Metallic Nanoparticles and Proton Conductivity

ECS Transactions, 45 (1) 143-154 (2012) 10.1149/1.3701303 © The Electrochemical Society

Metallic Nanoparticles and Proton Conductivity: Improving Proton Conductivity of BaCe0.9Y0.1O3-δ and La0.75Sr0.25Cr0.5Mn0.5O3-δ by Ni-doping M.T. Caldesa, K.V. Kravchyka, M. Benamiraa, N. Besnarda, O. Jouberta, O.Bohnkeb, V.Gunesb, A. Jarrya, N. Dupréa a

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2, rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France b Institut des Molécules et Matériaux du Mans (IMMM), Université du Maine, CNRS, Av. O. Messiaen, 72085 Le Mans Cedex 9, France In this work we have used metallic nanoparticles to improve proton conductivity of the ceramic electrolyte BaCe0.9Y0.1O3-δ (BCY) and of the electrode material La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM). Instead of adding metallic nanoparticles as a separate phase, they were dissolved in the compounds as their oxidized form (Ni2+). The metal nanoparticles precipitated from compounds upon heating under reducing atmosphere. Below 600°C under reducing atmosphere, BaCe0.9-xY0.1NixO3-δ compounds exhibit higher conductivity than BCY. Thus, an increase of one order of magnitude was observed for BaCe0.7Y0.1Ni0.2O3-δ. A protonic contribution to the total conductivity is observed below 600°C. This phenomenon is more pronounced for the compounds containing more nickel in surface which can facilitate the dissociation of hydrogen and the incorporation of protons in the structure. La0.75Sr0.25Cr0.5Mn0.5-xNixO3-δ compounds show also a protonic contribution to the total conductivity below 400°C. NMR results confirmed that these compounds contain protons. Introduction Improvement of the performances of Protonic Ceramic Fuel Cells (PCFC) requires both the enhancement of the protonic conductivity of the electrolyte and the design of new mixed protonic-electronic conducting electrodes. In order to improve the protonic conductivity of the electrolyte a structural approach is mostly followed by screening oxygen-deficient oxides, inherently or acceptor-doped exhibiting a high-symmetric structure and a good oxygen dynamics (1). In the case of the electrodes a composite approach is often used. Indeed a CERCER (ceramic-ceramic) made up of a MIEC (conducting oxygen ions and electrons) and a proton conductor is commonly used as electrode (2). Nevertheless, a catalytic approach could also be used to improve proton conductivity. Metallic nanoparticles (Ni, Ru) catalyze the hydrogen dissociation and can consequently facilitate the incorporation of protons in ceramic oxides following this reaction: 1 ( H 2 )( g ) + OOx ↔ (OH ) •O + e ' 2

[1]

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ECS Transactions, 45 (1) 143-154 (2012)

Metallic nanoparticles can be added to oxides as a separate phase by impregnation with a metallic salt or a nanopowder suspension. However in this study metallic nanoparticles were dissolved in the compounds as their oxidized form (Ni2+) instead to be added as a separate phase. The metal nanoparticles precipitated from compounds upon heating under reducing atmosphere (3). By this way, a stronger interaction between support and metallic nanoparticles is expected. This strength can avoid coarsening while facilitating the incorporation of protons. We have used this approach to improve proton conductivity of both the ceramic electrolyte BaCe0.9Y0.1O3-δ (BCY) and the electrode material La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM). Experimental XRD data were obtained using a Brüker “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 K alpha1 radiation) and a 1D position-sensitive detector (“Vantec” detector). The active area of the detector was restricted to 3° 2theta to improve the angular resolution of XRD diagrams. The experimental data were refined using the FullProf 2k program (4) and its graphical interface WinPLOTR (5). Transmission Electron Microscopy study was carried out with a Hitachi H9000NAR electron microscope, operating at 300 kV with a Scherzer resolution of 1.8 Å. Energy Dispersive X-ray (EDX) analyses were also performed on several crystals. 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 700×300 μm2. The hemispherical analyzer was used in constant analyzer energy (CAE) mode for all spectra. The pass energy was 160 eV and 40 eV for wide and narrow scan spectra respectively. Quantification was performed using CasaXPS (CasaXPS 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. 1

H MAS NMR spectrum was recorded on a Bruker “Avance” 200 MHz spectrometer (TMS reference, MAS rate 25 kHz, 60 s repetition time). Total conductivity measurements (4-probe DC) were carried out on Au electroded bars in air between 350°C and 750°C, each 50°C after stabilization time of 2 h. DC conductivity was also studied under dry and wet (PH2O=0.023 atm) 5% H2/95% Ar. Sintered pellets densified to over 95% of theoretical density were cut to rectangularshaped samples with dimensions 2x2x10 mm. In order to promote Ni exsolution, the samples were maintained at 800 °C in reducing atmosphere during 12 h before beginning measurements. Electrical measurements were also performed by electrochemical impedance spectroscopy (EIS). The measurements were realized using a frequency response analyzer Solartron 1260. The impedance spectra were recorded over a frequency range 2MHz to 0.01 Hz with signal amplitude of 100 mV and with 10 points per decade under



open circuit conditions. 2 h stabilisation time was considered between each temperature change Hebb–Wagner ion blocking method was used to determine the electronic transport number (6-7). This method is based on an electrochemical cell (see Figure 1) consisting in an ion-blocking electrode (platinum micro contact 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.

Figure 1. Schematic representation of the ion-blocking electrochemical cell Results and Discussion BaCe0.9-xY0.1NixO3-δ Compounds (BCYNi) Ni-doped compounds BaCe0.9-xY0.1NixO3-δ were synthesized by solid state reaction of BaCO3, Ce2O3, Y2O3 and NiO. Reactants were mixed in stoichiometric proportions, ground thoroughly in acetone and calcinated in air at 1350 °C for 10 h. Cerium was partially substituted by Ni. Ni rates on B-site (from ABO3) vary from 2% to 20% (0≤x≤0.2). As shown in Figure 2 all compounds exhibited x-ray diffraction (XRD) patterns similar to BaCe0.9Y0.1O3 (S.G Pmcn). However a single phase was only obtained for 2% of Ni: BaCe0.9-xY0.1Ni0.02O3-δ (BCYNi02). For all the other compositions the nickelate BaNi0.83O2.5 appears as an impurity



As-prepared Intensity (a.u)

Ba(Ce)Ni0.8302.5

2θ Figure 2. XRD patterns of as prepared BaCe0.9-xY0.1NixO3-δ compounds

Ni Exsolution. To induce Ni exsolution compounds were reduced under 5% H2/Ar at 800 °C for 12h. In all cases, the perovskite-type structure of BCYNi compounds was preserved (S.G Pmcn) but BaNi0.83O2.5 was decomposed. Partial exsolution of Ni and decomposition of the barium nickelate were confirmed by transmission electron microscopy (TEM). Nickelate decomposition induces also the formation of Ni nanoparticles supported on Barium oxide as shown in Figure 3.

BCYNi05 BCYNi05 BaNi0.83O2.5

Ni

20 nm Figure 3. HREM images of BCYNi05 and BaNi0.83O2.5 attesting of partial exsolution of Ni under reducing atmosphere. The electron diffraction pattern included as an inset testifies of nickelate decomposition.



X-ray photoelectrons spectroscopy (XPS) was used to analyze surface composition of BCYNi compounds, as prepared and reduced ones. Reduction process enhances surface concentration of Ni, Ba and Ce. Moreover, the concentration of Ni at the surface increases with increasing nominal doping rate (see Table I). In fact, the presence of barium nickelate as impurity could be considered as an advantage from of the point of view of catalysis, because its decomposition leads to an increase of the number of Ni nanoparticles on the surface. Therefore, the formation of nickel metal is also confirmed by XPS. Indeed, after reduction a component attributed to Ni0 is clearly observed in the Ni 2p core level signal (see Figure 4). TABLE I. Surface concentrations estimated by XPS BCYNix % Ni % Ba BCYNi05 0.5 10.2 BCYNi05 reduced 0.6 18.5 BCYNi10 0.7 11.8 BCYNi10 reduced 1 15.9

As-prepared

% Ce 8 11.2 7.6 12.8

%Y 2.2 2.3 2 2.3

%O 79.1 67.4 78 68

After reduction

Figure 4. Ni 2p3/2 core level signal corresponding to BCYNi10 as-prepared and reduced. Electrical Properties. The electrical properties of BaCe0.9-xY0.1NixO3-δ were measured by electrochemical impedance spectroscopy (EIS) under air, dry and wet 5%H2/95%Ar. In Figures 5 and 6 the temperature dependence of the total conductivity of BCYNi compounds under wet and dry reducing atmosphere are presented and compared to that of BCY. All Ni-doped compounds exhibiting the same compacity. i.e. 95%.



Figure 5. Total conductivity of BCYNi compounds under dry 5% H2/Ar.

Figure 6. Total conductivity of BCYNi compounds under wet 5% H2/Ar



As observed any compound does not present a linear dependence of conductivity with the temperature. The curvature of the plots above 600°C suggests a protonic contribution to the total conductivity at lower temperatures and is related to loss of protonic defects as described extensively in literature (8). This phenomenon is more pronounced for the compounds with a higher Ni rate. In fact, as we determine by XPS these materials exhibit a concentration of Ni metal more important on the surface, which can facilitates the dissociation of hydrogen and the incorporation of protons in the structure. Below 600°C the conductivity increases with Ni content (see Table II). For instance under dry reducing atmosphere BCYNi20 exhibits at 500°C a conductivity three times higher than that of BCY (1.7 10-2 S/cm vs 0.6 10-2 S/cm). TABLE II. Total conductivity of BaCe0.9-xY0.1NixO3-δ compounds (10-3 S/cm) Dry 5%H2/Ar Wet 5%H2/Ar Wet air 700°C 500°C 700°C 500°C 700°C 500°C 12.4 6.4 12 6.5 18.6 6 BCY 12.7 7.1 11.7 6.8 20.2 3.8 BCYNi02 16.4 9.5 17.1 9.3 23.6 8.3 BCYNi05 13.1 11.7 16.1 11 21.2 6.7 BCYNi10 7.1 17.2 20.2 15.7 25 6.5 BCYNi20

The beneficial effect of Ni on conductivity below 600°C seems less clear under wet atmospheres mainly in the case of wet air (see Figure 7). In fact, during measurements under air nickel nanoparticules are oxidized thus induces a decrease in their catalytic activity towards hydrogen dissociation. This result seems to confirm that the metallic nanoparticules play an essential role in the improvement of total conductivity of these materials.

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



Nevertheless it must be checked if the enhancement of total conductivity of BCY under reducing atmospheres by Ni-doping is only due to the protonic contribution or if an electronic contribution must be also taken into account. Thus, the electronic conductivity of BCYNi10 was evaluated by the Hebb-Wagner ion blocking method as a function of oxygen activity for different temperatures. As shown in Figure 8, the electronic contribution to total conductivity is negligible between 400°C and 600°C. Thus, the increase in the total conductivity observed below 600°C corresponds well to an increase of the protonic conductivity what seems to prove metal nickel supports proton incorporation in the structure. By contrast, beyond 600°C the electronic contribution starts to be significant. In fact, the activation energy measured beyond 600°C under reducing atmosphere (~0.3 eV) could correspond to a electronictype conduction mechanism due to the partial reduction of Ce4+ into Ce3+.

Figure 8. Electronic conductivity of BCYNi10 as a function of oxygen activity for different temperatures. La0.75Sr0.25Cr0.5Mn0.5-xNixO3-δ compounds (LSCMNi) Ni-doped LSCM compounds La0.75Sr0.25Cr0.5Mn0.5-xNixO3-δ (0≤x≤0.2) were previously studied by T. Jardiel et al (9). A LSCM-type single phase was obtained for all compositions under air (SG R-3c) and different reducing atmospheres (SG Pm-3 m). Partial exsolution of Ni was obtained by reduction of LSCMNi compounds under 5% H2/Ar at 800 °C for 12h. These compounds are p-type semiconductors. However, under reducing atmosphere and below 500 °C the total conductivity increases with respect to that observed at higher temperatures. A suggestive protonic contribution to the total conductivity is announced by authors. Here we present a complementary study relating to this phenomenon has been undertaken. Ni-doped compounds La0.75Sr0.25Cr0.5Mn0.5-xNixO3-δ (LSCMNix x=0, 0.06 and 0.2) were synthesized by solid state reaction of La2O3, SrCO3, Cr2O3, Mn2O3 and NiO.



Reactants were mixed in stoichiometric proportions, ground thoroughly in acetone and pre-heated at 800°C for 4 h to decompose carbonates, and finally calcined at 1450 °C for 60 h in air with intermediate grindings. Powders were uniaxially pressed into disks and sintered in air at 1450 °C for 12. To induce Ni exsolution compounds were reduced under 5% H2/Ar at 800 °C for 12h. The perovskite-type structure is preserved under reducing atmosphere but a structural transition from rhombohedral symmetry R-3c to cubic symmetry Pm-3 m is observed. Ni exsolution was confirmed by TEM (see Figure 9).

Figure 9. HREM image of LSCMNi20 attesting of partial exsolution of Ni under reducing atmosphere Electrical properties. The electrical properties of LSCM, LSCMNi06 and LSCMNi20 were measured by 4-probe DC conductivity measurements under dry and wet 5%H2/95%Ar (see Figures 10-11). All compounds exhibiting the same compacity. i.e. 95%. For La0.75Sr0.25Cr0.5Mn0.5-xNixO3-δ compounds, the partial substitution of manganese by nickel induces a lowering of the total conductivity over 400°C. However, considering only substituted compounds, total conductivity seems to increase with the Ni rate.

Figure 10. Total conductivity of LSCMNix under dry 5%H2/Ar



Figure 11. Total conductivity of LSCMNix under wet and dry 5%H2/Ar Under wet reducing atmosphere any compound does not present a linear dependence of conductivity with the temperature. The curvature of the plots below 400°C suggests a protonic contribution to the total conductivity. However under dry 5%H2/Ar only Nidoped compounds exhibit this behaviour. In fact in this atmosphere only reaction [1] can take place. Ni nanoparticles could facilitate hydrogen dissociation and proton incorporation. This can explain why under dry reducing atmosphere only LSCMNi compounds exhibited protonic conductivity. Nevertheless under wet 5% H2, reaction [2] can also occur and LSCM can incorporate protons by this way. LSCMNi compounds can incorporate protons following both reactions [1] and [2]: H 2O + OOX + V O°° → 2 OH °

[2]

Since conductivity results under wet reducing atmosphere were obtained by decreasing the temperature, all compounds must contain protons at the end of measurements. Thus, to prove proton incorporation phenomenon compounds were analysed by 1H MAS NMR spectroscopy. Figure 12 shows the NMR spectra corresponding to LSCM and LSCMNi compounds. The spectra exhibit signals in the 0 to 10 ppm region. These resonances typically correspond to adsorbed protons on the surface of the materials well as the few residual protons in the probe. A set of signals is also observed in the -8 to -20 ppm region. This is an unusual chemical shift range for protons, and it may correspond to species inserted in the bulk of the material and submitted to a Fermi contact interaction from the electron spins of the transition metals of the compounds. These results confirm that all compounds contain protons.



0.45

0.4

1

H NMR 200 MHz

0.35 LSCM-Ni02

Intensity (a.u.)

0.3 LSCM-Ni006 0.25

0.2

0.15 LSCM

0.1

0.05 100

50

0

-50

-100

δ (ppm)

Figure 12. 1H MAS NMR spectra of LSCM and LSCMNi compounds

Conclusion Ni nanoparticles have been used to improve proton conductivity of the ceramic electrolyte BaCe0.9Y0.1O3-δ and the electrode material La0.75Sr0.25Cr0.5Mn0.5O3-δ. Metallic nanoparticles seem facilitate the dissociation of hydrogen and the incorporation of protons in the structure. Thus, below 500°C, proton conductivity is enhanced under reducing atmosphere. However further investigations are still necessary to better understand relationship between metallic nanoparticles and proton conductivity. Therefore, EMF measurements of LSCMNi compounds, conductivity measurements under D2 atmosphere and an evaluation by SIMS of proton diffusion coefficients in BCYNi and LSCMNi compounds are in progress.



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