Infiltration of Lan+1NinO3n+1 and La0.6Sr0.4Co0.2Fe0.8 in LSGM Cathodes for Intermediate Temperature SOFCs

Timothy V. Flavin

Northwestern University, Materials Science and Engineering

Mentor: Justin G. Railsback

Advisor: Scott A. Barnett

June 11, 2014

LNO and LSCF in Solid Oxide Fuel Cells

2

Infiltration of Lan+1NinO3n+1 and La0.6Sr0.4Co0.2Fe0.8 in LSGM Cathodes for Intermediate Temperature SOFCS

Timothy Flavin Materials Science & Engineering Northwestern University June 11, 2014 Abstract

Solid oxide fuel cells with infiltrated La0.9Sr0.1Ga0.8Mg0.2O3- (LSGM) cathodes have the potential to provide low polarization resistances at intermediate temperatures between 500-650 C. The present study examines LSGM cathodes infiltrated with Lan+1NinO3n+1 (LNO) and La0.6Sr0.4Co0.2Fe0.8O3- (LSCF). Symmetrical LSGM cathodes were loaded with varying ratios of LNO and LSCF infiltrates on both sides of bulk LSGM electrolytes. Each infiltration was fired at 900 C to evaporate the infiltrate solution and allow LNO to form Ruddlesden-Popper phases. X-ray diffraction determined this temperature ideal for formation of the La2Ni1O4 and a La8Ni4O17 ordered secondary phase. Cathode polarization resistance Rp, tested using impedance spectroscopy, was highest in the cells with an even ratio of LNO:LSCF, whereas the Rp was lowest in the cells primarily loaded with LSCF. At 650 C, cells infiltrated only with LSCF had a polarization resistance as low as 0.21 cm2, which compares well to a previous LSCF/LSGM cathode study that found Rp of 0.18 cm2. Cells infiltrated only with LNO had a polarization resistance as low as 0.33 cm2, which is the lowest recorded Rp for LNO any LNO based electrode. The temperature dependence of Rp yielded an activation energy of 1.5 eV for all cells infiltrated with LSCF, and an activation energy of 1.2 eV for cells infiltrated with LNO only.

LNO and LSCF in Solid Oxide Fuel Cells

3

Table of Contents

1. Introduction ............................................................................................................................... 4

1.1 Solid Oxide Fuel Cell Operation ........................................................................................... 4

1.2 Cathode Oxygen Reduction Reaction ................................................................................... 5

1.3 Cell Infiltration ...................................................................................................................... 5

2. Experimental ............................................................................................................................. 7

2.1 Fabrication of LSGM Electrolytes ........................................................................................ 7

2.2 Preparation of Symmetrical Cells ......................................................................................... 7

2.3 Experimental Design ............................................................................................................. 9

2.4 SOFC Test Preparation ......................................................................................................... 9

2.5 Cathode Characterization Methods ..................................................................................... 10

3. Results and Discussion ............................................................................................................ 10

3.1 Composite Cathode Microstructure ................................................................................. 10

3.2 Impedance Spectra .......................................................................................................... 12

3.3 Polarization Resistance: Infiltrate Composition Effect ................................................... 13

3.4 Activation Energy Calculation ........................................................................................ 14

4. Conclusions .............................................................................................................................. 15

Acknowledgements ..................................................................................................................... 15

References .................................................................................................................................... 16

LNO and LSCF in Solid Oxide Fuel Cells

4

1. Introduction Solid oxide fuel cells (SOFC) are an emerging alternative energy device capable of achieving very high efficiency, high power densities, and low emissions[1]. SOFCs convert chemical energy to electricity directly, by separating a combustion reaction into two half reactions mediated by an oxygen conducting electrolyte and electrocatalytic electrodes. Due to their fuel flexibility, SOFCs can use commonly available fuels, which could reduce fossil fuel use for electricity production and mitigate global climate change[2]. As fuel cell technology continues to grow, improvements in cost, power density, and efficiency can also prove to make SOFCs a very cost-effective energy source. However, despite the numerous benefits of SOFCs, their high temperature demands limit them and prohibit fuel cells from becoming a viable option for widespread energy production.

High operating temperatures in SOFCs can lead to material incompatibilities, and degradation[3]. Additionally, higher temperatures lead to higher operating and balance of plant costs, limiting the financial feasibility of fuel cell use. In order to reduce operating costs and improve cell longevity, it is advantageous to create cells that can function at lower temperatures (500-650 C) called intermediate-temperature solid oxide fuel cells (IT-SOFCs).

1.1 Solid Oxide Fuel Cell Operation SOFCs are comprised of three principle components: the anode, cathode, and electrolyte. At the anode, hydrogen fuel is oxidized by O2- ions coming from the electrolyte layer. The electrolyte serves to electrically insulate the anode from the cathode while conducting O2- ions from the cathode to the anode. At the cathode, oxygen is reduced using current flowing from the anode, which completes the cycle of oxygen ions and electrons through the cell to produce electricity as shown in fig. 1.

Figure 1: Solid oxide fuel cell schematic[1]

LNO and LSCF in Solid Oxide Fuel Cells

5

1.2 Cathode Oxygen Reduction Reaction At the cathode, oxygen is reduced according to the following Kroger-Vink reaction (equation1) in perovskite materials[5]:

(Eq. 1)

Whereas in Ruddlesden-Popper materials such as LNO that conduct oxygen ions interstitially, the ORR proceeds overall as:

(Eq. 2) [5]

Due to the slow ORR kinetics, overpotentials, and corresponding polarization resistances arise during operation. These exponentially grow in magnitude as temperature decreases[5]. By minimizing this resistance to the oxygen reduction reaction (ORR) within the fuel cell cathode, it is possible to operate the cell even at intermediate temperatures. ORR polarization resistance is a prime contributor to loss of efficiency in SOFCs, particularly at lower temperatures[4]. One way to reduce this resistance is through cathode infiltration. An infiltrate solution can be absorbed into a porous cathode scaffold to modify the surface of the cathode particles. The nanostructuring and large surface area resultant from infiltration corresponds to a higher volumetric density of active sites, leading to a smaller polarization resistance[4]. Catalysts in the infiltrate such as Sm0.5Sr0.5CoO3- (SSC), La0.6Sr0.4Co0.2Fe0.8O2- (LSCF), and La2NiO4 (LNO) have been proven to improve fuel cell performance when combined with mixed ionic-electronic conductor (MIEC) cathode materials[4,5,1]. Because the scaffold material of the cathode is an MIEC material, the cathode has excellent ionic and electric conductivity, while the infiltrated particles on the surface of the scaffold allow for high catalytic activity and minimal ORR resistance[4]. This combination of properties indicate that infiltrated cathodes can help to lower cell polarization resistance and make IT-SOFCs a realistic option for power application.

1.3 Cell Infiltration The method of infiltrating catalytically active particles onto the surface of SOFC cathode has been proven to improve polarization resistance at low temperatures[4]. The process is sometimes referred to as impregnation, during which a liquid solution containing the stoichiometric metal salt precursors of the chosen infiltrate is dropped onto a porous cathode scaffold. After soaking into the cathode, the cell is then fired at high temperatures at which the precursors can combine and bond to the surface of the cathode. Fig. 2 provides a schematic for the typical infiltration process, taken from Dong Dings Enhancing SOFC cathode performance by surface modification through infiltration[4]. During infiltration, the cathode backbone material is soaked in infiltrate solution. When it is treated at high temperatures, the infiltrate material bonds to the cathode as a distribution of particles or a thin film coating. When the infiltrate is distributed throughout the cathode, the cell is considered to a composite cathode material cell, which consists of the original scaffold material as well as any particles infiltrated into the cell.

LNO and LSCF in Solid Oxide Fuel Cells

6

Figure 2: Schematic of infiltration process[4]

In the present study, both Lan+1NinO3n+1 (LNO) and La0.6Sr0.4Co0.2Fe0.8O3- (LSCF) were infiltrated into a La0.9Sr0.1Ga0.8Mg0.2O3- (LSGM) cathode scaffold. LSGM is the base scaffold due to its high ionic conductivity at low temperatures[6,7]. LNO has very high oxygen self-diffusion and surface exchange values, but is limited in performance by its low electrical conductivity[1]. LSCF is an excellent electronic conductor with good oxygen surface exchange and ionic conductivity[8].

At processing temperature, the LSCF infiltrated nanoparticles will be of standard perovskite structure[9]. At elevated temperatures, LNO takes on an ordered Ruddlesden-Popper (RP) phase, in which perovskite units layer with rock salt layers, shown in fig. 3[1]. In this Ruddlesden-Popper phase, LNO takes on the general form Lan+1NinO3n+1, with n representative of the number of perovskite layers that lie between each rock salt layer in the unit cell. Higher n values correspond to higher order Ruddlesden-Popper LNO molecules. In the present study the n=1 phase is the primary focus, as it has the best oxygen exchange and self-diffusion properties of the LNO RP series[10].

Figure 3: Ruddlesden-Popper structure, with n = 1, 2, and 3 shown left to right[1]

LNO and LSCF in Solid Oxide Fuel Cells

7

2. Experimental

2.1 Fabrication of LSGM Electrolytes

La0.9Sr0.1Ga0.8Mg0.2O3- (LSGM) powder was ground and die-pressed at 120 MPa into 0.5g pellets, 1.9 cm in diameter. The resulting thickness was 0.5 mm. The pellets were sintered at 1450 C for four hours to allow them to achieve dense structure.

2.2 Preparation of Symmetrical Cells

To fabricate the cathode scaffold, LSGM ink was screen printed in 3 layers onto the dense LSGM electrolyte. Each layer was a circle 0.8 cm in diameter, and the pellets were dried at 150 C after each layer was printed. After all layers were printed, the pellets were sintered at 1425 C for 4 hours. Next, a single layer of LSCF ink was printed atop the LSGM layers to serve as a current collector, and was fired at 1100 C for 1 hours. At the end of the printing/sintering process, each cell had 3 LSGM layers and 1 LSCF current collector layer symmetrically sintered on each side, as shown in fig. 4.

Figure 4: The LSGM cathode (black) sintered upon the dense LSGM electrolyte

To prepare the infiltrate solutions, stoichiometric quantities of nitrate salt precursors were measured and dissolved in water as follows:

Lan+1NinO3n+1 (LNO): 2La(NO3)3 + Ni(NO3)2

La0.6Sr0.4Co0.2Fe0.8O3- (LSCF): 3La(NO3)3 + 2Sr(NO3)2 + Co(NO3)2 + 4Fe(NO3)3

The cathode scaffold on both sides of each cell was dropped with 7 L of infiltrate solution for each step, as shown in fig. 5. After each infiltration step, the cells were dried of any excess solution and fired at 900 C for 4 hours to burn out the nitrate solutions and to allow the precursors to form the ordered perovskite phases mentioned previously. After 12 full infiltration steps, the cathodes were loaded with 30-50% infiltrate mass by weight.

LNO and LSCF in Solid Oxide Fuel Cells

8

Figure 5: Cell infiltration process

In order to ensure the LNO precursors combined to form the proper Ruddlesden-Popper phase, the three samples of the solution was fired at 800, 900, and 950 C for 4 hours and tested using x-ray diffraction to find which phase is present. As shown in fig. 6, a pronounced peak of La2NiO4 at ~31 did not form at 800 C. At both 900 and 950 C, peaks for La2NiO4 and an ordered Ruddlesden-Popper phase (La8Ni4O17) were observed. However, at 950 C, a strong peak of the La3O3 rock salt structure appeared in the sample. Thus, in order to achieve the ORR catalytic properties of LNO, it was concluded that 900 C was the ideal firing temperature for the infiltration steps.

Figure 6: Verification of 900 C as optimum firing temperature for infiltrates. Lanthanum oxide (non perovskite) peak disappears above 800 C, but reappears at 950 C.

LNO and LSCF in Solid Oxide Fuel Cells

9

2.3 Experimental Design

Prior to infiltration, each cell is identical. Then cells were each infiltrated 12 times with varying ratios of LNO to LSCF. Group A, a control group, was infiltrated 12 times only with LNO. Group B was infiltrated 11 times with LNO and once with LSCF. Group C was infiltrated evenly, with 6 infiltrations of LNO and LSCF each. Group E was infiltrated 11 times with LSCF and once with LNO. Group F, another control, was infiltrated with only LSCF. This infiltration scheme can be more clearly visualized in table 1. Each of the 5 test groups consisted of 3 replicate cells.

Table 1: Cathode infiltration scheme

Group A Group B Group C Group D Group E

LNO infiltrations 12 11 6 1 0

LSCF infiltrations 0 1 6 11 12

2.4 SOFC Test Preparation

After all infiltrations were complete, each cell was printed with silver ink grids to serve as current collectors, and silver paste was used to connect silver wire to the grid (shown in fig. 7). All silver was dried and cured at 150 C.

Figure 7: Silver grid printed (left) and silver wire with silver paste to wire the cell (right)

After the cells were wired, they were connected in parallel to a testing rig (shown in fig. 8) and inserted in into a testing furnace for electrochemical impedance spectroscopy (EIS).

LNO and LSCF in Solid Oxide Fuel Cells

10

Figure 8: Cells wired and connected to test rig.

2.5 Cathode Characterization Methods

The cathode microstructure was observed after testing using scanning electron microscopy (SEM) and analyzed using energy dispersive spectroscopy (EDS) in a Hitachi 8030 microscope. Samples were coated with osmium to limit charging during imaging. X-ray diffraction was done in a Scintag powder diffractometer using a Cu K source (1.54 ).

The electrochemical impedance spectra (EIS) were taken with an electrochemical workstation (IM6, ZAHNER) from 500 to 650 C over the frequency range from 0.1 Hz to 1 MHz with signal amplitude of 5mV.

3. Results and Discussion

3.1 Composite Cathode Microstructure

Fig. 9(a) and (b) are fracture cross-sectional SEM images of a Group E cell, infiltrated only with LSCF. At 800x the interface between the dense LSGM electrolyte (top) and the porous composite cathode (bottom) is clear. When magnification is intensified to 5000k, the LSGM porosity of the scaffold is evident, as well as apparent surface modifications from the infiltrates.

LNO and LSCF in Solid Oxide Fuel Cells

11

Figure 9: Group F cathode infiltrated with LSCF at (a) 800x and (b) 5000x.

In order to ensure complete penetration of the infiltrate solutions into the cathode, an EDS line scan was completed across a Group A cathode infiltrated with only LNO. The scan shown in fig. 10 illustrates the presence of nickel as a function of position in the cell. In the dense LSGM electrolyte which is void of nickel, EDS detected negligible traces of Ni. However, at the electrolyte/cathode interface, detected nickel spikes. This verifies that infiltrate particles infiltrated completely though the cathode, rather than remaining at the surface.

(a)

(b)

LNO and LSCF in Solid Oxide Fuel Cells

12

Figure 10: EDS line scan across a Group A cell infiltrated with LNO only.

3.2 Impedance Spectra

Fig. 11 shows the results of electrochemical impedance spectra (EIS) of the composite cathodes at 650 C. When imaginary impedance is plotted vs. real resistance at a range of discrete frequencies in the form of a Nyquist plot, the arc-like plots can be used to calculate Rp, which is the distance on the real axis between each arcs two intersections with the real axis. A wider arc is indicative of higher Rp, and smaller indicates a low Rp.

Figure 11: Nyquist plot of each cell type, plotting imaginary impedance (y-axis) vs. real impedance (x-axis)

LNO and LSCF in Solid Oxide Fuel Cells

13

Fig. 12 shows the EIS data in terms of imaginary impedance as a function of frequency in the form of a Bode plot. These plots can be indicative of the mechanisms at work in the cathode (i.e. electron scattering, ORR resistance, kinetics, etc.) depending on the position of each curves peak in relation to frequency. It is interesting to note that the evenly mixed 6:6 ratio of LNO:LSCF observed its peak imaginary impedance around 24 Hz, whereas the other cells saw peak imaginary impedance near 40 Hz. Unfortunately, further investigations of the mechanisms at work are currently beyond the scope of this project.

Figure 12: Bode plot of each cell type, plotting imaginary impedance (y-axis) vs. frequency

3.3 Polarization Resistance: Infiltrate Composition Effect

Table 2 shows the total area-specific polarization resistances (i.e., the difference between the real axis intercepts of the impedance arcs) from EIS data in fig. 11 for cathodes of each infiltrate composition. Rp values were markedly higher in samples with maximum infiltrate mixing (i.e. LNO:LSCF ratio 6:6). The lowest Rp was observed in the cells loaded only with LSCF, and cells loaded with a 1:11 ratio of LNO:LSCF varied little from those of a 12:0 ratio. This data can be more clearly visualized in fig. 13.

Table 2: Average polarization resistance values of each cell group obtained for fits to the impedance spectra in fig. 11

Measurement temperature

LNO:LSCF Ratio

12:0 (Group A) 11:1 (Group B) 6:6 (Group C) 1:11 (Group D) 0:12 (Group E)

650 C 0.401 cm2 0.565 cm2 0.817 cm2 0.240 cm2 0.220 cm2

600 C 0.985 cm2 1.36 cm2 3.83 cm2 0.681 cm2 0.653 cm2

550 C 2.68 cm2 4.27 cm2 12.0 cm2 2.24 cm2 2.27 cm2

500 C 9.47 cm2 9.98 cm2 40.4 cm2 8.46 cm2 9.13 cm2

LNO and LSCF in Solid Oxide Fuel Cells

14

Figure 13: Polarization resistance as a function of LSCF composition at varied temperature

The best Rp was found in a cell in Group E which achieved an Rp = 0.21 cm2 at 650 C. This resistance is comparable to that of previous LSCF-LSGM composite cathodes, which have reached Rp as low as 0.19 cm2 [8]. The best performing cell infiltrated primarily with LNO was found in group A and achieved a polarization resistance Rp = 0.33 cm2. This resistance is significantly lower than that of previous LNO-based composite cathodes, which reached Rp no lower than 1.37 cm2 at 650 C [10] .

3.4 Activation Energy Calculation

Table 3 shows the activation energy calculated for the different LNO:LSCF ratios. The cells containing significant amounts of LSCF (Groups C, D, and E) showed activation energies ranging from 1.47-1.58 eV, comparable to previous LSCF-LSGM composite cells which showed activation energies ranging from 1.63-1.71 eV [8]. Cells primarily infiltrated with LNO saw activation energies significantly lower, ranging from 1.20-1.26 eV.

Table 3: Activation energy LNO: LSCF Ratio Activation Energy (eV)

12:0 1.26 11:1 1.20 6:6 1.58

1:11 1.47 0:12 1.54

LNO and LSCF in Solid Oxide Fuel Cells

15

Figure 14: log Rp vs. 1/T for each cathode composition In fig. 14, the slopes of the log Rp vs. 1/T data (fig. 14) yield the activation energies of each cathode composition. Cells containing 6, 11, and 12 infiltrations of LSCF appear parallel. However, cells containing primarily LNO (i.e. 0 or 1 infiltrations of LSCF) had smaller slopes, indicating lower activation energy.

4. Conclusions LNO-LSCF infiltrated LSGM cathodes were investigated. LSGM cathodes were loaded between 30 and 50 wt.% LNO and LSCF, and cells loaded only with LSCF yielded the best electrochemical performance. The cells loaded with a 50% ratio of LNO-LSCF showed markedly worse performance. The composite cathode polarization resistance showed an Arrhenius dependence on temperature with activation energy of 1.5 eV for cells loaded primarily with LSCF, and 1.2 eV for cells loaded primarily with LNO.

Acknowledgements I sincerely thank Dr. Scott Barnett and Dr. Kathleen Stair for making this research and this project possible. I also thank my mentor Justin Railsback for his careful guidance and tireless enthusiasm, as well as the entire Barnett group for making my past three years of research memorable.

LNO and LSCF in Solid Oxide Fuel Cells

16

References

1. Choi, S., Yoo, S., Shin, J.-Y., Kim, G. "High Performance SOFC Cathode Prepared by Infiltration of Lan+1NinO3n+1 (n=1, 2, and 3) in Porous YSZ." Journal of The Electrochemical Society 158.8 (2011): B995.

2. Atkinson, A., Barnett, S., Gorte, R., Irvine, J., McEvoy, A., Mogensen, M., Singhal, S.,

Vohs, J. Advanced anodes for high-temperature fuel cells. Nature Materials 3.1 (2004): 17-27.

3. Brandon, N., Skinner, S., Steele, B. Recent Advances in Materials for Fuel Cells.

Annual Review of Materials Research 33.1 (2003): 183-213.

4. Ding, D., Li, X., Lai, Y., Gerdes, K., Liu, M. Enhancing SOFC cathode performance by surface modification through infiltration. Energy & Environmental Science Review.

5. O'Hayre, R., Cha, S.-W., Colella, W., Prinz, F. Fuel Cell Fundamentals. Hoboken, NJ: John Wiley & Sons, 2009.

6. Zhang, N., Kening, S., Zhou, D., Jia, D. Study on Properties of LSGM Electrolyte Made

by Tape Casting Method and Applications in SOFC Journal of Rare Earths 24.1 (2006): 90-92.

7. Huang, K. Wan, J.-H., Goodenough, J. Increasing Power Density of LSGM-Based Solid

Oxide Fuel Cells Using New Anode Materials Journal of The Electrochemical Society 148.7 (2001): A788.

8. Lin, Y., Barnett, S. "La0.9Sr0.1Ga0.8Mg0.2O3-La0.6Sr0.4Co0.2Fe0.8O3 Composite Cathodes for Intermediate-temperature Solid Oxide Fuel Cells." Solid State Ionics 179.11-12 (2008): 420-27.

9. Qiang, F., Sun, K., Zhang, N., Zhu, X., Le, S., Zhou, D. Characterization of electrical

properties of GDC doped A-site deficient LSCF based composite cathode using impedance spectroscopy Journal of Power Sources 168.2 (2007) 338-345.

10. Woolley, R., Skinner, S. Novel La2NiO4+ and La4Ni3O10- composites for solid oxide

fuel cell cathodes. Journal of Power Sources 243 (2013) 790-795.