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Effect of contact between electrode and current collector on the performance of solid oxide fuel cells
Solid State Ionics 160 (2003) 15 – 26 www.elsevier.com/locate/ssi
Effect of contact between electrode and current collector on the performance of solid oxide fuel cells S.P. Jiang a,*, J.G. Love b, L. Apateanu b a
Fuel Cells Strategic Research Program, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Ceramic Fuel Cells Limited, 170 Browns Road, Noble Park, Victoria 3174, Australia Received 4 September 2002; received in revised form 9 December 2002; accepted 4 February 2003
Abstract The effect of contact area between electrode and current collector (i.e., the interconnect) on the performance of anodesupported solid oxide fuel cells (SOFC) has been investigated using current collector with various contact area on the (Pr,Sr)MnO3 (PSM) cathode side. The cell resistance decreased significantly with the increase in the contact area between the PSM cathode and the current collector. When the contact area of the current collector increased from 4.6% to 27.2%, the cell resistance decreased from 1.43 to 0.19 V cm2 at 800 jC, a reduction of more than 80%. Furthermore, the polarization losses of the cell were also significantly reduced with the increase in the contact area of the current collector. The results indicate that there is close correlation between the contact area of the current collector and the cell performance. This shows that the constriction effect as frequently observed in solid electrolyte cells not only occurs at the electrode/electrolyte interface but also at the interface of the electrode/current collector. A hypothesis on the effect of the discrete contact between current collector and electrode on the current distribution in the electrolyte cell has been proposed. D 2003 Published by Elsevier Science B.V. Keywords: Solid oxide fuel cells; Performance; Current collector; Contact; Constriction effect
1. Introduction To develop solid oxide fuel cells (SOFC) with high power output and operating at reduced temperatures (600 – 800 jC), it is essential to reduce both the polarization and resistance losses of the cell. Low polarization losses can be achieved by using materials with high electrochemical activity for electrode reac-
* Corresponding author. Tel.: +65-6790-5010; fax: +65-67911859. E-mail address: [email protected] (S.P. Jiang).
tions at the cathode and anode sides and by optimizing the microstructure at the electrode and electrolyte interface region. For example, electrode materials with high ionic and electronic conductivity such as (La,Sr)(Co,Fe)O3 can significantly reduce the polarization losses for the O2 reduction reaction compared to the electrodes with predominant electronic conductivity such as Sr-doped LaMnO3 (LSM) [1– 3]. Polarization losses of Ni/Y2O3 – ZrO2 (Ni/YSZ) cermet anodes for the H2 oxidation reaction can be reduced through the optimization of the electrode fabrication process and the enhancement of electrode/electrolyte interface microstructure and the Ni and zirconia phase
0167-2738/03/$ - see front matter D 2003 Published by Elsevier Science B.V. doi:10.1016/S0167-2738(03)00127-9
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distribution by processes such as polarized electrochemical vapor deposition (PEVD) and ion impregnation techniques [4 – 7]. Use of various thin film techniques has also led to the substantial reduction in the electrolyte thickness and thus the decrease in the overall cell resistance [8,9]. However, in SOFC stacks where individual cells are connected through interconnect materials, the overall stack performance depends not only on the performance of individual cells but also on the contact between the electrode and the interconnect on both anode and cathode sides. Thus, to minimize overall stack resistance, one needs to have the cell structure, electrode and electrolyte materials with optimum performance and to have good electronic contact between the porous electrode coating and the current collector (i.e., the interconnect). In molten carbonate fuel cells, the contact resistance between the cathode and current collector contributes substantially to the overall cathode performance due to the corrosion of the current collector. In the case of Ni cathode, the contact resistance between the Ni cathode and stainless steel current collector was as high as 0.5 V cm2 and the contact resistance was even higher for LiCoO2 cathode due to the poor conductivity of LiCoO2 materials [10]. By using special voltage probes which are not physically connected to the current collector, the resistance associated with the contact between the electrode coating and the current collector can be separated from the total cell resistance and the results indicate that the contact resistance between the electrode and current collector can contribute significantly to the total cell resistance in SOFCs [11]. Sasaki et al. [12] showed that both cell resistance and overpotential losses of LSM cathodes increased with the spacing of the Pt mesh current collector embedded in the LSM electrode. Fo¨ger et al. [13] recently showed that by improving the contact between the electrode and the interconnect with additional contact layer and Ag mesh, the overall cell resistance decreased from 1.09 to 0.30 V cm2 for the cell at 750 jC, a reduction of more than 70%. The impedance behavior of Ni/YSZ cermet anodes can also be affected by the nature of the current collector used [14]. This indicates that the effect of the contact between the porous electrode coating and the current collector (i.e., the interconnect) on the performance of SOFC stacks can be substantial. In this paper, the effect of the contact
areas between Sr-doped PrMnO3 cathode and current collector on the overall cell performance has been investigated on anode-supported thin Y2O3 – ZrO2 electrolyte cells at 800 jC. The result demonstrated that there is close correlation between the cell resistance, polarization losses and the contact areas between the electrode and the current collector.
2. Experimental Anode-supported cells were prepared by tape-casting Ni/8 mol%Y2O3-doped ZrO2 (TZ8Y, Tosoh, Japan) (Ni/YSZ) anode layers of 1– 2 mm thick and YSZ electrolyte layers of 40– 100 Am thick, followed by lamination, rolling and sintering at 1400 jC [15]. The final thickness of the YSZ electrolyte was 25 F 4 Am and that of the Ni/YSZ anode substrates was 625 F 100 Am. Anode-supported cells were 50 50 mm in size. Pr0.80Sr0.20MnO3 (PSM) powder was prepared by co-precipitation and coarsened at 1000 jC in air for 4 h. PSM cathode was applied onto the electrolyte side by screenprinting and fired at 1150 jC in air. The PSM electrode coating thickness was f 95 Am. Ni/YSZ anode substrate area was 25 cm2 and PSM cathode area was 14.4 cm2. Under the conditions of the present study, the interfacial reactions between PSM and YSZ electrolyte and the formation of insulating praseodymium zirconate phase would be negligible [16]. Four current collectors with different contact areas were used on the cathode side. They were Pt waven mesh, Ag waven mesh, Ag foil#1 and Ag foil#2. Pt and Ag waven meshes had wire diameter of 180 and 110 Am, respectively and the contact to the electrode coating was made through the crossover points of the wire. Ag foil#1 and Ag foil#2 current collectors were made in-house. The contact points of Ag foils (100 – 120 Am thick) were made by punching and pressing the foil using a special homemade die. Fig. 1 shows the schematic diagram of designs of Ag foil#1 and Ag foil#2 current collector. The unit area and the open area were identical in both foils. The contact area of single contact point of Ag foil#1 was 0.2 mm2 while that of Ag foil#2 was 1 mm2. The compliance of the Ag foil current collector was provided by the louvers bent back built into the foil. Meulenberg et al. [17] showed that Ag contact pins and meshes are good and
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Fig. 1. Schematic diagram of made-in-house Ag foil#1 and Ag foil#2 current collector.
stable interconnect materials on the cathode side for SOFC operation temperatures up to 800 jC. In all cases, flattened fine Ni waven mesh was used as current collector on the anode side. The contact area between the flattened Ni waven mesh and the Ni/YSZ cermet substrate was estimated to be f 27%. The weight to the cell was kept as f 2 kg. In this study, the cathode area (14.4 cm2) was used for the calculation of the cell resistance and power density. The flatness of the anode-supported cells and the assembly conditions of the cell were carefully controlled to ensure the reproducibility and reliability of the testing results. Hydrogen gas (industrial grade, BOG) with 3% H2O introduced through a humidifier was used as fuel on the anode side and air (industrial grade, CIG) was used as oxidant on the cathode side. The anode side was sealed with a high-temperature glass gasket. The gas was distributed to the electrode coatings through channels of alumina blocks on both sides. The flow rate for both air and fuel was 1 l min 1. The electrochemical performance of the cells was measured by galvanostatic current interruption (GCI) [18]. GCI measurements were made under current-generating mode (i.e., the cell supplies current to the external load) at 800 jC. Current probe was made of hightemperature Ni alloy rod and additional Pt wires were used as voltage probes. As there was no reference electrode in the present study, all the electrochemical measurements were carried out on two-electrode systems. Cell resistance (R) and cell polarization losses (g) were directly measured from the current interruption curves. The variation in the separation of the overpotential component from the iR component was typically in the range of F 6 mV by the current
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interruption technique used (the sampling rate of the Keithley High Speed Voltmeter used was 10 AS). The maximum power output of the cell was measured at a cell voltage of 0.5 V. To assess the lateral conductivity of the porous electrode coating, a four-probe technique was adopted in order to be compatible with the fuel cell testing configuration. Fig. 2 shows the modified four-probe conductivity measurement arrangement. In this test, electrolyte-supported cells were used. Y2O3 – ZrO2 (TZ3Y, Tosoh) electrolyte plates (3 mol%) were prepared by tape-casting and fired at 1500 jC. TZ3Y electrolyte substrates were 50 50 mm in dimension and f 100 Am thick. Ni (50 vol.%)/ TZ3Y (50 vol.%) cermet electrode was prepared by screenprinting method onto TZ3Y electrolyte plate, followed by sintering at 1400 jC. The porosity of the coating prepared was in the range of 50– 60%, based on the coating thickness and the Ni and TZ3Y volume distribution. The dimension of the electrode coating was 34 34 mm and the thickness of the coating was around 50 Am. There was no cathode coating on the other side of the electrolyte. The hydrogen gas at the inlet and distributed across the cell were the same as in fuel cell test. On the reverse side, the plate was open to air with an alumina felt to evenly distribute the load. The load to the cell was 1 kg. The electrical contact for the current and voltage probes was made by Ni mesh along the 34 mm length of the Ni/zirconia cermet electrode coating and 3 mm in width. The average distance between the edges of the voltage probes was 4.7 mm. Current was passed through the two outside meshes (current probe) and the voltage was measured using the two inner meshes (voltage probes). Measurement was carried out in 97%H2/ 3%H2O at 800 jC. The calculation of the conductivity was made using the formula: I d r¼ ð1Þ V lt where t is the thickness of the coating, l the length of the Ni mesh, d the distance between the Ni meshes (i.e., 4.7 mm in the case of Ni/zirconia cermet anodes), I and V the applied current and measured voltage, respectively. The conductivity of porous PSM and La0.72Sr0.18 MnO3 (LSM) coatings was also measured using the
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Fig. 2. The four-probe conductivity and potential distribution measurement for porous electrode coating. (a) Arrangement of current and voltage probes, (b) the setup of the conductivity cell, and (c) setup of the potential distribution cell.
same four-probe measurement setup as described above. The cathode was screenprinted onto TZ3Y electrolyte and fired at 1150 jC. There was no Ni/ TZ3Y cermet anode coating on the other side of the electrolyte in the case of cathode conductivity measurement. Pt mesh was used as electrical contact with the same dimension as Ni meshes. Measurement was carried out at 800 jC in air. To study the effect of the contact of the current collector on the potential distribution at the electrode coating in the vicinity of the electrode/electrolyte interface, a special experiment was designed. The cell dimension and the setup were similar to that described for the conductivity measurement described above, as shown in Fig. 2c. Different to the half-cell used in the conductivity measurement, an electrolyte supported cell (50 50 mm) with Ni/TZ3Y cermet anode and LSM cathode was used. The dimension of the electrode coating was 34 34 mm and the thickness of the TZ3Y electrolyte was f 100 Am. Ni mesh strip (3 34 mm) was used on the anode side and Pt mesh strip (3 34 mm) on the cathode side, respectively. Ni and Pt mesh probes were arranged in symmetry and the average distance between the edges of the meshes (d) was 3.0 mm. Current was passed through the first pair of Ni and Pt mesh probes (current probe) and the
potential was measured at each point of the four voltage probes of Ni and Pt meshes. Measurement was carried out in 97%H2/3%H2O and air at 800 jC.
3. Results and discussion Prior to passing hydrogen gas with the as-fired Ni/ zirconia coating, the measured resistance between the two voltage probes was 10.6 kV. This dropped to near zero on immediate addition of hydrogen gas with 3% H2O. A leak test was made and very good bubbling was observed at very low gas flows, indicating good sealing. Fig. 3 shows the change of the conductivity of a Ni/zirconia cermet anode with the time measured at 800 jC. The initial conductivity value was 197 S cm 1 and reached a near steady state value of 155 S cm 1 after passing the hydrogen gas for about 3 h. The possible error in the estimation of the conductivity value seems to be the distance between the two voltage probes. The value used in the calculation (i.e., 4.7 mm as shown in Fig. 2) was the minimum distance between the two Ni voltage probe meshes. Assuming that the maximum distance is between the outer edge of the Ni voltage probe meshes, the maximum distance would be 10.7 mm. This would give the maximum conduc-
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Fig. 3. Plot of the conductivity of a porous Ni/zirconia cermet anode measured in 97%H2/3%H2O at 800 jC.
tivity value of f 353 S cm 1. On separate experiments of conventional four-probe conductivity measurement of tape-casted porous Ni/YSZ tapes with thickness of f 600 Am, the electric conductivity was in the range of 400 –600 S cm 1 at 800 jC. This value compares favourably with those reported on porous Ni/zirconia coating prepared by tape-casting with different zirconia powders [19]. Itoh et al. [20] reported the very wide range of conductivity value between 1 and 1000 S cm 1 of Ni/zirconia cermet anode coatings at 1000 jC, depending on the content of coarse zirconia in the cermet. It was shown by Huebner et al. [21] that mechanical milling improved the electrical conductivity from 100 to about 300 S cm 1 at 1000 jC for Ni/zirconia cermet prepared from commercial NiO powders. This indicates that the electrical conductivity of Ni/zirconia cermet is very much dependent on the distribution of the Ni and zirconia phase and the microstructure. On porous PSM electrode coating, the electrical conductivity measured using similar four-probe arrangement as shown in Fig. 2 was between 3 and 5 S cm 1, and for LSM, it was 34– 56 S cm 1 in air at 800 jC. This is considerably lower than the conductivity reported for the PSM and LSM materials. For example, for LSM materials the electrical conductivity is in the range of 175 –300 S cm 1 between 700 and 1000 jC [22] and for PSM with composition of Pr0.8Sr0.2MnO3, the conductivity of dense pellet was found to be 100 S cm 1 at 800 jC [23]. However, it has been shown that porosity has significant influence on the electrical conductivity of the LSM electrode. Otoshi et al. [24]
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studied the effect of porosity on the electrical conductivity of LSM coating at 800 jC and found that the electrical conductivity decreased from f 148 S cm 1 for 10% porosity to f 45 S cm 1 for 41% porosity. Lower value of conductivity of 21 S cm 1 was also reported for LSM electrode coating with 40% porosity at 1000 jC [25]. Similar effects of porosity on the conductivity would also be expected on porous PSM electrode coatings. This shows that porous electrode coatings have generally much lower electrical conductivity than the dense materials and this may have significant implication for the design of the current collector or interconnect in fuel cells. Fig. 4 shows the optical pictures of the Pt and Ag waven meshes used in the present study. For the purpose of simplicity, a square area of the wire diameter was taken as the contact area of individual contact points as shown in the figure. The area of an individual contact point of Pt and Ag mesh was thus
Fig. 4. Optical micrographs of Pt and Ag waven mesh current collectors.
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0.032 and 0.012 mm2, respectively. The contact is made through crossover points and the rest of the mesh area is designated as ‘‘dead’’ area used to simply carry current laterally out of the system. As shown by the difference in the contrast of the crossover points of the waven mesh, not all crossover points of the waven mesh were in the same level and thus not all crossover points were actually in contact with the electrode coating initially. Though it was difficult to measure exact contact areas of the waven mesh with the porous electrode, it has been observed that by examining the surface of the mesh wire after the fuel cell testing, contact points of the waven mesh could be accurately estimated. For Pt waven mesh used in the present study, the contact was primarily made through alternate crossover points as indicated by the roughed surface of majority of the alternate crossover points. This indicates that only about half of the crossover points were in contact with the PSM cathode during the fuel cell testing. In contrast, all the crossover points of Ag waven mesh showed the contact mark with the electrode. In some case, the whole Ag mesh wire had the sign of being contacted. The difference in the contact of Pt and Ag meshes may not be surprising if we consider their very different melting temperature and softening properties. For Ag, the melting temperature is 961.93 jC, significantly lower than 1772 jC of Pt [26]. Thus, at testing temperatures of 800 jC, the operating temperature to melting temperature ratio (T/ Tm) is 0.87 for Ag, much higher than 0.53 for Pt. This indicates that Ag would be much softer than Pt at testing temperature of 800 jC. This may explain the increased contact area of Ag waven mesh in comparison to that of Pt waven mesh as observed after the fuel cell testing. Therefore, for Ag waven mesh, the contacts were made through all crossover points under
the present testing conditions. Thus, based on the situation where the contacts are made mainly through alternate crossover points, the contact area of Pt and Ag mesh would be 4.62% and 6.74%, respectively. Taking into account the observation that contacts were made through all crossover points for Ag mesh, the actual contact area for Ag waven mesh would be 13.48%, significantly higher than that of Pt waven mesh. For Ag foil#1 and foil#2 current collectors, the original assigned overall contact areas were 5.43% and 27.2%, respectively. Similar to the Ag mesh, a certain degree of the increase of the contact area was also observed for the Ag foils. For Ag foil#1, the increase in the contact area after examining the contact surface of the foil was estimated to be between 1.5 and 2 times while the increase in the contact area for Ag foil#2 was very small probably due to the restriction of the limited surrounding areas. To simplify the calculation, a multiplying factor of 1.75 was used for the estimation of the actual contact area of Ag foil#1. This gave the contact area of f 9.5% for Ag foil#1 during the fuel cell testing. Table 1 lists the contact areas of the current collectors used for the 50 50 mm cells. Fig. 5 shows the performance curves of 50 50 mm anode-supported cells measured under 97%H2/ 3%H2O fuel and air at 800 jC with different current collectors on the PSM cathode side. The open circuit voltage was 1.082 –1.097 V and was stable during the test. In the cells tested, electrode and electrolyte materials were identical and the only significant difference was the current collector used on the cathode side. Therefore, the difference in the cell performance would be mainly related to the contact areas between the cathode and the current collector. This is supported by the observation of the significant
Table 1 Contact area of current collectors used on the cathode sides of the 50 50 mm anode-supported cells and cell performance at 800 jC Current collector
Unit area (mm2)
Area of each contact (mm2)
Contact area Designed (%)
Observed (%)
Pt mesh Ag foil#1 Ag mesh Ag foil#2
0.346 3.68 0.089 3.68
0.032 0.2 0.012 1.0
4.62 5.43 6.74 27.2
4.62 9.50a 13.48b 27.2
a b
Contact area was obtained by multiplying 1.75 (see text for explanation). Contact area was obtained by multiplying 2 (see text for explanation).
Pmax (mW cm 2)
RV (V cm 2)
g at 250 mA cm 2 (mV)
125 300 420 520
1.43 0.48 0.26 0.19
225 205 163 119
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
Fig. 5. Performance curves of anode-supported cells measured at 97%H2/3%H2O fuel and air and at 800 jC with different cathode current collectors.
increase in the cell performance (i.e., high cell voltage and thus high power density) with the contact area of the current collector. As shown in the figure, the best cell performance was obtained on the cell with Ag foil#2 current collector, which has the highest contact area (27.2%) among the current collectors used. For the cell using Ag foil#2 current collector, the overall cell resistance was 0.19 V cm2, much lower than 1.43 V cm2 for the similar cell but using Pt waven mesh current collector. The contact area of current collector not only affects the cell resistance but also the polarization losses. The overall cell overpotential losses (iR losses not included) decreased with the increase of the contact area of cathode current collector. At 250 mA cm 2 and 800 jC, cell overpotential (g) was 225 mV for the cell with Pt waven mesh and decreased to 119 mV for the cell with Ag foil#2 current collectors, a reduction in overpotential losses of almost 50%. Cell performance with different current collectors measured at 800 jC is also given in Table 1. Fig. 6 shows the cell resistance and maximum power density as a function of the contact area between the PSM cathode and current collector measured at 800 jC. The cell resistance was significantly affected by the contact area of the current collector. The higher the contact area between the cathode and the current collector, the lower the cell resistance. Based on the original designed contact area (i.e., contacts mainly through alternate crossover contact points and no change of the contact areas of the current collector at high temperatures), the increase
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in the cell resistance with the decrease of the contact area becomes very sharp as the contact areas approach 5% (see Fig. 6a). However, by taking into account the increased contact areas of Ag mesh and Ag foil#1 current collector under the present operation conditions, the dependence of the cell resistance on the contact areas became much more moderate (Fig. 6b). Such gradual relationships between the cell resistance and the contact area between current collector and cathode may be more realistic and would be similar to the geometrical effect of the discrete contacts on the current distribution and polarization resistance of the electrode [27]. On the other hand, it was found that replacing the flattened fine Ni waven with expanded fine Ni mesh had little on the overall cell resistance and cell performance despite the fact the expanded fine Ni mesh had contact area of 56%, significantly
Fig. 6. Plots of cell resistance and maximum power density of anode-supported cells measured at 97%H2/3%H2O fuel and air and at 800 jC as a function of (a) designed contact area and (b) observed contact area under fuel cell operation conditions of cathode current collector. See text for explanation.
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higher than 27% of the flattened fine Ni waven mesh. This may be due to the much higher electrical conductivity of porous Ni/YSZ cermet coating ( f 254 S cm 1) than f 4 S cm 1 of the PSM electrode coating. The actual dependence of the cell resistance on the contact area of the current collector is most likely dependent on the conductivity of the porous electrode coating as well. Fig. 7 shows the potential distribution as a function of the distance between the current (i) and voltage (V) probes at different currents measured with 97%H2/ 3%H2O on the anode side and air on the cathode side at 800 jC. It should be pointed out here that the potentials in Fig. 7 are the average values measured over 3 mm wide voltage probes. At zero current, the observed OCP was 1.100 V and was the same at all four positions of the voltage probes as expected. The OCP was close to the theoretical value of 1.098 V under the conditions studied. Under fuel cell operation conditions with currents passing through the current probe, the potential measured between Ni and Pt voltage probes varied significantly with the position of the voltage probes. At a current of 1.01 A, the potential at the electrode/electrolyte interface areas directly under the current probe or contact points of current collector (d = 0 mm) was 0.571 V and this corresponded to an overall local overpotential (the sum of polarization and ohmic overpotentials) of 0.529 V, i.e., g = OCP 0.571 = 0.529 V. As the distance between the voltage and current probe
Fig. 7. Potential distribution as a function of the distance between the current and voltage probes for a cell with Ni/YSZ cermet anode and LSM cathode, measured under different current at 97%H2/ 3%H2O fuel and air and at 800 jC.
increased to 3 mm, the potential increased to 0.996 V. This indicates that g at the electrode/electrolyte interface areas under the open area of the current probe (3 mm from the current probe) was 0.104 V, much smaller than 0.529 V measured directly under the current probe. Increasing the distance of the voltage probe to 6 mm increased the potential to 1.085 V and g became very small (0.015 V). This clearly demonstrated macroscopically that the polarization potential at the electrode region close to the electrode/electrolyte interface is not uniform and decreases with the increase of the distance to the current source, i.e., the contact point between the current collector and the electrode interface. This is consistent with the observed significant dependence of the cell performance on the contact areas of the current collector. It has been known in solid electrolyte cells that measured resistance is generally much higher than the calculated resistance of the solid electrolyte based on the thickness and resistivity of the electrolyte material [28,29]. As shown by Tannenberger and Siegert [28], silver electrode (5 Am thick) would only be active on discrete spots and this causes the loss of effective cross-section for the current flow through the electrolyte, leading to the higher measured resistance compared to the calculated resistance based on electrolyte thickness. Such constriction effect due to the discontinuous nature of the geometric effect of the discrete contact between electrode and electrolyte has also been observed on electrode materials with predominant electronic conductivity and negligible ionic conductivity such as Au, Pt and LSM [29,30]. As the electrochemical reactions occur primarily at regions where electrode, electrolyte and reactant gas meet (i.e., three phase boundary) [30 – 32], the loss of the electrolyte areas due to the discontinuous geometric contact at the electrode and electrolyte interface can also result in the significant loss of the electrode polarization performance as shown by van Berkel et al. [33] and in the increase of polarization resistance as shown by Kenjo and Kanehira [27] and Fleig and Maier [34]. In addition to the significance of the electrode/ electrolyte interface contact, the contact between the electrode and current collector also plays an important role in the cell resistance and performance, as shown in the present study. This is clearly demonstrated by the
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observed high cell overpotential and high cell resistance with cathode current collector with low contact area. For example, Pt waven mesh current collector has the lowest contact area and thus the cell with the Pt waven mesh as the current collector has highest cell resistance and cell overpotential (see Table 1 and Fig. 6). Lower contact area of the current collector will have fewer contact points between the current collector and electrode coating. This would lead to an uneven current distribution in the electrode coating and thus high cell overpotentials as the cell potential measured is an average value. The suggested uneven current distribution in the electrode coating due to the discrete contact between current collector and electrode coating is also supported by the observed significant dependence of the potential on the distance between the current and voltage probes in the specially designed experiment (Fig. 7). The overpotential (i.e., the difference between the OCP and the cell potential) measured between Ni and Pt voltage probes decreases significantly with the increase of the distance between the current and voltage probes with the current passing through the current probe. At a current of 1.01 A, the g at the electrode/electrolyte interface areas directly under the current probe (d = 0 mm) was 0.529 V. At d = 3 mm, g decreased to 0.104 V, implying that the current at the electrode/electrolyte interface areas under the open area of the current probe would be much smaller than that at the interface areas directly under the current probe (high g implies high current). Increasing the distance of the voltage probe to 6 mm, g was reduced to 0.015 V, indicating that current at the electrode/electrolyte interface region under the voltage probes is very small. At d = 9 mm, the measured potential is the same as that at open circuit, indicating that there is no current flow in this region of the electrode coating. The results clearly demonstrated that current distribution at the electrode/electrolyte interface region is not uniform and is strongly affected by the discrete contact between the current collector and the electrode coating. However, there are difficulties in the explanation of the observed phenomenon purely based on the magnitude and differences in the calculated crossplane and in-plane resistance of the electrode coatings. In the present study, the thickness of the PSM air electrode coating was generally in the range of f 95 Am, while the shortest distance from the middle
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of the open areas to the nearest alternate crossover points of the current collector was f 500 Am for Pt waven mesh (Fig. 4). The average conductivity of PSM electrode coating in the present study was f 4 S cm 1 at 800 jC (see conductivity measurement sections). Therefore, the cross-plane resistance for the current to travel to the electrode/electrolyte interface region would be 2.4 10 3 V cm2, while the inplane resistance for the current to reach the open area between the alternate crossover contact points of the current collector would be 1.3 10 2 V cm2. The cross-plane resistance for the current to travel to the electrode/electrolyte interface region is relatively smaller compared to the in-plane resistance for the current to reach the open area between the alternate crossover contact points of the current collector. Based on the average conductivity of f 45 S cm 1 measured for the LSM electrode at 800 jC, the electrode resistance across the electrode coating (coating thickness was 50 Am) would be 1.1 10 4 V cm2. The corresponding in-plane resistance for d = 3 mm and d = 6 mm would be 6.6 10 3 and 1.3 10 2 V cm2, respectively, similar in the magnitude as compared to the in-plane resistance of the PSM electrode coating. On the other hand, the overall polarization resistance for the H2 oxidation and O2 reduction reactions is in the range of 6– 9 V cm2 under similar testing conditions on almost identical Ni/YSZ anode and LSM cathode at 800 jC [35,36]. For TZ3Y electrolyte with f 100 Am thickness, the calculated electrolyte resistance is 0.56 V cm 2 according to the conductivity of 0.018 S cm 1 reported for TZ3Y materials at 800 jC [37]. This shows that calculated cross- and in-plane resistance values of the PSM and LSM electrode coatings are very small indeed as compared to the polarization resistance of the electrode reactions at the electrode/ electrolyte interface and the electrolyte resistance. Consequently, the current distribution should be uniform in the electrode coating and the effect of the contact between the current collector and the electrode on the cell performance should be negligible. However, this is not the case as shown clearly in the present study and is also contradictory with the observation reported [12,13]. The answer to the contradictory as outlined above may be related to the nature of the solid oxide fuel cell system. Fig. 8 shows schematically the current distri-
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Fig. 8. Schematic diagram of current distribution in (a) a pure electronic conducting (or semiconducting) medium, (b) a material with different conductivity, and (c) a solid electrolyte cell. The dotted and dashed lines indicate the current lines with weak strength as compared with the current lines with solid lines.
bution in a pure electronic (or semiconductor) conducting medium and a solid electrolyte cell. In a pure electronic or semiconducting medium such as porous LSM electrode, the conductivity of the medium is reasonable (e.g., 4 S cm 1 for PSM and 45 S cm 1 for LSM porous coatings at 800 jC, respectively) and the resistance of the medium in any location (R1) should not change with the current as would be expected. Thus the current distribution inside the medium would be uniform, shown by the solid lines in Fig. 8a. In the case of a material with different conductivity (e.g., YSZ materials sandwiched by electrode materials), the current distribution inside the medium would also be uniform as the resistance of the materials (R1 and R2) do not change with the current, assuming uniform contact and no interface resistance between these two materials (Fig. 8b). However, if there is a polarization resistance at the electrode/electrolyte interface and it changes with the current, the situation would be different. Fig. 8c shows a simplified SOFC system in which the electrode resistance (R1) and polarization resistance at the elec-
trode/electrolyte interface (R3) are assumed to be the same on both sides. Different to R1 and R2, it is well known that the polarization resistance (R3) for both H2 oxidation and O2 reduction reaction in SOFC is significantly dependent on the current and polarization potential [2,23,38 – 42]. For example, at 1000 jC, the polarization resistance (R3) was 4.4 V cm2 at open circuit and decreased to 3.1 V cm2 at overpotential of 50 mV for H2 oxidation on Ni anode and for H2 oxidation on Ni/YSZ anode R3 was reduced by more than 60% at g = 50 mV [39]. For O2 reduction on LSM electrode at 800 jC, the polarization resistance was 132 V at open circuit and reduced to 56 and 10 V under polarization potential of 100 and 300 mV, respectively [41]. Similar dependence of the polarization resistance on the polarization potential (or current) was also reported for the O2 reduction on Pr0.8Sr0.2Mn0.5Co0.5 electrode at 800 jC [23]. This indicates that polarization resistance (R3) for the electrode reaction on the Ni/YSZ cermet anode, PSM and LSM cathodes is strongly dependent on the current and polarization potential. The sensitivity of the polariza-
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
tion resistance on the current and polarization potential has also been reported for the electrode reaction on different electrode materials [2,43,44]. Consequently, under fuel cell operation conditions, the current directly under the contact areas of the current collector will be slightly higher than that under the open areas of the current collector as the crossplane resistance of the electrode coating is lower than the in-plane resistance. Due to the sensitivity of the polarization resistance on the current, the polarization resistance (R3) directly under the contact areas of the current collector would be reduced more as compared to that under the open areas of the current collector. As R3 is orders of magnitude higher than the resistance of the electrode coating (R1), the small change in R3 will have significant effect on the overall local cell resistance (2R1 + R2 + 2R3). This would lead to the considerable increase of the current and thus more reduction of R3 directly under the contact area of the current collector, compared to that under the open area of the current collector. However, further reduction of R3 would be limited by the kinetics of the reaction (e.g., the increase of the local polarization potential) and the increased dominance of the electrolyte resistance. An equilibrium will be reached and at this stage the polarization resistance, R3, would be much less sensitive to the change in current. Therefore, under fuel cell operation conditions, the local cell resistance directly under the contact areas of current collector (region a in Fig. 8c) would be noticeably smaller than that under the open areas of the current collector (region b in Fig. 8c). This results in the high current at the electrode/electrolyte interface region a as compared to that at the interface region b. 2R1 þ 2R3a þ R2 < 2R1 þ 2R3b þ R2
ð2Þ
ia > ib
ð3Þ
where R3a and R3b represent the polarization resistance under polarization at regions a and b (Fig. 8c) and R3a < R3b under fuel cell operation conditions. Therefore, the polarization potential at the electrode/ electrolyte interface region a would be higher than that at the interface region b. Lower contact area of the current collector will have fewer contact points between the current collector and electrode coating.
25
This will lead to lower uniformity of current distribution in the electrode coating, magnified by the constriction effect due to the discrete contact at the electrode/electrolyte interface and thus poorer performance. This explains the observation of high cell overpotential and high cell resistance with cathode current collector with low contact area (Table 1 and Fig. 6). The uneven current distribution in the porous electrode due to the discrete and inhomogeneous contacts between the current collector and electrode coating would also be affected by the electrical conductivity of porous electrodes, particularly in the case of electrode materials with low conductivity such as PSM. Therefore, it would be expected that the lower the contact area or density of contact points and the lower electric conductivity of the electrode coating, the poorer the current distribution would be and this would lead to higher cell resistance and higher cell polarization losses. The increased overpotential losses of the cell with the decrease in the contact area between the current collector and electrode clearly demonstrated that the constriction effect due to the uneven current distribution in the solid electrolyte cells is not only dependent on the discrete contact between electrode and electrolyte [27 – 30,34] but also on the discrete contact between electrode and current collector. However, the spread of the current lines inside the electrode area would be dependent on the relative magnitude of the polarization resistance in overall cell resistance and its sensitivity on the polarization current or potential. The hypothesis proposed in Fig. 8c indicates that the assumption of uniform potential distribution at the electrode area in the vicinity of the electrode/electrolyte interface [27] may not be valid in the case of discrete contact between the current collector and electrode coating and particularly in the case of electrode coatings with low electrical conductivity.
4. Conclusions The effect of contact area between electrode and current collector has been studied on anode-supported cells at 800 jC in 97%H2/3%H2O and air, using current collectors with different contact area on the PSM cathode side. The results clearly demonstrated
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that the contact area between electrode and current collector (i.e., the interconnect) affects both the cell resistance and polarization losses. Increase of the contact area between the PSM cathode and the current collector reduced both cell resistance and cell overpotential losses. This indicates that the constriction effect observed at the electrode/electrolyte interface due to the discrete contact also occurs at the interface of the electrode/current collector.
References [1] B.C.H. Steele, Solid State Ionics 134 (2000) 3. [2] S.P. Jiang, Solid State Ionics 146 (2002) 1. [3] A. Endo, S. Wada, C.-J. Wen, H. Komiyama, K. Yamada, J. Electrochem. Soc. 145 (1998) L35. [4] M. Brown, S. Primdahl, M. Mogensen, J. Electrochem. Soc. 147 (2000) 475. [5] S.P. Jiang, P.J. Callus, S.P.S. Badwal, Solid State Ionics 132 (2000) 1. [6] S.P. Jiang, Y.Y. Duan, J.G. Love, J. Electrochem. Soc. 149 (2002) A1175. [7] E.Z. Tang, T.H. Etsell, D.G. Ivey, J. Am. Ceram. Soc. 83 (2000) 1626. [8] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79. [9] P.K. Srivastava, T. Quach, Y.Y. Duan, R. Donelson, S.P. Jiang, F.T. Ciacchi, S.P.S. Badwal, Solid State Ionics 99 (1997) 311. [10] B. Bergman, C. Lagergren, G. Lindbergh, S. Schwartz, B. Zhu, J. Electrochem. Soc. 148 (2001) A38. [11] S.P. Jiang, J. Electrochem. Soc. 148 (2001) A887. [12] K. Sasaki, J.-P. Wurth, R. Gschwend, M. Go¨dickemeier, L.J. Gauckler, J. Electrochem. Soc. 143 (1996) 530. [13] K. Fo¨ger, R. Donelson, R. Ratnaraj, in: S.C. Singhal, M. Dokiya (Eds.), SOFC-VI, PV99-19, The Electrochemical Society, Pennington, NJ, 1999, p. 95. [14] M. Guillodo, P. Vernoux, J. Fouletier, Solid State Ionics 127 (2000) 99. [15] R. Donelson, S. Amarasinghe, D. Goble, D. Hickey, S.P. Jiang, J. Love, T. Quach, in: P. Stevens (Ed.), Proc. 3rd Euro. SOFC Forum, The European Fuel Cells Group, 1998, p. 151. [16] J.-P. Zhang, S.P. Jiang, J.G. Love, K. Foger, S.B.S. Badwal, J. Mater. Chem. 8 (1998) 2787. [17] W.A. Meulenberg, O. Teller, U. Flesch, H.P. Buchkremer, D. Sto¨ver, J. Mater. Sci. 36 (2001) 3189.
[18] S.P. Jiang, S.P.S. Badwal, J. Electrochem. Soc. 144 (1997) 3777. [19] D.W. Dees, T.D. Claar, T.E. Easler, D.C. Fee, F.C. Mrazek, J. Electrochem. Soc. 134 (1987) 2141. [20] H. Itoh, T. Yamamoto, M. Mori, T. Horita, N. Sakai, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 144 (1997) 641. [21] W. Huebner, H.U. Anderson, D.M. Reed, S.R. Sehlin, X. Deng, in: M. Dokiya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), SOFC-IV, The Electrochemical Society, Pennington, NJ, 1995, p. 696. [22] S.P.S. Badwal, K. Foger, Mater. Forum 21 (1997) 187. [23] R. Chiba, T. Ishii, in: M. Dokiya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), SOFC-IV, The Electrochemical Society, Pennington, NJ, 1995, p. 482. [24] S. Otoshi, H. Sasaki, H. Ohnishi, M. Hase, K. Ishimaru, M. Ippommatsu, T. Higuchi, M. Miyayama, H. Yanagida, J. Electrochem. Soc. 138 (1991) 1519. [25] F.P. Mattiot, G. Giunta, A. Selvaggi, in: U. Bossel (Ed.), Proceedings of 1st European Solid oxide Fuel Cell Forum, The European Fuel Cells Group, 1994, p. 735. [26] CRC Handbook of Chemistry and Physics, 67th ed., CRC Press, Florida, 1987. [27] T. Kenjo, Y. Kanehira, Solid State Ionics 148 (2002) 1. [28] H. Tannenberger, H. Siegert, Adv. Chem. 90 (1969) 281. [29] T. Kenjo, T. Nakagawa, J. Electrochem. Soc. 143 (1996) L92. [30] F.H. van Heuveln, H.J.M. Bouwmeester, F.P.F. van Berkel, J. Electrochem. Soc. 144 (1997) 126. [31] T. Horita, K. Yamaji, M. Ishikawa, N. Sakai, H. Yokokawa, T. Kawada, T. Kato, J. Electrochem. Soc. 145 (1998) 3196. [32] H. Fukunaga, M. Ihara, K. Sakaki, K. Yamada, Solid State Ionics 86 – 88 (1996) 1179. [33] F.P.F. van Berkel, F.H. van Heuveln, J.P.P. Huijsmans, Solid State Ionics 72 (1994) 240. [34] J. Fleig, J. Maier, J. Electrochem. Soc. 144 (1997) L302. [35] S.P. Jiang, Y. Ramprakash, Solid State Ionics 122 (1999) 211. [36] S.P. Jiang, J.G. Love, Y. Ramprakash, J. Power Sources 110 (2002) 201. [37] F.T. Ciacchi, K.M. Crane, S.P.S. Badwal, Solid State Ionics 73 (1994) 49. [38] S. Primdahl, M. Mogensen, J. Electrochem. Soc. 144 (1997) 3409. [39] S.P. Jiang, S.P.S. Badwal, Solid State Ionics 123 (1999) 209. [40] S.P. Jiang, J.G. Love, Solid State Ionics 138 (2001) 183. [41] B. Gharbage, T. Pagnier, A. Hammou, J. Electrochem. Soc. 141 (1994) 2118. [42] H.Y. Lee, W.S. Cho, S.M. Oh, H.-D. Wiemho¨fer, W. Go¨pel, J. Electrochem. Soc. 142 (1995) 2659. [43] H. Hu, M. Liu, Solid State Ionics 109 (1998) 259. [44] M. Liu, Z.L. Wu, Solid State Ionics 107 (1998) 105.
Effect of contact between electrode and current collector on the performance of solid oxide fuel cells S.P. Jiang a,*, J.G. Love b, L. Apateanu b a
Fuel Cells Strategic Research Program, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Ceramic Fuel Cells Limited, 170 Browns Road, Noble Park, Victoria 3174, Australia Received 4 September 2002; received in revised form 9 December 2002; accepted 4 February 2003
Abstract The effect of contact area between electrode and current collector (i.e., the interconnect) on the performance of anodesupported solid oxide fuel cells (SOFC) has been investigated using current collector with various contact area on the (Pr,Sr)MnO3 (PSM) cathode side. The cell resistance decreased significantly with the increase in the contact area between the PSM cathode and the current collector. When the contact area of the current collector increased from 4.6% to 27.2%, the cell resistance decreased from 1.43 to 0.19 V cm2 at 800 jC, a reduction of more than 80%. Furthermore, the polarization losses of the cell were also significantly reduced with the increase in the contact area of the current collector. The results indicate that there is close correlation between the contact area of the current collector and the cell performance. This shows that the constriction effect as frequently observed in solid electrolyte cells not only occurs at the electrode/electrolyte interface but also at the interface of the electrode/current collector. A hypothesis on the effect of the discrete contact between current collector and electrode on the current distribution in the electrolyte cell has been proposed. D 2003 Published by Elsevier Science B.V. Keywords: Solid oxide fuel cells; Performance; Current collector; Contact; Constriction effect
1. Introduction To develop solid oxide fuel cells (SOFC) with high power output and operating at reduced temperatures (600 – 800 jC), it is essential to reduce both the polarization and resistance losses of the cell. Low polarization losses can be achieved by using materials with high electrochemical activity for electrode reac-
* Corresponding author. Tel.: +65-6790-5010; fax: +65-67911859. E-mail address: [email protected] (S.P. Jiang).
tions at the cathode and anode sides and by optimizing the microstructure at the electrode and electrolyte interface region. For example, electrode materials with high ionic and electronic conductivity such as (La,Sr)(Co,Fe)O3 can significantly reduce the polarization losses for the O2 reduction reaction compared to the electrodes with predominant electronic conductivity such as Sr-doped LaMnO3 (LSM) [1– 3]. Polarization losses of Ni/Y2O3 – ZrO2 (Ni/YSZ) cermet anodes for the H2 oxidation reaction can be reduced through the optimization of the electrode fabrication process and the enhancement of electrode/electrolyte interface microstructure and the Ni and zirconia phase
0167-2738/03/$ - see front matter D 2003 Published by Elsevier Science B.V. doi:10.1016/S0167-2738(03)00127-9
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distribution by processes such as polarized electrochemical vapor deposition (PEVD) and ion impregnation techniques [4 – 7]. Use of various thin film techniques has also led to the substantial reduction in the electrolyte thickness and thus the decrease in the overall cell resistance [8,9]. However, in SOFC stacks where individual cells are connected through interconnect materials, the overall stack performance depends not only on the performance of individual cells but also on the contact between the electrode and the interconnect on both anode and cathode sides. Thus, to minimize overall stack resistance, one needs to have the cell structure, electrode and electrolyte materials with optimum performance and to have good electronic contact between the porous electrode coating and the current collector (i.e., the interconnect). In molten carbonate fuel cells, the contact resistance between the cathode and current collector contributes substantially to the overall cathode performance due to the corrosion of the current collector. In the case of Ni cathode, the contact resistance between the Ni cathode and stainless steel current collector was as high as 0.5 V cm2 and the contact resistance was even higher for LiCoO2 cathode due to the poor conductivity of LiCoO2 materials [10]. By using special voltage probes which are not physically connected to the current collector, the resistance associated with the contact between the electrode coating and the current collector can be separated from the total cell resistance and the results indicate that the contact resistance between the electrode and current collector can contribute significantly to the total cell resistance in SOFCs [11]. Sasaki et al. [12] showed that both cell resistance and overpotential losses of LSM cathodes increased with the spacing of the Pt mesh current collector embedded in the LSM electrode. Fo¨ger et al. [13] recently showed that by improving the contact between the electrode and the interconnect with additional contact layer and Ag mesh, the overall cell resistance decreased from 1.09 to 0.30 V cm2 for the cell at 750 jC, a reduction of more than 70%. The impedance behavior of Ni/YSZ cermet anodes can also be affected by the nature of the current collector used [14]. This indicates that the effect of the contact between the porous electrode coating and the current collector (i.e., the interconnect) on the performance of SOFC stacks can be substantial. In this paper, the effect of the contact
areas between Sr-doped PrMnO3 cathode and current collector on the overall cell performance has been investigated on anode-supported thin Y2O3 – ZrO2 electrolyte cells at 800 jC. The result demonstrated that there is close correlation between the cell resistance, polarization losses and the contact areas between the electrode and the current collector.
2. Experimental Anode-supported cells were prepared by tape-casting Ni/8 mol%Y2O3-doped ZrO2 (TZ8Y, Tosoh, Japan) (Ni/YSZ) anode layers of 1– 2 mm thick and YSZ electrolyte layers of 40– 100 Am thick, followed by lamination, rolling and sintering at 1400 jC [15]. The final thickness of the YSZ electrolyte was 25 F 4 Am and that of the Ni/YSZ anode substrates was 625 F 100 Am. Anode-supported cells were 50 50 mm in size. Pr0.80Sr0.20MnO3 (PSM) powder was prepared by co-precipitation and coarsened at 1000 jC in air for 4 h. PSM cathode was applied onto the electrolyte side by screenprinting and fired at 1150 jC in air. The PSM electrode coating thickness was f 95 Am. Ni/YSZ anode substrate area was 25 cm2 and PSM cathode area was 14.4 cm2. Under the conditions of the present study, the interfacial reactions between PSM and YSZ electrolyte and the formation of insulating praseodymium zirconate phase would be negligible [16]. Four current collectors with different contact areas were used on the cathode side. They were Pt waven mesh, Ag waven mesh, Ag foil#1 and Ag foil#2. Pt and Ag waven meshes had wire diameter of 180 and 110 Am, respectively and the contact to the electrode coating was made through the crossover points of the wire. Ag foil#1 and Ag foil#2 current collectors were made in-house. The contact points of Ag foils (100 – 120 Am thick) were made by punching and pressing the foil using a special homemade die. Fig. 1 shows the schematic diagram of designs of Ag foil#1 and Ag foil#2 current collector. The unit area and the open area were identical in both foils. The contact area of single contact point of Ag foil#1 was 0.2 mm2 while that of Ag foil#2 was 1 mm2. The compliance of the Ag foil current collector was provided by the louvers bent back built into the foil. Meulenberg et al. [17] showed that Ag contact pins and meshes are good and
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
Fig. 1. Schematic diagram of made-in-house Ag foil#1 and Ag foil#2 current collector.
stable interconnect materials on the cathode side for SOFC operation temperatures up to 800 jC. In all cases, flattened fine Ni waven mesh was used as current collector on the anode side. The contact area between the flattened Ni waven mesh and the Ni/YSZ cermet substrate was estimated to be f 27%. The weight to the cell was kept as f 2 kg. In this study, the cathode area (14.4 cm2) was used for the calculation of the cell resistance and power density. The flatness of the anode-supported cells and the assembly conditions of the cell were carefully controlled to ensure the reproducibility and reliability of the testing results. Hydrogen gas (industrial grade, BOG) with 3% H2O introduced through a humidifier was used as fuel on the anode side and air (industrial grade, CIG) was used as oxidant on the cathode side. The anode side was sealed with a high-temperature glass gasket. The gas was distributed to the electrode coatings through channels of alumina blocks on both sides. The flow rate for both air and fuel was 1 l min 1. The electrochemical performance of the cells was measured by galvanostatic current interruption (GCI) [18]. GCI measurements were made under current-generating mode (i.e., the cell supplies current to the external load) at 800 jC. Current probe was made of hightemperature Ni alloy rod and additional Pt wires were used as voltage probes. As there was no reference electrode in the present study, all the electrochemical measurements were carried out on two-electrode systems. Cell resistance (R) and cell polarization losses (g) were directly measured from the current interruption curves. The variation in the separation of the overpotential component from the iR component was typically in the range of F 6 mV by the current
17
interruption technique used (the sampling rate of the Keithley High Speed Voltmeter used was 10 AS). The maximum power output of the cell was measured at a cell voltage of 0.5 V. To assess the lateral conductivity of the porous electrode coating, a four-probe technique was adopted in order to be compatible with the fuel cell testing configuration. Fig. 2 shows the modified four-probe conductivity measurement arrangement. In this test, electrolyte-supported cells were used. Y2O3 – ZrO2 (TZ3Y, Tosoh) electrolyte plates (3 mol%) were prepared by tape-casting and fired at 1500 jC. TZ3Y electrolyte substrates were 50 50 mm in dimension and f 100 Am thick. Ni (50 vol.%)/ TZ3Y (50 vol.%) cermet electrode was prepared by screenprinting method onto TZ3Y electrolyte plate, followed by sintering at 1400 jC. The porosity of the coating prepared was in the range of 50– 60%, based on the coating thickness and the Ni and TZ3Y volume distribution. The dimension of the electrode coating was 34 34 mm and the thickness of the coating was around 50 Am. There was no cathode coating on the other side of the electrolyte. The hydrogen gas at the inlet and distributed across the cell were the same as in fuel cell test. On the reverse side, the plate was open to air with an alumina felt to evenly distribute the load. The load to the cell was 1 kg. The electrical contact for the current and voltage probes was made by Ni mesh along the 34 mm length of the Ni/zirconia cermet electrode coating and 3 mm in width. The average distance between the edges of the voltage probes was 4.7 mm. Current was passed through the two outside meshes (current probe) and the voltage was measured using the two inner meshes (voltage probes). Measurement was carried out in 97%H2/ 3%H2O at 800 jC. The calculation of the conductivity was made using the formula: I d r¼ ð1Þ V lt where t is the thickness of the coating, l the length of the Ni mesh, d the distance between the Ni meshes (i.e., 4.7 mm in the case of Ni/zirconia cermet anodes), I and V the applied current and measured voltage, respectively. The conductivity of porous PSM and La0.72Sr0.18 MnO3 (LSM) coatings was also measured using the
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Fig. 2. The four-probe conductivity and potential distribution measurement for porous electrode coating. (a) Arrangement of current and voltage probes, (b) the setup of the conductivity cell, and (c) setup of the potential distribution cell.
same four-probe measurement setup as described above. The cathode was screenprinted onto TZ3Y electrolyte and fired at 1150 jC. There was no Ni/ TZ3Y cermet anode coating on the other side of the electrolyte in the case of cathode conductivity measurement. Pt mesh was used as electrical contact with the same dimension as Ni meshes. Measurement was carried out at 800 jC in air. To study the effect of the contact of the current collector on the potential distribution at the electrode coating in the vicinity of the electrode/electrolyte interface, a special experiment was designed. The cell dimension and the setup were similar to that described for the conductivity measurement described above, as shown in Fig. 2c. Different to the half-cell used in the conductivity measurement, an electrolyte supported cell (50 50 mm) with Ni/TZ3Y cermet anode and LSM cathode was used. The dimension of the electrode coating was 34 34 mm and the thickness of the TZ3Y electrolyte was f 100 Am. Ni mesh strip (3 34 mm) was used on the anode side and Pt mesh strip (3 34 mm) on the cathode side, respectively. Ni and Pt mesh probes were arranged in symmetry and the average distance between the edges of the meshes (d) was 3.0 mm. Current was passed through the first pair of Ni and Pt mesh probes (current probe) and the
potential was measured at each point of the four voltage probes of Ni and Pt meshes. Measurement was carried out in 97%H2/3%H2O and air at 800 jC.
3. Results and discussion Prior to passing hydrogen gas with the as-fired Ni/ zirconia coating, the measured resistance between the two voltage probes was 10.6 kV. This dropped to near zero on immediate addition of hydrogen gas with 3% H2O. A leak test was made and very good bubbling was observed at very low gas flows, indicating good sealing. Fig. 3 shows the change of the conductivity of a Ni/zirconia cermet anode with the time measured at 800 jC. The initial conductivity value was 197 S cm 1 and reached a near steady state value of 155 S cm 1 after passing the hydrogen gas for about 3 h. The possible error in the estimation of the conductivity value seems to be the distance between the two voltage probes. The value used in the calculation (i.e., 4.7 mm as shown in Fig. 2) was the minimum distance between the two Ni voltage probe meshes. Assuming that the maximum distance is between the outer edge of the Ni voltage probe meshes, the maximum distance would be 10.7 mm. This would give the maximum conduc-
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
Fig. 3. Plot of the conductivity of a porous Ni/zirconia cermet anode measured in 97%H2/3%H2O at 800 jC.
tivity value of f 353 S cm 1. On separate experiments of conventional four-probe conductivity measurement of tape-casted porous Ni/YSZ tapes with thickness of f 600 Am, the electric conductivity was in the range of 400 –600 S cm 1 at 800 jC. This value compares favourably with those reported on porous Ni/zirconia coating prepared by tape-casting with different zirconia powders [19]. Itoh et al. [20] reported the very wide range of conductivity value between 1 and 1000 S cm 1 of Ni/zirconia cermet anode coatings at 1000 jC, depending on the content of coarse zirconia in the cermet. It was shown by Huebner et al. [21] that mechanical milling improved the electrical conductivity from 100 to about 300 S cm 1 at 1000 jC for Ni/zirconia cermet prepared from commercial NiO powders. This indicates that the electrical conductivity of Ni/zirconia cermet is very much dependent on the distribution of the Ni and zirconia phase and the microstructure. On porous PSM electrode coating, the electrical conductivity measured using similar four-probe arrangement as shown in Fig. 2 was between 3 and 5 S cm 1, and for LSM, it was 34– 56 S cm 1 in air at 800 jC. This is considerably lower than the conductivity reported for the PSM and LSM materials. For example, for LSM materials the electrical conductivity is in the range of 175 –300 S cm 1 between 700 and 1000 jC [22] and for PSM with composition of Pr0.8Sr0.2MnO3, the conductivity of dense pellet was found to be 100 S cm 1 at 800 jC [23]. However, it has been shown that porosity has significant influence on the electrical conductivity of the LSM electrode. Otoshi et al. [24]
19
studied the effect of porosity on the electrical conductivity of LSM coating at 800 jC and found that the electrical conductivity decreased from f 148 S cm 1 for 10% porosity to f 45 S cm 1 for 41% porosity. Lower value of conductivity of 21 S cm 1 was also reported for LSM electrode coating with 40% porosity at 1000 jC [25]. Similar effects of porosity on the conductivity would also be expected on porous PSM electrode coatings. This shows that porous electrode coatings have generally much lower electrical conductivity than the dense materials and this may have significant implication for the design of the current collector or interconnect in fuel cells. Fig. 4 shows the optical pictures of the Pt and Ag waven meshes used in the present study. For the purpose of simplicity, a square area of the wire diameter was taken as the contact area of individual contact points as shown in the figure. The area of an individual contact point of Pt and Ag mesh was thus
Fig. 4. Optical micrographs of Pt and Ag waven mesh current collectors.
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0.032 and 0.012 mm2, respectively. The contact is made through crossover points and the rest of the mesh area is designated as ‘‘dead’’ area used to simply carry current laterally out of the system. As shown by the difference in the contrast of the crossover points of the waven mesh, not all crossover points of the waven mesh were in the same level and thus not all crossover points were actually in contact with the electrode coating initially. Though it was difficult to measure exact contact areas of the waven mesh with the porous electrode, it has been observed that by examining the surface of the mesh wire after the fuel cell testing, contact points of the waven mesh could be accurately estimated. For Pt waven mesh used in the present study, the contact was primarily made through alternate crossover points as indicated by the roughed surface of majority of the alternate crossover points. This indicates that only about half of the crossover points were in contact with the PSM cathode during the fuel cell testing. In contrast, all the crossover points of Ag waven mesh showed the contact mark with the electrode. In some case, the whole Ag mesh wire had the sign of being contacted. The difference in the contact of Pt and Ag meshes may not be surprising if we consider their very different melting temperature and softening properties. For Ag, the melting temperature is 961.93 jC, significantly lower than 1772 jC of Pt [26]. Thus, at testing temperatures of 800 jC, the operating temperature to melting temperature ratio (T/ Tm) is 0.87 for Ag, much higher than 0.53 for Pt. This indicates that Ag would be much softer than Pt at testing temperature of 800 jC. This may explain the increased contact area of Ag waven mesh in comparison to that of Pt waven mesh as observed after the fuel cell testing. Therefore, for Ag waven mesh, the contacts were made through all crossover points under
the present testing conditions. Thus, based on the situation where the contacts are made mainly through alternate crossover points, the contact area of Pt and Ag mesh would be 4.62% and 6.74%, respectively. Taking into account the observation that contacts were made through all crossover points for Ag mesh, the actual contact area for Ag waven mesh would be 13.48%, significantly higher than that of Pt waven mesh. For Ag foil#1 and foil#2 current collectors, the original assigned overall contact areas were 5.43% and 27.2%, respectively. Similar to the Ag mesh, a certain degree of the increase of the contact area was also observed for the Ag foils. For Ag foil#1, the increase in the contact area after examining the contact surface of the foil was estimated to be between 1.5 and 2 times while the increase in the contact area for Ag foil#2 was very small probably due to the restriction of the limited surrounding areas. To simplify the calculation, a multiplying factor of 1.75 was used for the estimation of the actual contact area of Ag foil#1. This gave the contact area of f 9.5% for Ag foil#1 during the fuel cell testing. Table 1 lists the contact areas of the current collectors used for the 50 50 mm cells. Fig. 5 shows the performance curves of 50 50 mm anode-supported cells measured under 97%H2/ 3%H2O fuel and air at 800 jC with different current collectors on the PSM cathode side. The open circuit voltage was 1.082 –1.097 V and was stable during the test. In the cells tested, electrode and electrolyte materials were identical and the only significant difference was the current collector used on the cathode side. Therefore, the difference in the cell performance would be mainly related to the contact areas between the cathode and the current collector. This is supported by the observation of the significant
Table 1 Contact area of current collectors used on the cathode sides of the 50 50 mm anode-supported cells and cell performance at 800 jC Current collector
Unit area (mm2)
Area of each contact (mm2)
Contact area Designed (%)
Observed (%)
Pt mesh Ag foil#1 Ag mesh Ag foil#2
0.346 3.68 0.089 3.68
0.032 0.2 0.012 1.0
4.62 5.43 6.74 27.2
4.62 9.50a 13.48b 27.2
a b
Contact area was obtained by multiplying 1.75 (see text for explanation). Contact area was obtained by multiplying 2 (see text for explanation).
Pmax (mW cm 2)
RV (V cm 2)
g at 250 mA cm 2 (mV)
125 300 420 520
1.43 0.48 0.26 0.19
225 205 163 119
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
Fig. 5. Performance curves of anode-supported cells measured at 97%H2/3%H2O fuel and air and at 800 jC with different cathode current collectors.
increase in the cell performance (i.e., high cell voltage and thus high power density) with the contact area of the current collector. As shown in the figure, the best cell performance was obtained on the cell with Ag foil#2 current collector, which has the highest contact area (27.2%) among the current collectors used. For the cell using Ag foil#2 current collector, the overall cell resistance was 0.19 V cm2, much lower than 1.43 V cm2 for the similar cell but using Pt waven mesh current collector. The contact area of current collector not only affects the cell resistance but also the polarization losses. The overall cell overpotential losses (iR losses not included) decreased with the increase of the contact area of cathode current collector. At 250 mA cm 2 and 800 jC, cell overpotential (g) was 225 mV for the cell with Pt waven mesh and decreased to 119 mV for the cell with Ag foil#2 current collectors, a reduction in overpotential losses of almost 50%. Cell performance with different current collectors measured at 800 jC is also given in Table 1. Fig. 6 shows the cell resistance and maximum power density as a function of the contact area between the PSM cathode and current collector measured at 800 jC. The cell resistance was significantly affected by the contact area of the current collector. The higher the contact area between the cathode and the current collector, the lower the cell resistance. Based on the original designed contact area (i.e., contacts mainly through alternate crossover contact points and no change of the contact areas of the current collector at high temperatures), the increase
21
in the cell resistance with the decrease of the contact area becomes very sharp as the contact areas approach 5% (see Fig. 6a). However, by taking into account the increased contact areas of Ag mesh and Ag foil#1 current collector under the present operation conditions, the dependence of the cell resistance on the contact areas became much more moderate (Fig. 6b). Such gradual relationships between the cell resistance and the contact area between current collector and cathode may be more realistic and would be similar to the geometrical effect of the discrete contacts on the current distribution and polarization resistance of the electrode [27]. On the other hand, it was found that replacing the flattened fine Ni waven with expanded fine Ni mesh had little on the overall cell resistance and cell performance despite the fact the expanded fine Ni mesh had contact area of 56%, significantly
Fig. 6. Plots of cell resistance and maximum power density of anode-supported cells measured at 97%H2/3%H2O fuel and air and at 800 jC as a function of (a) designed contact area and (b) observed contact area under fuel cell operation conditions of cathode current collector. See text for explanation.
22
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
higher than 27% of the flattened fine Ni waven mesh. This may be due to the much higher electrical conductivity of porous Ni/YSZ cermet coating ( f 254 S cm 1) than f 4 S cm 1 of the PSM electrode coating. The actual dependence of the cell resistance on the contact area of the current collector is most likely dependent on the conductivity of the porous electrode coating as well. Fig. 7 shows the potential distribution as a function of the distance between the current (i) and voltage (V) probes at different currents measured with 97%H2/ 3%H2O on the anode side and air on the cathode side at 800 jC. It should be pointed out here that the potentials in Fig. 7 are the average values measured over 3 mm wide voltage probes. At zero current, the observed OCP was 1.100 V and was the same at all four positions of the voltage probes as expected. The OCP was close to the theoretical value of 1.098 V under the conditions studied. Under fuel cell operation conditions with currents passing through the current probe, the potential measured between Ni and Pt voltage probes varied significantly with the position of the voltage probes. At a current of 1.01 A, the potential at the electrode/electrolyte interface areas directly under the current probe or contact points of current collector (d = 0 mm) was 0.571 V and this corresponded to an overall local overpotential (the sum of polarization and ohmic overpotentials) of 0.529 V, i.e., g = OCP 0.571 = 0.529 V. As the distance between the voltage and current probe
Fig. 7. Potential distribution as a function of the distance between the current and voltage probes for a cell with Ni/YSZ cermet anode and LSM cathode, measured under different current at 97%H2/ 3%H2O fuel and air and at 800 jC.
increased to 3 mm, the potential increased to 0.996 V. This indicates that g at the electrode/electrolyte interface areas under the open area of the current probe (3 mm from the current probe) was 0.104 V, much smaller than 0.529 V measured directly under the current probe. Increasing the distance of the voltage probe to 6 mm increased the potential to 1.085 V and g became very small (0.015 V). This clearly demonstrated macroscopically that the polarization potential at the electrode region close to the electrode/electrolyte interface is not uniform and decreases with the increase of the distance to the current source, i.e., the contact point between the current collector and the electrode interface. This is consistent with the observed significant dependence of the cell performance on the contact areas of the current collector. It has been known in solid electrolyte cells that measured resistance is generally much higher than the calculated resistance of the solid electrolyte based on the thickness and resistivity of the electrolyte material [28,29]. As shown by Tannenberger and Siegert [28], silver electrode (5 Am thick) would only be active on discrete spots and this causes the loss of effective cross-section for the current flow through the electrolyte, leading to the higher measured resistance compared to the calculated resistance based on electrolyte thickness. Such constriction effect due to the discontinuous nature of the geometric effect of the discrete contact between electrode and electrolyte has also been observed on electrode materials with predominant electronic conductivity and negligible ionic conductivity such as Au, Pt and LSM [29,30]. As the electrochemical reactions occur primarily at regions where electrode, electrolyte and reactant gas meet (i.e., three phase boundary) [30 – 32], the loss of the electrolyte areas due to the discontinuous geometric contact at the electrode and electrolyte interface can also result in the significant loss of the electrode polarization performance as shown by van Berkel et al. [33] and in the increase of polarization resistance as shown by Kenjo and Kanehira [27] and Fleig and Maier [34]. In addition to the significance of the electrode/ electrolyte interface contact, the contact between the electrode and current collector also plays an important role in the cell resistance and performance, as shown in the present study. This is clearly demonstrated by the
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
observed high cell overpotential and high cell resistance with cathode current collector with low contact area. For example, Pt waven mesh current collector has the lowest contact area and thus the cell with the Pt waven mesh as the current collector has highest cell resistance and cell overpotential (see Table 1 and Fig. 6). Lower contact area of the current collector will have fewer contact points between the current collector and electrode coating. This would lead to an uneven current distribution in the electrode coating and thus high cell overpotentials as the cell potential measured is an average value. The suggested uneven current distribution in the electrode coating due to the discrete contact between current collector and electrode coating is also supported by the observed significant dependence of the potential on the distance between the current and voltage probes in the specially designed experiment (Fig. 7). The overpotential (i.e., the difference between the OCP and the cell potential) measured between Ni and Pt voltage probes decreases significantly with the increase of the distance between the current and voltage probes with the current passing through the current probe. At a current of 1.01 A, the g at the electrode/electrolyte interface areas directly under the current probe (d = 0 mm) was 0.529 V. At d = 3 mm, g decreased to 0.104 V, implying that the current at the electrode/electrolyte interface areas under the open area of the current probe would be much smaller than that at the interface areas directly under the current probe (high g implies high current). Increasing the distance of the voltage probe to 6 mm, g was reduced to 0.015 V, indicating that current at the electrode/electrolyte interface region under the voltage probes is very small. At d = 9 mm, the measured potential is the same as that at open circuit, indicating that there is no current flow in this region of the electrode coating. The results clearly demonstrated that current distribution at the electrode/electrolyte interface region is not uniform and is strongly affected by the discrete contact between the current collector and the electrode coating. However, there are difficulties in the explanation of the observed phenomenon purely based on the magnitude and differences in the calculated crossplane and in-plane resistance of the electrode coatings. In the present study, the thickness of the PSM air electrode coating was generally in the range of f 95 Am, while the shortest distance from the middle
23
of the open areas to the nearest alternate crossover points of the current collector was f 500 Am for Pt waven mesh (Fig. 4). The average conductivity of PSM electrode coating in the present study was f 4 S cm 1 at 800 jC (see conductivity measurement sections). Therefore, the cross-plane resistance for the current to travel to the electrode/electrolyte interface region would be 2.4 10 3 V cm2, while the inplane resistance for the current to reach the open area between the alternate crossover contact points of the current collector would be 1.3 10 2 V cm2. The cross-plane resistance for the current to travel to the electrode/electrolyte interface region is relatively smaller compared to the in-plane resistance for the current to reach the open area between the alternate crossover contact points of the current collector. Based on the average conductivity of f 45 S cm 1 measured for the LSM electrode at 800 jC, the electrode resistance across the electrode coating (coating thickness was 50 Am) would be 1.1 10 4 V cm2. The corresponding in-plane resistance for d = 3 mm and d = 6 mm would be 6.6 10 3 and 1.3 10 2 V cm2, respectively, similar in the magnitude as compared to the in-plane resistance of the PSM electrode coating. On the other hand, the overall polarization resistance for the H2 oxidation and O2 reduction reactions is in the range of 6– 9 V cm2 under similar testing conditions on almost identical Ni/YSZ anode and LSM cathode at 800 jC [35,36]. For TZ3Y electrolyte with f 100 Am thickness, the calculated electrolyte resistance is 0.56 V cm 2 according to the conductivity of 0.018 S cm 1 reported for TZ3Y materials at 800 jC [37]. This shows that calculated cross- and in-plane resistance values of the PSM and LSM electrode coatings are very small indeed as compared to the polarization resistance of the electrode reactions at the electrode/ electrolyte interface and the electrolyte resistance. Consequently, the current distribution should be uniform in the electrode coating and the effect of the contact between the current collector and the electrode on the cell performance should be negligible. However, this is not the case as shown clearly in the present study and is also contradictory with the observation reported [12,13]. The answer to the contradictory as outlined above may be related to the nature of the solid oxide fuel cell system. Fig. 8 shows schematically the current distri-
24
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
Fig. 8. Schematic diagram of current distribution in (a) a pure electronic conducting (or semiconducting) medium, (b) a material with different conductivity, and (c) a solid electrolyte cell. The dotted and dashed lines indicate the current lines with weak strength as compared with the current lines with solid lines.
bution in a pure electronic (or semiconductor) conducting medium and a solid electrolyte cell. In a pure electronic or semiconducting medium such as porous LSM electrode, the conductivity of the medium is reasonable (e.g., 4 S cm 1 for PSM and 45 S cm 1 for LSM porous coatings at 800 jC, respectively) and the resistance of the medium in any location (R1) should not change with the current as would be expected. Thus the current distribution inside the medium would be uniform, shown by the solid lines in Fig. 8a. In the case of a material with different conductivity (e.g., YSZ materials sandwiched by electrode materials), the current distribution inside the medium would also be uniform as the resistance of the materials (R1 and R2) do not change with the current, assuming uniform contact and no interface resistance between these two materials (Fig. 8b). However, if there is a polarization resistance at the electrode/electrolyte interface and it changes with the current, the situation would be different. Fig. 8c shows a simplified SOFC system in which the electrode resistance (R1) and polarization resistance at the elec-
trode/electrolyte interface (R3) are assumed to be the same on both sides. Different to R1 and R2, it is well known that the polarization resistance (R3) for both H2 oxidation and O2 reduction reaction in SOFC is significantly dependent on the current and polarization potential [2,23,38 – 42]. For example, at 1000 jC, the polarization resistance (R3) was 4.4 V cm2 at open circuit and decreased to 3.1 V cm2 at overpotential of 50 mV for H2 oxidation on Ni anode and for H2 oxidation on Ni/YSZ anode R3 was reduced by more than 60% at g = 50 mV [39]. For O2 reduction on LSM electrode at 800 jC, the polarization resistance was 132 V at open circuit and reduced to 56 and 10 V under polarization potential of 100 and 300 mV, respectively [41]. Similar dependence of the polarization resistance on the polarization potential (or current) was also reported for the O2 reduction on Pr0.8Sr0.2Mn0.5Co0.5 electrode at 800 jC [23]. This indicates that polarization resistance (R3) for the electrode reaction on the Ni/YSZ cermet anode, PSM and LSM cathodes is strongly dependent on the current and polarization potential. The sensitivity of the polariza-
S.P. Jiang et al. / Solid State Ionics 160 (2003) 15–26
tion resistance on the current and polarization potential has also been reported for the electrode reaction on different electrode materials [2,43,44]. Consequently, under fuel cell operation conditions, the current directly under the contact areas of the current collector will be slightly higher than that under the open areas of the current collector as the crossplane resistance of the electrode coating is lower than the in-plane resistance. Due to the sensitivity of the polarization resistance on the current, the polarization resistance (R3) directly under the contact areas of the current collector would be reduced more as compared to that under the open areas of the current collector. As R3 is orders of magnitude higher than the resistance of the electrode coating (R1), the small change in R3 will have significant effect on the overall local cell resistance (2R1 + R2 + 2R3). This would lead to the considerable increase of the current and thus more reduction of R3 directly under the contact area of the current collector, compared to that under the open area of the current collector. However, further reduction of R3 would be limited by the kinetics of the reaction (e.g., the increase of the local polarization potential) and the increased dominance of the electrolyte resistance. An equilibrium will be reached and at this stage the polarization resistance, R3, would be much less sensitive to the change in current. Therefore, under fuel cell operation conditions, the local cell resistance directly under the contact areas of current collector (region a in Fig. 8c) would be noticeably smaller than that under the open areas of the current collector (region b in Fig. 8c). This results in the high current at the electrode/electrolyte interface region a as compared to that at the interface region b. 2R1 þ 2R3a þ R2 < 2R1 þ 2R3b þ R2
ð2Þ
ia > ib
ð3Þ
where R3a and R3b represent the polarization resistance under polarization at regions a and b (Fig. 8c) and R3a < R3b under fuel cell operation conditions. Therefore, the polarization potential at the electrode/ electrolyte interface region a would be higher than that at the interface region b. Lower contact area of the current collector will have fewer contact points between the current collector and electrode coating.
25
This will lead to lower uniformity of current distribution in the electrode coating, magnified by the constriction effect due to the discrete contact at the electrode/electrolyte interface and thus poorer performance. This explains the observation of high cell overpotential and high cell resistance with cathode current collector with low contact area (Table 1 and Fig. 6). The uneven current distribution in the porous electrode due to the discrete and inhomogeneous contacts between the current collector and electrode coating would also be affected by the electrical conductivity of porous electrodes, particularly in the case of electrode materials with low conductivity such as PSM. Therefore, it would be expected that the lower the contact area or density of contact points and the lower electric conductivity of the electrode coating, the poorer the current distribution would be and this would lead to higher cell resistance and higher cell polarization losses. The increased overpotential losses of the cell with the decrease in the contact area between the current collector and electrode clearly demonstrated that the constriction effect due to the uneven current distribution in the solid electrolyte cells is not only dependent on the discrete contact between electrode and electrolyte [27 – 30,34] but also on the discrete contact between electrode and current collector. However, the spread of the current lines inside the electrode area would be dependent on the relative magnitude of the polarization resistance in overall cell resistance and its sensitivity on the polarization current or potential. The hypothesis proposed in Fig. 8c indicates that the assumption of uniform potential distribution at the electrode area in the vicinity of the electrode/electrolyte interface [27] may not be valid in the case of discrete contact between the current collector and electrode coating and particularly in the case of electrode coatings with low electrical conductivity.
4. Conclusions The effect of contact area between electrode and current collector has been studied on anode-supported cells at 800 jC in 97%H2/3%H2O and air, using current collectors with different contact area on the PSM cathode side. The results clearly demonstrated
26
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that the contact area between electrode and current collector (i.e., the interconnect) affects both the cell resistance and polarization losses. Increase of the contact area between the PSM cathode and the current collector reduced both cell resistance and cell overpotential losses. This indicates that the constriction effect observed at the electrode/electrolyte interface due to the discrete contact also occurs at the interface of the electrode/current collector.
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