Article pubs.acs.org/JPCC
Extended X-ray Absorption Fine Structure and X-ray Diffraction Examination of Sputtered Nickel Carbon Binary Thin Films for Fuel Cell Applications Bridget Ingham,*,†,‡ Nicola Gaston,†,‡ Kieran Fahy,§ Xiao Yao Chin,§ Christian J. Dotzler,† Eric Rees,§ Gareth Haslam,§ Zoe H. Barber,§ G. Timothy Burstein,§ and Mary P. Ryan∥ †
Industrial Research Ltd., 69 Gracefield Road, Lower Hutt 5011, New Zealand The MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand § Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom ∥ Department of Materials and London Centre for Nanotechnology, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, United Kingdom ‡
S Supporting Information *
ABSTRACT: Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) are used to study the structure of sputtered binary nickel carbon alloy films (5− 44 atom % Ni) for use as potential electrocatalysts in acidic solutions. Three compositional regions are identified: “low” (5 atom % Ni), where the structure consists mainly of isolated Ni atoms or dimers in a carbon matrix; “medium” (11−24 atom % Ni), where the Ni−Ni nearest-neighbor coordination is increased but there is little longer-range order; and “high” (35−44 atom % Ni), where crystalline Ni3C is formed. This indicates a threshold concentration of Ni of between 25 and 35 at% before Ni3C starts to form.
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rates than WC.7,8 In addition, much work has been done using WC as a support material with a small amount of Pt covering the surface; this combination has been found to exhibit superior electrocatalytic performance and corrosion resistance and also appears to circumvent the CO poisoning problem of Pt.2,9,10,20−30 Such an electrode uses low Pt quantities but is not Pt-free. Some authors surmise that submonolayer coverages of Pt on WC may be necessary for practical applications.9 Other transition metal carbide systems that have been studied include molybdenum carbides10,31 and carburized single crystals of V,32 Mo,33,34 Ti,35 and W.2,36−39 Ternary systems such as Fe−W−C,40,41 Co−W−C,8,41−43 Co−Mo−C,31 Ni− Ta−C,44,45 Ni−W−C,8,40,41,46,47 Ni−Mo−C,40 Mo−W−C,42,48 and W−Ta−C49 have also been studied. A Co−W−C electrode was reported to have a power density of around 14% that of a 20% Pt/C catalyst.42 Therefore, exploring transition metal carbides other than WC may yet lead to a material suitable for replacing Pt as an electrocatalyst.
INTRODUCTION Transition metal carbides are promising electrocatalytic materials that could be used in place of Pt and other precious metal catalysts in low-temperature fuel cells to alleviate the problems of high cost and limited abundance. A number of review articles in the literature describe recent progress in various transition metal carbide systems.1−3 The majority of work in this field focuses on the tungsten carbide system since WC has similar electronic properties near the Fermi level to Pt.4,5 WC is stable as an anode in acidic solutions due to its passivity against corrosion.3,6,7 It is also much less susceptible than Pt to CO poisoning.1,8,9 However, in aqueous solutions, WC tends to form oxide species which may be detrimental to the electrocatalytic activity.10−16 When used as an anode for methanol oxidation, it has been reported that methane comprises a significant fraction of the decomposition product, which is undesirable.17,18 In contrast, Mo2C does not produce any methane as a decomposition product in methanol oxidation.19 However, Mo2C is less passive against corrosion in acidic solution than WC.10 Other tungsten carbide stoichiometries have also been studied. A number of reports explored W2C as the electrocatalyst; however, this has lower activity and higher corrosion © 2012 American Chemical Society
Received: July 31, 2011 Revised: February 16, 2012 Published: February 21, 2012 6159
dx.doi.org/10.1021/jp207308g | J. Phys. Chem. C 2012, 116, 6159−6165
The Journal of Physical Chemistry C
Article
being made through the sputtered Au film) and as the window through which X-rays can penetrate to probe the Ni−C film. The purpose of the Au is solely to provide electrical contact to the sample material for the polarization studiesit does not interfere with the electrochemistry (based on similar measurements performed on a variety of systems52,53). A conventional three-electrode geometry was used in the electrochemical cell, with a Au counter electrode and Ag/AgCl/ KCl (3 M) reference electrode. The electrolyte was 1.5 M H2SO4, and the solution was at 23 ± 2 °C. The surface area of the sample exposed to the electrolyte is determined by the window in the in situ electrochemical cell as ∼0.75 cm2. Each experiment consisted of first measuring the EXAFS spectrum of the as-deposited film in air and then introducing the electrolyte to the cell. A series of short XAS scans were recorded to monitor the dissolution rate. The following electrochemical procedure was used: Step 1. Polarization at −150 mV (vs Ag/AgCl) until the XAS intensity stabilized. Step 2. Polarization at +200 mV (vs Ag/AgCl) until the XAS intensity stabilized (or in the case of the higher Ni content samples, a 50% drop in intensity was observed). Step 3. Polarization at −230 mV (vs Ag/AgCl) until the XAS intensity stabilized. After the XAS intensity stabilized in step 3, a second EXAFS spectrum was recorded with potential control being maintained throughout the measurement. In addition to the samples, a Ni foil standard was measured in a transmission geometry. The EXAFS data from the samples and standards were prepared and analyzed using Athena and Artemis54 with FEFF6.55 The averaging, background subtraction, and normalization were performed using standard routines. The data were fitted in R-space with k1, k2, and k3 weightings simultaneously. X-ray Diffraction. X-ray diffraction measurements were performed using the Powder Diffraction beamline at the Australian Synchrotron. The samples deposited on Mylar (as described above) were mounted in air in a transmission geometry. The beam size was 3 × 1 mm (h × v), and the X-ray wavelength was 0.6196 Å (20 keV), calibrated using a LaB6 NIST 660b standard. Data were collected for exposure times up to 1 h using a Mythen detector spanning 80° of arc. The data were analyzed using Topas 2.0 (Bruker). Density Functional Theory Calculations. Density functional theory (DFT) calculations56 were performed using VASP (Vienna Ab-initio Simulation Package)57−61 employing the PBE (Purdue, Becker, Erzenhof) functional.62 Ultrasoft pseudopotentials were used for Ni and C, with a plane wave cutoff of 286 eV and a simple cubic cell allowing a minimum of 10 Å between repeated cluster images (the cell size was scaled with the size of the cluster). Spin-polarized calculations were performed resulting in nonzero magnetic moments.
Most crystalline transition metal carbides are metal rich, e.g., Fe3C, Ni3C. These would not be suitable for anode materials in acidic solutions because the materials corrode rapidly. However, thin film fabrication techniques such as sputtering enable nonstoichiometric materials to be produced, by which the metal/carbon ratio can be varied continuously to alleviate these corrosion problems. Amorphous nickel−carbon sputtered films with Ni/C ratios of 1:3 have been shown to have good electrocatalytic activity toward the hydrogen oxidation reaction while also being passive against corrosion in acidic solutions.50 A high-resolution transmission electron microscopy study of a film of similar composition showed that the film consisted of small (
