside the fume hood. After the GCE was removed from the ultrasonicator it was placed inside
the fume hood facing down inside the vial of Aqua regia so that the surface of the Glassy Carbon
came into contact with the Aqua regia. The electrode was left to sit in the Aqua regia between
15 minutes to an hour. The GCE was then removed from the vial and rinsed with deionized
water. After rinsing the electrode it was placed in a 150 mL beaker half filled with deionized
water and put in the ultrasonicator again. The GCE was removed from the beaker and rinsed
with deionized water one last time. The electrode was then connected to the cyclic voltammeter
to check for electrochemical impurities.
It is important to note that for some of the early tests, the electrode showed very small Bismuth
oxidation peaks after multiple intensive cleanings using the Aqua regia step. These small peaks
were assumed negligible. Calibrated cleaning scans were used as the baseline for analysis of the
tested electrode.
Cyclic Voltammetry
Cyclic Voltammetry was the method of analysis for electroactivity of the GCE. Cyclic
Voltammetry involved three electrodes: a reference, a working, and a counter. The reference in
these experiments was an Ag/AgCl electrode. The counter in these experiments was a standard
Platinum electrode. The working electrode was the assembled GCE.
Two different types of electrolytes were used throughout this project. The electrolyte used to
electrochemically characterize Bismuth was a 0.5M solution of H2SO4. When experiments were
in this solution, they were typically scanned between -650 mV and 650 mV with respect to
Ag/AgCl. There were various scan rates used with H2SO4 solution but the main scan rate used
for analysis was 50mV/s. The main focal point for analysis using this solution were the peak
current magnitude, and the stability of the peak currents after being cycled. The peak currents
can be observed in any scan but to analyze the stability the 2nd and 25th Bismuth peaks were
used. The second peak current was used instead of the first peak, because the first peak always
had a lot of noise and nothing could be interpreted from it. The 25th cycle was used to represent
a stable system.
The other electrolyte used was a 2M solution of H2SO4 with 0.1M V3+ ions. This electrolyte was
used to test the electrocatalytic effects of Bismuth on the Vanadium redox reaction between V3+
and V2+. The GCE in this electrolyte was scanned between -100 mV and -1100 mV with respect
to Ag/AgCl to get the Vanadium to react. It is important to keep the window as small as possible
because large potentials in the negative range reduces Hydrogen, which would damage the
integrity of the experiment. The electrodes were also scanned at different scan rates. For the
Vanadium solution the scan rates were either 10, 20, 30, or 40mV/s. The main focus for these
tests was to observe how large the peak currents separation between cathodic and anodic peaks,
and the magnitudes of the peak current. For the Vanadium solution the 25th cycle of the scan
was used for analysis.
Standard Layer by Layer
Standard Layer by Layer (sLbL) was a method that allowed for the controlled application of an
electrocatalyst on an electrode surface by using electrostatic attractions. Advantage is taken of
the positive surface charge of the cationic polymer and the negative surface charge the Bi-Sn
nanoparticles. They were be stacked in such a way that they formed layers. Both the cationic
polymer and the Bi-Sn nanoparticles were in liquid form, and they were applied by a
micropipette drop-wise until covering the surface of the GCE. The first layer applied to the GCE
was the cationic polymer. The positive surface charge stuck to the negative surface charge of the
carbon on the GCE. Polymer was left on the GCE for two minutes. The GCE was then washed
in a beaker full of DI water to get rid of the excess polymer. The Bi-Sn nanoparticles were then
applied on top of the polymer and left on for two minutes. The GCE was then washed in a
beaker full of DI water to get rid of the excess nanoparticles. The nanoparticles are coated with a
thin shell of Sn, which should theoretically give the nanoparticles a negative surface charge.
Once the Bismuth and polymer were applied and washed, one layer was complete.
A control was used to test against the Layer by Layer method and to understand effect of the
cationic polymer. This control electrode was constructed by simply layering the Bi-Sn
nanoparticles on top of each other. Bismuth was applied to the surface of the GCE and left on
for two minutes before being washed. This was equivalent to one layer.
Another variable in the electrode assembly involved washing the assembled electrode in 0.25M
Sodium Hydroxide (NaOH). The purpose of this was to test if some of the Tin shell on the
nanoparticles could be removed to expose more Bismuth. The assembled electrode was washed
in 0.25M NaOH for two minutes, was briefly rinsed, and then left to dry in air before being
tested.
Once testing was completed on the Bismuth electrodes, Carbon nanoplatelets were also
implemented into the sLbL assemblies. Based on previous studies, Carbon had a negative
surface charge and was a strong electrocatalyst. The Carbon nanoplatelets were suspended in
solution and were applied in a similar manner as the polymer and Bi-Sn nanoparticles. The
Carbon would be applied until covering the surface of the GCE and left on for two minutes
before washing. Since both the Carbon nanoplatelets and Bi-Sn nanoparticles had a negative
surface charge they were separated by a layer of cationic polymer to meet sLbL requirements.
The layering order was polymer, carbon, polymer, Bi-Sn. Applying and washing those in order
would complete one layer. Each electrode with Carbon nanoplatelets and Bi-Sn nanoparticles
was washed in NaOH as described earlier in this section.
Directed Layer by Layer
Directed Layer by Layer (dLbL) was another Layer by Layer method for the controlled
application of an electrocatalyst. It was similar to sLbL in that it took advantage of electrostatic
charges, but the difference came from how these charges were layered. In dLbL the positive
polymer was still applied first to the base of the Glassy Carbon. The next layer applied was the
negatively charged carbon nanoplatelets. The next layer involved applying the negatively
charged Bi-Sn nanoparticles. The repulsion forces of like charges from the Carbon nanoparticles
“direct” the Bi-Sn nanoparticles onto any exposed polymer that the Carbon nanoparticles haven’t
covered. This technique was used because, in theory, it should minimize the insulative
properties of the polymer by packing the Bi-Sn into the spaces created by the polymer. After the
Bi-Sn layer was applied, one layer of dLbL had been applied and the process was repeated. The
dLbL method was being tested to compare to the sLbL method to determine which was more
effective for assembling a Carbon-Bismuth hybrid electrode. They were compared using 8
layers for both methods.
5. Results and Discussion.
4-Layers vs. 8-Layers without Carbon
The first goal in understanding Bismuth is to investigate whether or not the peak current grows
linearly with the increasing of the layers. This is done by doubling the layers from four to eight
to observe if the peak height will double as well. The two assemblies compared are the 4-layer
sLbL assembly without NaOH wash and the 8-layer sLbL assembly without NaOH wash. The
results are not very stable, but they do indicate that increasing the number of layers increases the
peak height. The 4-layers produces an initial peak of 0.62 mA/cm2 while the 8-layers produces
an initial peak of 1.0 mA/cm2. The stability issue can plainly be seen especially in the 8 layer
test where irregularities are happening all over in the course of 25 cycles. Both tests clearly
show that the peak current dwindles down to a fraction of the initial peak.
Figure 1. This shows the stability and reactivity of a 4-layer Bismuth electrode over 25 cycles.
Figure 2. This shows the stability and reactivity of an 8-layer Bismuth electrode over 25 cycles.
NaOH Wash vs. No NaOH Wash
Pure Bismuth nanoparticles currently cannot be produced due to the fact that Bismuth
nanoparticles need stabilization. The Bismuth nanoparticles used for this experiment are coated
with a thin Tin shell. Once the Bismuth is layered on the electrode there is no need for the shell.
The hypothesis is that removing some of the Tin will expose more Bismuth and make it more
electroactive. A test is conducted that includes a step treating the electrode in a solution that will
remove Tin but not Bismuth. For this experiment NaOH is used to treat the electrode. This
solvent is chosen because it is a strong base and Pourbaix phase diagrams indicate Tin will be
ionized and not Bismuth. Sodium is used as the support ion based on previous successful usage.
The results from this experiment are shown in figures three and four below. The initial peak
density without the NaOH wash is 0.62 mV/cm2 while the peak with the NaOH is 2.9 mV/cm2.
This data isn’t conclusive because both results show peak activity significantly decreasing after
each cycle and the results aren’t able to be reproduced. However, a general trend is observed
that showed electrodes treated with NaOH are more electroactive than electrodes without NaOH.
From this trend it is safe to conclude that NaOH wash has a positive effect on the electroactivity
of Bismuth on an electrode. The extent of this effect is currently unknown due to the instability
of the experiments and lack of reproducibility with Bismuth only sLbL experiments.
Figure 3. This figure shows the reactivity of a 4-layer electrode without the NaOH wash.
Figure 4. This figure shows the reactivity of a 4-layer electrode with the NaOH wash.
Polymer vs. No Polymer
Experiments are conducted without polymer to test the effect of the polymer and the Layer by
Layer process on the electroactivity of the Bismuth nanoparticles. This is done by layering
Bismuth on top of itself. Theoretically the polymer should keep the Bismuth nanoparticles
spread out evenly in each layer. It should also provide a surface charge that allows the Bismuth
to stack resulting in a higher quantity of Bismuth applied. This should enable the reaction to
happen quickly and last longer. The results of these experiments show that the polymer does
improve the performance of the electrode by increasing the peak current and making the
electrode more stable.
Carbon vs. No Carbon
Carbon is used to study a sLbL hybrid with Bismuth nanoparticles. The Carbon nanoplatelets
that are used have been shown to be an effective electrocatalyst in the V4+/V5+ reaction. After
failing to make a solid conclusion about Bismuth nanoparticles, the Carbon nanoplatelets are
introduced into testing. The comparison between a Carbon and no Carbon electrode is seen in
figures six and seven. In figure six the current density peak decreases a significant amount with
each cycle. In figure seven the current density peak stabilizes fairly quickly and remains stable.
Figure six does have higher peak currents than figure eight, but peak heights decayed to similar
values within a few cycles. Figure seven is a very important test result, because it is the first that
can be easily reproduced. It is observed that when Carbon is introduced the Bismuth peaks
stabilize more quickly and sooner. Future research in this area will investigate why Carbon and
Bismuth nanoparticles have this favorable interaction.
Figure 6. This figure shows the reactivity of an electrode that doesn’t include Carbon.
Figure 7. This figure shows the reactivity of an electrode that includes Carbon.
4-Layers vs. 8-Layers with Carbon
Previously it is discussed that there is a general trend in the peak current when the number of
layers is increased from four to eight. This trend is confirmed by comparing the more stable
Carbon-Bismuth hybrid with four and eight layers using the sLbL method. The result can clearly
be seen in figure eight below. The 8-layer electrode has an initial peak current of 0.19 mA/cm2
and the 4-layer peak current is 0.1 mA/cm2, a near doubling in the peak current from the
fourth layer to the eighth layer. This makes sense because the electroactive Bismuth is
applied in the same manner for all tests. The electrode with eight layers should have
about twice as much Bismuth as the one with four layers. Also, the 8-layer electrode
shows greater stability than the 4-layer electrode over the course of 25 cycles. This is
important because it indicates that the electrode will increase in stability as more layers
are added with Carbon nanoplatelets present.
Figure 8. This figure compares the initial peaks of a 4-layer and an 8-layer electrode that involved Carbon.
dLbL vs. sLbL
The differences between the dLbL and sLbL Layer by Layer techniques are explained earlier in
the methods section. It is important to find out which method is most effective for assembling
the Carbon-Bismuth hybrid electrode. An eight layer dLbL and an eight layer sLbL are
compared in figures seven and eight. The sLbL technique displayed a more stable system than
the dLbL technique over 25 cycles. This is because sLbL uses electrostatic attractions to hold
each sublayer together while dLbL has some sublayers put together that have the same charge
causing them to want to repel each other. This repulsion hurts the integrity of the structure
causing it to not be stable. The initial current density peak for sLbL is approximately 0.2
mA/cm2 while it is approximately 0.16 mA/cm2 for dLbL. More importantly, the sLbL method
shows greater stability over the course of 25 cycles than the dLbL method, with the sLbL peak
decaying to only 0.16 mA/cm2 and the dLbL peak decaying to 0.076 mA/cm2. This test clearly
indicates that the sLbL method is the most effective assembly method for the Carbon-Bismuth
hybrid electrode.
Figure 9. This figure shows the reactivity of an electrode constructed by using the standard layer by layer technique.
Figure 10. This figure shows the reactivity of an electrode constructed using the direct layer by layer technique.
Carbon-Bismuth Relationship
Through this project it has been discovered that Carbon and Bismuth have a very interesting
relationship with each other. As mentioned earlier, it was observed that Carbon acts like a
stabilizing agent for the Bismuth. It has been seen from figures six and seven, when the
assembly doesn’t have Carbon in it, it doesn’t reach a stable state before the current is essentially
zero. This relationship between Carbon and Bismuth nanoparticles will need to be investigated
in future research. Currently it is too early to speculate on this interaction.
Vanadium Studies
All previous experiments have been completed in a 0.5M H2SO4 electrolyte due to the need to
have an understanding of the electrochemical properties of Bismuth nanoparticles. Once this
understanding is reached, experiments are completed in a 0.1M V3+ solution. This is done so
there can be an understanding of how the Carbon-Bismuth hybrid electrode affects the
performance of the negative side of a Vanadium Redox flow battery. Previous studies have
shown that the application of Bismuth to the positive cell has no significant effect so the focus
for this project is on the negative side.
Different experiments are completed so that the behavior of both Carbon and Bismuth in
Vanadium can be observed and analyzed. The first experiment is the Glassy Carbon control test.
The next experiment involves eight layers of Carbon and cationic polymer, which serves as a
control for Carbon. The eight layer Carbon sLbL assembly is compared to the Glassy Carbon
control to give an understanding of Carbon’s effect on the Vanadium reaction. The next
experiment involves eight layers using sLbL with Bismuth and Carbon. The results for the
Bismuth-Carbon hybrid are compared to the results of the Carbon sLbL control to analyze the
effect of Bismuth on the Vanadium reaction. The final experiment that is completed involves
four layers using sLbL. This shows how the quantity of Bismuth will affect the Vanadium
reaction. Figure 11 shows the comparison of the four layer sLbL Bismuth-Carbon hybrid to the
eight layer sLbL Bismuth-Carbon hybrid. From this comparison it can be concluded that the
eight layer Bismuth-Carbon hybrid improved peak current density as well as shifting the
oxidation and reduction peaks closer together. The significance of the peak shifting is especially
important for the V3+/V2+ reduction reaction. This causes the negative peaks seen on the left side
of the graph below. This is important because H+ reduction in the electrolyte begins to happen
near the reduction peak and causes the peak to be less prominent. When the peak is shifted to a
more positive value it is shifted away from the H+ reduction. This improves energy conversion
efficiency since less energy is wasted on H+ reduction. Figure 12 shows the comparison of the
eight layer sLbL Carbon control and the eight layer sLbL Bismuth-Carbon hybrid electrode.
This graph distinctly shows the H+ reduction slope on the Carbon Control plot after -1V
referencing Ag/AgCl. The H+ reduction is also a contributing factor the cathodic peak being
greater in the Carbon control than the Carbon-Bismuth hybrid. There is also a distinct difference
in peak separation. This is an important result because it demonstrates Bismuth’s role as an
electrocatalyst in the Vanadium reaction. The Glassy Carbon control test fails to produce a
Vanadium reduction peak and goes directly towards H+ reduction when sweeping in the negative
range.
Figure 11. This figure compares the electroactivity of a 4-layer electrode and an 8-layer electrode in a 0.1M V3+ electrolyte solution.
Figure 12. This figure compares the electroactivity of an 8-layer electrode that just involves Carbon to one that involves Carbon and Bismuth.
6. Conclusion.
Making solar energy economical is one of the 14 NAE grand challenges. This study focused on
energy storage, particularly the VRFB. Making the VRFB economically viable would be a step
towards making not just solar energy economically viable but all renewable energy. The
purpose is to investigate the application of a hybrid Carbon-Bismuth electrode in the negative
cell of the VRFB.
The first experiments that have been to understand the basics of Bismuth nanoparticle
electrochemistry and the sLbL application method. The results from the initial Bismuth studies
have shown that Bismuth nanoparticles are not very stable. These studies have provided a
foundation for future experiments. These studies indicate that polymer is necessary to stack
Bismuth layers and washing the electrode in NaOH enhances electroactivity. The next
experiments have introduced Carbon nanoplatelets and a new type of application process: dLbL.
When Carbon was introduced, it was found that it stabilizes the system enough that the data was
reproducible. From these experiments it can be concluded that increasing layers increases
electroactivity, and sLbL is the most effective application method for the Carbon-Bismuth hybrid
electrode. The final set of experiments are centered on the V2+/V3+ reaction. From these
experiments show that the Bismuth-Carbon hybrid is the most effective at shifting the cathodic
peak away from H+ reduction and amplifying the cathodic and anodic peak currents. This also
shows that increasing layers enhances electrocatalytic effect, and that Bismuth nanoparticles play
an important role as an electrocatalyst.
The results of this research have a lot to be expanded upon. Future research will need to
investigate why Carbon and Bismuth have such positive interactions. Scanning Electron
Microscopy (SEM) will be used to visually understand what is going on at the surface of the
electrode. It will also be necessary to quantify the layering of Carbon-Bismuth hybrid electrode
and determine the exact mathematical relationship between the amount of layers and peak
current density. This information will be used to optimize an electrode for use in the negative
cell of a VRFB. Further testing will need to be conducted to find consistent electrochemical
properties of Bismuth nanoparticles. Nonetheless, these initial findings have shown promise for
the use of the hybrid Carbon-Bismuth nanoparticle assemblies as electrocatalysts in the negative
cells of VRFBs.
7. References.
1. Evans, Dennis H.; O'Connell, Kathleen M.; et al. (1983). “Cyclic voltammetry,” Journal of
Chemical Education, Vol. 60, No. 4, pp. 290-293.
2. Li, Bin; Gu, Meng; et al. (2013). “Bismuth nanoparticle decorating graphite felt as a high-
performance electrode for an all-vanadium redox flow battery,” Nano Letters Vol. 13, No. 3, pp.
1130-1335.
3. Mabbott, Gary A. (1983). “An introduction to cyclic voltammetry,” Journal of Chemical Education,
Vol. 60, No.9, pp. 697-702.
4. St. John, Samuel; Dutta, Indrajit; Angelopoulos, Anastasios P. (2010). “Synthesis and
Characterization of Electrocatalytically Active Platinum Atom Clusters and Monodisperse Single
Crystals,” Journal of Physical Chemistry, Vol. 114, No. 32, pp. 13515-13525.
5. Suarez, David J.; Gonzalez, Zoraida; et al. (2014). “Graphite Felt Modified with Bismuth
Nanoparticles as Negative Electrode in a Vanadium Redox Flow Battery,” ChemSusChem, Vol.7,
No.3, pp.914-918.
6. Independent Statistics & Analysis United States Energy Information Association (EIA). Available
