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Synthesis, characterization of a CoSe2 catalyst for the oxygen reduction reaction
Applied Catalysis A: General 386 (2010) 157–165
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Synthesis, characterization of a CoSe2 catalyst for the oxygen reduction reaction L. Zhu a , M. Teo a , P.C. Wong a , K.C. Wong a , I. Narita b , F. Ernst b , K.A.R. Mitchell a,∗ , S.A. Campbell c,∗∗ a
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA c Ballard Power Systems Inc., 9000 Glenlyon Parkway, Burnaby, British Columbia, Canada V5J 5J9 b
a r t i c l e
i n f o
Article history: Received 21 April 2010 Received in revised form 22 July 2010 Accepted 25 July 2010 Available online 3 August 2010 Keywords: CoSe2 catalyst Oxygen reduction reaction Materials characterization Surface characterization Electrochemical surface modification
a b s t r a c t Crystalline CoSe2 , with the pyrite structure, supported on carbon powder has been synthesized by a wet chemical method. Samples were characterized by a range of techniques, including X-ray diffraction and transmission electron microscopy for structure, and X-ray photoelectron spectroscopy and scanning Auger microscopy (SAM) for surface composition. Electrochemical dynamic polarization measurements were carried out using the CoSe2 -decorated carbon powder as part of a rotating disk electrode to assess its catalytic activity for the oxygen reduction reaction (ORR), and the surface analysis methods were used to identify changes in surface composition. Comparisons of ORR activities were made with CoSe and Pt powder catalysts prepared by comparable methods. Stationary cyclic voltammetry was used to assess the stability of the as-prepared CoSe2 powder catalyst in an acidic electrochemical environment. A preliminary investigation was also made of an electrochemical modification treatment for CoSe2 . The surface formed was characterized by the rotating ring-disk electrode technique and by micro-Raman spectroscopy, backscattered-electron imaging and SAM. The latter combination of characterization techniques helped to relate an observed passivation to a thin layer of Se formed by the modification treatment. © 2010 Elsevier B.V. All rights reserved.
1. Introduction While the proton exchange membrane (PEM) fuel cell provides a potentially promising approach for vehicle propulsion due to its low greenhouse gas emissions, high efficiency and high energy density [1–3], a number of major issues still need to be resolved before widespread use in this direction is likely. Catalyst cost represents one obstacle to progress, although in principle that could be mitigated by the use of non-precious metal catalysts, instead of those based on Pt. Transition-metal chalcogenides provide one direction of opportunity [4–8], and the study reported in this paper forms part of a broader Department of Energy program to investigate the performance of transition-metal chalcogen combinations as model catalysts for the oxygen reduction reaction (ORR). Initial work considered Co–Se catalysts as thin films [5] since that form enabled a direct comparison with the performance of Pt, still the catalyst of choice for use in PEM fuel cells. Evidence was presented to show that thin films formed by the Co–Se system can be catalytically active for ORR, with a Tafel slope of −130 mV decade−1 which is close to that of Pt (−120 mV decade−1 ) [5,9]. A powder catalyst
∗ Corresponding author. Tel.: +1 6048225831. ∗∗ Corresponding author. Present address: AFCC Automotive Fuel Cell Cooperation Corp., 9000 Glenlyon Parkway, Burnaby, British Columbia, Canada V5J 5J8. E-mail addresses: [email protected] (K.A.R. Mitchell), [email protected] (S.A. Campbell). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.07.048
based on CoSe prepared on dispersed carbon powder showed a similar performance. Subsequent work with thin films based on the Ni–S, Co–S and Fe–S combinations showed larger ORR activities for the dichalcogenides with the pyrite structure [6,7], which, in the Co–Se system, is adopted by CoSe2 . However, the early work on the Co–Se thin-film system [5], prepared by magnetron sputtering using elemental Co and Se as targets, did not show evidence for a significant presence of CoSe2 , but a more recent study by Feng et al. has specifically investigated ORR performance on CoSe2 powders with different loadings [10]. The present study extends our knowledge in this area by preparing crystalline CoSe2 supported on carbon, and investigating its activity for ORR, which is compared against those of CoSe and Pt. Detailed assessments of the CoSe2 /C structure, under a variety of conditions including in tests for stability in electrochemical environments, were made with a range of characterization techniques including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, scanning Auger microscopy, micro-Raman spectroscopy and backscattered-electron imaging. The aim is to gain more understanding of how this type of material behaves at the microscopic level where the catalytic reactions occur. 2. Experimental The following procedure for preparing CoSe2 supported on carbon powder derived from that described previously for sup-
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ported CoSe [5]: 1 g of carbon powder (Vulcan XC72R, Cabot Corp.) was dispersed in 1 L of deionized water (NCCLS Type 1, ultra-pure, 18.2 M cm), 50 mL of isopropanol (Fischer Scientific) was introduced to aid wetting, followed by the addition of 10.2 g of CoSO4 ·7H2 O (Sigma, 99% purity) and 8.2 g of SeO2 (Aldrich, 99.9+% purity). After magnetic stirring for 1 h, 3.2 g of NaBH4 (Sigma–Aldrich, 99% purity) was added slowly to the solution, and this was followed by stirring for another 1 h. The precipitated powder was filtered, washed twice with ultra-pure deionized water, and then dried under an Ar atmosphere in a tube furnace (4 h, 600 ◦ C). Such samples were characterized directly by X-ray diffraction (XRD) using a Bruker D8 advanced diffractometer (Cu K␣ radiation) with the scattering angle (2) scanned from 5◦ to 90◦ (rate 0.02◦ s−1 ). Prior to measurements, the XRD instrument was calibrated with a NIST 1976 XRD flat-plate intensity corundum standard (Bruker). For the other characterizations, the supported CoSe2 as prepared above was re-deposited according to the subsequent test being made. For measurements with transmission electron microscopy (TEM), the CoSe2 on carbon powder was ultrasonically dispersed in ethanol. A drop of the dispersion was deposited onto an ultrathin amorphous carbon film, supported by a thicker holey carbon film residing on a Cu grid, and dried. TEM was mostly carried out using an analytical Tecnai F30 instrument (FEI) with a 300 kV fieldemission gun; this instrument is equipped with a scanning TEM (STEM) unit, a high-angle angular dark-field (HAADF) detector, suitable for recording “Z-contrast” images, and a high-resolution Si–Li detector (129 eV at Mn K␣) for X-ray energy-dispersive spectrometry (XEDS). The magnification of TEM images and the camera length were calibrated with a standard of gold nanoparticles on amorphous carbon. For measurements with convergent-beam electron diffraction (CBED), we used a CM20 instrument (Philips), equipped with a LaB6 emitter and operated at 200 kV. All electrochemical characterizations were made with the CoSe2 powder deposited onto a polished glassy carbon (GC) electrode (Tokai Carbon). The deposition started with 20 mg of the powder sample being ultrasonically dispersed in glacial acetic acid (2 mL), and 10 L of that dispersion was pipetted onto the electrode surface and dried in hot air. The coated GC electrode described above formed the working electrode in a standard three-electrode glass cell with a reversible hydrogen electrode (RHE) for reference (which was connected to the Luggin capillary) and a Pt wire as the counter electrode. All potentials are quoted with respect to the RHE. The electrochemical set-up was described in our previous study for the Co–Se thin film [5]. The electrode design, with a “top-hat” geometry, allowed it to act as a rotating disk electrode (RDE) and to facilitate direct sample transfer for the surface characterizations, as described previously [5–8]. Unless otherwise stated, RDE measurements reported here were made at 2000 rpm. Typically, after an electrochemical characterization, a sample was washed in doubledeionized water, dried with Ar gas, and stored in an Ar atmosphere prior to the transfer for surface characterization. A rotating ring-disk electrode (RRDE, from Pine Instruments) was employed to investigate electron transfer number and H2 O2 yield experienced by the CoSe2 /C catalyst. All RRDE measurements were performed in parallel with RDE measurements using a jacketed three-compartment electrochemical cell, a rotator (ARS, Pine Instruments) and a computer-controlled Solartron 1408 multi-channel potentiostat (Ametek). Areas of the disk electrode and ring-disk electrode were 0.164 and 0.247 cm2 respectively. Before making RRDE measurements, each prepared electrode was inspected with a digital optical microscope (ImagingSource) to ensure sufficient uniformity for the coating layer, with no catalyst deposited on the ring. In addition, assessments were made for the thickness of the catalyst coating. This was evaluated using the method of Schmidt et al. [11] to ensure that the binding layer resis-
tance was negligible and that the catalyst layer was thin enough to allow the geometric area to be used directly in electrochemical calculations for assessing ORR mechanism. The H2 O2 yield was determined with the potential of the Pt ring electrode set to 1.2 V, at which value the detection of peroxide is diffusion limited. Polarization curves with the RRDE were recorded with a sweep rate of 2 mV s−1 and a rotation speed of 900 rpm. The collection efficiency of the RRDE was measured to be 0.392 by using the method of Paulus et al. with a 10 mM solution of K3 [Fe(CN)6 ] in N2 -saturated KOH [12]. The CoSe2 powder surface was characterized by X-ray photoelectron spectroscopy (XPS) using a Leybold MAX200 spectrometer operated with the Mg K␣ source (1253.6 eV) at 10 kV, 20 mA, with a 48 eV pass energy, and the analytical chamber at 2 × 10−9 mbar; all binding energies were referenced to the adventitious hydrocarbon C 1s peak at 285.0 eV. Scanning Auger microscopy (SAM) was used for characterizing local regions of the supported CoSe2 powder samples using a Microlab 350 system (Thermo Electron Corp.) equipped with a field-emission source (10 keV, 3.5 nA) and hemispherical energy analyzer; this equipment also allowed characterization with scanning electron microscopy (SEM) and backscattered-electron (BSE) imaging. Survey-scan Auger electron spectra and those at higher-energy resolution were measured using a constant retarding ratio (CRR) of 4.0 and 40.0 respectively. MicroRaman spectra were acquired using the 632.8 nm (HeNe) laser on a Renishaw inVia Raman Microscope using a 20× objective (Numerical Aperture = 0.40). Each spectrum was acquired using 10% of the maximum output laser power (<0.5 mW on the sample surface) by co-adding signals from a 100 s exposure. Before each Raman measurement, the instrument was calibrated with a standard Si sample (wavenumber 520 cm−1 ).
3. Results and discussion 3.1. XRD and TEM analyses for the as-prepared sample Fig. 1 compares the diffractogram measured by XRD for the sample designated above as CoSe2 supported on the XC72R carbon powder with the diffractogram from a reference standard sample of crystalline cubic CoSe2 [13]. The close match in peak positions between the two diffractograms confirms that a successful synthesis of supported CoSe2 with the pyrite structure was accomplished. The sharpness of the peaks and the low background in Fig. 1(a) suggest that the supported CoSe2 has been prepared with a high degree of crystalline perfection. The measured peak widths correspond to an average particle size of about 130 nm. Further, the measured diffractogram shows only weak peaks from carbon [14,15], and no additional species were detected either by XRD or XEDS. Together these observations indicate that our CoSe2 powder has been prepared with a good state of purity, and apparently a high mass loading. Fig. 2(a) shows a conventional TEM bright-field image recorded from the same CoSe2 powder sample. The image reveals two kinds of particles: (i) larger dark particles with diameters of order 100 nm, and (ii) smaller light-gray particles with diameters of order 50 nm. Fig. 2(b) presents a Z-contrast STEM image of the same area. In this image, the intensity is proportional to the square of the average local projected atomic number Z. Only the dark particles seen in (a) show up with bright contrast in this image, indicating they contain heavier elements, such as Co and Se. The light-gray particles in Fig. 2(a) do not show up in Fig. 2(b), consistently with their being composed of light material, such as the amorphous carbon. Elemental mapping by XEDS in the STEM mode confirmed this interpretation. Fig. 2(c) presents an XEDS spectrum recorded with a primary beam diameter of 10 nm at the location marked ‘XEDS’ in
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cubo-octahedron faceted on {1 1 1} planes and truncated on {2 0 0} planes. 3.2. Evaluation of supported CoSe2 powders
Fig. 1. X-ray diffractograms (a) measured in this work for as-prepared CoSe2 supported on XC72R carbon powder and (b) reference diffractogram for cubic CoSe2 [13].
(b). A quantification of the spectrum, not using standards, yielded a Se/Co ratio of 2.0. This is consistent with CoSe2 , although uncertainties in the analysis do prevent a definitive assignment on this evidence alone. The carbon and Cu peaks in Fig. 2(c) originate from the carbon support, the holey carbon film, and the Cu TEM grid. Fig. 2(d) presents a histogram for the CoSe2 particle diameter, obtained by evaluating about 300 particles in Z-contrast images like that in (b). The average particle diameter is 152 ± 5 nm. The standard deviation of the distribution is 81 nm, and the distribution is significantly skewed towards larger particle size. The particle sizes estimated for local regions by TEM are comparable with the estimate given previously by XRD. According to these data, the CoSe2 particles are much larger than common Pt catalyst nanoparticles, which can be as small as 3 nm [16]. The upper half of Fig. 2(e) shows an experimental convergentbeam electron diffraction (CBED) pattern recorded from one of the particles that show the dark contrast in Fig. 2(a) (for better visibility of fine detail, the contrast and brightness in the center and in the periphery were adjusted differently). For comparison with the experimental pattern, the lower half of Fig. 2(e) shows a simulated CBED pattern of CoSe2 with the pyrite structure. The simulation was carried out with the software package JEMS (the Java version of EMS [17]) for dynamical multibeam conditions and a crystal thickness of 180 nm, that is close to the average crystal thickness displayed in Fig. 2(d). The comparison between the experimental and simulated pattern in (e) reveals excellent agreement. In the zero-order Laue zone (the inner part of the pattern), the simulated pattern satisfactorily reproduces the spot pattern, spot positions, and relative spot brightness. Further, the simulation nicely reproduces the main Kikuchi lines and the first-order Laue zone ring in the experimental pattern. Based on this excellent agreement, there is no doubt that the synthesized nanoparticles are indeed CoSe2 with the pyrite structure. As seen for several examples in Fig. 2(a) and (b), most of the CoSe2 particles are faceted. Apparently, the “ideal” shape (from which the real particles may substantially deviate) corresponds to a
3.2.1. Catalytic activity for ORR Fig. 3 compares electrochemical measurements of current as a function of potential applied to CoSe2 supported on carbon powder that has in turn been deposited on the GC rotating disk electrode. These measurements, done separately in N2 -saturated and O2 saturated 0.5 M H2 SO4 electrolyte solutions, were made from the open circuit potential (OCP) to 0.1 V vs. RHE. The OCP was measured potentiometrically as the voltage between the working electrode of interest and the counter electrode when no current was drawn in that circuit. In the N2 -saturated electrolyte (i.e. O2 -free), the OCP of the CoSe2 powder is 0.74 V vs. RHE, while it is increased to 0.81 V vs. RHE for the O2 -saturated solution. The current flowing in the latter case, in the potential window 0.0–0.4 V vs. RHE, is about 40 times larger than that in the N2 -saturated solution (Fig. 3). This provides direct evidence for the catalytic ability of supported CoSe2 powder to sustain cathodic current with the presence of O2 in an acidic environment. Fig. 4 compares dynamic cathodic polarization curves measured in O2 -saturated 0.5 M H2 SO4 solution, in a potential vs. current density plot, for a blank GC substrate, the CoSe2 on XC72R carbon, and 40 wt% of Pt (HiSpec 4000, Johnson Matthey) on XC72R powder; in addition a comparison is included with earlier measurements for CoSe supported on carbon [5]. The polarization curves in Fig. 4 have been background-corrected insofar as the contribution from the GC (known area) has been subtracted, and constant loadings of the XC72R carbon are used in each case; with the latter we believe that these curves enable an indication of the relative activities of the different catalysts. Supported CoSe2 powder clearly shows a systematic increase in current compared with the GC substrate alone which itself does not contribute much catalytic activity (its OCP is 0.70 V vs. RHE). Furthermore, the polarization curve for supported CoSe2 has a comparable shape to that for Pt in the supported state, although the two curves are displaced so that within the kinetic control region (0.78–0.72 V vs. RHE), the current for CoSe2 is about 1.5 orders of magnitude less than that of Pt, and its OCP is 0.19 V less than for Pt. Incidentally the Tafel slope measured for CoSe2 is −113 mV decade−1 , in close correspondence with that of Pt (−120 mV decade−1 ) [9]. Overall CoSe2 does demonstrate a significant intrinsic catalytic activity, and in the mass-transfer region (below 0.3 V) its polarization curve matches closely with that for Pt. That may suggest a common ORR mechanism for electron transfer. By contrast, given differences in the measured polarization curves, the mechanisms are likely different for the CoSe [5] and CoSe2 powders supported on GC, and the latter is clearly the more active for ORR. Thus it manifests currents approximately one order of magnitude greater than that for CoSe, and its OCP is about 0.09 V greater. Such observations clearly demonstrate the enhanced ORR activity from the pyrite structure (i.e. CoSe2 ) compared with that for CoSe (for which a monoclinic non-stoichiometric form of Co1−x Se (x = 0.125–0.25) had been proposed previously [8]). 3.2.2. XPS and SAM analysis for as-prepared and acid treated samples Since surface composition is likely to be the prime determinant of electrochemical catalytic behavior [5–8], SAM and XPS were used to characterize surfaces associated with CoSe2 powder in the asprepared state, after acid immersion for 30 min in 0.5 M H2 SO4 , and after electrochemical ORR tests. It will be argued later that the acid immersion treatment helps to simulate the nature of the electrode surface in an active electrochemical environment, specif-
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Fig. 2. Electron microscopic characterizations for CoSe2 particles supported on XC72R carbon powder: (a) bright-field TEM image showing CoSe2 particles (dark) and carbon particles (lighter); (b) Z-contrast image of the region in (a) recorded with the high-angle angular dark-field detector; (c) X-ray energy-dispersive spectrum from the location marked “XEDS” in (b); (d) size histogram from about 300 CoSe2 particles; and (e) convergent-beam electron diffraction pattern obtained from a CoSe2 particle (upper half) and a simulation for CoSe2 with the pyrite structure (lower half).
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Fig. 3. Dynamic cathodic polarization measurements for supported CoSe2 powder (loading 100 g) in O2 -saturated 0.5 M H2 SO4 and compared against the equivalent measurements in N2 -saturated solution for a sweep rate of 5 mV s−1 and rotation speed 2000 rpm.
ically insofar as it helps remove surface oxide formed by ambient air exposure. For the as-prepared supported CoSe2 powder, Auger point analysis detects cobalt oxide on the surface. This is evidenced by O KLL Auger signals present in the survey-scan spectrum (not shown) and a peak at 750.0 eV associated with oxidized cobalt in the Co LMM spectrum (Fig. 5(a), spectrum (i)). This peak is removed by the acid immersion treatment (Fig. 5(a), spectrum (ii)), which leaves a single Co component at 754.5 eV, and correspondingly the Auger survey spectrum no longer shows evidence for an O signal. Fig. 5(b) compares Se LMM spectra for elemental Se (spectrum (i)), for which the most intense peak is at 1306.9 eV, with that for the sample that had been immersed in acid (spectrum (ii)) which has the corresponding peak at 1309.1 eV. The shift of 2.2 eV compared with elemental Se does not appear to be an artifact, since other Se LMM peaks are unchanged; earlier such a shift was taken
Fig. 4. Comparison of dynamic polarization curves in O2 -saturated 0.5 M H2 SO4 for blank glassy carbon (GC) and for CoSe, CoSe2 and Pt each supported on XC72R carbon powder (see text) with all loadings close to 100 g. Each set of measurements was for a sweep rate of 5 mV s−1 and rotation speed 2000 rpm. Tafel lines are shown for the curves measured for CoSe2 and Pt.
as indicative of a Co–Se bonding interaction [5]. Nevertheless the shift of 2.2 eV found here is greater than the value of 1.2 eV identified previously for the CoSe sample [5], an observation that seems likely to be associated with their different crystal structures. Quantitative Auger analysis from the as-prepared CoSe2 powders gave Se/Co atomic ratios as low as 1.7, apparently because of surface oxidation which results in the presence of some CoO, but after the acid immersion treatment that ratio reverts to the stoichiometric value expected for the CoSe2 sample (i.e. 2.0).
Fig. 5. Auger electron spectra: (a) Co LMM spectra from supported CoSe2 powder: (i) as-prepared and (ii) after acid immersion treatment (see text); and (b) Se LMM spectra: (i) from high-purity Se, and from supported CoSe2 after: (ii) acid immersion treatment and (iii) ORR measurement.
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Fig. 6. Cyclic voltammetry plots for as-prepared supported CoSe2 powder in O2 saturated 0.5 M H2 SO4 showing measurements for the 1st, 2nd and 10th cycles to assess electrochemical stability. In these measurements the electrode is stationary and the sweep rate is 10 mV s−1 .
Comparative information was also obtained by XPS, but Auger electron spectra are emphasized in this paper because they are needed later in a context where the high spatial resolution with SAM is beneficial. The observations from the Auger measurements that the as-prepared supported CoSe2 powder has some oxide on its surface, which is removed by the acid immersion treatment, are also confirmed with XPS. With the latter technique, it is concluded that the Co 2p3/2 and Se 3d peaks, observed at 779.0 and 55.7 eV respectively, together provide a signature of a cleaned CoSe2 surface. Fig. 5(b) part (iii) shows the Se LMM spectrum measured from the supported CoSe2 powder after an ORR measurement. The similarity to the spectrum in part (ii), for after the acid immersion treatment, along with other comparable observations made with Auger electron spectroscopy and XPS, confirms that the nature of the surface after the acid immersion test is comparable to that after an electrochemical characterization. The evidence is that any oxide from air exposure is removed in either case, and that after one of those treatments the Se/Co ratio reverts to the value expected for stoichiometric CoSe2 at the surface. 3.2.3. Electrochemical stability test for CoSe2 powder Fig. 6 compares cyclic voltammograms measured through ten successive cycles for the as-prepared supported CoSe2 powder sample, in O2 -saturated 0.5 M H2 SO4 solution, while the potential is swept between the OCP and 0.0 V vs. RHE. In this case the electrode is stationary (in order to reduce the possibility of particle detachment), and the scanning rate is 10 mV s−1 . The structure observed for the first cycle is reproducible for these conditions. The features at 0.55 and 0.75 V vs. RHE correlate with the effect of surface oxide, which was identified above by surface analysis, and also with some contribution from functional groups on the carbon powder [9,18]. After the first cycle, the behaviour seems largely determined by the CoSe2 surface, although some detachment of carbon powder does occur which causes a degree of shrinkage in the cyclic voltammogram. In any event, after the first cycle, there is a good level of match between the different voltammograms through to the tenth cycle. The match appears complete for potentials above 0.4 V, while deviations noted in the lower potential region are likely associated with non-equilibrium effects from hydrogen evolution [9]. The first cycle for the as-prepared sample showed an OCP at 0.81 V vs. RHE, but for all other cycles the OCP remains constant at 0.80 V.
Fig. 7. The first and fourth cyclic voltammograms measured for CoSe2 powder in O2 saturated 0.5 M H2 SO4 to test the electrochemical modification approach described in the text. In these measurements, the electrode was stationary with a scanning rate of 100 mV s−1 .
This simple test suggests that supported CoSe2 may have stability under ORR working conditions, although for an application to fuel cells there remains concern about surface corrosion in an atmospheric environment. Following earlier work on thin-film catalysts based on CoSe [5], and the passivation possible with surface layers of Se, the next section considers a possible electrochemical process for surface modification of the supported CoSe2 powder. 3.3. Surface modification of supported CoSe2 powder 3.3.1. Electrochemical characterizations A cyclic voltammetric (CV) procedure was used to test whether some surface CoSe2 can be sacrificed to form a controlled Se-rich region on the supported CoSe2 powder, in order to increase further the corrosion resistance of this catalyst. The process used involved relatively fast cycling (sweep rate 100 mV s−1 ) on the stationary working electrode in O2 -saturated H2 SO4 solution, where, starting at 0.5 V, the potential was increased to 1.2 V, then reduced to 0.0 V and taken back to 0.5 V. Factors in the choice of this procedure include: (i) the Pourbaix diagram [19], which consistently with information in the Electrochemistry Encyclopedia [20] for use of 0.5 M H2 SO4 , suggests that this process will take through various oxidation states of Se (from −1 for CoSe2 to +6) while Co2+ remains stable; (ii) chances of photochemical effects with metallic Se can be reduced by starting at 0.5 V [20]; and (iii) empirically the conditions allow some Se to be formed at the surface without extensive depletion of the CoSe2 . Fig. 7 shows the CVs resulting from the first and fourth cycles, and it is clear they have similar shapes. Between 0.0 and 0.81 V, the CV curves are flat with no significant evidence for H2 desorption or oxidation of water-related species (e.g. OH− in the potential range 0.08–0.3 V [21]). Within this potential range, the curves for the four cycles overlap and that is consistent with the CoSe2 structure being basically maintained. However, with increase in potential from 0.81 to 1.2 V, the current increases rapidly consistently with oxidation first to Se(4+) and then to Se(6+). Correspondingly, as the potential cycles back from 1.2 to 0.0 V, the current decrease corresponds to reduction to elemental Se (in the range 0.87–0.74 V [19,20]). The Se deposition is assessed by surface analysis below, but it does affect the CV curves insofar as: (i) there is a small decrease in oxidation current from the first to the fourth cycle; (ii) there is some suppression of H2 desorption in the range 0.05–0.2 V (seen by comparing the shapes of the first and fourth curves in this potential range); and (iii) the onset potential for the surface oxidation current in the
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Fig. 8. ORR currents measured for the CoSe2 /C catalyst on (a) the ring electrode and (b) the disk electrode as a function of potential before and after the electrochemical surface modification described in Section 3.3.1. In each case the continuous line applies to before the modification and the dashed line is for after the modification. The measurements were made in O2 -saturated 0.5 M H2 SO4 with a catalyst loading of 100 g and a rotation rate of 900 rpm (see text for other details).
fourth cycle is delayed compared with the first cycle (the upswing in this current starts at 0.85 V for the fourth cycle instead of 0.81 V for the first cycle). Point (iii) apparently relates to the passivation effect of elemental Se, and so adds to the earlier evidence [5,8] that provided the motivation for undertaking this part of the project. More quantitative information for ORR on the CoSe2 /C catalyst was obtained using RRDE, as shown by the measured ORR currents in Fig. 8 before and after the surface modification. The electron transfer number and the percentage H2 O2 yield for the CoSe2 /C electrode surface were calculated with the equations n=
4Id Id + (Ir /N)
%H2 O2 =
100(2Ir /N) Id + (Ir /N)
(1) (2)
where N is the collection efficiency of the RRDE, and Id and Ir are the faradic currents at the disk and ring respectively [12]. The H2 O2 yield and the electron transfer number were determined to be 40% and 3.3 respectively for the electrode after surface modification, while without the electrochemical modification the corresponding values were found to be 50% and 3.5. The latter values are consistent with those reported by Feng et al. [10]. In principle the high H2 O2 yield can be suppressed by increasing the CoSe2 loading, which in general is proportional to the thickness of the catalyst layer. The differences seen in the two pairs of curves in Fig. 8, before and after the surface modification, appear as a consequence of changes in the structure of the catalytically active sites. This is assessed further by the surface characterizations reported in Section 3.3.2, but additional comments can be made now. The shape of the polarization curve for CoSe2 /C after the electrochemical surface modification (Fig. 8b) shows a minor peak at about 0.52 V, while before the surface modification there is a slope change (or kink) at around 0.59 V. These observations suggest that the ORR on the CoSe2 /C electrode after modification may occur as two consecutive processes, the first to reduce O2 to peroxide, and the second to take to water, with two electrons transferred in each process. The polarization curve of CoSe2 /C prior to electrochemical modification suggests the direct reduction of oxygen to water, possibly by the four-electron pathway. However, no well-defined plateau is observed in the polarization curve, indicating that at least one more reduction process must be occurring in parallel. The observation from this analysis, that the electron transfer number for these
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Fig. 9. Micro-Raman spectra measured for supported CoSe2 powder: (a) after application of the electrochemical surface modification procedure; (b) after the acid immersion treatment applied to the as-prepared CoSe2 sample.
electrodes is between 2 and 4, is consistent with that concept. A similar result was reported in independent work by Feng et al. [10] for an unmodified CoSe2 /C electrode, although that surface was not subjected to detailed characterization. 3.3.2. Spectroscopic characterizations Fig. 9(a) reports a Raman spectrum measured from the CoSe2 powder sample after it has been given this four-stage cycling treatment. Characteristic peaks are observed: that at 188 cm−1 is associated with CoSe2 [22,23], while that at 230 cm−1 is attributed to trigonal-Se0 [24–28]. The latter Raman peak was not detected before the electrochemical surface modification (e.g. Fig. 9(b) shows a comparable spectrum after the 30 min acid immersion treatment applied to the as-prepared sample), so suggesting that it truly results from Se0 generated during the cycling. The question of how the Se, which is not bonded to Co, is distributed across the powder surface was considered by first using BSE imaging [29,30], where the variation in brightness corresponds to variation of composition on the micro-regions of the treated CoSe2 powder, and then to use Auger point analysis to identify the nature of these surface regions. The BSE composition image in Fig. 10(a) shows three distinct regions, which are identified according to their relative brightness as brighter, darker and intermediate regions. Auger point analysis from the brightest areas detects the presence of Co LMM, Se LMM and C KLL transitions but no O KLL (Fig. 10(b)); the strongest transition in the Se LMM spectrum is still positioned at 1309.1 eV, which supports the predominance of Se2 2− , and therefore it is concluded that the brightest regions correspond to the supported CoSe2 particles. However, the Se/Co ratio from Auger analyses for this region is 2.4, and hence corresponds to more Se than for the expected stoichiometric ratio of 2.0. The Auger electron spectra from the intermediate regions reveal mainly carbon and selenium (Fig. 10(b)), while the dark regions are essentially just carbon, as expected for the exposed GC substrate. In relation to Fig. 10(a), the bright particles are thus shown to be CoSe2 with deposited Se, and the off-particle regions of intermediate brightness in the BSE image are carbon particles with some extra Se. Traces of Co and Se are detected from the intermediate (Fig. 10(b)) and dark regions respectively. This study, which depends on a combined approach of BSE imaging and micro-Raman and Auger spectroscopies, therefore establishes that the surface modification
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CoSe2 with the pyrite structure has superior performance for the oxygen reduction reaction compared with the monoselenides. Earlier studies have also indicated that the catalytic performance of some pure transition-metal disulphides can be increased by doping with a different disulphide [5–7] and such considerations may also apply for CoSe2 . However, an immediate point here was to show that, while high-purity CoSe2 can be susceptible to corrosion under atmospheric conditions, the material is basically stable in an electrochemical environment. Preliminary indications are given that the CoSe2 surface can be passivated by a simple cyclic voltammetric procedure to enhance corrosion resistance, without adding reactant to the electrolyte, while keeping a level of ORR activity. That part of the work emphasizes the need for specific characterizations of the catalyst surface, as opposed to the bulk, particularly since the electrochemical behaviour occurs at the surface. There was an emphasis to explore the basic science for a CoSe2 catalyst without, at this stage, optimizing any engineering aspects, such as the binding to carbon. Considerations of cost, compared with Pt, suggest it appears timely to start testing such alternative catalytic materials, as considered in this work, for ORR in actual unit and stack fuel cells. Acknowledgements
Fig. 10. Assessments of supported CoSe2 powder after the electrochemical surface modification: (a) a BSE image showing bright areas (region 1), intermediate areas (region 2) and dark areas (not marked) which correspond to the uncovered glassy carbon (see text); (b) Auger electron survey spectra from (i) region 1, and (ii) region 2.
from the cycling procedure leads to Se deposition over the active surface and to an absence of O after air exposure. In summary, the approaches described in this section appear effective for demonstrating some passivation, as would be required by an actual CoSe2 catalyst, and for examining the nature of the surface in order to identify the origin of the passivation. 4. Concluding remarks The work described in this paper represents part of a wider program to investigate different types of non-precious metal chalcogenide catalysts [5–8]. This project has been particularly motivated by the search for lower-cost alternatives to platinum which, since the early presentations in the 1840s, has been the catalyst of choice for the acid fuel cell [31], especially for the oxygen reduction reaction at the cathode. The research here prepared and characterized, in considerable detail, a CoSe2 catalyst supported on carbon powder, which was itself deposited on an electrode made of glassy carbon. This CoSe2 sample was shown to be capable of catalyzing the oxygen reduction reaction, although its performance remains less than that of Pt. For example, the OCP for supported CoSe2 is 0.81 V, about 0.19 V below Pt, and the cathodic current is one to two orders of magnitude less. Even so, the Tafel slope of CoSe2 (−113 mV decade−1 ) is similar to that of Pt (−120 mV decade−1 ) under comparable conditions, and that reflects some correspondence in shape of the measured polarization curves. Although an initial study in this program used materials based around the CoSe stoichiometry [8], this work makes very clear that
This research was supported jointly by Ballard Power Systems and the United States Department of Energy under contract DEFC36-03GO13107. The authors acknowledge the help of Ms. A. Lam in the Structural Chemistry Lab (Chemistry, UBC) for the XRD work, and thank the Natural Sciences and Engineering Research Council of Canada for support to M. Teo. References [1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B 56 (2005) 9–35. [2] D.S. Cameron, Platinum Met. Rev. 49 (2005) 16–20. [3] D. Hart, J. Power Sources 86 (2000) 23–27. [4] H. Behret, H. Binder, G. Sandstede, Electrochim. Acta 20 (1975) 111–117. [5] D. Susac, A. Sode, L. Zhu, P.C. Wong, M. Teo, D. Bizzotto, K.A.R. Mitchell, R.R. Parsons, S.A. Campbell, J. Phys. Chem. B 110 (2006) 10762–10770. [6] D. Susac, L. Zhu, M. Teo, A. Sode, K.C. Wong, P.C. Wong, R.R. Parsons, D. Bizzotto, K.A.R. Mitchell, S.A. Campbell, J. Phys. Chem. C 111 (2007) 18715–18723. [7] L. Zhu, D. Susac, M. Teo, K.C. Wong, P.C. Wong, R.R. Parsons, D. Bizzotto, K.A.R. Mitchell, S.A. Campbell, J. Catal. 258 (2008) 235–242. [8] L. Zhu, D. Susac, A. Lam, M. Teo, P.C. Wong, D. Bizzotto, S.A. Campbell, R.R. Parsons, K.A.R. Mitchell, J. Solid State Chem. 179 (2006) 3905–3911. [9] M. Gattrell, M. MacDougall, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cells: Fundamentals, Technology and Applications, vol. 2, John Wiley and Sons, New York, 2003, p. 443. [10] Y.J. Feng, T. He, N. Alonso-Vante, Electrochim. Acta 54 (2009) 5252. [11] T.J. Schmidt, H.A. Gasteiger, G.D. Stäb, P.M. Urban, D.M. Kolb, R.J. Behm, J. Electrochem. Soc. 145 (1998) 2354. [12] U.A. Paulus, T.J. Schmidt, H.A. Gasteiger, R.J. Behm, J. Electroanal. Chem. 495 (2001) 134. [13] International Center for Diffraction Database, PDF 03-065-3327 (CoSe2 ). [14] International Center for Diffraction Database, PDF 00-026-1076 (Carbon). [15] International Center for Diffraction Database, PDF 00-026-1080F (Carbon). [16] K. Du, F. Ernst, Int. J. Mater. Res. (2006/2007) 928–933 (Germany). [17] P.A. Stadelmann, Ultramicroscopy 21 (1987) 131–145. [18] N.P. Subramanian, S.P. Kumaraguru, H. Colon-Mercado, H. Kim, B.N. Popov, T. Black, D.A. Chen, J. Power Sources 157 (2006) 56–63. [19] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, TX, 1974, pp. 325, 557. [20] A.J. Bard (Eds), Encyclopedia of Electrochemistry of the Element, Marcel Dekker Inc., New York, 1975 (vol. III, pp. 43–200 (for Co) and vol. IV, pp. 361–392 (for Se)). [21] D.X. Cao, A. Wieckowski, J.J. Inukai, N. Alonso-Vante, J. Electrochem. Soc. 153 (2006) A869–A874. [22] E. Anastassakis, C.H. Perry, J. Chem. Phys. 64 (1976) 3604. [23] E. Anastassakis, Solid State Commun. 13 (1973) 1297. [24] F.Q. Guo, K. Lu, Phys. Rev. B 57 (1998) 10414–10420. [25] V.V. Poborchii, A.V. Kolobov, K. Tanaka, Appl. Phys. Lett. 72 (1998) 1167– 1169. [26] V.N. Bogomolov, V.V. Poborchy, S.G. Romanov, S.I. Shagin, J. Phys. C: Solid State 18 (1985) L313–317.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Synthesis, characterization of a CoSe2 catalyst for the oxygen reduction reaction L. Zhu a , M. Teo a , P.C. Wong a , K.C. Wong a , I. Narita b , F. Ernst b , K.A.R. Mitchell a,∗ , S.A. Campbell c,∗∗ a
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA c Ballard Power Systems Inc., 9000 Glenlyon Parkway, Burnaby, British Columbia, Canada V5J 5J9 b
a r t i c l e
i n f o
Article history: Received 21 April 2010 Received in revised form 22 July 2010 Accepted 25 July 2010 Available online 3 August 2010 Keywords: CoSe2 catalyst Oxygen reduction reaction Materials characterization Surface characterization Electrochemical surface modification
a b s t r a c t Crystalline CoSe2 , with the pyrite structure, supported on carbon powder has been synthesized by a wet chemical method. Samples were characterized by a range of techniques, including X-ray diffraction and transmission electron microscopy for structure, and X-ray photoelectron spectroscopy and scanning Auger microscopy (SAM) for surface composition. Electrochemical dynamic polarization measurements were carried out using the CoSe2 -decorated carbon powder as part of a rotating disk electrode to assess its catalytic activity for the oxygen reduction reaction (ORR), and the surface analysis methods were used to identify changes in surface composition. Comparisons of ORR activities were made with CoSe and Pt powder catalysts prepared by comparable methods. Stationary cyclic voltammetry was used to assess the stability of the as-prepared CoSe2 powder catalyst in an acidic electrochemical environment. A preliminary investigation was also made of an electrochemical modification treatment for CoSe2 . The surface formed was characterized by the rotating ring-disk electrode technique and by micro-Raman spectroscopy, backscattered-electron imaging and SAM. The latter combination of characterization techniques helped to relate an observed passivation to a thin layer of Se formed by the modification treatment. © 2010 Elsevier B.V. All rights reserved.
1. Introduction While the proton exchange membrane (PEM) fuel cell provides a potentially promising approach for vehicle propulsion due to its low greenhouse gas emissions, high efficiency and high energy density [1–3], a number of major issues still need to be resolved before widespread use in this direction is likely. Catalyst cost represents one obstacle to progress, although in principle that could be mitigated by the use of non-precious metal catalysts, instead of those based on Pt. Transition-metal chalcogenides provide one direction of opportunity [4–8], and the study reported in this paper forms part of a broader Department of Energy program to investigate the performance of transition-metal chalcogen combinations as model catalysts for the oxygen reduction reaction (ORR). Initial work considered Co–Se catalysts as thin films [5] since that form enabled a direct comparison with the performance of Pt, still the catalyst of choice for use in PEM fuel cells. Evidence was presented to show that thin films formed by the Co–Se system can be catalytically active for ORR, with a Tafel slope of −130 mV decade−1 which is close to that of Pt (−120 mV decade−1 ) [5,9]. A powder catalyst
∗ Corresponding author. Tel.: +1 6048225831. ∗∗ Corresponding author. Present address: AFCC Automotive Fuel Cell Cooperation Corp., 9000 Glenlyon Parkway, Burnaby, British Columbia, Canada V5J 5J8. E-mail addresses: [email protected] (K.A.R. Mitchell), [email protected] (S.A. Campbell). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.07.048
based on CoSe prepared on dispersed carbon powder showed a similar performance. Subsequent work with thin films based on the Ni–S, Co–S and Fe–S combinations showed larger ORR activities for the dichalcogenides with the pyrite structure [6,7], which, in the Co–Se system, is adopted by CoSe2 . However, the early work on the Co–Se thin-film system [5], prepared by magnetron sputtering using elemental Co and Se as targets, did not show evidence for a significant presence of CoSe2 , but a more recent study by Feng et al. has specifically investigated ORR performance on CoSe2 powders with different loadings [10]. The present study extends our knowledge in this area by preparing crystalline CoSe2 supported on carbon, and investigating its activity for ORR, which is compared against those of CoSe and Pt. Detailed assessments of the CoSe2 /C structure, under a variety of conditions including in tests for stability in electrochemical environments, were made with a range of characterization techniques including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, scanning Auger microscopy, micro-Raman spectroscopy and backscattered-electron imaging. The aim is to gain more understanding of how this type of material behaves at the microscopic level where the catalytic reactions occur. 2. Experimental The following procedure for preparing CoSe2 supported on carbon powder derived from that described previously for sup-
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ported CoSe [5]: 1 g of carbon powder (Vulcan XC72R, Cabot Corp.) was dispersed in 1 L of deionized water (NCCLS Type 1, ultra-pure, 18.2 M cm), 50 mL of isopropanol (Fischer Scientific) was introduced to aid wetting, followed by the addition of 10.2 g of CoSO4 ·7H2 O (Sigma, 99% purity) and 8.2 g of SeO2 (Aldrich, 99.9+% purity). After magnetic stirring for 1 h, 3.2 g of NaBH4 (Sigma–Aldrich, 99% purity) was added slowly to the solution, and this was followed by stirring for another 1 h. The precipitated powder was filtered, washed twice with ultra-pure deionized water, and then dried under an Ar atmosphere in a tube furnace (4 h, 600 ◦ C). Such samples were characterized directly by X-ray diffraction (XRD) using a Bruker D8 advanced diffractometer (Cu K␣ radiation) with the scattering angle (2) scanned from 5◦ to 90◦ (rate 0.02◦ s−1 ). Prior to measurements, the XRD instrument was calibrated with a NIST 1976 XRD flat-plate intensity corundum standard (Bruker). For the other characterizations, the supported CoSe2 as prepared above was re-deposited according to the subsequent test being made. For measurements with transmission electron microscopy (TEM), the CoSe2 on carbon powder was ultrasonically dispersed in ethanol. A drop of the dispersion was deposited onto an ultrathin amorphous carbon film, supported by a thicker holey carbon film residing on a Cu grid, and dried. TEM was mostly carried out using an analytical Tecnai F30 instrument (FEI) with a 300 kV fieldemission gun; this instrument is equipped with a scanning TEM (STEM) unit, a high-angle angular dark-field (HAADF) detector, suitable for recording “Z-contrast” images, and a high-resolution Si–Li detector (129 eV at Mn K␣) for X-ray energy-dispersive spectrometry (XEDS). The magnification of TEM images and the camera length were calibrated with a standard of gold nanoparticles on amorphous carbon. For measurements with convergent-beam electron diffraction (CBED), we used a CM20 instrument (Philips), equipped with a LaB6 emitter and operated at 200 kV. All electrochemical characterizations were made with the CoSe2 powder deposited onto a polished glassy carbon (GC) electrode (Tokai Carbon). The deposition started with 20 mg of the powder sample being ultrasonically dispersed in glacial acetic acid (2 mL), and 10 L of that dispersion was pipetted onto the electrode surface and dried in hot air. The coated GC electrode described above formed the working electrode in a standard three-electrode glass cell with a reversible hydrogen electrode (RHE) for reference (which was connected to the Luggin capillary) and a Pt wire as the counter electrode. All potentials are quoted with respect to the RHE. The electrochemical set-up was described in our previous study for the Co–Se thin film [5]. The electrode design, with a “top-hat” geometry, allowed it to act as a rotating disk electrode (RDE) and to facilitate direct sample transfer for the surface characterizations, as described previously [5–8]. Unless otherwise stated, RDE measurements reported here were made at 2000 rpm. Typically, after an electrochemical characterization, a sample was washed in doubledeionized water, dried with Ar gas, and stored in an Ar atmosphere prior to the transfer for surface characterization. A rotating ring-disk electrode (RRDE, from Pine Instruments) was employed to investigate electron transfer number and H2 O2 yield experienced by the CoSe2 /C catalyst. All RRDE measurements were performed in parallel with RDE measurements using a jacketed three-compartment electrochemical cell, a rotator (ARS, Pine Instruments) and a computer-controlled Solartron 1408 multi-channel potentiostat (Ametek). Areas of the disk electrode and ring-disk electrode were 0.164 and 0.247 cm2 respectively. Before making RRDE measurements, each prepared electrode was inspected with a digital optical microscope (ImagingSource) to ensure sufficient uniformity for the coating layer, with no catalyst deposited on the ring. In addition, assessments were made for the thickness of the catalyst coating. This was evaluated using the method of Schmidt et al. [11] to ensure that the binding layer resis-
tance was negligible and that the catalyst layer was thin enough to allow the geometric area to be used directly in electrochemical calculations for assessing ORR mechanism. The H2 O2 yield was determined with the potential of the Pt ring electrode set to 1.2 V, at which value the detection of peroxide is diffusion limited. Polarization curves with the RRDE were recorded with a sweep rate of 2 mV s−1 and a rotation speed of 900 rpm. The collection efficiency of the RRDE was measured to be 0.392 by using the method of Paulus et al. with a 10 mM solution of K3 [Fe(CN)6 ] in N2 -saturated KOH [12]. The CoSe2 powder surface was characterized by X-ray photoelectron spectroscopy (XPS) using a Leybold MAX200 spectrometer operated with the Mg K␣ source (1253.6 eV) at 10 kV, 20 mA, with a 48 eV pass energy, and the analytical chamber at 2 × 10−9 mbar; all binding energies were referenced to the adventitious hydrocarbon C 1s peak at 285.0 eV. Scanning Auger microscopy (SAM) was used for characterizing local regions of the supported CoSe2 powder samples using a Microlab 350 system (Thermo Electron Corp.) equipped with a field-emission source (10 keV, 3.5 nA) and hemispherical energy analyzer; this equipment also allowed characterization with scanning electron microscopy (SEM) and backscattered-electron (BSE) imaging. Survey-scan Auger electron spectra and those at higher-energy resolution were measured using a constant retarding ratio (CRR) of 4.0 and 40.0 respectively. MicroRaman spectra were acquired using the 632.8 nm (HeNe) laser on a Renishaw inVia Raman Microscope using a 20× objective (Numerical Aperture = 0.40). Each spectrum was acquired using 10% of the maximum output laser power (<0.5 mW on the sample surface) by co-adding signals from a 100 s exposure. Before each Raman measurement, the instrument was calibrated with a standard Si sample (wavenumber 520 cm−1 ).
3. Results and discussion 3.1. XRD and TEM analyses for the as-prepared sample Fig. 1 compares the diffractogram measured by XRD for the sample designated above as CoSe2 supported on the XC72R carbon powder with the diffractogram from a reference standard sample of crystalline cubic CoSe2 [13]. The close match in peak positions between the two diffractograms confirms that a successful synthesis of supported CoSe2 with the pyrite structure was accomplished. The sharpness of the peaks and the low background in Fig. 1(a) suggest that the supported CoSe2 has been prepared with a high degree of crystalline perfection. The measured peak widths correspond to an average particle size of about 130 nm. Further, the measured diffractogram shows only weak peaks from carbon [14,15], and no additional species were detected either by XRD or XEDS. Together these observations indicate that our CoSe2 powder has been prepared with a good state of purity, and apparently a high mass loading. Fig. 2(a) shows a conventional TEM bright-field image recorded from the same CoSe2 powder sample. The image reveals two kinds of particles: (i) larger dark particles with diameters of order 100 nm, and (ii) smaller light-gray particles with diameters of order 50 nm. Fig. 2(b) presents a Z-contrast STEM image of the same area. In this image, the intensity is proportional to the square of the average local projected atomic number Z. Only the dark particles seen in (a) show up with bright contrast in this image, indicating they contain heavier elements, such as Co and Se. The light-gray particles in Fig. 2(a) do not show up in Fig. 2(b), consistently with their being composed of light material, such as the amorphous carbon. Elemental mapping by XEDS in the STEM mode confirmed this interpretation. Fig. 2(c) presents an XEDS spectrum recorded with a primary beam diameter of 10 nm at the location marked ‘XEDS’ in
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cubo-octahedron faceted on {1 1 1} planes and truncated on {2 0 0} planes. 3.2. Evaluation of supported CoSe2 powders
Fig. 1. X-ray diffractograms (a) measured in this work for as-prepared CoSe2 supported on XC72R carbon powder and (b) reference diffractogram for cubic CoSe2 [13].
(b). A quantification of the spectrum, not using standards, yielded a Se/Co ratio of 2.0. This is consistent with CoSe2 , although uncertainties in the analysis do prevent a definitive assignment on this evidence alone. The carbon and Cu peaks in Fig. 2(c) originate from the carbon support, the holey carbon film, and the Cu TEM grid. Fig. 2(d) presents a histogram for the CoSe2 particle diameter, obtained by evaluating about 300 particles in Z-contrast images like that in (b). The average particle diameter is 152 ± 5 nm. The standard deviation of the distribution is 81 nm, and the distribution is significantly skewed towards larger particle size. The particle sizes estimated for local regions by TEM are comparable with the estimate given previously by XRD. According to these data, the CoSe2 particles are much larger than common Pt catalyst nanoparticles, which can be as small as 3 nm [16]. The upper half of Fig. 2(e) shows an experimental convergentbeam electron diffraction (CBED) pattern recorded from one of the particles that show the dark contrast in Fig. 2(a) (for better visibility of fine detail, the contrast and brightness in the center and in the periphery were adjusted differently). For comparison with the experimental pattern, the lower half of Fig. 2(e) shows a simulated CBED pattern of CoSe2 with the pyrite structure. The simulation was carried out with the software package JEMS (the Java version of EMS [17]) for dynamical multibeam conditions and a crystal thickness of 180 nm, that is close to the average crystal thickness displayed in Fig. 2(d). The comparison between the experimental and simulated pattern in (e) reveals excellent agreement. In the zero-order Laue zone (the inner part of the pattern), the simulated pattern satisfactorily reproduces the spot pattern, spot positions, and relative spot brightness. Further, the simulation nicely reproduces the main Kikuchi lines and the first-order Laue zone ring in the experimental pattern. Based on this excellent agreement, there is no doubt that the synthesized nanoparticles are indeed CoSe2 with the pyrite structure. As seen for several examples in Fig. 2(a) and (b), most of the CoSe2 particles are faceted. Apparently, the “ideal” shape (from which the real particles may substantially deviate) corresponds to a
3.2.1. Catalytic activity for ORR Fig. 3 compares electrochemical measurements of current as a function of potential applied to CoSe2 supported on carbon powder that has in turn been deposited on the GC rotating disk electrode. These measurements, done separately in N2 -saturated and O2 saturated 0.5 M H2 SO4 electrolyte solutions, were made from the open circuit potential (OCP) to 0.1 V vs. RHE. The OCP was measured potentiometrically as the voltage between the working electrode of interest and the counter electrode when no current was drawn in that circuit. In the N2 -saturated electrolyte (i.e. O2 -free), the OCP of the CoSe2 powder is 0.74 V vs. RHE, while it is increased to 0.81 V vs. RHE for the O2 -saturated solution. The current flowing in the latter case, in the potential window 0.0–0.4 V vs. RHE, is about 40 times larger than that in the N2 -saturated solution (Fig. 3). This provides direct evidence for the catalytic ability of supported CoSe2 powder to sustain cathodic current with the presence of O2 in an acidic environment. Fig. 4 compares dynamic cathodic polarization curves measured in O2 -saturated 0.5 M H2 SO4 solution, in a potential vs. current density plot, for a blank GC substrate, the CoSe2 on XC72R carbon, and 40 wt% of Pt (HiSpec 4000, Johnson Matthey) on XC72R powder; in addition a comparison is included with earlier measurements for CoSe supported on carbon [5]. The polarization curves in Fig. 4 have been background-corrected insofar as the contribution from the GC (known area) has been subtracted, and constant loadings of the XC72R carbon are used in each case; with the latter we believe that these curves enable an indication of the relative activities of the different catalysts. Supported CoSe2 powder clearly shows a systematic increase in current compared with the GC substrate alone which itself does not contribute much catalytic activity (its OCP is 0.70 V vs. RHE). Furthermore, the polarization curve for supported CoSe2 has a comparable shape to that for Pt in the supported state, although the two curves are displaced so that within the kinetic control region (0.78–0.72 V vs. RHE), the current for CoSe2 is about 1.5 orders of magnitude less than that of Pt, and its OCP is 0.19 V less than for Pt. Incidentally the Tafel slope measured for CoSe2 is −113 mV decade−1 , in close correspondence with that of Pt (−120 mV decade−1 ) [9]. Overall CoSe2 does demonstrate a significant intrinsic catalytic activity, and in the mass-transfer region (below 0.3 V) its polarization curve matches closely with that for Pt. That may suggest a common ORR mechanism for electron transfer. By contrast, given differences in the measured polarization curves, the mechanisms are likely different for the CoSe [5] and CoSe2 powders supported on GC, and the latter is clearly the more active for ORR. Thus it manifests currents approximately one order of magnitude greater than that for CoSe, and its OCP is about 0.09 V greater. Such observations clearly demonstrate the enhanced ORR activity from the pyrite structure (i.e. CoSe2 ) compared with that for CoSe (for which a monoclinic non-stoichiometric form of Co1−x Se (x = 0.125–0.25) had been proposed previously [8]). 3.2.2. XPS and SAM analysis for as-prepared and acid treated samples Since surface composition is likely to be the prime determinant of electrochemical catalytic behavior [5–8], SAM and XPS were used to characterize surfaces associated with CoSe2 powder in the asprepared state, after acid immersion for 30 min in 0.5 M H2 SO4 , and after electrochemical ORR tests. It will be argued later that the acid immersion treatment helps to simulate the nature of the electrode surface in an active electrochemical environment, specif-
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Fig. 2. Electron microscopic characterizations for CoSe2 particles supported on XC72R carbon powder: (a) bright-field TEM image showing CoSe2 particles (dark) and carbon particles (lighter); (b) Z-contrast image of the region in (a) recorded with the high-angle angular dark-field detector; (c) X-ray energy-dispersive spectrum from the location marked “XEDS” in (b); (d) size histogram from about 300 CoSe2 particles; and (e) convergent-beam electron diffraction pattern obtained from a CoSe2 particle (upper half) and a simulation for CoSe2 with the pyrite structure (lower half).
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Fig. 3. Dynamic cathodic polarization measurements for supported CoSe2 powder (loading 100 g) in O2 -saturated 0.5 M H2 SO4 and compared against the equivalent measurements in N2 -saturated solution for a sweep rate of 5 mV s−1 and rotation speed 2000 rpm.
ically insofar as it helps remove surface oxide formed by ambient air exposure. For the as-prepared supported CoSe2 powder, Auger point analysis detects cobalt oxide on the surface. This is evidenced by O KLL Auger signals present in the survey-scan spectrum (not shown) and a peak at 750.0 eV associated with oxidized cobalt in the Co LMM spectrum (Fig. 5(a), spectrum (i)). This peak is removed by the acid immersion treatment (Fig. 5(a), spectrum (ii)), which leaves a single Co component at 754.5 eV, and correspondingly the Auger survey spectrum no longer shows evidence for an O signal. Fig. 5(b) compares Se LMM spectra for elemental Se (spectrum (i)), for which the most intense peak is at 1306.9 eV, with that for the sample that had been immersed in acid (spectrum (ii)) which has the corresponding peak at 1309.1 eV. The shift of 2.2 eV compared with elemental Se does not appear to be an artifact, since other Se LMM peaks are unchanged; earlier such a shift was taken
Fig. 4. Comparison of dynamic polarization curves in O2 -saturated 0.5 M H2 SO4 for blank glassy carbon (GC) and for CoSe, CoSe2 and Pt each supported on XC72R carbon powder (see text) with all loadings close to 100 g. Each set of measurements was for a sweep rate of 5 mV s−1 and rotation speed 2000 rpm. Tafel lines are shown for the curves measured for CoSe2 and Pt.
as indicative of a Co–Se bonding interaction [5]. Nevertheless the shift of 2.2 eV found here is greater than the value of 1.2 eV identified previously for the CoSe sample [5], an observation that seems likely to be associated with their different crystal structures. Quantitative Auger analysis from the as-prepared CoSe2 powders gave Se/Co atomic ratios as low as 1.7, apparently because of surface oxidation which results in the presence of some CoO, but after the acid immersion treatment that ratio reverts to the stoichiometric value expected for the CoSe2 sample (i.e. 2.0).
Fig. 5. Auger electron spectra: (a) Co LMM spectra from supported CoSe2 powder: (i) as-prepared and (ii) after acid immersion treatment (see text); and (b) Se LMM spectra: (i) from high-purity Se, and from supported CoSe2 after: (ii) acid immersion treatment and (iii) ORR measurement.
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Fig. 6. Cyclic voltammetry plots for as-prepared supported CoSe2 powder in O2 saturated 0.5 M H2 SO4 showing measurements for the 1st, 2nd and 10th cycles to assess electrochemical stability. In these measurements the electrode is stationary and the sweep rate is 10 mV s−1 .
Comparative information was also obtained by XPS, but Auger electron spectra are emphasized in this paper because they are needed later in a context where the high spatial resolution with SAM is beneficial. The observations from the Auger measurements that the as-prepared supported CoSe2 powder has some oxide on its surface, which is removed by the acid immersion treatment, are also confirmed with XPS. With the latter technique, it is concluded that the Co 2p3/2 and Se 3d peaks, observed at 779.0 and 55.7 eV respectively, together provide a signature of a cleaned CoSe2 surface. Fig. 5(b) part (iii) shows the Se LMM spectrum measured from the supported CoSe2 powder after an ORR measurement. The similarity to the spectrum in part (ii), for after the acid immersion treatment, along with other comparable observations made with Auger electron spectroscopy and XPS, confirms that the nature of the surface after the acid immersion test is comparable to that after an electrochemical characterization. The evidence is that any oxide from air exposure is removed in either case, and that after one of those treatments the Se/Co ratio reverts to the value expected for stoichiometric CoSe2 at the surface. 3.2.3. Electrochemical stability test for CoSe2 powder Fig. 6 compares cyclic voltammograms measured through ten successive cycles for the as-prepared supported CoSe2 powder sample, in O2 -saturated 0.5 M H2 SO4 solution, while the potential is swept between the OCP and 0.0 V vs. RHE. In this case the electrode is stationary (in order to reduce the possibility of particle detachment), and the scanning rate is 10 mV s−1 . The structure observed for the first cycle is reproducible for these conditions. The features at 0.55 and 0.75 V vs. RHE correlate with the effect of surface oxide, which was identified above by surface analysis, and also with some contribution from functional groups on the carbon powder [9,18]. After the first cycle, the behaviour seems largely determined by the CoSe2 surface, although some detachment of carbon powder does occur which causes a degree of shrinkage in the cyclic voltammogram. In any event, after the first cycle, there is a good level of match between the different voltammograms through to the tenth cycle. The match appears complete for potentials above 0.4 V, while deviations noted in the lower potential region are likely associated with non-equilibrium effects from hydrogen evolution [9]. The first cycle for the as-prepared sample showed an OCP at 0.81 V vs. RHE, but for all other cycles the OCP remains constant at 0.80 V.
Fig. 7. The first and fourth cyclic voltammograms measured for CoSe2 powder in O2 saturated 0.5 M H2 SO4 to test the electrochemical modification approach described in the text. In these measurements, the electrode was stationary with a scanning rate of 100 mV s−1 .
This simple test suggests that supported CoSe2 may have stability under ORR working conditions, although for an application to fuel cells there remains concern about surface corrosion in an atmospheric environment. Following earlier work on thin-film catalysts based on CoSe [5], and the passivation possible with surface layers of Se, the next section considers a possible electrochemical process for surface modification of the supported CoSe2 powder. 3.3. Surface modification of supported CoSe2 powder 3.3.1. Electrochemical characterizations A cyclic voltammetric (CV) procedure was used to test whether some surface CoSe2 can be sacrificed to form a controlled Se-rich region on the supported CoSe2 powder, in order to increase further the corrosion resistance of this catalyst. The process used involved relatively fast cycling (sweep rate 100 mV s−1 ) on the stationary working electrode in O2 -saturated H2 SO4 solution, where, starting at 0.5 V, the potential was increased to 1.2 V, then reduced to 0.0 V and taken back to 0.5 V. Factors in the choice of this procedure include: (i) the Pourbaix diagram [19], which consistently with information in the Electrochemistry Encyclopedia [20] for use of 0.5 M H2 SO4 , suggests that this process will take through various oxidation states of Se (from −1 for CoSe2 to +6) while Co2+ remains stable; (ii) chances of photochemical effects with metallic Se can be reduced by starting at 0.5 V [20]; and (iii) empirically the conditions allow some Se to be formed at the surface without extensive depletion of the CoSe2 . Fig. 7 shows the CVs resulting from the first and fourth cycles, and it is clear they have similar shapes. Between 0.0 and 0.81 V, the CV curves are flat with no significant evidence for H2 desorption or oxidation of water-related species (e.g. OH− in the potential range 0.08–0.3 V [21]). Within this potential range, the curves for the four cycles overlap and that is consistent with the CoSe2 structure being basically maintained. However, with increase in potential from 0.81 to 1.2 V, the current increases rapidly consistently with oxidation first to Se(4+) and then to Se(6+). Correspondingly, as the potential cycles back from 1.2 to 0.0 V, the current decrease corresponds to reduction to elemental Se (in the range 0.87–0.74 V [19,20]). The Se deposition is assessed by surface analysis below, but it does affect the CV curves insofar as: (i) there is a small decrease in oxidation current from the first to the fourth cycle; (ii) there is some suppression of H2 desorption in the range 0.05–0.2 V (seen by comparing the shapes of the first and fourth curves in this potential range); and (iii) the onset potential for the surface oxidation current in the
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Fig. 8. ORR currents measured for the CoSe2 /C catalyst on (a) the ring electrode and (b) the disk electrode as a function of potential before and after the electrochemical surface modification described in Section 3.3.1. In each case the continuous line applies to before the modification and the dashed line is for after the modification. The measurements were made in O2 -saturated 0.5 M H2 SO4 with a catalyst loading of 100 g and a rotation rate of 900 rpm (see text for other details).
fourth cycle is delayed compared with the first cycle (the upswing in this current starts at 0.85 V for the fourth cycle instead of 0.81 V for the first cycle). Point (iii) apparently relates to the passivation effect of elemental Se, and so adds to the earlier evidence [5,8] that provided the motivation for undertaking this part of the project. More quantitative information for ORR on the CoSe2 /C catalyst was obtained using RRDE, as shown by the measured ORR currents in Fig. 8 before and after the surface modification. The electron transfer number and the percentage H2 O2 yield for the CoSe2 /C electrode surface were calculated with the equations n=
4Id Id + (Ir /N)
%H2 O2 =
100(2Ir /N) Id + (Ir /N)
(1) (2)
where N is the collection efficiency of the RRDE, and Id and Ir are the faradic currents at the disk and ring respectively [12]. The H2 O2 yield and the electron transfer number were determined to be 40% and 3.3 respectively for the electrode after surface modification, while without the electrochemical modification the corresponding values were found to be 50% and 3.5. The latter values are consistent with those reported by Feng et al. [10]. In principle the high H2 O2 yield can be suppressed by increasing the CoSe2 loading, which in general is proportional to the thickness of the catalyst layer. The differences seen in the two pairs of curves in Fig. 8, before and after the surface modification, appear as a consequence of changes in the structure of the catalytically active sites. This is assessed further by the surface characterizations reported in Section 3.3.2, but additional comments can be made now. The shape of the polarization curve for CoSe2 /C after the electrochemical surface modification (Fig. 8b) shows a minor peak at about 0.52 V, while before the surface modification there is a slope change (or kink) at around 0.59 V. These observations suggest that the ORR on the CoSe2 /C electrode after modification may occur as two consecutive processes, the first to reduce O2 to peroxide, and the second to take to water, with two electrons transferred in each process. The polarization curve of CoSe2 /C prior to electrochemical modification suggests the direct reduction of oxygen to water, possibly by the four-electron pathway. However, no well-defined plateau is observed in the polarization curve, indicating that at least one more reduction process must be occurring in parallel. The observation from this analysis, that the electron transfer number for these
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Fig. 9. Micro-Raman spectra measured for supported CoSe2 powder: (a) after application of the electrochemical surface modification procedure; (b) after the acid immersion treatment applied to the as-prepared CoSe2 sample.
electrodes is between 2 and 4, is consistent with that concept. A similar result was reported in independent work by Feng et al. [10] for an unmodified CoSe2 /C electrode, although that surface was not subjected to detailed characterization. 3.3.2. Spectroscopic characterizations Fig. 9(a) reports a Raman spectrum measured from the CoSe2 powder sample after it has been given this four-stage cycling treatment. Characteristic peaks are observed: that at 188 cm−1 is associated with CoSe2 [22,23], while that at 230 cm−1 is attributed to trigonal-Se0 [24–28]. The latter Raman peak was not detected before the electrochemical surface modification (e.g. Fig. 9(b) shows a comparable spectrum after the 30 min acid immersion treatment applied to the as-prepared sample), so suggesting that it truly results from Se0 generated during the cycling. The question of how the Se, which is not bonded to Co, is distributed across the powder surface was considered by first using BSE imaging [29,30], where the variation in brightness corresponds to variation of composition on the micro-regions of the treated CoSe2 powder, and then to use Auger point analysis to identify the nature of these surface regions. The BSE composition image in Fig. 10(a) shows three distinct regions, which are identified according to their relative brightness as brighter, darker and intermediate regions. Auger point analysis from the brightest areas detects the presence of Co LMM, Se LMM and C KLL transitions but no O KLL (Fig. 10(b)); the strongest transition in the Se LMM spectrum is still positioned at 1309.1 eV, which supports the predominance of Se2 2− , and therefore it is concluded that the brightest regions correspond to the supported CoSe2 particles. However, the Se/Co ratio from Auger analyses for this region is 2.4, and hence corresponds to more Se than for the expected stoichiometric ratio of 2.0. The Auger electron spectra from the intermediate regions reveal mainly carbon and selenium (Fig. 10(b)), while the dark regions are essentially just carbon, as expected for the exposed GC substrate. In relation to Fig. 10(a), the bright particles are thus shown to be CoSe2 with deposited Se, and the off-particle regions of intermediate brightness in the BSE image are carbon particles with some extra Se. Traces of Co and Se are detected from the intermediate (Fig. 10(b)) and dark regions respectively. This study, which depends on a combined approach of BSE imaging and micro-Raman and Auger spectroscopies, therefore establishes that the surface modification
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CoSe2 with the pyrite structure has superior performance for the oxygen reduction reaction compared with the monoselenides. Earlier studies have also indicated that the catalytic performance of some pure transition-metal disulphides can be increased by doping with a different disulphide [5–7] and such considerations may also apply for CoSe2 . However, an immediate point here was to show that, while high-purity CoSe2 can be susceptible to corrosion under atmospheric conditions, the material is basically stable in an electrochemical environment. Preliminary indications are given that the CoSe2 surface can be passivated by a simple cyclic voltammetric procedure to enhance corrosion resistance, without adding reactant to the electrolyte, while keeping a level of ORR activity. That part of the work emphasizes the need for specific characterizations of the catalyst surface, as opposed to the bulk, particularly since the electrochemical behaviour occurs at the surface. There was an emphasis to explore the basic science for a CoSe2 catalyst without, at this stage, optimizing any engineering aspects, such as the binding to carbon. Considerations of cost, compared with Pt, suggest it appears timely to start testing such alternative catalytic materials, as considered in this work, for ORR in actual unit and stack fuel cells. Acknowledgements
Fig. 10. Assessments of supported CoSe2 powder after the electrochemical surface modification: (a) a BSE image showing bright areas (region 1), intermediate areas (region 2) and dark areas (not marked) which correspond to the uncovered glassy carbon (see text); (b) Auger electron survey spectra from (i) region 1, and (ii) region 2.
from the cycling procedure leads to Se deposition over the active surface and to an absence of O after air exposure. In summary, the approaches described in this section appear effective for demonstrating some passivation, as would be required by an actual CoSe2 catalyst, and for examining the nature of the surface in order to identify the origin of the passivation. 4. Concluding remarks The work described in this paper represents part of a wider program to investigate different types of non-precious metal chalcogenide catalysts [5–8]. This project has been particularly motivated by the search for lower-cost alternatives to platinum which, since the early presentations in the 1840s, has been the catalyst of choice for the acid fuel cell [31], especially for the oxygen reduction reaction at the cathode. The research here prepared and characterized, in considerable detail, a CoSe2 catalyst supported on carbon powder, which was itself deposited on an electrode made of glassy carbon. This CoSe2 sample was shown to be capable of catalyzing the oxygen reduction reaction, although its performance remains less than that of Pt. For example, the OCP for supported CoSe2 is 0.81 V, about 0.19 V below Pt, and the cathodic current is one to two orders of magnitude less. Even so, the Tafel slope of CoSe2 (−113 mV decade−1 ) is similar to that of Pt (−120 mV decade−1 ) under comparable conditions, and that reflects some correspondence in shape of the measured polarization curves. Although an initial study in this program used materials based around the CoSe stoichiometry [8], this work makes very clear that
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