Synthesis and Evaluation of Graphene Aerogel-Supported MnxFe3?xO4 for Oxygen Reduction in Urea/O2 Fuel Cells

Article abstract of DOI:10.1002/open.201900105

Graphene aerogel-supported manganese ferrite (Mn_xFe_3?xO₄/GAs) and reduced-graphene oxide/manganese ferrite composite (MnFe₂O₄/rGO) were synthesized and studied as cathode catalysts for oxygen reduction reactions in urea/O₂ fuel cells. MnFe₂O₄/GAs exhibited a 3D framework with a continuous macroporous structure. Among the investigated Fe/Mn ratios, the more positive oxygen reduction onset potential was observed with Fe/Mn=2/1. The half-wave potential of MnFe₂O₄/GAs was considerably more positive than that of MnFe₂O₄/rGO and comparable with that of Pt/C, while the stability of MnFe₂O₄/GAs significantly higher than that of Pt/C. The best urea/O₂ fuel cell performance was also observed with the MnFe₂O₄/GAs. The MnFe₂O₄/GAs exhibited an OCV of 0.713 V and a maximum power density of 1.7 mW cm^?2 at 60 °C. Thus, this work shows that 3D structured graphene aerogel-supported MnFe₂O₄ catalysts can be used as an efficient cathode material for alkaline fuel cells.

Full text of DOI:10.1002/open.201900105

DOI: 10.1002/open.201900105
Full Papers
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Synthesis and Evaluation of Graphene Aerogel-Supported
Mn Fe O for Oxygen Reduction in Urea/O Fuel Cells
x
3À x 4
2
[a]
[b]
[a]
[a]
Keyru Serbara Bejigo, Bang Ju Park, Ji Hyeon Kim, and Hyon Hee Yoon*
Graphene aerogel-supported manganese ferrite (Mn Fe O4/
MnFe O /rGO and comparable with that of Pt/C, while the
2 4
x
3À x
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GAs) and reduced-graphene oxide/manganese ferrite compo-
site (MnFe O /rGO) were synthesized and studied as cathode
stability of MnFe O /GAs significantly higher than that of Pt/C.
2 4
The best urea/O fuel cell performance was also observed with
2
4
2
catalysts for oxygen reduction reactions in urea/O fuel cells.
the MnFe O /GAs. The MnFe O /GAs exhibited an OCV of
2
2
4
2
4
À 2
MnFe O /GAs exhibited a 3D framework with a continuous
0.713 V and a maximum power density of 1.7 mWcm at 60°C.
Thus, this work shows that 3D structured graphene aerogel-
supported MnFe O catalysts can be used as an efficient
2
4
macroporous structure. Among the investigated Fe/Mn ratios,
the more positive oxygen reduction onset potential was
observed with Fe/Mn=2/1. The half-wave potential of
MnFe O /GAs was considerably more positive than that of
2
4
cathode material for alkaline fuel cells.
2
4
À
1
. Introduction
OH, HO , and H O as reaction intermediates, which makes it
2
2
2
[
5,6]
slower. Currently, Pt is the most active ORR catalyst. However,
its high cost is a critical barrier for practical implementation. To
reduce Pt consumption, it has been alloyed with non-precious
metals such as Co, Cr, and Ni, which are reported to be efficient
Recently, anion exchange membrane fuel cells (AEMFCs) have
received considerable attention in area of fuel technology with
promising output. AEMFCs have benefits over proton exchange
membrane fuel cells (PEMFCs) as operated in alkaline media,
which boosts oxygen reduction kinetics and allows the use of
non-precious metal catalysts. Other benefits of AEMFCs are
lower fuel cross-over due to the movement of anions against
fuel and fuel flexibility; various fuels such as H , methanol,
[7]
catalyst for ORR. As an alternative to Pt, non-noble catalysts
[8]
for ORR including transition metal oxides, transition metal
[1]
[9]
[10]
nitrides, and their chalcogenides have been studied and
reported as promising catalysts for ORR. Among the new
approaches, oxides of transition metals exhibited outstanding
[2]
2
[11]
ethanol, and glucose can be used in AEMFCs. Urea (CO(NH ) ), is
performance for ORR.
For instance, manganese ferrite
2
2
an industrial product mainly used as an agricultural fertilizer,
can also be used as a fuel in AEMFCs. Urea is a non-toxic, non-
flammable, and biodegradable compound, and is relatively
cheap and convenient to store and transport compared with
hydrogen. Furthermore, urine and urea-containing wastes can
be purified with electricity generation using AEMFCs.
(MnFe O ), which has an inverse spinel structure with multiple
2
4
valance electrons, has been proved to be a good ORR catalyst
in alkaline media. Zhu and coworkers also reported that
manganese-substituted ferrite outperformed over others (Cu-
and Co-substituted ferrite) and was even comparable to Pt in
[3]
[12]
basic media. However, MnFe O is a semi-conductive material
2
4
In AEMFCs, anode reaction oxidizes the fuel with the release
of electrons, which pass through an external circuit, while the
that leads to insufficient performance resulting from poor ion
[13]
and electron transfer. The catalytic activity of MnFe O was
2
4
À
electrolyte membrane allows the transfer of OH produced
improved by integrating it with other materials that are capable
of boosting conductivity in addition to the reduction in
agglomeration of active catalyst. MnFe O -supported conduc-
[4]
from the oxygen reduction reaction (ORR) at the cathode. ORR
is known to be multifaceted owing to its multistep and multi-
2
4
electron transfer behavior involving numerous adsorption/
tive materials such as graphene and polyaniline composites
were studied for ORR and exhibited higher catalytic activity
À
desorption stages for oxygen-containing species such as O, O ,
2
[14]
than MnFe O did.
2
4
Graphene is one of the carbon-based nanomaterials with a
high electrical conductivity, large surface area, and good
mechanical strength, which make it an ideal support for catalyst
materials. Incorporating metal and their oxide nanoparticles
into graphene creates porous networks that enhance both
[
a] K. S. Bejigo, Prof. J. H. Kim, Prof. H. H. Yoon
Department of Chemical and Biological Engineering
Gachon University
1342 Seongnam-daro, Seongnam, S. Korea
E-mail: hhyoon@gachon.ac.kr
[b] Prof. B. J. Park
[15,16]
Department of Electronic Engineering
Gachon University
catalyst activity and its stability.
Graphene-supported tran-
sition metal oxides such as MnCo O and Mn O nanoparticles
2
4
3
4
1342 Seongnam-daro, Seongnam, S. Korea
[
17,18]
exhibited good ORR performance in alkaline media.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.201900105
Recently, graphene aerogel having a three-dimensional meso-
porous structure has attracted the most attention owing to its
high surface area, light weight, and high porosity, which allow
©
201x The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
[19–21]
sufficient electron transfer pathways.
Wang et al. developed
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ferric oxide on a graphene aerogel for ORR, which out-
reflection peak corresponding to the (001) plane. This peak
originates from the inter planner spacing of graphene oxide
due to the presence of different oxygenated functionalities on
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[22]
performed over commercial Pt/C.
In this study, a manganese ferrite-decorated graphene
aerogel (Mn Fe O /GAs) composite was synthesized by a
the surface. In the XRD pattern of MnFe O /GAs (Figure 1a) and
x
3À x
4
2
4
reducing agent-assisted hydrothermal self-assembly process
Mn Fe O /GAs (Suppl. Figure S1), the disappearance of (001)
0.5 2.5 4
and was studied as a cathode material in a urea/O fuel cell. The
reflection peak suggested the reduction of GO into graphene
sheets. Additionally, in the diffraction spectrum of MnFe O /
2
structural and morphological properties of the Mn Fe O /GAs
x
3À x
4
2
4
catalyst were characterized. The electrochemical activity of the
Mn Fe O /GAs-modified electrodes were studied towards ORR
GAs, peaks corresponding to the diffraction planes viz., (220),
[23]
(311), (400), (511), and (440) [JCPDS-10-0319] were also seen.
Both MnFe O /GAs and MnFe O displayed similar diffraction
x
3À x
4
using cyclic voltammetry and linear sweep voltammetry.
2
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In addition, the performances of urea/O fuel cells compris-
pattern. The diffraction peaks of the two samples can be traced
to a face-centered cubic crystal structure, indicating a phase
pure synthesis of MnFe O .
2
ing MnFe O /GAs as a cathode material was evaluated.
2
4
2
4
The functional groups in GO, MnFe O , and MnFe O /GAs
2
4
2
4
were analyzed by FT-IR spectroscopy, as shown Figure 1b. The
2. Results and Discussion
À 1
characteristic peaks of GO appeared at 1730 cm (stretching
À 1
2
.1. Characterization of GO, MnFe O , MnFe O /rGO, and
vibration of C=O), 1622 cm (skeletal stretching vibrations of
2
4
2
4
À 1
MnFe O /GAs
C=C), and 1100 cm (CÀ O stretching vibrations). On the other
2
4
hand, for MnFe O /GAs, the characteristic peak of GO at
2
4
À 1
À 1
The structures and crystallographic phases of graphene oxide
GO), MnFe O , and MnFe O /GAs particles were studied by XRD
1730 cm shifted to 1558 cm , implying that the MnFe O
2 4
(
particles were strongly adsorbed onto the GO surface by
2
4
2
4
[24–26]
as plotted in Figure 1a. The pristine GO showed a characteristic
chemical reduction.
From the FTIR spectrum of MnFe O /
2 4
GAs, the vibration peaks of the most oxygen-containing group
disappeared, indicating that GO reduced during the hydro-
thermal and self-assembly processes; this agrees with the XRD
[27]
result.
Figure 2 shows the SEM images and EDX elemental maps of
MnFe O /GAs and MnFe O /rGO. For MnFe O /GAs, 3D
a
2
4
2
4
2
4
2 4 2 4
Figure 2. SEM images of MnFe O /GAs (a-1, a-2) and MnFe O /rGO (b-1, b-2),
and EDX elemental maps corresponding to SEM images (a-3, b-3).
Figure 1. XRD patterns (a) and FTIR spectra (b) of GO, MnFe
MnFe /GAs.
2 4
O , and
2
O
4
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2 4 2 4
Figure 4. ORR polarization plots of MnFe O /GAs (a) and MnFe O /rGO (b) at
different rpm in O
different potentials) at a scan rate of 20 mVs
2
-saturated 0.1 M KOH (insets: Koutecky-Levich plots at
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À 1
.
Figure 3. CV curves of MnFe O /GAs, MnFe O /rGO, and Pt/C (a) and
2
4
2
4
Mn
x
Fe3À x
O
4
/GAs (b) in O
2
(solid) and N
2
(dashed) saturated 0.1 M KOH
À 1
electrolyte at a scan rate of 20 mVs
.
graphene aerogel framework with a continuous macroporous
structure is clearly seen in Figure 2a. The graphene sheets were
Figure 5. LSV of MnFe O /GAs in O saturated 0.1 M KOH at 900 rpm under
2
4
2
interconnected together forming
a corrugated structures
different temperatures (a), and current density at 0.2 V vs. temperature (b).
(
Suppl. Figure S2). The MnFe O nanoparticles were uniformly
2
4
dispersed over the graphene aerogel matrix. On the other hand,
the SEM images of MnFe O /rGO exhibit uniform-sized MnFe O
4
2
4
2
nanoparticles formed on the graphene surface, as shown in
Figure 2b. In addition, the elemental maps of both the catalysts
revealed uniform distributions of Mn and Fe, and the elemental
spectra showed that the Mn/Fe ratio was close to the
theoretical loading ratio (Suppl. Figure S3).
GAs for the ORR. In addition, ORR onset potentials of MnFe O /
2
4
GAs and commercial Pt/C appeared at a similar position,
suggesting that the catalytic activity of MnFe O /GAs is
2
4
comparable with that of the Pt/C catalyst. In addition, the CV
curves of Mn Fe O /GAs with different x values are shown in
x
3À x
4
The 3D-structured MnFe O /GAs exhibited a high BET sur-
Figure 3b. The results indicated that the presence of Mn oxide
increased the ORR activity, mainly owing to the facilitation of
the adsorption of oxygen by the corresponding redox of Mn
2
4
2
À 1
face area of 169 m g with an average pore size of 3.6 nm,
whereas MnFe O /rGO had a BET surface area of 158 m g with
2
À 1
2
4
[17]
an average pore size of 5.03 nm, as measured by nitrogen
adsorption (Suppl. Figure S4). Catalyst pore sizes of 2–10 nm
range is known to be preferable for electrochemical applica-
tions because not only do these pores increase the active
oxide. Among the studied Fe/Mn ratios, the more positive
ORR onset potential was observed for Fe/Mn=2/1.
The ORR kinetics of the MnFe O /GAs and MnFe O /rGO
2
4
2
4
catalysts were examined by rotating disc electrode (RDE)
measurements with different electrode rotation rates. The half-
wave potential of MnFe O /GAs at 900 rpm was 0.09 V, which
[
28]
reaction sites, they also decrease mass-transfer resistance.
The nitrogen adsorption-desorption isotherms MnFe O /GAs
2
4
2
4
and MnFe O /rGO were observed to be type IV (according to
was shifted positively as compared to that of MnFe O /rGO
2 4
2
4
[29]
IUPAC classification), indicating the presence of mesopores,
(À 0.01 V), further indicating enhanced ORR activity of MnFe O /
2
4
which was consistent with pore size analysis. These isotherms
showed H3 hysteresis loops, suggesting that slit shaped pores
were formed by the aggregation of nonuniform sized and/or
shaped graphene nanosheets.
GAs probably due to its 3D structure. The insets in Figure 4a
and 4b show the Koutecky-Levich (K-L) plots at different
potentials. The linearity and parallel profiles revealed that the
ORR on the catalyst surface of the electrode occurred by the
first-order kinetics and a similar number of electron transfer,
[18]
respectively. From the slope of the K-L plots, the number of
electrons transferred on the catalysts was estimated to be
2
.2. Electrocatalytic Properties of MnFe O NPs, MnFe O /rGO,
2 4 2 4
and MnFe O /GAs
~3.73 for MnFe
2
O
4
/rGO and ~3.97 for MnFe
2
O
4
/GAs, suggesting
2
4
À
À
a 4–e transfer process of ORR as similar to the 4–e process of
[2]
The CV curves of MnFe O /rGO, MnFe O /GAs, and Pt/C
ORR on Pt/C.
2
4
2
4
obtained in O - and N -saturated 0.1 M KOH aqueous solution
Figure 5a presents the ORR polarization curves of the
MnFe O /GAs catalyst at 900 rpm with different temperatures
2
2
are shown in Figure 3a. The reduction peak potentials of
2
4
MnFe O /GAs and MnFe O /rGO appeared at À 0.01 V and
from 25 to 80°C. The current density measured at 0.2 V
increased with temperature up to 60°C and then decreased, as
shown in Figure 5b. According to Arrhenius equation, the ORR
rate enhances with temperature. However, gaseous oxygen
2
4
2
4
À 0.1 V, respectively, indicating a considerable positive potential
shift from MnFe O /rGO to MnFe O /GAs, and thus, a higher
2
4
2
4
catalytic efficiency with a reduced overpotential of MnFe O /
2
4
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Figure 6. Chronoamperometric responses of MnFe
saturated 0.1 M KOH at 0.1 V (a) and Nyquist plots of urea/O fuel cell with
2 4 2
O /GAs and Pt/C in O -
2
2 4 2 4
MnFe O /GAs and MnFe O /rGO cathode catalysts from 10 Hz to 5 MHz
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frequency.
needs to be dissolved in an aqueous KOH solution before it is
used in the ORR; therefore, a high temperature adversely
affected the ORR rate because the solubility of oxygen in water
decreased with temperature.
The stabilities of MnFe O /GAs and Pt/C were studied by
2
4
chronoamperometric measurements at a constant potential of
À 0.1 V, as shown Figure 6a. The MnFe O /GAs showed a slower
2
4
current decay than the commercial Pt/C did. MnFe O /GAs
2
4
retained 77% of its initial current density after 150 min of
continuous running. The deactivation of Pt/C in an alkaline
solution is known to occur by the formation of Pt hydroxide on
[30]
its surface.
Electrochemical impedance spectroscopy measurement was
carried out to further examine ORR on both MnFe O /GAs and
2
4
Figure 7. Performances of urea/O
(MnFe /rGO MnFe
anolyte and humidified O
2
fuel cells with various cathode materials
O /GAs, and Pt/C) in 0.33 M urea in 1.0 M KOH as an
2 4
MnFe O /rGO catalysts, as shown in Figure 6b. From the Nyquist
2
4
2
O
4
,
plots, the charge transfer resistance was estimated from the
diameter of the semicircle. The charge transfer resistance of
°
°
2
as a catholyte at 25 C (a) and 60 C (b).
2
2
MnFe O /rGO was 30.5 Ωcm , and decreased to 17.5 Ωcm for
2
4
MnFe O /GAs, further indicating that the MnFe O /GAs catalyst
2
4
2
4
exhibited better charge-transfer kinetics towards ORR.
deposited on the highly porous 3D network-structured
MnFe O /GAs composite materials. The MnFe O /GAs catalysts
2
4
2
4
exhibited a distinct electrocatalytic activity toward ORR, which
was higher than that of MnFe /rGO, with an enhanced
stability, possibly because of its mesoporous 3D network
structure with a high BET surface area. The MnFe O /GAs
2
.3. Performances of uUea/O Fuel Cells with MnFe O /rGO
O
2 4
2
2
4
and MnFe O /GAs
2
4
2
4
Urea/O fuel cells were fabricated using MnFe O /rGO, MnFe O /
exhibited an OCV of 0.713 V and a maximum power density of
2
2
4
2
4
À 2
GAs, and Pt/C as cathode materials, separatively. The I-V
polarization and power density curves of these cells with 0.33 M
urea in 1.0 M KOH feed as an anolyte and dissolved O bubbled
as a catholyte at different temperatures are shown Figure 7. The
best fuel cell performance was observed for the MnFe O /GAs
1.7 mwcm at 60°C, which was even higher than that of the
commercial Pt/C. The results demonstrated that the 3D
structured graphene aerogel-supported MnFe O can be a
2
2
4
promising ORR catalyst.
2
4
cathode catalyst, mainly because of its mesoporous 3D network
structure with a high BET surface area, as discussed earlier. Experimental Section
MnFe O /GAs exhibited an OCV of 0.713 V and a maximum
2
4
À 2
power density of 1.7 mwcm at 60°C, which was even higher
Synthesis of Graphene Oxide
Graphene oxide (GO) was synthesized from graphite flakes by a
than that of commercial Pt/C.
[31]
modified Hummers process. Briefly, 3.0 g of graphite flakes was
added into 400 mL of concentrated H SO /H PO (9:1). Then, 18 g
2
4
3
4
3. Conclusions
of KMnO
4
(18.0 g) was added and stirred for 12 h at 50°C. After the
reaction completed, it was cooled and poured into an ice water
(400 mL) containing 6 mL of 30% H O . The final product was
2 2
Manganese ferrite decorated on a graphene aerogel was
synthesized and studied as a cathode catalyst for a urea/O fuel
centrifuged, washed to remove excess acid, and freeze-dried at
À 60°C for 72 h.
2
cell. GO was reduced, and MnFe O nanoparticles were
2
4
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Synthesis of Mn Fe O -Decorated Graphene Aerogel
background current was collected by bubbling nitrogen. All current
densities were normalized to the respective surface areas.
x
3À x
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Mn
Fe3À xO was impregnated into a graphene aerogel using metal
4
x
salts and GO as precursors and hydrazine monohydrate as a
reducing agent. First, 140 mg of GO was dispersed in 90 mL of
[20]
deionized (DI) water by sonication for 30 min. To this suspension, Acknowledgements
stoichiometric amounts of Fe(Cl) ·6H O and MnCl ·4H O were
3
2
2
2
added (Suppl. Table S1). The mixture was neutralized using 5 M
NaOH, followed by the addition of 2 mL of hydrazine monohydrate
as a reducing agent and stirred continuously for 30 min. It was then
transferred to a 100-mL autoclave reactor and kept at 180°C under
stationary condition for 12 h. The resulting black hydrogel was
freeze-dried to obtain graphene aerogel-supported manganese
This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology
(2017R1AB4002083).
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ferrite oxides (Mn Fe O /GAs).
x
3À x
4
MnFe O4 nanoparticles (MnFe O4 NPs) were also synthesized by Conflict of Interest
2
2
reducing the precursor salts with hydrazine as described above.
MnFe O supported on reduced GO (MnFe O /rGO) was prepared
2
4
2
4
The authors declare no conflict of interest.
by mixing MnFe O4 NPs with GO suspention and reducing the
2
[
32]
mixture with hydrazine, as described elsewhere.
Keywords: oxygen reduction reaction · urea fuel cell · anode
catalyst · manganese ferrite · graphene aerogel
Preparation of Electrodes and Urea/O Fuel Cell Testing
2
The as-prepared Mn Fe O /GAs and MnFe O /rGO catalyst pow-
x
3À x
4
2
4
ders and the commercial Pt/C (20%, E-TEK) powder were dispersed
in 5% Nafion solution in isopropanol, respectively, and sonicated
for 30 min. The resulting inks were coated on a glassy carbon
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Analysis
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examined using an X-ray diffraction analyzer (XRD, Rigaku D/MAX-
[
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2002, Japan) with Cu K radiation with a wavelength of 1.5406 Å by
α
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°min . The morphology of the samples was studied by scanning
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