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. 2019 May 14;8(5):615–620. doi: 10.1002/open.201900105

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

Keyru Serbara Bejigo 1, Bang Ju Park 2, Ji Hyeon Kim 1, Hyon Hee Yoon 1,✉
PMCID: PMC6515475  PMID: 31114747

Abstract

Graphene aerogel‐supported manganese ferrite (MnxFe3−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.

Keywords: oxygen reduction reaction, urea fuel cell, anode catalyst, manganese ferrite, graphene aerogel

1. Introduction

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.1 Other benefits of AEMFCs are lower fuel cross‐over due to the movement of anions against fuel and fuel flexibility;2 various fuels such as H2, methanol, ethanol, and glucose can be used in AEMFCs. Urea (CO(NH2)2), is 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.3 Furthermore, urine and urea‐containing wastes can be purified with electricity generation using AEMFCs.

In AEMFCs, anode reaction oxidizes the fuel with the release of electrons, which pass through an external circuit, while the electrolyte membrane allows the transfer of OH− produced from the oxygen reduction reaction (ORR) at the cathode.4 ORR is known to be multifaceted owing to its multistep and multi‐electron transfer behavior involving numerous adsorption/desorption stages for oxygen‐containing species such as O, O2 −, OH, HO2 −, and H2O2 as reaction intermediates, which makes it slower.5,6 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 catalyst for ORR.7 As an alternative to Pt, non‐noble catalysts for ORR including transition metal oxides,8 transition metal nitrides,9 and their chalcogenides10 have been studied and reported as promising catalysts for ORR. Among the new approaches, oxides of transition metals exhibited outstanding performance for ORR.11 For instance, manganese ferrite (MnFe2O4), which has an inverse spinel structure with multiple 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 basic media.12 However, MnFe2O4 is a semi‐conductive material that leads to insufficient performance resulting from poor ion and electron transfer.13 The catalytic activity of MnFe2O4 was improved by integrating it with other materials that are capable of boosting conductivity in addition to the reduction in agglomeration of active catalyst. MnFe2O4‐supported conductive materials such as graphene and polyaniline composites were studied for ORR and exhibited higher catalytic activity than MnFe2O4 did.14

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 catalyst activity and its stability.15,16 Graphene‐supported transition metal oxides such as MnCo2O4 and Mn3O4 nanoparticles exhibited good ORR performance in alkaline media.17,18 Recently, graphene aerogel having a three‐dimensional mesoporous structure has attracted the most attention owing to its high surface area, light weight, and high porosity, which allow sufficient electron transfer pathways.19, 20, 21 Wang et al. developed ferric oxide on a graphene aerogel for ORR, which outperformed over commercial Pt/C.22

In this study, a manganese ferrite‐decorated graphene aerogel (MnxFe3−xO4/GAs) composite was synthesized by a reducing agent‐assisted hydrothermal self‐assembly process and was studied as a cathode material in a urea/O2 fuel cell. The structural and morphological properties of the MnxFe3−xO4/GAs catalyst were characterized. The electrochemical activity of the MnxFe3−xO4/GAs‐modified electrodes were studied towards ORR using cyclic voltammetry and linear sweep voltammetry.

In addition, the performances of urea/O2 fuel cells comprising MnFe2O4/GAs as a cathode material was evaluated.

2. Results and Discussion

2.1. Characterization of GO, MnFe2O4, MnFe2O4/rGO, and MnFe2O4/GAs

The structures and crystallographic phases of graphene oxide (GO), MnFe2O4, and MnFe2O4/GAs particles were studied by XRD as plotted in Figure 1a. The pristine GO showed a characteristic 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 the surface. In the XRD pattern of MnFe2O4/GAs (Figure 1a) and Mn0.5Fe2.5O4/GAs (Suppl. Figure S1), the disappearance of (001) reflection peak suggested the reduction of GO into graphene sheets. Additionally, in the diffraction spectrum of MnFe2O4/GAs, peaks corresponding to the diffraction planes viz., (220), (311), (400), (511), and (440) [JCPDS‐10‐0319]23 were also seen. Both MnFe2O4/GAs and MnFe2O4 displayed similar diffraction pattern. The diffraction peaks of the two samples can be traced to a face‐centered cubic crystal structure, indicating a phase pure synthesis of MnFe2O4.

Figure 1.

Figure 1

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XRD patterns (a) and FTIR spectra (b) of GO, MnFe2O4, and MnFe2O4/GAs.

The functional groups in GO, MnFe2O4, and MnFe2O4/GAs were analyzed by FT‐IR spectroscopy, as shown Figure 1b. The characteristic peaks of GO appeared at 1730 cm−1 (stretching vibration of C=O), 1622 cm−1 (skeletal stretching vibrations of C=C), and 1100 cm−1 (C−O stretching vibrations). On the other hand, for MnFe2O4/GAs, the characteristic peak of GO at 1730 cm−1 shifted to 1558 cm−1, implying that the MnFe2O4 particles were strongly adsorbed onto the GO surface by chemical reduction.24, 25, 26 From the FTIR spectrum of MnFe2O4/GAs, the vibration peaks of the most oxygen‐containing group disappeared, indicating that GO reduced during the hydrothermal and self‐assembly processes; this agrees with the XRD result.27

Figure 2 shows the SEM images and EDX elemental maps of MnFe2O4/GAs and MnFe2O4/rGO. For MnFe2O4/GAs, a 3D graphene aerogel framework with a continuous macroporous structure is clearly seen in Figure 2a. The graphene sheets were interconnected together forming a corrugated structures (Suppl. Figure S2). The MnFe2O4 nanoparticles were uniformly dispersed over the graphene aerogel matrix. On the other hand, the SEM images of MnFe2O4/rGO exhibit uniform‐sized MnFe2O4 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).

Figure 2.

Figure 2

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SEM images of MnFe2O4/GAs (a‐1, a‐2) and MnFe2O4/rGO (b‐1, b‐2), and EDX elemental maps corresponding to SEM images (a‐3, b‐3).

The 3D‐structured MnFe2O4/GAs exhibited a high BET surface area of 169 m2 g−1 with an average pore size of 3.6 nm, whereas MnFe2O4/rGO had a BET surface area of 158 m2 g−1 with 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 applications because not only do these pores increase the active reaction sites, they also decrease mass‐transfer resistance.28 The nitrogen adsorption‐desorption isotherms MnFe2O4/GAs and MnFe2O4/rGO were observed to be type IV (according to IUPAC classification), indicating the presence of mesopores,29 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.

2.2. Electrocatalytic Properties of MnFe2O4 NPs, MnFe2O4/rGO, and MnFe2O4/GAs

The CV curves of MnFe2O4/rGO, MnFe2O4/GAs, and Pt/C obtained in O2‐ and N2‐saturated 0.1 M KOH aqueous solution are shown in Figure 3a. The reduction peak potentials of MnFe2O4/GAs and MnFe2O4/rGO appeared at −0.01 V and −0.1 V, respectively, indicating a considerable positive potential shift from MnFe2O4/rGO to MnFe2O4/GAs, and thus, a higher catalytic efficiency with a reduced overpotential of MnFe2O4/GAs for the ORR. In addition, ORR onset potentials of MnFe2O4/GAs and commercial Pt/C appeared at a similar position, suggesting that the catalytic activity of MnFe2O4/GAs is comparable with that of the Pt/C catalyst. In addition, the CV curves of MnxFe3−xO4/GAs with different x values are shown in 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 oxide.17 Among the studied Fe/Mn ratios, the more positive ORR onset potential was observed for Fe/Mn=2/1.

Figure 3.

Figure 3

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CV curves of MnFe2O4/GAs, MnFe2O4/rGO, and Pt/C (a) and MnxFe3−xO4/GAs (b) in O2 (solid) and N2 (dashed) saturated 0.1 M KOH electrolyte at a scan rate of 20 mVs−1.

The ORR kinetics of the MnFe2O4/GAs and MnFe2O4/rGO catalysts were examined by rotating disc electrode (RDE) measurements with different electrode rotation rates. The half‐wave potential of MnFe2O4/GAs at 900 rpm was 0.09 V, which was shifted positively as compared to that of MnFe2O4/rGO (−0.01 V), further indicating enhanced ORR activity of MnFe2O4/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, respectively.18 From the slope of the K‐L plots, the number of electrons transferred on the catalysts was estimated to be ∼3.73 for MnFe2O4/rGO and ∼3.97 for MnFe2O4/GAs, suggesting a 4–e− transfer process of ORR as similar to the 4–e− process of ORR on Pt/C.2

Figure 4.

Figure 4

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ORR polarization plots of MnFe2O4/GAs (a) and MnFe2O4/rGO (b) at different rpm in O2‐saturated 0.1 M KOH (insets: Koutecky‐Levich plots at different potentials) at a scan rate of 20 mV s−1.

Figure 5a presents the ORR polarization curves of the MnFe2O4/GAs catalyst at 900 rpm with different temperatures 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 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.

Figure 5.

Figure 5

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LSV of MnFe2O4/GAs in O2 saturated 0.1 M KOH at 900 rpm under different temperatures (a), and current density at 0.2 V vs. temperature (b).

The stabilities of MnFe2O4/GAs and Pt/C were studied by chronoamperometric measurements at a constant potential of −0.1 V, as shown Figure 6a. The MnFe2O4/GAs showed a slower current decay than the commercial Pt/C did. MnFe2O4/GAs 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 its surface.30

Figure 6.

Figure 6

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Chronoamperometric responses of MnFe2O4/GAs and Pt/C in O2‐saturated 0.1 M KOH at 0.1 V (a) and Nyquist plots of urea/O2 fuel cell with MnFe2O4/GAs and MnFe2O4/rGO cathode catalysts from 10 Hz to 5 MHz frequency.

Electrochemical impedance spectroscopy measurement was carried out to further examine ORR on both MnFe2O4/GAs and MnFe2O4/rGO catalysts, as shown in Figure 6b. From the Nyquist plots, the charge transfer resistance was estimated from the diameter of the semicircle. The charge transfer resistance of MnFe2O4/rGO was 30.5 Ω cm2, and decreased to 17.5 Ω cm2 for MnFe2O4/GAs, further indicating that the MnFe2O4/GAs catalyst exhibited better charge‐transfer kinetics towards ORR.

2.3. Performances of uUea/O2 Fuel Cells with MnFe2O4/rGO and MnFe2O4/GAs

Urea/O2 fuel cells were fabricated using MnFe2O4/rGO, MnFe2O4/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 O2 bubbled as a catholyte at different temperatures are shown Figure 7. The best fuel cell performance was observed for the MnFe2O4/GAs cathode catalyst, mainly because of its mesoporous 3D network structure with a high BET surface area, as discussed earlier. MnFe2O4/GAs exhibited an OCV of 0.713 V and a maximum power density of 1.7 mw cm−2 at 60 °C, which was even higher than that of commercial Pt/C.

Figure 7.

Figure 7

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Performances of urea/O2 fuel cells with various cathode materials (MnFe2O4/rGO, MnFe2O4/GAs, and Pt/C) in 0.33 M urea in 1.0 M KOH as an anolyte and humidified O2 as a catholyte at 25 °C (a) and 60 °C (b).

3. Conclusions

Manganese ferrite decorated on a graphene aerogel was synthesized and studied as a cathode catalyst for a urea/O2 fuel cell. GO was reduced, and MnFe2O4 nanoparticles were deposited on the highly porous 3D network‐structured MnFe2O4/GAs composite materials. The MnFe2O4/GAs catalysts exhibited a distinct electrocatalytic activity toward ORR, which was higher than that of MnFe2O4/rGO, with an enhanced stability, possibly because of its mesoporous 3D network structure with a high BET surface area. The MnFe2O4/GAs exhibited an OCV of 0.713 V and a maximum power density of 1.7 mw cm−2 at 60 °C, which was even higher than that of the commercial Pt/C. The results demonstrated that the 3D structured graphene aerogel‐supported MnFe2O4 can be a promising ORR catalyst.

Experimental Section

Synthesis of Graphene Oxide

Graphene oxide (GO) was synthesized from graphite flakes by a modified Hummers process.31 Briefly, 3.0 g of graphite flakes was added into 400 mL of concentrated H2SO4/H3PO4 (9 : 1). Then, 18 g of KMnO4 (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 % H2O2. The final product was centrifuged, washed to remove excess acid, and freeze‐dried at −60 °C for 72 h.

Synthesis of MnxFe3−xO4‐Decorated Graphene Aerogel

MnxFe3−xO4 was impregnated into a graphene aerogel using metal salts and GO as precursors and hydrazine monohydrate as a reducing agent.20 First, 140 mg of GO was dispersed in 90 mL of deionized (DI) water by sonication for 30 min. To this suspension, stoichiometric amounts of Fe(Cl)3 ⋅ 6H2O and MnCl2 ⋅ 4H2O were 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 ferrite oxides (MnxFe3−xO4/GAs).

MnFe2O4 nanoparticles (MnFe2O4 NPs) were also synthesized by reducing the precursor salts with hydrazine as described above. MnFe2O4 supported on reduced GO (MnFe2O4/rGO) was prepared by mixing MnFe2O4 NPs with GO suspention and reducing the mixture with hydrazine, as described elsewhere.32

Preparation of Electrodes and Urea/O2 Fuel Cell Testing

The as‐prepared MnxFe3−xO4/GAs and MnFe2O4/rGO catalyst powders 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 electrode with a loading of 20 μg cm−2 and the electrochemical properties were measured. The catalyst ink was also coated on a 5.0‐cm2 carbon paper to prepare the cathode with a catalyst loading of 1 mg cm−2, and the anode was prepared using a commercial Ni/C (20 %, E‐Tek) with the same loading. An anion exchange membrane (AEM; Fumasep FAA‐3‐PK‐130, Germany) was used as a polymer electrolyte separating the anode and cathode compartments. Membrane electrode assemblies (MEAs) for fuel cell tests were fabricated from both the electrodes and AEM by hot‐pressing. A single‐cell bipolar plate was set using graphite with serpentine flow channels. A urea solution of 0.33 M in 1 M KOH was pumped into the anode side by a peristaltic pump at 2 mL min−1, and humidified O2 was supplied to the cathode.

Analysis

The crystallographic phases and structures of the samples were examined using an X‐ray diffraction analyzer (XRD, Rigaku D/MAX‐2002, Japan) with Cu Kα radiation with a wavelength of 1.5406 Å by scanning the samples in the 2Θ range of 5° to 80° at a rate of 2° min−1. The morphology of the samples was studied by scanning electron microscopy (SEM, Hitachs‐4700, Japan). Functional groups in the powder were analyzed using a Fourier transform infrared (FTIR) spectrometer (Bruker, Saarbrucken, Germany). The Brunauer‐Emmett‐Telle (BET) surface area was measured from nitrogen adsorption and desorption isotherms, which were recorded at 77 K after degassing the analyte at 250 °C with a surface area analyzer (Micromeritics ASAP 2020, USA).

The ORR catalytic activities of the prepared samples were measured by cyclic voltammetry (CV), chronoamperometry (CA), and linear sweep voltammetry (LSV) by using a potentiostat (Biologic Sp‐240) and a rotating ring disc electrode apparatus (RRDE‐3A, ALS Company, Japan) with a three‐electrode configuration. A glassy carbon‐supported active material was used as the working electrode, Ag/AgCl filled with saturated KCl was used as the reference electrode, and Pt wire was used as the counter electrode. CV measurements were carried out with a supply of oxygen, while background current was collected by bubbling nitrogen. All current densities were normalized to the respective surface areas.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

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Acknowledgements

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).

K. S. Bejigo, B. J. Park, J. H. Kim, H. H. Yoon, ChemistryOpen 2019, 8, 615.

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Supplementary

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