Broad Street Scientific Journal 2020

The sesamol fuel cell saw the greatest increase in peak
height over the ten minutes voltage was applied (Fig. 10).
Although this does not directly correlate to an increase in
fuel cell function, since FTIR testing cannot quantify efficiency,
it does indicate that sesamol is a functional redox
mediator that is qualitatively on par with quinone.
Figure 10. Graph representing peak height vs. time
the voltage has been applied measured from the initial
peak height. Compiled data from bicarbonate
fuel cell (green), quinone fuel cell (blue), and sesamol
fuel cell (yellow).
4. Discussion
The cyclic voltammograms of sesamol above (Fig. 4
and 5) support the proposed scheme for the oxidation of
sesamol, which involves the creation of a quinone species.
The presence of shoulder peaks points to the creation of
a new compound (2-hydroxymethoxybenzoquinone) in
the first step of the reaction, and the subsequent reversible
reaction these compound(s) undergo. When the sample
is purged with argon, (Fig. 4b and 5b), none of the voltammograms
change shape dramatically, suggesting that
none of the peaks were due to the presence of oxygen in
the sample. Additionally, carbon dioxide does not interfere
with the progression of the reaction, as when the samples
are purged with carbon dioxide, the voltammograms
retain their general shape (Fig. 4c and 5c). Although the
cyclic voltammograms in bicarbonate have less defined
peaks (see Fig. 3), this may be attributed in part to the increased
pH and in part to the drift that Ag/AgCl reference
electrodes undergo as they age. Further research could be
done into the impact of pH on sesamol’s redox reaction to
determine if the movement of the peaks represents the impact
of an increased concentration of OH- ions available,
as these are required for the first irreversible oxidation
step of the reaction, and decreased concentration of H+
ions available, as these are required for the reduction of
2-HMBQ to 2-HMHQ. By examining how the pH impacts
the progression of the redox reaction, the optimal pH of a
fuel cell could be determined for maximum carbon dioxide
transport [3].
Half-cell testing results fully support the hypothesis that
sesamol undergoes a proton-coupled electron transfer reaction
because in sodium bicarbonate, a carbon dioxide saturated
solution saw gas evolution at a 0.5V potential. This
gas evolution would not be possible without the transfer
of protons creating an acidic pH at the anode, subsequently
driving the release of carbon dioxide dissolved in solution
as a gas, observed on the electrode. It is important to note
that since only a 0.5V potential was applied, none of the
gas evolution would be expected to be due to water-splitting
[7]. Additionally, the platinum catalyst was chosen
because it does not contribute heavily to water-splitting,
even at higher potentials [3].
Fuel cell testing suggests that quinone and sesamol redox
mediators, rather than the simple application of voltage,
are responsible for moving carbon dioxide across the
fuel cell. Additionally, the decrease in concentration observed
when the voltage is removed dispels the theory that
carbon dioxide is leaking over to the permeate side of the
cell over time. Further work is necessary to evaluate the
efficiency of each fuel cell, as this project’s set-up was not
equipped to evaluate percent carbon dioxide transported,
only observe relative decreases or increases in concentration.
Plotting peak height against time reveals that sesamol
and quinone have similar efficiencies in this setting. All evidence
of carbon dioxide transport was qualitative. Percent
carbon dioxide efficiency can be evaluated with different
applied potentials as well, not just 2.5V. Further research
can also be done to verify that the mechanism is selective
for carbon dioxide by pumping a mixture of nitrogen, oxygen
and carbon dioxide, rather than pure carbon dioxide,
across a fuel cell to further emulate flue gas conditions.
One area of concern is that the platinum catalyst will catalyze
water-splitting and that oxygen could be released on
the permeate side with carbon dioxide, although previous
works have found that platinum on carbon black does not
generate oxygen in large amounts as other similar metal
catalysts might [3].
5. Conclusion and Future Work
It has been shown that sesamol undergoes a quasi-reversible
redox reaction in sodium bicarbonate; it is assumed
that two non-toxic quinones are created, 2-hydroxymethoxybenzoquinone
and 2-hyroxymethoxyhydroquinone,
that further reduce and oxidize reversibly while transfering
protons. The appearance of shoulder peaks in the cyclic
voltammograms supports this reaction scheme, and their
relative size and placement indicate that such a reaction is
reversible and repeatable. Through half-cell testing, it was
confirmed that protons are transferred, thus creating an in
situ pH gradient that releases carbon dioxide that has been
dissolved in solution at the anode. Based on the results of
preliminary fuel cell testing, it can be concluded that sesamol’s
quasi-reversible proton-coupled electron transfer
reaction functions to transport carbon dioxide across a fuel
cell like that of a quinone, and can be used as a more environmentally-friendly
choice of mediator to separate CO 2
38 | 2019-2020 | Broad Street Scientific CHEMISTRY