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Influence of nano zirconia on NiAl anodes for molten carbonate fuel cell: Characterization, cell tests and post-analysis
Journal of Power Sources xxx (2016) 1e8
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Influence of nano zirconia on NiAl anodes for molten carbonate fuel cell: Characterization, cell tests and post-analysis Grazia Accardo a, Domenico Frattini a, *, Angelo Moreno b, Sung Pil Yoon c, Jong Hee Han c, Suk Woo Nam c a b c
Department of Engineering, University of Naples Parthenope, Centro Direzionale Napoli Isola C4, 80143 Naples, Italy ENEA e Italian National Agency for New Technologies, Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy Fuel Cell Research Center, KIST - Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NiAl anodes with nano ZrO2 are tested in MCFC single cell after characterization. Results show that nanoparticles improve mechanical and creep properties. The anode with ZrO2 3% wt. has the best performance over 1000 h operations. Post analysis shows zirconates formation at cathode for ZrO2 contents over 5% wt. A high content of nanoparticles can diminish too much porosity and pore size.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2016 Accepted 7 November 2016 Available online xxx
Anode materials in Molten Carbonate Fuel Cells should have high creep resistance and good mechanical behavior to endure in high temperature-corrosive environments. In this work, zirconia nanoparticles (1 e10% wt.) are added to NiAl anodes in order to investigate their effects on mechanical properties and single cell performances. Results show that nanoparticles strongly adhere to metal particles and bending strength increases from 6.08 to 11.33 kgf cm2 while creep strain is reduced from 7.55% to 3.25%. In the case of the anode with ZrO2 3% wt., the stable and high output voltage of 0.81 V at 150 mA cm2 is a promising result, compared to the literature. In addition, the solid contact angles between melted electrolyte and anode, for the NiAl reference sample and the ZrO2 3% wt. are 37.6 and 17, respectively, showing the improved wettability of the modified anode. However, it seems to be a limit to the effective zirconia content as the contact angle of the anode with ZrO2 10% wt. is 58.1, which indicates a low wetting ability. When zirconia content is too high, single cells have low performances due to high internal resistance and porosity reduction. The formation of a zirconate phase also occurs during operations. © 2016 Elsevier B.V. All rights reserved.
Keywords: Anode Cell test Characterization Nanoparticle Post-analysis Wettability
1. Introduction * Corresponding author. E-mail address: [email protected] (D. Frattini).
Molten Carbonate Fuel Cells (MCFCs) are considered as a clean
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power generation technology and it is in the early stage of market diffusion. However, the actual lifetime of MCFCs is not long enough, namely 4 years of operation, and this short lifetime causes high maintenance costs and a limited diffusion. The major causes of short lifetime and performance decay are related to microstructural instability of electrodes, electrolyte losses, gas crossover, poisoning due to sulfur compounds, creep and micro-cracks formation [1e3]. The development of durable components, in particular the anode, is strategic to compete in the power generation market [4]. State-ofthe-art anodes for MCFC are represented by Ni alloys, based on Chromium, Aluminum or both, to obtain a sintered and porous material with high performances [5e7]. Chromium can react with the electrolyte, is susceptible to lithiation and consumes carbonates. Anyway, an anode of pure Ni has a creep strain of 40% after 100 h of operation, but the addition of Cr, up to 8% wt., has a significant effect on preventing the crack-growth phenomena and can cut by half creep strain [1,8,9]. Actually, porous NiAl alloy is the most diffused anode material as it has acceptable physical properties but presents some drawbacks similar to NiCr electrodes for long term MCFC operations [10,11]. Nguyen et al. [7] have reported that creep deformation in the Ni-Cr system is higher than that of a ternary Ni-Al-Cr system with various amount of Al. By adding 5% wt. of Al the same authors reduced the creep strain from 7% to 3%. To improve the durability of MCFC anode materials, some transition metals, as Nb, Ce, Gd, Mg, Cu, Zr, and their oxides can be used as additives in Ni alloys [12]. In particular, mechanical properties are enhanced thanks to crystal structure strengthening techniques, using alien atoms or hard oxide particles to block dislocation movements [13e15]. In their works, Baizeng et al. [12] and Fang et al. [16] have verified that NiNb alloys are chemically stable and insoluble in melted electrolyte due to the fact that niobium oxide cannot be reduced by CO and H2 in the anode atmosphere. However, limited mechanical tests have been reported for this type of anode material and preliminary cell tests, in which several parameters were not strictly controlled, showed that these electrodes exhibit poor performance (1.03e1.05 V for OCV) and could not operate above 100 mA cm2. Apart the mechanical resistance, an improvement of the electrochemical behavior of the anode is required [17e19]. Anodes with large surface area, high wettability, small thickness and stable structure are usually preferred because they can decrease charge and mass transport resistance. Nguyen et al. [20] for example, studied the effects of anode thickness on electrochemical performance and cell voltage stability. They found that the anodes with small pore size could effectively reduce the mass transfer resistance up to 0.35 mU cm2 with a nitrogen cross over of 0.5%. At the same time, under reaction conditions, the wetting behavior of the electrodes, e.g. contact angle, mainly at the anode side, has a high impact on cell performance and it is a function of anode porous structure and properties of the phases involved [21e23]. Generally, wetting angle in standard MCFC anodes is around 50 with (Li/ Na)2CO3 and 31 with (Li/K)2CO3 electrolytes [1] but, as reported by Yoon et al. [24], when the contact angle is lower than 20 , wettability is high and it is possible to improve long-term stability of the electrodes. Electrochemical reactions in MCFC are faster and sintering slower when the anode is more wetted by electrolyte. On the contrary, a contact angle larger than 50 or 90 is a consequence of poor and very poor electrode wettability, respectively [25]. In this work, differently from the current state-of-the-art, the use of the transition metal oxide ZrO2 is pretty new and it is proposed for the first time in anode fabrication. In fact, ZrO2 is used as a starting component of the anode and not as a coating material. Zirconia nanoparticles were added directly to NiAl alloy powders during fabrication. Anodes with different ZrO2 content (1e10% wt.)
were prepared by means of tape casting and the effects on physical and mechanical properties were studied. Cell performances of modified anodes have been investigated in long-term single cell tests and a post analysis on used components was carried out. Polarization and power curves, output voltage and impedance behavior have been compared to standard NiAl anodes. In addition, the contact angles under MCFC operating condition have been measured in order to find a correlation between nano zirconia addition, wettability and cell performances. 2. Experimental 2.1. Fabrication and characterization of anodes NiAl alloy powders (Chang Sung Co., Al 5.0% wt., 5 mm) and ZrO2 nanopowders (Alfa Caesar, 20 nm) were used as raw materials. Firstly, zirconia nanopowders, dispersant (BKY110, Altana Chemie), defoamer (SN-D348, San Nopco) and a mixture of Ethanol/Toluene (30/70% wt.) were homogeneously mixed in a ball miller for one day. After this first step, binder (Butvar B72, Monsanto Co.), plasticizer (Dibutyl Phthalate, Junsei Chemical Co.) and the NiAl powders were added and ball milled over two days to obtain a viscous slurry. In this step, a small amount of Ni powders (INCO 255, 5 mm) was used as sintering aid to promote metal particle aggregation. The amount was the same for each sample prepared. Secondly, a de-airing step under vacuum at 40 C was used to remove solvent and trapped air. After de-airing, the slurry was tape casted with a doctor blade on a glass support coated with a polyethylene film, and then dried in oven at 90 C for 3 h to obtain the green sheet. Finally, the green sheet was sintered in a furnace at 650 C for 2 h under a N2/H2 (90/10% vol.) atmosphere to eliminate the organic part. Anodes with four different nano zirconia compositions, ranging from 1% to 10% wt., were prepared and characterized. The composition of zirconia is intended as the weight of nanoparticles with respect to total weight. A zirconia-free reference sample, named Ni5Al, was also prepared as reference benchmark. The porosity and the Median Pore Diameter (MPD) were measured according to the Archimedes' principle (ASTM C373-88) on 3 specimens and by means of a Mercury Intrusion Porosimeter (Autopore IV, Micrometrics Co.), respectively. The crystalline structure was determined via XRD technique (Miniflex II, Rigaku Co.) in the 2q range 5 e80 and the morphology was examined with SEM technique (InspectF50, FEI Co.). Bending strength was determined according to the ASTM E855-90 standard by a 3-point bending strength testing machine, equipped with a 10 N load cell, (QC-508E, Techcome Ltd) on 5 rectangular specimens (5 2 0.06 cm3) to obtain a reliable average value. Creep tests on square specimens (1 1 0.06 cm3) were performed at 650 C and 100 psi for 100 h under hydrogen atmosphere in order to calculate the creep strain ratio by measuring the thickness before and after creep [7]. Finally, wettability measurements were carried out on square specimens (1 1 0.06 cm3) using an on-line Optical Contact Angle device (OCA, Dataphysics) with a quartz pipe in a tubular reactor, an adjustable metal support for alignment, a CCD camera with a 2 optical zoom and a contact angle software analyzer (SCA 20, Dataphysics). An equal amount of (Li0.7K0.3)2CO3 electrolyte powder was placed on each specimen and the reactor was heated at 650 C under nitrogen atmosphere to acquire the physical contact angle between electrolyte and electrode. 2.2. Long-term single cell tests and post analysis Single cell tests, up to 1000 h of operation, were carried out to compare the performance of modified anodes. Polarization and power curves, nitrogen crossover and output voltage at
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150 mA cm2 were recorded for each cell. Single cells, with an electrode area of 26 cm2, were fabricated using standard NiO cathode, gLiAlO2 matrix and (Li0.7K0.3)2CO3 electrolyte sheets (Twin Energy Co., South Korea). A sealing pressure of 0.2 MPa and an operating temperature of 650 C were applied for the experiments. The anode gas composition was H2:CO2:H2O ¼ 72:18:10 while the cathode gas composition was Air:CO2 ¼ 70:30. An electrical loader (ELS300Z, ELTO DC Electronics Co.) was used to impose the external current and to collect the cell voltage; the nitrogen crossover at the anode outlet was monitored by gas chromatography (789A, Agilent Tech.). In order to evaluate the contributions to electrodes polarization, Electrochemical Impedance Spectroscopy (EIS) was used to analyze the frequency response of cells (1287 Electrochemical Interface, Solartron Co.) within the frequency range 10 kHz e 0.01 Hz (FRA 1255B, Solartron Co.). After long-term tests, a post analysis was conducted in order to evaluate the degradation of modified anodes. Morphology was investigated with SEM technique (InspectF50, FEI Co.), crystal structure of anode and cathode was evaluated with XRD in the 2q range 5 e90 (Miniflex II, Rigaku Co.) and the change between initial and final porosity, after electrolyte removal, was evaluated according to the ASTM C373-88 standard on 3 samples. Residual carbonate in the anode was dissolved and removed by immersion in hot acetic acid for 8 h and then in boiling water for 5 h.
3. Results and discussion 3.1. Characterization of modified anodes Table 1 shows porosity and MPD of anodes modified with different amounts of ZrO2. The standard porosity of MCFC anodes is usually in the range 50e60%, while the pore size is around 3 mm [1]. These results suggest that fabricated anodes have an acceptable porosity for MCFC application but a slightly reduced pore size. This can be explained by considering that the porosity after sintering is the result of the organic part elimination and particle coalescence. Theoretically, porosities should be similar when the organic/inorganic proportion is the same or when alien atoms can diffuse to form solid solutions, as observed by Nguyen et al. [7]. The different physical properties observed between the Ni5Al reference sample and ZrO2 samples can be ascribed to the increasing amount of nanoparticles which tend to adhere on metal particles or to fill voids between them. XRD spectra for all samples are shown in Fig. 1. The spectra show a high crystalline structure in all cases. The patterns confirm the presence of ZrO2 together with two characteristic NiAl phases, namely Ni3Al and Ni2Al3, which are formed after sintering, in agreement with the literature [5,10]. The typical peaks of Ni, around 44.56 and 51.91 are also visible. The microstructure of anodes modified with nano zirconia can be observed in SEM micrographs of Fig. 2. The Ni5Al sample has a
Table 1 Physical properties of modified anodes. Samples
MPD (mm)
Porosity (%)
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
1.8 1.2 1.5 1.3 1.0
54.01 54.87 58.77 57.97 52.72
± ± ± ± ±
0.32 0.58 0.28 0.18 0.06
3
Fig. 1. XRD spectra of anodes with ZrO2 nanoparticles.
microstructure composed by spherical particles related to the NiAl powders and some irregular shaped particles ascribed to Ni powders and the formation of Ni2Al3 phase due to mechanical alloying and sintering, as confirmed by XRD spectra. The addition of zirconia is clearly visible in the micrographs due to the deposition of the nanoparticles on the surface of larger NiAl particles, promoted by the vigorous ball milling during fabrication. The porous structure obtained is similar in all samples, as the morphology presents many pores with a quite round shape. However, for the ZrO2 10% sample, zirconia covers largely the surface of metal particles, as showed in Fig. 2e. In this case, surface seems to be quite clogged by zirconia, and the initial porosity and MPD are therefore reduced as observed in Table 1. On the other hand, the sample ZrO2 3% has a good compromise between MPD and porosity as showed in Fig. 2c. At macroscale level, these results can be correlated to the mechanical behavior analyzed by creep and 3-point bending tests. In addition to creep ratio and bending strength, creep rate of secondary creep and modulus of elasticity (in bending) have been calculated from the slope of the curves. The mechanical properties of anodes are summarized in Table 2. In contrast with the standard Ni5Al anode, bending strength of zirconia anodes is considerably higher than that of the pure anode, as visible in Table 2. A lower steady-state creep rate indicates an improved resistance to creep deformation after primary creep. The stiffness of modified anodes is also higher than Ni5Al due to the increase in modulus of elasticity and therefore a higher stress resistance. The increase in the zirconia content denotes a characteristic mechanical improvement but at high zirconia content the increment of mechanical properties is marginal as ZrO2 5% and ZrO2 10% have similar properties. These values are encouraging compared with the literature, where the range of creep strain is 3e35% by changing the chromium and/or aluminum content in the anode composition up to 10% wt [7,10,26,27]. Finally, the wettability of the various anodes, in terms of contact angle, is showed in Fig. 3. The images show the angle established at the electrolyte/anode interface, derived from the intersection between a reference baseline, which represents the flat anode surface, and an elliptical line, which represents the profile of the carbonate melt. The sample ZrO2 10% wt. has a low wettability towards the carbonate melt due to the high contact angle compared to other
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Fig. 2. Microstructure of modified anodes: a) Ni5Al; b) ZrO2 1%; c) ZrO2 3%; d) ZrO2 5%; e) ZrO2 10%.
Table 2 Mechanical properties of modified anodes. Samples
Creep strain ratio (%)
Steady-state creep rate (s1)
Bending strength (kgf cm2)
Modulus of Elasticity (MPa)
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
7.55 5.15 4.42 3.51 3.25
2.00$108 1.81$108 1.72$108 1.75$108 1.78$108
6.08 ± 0.32 6.93 ± 0.57 8.58 ± 0.56 10.63 ± 0.56 11.33 ± 0.68
17.63 24.19 30.64 38.06 39.20
± ± ± ± ±
0.43 0.62 0.64 0.66 0.74
Fig. 3. Contact angle of a) Ni5Al; b) ZrO2 1%; c) ZrO2 3%; d) ZrO2 5%; e) ZrO2 10%.
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Fig. 5. Polarization and power curves after 1000 h in single cell tests. Fig. 4. Cell voltage over 1000 h in single cell test with modified anodes.
samples. The ZrO2 3% wt. anode established the best wettability, among the five samples. Nano zirconia addition in the range 1e5% wt. promoted the enhancement of both wettability and mechanical properties. This positive effect seems limited to 5% wt. In fact, wettability decreases for the sample with 10% of zirconia due to an unsuitable pore structure because this sample had the lowest porosity (52.72%) and the lowest MDP (1 mm). Therefore, the large mechanical properties of ZrO2 10% wt. are not compensated by an enhancement of physical and wetting properties for MCFC applications. 3.2. Single cell performance and post analysis Fig. 4 shows the output voltage vs time at constant current density value of 150 mA cm2 of single cells assembled with the modified anodes. An initial increase in cell voltages can be observed during the first 100 h of operation. This evidence is due to the stabilization of cell components, included the electrolyte distribution in the matrix, and the lithiation reaction at cathode. A very stable voltage is observed for each cell, especially for cells with the ZrO2 3% and 5% anodes, and voltage losses are calculated to be <8e10 mV during operation. The output voltage of cell with the ZrO2 10% anode is found to be very low during all the experiment, compared to the reference. For each cell, the nitrogen crossover during operation was monitored via GC at anode outlet and the values measured are resumed in Table 3. The results show an improved resistance to nitrogen leakage of ZrO2 modified anodes. The increase of crossover over time is around 0.14e0.15% compared to 0.30% of the reference cell with the standard anode. The lower crossover can be associated to the improved mechanical properties of modified anodes as the
Table 3 Nitrogen crossover during single cell tests. Sample
100 h
500 h
1000 h
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
0.35 0.38 0.29 0.33 0.32
0.40 0.50 0.35 0.40 0.32
0.65 0.53 0.43 0.47 0.45
Fig. 6. Impedance spectra for Ni5Al, ZrO2 3% and ZrO2 10% anode after 1000 h in single cell tests.
formation of micro cracks due to bending and creep is reduced [7,20]. The effect of zirconia is also visible from polarization curves after 1000 h of operation, as reported in Fig. 5. The OCV is the same for each cell, around 1.065 V. When the electrical load is applied to the cell, in order to obtain the desired current density, the cell voltage decreases but some differences are observed. The lowest values are achieved by the cell with ZrO2 10% anode and its performance falls at 150e200 mA cm2. For the other cells the polarization behavior is more stable, even at high current density. At low current densities, the low performances of ZrO2 10% wt. are ascribed to the high internal resistance as showed in Fig. 5, while at high current densities, the excess of zirconia on particle surface may limit the transfer of ions and of electrons due to concentration losses. All other experimental conditions being the same, the lower performance of ZrO2 10% anode could be ascribed to the insufficient electrochemical behavior of the cell as confirmed by the Nyquist plot of cells with the reference anode and the 3 and 10% nano ZrO2 wt. anode, reported in Fig. 6. The impedance curve of porous electrode of MCFC usually displays two semi-circles under a wide range of sweeping frequency.
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Table 4 Charge transfer resistance (Rct) and mass transfer resistance (Rmt) after 500 h and 1000 h. Sample
Ni5Al ZrO2 3% ZrO2 10%
500 h
1000 h
Rct (mU cm2)
Rmt (mU m2)
Rct (mU cm2)
Rmt (mU cm2)
0.711 0.896 0.594
0.819 0.623 0.947
0.576 0.837 0.556
0.957 0.676 1.073
Table 5 Porosity decrease after 1000 h. Sample
Initial porosity (%)
Final porosity (%)
Variation (%)
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
54.01 54.87 58.77 57.97 52.72
48.33 49.01 55.80 53.40 44.66
10.52 10.68 5.05 7.89 15.28
The first semi-circle, at high frequencies, relates the kinetic of electrode reactions and the second one, at low frequencies, describes the diffusion phenomena inside the porous structure of electrodes. From Fig. 6 the increase of internal resistance for the sample with 10% wt. of zirconia is clearly visible, as well as the intensification of mass transfer resistance at low frequencies, which are responsible for losses inside MCFC. This is quantitatively confirmed for the anode with 10% wt. of zirconia by the deconvolution of previous impedance spectra into the charge transfer resistance, Rct, and the mass transfer resistance, Rmt, showed in Table 4. The 3% wt. zirconia sample has the lower internal resistance and a charge transfer resistance higher than the mass transfer resistance. This indicates that this composition increases the effective three phase boundary area at the anode side, as confirmed by literature [6,20]. In fact, the standard anode and the anode with 10% wt. of zirconia maintain a charge transfer lower than mass transfer resistance during time. However, at longer operation, for all samples investigated Rct decreases while Rmt rapidly increases, while for the ZrO2 3% this is almost unchanged. After single cell operations, the porosity reduction is summarized in Table 5. The high porosity reduction for the anode with 10% wt. of zirconia contributes to decrease the electrochemical activity and consequently cell performance. The change of porosity for the ZrO2 3% wt. is relatively lower if compared to the other anodes, and
indicates that this sample has a good structural stability and durability for long-term operations. If we compare residual porosities and impedance data, it is possible to realize the different behaviors. The samples with 3% and 5% have the lowest porosity reduction, while 10% the highest one. These behaviors are confirmed by SEM investigation on the anodes after tests. As visible in Fig. 7, after 1000 h of operation, each sample show a degradation of the initial microstructure due to sintering, with a different pore structure. The ZrO2 10% wt. is characterized by a dense surface microstructure with very small pores of regular shape: even if the large amount of zirconia nanoparticles can improve mechanical properties, the resulting pore structure tends easily to clog and the impregnation of anode with the electrolyte is reduced, thus affecting severely the performance of MCFC [28]. This result is also in agreement with the contact angle measurements. For the ZrO2 5% wt. the behavior is similar but the residual porosity and the pore size are still sufficient for MCFCs. Finally, for the ZrO2 3% wt., an unavoidable degradation due to sintering is present, but a larger residual porosity is retained after cell operation. Anyway, a lower porosity reduction due to the presence of hard nanoparticles, which limit sintering, together with wettability, allow a better electrolyte distribution. On the other hand, a high amount of nanoparticles can reduce too much porosity and pore size, by clogging, thus creating bottlenecks which can affect electrolyte distribution. Post-analysis was also useful to detect another effect of zirconia content. In XRD spectra of Fig. 8 can be observed the crystalline phases formed at the anode and cathode side for cells with ZrO2 3% wt. and 10% wt. samples. Comparing these four spectra, some relevant differences are observed between anode and cathode side, especially for the ZrO2 10% wt. After cell operation, the anodes are essentially formed by nickel and zirconium dioxide, while cathode is nickel oxide impregnated with the electrolyte, i.e. potassium/lithium carbonate, as expected. However, after 1000 h, in the sample with 10% wt. of zirconia the peaks related to zirconium dioxide are weak and it is visible the formation of a new crystalline phases, i.e. dipotassium zirconate, K2ZrO3. The missing zirconia at anode is ascribed to the dissolution and migration of particles from anode to the cathode side, because the zirconia particles in excess in the anode can dissolve and react with the ions present in the electrolyte, thus altering the mass transfer and the electrochemical reactions inside the cell. The presence of this new phase is not detected in other samples with zirconia. This means that there is a limit to the zirconia that can be added to the nickel aluminum alloy.
Fig. 7. Microstructure of modified anode after single cell test a) ZrO2 3%; b) ZrO2 5%; c) ZrO2 10%.
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Fig. 8. XRD post analysis of electrodes a) ZrO2 3% anode; b) ZrO2 3% cathode; c) ZrO2 10%; anode; d) ZrO2 10% cathode.
4. Conclusions
Acknowledgement
In this work NiAl anode for MCFCs has been modified with nano ZrO2 to improve anode properties. Results show that zirconia effectively increases bending strength and reduces creep strain. The reason is that nanoparticles adhere strongly to metal particle surface and tend to reinforce grain boundaries against creep deformation. However, large addition of ZrO2 can reduce too much porosity and MPD for MCFC application. These reductions can limit some electrochemical phenomena related to charge and mass transfer. So, even if bending, creep and mechanical resistance increase, a tradeoff between resistance and ZrO2 content is required for long term operation. In fact, anodes with 3 and 5% wt. of zirconia have better cell performances compared to the reference cell with a standard NiAl anode. Above all, the ZrO2 3% cell has the highest output voltage at 150 mA cm2, i.e. 0.81 V, the lowest nitrogen crossover, equal to 0.43% after 1000 h, and the minimum contact angle between solid anode and liquid electrolyte, resulting in the best sample. In addition, the low internal resistance and the low mass transfer resistance, measured with EIS, seem to be the main reason of the good performance of the ZrO2 3% anode and the variation in the charge and mass transfer resistances is lower for this sample. This can be ascribed to the stable microstructure and improved mechanical properties. Post analysis of cathode used in the cell with the ZrO2 10% anode reveals that an undesired crystalline phase, i.e. dipotassium zirconate, can be formed within the electrolyte. This phenomenon consumes electrolyte and is another explanation of the low performance of this anode.
This work was supported by GRL e Global Research Laboratory Program through the National Research Foundation of Korea, funded by the Ministry of Science, ICT and Future Planning of Republic of Korea (Grant Number NRF-2009-00406). References [1] A. Kulkarni, S. Giddey, Materials issues and recent developments in molten carbonate fuel cells, J. Solid State Electrochem 16 (2012) 3123e3146, http:// dx.doi.org/10.1007/s10008-012-1771-y. [2] M. Kawase, Manufacturing method for tubular molten carbonate fuel cells and basic cell performance, J. Power Sources 285 (2015) 260e265, http:// dx.doi.org/10.1016/j.jpowsour.2015.03.117. [3] S.J. McPhail, A. Aarva, H. Devianto, R. Bove, A. Moreno, SOFC and MCFC: commonalities and opportunities for integrated research, Int. J. Hydrogen Energy 36 (2011) 10337e10345, http://dx.doi.org/10.1016/ j.ijhydene.2010.09.071. [4] A.H. Mamaghani, B. Najafi, A. Shirazi, F. Rinaldi, Exergetic, economic, and environmental evaluations and multi-objective optimization of a combined molten carbonate fuel cell-gas turbine system, Appl. Therm. Eng. 77 (2015) 1e11, http://dx.doi.org/10.1016/j.applthermaleng.2014.12.016. [5] S.-C. Jang, B.Y. Lee, S.W. Nam, H.C. Ham, J. Han, S.P. Yoon, S.-G. Oh, New method for low temperature fabrication of NieAl alloy powder for molten carbonate fuel cell applications, Int. J. Hydrogen Energy 39 (2014) 12259e12265, http://dx.doi.org/10.1016/j.ijhydene.2014.01.097. [6] H.V.P. Nguyen, M.R. Othman, D. Seo, S.P. Yoon, H.C. Ham, S.W. Nam, J. Han, J. Kim, Nano Ni layered anode for enhanced MCFC performance at reduced operating temperature, Int. J. Hydrogen Energy 39 (2014) 12285e12290, http://dx.doi.org/10.1016/j.ijhydene.2014.03.253. [7] H.V.P. Nguyen, S.A. Song, D. Seo, D.-N. Park, H.C. Ham, I.-H. Oh, S.P. Yoon, J. Han, S.W. Nam, J. Kim, Fabrication of NieAleCr alloy anode for molten carbonate fuel cells, Mater. Chem. Phys. 136 (2012) 910e916, http:// dx.doi.org/10.1016/j.matchemphys.2012.08.018.
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Influence of nano zirconia on NiAl anodes for molten carbonate fuel cell: Characterization, cell tests and post-analysis Grazia Accardo a, Domenico Frattini a, *, Angelo Moreno b, Sung Pil Yoon c, Jong Hee Han c, Suk Woo Nam c a b c
Department of Engineering, University of Naples Parthenope, Centro Direzionale Napoli Isola C4, 80143 Naples, Italy ENEA e Italian National Agency for New Technologies, Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy Fuel Cell Research Center, KIST - Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NiAl anodes with nano ZrO2 are tested in MCFC single cell after characterization. Results show that nanoparticles improve mechanical and creep properties. The anode with ZrO2 3% wt. has the best performance over 1000 h operations. Post analysis shows zirconates formation at cathode for ZrO2 contents over 5% wt. A high content of nanoparticles can diminish too much porosity and pore size.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2016 Accepted 7 November 2016 Available online xxx
Anode materials in Molten Carbonate Fuel Cells should have high creep resistance and good mechanical behavior to endure in high temperature-corrosive environments. In this work, zirconia nanoparticles (1 e10% wt.) are added to NiAl anodes in order to investigate their effects on mechanical properties and single cell performances. Results show that nanoparticles strongly adhere to metal particles and bending strength increases from 6.08 to 11.33 kgf cm2 while creep strain is reduced from 7.55% to 3.25%. In the case of the anode with ZrO2 3% wt., the stable and high output voltage of 0.81 V at 150 mA cm2 is a promising result, compared to the literature. In addition, the solid contact angles between melted electrolyte and anode, for the NiAl reference sample and the ZrO2 3% wt. are 37.6 and 17, respectively, showing the improved wettability of the modified anode. However, it seems to be a limit to the effective zirconia content as the contact angle of the anode with ZrO2 10% wt. is 58.1, which indicates a low wetting ability. When zirconia content is too high, single cells have low performances due to high internal resistance and porosity reduction. The formation of a zirconate phase also occurs during operations. © 2016 Elsevier B.V. All rights reserved.
Keywords: Anode Cell test Characterization Nanoparticle Post-analysis Wettability
1. Introduction * Corresponding author. E-mail address: [email protected] (D. Frattini).
Molten Carbonate Fuel Cells (MCFCs) are considered as a clean
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power generation technology and it is in the early stage of market diffusion. However, the actual lifetime of MCFCs is not long enough, namely 4 years of operation, and this short lifetime causes high maintenance costs and a limited diffusion. The major causes of short lifetime and performance decay are related to microstructural instability of electrodes, electrolyte losses, gas crossover, poisoning due to sulfur compounds, creep and micro-cracks formation [1e3]. The development of durable components, in particular the anode, is strategic to compete in the power generation market [4]. State-ofthe-art anodes for MCFC are represented by Ni alloys, based on Chromium, Aluminum or both, to obtain a sintered and porous material with high performances [5e7]. Chromium can react with the electrolyte, is susceptible to lithiation and consumes carbonates. Anyway, an anode of pure Ni has a creep strain of 40% after 100 h of operation, but the addition of Cr, up to 8% wt., has a significant effect on preventing the crack-growth phenomena and can cut by half creep strain [1,8,9]. Actually, porous NiAl alloy is the most diffused anode material as it has acceptable physical properties but presents some drawbacks similar to NiCr electrodes for long term MCFC operations [10,11]. Nguyen et al. [7] have reported that creep deformation in the Ni-Cr system is higher than that of a ternary Ni-Al-Cr system with various amount of Al. By adding 5% wt. of Al the same authors reduced the creep strain from 7% to 3%. To improve the durability of MCFC anode materials, some transition metals, as Nb, Ce, Gd, Mg, Cu, Zr, and their oxides can be used as additives in Ni alloys [12]. In particular, mechanical properties are enhanced thanks to crystal structure strengthening techniques, using alien atoms or hard oxide particles to block dislocation movements [13e15]. In their works, Baizeng et al. [12] and Fang et al. [16] have verified that NiNb alloys are chemically stable and insoluble in melted electrolyte due to the fact that niobium oxide cannot be reduced by CO and H2 in the anode atmosphere. However, limited mechanical tests have been reported for this type of anode material and preliminary cell tests, in which several parameters were not strictly controlled, showed that these electrodes exhibit poor performance (1.03e1.05 V for OCV) and could not operate above 100 mA cm2. Apart the mechanical resistance, an improvement of the electrochemical behavior of the anode is required [17e19]. Anodes with large surface area, high wettability, small thickness and stable structure are usually preferred because they can decrease charge and mass transport resistance. Nguyen et al. [20] for example, studied the effects of anode thickness on electrochemical performance and cell voltage stability. They found that the anodes with small pore size could effectively reduce the mass transfer resistance up to 0.35 mU cm2 with a nitrogen cross over of 0.5%. At the same time, under reaction conditions, the wetting behavior of the electrodes, e.g. contact angle, mainly at the anode side, has a high impact on cell performance and it is a function of anode porous structure and properties of the phases involved [21e23]. Generally, wetting angle in standard MCFC anodes is around 50 with (Li/ Na)2CO3 and 31 with (Li/K)2CO3 electrolytes [1] but, as reported by Yoon et al. [24], when the contact angle is lower than 20 , wettability is high and it is possible to improve long-term stability of the electrodes. Electrochemical reactions in MCFC are faster and sintering slower when the anode is more wetted by electrolyte. On the contrary, a contact angle larger than 50 or 90 is a consequence of poor and very poor electrode wettability, respectively [25]. In this work, differently from the current state-of-the-art, the use of the transition metal oxide ZrO2 is pretty new and it is proposed for the first time in anode fabrication. In fact, ZrO2 is used as a starting component of the anode and not as a coating material. Zirconia nanoparticles were added directly to NiAl alloy powders during fabrication. Anodes with different ZrO2 content (1e10% wt.)
were prepared by means of tape casting and the effects on physical and mechanical properties were studied. Cell performances of modified anodes have been investigated in long-term single cell tests and a post analysis on used components was carried out. Polarization and power curves, output voltage and impedance behavior have been compared to standard NiAl anodes. In addition, the contact angles under MCFC operating condition have been measured in order to find a correlation between nano zirconia addition, wettability and cell performances. 2. Experimental 2.1. Fabrication and characterization of anodes NiAl alloy powders (Chang Sung Co., Al 5.0% wt., 5 mm) and ZrO2 nanopowders (Alfa Caesar, 20 nm) were used as raw materials. Firstly, zirconia nanopowders, dispersant (BKY110, Altana Chemie), defoamer (SN-D348, San Nopco) and a mixture of Ethanol/Toluene (30/70% wt.) were homogeneously mixed in a ball miller for one day. After this first step, binder (Butvar B72, Monsanto Co.), plasticizer (Dibutyl Phthalate, Junsei Chemical Co.) and the NiAl powders were added and ball milled over two days to obtain a viscous slurry. In this step, a small amount of Ni powders (INCO 255, 5 mm) was used as sintering aid to promote metal particle aggregation. The amount was the same for each sample prepared. Secondly, a de-airing step under vacuum at 40 C was used to remove solvent and trapped air. After de-airing, the slurry was tape casted with a doctor blade on a glass support coated with a polyethylene film, and then dried in oven at 90 C for 3 h to obtain the green sheet. Finally, the green sheet was sintered in a furnace at 650 C for 2 h under a N2/H2 (90/10% vol.) atmosphere to eliminate the organic part. Anodes with four different nano zirconia compositions, ranging from 1% to 10% wt., were prepared and characterized. The composition of zirconia is intended as the weight of nanoparticles with respect to total weight. A zirconia-free reference sample, named Ni5Al, was also prepared as reference benchmark. The porosity and the Median Pore Diameter (MPD) were measured according to the Archimedes' principle (ASTM C373-88) on 3 specimens and by means of a Mercury Intrusion Porosimeter (Autopore IV, Micrometrics Co.), respectively. The crystalline structure was determined via XRD technique (Miniflex II, Rigaku Co.) in the 2q range 5 e80 and the morphology was examined with SEM technique (InspectF50, FEI Co.). Bending strength was determined according to the ASTM E855-90 standard by a 3-point bending strength testing machine, equipped with a 10 N load cell, (QC-508E, Techcome Ltd) on 5 rectangular specimens (5 2 0.06 cm3) to obtain a reliable average value. Creep tests on square specimens (1 1 0.06 cm3) were performed at 650 C and 100 psi for 100 h under hydrogen atmosphere in order to calculate the creep strain ratio by measuring the thickness before and after creep [7]. Finally, wettability measurements were carried out on square specimens (1 1 0.06 cm3) using an on-line Optical Contact Angle device (OCA, Dataphysics) with a quartz pipe in a tubular reactor, an adjustable metal support for alignment, a CCD camera with a 2 optical zoom and a contact angle software analyzer (SCA 20, Dataphysics). An equal amount of (Li0.7K0.3)2CO3 electrolyte powder was placed on each specimen and the reactor was heated at 650 C under nitrogen atmosphere to acquire the physical contact angle between electrolyte and electrode. 2.2. Long-term single cell tests and post analysis Single cell tests, up to 1000 h of operation, were carried out to compare the performance of modified anodes. Polarization and power curves, nitrogen crossover and output voltage at
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150 mA cm2 were recorded for each cell. Single cells, with an electrode area of 26 cm2, were fabricated using standard NiO cathode, gLiAlO2 matrix and (Li0.7K0.3)2CO3 electrolyte sheets (Twin Energy Co., South Korea). A sealing pressure of 0.2 MPa and an operating temperature of 650 C were applied for the experiments. The anode gas composition was H2:CO2:H2O ¼ 72:18:10 while the cathode gas composition was Air:CO2 ¼ 70:30. An electrical loader (ELS300Z, ELTO DC Electronics Co.) was used to impose the external current and to collect the cell voltage; the nitrogen crossover at the anode outlet was monitored by gas chromatography (789A, Agilent Tech.). In order to evaluate the contributions to electrodes polarization, Electrochemical Impedance Spectroscopy (EIS) was used to analyze the frequency response of cells (1287 Electrochemical Interface, Solartron Co.) within the frequency range 10 kHz e 0.01 Hz (FRA 1255B, Solartron Co.). After long-term tests, a post analysis was conducted in order to evaluate the degradation of modified anodes. Morphology was investigated with SEM technique (InspectF50, FEI Co.), crystal structure of anode and cathode was evaluated with XRD in the 2q range 5 e90 (Miniflex II, Rigaku Co.) and the change between initial and final porosity, after electrolyte removal, was evaluated according to the ASTM C373-88 standard on 3 samples. Residual carbonate in the anode was dissolved and removed by immersion in hot acetic acid for 8 h and then in boiling water for 5 h.
3. Results and discussion 3.1. Characterization of modified anodes Table 1 shows porosity and MPD of anodes modified with different amounts of ZrO2. The standard porosity of MCFC anodes is usually in the range 50e60%, while the pore size is around 3 mm [1]. These results suggest that fabricated anodes have an acceptable porosity for MCFC application but a slightly reduced pore size. This can be explained by considering that the porosity after sintering is the result of the organic part elimination and particle coalescence. Theoretically, porosities should be similar when the organic/inorganic proportion is the same or when alien atoms can diffuse to form solid solutions, as observed by Nguyen et al. [7]. The different physical properties observed between the Ni5Al reference sample and ZrO2 samples can be ascribed to the increasing amount of nanoparticles which tend to adhere on metal particles or to fill voids between them. XRD spectra for all samples are shown in Fig. 1. The spectra show a high crystalline structure in all cases. The patterns confirm the presence of ZrO2 together with two characteristic NiAl phases, namely Ni3Al and Ni2Al3, which are formed after sintering, in agreement with the literature [5,10]. The typical peaks of Ni, around 44.56 and 51.91 are also visible. The microstructure of anodes modified with nano zirconia can be observed in SEM micrographs of Fig. 2. The Ni5Al sample has a
Table 1 Physical properties of modified anodes. Samples
MPD (mm)
Porosity (%)
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
1.8 1.2 1.5 1.3 1.0
54.01 54.87 58.77 57.97 52.72
± ± ± ± ±
0.32 0.58 0.28 0.18 0.06
3
Fig. 1. XRD spectra of anodes with ZrO2 nanoparticles.
microstructure composed by spherical particles related to the NiAl powders and some irregular shaped particles ascribed to Ni powders and the formation of Ni2Al3 phase due to mechanical alloying and sintering, as confirmed by XRD spectra. The addition of zirconia is clearly visible in the micrographs due to the deposition of the nanoparticles on the surface of larger NiAl particles, promoted by the vigorous ball milling during fabrication. The porous structure obtained is similar in all samples, as the morphology presents many pores with a quite round shape. However, for the ZrO2 10% sample, zirconia covers largely the surface of metal particles, as showed in Fig. 2e. In this case, surface seems to be quite clogged by zirconia, and the initial porosity and MPD are therefore reduced as observed in Table 1. On the other hand, the sample ZrO2 3% has a good compromise between MPD and porosity as showed in Fig. 2c. At macroscale level, these results can be correlated to the mechanical behavior analyzed by creep and 3-point bending tests. In addition to creep ratio and bending strength, creep rate of secondary creep and modulus of elasticity (in bending) have been calculated from the slope of the curves. The mechanical properties of anodes are summarized in Table 2. In contrast with the standard Ni5Al anode, bending strength of zirconia anodes is considerably higher than that of the pure anode, as visible in Table 2. A lower steady-state creep rate indicates an improved resistance to creep deformation after primary creep. The stiffness of modified anodes is also higher than Ni5Al due to the increase in modulus of elasticity and therefore a higher stress resistance. The increase in the zirconia content denotes a characteristic mechanical improvement but at high zirconia content the increment of mechanical properties is marginal as ZrO2 5% and ZrO2 10% have similar properties. These values are encouraging compared with the literature, where the range of creep strain is 3e35% by changing the chromium and/or aluminum content in the anode composition up to 10% wt [7,10,26,27]. Finally, the wettability of the various anodes, in terms of contact angle, is showed in Fig. 3. The images show the angle established at the electrolyte/anode interface, derived from the intersection between a reference baseline, which represents the flat anode surface, and an elliptical line, which represents the profile of the carbonate melt. The sample ZrO2 10% wt. has a low wettability towards the carbonate melt due to the high contact angle compared to other
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Fig. 2. Microstructure of modified anodes: a) Ni5Al; b) ZrO2 1%; c) ZrO2 3%; d) ZrO2 5%; e) ZrO2 10%.
Table 2 Mechanical properties of modified anodes. Samples
Creep strain ratio (%)
Steady-state creep rate (s1)
Bending strength (kgf cm2)
Modulus of Elasticity (MPa)
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
7.55 5.15 4.42 3.51 3.25
2.00$108 1.81$108 1.72$108 1.75$108 1.78$108
6.08 ± 0.32 6.93 ± 0.57 8.58 ± 0.56 10.63 ± 0.56 11.33 ± 0.68
17.63 24.19 30.64 38.06 39.20
± ± ± ± ±
0.43 0.62 0.64 0.66 0.74
Fig. 3. Contact angle of a) Ni5Al; b) ZrO2 1%; c) ZrO2 3%; d) ZrO2 5%; e) ZrO2 10%.
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5
Fig. 5. Polarization and power curves after 1000 h in single cell tests. Fig. 4. Cell voltage over 1000 h in single cell test with modified anodes.
samples. The ZrO2 3% wt. anode established the best wettability, among the five samples. Nano zirconia addition in the range 1e5% wt. promoted the enhancement of both wettability and mechanical properties. This positive effect seems limited to 5% wt. In fact, wettability decreases for the sample with 10% of zirconia due to an unsuitable pore structure because this sample had the lowest porosity (52.72%) and the lowest MDP (1 mm). Therefore, the large mechanical properties of ZrO2 10% wt. are not compensated by an enhancement of physical and wetting properties for MCFC applications. 3.2. Single cell performance and post analysis Fig. 4 shows the output voltage vs time at constant current density value of 150 mA cm2 of single cells assembled with the modified anodes. An initial increase in cell voltages can be observed during the first 100 h of operation. This evidence is due to the stabilization of cell components, included the electrolyte distribution in the matrix, and the lithiation reaction at cathode. A very stable voltage is observed for each cell, especially for cells with the ZrO2 3% and 5% anodes, and voltage losses are calculated to be <8e10 mV during operation. The output voltage of cell with the ZrO2 10% anode is found to be very low during all the experiment, compared to the reference. For each cell, the nitrogen crossover during operation was monitored via GC at anode outlet and the values measured are resumed in Table 3. The results show an improved resistance to nitrogen leakage of ZrO2 modified anodes. The increase of crossover over time is around 0.14e0.15% compared to 0.30% of the reference cell with the standard anode. The lower crossover can be associated to the improved mechanical properties of modified anodes as the
Table 3 Nitrogen crossover during single cell tests. Sample
100 h
500 h
1000 h
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
0.35 0.38 0.29 0.33 0.32
0.40 0.50 0.35 0.40 0.32
0.65 0.53 0.43 0.47 0.45
Fig. 6. Impedance spectra for Ni5Al, ZrO2 3% and ZrO2 10% anode after 1000 h in single cell tests.
formation of micro cracks due to bending and creep is reduced [7,20]. The effect of zirconia is also visible from polarization curves after 1000 h of operation, as reported in Fig. 5. The OCV is the same for each cell, around 1.065 V. When the electrical load is applied to the cell, in order to obtain the desired current density, the cell voltage decreases but some differences are observed. The lowest values are achieved by the cell with ZrO2 10% anode and its performance falls at 150e200 mA cm2. For the other cells the polarization behavior is more stable, even at high current density. At low current densities, the low performances of ZrO2 10% wt. are ascribed to the high internal resistance as showed in Fig. 5, while at high current densities, the excess of zirconia on particle surface may limit the transfer of ions and of electrons due to concentration losses. All other experimental conditions being the same, the lower performance of ZrO2 10% anode could be ascribed to the insufficient electrochemical behavior of the cell as confirmed by the Nyquist plot of cells with the reference anode and the 3 and 10% nano ZrO2 wt. anode, reported in Fig. 6. The impedance curve of porous electrode of MCFC usually displays two semi-circles under a wide range of sweeping frequency.
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Table 4 Charge transfer resistance (Rct) and mass transfer resistance (Rmt) after 500 h and 1000 h. Sample
Ni5Al ZrO2 3% ZrO2 10%
500 h
1000 h
Rct (mU cm2)
Rmt (mU m2)
Rct (mU cm2)
Rmt (mU cm2)
0.711 0.896 0.594
0.819 0.623 0.947
0.576 0.837 0.556
0.957 0.676 1.073
Table 5 Porosity decrease after 1000 h. Sample
Initial porosity (%)
Final porosity (%)
Variation (%)
Ni5Al ZrO2 1% ZrO2 3% ZrO2 5% ZrO2 10%
54.01 54.87 58.77 57.97 52.72
48.33 49.01 55.80 53.40 44.66
10.52 10.68 5.05 7.89 15.28
The first semi-circle, at high frequencies, relates the kinetic of electrode reactions and the second one, at low frequencies, describes the diffusion phenomena inside the porous structure of electrodes. From Fig. 6 the increase of internal resistance for the sample with 10% wt. of zirconia is clearly visible, as well as the intensification of mass transfer resistance at low frequencies, which are responsible for losses inside MCFC. This is quantitatively confirmed for the anode with 10% wt. of zirconia by the deconvolution of previous impedance spectra into the charge transfer resistance, Rct, and the mass transfer resistance, Rmt, showed in Table 4. The 3% wt. zirconia sample has the lower internal resistance and a charge transfer resistance higher than the mass transfer resistance. This indicates that this composition increases the effective three phase boundary area at the anode side, as confirmed by literature [6,20]. In fact, the standard anode and the anode with 10% wt. of zirconia maintain a charge transfer lower than mass transfer resistance during time. However, at longer operation, for all samples investigated Rct decreases while Rmt rapidly increases, while for the ZrO2 3% this is almost unchanged. After single cell operations, the porosity reduction is summarized in Table 5. The high porosity reduction for the anode with 10% wt. of zirconia contributes to decrease the electrochemical activity and consequently cell performance. The change of porosity for the ZrO2 3% wt. is relatively lower if compared to the other anodes, and
indicates that this sample has a good structural stability and durability for long-term operations. If we compare residual porosities and impedance data, it is possible to realize the different behaviors. The samples with 3% and 5% have the lowest porosity reduction, while 10% the highest one. These behaviors are confirmed by SEM investigation on the anodes after tests. As visible in Fig. 7, after 1000 h of operation, each sample show a degradation of the initial microstructure due to sintering, with a different pore structure. The ZrO2 10% wt. is characterized by a dense surface microstructure with very small pores of regular shape: even if the large amount of zirconia nanoparticles can improve mechanical properties, the resulting pore structure tends easily to clog and the impregnation of anode with the electrolyte is reduced, thus affecting severely the performance of MCFC [28]. This result is also in agreement with the contact angle measurements. For the ZrO2 5% wt. the behavior is similar but the residual porosity and the pore size are still sufficient for MCFCs. Finally, for the ZrO2 3% wt., an unavoidable degradation due to sintering is present, but a larger residual porosity is retained after cell operation. Anyway, a lower porosity reduction due to the presence of hard nanoparticles, which limit sintering, together with wettability, allow a better electrolyte distribution. On the other hand, a high amount of nanoparticles can reduce too much porosity and pore size, by clogging, thus creating bottlenecks which can affect electrolyte distribution. Post-analysis was also useful to detect another effect of zirconia content. In XRD spectra of Fig. 8 can be observed the crystalline phases formed at the anode and cathode side for cells with ZrO2 3% wt. and 10% wt. samples. Comparing these four spectra, some relevant differences are observed between anode and cathode side, especially for the ZrO2 10% wt. After cell operation, the anodes are essentially formed by nickel and zirconium dioxide, while cathode is nickel oxide impregnated with the electrolyte, i.e. potassium/lithium carbonate, as expected. However, after 1000 h, in the sample with 10% wt. of zirconia the peaks related to zirconium dioxide are weak and it is visible the formation of a new crystalline phases, i.e. dipotassium zirconate, K2ZrO3. The missing zirconia at anode is ascribed to the dissolution and migration of particles from anode to the cathode side, because the zirconia particles in excess in the anode can dissolve and react with the ions present in the electrolyte, thus altering the mass transfer and the electrochemical reactions inside the cell. The presence of this new phase is not detected in other samples with zirconia. This means that there is a limit to the zirconia that can be added to the nickel aluminum alloy.
Fig. 7. Microstructure of modified anode after single cell test a) ZrO2 3%; b) ZrO2 5%; c) ZrO2 10%.
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Fig. 8. XRD post analysis of electrodes a) ZrO2 3% anode; b) ZrO2 3% cathode; c) ZrO2 10%; anode; d) ZrO2 10% cathode.
4. Conclusions
Acknowledgement
In this work NiAl anode for MCFCs has been modified with nano ZrO2 to improve anode properties. Results show that zirconia effectively increases bending strength and reduces creep strain. The reason is that nanoparticles adhere strongly to metal particle surface and tend to reinforce grain boundaries against creep deformation. However, large addition of ZrO2 can reduce too much porosity and MPD for MCFC application. These reductions can limit some electrochemical phenomena related to charge and mass transfer. So, even if bending, creep and mechanical resistance increase, a tradeoff between resistance and ZrO2 content is required for long term operation. In fact, anodes with 3 and 5% wt. of zirconia have better cell performances compared to the reference cell with a standard NiAl anode. Above all, the ZrO2 3% cell has the highest output voltage at 150 mA cm2, i.e. 0.81 V, the lowest nitrogen crossover, equal to 0.43% after 1000 h, and the minimum contact angle between solid anode and liquid electrolyte, resulting in the best sample. In addition, the low internal resistance and the low mass transfer resistance, measured with EIS, seem to be the main reason of the good performance of the ZrO2 3% anode and the variation in the charge and mass transfer resistances is lower for this sample. This can be ascribed to the stable microstructure and improved mechanical properties. Post analysis of cathode used in the cell with the ZrO2 10% anode reveals that an undesired crystalline phase, i.e. dipotassium zirconate, can be formed within the electrolyte. This phenomenon consumes electrolyte and is another explanation of the low performance of this anode.
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