Microwave Synthesis of High Activity FeSe 2 / C Catalyst toward Oxygen Reduction Reaction

The carbon supported iron selenide catalysts (FeSe2/C) were prepared with various selenium to iron ratios (Se/Fe), namely, Se/Fe = 2.0, 2.5, 3.0, 3.5 and 4.0, through facile microwave route by using ferrous oxalate (FeC2O4·2H2O) and selenium dioxide (SeO2) as precursors. Accordingly, effects of Se/Fe ratio on the crystal structure, crystallite size, microstructure, surface composition and electrocatalytic activity for oxygen reduction reaction (ORR) of FeSe2/C in an alkaline medium were systematically investigated. The results revealed that all the FeSe2/C catalysts obtained with the Se/Fe ratios of 2.0–4.0 exhibited almost pure orthogonal FeSe2 structure with the estimated mean crystallite sizes of 32.9–36.2 nm. The electrocatalytic activities in potassium hydroxide solutions were higher than those in perchloric acid solutions, and two peak potentials or two plateaus responded to ORR were observed from cyclic voltammograms and polarization curves, respectively. The ORR potentials of 0.781–0.814 V with the electron transfer numbers of 3.3–3.9 at 0.3 V could be achieved as the Se/Fe ratios varied from 2.0 to 4.0. The Fe and Se were presented at the surface of FeSe2/C upon further reduction on FeSe2. The Se/Fe ratios slightly influenced the degree of graphitization in carbon support and the amount of active sites for ORR. OPEN ACCESS Catalysts 2015, 5 1080


Introduction
Alkaline fuel cells have been attracting extensive attention due to their great advantages in cathode dynamics and with the reduction of ohmic polarization [1].Oxygen reduction reaction (ORR) is an important process in an electrochemical energy conversion and a four-electron reaction is desirable to take place for a given catalyst in order to achieve good electrocatalytic performance.In recent years, the transition metals such as Mn [2], Fe [3,4], Co [5], Ni [6], Cu [7] and the heteroatom dopants B [8], N [9,10], P [11,12], S [13,14], Se [15] have been reported to modify the catalytic properties of various carbon materials including amorphous carbon, carbon nanotubes, and graphene, which arouse a great deal of interest for the research and development of non-noble catalysts.
Chalcogenides are promising for the potential replacement Pt based cathode catalysts because of their good electrocatalytic activity and high selectivity toward ORR in both acidic and basic media.The onset potential of 0.823 V for ORR could be attained in H2SO4 solutions with CoSe/C synthesized through microwave assisted routes [16], while the ORR potentials of 0.6-0.7 V and the electron transfer numbers of 3.1 Oxygen reduction reaction 4.0 could be obtained for CoSe2/C prepared with the Se/Co ratios of 2.5-4.0 [17].In addition, CoSe2 nanoparticles showed a higher ORR activity in KOH than in H2SO4 solutions and a higher tolerance to methanol as compared with a commercial 20 wt % Pt/C catalyst [18].The tetragonal and cubic Cu2Se nanowires were found to have the four-electron mechanism, while the cubic nanowires were a dual-path mode in KOH solution [7].Although FeSe and FeSe2 have been reported for the applications in superconductors or magnetic semiconductors [19][20][21], their ORR activities have not been investigated so far.
In this work, a series of carbon supported FeSe2 nanoparticles were synthesized using the microwave method with different molar ratios of Se/Fe.The crystal phases, microstructures, chemical compositions and electrocatalytic activities of the as-prepared FeSe2/C catalysts were explored by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry and rotating disk electrode (RDE) techniques.The effect of Se/Fe ratio on ORR activity in an alkaline medium is discussed in terms of ORR active site and carbon graphitization.

Results and Discussion
Typical powder XRD patterns of FeSe2/C catalysts prepared with different Se/Fe ratios are shown in Figure 1.Compared with the standard lines of orthogonal FeSe2 phase (PDF#65-2570) included in the bottom of Figure 1a, the major characteristic diffraction peaks appeared at 2θ ≈ 35.0°, 36.4° and 48.4° belonged to (111), (120) and (211) planes, while a pair of twin peaks near 31° and 50° to (101)/(020) and (031)/(130) planes.A closer examination in the range of 25°-50° revealed that the diffraction peak corresponded to (120) shifted to smaller Bragg angles as evident in Figure 1b.Despite this, the formation of orthogonal FeSe2 structure is strongly indicated.The average crystallite sizes were evaluated using Scherrer equation described below: where d is the mean crystallite size; K is a dimensionless shape factor and has a typical value of 0.89, λ is the X-ray wavelength, λ = 0.1546 nm; β is the full width at half maximum (FWHM); θ is the Bragg scattering angle.The results are given in Table 1.Apparently, the Se/Fe ratios did not significantly influence the average crystallite sizes of the as-prepared FeSe2/C catalysts, which ranged 36.2-32.9nm with Se/Fe ratios of 2.0-4.0.The empirical Se/Fe ratios, also included in Table 1, were roughly evaluated from EDS data and agreed reasonably well with those nominal ones.The electrocatalytic activities of FeSe2/C prepared with different Se/Fe ratios toward ORR were studied in both acidic and alkaline media.Figure 2 presents the cyclic voltammograms obtained in N2 (dashed lines) and O2 (solid lines) saturated 0.1 mol•L −1 HClO4 solutions at 50 mV•s −1 .No apparent peaks were observed in N2 atmosphere, while one or two reduction peaks in O2 atmosphere.Two peak potentials of 0.211 V, −0.045 V and 0.204 V, −0.028 V were obtained only for FeSe2/C prepared with Se/Fe = 2.5 and Se/Fe = 4.0.Obviously, the ORR activity of FeSe2/C in an acidic medium was poor.
The cyclic voltammograms and RDE polarization curves for FeSe2/C prepared with different Se/Fe ratios measured in N2 (dashed lines) and O2 (solid lines) saturated 0.1 mol•L −1 KOH solutions are illustrated in Figure 3.A large reduction peak I was observed near 0.7 V, and followed by a small reduction peak II around 0.5 V in O2 saturated KOH solutions (solid lines in Figure 3a).This phenomenon was also observed for vertically aligned carbon nanotubes in a KOH solution [22].The peak potentials (EP) could be obtained from Figure 3a and are also summarized in Table 1.The EP values ranged from 0.699-0.773V for Peak I and 0.499-0.514V for Peak II, suggesting an enhanced ORR activity in an alkaline medium.As can be seen in Figure 3b, the plateaus observed for FeSe2/C were not well defined in KOH solutions, which significantly differed from those observed for CoSe2/C in H2SO4 solutions [17].Similarly, the electron transfer numbers (n) of FeSe2/C prepared for different Se/Fe ratios could be determined from the slops of Koutecky-Levich plots at 0.3 V as shown in Figure 4a.The dashed lines indicated the slopes corresponding to two-electron and four-electron reactions.The calculated results are also included in Table 1.The n values varied from 3.3-3.9at 0.3 V in KOH solutions for FeSe2/C prepared with Se/Fe ratios of 2.0-4.0, while they were 3.1-4.0 in H2SO4 solutions for CoSe2/C prepared with Se/Co ratios of 2.0-4.0 [17].
The polarization curves of FeSe2/C prepared with different Se/Fe ratios at 1600 rpm are compared with that of a commercial 20% Pt/C in Figure 4b.The potential at the current density of −0.5 mA•cm −2 is defined as EORR (the inset in Figure 4b) and the values are also provided in Table 1.The EORR value for 20% Pt/C was 0.992 V, while those for FeSe2/C ranged 0.781-0.814V with the Se/Fe ratios varying 2.0-4.0.However, two platforms observed with FeSe2/C resulted in slightly larger limiting current densities than 20% Pt/C.The information in surface species and carbon support could be further studied by obtaining Raman spectra using Ar ion laser excitation of 532 nm as shown in Figure 5.The presences of Fe-Se near 219 cm −1 , 284 cm −1 and 597 cm −1 , as well as Fe-O at 400 cm −1 [23,24] are identified in Figure 5a for all the Se/Fe ratios.In Figure 5b, the Raman bands appeared at 1336 cm −1 and 1593 cm −1 corresponded to D-band of sp 2 type carbon ascribed to the finite-sized crystals of graphite due to the reduction in symmetry and G-band of all sp 2 bonds in an ideal graphitic layer, respectively.Figure 5c illustrates the curve fitting plots for the Raman data given in Figure 5b.Two additional weak bands at 1191 cm −1 and 1499 cm −1 belonged to sp 3 type carbon.The relative intensity of the D band over G band (ID/IG) and the relative ratio under the areas of sp 3 and sp 2 types of carbon (Asp 3 /Asp 2 ) were calculated, and the results are compared in Table 1.The ID/IG and Asp 3 /Asp 2 values ranged 1.64-1.90and 0.37-0.51,respectively, for the Se/Fe ratios of 2.0-4.0.The small difference of ID/IG valves might mean that the carbon surface was partially oxygenated without significant structural deformation [25].The least ID/IG and Asp 3 /Asp 2 values were obtained for the FeSe2/C prepared with Se/Fe = 2.0, implying the presence of less defect and higher degree graphitization in carbon support.Contrarily, FeSe2/C prepared with Se/Fe = 3.5 showed the largest ID/IG and Asp 3 /Asp 2 values, and resulted in more defect and lower degree graphitization.Similar ID/IG and Asp 3 /Asp 2 values were observed for FeSe2/C prepared with Se/Fe = 2.5, 3.0 and 4.0 as evident in Table 1.The high resolution TEM images and SAED patterns of FeSe2/C prepared with different Se/Fe ratios are supplied in Figures 6 and 7, respectively.The particles sized about 3-12 nm were observed in Figure 6, which are much smaller than those calculated from XRD data (32.9-36.2nm in T -1) because XRD gives volume-weighted measurements that tend to overestimate the geometric particle size [26].The formation of orthogonal FeSe2 nanoparticles by microwave synthesis was verified by combining both TEM and SAED data, which is consistent with the XRD results in Figure 1.High resolution XPS spectra of C1s, O1s, Fe2p and Se3d were obtained for FeSe2/C prepared with different Se/Fe ratios and are presented in Figure 8.The surface compositions could be evaluated by performing multi-peak fitting analysis of every spectrum, and the deconvoluted XPS spectra are also illustrated in Figure 8.The C1 peaks in Figure 8A were fitted with four components centered at 284.6, 285.0, 286.0 and 290.5 eV, which were attributed to C=C, C-C, C-O and C=O, respectively.The surfaces of carbon support (BP2000) consisted of C=C (sp 2 type) and C-C (sp 3 type) bonding.The origins of C=O and C-O might possibly be from oxygenation of the carbon surface during the preparation [27].The O1s spectra in Figure 8B were deconvoluted into oxide oxygen species mainly associated with Fe oxide (Fe-O) at 530.0 eV and Se oxide (Se-O) at 532.0 eV, as well as C=O at 531.1 eV, C-O at 532.9 eV, and adsorbed water molecule (H-O-H) at 533.8 eV.The presence of Fe-O was also indicated by Raman spectra in Figure 5a, and the Se-O mainly came from the non-reacted raw material.The Fe2p spectra in Figure 8C suggested the existences of Fe 2+ related to the formation of major compound of FeSe2 with possible formations of FeSe and Fe oxide (Fe-O).Possible formation of Fe 0 could be resulted from further reduction in FeSe2 (or FeSe) during the microwave preparation.Furthermore, the surface of FeSe2 might become oxidized during the preparation and characterization, which was clearly indicated as Fe-O in Raman spectra (Figure 5a).However, the Fe2p spectra in Figure 8C could not directly differentiated the Fe3O4 (Fe 2+ and Fe 3+ ), Fe2O3 (Fe 3+ ) and FeSe, because the binding energies of Fe3O4 (711.4 eV), Fe2O3 (711.0 eV) and FeSe (711.5 eV) are very close.Similarly, they could not be readily differentiated in O1s based on the binding energy O-Fe-O (530.1 eV) and Fe-O (530.0 eV).The formation of FeSe2 (Se-Fe-Se) could be accompanied by over reduction of FeSe2 since the strong reducing environment was created by using ethylene glycol and glycerol during microwave preparation.Further reduction on FeSe2 occurred for all the Se/Fe ratios at the surface of FeSe2 and led to formations of FeSe, Fe and Se.In addition, other species such as FeSe4 could also exist for the excess Se, but the exact verification required more detailed study.It has been found that the appropriate excess amounts of SeO2 could prevent the CoSe2/C nanoparticles from agglomeration and dissolution, which contributed to the improved ORR activity and good stability [17].The Se3d peaks in Figure 8D indicated the presences of Fe-Se, Se-Fe-Se, Se-Se and Se-O.The relative amounts (R. A.) of surface species could be obtained by multi-peak fitting the XPS data and are summarized in Table 2.The FeSe2 and FeSe were ORR active sites, and the total amounts of FeSe2 and FeSe were 47.2%, 37.1%, 47.1%, 38.2% and 48.2% for Se/Fe = 2.0, 2.5, 3.0, 3.5, and 4.0, respectively, which were consistent with the ORR activities indicated by EORR values in Table 1.The FeSe2/C catalysts prepared with Se/Fe = 2.5 and Se/Fe = 3.5 showed relatively smaller EORR while those with Se/Fe = 2.0, Se/Fe = 3.0, and Se/Fe = 4.0 had larger EORR values.

Materials
The chemical reagents of ferrous oxalate (FeC2O4•2H2O), selenium dioxide (SeO2), ethylene glycol and glycerol of analytic grade were purchased from Sinopharm Chemical Reagent Co. Ltd. in China.The carbon support material of Black Pearls 2000 (BP2000) was purchased from Cabot Co.The mean grain size and specific surface area (Brunauer-Emmet-Teller, BET) were 12 nm and 1500 m 2 •g −1 , respectively.

Catalyst Synthesis
The amounts of 40.0 mg FeC2O4•2H2O were dissolved in 2 mL ethylene glycol and a certain amount of 0.161 mmol/mL SeO2 aqueous solution with the different molar ratios of Se/Fe, namely, 2.0, 2.5, 3.0, 3.5, 4.0.Then, a certain amount of glycerol was added, and the mixed solution was agitated with a glass rod and homogenized in an ultrasonic bath for 30 min.The BP2000 was continually added during the ultrasonic processing.The loading amount of FeSe2 on carbon was about 36% according to the weight ratio at the start of the feeding.The homogeneous solution was placed in a microwave oven by using 800 W for 180 s while the solution cooled to room temperature, which was sonicated and stirred for 4 h.The product was finally centrifuged, washed with ethanol and deionized water, and dried in a vacuum oven at 338 K for 12 h.

Electrochemical Characterization
The electrochemical measurements were carried out by using the electrochemical test station (Autolab-PGSTAT30) with rotation disc electrode (RDE) system (Pine Research Instrument) in a conventional three-electrode cell.A glassy carbon RDE was a working electrode, a Pt mesh (2 cm × 2 cm) a counter electrode and a Ag/AgCl a reference electrode.Catalyst ink was prepared by homogeneously dispersing 2 mg of the as-prepared FeSe2/C powder ultrasonically in a solution mixture containing 0.5 mL isopropanol and 10 μL 5 wt.% Nafion solution.Then, 10 μL of the mixture was transferred onto the 0.196 cm −2 polished glassy carbon electrode surface and dried at room temperature.The catalyst loadings on the electrodes were evaluated to be 0.2 mg•cm −2 (including the support).
The cyclic voltammetry (CV) and RDE measurements were done at 20 °C in either nitrogen purged or oxygen saturated 0.1 mol•L −1 HClO4 and 0.1 mol•L −1 KOH solutions.Prior to the measurement, the electrolyte was deaerated by nitrogen or oxygen throughout the 30 min.The scanning potentials started from -0.160 V to 1.034 V in HClO4 or 0.413 V to 1.410 V in KOH at a sweep rate of 50 mV•s −1 .The linear sweep voltammetry (LSV) curves were recorded in the potential range of 0.107-1.105V with 5 mV•s −1 over a rotation rate of 0-2000 rpm in oxygen saturated electrolyte.All the potentials in this work were reported with respect to reversible hydrogen electrode (RHE).

Physicochemical Characterization
XRD analysis of the catalyst nanoparticles was performed with a powder diffractometer (Rigaku Ultima IV XRD) using Cu Kα radiation (λ = 0.1546 nm).Raman spectra were acquired using the 532 nm laser on a Princeton TriVista CRS557 Raman spectrometer.A high resolution transmission electron microscope (TEM) (JEOL JEM-2100), field emission scanning electron microscope (SEM) with built-in energy dispersive X-ray spectroscope (EDS) (Zeiss Sigma SEM) and X-ray photoelectron spectroscope (XPS) (PHI Quantum 2000) using Al Kα radiation were used to examine the microstructures and chemical compositions of the as-prepared catalyst nanoparticles, respectively.

Conclusions
The FeSe2/C catalysts could be rapidly prepared through a simple microwave method by using various Se/Fe ratios.The formation of the orthogonal FeSe2 structure was confirmed by XRD, TEM and SAED analyses.The estimated average crystallite sizes were 32.9-36.2nm for the Se/Fe ratios of 2.0-4.0.The catalysts exhibited the enhanced ORR activities in alkaline media rather than in acidic media.The ORR potentials of 0.781-0.814V with the electron transfer numbers of 3.3-3.9at 0.3 V could be achieved in KOH solutions as the Se/Fe ratios varied from 2.0 to 4.0.The Se/Fe ratios slightly influenced the amounts of ORR active sites and the defects of carbon support, as well as the degrees of graphitization, which together affected the ORR activities.

Figure 4 .
Figure 4. (a) Koutecky-Levich plots at 0.3 V; (b) RDE polarization curves of Pt/C (dashed line) and FeSe2/C (solid lines) prepared with different Se/Fe ratios in O2 saturated 0.1 mol•L −1 KOH solutions at 1600 rpm.The inset in (b) illustrates the potential corresponding to ORR at the current density of −0.5 mA•cm −2 (EORR).

Table 2 .
Surface compositions of FeSe2/C determined based on XPS analyses.