Yttria

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Janus-Faced SiO2: Activation and Passivation in the Electrode System Platinum/Yttria-Stabilized Zirconia €rgen Janek Eva Mutoro,*,† Nils Baumann,‡ and Ju Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany

ABSTRACT Small amounts of impurities can strongly impact the electrode kinetics of electrochemical cells and devices, leading to an improved or declined performance. This study shows that one material, silica, can influence the electrochemical behavior of the basic solid-state electrode system Pt(O2)/YSZ (yttriastabilized zirconia) in opposite directions, depending on its local distribution in the system. Compared to a silica-free electrode as the reference, SiO2 located at the interface Pt/YSZ caused a reduced performance, while SiO2 on the surface of the Pt electrode caused a small enhancement. The passivation can be explained by blocking of active three-phase boundary sites (tpb, Pt/YSZ/O2), while silica surface doping is hypothesized to influence the oxygen adsorption/desorption and surface diffusion. SECTION Surfaces, Interfaces, Catalysis

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ontaminants may act either as poisoning or activating agents in catalyst, electrode, or sensor systems. Regarding high-temperature solid-state electrochemical systems, silica (SiO2) and related glass phases are often found at the electrolyte/electrode interfaces1-4 and are well-known to deactivate the electrode kinetics.5,6 As small amounts of silica are ubiquitous in oxide ceramics, this is an inherent problem in view of the performance of high-temperature applications, such as solid oxide fuel cells (SOFC).1,7 A Si-rich surface layer is usually formed during annealing of YSZ (yttriastabilized zirconia, the most important oxygen solid electrolyte) in air at temperatures between 1073 and 1773 K8-10 and under working conditions of SOFCs.11 Silica-free Pt/YSZ interfaces show improved electrode kinetics,12 and glass particles in sintered Pt paste electrodes decrease the current density.13 Here, we demonstrate that SiO2 can either activate or passivate Pt/YSZ electrodes, dependent on its spatial distribution in the Pt phase. We contaminated Pt/YSZ electrodes systematically at different regions with SiO2 by pulsed laser deposition (PLD); Pt paste electrodes on YSZ single crystals were prepared (a) without SiO2, (b) with SiO2 doping of the surface, and (c) with a SiO2 interlayer between Pt and YSZ; see also Figure 1. In order to exclude the influence of morphology changes caused by the electrochemical and temperature treatment, sintered porous electrodes have been used, which are inherently stable under the experimental conditions. The effect of native impurities, originating from the Pt paste (Supporting Information SI 1) or the YSZ single crystal,13 is identical in all samples and thus does not affect the results. The Pt electrodes consisted of porous, polycrystalline (Figure 2a-c), and nontextured films on the YSZ(111) single crystals (Supporting Information SI 2). A film thickness of several μm (Figure 2b) was determined by a cross section.

r 2010 American Chemical Society

Figure 1. Electrode reactions, geometry, and cell setup; (a) reference system Pt/YSZ/Pt, (b) SiO2 surface-doped system, and (c) SiO2 interface-doped system. WE, CE, and RE = working, counter, and reference electrode, respectively; tpb = three-phase boundary.

EDX line scans (Supporting Information SI 1) indicate a few iron oxide particles on the undoped Pt paste surface. Neither XRD nor EDX was sensitive enough to detect the small amounts of deposited SiO2 (Supporting Information SI 1,2). SEM images show some small SiO2 particles (droplets, intrinsic imperfection of PLD) on the surface (Figure 2c,d). XPS and SIMS prove the surface doping; Figure 3 depicts the Si surface enrichment (see Supporting Information SI 3 for more details). XPS (Figure 4) shows the Si 2p peak at a binding energy Ebind of 103.40 eV, and the Pt 4f peak is at Ebind = 70.55 eV, which agrees with SiO2 and Pt. All samples showed Butler-Volmer-type electrode kinetics. Silica interface doping led to decreased exchange current Received Date: May 22, 2010 Accepted Date: July 6, 2010 Published on Web Date: July 12, 2010

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DOI: 10.1021/jz1006884 |J. Phys. Chem. Lett. 2010, 1, 2322–2326

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Figure 2. SEM images of (a) the reference WE (top view), (b) a cross section (breaking edge), (c) a surface-doped sample (160 laser pulses SiO2), and (d) 3000 pulses of Si on YSZ.

Figure 4. XPS of a surface-doped sample (after electrochemical studies, 450 pulses of SiO2), (a) Pt 4f and (b) Si 2p.

Figure 3. TOF-SIMS depth profile of a surface-doped sample (after electrochemical studies, 450 pulses of SiO2). Note that the relative SIMS intensities do not offer absolute quantitative information.

densities j0, as depicted in Figure 5. This result is in accordance with the literature12 and is usually explained by blocking of active sites4,6 at the tpb (cf. Figure 1). The j0 values of identically prepared samples (0 laser pulses of SiO2) typically scatter, which reflects the statistical character of the porous electrode/electrolyte contact (Supporting Information SI 4). In order to eliminate these effects, we investigated individual samples for the surface doping and subsequently increased the doping level of silica. Detailed information on the data analysis can be found in the Experimental Methods section and the Supporting Information (SI 5). Surprisingly, the surface-doped samples showed an increasing j0 with increasing silica contamination, going through a maximum and decreasing again when the silica layer became thicker. Consistent with that, few μm thick and covering SiO2 films (Supporting Information SI 6) prepared by thermal decomposition of HMDSO (hexamethyldisiloxane) showed either strong passivation or even no current at all. Figure 6 shows the cathodic (a) and anodic (b) regions of the measured steady-state I-V characteristics. The influence of SiO2 surface doping on the exchange current densities j0 is


Figure 5. Exchange current densities j0 versus the number of laser pulses of SiO2 deposited at the interface of Pt/YSZ electrodes. The reference electrode used for comparison in Figure 6d is marked green. The j0 values were determined from the cathodic low-field region.

shown in Figure 6c,d. The labeling 1 and 2 corresponds to two different electrodes; the surface of electrode 1 is covered with SiO2 in 50 laser pulse increments, and electrode 2 is covered in 20 laser pulse increments. Here, j0 values have been determined by the low-voltage approximation in the cathodic range (Supporting Information SI 5c). For analyses of the anodic ranges and a comparison of the current densities j at

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Figure 6. Change of electrode performance with SiO2 surface doping; steady-state measurements of the (a) cathodic region and (b) anodic region, (c) influence of SiO2 doping on the exchange current density (two different tested electrodes, labeled 1 and 2, are depicted;( a) and (b) correspond to electrode 2), and (d) comparison with the undoped reference. For the surface-doped samples, every second measurement has been plotted; for other measurements, see Supporting Information SI 5. Error bars were determined from linear regression; lines only provide optic guidance.

higher applied voltage (VWR = (0.5 V), we refer to Supporting Information SI 5. We checked the electrochemical behavior of a nondoped sample under comparable conditions (temperature, heating/ cooling). The results are also shown in Figure 6d and prove that surface doping is responsible for the observed activation. Interestingly, the activation effect usually became smaller in a subsequent second measurement (see Supporting Information SI 5a). Electrode activation over time is a frequently observed phenomenon in solid-state electrochemistry. Usually, the effect is explained in rather general terms by morphological changes (increase of active sites, e.g., tpb sites) or compositional changes (e.g., segregation of electrocatalytically active elements and their accumulation at active sites). In the present case, it is unlikely that the small amount of SiO2 on top of the surface of a several μm thick Pt film affects the active tpb. Morphology changes were not taken into account in this study either, as they should have appeared as well in the undoped reference system. Moreover, we could not detect any morphological instability due to the experimental treatment by SEM. In liquid electrochemistry, it is well-known that metal electrode surfaces modified with small amounts of a different


metal (usually only (sub)monolayers) can affect their kinetics, for example, the oxygen reduction reaction.14,15 Activation, passivation, and changes in the reaction pathway have been observed. At this point, we can only hypothesize about the possible origin of the activation effect in our study. We tried to exclude all possible errors. The only difference in the conditioning of the SiO2-free reference and the SiO2 surface-doped electrodes was that only the latter have been (dis)mounted between two measurements in order to deposit the silica. However, all heating and cooling procedures and electrical contacts were identical. Admittedly, scratches caused by the electrode contacting slightly decreased the electrode areas, but the effect has been estimated to be only 1.5-2.7%, and in addition, it would only explain a reduced exchange current density. Obviously, small amounts of SiO2 on Pt influence the ratedetermining step of the oxygen reduction or oxidation reaction. Oxygen adsorption/desorption, dissociation/association, oxygen surface diffusion, and electron transfer contribute to the overall reaction. Assuming that the electron transfer occurs at or close to the tpb,1,16 silica at the outer Pt surface cannot influence this step directly. Considering the exponential I-V characteristics, diffusion limitation does not occur. Most probably, silica on the electrode surface influences the

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Present Addresses:

oxygen adsorption/desorption. However, the observed activation effect is relatively small. Further detailed studies using model-type electrodes and in situ spectroscopic investigations will be necessary for a complete understanding of this unusual effect of SiO2.

†

Electrochemical Energy Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139. Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT 84112-0850. ‡

ACKNOWLEDGMENT The authors thank S. O. Steinm€uller (SIMS) and T. Leichtweiss (XPS) for their assistance and are grateful for financial support from the German Research Foundation. N.B. acknowledges a scholarship of the Studienstiftung des Deutschen Volkes e.V.

EXPERIMENTAL METHODS Sample Preparation. Three types of Pt/YSZ/(Si) electrodes were prepared; see Figure 1. We used YSZ(111) single crystals (CrysTec, Germany, one-side-polished, surface rms roughness of 0.165 nm, thickness 0.5 mm, square 1 cm2) and fluxless Pt paste (Hereaus, no. 3683151626, diluted in HVS100; sintering parameters: 17.5 h at 1273 K, heating rate 5 K/min). The film thickness as determined by SEM cross section was about 3-3.5 μm (cf. Figure 2b). For silica doping of the WE, we used PLD (SiO2 target, laser energy 170 mJ, different number of laser pulses, repetition rate 10 Hz, 10-2 mbar of O2, substrate-target distance 4 cm, T = 298 K). The rms roughness of 3000 laser pulses on YSZ was 28.9 nm (atomic force microscopy, AFM). Energy dispersive X-ray spectroscopy (EDX) showed an Al impurity of less than 0.18 atom % in the SiO2 target. SIMS analysis on a thicker SiO2 PLD film on YSZ revealed Na, Al, Li, and C, however, with very low concentrations. The X-ray diffractogram (XRD) of the silica powder indicated quartz, as depicted in Supporting Information SI 2. Low silica coverages on Pt are not easy to quantify,17 and we used a SIMS depth profile of a YSZ single crystal with a thick SiO2 PLD film in order to correlate the number of PLD pulses to a film thickness (Supporting Information SI 7). This resulted in approximately 1.13 10-11 m per 10 pulses. Electrode Characterization. XRD: Siemens Daco-MP, Θ/2 Θ scans in the range of 10-90°, Cu KR radiaton. (HR)SEM/EDX: Leo Gemini 982, 5 kV/10 kV. XPS: VG ESCALAB MK II; detector: CLAM 100; radiation: Mg KR with 1253.6 eV; pass energy: 50 eV. TOF-SIMS: TOF SIMS 5, ION-TOF GmbH, Bi ion sputtering, Cs ion analyzing. Profilometer: KLA Tencor, Alpha-Step IQ. AFM: Quesant QScope. Steady state I-V characteristics: voltage range: ( 0.5 V; resolution: 5 mV; current tolerance: 1 nA (abs)/5 mA (rel); delay: 1 s min/60 s max (IM6, Zahner electronics) at 723 K in air. The working and reference electrodes were electrically contacted with a Pt wire pressed to the Pt paste, and the counter electrode was contacted with an Ag wire. The current densities refer to the macroscopic WE area, as determined by photography and image processing software (Adobe PS CS4 extended).

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SUPPORTING INFORMATION AVAILABLE Electrode characterization by EDX, XRD, and SIMS; determination of the number of PLD pulses to SiO2 film thickness; typical current densities of porous Pt/YSZ electrodes; additional information on electrochemical measurements/analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. Fax: (þ)49 0641 9934509. E-mail: [email protected].


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Adler, S. B. Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104, 4791–4844. Backhaus-Ricoult, M.; Trichet, M.-F. Interfacial Chemistry at Metal Electrode-Oxide Electrolyte Contacts. Solid State Ionics 2002, 150, 143–156. Gozzi, D.; Tomellini, M.; Bianconi, A.; Fanfoni, M. Effect of SiO2 on Charge-Transfer at the Zirconia Electrode Interface. J. Electroanal. Chem. 1986, 198, 53–63. Mogensen, M.; Jensen, K. V.; Jorgensen, M. J; Primdahl, S. Progress in Understanding SOFC Cathodes. Solid State Ionics 2002, 150, 123–129. Viitanen, M. M.; v Welzenis, R. G.; Brongersma, H. H.; van Berkel, F. P. F. Silica Poisoning of Oxygen Menbranes. Solid State Ionics 2002, 150, 223–228. Hauch, A.; Jensen, S. H.; Bilde-Sorensen, J. B.; Mogensen, M. Silica Segregation in the Ni/YSZ Electrode. J. Electrochem. Soc. 2007, 154, A619–A626. Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345–352. De Ridder, M.; Vervoort, A.G. J.; van Welzenis, R. G.; Brongersma, H. H. The Limiting Factor for Oxygen Exchange at the Surface of Fuel Cell Electrolytes. Solid State Ionics 2003, 156, 255–262. Bernasik, A.; Kowalski, K.; Sadowski, A. Surface Segregation in Yttria-Stabilized Zirconia by Means of Angle Resolved X-ray Photoelectron Spectroscopy. J. Phys. Chem. Solids 2002, 63, 233–239. Hughes, A. E.; Badwal, S. P. S. Impurity Segregation Study at the Surface of Yttria-Zirconia Electrolytes by XRS. Solid State Ionics 1990, 40-41, 312–315. Brongersma, H. H.; de Ridder, M.; Gildenpfennig, A.; Viitanen, M. M. Insight in the Outside: Materials Science at the Atomic Level Using LEIS. J. Eur. Ceram. Soc. 2003, 23, 2761– 2767. Hertz, J. L.; Rothschild, A.; Tuller, H. L. Highly Enhanced Electrochemical Performance of Silicon-Free Platinum-Yttria Stabilized Zirconia Interfaces. J. Electroceram. 2009, 22, 428–435. Mutoro, E.; Luerssen, B.; G€ unther, S.; Janek, J. The Electrode Model System Pt(O2)/YSZ: Influence of Impurities and Electrode Morphology on Cyclic Voltammograms. Solid State Ionics 2009, 180, 1019–1033. Stamenkovic, V.; Markovic, N. M. Oxygen Reduction and Hydrogen Oxidation Reactions on Pt(111) and Pt(100) in Solutions Containing Copper Ions. Langmuir 2001, 17, 2388–2394. Miah, M. R.; Ohsaka, T. Kinetics of Oxygen Reduction Reaction at Tin-Adatoms-Modified Gold Electrodes in Acidic Media. Electrochim. Acta 2009, 54, 5871–5876.


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