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Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene
G Model EA 24454 No. of Pages 8
Electrochimica Acta xxx (2015) xxx–xxx
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene Willyam R.P. Barros a,b , Qiliang Wei b , Gaixia Zhang b , Shuhui Sun b,1, Marcos R.V. Lanza a,1, Ana C. Tavares b,1, * a b
Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-Carlense 400, São Carlos, SP, 13566-590, Brazil Institut National de la Recherche Scientifique – Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 August 2014 Received in revised form 20 February 2015 Accepted 20 February 2015 Available online xxx
Fe3O4 nanoparticles supported on graphene (Fe3O4/graphene) and Printex carbon (Fe3O4/Printex) are reported as promising catalysts for oxygen reduction reaction (ORR) to H2O2 in alkaline medium. The catalysts were synthesized by a simple precipitation method using NaBH4 as reducing agent to favor the formation of the magnetite phase. The structure and morphology of the catalysts was evaluated by x-ray diffraction, transmission electron microscopy and small angle electron diffraction. The electrocatalytic activity towards ORR was investigated by means of cyclic and linear voltammetries in 1 mol L1 KOH. The linear polarization curves highlighted the synergetic effect between the oxide and the carbon supports. Analysis of the polarization curves showed that electro-catalytic activity of the materials towards oxygen reduction expressed by the current density at 0.3 V vs SCE increases from Printex (0.29 mA cm2) < Fe3O4/Printex (0.38 mA cm2) < graphene (0.85 mA cm2) < Fe3O4/graphene (1.12 mA cm2). The number of exchanged electrons was close to 2.7 for both catalysts, and the % H2O2 electro-generated above 60% in the 0.2 to 0.7 V (vs SCE) potential range. Furthermore, both Fe3O4/graphene and Fe3O4/ Printex catalysts show excellent durability. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Fe3O4 oxygen reduction reaction 2 electron pathway Printex 6L carbon graphene
1. Introduction Electrochemical reactions involving oxygen reduction reaction (ORR) occurring at an electrocatalytic surface [1–4], particularly in acidic medium [2,3], aiming at energy [5,6] and environmental [7–11] applications have been extensively studied in literature. Hydrogen peroxide (H2O2) is an important chemical used in industry namely for waste water treatment and pulp- and paperbleaching [12–15]. In this context, ORR in alkaline medium to generate H2O2 involving two electrons pathway (Eq. (1)) is attracting increasing interest, because the shift in the potential to less negative values with respect to acid medium, leads to a decrease in the energy demand and cost of the process. O2 + H2O + 2 e ! HO2 + OH E = 0.065 V (vs. SHE) (1) Catalysts for the ORR are at the core of the process. It is well known that carbon materials are excellent catalysts for H2O2 generation in alkaline medium [15–19]. Among them, graphene
* Corresponding author. Tel.: +1 514 4096327. E-mail address: [email protected] (A.C. Tavares). ISE Member
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has emerged as a very promising carbon material because of its high electrical conductivity, high surface area, and excellent chemical and environmental stability. Also, ORR on graphene in alkaline medium proceeds with the two electrons pathway [19]. On the other hand, recent studies on low cost carbon blacks demonstrated that Printex 6L carbon is also an excellent electrode material for the O2/H2O2 process in alkaline solution [17]. In the past decades, transition metal oxides (Co3O4, MnO2, Fe3O4 etc.) have also been widely studied as ORR catalysts in alkaline solution because of their low cost, abundance and environmental compatibility [20–24]. Nevertheless, pristine transition metal oxides usually exhibit limited ORR activities and insufficient durability, probably due to their low electrical conductivity and agglomeration during operation. The loading of transition metal oxides on conducting high surface carbon supports (carbon blacks, activated carbon, porous carbon, carbon nanotubes, graphene and N-doped graphene) has been used to overcome these limitations [19,21,25– 31]. In this way, considering that both carbon materials and transition metal oxides have considerable ORR activities, it is attractive to study the ORR performances of their composites. To date, most of the works report on metal oxide/carbon composites as catalysts for the 4e ORR to replace noble metals in fuel cells and for Zn-air batteries. Less efforts have been put on the development
http://dx.doi.org/10.1016/j.electacta.2015.02.175 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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of highly efficient catalysts for 2e ORR [19,25,26] which are highly desirable for the H2O2 electro-synthesis with high yield and minimal energy consumption. In this work, we demonstrate the fabrication of Fe3O4 supported on graphene (Fe3O4/graphene) and on Printex 6L (Fe3O4/Printex) as catalysts for the 2e ORR in alkaline medium. The Fe3O4 (magnetite) unsupported and supported nanoparticles (NPs) were synthesized by a modified precipitation method, where for the first time NaBH4 was used as the reducing agent to promote the formation of magnetite versus maghemite (g-Fe2O3) [32,33]. The addition of Fe3O4 NPs to the carbon support enhances the production of H2O2, and the percentage of electro-generated H2O2 was higher on Fe3O4/Printex catalyst. In terms of the effect of the carbon support on the Fe3O4 NPs for the ORR, we found that Fe3O4/ graphene is more active for ORR than Fe3O4/Printex as it shows a more positive onset potential and higher current density. Furthermore, both Fe3O4/Printex and Fe3O4/graphene catalysts show excellent durability.
neodymium magnet. After decantation, the NPs were washed several times with deionized water until reach pH 7.5-7.0 and then dried in an oven at 100 C under N2 flow for 3 h. For the synthesis of Fe3O4/carbon catalysts, 0.9 g of carbon were dispersed in 10 mL of water, and 0.054 g of FeCl2.4H2O and 0.150 g of FeCl3.6H2O were dissolved in the dispersion. The precipitation, washing and separation steps proceeded as described above for the unsupported NPs. However, prior to the addition of the chlorides precursors, the carbon/water suspensions were sonicated for 10 min in the case of Printex carbon and 30 min in the case of graphene, to get good dispersion of carbon. The Fe3O4 mass loading on Printex and graphene was determined gravimetrically by burning the samples in air at 800 C, and was found to be 5 wt%.
2. Experimental 2.1. Materials Graphite powder (+100 mesh, 99.99%), iron (II) chloride (FeCl2.4H2O, 99.0%), iron (III) chloride (FeCl3.6H2O, 98.0%), sodium borohydride (NaBH4, 98.0%), sulphuric acid (H2SO4, 96.0%), potassium permanganate (KMnO4, 99.0%), sodium nitrate (NaNO3, 99.0%), hydrogen peroxide (H2O2, 30 wt% aqueous solution) were purchased from Sigma–Aldrich; Printex 6L carbon was purchased from Evonik-Degussa; sodium hydroxide (NaOH, 95.0%) and potassium hydroxide (KOH, 85.0%) were purchased from Fischer. Nafion solution (5 wt%) was purchased from Ion Power. All chemicals were used as received and solutions were prepared using deionized water (Millipore Milli-Q, 18.2 MV cm). 2.2. Synthesis of graphene nanosheets The graphene nanosheets support was prepared using the procedure described previously [34] which involves graphite oxidation, thermal exfoliation, and chemical reduction. Natural flake graphite (+100 mesh) was used as the starting material. In detail, 1 g of natural graphite powder was first stirred in 23 mL of concentrated H2SO4, followed by the addition of 0.5 g of NaNO3 at room temperature. The stirring lasted for 16 h and after that the mixture was cooled down to 0 C. Then, 3 g of KMnO4 were added into the solution. After 2 h, a green slurry was formed at around 35 C, which was then stirred for another 3 h. Then, 46 mL of H2O was slowly added into the solution at around 98 C. The suspension was kept at this temperature for 30 min before it was further diluted with another addition of water and 3 g of H2O2. The suspension was subsequently filtered and washed until reaching a neutral pH, then dried in a vacuum oven at 60 C to obtain graphite oxide (GO). To obtain graphene, the as-synthesized GO was heated at around 1050 C in Ar atmosphere for 30 s in a tube furnace. 2.3. Synthesis of Fe3O4 nanoparticles and Fe3O4–carbon supported catalysts The Fe3O4 NPs were synthesized using a simple co-precipitation method. FeCl2.4H2O (5.34 g) and FeCl3.6H2O (14.53 g) were dissolved in 100 mL of deionized water and 2.0 mL of 0.5 mol L1 NaBH4 solution were carefully added at room temperature. After 20 min. under stirring, the temperature was increased to 90 C and 100 mL of 0.5 mol L1 NaOH (pre-heated to 90 C) were added to the solution occurring the precipitation the Fe3O4 NPs. The separation of the NPs from the solution was performed using a
Fig. 1. (A) XRD pattern, (B) and (C) TEM images at different magnifications of unsupported Fe3O4 nanoparticles. The insert in (B) shows the SAED pattern, and the insert in (C) shows the HR-TEM image with a scale bar of 2 nm.
Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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2.4. Physicochemical characterization of the catalysts XRD characterization of the samples was performed using Bruker D8 Advance X-ray diffractometer equipped with CuKa source (l = 1.5406 Å) operating at 40 kV and 40 mA. The patterns were collected in the 20 to 80 (2u) range with a 0.01 step mode and 8 s step duration. The values of cell parameter (a) and the full weight half medium (FWHM) were calculated using EVA V14 software. N2-physisorption analysis was performed using Quantachrome Instrument Autosorb-1 and the isotherm was measured at 196 C. The samples were weighed, placed on the analysis port, and prior to analysis they were heated at 200 C for 2 h under vacuum. The specific surface area data was obtained using the BET theory. The catalysts morphology was investigated with a transmission electron microscope (TEM) JEOS-2100F operated at 200 kV (Center for Characterization of Microscopic Materials, at École Polytechnique de Montréal). Selected area electron diffraction (SAED) patterns were taken on selected areas under TEM for structural analysis of the catalysts. The TEM samples were prepared by dipping copper grids into dispersions of the sample powders in methanol. 2.5. Electrochemical studies Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements were performed using a conventional threeelectrode glass cell at room temperature and a computer controlled Autolab 302 N PGSTAT potentiostat coupled to a Pine
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Instruments AFMSRCE rotator. The reference electrode was a saturated calomel electrode (SCE) and a coiled Pt wire was used as counter electrode. Cyclic voltammograms were recorded in N2- or O2-saturated 1 mol L1 KOH solution at 50 mV s1. The O2 was bubbled for at least 30 min before starting the tests and an O2 atmosphere was kept during the measurements. Linear sweep voltammograms (LV) were also recorded in N2- or O2-saturated electrolyte at 10 mV s1 from the highest to the lowest potential, and at various rotation speeds (from 0 to 2500 rpm). Catalyst inks were prepared by dispersing 1 mg of catalyst (unsupported Fe3O4, graphene, Printex, Fe3O4/Printex and Fe3O4/ graphene) in 1 mL of isopropyl alcohol, followed by sonication for 10 min. Then, 5 mL of the catalyst ink were carefully drop-casted on the surface of a clean glassy carbon (GC) disk electrode (5 mm diameter, PineChem Inc) and dried under N2 flow. This operation was repeated 2 more times to reach a catalyst loading of 76.5 mg/ cm2. Finally 15 mL of a Nafion solution (diluted 1:100) were dropcasted on the top of the catalyst layer and allowed to dry. Prior to the catalyst deposition, the surface of the GC electrode was polished to mirror finish using 1.0 and 0.05 mm alumina slurries, and subsequently cleaned in an ultrasonic bath in deionised water for 5 min. The number of electrons exchanged during ORR and the percentage of peroxide produced by the reaction were evaluated for each catalyst by rotating ring disk electrode (RRDE) measurements. The diameter of the glassy carbon disk was 5.61 mm, and it was surrounded by a Pt ring (6.25 and 7.92 mm inner and outer diameters, respectively). The catalytic film was formed on the disk electrode according to the procedure described above for the RDE
Fig. 2. TEM images of Fe3O4 nanoparticles (dark spots) on different carbon supports at different magnifications. (A, C) Fe3O4/Printex catalyst; the inserts in (A) and (C) show the SAED patterns of the catalyst and the HR-TEM image of Fe3O4 nanoparticle, respectively. (B, D) Fe3O4/graphene catalysts; inserts in (B) and (D) show the SAED patterns of the catalyst and the HR-TEM image of Fe3O4 nanoparticles, respectively. Both scale bars in the inserts in (C) and (D) are 2 nm.
Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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measurements but a total 19 mL catalyst ink and 19 mL Nafion solution were deposited on the electrode in order to reach the same loadings as in the RDE. The ring potential was set at 1.3 V and the voltammograms were recorded at 10 mV s1 with a Pine potentiostat (model AFRDE4). The RRDE was rotated at 1600 rpm. The collection efficiency of the ring-disk electrode was N = 0.37. The equations used to calculate n (the number of electrons transferred during ORR) and %H2O2 (the percentage of H2O2 released during ORR) are the following: n¼
4ID ID þ IR =N
%H2 O2 ¼ 100
(2) 2IR =N ID þ IR =N
(3)
where ID is the current at the disk, IR the current at the ring and N = 0.37 is the RRDE collection efficiency. The stability of Fe3O4/Printex and Fe3O4/graphene catalysts was tested on fresh electrodes by chronoamperometry at 0.3 V for 5.5 hours in O2-satured 1 mol L1 KOH and at a rotation speed of 1600 rpm. Before and after the tests, LVs were recorded in a N2saturated and an O2-saturated 1 mol L1 KOH electrolyte at a scan rate of 10 mV s1 and 1600 rpm. Unless otherwise stated, all O2 reduction linear polarization curves presented in this work were corrected by subtracting of the background current recorded in N2-saturated 1 mol L1 KOH.
3. Results and discussion The X-ray diffraction (XRD) of the unsupported Fe3O4 is presented in Fig. 1A. The XRD patterns confirmed the formation of Fe3O4 (JCPDS #19-0629). The lattice parameter calculated from (3 11) crystalline plane is a = 8.397 Å and in good agreement with the values reported in the literature for bulk stoichiometric Fe3O4 [33]. The unsupported oxide is composed of agglomerates of cubicshaped NPs with a particle size ranging from 5 to 40 nm, with an average particle size around 20 nm, as shown in Fig. 1B. A typical high-resolution TEM (HR-TEM) image is shown in Fig. 1C. Lattice fringes with spacing of 2.97 Å assigned to (2 2 0) plane are clearly observed in the inset. As demonstrated elsewhere, there is a variation of the composition of the iron oxide nanoparticles with the particle size, being this oxide better described as a solid solution of both magnetite (Fe3O4,a 8.392 Å) and g -maghemite (g -Fe2O3, a 8.342 Å) [33]. In general, for nanoparticles smaller than 40 nm the lattice parameter of the iron oxide decreases with the decrease of the particle size because more Fe2O3 is formed [32,33]. Interestingly, with the modified method used in this work the nanoparticles with size 20 nm in average have a lattice parameter close to bulk Fe3O4. The specific surface area of the Fe3O4 NPs, determined through N2-physisorption analysis (isotherm not shown), is 70 m2 g1 which compares very well with the value (72 m2 g1) estimated from the average crystallite size of 16 nm estimated from XRD.
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Fig. 3. Cyclic voltammograms of (A) Fe3O4 nanoparticles, (B) Printex 6 L carbon, (C) graphene, recorded in N2- and O2- saturated 1 mol L1 KOH at 21 C and 50 mV s1. (D) Comparison of the O2 reduction waves recorded for the three materials after subtraction of the background current recorded in N2-saturated solution.
Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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TEM observations of Fe3O4/Printex sample revealed an uneven distribution of the oxide NPs on the carbon and the formation of localized oxide clusters as illustrated in Fig. 2A. In contrast, the distribution of the oxide NPs on the graphene sheets was more uniform as shown in Fig. 2B. Printex carbon has a much lower
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specific surface area (230 m2 g1 measured by BET analysis) compared to graphene (450 m2 g1 by BET analysis) and are morphologically quite different (bundles vs sheets). Most probably, the larger number of functional groups on the surface of the graphene sheets with respect to Printex provides more anchoring sites for the nucleation and growth of the Fe3O4 NPs. The SAED confirmed the formation of Fe3O4-type structure for both samples, as illustrated by the respective patterns and lattice fringes (2.97 Å for (2 2 0), 4.85 Å for (111), and 2.53 Å for (3 11)) in the insets of Fig. 2. The HRTEM revealed that the shape and size of the Fe3O4 NPs are not the same on both samples. In the case of Fe3O4/Printex, the particles have an ill-defined contour and their sizes are typically equal or bellow than 10 nm. In the case of Fe3O4/ graphene, the NPs have a more squared-shape as the unsupported oxide and their average size varies between 20 and 30 nm. Shape change from cubic to spherical accompanying a particle size reduction was previously observed for unsupported Fe3O4 NPs [33]. Figs. 3A to C show the CV recorded in N2 and O2- saturated 1 mol L1 KOH for unsupported Fe3O4, Printex carbon and graphene at 50 mV s1. As shown in Fig. 3A, the CV of unsupported Fe3O4 in N2-saturated 1 mol L1 KOH has two cathodic ( 1.1 V and 0.72 V) and two anodic peaks ( 0.96 V and 0.71 V). The cathodic peaks are related to the reduction of surface Fe (III)
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Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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oxide/hydroxide to surface Fe (II) hydroxide and to metallic Fe. Accordingly, the two anodic peaks develop upon the reoxidation of the species formed during the cathodic sweep. Inner Fe (II) ions are oxidized to Fe (III) ions above 0.6 V, and a layer of Fe2O3 is formed above this potential [22,35]. Oxygen reduction on Fe3O4 starts at 0.32 V, and according to Vago et al. [22] at this potential the Fe2O3 surface oxide is reduced. The CVs of Printex 6L (Fig. 3B) and graphene (Fig. 3C) in the O2 free electrolyte solution are featureless as expected [17]. The
80
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current and the charge under the voltammogram recorded for the graphene electrode (qtotal = 0.472 mC) are much higher than for Printex (qtotal = 0.0453 mC) electrode as expected from its larger surface area. In O2-saturated 1 mol L1 KOH, the onset potential for O2 reduction is 0.23 V for Printex and 0.16 V for graphene, and the peak currents 0.76 mA cm2 and 1.32 mA cm2 suggesting the higher ORR activity of the latter. Fig. 3D compares the oxygen reduction waves after the subtraction of the background current recorded in N2-saturated 1 mol L1 KOH. Among the three materials, graphene shows the most positive on-set (0.08 V) and peak (0.28 V) potentials and 0.68 mA cm2 peak current. For Printex, the on-set and peak potentials at 0.19 V and 0.36 V, respectively, and 0.70 mA cm2 peak current. Finally, the unsupported Fe3O4 shows the on-set and
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Fig. 6. Rotating ring disk experiments for graphene, Printex, Fe3O4, Fe3O4/Printex and Fe3O4/graphene in O2–saturated 1 mol L1 KOH at 10 mV s1 and at a rotation rate of 1600 rpm. (A) ring (top) and disk (bottom) current (corrected for the current in N2-saturated 1 mol L1 KOH). (B) Percentage of produced peroxide and (C) number of exchanged electrons as a function of the applied potential. The polarization curves recorded for the glassy carbon substrate are included for reference.
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Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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peak potentials at 0.27 V and 0.47 V, respectively, and a peak current of 0.26 mA cm2. The CVs recorded at 50 mV s1 for the Fe3O4/Printex and Fe3O4/ graphene composites are displayed in Figs. 4A to C. The voltammograms are similar to those recorded for the respective pristine carbon substrates and to the ones reported earlier by Wu et al. for 46.2 wt% Fe3O4 supported on nitrogen doped graphene aerogels in 0.1 mol L1 KOH [30]. The kinetic of ORR on Printex, graphene, Fe3O4/Printex and Fe3O4/graphene was examined by recording LVs at 10 mV s1 using the rotating disk electrode. Fig. 5 shows the series of polarization curves obtained at various rotation rates with Fe3O4/ Printex and Fe3O4/graphene. Similar profiles were recorded for Printex 6L and graphene. The profiles consist of two reduction waves and are similar to those reported by Wu et al. for 46.2 wt% Fe3O4 supported on nitrogen doped graphene aerogels in 0.1 mol L1 KOH [30]. The limiting current appears between 0.4 and .7 V, depending on the rotation speed, followed by a second reduction wave at more negative potential. This type of profile is a strong indication that ORR is proceeding through the 2 electron process with the formation of hydroperoxide anion as intermediate (Eq. (1)) followed by it reduction to hydroxyl anion (Eq. (2)) [5], [30]. HO2 + H2O + 2 e ! 3 OH (4) Fig. 6A compares the RRDE polarization curves obtained at 1600 rpm for the four catalysts and the unsupported Fe3O4. The polarization curves of the GC electrode were also included as a reference for the two electron process [36]. The electro-catalytic activity expressed by the disk current density at 0.3 V increases in the following order: Printex (0.29 mA cm2) < Fe3O4/Printex (0.38 mA cm2) < graphene (0.85 mA cm2) < Fe3O4/graphene 2 (1.12 mA cm ). The disk currents at 0.3 V for unsupported Fe3O4 and the GC electrode are negligible. The ring current, proportional to the produced hydrogen peroxide, increases in the same order. With respect to the GC electrode, the ring current of the four catalysts is much higher and appears at significantly lower overvoltage. On the other hand, the ring current for the unsupported Fe3O4 is lower than that of the GC electrode, which emphasizes the poor capability of these nanoparticles for the electro-generation of hydrogen peroxide. As evidenced in Figs. 6B and C, all catalysts are characterized by a high peroxide yield (55%) and a number of exchanged electrons close to 2.7 in the potential range comprised between 0.4 and 0.8 V vs SCE. Moreover, the functionalization of graphene and Printex carbons with the Fe3O4 NPs further increases the H2O2 yield with respect to the pristine carbons. For example, at 0.6 V the % H2O2 is 68% for Fe3O4/Printex, 62% for Fe3O4/graphene, while 59% for the two carbon supports. For potentials more negative than 0.8 V the 4 electron process starts to dominate and the % H2O2 decreases. It should be noted that Printex 6L keeps a % H2O2 close to 50% in this potential range. Finally, the performance of Fe3O4/Printex and Fe3O4/graphene catalysts was tested at constant potential for 5.5 hours. Fig. 7A displays the chronoamperograms recorded under RDE mode (1600 rpm) at 0.3 V. After an initial transient period, the current was remarkable stable in both cases. The LVs recorded for Fe3O4/ Printex and Fe3O4/graphene before and after the stability test are shown in Fig. 7B and C. The LVs overlap completely in the activation and mixed control regions, but the limiting current for Fe3O4/ graphene was even higher after the stability test. Clearly, these results indicate that Fe3O4/graphene and Fe3O4/Printex are promising catalysts for H2O2 production in-situ. The former is preferred in terms of lower on-set potential and higher current, whereas the second is preferred in terms of selectivity towards
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H2O2 electrogeneration. The influence of other parameters such as electrolyte concentration (pH) and catalyst loading on the selectivity of the oxygen reduction reaction towards the H2O2 formation [37,38] will be subject of a future study. 4. Conclusions We have successfully fabricated unsupported Fe3O4, Fe3O4/ Printex and Fe3O4/graphene by a simple precipitation method using NaBH4 as reducing agent. Fe3O4 NPs show a better distribution on graphene (specific surface area 450 m2 g1) than on Printex (specific surface area 230 m2 g1). Both Fe3O4/Printex and Fe3O4/graphene reduce oxygen through the 2-electron pathway in 1 mol L1 KOH and are very promising catalysts for H2O2 production in alkaline medium. Fe3O4/graphene exhibits higher catalytic activity for the ORR, including a more positive onset potential and higher current density, as well as excellent durability. However, Fe3O4/Printex exhibits higher selectivity for H2O2 electrogeneration. Acknowledgements The authors wish to acknowledge the “Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)” and “Fonds de Recherche du Québec - Nature et Technologies (FRQNT)” for financial support. Q. Wei gratefully acknowledges the scholarship from China Scholarship Council (CSC). References [1] K. Kinoshita, Carbon: electrochemical and physicochemical properties, Jonh Wiley, New York, 1998. [2] E. Yeager, Electrocatalysts for O2 reduction, Electrochimica Acta 29 (1984) 1527–1537. [3] E. Yeager, Dioxygen electrocatalysis: mechanisms in relation to catalyst structure, Journal of Molecular Catalysis 38 (1986) 5–25. [4] C. Zhang, K. Yanagisawa, H. Tao, A. Onda, T. Shou, S. Kamiya, Oxygen Reduction Activity and Methanol Resistance of Ru-based Catalysts Prepared by Solvothermal Reaction, Catal Lett 142 (2012) 1128–1133. [5] S. Guo, S. Zhang, S. Sun, Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction, Angewandte Chemie International Edition 52 (2013) 8526–8544. [6] Y. Li, H. Dai, Recent advances in zinc-air batteries, Chemical Society Reviews 43 (2014) 5257–5275. [7] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton’s Reaction Chemistry, Chemical Reviews 109 (2009) 6570–6631. [8] B.E. Logan, K. Rabaey, Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies, Science 337 (2012) 686–690. [9] W.R.P. Barros, R.M. Reis, R.S. Rocha, M.R.V. Lanza, Electrogeneration of hydrogen peroxide in acidic medium using gas diffusion electrodes modified with cobalt (II) phthalocyanine, Electrochimica Acta 104 (2013) 12–18. [10] F.L. Silva, R.M. Reis, W.R.P. Barros, R.S. Rocha, M.R.V. Lanza, Electrogeneration of hydrogen peroxide in gas diffusion electrodes: Application of iron (II) phthalocyanine as a modifier of carbon black, Journal of Electroanalytical Chemistry 722–723 (2014) 32–37. [11] J.C. Forti, R.S. Rocha, M.R.V. Lanza, R. Bertazzoli, Electrochemical synthesis of hydrogen peroxide on oxygen-fed graphite/PTFE electrodes modified by 2ethylanthraquinone, Journal of Electroanalytical Chemistry 601 (2007) 63–67. [12] A. Alvarez-Gallegos, D. Pletcher, The removal of low level organics via hydrogen peroxide formed in a reticulated vitreous carbon cathode cell, Part 1. The electrosynthesis of hydrogen peroxide in aqueous acidic solutions, Electrochimica Acta 44 (1998) 853–861. [13] P.C. Foller, R.T. Bombard, Processes for the production of mixtures of caustic soda and hydrogen peroxide via the reduction of oxygen, J Appl Electrochem 25 (1995) 613–627. [14] M. Panizza, G. Cerisola, Electrochemical generation of H2O2 in low ionic strength media on gas diffusion cathode fed with air, Electrochimica Acta 54 (2008) 876–878. [15] A. Wang, A. Bonakdarpour, D.P. Wilkinson, E. Gyenge, Novel organic redox catalyst for the electroreduction of oxygen to hydrogen peroxide, Electrochimica Acta 66 (2012) 222–229. [16] F. Alcaide, E. Brillas, P.-L.s. Cabot, Oxygen Reduction on Uncatalyzed CarbonPTFE Gas Diffusion Cathode in Alkaline Medium, Journal of the Electrochemical Society 149 (2002) E64–E70.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene Willyam R.P. Barros a,b , Qiliang Wei b , Gaixia Zhang b , Shuhui Sun b,1, Marcos R.V. Lanza a,1, Ana C. Tavares b,1, * a b
Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-Carlense 400, São Carlos, SP, 13566-590, Brazil Institut National de la Recherche Scientifique – Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 August 2014 Received in revised form 20 February 2015 Accepted 20 February 2015 Available online xxx
Fe3O4 nanoparticles supported on graphene (Fe3O4/graphene) and Printex carbon (Fe3O4/Printex) are reported as promising catalysts for oxygen reduction reaction (ORR) to H2O2 in alkaline medium. The catalysts were synthesized by a simple precipitation method using NaBH4 as reducing agent to favor the formation of the magnetite phase. The structure and morphology of the catalysts was evaluated by x-ray diffraction, transmission electron microscopy and small angle electron diffraction. The electrocatalytic activity towards ORR was investigated by means of cyclic and linear voltammetries in 1 mol L1 KOH. The linear polarization curves highlighted the synergetic effect between the oxide and the carbon supports. Analysis of the polarization curves showed that electro-catalytic activity of the materials towards oxygen reduction expressed by the current density at 0.3 V vs SCE increases from Printex (0.29 mA cm2) < Fe3O4/Printex (0.38 mA cm2) < graphene (0.85 mA cm2) < Fe3O4/graphene (1.12 mA cm2). The number of exchanged electrons was close to 2.7 for both catalysts, and the % H2O2 electro-generated above 60% in the 0.2 to 0.7 V (vs SCE) potential range. Furthermore, both Fe3O4/graphene and Fe3O4/ Printex catalysts show excellent durability. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Fe3O4 oxygen reduction reaction 2 electron pathway Printex 6L carbon graphene
1. Introduction Electrochemical reactions involving oxygen reduction reaction (ORR) occurring at an electrocatalytic surface [1–4], particularly in acidic medium [2,3], aiming at energy [5,6] and environmental [7–11] applications have been extensively studied in literature. Hydrogen peroxide (H2O2) is an important chemical used in industry namely for waste water treatment and pulp- and paperbleaching [12–15]. In this context, ORR in alkaline medium to generate H2O2 involving two electrons pathway (Eq. (1)) is attracting increasing interest, because the shift in the potential to less negative values with respect to acid medium, leads to a decrease in the energy demand and cost of the process. O2 + H2O + 2 e ! HO2 + OH E = 0.065 V (vs. SHE) (1) Catalysts for the ORR are at the core of the process. It is well known that carbon materials are excellent catalysts for H2O2 generation in alkaline medium [15–19]. Among them, graphene
* Corresponding author. Tel.: +1 514 4096327. E-mail address: [email protected] (A.C. Tavares). ISE Member
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has emerged as a very promising carbon material because of its high electrical conductivity, high surface area, and excellent chemical and environmental stability. Also, ORR on graphene in alkaline medium proceeds with the two electrons pathway [19]. On the other hand, recent studies on low cost carbon blacks demonstrated that Printex 6L carbon is also an excellent electrode material for the O2/H2O2 process in alkaline solution [17]. In the past decades, transition metal oxides (Co3O4, MnO2, Fe3O4 etc.) have also been widely studied as ORR catalysts in alkaline solution because of their low cost, abundance and environmental compatibility [20–24]. Nevertheless, pristine transition metal oxides usually exhibit limited ORR activities and insufficient durability, probably due to their low electrical conductivity and agglomeration during operation. The loading of transition metal oxides on conducting high surface carbon supports (carbon blacks, activated carbon, porous carbon, carbon nanotubes, graphene and N-doped graphene) has been used to overcome these limitations [19,21,25– 31]. In this way, considering that both carbon materials and transition metal oxides have considerable ORR activities, it is attractive to study the ORR performances of their composites. To date, most of the works report on metal oxide/carbon composites as catalysts for the 4e ORR to replace noble metals in fuel cells and for Zn-air batteries. Less efforts have been put on the development
http://dx.doi.org/10.1016/j.electacta.2015.02.175 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
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of highly efficient catalysts for 2e ORR [19,25,26] which are highly desirable for the H2O2 electro-synthesis with high yield and minimal energy consumption. In this work, we demonstrate the fabrication of Fe3O4 supported on graphene (Fe3O4/graphene) and on Printex 6L (Fe3O4/Printex) as catalysts for the 2e ORR in alkaline medium. The Fe3O4 (magnetite) unsupported and supported nanoparticles (NPs) were synthesized by a modified precipitation method, where for the first time NaBH4 was used as the reducing agent to promote the formation of magnetite versus maghemite (g-Fe2O3) [32,33]. The addition of Fe3O4 NPs to the carbon support enhances the production of H2O2, and the percentage of electro-generated H2O2 was higher on Fe3O4/Printex catalyst. In terms of the effect of the carbon support on the Fe3O4 NPs for the ORR, we found that Fe3O4/ graphene is more active for ORR than Fe3O4/Printex as it shows a more positive onset potential and higher current density. Furthermore, both Fe3O4/Printex and Fe3O4/graphene catalysts show excellent durability.
neodymium magnet. After decantation, the NPs were washed several times with deionized water until reach pH 7.5-7.0 and then dried in an oven at 100 C under N2 flow for 3 h. For the synthesis of Fe3O4/carbon catalysts, 0.9 g of carbon were dispersed in 10 mL of water, and 0.054 g of FeCl2.4H2O and 0.150 g of FeCl3.6H2O were dissolved in the dispersion. The precipitation, washing and separation steps proceeded as described above for the unsupported NPs. However, prior to the addition of the chlorides precursors, the carbon/water suspensions were sonicated for 10 min in the case of Printex carbon and 30 min in the case of graphene, to get good dispersion of carbon. The Fe3O4 mass loading on Printex and graphene was determined gravimetrically by burning the samples in air at 800 C, and was found to be 5 wt%.
2. Experimental 2.1. Materials Graphite powder (+100 mesh, 99.99%), iron (II) chloride (FeCl2.4H2O, 99.0%), iron (III) chloride (FeCl3.6H2O, 98.0%), sodium borohydride (NaBH4, 98.0%), sulphuric acid (H2SO4, 96.0%), potassium permanganate (KMnO4, 99.0%), sodium nitrate (NaNO3, 99.0%), hydrogen peroxide (H2O2, 30 wt% aqueous solution) were purchased from Sigma–Aldrich; Printex 6L carbon was purchased from Evonik-Degussa; sodium hydroxide (NaOH, 95.0%) and potassium hydroxide (KOH, 85.0%) were purchased from Fischer. Nafion solution (5 wt%) was purchased from Ion Power. All chemicals were used as received and solutions were prepared using deionized water (Millipore Milli-Q, 18.2 MV cm). 2.2. Synthesis of graphene nanosheets The graphene nanosheets support was prepared using the procedure described previously [34] which involves graphite oxidation, thermal exfoliation, and chemical reduction. Natural flake graphite (+100 mesh) was used as the starting material. In detail, 1 g of natural graphite powder was first stirred in 23 mL of concentrated H2SO4, followed by the addition of 0.5 g of NaNO3 at room temperature. The stirring lasted for 16 h and after that the mixture was cooled down to 0 C. Then, 3 g of KMnO4 were added into the solution. After 2 h, a green slurry was formed at around 35 C, which was then stirred for another 3 h. Then, 46 mL of H2O was slowly added into the solution at around 98 C. The suspension was kept at this temperature for 30 min before it was further diluted with another addition of water and 3 g of H2O2. The suspension was subsequently filtered and washed until reaching a neutral pH, then dried in a vacuum oven at 60 C to obtain graphite oxide (GO). To obtain graphene, the as-synthesized GO was heated at around 1050 C in Ar atmosphere for 30 s in a tube furnace. 2.3. Synthesis of Fe3O4 nanoparticles and Fe3O4–carbon supported catalysts The Fe3O4 NPs were synthesized using a simple co-precipitation method. FeCl2.4H2O (5.34 g) and FeCl3.6H2O (14.53 g) were dissolved in 100 mL of deionized water and 2.0 mL of 0.5 mol L1 NaBH4 solution were carefully added at room temperature. After 20 min. under stirring, the temperature was increased to 90 C and 100 mL of 0.5 mol L1 NaOH (pre-heated to 90 C) were added to the solution occurring the precipitation the Fe3O4 NPs. The separation of the NPs from the solution was performed using a
Fig. 1. (A) XRD pattern, (B) and (C) TEM images at different magnifications of unsupported Fe3O4 nanoparticles. The insert in (B) shows the SAED pattern, and the insert in (C) shows the HR-TEM image with a scale bar of 2 nm.
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2.4. Physicochemical characterization of the catalysts XRD characterization of the samples was performed using Bruker D8 Advance X-ray diffractometer equipped with CuKa source (l = 1.5406 Å) operating at 40 kV and 40 mA. The patterns were collected in the 20 to 80 (2u) range with a 0.01 step mode and 8 s step duration. The values of cell parameter (a) and the full weight half medium (FWHM) were calculated using EVA V14 software. N2-physisorption analysis was performed using Quantachrome Instrument Autosorb-1 and the isotherm was measured at 196 C. The samples were weighed, placed on the analysis port, and prior to analysis they were heated at 200 C for 2 h under vacuum. The specific surface area data was obtained using the BET theory. The catalysts morphology was investigated with a transmission electron microscope (TEM) JEOS-2100F operated at 200 kV (Center for Characterization of Microscopic Materials, at École Polytechnique de Montréal). Selected area electron diffraction (SAED) patterns were taken on selected areas under TEM for structural analysis of the catalysts. The TEM samples were prepared by dipping copper grids into dispersions of the sample powders in methanol. 2.5. Electrochemical studies Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements were performed using a conventional threeelectrode glass cell at room temperature and a computer controlled Autolab 302 N PGSTAT potentiostat coupled to a Pine
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Instruments AFMSRCE rotator. The reference electrode was a saturated calomel electrode (SCE) and a coiled Pt wire was used as counter electrode. Cyclic voltammograms were recorded in N2- or O2-saturated 1 mol L1 KOH solution at 50 mV s1. The O2 was bubbled for at least 30 min before starting the tests and an O2 atmosphere was kept during the measurements. Linear sweep voltammograms (LV) were also recorded in N2- or O2-saturated electrolyte at 10 mV s1 from the highest to the lowest potential, and at various rotation speeds (from 0 to 2500 rpm). Catalyst inks were prepared by dispersing 1 mg of catalyst (unsupported Fe3O4, graphene, Printex, Fe3O4/Printex and Fe3O4/ graphene) in 1 mL of isopropyl alcohol, followed by sonication for 10 min. Then, 5 mL of the catalyst ink were carefully drop-casted on the surface of a clean glassy carbon (GC) disk electrode (5 mm diameter, PineChem Inc) and dried under N2 flow. This operation was repeated 2 more times to reach a catalyst loading of 76.5 mg/ cm2. Finally 15 mL of a Nafion solution (diluted 1:100) were dropcasted on the top of the catalyst layer and allowed to dry. Prior to the catalyst deposition, the surface of the GC electrode was polished to mirror finish using 1.0 and 0.05 mm alumina slurries, and subsequently cleaned in an ultrasonic bath in deionised water for 5 min. The number of electrons exchanged during ORR and the percentage of peroxide produced by the reaction were evaluated for each catalyst by rotating ring disk electrode (RRDE) measurements. The diameter of the glassy carbon disk was 5.61 mm, and it was surrounded by a Pt ring (6.25 and 7.92 mm inner and outer diameters, respectively). The catalytic film was formed on the disk electrode according to the procedure described above for the RDE
Fig. 2. TEM images of Fe3O4 nanoparticles (dark spots) on different carbon supports at different magnifications. (A, C) Fe3O4/Printex catalyst; the inserts in (A) and (C) show the SAED patterns of the catalyst and the HR-TEM image of Fe3O4 nanoparticle, respectively. (B, D) Fe3O4/graphene catalysts; inserts in (B) and (D) show the SAED patterns of the catalyst and the HR-TEM image of Fe3O4 nanoparticles, respectively. Both scale bars in the inserts in (C) and (D) are 2 nm.
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measurements but a total 19 mL catalyst ink and 19 mL Nafion solution were deposited on the electrode in order to reach the same loadings as in the RDE. The ring potential was set at 1.3 V and the voltammograms were recorded at 10 mV s1 with a Pine potentiostat (model AFRDE4). The RRDE was rotated at 1600 rpm. The collection efficiency of the ring-disk electrode was N = 0.37. The equations used to calculate n (the number of electrons transferred during ORR) and %H2O2 (the percentage of H2O2 released during ORR) are the following: n¼
4ID ID þ IR =N
%H2 O2 ¼ 100
(2) 2IR =N ID þ IR =N
(3)
where ID is the current at the disk, IR the current at the ring and N = 0.37 is the RRDE collection efficiency. The stability of Fe3O4/Printex and Fe3O4/graphene catalysts was tested on fresh electrodes by chronoamperometry at 0.3 V for 5.5 hours in O2-satured 1 mol L1 KOH and at a rotation speed of 1600 rpm. Before and after the tests, LVs were recorded in a N2saturated and an O2-saturated 1 mol L1 KOH electrolyte at a scan rate of 10 mV s1 and 1600 rpm. Unless otherwise stated, all O2 reduction linear polarization curves presented in this work were corrected by subtracting of the background current recorded in N2-saturated 1 mol L1 KOH.
3. Results and discussion The X-ray diffraction (XRD) of the unsupported Fe3O4 is presented in Fig. 1A. The XRD patterns confirmed the formation of Fe3O4 (JCPDS #19-0629). The lattice parameter calculated from (3 11) crystalline plane is a = 8.397 Å and in good agreement with the values reported in the literature for bulk stoichiometric Fe3O4 [33]. The unsupported oxide is composed of agglomerates of cubicshaped NPs with a particle size ranging from 5 to 40 nm, with an average particle size around 20 nm, as shown in Fig. 1B. A typical high-resolution TEM (HR-TEM) image is shown in Fig. 1C. Lattice fringes with spacing of 2.97 Å assigned to (2 2 0) plane are clearly observed in the inset. As demonstrated elsewhere, there is a variation of the composition of the iron oxide nanoparticles with the particle size, being this oxide better described as a solid solution of both magnetite (Fe3O4,a 8.392 Å) and g -maghemite (g -Fe2O3, a 8.342 Å) [33]. In general, for nanoparticles smaller than 40 nm the lattice parameter of the iron oxide decreases with the decrease of the particle size because more Fe2O3 is formed [32,33]. Interestingly, with the modified method used in this work the nanoparticles with size 20 nm in average have a lattice parameter close to bulk Fe3O4. The specific surface area of the Fe3O4 NPs, determined through N2-physisorption analysis (isotherm not shown), is 70 m2 g1 which compares very well with the value (72 m2 g1) estimated from the average crystallite size of 16 nm estimated from XRD.
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Fig. 3. Cyclic voltammograms of (A) Fe3O4 nanoparticles, (B) Printex 6 L carbon, (C) graphene, recorded in N2- and O2- saturated 1 mol L1 KOH at 21 C and 50 mV s1. (D) Comparison of the O2 reduction waves recorded for the three materials after subtraction of the background current recorded in N2-saturated solution.
Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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TEM observations of Fe3O4/Printex sample revealed an uneven distribution of the oxide NPs on the carbon and the formation of localized oxide clusters as illustrated in Fig. 2A. In contrast, the distribution of the oxide NPs on the graphene sheets was more uniform as shown in Fig. 2B. Printex carbon has a much lower
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specific surface area (230 m2 g1 measured by BET analysis) compared to graphene (450 m2 g1 by BET analysis) and are morphologically quite different (bundles vs sheets). Most probably, the larger number of functional groups on the surface of the graphene sheets with respect to Printex provides more anchoring sites for the nucleation and growth of the Fe3O4 NPs. The SAED confirmed the formation of Fe3O4-type structure for both samples, as illustrated by the respective patterns and lattice fringes (2.97 Å for (2 2 0), 4.85 Å for (111), and 2.53 Å for (3 11)) in the insets of Fig. 2. The HRTEM revealed that the shape and size of the Fe3O4 NPs are not the same on both samples. In the case of Fe3O4/Printex, the particles have an ill-defined contour and their sizes are typically equal or bellow than 10 nm. In the case of Fe3O4/ graphene, the NPs have a more squared-shape as the unsupported oxide and their average size varies between 20 and 30 nm. Shape change from cubic to spherical accompanying a particle size reduction was previously observed for unsupported Fe3O4 NPs [33]. Figs. 3A to C show the CV recorded in N2 and O2- saturated 1 mol L1 KOH for unsupported Fe3O4, Printex carbon and graphene at 50 mV s1. As shown in Fig. 3A, the CV of unsupported Fe3O4 in N2-saturated 1 mol L1 KOH has two cathodic ( 1.1 V and 0.72 V) and two anodic peaks ( 0.96 V and 0.71 V). The cathodic peaks are related to the reduction of surface Fe (III)
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Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175
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oxide/hydroxide to surface Fe (II) hydroxide and to metallic Fe. Accordingly, the two anodic peaks develop upon the reoxidation of the species formed during the cathodic sweep. Inner Fe (II) ions are oxidized to Fe (III) ions above 0.6 V, and a layer of Fe2O3 is formed above this potential [22,35]. Oxygen reduction on Fe3O4 starts at 0.32 V, and according to Vago et al. [22] at this potential the Fe2O3 surface oxide is reduced. The CVs of Printex 6L (Fig. 3B) and graphene (Fig. 3C) in the O2 free electrolyte solution are featureless as expected [17]. The
80
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current and the charge under the voltammogram recorded for the graphene electrode (qtotal = 0.472 mC) are much higher than for Printex (qtotal = 0.0453 mC) electrode as expected from its larger surface area. In O2-saturated 1 mol L1 KOH, the onset potential for O2 reduction is 0.23 V for Printex and 0.16 V for graphene, and the peak currents 0.76 mA cm2 and 1.32 mA cm2 suggesting the higher ORR activity of the latter. Fig. 3D compares the oxygen reduction waves after the subtraction of the background current recorded in N2-saturated 1 mol L1 KOH. Among the three materials, graphene shows the most positive on-set (0.08 V) and peak (0.28 V) potentials and 0.68 mA cm2 peak current. For Printex, the on-set and peak potentials at 0.19 V and 0.36 V, respectively, and 0.70 mA cm2 peak current. Finally, the unsupported Fe3O4 shows the on-set and
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Fig. 6. Rotating ring disk experiments for graphene, Printex, Fe3O4, Fe3O4/Printex and Fe3O4/graphene in O2–saturated 1 mol L1 KOH at 10 mV s1 and at a rotation rate of 1600 rpm. (A) ring (top) and disk (bottom) current (corrected for the current in N2-saturated 1 mol L1 KOH). (B) Percentage of produced peroxide and (C) number of exchanged electrons as a function of the applied potential. The polarization curves recorded for the glassy carbon substrate are included for reference.
after 5.5 h
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peak potentials at 0.27 V and 0.47 V, respectively, and a peak current of 0.26 mA cm2. The CVs recorded at 50 mV s1 for the Fe3O4/Printex and Fe3O4/ graphene composites are displayed in Figs. 4A to C. The voltammograms are similar to those recorded for the respective pristine carbon substrates and to the ones reported earlier by Wu et al. for 46.2 wt% Fe3O4 supported on nitrogen doped graphene aerogels in 0.1 mol L1 KOH [30]. The kinetic of ORR on Printex, graphene, Fe3O4/Printex and Fe3O4/graphene was examined by recording LVs at 10 mV s1 using the rotating disk electrode. Fig. 5 shows the series of polarization curves obtained at various rotation rates with Fe3O4/ Printex and Fe3O4/graphene. Similar profiles were recorded for Printex 6L and graphene. The profiles consist of two reduction waves and are similar to those reported by Wu et al. for 46.2 wt% Fe3O4 supported on nitrogen doped graphene aerogels in 0.1 mol L1 KOH [30]. The limiting current appears between 0.4 and .7 V, depending on the rotation speed, followed by a second reduction wave at more negative potential. This type of profile is a strong indication that ORR is proceeding through the 2 electron process with the formation of hydroperoxide anion as intermediate (Eq. (1)) followed by it reduction to hydroxyl anion (Eq. (2)) [5], [30]. HO2 + H2O + 2 e ! 3 OH (4) Fig. 6A compares the RRDE polarization curves obtained at 1600 rpm for the four catalysts and the unsupported Fe3O4. The polarization curves of the GC electrode were also included as a reference for the two electron process [36]. The electro-catalytic activity expressed by the disk current density at 0.3 V increases in the following order: Printex (0.29 mA cm2) < Fe3O4/Printex (0.38 mA cm2) < graphene (0.85 mA cm2) < Fe3O4/graphene 2 (1.12 mA cm ). The disk currents at 0.3 V for unsupported Fe3O4 and the GC electrode are negligible. The ring current, proportional to the produced hydrogen peroxide, increases in the same order. With respect to the GC electrode, the ring current of the four catalysts is much higher and appears at significantly lower overvoltage. On the other hand, the ring current for the unsupported Fe3O4 is lower than that of the GC electrode, which emphasizes the poor capability of these nanoparticles for the electro-generation of hydrogen peroxide. As evidenced in Figs. 6B and C, all catalysts are characterized by a high peroxide yield (55%) and a number of exchanged electrons close to 2.7 in the potential range comprised between 0.4 and 0.8 V vs SCE. Moreover, the functionalization of graphene and Printex carbons with the Fe3O4 NPs further increases the H2O2 yield with respect to the pristine carbons. For example, at 0.6 V the % H2O2 is 68% for Fe3O4/Printex, 62% for Fe3O4/graphene, while 59% for the two carbon supports. For potentials more negative than 0.8 V the 4 electron process starts to dominate and the % H2O2 decreases. It should be noted that Printex 6L keeps a % H2O2 close to 50% in this potential range. Finally, the performance of Fe3O4/Printex and Fe3O4/graphene catalysts was tested at constant potential for 5.5 hours. Fig. 7A displays the chronoamperograms recorded under RDE mode (1600 rpm) at 0.3 V. After an initial transient period, the current was remarkable stable in both cases. The LVs recorded for Fe3O4/ Printex and Fe3O4/graphene before and after the stability test are shown in Fig. 7B and C. The LVs overlap completely in the activation and mixed control regions, but the limiting current for Fe3O4/ graphene was even higher after the stability test. Clearly, these results indicate that Fe3O4/graphene and Fe3O4/Printex are promising catalysts for H2O2 production in-situ. The former is preferred in terms of lower on-set potential and higher current, whereas the second is preferred in terms of selectivity towards
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H2O2 electrogeneration. The influence of other parameters such as electrolyte concentration (pH) and catalyst loading on the selectivity of the oxygen reduction reaction towards the H2O2 formation [37,38] will be subject of a future study. 4. Conclusions We have successfully fabricated unsupported Fe3O4, Fe3O4/ Printex and Fe3O4/graphene by a simple precipitation method using NaBH4 as reducing agent. Fe3O4 NPs show a better distribution on graphene (specific surface area 450 m2 g1) than on Printex (specific surface area 230 m2 g1). Both Fe3O4/Printex and Fe3O4/graphene reduce oxygen through the 2-electron pathway in 1 mol L1 KOH and are very promising catalysts for H2O2 production in alkaline medium. Fe3O4/graphene exhibits higher catalytic activity for the ORR, including a more positive onset potential and higher current density, as well as excellent durability. However, Fe3O4/Printex exhibits higher selectivity for H2O2 electrogeneration. Acknowledgements The authors wish to acknowledge the “Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)” and “Fonds de Recherche du Québec - Nature et Technologies (FRQNT)” for financial support. Q. Wei gratefully acknowledges the scholarship from China Scholarship Council (CSC). References [1] K. Kinoshita, Carbon: electrochemical and physicochemical properties, Jonh Wiley, New York, 1998. [2] E. Yeager, Electrocatalysts for O2 reduction, Electrochimica Acta 29 (1984) 1527–1537. [3] E. Yeager, Dioxygen electrocatalysis: mechanisms in relation to catalyst structure, Journal of Molecular Catalysis 38 (1986) 5–25. [4] C. Zhang, K. Yanagisawa, H. Tao, A. Onda, T. Shou, S. Kamiya, Oxygen Reduction Activity and Methanol Resistance of Ru-based Catalysts Prepared by Solvothermal Reaction, Catal Lett 142 (2012) 1128–1133. [5] S. Guo, S. Zhang, S. Sun, Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction, Angewandte Chemie International Edition 52 (2013) 8526–8544. [6] Y. Li, H. Dai, Recent advances in zinc-air batteries, Chemical Society Reviews 43 (2014) 5257–5275. [7] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton’s Reaction Chemistry, Chemical Reviews 109 (2009) 6570–6631. [8] B.E. Logan, K. Rabaey, Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies, Science 337 (2012) 686–690. [9] W.R.P. Barros, R.M. Reis, R.S. Rocha, M.R.V. Lanza, Electrogeneration of hydrogen peroxide in acidic medium using gas diffusion electrodes modified with cobalt (II) phthalocyanine, Electrochimica Acta 104 (2013) 12–18. [10] F.L. Silva, R.M. Reis, W.R.P. Barros, R.S. Rocha, M.R.V. Lanza, Electrogeneration of hydrogen peroxide in gas diffusion electrodes: Application of iron (II) phthalocyanine as a modifier of carbon black, Journal of Electroanalytical Chemistry 722–723 (2014) 32–37. [11] J.C. Forti, R.S. Rocha, M.R.V. Lanza, R. Bertazzoli, Electrochemical synthesis of hydrogen peroxide on oxygen-fed graphite/PTFE electrodes modified by 2ethylanthraquinone, Journal of Electroanalytical Chemistry 601 (2007) 63–67. [12] A. Alvarez-Gallegos, D. Pletcher, The removal of low level organics via hydrogen peroxide formed in a reticulated vitreous carbon cathode cell, Part 1. The electrosynthesis of hydrogen peroxide in aqueous acidic solutions, Electrochimica Acta 44 (1998) 853–861. [13] P.C. Foller, R.T. Bombard, Processes for the production of mixtures of caustic soda and hydrogen peroxide via the reduction of oxygen, J Appl Electrochem 25 (1995) 613–627. [14] M. Panizza, G. Cerisola, Electrochemical generation of H2O2 in low ionic strength media on gas diffusion cathode fed with air, Electrochimica Acta 54 (2008) 876–878. [15] A. Wang, A. Bonakdarpour, D.P. Wilkinson, E. Gyenge, Novel organic redox catalyst for the electroreduction of oxygen to hydrogen peroxide, Electrochimica Acta 66 (2012) 222–229. [16] F. Alcaide, E. Brillas, P.-L.s. Cabot, Oxygen Reduction on Uncatalyzed CarbonPTFE Gas Diffusion Cathode in Alkaline Medium, Journal of the Electrochemical Society 149 (2002) E64–E70.
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Please cite this article in press as: W.R.P. Barros, et al., Oxygen reduction to hydrogen peroxide on Fe3O4 nanoparticles supported on Printex carbon and Graphene, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.175