Electrochemical Oxidation of Alcohols at Mixed Metal Polymer Films

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Electrochemical Oxidation of Alcohols at Mixed
Metal Polymer Films
Charlene Ricciardi, S. Michelle Rogers and Dr.
Brett K. Simpson Department of Chemistry and
Physics, Coastal Carolina University, PO Box
261954, Conway, SC 29528-6054
Abstract The electrocatalytic properties of metal
doped Nafion-117 membrane films with respect to
the oxidation of alcohols were studied. Various
binary combinations of cobalt and nickel were
looked at including binary mixtures of cobalt
with nickel. The films were prepared by allowing
a portion of prefabricated Nafion-117 to
equilibrate in a salt solution of the desired
metal content for a minimum of 24 hours. Prior
to electrochemical studies, the metal ions in the
film were then reduced to the zero valent state
using sodium borohydride. Results indicate that
the films are electroactive for the oxidation of
simple alcohols. The binary metal films showed
improved electrocatalytic activity in comparison
to their pure metal counterparts. The improved
response suggests that the oxidation process
occurs by adsorption of the analyte of interest
at a specific metal site while formation or
adsorption of hydroxyl radicals occurs at the
other metal site in the binary films.
Results The Fe and Co/Fe films required removal
of oxygen from all preparation solutions used due
to the Fe precipitating out of solution and
subsequently being removed from the Nafion
membrane. The films prepared without oxygen
removal showed poor results due to the loss of
iron from the film in the preparation process.
The reduced Co film was found to have a larger
current for oxygen evolution and a larger change
in response with the addition of ethanol as
compared to the nonreduced Co film. The current
response at the Co, Fe and Co/Fe films showed a
loss in current that was proportional to the
concentration of ethanol in solution. The
mechanism associated with this process is yet to
be determined. The Co/Fe film showed a larger
current for oxygen evolution than either of the
two pure metals by themselves. The Co/Fe showed
an increase in current for glucose in 30 mM NaOH
suggesting that glucose oxidation was occurring
at the surface. Further increase in glucose
concentration resulted in loss of signal yet
never below the background current obtained as
was seen with the ethanol.
Objective The oxidation of alcohols has seen an
Binary metal alloy electrodes have been found to
be useful for various electrochemical methods.
One possible method for the construction of these
alloy electrodes is to use a high temperature
process. The high temperature process requires a
furnace requires the initial melting of the
metals, mixing them in their liquid state and
then cooling down to produce the desired
electrode. High temperature furnaces are
required to carry out this process. Also, not
all metals will easily (if at all) alloy
together. Additionally, after the formation of
the alloy slug, you then have to process the slug
to obtain the desired shape of the electrode of
interest. Often binary metal alloys are brittle
and difficult to machine thus making it
problematic to create such electrode. An
additional The doping of Nafion membranes is a
proposed solution to some of the difficulties
associated with preparation of these binary metal
alloy electrodes. The preparation process
involves simply soaking the desired shaped and
size of a Nafion membrane in a solution
containing the desired mixture of metals followed
by a reduction process using sodium borohydride.
The current study looks at the electrochemical
properties of various metal doped Nafion film
electrodes. The films show possible use for
detection of simple alcohols as well as larger
organic molecules with alcohol functional groups
such as glucose.
Future Plans The metal salt ratios of the
doping solutions will be varied to see if the
properties of the films will change with a change
in solution composition. The metal salt ratio
of the solutions will be looked at to determine
if they reflect the amounts of metals in the
doped Nafion film. Further studies with the
Co, Fe and Co/Fe Nafion films will be carried out
to determine the process involved with the
current response of ethanol seen at these films
in 0.1M NaOH. The electrochemical properties of
these films in 30mM NaOH with respect to
oxidizing simple alcohols will be looked at to
determine the effect of electrolyte
concentration. Additional binary mixtures
containing Co in the Nafion films will be looked
at to determine their ability at oxidizing simple
alcohols to see if improved response can be
obtained using metals other than Fe.
Data Collection A Pine RDE4 bipotentiostat was
used to collect all cyclic voltammograms. A three
electrode system was utilized in which the
reference electrode was a saturated calomel
electrode (SCE), the auxiliary electrode was a
platinum wire, and the working electrode was the
doped Nafion film. The working electrode was
connected through use of an alligator clip. Only
the lower half of the film was exposed to the
solution of interest and care was taken to
prevent exposure of the alligator clip to
solution. At the end of each experiment, the
surface area of the exposed film was recorded.
Each experiment was carried out in 10 mL of 0.1M
NaOH solution. All experiments were carried out
at a scan rate of 100mV/s.
Fig 7 Glucose concentration dependence of a
Co/Fe film.
Fig 6 Typical examples of the types of films
used. (A) pure Nafion film, (B) non-reduced
Co film, (C) reduced Co film.
Film Preparation Each film was prepared using a
piece of Nafion-117 cation exchange membrane that
was approximately 2.0 cm long and 0.40 cm wide.
The membrane was soaked in a small vial
containing approximately 5 mL of the solution of
interest for a minimum of 24 hours. The film was
then rinsed with deionized water to remove any
remaining solution. The films identified as
non-reduced were immediately used after the
rinsing step. The reduced films were allowed to
soak in a sodium borohydride solution for a
minimum of 30 minutes. They were then removed
from the borohydride solution and rinsed with
deionized water before use. The cobalt films
were prepared using a 0.10M cobalt (II) nitrate
solution. The iron films were prepared using a
0.025M iron (II) sulfate heptahydrate solution.
The iron solution was bubbled with nitrogen to
remove any dissolved oxygen and sealed
immediately upon preparation. To properly
prepare the films that contained iron, it was
necessary to also bubble the solution immediately
prior to soaking the Nafion membrane. The
cobalt-iron binary metal films were prepared by
soaking the Nafion membrane in a 5050 mixture of
0.1M cobalt (II) nitrate and 0.1M nickel (II)
sulfate heptahydrate.
Acknowledgements The authors would like to thank
Coastal Carolina University for financial support
of this project. The authors would also like to
thank Jennifer Coor and Dr. John Goodwin for
initial help with preparation of the Nafion
films.