Metallic Nanoparticles and Proton Conductivity ... - ACS Publications

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Metallic Nanoparticles and Proton Conductivity: Improving Proton Conductivity of BaCe0.9Y0.1O3−δ Using a Catalytic Approach M. T. Caldes,†,* K. V. Kravchyk,† M. Benamira,† N. Besnard,† V. Gunes,‡ O. Bohnke,‡ and O. Joubert† †

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2, rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France ‡ Institut des Molécules et Matériaux du Mans (IMMM UMR 6283 CNRS), Université du Maine, Av. O. Messiaen, 72085 Le Mans Cedex 9, France S Supporting Information *

ABSTRACT: In this work, we have used nickel nanoparticles to improve proton conductivity of the electrolyte BaCe0.9Y0.1O3−δ (BCY). Ni nanoparticles were dissolved into the compounds as their oxidized form (BaCe0.9−xY0.1NixO3−δ) and precipitated upon heating under a reducing atmosphere. Below 700 °C, proton conductivity is enhanced under a reducing atmosphere. An increase of 1 order of magnitude, with respect to BCY, was observed for BaCe0.7Y0.1Ni0.2O3−δ (1.7 × 10−2 S/cm at 500 °C). This phenomenon is more pronounced for the compounds containing more nickel on the surface, which can facilitate the dissociation of hydrogen and the incorporation of protons in the structure. Under reducing atmosphere, nickel doping enhances both bulk and grain boundaries conductivities and the blocking effect appear at lower temperature. KEYWORDS: protonic ceramic fuel cells, proton conductivity, BCY, Ni exsolution, Hebb-Wagner ion blocking method

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INTRODUCTION Protonic ceramic fuel cells (PCFCs) seem to be a promising technology for operation at 600 °C and below. Indeed, in these devices, a high-temperature proton conductor is used as an electrolyte. Improvement of the performances of PCFCs requires both enhancement of the protonic conductivity of the electrolyte and the design of new mixed protonic-electronic conducting electrodes. Focusing on the electrolyte, its ionic conductivity can be improved by increasing concentration of charge carriers or/and their mobility. In most cases, a structural approach is used, i.e., screening oxygen-deficient oxides, inherently or acceptordoped, exhibiting a high-symmetric structure and good oxygen dynamics.1 However, a catalytic approach could be also used to improve proton conductivity. Metallic nanoparticles (Ni, Ru) are well-known as catalysts for hydrogen dissociation. This property has been extensively used to improve the electrocatalytic properties of SOFC electrodes toward direct oxidation of fuel.2,3 Thus, one can also expect to improve H+ concentration in ceramic oxides by catalyzing the following reaction: 1 H 2(g ) + OOx ↔ (OH)•O + e′ (1) 2

coarsening. Instead of adding metallic nanoparticles as a separate phase, they can be dissolved into the compounds as their oxidized form (i.e., Ni2+). Then, metal nanoparticles precipitated from grains upon heating under a reducing atmosphere.4,5 By this way, a stronger interaction between support (electrolyte) and metallic nanoparticles is expected, which can avoid coarsening while facilitating the incorporation of protons. In this work, we have used, for the first time, a catalytic approach to improve proton conductivity of the PCFC electrolyte BaCe0.9Y0.1O3−δ (BCY10), which is one of the best high-temperature proton conductors described in the literature.6

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Materials. Ni-doped compounds BaCe0.9−xY0.1NixO3−δ (BCYNi) were prepared by solid-state reaction. Ce was partially substituted by Ni. Ni substitution levels on the B-site (from ABO3) vary from 2% to 20% (0 ≤ x ≤ 0.2). X-ray Analysis. X-ray diffraction (XRD) data were obtained using a Brüker Model D8 Advance powder diffractometer operated in Bragg−Brentano reflection geometry with a Cu anode X-ray source, a focusing Ge(111) primary monochromator (selecting the Cu Kα1 radiation) and a one-dimensional (1-D) position-sensitive detector (“Vantec” detector). The active area of the detector was restricted to 3° 2θ, to improve the angular resolution of XRD diagrams. The

Metallic nanoparticles can be added to oxides by impregnation with a metallic salt or a nanopowder suspension. Nevertheless, the use of such techniques applied to high-temperature operating systems can sometimes give rise to an inhomogeneous repartition of the metallic particles, as well as their © 2012 American Chemical Society

EXPERIMENTAL SECTION

Received: June 1, 2012 Revised: November 29, 2012 Published: December 4, 2012 4641

dx.doi.org/10.1021/cm301685x | Chem. Mater. 2012, 24, 4641−4646

Chemistry of Materials

Article

experimental data were refined using the FullProf 2k program and its graphical interface WinPLOTR. TEM Analysis. Transmission electron microscopy (TEM) study was carried out with a Hitachi Model H9000NAR electron microscope, operating at 300 kV. Energy-dispersive X-ray (EDX) analyses were also performed on several crystals. XPS Analysis. X-ray photoemission spectroscopy (XPS) studies were performed with a Kratos AXIS Ultra spectrometer, using a monochromatic Al Kα X-ray source (1486.6 eV), operating at 150 W. The base pressure in the analysis chamber was 10−8 Pa, and the analyzed area was 700 μm × 300 μm. The hemispherical analyzer was used in constant analyzer energy (CAE) mode for all spectra. The pass energy was 160 and 40 eV for wide and narrow scan spectra, respectively. Quantification was performed using CasaXPS software (Copyright 2005, Casa Software, Ltd.) from the photoelectron peak areas using Shirley background subtraction. Spectra were calibrated in binding energy with C 1s, assumed at 284.7 eV. Electrical Properties. Electrical measurements were performed by electrochemical impedance spectroscopy (EIS). The measurements were realized using a Solartron Model 1260 frequency response analyzer. The impedance spectra were recorded over a frequency range of 2 MHz to 0.01 Hz, with a signal amplitude of 100 mV and with 10 points per decade under open-circuit conditions. A stabilization time of 2 h was considered between each temperature change. In order to promote Ni exsolution, the samples were maintained at 800 °C in a reducing atmosphere for 12 h before beginning measurements. The Hebb−Wagner ion blocking method was used to determine the electronic transport number. This method is based on an electrochemical cell (see Figure S1 in the Supporting Information (SI)) that consisted of an ion-blocking electrode (platinum microcontact totally isolated from O2 from the ambient atmosphere with glue), an electrolyte (studied sample), and a reversible electrode (composed of CuO and Cu2O oxides). When a DC voltage is applied to the cell (which corresponds to a certain oxygen partial pressure), the O2− ions move to or from the blocking electrode, depending on the sign of the voltage, and the current through the sample is ionic and electronic. After a certain time, steady-state conditions are reached and the remaining current becomes only electronic. The results (voltage, steady-state current) are recorded and this is repeated for different voltages. Finally, electronic conductivities, as a function of oxygen partial pressure, are deduced.

cell volume increases with Ni content (see Figure S2 in the SI), as expected from an increase in oxygen vacancy concentration through the reaction × NiO → Ni″Ce + V O•• + OO

(2)

In contrast, beyond 5% doping, the unit-cell volume decreases as cationic vacancies were created on the A-site.7 The cell volume dependence on Ni content indicates that cationic substitution Ni2+→Ce4+ occurs in the BCY structure. Moreover, EDX analysis of doped compounds confirmed the presence of nickel into the bulk (see Figure S3 in the SI). To induce Ni exsolution, BCYNi compounds were reduced under 5% H2/Ar at 800 °C for 12 h. In all cases, the perovskite-type structure of BCYNi compounds is preserved but BaNi0.83O2.5 is decomposed. Partial exsolution of Ni and decomposition of the barium nickelate were confirmed by transmission electron microscopy (TEM). Nickelate decomposition also induces the formation of Ni nanoparticles supported on barium oxide, as shown in Figure 2. As shown in TEM images, metal nanoparticles diffuse

Figure 2. High-resolution electron microscopy (HREM) images of BCYNi05 and BaNi0.83O2.5, attesting to the partial exsolution of Ni under a reducing atmosphere.

toward the crystal surface, which suggests that Ni nanoparticles can be located in grain boundaries. The chemical composition of crystal bulk was analyzed by EDX. Ni was still detected (see Figure S4 in the SI), indicating that Ni exsolution is only partial. The electron diffraction pattern included as an inset confirms the decomposition of BaNi0.83O2.5. Therefore, the presence of a BaNi0.83O2.5-like impurity could be considered to be an advantage from a catalysis point of view, because its decomposition leads to an increase of the number of Ni nanoparticles on the surface. X-ray photoelectron spectroscopy (XPS) was used to analyze surface composition of BCYNi compounds, as-prepared and reduced ones. The reduction process enhances the Ni, Ba and Ce contents on the surface (see Table S1 in the SI). Moreover, the concentration of Ni at the surface increases with increasing nominal doping content. The formation of metallic nickel is also confirmed by XPS.8 Indeed, after reduction, a component attributed to Ni0 is clearly observed in the Ni 2p core level signal (see Figure 3). Electrical properties of BCYNi compounds were measured by electrochemical impedance spectroscopy (EIS) under air, dry, and wet 5%H2/95%Ar (PH2O = 0.025 atm). In Figure 4, the temperature dependence of the total conductivity of BCYNi compounds under wet and dry reducing atmospheres are presented and compared to that of BCY. All compounds exhibit the same compacity (i.e., 95%). The conductivity increases mostly for the doped compositions (see Table S2 in the SI). For instance, BCYNi20, under diluted dry hydrogen (5% in

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RESULTS AND DISCUSSION Ni-doped compounds BaCe0.9−xY0.1NixO3−δ (BCYNi) were prepared as described in the Experimental Section. As shown in Figure 1, a single phase exhibiting an orthorhombic symmetry (space group Pmcn) was only obtained for 2% Ni (a compound named BCYNi02). For all of the other compositions, BaNi0.83O2.5 appears as an impurity. Up to 5% Ni, the unit-

Figure 1. XRD patterns of as-prepared BaCe 0.9−x Y 0.1 Ni x O 3−δ compounds. Main reflections of BaNi0.83O2.5 are marked (denoted by black stars, ★). 4642



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dissociation of hydrogen and the incorporation of protons in the structure. In order to confirm proton incorporation, thermogravimetric analysis−mass spectroscopy (TGA-MS) measurements were done under wet 5% H2/Ar atmosphere: a first heating ramp up to 800 °C at 10 °C/min, followed by a dwell of 12 h in order to promote Ni exsolution. The temperature then was slowly decreased (0.5 °C/min) down to 25 °C to facilitate the incorporation of the protons in material and to reach equilibrium. Finally, a second heating ramp to 800 °C was done. Figure 5 shows the weight loss measured for BCY and

Figure 3. Ni 2p3/2 core level signal, corresponding to BCYNi10 asprepared (left) and reduced (right).

Figure 5. TGA curves of BCY and BCYNi10 compounds recorded during the first heating ramp and after a 12-h long dwell (half-filled symbols).

BCYNi10 during both the first ramp and the dwell at 800 °C. The half-full symbols correspond to the masses measured at the end of the dwell at 800 °C. The weight loss observed for BCYNi10 (11%) is higher than that observed for BCY (5%), probably due to the partial exsolution of Ni (Ni2+ → Ni0 reduction) and the decomposition of the nickelate BaNi0.83O2.5, which appears as a minor impurity. Obviously, for both compounds, a water loss and a partial reduction of Ce4+ must be also taken into account to explain the weight loss observed. Figure 6 shows the weight loss measured for BCY and BCYNi10 during the second heating ramp. The mass losses Figure 4. Total conductivity of BCYNi compounds under (top) dry and (bottom) wet 5% H2/Ar.

argon), shows, at 500 °C, a conductivity level three times higher than that of BCY (1.7 × 10−2 S/cm vs 6 × 10−3 S/cm). Although for x ≥ 0.05, the BCYNi compounds probably exhibit a cationic nonstoichiometry on the A-site, the increase of the total conductivity with x cannot be attributed to this fact. Indeed, for BCY,7 the nonstoichiometry leads rather to a reduction of total conductivity, contrary to that observed for Ni-doped compounds. None of the compounds presents a linear dependence of conductivity with the temperature. The electrical conductivity increases with temperature from 300 °C to 500 °C and then reaches a plateau followed by increasing again with temperature. The existence of a plateau suggests a protonic contribution to the total conductivity and it is due to a loss of protonic defects, the increased thermal activation energy, and mobility. This phenomenon is more pronounced for the compounds with high nickel content. In fact, as we determined by XPS, these materials exhibit a high amount of Ni nanoparticles on the surface, which can facilitate the

Figure 6. Thermogravimetric analysis (TGA) curves of BCY and BCYNi10 compounds recorded during the second heating ramp.

measured correspond mostly to water uptake as confirmed by the MS analysis of evolved gas. The mass loss observed for BCYNi10 (0.9%) is higher than that observed for BCY (0.5%), which suggests that BCYNi10 incorporates more protons than BCY. Thus, the presence of Ni/BCY interfaces at the nanoscale may change the thermodynamics of the reaction, not just the 4643



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kinetics, even if it is necessary to keep in mind that cation substitution Ni2+ → Ce4+ in BCY occurs and since, during the reduction process, Ni exsolution is only partial, atomic structures of BCY and BCYNi must be, at least locally, slightly different. This feature could also change the thermodynamics of protons incorporation reaction. The beneficial effect of Ni substitution on BCY conductivity seems less clear under wet atmospheres mainly in the case of wet air (see Figure 7). In fact, during measurements under air,

Figure 7. Total conductivity of BCYNi compounds under wet air.

nickel nanoparticles were oxidized which induces a lowering of their catalytic activity toward proton incorporation reactions. This result seems to confirm that the metallic nanoparticles play an essential role in the improvement of total conductivity of these materials. In Figure 8, the bulk, grain boundaries, and total conductivities under dry and wet hydrogen for BCY and

Figure 9. Grains boundaries and bulk conductivity of BCYNi compounds under dry H2/Ar.

grain boundaries could modify the space charge layer,9 which could explain the enhancement of the observed conductivity. For bulk conductivity, this effect is much more visible under dry hydrogen. However, it must be checked if the enhancement of total conductivity of BCY obtained by Ni substitution under reducing atmospheres, especially below 600 °C, is only due to a protonic contribution or if, in contrast, an electronic contribution must be also considered. The electronic conductivity of BCY and BCYNi10 were evaluated by the Hebb−Wagner ion blocking method,10 as a function of oxygen partial pressures for different temperatures between 400 °C and 700 °C. As shown in Figure 10, for BCYNi10, the electronic contribution to total conductivity is minor between 400 °C and 600 °C. However, beyond 600 °C, it starts to be significant. In Figure 11, the electronic conductivity of BCY and BCYNi10 are compared at 400 and 500 °C. Results corresponding to 600 °C are shown in Figure 12. For 400 °C, the electronic conductivity of BCY under a reducing atmosphere is higher than that of BCYNI10. In contrast, at 500 °C, the electronic conductivity of BCYNi10 is superior but its contribution to total conductivity is minor. However, beyond 600 °C, it starts to become significant. These results might be explained by the defect equilibrium chemistry described below: 1 × V O° ° + O2 ↔ OO + 2h° (3) 2

Figure 8. Bulk, grain boundaries, and total conductivity of BCY and BCYNi10 under dry 5% H2/Ar.

BCYNi10 are shown. The conductivities measured under wet hydrogen are reported as Figure S5 in the SI. Total conductivity is bulk dominated at high temperatures, but the temperature for which blocking effect is prevailing decreases with Ni content. Activation energies in both atmospheres are in good agreement with those reported in the literature: 0.4 eV for bulk conductivity and 0.6−0.7 eV for grain-boundary blocking effect. Moreover, Ni doping has a beneficial influence on the grain boundary conductivity, which increases with Ni content under both atmospheres, dry (Figure 9) and wet hydrogen (Figure S6 in the SI). It must be noted that Ni exsolution at

× H 2O + V O°° + OO ↔ 2OH°

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is the reason why, below 600 °C, the conductivity measured under a wet or dry reducing atmosphere is higher than that measured under wet air (see Table S2 in the SI). Even if Ni doping seems to increase electronic conductivity, its contribution to total conductivity is negligible below 600 °C. Thus, the enhancement of total conductivity of BCYNi compounds under reducing atmospheres could be mainly attributed to a protonic contribution. Above 600 °C, the electronic contribution starts to become significant. A n-type behavior is deduced from ion blocking measurements. It is related to Ce4+ → Ce3+ reduction, as confirmed by XPS. Compounds derived from BCY with transition-metal doping on the Ce-site have not been extensively studied in the literature.11−14 Moreover, the addition or substitution by Ni was mainly considered to facilitate sintering and never from catalysis point of view. Indeed, in our case, the addition of Ni was also beneficial for sinterability. A reduction of the sintering temperature of 300 °C was observed with the increase of the nickel content (see Figure S7 in the SI). Besides, in most of the papers that involve transition-metal doping in BCY, a decrease of the total conductivity with the substitution level is observed, especially in air. Under a reducing atmosphere, cation substitution slightly influences the transport properties, but with regard to experimental conditions, the compounds studied were not reduced for a sufficiently long time to induce Ni exsolution. The divergence between our results and those described in the literature concerning the Ni-doping effect on transport properties are probably attributable to this reason.

Figure 10. Electronic conductivity of BCYNi10, as a function of oxygen activity for different temperatures.

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CONCLUSION For the first time, metal nanoparticles obtained by exsolution were used to facilitate the incorporation of the protons at high temperature in oxides, thus improving their protonic conductivity. Ni substitution leads to an increase of the total conductivity by 1 order of magnitude. Both grain boundaries and bulk conductivities are improved and the blocking effect appears at lower temperature. The enhancement of the electrical performances of BCY electrolyte by a catalytic approach seems to be promising to optimize the performance of the electrochemical devices comprising proton-conducting ceramic oxides.

Figure 11. Electronic conductivity of BCY and BCYNi10 at 400 and 500 °C.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data, TGA, XPS, and EIS results, such as tables summarizing data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 12. Electronic conductivity of BCY and BCYNi10 at 600 °C.

H 2O + 2h° ↔

1 O2 + 2H° 2

(5)

H 2 + 2h° ↔ 2H°

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

(6)

The authors declare no competing financial interest.

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O×O

where VO°° is the oxygen vacancy, the lattice oxygen ion, H° a proton, and h° a hole. These defect equilibria are described for a negligible concentration of free electrons. In air, BCYNi compounds are mainly ionic conductors with a small p-type electronic contribution to the total conductivity, according to reaction 3. Below 600 °C, proton conductivity appears and the p-type contribution can decrease, according to reactions 4 and 5. Under a reducing atmosphere, H2 can also react with holes to form proton defects, according to reaction 1 or reaction 6. This

ACKNOWLEDGMENTS The authors thank Jonathan Hamon, Stephane Grolleau and Angelique Jarry for their help during XPS and TG measurements.

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REFERENCES

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