TEMPO-Modified Linear Poly(ethylenimine) for Immobilization-Enhanced Electrocatalytic Oxidation of Alcohols

Article abstract of DOI:10.1021/acscatal.5b01668

We demonstrate a method to simultaneously immobilize the oxidation catalyst, TEMPO, while dramatically enhancing its electrocatalytic activity toward several biologically available alcohols. TEMPO is covalently immobilized onto linear poly(ethylenimine), which is then cross-linked onto the surface of a glassy carbon electrode to form a hydrogel through which substrates can readily diffuse. The TEMPO-LPEI electrode is used as an anode capable of generating currents from 0.41 ± 0.06 mA cm^-2 in the presence of 250 mM sucrose to 8.20 ± 0.04 mA cm^-2 in the presence of 2 M methanol and 33.4 ± 9.4 mA cm^-2 in the presence of 500 mM formate under neutral pH and at 25 °C. The newly described anode is combined with an enzymatic biocathode to construct a hybrid biofuel cell to produce 0.38 ± 0.04 mA cm^-2 while using 2 M methanol as a fuel source.

Full text of DOI:10.1021/acscatal.5b01668

Research Article
pubs.acs.org/acscatalysis
TEMPO-Modified Linear Poly(ethylenimine) for Immobilization-
Enhanced Electrocatalytic Oxidation of Alcohols
David P. Hickey, Ross D. Milton, Dayi Chen, Matthew S. Sigman,* and Shelley D. Minteer*
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, United States
S
* Supporting Information
ABSTRACT: We demonstrate a method to simultaneously
immobilize the oxidation catalyst, TEMPO, while dramatically
enhancing its electrocatalytic activity toward several biologically
available alcohols. TEMPO is covalently immobilized onto
linear poly(ethylenimine), which is then cross-linked onto the
surface of a glassy carbon electrode to form a hydrogel through
which substrates can readily diffuse. The TEMPO-LPEI
electrode is used as an anode capable of generating currents
from 0.41 0.06 mA cm−2 in the presence of 250 mM sucrose
to 8.20 0.04 mA cm−2 in the presence of 2 M methanol and
33.4 9.4 mA cm−2 in the presence of 500 mM formate under
neutral pH and at 25 °C. The newly described anode is
combined with an enzymatic biocathode to construct a hybrid
biofuel cell to produce 0.38 0.04 mA cm−2 while using 2 M methanol as a fuel source.
KEYWORDS: hybrid fuel cell, oxidation, alcohols, TEMPO, electrocatalysis
For practical application in a fuel cell, it is desirable to
immobilize TEMPO to prevent cross-contamination at the
cathode, simplify the diffusional kinetics, and allow for
reusability of the catalyst. TEMPO derivatives have been
heterogenized onto a polymer matrix at an electrode inter-
face;4,10 however, previous studies have demonstrated that the
immobilization of TEMPO results in a significant decrease in
catalytic activity as a result of diffusional restrictions caused by
the immobilizing matrix.11−13 Previously, redox-active hydro-
gels based on either poly(vinylpyridine) (PVP) or poly-
(ethylenimine) (PEI) have been used to entrap enzymatic
catalysts because their ability to swell allows for rapid substrate
diffusion throughout the gel matrix.14−16 In such systems,
polymers are doped or modified with redox moieties to
facilitate rapid electron transfer to the active site of an
oxidoreductase enzyme. We sought to modify this construct to
allow for the redox mediator to also act as the catalyst. In
addition, we have recently demonstrated the enhancement in
electrocatalytic activity of TEMPO toward a simple bioavailable
alcohol by the presence of a covalently linked basic amine
functionality.17 On the basis of this discovery, we envisioned
using an amine-containing polymer to simultaneously enhance
the catalytic rate of a relatively unreactive TEMPO species and
act as an anchor for immobilization onto an electrode surface.
Herein, we demonstrate a TEMPO-modified linear poly-
(ethylenimine) (TEMPO-LPEI) anode material that can be
1. INTRODUCTION
Biologically available sugars and short-chain alcohols are seen as
attractive potential fuel sources because they have high energy
density and low toxicity. Recently, there has been great interest
in the development of catalytic cascades that can facilitate
multiple oxidative steps to increase the amount of energy
extracted per molecule of fuel (energy density) at the anode of
a fuel cell.1−4 Organic alcohol-containing fuels can be
electrochemically oxidized by transition metal catalysts to
generate current densities in the range of milliamperes per
square centimeter;5 however, these catalysts often require
alkaline conditions with elevated temperature and pressure to
achieve high rates of oxidation (current density) and are rarely
capable of deep oxidation. Therefore, there remains a
significant need to develop environmentally friendly oxidation
catalysts that are capable of facilitating multiple oxidative steps
and are suitable for use in a fuel cell.
As a promising alternative to precious metals, TEMPO is a
simple, organic oxidation catalyst capable of oxidizing a wide
range of alcohols and aldehydes at room temperature and under
mild aqueous conditions.6 TEMPO has previously been shown
to electrochemically oxidize primary and, to a lesser extent,
secondary alcohols to the corresponding aldehydes and
ketones.7−9 The mechanism of TEMPO proceeds through an
initial electrochemical oxidation step from a stable nitroxyl
radical to the catalytically active oxoammonium salt; sub-
sequent oxidation of the substrate results in a hydroxylamine,
which is oxidized to the nitroxyl radical, completing the
catalytic cycle.
Received: July 31, 2015
Revised: August 13, 2015
© XXXX American Chemical Society
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cross-linked onto the surface of a carbon electrode to
electrocatalytically oxidize a wide range of saccharides and
short chain alcohols under neutral aqueous conditions at room
temperature. The catalytic activity of TEMPO-LPEI is
compared with that of an analogous nonimmobilized
TEMPO derivative to reveal a dramatic increase in the catalytic
current density as a result of immobilization. In addition, we
demonstrate how this immobilized organic oxidation catalyst
can be combined with a previously reported enzymatic
biocathode18 to operate as a hybrid biofuel cell.
allow for stable and quasi-reversible redox peaks; 20 scans at 50
mV s−1 from 0.0 to 0.8 V vs SCE. Electrochemical
characterization of cross-linked TEMPO-LPEI films was
performed using constant potential amperometry. Substrate
kinetics were determined by measuring the change in current at
a fixed potential of 0.8 V vs SCE while injecting various aliquots
of substrate using a 150 mM phosphate buffer at pH 7.0 and 25
°C. Catalytic pH profiles were determined by monitoring the
change in current at a fixed potential of 0.8 V vs SCE with the
injection of various aliquots of 5 M glycerol (pre-equilibrated to
the desired pH) using 200 mM phosphate/citrate buffer at 25
°C and various pHs.
2. EXPERIMENTAL SECTION
2.7. Hybrid Biofuel Cell Characterization. Fuel cells
consisted of a MWCNT-containing TEMPO-LPEI film on a 3
mm glassy carbon electrode (as described above) and a laccase
biocathode. Electrochemical experiments were performed using
a standard three-electrode cell with the reference and counter
electrode connected to the TEMPO-LPEI anode and the
working electrode connected to the laccase biocathode. Current
and power density curves were generated using linear sweep
voltammetry at 1 mV s−1 in the absence and presence of 2 M
methanol using stirred 200 mM phosphate/citrate buffer at pH
5.0 and 25 °C with ambient O2 concentration.
2.1. Chemicals and Solutions. All chemicals were
purchased from Sigma-Aldrich unless otherwise specified.
Catalytic pH profile experiments and fuel cell experiments
were performed using 200 mM phosphate/citrate buffer
solutions at variable pH for the pH profiles and pH 5.0 for
the fuel cell experiments, both at 25 °C. All other electro-
chemical experiments were performed using a 150 mM
phosphate buffer at pH 7.0 and 25 °C.
2.2. Scanning Electron Microscopy. Samples were
imaged by a FEI Quanta 600 FEG scanning electron
microscope with true environmental (water vapor ambient)
mode. The vapor pressure was controlled to be 0.24−0.31 Torr.
The accelerating voltage was 15 kV with spot size 3.0 and dwell
time 10 μs. Both secondary and back-scattered electron images
were captured.
2.3. Electrochemistry. Electrochemical experiments were
performed using a CH Instruments 611C potentiostat. Cyclic
voltammetry (CV) and constant potential amperometry
experiments were performed with a standard three-electrode
cell with a SCE reference electrode, a Pt mesh counter
electrode, and a 3 mm glassy carbon electrode. Glassy carbon
electrodes were polished with successive 1 and 0.5 μm alumina
slurries and rinsed with 18 MΩ water prior to use.
2.4. TEMPO-LPEI Electrode Film Preparation. Immobi-
lized TEMPO-LPEI electrode films were prepared by dissolving
the TEMPO-modified polymer into 0.05 M HCl (10 mg/mL).
The polymer solution (80 μL) was then combined with 2.5 M
glutaraldehyde (0.25 μL), and the mixture was stirred using a
vortex generator for 10 s. For MWCNT-infused films, the
polymer solution (80 μL) was combined with carboxylated
MWCNTs and stirred using a vortex generator for 15 s; to that
mixture was added 2.5 M glutaraldehyde (0.25 μL), which was
then sonicated for 1 min and then stirred using a vortex
generator for 20 s. For both film formulations, the electrode
film solution mixture (3 μL) was coated onto the surface of a
polished 3 mm glassy carbon electrode using a pipet tip to
evenly distribute the solution across the electrode. The
electrode films were allowed to cure under ambient conditions
for 12 h prior to use.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization. TEMPO was
immobilized onto linear poly(ethylenimine) via a glycidyl
linkage. Hydroxy-TEMPO (1) was combined with epichlor-
ohydrin in the presence of a phase transfer catalyst and NaOH
to produce 4-glycidyl-TEMPO (2), which was subsequently
treated with LPEI overnight at room temperature to yield the
desired TEMPO-LPEI polymer (3) (Scheme 1). Initially,
Scheme 1. Synthesis of TEMPO-LPEI (3) Where 4-hydroxy-
TEMPO (1) Is Reacted with Epichlorohydrin To Form 4-
Glycidyl-TEMPO, Which Subsequently Is Reacted with
Linear Poly(ethylenimine) To Result in 27% Substitution of
the Polymer Backbone
2.5. Homogeneous 4-Methoxy-TEMPO Electrochem-
istry. Homogeneous TEMPO electrochemical characterization
was done by performing CVs on a 2.5 mM solution of 4-
methoxy-TEMPO at 10 mV s−1 at 25 °C in the absence and
presence of substrate using 150 mM phosphate buffer, pH 7.0
and 25 °C. Catalytic pH profiles were generated by determining
the catalytic current density (jmax) in the absence and presence
of 1 M glycerol at various values of pH using a 200 mM
phosphate/citrate buffer at 25 °C.
TEMPO-LPEI was cross-linked with glutaraldehyde onto the
surface of a glassy carbon electrode; however, the resulting film
proved to be physically unstable in H2O and could be observed
delaminating from the electrode. This problem was addressed
by the addition of carboxylated multiwalled carbon nanotubes
(COOH-MWCNTs) into the films to enhance structural
rigidity and improve active surface area. SEM micrographs are
shown in Figure 1 of the TEMPO-LPEI films with and without
MWCNTs before and after being probed by cyclic
2.6. Electrochemistry of Cross-Linked Films of
TEMPO-LPEI. All films were electrochemically equilibrated
prior to use by running CV scans in the absence of substrate to
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Figure 1. SEM micrographs of cross-linked TEMPO-LPEI films
without MWCNTs (1) and with carboxylated MWCNTs (2) prior to
use (a) and after multiple CVs (b).
voltammetry. In the absence of MWCNTs, the cross-linked
films undergo rapid delamination that corresponds to the loss
in electrochemical signal. However, films containing MWCNTs
appear to exhibit swelling, but remain adhered to the electrode
surface and maintain their electrochemical signal. This indicates
that the incorporation of the MWCNTs adds a degree of
structural rigidity, which in turn results in an enhanced
electrochemical stability.
Figure 2. Comparative CVs of 2.5 mM 4-methoxy-TEMPO (A) and a
cross-linked film of TEMPO-LPEI (B) in the absence (dashed) and
presence (solid) of 1 M glycerol. CVs were performed using a 150 mM
phosphate buffer at 10 mV s−1, pH 7.0, and 25 °C.
After synthesizing the TEMPO-modified polymer, we sought
to determine how the electrocatalytic characteristics of the
immobilized TEMPO compared with that of an analogous
homogeneous TEMPO derivative. The catalytic activity of both
the TEMPO-LPEI films and 4-methoxy-TEMPO (homoge-
neous catalyst) were determined with a combination of slow
scan rate cyclic voltammetry and constant potential amperom-
etry. Representative catalytic CVs of TEMPO-OMe and
TEMPO-LPEI in the absence and presence of glycerol are
shown in Figure 2. The reversible oxidation and reduction
peaks correspond to the nitroxyl radical species and
oxoammonium salt being oxidized and reduced, respectively.
In the presence of substrate, the oxoammonium salt is
chemically converted into the hydroxylamine while producing
the oxidized substrate; the hydroxylamine is then electrochemi-
cally oxidized back to the nitroxyl radical and subsequently to
the active oxoammonium salt. Electrocatalysis in the CV is thus
observed as an increase in the peak oxidative current density,
with a corresponding decrease in the peak reductive current
density, and the catalytic current density (jmax) is defined by the
difference between the peak oxidation current in the presence
of saturating substrate concentrations and the oxidative current
in the absence of substrate. Because of the swelling and
shrinking of the TEMPO-LPEI hydrogel film, it is exceptionally
difficult to determine the exact concentration of TEMPO
within any given film. Thus, a concentration of homogeneous
methoxy-TEMPO was selected to result in a comparable
capacitance-corrected peak anodic current (ipa) to that of the
immobilized TEMPO species.
biofuels, including methanol, glycerol, and sucrose; the
resulting calibration curves are shown in Figure 3. Values of
jmax for TEMPO-LPEI were between 2× and 50× higher than
that of the homogeneous TEMPO analogue for all of the
substrates examined, while reaching jmax values as high as 8.20
0.04 mA cm−2 in methanol compared with 0.72
0.06 mA
cm−2 for the homogeneous catalyst. The largest increase in jmax
between the immobilized and homogeneous catalysts is
observed in the presence of isopropyl alcohol. Where the
homogeneous methoxy-TEMPO is virtually unreactive toward
the simple secondary alcohol, TEMPO-LPEI is able to generate
3.5 0.9 mA cm−2 in the presence of 5 M isopropyl alcohol in
150 mM phosphate buffer at pH 7 and 25 °C. This result runs
counter to previous reports and conventional wisdom, which
indicates that immobilization of a TEMPO catalyst will
necessarily result in decreased activity due to diffusional
restrictions induced by immobilization.11 Evidence from our
previous work suggests that the presence of a covalently linked
amine can enhance the rate of electrocatalytic alcohol oxidation
by TEMPO.17 Although the exact nature of this rate
enhancement is still under investigation, our present results
indicate that the localized abundance of amines from the LPEI
backbone may be inducing a similar effect. Such a result is not
trivial because it implies that any TEMPO catalyst could be
immobilized in a polyamine network to enhance the catalytic
rate while simultaneously localizing the electrochemical
catalysis. A further discussion on the possibility of self-reactivity
between the TEMPO catalyst and the polyamine backbone as
3.2. Electrocatalytic Activity Comparison. The catalytic
activity of both catalysts was determined for a variety of
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●
○
Figure 3. Electrocatalytic calibration curves of cross-linked TEMPO-LPEI films ( ) and in situ 2.5 mM 4-methoxy-TEMPO ( ) for methanol,
ethanol, isopropyl alcohol, glycerol, fructose, and sucrose. Concentration profiles were determined using 150 mM phosphate buffer at pH 7.0 and 25
°C.
well as overall film stability can be found in the Supporting
Information.
It should be noted that the kinetics of the catalytic reaction
are altered upon immobilization onto a PEI hydrogel. Because
of the complex nature of substrate diffusion through hydrogel
films, all kinetic parameters presented here refer to apparent
values that do not necessarily correspond to the absolute
kinetics of the immobilized TEMPO catalyst but, rather, the
observed combination of diffusional and kinetic effects
experienced by the catalyst within the hydrogel matrix. A
complete list of kinetic parameters as well as a preliminary
product analysis is provided in the Supporting Information
(Table S1 and Scheme S1). The apparent Michaelis−Menten
constant (Km*) for TEMPO-LPEI is approximately double that
of the homogeneous methoxy-TEMPO catalyst for all of the
substrates examined. In addition, the magnitude of catalytic
current densities for TEMPO-LPEI appears to be dependent
on the relative size of the substrate. These combined results
indicate that the immobilized TEMPO catalyst is, in fact,
limited by diffusion, but to a significantly lesser extent than
previously studied methods for TEMPO immobilization.11 This
is likely a result of the hydrogel nature of cross-linked LPEI
films, which allows for dramatic swelling of the polymer film
upon electrochemical oxidation of TEMPO and subsequently
facilitates rapid diffusion of the substrate through the polymer
network.
Figure 4. Comparative pH profile of cross-linked TEMPO-LPEI films
●
○
( ) and homogeneous 2.5 mM 4-methoxy-TEMPO ( ). Profiles
were determined using 200 mM phosphate/citrate buffer with 1 M
glycerol at variable pH and at 25 °C.
TEMPO-LPEI films are more compatible with a wide range of
biological catalysts that can be used to facilitate deeper biofuel
oxidation.
3.3. Reaction Cascade for the Complete Oxidation of
Methanol. To determine the feasibility of using TEMPO-LPEI
films to catalyze multiple oxidations for a single fuel source, our
focus shifted toward methanol as a simplified substrate for
which the pathways to complete oxidation are limited, with the
predominant oxidation pathway going through formaldehyde
and formate intermediates prior to formation of CO2. The
catalytic current densities of TEMPO-LPEI films with each
intermediate, methanol, formaldehyde, and formate were
determined using constant potential amperometry and
confirmed using cyclic voltammetry. Reaction profiles for
formaldehyde and formate are provided in the Supporting
Information (Figure S3) and representative voltammograms are
In addition to enhanced catalytic activity and rapid diffusion,
LPEI provides a localized buffer region to facilitate TEMPO
catalytic activity under mildly acidic conditions.19,20
A
comparative pH profile shown in Figure 4 for the catalytic
activity of each catalyst toward 1 M glycerol highlights the
expanded pH range of TEMPO-LPEI films compared with
homogeneous methoxy-TEMPO. This characteristic allows for
much more mild reaction conditions than those required for
many transition metal electrooxidation catalysts, and thus,
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shown in Figure 5. Values of jmax generated by TEMPO-LPEI
films were found to be 8.20
enzymatic/organic catalyst biofuel cell. Such enzymatic
cathodes are not catalytically active under basic conditions
but are capable of reducing O2 under mild acidic conditions at
room temperature. A previously reported biocathode was
selected that utilizes the direct electron transfer of a laccase
enzyme that can generate nonlimiting current densities under
the desired conditions.18 Methanol was chosen as a fuel source
for the hybrid biofuel cell for the high catalytic current densities
achieved by the TEMPO-LPEI anode. A representative power
curve is shown in Figure 6. The maximum current density for
0.04 mA cm−2 in methanol
Figure 6. Current density (black) and power density curves (blue) for
a fuel cell consisting of a cross-linked TEMPO-LPEI anode and a DET
laccase cathode. Curves were generated using 200 mM phosphate/
citrate buffer with 2 M methanol at pH 5.0 and 25 °C. The
corresponding controls were run in the absence of methanol and are
shown as dashed lines.
Figure 5. Representative cyclic voltammograms of MWCNT-infused
TEMPO-LPEI films in the absence (black) and presence (blue) of 400
mM formaldehyde (A) and 500 mM sodium formate (B). Experiments
were performed on 3 mm glassy carbon electrodes using 150 mM
phosphate buffer at 10 mV s−1, pH 7.0, and 25 °C.
the resulting fuel cell was 378 39 μA cm−2, with a maximum
power density of 8
1 μW cm−2 using a solution of 2 M
methanol at pH 5.0 and 25 °C. The lack of a diffusion-limited
region of the current density curve suggests that the hybrid fuel
cell is presently limited by the low overall cell potential (Ecell),
which is largely due to the relatively high oxidation potential of
the TEMPO-LPEI anode material. Despite a low overall cell
potential, these results provide an initial proof of concept for
the use of TEMPO derivatives as organic electrocatalysts in the
context of a fuel cell.
(described above), 4.8 0.4 mA cm−2 in formaldehyde, and
33.4 9.4 mA cm−2 in a solution of formate at pH 7.0 and 25
°C. These results indicate that a catalytic oxidative cascade of
methanol with TEMPO-LPEI should be limited by the
oxidation of formaldehyde, which could be the result of
equilibrium between formaldehyde and its corresponding
monohydrate under aqueous conditions. The equilibrium
between formaldehyde and formaldehyde monohydrate lies
heavily in favor of the monohydrate, which is the form that is
active toward oxidation by TEMPO; however, the small
amount of the substrate in the nonhydrated form could be
responsible for the relatively lower reactivity when compared
with that of methanol. The extraordinary catalytic current
density generated by TEMPO-LPEI in the presence of formate
is largely attributed to the formation of CO2 gas, which both
allows for the rapid diffusion of products away from the
electrode surface and subsequently skews the catalytic
equilibrium to favor continued oxidation. The successful
oxidation of each intermediate of the methanol oxidation
cascade indicates that TEMPO-LPEI films are capable of the
complete oxidation of methanol under low temperature
conditions and at neutral pH.
4. CONCLUSION
We have demonstrated a method of covalent immobilization of
4-glycidyl-TEMPO onto a LPEI backbone, which, when cross-
linked onto a carbon electrode, can result in a dramatic
enhancement in the catalytic current density (as high as 8.20
mA cm−2) when compared with an analogous homogeneous
TEMPO catalyst. It is currently thought that the enhanced
activity is derived from the cationic nature of the polymer
backbone; however, the exact nature of the rate enhancement is
currently being investigated. This new electrocatalytic material
was shown to oxidize several bioavailable alcohols as well as
several short-chain alcohols under neutral aqueous conditions
at 25 °C. Such a broad substrate range allows for the possibility
of utilizing an immobilized TEMPO catalyst to facilitate a wide
range of oxidative cascades. TEMPO-LPEI was coupled with a
DET laccase cathode to construct a hybrid enzymatic/organic
catalyst biofuel cell capable of generating 378 μA cm−2 in a 2 M
aqueous methanol solution at pH 5.0 and 25 °C. Although the
fuel cell is limited by a low cell potential, it provides a proof of
concept demonstrating the possibility of using an immobilized
3.4. TEMPO-LPEI Films in a Hybrid Fuel Cell. The
expanded pH range of TEMPO-LPEI allowed us to consider
the possibility of coupling our new anodic material with an
enzymatic biocathode to construct a compartmentless hybrid
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TEMPO species as the anodic catalyst in a fuel cell operating at
near neutral pH. Furthermore, ongoing research is focused on
the development of TEMPO catalysts that can operate at lower
oxidation potentials while maintaining high catalytic activity
under mild aqueous conditions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01668.
■
S
Experimental procedures, synthesis and characterization
of TEMPO-LPEI, preliminary product analysis, electro-
chemical film stability analysis, and substrate kinetics
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: sigman@chem.utah.edu.
*E-mail: minteer@chem.utah.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors would like to thank the Army Research Office
MURI (No. W911NF1410263).
■
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