ARTICLE pubs.acs.org/JPCC
Enhanced Electrocatalysis of Oxygen Reduction on Pt/TaOx/GC Zaenal Awaludin, Masatoshi Suzuki, Jahangir Masud, Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ABSTRACT: Here we demonstrate a remarkable enhancement of oxygen reduction reaction (ORR) activity on a novel Pt/ TaOx/GC electrocatalyst where at first tantalum oxide (TaOx) and next Pt were deposited electrochemically on a glassy carbon (GC) surface. An excellent electrocatalytic activity of the Pt/TaOx/ GC electrocatalyst for ORR was found to be more than 12 times that of the unmodified Pt/GC one as evaluated from the kinetic currents at 0.80 V. SEM images showed no significant differences in the size and distribution of Pt nanoparticles between these two electrocatalysts, indicating that these are not factors causing the observed ORR activity. The spillover of oxygen-containing species resulting from the electronic interaction between Pt and TaOx, which is evidently demonstrated from the XPS analysis, is strongly suggested as the crucial factor for the ORR enhancement. Interestingly, the spillover effect also results in a remarkable increase in the electrochemically active “apparent” surface area of Pt on the Pt/TaOx/GC electrocatalyst. Moreover, the rotating ring-disk electrode voltammetric measurements obviously showed the increase in limiting current as well as the decrease in ring current on this novel electrocatalyst relative to the unmodified one, confirming a complete four-electron reduction pathway. On the basis of these findings a plausible mechanism has been proposed for the observed enhancement in ORR where the role of TaOx is to reduce the formation of OH on the Pt surface by spillover effect and to promote d orbital vacancy of Pt for oxygen adsorption by electron donation to Ta.
1. INTRODUCTION A polymer electrolyte membrane fuel cell (PEMFC) is one of promising devices in energy conversion and greenhouse gas reduction.14 This device converts chemical energy of fuel (H2) directly into electrical energy, generally having a higher efficiency than conventional combustion engine.1 However, the commercialization of PEMFC is predominantly hindered by a high content of Pt and slow kinetics of oxygen reduction reaction (ORR).1,2,57 State-of-the-art Pt/C catalysts possess high activity of ORR at Pt loading of ca. 0.4 mg cm2, but lowering this Pt content causes a poor activity of the catalyst for ORR.1,6 The agglomeration of Pt in the Pt/C catalyst811 is also a serious problem to be resolved for achieving a highly active and durable catalyst for cathode in PEMFC. The modification of Pt with transition metals (M) (M = Ni, Co, Fe, V, Ti) has resulted in a higher electrocatalytic activity toward ORR than pure Pt catalyst.14,1113 There have been several hypotheses to explain the enhancement of ORR on Ptalloy catalysts. A lattice contraction of PtPt bond distance is proposed to promote ORR by producing more sites for dissociative adsorption of oxygen.1416 An electronic interaction resulting in promoting the Pt-d band vacancy is also expected to play an important role for the enhancement of ORR on Pt alloys.4,17 Meanwhile, other studies suggest that the increased ORR activity of Pt alloys is attributed to the inhibition of adsorbed OH (OHad) formation, which is confirmed from electrochemical and XANES data.13,16 Practically, the use of Ptalloys/C has resulted in a positive shift (ca. 2040 mV) of the onset potential of ORR compared with Pt/C catalyst.1,6 r 2011 American Chemical Society
Therefore, this modification significantly increases the ORR activity of Pt, but the dissolution of the alloys is unhindered, particularly in acidic media.18 The effects of strong metal oxide-support interaction have attracted much attention in many catalytic reactions including ORR.1923 A spillover phenomenon, which was originally noticed and defined by Boudart et al.2426 for the heterogeneous catalysis of the Pt/WO3 system is one of the effects introduced by this interaction that alter catalytic activity of noble metal.2129 The spillover of oxygencontaining species is also suggested as a main factor that enhances the electrocatalysis of ORR in the composites of Pt with metal oxide, MOx (M = Mo, W, Ti).23,3037 The previous investigation by Bockris and McHardy predicted that the spillover of adsorbed species from platinum to NaxWO3 results in the increased velocity of the rate-determining species (RDS) of ORR on Pt surface and leads to the increase in reaction rate by 150 times compared with pure Pt catalyst.34 Elezovic and coworkers33 have also mentioned that the increase in catalytic activity of MoOxPt/C, in comparison with Pt/C catalyst, could be explained through the increase in active sites for the ORR or by synergetic effects due to the formation of the interface between the platinum and oxide materials and by spillover due to surface diffusion of oxygen reaction intermediates. In addition, these metal oxide supports have a high stability in acidic environments and also help in preventing the supported Pt particles from aggregation.8,3842 Received: September 27, 2011 Revised: November 6, 2011 Published: November 08, 2011 25557
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The Journal of Physical Chemistry C Tantalum and its oxides (TaOx) have been regarded as very stable materials in acidic media due to the formation of a passive layer of Ta2O5.4145 TaOx has also attracted much attention in many catalytic reactions such as esterification, alkylation, and oxidation of organic molecules.4648 To our knowledge, there have been only a few reports regarding the effect of TaOx as insulator on ORR.49,50 Baturina et al.51 have reported that the Ta2O5/Pt/VC and Pt/Ta2O5/VC electrodes, in which the former was prepared by deposition of Pt colloids on Vulcan carbon (VC) and then Ta2O5, whereas the latter by the deposition of Ta2O5 on VC and then Pt colloids, give a decreased electrochemically active surface area (ECSA) resulting in a lower catalytic activity of ORR compared with the Pt-deposited VC electrode. A similar result was also reported by Bonakdarpour et al.43 where the addition of Ta also caused a negative impact on the electrocatalytic activity of Pt toward ORR. Very recently, Wesselmark et al.52 also reported that the Pt-deposited TaOx electrode, in which TaOx layer and then Pt layer were deposited on a gas diffusion layer (GDL) by thermal evaporation in vacuum, resulted in a significant loss of ECSA and ORR activity. In contrast with these reports, recently, our group53 has demonstrated that amorphous TaOx electrodeposited Pt (TaOx/Pt) electrode, which was prepared by electrodeposition of TaOx on Pt electrode in propylene carbonate (PC), possesses a remarkably enhanced catalytic activity toward ORR in acidic media. The onset potential of ORR at the TaOx/Pt electrode positively shifted with a decrease in production percentage of hydrogen peroxide (H2O2) in comparison with that at the pure Pt. The different electrocatalysis of TaOx composites for ORR, which largely depends on the procedures of their preparation, has not been essentially clarified yet. In the present study, we have successfully developed a novel composite electrocatalyst of Pt nanoparticles-deposited TaOx modified glassy carbon (Pt/TaOx/GC) with a high electroactivity for ORR with a low Pt content. The Pt/TaOx/GC electrocatalyst was prepared by the electrodeposition of TaOx on GC substrate, followed by the electrodeposition of Pt nanoparticles. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the electrochemical and electrocatalytic properties of this composite electrocatalyst, together with the surface characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The so-called spillover phenomena of hydrogen and oxygen-containing species have been observed, demonstrating a remarkable increase in the real surface area of Pt. The XPS analysis on Pt and TaOx suggested strong evidence of the metal oxidesupport interaction. On the basis of the results obtained, a plausible mechanism has been proposed for the observed enhancement in ORR.
2. EXPERIMENTAL SECTION The electrodeposition of TaOx and Pt and electrochemical measurements was carried out in a standard three-compartment electrochemical cell using Pt spiral for counter electrode. All electrochemical experiments were conducted at room temperature (set at 25 °C) using an ALS/CH electrochemical analyzer (model 760 Ds). A reference electrode of Ag|AgCl|KCl(sat) was used where the cell was separated by a salt bridge from the reference compartment. Exceptionally, we used Ag wire as a reference electrode in the electrodeposition of TaOx. All potentials are reported with respect to a reversible hydrogen electrode (RHE), unless otherwise noted.
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2.1. Preparation of Electrocatalysts. Prior to the electrodeposition, a bare GC electrode (6 mm in diameter, 0.283 cm2 of geometric surface area) was mechanically and electrochemically pretreated to produce a clean GC surface. First, a GC electrode was polished mechanically on emery paper with grid no. 2000. The polishing was followed with aqueous slurries of successively finer alumina powder (1 μm down to 0.06 μm) with the help of a polishing microcloth. To remove the slurries, the electrode was washed ultrasonically in >18 MΩ Millipore water for 10 min and electrochemically pretreated by polarizing the electrode at 2.0 V versus Ag|AgCl|KCl(sat) in Ar-saturated 0.1 M H2SO4 solution for 20 s to enhance the stability of Pt particles.54,55 The TaOx was first electrodeposited on the pretreated GC electrode. The electrodeposition of TaOx on the GC electrode was basically carried out according to our preceding report.53 All chemicals were used as received either from Wako Pure Chemical Industries or Kanto Chemical without any further purification. PC solution containing 1 M lithium perchlorate (LiClO4) and 0.05 M tantalum pentachloride (TaCl5) were used as electrolyte and source of Ta, respectively. A cyclic potential scan technique was employed to deposit TaOx on the GC in the potential range between 0.3 and 2.0 V versus Ag wire for two cycles at a scan rate of 20 mV s1. Just after the deposition of TaOx, the electrode was soaked subsequently in methanol and water for 1 min to remove organic impurities on the GC surface. This electrode was then transferred to the electrochemical cell for the post-treatment, which was conducted by repeating the potential scan in the potential range between 0.06 and 1.46 V at a scan rate of 0.20 V s1 for ca. 30 cycles or until a steady CV was obtained. The thus-prepared electrode will be hereinafter denoted as TaOx/GC electrode. To complete the fabrication of Pt/TaOx/GC electrocatalyst, we carried out the electrodeposition of Pt on the TaOx/GC electrode in 0.1 M hydrochloric acid (HCl) solution containing 3 mM potassium hexachloroplatinate (K2PtCl6). A bipotential step technique was used to obtain nanosized particles of Pt according to the previous report.56 First, the electrode was potentiostated at 1.24 V for 0.1 s, followed by the deposition of Pt at 0.02 V for 10 to 20 s. This method was expected to yield Pt particles with ca. 20 nm of size on the GC surface.56 The amount of charge of Pt deposition was set so as to obtain ∼6 μg Pt cm2. After this deposition, the electrode was electrochemically pretreated by repeating the potential scan in the potential range between 0.06 and 1.30 V in Ar-saturated 0.1 M HClO4 electrolyte solution at scan rate of 0.20 V s1 for several cycles until a steady CV was obtained. The thus-prepared electrocatalyst will be hereinafter denoted as Pt/ TaOx/GC. The same technique of Pt electrodeposition was also applied to the bare GC substrate, and the prepared electrode will be denoted as Pt/GC electrocatalyst. 2.2. Surface Characterizations. In XPS measurements, the prepared electrocatalysts were placed under an ultra high vacuum (UHV) condition (1 106 Pa) where an ESCA-3400 electron spectrometer (SHIMADZU) with unmonochromatized X-ray source (Mg Kα (hυ = 1253.6 eV) anode, emission current: 20 mA, and acceleration voltage: 10 kV) was employed. For each measurement, all XPS spectra were corrected to the internal reference spectra of C 1s at 284.5 eV to compensate the electrostatic charging. Then, the core-level spectra were carefully deconvoluted to investigate the chemical states of elements on the electrocatalyst surface. For Ta 4f7/2 and 4f5/2 spectra, the binding energy gap and area ratio were kept constant at 1.9 and 1.4 eV, respectively.57 For deconvolation of C 1s spectra, the 25558
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Figure 1. SEM images of TaOx/GC (a,a0 ), Pt/TaOx/GC (b,b0 ), Pt/GC (c,c0 ) electrocatalysts taken at two different magnifications.
value of full with at half-maximum (fwhm) was also fitted to 1.3 eV for sp2 hybridization of CC in pyrolytic graphite.58 To take insight into the depth-profile of TaOx, a pumped Ar+ ion gun was sputtered to remove the oxide layer on the electrode surface. For SEM characterizations, the electrode surface was examined using an SEM S4700 (Hitachi). The XRD patterns were obtained from a diffractometer (Philips X0 pert pro MRD), where the radiation source was obtained from Cu Kα. The measurements were performed from 30 to 90° and 35 to 50° at different scanning rates. 2.3. Electrochemical Measurements. The electrochemical characterizations were performed with the electrocatalyst immersed in 0.1 M HClO4 solution where prior to each measurement the electrolyte solution was purged with either Ar or O2 gas (99.999% of purity) for 25 min. In the ORR study, the RRDE voltammetric measurements were carried out using Nikko Keisoku Motor speed controller in O2-saturated solution and recorded at various rotation rates. In this case, the Pt ring electrode was potentiostated at 1.2 V, where hydrogen peroxide was completely oxidized under a diffusion control. All current densities were normalized by the geometric surface area of electrode. The real Pt surface area of Pt/GC electrocatalyst, which is usually defined as ECSA, and the “apparent” Pt surface
area of Pt/TaOx/GC electrocatalyst were calculated by the conventional manner based on an electric charge for the hydrogen desorption,59,60 supposing 210 μC cm2 for a smooth polycrystalline Pt surface. In the case of the latter electrocatalyst, the thus-calculated Pt surface area reflects the so-called hydrogen spillover, as demonstrated in the present study and thus will hereinafter be denoted as “apparent” surface area.
3. RESULTS AND DISCUSSION 3.1. SEM, EDX, and XRD Analyses. The SEM images of TaOx/GC, Pt/GC, and Pt/TaOx/GC electrocatalysts were taken with different magnifications and are shown in Figure 1. In the case of the TaOx/GC electrocatalyst (Figure 1a,a0 ), the TaOx particles, estimated as about 13 nm in size, disperse homogenously on the GC surface. Figure 1b,b0 ,c,c0 shows a distribution of Pt particles on the Pt/TaOx/GC and Pt/GC elctrocatalysts, respectively. As seen from these Figures, there is no marked difference in the distribution and morphology of Pt particles and TaOx appears to have no significant effect on the Pt electrodeposition. However, interestingly, the Pt ion complex (PtCl62) was reduced at the TaOx/GC electrode at more 25559
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Figure 2. Histograms of size distribution of Pt particles in the Pt/GC (a) and Pt/TaOx/GC (b) electrocatalysts obtained from random measurements of 100 particles (in Figure 1).
Figure 3. EDX spectrum of the surface of Pt/TaOx/GC electrocatalyst.
positive potential compared with the bare GC electrode and for depositing the same amount of Pt, the time of electrolysis for the Pt deposition on the former electrode was shorter than that on the latter electrode. The detailed electrochemical behaviors will be reported elsewhere. A distribution of Pt or Pt/TaOx particle size on the Pt/GC and Pt/TaOx/GC electrocatalysts is shown in Figure 2 in which 100 particles in the SEM images of both electrocatalysts were randomly selected and their size was measured. The mean sizes of particles are 19.8 and 21.9 nm for the Pt/GC and Pt/TaOx/ GC electrocatalysts, respectively. In addition, the particle size distribution of the Pt/TaOx/GC electrocatalyst is narrower than that of the Pt/GC electrocatalyst. This may reflect the effect of TaOx on the GC surface in the growth of Pt nuclei during the Pt deposition as anchor sites.55 The EDX pattern of the Pt/TaOx/GC electrocatalyst (Figure 3) clearly shows an elemental characteristic of the surface, that is, the presence of C, O, and a small amount of Ta and Pt (extremely
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Figure 4. XRD patterns of the Pt/GC (a,a0 ) and Pt/TaOx/GC (b,b0 ), electrocatalysts. The peaks indicated by Δ and 2 correspond to the graphite (100) and Pt (111), respectively. The inset figure shows the XRD patterns of these electrocatalysts at a slower scan rate in the range of 2θ between 35 and 50°.
low intensities). A quantitative EDAX ZAF analysis revealed that the weight fractions of Ta and Pt are 0.88 and 0.61%, respectively. The XRD pattern shows the diffraction peak corresponding to the Pt (111) at 39.8°, but no diffraction peak corresponding to the Ta2O5 is observed (Figure 4).53,61 The inset figure gives more apparent diffraction peaks, which were measured at lower scan rate. However, we could not observe any diffraction peak corresponding to Ta2O5 or TaOx, which confirms an amorphous phase of TaOx in the Pt/TaOx/GC surface. 3.2. XPS Analysis. The electronic state of Ta after the electrodeposition on the GC surface was comprehensively studied by XPS, as shown in Figure 5. From the curve fitting after deconvolution (Figure 5a), it is clearly seen that Ta exists in one chemical state shown by a doublet peak at the binding energies of 26.4 and 28.3 eV corresponding to Ta 4f7/2 and Ta 4f5/2, respectively.53,61,62 This means that these binding energies shift by 4.8 eV to the higher values with respect to the Ta metal; that is, the spectra correspond to the 5+ oxidation state of Ta, which is the most stable oxidation state of Ta.57 The only oxidation state of 5+ of Ta as Ta2O5 was also observed in our previous studies.53,62 To examine the oxidation states of Ta under the surface Ta2O5 layer, we performed a controlled erosion of the top layer of Ta2O5 via Ar+etching. The thusobtained XPS spectrum is shown in Figure 5b. As demonstrated in this Figure, even after 1 min of Ar+ etching, Ta0 could not be observed: after deconvolution of the spectrum the lowest binding energy of Ta 4f7/2 was found to still be 2.9 eV higher than that of elemental Ta. So, we convincingly confirm that Ta metal does not exist in the deposit; that is, it is expressed as TaOx (0 < x e 5). In addition, the observation of various peaks of TaOx other than that corresponding to the Ta2O5 also suggests that Ta species of various oxidation states may contribute in the electrocatalytic activity of the Pt/TaOx/GC for the ORR (mentioned below). Figure 6 shows the typical XPS spectra of Pt 4f obtained at the Pt/TaOx/GC and Pt/GC electrocatalysts in which the XPS spectrum for Pt plate is also given for comparison. The corelevel spectra of Pt 4f7/2 and Pt 4f5/2 for the bulk Pt are 70.8 and 25560
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Figure 5. Ta (4f) XPS spectra of the TaOx/GC electrocatalyst before (a) and after (b) Ar+ etching for 1 min to observe the chemical states of Ta under the surface of Ta2O5 layer. The dotted lines correspond to the binding energies of 4f7/2 and 4f5/2 of elemental Ta.
74.1 eV, respectively. The fitted spectra of Pt 4f7/2 and Pt 4f5/2 for both Pt/TaOx/GC and Pt/GC electrocatalysts shift to the higher binding energies (71.1 and 74.4 eV) with respect to those for the bulk Pt. This result strongly indicates an electronic interaction of Pt and the substrates.55,63,64 In the case of the Pt/GC electrocatalyst, we suggested that the size effect of Pt particles could alter the binding energy of Pt 4f according to the explanation by Takasu and coworkers:65 small particle size of Pt, that is,
