Mixed Proton–Electron Conducting Chromite Electrocatalysts as Anode Materials for LWO‐Based Solid Oxide Fuel Cells (2012)

Chromite anodes in LWO PC-SOFC: Chemically compatible LSCN/LWO symmetric cells show the high protonic conductivity and p-type electronic conductivity of LSCN in reducing atmospheres. This allows to avoid blending with other protonic or electronic materials and enlarges the surface area. An Rp of 2.68 Ω cm2 at 750 °C and performance in LF-limited processes demonstrate the potential of LSCN as anode material. Recent research on solid oxide fuel cells (SOFCs) has focused on the development of proton-conducting SOFCs (PC-SOFCs). These allow lower operating temperatures and achieving very high fuel utilization efficiencies, because water is formed at the air electrode and, hence, the fuel is not diluted.1 However, there is still a need to further develop protonic electrolytes and electrode materials, in order to achieve a combination of excellent proton transport properties, compatibility with other cell components, and thermochemical stability. La6−xWO12−δ (LWO), with 0.4<x<0.7, is a promising proton conductor. Under typical fuel cell operating conditions it shows a high proton conductivity as well as a high stability in wet CO2 environments.2 However, the use of this appealing PC-SOFC electrolyte requires the development of tailor-made, suitable electrodes. A recent publication addressed the development of a cathode specific for LWO-based cells, featuring a composite of LSM (La0.8Sr0.2MnO3) and LWO.3 The development of an anode requires alternative materials because NiO reacts with LWO.4 Herein, the use of chromite perovskites (e.g., La0.85Sr0.15Cr0.9Ni0.1O3−δ; LSCN) as anode material is proposed. Indeed, doped LaCrO3 materials have been tested as anodes for conventional SOFCs, because they combine redox stability and sufficient p-type electronic5 and oxygen-ion conductivity under reducing conditions while their electrochemical activity can be fine-tuned through the incorporated catalytic doping elements, for example, nickel or ruthenium.6 Furthermore, doped LaCrO3 compounds have been claimed as hydrogen permeation membrane materials, and this particular function suggests the presence of mixed protonic–electronic conductivity in this material.7 This work illustrates the potential of chromite perovskites as anodes in LWO-based cells by (1) evidencing the compatibility of LSCN and LWO materials, (2) proving the mixed protonic–electronic high conductivity of LSCN in wet reducing atmospheres, and (3) analyzing the electrochemical performance of LSCN electrodes on LWO employing LSCN/LWO/LSCN symmetrical cells and by means of impedance spectroscopy. To test the chemical compatibility of LSCN as anode and LWO as electrolyte, an X-ray diffraction (XRD) analysis of a mixture of both phases was performed upon sintering of the powder mixture at high temperature. Figure 1 a shows the XRD pattern of a mixture of LSCN and LWO (50:50 v/v %) after treatment for 5 h at 1150 °C in air. No new crystalline phases appeared, and all diffraction peaks could be assigned to either the LSCN or LWO. This is in contrast with the high reactivity of LWO with NiO, which led to the formation of several perovskite-related products (also shown in Figure 1 a). The lack of reactivity between LWO and metallic Ni was confirmed by recording the XRD pattern of LWO+NiO (50:50 v/v %) after treatment at 900 °C for 48 h in H2: no reaction products were visible. Moreover, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the LSCN anode on LWO electrolyte (Figure 1 b and c, respectively) revealed that the integrity of the interface between both materials was preserved after sintering, and no reaction layers appeared (Supporting Information, Figure S1). In addition, the porosity of the anode appeared to be sufficient for gas exchange, with a relatively high surface area available for the electrochemical reaction, while the LWO electrolyte was dense and only showed a “closed” porosity. a) XRD patterns of (top) LSCN and LWO 50:50 v/v % treated at 1150 °C in air for 5 h, and (bottom) LWO+NiO 50:50 v/v % treated at 1150 °C in air for 5 h, LWO+NiO treated at 900 °C in H2 for 48 h, and LWO+NiO at RT. ▪: La5.5WO12, •: LaCrO3, □: NiO, ▵: Ni, ▿: LaNi0.8W0.2O3. b) SEM image and c) TEM image of a LSCN anode on LWO electrolyte sintered at 1150 °C. d) TEM images of as-sintered LSCN on LWO electrolyte, revealing the good bonding between electrolyte and porous electrode particles. e) LSCN powder after reduction at 900 °C. Under operational conditions, that is, at high temperature and under reducing conditions, LSCN splits up into metallic Ni nanoparticles. This is visible in Figure 1 d and e, where TEM images of as-sintered LSCN anode on LWO electrolyte and reduced LSCN powder are presented, respectively.8 These Ni nanoparticles (19.7±3 nm, when material is reduced at 900 °C8a) are active, catalyzing the anodic hydrogen oxidation reaction. Furthermore, the Ni particles are chemically compatible with LWO, as inferred from Figure 1 a. Although a TEM analysis suggested that a higher Ni loading would increase the surface concentration of metallic particles, a moderate amount of Ni (10 % Cr substitution) was used in this study to avoid any compatibility issues related to the reactivity with NiO. The stability of the reduced LSCN anode on LWO electrolyte was confirmed by XRD analysis of the spent anode after electrochemical cell testing (Figure S2). The formation of metallic Ni nanoparticles upon reduction in hydrogen was monitored by temperature-programmed reduction (TPR) measurements (Figure 2 a). While no reduction took place in the undoped LaCrO3, a reduction peak centered at T=380 °C appeared when Cr was partially substituted by Ni. This reduction is related to two reduction processes: the reduction of Ni+2 to metallic nickel, and the reduction of Cr+4 to Cr+3 and more reduced species (Cr+δ), as supported by XPS measurements (Figure S3), which involves the concomitant formation of new oxygen vacancies. This reduction peak increased substantially when Cr was further substituted by Sr, because the concentration of Cr+4 is higher to compensate for the presence of Sr+2 and Ni+2 in oxidizing conditions. This additional reduction is attributed to (1) the formation of metallic Ni nanoparticles active in the anodic reactions; and (2) the formation of oxygen vacancies, indispensable for the oxide hydration and protonic transport, through Cr+4 reduction. The kinetics of Ni particle formation was studied by using a LSCN porous electrode on LWO. The anode polarization resistance (Rp) was monitored at 900 °C upon feeding pure moist hydrogen. The total reduction of the LSCN material with the consequent formation of metallic Ni lasted for 6.6 h, as can be inferred from Figure 2 b, while promotion of the electrochemical reaction by the Ni nanoparticles was confirmed. a) TPR analysis of LaCrO3−δ, LaCr0.9Ni0.1O3−δ, and La0.85Sr0.15Cr0.9Ni0.1O3−δ. b) Rp of La0.85Sr0.15Cr0.9Ni0.1O3−δ as a function of time at 900 °C in 100 % H2. c) Total conductivity of La0.85Sr0.15Cr0.9Ni0.1O3−δ in ▪: 5 % H2, ▴: 5 % D2, □: 5 % H2+H2O, and ▵5 %: D2+D2O. d) Rp of La0.85Sr0.15Cr0.9Ni0.1O3−δ as a function of temperature in moist H2. The transport properties of reduced LSCN in 5 % H2 and 5 % D2 dry and wet (where wet means 2.5 % atm of H2O and D2O, respectively: Figure 2 c) were studied by dc measurements. The results suggested that LSCN presents both protonic and p-type electronic conductivity under typical anode operating conditions. In fact, the conductivity curves had the expected hydration and isotopic effects for a proton conductor while electron hole conduction was affected similarly by the corresponding pO2 but following a 1/4 power dependency. Explicitly, (i) the conductivity in wet atmosphere is always higher than in dry atmosphere, although the exothermal oxide dehydration at high temperature results in a drop of the protonic conductivity and thus the conductivity in wet and dry conditions tend to equal, for example, at 800 °C in wet and dry H2; this effect is expected at higher temperatures in D2 due to slightly different oxide hydration equilibrium for D2O;9 (ii) the conductivities in H2 are always higher than in D2 (very close to the theoretical √2 factor) in both dry and wet atmosphere. The isotopic effect in dry atmosphere is ascribed to the presence of residual amounts of H2O/D2O (ca. 30 ppm); and (iii) the dependency of conductivity with pO2 (Figure S5, at 600 and 800 °C) shows that the conductivity rises with increasing pO2, especially at temperatures below 800 °C, and the slope is significantly lower than the value 1/4, expected for pure p-type conduction. These results, at 600 °C, illustrate the mixed protonic–electronic character of LSCN showing a clear isotopic effect. However, at 800 °C, the pO2 dependency becomes small although the isotopic effect is visible, which suggests a growing contribution of oxide-ion conductivity at high temperature. With the compatibility of LSCN and LWO proved, and after checking that the transport properties and catalytic activity of LSCN are adequate, its suitability as PC-SOFC anode material was analyzed in symmetrical cells on LWO electrolyte. Figure 2 d depicts Rp as a function of temperature for moist H2. As expected, Rp increased with decreasing temperature, following Arrhenius behavior. Impedance spectra recorded in wet 100 % H2 at different temperatures is represented, after subtracting electrolyte contribution, in Figure 3 a and b (Nyquist and Bode plots, respectively). The spectra could be fitted to an equivalent circuit (Figure 3 a inset) comprising two depressed arcs, one observed at low frequencies (LF, 10–100 Hz) and another at medium frequency (MF, 0.1–1 kHz), which are more or less overlapped depending on the operating conditions. LF and MF resistances (R) measured in 100 % moist H2 are represented as a function of temperature in Arrhenius arrangement (Figure 3 c). Above 850 °C both LF and MF contributions are similar but below 850 °C the rate limiting mechanism corresponds to the LF associated processes, as LF resistances are twice MF contributions. Resistance increases when reducing pH2, and at 750 °C, both LF and MF associated mechanisms have a pH2 dependency close to 1/4 (Figure 3 d). The LF process (with associated capacitances showed in Supporting Figure S6) may be related to surface processes: catalytic reactions between chemisorbed species, adsorption/desorption reactions from/to the gas phase and surface diffusion of intermediate species.10 This important contribution to the anode impedance of the adsorption/desorption of H2O and OH− species has been previously observed in similar frequency ranges and activation energies for Ni-YSZ/Ni-CGO anodes.10b Although in this material higher amounts of Ni were not considered due to its possible reaction with LWO, it can anticipated that the further development of chromite-based PC-SOFC anodes should concentrate on the optimization of surface activity, either by increasing the intrinsic activity of the catalyst or by improving the catalyst dispersion, in order to diminish LF associated processes. Furthermore, MF processes (with capacitances shown in Figure S6) might be associated to charge transfer reactions,11 that is, (i) the charge transfer between Ni nanoparticles and the oxide and (ii) the diffusion charged species to the three phase boundary.12 Figure 3 e presents an SEM analysis corresponding to the fracture cross-section of the measured LSCN/LWO symmetrical cell. a) Nyquist, and b) Bode plots of impedance spectra recorded in wet (2.5 % vol. H2O) H2 at different temperatures (inset: equivalent circuit). □: 900 °C, ○: 850 °C, ▵: 800 °C, ▿: 750 °C. Modeled LF and MF resistances c) as a function of the temperature, and d) as a function of the pH2. □: LF (10–100 Hz), ○; MF (0.1–1 kHz). e) SEM cross-sectional image of the measured LSCN/LWO symmetrical cell. In conclusion, this work demonstrates the potential of chromite-based anodes in LWO-based PC-SOFCs. Aside from the chemical compatibility of LSCN and LWO, the high proton conductivity of reduced LSCN together with satisfactory p-type electron conductivity of LSCN in reducing atmospheres is illustrated. These properties are requisites for protonic anodes and make it possible to avoid the blending with other protonic or electronic materials, as classically happens with protonic composite cathodes for PC-SOFC3, 13 and Ni-based cermet anodes.14 Furthermore, the mixed conductivity allows enlarging the surface area, that is, TPB (three phase boundary), available for the H2 oxidation reactions and proton incorporation. The present study of the LSCN anode on LWO, in symmetrical cell configuration, demonstrates that the anode performance is reasonably good (2.68 Ω cm2 at 750 °C) while the electrode performance is principally limited by LF processes, ascribed to surface reactions, below 850 °C. Therefore, future anode optimizations should focus on tailoring the surface properties of the (mixed H+/e− conducting) chromite backbone structure, that is, catalytic activity and catalyst surface dispersion, in order to enable the practical operation in the range 600–750 °C, where LWO electrolyte behaves as a pure proton conductor. La0.85Sr0.15Cr0.9Ni0.1O3−δ (LSCN) powders were prepared by solid-state reaction technique as described elsewhere, with a final sintering temperature of 1200 °C.8a La5.6WO12−δ (LWO) powder was commercial powder provided by Cerpotech (Norway). SEM images of both powders are shown in Figure S7. Crystalline phases were identified by X-ray diffraction (XRD) performed in a PANalytical Cubix fast diffractometer, using CuKα1 radiation (λ=1.5406 Å) and an X′Celerator detector in Bragg–Brentano geometry. XRD patterns were recorded in the range 2θ=10°–90° and analyzed by using X′Pert Highscore Plus software. A Micromeritics system was used to carry out temperature-programmed reduction (TPR). 100 mg of sample was degassed under Ar flow for 1 h and then subjected to reduction under H2/Ar (1:9) flow. The heating rate was 10 °C min−1 till 1000 °C. The H2 consumption was measured by a TCD. The reported results were normalized by the sample weight and the consumption values were comparable among samples. Total conductivity measurements were carried out by standard four-point DC technique on sintered rectangular bars (1500 °C) by using silver contacts and wires. The constant current was supplied by a programmable current source (Keithley 2601), and the voltage was measured by a multimeter (Keithley 3706). The measurements were performed in cooling down (after two hours stabilization at 800 °C) under different atmospheres (dry 5 % H2 and D2 and 5 % H2 and D2 saturated with H2O and D2O at 20 °C, respectively) Dense, ca. 1 mm thick LWO disks were obtained by uniaxially pressing the ball-milled LWO powder at ca. 120 MPa and final firing at 1500 °C for 5 h. Porous ca. 18 μm electrodes were obtained by screen-printing the inks on both sides of LWO electrolyte disks. Firing temperature of the screen-printed cells was 1150 °C for 2 h. The final size of symmetrical cells was 15.5 mm in diameter, whereas anodes were ca. 9 mm in diameter. A top screen-printed gold mesh was applied on the electrodes to ensure proper current collection. LSCN/LWO/LSCN symmetrical cells were tested by electrochemical impedance spectroscopy (EIS) in two-point configuration. Input signal was 0 V DC–20 mV AC in the 0.01–3⋅105 Hz frequency range. This signal was generated by a Solartron 1470E and a 1455 A FRA module equipment. EIS measurements were performed in the 650–900 °C range, under moistened atmospheres (2.5 % vol. H2O) at different pH2 (100 %, 50 % and 5 % H2) and in wet 5 % D2 (2.5 % vol. D2O). In all the cases, the total flow remained constant (100 mL min−1). All the impedance spectra have been corrected by removing the contribution of the LWO electrolyte, that is, the high frequency real axis cut-off in the Nyquist diagrams. The microstructure was investigated using scanning electron microscopy (SEM) (Zeiss Ultra 55), and elemental analysis was carried out with energy-dispersive X-ray spectroscopy (EDS) (INCA, Oxford). Transmission electron microscope (TEM) investigation was carried out by using a FEI Tecnai F20 (with acceleration voltage of 200 kV), equipped with an EDAX energy dispersive X-ray spectrometer. TEM lamellas were prepared by means of a FIB (Focused-Ion Beam, FEI Helios Nanolab 400 s) and subsequently thinned by standard argon-ion milling techniques. Funding from the Spanish Government (ENE2011-24761 grant) and the European Union (FP7 Project EFFIPRO, Grant Agreement 227560) is acknowledged. The authors are indebted to S. Jiménez and M. Fabuel for sample preparation. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.