Advanced Symmetric Solid Oxide Fuel Cell with an Infiltrated K2

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Advanced Symmetric Solid Oxide Fuel Cell with an Infiltrated K2NiF4‑Type La2NiO4 Electrode Guangming Yang,† Chao Su,*,† Ran Ran,† Moses O. Tade,‡ and Zongping Shao*,‡ †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Number 5 Xin Mofan Road, Nanjing, Jiangsu 210009, People’s Republic of China ‡ Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia ABSTRACT: Advanced symmetric solid oxide fuel cells (SOFCs) with a reducible electrode were proposed. Specifically, La2NiO4 + La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) [or Sm0.2Ce0.8O1.9 (SDC)] composite electrodes were successfully fabricated by an infiltration method and tested for power generation. X-ray diffraction (XRD) results demonstrated there was no noticeable phase reaction between infiltrated La2NiO4 and LSGM (or SDC) scaffold, and scanning electron microscopy (SEM) analysis indicated that the La2NiO4 phase formed as nanoparticles that decorated the surface of the scaffold. Different from conventional symmetric SOFCs, the electrode material La2NiO4 of current cells was reduced under an anode atmosphere to form metallic nickel as a high active catalyst for fuel oxidation. After the reduction, the electrode morphology and geometric integrity were maintained for the infiltrated electrode. For thick electrolyte-supported symmetric SOFCs with infiltrated La2NiO4 electrodes, an attractive maximum power density of ∼550 mW cm−2 was achieved at 800 °C operating on hydrogen fuel, significantly higher than similar cells with stable perovskite oxide electrodes, as reported in the literature. It suggested that the unreduced and reduced La2NiO4 performed well as a cathode for the oxygen reduction reaction and as an anode for fuel electro-oxidation, respectively. In addition, a favorable operating stability was demonstrated for a symmetric SOFC with an infiltrated La2NiO4 electrode. It provides a new way for developing cost-effective SOFCs with huge application opportunities.

1. INTRODUCTION With a fast expanding energy demand, worsening environmental pollution, and an increasing concentration of carbon dioxide in the atmosphere from low-efficient use of fossil fuels, currently, there are considerable research activities in the development of new energy materials and alternative ways for more efficient use of available energy resources with less environmental impact.1,2 Solid oxide fuel cell (SOFC) is a type of high-temperature energy conversion device, which converts chemical energy to electric power through an electrochemical way without the limitation by the Carnot cycle. SOFCs have received great attention in recent years as an alternative way for clean power generation because of their high system efficiency, low emissions, and fuel flexibility.3,4 A typical SOFC is composed of a porous cathode, a porous anode, and a dense electrolyte sandwiching them. The cathode is the place where the reduction of oxygen to oxygen ions occurs. As a pure ionic conductor, the electrolyte acts as a diffusion block for the electron as well as reaction gases. The oxygen ions diffuse through the electrolyte to the anode side, where they react with fuel and release electrons, which are transported to the cathode side through the external circuit. Conventional SOFC is composed of a nickel cermet anode, a stabilized zirconia electrolyte, and a La0.8Sr0.2MnO3 (LSM) cathode, which was typically operated at around 1000 °C; the anode and cathode layers were typically fabricated separately, resulting in a high cost related to cell materials, fabrication, and maintenance and insufficient operation lifetime. Thus, the widespread commercial application of SOFCs has still not been realized. To be competitive with currently matured power generation technologies based on fossil fuel combustion, the © 2013 American Chemical Society

cost as well as operation lifetime of SOFCs should still be significantly improved. To reduce the operation cost and prolong the cell lifetime, one way is to reduce the operation temperature to the intermediate range,5,6 thus mediating the interfacial reaction between cell components, lessening electrode sintering, and allowing flexible sealing. Another effective way is the application of symmetric SOFCs,7−9 in which the same material is used for both electrodes, allowing for the preparation of the anode and cathode in one single thermal step and minimizing compatibility problems because of two identical electrode−electrolyte interfaces in comparison to the typical anode−electrolyte and cathode−electrolyte interfaces in traditional SOFCs. In addition, any performance deterioration of the fuel cell anode because of carbon deposition or sulfur poisoning may be recovered by reversing the gas streams between the two electrode chambers. However, the requirements for the electrode materials of conventional symmetric SOFCs are rather restrictive. The materials should have (1) acceptable electronic conductivity and chemical stability in both oxidizing and reducing conditions, (2) enough electrocatalytic activity toward oxygen reduction as well as fuel oxidation, and (3) chemical and physical compatibility with other fuel cell components. Because the electrode materials should be structurally stable under both Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 29, 2013 Revised: September 30, 2013 Published: September 30, 2013 356

dx.doi.org/10.1021/ef401473w | Energy Fuels 2014, 28, 356−362

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in isopropyl alcohol) was spray-deposited onto both surfaces of the prepared SDC electrolyte, creating an effective electrode area of 0.48 cm2, and then fired at 1000 °C for 2 h to make electrodes adhere to the electrolyte. 2.2. Electrochemical Test. Silver wires were attached to both electrodes as the leads, and silver was selected as the current collector. The symmetric cell was sealed onto a quartz tube by silver paste, then placed inside a furnace, and heated to the test temperature (600−800 °C) at a rate of 10 °C min−1. The anode and cathode sides were supplied with humidified hydrogen (3% H2O) at a flow rate of 80 mL min−1 [standard temperature and pressure (STP)] and ambient air, respectively. The I−V polarization measurement was performed using a Keithley 2420 digital sourcemeter, interfaced with a computer for data acquisition. A four-terminal configuration was employed for the measurement. The electrochemical impedance spectra (EIS) of the cells were collected by a Solartron 1287 potentiostat in combination with a 1260A frequency response analyzer. 2.3. Basic Characterizations. The crystalline structure of the prepared samples was monitored by X-ray diffraction (XRD, D8 Advance, Bruker, with Cu Kα radiation) with the scan rate of 10° min−1. The microstructure of La2NiO4-infiltrated electrodes was characterized by a field emission scanning electron microscope (FESEM, JEOL-S4800).

atmospheres, they are typically in perovskite lattice structure and composed of Cr, Mn, Mo, and Ti elements in the B site of the perovskite lattice, while the highly reducible elements, such as cobalt and iron, are totally avoided. Up to now, doped LaCrO3, LaMnO3, and SrTiO3 perovskite oxides and doped Sr2Mo2O5 double perovskite oxide are mainly investigated as the electrodes of symmetric SOFCs.7,10−12 However, these materials have low oxygen vacancy concentration, especially at reduced temperatures and an oxidizing atmosphere, resulting in poor activity for the oxygen reduction reaction (ORR). Moreover, such perovskite oxides usually show low electronic conductivity, with the value of typically less than 1 S cm−1, under a reducing atmosphere, because of a n-type electronconducting mechanism, which could introduce a large contact resistance at a high polarization current.13 In this study, an advanced symmetric SOFC with infiltrated K2NiF4-type La2NiO4 as both anode and cathode materials was reported. Under an oxidizing atmosphere, it showed a K2NiF4 lattice structure and performed as an oxygen reduction electrode, while La2NiO4 was reduced to a nanosized Ni and La2O3 mixture, which acted as highly active catalysts for fuel electro-oxidation. Both single-phase La2NiO4 and infiltrating La 2 NiO 4 into La 0 . 9 Sr 0 . 1 Ga 0 . 8 Mg 0 . 2 O 3 − δ (LSGM) or Sm0.2Ce0.8O1.9 (SDC) scaffolds as electrodes of SOFCs were tried to generate power, and promising power outputs and stability were obtained for the cells with infiltrated La2NiO4 electrodes.

3. RESULTS AND DISCUSSION During the past several years, the composite oxides of the Ruddlesden−Popper homologous series, as shown in Figure 1,

2. EXPERIMENTAL SECTION 2.1. Fabrication of Symmetric SOFCs. The doped lanthanum gallate perovskite, LSGM, was prepared by a standard solid-state reaction method.14 The SDC powder was synthesized via a combined ethylenediaminetetraacetic acid (EDTA)−citrate complexing sol−gel process, as reported elsewhere.15 The single-phase La2NiO4 was prepared by a glycine nitrate process (GNP).16 The 0.8 mol L−1 La2NiO4 precursor solution for the infiltration was prepared by mixing the stoichiometric amounts of La(NO3)3·6H2O (analytical reagent) and Ni(NO3)3·6H2O (analytical reagent) into the distilled water to form aqueous solution, and glycine as the complexing agent was also added to the aqueous solution at a molar ratio of 2:1 to the total amount of cations. To modify the liquid surface tension, ethanol was mixed with the aqueous solution at a volume ratio of 1:3. The symmetric cells with the configuration of La2NiO4-infiltrated LSGM (or SDC)|LSGM (or SDC)|La2NiO4-infiltrated LSGM (or SDC) were fabricated as follows. The prepared LSGM or SDC powders were uniaxially pressed into disk-shaped pellets at the pressure of 200 MPa and then calcined at 1400 °C with a heating rate of 5 °C min−1 and a holding time of 5 h. Both sides of the sintered LSGM or SDC electrolyte pellets were then polished using the attrition paper for reaching a thickness of approximately 600 μm for LSGM and 300 μm for SDC, respectively. Then, proper LSGM or SDC powders and 10 wt % soluble starch served as the pore former were dispersed in isopropyl alcohol using a high-energy ball mill (Pulverisette 6, Fritsch) at 400 rpm for 30 min to form a colloidal suspension, which was sprayed onto both surfaces of the sintered electrolyte with the effective surface area of 0.48 cm2 and sintered in air at 1300 °C for 5 h to form the porous LSGM or SDC scaffolds with a thickness of ∼30 μm. Finally, the prepared 0.8 mol L−1 La2NiO4 precursor solution was infiltrated into scaffolds. After each infiltration, the pellet was calcined in air at 500 °C for 30 min for the decompositions of the metal nitrates and glycine. Multiple steps were required to reach the final loading of ∼20 wt % during the overall infiltration. Afterward, the composite was calcined in air at 850 °C for 5 h to form the targeted Ruddlesden−Popper structure. To prepare the SDC-supported symmetric cell with pure La2NiO4 as the electrode, the single-phase La2NiO4 slurry (La2NiO4 dispersed

Figure 1. Crystal structure of La2NiO4 oxide.

have received considerable attention as the electrode materials of SOFCs. Such oxides have a general formula Lan+1NinO3n+1 and are composed of alternating perovskite layers (LaNiO3) and rock-salt layers (LaO) along the crystallographic c direction, where n represents the number of perovskite layers in a formula unit. La2NiO4 is a type of mixed oxygen ion and electronic conducting material, which is the first member of the Ruddlesden−Popper homologous series (n = 1). It was reported to have high oxygen ionic conductivity, attractive electronic conductivity, moderate thermal expansion coefficients (TECs), and high electrocatalytic activity under oxidizing conditions.17,18 On the other hand, La2NiO4 will convert to a mixture of La2O3 and metallic nickel after reduction. It is wellknown that nickel is a good electronic conductor and highly active catalyst for fuel electro-oxidation, and La2O3 + Ni catalyst 357


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Figure 2. XRD patterns of (a) La2NiO4, LSGM, and La2NiO4-infiltrated LSGM and (b) La2NiO4, SDC, and La2NiO4-infiltrated SDC.

Figure 3. FESEM images of the as-prepared La2NiO4 infiltrated electrodes, (a) La2NiO4/LSGM and (b) La2NiO4/SDC, and the electrodes after reduction in H2, (c) La2NiO4/LSGM and (d) La2NiO4/SDC.

scaffold after firing at 850 °C in air. For comparison, the XRD patterns of LSGM (Figure 2a) and SDC (Figure 2b) were also presented. The diffraction peak of as-prepared La2NiO4 can be indexed well based on a K2NiF4-type structure with space group I4/mmm, and the lattice parameters are a = 3.869 Å, b = 3.869 Å, and c = 12.60 Å, which matched well with the literature, suggesting the formation of phase-pure La2NiO4.18 With regard to the as-prepared La2NiO4-infiltrated LSGM (or SDC) scaffold electrode, the XRD pattern can be indexed on the basis of a physical mixture of perovskite-type LSGM (or fluorite-type SDC) phase and a K2NiF4-type La2NiO4 phase. It suggests that the La2NiO4 phase was also successfully formed after the infiltration and calcination at 850 °C on the one hand and no noticeable phase reaction between La2NiO4 and LSGM (or SDC) on the other hand. The formation of the crystalline phase in the LSGM (or SDC) scaffold at a relatively low calcination temperature (850 °C) can be attributed to the adoption of complexing agent (glycine) during the infiltration, which effectively reserved the atomic level homogeneous mixing of La3+ and Ni2+ in the solution to the solid precursor after the infiltration and drying; thus, a relatively low calcination

was also reported to have high activity for fuel oxidation.19,20 It suggests that it is possible to apply La2NiO4 as both cathode and anode materials of SOFCs. The formation of pure-phase La2NiO4 is critical for cathode application because the oxygen reduction reaction is closely related to its phase structure, while for the anode, it is less important because it will be further reduced. However, the formation of the La2NiO4 composite realizes the homogeneous mixing of La and Ni in the atomic level, which will allow for the well dispersal of Ni in the La2O3 + Ni composite after reduction, benefiting the catalytic activity.21 Thus, the synthesis of pure-phase La2NiO4 is important for its use as an electrode of the symmetric cell. As a component of the electrode prepared by the infiltration method, it is more difficult for the formation of the pure phase because the firing temperature could not be too high; otherwise, it may lead to the bad sintering of the electrode to result in insufficient electrode porosity. Figure 2 presents the XRD patterns of the as-prepared singlephase La2NiO4 by GNP calcined at 1000 °C and the infiltrated La2NiO4 electrode with LSGM (Figure 2a) or SDC (Figure 2b) 358


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Figure 4. Typical I−V and I−P polarization curves of the electrolyte-supported cells with the configurations of (a) La2NiO4/LSGM|LSGM| La2NiO4/LSGM, (b) La2NiO4/SDC|SDC|La2NiO4/SDC, and (c) La2NiO4|SDC|La2NiO4.

Table 1. Comparison of the Performance of Our Fuel Cells to Other Symmetric Cells Reported in the Literature

a

electrode

electrolyte

thickness (μm)

temperature (°C)

PPD (mW cm−2)

reference

La2NiO4-infiltrated LSGM La2NiO4-infiltrated SDC Ag-infiltrated SDC La0.8Sr0.2Sc0.2Mn0.8O3−δ Pr0.7Ca0.3Cr0.6Mn0.4O3−δ Pt-infiltrated YSZ YSr2Fe3O8−δ La0.75Sr0.25Cr0.5Mn0.5O3−δ-infiltrated YSZ La0.75Sr0.25Cr0.5Mn0.5O3−δ La0.75Sr0.25Cr0.5Al0.5O3−δ La0.3Sr0.7Fe0.7Cr0.3O3−δ Sr2Fe1.5Mo0.5O6−δ Sr2Co1.15Mo0.85O6

LSGM SDC SDC ScSZa YSZ YSZ YSZ YSZ YSZ LSGM LSGM LSGM LSGM

600 300 400 300 370 80 70 20 20 1500 500 265 300

800 800 750 900 950 800 900 900 900 800 800 800 800

520 550 200 310 250 450 35 333 300 45 300 500 460

this work this work 22 10 23 24 25 26 7 27 28 29 30

ScSZ = (Sc2O3)0.1(ZrO2)0.9

sintered than the La2NiO4 phase in the La2NiO4/LSGM electrode. It suggests that the scaffold had some effect on the grain growth of the La2NiO4 phase during the high-temperature fabrication. After the reduction in hydrogen (performing as an anode), the infiltrated electrodes still maintained well geometric integrity and good mechanical strength and no pulverization of the electrode was experienced, while this often happened for the reduction of single-phase composite oxides. If we had a comparison of the LSGM or SDC phase in the infiltrated electrodes before and after the reduction (panels a and b of Figure 3 versus panels c and d of Figure 3), the LSGM and SDC particles in the reduced electrodes were less exposed than those in the unreduced electrodes. It suggests that, during reduction, the newly formed La2O3 and Ni phases likely

temperature was needed for the formation of the La2NiO4 pure phase. Figure 3 shows the SEM images of the as-prepared La2NiO4infiltrated electrodes (Figure 3a for La2NiO4/LSGM and Figure 3b for La2NiO4/SDC) and the electrodes after the reduction in a hydrogen atmosphere (Figure 3c for La2NiO4/LSGM and Figure 3d for La2NiO4/SDC). All of the electrodes showed porous morphological structure. By applying LSGM as the scaffold, we can clearly observe two phases in the unreduced infiltrated electrode, i.e., sintered LSGM grains with the size of around 500 nm and La2NiO4 nanoparticles, which modified the surface of LSGM, with the particle size of around 100 nm. For the electrode using SDC as the scaffold, the La2NiO4 particles had a smaller size of around 80 nm, which were slightly more 359


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Figure 5. EIS of the electrolyte-supported cells with the configurations of (a) La2NiO4/LSGM|LSGM|La2NiO4/LSGM, (b) La2NiO4/SDC|SDC| La2NiO4/SDC, and (c) La2NiO4|SDC|La2NiO4.

electrochemical reduction of O2 to O2− and the reduced La2NiO4/LSGM (or SDC) electrode, that is, La2O3 + Ni/ LSGM (or SDC), performed well as an anode for the fuel electro-oxidation. Listed in Table 1 is a comparison of the performance of our fuel cells and other symmetric cells reported in the literature.7,10,22−30 It shows that our cells delivered excellent power outputs. It thus greatly appreciates the infiltrated La2NiO4 as potential electrodes of symmetric SOFCs. From Figure 4c, a PPD of only ∼250 mW cm−2 was achieved at 800 °C, which is only about half of the cell with La2NiO4-infiltrated SDC electrode. It can be explained as follows. La2NiO4 has relatively poor oxygen ion conductivity, as compared to SDC. By applying single-phase La2NiO4 as the electrode, lower cathode performance is expected than the La2NiO4/SDC composite electrode. To perform as an anode, La2NiO4 was converted to La2O3 and Ni composite after the reduction, which had negligible oxygen ion conductivity. By forming a composite with the SDC phase in the anode, oxygen ion conductivity was introduced into the anode; thus, the active reaction sites for fuel electro-oxidation were effectively extended. Both the improvement in anodic and cathodic performance led to the much higher power output for the cell with La2NiO4-infiltrated electrodes than the similar cell with single-phase La2NiO4 electrode.

diffused onto the exposed scaffold surface and such diffusion turned out to be more serious in the electrode with the SDC scaffold. By applying LSGM scaffold, after reduction, we can clearly observe the metallic nickel phase with the particle size of ∼20 nm, which distributed homogeneously among the original La2NiO4 phase. The high dispersity of the nickel phase inside the electrode can be explained by the homogeneous mixing of La and Ni in the atomic level in the La2NiO4 oxide. To test the performance of the symmetric SOFCs with the infiltrated La2NiO4 electrode for power generation, two different electrolyte-supported SOFCs with the configurations of La2NiO4/LSGM|LSGM (600 μm)|La2NiO4/LSGM and La2NiO4/SDC|SDC (300 μm)|La2NiO4/SDC were tried, respectively. For comparison, the cell with the configuration of La2NiO4|SDC (300 μm)|La2NiO4 was also tested. The typical I−V and I−P polarization curves of these three cells at various temperatures are shown in Figure 4. For both cells with La2NiO4-infiltrated LSGM (or SDC) electrodes, no concentration polarization appeared at high current density (panels a and b of Figure 4), suggesting the sufficient porosity of the electrodes for free gas diffusion. A peak power density (PPD) of ∼550 mW cm−2 was achieved at 800 °C for both cells, which is a highly attractive result as the thick electrolytes were applied in this study. It suggests that the unreduced La2NiO4-infiltrated LSGM (or SDC) electrode performed well as a cathode for 360


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4. CONCLUSION Advanced symmetric SOFCs with La2NiO4/LSGM (or SDC) composite oxide electrode were successfully prepared by infiltrating La2NiO4 into LSGM or SDC scaffold. As for the cathode, the infiltrated nanosized La2NiO4 electrode delivered a high oxygen reduction activity, while as for the anode, the nanosized La2NiO4 was decomposed into La2O3 and metal nickel particle with a smaller size, which distributed homogeneously and presented an excellent catalytic activity for fuels. The cells with different electrolytes of LSGM (600 μm) and SDC (300 μm) both achieved a peak power density of ∼550 mW cm−2 at 800 °C by applying hydrogen as the fuel, which was higher than the cell with pure La2NiO4 as the electrode material at the same condition. Furthermore, the cell with the configuration of La2NiO4/SDC|SDC|La2NiO4/SDC was successfully operated for a period of 210 h without any obvious degradation at 650 °C. Therefore, the infiltrated K2NiF4-type La2NiO4 was a promising electrode material for the development of the symmetric SOFCs. The use of a reducible electrode for symmetric SOFCs can solve the problem of low electrical conductivity and poor electrochemical activity of conventional LaCrO3-based electrodes under an anode atmosphere. It provides a new way for the development of cost-effective and high-performance SOFCs that may then accelerate the commercialization of this fascinating technology.

Figure 5 shows the corresponding EIS of the cells under open circuit voltage (OCV) conditions. It is interesting that both the ohmic resistance (Ro) that is mainly contributed from the electrolyte and the polarization resistance (Rp) that is a sum of the anode and cathode are smaller for the cell with the SDC electrolyte (Figure 5b) than that with the LSGM electrolyte (Figure 5a), but comparable PPDs were achieved for both cells. Such discrepancy can be explained by the following reasons. Assuming that the overall resistance of the cell is insensitive to the polarization current, in fact, which is reasonably true for both cells, as demonstrated by roughly the linear response of cell voltage to polarization current (panels a and b of Figure 4), the peak power of a cell can be estimated by the equation: P = U2/4R, where U is the OCV and R is the overall resistance of the cell. At the same temperature, the OCV of the cell with the SDC electrolyte is lower than the cell with the LSGM electrolyte because of the leaked current in the SDC electrolyte. Therefore, the smaller cell resistance (both Ro and Rp) for the cell with the SDC electrolyte than that with the LSGM electrolyte compensated for the lower OCV, resulting in the similar power outputs for both cells. By comparing panels b and c of Figure 5, we found that smaller Ro and Rp were obtained for the cell with the La2NiO4-infiltrated electrode than that with the single-phase La2NiO4 electrode, although the same electrolyte, SDC, was applied. It is due to the introduction of SDC in the composite electrode, which resulted in a good connection between the electrode and electrolyte. Meanwhile, SDC has a higher oxygen ion conductivity than La2NiO4, which improved the electrode activity. As a result, the smaller resistances (Ro and Rp) were expected for the cell with the La2NiO4-infiltrated SDC electrode. Operational stability is a big concern for the practical application of SOFCs. Figure 6 shows an operational stability

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-25-83172242. E-mail: [email protected]. *Telephone: +61-8-92664702. E-mail: zongping.shao@curtin. edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Zongping Shao acknowledges the support from the Australian Research Council Future Fellowships. REFERENCES

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Figure 6. Cell with the configuration of La2NiO4/SDC|SDC|La2NiO4/ SDC tested at 650 °C for 210 h under the polarization current density of 150 mA cm−2.

test of the cell with the configuration of La2NiO4/SDC|SDC| La2NiO4/SDC using 3% water humidified hydrogen. The cell was maintained under a constant polarization current density of 150 mA cm−2 at 650 °C for a period over 200 h. After the operation for 210 h, the test was mandatorily stopped. The cell voltage was kept stable at around 0.67 V during the test period of 210 h, suggesting that this type of symmetrical SOFC presented a relatively stable performance. 361


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Advanced Symmetric Solid Oxide Fuel Cell with an Infiltrated K2

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