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Impregnated LaCo0.3Fe0.67Pd0.03O3-δ as a promising electrocatalyst for “symmetrical” intermediate-temperature solid oxide fuel cells
Journal of Power Sources 306 (2016) 92e99
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Impregnated LaCo0.3Fe0.67Pd0.03O3-d as a promising electrocatalyst for “symmetrical” intermediate-temperature solid oxide fuel cells b, Jian Shen a, Yubo Chen a, Guangming Yang a, Wei Zhou a, Moses O. Tade Zongping Shao a, b, c, * a College of Chemistry & Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing, 210009, China b Department of Chemical Engineering, Curtin University, Perth, WA, 6845, Australia c College of Energy, Nanjing Tech University, Nanjing, 210009, China
h i g h l i g h t s LaCo0.3Fe0.67Pd0.03O3-d is developed as electrode material for symmetrical SOFC. Pd-doping remarkably enhances the catalytic activity of LaCo0.3Fe0.7O3-d material. A low area specific resistance of 0.23 U cm2 was achieved at 600 C for LCFPd. Peak power density of 535 mW cm2 for LCFPd as both electrodes at 750 C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2015 Received in revised form 4 December 2015 Accepted 7 December 2015 Available online xxx
The higher cost of solid oxide fuel cells (SOFCs) compared with the cost of conventional energy conversion devices has greatly hindered their wide application. The symmetrical SOFCs that use identical material as both cathode and anode can greatly reduce the fabrication cost. The key point for the development of symmetrical SOFCs is to find a promising electrode catalyst. Herein, we report a LaCo0.3Fe0.67Pd0.03O3-d (LCFPd) material with superior catalytic activity under both oxidizing and reducing atmospheres due to the slight Pd-doping. An LCFPd-infiltrated Sm0.2Ce0.8O1.9 (SDC) electrode possesses competitive oxygen reduction activity compared with a Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) cathode and exhibits even better fuel oxidation activity than the state-of-the-art Ni-SDC composite anode. In addition, the superiority of LCFPd is demonstrated through the high and stable power outputs that can be obtained from a symmetrical SOFC with an LCFPd-based electrode as both cathode and anode. © 2015 Published by Elsevier B.V.
Keywords: LaCo0.3Fe0.67Pd0.03O3-d Impregnation Symmetrical electrode Solid oxide fuel cells
1. Introduction Solid oxide fuel cells (SOFCs) continue to attract significant attention as an ideal power generation technology for the future because of several important advantages, such as high efficiency of energy conversion, low emission of CO2 greenhouse gas and NOx/ SOx harmful gases, fuel flexibility, and high quality of exhaust heat [1,2]. Reduction of fabrication and operation costs and cell lifetime increase are critical for widespread SOFC technology
* Corresponding author. College of Chemistry & Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing, 210009, China. E-mail address: [email protected] (Z. Shao). http://dx.doi.org/10.1016/j.jpowsour.2015.12.021 0378-7753/© 2015 Published by Elsevier B.V.
commercialization. Decrease of operation temperature to an intermediate range of 500e800 C may result in several important benefits, such as increased morphological stability of the electrodes, mediated interfacial reaction between cell components and increased versatility in cell material selection, which will contribute to both a reduction in cell fabrication/operation costs and a prolonged cell lifetime [3e6]. During the past decades, tremendous research activities have been directed to the development of intermediate temperature (IT) SOFCs. However, the main challenge is the quick decrease in catalytic activity of the cathode for the oxygen reduction reaction (ORR) alongside with the drop of operation temperature [7e11]. Therefore, the development of innovative electrocatalysts that maintain high ORR activity at intermediate temperatures is urgently needed and has become a
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prominent research area in the SOFC community. Another effective way to reduce cell fabrication cost and to prolong cell lifetime may be the adoption of symmetrical SOFCs configuration [12e18]. It is known that a typical SOFC is in an asymmetrical configuration with a nickel-ceramic (YSZ or SDC) anode, an oxide electrolyte (YSZ, SDC) and an oxide cathode (perovskite), in which separate fabrication processes are usually required for integrating the anode and the cathode layers into a single cell because of their different properties and functionalities in an SOFC. In a symmetrical SOFC, because the same material is adopted for both electrodes, one-step fabrication is possible. More importantly, the functional roles of both electrodes can be reversed by swapping their atmospheres. Using this approach, any anode performance degradation due to coking can be readily regenerated. Therefore, an increase in lifetime of SOFCs can be realized by adopting a symmetrical cell configuration. The combination of symmetrical cell configuration and intermediate-temperature operation is of particular interest because it promotes commercialization of SOFC technology by significantly reducing cost and prolonging cell lifetime. The development of proper electrode materials is the key for realizing intermediate-temperature symmetrical SOFCs. However, this is a significant challenge. Because the electrode material needs to meet both the strict requirements of anode and cathode, it should show sufficiently high phase stability under both reducing and oxidizing atmospheres. In addition, the material needs to be electrocatalytically active for both ORR and fuel oxidation reaction [12,13]. During the past several years, LaCrO3-d and SrTiO3-d-based perovskite oxides or Sr2Mo2O5þd-based double perovskite oxides have been primarily used as electrode materials in symmetrical SOFCs [16,19,20]. Unfortunately, these materials showed poor activity for the ORR at reduced temperature and poor electronic conductivity under an anode atmosphere. Thus, the derived symmetrical SOFCs can deliver affordable power output only at high temperature. However, to achieve the affordable activity of electrode for the ORR, several alkaline earth metal elements (Ca, Sr, Mg) have been often doped into the oxide lattice, which made the electrodes prone to poisoning with CO2 from the environment [21,22]. Recently, some nano-sized precious metal-modified electrodes, such as Pd-modified YSZ, La0.84Sr0.16MnO3-d or La0.6Sr0.4Co0.2Fe0.8O3-d, have demonstrated favorable catalytic activity for the ORR [23e26]. Additionally, because precious metals are excellent catalysts for fuel oxidation, such precious metal-modified materials may also be used as anodes in SOFCs [27e31]. Specifically, they may be used as electrodes in symmetrical SOFCs. Recently, we have reported for the first time the successful development of an intermediate-temperature symmetrical SOFC using silverinfiltrated porous SDC as both anode and cathode, which achieved an encouraging power output [32]. Taking into account how easily precious metals sintered at elevated temperatures and the large amount of precious metals required for the electrodes, further material composition and configuration optimization are necessary for precious metal-contained electrodes to reduce the amount of precious metal catalyst and to increase the operational stability. Herein, we have demonstrated that LaCo0.3Fe0.67Pd0.03O3d (LCFPd) perovskite oxide, a typical material of the LaCo1-x-yFexPdyO3-d series with the Pd element egress-ingress capability that have been previously developed as typical smart catalysts for automobile exhaust treatment [33e36], can also be developed into an effective electrocatalyst for symmetrical IT-SOFCs with attractive performance. More specifically, the as-fabricated LCFPd þ SDC composite electrode with infiltrated LCFPd showed favorable activity for the ORR at intermediate temperatures, even though it has previously operated in an anode mode. In addition, the LCFPd þ SDC electrode demonstrated superior activity for hydrogen
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oxidation compared with the state-of-the-art Ni þ SDC anode. Promising power output and relatively stable performance at intermediate temperatures have been demonstrated for symmetrical SOFCs with infiltrated LCFPd þ SDC electrodes. The very low actual Pd loading (0.02 mg cm2), the favorable power output and its good stability make the symmetrical cell with infiltrated LCFPd a highly promising energy generation device with wide application potential. 2. Experimental 2.1. Sample preparation Single phase LaCo0.3Fe0.7O3-d (LCF), LaCo0.3Fe0.67Pd0.03O3d (LCFPd) and Sm0.2Ce0.8O1.9 (SDC) Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) powders were prepared via a combined EDTA-citrate complexing method, as reported elsewhere [17]. The powders were calcined at 800 C or 1000 C for 5 h. Aqueous nitrate solutions of LCF and LCFPd precursors for infiltration were prepared by dissolving stoichiometric amounts of La(NO3)3$6H2O, Co(NO3)2$6H2O, Fe(NO3)3$9H2O, and Pd(NO3)2$2H2O (all analytical grade, Sinopharm Chemical Reagent Co., Ltd.) in deionized water with a concentration of 0.6 mol L1. Glycine was also added to the solution as a complexing agent to form the desired perovskite phase, and ethanol was added to modify the liquid surface tension. To assemble the electrolyte-supported symmetrical SOFC with the structure of (anode) LCF (Pd)-infiltrated SDCjSDCjLCF (Pd)infiltrated SDC (cathode) for the IeV tests, the symmetrical cells with the above mentioned structure were prepared as follows. First, at the pressure of 200 MPa, the as-synthesized SDC powder was pressed into disk-shaped pellets. These SDC pellets were sintered at 1500 C in air for 5 h, and then selected pellets with intact structure were surface polished with sand paper to reach 300 mm thicknesses. The appropriate SDC powder and 10 wt.% soluble starch (pore former) were dispersed in isopropyl alcohol to form a suspension that was sprayed onto both surfaces of the as-synthesized SDC electrolyte with an effective surface area of 0.48 cm2. The composite was calcined at 1300 C in air for 5 h to yield the porous SDC scaffold with a thickness of ~30 mm. Finally, the as-prepared LCF and LCFPd precursor solutions were infiltrated into the SDC scaffold several times until the final loading reached ~20 wt.%. The composites were calcined at 800 C in air for 2 h to form the target lattice structure. Other preparation processes for impedance tests were the same as described in the previous section [18]. Silver ink (current collector) was applied to both electrode surfaces, and silver wires were selected as the current leads. The symmetrical single cells of LCF and LCFPd were assessed over the temperature range of 600e750 C. Cathode and anode sides were exposed to ambient air and humidified hydrogen (3% H2O) at a flow rate of 80 mL min1 [Standard Temperature and Pressure (STP)]. IeV polarization measurement was performed using a Keithley 2420 digital sourcemeter. The electrochemical impedance spectra (EIS) of the cells were recorded using a Solartron 1287 potentiostat in combination with a 1260 A frequency response analyzer. The impedance measurement frequency range was 0.1 Hze100 kHz with a 10 mV AC amplitude. 2.2. Characterizations Phase identification was determined using X-ray diffraction (XRD, Rigaku Smartlab) operated at 40 kV and 200 mA using Cu-Ka radiation over a range of 20e90 with an interval of 0.02 . The obtained diffraction profiles were analyzed using the Rietveld method with the GSAS program and the EXPGUI interface. The Pd states were determined using X-ray photoelectron spectroscopy
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(XPS, PHI5000 VersaProbe spectrometer equipped with an Al Ka Xray source). The bright-field scanning transmission electron microscopy (STEM) images and the corresponding energy-dispersive X-ray (EDX) mapping were acquired using an FEI Tecnai G2 T20 electron microscope. The morphologies of the electrodes were observed using a field emission scanning electron microscope (FESEM, HITACHI-S4800). 3. Results and discussion Infiltration has been widely used to prepare composite electrodes for SOFCs [37e42]. Compared to conventional composite electrodes prepared via physical mixing, improved electrode activity and stability have been often observed for the infiltrated electrodes with the same material composition. This has been attributed to the unique electrode architecture created using the infiltration method. For the fabrication of infiltrated electrodes, the adoption of liquid precursor is preferred because of the finer particle size and easier introduction of the second phase into the scaffold [40e42]. Because of the complex composition of LCFPd perovskite oxide, phase formation is a significant concern during fabrication using a liquid precursor. Furthermore, a lower calcination temperature is preferred for practical application because lower temperature can result in smaller particle size of the infiltrated phase, consequently providing larger active sites. However, to perform as an electrode material for symmetrical SOFCs, LCFPd will experience not only a highly oxidizing atmosphere of air and a very reducing atmosphere of anode with an oxygen partial pressure as low as 1021 atm [12,13]. Thus, phase stability is crucial for stable operation of LCFPd in symmetrical SOFC because large volume changes associated with phase transition can result in pulverization of the electrode material, thus, leading to cell failure. Shown in Fig. 1 are XRD patterns of the LCFPd-impregnated SDC electrode synthesized at 800 C for 5 h and the electrode after a further treatment in 10% H2eAr atmosphere at 700 C for 10 h. For comparison, the XRD patterns of SDC and single-phase LCFPd are also presented. In the XRD patterns of as-prepared LCFPd-infiltrated SDC electrode calcined at 800 C for 5 h, in addition to the characteristic diffraction peaks of SDC scaffold, the diffraction peaks assignable to perovskite were also observed. The perovskite phase
Fig. 1. XRD patterns of SDC, LCFPd, LCFPd-impregnated SDC and LCFPd-impregnated SDC electrode after the 10% H2eAr atmosphere treatment.
diffraction peaks in the LCFPd-infiltrated SDC overlapped well with the peaks of the single-phase LCFPd. This suggests that the LCFPd perovskite phase was successfully formed inside the pores of SDC scaffold at a relatively low calcination temperature of 800 C for the LCFPd-infiltrated SDC sample. Furthermore, the results suggest that there was no obvious solidestate reaction between LCFPd phase and SDC scaffold in the infiltrated electrodes at 800 C. The successful formation of LCFPd phase at a low calcination temperature of 800 C was supported by the slightly larger lattice parameters of the LCFPd phase (a ¼ 5.49(1), b ¼ 7.78(5), c ¼ 5.52(9) Å, V ¼ 236.0(4) Å3) compared with those of the Pd-free LCF phase calcined at the same temperature of 800 C (a ¼ 5.490(5), b ¼ 7.77(7), c ¼ 5.526(1) Å, V ¼ 235.0(9) Å3). The parameters were derived based on the Retvield refinement of the X-ray diffraction patterns (Fig. S1). It is known that Pd3þ/4þ has an ionic size of 0.76/ 0.615 Å, which is larger than that of Co3þ and Fe3þ with ionic sizes of 0.545 and 0.55 Å, respectively. Therefore, doping of a larger Pdxþ cation into the LCF oxide lattice should increase the lattice parameter, which is in agreement with the experimental results. Additionally, the successful doping of Pd into the LCF oxide lattice was supported by the homogeneous distribution of various compositional elements within the LCFPd oxide, as determined by EDX mapping and shown in Fig. 2. Furthermore, it is encouraging that XRD patterns of the sample after the hydrogen atmosphere treatment can still be well indexed based on a physical mixture of LCFPd and SDC. This indicates that the main LCFPd perovskite structure was stable even under the highly reducing hydrogen atmosphere due to the stable framework of LCF. Nevertheless, all perovskite phase diffraction peaks slightly shifted to lower angles compared with those of the fresh LCFPd. This suggests the partial reduction of Fe and Co cations in oxide lattice under reducing atmosphere. Shown in Fig. 3 are the SEM images of the SDC scaffold, the asprepared LCFPd-impregnated SDC electrode, and the electrode after further reduction in 10% H2eAr atmosphere at 700 C for 10 h. The SDC showed a large number of pores with size ranges of 0.2e1 mm that were built up from well-connected SDC grains with sizes of 200e400 nm. After LCFPd precursor infiltration and further calcination at 800 C, many nanoparticles with sizes of 50e200 nm were deposited on the wall of porous SDC scaffold and formed a well-connected network. Based on the XRD results, these nanoparticles are the newly formed LCFPd perovskite. This demonstrates that LCFPd was successfully infiltrated into the SDC scaffold and presented as nanoparticles decorating the SDC scaffold wall. After the hydrogen atmosphere treatment (to simulate the anode environment), no obvious changes in the electrode morphology were observed. Furthermore, the LCFPd nanoparticles remained firmly attached to the SDC scaffold wall without the appearance of electrode pulverization. This indicates the high mechanical stability of electrode material, which is strongly connected with the high structural stability of the LCF perovskite oxide lattice as demonstrated by XRD results. LCFPd has been initially developed as a smart catalyst to treat the automobile exhaust [33]. The Pd has been reported to incorporate into the oxide lattice under oxidizing atmosphere, while Pd egresses from the oxide bulk as metallic Pd to decorate the oxide surface with very small nanoparticles. As shown in Fig. 3f, many small particles with sizes of approximately 5e10 nm appeared, which were homogeneously distributed over the surface of LCFPd nanoparticles. It is well known that the perovskite structure can stabilize some unusual high oxidation states of transition metals and other elements. Typically, Pd is 2 þ in palladium oxide. However, it has been reported that Pd has an unusually high oxidation state (Pdxþ, x > 2) in the perovskite lattice of LCFPd [34,35]. Therefore, the electronic structure of Pd provides indirect information of the state of Pd
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Fig. 2. STEM images and EDX elemental distributions of La, Co, Fe, Pd, and O in the LCFPd particles.
Fig. 3. SEM images and magnifications of the SDC scaffold (a, b), the as-prepared LCFPd-impregnated SDC electrode (c, d), and the electrode after further reduction in 10% H2eAr atmosphere (e, f).
presented in LCFPd samples. Three different samples, i.e., the freshly prepared LCFPd (LCFPd-a), the LCFPd sample after the treatment in 10% H2eAr at 800 C for 10 h (LCFPd-b), and the re-
oxidation of LCFPd-b in air at 800 C for 5 h (LCFPd-c), were subjected for XPS characterization. As shown in Fig. 4, the freshly prepared LCFP exhibited two main peaks with different binding
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Fig. 4. XPS patterns of Pd in the LCFPd sample calcined at 800 C (a), the LCFPd sample after the treatment in 10% H2eAr at 800 C for 10 h (b), and re-oxidation of LCFPd-b in air at 800 C for 5 h (c).
energies in the Pd 3d5/2 spectra region. The low intensity peak with a lower binding energy clearly indicated that Pd is in a þ2 oxidation state (Pd2þ). However, the high intensity peak shifted to a higher binding energy than Pd2þ. This suggests that some Pd was in unusually high oxidation states (Pdxþ, x > 2), which, as has been previously reported, is attributed to a special ionic environment located in the B-site lattice of the perovskite network [33e36]. Based on the XPS analysis, we suggest that majority of Pd was successfully doped into the perovskite lattice, and a small amount of PdO still existed as a separate phase in the freshly prepared LCFPd sample. After the treatment in hydrogen, Pd was in the oxidation state of Pd0, which suggests a successful segregation of Pd from the oxide bulk. Once the reduced sample was treated in air, the XPS analysis of the sample suggested that the oxidation state of Pd appeared to be primarily >2þ, which indicated a successful incorporation of Pd into the perovskite lattice after the oxidizing atmosphere treatment. 3.1. Electrocatalytic activity for the ORR In the above analysis, we have demonstrated that the LCFPdinfiltrated SDC electrode can be stably present in phase structure under anode operation conditions. To test the electrocatalytic activity of impregnated LCFPd electrode for fuel oxidation, we fabricated and comparatively studied two asymmetrical fuel cells with the same SDC electrolyte and BSCF cathode but different anodes: one with the LCFPd impregnated SDC anode and the other with a conventional Ni þ SDC anode. BSCF has been reported to have high activity for the ORR at intermediate temperatures, while Ni þ SDC is an excellent anode that showed high catalytic activity for hydrogen electro-oxidation at intermediate temperatures. Therefore, any power output differences between the two cells should be primarily attributed to the anode. Fig. 5 demonstrates the typical IeV curves of the two cells for the temperature range of 600e750 C. The Ni þ SDCjSDCjBSCF cell delivered peak powder densities (PPDs) of 222, 310, 395 and 471 mW cm2 at 600, 650, 700 and 750 C, respectively. The corresponding values for the LCFPd þ SDCjSDCjBSCF cell are 321, 431, 540 and 646 mW cm2. This suggests that the LCFPd þ SDC anode had an even better electrocatalytic activity for hydrogen oxidation compared with that of the Ni þ SDC anode. This conclusion was further supported by
the measured EIS of both cells at different temperatures under open circuit voltage (OCV), and the results are listed in Table S1. The cell with an impregnated LCFPd þ SDC anode had a lower electrode polarization resistance than that of the cell with a Ni þ SDC anode. Here, the electrode polarization resistance should be the sum of anode and cathode. Because the same BSCF cathode was used in both cells, the difference in electrode polarization resistance is primarily attributed to the anode side. It is known that Pd is a more active catalyst for fuel oxidation than nickel, which may explain the superior anode performance of LCFPd þ SDC compared with that of Ni þ SDC. In addition to high activity of fuel oxidation, to achieve high power output from symmetrical SOFCs, the electrodes should have high electrocatalytic activity for the ORR. The poor activity of electrodes for the ORR at intermediate temperatures has become the main obstacle for reducing the operation temperature of many symmetrical SOFCs. Most investigated perovskite materials for symmetrical SOFCs in the literature are primarily based on doped LaCrO3-d, LaMnO3-dor doped SrTiO3-d perovskites, and all of them showed poor activity for the ORR at intermediate temperatures [43e46]. Thus, most of the above-mentioned symmetrical SOFCs have been operated at temperatures higher than 850 C. The electrocatalytic activity of LCFPd-infiltrated SDC electrode for the ORR at intermediate temperatures was investigated using EIS on the cell with an SDC electrolyte and symmetrical LCFPd-infiltrated SDC electrodes. In addition, a symmetrical cell with LCF-infiltrated SDC electrodes was fabricated and tested, using EIS with air as the atmosphere for both electrodes, to obtain information about the beneficial effect of Pd for the ORR. Fig. 6 shows the typical EIS of the infiltrated LCFPd þ SDC composite electrode for the ORR that was calcined at 800 C and operated at intermediate temperatures between 600 and 800 C. The superior catalytic activity of infiltrated LCFPd þ SDC for the ORR compared with that of LCF þ SDC is clearly demonstrated. For example, at 600 C, the area specific resistance (ASR) of the LCFPd þ SDC electrode is approximately 0.23 U cm2, while a larger value of 0.99 U cm2 was observed for the LCF þ SDC electrode. Because the fabrication process and parameters are similar for both electrodes, the differences in their ASRs primarily resulted from the electrode perovskite phase. Pd has been widely used to improve perovskite oxides for the ORR because Pd itself is a good electrocatalyst for the ORR [23e26]. Recently, we have demonstrated that the catalytic activity of Pd for the ORR at room temperature was closely related to the oxidation state of Pd, i.e., the higher the oxidation state of Pd is, the higher the activity [47]. Because there is no significant difference in mechanism of the ORR at low and high temperatures, the same trend may be applicable for Pd operating at elevated temperatures. Thus, it is reasonable to conclude that the superior activity of LCFPd þ SDC compared with that of LCF þ SDC for the ORR at intermediate temperatures resulted from the doping of unusually high Pdxþ (x > 2) into the perovskite lattice, which might have modified the surface properties of the electrode. Furthermore, to demonstrate the beneficial effect of higher oxidation state Pd for the ORR, we tested the electrocatalytic activity for the ORR of the electrode that was first reduced with hydrogen and then was re-oxidized with air. As shown in Fig. S2, the electrode performance was slightly worse compared with the performance of the unreduced electrode. At 600 C, an ASR of 0.35 U cm2 was achieved, which is approximately 1/2 larger than that of fresh LCFPd (0.23 U cm2), but it is still significantly lower than the ASR of Pd-free LCF (0.99 U cm2). This further confirmed that Pd in higher oxidation state is beneficial for the ORR. Nevertheless, the LCFPd þ SDC electrode that had previously operated in the anode mode can still deliver an attractive performance as a cathode for the ORR. This suggests that both electrodes can be swapped if necessary.
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Fig. 5. IeV and IeP curves of the cell with configurations of (a) Ni þ SDCjSDCjBSCF and (b) LCFPd þ SDCjSDCjBSCF operating on hydrogen.
Fig. 6. Nyquist plots of the LCF (Pd)-infiltrated SDC electrodes at 600, 650, 700, 750 and 800 C.
Symmetrical SOFCs with infiltrated LCF (LCFPd)þSDC electrodes and in the electrolyte-supported configuration were then tested for power generation. Here, the thickness of the SDC electrolyte was controlled at 0.3 mm, the calcination temperature for the electrodes was set at 800 C, and ambient air and hydrogen with fluxes of 80 mL min1 were used as the cathode atmosphere and anode fuel. Shown in Fig. 7a are the typical IeV polarization curves of an LCFPd þ SDCjSDCjLCFPd þ SDC symmetrical SOFC. The linear response of the current to voltage at high polarization current demonstrated that the electrodes had sufficient porosity to avoid the appearance of concentration polarization. This implies that the infiltrated LCFPd phase did not cause a detrimental effect on the gas diffusion channels inside the electrode. Favorable power outputs were demonstrated at intermediate temperatures for the cell. For example, the PPDs reached 132, 172, 246, 309, 394, 467 and 535 mW cm2 at operation temperatures of 600, 625, 650, 675, 700, 725 and 750 C, respectively. These values are highly attractive when considering the thick electrolyte configuration of the cell. The
above-mentioned values are comparable to an asymmetrical BSCFjSDCjLCFPd þ SDC (anode) cell and are even higher than the values for LCFPd þ SDCjSDCjNi þ SDC (anode) (Table S2). Furthermore, this strongly demonstrates the high electrocatalytic activity of the infiltrated LCFPd þ SDC electrode for both the ORR and hydrogen oxidation at intermediate temperatures. For comparison, a similar cell with an LCF-infiltrated SDC electrode has been tested and the IeV polarization curves are presented in Fig. 7b. The PPDs of the LCF þ SDCjSDCjLCF þ SDC symmetrical SOFC are 100, 122, 151, 182, 212, 245 and 291 mW cm2, at operation temperatures of 600, 625, 650, 675, 700 725 and 750 C, respectively, which are clearly lower than the values of an LCFPd þ SDCjSDCjLCFPd þ SDC symmetrical SOFC at corresponding temperatures. Because the electrolyte material and membrane thickness were the same, the different power outputs of the two cells primarily resulted from differences in electrode performance. As demonstrated using EIS, the LCFPd þ SDC electrode also had a superior activity than LCF þ SDC electrode for the ORR when both were used as cathodes.
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Fig. 7. IeV and IeP curves of the cells with configurations of (a) LCFPd þ SDCjSDCjLCFPd þ SDC and (b) LCF þ SDCjSDCjLCF þ SDC operating on hydrogen.
In addition, under reducing atmosphere, the nanoparticle Pd was segregated from the oxide bulk, which had a high activity for hydrogen [27e30]. Therefore, the symmetrical cell with the infiltrated LCFPd þ SDC electrode had higher electrocatalytic activity than LCF þ SDC electrode for both the ORR and hydrogen oxidation. Sintering of the electrode, the collapse of perovskite lattice, and the delamination of the electrode layer from the electrolyte surface could also lead to deterioration of the cell performance with operation time. Then, the operational stability of the LCFPd þ SDCjSDCjLCFPd þ SDC symmetrical SOFC was investigated at 650 C under the constant polarization current density of 300 mA cm2 for a total period of approximately 120 h. Shown in Fig. 8 is the operation time dependence of the cell voltage. After the initial 20 h, a slight decrease in cell voltage from the initial value of 0.67 Ve0.65 V was observed, which could have resulted from stabilization of the electrode and current collector. Then, a very stable cell voltage was observed during the next 100 h of test. This suggests that the infiltrated LCFPd þ SDC electrode can be reliably operated under both the anode and cathode conditions. The redox (reduction-oxidation) cycling test was also performed for the “symmetrical” SOFC with LCFPd-infiltrated SDC electrode. The cell was firstly heated up to 700 C, and the process of a redox cycle was described in Fig. S3. The OCV and PPD of the cell decreased slowly with each complete redox cycle. After several cycling tests, the OCV and PPD of the cell stayed around 0.82 V and 370 mW cm2,
respectively, indicating that the single cell with LCFPd-infiltrated SDC electrode exhibited a good redox stability. Considering that the possible pulverization of perovskite oxide due to deep reduction under reducing atmosphere could be the main problem for the deterioration of symmetrical SOFCs with perovskite oxide-based electrodes, the anode layer of symmetrical SOFCs after a long-term stability test was analyzed using SEM, and the typical images are shown in Fig. S4. After the test, the connection between the LCFPd þ SDC electrode layer and the SDC electrolyte remained perfect without the appearance of anode delimitation. The LCFPd nanoparticles were still clearly observed and were firmly attached to the surface of SDC scaffold without the appearance of any electrode material pulverization. The particle size of LCFPd nanoparticles did not show any obvious growth compared to the fresh sample. The stable phase structure of the main LCF perovskite lattice and the comparable thermal expansion coefficient of the electrode, prepared using the infiltrated method, to that of electrolyte may explain the good morphological stability of the electrode under anodic operation conditions. This might have contributed to excellent operation stability of symmetrical SOFCs under real operation conditions.
4. Conclusion In this study, we have reported an LCFPd perovskite material as a promising electrocatalyst for intermediate-temperature symmetrical solid oxide fuel cells. The LCFPd material and the LCFPdinfiltrated SDC electrode show high structural/microstructural stability under both oxidizing and reducing atmospheres. Additional doping with Pd has greatly improved the catalytic activity of LCF for oxygen reduction as well as fuel oxidation. As expected, high power outputs at intermediate temperatures and long-term performance stability are obtained from symmetrical solid oxide fuel cells with LCFPd-infiltrated SDC as both anode and cathode. These favorable results highlight the practical application of LCFPd-based materials in symmetrical intermediate-temperature solid oxide fuel cells.
Acknowledgments
Fig. 8. Cell voltage time-dependence with 3% H2O humidified H2 fuel under a constant polarization current of 300 mA cm2 at 650 C.
This work was financially supported by the Key Projects in Nature Science Foundation of Jiangsu Province under contract No. BK2011030, the Major Project of Educational Commission of Jiangsu Province of China under contract No. 13KJA430004, the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Australia Research Council under contracts DP150104365 and DP160104835.
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Impregnated LaCo0.3Fe0.67Pd0.03O3-d as a promising electrocatalyst for “symmetrical” intermediate-temperature solid oxide fuel cells b, Jian Shen a, Yubo Chen a, Guangming Yang a, Wei Zhou a, Moses O. Tade Zongping Shao a, b, c, * a College of Chemistry & Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing, 210009, China b Department of Chemical Engineering, Curtin University, Perth, WA, 6845, Australia c College of Energy, Nanjing Tech University, Nanjing, 210009, China
h i g h l i g h t s LaCo0.3Fe0.67Pd0.03O3-d is developed as electrode material for symmetrical SOFC. Pd-doping remarkably enhances the catalytic activity of LaCo0.3Fe0.7O3-d material. A low area specific resistance of 0.23 U cm2 was achieved at 600 C for LCFPd. Peak power density of 535 mW cm2 for LCFPd as both electrodes at 750 C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2015 Received in revised form 4 December 2015 Accepted 7 December 2015 Available online xxx
The higher cost of solid oxide fuel cells (SOFCs) compared with the cost of conventional energy conversion devices has greatly hindered their wide application. The symmetrical SOFCs that use identical material as both cathode and anode can greatly reduce the fabrication cost. The key point for the development of symmetrical SOFCs is to find a promising electrode catalyst. Herein, we report a LaCo0.3Fe0.67Pd0.03O3-d (LCFPd) material with superior catalytic activity under both oxidizing and reducing atmospheres due to the slight Pd-doping. An LCFPd-infiltrated Sm0.2Ce0.8O1.9 (SDC) electrode possesses competitive oxygen reduction activity compared with a Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) cathode and exhibits even better fuel oxidation activity than the state-of-the-art Ni-SDC composite anode. In addition, the superiority of LCFPd is demonstrated through the high and stable power outputs that can be obtained from a symmetrical SOFC with an LCFPd-based electrode as both cathode and anode. © 2015 Published by Elsevier B.V.
Keywords: LaCo0.3Fe0.67Pd0.03O3-d Impregnation Symmetrical electrode Solid oxide fuel cells
1. Introduction Solid oxide fuel cells (SOFCs) continue to attract significant attention as an ideal power generation technology for the future because of several important advantages, such as high efficiency of energy conversion, low emission of CO2 greenhouse gas and NOx/ SOx harmful gases, fuel flexibility, and high quality of exhaust heat [1,2]. Reduction of fabrication and operation costs and cell lifetime increase are critical for widespread SOFC technology
* Corresponding author. College of Chemistry & Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing, 210009, China. E-mail address: [email protected] (Z. Shao). http://dx.doi.org/10.1016/j.jpowsour.2015.12.021 0378-7753/© 2015 Published by Elsevier B.V.
commercialization. Decrease of operation temperature to an intermediate range of 500e800 C may result in several important benefits, such as increased morphological stability of the electrodes, mediated interfacial reaction between cell components and increased versatility in cell material selection, which will contribute to both a reduction in cell fabrication/operation costs and a prolonged cell lifetime [3e6]. During the past decades, tremendous research activities have been directed to the development of intermediate temperature (IT) SOFCs. However, the main challenge is the quick decrease in catalytic activity of the cathode for the oxygen reduction reaction (ORR) alongside with the drop of operation temperature [7e11]. Therefore, the development of innovative electrocatalysts that maintain high ORR activity at intermediate temperatures is urgently needed and has become a
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prominent research area in the SOFC community. Another effective way to reduce cell fabrication cost and to prolong cell lifetime may be the adoption of symmetrical SOFCs configuration [12e18]. It is known that a typical SOFC is in an asymmetrical configuration with a nickel-ceramic (YSZ or SDC) anode, an oxide electrolyte (YSZ, SDC) and an oxide cathode (perovskite), in which separate fabrication processes are usually required for integrating the anode and the cathode layers into a single cell because of their different properties and functionalities in an SOFC. In a symmetrical SOFC, because the same material is adopted for both electrodes, one-step fabrication is possible. More importantly, the functional roles of both electrodes can be reversed by swapping their atmospheres. Using this approach, any anode performance degradation due to coking can be readily regenerated. Therefore, an increase in lifetime of SOFCs can be realized by adopting a symmetrical cell configuration. The combination of symmetrical cell configuration and intermediate-temperature operation is of particular interest because it promotes commercialization of SOFC technology by significantly reducing cost and prolonging cell lifetime. The development of proper electrode materials is the key for realizing intermediate-temperature symmetrical SOFCs. However, this is a significant challenge. Because the electrode material needs to meet both the strict requirements of anode and cathode, it should show sufficiently high phase stability under both reducing and oxidizing atmospheres. In addition, the material needs to be electrocatalytically active for both ORR and fuel oxidation reaction [12,13]. During the past several years, LaCrO3-d and SrTiO3-d-based perovskite oxides or Sr2Mo2O5þd-based double perovskite oxides have been primarily used as electrode materials in symmetrical SOFCs [16,19,20]. Unfortunately, these materials showed poor activity for the ORR at reduced temperature and poor electronic conductivity under an anode atmosphere. Thus, the derived symmetrical SOFCs can deliver affordable power output only at high temperature. However, to achieve the affordable activity of electrode for the ORR, several alkaline earth metal elements (Ca, Sr, Mg) have been often doped into the oxide lattice, which made the electrodes prone to poisoning with CO2 from the environment [21,22]. Recently, some nano-sized precious metal-modified electrodes, such as Pd-modified YSZ, La0.84Sr0.16MnO3-d or La0.6Sr0.4Co0.2Fe0.8O3-d, have demonstrated favorable catalytic activity for the ORR [23e26]. Additionally, because precious metals are excellent catalysts for fuel oxidation, such precious metal-modified materials may also be used as anodes in SOFCs [27e31]. Specifically, they may be used as electrodes in symmetrical SOFCs. Recently, we have reported for the first time the successful development of an intermediate-temperature symmetrical SOFC using silverinfiltrated porous SDC as both anode and cathode, which achieved an encouraging power output [32]. Taking into account how easily precious metals sintered at elevated temperatures and the large amount of precious metals required for the electrodes, further material composition and configuration optimization are necessary for precious metal-contained electrodes to reduce the amount of precious metal catalyst and to increase the operational stability. Herein, we have demonstrated that LaCo0.3Fe0.67Pd0.03O3d (LCFPd) perovskite oxide, a typical material of the LaCo1-x-yFexPdyO3-d series with the Pd element egress-ingress capability that have been previously developed as typical smart catalysts for automobile exhaust treatment [33e36], can also be developed into an effective electrocatalyst for symmetrical IT-SOFCs with attractive performance. More specifically, the as-fabricated LCFPd þ SDC composite electrode with infiltrated LCFPd showed favorable activity for the ORR at intermediate temperatures, even though it has previously operated in an anode mode. In addition, the LCFPd þ SDC electrode demonstrated superior activity for hydrogen
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oxidation compared with the state-of-the-art Ni þ SDC anode. Promising power output and relatively stable performance at intermediate temperatures have been demonstrated for symmetrical SOFCs with infiltrated LCFPd þ SDC electrodes. The very low actual Pd loading (0.02 mg cm2), the favorable power output and its good stability make the symmetrical cell with infiltrated LCFPd a highly promising energy generation device with wide application potential. 2. Experimental 2.1. Sample preparation Single phase LaCo0.3Fe0.7O3-d (LCF), LaCo0.3Fe0.67Pd0.03O3d (LCFPd) and Sm0.2Ce0.8O1.9 (SDC) Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) powders were prepared via a combined EDTA-citrate complexing method, as reported elsewhere [17]. The powders were calcined at 800 C or 1000 C for 5 h. Aqueous nitrate solutions of LCF and LCFPd precursors for infiltration were prepared by dissolving stoichiometric amounts of La(NO3)3$6H2O, Co(NO3)2$6H2O, Fe(NO3)3$9H2O, and Pd(NO3)2$2H2O (all analytical grade, Sinopharm Chemical Reagent Co., Ltd.) in deionized water with a concentration of 0.6 mol L1. Glycine was also added to the solution as a complexing agent to form the desired perovskite phase, and ethanol was added to modify the liquid surface tension. To assemble the electrolyte-supported symmetrical SOFC with the structure of (anode) LCF (Pd)-infiltrated SDCjSDCjLCF (Pd)infiltrated SDC (cathode) for the IeV tests, the symmetrical cells with the above mentioned structure were prepared as follows. First, at the pressure of 200 MPa, the as-synthesized SDC powder was pressed into disk-shaped pellets. These SDC pellets were sintered at 1500 C in air for 5 h, and then selected pellets with intact structure were surface polished with sand paper to reach 300 mm thicknesses. The appropriate SDC powder and 10 wt.% soluble starch (pore former) were dispersed in isopropyl alcohol to form a suspension that was sprayed onto both surfaces of the as-synthesized SDC electrolyte with an effective surface area of 0.48 cm2. The composite was calcined at 1300 C in air for 5 h to yield the porous SDC scaffold with a thickness of ~30 mm. Finally, the as-prepared LCF and LCFPd precursor solutions were infiltrated into the SDC scaffold several times until the final loading reached ~20 wt.%. The composites were calcined at 800 C in air for 2 h to form the target lattice structure. Other preparation processes for impedance tests were the same as described in the previous section [18]. Silver ink (current collector) was applied to both electrode surfaces, and silver wires were selected as the current leads. The symmetrical single cells of LCF and LCFPd were assessed over the temperature range of 600e750 C. Cathode and anode sides were exposed to ambient air and humidified hydrogen (3% H2O) at a flow rate of 80 mL min1 [Standard Temperature and Pressure (STP)]. IeV polarization measurement was performed using a Keithley 2420 digital sourcemeter. The electrochemical impedance spectra (EIS) of the cells were recorded using a Solartron 1287 potentiostat in combination with a 1260 A frequency response analyzer. The impedance measurement frequency range was 0.1 Hze100 kHz with a 10 mV AC amplitude. 2.2. Characterizations Phase identification was determined using X-ray diffraction (XRD, Rigaku Smartlab) operated at 40 kV and 200 mA using Cu-Ka radiation over a range of 20e90 with an interval of 0.02 . The obtained diffraction profiles were analyzed using the Rietveld method with the GSAS program and the EXPGUI interface. The Pd states were determined using X-ray photoelectron spectroscopy
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(XPS, PHI5000 VersaProbe spectrometer equipped with an Al Ka Xray source). The bright-field scanning transmission electron microscopy (STEM) images and the corresponding energy-dispersive X-ray (EDX) mapping were acquired using an FEI Tecnai G2 T20 electron microscope. The morphologies of the electrodes were observed using a field emission scanning electron microscope (FESEM, HITACHI-S4800). 3. Results and discussion Infiltration has been widely used to prepare composite electrodes for SOFCs [37e42]. Compared to conventional composite electrodes prepared via physical mixing, improved electrode activity and stability have been often observed for the infiltrated electrodes with the same material composition. This has been attributed to the unique electrode architecture created using the infiltration method. For the fabrication of infiltrated electrodes, the adoption of liquid precursor is preferred because of the finer particle size and easier introduction of the second phase into the scaffold [40e42]. Because of the complex composition of LCFPd perovskite oxide, phase formation is a significant concern during fabrication using a liquid precursor. Furthermore, a lower calcination temperature is preferred for practical application because lower temperature can result in smaller particle size of the infiltrated phase, consequently providing larger active sites. However, to perform as an electrode material for symmetrical SOFCs, LCFPd will experience not only a highly oxidizing atmosphere of air and a very reducing atmosphere of anode with an oxygen partial pressure as low as 1021 atm [12,13]. Thus, phase stability is crucial for stable operation of LCFPd in symmetrical SOFC because large volume changes associated with phase transition can result in pulverization of the electrode material, thus, leading to cell failure. Shown in Fig. 1 are XRD patterns of the LCFPd-impregnated SDC electrode synthesized at 800 C for 5 h and the electrode after a further treatment in 10% H2eAr atmosphere at 700 C for 10 h. For comparison, the XRD patterns of SDC and single-phase LCFPd are also presented. In the XRD patterns of as-prepared LCFPd-infiltrated SDC electrode calcined at 800 C for 5 h, in addition to the characteristic diffraction peaks of SDC scaffold, the diffraction peaks assignable to perovskite were also observed. The perovskite phase
Fig. 1. XRD patterns of SDC, LCFPd, LCFPd-impregnated SDC and LCFPd-impregnated SDC electrode after the 10% H2eAr atmosphere treatment.
diffraction peaks in the LCFPd-infiltrated SDC overlapped well with the peaks of the single-phase LCFPd. This suggests that the LCFPd perovskite phase was successfully formed inside the pores of SDC scaffold at a relatively low calcination temperature of 800 C for the LCFPd-infiltrated SDC sample. Furthermore, the results suggest that there was no obvious solidestate reaction between LCFPd phase and SDC scaffold in the infiltrated electrodes at 800 C. The successful formation of LCFPd phase at a low calcination temperature of 800 C was supported by the slightly larger lattice parameters of the LCFPd phase (a ¼ 5.49(1), b ¼ 7.78(5), c ¼ 5.52(9) Å, V ¼ 236.0(4) Å3) compared with those of the Pd-free LCF phase calcined at the same temperature of 800 C (a ¼ 5.490(5), b ¼ 7.77(7), c ¼ 5.526(1) Å, V ¼ 235.0(9) Å3). The parameters were derived based on the Retvield refinement of the X-ray diffraction patterns (Fig. S1). It is known that Pd3þ/4þ has an ionic size of 0.76/ 0.615 Å, which is larger than that of Co3þ and Fe3þ with ionic sizes of 0.545 and 0.55 Å, respectively. Therefore, doping of a larger Pdxþ cation into the LCF oxide lattice should increase the lattice parameter, which is in agreement with the experimental results. Additionally, the successful doping of Pd into the LCF oxide lattice was supported by the homogeneous distribution of various compositional elements within the LCFPd oxide, as determined by EDX mapping and shown in Fig. 2. Furthermore, it is encouraging that XRD patterns of the sample after the hydrogen atmosphere treatment can still be well indexed based on a physical mixture of LCFPd and SDC. This indicates that the main LCFPd perovskite structure was stable even under the highly reducing hydrogen atmosphere due to the stable framework of LCF. Nevertheless, all perovskite phase diffraction peaks slightly shifted to lower angles compared with those of the fresh LCFPd. This suggests the partial reduction of Fe and Co cations in oxide lattice under reducing atmosphere. Shown in Fig. 3 are the SEM images of the SDC scaffold, the asprepared LCFPd-impregnated SDC electrode, and the electrode after further reduction in 10% H2eAr atmosphere at 700 C for 10 h. The SDC showed a large number of pores with size ranges of 0.2e1 mm that were built up from well-connected SDC grains with sizes of 200e400 nm. After LCFPd precursor infiltration and further calcination at 800 C, many nanoparticles with sizes of 50e200 nm were deposited on the wall of porous SDC scaffold and formed a well-connected network. Based on the XRD results, these nanoparticles are the newly formed LCFPd perovskite. This demonstrates that LCFPd was successfully infiltrated into the SDC scaffold and presented as nanoparticles decorating the SDC scaffold wall. After the hydrogen atmosphere treatment (to simulate the anode environment), no obvious changes in the electrode morphology were observed. Furthermore, the LCFPd nanoparticles remained firmly attached to the SDC scaffold wall without the appearance of electrode pulverization. This indicates the high mechanical stability of electrode material, which is strongly connected with the high structural stability of the LCF perovskite oxide lattice as demonstrated by XRD results. LCFPd has been initially developed as a smart catalyst to treat the automobile exhaust [33]. The Pd has been reported to incorporate into the oxide lattice under oxidizing atmosphere, while Pd egresses from the oxide bulk as metallic Pd to decorate the oxide surface with very small nanoparticles. As shown in Fig. 3f, many small particles with sizes of approximately 5e10 nm appeared, which were homogeneously distributed over the surface of LCFPd nanoparticles. It is well known that the perovskite structure can stabilize some unusual high oxidation states of transition metals and other elements. Typically, Pd is 2 þ in palladium oxide. However, it has been reported that Pd has an unusually high oxidation state (Pdxþ, x > 2) in the perovskite lattice of LCFPd [34,35]. Therefore, the electronic structure of Pd provides indirect information of the state of Pd
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Fig. 2. STEM images and EDX elemental distributions of La, Co, Fe, Pd, and O in the LCFPd particles.
Fig. 3. SEM images and magnifications of the SDC scaffold (a, b), the as-prepared LCFPd-impregnated SDC electrode (c, d), and the electrode after further reduction in 10% H2eAr atmosphere (e, f).
presented in LCFPd samples. Three different samples, i.e., the freshly prepared LCFPd (LCFPd-a), the LCFPd sample after the treatment in 10% H2eAr at 800 C for 10 h (LCFPd-b), and the re-
oxidation of LCFPd-b in air at 800 C for 5 h (LCFPd-c), were subjected for XPS characterization. As shown in Fig. 4, the freshly prepared LCFP exhibited two main peaks with different binding
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Fig. 4. XPS patterns of Pd in the LCFPd sample calcined at 800 C (a), the LCFPd sample after the treatment in 10% H2eAr at 800 C for 10 h (b), and re-oxidation of LCFPd-b in air at 800 C for 5 h (c).
energies in the Pd 3d5/2 spectra region. The low intensity peak with a lower binding energy clearly indicated that Pd is in a þ2 oxidation state (Pd2þ). However, the high intensity peak shifted to a higher binding energy than Pd2þ. This suggests that some Pd was in unusually high oxidation states (Pdxþ, x > 2), which, as has been previously reported, is attributed to a special ionic environment located in the B-site lattice of the perovskite network [33e36]. Based on the XPS analysis, we suggest that majority of Pd was successfully doped into the perovskite lattice, and a small amount of PdO still existed as a separate phase in the freshly prepared LCFPd sample. After the treatment in hydrogen, Pd was in the oxidation state of Pd0, which suggests a successful segregation of Pd from the oxide bulk. Once the reduced sample was treated in air, the XPS analysis of the sample suggested that the oxidation state of Pd appeared to be primarily >2þ, which indicated a successful incorporation of Pd into the perovskite lattice after the oxidizing atmosphere treatment. 3.1. Electrocatalytic activity for the ORR In the above analysis, we have demonstrated that the LCFPdinfiltrated SDC electrode can be stably present in phase structure under anode operation conditions. To test the electrocatalytic activity of impregnated LCFPd electrode for fuel oxidation, we fabricated and comparatively studied two asymmetrical fuel cells with the same SDC electrolyte and BSCF cathode but different anodes: one with the LCFPd impregnated SDC anode and the other with a conventional Ni þ SDC anode. BSCF has been reported to have high activity for the ORR at intermediate temperatures, while Ni þ SDC is an excellent anode that showed high catalytic activity for hydrogen electro-oxidation at intermediate temperatures. Therefore, any power output differences between the two cells should be primarily attributed to the anode. Fig. 5 demonstrates the typical IeV curves of the two cells for the temperature range of 600e750 C. The Ni þ SDCjSDCjBSCF cell delivered peak powder densities (PPDs) of 222, 310, 395 and 471 mW cm2 at 600, 650, 700 and 750 C, respectively. The corresponding values for the LCFPd þ SDCjSDCjBSCF cell are 321, 431, 540 and 646 mW cm2. This suggests that the LCFPd þ SDC anode had an even better electrocatalytic activity for hydrogen oxidation compared with that of the Ni þ SDC anode. This conclusion was further supported by
the measured EIS of both cells at different temperatures under open circuit voltage (OCV), and the results are listed in Table S1. The cell with an impregnated LCFPd þ SDC anode had a lower electrode polarization resistance than that of the cell with a Ni þ SDC anode. Here, the electrode polarization resistance should be the sum of anode and cathode. Because the same BSCF cathode was used in both cells, the difference in electrode polarization resistance is primarily attributed to the anode side. It is known that Pd is a more active catalyst for fuel oxidation than nickel, which may explain the superior anode performance of LCFPd þ SDC compared with that of Ni þ SDC. In addition to high activity of fuel oxidation, to achieve high power output from symmetrical SOFCs, the electrodes should have high electrocatalytic activity for the ORR. The poor activity of electrodes for the ORR at intermediate temperatures has become the main obstacle for reducing the operation temperature of many symmetrical SOFCs. Most investigated perovskite materials for symmetrical SOFCs in the literature are primarily based on doped LaCrO3-d, LaMnO3-dor doped SrTiO3-d perovskites, and all of them showed poor activity for the ORR at intermediate temperatures [43e46]. Thus, most of the above-mentioned symmetrical SOFCs have been operated at temperatures higher than 850 C. The electrocatalytic activity of LCFPd-infiltrated SDC electrode for the ORR at intermediate temperatures was investigated using EIS on the cell with an SDC electrolyte and symmetrical LCFPd-infiltrated SDC electrodes. In addition, a symmetrical cell with LCF-infiltrated SDC electrodes was fabricated and tested, using EIS with air as the atmosphere for both electrodes, to obtain information about the beneficial effect of Pd for the ORR. Fig. 6 shows the typical EIS of the infiltrated LCFPd þ SDC composite electrode for the ORR that was calcined at 800 C and operated at intermediate temperatures between 600 and 800 C. The superior catalytic activity of infiltrated LCFPd þ SDC for the ORR compared with that of LCF þ SDC is clearly demonstrated. For example, at 600 C, the area specific resistance (ASR) of the LCFPd þ SDC electrode is approximately 0.23 U cm2, while a larger value of 0.99 U cm2 was observed for the LCF þ SDC electrode. Because the fabrication process and parameters are similar for both electrodes, the differences in their ASRs primarily resulted from the electrode perovskite phase. Pd has been widely used to improve perovskite oxides for the ORR because Pd itself is a good electrocatalyst for the ORR [23e26]. Recently, we have demonstrated that the catalytic activity of Pd for the ORR at room temperature was closely related to the oxidation state of Pd, i.e., the higher the oxidation state of Pd is, the higher the activity [47]. Because there is no significant difference in mechanism of the ORR at low and high temperatures, the same trend may be applicable for Pd operating at elevated temperatures. Thus, it is reasonable to conclude that the superior activity of LCFPd þ SDC compared with that of LCF þ SDC for the ORR at intermediate temperatures resulted from the doping of unusually high Pdxþ (x > 2) into the perovskite lattice, which might have modified the surface properties of the electrode. Furthermore, to demonstrate the beneficial effect of higher oxidation state Pd for the ORR, we tested the electrocatalytic activity for the ORR of the electrode that was first reduced with hydrogen and then was re-oxidized with air. As shown in Fig. S2, the electrode performance was slightly worse compared with the performance of the unreduced electrode. At 600 C, an ASR of 0.35 U cm2 was achieved, which is approximately 1/2 larger than that of fresh LCFPd (0.23 U cm2), but it is still significantly lower than the ASR of Pd-free LCF (0.99 U cm2). This further confirmed that Pd in higher oxidation state is beneficial for the ORR. Nevertheless, the LCFPd þ SDC electrode that had previously operated in the anode mode can still deliver an attractive performance as a cathode for the ORR. This suggests that both electrodes can be swapped if necessary.
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Fig. 5. IeV and IeP curves of the cell with configurations of (a) Ni þ SDCjSDCjBSCF and (b) LCFPd þ SDCjSDCjBSCF operating on hydrogen.
Fig. 6. Nyquist plots of the LCF (Pd)-infiltrated SDC electrodes at 600, 650, 700, 750 and 800 C.
Symmetrical SOFCs with infiltrated LCF (LCFPd)þSDC electrodes and in the electrolyte-supported configuration were then tested for power generation. Here, the thickness of the SDC electrolyte was controlled at 0.3 mm, the calcination temperature for the electrodes was set at 800 C, and ambient air and hydrogen with fluxes of 80 mL min1 were used as the cathode atmosphere and anode fuel. Shown in Fig. 7a are the typical IeV polarization curves of an LCFPd þ SDCjSDCjLCFPd þ SDC symmetrical SOFC. The linear response of the current to voltage at high polarization current demonstrated that the electrodes had sufficient porosity to avoid the appearance of concentration polarization. This implies that the infiltrated LCFPd phase did not cause a detrimental effect on the gas diffusion channels inside the electrode. Favorable power outputs were demonstrated at intermediate temperatures for the cell. For example, the PPDs reached 132, 172, 246, 309, 394, 467 and 535 mW cm2 at operation temperatures of 600, 625, 650, 675, 700, 725 and 750 C, respectively. These values are highly attractive when considering the thick electrolyte configuration of the cell. The
above-mentioned values are comparable to an asymmetrical BSCFjSDCjLCFPd þ SDC (anode) cell and are even higher than the values for LCFPd þ SDCjSDCjNi þ SDC (anode) (Table S2). Furthermore, this strongly demonstrates the high electrocatalytic activity of the infiltrated LCFPd þ SDC electrode for both the ORR and hydrogen oxidation at intermediate temperatures. For comparison, a similar cell with an LCF-infiltrated SDC electrode has been tested and the IeV polarization curves are presented in Fig. 7b. The PPDs of the LCF þ SDCjSDCjLCF þ SDC symmetrical SOFC are 100, 122, 151, 182, 212, 245 and 291 mW cm2, at operation temperatures of 600, 625, 650, 675, 700 725 and 750 C, respectively, which are clearly lower than the values of an LCFPd þ SDCjSDCjLCFPd þ SDC symmetrical SOFC at corresponding temperatures. Because the electrolyte material and membrane thickness were the same, the different power outputs of the two cells primarily resulted from differences in electrode performance. As demonstrated using EIS, the LCFPd þ SDC electrode also had a superior activity than LCF þ SDC electrode for the ORR when both were used as cathodes.
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Fig. 7. IeV and IeP curves of the cells with configurations of (a) LCFPd þ SDCjSDCjLCFPd þ SDC and (b) LCF þ SDCjSDCjLCF þ SDC operating on hydrogen.
In addition, under reducing atmosphere, the nanoparticle Pd was segregated from the oxide bulk, which had a high activity for hydrogen [27e30]. Therefore, the symmetrical cell with the infiltrated LCFPd þ SDC electrode had higher electrocatalytic activity than LCF þ SDC electrode for both the ORR and hydrogen oxidation. Sintering of the electrode, the collapse of perovskite lattice, and the delamination of the electrode layer from the electrolyte surface could also lead to deterioration of the cell performance with operation time. Then, the operational stability of the LCFPd þ SDCjSDCjLCFPd þ SDC symmetrical SOFC was investigated at 650 C under the constant polarization current density of 300 mA cm2 for a total period of approximately 120 h. Shown in Fig. 8 is the operation time dependence of the cell voltage. After the initial 20 h, a slight decrease in cell voltage from the initial value of 0.67 Ve0.65 V was observed, which could have resulted from stabilization of the electrode and current collector. Then, a very stable cell voltage was observed during the next 100 h of test. This suggests that the infiltrated LCFPd þ SDC electrode can be reliably operated under both the anode and cathode conditions. The redox (reduction-oxidation) cycling test was also performed for the “symmetrical” SOFC with LCFPd-infiltrated SDC electrode. The cell was firstly heated up to 700 C, and the process of a redox cycle was described in Fig. S3. The OCV and PPD of the cell decreased slowly with each complete redox cycle. After several cycling tests, the OCV and PPD of the cell stayed around 0.82 V and 370 mW cm2,
respectively, indicating that the single cell with LCFPd-infiltrated SDC electrode exhibited a good redox stability. Considering that the possible pulverization of perovskite oxide due to deep reduction under reducing atmosphere could be the main problem for the deterioration of symmetrical SOFCs with perovskite oxide-based electrodes, the anode layer of symmetrical SOFCs after a long-term stability test was analyzed using SEM, and the typical images are shown in Fig. S4. After the test, the connection between the LCFPd þ SDC electrode layer and the SDC electrolyte remained perfect without the appearance of anode delimitation. The LCFPd nanoparticles were still clearly observed and were firmly attached to the surface of SDC scaffold without the appearance of any electrode material pulverization. The particle size of LCFPd nanoparticles did not show any obvious growth compared to the fresh sample. The stable phase structure of the main LCF perovskite lattice and the comparable thermal expansion coefficient of the electrode, prepared using the infiltrated method, to that of electrolyte may explain the good morphological stability of the electrode under anodic operation conditions. This might have contributed to excellent operation stability of symmetrical SOFCs under real operation conditions.
4. Conclusion In this study, we have reported an LCFPd perovskite material as a promising electrocatalyst for intermediate-temperature symmetrical solid oxide fuel cells. The LCFPd material and the LCFPdinfiltrated SDC electrode show high structural/microstructural stability under both oxidizing and reducing atmospheres. Additional doping with Pd has greatly improved the catalytic activity of LCF for oxygen reduction as well as fuel oxidation. As expected, high power outputs at intermediate temperatures and long-term performance stability are obtained from symmetrical solid oxide fuel cells with LCFPd-infiltrated SDC as both anode and cathode. These favorable results highlight the practical application of LCFPd-based materials in symmetrical intermediate-temperature solid oxide fuel cells.
Acknowledgments
Fig. 8. Cell voltage time-dependence with 3% H2O humidified H2 fuel under a constant polarization current of 300 mA cm2 at 650 C.
This work was financially supported by the Key Projects in Nature Science Foundation of Jiangsu Province under contract No. BK2011030, the Major Project of Educational Commission of Jiangsu Province of China under contract No. 13KJA430004, the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Australia Research Council under contracts DP150104365 and DP160104835.
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