Assessing Substitution Effects on Surface Chemistry by In Situ

Oct 3, 2018 - Performance of Proton-Solid Oxide Fuel Cells (H+-SOFC) is governed by ion transport through solid/gas interfaces. Major breakthroughs ar...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 37661−37670

Assessing Substitution Effects on Surface Chemistry by in Situ Ambient Pressure X‑ray Photoelectron Spectroscopy on Perovskite Thin Films, BaCexZr0.9−xY0.1O2.95 (x = 0; 0.2; 0.9)

Angelique Jarry,*,†,‡,§ Sandrine Ricote,⊥ Aaron Geller,† Christopher Pellegrinelli,∥ Xiaohang Zhang,§ David Stewart,§ Ichiro Takeuchi,§ Eric Wachsman,∥ Ethan J. Crumlin,*,‡ and Bryan Eichhorn*,†

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Department of Chemistry and Biochemistry, §Department of Materials Science and Engineering, and ∥Energy Research Center, University of Maryland, College Park, Maryland 20742, United States ‡ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Mechanical Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Performance of proton-solid oxide fuel cells (H+SOFC) is governed by ion transport through solid/gas interfaces. Major breakthroughs are then intrinsically linked to a detailed understanding of how parameters tailoring bulk proton conductivity affect surface chemistry in situ, at an early stage. In this work, we studied proton and oxygen transport at the interface between H+SOFC electrolyte BaCexZr0.9−xY0.1O2.95 (x = 0; 0.2; 0.9) thin films and the gas (100 mTorr of H2O and O2) by using synchrotron-based ambient pressure X-ray photoelectron spectroscopy at operating temperature (>400 °C). We developed highly textured BaCexZr0.9−xY0.1O2.95 epitaxial thin films, which exhibit high level of in-plane proton conductivity, that is, up to 0.08 S cm−1 at 500 °C for x = 0.9. Upon applying 100 mTorr water partial pressure above 300 °C, major changes are observed only in the O 1s and Y 3d core level spectra, with a clear Zr/Ce ratio dependency. OH− formation is favored by Ce content while initiated near Y. Hydration is also associated with surface secondary phase growth comprising oxygen-under-coordinated yttrium and/or yttrium hydroxide. With BaCe0.2Zr0.7Y0.1O2.95, high levels of ionic conductivities and chemical stability are obtained as a result of the optimized surface reaction kinetics, with low activation energy barrier for proton transport while restraining formation of OH−/SO42− adsorb species. KEYWORDS: ambient pressure X-ray photoelectron spectroscopy, protonic ceramic fuel cells, perovskite, thin films


of this impact at the molecular level on the surface is not fully understood which impedes further PCFC material development.8,14−16 In Ba(Zr/Ce/Y)O3−δ electrolytes, substitutions of the tetravalent cations (Ce/Zr) on the perovskite B site by a trivalent cation (Y) creates oxygen vacancies by charge compensation. Hydration occurs via dissociation of water by filling of these intrinsic oxygen vacancies (VO) forming hydroxyl protons (OH−) given as H2O + VO + O2− → 2OH−. Protons will subsequently hop to the nearest adjacent oxygen available via the Grotthuss mechanism.17 H+-SOFC performance is thus highly influenced by the hydration process and in particular, proton and oxygen ion transport through the solid oxide host lattice/humid gas interfaces. Encouraging results toward establishing the interplay between surface oxide

Implementation of solid oxide fuel cell (SOFC) technology is hindered by their operating temperature, typically above 750 °C.1,2 Because of their high level of protonic conductivity at intermediate temperatures (>400 °C), yttrium-doped barium perovskite materials Ba(Zr/Ce/Y)O3−δ appear as one of the most promising response to this issue.3−9 However, the poor grain boundary conductivity of BaZr0.9Y0.1O2.95 (BZY) and the lack of chemical stability of BaCe0.9Y0.1O2.95 (BCY) impede their application. The first issue can be addressed by the development of grain boundary-free thin films that exhibit conductivity levels up to 3 orders of magnitude higher than standard sintered pellets.10,11 The second can be partially resolved with the solid solution of BZY and BCY, leading to BaCe0.2Zr0.7Y0.1O2.95 (BCZY), for example, which demonstrates superior chemical stability.12,13 The strong correlation between composition, stability, and transport properties is well established.20,25−30 Yet, the origin © 2018 American Chemical Society

Received: July 24, 2018 Accepted: October 3, 2018 Published: October 3, 2018 37661

DOI: 10.1021/acsami.8b12546 ACS Appl. Mater. Interfaces 2018, 10, 37661−37670

Research Article

ACS Applied Materials & Interfaces

Pulsed laser deposition (PLD) targets of 25 mm in diameter were obtained by uniaxial pressing of the powders at 350 MPa and sintered at 1700 °C in air for 6 h in a bed of powder with 20% wt of BaCO3 to prevent barium evaporation.34 Highly epitaxial thin films were grown by PLD on (100)-oriented single crystals MgO wafers, two-side polished (surface roughness < 0.5 nm) with the dimensions of 5 mm × 10 mm × 0.5 mm (SPI, USA). During the PLD of the BaCexZr0.9−xY0.1O2.95 (x = 0; 0.2; 0.9) targets, the distance between the target and the substrate was kept at 5.0 cm. A pulsed excimer laser (KrF; λ = 248 nm) with an energy density of 0.25 J/cm2 and a repetition rate of 5 Hz was used. The films were deposited on the substrates in 100 mTorr of oxygen at 750 °C with the deposition rate of ∼0.3 Å/pulse. X-ray reflectivity (XRR) and atomic force microscopy (AFM, Vecco, USA) were used to determine the thickness and surface roughness, respectively. Poles figures were acquired to investigate the crystalline structure of the films (PANalytical X’Pert Pro MPD). Electrochemical Characterization. In-plane ac impedance conductivity measurements were performed on the thin films using a 1260 frequency response analyzer of Solartron (Schlumberger, UK). Parallel strip-shaped Au electrodes of a few tens of nanometers thick were deposited in vacuum by sputtering on the film surface at a distance of ∼1 mm and wired to the Solartron using Au paste and wires. Proton conductivities were determined by electrochemical impedance spectroscopy measurements in moist air (pH2O = 0.023 atm) using a 100 mV ac perturbation, from 2 MHz to 1 Hz, between 400 and 500 °C, with a 4 h dwell time before each measurement under moist air. The impedance spectra were analyzed with ZView software. In Situ APXPS Characterization. High-temperature APXPS data were collected at the beamline 9.3.2 at Lawrence Berkeley National Laboratory’s (LBNL) Advanced Light Source (ALS).35 Thin films were placed onto a ceramic heater and held in place by clips and ceramic spacers. For accurate surface temperature measurements, a thermocouple was placed directly onto the sample surface. All samples were investigated separately. An initial preheating under 100 mTorr of Ar or UHV at 500 °C for 2 h and under 100 mTorr of O2 at 300 °C for 2 h was used as dehydration process and precleaning, to remove adventitious carbon. To account for the charging effect, the binding energy (BE) for the collected spectra was then subsequently calibrated to the Ba 4d photoemission peak of barium perovskite at 88.9 eV. The spectra were collected in the following sequence: at a PE of 850 eV, low-resolution survey (BE = −10 to 750 eV), highresolution O 1s, C 1s, Ba 4d, Y 3d, Zr 3d. At 710 eV, low-resolution survey (BE = −10 to 600 eV), high-resolution O 1s, C 1s, Ba 4d, Y 3d, Zr 3d, Ce 4d. At 490 eV low-resolution survey (BE = −10 to 370 eV), high-resolution C 1s, Ba 4d, Y 3d, Zr 3d, Ce 4d, and at 300 eV, highresolution Ba 4d. These spectra were collected at the following conditions, in this order: 100 mTorr of argon at T = 300 and 500 °C, 100 mTorr of steam at T = 300 and 500 °C and 100 mTorr of O2 at T = 300 and 500 °C. The heating rate was about 10 °C/min, and the temperature was held constant for 20 min before beginning to collect data at each temperature. The inelastic mean free path (IMFP) for each core level was calculated using the QUASES-IMFP-TPP2M version 3.0 software. Spectra were fitted with Casa-XPS software using a combined Gaussian−Lorentzian line shape and a Shirley background. Additional information regarding APXPS fitting and normalization procedure are given in the Supporting Information.

chemistry, proton conduction, and degradation mechanisms have been reported.4,18,19 However, because of low concentration of protons, surface gas sensitivity, and high operating temperatures, the understanding of the surface processes at relevant conditions remains limited. Available knowledge relies mostly on a combination of ex situ surface [X-ray photoelectron spectroscopy (XPS), low-energy ion scattering, electron energy loss spectroscopy, scanning electron microscopy]5,18,20−22 and in situ bulk [X-ray diffraction (XRD), neutron powder diffraction, Raman]23,24 techniques at low temperature and computational studies.25−28 Because of these shortcomings, these studies provide limited information on surface and bulk states at the same time in operating conditions. Recent progress in the development of in situ characterization surface techniques offers unique opportunities toward establishing the behavior of surface reactive species in realistic operating conditions as a function of composition. In particular, in situ synchrotron-based ambient pressure XPS (APXPS) has been successfully used to observe oxygen vacancy filling upon hydration at the surface of a BCY pellet at 272 °C.20 Using variable photon energy (PE) allows for depth profiling where one can attempt to correlate the protonic conduction bulk properties to the surface proton incorporation. Of particular interest is to understand how the composition affects surface chemistry in epitaxial thin films presenting low roughness and enhanced protonic conductivity levels. A critical feature governing H+-SOFC electrolyte performance is the hydration level of the electrolyte, which dictates the concentrations of protons and oxide ion vacancies. Because the hydration of the electrolyte is a post synthetic process, the mechanism of water uptake is essential to understand how to optimize performance. It is well known that perovskite surface structures and compositions change at high temperatures and under different environments.29−31 As such, the most effective means of studying the mechanism of H+-SOFC electrolyte hydration is through the use of in situ spectroscopic probes that can measure compositions and chemical states at the water vapor−solid interface under relevant temperatures and atmospheres. In this work, we employ synchrotron-based APXPS to investigate hydration and degradation mechanisms at the interface between state of the art H+-SOFC perovskite electrolyte BaCexZr0.9−xY0.1O2.95 (x = 0; 0.2; 0.9) thin films and the gas. Knowing that the energy of water uptake is exothermic in doped barium perovskites,32 we explored substitution effects on early stage surface reaction in fuel cell environments (under 100 mTorr of Ar, H2O and O2) at operating temperatures (500 °C) and at 300 °C in humid environments. We exploit the tuneable PE source, from 300 to 850 eV, of beamline 9.3.2 at the Advanced Light Source (ALS) to collect information related to the surface and the bulk with high resolution. The correlation between surface activity and composition as well as associated new insights into the composition effect on hydration and degradation mechanisms will be discussed.

RESULTS AND DISCUSSION Structural and Electrochemical Characterization. The crystalline structure, preferential orientation, and low surface roughness of BaCexZr0.9−xY0.1O2.95 [x = 0 (BZY); 0.2 (BCZY); 0.9 (BCY)] thin films grown on (100) MgO substrates were confirmed by XRD (Figures S1 and S2), pole figures (Figure S3), and AFM (Figure S4) studies. BCY and BZY epitaxial thin films oriented along the 110 and 100 directions of the pseudocubic system, respectively, while the BCZY films grew along


Thin-Film Preparation. BaCexZr0.9−xY0.1O2.95 (x = 0; 0.2; 0.9) powders were synthesized by standard solid-state reaction from stoichiometric mixtures of BaCO3, CeO2, Y2O3, and ZrO2 (Aldrich, USA) as described previously.33 Phase purity was confirmed by X-ray powder diffraction with a Bruker D8 ADVANCE diffractometer. 37662

DOI: 10.1021/acsami.8b12546 ACS Appl. Mater. Interfaces 2018, 10, 37661−37670

Research Article

ACS Applied Materials & Interfaces both 100 and 110 orientations. XRR confirmed each film thickness to be 65 ± 10 nm (Figure S5). ac impedance measurements were performed to evaluate the conductivities of the BaCexZr0.9−xY0.1O2.95 thin films. Figure 1 shows a typical Nyquist impedance spectrum for BCZY recorded at 500 °C under moist air with a p(H2O) = 0.023 atm.

Figure 2. Arrhenius plots comparing the conductivity in moist air of ∼65 nm BaCexZr0.9−xY0.1O2.95 (x = 0; 0.2; 0.9) thin films grown on MgO(100) substrates by PLD . For comparison, literature conductivity values collected on pellets are added.15,38,39

°C temperature range. This plot was used to extract the activation energies, with values ranging from 0.49 to 0.79 eV for the proton transport, in agreement with the literature reports.13,40,41 While both ionic and electronic conductivities can generally be present in these conditions, the electronic contribution becomes negligible at lower temperatures (