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Effect of impurities on structural and electrochemical properties of the Ni–YSZ interface
Solid State Ionics 160 (2003) 27 – 37 www.elsevier.com/locate/ssi
Effect of impurities on structural and electrochemical properties of the Ni–YSZ interface Karin Vels Jensen a,b,*, Reine Wallenberg c, Ib Chorkendorff b, Mogens Mogensen a b
a Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark ICAT, Department of Physics and Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark c Materials Chemistry, Center for Chemistry and Chemical Engineering, Box 124, Lund University, S-22100, Lund, Sweden
Received 6 June 2002; received in revised form 22 October 2002; accepted 5 March 2003
Abstract The changes in interface structure and chemical composition of a 99.995% pure nickel/yttria-stabilised zirconia (YSZ) interface were examined after heat treatment at 1000 jC in 97% H2/3% H2O with and without polarisation. Impedance spectroscopy was used for electrochemical characterisation. The results were compared to earlier investigations of a less pure nickel/YSZ interface. The pure interface developed different structures depending on whether or not the samples were polarised. Despite the purity of the nickel, impurities were found in the interfacial region. The pure electrodes/interfaces showed area specific polarisation resistances 10 times lower than the impure interfaces. D 2003 Elsevier Science B.V. All rights reserved. Keywords: SOFC; Anode; Ni – YSZ interface; Foreign phases; Polarisation resistance
1. Introduction In present day solid oxide fuel cells (SOFC), a nickel-YSZ cermet is commonly used as the anode. For optimisation purposes, the understanding of interface phenomena between nickel and YSZ is important as the interface appears both between the electrolyte and the electrode and internally in the anode. The task of the cermet is to provide reaction sites for the oxidation of the fuel and to conduct electrons to the
* Corresponding author. Materials Research Department, Risø National Laboratory, PO-Box 49, DK-4000 Roskilde, Denmark. Tel.: +45-46-77-57-96; fax: +45-46-77-57-58. E-mail address: [email protected] (K.V. Jensen).
external circuit. It has been proposed that foreign phases present at SOFC interfaces are likely to influence the performance of the cells [1]. In a previous paper [2], the structural and chemical changes of interfaces between 8 mol% yttria stabilised zirconia and point electrodes of 99.8% pure nickel were presented. Changes in the interface morphology were found to be independent of whether or not current was passed through the interface. The structural changes manifested itself as the development of a hill and valley structure throughout the nickel/YSZ interface and an accumulation of impurities, both as particles in the interfacial region and as the so-called rim ridge at the contact area border (Fig. 1). In the present investigation, 99.995% nickel was used. Structural changes were also found to occur
0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(03)00147-4
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Fig. 1. SEM image of the characteristic structures that developed on samples with an impure nickel electrode (sample 51o) (from Ref. [2]). The sample was tested for 255 h in H2 with 3% H2O at 1000 jC.
when using the pure nickel as an electrode, although the changes were clearly dependent on current passage and polarisation direction. Impedance spectroscopy was used for both the samples with impure [2] and pure nickel electrodes to characterise the samples with respect to the series and polarisation resistance. In the discussion, a comparison of the influence of the amount of impurities on the structure development as well as on the electrochemical properties of the interface is presented.
2. Experimental An electropolished nickel wire (99.995%, 0.5 mm in diameter, Johnson Matthey) was used as the working electrode. The set-up is similar to earlier experiments [2]. A simplified drawing of the Ni wire –YSZ pellet configuration with electrodes is shown in Fig. 2. Table 1 shows the impurity content of both the nickel and the YSZ. After heating to 1000 jC in 9% H2/3% H2O, the atmosphere was changed to 97% H2/3% H2O. The samples for electrochemical measurements were stabilised at open circuit voltage (OCV) then polarised either anodically or cathodically (100 mV), and after that, they were left at OCV for some time.
Fig. 2. (A) The contact area between nickel wires and YSZ pellets. (B) The three-electrode set-up.
Impedance spectroscopy was performed with intervals of 0.5, 1 or 2 h. For instrumentation, please refer to Ref. [2]. The experiments lasted up to 2 weeks, where each sample was polarised 2 –3 days. Experiments were also performed with samples, which were not electrically connected. For comparison, a polished YSZ pellet was thermally annealed for 500 h in air at 1200 jC to accelerate any structures developed as a result of the heat treatment only. Table 2 lists all the samples and experimental conditions. Microstructures were examined with a scanning electron microscope (SEM: JEOL JSM-840 and JEOL JSM-840A), all SEM micrographs are secondary electron images. Chemical analyses were performed with energy dispersive X-ray spectroscopy (EDS: Noran). Table 1 The impurity content of the nickel wire and the YSZ pellets as given by Johnson Matthey (nickel) and Tosoh (YSZ) Nickel, 99.995%
YSZ
Element
ppm
Oxide
ppm
Al Ca Cu Fe Mg Ag Mo Si Pb
<1 <1 <1 <3 <1 <1 2 1 1
Al2O3 SiO2 Fe2O3 Na2O
< 50 f 50 f 50 800
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Table 2 The samples and experimental conditions Sample treatment
Sample code
Anodic polarisation, 100 mV – – – – Cathodic polarisation, 100 mV – – Not electrically connected – Thermally annealed
212a 213a 221a 222a 224a 241c 242c 243c 231n* 233n 151
* Sample has been analysed with TOF-SIMS.
Fig. 3. SEM image of the structures developed on non-polarised samples (233). The sample was tested for 288 h in H2 with 3% H2O at 1000 jC.
Time of flight secondary ion mass spectrometry (TOFSIMS IV, Ion-Tof) was later used to examine the chemical composition of the contact areas [3]. Atomic force microscopy (AFM: Digital Instruments, Dimension 3000) were used to obtain 3D images and measure the heights of the surface structures.
3. Results The nickel wires separated in most cases from the YSZ without major deformation of the interface, and so it was possible to identify contact areas on both the nickel and the YSZ. Single YSZ grains were ripped out of the polarised samples and only two samples (212a and 221a) showed a continuous fracture in the YSZ. The nickel wires showed contact areas with imprints of the structures developed in the YSZ contact areas. The contact areas were elliptical with an average aspect ratio of 2.45 F 0.4. The average size of the contact areas was 0.122 F 0.042 mm2. The contact areas were measured by approximation to an ellipse whereas the winding circumferences were measured by hand. Outside the contact area, thermal annealing of the YSZ created only minor changes in roughness of the grains and grooves at the grain boundaries. 3.1. Developed structures All samples showed a change of the structures in the interfacial region. Before the experiments, the interfaces consisted of two polished surfaces. Post-
Fig. 4. (A) 2D-AFM image of the contact area border with the rim ridge of a non-polarised sample (124n). Light areas are high and dark areas are low. The hill and valley structure and the rim ridge (arrow) are seen. The height of the rim ridge is 20 nm. The sample was tested for 170 h in H2 with 3% H2O at 1000 jC. A line profile along the white line is seen in B. The rim ridge (arrow) is 18 nm high.
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contact and each grain showed its own version of a hill and valley structure. In some grains, the structures were more developed than in other grains and different orientations of the hills were found in different grains. A thin rim ridge was locally present along the contact area boundary (Fig. 3). Impurity particles were
Fig. 5. SEM image of the complete contact area of sample 224a. Area a is seen in Fig. 6. Area h is seen in Fig. 8 (nickel side) and Fig. 10. Notice the winding contact area border. The sample was tested for 332 h in H2 with 3% H2O at 1000 jC. Fifty-three hours were at 100 mV anodic polarisation.
experimental analyses revealed three types of structures. 3.1.1. Samples subjected to OCV At high magnifications, the contact areas were easily distinguished from the surrounding YSZ. All grains and grain boundaries within the contact area were visible (Fig. 3). The surfaces of the YSZ grains were rough compared to YSZ grains outside the
Fig. 6. SEM image showing the two types of structures (A and B) in area a found on anodically polarised samples (224a). The sample was tested for 332 h in H2 with 3% H2O at 1000 jC. Fifty-three hours were at 100 mV anodic polarisation.
Fig. 7. Close-up images of A and B surfaces. (A) SEM image of the A surface. The grain boundaries seem very wide and bright Ni particles are seen on the grain surfaces. (B) The morphology of the B surface indicates a homogeneous structure consisting of nanosized particles. Nickel particles are also found on top of the layer. Both images are from sample 224a.
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present in the interfacial region (Fig. 3). The grain boundaries were characterised by grooves and shoulders. Atomic force microscopy showed a rim ridge up to 250 nm wide and 13 – 50 nm high (Fig. 4). The amplitude of the hill and valley structure was 30 – 40 nm. A close-up of the hill and valley structure shows a structure with pyramid-shaped hills.
Fig. 6 shows the characteristic structures, in the area a (Fig. 5), developed on anodically polarised samples. Relatively deep holes after the ripped-out YSZ grains can be seen. Two different kinds of morphologies can be distinguished (A and B). Fig. 7 shows a close-up of the two types of morphologies. In A areas, the underlying YSZ grains are visible and bright Ni particles up to 1 Am in diameter, positioned on top of the rough grain surfaces, are present all over the area. The B areas are characterised by a structure of nano-sized particles where YSZ grain boundaries are not visible (Fig. 7). The imprints in the nickel wire also reveal this layered structure (Fig. 8). The framed area (nickel contact) in Fig. 8 compares to the area h in Fig. 5 (YSZ contact). The C areas are believed to represent the bottom level of the interface layer and D areas probably represent the top of the layer. Fig. 9 shows a schematic drawing of the interface layer and A, B, C and D surfaces on both the YSZ and nickel. The AFM image of a contact area boundary in Fig. 10 shows areas of both A and B structures and YSZ (E areas) outside the contact area. The image shows topographic details on the morphology of the structures. The line profile of the surface (Fig. 10) shows that the thickness of the interface layer is 500– 700 nm but a thickness of up to 1 Am has been measured.
3.1.2. Anodically polarised samples A complete contact area with a winding circumference is shown in Fig. 5. The anodically polarised samples clearly show a different and topographically higher structure than the cathodically polarised and non-polarised samples. The contact area boundary is winding and well defined.
3.1.3. Cathodically polarised samples The structures found on cathodically polarised samples partly resembled the structures found at OCV with visible grain boundaries and a hill and valley structure but additionally areas with a granulated structure was found (Fig. 11). The topography of these structures is varying, from 130 nm below to 80
Fig. 8. SEM image of the nickel part (sample 224a) of the h-area in Fig. 5. The C and D surfaces correspond to A and B surfaces (from Fig. 6), respectively. In Fig. 10, an AFM image of the YSZ part of the framed area is seen.
Fig. 9. Schematic drawing of the geometry of the interface layer between the YSZ and nickel. The layer is formed on samples, which have been anodically polarised at 100 mV for 50 – 70 h in H2 with 3% H2O at 1000 jC.
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nm above the YSZ surface. The hill and valley structure areas are surrounded by a thin rim ridge which is 10 – 60 nm high. The nickel wire showed structures corresponding to both types of structure. The rim ridges were coincident with the circumference of the nickel contact areas. 3.1.4. Thermally annealed YSZ After 500 h at 1200 jC, the polished YSZ surface developed structures. Fig. 12 shows that some grains have a rougher surface than others. The line profile in Fig. 13 shows the variation in topography of different grains. The amplitude of the structures within each grain is 10 –50 nm. AFM examinations show that the surfaces of the rougher grains consist of pyramids. 3.2. Chemical composition of the interface
Fig. 10. 2D-AFM image of an anodically polarised sample (224a) (part of area h). A and B refers to the two structure types and E is outside the contact area. Light areas are high and dark areas are low. The profile along the white line shows the height relations between the A structure and the B structure. The arrows mark corresponding points. The height of the interface layer is 500 – 700 nm.
Fig. 11. SEM image of the structures on cathodically polarised samples (241c). Both the granulated structure and the hill and valley type structure with a rim ridge are seen. The sample was tested for 279 h in H2 with 3% H2O at 1000 jC. Sixty-five hours were at 100mV cathodic polarisation.
The rim ridge on non-polarised and cathodically polarised samples is generally too small to be analysed with EDS, since the excitation volume is in the order of a 1 Am3 in these materials at 15 kV. It was thus only possible to measure a trustworthy composition in a few places. Yttrium and zirconium from below the rim ridge account for f 95% of the calculated elemental composition. It was found that the rim ridge and impurity particles contained 30 –50 wt.% Si, 25– 35 wt.% Al, up to 20% Ti and 5 – 10% Na. Furthermore, the analysis indicated a minor fraction of K, Mn and Ca.
Fig. 12. SEM image of the YSZ surface structures after thermal annealing of sample (151). Different grains show different surface morphologies. The sample was annealed at 1200 jC for 500 h in air.
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Fig. 13. Line profile showing the topography of four grains with different surface morphologies on the thermally annealed sample (151). The arrows point at two of the grain boundaries.
TOF-SIMS analyses [3] of the contact area on the non-polarised sample 231n confirmed the presence of various elements in the contact area: Si, Ca, Ti, Mg, Mn, Al, Ni and Na. Ti and Mn are not accounted for in the chemical analyses of the YSZ and the nickel. Outside the contact area, Na, Si and Al are seen but also Ca and Mg are detected. A more thorough analysis of TOF-SIMS data combined with XPS measurements in contact areas on samples with pure and impure nickel electrodes and on the YSZ surfaces is in preparation [3]. The interface layer on anodically polarised samples contains on average 20 wt.% Ni, 10 wt.% Y and 70 wt.% Zr. Some of the measurements are shown in Table 3. EDS maps revealed no difference in the chemical composition between the A and B structure. The contact area boundaries in anodically polarised contact areas show no indications of their chemical composition being different from the rest of the contact area. The interface layer on the nickel wire contained in average 7 wt.% Y, 46 wt.% Zr and 46 wt.% Ni. EDS measurements of the nickel wires from cathodically polarised samples reveal up to 15 wt.% Zr in
areas corresponding to the granulated areas of the YSZ part of the contact. Only Y and Zr were detected on the YSZ pellet. 3.3. Impedance spectroscopy The conditions of the samples at OCV before polarisation are similar for all samples. The OCV data for pure electrodes mentioned in this paper are taken from the pre-polarisation period of the anodically polarised samples. Figs. 14 and 15 present the series resistance, Rs, which originates mainly from the electrolyte, for anodically and cathodically polarised samples, respectively. A correlation of Rs with the electrical contact area is possible [2,4,5]. Rs decreases with time and after 24– 48 h the Rs and the contact area reach a constant value.
Table 3 EDS measurements in wt.% of the interface layer on sample 221a tested for 332 h in H2 with 3% H2O at 1000 jC Layer on YSZ Y Zr Ni
7.9 63.7 28.4
9.8 70.1 20.2
11.8 74.8 13.3
Layer on nickel 10.0 70.8 19.3
10.5 71.9 17.6
9.1 72.2 18.7
46 h were at 100 mV anodic polarisation.
7.2 49.3 43.5
5.7 44.1 50.2
6.9 50.4 42.7
Fig. 14. Rs as a function of time for samples subjected to anodic polarisation of 100 mV. Zero time is when the temperature of the set-up reached 1000 jC.
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Fig. 15. Rs as a function of time for samples subjected to cathodic polarisation of 100 mV. Zero time is when the temperature of the set-up reached 1000 jC.
Table 4 shows contact area sizes estimated from SEM micrographs and calculated from electrical data including deviations for polarised and OCV samples. The contact areas in the SEM micrographs are approximated to an ellipse of which the area is calculated. A 10 –15% uncertainty is estimated, as the geometry of the areas is not always strictly elliptical. Assuming an uncertainty of the measured axes of the ellipse of each 10 Am, of Rs of 5 V and that rYSZ is constant at 0.15 S/cm, the resulting standard deviation of the electrical area is 0.015 mm2. Table 4 The measured (SEM) and electrical areas in mm2 Sample code
Area estimated in SEM (mm2)
Calculated electrical area (mm2)
212a 213a 221a 222a 224a 241c 242c 243c 212* 213* 221* 222* 224*
0.058 0.128 0.133 0.105 0.144 0.064 0.125 0.175 0.058 0.128 0.113 0.105 0.144
0.062 0.087 0.071 0.026 0.105 0.071 0.147 0.168 0.061 0.139 0.104 0.113 0.120
Deviation (%)
Fig. 16. Rp as a function of time for samples subjected to anodic polarisation (100 mV). Open and closed symbols indicate measurements during OCV and polarised conditions. Zero time is when the temperature of the set-up reached 1000 jC.
For all samples, the calculated electrical area is smaller than or very close to the measured area, indicating that practically the entire contact area was electrically in contact. The electrode polarisation resistance (Rp) at OCV conditions shows a decrease, often from well above 100 kV to a constant value below 50 kV during the first 24– 48 h (Figs. 16 and 17). Subsequent anodic polarisation causes the Rp to drop, usually below 1000 V (Fig. 16). Rp increases again upon the change back to OCV. For some samples, this constitutes only a few hundred ohms. For other samples, Rp increases approximately to the level prior to the polarisation. Cathodic polarisation also causes Rp to decrease but the drop is less than for anodic polarisation, typically to resistances around 5– 15 kV (Fig. 17). Contrary to
7 32 37 76 27 11 18 4 5 8 8 8 17
The last column shows the deviations of the electrical areas from the SEM areas. * The areas are calculated from Rs at OCV before samples were polarised.
Fig. 17. Rp as a function of time for samples subjected to cathodic polarisation (100 mV). Open and closed symbols indicate measurements during OCV and polarised conditions. Zero time is when the temperature of the set-up reached 1000 jC.
K.V. Jensen et al. / Solid State Ionics 160 (2003) 27–37 Table 5 The polarisation resistance (Rp), the area specific resistance (ASR) and the length specific resistance (LSR) for the relevant electrically connected samples Sample code
Rp (V)
ASR (V cm2)
LSR (V cm)
212* 213* 221* 222* 224* 212a 213a 221a 222a 224a 241c 242c 243c
17 000 16 000 30 000 29 000 24 000 6500 500 770 250 280 16 000 5600 9800 Rp at OCV after pol. 5400 1700 2200
10 21 34 30 35 3.8 0.64 0.87 0.26 0.40 10 7 17
1719 4925 4860 7189 6617 657 154 125 62 77 4344 2169 3528
3.4 2.1 3.9
1466 659 792
241c 242c 243c
* Marks the OCV values taken from the pre-polarisation period of anodically polarised samples.
anodically polarised samples, the polarisation resistance for cathodically polarised samples drops further upon the change back to OCV conditions. Table 5 shows the stable Rp values together with the area specific resistance (ASR), calculated as SEM contact area times the polarisation resistance. The anodically polarised samples clearly produce the smallest resistances. The last column in Table 5 shows the length specific resistance (LSR) with respect to the measured circumference. Again, the anodically polarised samples show the lowest values.
4. Discussion 4.1. Development of structures When two nickel wires of different purity is put in contact with a polished YSZ surface, very different results are seen. The contact areas on samples with impure electrodes show well-developed hill and valley structures with some crystallographic influence [2]. In contact areas on samples with pure electrodes, a much more distinct grain orientation dependence on
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the structure appearance is found. These structures, however, have amplitudes and heights, which are about five times smaller than the structures found in contact areas on samples with impure electrodes. The YSZ away from the contact area do not show any development of structures after 1 week of exposure to 1000 jC. By thermal annealing, a sample at a higher temperature and for a longer period, structures resembling the structures found on non-polarised samples with pure electrodes were developed. Thermodynamically unstable surfaces attempt to reduce their surface free energy by decomposing into a hill and valley structure. This is well described for alumina [6]. For metals (Mo), it has been shown [7] that ultrathin overlayers of some metals cause the surface to facet after annealing whereas other metals do not. The authors argue that some adsorbed species lower the surface formation energy and make it more energetically favourable to form facets. First principle calculations confirm their findings. The structures found on the thermally annealed YSZ sample are equilibrium structures developed by the YSZ to lower the surface free energy, and it is concluded that the impurities or some of the impurities significantly promote the further hill and valley structure development. The structure development in contact areas on samples with pure electrodes was current dependent. The structures on anodically polarised samples differ clearly from structures on non-polarised and cathodically polarised samples. For the contact areas on samples with impure electrodes current did not seem to affect the structure development. It is probable that the same current effects occur in the contact areas of polarised, impure samples but that the impurity phase interferes with the effect of current. Perhaps longer polarisation periods may change the appearance of the structures in the contacts between impure nickel and YSZ. The current dependent structure found on anodically polarised samples with pure electrodes shows that an interface layer exists at the Ni –YSZ interface causing the separation of Ni and YSZ to occur at different levels. Cathodically polarised samples with pure electrodes show two different types of structures. A difference in either current density or local pressure may be responsible for the different structures.
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Especially for anodically but also for cathodically polarised samples with pure electrodes, a different structure forms and some redistribution of Ni and/or YSZ take place. As this only happens during polarisation, it is reasonable to suggest that electromigration is the responsible mechanism. Electromigration is the process of mass transport driven by an externally applied electrical force and is commonly found in metal/semiconductor systems [8]. 4.2. Rim ridge and other impurities TOF-SIMS analyses [3] show that despite a low impurity content in the nickel and YSZ, the impurities readily migrate to the surface and interfaces. The segregation of Y, Na, Al and Si from the bulk of the YSZ to the surface is well known from the literature [9– 11]. A number of the detected impurities were not present in the YSZ or nickel. The Mn, Ti and K could be supplied from the laboratory environment or they could be present in the nickel in concentrations below the detection limit. Ca and Mg are constituents of the nickel but were found on the YSZ surface away from the contact area. This implies that impurities in the nickel may migrate a lot further than to the interface. The chemical composition of the rim ridge and the impurity particles found on samples with impure electrodes suggest that the impurities originate from the impurities in the nickel and the YSZ [2]. Cathodically polarised and non-polarised samples with pure electrodes also display a rim ridge and some impurity particles. Even though only a couple of EDS measurements were performed, it seems probable that the rim ridge consists of impurities from the nickel, mixed with impurities from the YSZ. Anodically polarised samples showed some mixing of Ni with the YSZ. During anodic polarisation, water is produced at the three-phase boundary zone. This could result in local oxygen partial pressures where Ni could be hydroxylated and transported through the gas phase as Ni(OH)2 into the YSZ interface layer. This is a mechanism already described by Aaberg et al. [12]. The missing YSZ in granulated areas on cathodically polarised samples is found on the nickel wire, indicating that in some areas, a stronger bonding between Ni and YSZ exists.
It is evident that impurities initially present in the nickel migrate to the interface and to the three-phase boundary. Larger concentrations of impurities result in large rim ridges (up to 1.6 Am) [2] whereas a purer nickel leaves only very small rim ridges (up to 50 nm), as seen especially on non-polarised samples. In the case of anodically polarised samples, no rim ridge was directly observed, but the dimensions of the interface layer compared to the height of the rim ridge prevent in any case an observation of the ridge. The distribution of impurities in the anodically polarised samples is not resolved yet. TOF-SIMS showed that in the case of non-polarised samples, some of the impurities covered the entire contact area. 4.3. Impedance spectroscopy The samples with pure nickel electrodes showed generally lower polarisation resistances than samples with impure electrodes. In Table 6, it is recognised that there is a factor of 10 between the best, impure and the best, pure electrode. Similarly, impure electrodes showed length specific polarisation resistances of 680 – 2450 V cm, and for pure electrodes, the values ranged between 62 and 657 V cm. The significant differences between the ASRs and LSRs, respectively, indicate that the impurities in the nickel wire influence the polarisation resistance. The polarisation resistances for cathodically polarised samples are inconclusive. The lowest Rps were found for samples with pure electrodes, but so was the highest Rp. An overlap exists for samples subjected to open circuit voltage, but again, the lowest Rp was found for a sample with a pure electrode. Comparison of the development of the polarisation resistance with time for impure and pure electrodes at Table 6 Comparison of area specific polarisation resistances for pure and impure nickel electrodes Treatment
ASR (V cm2) Impure
Pure
OCV Anodically polarised Cathodically polarised
24 – 135 3 – 13 12 – 14
10 – 35 0.26 – 3.8 7 – 17
The values for impure electrodes are from Ref. [2] or from later unpublished results.
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open circuit voltage shows two very different trends. As described in Ref. [2], impure electrodes showed an initial increase in the polarisation resistance and a subsequent stabilisation at a high value. The polarisation resistances for pure electrodes show an initial decrease and a subsequent stabilisation at a relatively low value. The major part of the decrease in Rp for pure samples is ascribed to the expansion of the contact area. For the impure electrodes, the increase in Rp with time is due to the distribution of the impurities in the interface and the rim ridge and possibly the formation of the hill and valley structure. The behaviour of Rp at polarisation may be related to electrically controlled changes in the coverage or distribution of impurities in the interfacial region. The fact that all the lowest polarisation resistances in the three groups were found for pure electrodes and the different in polarisation resistance trends both imply that impurities have an important influence on the polarisation resistance and thereby on the performance of the electrode/electrolyte interfaces. For the anodically polarised samples, it may further be speculated that the interface layer expands the three-phase boundary and thereby improves the performance. If the phenomena, which are taking place at the Ni –YSZ interface are transferred to cermet anodes and anode-electrolyte interfaces, it is possible that impurities, dependent on the initial purity of the materials, influence the performance of the anodes.
5. Conclusion The pure Ni – YSZ interface develops structures, which are dependent on whether or not current is passed through the interface. An impurity rim ridge developed along the contact area border for nonpolarised and cathodically polarised samples. Anodically polarised samples develop an interface layer. The rim ridge size depends on the amount of impur-
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ities in the nickel wire and so does the electrochemical performance. Polarisation resistances for the pure electrodes are an order of magnitude lower than for the impure electrodes.
Acknowledgements The PhD project of Karin Vels Jensen, which this work is a part of, is sponsored by the Nordic Energy Research Programme. The AFM and the TOF-SIMS measurements were performed in cooperation with the Danish Polymer Centre at Risø National Laboratory. The main part of the electron microscopy was performed at the Department of Materials Chemistry at Lund University.
References [1] M. Mogensen, S. Primdahl, K. Vels Jensen, M.J. Jørgensen, C. Bagger, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. of SOFCVII, June 3 – 8, Tsukuba, Japan, vol. 16, The Electrochemical Society, Pennington, NJ, 2001, pp. 521 – 528. [2] K. Vels Jensen, S. Primdahl, I. Chorkendorff, M. Mogensen, Solid State Ionics 144 (2001) 197 – 209. [3] K. Norrman, K. Vels Jensen, M. Mogensen, in preparation. [4] J. Newman, J. Electrochem. Soc. 113 (1966) 501 – 503. [5] R. Holm, Electric Contacts, Chapter 1, Springer-Verlag, Berlin, 1967. [6] J.R. Heffelfinger, C.B. Carter, Surf. Sci. 389 (1997) 188 – 200. [7] C.T. Chan, J.G. Che, T.C. Leung, Prog. Surf. Sci. 59 (1998) 1 – 11. [8] H. Yasunaga, A. Natori, Surf. Sci. Rep. 15 (1992) 205 – 280. [9] S.P.S. Badwal, A.E. Hughes, in: F. Grosz, P. Segers, S.C. Singhal, O. Yamamoto (Eds.), Proc. of the 2nd. International Symposium on Solid Oxide Fuel Cells, Athens, Greece, Commission of the European Communities, Luxembourg, EUR 13564 EN, 1991, pp. 445 – 454. [10] S.P.S. Badwal, J. Drennan, Sci. Ceram. Interfaces II (1994) 71 – 111. [11] A.E. Hughes, Sci. Ceram. Interfaces II (1994) 183 – 238. [12] R.J. Aaberg, R. Tunold, M. Mogensen, R.W. Berg, R. Ødega˚rd, J. Electrochem. Soc. 145 (1998) 2244 – 2252.
Effect of impurities on structural and electrochemical properties of the Ni–YSZ interface Karin Vels Jensen a,b,*, Reine Wallenberg c, Ib Chorkendorff b, Mogens Mogensen a b
a Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark ICAT, Department of Physics and Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark c Materials Chemistry, Center for Chemistry and Chemical Engineering, Box 124, Lund University, S-22100, Lund, Sweden
Received 6 June 2002; received in revised form 22 October 2002; accepted 5 March 2003
Abstract The changes in interface structure and chemical composition of a 99.995% pure nickel/yttria-stabilised zirconia (YSZ) interface were examined after heat treatment at 1000 jC in 97% H2/3% H2O with and without polarisation. Impedance spectroscopy was used for electrochemical characterisation. The results were compared to earlier investigations of a less pure nickel/YSZ interface. The pure interface developed different structures depending on whether or not the samples were polarised. Despite the purity of the nickel, impurities were found in the interfacial region. The pure electrodes/interfaces showed area specific polarisation resistances 10 times lower than the impure interfaces. D 2003 Elsevier Science B.V. All rights reserved. Keywords: SOFC; Anode; Ni – YSZ interface; Foreign phases; Polarisation resistance
1. Introduction In present day solid oxide fuel cells (SOFC), a nickel-YSZ cermet is commonly used as the anode. For optimisation purposes, the understanding of interface phenomena between nickel and YSZ is important as the interface appears both between the electrolyte and the electrode and internally in the anode. The task of the cermet is to provide reaction sites for the oxidation of the fuel and to conduct electrons to the
* Corresponding author. Materials Research Department, Risø National Laboratory, PO-Box 49, DK-4000 Roskilde, Denmark. Tel.: +45-46-77-57-96; fax: +45-46-77-57-58. E-mail address: [email protected] (K.V. Jensen).
external circuit. It has been proposed that foreign phases present at SOFC interfaces are likely to influence the performance of the cells [1]. In a previous paper [2], the structural and chemical changes of interfaces between 8 mol% yttria stabilised zirconia and point electrodes of 99.8% pure nickel were presented. Changes in the interface morphology were found to be independent of whether or not current was passed through the interface. The structural changes manifested itself as the development of a hill and valley structure throughout the nickel/YSZ interface and an accumulation of impurities, both as particles in the interfacial region and as the so-called rim ridge at the contact area border (Fig. 1). In the present investigation, 99.995% nickel was used. Structural changes were also found to occur
0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(03)00147-4
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Fig. 1. SEM image of the characteristic structures that developed on samples with an impure nickel electrode (sample 51o) (from Ref. [2]). The sample was tested for 255 h in H2 with 3% H2O at 1000 jC.
when using the pure nickel as an electrode, although the changes were clearly dependent on current passage and polarisation direction. Impedance spectroscopy was used for both the samples with impure [2] and pure nickel electrodes to characterise the samples with respect to the series and polarisation resistance. In the discussion, a comparison of the influence of the amount of impurities on the structure development as well as on the electrochemical properties of the interface is presented.
2. Experimental An electropolished nickel wire (99.995%, 0.5 mm in diameter, Johnson Matthey) was used as the working electrode. The set-up is similar to earlier experiments [2]. A simplified drawing of the Ni wire –YSZ pellet configuration with electrodes is shown in Fig. 2. Table 1 shows the impurity content of both the nickel and the YSZ. After heating to 1000 jC in 9% H2/3% H2O, the atmosphere was changed to 97% H2/3% H2O. The samples for electrochemical measurements were stabilised at open circuit voltage (OCV) then polarised either anodically or cathodically (100 mV), and after that, they were left at OCV for some time.
Fig. 2. (A) The contact area between nickel wires and YSZ pellets. (B) The three-electrode set-up.
Impedance spectroscopy was performed with intervals of 0.5, 1 or 2 h. For instrumentation, please refer to Ref. [2]. The experiments lasted up to 2 weeks, where each sample was polarised 2 –3 days. Experiments were also performed with samples, which were not electrically connected. For comparison, a polished YSZ pellet was thermally annealed for 500 h in air at 1200 jC to accelerate any structures developed as a result of the heat treatment only. Table 2 lists all the samples and experimental conditions. Microstructures were examined with a scanning electron microscope (SEM: JEOL JSM-840 and JEOL JSM-840A), all SEM micrographs are secondary electron images. Chemical analyses were performed with energy dispersive X-ray spectroscopy (EDS: Noran). Table 1 The impurity content of the nickel wire and the YSZ pellets as given by Johnson Matthey (nickel) and Tosoh (YSZ) Nickel, 99.995%
YSZ
Element
ppm
Oxide
ppm
Al Ca Cu Fe Mg Ag Mo Si Pb
<1 <1 <1 <3 <1 <1 2 1 1
Al2O3 SiO2 Fe2O3 Na2O
< 50 f 50 f 50 800
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Table 2 The samples and experimental conditions Sample treatment
Sample code
Anodic polarisation, 100 mV – – – – Cathodic polarisation, 100 mV – – Not electrically connected – Thermally annealed
212a 213a 221a 222a 224a 241c 242c 243c 231n* 233n 151
* Sample has been analysed with TOF-SIMS.
Fig. 3. SEM image of the structures developed on non-polarised samples (233). The sample was tested for 288 h in H2 with 3% H2O at 1000 jC.
Time of flight secondary ion mass spectrometry (TOFSIMS IV, Ion-Tof) was later used to examine the chemical composition of the contact areas [3]. Atomic force microscopy (AFM: Digital Instruments, Dimension 3000) were used to obtain 3D images and measure the heights of the surface structures.
3. Results The nickel wires separated in most cases from the YSZ without major deformation of the interface, and so it was possible to identify contact areas on both the nickel and the YSZ. Single YSZ grains were ripped out of the polarised samples and only two samples (212a and 221a) showed a continuous fracture in the YSZ. The nickel wires showed contact areas with imprints of the structures developed in the YSZ contact areas. The contact areas were elliptical with an average aspect ratio of 2.45 F 0.4. The average size of the contact areas was 0.122 F 0.042 mm2. The contact areas were measured by approximation to an ellipse whereas the winding circumferences were measured by hand. Outside the contact area, thermal annealing of the YSZ created only minor changes in roughness of the grains and grooves at the grain boundaries. 3.1. Developed structures All samples showed a change of the structures in the interfacial region. Before the experiments, the interfaces consisted of two polished surfaces. Post-
Fig. 4. (A) 2D-AFM image of the contact area border with the rim ridge of a non-polarised sample (124n). Light areas are high and dark areas are low. The hill and valley structure and the rim ridge (arrow) are seen. The height of the rim ridge is 20 nm. The sample was tested for 170 h in H2 with 3% H2O at 1000 jC. A line profile along the white line is seen in B. The rim ridge (arrow) is 18 nm high.
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contact and each grain showed its own version of a hill and valley structure. In some grains, the structures were more developed than in other grains and different orientations of the hills were found in different grains. A thin rim ridge was locally present along the contact area boundary (Fig. 3). Impurity particles were
Fig. 5. SEM image of the complete contact area of sample 224a. Area a is seen in Fig. 6. Area h is seen in Fig. 8 (nickel side) and Fig. 10. Notice the winding contact area border. The sample was tested for 332 h in H2 with 3% H2O at 1000 jC. Fifty-three hours were at 100 mV anodic polarisation.
experimental analyses revealed three types of structures. 3.1.1. Samples subjected to OCV At high magnifications, the contact areas were easily distinguished from the surrounding YSZ. All grains and grain boundaries within the contact area were visible (Fig. 3). The surfaces of the YSZ grains were rough compared to YSZ grains outside the
Fig. 6. SEM image showing the two types of structures (A and B) in area a found on anodically polarised samples (224a). The sample was tested for 332 h in H2 with 3% H2O at 1000 jC. Fifty-three hours were at 100 mV anodic polarisation.
Fig. 7. Close-up images of A and B surfaces. (A) SEM image of the A surface. The grain boundaries seem very wide and bright Ni particles are seen on the grain surfaces. (B) The morphology of the B surface indicates a homogeneous structure consisting of nanosized particles. Nickel particles are also found on top of the layer. Both images are from sample 224a.
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present in the interfacial region (Fig. 3). The grain boundaries were characterised by grooves and shoulders. Atomic force microscopy showed a rim ridge up to 250 nm wide and 13 – 50 nm high (Fig. 4). The amplitude of the hill and valley structure was 30 – 40 nm. A close-up of the hill and valley structure shows a structure with pyramid-shaped hills.
Fig. 6 shows the characteristic structures, in the area a (Fig. 5), developed on anodically polarised samples. Relatively deep holes after the ripped-out YSZ grains can be seen. Two different kinds of morphologies can be distinguished (A and B). Fig. 7 shows a close-up of the two types of morphologies. In A areas, the underlying YSZ grains are visible and bright Ni particles up to 1 Am in diameter, positioned on top of the rough grain surfaces, are present all over the area. The B areas are characterised by a structure of nano-sized particles where YSZ grain boundaries are not visible (Fig. 7). The imprints in the nickel wire also reveal this layered structure (Fig. 8). The framed area (nickel contact) in Fig. 8 compares to the area h in Fig. 5 (YSZ contact). The C areas are believed to represent the bottom level of the interface layer and D areas probably represent the top of the layer. Fig. 9 shows a schematic drawing of the interface layer and A, B, C and D surfaces on both the YSZ and nickel. The AFM image of a contact area boundary in Fig. 10 shows areas of both A and B structures and YSZ (E areas) outside the contact area. The image shows topographic details on the morphology of the structures. The line profile of the surface (Fig. 10) shows that the thickness of the interface layer is 500– 700 nm but a thickness of up to 1 Am has been measured.
3.1.2. Anodically polarised samples A complete contact area with a winding circumference is shown in Fig. 5. The anodically polarised samples clearly show a different and topographically higher structure than the cathodically polarised and non-polarised samples. The contact area boundary is winding and well defined.
3.1.3. Cathodically polarised samples The structures found on cathodically polarised samples partly resembled the structures found at OCV with visible grain boundaries and a hill and valley structure but additionally areas with a granulated structure was found (Fig. 11). The topography of these structures is varying, from 130 nm below to 80
Fig. 8. SEM image of the nickel part (sample 224a) of the h-area in Fig. 5. The C and D surfaces correspond to A and B surfaces (from Fig. 6), respectively. In Fig. 10, an AFM image of the YSZ part of the framed area is seen.
Fig. 9. Schematic drawing of the geometry of the interface layer between the YSZ and nickel. The layer is formed on samples, which have been anodically polarised at 100 mV for 50 – 70 h in H2 with 3% H2O at 1000 jC.
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nm above the YSZ surface. The hill and valley structure areas are surrounded by a thin rim ridge which is 10 – 60 nm high. The nickel wire showed structures corresponding to both types of structure. The rim ridges were coincident with the circumference of the nickel contact areas. 3.1.4. Thermally annealed YSZ After 500 h at 1200 jC, the polished YSZ surface developed structures. Fig. 12 shows that some grains have a rougher surface than others. The line profile in Fig. 13 shows the variation in topography of different grains. The amplitude of the structures within each grain is 10 –50 nm. AFM examinations show that the surfaces of the rougher grains consist of pyramids. 3.2. Chemical composition of the interface
Fig. 10. 2D-AFM image of an anodically polarised sample (224a) (part of area h). A and B refers to the two structure types and E is outside the contact area. Light areas are high and dark areas are low. The profile along the white line shows the height relations between the A structure and the B structure. The arrows mark corresponding points. The height of the interface layer is 500 – 700 nm.
Fig. 11. SEM image of the structures on cathodically polarised samples (241c). Both the granulated structure and the hill and valley type structure with a rim ridge are seen. The sample was tested for 279 h in H2 with 3% H2O at 1000 jC. Sixty-five hours were at 100mV cathodic polarisation.
The rim ridge on non-polarised and cathodically polarised samples is generally too small to be analysed with EDS, since the excitation volume is in the order of a 1 Am3 in these materials at 15 kV. It was thus only possible to measure a trustworthy composition in a few places. Yttrium and zirconium from below the rim ridge account for f 95% of the calculated elemental composition. It was found that the rim ridge and impurity particles contained 30 –50 wt.% Si, 25– 35 wt.% Al, up to 20% Ti and 5 – 10% Na. Furthermore, the analysis indicated a minor fraction of K, Mn and Ca.
Fig. 12. SEM image of the YSZ surface structures after thermal annealing of sample (151). Different grains show different surface morphologies. The sample was annealed at 1200 jC for 500 h in air.
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Fig. 13. Line profile showing the topography of four grains with different surface morphologies on the thermally annealed sample (151). The arrows point at two of the grain boundaries.
TOF-SIMS analyses [3] of the contact area on the non-polarised sample 231n confirmed the presence of various elements in the contact area: Si, Ca, Ti, Mg, Mn, Al, Ni and Na. Ti and Mn are not accounted for in the chemical analyses of the YSZ and the nickel. Outside the contact area, Na, Si and Al are seen but also Ca and Mg are detected. A more thorough analysis of TOF-SIMS data combined with XPS measurements in contact areas on samples with pure and impure nickel electrodes and on the YSZ surfaces is in preparation [3]. The interface layer on anodically polarised samples contains on average 20 wt.% Ni, 10 wt.% Y and 70 wt.% Zr. Some of the measurements are shown in Table 3. EDS maps revealed no difference in the chemical composition between the A and B structure. The contact area boundaries in anodically polarised contact areas show no indications of their chemical composition being different from the rest of the contact area. The interface layer on the nickel wire contained in average 7 wt.% Y, 46 wt.% Zr and 46 wt.% Ni. EDS measurements of the nickel wires from cathodically polarised samples reveal up to 15 wt.% Zr in
areas corresponding to the granulated areas of the YSZ part of the contact. Only Y and Zr were detected on the YSZ pellet. 3.3. Impedance spectroscopy The conditions of the samples at OCV before polarisation are similar for all samples. The OCV data for pure electrodes mentioned in this paper are taken from the pre-polarisation period of the anodically polarised samples. Figs. 14 and 15 present the series resistance, Rs, which originates mainly from the electrolyte, for anodically and cathodically polarised samples, respectively. A correlation of Rs with the electrical contact area is possible [2,4,5]. Rs decreases with time and after 24– 48 h the Rs and the contact area reach a constant value.
Table 3 EDS measurements in wt.% of the interface layer on sample 221a tested for 332 h in H2 with 3% H2O at 1000 jC Layer on YSZ Y Zr Ni
7.9 63.7 28.4
9.8 70.1 20.2
11.8 74.8 13.3
Layer on nickel 10.0 70.8 19.3
10.5 71.9 17.6
9.1 72.2 18.7
46 h were at 100 mV anodic polarisation.
7.2 49.3 43.5
5.7 44.1 50.2
6.9 50.4 42.7
Fig. 14. Rs as a function of time for samples subjected to anodic polarisation of 100 mV. Zero time is when the temperature of the set-up reached 1000 jC.
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Fig. 15. Rs as a function of time for samples subjected to cathodic polarisation of 100 mV. Zero time is when the temperature of the set-up reached 1000 jC.
Table 4 shows contact area sizes estimated from SEM micrographs and calculated from electrical data including deviations for polarised and OCV samples. The contact areas in the SEM micrographs are approximated to an ellipse of which the area is calculated. A 10 –15% uncertainty is estimated, as the geometry of the areas is not always strictly elliptical. Assuming an uncertainty of the measured axes of the ellipse of each 10 Am, of Rs of 5 V and that rYSZ is constant at 0.15 S/cm, the resulting standard deviation of the electrical area is 0.015 mm2. Table 4 The measured (SEM) and electrical areas in mm2 Sample code
Area estimated in SEM (mm2)
Calculated electrical area (mm2)
212a 213a 221a 222a 224a 241c 242c 243c 212* 213* 221* 222* 224*
0.058 0.128 0.133 0.105 0.144 0.064 0.125 0.175 0.058 0.128 0.113 0.105 0.144
0.062 0.087 0.071 0.026 0.105 0.071 0.147 0.168 0.061 0.139 0.104 0.113 0.120
Deviation (%)
Fig. 16. Rp as a function of time for samples subjected to anodic polarisation (100 mV). Open and closed symbols indicate measurements during OCV and polarised conditions. Zero time is when the temperature of the set-up reached 1000 jC.
For all samples, the calculated electrical area is smaller than or very close to the measured area, indicating that practically the entire contact area was electrically in contact. The electrode polarisation resistance (Rp) at OCV conditions shows a decrease, often from well above 100 kV to a constant value below 50 kV during the first 24– 48 h (Figs. 16 and 17). Subsequent anodic polarisation causes the Rp to drop, usually below 1000 V (Fig. 16). Rp increases again upon the change back to OCV. For some samples, this constitutes only a few hundred ohms. For other samples, Rp increases approximately to the level prior to the polarisation. Cathodic polarisation also causes Rp to decrease but the drop is less than for anodic polarisation, typically to resistances around 5– 15 kV (Fig. 17). Contrary to
7 32 37 76 27 11 18 4 5 8 8 8 17
The last column shows the deviations of the electrical areas from the SEM areas. * The areas are calculated from Rs at OCV before samples were polarised.
Fig. 17. Rp as a function of time for samples subjected to cathodic polarisation (100 mV). Open and closed symbols indicate measurements during OCV and polarised conditions. Zero time is when the temperature of the set-up reached 1000 jC.
K.V. Jensen et al. / Solid State Ionics 160 (2003) 27–37 Table 5 The polarisation resistance (Rp), the area specific resistance (ASR) and the length specific resistance (LSR) for the relevant electrically connected samples Sample code
Rp (V)
ASR (V cm2)
LSR (V cm)
212* 213* 221* 222* 224* 212a 213a 221a 222a 224a 241c 242c 243c
17 000 16 000 30 000 29 000 24 000 6500 500 770 250 280 16 000 5600 9800 Rp at OCV after pol. 5400 1700 2200
10 21 34 30 35 3.8 0.64 0.87 0.26 0.40 10 7 17
1719 4925 4860 7189 6617 657 154 125 62 77 4344 2169 3528
3.4 2.1 3.9
1466 659 792
241c 242c 243c
* Marks the OCV values taken from the pre-polarisation period of anodically polarised samples.
anodically polarised samples, the polarisation resistance for cathodically polarised samples drops further upon the change back to OCV conditions. Table 5 shows the stable Rp values together with the area specific resistance (ASR), calculated as SEM contact area times the polarisation resistance. The anodically polarised samples clearly produce the smallest resistances. The last column in Table 5 shows the length specific resistance (LSR) with respect to the measured circumference. Again, the anodically polarised samples show the lowest values.
4. Discussion 4.1. Development of structures When two nickel wires of different purity is put in contact with a polished YSZ surface, very different results are seen. The contact areas on samples with impure electrodes show well-developed hill and valley structures with some crystallographic influence [2]. In contact areas on samples with pure electrodes, a much more distinct grain orientation dependence on
35
the structure appearance is found. These structures, however, have amplitudes and heights, which are about five times smaller than the structures found in contact areas on samples with impure electrodes. The YSZ away from the contact area do not show any development of structures after 1 week of exposure to 1000 jC. By thermal annealing, a sample at a higher temperature and for a longer period, structures resembling the structures found on non-polarised samples with pure electrodes were developed. Thermodynamically unstable surfaces attempt to reduce their surface free energy by decomposing into a hill and valley structure. This is well described for alumina [6]. For metals (Mo), it has been shown [7] that ultrathin overlayers of some metals cause the surface to facet after annealing whereas other metals do not. The authors argue that some adsorbed species lower the surface formation energy and make it more energetically favourable to form facets. First principle calculations confirm their findings. The structures found on the thermally annealed YSZ sample are equilibrium structures developed by the YSZ to lower the surface free energy, and it is concluded that the impurities or some of the impurities significantly promote the further hill and valley structure development. The structure development in contact areas on samples with pure electrodes was current dependent. The structures on anodically polarised samples differ clearly from structures on non-polarised and cathodically polarised samples. For the contact areas on samples with impure electrodes current did not seem to affect the structure development. It is probable that the same current effects occur in the contact areas of polarised, impure samples but that the impurity phase interferes with the effect of current. Perhaps longer polarisation periods may change the appearance of the structures in the contacts between impure nickel and YSZ. The current dependent structure found on anodically polarised samples with pure electrodes shows that an interface layer exists at the Ni –YSZ interface causing the separation of Ni and YSZ to occur at different levels. Cathodically polarised samples with pure electrodes show two different types of structures. A difference in either current density or local pressure may be responsible for the different structures.
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Especially for anodically but also for cathodically polarised samples with pure electrodes, a different structure forms and some redistribution of Ni and/or YSZ take place. As this only happens during polarisation, it is reasonable to suggest that electromigration is the responsible mechanism. Electromigration is the process of mass transport driven by an externally applied electrical force and is commonly found in metal/semiconductor systems [8]. 4.2. Rim ridge and other impurities TOF-SIMS analyses [3] show that despite a low impurity content in the nickel and YSZ, the impurities readily migrate to the surface and interfaces. The segregation of Y, Na, Al and Si from the bulk of the YSZ to the surface is well known from the literature [9– 11]. A number of the detected impurities were not present in the YSZ or nickel. The Mn, Ti and K could be supplied from the laboratory environment or they could be present in the nickel in concentrations below the detection limit. Ca and Mg are constituents of the nickel but were found on the YSZ surface away from the contact area. This implies that impurities in the nickel may migrate a lot further than to the interface. The chemical composition of the rim ridge and the impurity particles found on samples with impure electrodes suggest that the impurities originate from the impurities in the nickel and the YSZ [2]. Cathodically polarised and non-polarised samples with pure electrodes also display a rim ridge and some impurity particles. Even though only a couple of EDS measurements were performed, it seems probable that the rim ridge consists of impurities from the nickel, mixed with impurities from the YSZ. Anodically polarised samples showed some mixing of Ni with the YSZ. During anodic polarisation, water is produced at the three-phase boundary zone. This could result in local oxygen partial pressures where Ni could be hydroxylated and transported through the gas phase as Ni(OH)2 into the YSZ interface layer. This is a mechanism already described by Aaberg et al. [12]. The missing YSZ in granulated areas on cathodically polarised samples is found on the nickel wire, indicating that in some areas, a stronger bonding between Ni and YSZ exists.
It is evident that impurities initially present in the nickel migrate to the interface and to the three-phase boundary. Larger concentrations of impurities result in large rim ridges (up to 1.6 Am) [2] whereas a purer nickel leaves only very small rim ridges (up to 50 nm), as seen especially on non-polarised samples. In the case of anodically polarised samples, no rim ridge was directly observed, but the dimensions of the interface layer compared to the height of the rim ridge prevent in any case an observation of the ridge. The distribution of impurities in the anodically polarised samples is not resolved yet. TOF-SIMS showed that in the case of non-polarised samples, some of the impurities covered the entire contact area. 4.3. Impedance spectroscopy The samples with pure nickel electrodes showed generally lower polarisation resistances than samples with impure electrodes. In Table 6, it is recognised that there is a factor of 10 between the best, impure and the best, pure electrode. Similarly, impure electrodes showed length specific polarisation resistances of 680 – 2450 V cm, and for pure electrodes, the values ranged between 62 and 657 V cm. The significant differences between the ASRs and LSRs, respectively, indicate that the impurities in the nickel wire influence the polarisation resistance. The polarisation resistances for cathodically polarised samples are inconclusive. The lowest Rps were found for samples with pure electrodes, but so was the highest Rp. An overlap exists for samples subjected to open circuit voltage, but again, the lowest Rp was found for a sample with a pure electrode. Comparison of the development of the polarisation resistance with time for impure and pure electrodes at Table 6 Comparison of area specific polarisation resistances for pure and impure nickel electrodes Treatment
ASR (V cm2) Impure
Pure
OCV Anodically polarised Cathodically polarised
24 – 135 3 – 13 12 – 14
10 – 35 0.26 – 3.8 7 – 17
The values for impure electrodes are from Ref. [2] or from later unpublished results.
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open circuit voltage shows two very different trends. As described in Ref. [2], impure electrodes showed an initial increase in the polarisation resistance and a subsequent stabilisation at a high value. The polarisation resistances for pure electrodes show an initial decrease and a subsequent stabilisation at a relatively low value. The major part of the decrease in Rp for pure samples is ascribed to the expansion of the contact area. For the impure electrodes, the increase in Rp with time is due to the distribution of the impurities in the interface and the rim ridge and possibly the formation of the hill and valley structure. The behaviour of Rp at polarisation may be related to electrically controlled changes in the coverage or distribution of impurities in the interfacial region. The fact that all the lowest polarisation resistances in the three groups were found for pure electrodes and the different in polarisation resistance trends both imply that impurities have an important influence on the polarisation resistance and thereby on the performance of the electrode/electrolyte interfaces. For the anodically polarised samples, it may further be speculated that the interface layer expands the three-phase boundary and thereby improves the performance. If the phenomena, which are taking place at the Ni –YSZ interface are transferred to cermet anodes and anode-electrolyte interfaces, it is possible that impurities, dependent on the initial purity of the materials, influence the performance of the anodes.
5. Conclusion The pure Ni – YSZ interface develops structures, which are dependent on whether or not current is passed through the interface. An impurity rim ridge developed along the contact area border for nonpolarised and cathodically polarised samples. Anodically polarised samples develop an interface layer. The rim ridge size depends on the amount of impur-
37
ities in the nickel wire and so does the electrochemical performance. Polarisation resistances for the pure electrodes are an order of magnitude lower than for the impure electrodes.
Acknowledgements The PhD project of Karin Vels Jensen, which this work is a part of, is sponsored by the Nordic Energy Research Programme. The AFM and the TOF-SIMS measurements were performed in cooperation with the Danish Polymer Centre at Risø National Laboratory. The main part of the electron microscopy was performed at the Department of Materials Chemistry at Lund University.
References [1] M. Mogensen, S. Primdahl, K. Vels Jensen, M.J. Jørgensen, C. Bagger, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. of SOFCVII, June 3 – 8, Tsukuba, Japan, vol. 16, The Electrochemical Society, Pennington, NJ, 2001, pp. 521 – 528. [2] K. Vels Jensen, S. Primdahl, I. Chorkendorff, M. Mogensen, Solid State Ionics 144 (2001) 197 – 209. [3] K. Norrman, K. Vels Jensen, M. Mogensen, in preparation. [4] J. Newman, J. Electrochem. Soc. 113 (1966) 501 – 503. [5] R. Holm, Electric Contacts, Chapter 1, Springer-Verlag, Berlin, 1967. [6] J.R. Heffelfinger, C.B. Carter, Surf. Sci. 389 (1997) 188 – 200. [7] C.T. Chan, J.G. Che, T.C. Leung, Prog. Surf. Sci. 59 (1998) 1 – 11. [8] H. Yasunaga, A. Natori, Surf. Sci. Rep. 15 (1992) 205 – 280. [9] S.P.S. Badwal, A.E. Hughes, in: F. Grosz, P. Segers, S.C. Singhal, O. Yamamoto (Eds.), Proc. of the 2nd. International Symposium on Solid Oxide Fuel Cells, Athens, Greece, Commission of the European Communities, Luxembourg, EUR 13564 EN, 1991, pp. 445 – 454. [10] S.P.S. Badwal, J. Drennan, Sci. Ceram. Interfaces II (1994) 71 – 111. [11] A.E. Hughes, Sci. Ceram. Interfaces II (1994) 183 – 238. [12] R.J. Aaberg, R. Tunold, M. Mogensen, R.W. Berg, R. Ødega˚rd, J. Electrochem. Soc. 145 (1998) 2244 – 2252.