A Comparative Study of Nickel, Cobalt, and Iron Oxyhydroxide Anodes for the Electrochemical Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

Article abstract of DOI:10.1021/acscatal.8b04003

2,5-Furandicarboxylic acid (FDCA) has received increasing attention as a near-market platform chemical that can potentially replace terephthalic acid in the production of commercial and high-performance polymers, such as polyethylene terephthalate. FDCA can be obtained from the oxidation of 5-hydroxymethylfurfural (HMF), which is produced from the dehydration of C-6 monosaccharides obtained from cellulosic biomass. Recently, various heterogeneous Ni- and Co-based electrocatalysts were reported that can efficiently oxidize HMF to FDCA. The actual catalytically active species of these catalysts are most likely NiOOH and CoOOH or species related to NiOOH and CoOOH. However, the intrinsic catalytic properties of NiOOH and CoOOH for HMF oxidation have yet to be carefully and systematically investigated. In this study, we prepared thin and thick sets of NiOOH, CoOOH, and FeOOH films having comparable numbers of metal sites to systematically and methodically compare the intrinsic catalytic activity of these materials for HMF oxidation in a 0.1 M KOH (pH 13) solution. Our investigation revealed that they have distinctively different catalytic abilities for HMF oxidation. The use of extremely thin MOOH films containing limited numbers of catalytic sites allowed us to resolve anodic currents that were generated from HMF oxidation by different oxidation pathways. By comparing the voltammetric results of thin and thick films, the effect of the film thickness on the current generated by different oxidation pathways could be observed. The thick set of MOOH films was also used to compare the performances of these films for constant potential HMF oxidation and product analysis. The work herein contributes to a better understanding of the mechanisms of HMF oxidation on Ni-, Co-, and Fe-containing heterogeneous electrocatalysts whose surfaces are covered by their hydroxide and oxyhydroxide phases.

Full text of DOI:10.1021/acscatal.8b04003

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Article
A Comparative Study of Nickel, Cobalt, and Iron
Oxyhydroxide Anodes for the Electrochemical Oxidation
of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid
Brandon James Taitt, Do-Hwan Nam, and Kyoung-Shin Choi
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04003 • Publication Date (Web): 07 Dec 2018
Downloaded from http://pubs.acs.org on December 7, 2018
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A Comparative Study of Nickel, Cobalt, and Iron Oxyhydroxide Anodes
for the Electrochemical Oxidation of 5-Hydroxymethylfurfural to 2,5-
Furandicarboxylic Acid
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†
†
Brandon J. Taitt , Do-Hwan Nam , and Kyoung-Shin Choi*
Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706, USA.
†Both authors contributed equally to this work.
*
Correspondence to: kschoi@chem.wisc.edu
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Abstract
2
,5-furandicarboxylic acid (FDCA) has received increasing attention as a near-
market platform chemical that can potentially replace terephthalic acid in the production
of commercial and high-performance polymers, such as polyethylene terephthalate.
FDCA can be obtained from the oxidation of 5-hydroxymethylfurfural (HMF), which is
produced from the dehydration of C-6 monosaccharides obtained from cellulosic biomass.
Recently, various heterogeneous Ni- and Co-based electrocatalysts were reported that can
efficiently oxidize HMF to FDCA. The actual catalytically active species of these
catalysts are most likely NiOOH and CoOOH or species related to NiOOH and CoOOH.
However, the intrinsic catalytic properties of NiOOH and CoOOH for HMF oxidation
have yet to be carefully and systematically investigated. In this study, we prepared thin
and thick sets of NiOOH, CoOOH, and FeOOH films having comparable numbers of
metal sites to systematically and methodically compare the intrinsic catalytic activity of
these materials for HMF oxidation in a 0.1 M KOH (pH 13) solution. Our investigation
revealed that they have distinctively different catalytic abilities for HMF oxidation. The
use of extremely thin MOOH films containing limited numbers of catalytic centers
allowed us to resolve anodic currents that were generated from HMF oxidation by
multiple different oxidation pathways. By comparing the voltammetric results of thin and
thick films, the effect of the film thickness on the current generated by different oxidation
pathways could be observed. The thick set of MOOH films was also used to compare the
performances of these films for constant potential HMF oxidation and product analysis.
The work herein contributes to a better understanding of the mechanisms of HMF
oxidation on Ni-, Co-, and Fe- containing heterogeneous electrocatalysts whose surfaces
are covered by their hydroxide and oxyhydroxide phases.
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Keywords: 5-hydroxymethylfurfural, 2,5-furandicarboxylic acid, nickel oxyhydroxide,
cobalt oxyhydroxide, iron oxyhydroxide, electrocatalyst, alcohol oxidation,
electrochemical oxidation
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1
. Introduction
The valorization of cellulosic biomass into building-block chemicals and fuels has
gained increasing attention as a potential carbon-neutral route toward achieving
independence from fossil fuels.1-5 Among the various useful molecules that can be
obtained from biomass, 2,5-furandicarboxylic acid (FDCA) is a versatile near-market
platform chemical with applications ranging from fine chemicals to pharmaceuticals to
agrochemicals.6-11 FDCA has also received particular attention as a promising
replacement for terephthalic acid (TA) in the production of commercial and high-
performance polymers, whose furan-containing analogues have been shown to exhibit
superior physicochemical properties to those containing petroleum-derived TA.12-15
FDCA can be obtained from the oxidation of 5-hydroxymethylfurfural (HMF), a product
of the dehydration of C-6 monosaccharides, via the oxidation of both the alcohol and
aldehyde groups to carboxylic acids (Figure 1).
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Figure 1. Two possible pathways for HMF oxidation to FDCA.
Conventional methods used to oxidize HMF to FDCA typically include the use of
high air or O pressures (3-20 bar), elevated temperatures (30-130 °C), and precious
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metal heterogeneous catalysts (Au, Pd, and Pt).7,10,16-21 Recently, the electrochemical
oxidation of HMF has received increasing attention due to its advantages of being
environmentally benign, using water as the oxygen source, and proceeding at ambient
temperatures and pressures without the need for chemical oxidizing agents.22-27
Electrochemical oxidation of HMF was investigated as early as 1991 using
Ni(OH) /NiOOH covered Ni mesh, achieving an isolated yield of 71% and a Faradaic
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efficiency (FE) of 84% for FDCA production in pH 14. More recently, Ni or Ni-
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containing compounds such as Ni P, Ni S , , and NiFe layered double hydroxides
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3 2
have been shown to achieve high to near-quantitative FDCA yields and FEs in pH 14. In
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addition, a recent report demonstrated an electrochemical FDCA production system
based on a flow reactor with a Ni/NiOOH electrode which achieved an FDCA yield of
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9% and a FE of 80% when using a highly concentrated HMF solution (~ 0.65 M) at pH
2.33 Co-based compounds such as Co-P34 and Co-metalloid alloys35 have also been
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shown to achieve an FDCA yield and FE of > 90%.
The focus of most previous studies was to achieve high performances for HMF
oxidation by using electrocatalysts deposited on high surface area 3D porous substrates
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(
e.g. 3D Ni foam29-31, 3D Cu foam , and carbon paper ). Also, many studies used
catalysts that may not maintain their initial composition and structure under strongly
anodic and alkaline conditions. In these studies, the actual catalytically active species
may have been NiOOH and CoOOH or species related to NiOOH and CoOOH.
Therefore, gaining in-depth understanding of the catalytic properties of NiOOH and
CoOOH for HMF oxidation would be highly beneficial to better understand the
properties of more complex, chemically unstable catalysts.
NiOOH has been reported to serve as a catalyst for alcohol oxidation through the
conversion between Ni(OH) and NiOOH; NiOOH is reduced to Ni(OH) by oxidizing
2
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various aliphatic and aromatic alcohols (Eq. 1), and NiOOH is regenerated by the
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6-37
electrochemical oxidation of Ni(OH) (Eq 2).
In this case, the anodic current obtained
2
during alcohol oxidation is not due to direct oxidation of alcohol on NiOOH but rather
due to the regeneration of NiOOH from Ni(OH) , which is formed as a consequence of
2
alcohol oxidation. This mechanism is referred to as indirect Ni(OH) /NiOOH-mediated
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alcohol oxidation. The mechanism of CoOOH for alcohol oxidation is also reported to
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involve the conversion between Co(OH) and CoOOH.
2
NiOOH (s) + alcohol (aq)  Ni(OH) (s) + alcohol
(aq)
(1)
(2)
2
oxidized
-
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Ni(OH) (s) + OH (aq)  NiOOH (s) + H O (l) + e
2
2
The purpose of the current study is to systematically investigate and compare the
intrinsic catalytic properties of NiOOH, CoOOH, and FeOOH for HMF oxidation. Since
MOOH (M = Ni, Co, and Fe) have often been investigated and compared as water
oxidation catalysts, it would also be interesting and informative to compare their
properties for HMF oxidation. For this purpose, NiOOH, CoOOH, and FeOOH films
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were prepared as extremely thin, featureless films so that their intrinsic catalytic
properties could be investigated without interferences caused by differences in
morphology, surface area, and resistivity of these film. The use of extremely thin MOOH
films containing limited numbers of catalytic sites allowed us to resolve anodic currents
that were generated by HMF oxidation through previously unreported pathways for
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alcohol oxidation as well as the well-known M(OH) /MOOH-mediated oxidation. The
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results discussed in this study will provide a better understanding of electrochemical
HMF oxidation on MOOH electrodes, which may be useful for designing better MOOH-
based catalysts for HMF oxidation or for employing MOOH-based catalysts for the
electrochemical oxidation of other organic molecules.
2
. Experimental Section
Materials. HMF (97%, Alfa Aesar), DFF (98%, TCI America), HMFCA (95%, Oxchem),
FFCA (98%, TCI America), FDCA (97%, AstaTech Inc.), Ni(NO ) • 6H O (99%,
3
2
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Acros), Co(NO ) • 6H O (98%, Sigma-Aldrich), FeSO • 7H O (99%, Acros), KNO
3
3
2
2
4
2
(99%, Alfa Aesar), and KOH (85%, Sigma-Aldrich) were used without further
purification. Deionized water (Barnstead E-pure water purification system, resistivity >18
MΩ cm) was used to prepare all solutions.
Electrodeposition of thin Ni(OH) , Co(OH) , and Fe(OH) films on FTO. Thin
2
2
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Ni(OH) , Co(OH) , and Fe(OH) electrodes were prepared via electrochemical nitrate
2
2
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reduction using an SP-200 potentiostat/EIS (BioLogic Science Instrument). A three-
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electrode setup consisting of an FTO working electrode (4 cm ), Pt counter electrode, and
Ag/AgCl (4 M KCl) reference electrode was used. Pt counter electrodes were prepared by
sputter coating a 20 nm Ti adhesion layer followed by 100 nm of Pt on pre-cleaned glass
slides. Thin Ni(OH) films were deposited from an aqueous plating solution containing
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100 mM Ni(NO ) • 6H O, and thin Co(OH) films were deposited from a plating solution
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containing 100 mM Co(NO ) • 6H O. Fe(OH) films were prepared from an N -purged
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solution containing 100 mM FeSO • 7H O and 200 mM KNO . The pH of each solution
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was not adjusted prior to deposition. Thin Ni(OH) , Co(OH) , and Fe(OH) films were
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prepared potentiostatically by applying -0.9 V vs. Ag/AgCl and passing 5 mC cm . After
deposition, the films were rinsed with deionized water and dried in air. The resulting
films contained the same amount of electrochemically active metal ions, which was
determined by comparing the charges necessary to oxidize the as-deposited thin M(OH)2
films to MOOH films (Eq. 3).
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M(OH) (s) + OH (aq)  MOOH (s) + H O (l) + e
(3)
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For this experiment, the oxidation of M(OH)2 films was performed
potentiostatically in a 0.1 M KOH solution using a three-electrode setup in an undivided
cell at 1.45 V vs. RHE, which is below the onset for water oxidation for all materials,
until the anodic current decayed to zero. Under this condition, the number of electrons
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+
required to complete the reaction equals the number of M ions present in the M(OH)
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film. The measured charge required to convert the as-deposited thin M(OH) films was
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approximately 0.9 mC cm for all films, confirming that the as-deposited films contain
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+
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the same amount of electrochemically active M ions. By dividing the coulombs/cm by
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the electric charge carried by a single electron, the number of metal sites/cm could be
easily calculated (Table S1). For Fe(OH) , which spontaneously oxidizes to FeOOH over
2
time under ambient conditions, special care was necessary; the electrochemical oxidation
of Fe(OH) was performed immediately after deposition of Fe(OH) in an N -purged 0.1
2
2
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M KOH solution under an N blanket. We note that typical electrical double layer
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capacitance measurements cannot be used to compare active metal sites present in
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M(OH) films because of the insulating nature of these films. The method described
2
here, which we report for the first time, provides a simple yet accurate way to measure
the active metal sites present in M(OH) films.
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Electrodeposition of thick Ni(OH) , Co(OH) , and Fe(OH) films on FTO. Thick
2
2
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Ni(OH) , Co(OH) , and Fe(OH) electrodes were prepared using the same three-electrode
2
2
2
setup used for the deposition of thin films. Thick Ni(OH) films were deposited from an
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aqueous plating solution containing 10 mM Ni(NO ) ·6H O and 30 mM KNO , and thick
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Co(OH)2 films were deposited from a plating solution containing 10 mM
Co(NO ) ·6H O and 30 mM KNO . Fe(OH) films were prepared from an N -purged
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solution containing 10 mM FeSO ·7H O and 50 mM KNO . We note that all the plating
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solutions used to deposit thicker films commonly contained less metal ion precursors than
those used to deposit thin films. This is because the use of more dilute solutions
facilitated diffusion-limited growth of M(OH) , resulting in the deposition of higher
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surface area films. The pH of each solution was not adjusted prior to deposition. Thick
-2
films were prepared galvanostatically by applying -0.25 mAcm for 12 min. After
deposition, the films were rinsed with deionized water and dried in air. As with the thin
M(OH) films, the resulting as-deposited thick M(OH) films contained the same amount
2
2
of electrochemically active metal ions, which was confirmed using the same method
described above. The charge necessary to convert the as-deposited thick M(OH) films to
2
-2
MOOH was approximately 0.15 C cm for all films (Table S2).
Characterization of prepared electrodes. The surface morphology of each film was
examined by scanning electron microscopy (SEM) using a LEO 1530 microscope with an
accelerating voltage of 2 kV. Powder X-ray diffraction (XRD) (Bruker D8 Advanced
PXRD, Ni- filtered Cu Kα radiation, λ = 1.5418 Å) was used to determine the
crystallinity of the M(OH) and MOOH films. UV-vis absorption spectra were measured
2
using a Cary 5000 UV-vis-NIR spectrophotometer (Agilent) fitted with an integrating
sphere attachment. In this setup, the electrode is located in the center of the integrating
sphere, and its absorbance is calculated from the measurement of the light that is both
reflected from and transmitted through the electrode.
Electrochemical experiments.
Linear sweep voltammetry and cyclic voltammetry experiments were performed
using a three-electrode setup in an undivided cell. M(OH) or MOOH electrodes were
2
used as working electrodes, while Ag/AgCl (4 M KCl) and Pt were used as reference and
counter electrodes, respectively. A 0.1 M KOH (pH 13) solution was used as the
electrolyte, and 5 mM HMF, DFF, HMFCA, or FFCA was added to the solution when
necessary. We selected the use of a pH 13 solution even though a higher catalytic
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performance of alcohol oxidation is expected at pH 14 because the instability of HMF
at pH 14 is known to result in undesirable humin formation, which makes this condition
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undesirable for practical purposes.41 Voltammetry experiments were conducted by
sweeping the potential in the positive direction from the open circuit potential (OCP) at a
-1
scan rate of 5 mV s at room temperature and without stirring. Experiments for the
constant potential oxidation of HMF were carried out using a three-electrode setup in an
H-shaped divided glass cell with fritted glass as a divider. Prior to constant potential
oxidation of HMF, freshly prepared thick Ni(OH) , Co(OH) , and FeOOH anodes were
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subjected to a single LSV in the positive direction (scan rate 5 mV s ) to ensure complete
oxidation of the film. The oxidized films were then used as working electrodes, while
Ag/AgCl (4 M KCl) and Pt were used as reference and counter electrodes, respectively.
The working electrode compartment consisted of 14 mL of a 0.1 M KOH solution
containing 5 mM HMF, while the counter electrode compartment contained 14 mL of a
0
.1 M KOH solution. The electrochemical oxidation was carried out at room temperature
with stirring at various potentials. The reaction was stopped after 40.52 C were passed,
which is the stoichiometric amount of coulombs necessary to convert the given amount of
HMF to FDCA. Although electrode potentials recorded in this study were measured
against an Ag/AgCl (4M KCl) reference electrode, all of the potentials in this work are
presented against the reversible hydrogen electrode (RHE) for ease of comparison with
other reports that use electrolytes with different pH conditions. The conversion between
potentials versus Ag/AgCl to potentials versus RHE was performed using Eq. 4. The
potentials used in this study were not corrected for uncompensated IR drop.
E(vs. RHE) = E(vs. Ag/AgCl) + 0.1976 V + 0.059 × pH
(4)
Product Analysis. After constant potential electrolyses, 150 μL of solution were taken
from the working electrode compartment and were analyzed using a Shimadzu
Prominence-i LC-2030C 3D HPLC system equipped with an internal UV-Vis detector to
calculate HMF conversion and oxidation product yields. An aqueous mobile phase
-1
composed of 0.1% sulfuric acid was used at a flow rate of 0.5 mL min in isocratic mode.
0 μL sample aliquots were injected directly onto a 300 mm × 7.8 mm ICSep ICE-
1
Coregel 87H3 column (Transgenomic), and the column oven temperature was maintained
at 65 °C. The identification of the products and calculation of their concentrations were
determined from calibration curves by applying standard solutions of known
concentration. Retention times of 20, 23, 27, 32 and 39 min correlated to FDCA,
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HMFCA, FFCA, HMF and DFF, respectively. Because each compound exhibits a
different light absorption profile, detection wavelengths of 262, 258, 281, 283 and 288
nm were chosen as λmax for FDCA, HMFCA, FFCA, HMF, and DFF, respectively.
The following equations were used to calculate HMF conversion, product yield,
1
1
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1
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and FE for FDCA production where F represents Faraday’s constant, 96485 C mol-1.
mol of HMF consumed
HMF conversion (%) =
× 100%
(5)
(6)
mol of initial HMF
mol of product formed
mol of initial HMF
Product yield (%) =
× 100%
charge used for FDCA production
total charge passed
FE for FDCA production (%) =
× 100% (7)
mol of FDCA formed x (6 x 퐹)
=
× 100%
total charge passed
3
. Results and discussion
The MOOH electrodes (M = Ni, Co, and Fe) used in this study were prepared
from M(OH) films that were electrodeposited onto FTO substrates by nitrate reduction
2
2+
42
from plating solutions containing M ions and nitrate ions. Nitrate reduction increases
2
+
the local pH at the working electrode (Eq. 8) and decreases the solubility of M at the
2
+
electrode surface. As a result, M ions precipitate and are deposited as M(OH) films on
2
the working electrode (Eq. 9). Fe(OH) films are not stable under ambient conditions and
2
spontaneously convert to FeOOH over time after deposition. The Co(OH) and Ni(OH)
2
2
films are stable at ambient conditions and were converted to CoOOH and NiOOH films
by applying an anodic potential when required.
-
-
-
-
NO (aq) + H O (l) + 2e  NO (aq) + 2OH (aq)
(8)
(9)
3
2
2
2
+
-
M (aq) + 2OH (aq)  M(OH) (s)
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Voltammetric study of the intrinsic catalytic activity of thin MOOH films for
electrochemical HMF oxidation
The electrocatalytic properties of NiOOH, CoOOH, and FeOOH were first
investigated using linear sweep voltammetry (LSV). The films used for these experiments
were composed of extremely thin, featureless, and uniform MOOH films that possess
comparable amounts of electrochemically active sites (Table S1). Since these extremely
thin films conformally coated the FTO substrate, the rough nanoparticle morphologies
shown in their SEM images (Figure 2a-c) are mainly of the underlying FTO substrate
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(Figure S1). The use of these films ensured that any differences in the measured catalytic
current between the three materials were strictly due to the materials’ intrinsic catalytic
ability for HMF oxidation rather than influences stemming from disparities in surface
morphologies or unequal electrochemically active sites in the films. Furthermore, the
small current densities generated by these thin films gave better resolution of currents
generated from different HMF oxidation mechanisms, which most likely could not be
resolved by previous studies due to their use of high surface area Ni- and Co-containing
catalysts.
Figure 2. SEM images of thin (a) NiOOH, (b) CoOOH, (c) FeOOH films used for
voltametic studies.
The LSVs of a thin NiOOH film obtained in 0.1 M KOH (pH 13) is shown in
Figure 3a. The first LSV was obtained by sweeping the potential from the OCP to the
positive direction using an as-deposited Ni(OH) film in the absence of HMF. Because
2
there was no HMF in solution, any oxidation features observed during the first sweep
must be due to the oxidation of the film and water. The oxidation peak centered at 1.43 V
vs. RHE is due to the oxidation of Ni(OH) to NiOOH, which is followed by the anodic
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current onset at 1.51 V for water oxidation (Figure 3a, black line). In order to confirm
that the oxidation of Ni(OH) to NiOOH was completed during the first LSV, a second
2
LSV was obtained immediately after the first scan. The second LSV no longer showed
the oxidation peak due to the oxidation of Ni(OH) to NiOOH and showed only anodic
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current associated with water oxidation, confirming that the complete conversion of
Ni(OH) to NiOOH was achieved by the first LSV (Figure 3a, blue line).
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Figure 3. LSVs of thin (a) Ni(OH) , (b) Co(OH) , and (c) FeOOH films in a 0.1 M KOH
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solution (scan rate: 5 mV s ). Three consecutive LSVs were obtained in the same
solution; the first (black) and second (blue) LSVs were performed in the absence of
HMF, while the third (red) was performed after the addition of 5 mM HMF (a1:
Ni(OH) /NiOOH-mediated indirect oxidation; a2: direct oxidation on MOOH; a3: direct
2
oxidation on M(OH)2).
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After the second LSV, 5 mM HMF was added to the solution and the OCP of the
NiOOH electrode was allowed to equilibrate, which took less than 3 min. The change in
OCP was due to electron transfer between the NiOOH electrode and HMF; NiOOH can
oxidize HMF as a chemical oxidant while being reduced to Ni(OH) under the open
2
circuit condition. Therefore, the third LSV, obtained after establishing equilibrium
between the electrode and the HMF solution, showed the reappearance of the oxidation
peak corresponding to the oxidation of Ni(OH) to NiOOH (Figure 3a, red line).
2
Furthermore, since the NiOOH that was electrochemically regenerated during the third
LSV could immediately participate in HMF oxidation and be reconverted to Ni(OH) on
2
the timescale of the LSV scan, more charge was passed for the conversion of Ni(OH) to
2
NiOOH when HMF was present in the solution. As a result, the Ni(OH) /NiOOH peak
2
was larger in the third LSV than in the first LSV, and this difference could be directly
related to the rate of the Ni(OH) /NiOOH-mediated HMF oxidation.
2
Previously, the Ni(OH) /NiOOH-mediated oxidation was reported to be the only
2
mechanism for alcohol oxidation by NiOOH (denoted as a1 in Figure 3a).36 The
mechanism in the a1 region is the same as when NiOOH is used as a chemical oxidant
under the open circuit condition. The rate determining step for the Ni(OH) /NiOOH-
2
mediated oxidation is known to be the extraction of a hydrogen radical at the alpha
position of the alcohol, which converts NiOOH to Ni(OH)2.36 The electrochemical
36
regeneration of NiOOH from Ni(OH) (Eq. 2) was reported to be fast. However, in
2
addition to the Ni(OH) /NiOOH-mediated oxidation, our LSV shows two additional
2
oxidation features related to HMF oxidation. One is the presence of oxidation current that
appears after the Ni(OH) /NiOOH peak but before the onset of water oxidation (denoted
2
as a2 in Figure 3a). The new oxidation mechanism in the a2 region appears to be enabled
only when a potential more positive than the redox potential for the Ni(OH) /NiOOH
2
conversion is applied to the NiOOH electrode. Most likely, this new mechanism achieves
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HMF oxidation on the NiOOH surface without involving the conversion of NiOOH to
Ni(OH) and will be referred to as direct oxidation of HMF in this study.
2
The third oxidation feature is the presence of oxidation current (denoted as a3 in
Figure 3a) appearing before the Ni(OH) /NiOOH oxidation peak. This means that HMF
2
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can be oxidized directly on the Ni(OH) surface without involving the formation of
2
NiOOH. The direct electrochemical oxidation of cysteine on a Ni(OH) electrode at
2
potentials well below the redox potential of the Ni(OH) /NiOOH conversion has been
2
previously reported.43
The LSVs of NiOOH obtained in the presence of 5 mM DFF, HMFCA, or FFCA
instead of 5 mM HMF are shown in Figure S2a. They all commonly show three oxidation
features (a1, a2, and a3). However, the anodic current densities generated in each region
vary depending on the substrate type because the functional groups attached to the furan
ring can affect the kinetics of the three different oxidation mechanisms to different
degrees.
The three LSVs obtained in the same manner with a thin Co(OH) film are shown
2
in Figure 3b. We note that before obtaining these LSVs, the as-deposited Co(OH) was
2
pre-treated with cyclic voltammograms (CVs). This is because not all of the
electrochemically active Co2+ sites in the as-deposited Co(OH) film can undergo
2
44
reversible oxidation and reduction between Co(OH) and CoOOH. In other words, the
2
Co(OH) /CoOOH peak obtained from the anodic scan of the first CV of the as-deposited
2
2
+
film, where all electrochemically active Co ions are oxidized, is always larger than the
2
+
Co(OH) /CoOOH peak obtained from subsequent CVs where only the Co that can be
2
reversibly oxidized and reduced are oxidized. Performing CVs until successive scans
were identical ensured that only the Co ions that can undergo reversible conversion
between Co(OH) and CoOOH participated in the LSVs shown in Figure 3b. This way,
2
any difference in the Co(OH) /CoOOH peak obtained with and without HMF could be
2
unambiguously attributed to the effect of HMF. The CVs used for the pretreatment of the
as-deposited Co(OH) film are shown in Figure S3a. The CV pretreatment revealed that
2
only approximately 34% of the Co2+ sites in the Co(OH) film are electrochemically
2
reversible (Table S1). This means that the number of active Co sites in the Co(OH) film
2
after the CV treatment is only ~1/3 of the active Ni sites in the Ni(OH) film since all of
2
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the as-deposited M(OH) films were prepared to have the same number of initial
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electrochemically active M sites.
The first LSV of Co(OH) in Figure 3b shows a peak for the Co(OH) /CoOOH
2
2
conversion centered at 1.14 V vs. RHE. This peak was followed by anodic current for
water oxidation, which has an onset at around 1.50 V vs. RHE. The second LSV that was
obtained immediately after the first scan in the same solution no longer shows the
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oxidation current corresponding to the conversion of Co(OH) to CoOOH, suggesting
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+
3+
that all of the Co ions that can be reversibly oxidized to Co were oxidized during the
first LSV.
Before obtaining the third LSV, 5 mM HMF was added to the solution and the
OCP of the CoOOH electrode was allowed to equilibrate. The third LSV showed a
reappearance of the Co(OH) /CoOOH peak (denoted as a1 in Figure 3b), suggesting that
2
under the open circuit condition, CoOOH oxidized HMF by serving as a chemical
oxidant and was reduced to Co(OH) . However, we note that the Co(OH) /CoOOH peak
2
2
shown in the third LSV is smaller than the peak shown in the first LSV. This is different
from the case of NiOOH where the Ni(OH) /NiOOH peak was enhanced when HMF was
2
present. The smaller Co(OH) /CoOOH peak in the third scan compared to that in the first
2
scan suggests two things. First, not all Co3+ ions that can reversibly undergo
electrochemical Co(OH) /CoOOH conversion are catalytically active for chemical HMF
2
3
+
oxidation and, as such, only a fraction of Co in CoOOH was converted to Co(OH) by
2
HMF oxidation under the open circuit condition. Second, the Co(OH) /CoOOH-mediated
2
HMF oxidation is not as fast as the Ni(OH) /NiOOH-mediated HMF oxidation. Therefore,
2
HMF in the solution cannot rapidly regenerate Co(OH) (which would re-oxidize to
2
CoOOH) during the time scale of the LSV scan. This results in a negligible amount of the
current enhancement for the Co(OH) /CoOOH peak during the LSV scan when HMF is
2
present.
CoOOH also generated anodic current for HMF oxidation in the potential region
between the Co(OH) /CoOOH peak and the onset of water oxidation (denoted as a2 in
2
Figure 3b). As in the case of NiOOH, the a2 mechanism likely achieves HMF oxidation
on the CoOOH surface without involving the conversion of CoOOH to Co(OH)2.
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However, unlike Ni(OH) , Co(OH) did not show any sign that it can directly oxidize
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2
HMF on the Co(OH) surface through the a3 mechanism.
2
We note that for both NiOOH and CoOOH, a slight current enhancement after the
onset of water oxidation is observed when HMF was present, meaning that HMF
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4+
oxidation can occur concurrently with water oxidation. Since the formation of Ni and
4
+
45-46
Co is possible in this potential region,
we expect that HMF oxidation in this region
can also be achieved through an additional mechanism that involves the formation of Ni4+
4
+
or Co sites.
The LSVs of CoOOH obtained in the presence of 5 mM DFF, HMFCA, or FFCA
instead of 5 mM HMF are shown in Figure S3b. They all commonly show anodic
currents for the Co(OH) /CoOOH conversion, meaning that CoOOH can oxidize DFF,
2
HMFCA, and FFCA under the open circuit condition. However, none of them showed an
enhancement for the Co(OH) /CoOOH peak current, suggesting that their rates of
2
Co(OH) /CoOOH-mediated oxidation are also very slow. They also commonly showed
2
anodic currents for their oxidation on the CoOOH surface through the a2 mechanism in
the potential region between the Co(OH) /CoOOH peak and the onset for water oxidation.
2
Again, the current densities in the a1 and a2 regions varied depending on the substrate
type because the functional groups attached to the furan ring affect the kinetics of the
different oxidation mechanisms to different degrees.
The LSVs of FeOOH show no catalytic activities for HMF oxidation (Figure 3c).
Since FeOOH remains as FeOOH under the open circuit condition, the first LSV shows
only anodic current for water oxidation, as does the second LSV. When HMF was added
to the solution, FeOOH could not serve as a chemical oxidant for HMF under the open
circuit condition as the redox potential of the Fe(OH) /FeOOH conversion is too low
2
(approximately 0.24 V vs. RHE). As a result, the LSV obtained with HMF present in
solution also did not show any anodic features related to the Fe(OH) /FeOOH
2
conversion. Furthermore, no sign of direct HMF oxidation on FeOOH was observed. This
result indicates that although FeOOH is catalytic for water oxidation, it does not
demonstrate any apparent catalytic ability for HMF oxidation. When the LSVs of FeOOH
were obtained in the presence of 5 mM DFF, HMFCA, or FFCA instead of 5 mM HMF
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(
Figure S3c), a current enhancement before water oxidation was observed only with DFF,
meaning that only the oxidation of DFF can occur before water oxidation on FeOOH.
Electrochemical HMF Oxidation Using Thick MOOH films
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Due to their extremely low surface area, the thin MOOH films used for the
voltammetric analysis could not be used to investigate the full conversion of HMF to
FDCA because it took too long to pass the stoichiometric amount of charge required for
the conversion. Extremely long electrolysis reaction times resulted in undesirable
complications during product analysis, such as water evaporation affecting product
concentrations, base-induced non-electrochemical humin formation, and crossover of
intermediates to the counter electrode compartment, which made it difficult to assess the
true intrinsic catalytic activity of each material during electrolysis.
Therefore, a set of thick MOOH films were prepared to perform constant potential
HMF oxidation and analyze the product yields and FEs for FDCA production. While the
morphologies of the thicker films varied depending on the material, the thickness of each
film was controlled such that each film possessed comparable amounts of initially
electrochemically active metal sites (Table S2). We also note that although the thick
MOOH films are thicker and have higher surface areas than the thin films used in the
previous section, they are still deposited on flat 2D FTO substrates and have limited
surface areas compared to electrodes prepared using 3D porous substrates that have
considerable thickness in the millimeter range.29-31,34 Therefore, the performances that
can be achieved by our flat, thick films cannot be as good as the catalysts that were
deposited on 3D substrates. Our goal of using thicker films was not to achieve high
performances, but rather to make thick enough samples with controlled electrochemically
active sites to examine the effect of film thickness on the LSVs of these films for HMF
oxidation and perform product analysis to accurately compare the performances of
NiOOH, CoOOH, and FeOOH for FDCA production.
The morphologies of the thick NiOOH, CoOOH and FeOOH electrodes are
shown in Figure 4, which are identical to those of as-deposited M(OH) films. These
2
films were commonly composed of 2D plates or sheets that roughen the surface, creating
porosities. The NiOOH film was composed of contoured nanoparticles, each of which
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was comprised of compactly packed nanoplates (Figure 4a). The CoOOH film was
composed of 2D sheets with micron-level lateral dimensions that were vertically aligned
against the substrate (Figure 4b). The morphology of the FeOOH was similar to that of
NiOOH, but the average size of the individual nanoplates in each cluster was slightly
larger, thereby increasing the size of each cluster. (Figure 4c). X-ray diffraction studies
showed that all MOOH films are amorphous (Figure S4).
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9
0
Figure 4. SEM images of thick (a) NiOOH, (b) CoOOH and (c) FeOOH films deposisted
on FTO substrates. The insets in (a)-(c) show a higher magnification image.
Prior to using these films for constant potential HMF oxidation, LSVs of the thick
NiOOH, CoOOH, and FeOOH films were obtained using the same procedures used to
analyze the thin films. This allowed us to observe how the increase in the surface area
and the presence of metal ions in more diverse structural environments affect the current
generation in the different catalytic regimes.
The first LSV of the thick Ni(OH) film shows a much broader Ni(OH) /NiOOH
2
2
peak than the thin Ni(OH) film (Figure 5a, black line). This is because the thick, high
2
2
+
surface area Ni(OH) film contains Ni ions in diverse structural environments, and their
2
structural environment directly affects their oxidation potential in Ni(OH)2.
2+
Consequently, a fraction of Ni ions in the thick Ni(OH) film is either easier or more
2
2
+
difficult to oxide than the Ni ions present in a thin, uniform Ni(OH) film. As a result,
2
the onset of Ni(OH) oxidation was as early as 1.15 V, and the conversion of Ni(OH) to
2
2
NiOOH was not completed before water oxidation occurred. Nonetheless, the complete
conversion of Ni(OH) to NiOOH occurred during the first LSV, which was confirmed
2
by the second LSV that no longer showed the Ni(OH) /NiOOH peak (Figure 5a, blue
2
line).
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Figure 5. LSVs of thick (a) Ni(OH) , (b) Co(OH) , and (c) FeOOH films in a 0.1 M
2
2
KOH solution. Three consecutive LSVs were obtained in the same solution: the first
black) and second (blue) LSVs were performed in the absence of HMF, while the third
(
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(red) was performed after the addition of 5 mM HMF (scan rate: 5 mV s ).
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The use of the thick films also allowed us to easily observe changes in the
appearance of the electrodes as the oxidation states of the films changed (Figure 6). The
as-deposited Ni(OH) was opaque white, but the NiOOH film obtained after the first LSV
2
was opaque brown (Figure 6a,c). Judging by the appearances of the films, it was evident
that not just the surface but rather the bulk of the thick films were oxidized, which is
consistent with the ion channel structure that permeates throughout the bulk of the
material.47
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Immediately after the second LSV was completed, 5 mM HMF was added to the
solution and the OCP was monitored. In the presence of HMF under open circuit
conditions, the color of the NiOOH electrode gradually changed from opaque brown to
opaque white over the course of 30 min as NiOOH was converted to Ni(OH) as a result
2
of oxidizing HMF (Figure 6c-d). After 30 min, the OCP had stabilized, suggesting that all
NiOOH was converted to Ni(OH) . The color of the resulting Ni(OH) was identical to
2
2
that of as-deposited Ni(OH) , visually confirming the complete conversion of NiOOH to
2
Ni(OH) by Ni(OH) /NiOOH mediated HMF oxidation. The corresponding UV-vis
2
2
absorption spectra of these films can be found in Figure S5.
Figure 6. Photographs showing changes in appearance of the thick films used in this
study after various treatments.
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A third LSV was obtained in the presence of HMF. As in the case of the thin film
analysis, the Ni(OH) /NiOOH peak reappeared and the size of the peak was greatly
2
enhanced. Since the width and magnitude of the Ni(OH) /NiOOH peak is so large, the
2
oxidation currents caused by the a2 and a3 mechanisms could no longer be resolved from
the oxidation current caused by the a1 mechanism. This result explains why the three
different oxidation mechanisms found in our voltammetric analysis using extremely thin
films could not be observed in previous studies.
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Similar to the thin Co(OH) film, the LSVs of the thick Co(OH) film were
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obtained after pretreating them with CVs. Although the as-deposited thick Co(OH) film
2
showed a light blue color, the Co(OH) film after the CV treatment changed to a red-black
2
2
+
3+
color due to the irreversibility of the Co /Co conversion during the first CV cycle, as
previously explained (Figure 6e-g). Consistent with the thin Co(OH) film, the CV
2
pretreatment showed that only approximately 34% of the initially electrochemically
2
+
active Co sites are electrochemically reversible (Figure S3b, Table S2). When this film
was used for the first LSV it turned an opaque black due to the formation of CoOOH
(
Figure 6h). The second LSV, which no longer showed the Co(OH) /CoOOH peak,
2
confirmed the complete conversion of Co(OH) to CoOOH during the first LSV. When
2
CoOOH was immersed in the HMF-containing solution, the color of the CoOOH
electrode slowly changed back to dark red over the course of 25 h (Figure 6i), which is
the same color of the Co(OH) film after the CV pretreatment. This conversion was much
2
slower than that of Ni(OH) /NiOOH, confirming the slow rate of HMF oxidation by the
2
Co(OH) /CoOOH conversion.
2
The LSV curve obtained in the presence of 5 mM HMF shows a
Co(OH) /CoOOH peak that is comparable to that shown in the first LSV obtained with
2
Co(OH) in the absence of HMF (Figure 5b). The fact that the Co(OH) /CoOOH peak in
2
2
the first and third LSVs look comparable suggest in this thicker and higher surface area
2+
3+
CoOOH film, the Co /Co ions that reversibly participate in the electrochemical
Co(OH) /CoOOH conversion are all catalytically active for HMF oxidation under the
2
open circuit condition. However, no catalytic enhancement of the Co(OH) /CoOOH peak
2
was observed, which was expected given the slow rate of HMF oxidation by CoOOH
under the open circuit condition. As in the case of the thin CoOOH, the thick CoOOH
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showed enhanced anodic current for HMF oxidation owing to the direct HMF oxidation
on the CoOOH surface at potentials between the Co(OH) /CoOOH peak and the onset of
2
water oxidation.
As with the thin FeOOH film, the thick FeOOH film showed only water oxidation
current during both the first and second scans (Figure 5c, black and blue lines). Also,
unlike NiOOH and CoOOH, the FeOOH film showed no change in color after the
addition of 5 mM HMF (Figure 6j-m) because FeOOH cannot serve as an oxidant for
alcohol oxidation under the open circuit condition due to the very low potential of the
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Fe(OH) /FeOOH couple. Consistent with the thin film study, the third LSV of FeOOH
2
obtained with HMF did not show any additional oxidation features caused by direct HMF
oxidation on FeOOH.
Constant potential oxidation of HMF to FDCA using thick MOOH films
Constant potential oxidation of HMF was performed with thick NiOOH, CoOOH,
and FeOOH films in a 0.1 M KOH solution containing 5 mM HMF at multiple potentials
to compare the effect of potential on HMF oxidation. HMF conversion, product yields,
and FE for FDCA production after passing the stoichiometric amount of change to
convert HMF to FDCA for each case are summarized in Table 1. We note that at the
stoichiometric amount of charge to convert HMF to FDCA, the yield for FDCA equals
the FE for FDCA production. (For electrochemical reactions, the relationship between the
yield and FE changes depending on the amount of charge passed relative to the
stoichiometric charge required to complete the reaction. The detailed explanation, along
with the performances of other Ni- and Co-containing electrocatalysts for HMF
oxidation, can be found in Table S3.)
Table 1. Results of HMF oxidation using NiOOH, CoOOH, and FeOOH at various
potentials after passing the stoichiometric amount of charge to convert HMF to FDCA.
FE
for
FDCA
HMF
Conversion
(%)
Potential
V. vs. RHE)
Time
(h)
DFF
(%)
HMFCA
(%)
FFCA
(%)
FDCA
(%)
Anode
(
(%)
NiOOH
1.47
4.7
99.8
0.03
1.31
1.79
96.0
96.0
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.56
1.62
.71
1.71
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CoOOH
FeOOH
4.6
1.7
2.3
1
The NiOOH electrode achieved the highest yield and FE for FDCA production
96.0%) at 1.47 V vs. RHE. As more positive potentials were used, water oxidation
(
became a competing anodic reaction and FDCA yield and FE gradually decreased, with
more HMF and HMF oxidation intermediates remaining in the solution at the end of the
electrolysis. Judging from the LSV shown in Figure 3a where the mechanism a2 is turned
on around ~1.45 V vs. RHE, we think that HMF oxidation at 1.47 V vs. RHE by the thick
NiOOH film is mostly achieved by Ni(OH) /NiOOH-mediated oxidation (a1). However,
2
considering the heterogeneity of Ni sites present in the thick NiOOH films, it is possible
that some Ni sites in the thick NiOOH film can oxidize HMF through mechanisms other
than the Ni(OH) /NiOOH-mediated oxidation at 1.47 V vs. RHE. We are currently
2
developing electrochemical methods that can deconvolute the currents obtained during
the constant potential HMF oxidation by the thick MOOH films into current by the a1
mechanism and that by the a2 mechanism.
The least positive potential that enabled CoOOH to generate an appreciable
amount of current density for HMF oxidation was 1.56 V vs. RHE. Below this potential,
extremely low levels of oxidation current were generated (Figure S6), and it was not
possible to convert a meaningful amount of HMF before base-induced humin formation
affected the product analysis. When HMF oxidation was performed at 1.56 V vs. RHE,
the FDCA yield and FE were limited to 35.1% since water oxidation can also occur at
this potential (Figure 5b). Since water oxidation by CoOOH involves the generation of
4
+
46
Co ions, we think that HMF oxidation at 1.56 V vs. RHE may involve the formation
4+
of Co ions in addition to oxidation through the a1 and a2 mechanisms. When HMF
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oxidation was achieved at more positive potentials, as with NiOOH, the FDCA yield and
FE decreased even further as water oxidation became an increasingly dominant reaction.
The results obtained for constant potential HMF oxidation with CoOOH suggests
that although the LSV results show that CoOOH has the ability to oxidize HMF at a less
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positive potential than NiOOH due to the lower redox potential for the Co(OH) /CoOOH
2
conversion, the slow kinetics of Co(OH) /CoOOH-mediated HMF oxidation along with
2
the fact that not all Co3+ sites in CoOOH can be used for reversible electrochemical
reactions, makes Co(OH) /CoOOH-mediated HMF oxidation less efficient than
2
Ni(OH) /NiOOH-mediated HMF oxidation. It appears that even the kinetics of direct
2
oxidation of HMF on CoOOH are not fast enough to be used for constant potential HMF
oxidation in the potential region where water oxidation does not occur. However,
although CoOOH is inferior to NiOOH when prepared with comparable metal sites, this
does not mean that CoOOH cannot achieve efficient HMF oxidation in high yield or FE.
If CoOOH is prepared as an extremely high surface area electrode that contains a higher
number of electrochemically reversible and catalytically active Co sites to enhance the
current for HMF oxidation before water oxidation becomes dominant, CoOOH may also
achieve HMF oxidation in a high yield and FE.
Using the FeOOH electrode, 1.71 V was the least positive potential that was able
to generate comparable oxidation current to NiOOH and CoOOH. At this potential, water
oxidation was already the dominant anodic reaction, and HMF conversion was low.
Nonetheless, the formation of DFF, HMFA, FFCA and FDCA was detected, although
their yields were not considerable (< 5%). This result suggests that although FeOOH has
the ability to oxidize HMF to FDCA in the high potential region, the combination of a
high overpotential requirement and the predominance of water oxidation will always
make the FE for HMF oxidation extremely low.
4
. Conclusion
We have comparatively investigated the catalytic activity of electrochemically-
prepared NiOOH, CoOOH, and FeOOH electrodes for the electrochemical oxidation of
HMF to FDCA in a pH 13 solution. The use of thin films possessing essentially no
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distinct surface morphologies and containing comparable metal sites enabled a systematic
comparison of the intrinsic catalytic activity of each material. Also, the use of thin films
containing a small number of uniform metal sites enabled us to observe evidence for the
presence of multiple pathways of HMF oxidation that have not been previously observed
for alcohol oxidation. Using thicker films, the effect of film thickness on the
voltammetric responses of the MOOH films by different oxidation pathways could be
examined, and the performances of the MOOH films for constant potential oxidation of
HMF could be investigated. The result showed that NiOOH is the most efficient catalyst
for electrochemical HMF oxidation, achieving up to 96.0% yield and FE for FDCA
production at 1.47 V vs. RHE. Although CoOOH has the ability to initiate HMF
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oxidation at a lower potential than NiOOH due to the Co(OH) /CoOOH conversion
2
occurring at a less positive potential, the rate of Co(OH) /CoOOH-mediated HMF
2
oxidation was too slow to generate sufficient current density for constant potential HMF
oxidation. When a more positive potential was applied to increase the rate of HMF
oxidation through different oxidation mechanisms, water oxidation was also enabled,
thereby lowering the FE for HMF oxidation. FeOOH did not exhibit any catalytic activity
for HMF oxidation at potentials below the onset for water oxidation during both thin and
thick film voltammetry experiments. However, the detection of trace amounts of FDCA
during constant potential HMF oxidation performed at 1.71 V suggests that FeOOH also
has the ability to oxidize HMF to FDCA, although it is not efficient. The results
contained in this study provide a better understanding of MOOH as electrocatalysts for
HMF oxidation, which can also be used as a foundation to design future studies for using
MOOH for general alcohol oxidation.
ASSOCIATED CONTENT
Supporting Information.
The supporting information is available free of charge.
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SEM of bare FTO, comparison of active metal sites in MOOH films, performance
comparison of Ni- and Co-containing electrocatalysts, XRD of M(OH) and MOOH, UV-
2
vis spectra of Ni(OH) and NiOOH, additional LSVs of M(OH) for the oxidation of
2
2
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DFF, HMFCA, and FFCA, CVs of thin and thick Co(OH) , and J-t plot of CoOOH at
2
1
.47 V vs. RHE (PDF)
AUTHOR INFORMATION
Corresponding Author
*
E-mail: kschoi@chem.wisc.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This study was supported by University of Wisconsin-Madison.
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References
(
1) Besson, M.; Gallezot, P.; Pinel, C. Conversion of Biomass into Chemicals Over
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2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from
Biomass: Chemistry, Catalysts, and Engineering. Chem. Rev. 2006, 106, 4044-4098.
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(4) Chatterjee, C.; Pong, F.; Sen, A. Chemical Conversion Pathways for Carbohydrates.
Green Chem. 2015, 17, 40-71.
(5) Teong, S. P.; Yi, G.; Zhang, Y. Hydroxymethylfurfural Production from
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(6) Grand View Research, Inc. Furandicarboxylic Acid (FDCA) Market Potential
Analysis and Segment Forecasts to 2020. Available at the following:
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7) Bozell, J. J.; Petersen, G. R. Technology Development for the Production of
Biobased Products from Biorefinery Carbohydrates— the US Department of
Energy’s “Top 10” Revisited. Green Chem. 2010, 12, 539-554.
(
8) Sousa, A. F.; Vilela, C.; Fonseca, A. C.; Matos, M.; Freire, C. S. R.; Gruter, J. F. J.
Coelho, G. -J. M.; Silvestre, A. J. D. Biobased Polyesters and Other Polymers from
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,5-Furandicarboxylic Acid: A Tribute to Furan Excellency. Polym. Chem. 2015, 6,
5961-5983.
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