J. Phys. Chem. C 2007, 111, 18715-18723
18715
Characterization of FeS2-Based Thin Films as Model Catalysts for the Oxygen Reduction Reaction D. Susac,† L. Zhu,† M. Teo,† A. Sode,† K. C. Wong,† P. C. Wong,† R. R. Parsons,‡ D. Bizzotto,† K. A. R. Mitchell,† and S. A. Campbell*,§ Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer, British Columbia, Canada V6T 1Z1, Department of Physics & Astronomy, UniVersity of British Columbia, 6224 Agricultural Road, VancouVer, British Columbia, Canada V6T 1Z1, and Ballard Power Systems Inc., 9000 Glenlyon Parkway, Burnaby, British Columbia, Canada V5J 5J8 ReceiVed: May 3, 2007; In Final Form: July 31, 2007
FeS2 and (Fe,Co)S2 thin films prepared by magnetron sputtering have been investigated as model catalysts for the oxygen reduction reaction (ORR), and their activities were compared against that of a sputtered thin film of Pt. Scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and micro-Raman spectroscopy have been used, in parallel with electrochemical activity measurements using the thin film as a rotating disk electrode (RDE), to assess how the electrochemical performances of the sulfide films relate to chemical composition and structure. Comparisons were also made against a mineral FeS2 pyrite whose open circuit potential (OCP) was 0.62 V and much less than the values of 0.78 and 0.80 V found for the FeS2 and (Fe,Co)S2 thin films, respectively (all potentials are given vs the reversible hydrogen electrode). There are indications that the ORR activities for these films may be associated with the presence of some polysulfides in addition to the expected S22bulk and surface sites.
1. Introduction Proton exchange membrane (PEM) fuel cells have been tested by Ballard Power Systems and other companies as electrical power generators in buses and experimental vehicles since 1995.1 Current PEM fuel cells use Pt and Pt alloys as catalysts for the anodic hydrogen oxidation and the cathodic oxygen reduction reactions, usages that contribute significantly to the cost of this technology.2 The development of less expensive materials is therefore needed, especially for the oxygen reduction reaction (ORR). To enable proton transfer from anode to cathode, PEM fuel cells use acidic ionomers with operating pHs less than 1.3 This implies that alternative cathode catalysts must be stable in an acidic environment, as well as having an open circuit potential (OCP) and activity comparable to those of Pt. A promising direction has been the development of ORR catalysts based upon Fe and Co with N, the precursors adsorbed on carbon supports being pyrolyzed at 600-900 °C.4-7 Tungsten carbide with addition of Ta has shown electrocatalytic activity and stability for ORR, although the OCP has not exceeded 0.8 V vs reversible hydrogen electrode (RHE).8 Cluster compounds based on Mo and Ru in combination with S, Se, and Te have demonstrated significant electrocatalytic activity,9-12 but economic limitations are likely with Ru as the cost is expected to increase with usage. Earlier, various chalcogenide-based transition metal spinels and pyrites were investigated for ORR catalytic activity,13,14 but that was before the opportunities for PEM fuel cells were fully realized. * To whom correspondence should be addressed. E-mail: stephen.
[email protected]. † Department of Chemistry, University of British Columbia. ‡ Department of Physics and Astronomy, University of British Columbia. § Ballard Power Systems.
The work presented in this paper is part of a broader program that is focused on the investigation of materials based on Fe, Co, and Ni combined with S and Se as catalysts for ORR. The methodology15 as recently demonstrated involves preparation of these materials, including a standard Pt reference, in the form of thin films deposited on to glassy carbon (GC) substrates to produce electrodes with similar surface areas. Such an approach allows the catalytic activity of a material based on a nonprecious metal compound to be compared with that of Pt, a comparison that is not possible with non-Pt catalysts supported on powders due to the difficulty in measuring their surface areas. In the previous work,15 a specially designed rotating disk electrode (RDE) holder made feasible the transfer of samples between the electrochemical and ultrahigh vacuum (UHV) surface characterization systems, thereby enabling measurement of thinfilm surface composition in an initial prepared state as well as after electrochemical characterization. The study of these model systems aims to provide chemical and structural knowledge needed to help guide the design and selection of ORR catalysts for industrial applications. This paper describes the preparation, surface characterization, and electrochemical ORR evaluation of FeS2-based thin films prepared by magnetron sputtering and comparisons with FeS2 in mineral form (pyrite), which is known to have activity for ORR in acidic environments.16-19 However, OCP values of natural samples, as well as of synthetic pyrite, have been reported to be no higher than 0.62 V vs RHE. Earlier work of Behret et al.13 indicated that mixed pyrites (e.g., solid solutions formed by CoS2 in FeS2) can show enhanced ORR activity, and this encouraged the extension of our study to the (Fe,Co)S2 system. FeS2 is a semiconducting material with a band gap of 0.95 eV, which makes it promising for photovoltaic and
10.1021/jp073395i CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007
18716 J. Phys. Chem. C, Vol. 111, No. 50, 2007 optoelectronic applications.20,21 For that work, FeS2 thin films have been prepared by sulfurization of iron thin films,22,23 by metal-organic chemical vapor deposition (MOCVD),24 and by reactive magnetron sputtering.25-27 The structural, thermoelectrical, and optical properties of such samples have been widely reported, but information on the chemical nature of their surfaces compared to natural minerals is limited. This work will show that the surface compositions of FeS2-like samples prepared by magnetron sputtering from a FeS2 target are different from those of natural minerals, which apparently influences the ORR activity. Studies of the effect of Co addition used a film prepared by sputtering from a target which was prepared by cold pressing FeS2 and CoS2 powders with elemental sulfur. The sputtered thin films were characterized by a range of techniques: energy dispersive X-ray (EDX) spectroscopy was used to obtain bulk chemical compositions, scanning electron microscopy (SEM) was used for surface topography, and surface composition was analyzed with X-ray photoelectron spectroscopy (XPS) and scanning Auger microscopy (SAM). In addition, X-ray diffraction (XRD) and micro-Raman spectroscopy were used to provide structural information, while catalytic activity for ORR was measured electrochemically using the RDE approach. 2. Experimental Section 2.1. Thin Film Preparation. The thin films were deposited on to glassy-carbon (GC) substrates (Tokai Carbon), which had been machined and polished according to the previously described procedure,15 to provide a working surface area of 1.23 cm2. The deposition used a sputter coater system (model V3T, Corona Vacuum Coaters Inc.) with pyrite target (FeS2 purity 99.9%, Superconductor Materials, Inc.) connected to a radio frequency (rf) (13.6 MHz) power supply operated at 38 W; the target to substrate distance was 11 cm. Similar operating conditions were used for preparing the mixed (Fe,Co)S2 thin film, which used a composite target prepared by cold pressing (pressure 35 MPa) equal weights of FeS2 powder (purity 99.9%, Sigma Aldrich) and CoS2 powder (purity 99.5%, Alfa Aesar) with some S powder (purity 99.98%, Sigma Aldrich). Prior to each deposition, the coater system was turbomolecular pumped to a base pressure of 3.5 × 10-6 Torr. The plasma discharge was operated at 3.5 × 10-3 Torr with a continuous throttled flow of argon (99.9%), and the targets were presputtered for at least 10 min prior to moving the substrate into the deposition zone. A bias of -180 V was applied to the substrate for the first minute of each deposition, with the objective of helping to promote subsequent film adhesion. A -75 V bias was applied for the remaining deposition, with the time adjusted to achieve film thicknesses of up to ∼0.3 µm. A natural pyrite sample (Zacatecas, Mexico) was cleaved in air, and the fresh surface was exposed to air for 1 h prior to introduction into the UHV chamber for surface analysis. This was done deliberately, since the sputtered thin films were exposed to air during handling before and after the electrochemical analysis. 2.2. Compositional and Structural Characterization of Thin Films. EDX spectra were measured with a Hitachi 3000 N/X spectrometer (20 keV incident electron beam), and the elemental peaks were converted to atomic percentages using Xone software. A Leybold MAX200 spectrometer with Mg KR source (hν ) 1253.6 eV) operated at 15 kV and 20 mA was used to measure XPS survey scans and higher resolution spectra with pass energies of 192 and 48 eV, respectively. Binding energies were referenced to the Au 4f7/2 peak at 84.0 eV.
Susac et al. Elemental amounts were estimated from peak areas using sensitivity factors provided by the manufacturer. A Microlab 350 spectrometer (Thermo Electron Corp.), equipped with a field emission source and a hemispherical energy analyzer, was used to measure local surface composition by SAM; this instrument also gave SEM images for assessing surface topography. The Auger electron spectroscopy (AES) measurements were performed on different surface microregions, with the primary electron beam set at 10 keV and 3.5 nA, and the analyzer operated in the constant retardation ratio mode with CRR values of 4 and 10 for the survey and higher resolution spectra, respectively. X-ray diffraction (XRD) measurements were performed using a Bruker D8 advanced diffractometer with Cu KR radiation source (λ ) 1.5418 Å), and intensities were recorded as the scattering angle, 2θ, varied from 5 to 90° at 0.02° s-1. Raman spectra were acquired using the 632.8 nm line of a heliumneon (HeNe) laser (Renishaw inVia Raman Microscope) with a 20× objective (numerical aperture of 0.40). Each spectrum was acquired using 10% of the maximum laser power (
