Performance of GDC-Impregnated Ni Anodes of

Electrochemical and Solid-State Letters, 7 共9兲 A282-A285 共2004兲

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1099-0062/2004/7共9兲/A282/4/$7.00 © The Electrochemical Society, Inc.

Performance of GDC-Impregnated Ni Anodes of SOFCs S. P. Jiang,z Sam Zhang, Y. D. Zhen, and A. P. Koh Fuel Cells Strategic Research Program, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798 Ni electrodes impregnated with nanosized electrocatalytic active (Gd, Ce)O2 共GDC兲 particles were fabricated and investigated as anodes for H2 oxidation reaction in solid oxide fuel cells 共SOFCs兲. Incorporation of nanosized ionic conducting GDC particles significantly reduced the electrode interface resistance and polarization potential. With the impregnation of 8.5 vol % GDC in Ni anodes, the electrode interface resistance was 0.71 ⍀ cm2 at 800°C, close to the good performance value of 0.24 ⍀ cm2 reported on Ni 共50 vol %兲/YSZ 共50 vol %兲 cermet anodes at the same temperature. Nanosized GDC particles were deposited on the surface of Ni grains, forming a thin and uniform GDC layer. Such a unique microstructure shows a potential application as direct methane oxidation anodes for SOFCs based on natural gas fuel. © 2004 The Electrochemical Society. 关DOI: 10.1149/1.1783112兴 All rights reserved. Manuscript submitted January 24, 2004; revised manuscript received February 5, 2004. Available electronically August 4, 2004.

In the development of solid oxide fuel cells 共SOFCs兲 based on thin electrolyte cells, the overall cell losses are increasingly dominated by the loss associated with electrode reactions at electrode and electrolyte interfaces due to significant reduction in the resistive losses associated with the electrolyte.1,2 Thus electrocatalytic activity and performance of electrode materials must be significantly enhanced to maintain the performance of SOFCs operating at reduced temperatures 共600-800°C兲. This may be achieved either by developing new electrode materials with high mixed ionic and electronic conductivity 共MIEC兲 and high electrocatalytic activity or by significantly improving the existing conventional electrode materials such as Sr-doped LaMnO3 cathodes and Ni/Y2 O3 -ZrO2 共Ni/YSZ兲 cermet anodes. The advantages of conventional electrode materials are their proven electrocatalytic activity at high temperatures and good stability with YSZ electrolyte materials.3,4 Ni/YSZ based cermets are the most commonly used anode materials for SOFCs. This is because Ni has excellent electrocatalytic activity for the H2 oxidation reaction as compared to other metals,5,6 high electrical conductivity, high stability with Y2 O3 -ZrO2 共YSZ兲 electrolyte,7 and is available in abundance. However, the electrocatalytic activities of Ni and Ni/YSZ cermet anodes for the H2 oxidation reaction are critically related to the three-phase boundary areas where Ni, YSZ, and H2 gas meet.8-11 This in turn strongly depends on the particle size and phase distribution of Ni and YSZ in the cermets.12,13 Usually, conventional Ni/YSZ cermet anodes are prepared using commercial NiO and YSZ powders and are homogenized by a ceramic mixing process.14-17 It has been shown previously that high-temperature sintering (⬃1400°C) and reducing 共900-1000°C兲 steps associated with conventional ceramic Ni/YSZ fabrication have a significant effect on grain growth of both Ni and YSZ phases and on the porosity of the anode.18 In the green state 共i.e., the stage of anode coating before sintering兲, distribution of NiO and YSZ phases is uniform with very fine NiO and YSZ particles. After sintering at 1400°C and reduction in H2 at 1000°C, Ni and YSZ particle size may be in the range of 2-5 and 1-3 ␮m, respectively. This places a serious limitation of the conventional fabrication process on the microstructure optimization of Ni/YSZ cermet anodes. Thus, avoiding the use of high temperatures during phase optimization would significantly reduce the coarsening process of the phase concerned and therefore increase the three-phase boundary areas and promote H2 oxidation reaction. Recently, it was shown that ion impregnation is an effective way to introduce nanosized oxide or precious metal particles into a SOFC electrode structure at relatively low temperatures.18,19,20 In this paper, Ni anodes impregnated with nanosized and electrocatalytic active Gd-doped ceria 共GDC兲 particles were fabricated and electrode performance and behavior of GDC-impregnated Ni anodes

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were investigated. The results indicate that it is feasible to develop high performance anodes based on low-temperature impregnation of nanosized electrocatalytic oxide particles. Experimental Electrolyte substrates were prepared by die pressing 8 mol % Y2 O3 -ZrO2 powder 共TZ8Y, Tosoh, Japan兲, followed by sintering at 1500°C for 4 h in air. Electrolyte thickness and diameter were 1 and 19 mm, respectively. NiO 共J.T. Baker兲 powders were precoarsened at 600°C for 2 h in air to reduce powder shrinkage and to improve coating quality,14 followed by ballmilling in propanol for 5 h. TZ3Y balls 共Tosoh, Japan兲 were used as the milling medium. NiO ink was prepared using polyethyleneglycol and applied to the YSZ electrolyte by slurry painting. NiO anodes were sintered at 1400°C for 2 h and electrode area was 0.5 cm2 . A three-electrode system was used for the electrochemical measurement. Pt paste was painted on the opposite side of the working electrode to make counter and reference electrodes. The counter electrode was positioned symmetrically to the working electrode and the reference electrode was painted as a ring around the counter electrode. The gap between the counter electrode and reference electrode was ⬃4 mm. Hydrogen gas humidified at room temperature (⬃3% H2 O) was used as fuel and air as oxidant. NiO was reduced in situ under H2 fuel environment. Electrochemical performance of Ni anodes was characterized by the polarization and electrochemical impedance spectroscopy 共EIS兲 techniques. EIS was carried out using a Solartron frequencyresponse analyzer 1250 in combination with a Solartron electrochemical interface with frequency range of 0.1 Hz to 1 MHz and amplitude of 10 mV. Electrode performance for H2 oxidation reaction was usually measured at 700-900°C. Electrode interface resistance (R E) was measured directly by the difference of the low- and high-frequency intercepts on the impedance curves. Gd0.2Ce0.8(NO3 ) x nitrate solution was used for the ion impregnation of Ni anodes. The solution was made from Gd(NO3 ) 3 • 6H2 O 共99.9%, Aldrich Chemical兲 and Ce(NO3 ) 3 • 6H2 O 共99.9%, Aldrich Chemical兲 for a 3 mol metal ion concentration. A dropper was used to deposit the solution on the top surface of Ni anodes and the solution was allowed to soak into the porous anode. The surface of the electrode coating was wiped with a soft tissue and dried again in open air. After impregnation the electrode was fired at 850°C for 1 h to decompose the Gd0.2Ce0.8(NO3 ) x nitrate solution, forming (GdCe)O2 oxide phase.18 The loading of the impregnated oxides was measured by the weight difference before and after impregnation treatment. The impregnation treatment was repeated to increase the loading of the oxide phase. Anode thickness was measured using a TalyScan 150 dual gauge system 共Taylor Hobson兲 and the microstructure of the Ni anodes was examined by scanning electron microscope 共SEM, Leica S360兲. Porosity of the anode was estimated by SEM image analysis21 of the cross section of the anodes. Based on the electrode thickness and porosity, the volume percent-


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age of the impregnated GDC in Ni anode can be calculated, according to the following equation vol % GDC ⫽ ⫽

V GDC V Ni ⫹ V GDC A ⫻ W GDC /d GDC ⫻ 100 A ⫻ ␦ ⫻ 共 1 ⫺ ␪ 兲 ⫹ A ⫻ W GDC /d GDC 关1兴

where A is electrode coating area, W GDC the loading of GDC, d GDC the density of GDC 共close to that of CeO2 : 7.132 g cm⫺3 ), ␦ the coating thickness and ␪ the porosity. Details of electrode materials preparation, cell configuration, and the electrochemical measurements may be found elsewhere.10,14,18 Results and Discussion After one impregnation of 3 M Gd0.2Ce0.8(NO3 ) x nitrate solution the GDC loading in Ni anode was 0.52 mg cm⫺2 . After repeated impregnation, the GDC loading increased to 1.7 mg cm⫺2 . Based on the image analysis of the cross-sectional SEM pictures, the porosity of Ni anode coating was ⬃30%, close to that measured by filtration method for Ni/YSZ cermet anodes.22 The coating thickness was in the range of 20-38 ␮m. Thus, according to Eq. 1, the volume percentage of impregnated GDC was 3.5 vol % for the Ni anode after one time impregnation of 3 M Gd0.2Ce0.8(NO3 ) x nitrate solution and 8.5 vol % after repeated impregnation. Figure 1 shows the impedance curves for H2 oxidation on pure Ni anode, Ni anode with 0.52 mg cm⫺2 GDC impregnation and 1.7 mg cm⫺2 GDC impregnation at different temperatures. Impedance curves were measured under open circuit. For the H2 oxidation reaction on pure Ni anodes, the impedance responses were characterized by a very large and depressed arc and there was no clear separation between low and high frequency arcs. This is similar to the impedance behavior of Ni anodes for the H2 oxidation reaction reported previously.6 After GDC impregnation, the overall impedance arcs were reduced significantly. For H2 oxidation on pure Ni anode, the overall electrode interface or polarization resistance (R E) was 1.9 ⍀ cm2 at 900°C. R E was reduced to 0.93 ⍀ cm2 for the reaction on Ni anode with impregnated GDC loading of 0.52 mg cm⫺2 and 0.22 ⍀ cm2 on Ni anode with impregnated GDC loading of 1.7 mg cm⫺2 . At 700°C, R E decreased from 10.7 ⍀ cm2 for the reaction on pure Ni anode to 4.9 ⍀ cm2 on 0.52 mg cm⫺2 GDC-impregnated Ni anode and 1.5 ⍀ cm2 for 1.7 mg cm⫺2 GDCimpregnated Ni anode. R E was reduced by seven to nine times for the H2 oxidation reaction on 1.7 mg cm⫺2 GDC-impregnated Ni anodes as compared to that on pure Ni anode. Ion impregnation of GDC phase into Ni anode has a significant effect not only on the reduction in size of the impedance arcs but also on the characteristics of the impedance arcs. There was a significant change in the distribution of low and high frequency arcs for the reaction on the GDC-impregnated Ni anodes and the low and high frequency arcs were clearly separated. This shows that the impregnated GDC phase may have a different effect on the electrode processes associated with low and high frequency arcs for the H2 oxidation reaction.10,23 The significant improvement in the electrode polarization resistance was also confirmed by the polarization performance. Figure 2 shows plots of polarization performance of pure Ni anode, Ni anode with 0.52 mg cm⫺2 GDC impregnation, and Ni anode with 1.7 mg cm⫺2 GDC impregnation at different temperatures. Impregnation of GDC phase substantially decreased the polarization potential for H2 oxidation reaction on Ni anodes. ␩ was reduced from 195 mV at 200 mA cm⫺2 and 900°C on pure Ni anode to 136 and 53 mV on 0.52 and 1.7 mg cm⫺2 GDC-impregnated Ni anodes, respectively, under the same conditions. Figure 3 compares the SEM pictures of the surface and cross sections of Ni anode with and without GDC impregnation. Ni par-

Figure 1. Impedance spectra for H2 oxidation on pure Ni and Ni anode with 0.52 and 1.7 mg cm⫺2 GDC impregnation at 共a兲 900, 共b兲 800, and 共c兲 700°C. Impedance curves were measured at open circuit.

ticle size was 1.6 ⫾ 0.48 ␮m for pure Ni anodes 共Fig. 3a and b兲. After repeated 3 M Gd0.2Ce0.8(NO3 ) x nitrite solution impregnation treatment (1.7 mg cm⫺2 GDC loading兲, except a few smooth Ni grains, the majority of Ni grains were covered by fine deposits 共Fig. 3c and d兲. The energy dispersive spectrometry 共EDS兲 pattern of the fine deposits showed the presence of Ce and Gd in addition to Ni and Zr 共Fig. 3e兲, indicating that the deposits were most likely (GdCe)O2 phase. The surface of GDC covered Ni grains was not smooth, indicating that GDC deposited layer was most likely composed of very fine GDC particles. As indicated in Fig. 3d, the size of the deposited GDC particles was in the range of 100-200 nm. Similar distribution and deposition of impregnated GDC particles were also observed on Ni anode with low GDC loading (0.52 mg cm⫺2 ). Due to the mixed ionic and electronic conduction behavior of GDC under reducing environment, nanosized GDC particles deposited on the Ni surface may also provide active sites for the H2 oxidation reaction in addition to the extension of the three-phase boundary areas where Ni electrode, YSZ/GDC electrolyte, and H2 gas phases meet. This appears to be supported by the observation that doped CeO2 -impregnated Ni/YSZ cermet anodes showed higher electrochemical performance than that of YSZ-impregnated Ni/YSZ cermet anodes.18 Electrochemical performance and microstructure results of GDC-impregnated Ni anodes demonstrate that the GDC impregnation is an effective way to significantly enhance the electrocatalytic activity of Ni-based anodes with low GDC loading.

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Figure 3. SEM pictures of 共a兲 surface and 共b兲 cross section of pure Ni anode, 共c兲 surface and 共d兲 cross section of 1.7 mg cm⫺2 GDC-impregnated Ni anode, and 共e兲 EDS pattern of deposits-covered Ni particles in 共c兲.

trocatalytic activity of the Ni anodes is significantly enhanced. R E of the 1.7 mg cm⫺2 GDC-impregnated Ni anode becomes 0.71 ⍀ cm2 at 800°C which is close to the good performance of 0.24 ⍀ cm2 reported on Ni 共50 vol %兲/YSZ 共50 vol %兲 cermet anodes at the same temperature.18

Figure 2. Polarization curves of the H2 oxidation reaction on pure Ni and GDC-impregnated Ni anodes at 共a兲 900, 共b兲 800, and 共c兲 700°C.

Figure 4 compares electrode interface resistance (R E) of GDC impregnated Ni anodes in the present study with those of Ni/YSZ cermet anodes reported in the literature.14-16,18,24-27 R E of Ni/YSZ cermet anodes was measured under open circuit by EIS in moist H2 . YSZ content in the cermet was in the range of 40-60 vol %. For H2 oxidation on Ni/YSZ cermet anodes, there are wide variations in the electrode polarization resistance (R E). For example, at 800°C R E values may vary from 7.6 ⍀ cm2 25 to as low as 0.24 ⍀ cm2 , 18 a difference by more than 31 times. The large difference in the reported electrode polarization resistance may be due to the significant difference in the anode structure prepared by different groups. For pure Ni anode, R E is in the upper region of the reported values, indicting the low electrocatalytic activities of pure Ni anode. With the impregnation of nanosized GDC phase into Ni anode, the elec-

Figure 4. Plots of R E of GDC-impregnated Ni anodes in this study in comparison with those of Ni/YSZ cermet anodes reported in the literature. R E of Ni/YSZ cermet anodes was measured in moist H2 under open circuit by electrochemical impedance spectroscopy. Numbers in the figure are cited references.

Electrochemical and Solid-State Letters, 7 共9兲 A282-A285 共2004兲 Activation energy of H2 oxidation reaction on pure Ni anode is 84 kJ mol⫺1 , close to that measured on Ni pattern electrode.28 Activation energy of GDC-impregnated Ni anodes is 80 kJ mol⫺1 for Ni anode with 0.52 mg cm⫺2 GDC loading and 73 kJ mol⫺1 for Ni anode with 1.7 mg cm⫺2 GDC, slightly lower than that on pure Ni anode. This appears to be very different from the activation energy behavior of Ni/CeO2 -based cermet anodes. Wang and Tatsumi29 investigated performance of Ni/Sm-doped CeO2 共Ni/SDC兲 anodes and found that addition of SDC improved electrochemical performance significantly but also led to an increase in activation energy. The increased activation energy resulted in the higher anodic overpotential for the reaction on Ni/SDC anodes at low temperatures. For H2 oxidation on Ni/YSZ cermet anodes, activation energy was in the range of 100-170 kJ mol⫺1 . 9,26,27,30 That the GDC-impregnated Ni anode gives rise to much lower activation energy as compared to that of Ni/YSZ cermet anodes indicates that GDC-impregnated Ni anodes render this process useful in the development of low temperature SOFC. MIEC oxides such as GDC have been investigated as potential anodes for intermediate temperature SOFCs. The major attraction of the MIEC is the possibility of the enhanced reaction zone over the three-phase boundary areas due to high electronic conductivity as the results of reduction of Ce4⫹ to Ce3⫹ under fuel-rich conditions. However, relatively large lattice expansion associated with the loss of oxygen under anodic conditions may result in coating spilling off the electrolyte.31 The electrode polarization resistance of pure GDC anode was also reported to be high (⬃11 ⍀ cm2 at 800°C兲 for H2 oxidation reaction.32 Ni/GDC cermet anodes showed better performance as compared to that of Ni/YSZ cermet anodes.33 However, volume change associated with GDC phase in the Ni/GDC cermet anode may potentially affect the performance stability of the anodes. As demonstrated in this study, ion impregnation process may achieve uniform distribution of GDC phase in the Ni anode with much lower volume of GDC (⬍10 vol %). The coverage of Ni grains with thin GDC fine deposits also minimize the direct exposure of Ni phase to the fuel gases. Such microstructure could enhance the structural stability of Ni-based anodes and minimize the carbon deposition when methane is used as fuel. Exploration of GDC-impregnated Ni anodes for natural gas fuel anodes is currently in progress. Conclusions This study shows that impregnation of ionic conducting oxides such as GDC in the Ni anode significantly enhances the electrochemical activity 共low electrode polarization resistance, low overpotential, and low activation energy兲 for H2 oxidation reactions as compared to that of pure Ni anodes. Despite the preliminary nature of this study, the performance of GDC-impregnated Ni anodes has been demonstrated close to that of the Ni/YSZ cermet anodes. The particle size of the impregnated GDC ionic conducting oxide through nitrite salt solution is in the range of 100-200 nm and is

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uniformly distributed on the surface of the Ni particles. The unique microstructure of GDC-impregnated Ni anodes opens up another avenue in the development of high performance and high stability Ni-based anodes for SOFCs operating in natural gas fuel environment. Nanyang Technological University assisted in meeting the publication costs of this article.

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