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Application of silver in microtubular solid oxide fuel cells

Application of silver in microtubular solid oxide fuel cells In this paper, the behaviour of silver as cathode conductive material, interconnect wire, and sealing for anode lead connection for microtubular solid oxide fuel cells (µSOFC) is reported. The changes in silver morphology are examined by scanning electron microscopy on cells that had been operated under reformed methane. It is found that using silver in an solid oxide fuel cell (SOFC) stack can improve the cell performance. However, it is also concluded that silver may be responsible for cell degradation. This report brings together and explains all the known problems with application of silver for SOFCs. The results show that silver is unstable in interconnect and in cathode environments. It is found that the process of cell passiva- tion/activation promotes silver migration. The difference in thermal expansion of silver and sealant results in damage to the glass. It is concluded that when silver is exposed to a dual atmosphere condition, high levels of porosity formation is seen in the dense silver interconnect. The relevance of application of silver in SOFC stacks is discussed. Keywords Silver · SOFC · Microtubular SOFC · SOFC stacks Introduction cobalt ferrite (LSCF) decreases at lower operating tempera- ture. High ohmic losses in the cathode result in reduced cell Several configurations of solid oxide fuel cells (SOFC) are performance. This can be overcome by the addition of a commercially available; however, the selection of suitable metal-based current collection layer that enhances the elec- materials and development techniques is still the subject of tronic conductivity. The high-conductive layer ensures adhe- current research. Within an SOFC cell, the cathode is a sig- sion and connectivity between the cathode and the intercon- nificant contributor to the cell overpotential caused by the nect. This consequently reduces the contact resistance of the slow oxygen reduction reaction [1], which occurs at the elec- cathode/interconnect interface. trolyte–cathode–air boundary phase (triple-phase boundary). In planar cells, a silver mesh can be applied to reduce the When the temperature of SOFC is low, the oxygen dissocia- contact resistance between the interconnect and the cathode tive adsorption slows further because of the decrease in cata- [2]. The presence of silver will also act as a catalyst for lytic activity of the cathode oxygen reduction. Key require- oxygen reduction [3]. Silver has been proven as a cathode ments for cathode materials are that they have to be highly material showing good results at intermediate temperatures conductive for electrons and oxygen ions, and they should in SOFC [1, 4–6]. In these tests, the limiting feature was the maintain thermal stability in SOFC operating temperature. length of the oxygen diffusion paths. Moreover, silver does The conductivity of modern cathode materials such as lan- not form a nonconductive oxide layer when exposed to air thanum strontium manganite (LSM) or lanthanum strontium at elevated temperatures. The high resistance to oxidation at elevated temperatures is thus beneficial for SOFC applica - tion. Silver also has one of the highest electrical conductivi- −1 * Artur J. Majewski ties of metals (6.30 × 107 S m at 20 °C), thus the addition a.j.majewski@bham.ac.uk of silver improves electron percolation. Silver is often used Aman Dhir as a conductive material for the cathode in microtubular aman.dhir@wlv.ac.uk solid oxide fuel cells (µSOFC) [7–10]. This has been imple- mented for planar [11] or honeycomb [12] cell designs, too. School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK Silver in a µSOFC can also function as a sealant [13, 14]. Infiltration of silver into the porous cathode is an alternative Present Address: School of Engineering, University of Wolverhampton, Wolverhampton WV1 1LY, UK Vol.:(0123456789) 1 3 16 Page 2 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 method to improve current collection [1, 9]. Infiltration of as an additional cathode conductive layer and for the seal- silver or an alternative highly conductive material such as ing of an anode/lead connection. The aim of this work is to lanthanum strontium cobalt (LSC) can significantly improve examine the degradation mechanisms of a µSOFC cell. current density of the standard LSM cathode [6, 10, 15]. Adding silver into the cathode system has shown to reduce the overpotential related to oxygen reduction [16]. Porous Experimental silver can also function as a cathode for low-temperature SOFC [17] Inert properties and stability in the oxidising The current interruption method was applied for internal atmosphere can make silver a good candidate as a cathode resistance measurements. Electrochemical impedance interconnect material for intermediate-temperature SOFC. spectra were performed using Solartron cell test system However, silver tends to agglomerate during annealing in 1400A/1470E (Potentiostat/Galvanostat). The cell response air or in ambient oxygen [18]. Porous silver tends to become was measured over a frequency range of 1 MHz to 0.1 Hz dense at elevated temperatures. For the mixed Ag/cathode with AC voltage amplitude 10 mV at the open circuit voltage systems of yttrium or erbium stabilized bismuth oxide, 1 h (OCV) condition. Electrochemical performance measure- of sintering was shown to be enough to result in porosity ments were made using the Solartron in galvanostatic mode. reduction, separation of compounds, and increase in Ag Long-term cell tests were conducted under potentiostatic phase [1]. Silver is also known to be prone to migration mode at a set voltage of 0.7 V. Every 24 h the test was inter- at elevated temperature. Compson et al. [19] suggested to rupted by I–V test and impedance scan. use silver as an interconnect for SOFC only at temperatures Analysis of cell morphology: post-cell test the cells were below 650 °C. Above this temperature, loss of silver caused fractured, cell surface and cell cross-section were character- by sublimation, evaporation, and diffusion transport may ised using a Hitachi TM3030Plus scanning electron micro- affect cell performance. scope (SEM). Using silver as a current collector can reduce cell per- The cell temperature was maintained at the temperature formance since the low-temperature melting point of silver of 650 or 700 °C in a tubular furnace (Vecstar HZ/split-tube) requires changes in cell preparation technique [12]. Moreo- and monitored by thermocouples flanking the test chamber. ver, silver can evaporate even at low (300–350 °C) tempera- tures [20]. Evaporation of silver increases with temperature Cell fabrication and is similar in the air and in the reducing environment [21]. Silver does not have high gas penetrability. However, The cell tests were conducted using commercial tubular, silver has high oxygen solubility. The solubility allows appli- anode-supported µSOFCs, of size 15.2 cm in length and cation of silver as an anode for direct carbon SOFC [22]. 6.6 mm in external diameter, of composition Ni-YSZ/YSZ/ Silver tends to migrate when submitted to an electric field SDC/LSCF (yttria-stabilized zirconia (YSZ), samaria-doped at high temperature with of oxygen [23–25]. Silver ions Ag ceria (SDC)). Silver was applied to improve cell’s perfor- can move under the influence of the electrical field [23]. mance. Silver wires (99.9% pure silver, Scientific Wire The presence of Cr can increase the Ag migration into the Company) 2 × 0.71 mm were used to collect current, with cathode. Formation of compounds such as AgCrO with additional six silver wires 2 × 0.20 mm to fasten the current higher evaporation rate than pure Ag increases Ag migra- collector to the cathode. Silver paste (ESL 9907, Electrosci- tion [15]. Migration of silver and formation of conductive ence UK) was used to connect the current collecting wire to filaments can cause short-circuit failures [23]. Singh et al. the exposed anode. The paste was dried at 105 °C for 1 h. [26] observed that exposure to the dual oxidation–reduction After attaching the silver wire, an additional layer of silver environment can damage the silver microstructure. They paste was applied to cover the wire and to improve sealing. observed the formation of pores and cracks on the fuel side This was sintered at 750 °C for 2 h. In addition, the silver of the solid silver barrier. They suggested that dissociation was covered by a commercial glass sealant, which was sin- and dissolution of H and O into silver and formation of tered at 850 °C for 8 h. The cathode was coated with silver steam cause development of pores in solid silver barriers. ink (producer, SPI 5001) to reduce the lateral resistivity and Using silver in SOFC cells and stacks has many benefits to improve the electrical connection between the cathode and for that reason many researchers are trying to improve and the interconnect. The cathode area was 16.6 cm con- SOFC performance using silver compounds. However, the sisting of two separate parts (Fig. 1a–b). The silver ink was incorporation of silver will affect the cell durability. The aim applied by ink brushing the cathode, then drying at 105 °C of this work is to discuss the plausibility of Ag as an SOFC for 1 h. material and to highlight the problems that may occur for The cell exhaust was cemented (using high-temperature application of silver in SOFC. In this work, silver is utilised cement) to a manifold, and the gas feed was connected in the µSOFC systems, as an interconnect, as a cathode or 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 3 of 13 16 Fig. 1 a A tested cell in the furnace; b the anode current connection (sealed by glass); c the surface of glass sealant; d glass sealant at the anode wire connection, visible cracks using a silicone tube so the cell was free to move in the Results and discussion axial direction. Cells were operated at 650 and 700  °C using either hydrogen or methane/air mixture. Inside the Benefits from using silver cell/tube, at the inlet, a partial oxidation catalyst (CPOX) (0.1 g) was inserted in the shape of a honeycomb structure. Usually, tubular cells have large cell voltage and ohmic −1 Hydrogen was introduced into the cell at 145 ml min , losses since electrons have to be transported through the and after 1 h of OCV, a polarisation curve was recorded cathode along the cell. The addition of silver can reduce this (galvanostatic mode) with 0.5 V as a lower safe limit. Then problem. Thus, the surface of the cathode was covered with the voltage was set to 0.7 V (potentiostatic mode) and a silver ink (SPI 5001) for these test cells. The primary role short 1 h constant voltage test was performed. Afterwards, of the silver layer was to deliver and distribute the flux of the fuel was changed to a mixture of CH :air (dry air) with electrons over the whole cathode area. This also helped to molar ratio CH :air of 1:2.4 and a long-term durability test reduce the area-specific resistance (ASR) of the connection −1 was conducted at 0.7 V (CH 48 ml min ). These condi- between the cathode and the interconnect (LSCF-Ag wire). tions were chosen to simulate the realistic conditions of a An additional benefit of adding silver was to support oxygen µSOFC auxiliary power unit (APU) [27]. The fuel gas con- reduction reaction. The silver ink formed a porous layer of nection was outside the hot zone, which eliminated prob- 10–30 μm. The high porosity of the silver layer allowed free lems with sealing. All tests were conducted under ambient gas diffusion. air-condition on the cathode side. Details about the cell For cells tested without Ag coating on the cathode sur- test system were described in previous papers [28, 29]. face, cathode lead wires (0.71 mmAg) were coated with LSCF to improve contact interconnect/cathode. For cells with the Ag cathode coating layer, the wires were coated with Ag ink. 1 3 16 Page 4 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 The additional coating by silver improved power den- overpotential. By improving the cathode, the cell perfor- sity. The power density for the cell increased from 0.05 mance was enhanced mostly by decrease in ohmic resist- −2 to 0.3 W cm at 0.6 V, after the cathode was coated with ance and the overall cell polarisation. silver ink. This was also reflected in the impedance char - To confirm and compare how the various cathode coat- acteristic where covering by silver reduced cell impedance ings affect the cell performance, a selection of cells was (Fig. 2a). OCV was not affected. The power loss for the coated with Pt ink, Ag ink and LSCF ink (to improve con- cell with the LSCF cathode can be attributed to the inter- nection between already sintered LSCF cathode and Ag nal resistance of the cathode/interconnect contact area and wires). These cells were tested at 700 °C. Increasing the to the deteriorated electronic current distribution. Results cell-operating temperature from 650 to 700 °C reduced the presented in Fig.  2 show that the application of a good cell overpotential, for the cell covered with Ag (Fig. 2a–b). electronic conductor such as Ag or Pt not only improves The possible reason for this was the increased conductivity the contact between the cathode and the interconnector, of the cathode and the electrolyte and higher catalytical but also improves electron distribution. Consequently, this activity of the cathode. Slightly larger impedance for Ag leads to a perceived and more homogeneous polarisation was obtained for the cell covered by Pt ink (Fig. 2b). In of the cathode. Without the Ag layer, the cathode was not contrast, for the LSCF coating, the resistance of the cell used to its full performance capability, the cause of this— increased at all frequencies. The cell ohmic resistance for in-plane electronic resistance. Without Ag or Pt, not only Ag (0.03 Ω) was slightly lower than for Pt (0.04 Ω) and is the high-frequency intercept higher, but also the elec- much lower than for LSCF (0.28 Ω). In addition, the size trode impedance is larger. The silver coating significantly of both impedance arcs was slightly smaller for Ag than reduced the internal resistant loss. The ohmic resistance for Pt and much smaller than for LSCF. The cell perfor- decreased from 0.23 to 0.03 Ω. The cell polarisation resist- mance depends on the used current collector. The low- ance still consisted of at least two arcs. Interestingly, the frequency semicircle for Pt and Ag was slightly larger than characteristic frequency of arcs changed only slightly from the high-frequency. The characteristic frequency for high 20 to 50 Hz for the high-frequency arc and stayed around and low-frequency arcs was independent of the type of 0.2–0.3 Hz for the low-frequency arc. The contact resist- cathode coating. ance between the cathode and interconnect was reduced The EIS spectra confirmed the positive aspect of intro- after silver was added. The anode side of the cell and the duction silver to the cathode structure. The data and electrolyte remained unchanged. Altering the composition results presented clearly show that SOFC with the Ag- at one electrode does not affect the impedance of the sec- coated cathode, operating in the temperature region of ond electrode at OCV. A significant change to the cathode 650–700 °C, showed improved performance compared to impedance may overlap the anode impedance. The LSCF similar cells without silver in the cathode system. cathode system had a significant contribution to the cell −1 Fig. 2 Electrochemical impedance spectroscopy: a cells with LSCF and LSCF + Ag cathode. 650 °C, H 145 ml min ; b cells with the cathode −1 coated with Ag, Pt and LSCF 700 °C, H 145 ml min 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 5 of 13 16 interconnect is followed by diffusion and reaction of dis - Anode Ag sealing interconnect solved species and water vapour formation. The hydrogen and oxygen concentration in silver increases up to a critical The whole area of the exposed anode was covered by silver paste to improve the electrical connection and to part-seal pressure and bubbles are generated. The increase in the size of the pores in the silver layer can finally result in fuel leak - the anode. The silver was densified by sintering. However, as the thermal expansion coefficient of silver is higher than age. According to Jackson et al. [30], 24 h of exposure to the dual atmosphere is enough to form pores across a 1-mm that of the ceramic cell, it is expected that the connection between the cell and the interconnect further improves thick silver membrane. The high temperature of fuel com- bustion and possible anode oxidation at the middle of the through the silver expansion during heating. Silver is duc- tile and should easily deform under thermal stress avoiding anode can result in crack formation and damage to the cell. The fact that porosity always began to form from the fuel delamination and keep the cell hermeticity. However, using only silver paste as a sealant resulted in poor sealing. Fuel side of the silver layer suggests faster solubility of oxygen than hydrogen. Also the ratio of O:H 1:2 in water promotes leakage was seen, indicated by high temperatures detected by the thermocouples around the anode current connec- nucleation closer to the hydrogen source. The diffusiv- ity and highest concentration in silver under dual atmos- tion. The cells with only silver as a sealant could operate −4 2 −1 for around 24–50 h. To investigate the reason for the weak phere at 800 °C is, respectively [30]: H 2.9 × 10  cm s ; −7 −3 −5 2 −1 6.1 × 10   mol  cm, O 1.1 × 10   cm  s ; sealing, the cell was tested at 700 °C with H (3% H O) 2 2 2 −5 −3 for 8 h under OCV. The H leak test conducted after silver 1.1 × 10  mol cm . The rapid degradation of silver mechanical integrity and sintering indicated gas tightness of the silver/anode joint at room temperature. hermeticity after exposure to dual atmosphere affects the cell performance. Formation of pores in the silver also has During this test, the temperature at the anode current con- nection increased approximately 100 °C above the operating an influence on the stability of the silver seal. Such degrada- tion of the structure leads to fuel leakage. For this system of temperature and the connection became red-hot even at a furnace temperature of 700 °C. After this test, the surface tubular cells with the anode connection at the middle of the cell, the silver seal would be exposed to the dual atmosphere. of silver exposed to the fuel side indicated degradation, and the dense silver developed into a porous structure. The layer Therefore, if silver is applied as a sealant/current collector for a µSOFC, it has to be isolated to avoid exposure to the of silver delaminated from the anode surface creating a gap (Fig. 3a) indicated the increase in cell impedance. Further to dual atmosphere. The additional sealing of the silver by glass was needed to extend the cell operation. This was achieved this, SEM scans showed that the degradation of solid silver started from the fuel side of the silver by developing poros- by coating the silver anode connection by a 10 µm layer of glass sealant. This extended the life of the µSOFC up to ity (Fig. 3b). Singh et al. [26] suggested that pores and voids in silver more than 800 h. However, due to the difference in thermal expansion are developed because of water vapour formation. Adsorp- tion and dissolution of H and O gaseous molecules in silver coefficient of silver and glass, there is a risk of damage to Fig. 3 SEM scans: a the cross-section of the anode current connection (Ag-YSZ-Ni/YSZ, 8 h at 700 °C); a visible gap formed between the anode and the silver sealant; b the cross-section of silver sealant near the air side 1 3 16 Page 6 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 the glass coating. The components that have much lower heat generated by fuel combustion could be enough to melt thermal expansion coefficient than silver are YSZ 10.5, Ni/ the glass and produce pinholes. For future work, the glass YSZ 12.5, SDC 12.8 and cathode 14.6, compared to silver coating technique has to be improved. −6 −1 18.9 × 10  K . The glass sealant’s thermal expansion coef- In this study, the long-term stability of the Ag-based ficient was selected to match the expansion of the cell com- anode current collection was examined. For the cells tested −6 −1 ponents and was around 12 × 10  K . With this mismatch for more than 850 h, not only the degradation of the silver in thermal expansion, significant internal stress is created. sealing layer (Ag paste) was observed, but additionally, After the glass sintering (at 850 °C for 8 h), there was no vis- a significant disintegration of the silver current collect- ible damage to the glass surface. However, the long-term cell ing wire fastened to the anode. The cell with the CPOX operation at elevated temperature or thermo-cycling resulted catalyst was fed with the mixture of CH and air in the in the crack formation in the glass structure (Fig. 1d). These molar ratio 1:2.4. Approximately, 70% of the fuel was studies suggest that the high silver coefficient of thermal converted on the catalyst before it reached the anode [28]. −2 expansion can create problems for application of silver as an The power of the cell was around 0.14 W cm at 0.7 V. anode interconnect (and sealant) for the µSOFCs even with Low operation temperature (650 °C) decreased the rate of a glass coating. For this specific application, the rapid start oxygen reduction and conductivity of the YSZ electrolyte. and cooling required by APU could favour crack formation Post-mortem analysis showed part of the wire close to the within the glass sealant. anode surface was damaged. The silver anode current col- The glass surface showed visible pinhole formation, lecting wire and all the silver interconnect had become approximately 1 μm in diameter. Several effects, including highly porous. The solid silver appeared in a sponge-like the application/deposition technique, can cause this effect. structure (Fig. 4a–b). In this case, it has been attributed to the irregular sur- Pores formed across all surface areas of Ag wire. The face of silver to which it was applied. Due to the complex silver wire at the anode connection became brittle. Sur- shape of wire/anode connection, it was difficult to obtain a prisingly, the cell performance was stable for more than smooth silver surface. Pinholes usually indicate localised 850 h of operation under such gas leakage conditions. For- microbubbles or dewetting that degassed during sintering. mation of silver crystals damaged the glass sealing and Contaminants like dust particles can cause a formation of increased the exposure of the wire to air. These results localised microbubbles. This could come from insulation did not confirm Compson’s conclusion [19] that for silver material. The glass coating thickness is in a micron range, interconnects 650 °C is a safe operating temperature. After and it is possible for the debris to be larger in diameter 850 h of operation at 650 °C, all the interconnect wire than the coating thickness. The electrostatic attraction is and sealing paste was significantly deformed and dam- enough to hold small particulates on the surface. Some aged. It can be concluded that silver paste should not be particulates could also have a high moisture level. In addi- used as a sealant for the anode connection for long-term tion, if there are existing cracks on the glass surface, the cell operation. −2 Fig. 4 SEM scans: a the cross-section of the anode current collection wire; b the cross-section of silver wire; after 850 h at 650 °C ~ 0.2 A cm with CH reformate as a fuel 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 7 of 13 16 (Fig. 5c). This part of the wire was only 10 mm from the Ag wires anode connection (presented in Fig. 4), and after damage to the glass the coating was exposed to the dual atmosphere. Samples of Ag wire (taken 1 cm away from the cell) from the anode and the cathode side were analysed to check the The damage to the silver wire microstructure was observed even 10 mm from the point of wire exposure to the dual stability of silver as an interconnect wire. These samples of wires after long-term cell testing were compared with air–fuel atmosphere after long operational time. After sev- eral hours of operation, silver wires became brittle. This unused wires to detect any visible microstructural changes that had occurred during cell operation. An erosion of the affected the mechanical strength and integrity. Even low physical stress could break the wire. This is important if silver wire surface is visible on the SEM scans (Fig. 5a–d). The initially smooth surface of tested wires developed facet- a stack with silver wires is considered for an APU unit in vehicles where the system must resist vibrational stresses ing and striation structures after being exposed to air at 650–700 °C. Faceting is related to surface energy minimisa- and strains. A cell with such brittle wires would be prone to damage caused by vibration. The risk of breaking the tion. According to Lu and Zhu [21], striation and faceting of the silver surface occur only in an oxygen atmosphere. Facet- wire connection is significant. All these problems with silver will lead to degradation of the cell structural stability and ing made of large structures developed on the surface of Ag wire at the cathode side after 850 h at 650 °C (Fig. 5a–b). in long-term operation to the degradation of the cell/stack performance. Zhong et al. [12] using Ag mesh for cathode At higher operating temperature (450 h at 700 °C) the structures formed were more intensive; all surfaces of the current collector did not observe this type of degradation. However, they used Pt paste to improve the strength of the wire had changed; however, the structures were smaller (Fig. 5d). Wires at the anode side behaved differently. Some connection, and the test time was short. Silver commercially available has the purity of 99.95%. Silver is rarely used by faceting was visible, but significant wire area became porous Fig. 5 SEM scans of the surface of Ag interconnect wire (1 cm from the cell) after test under constant current for 850 h at 650 °C: a, b cathode lead; c anode lead; d the surface of Ag cathode wire after 450 h at 700 °C 1 3 16 Page 8 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 industry in pure form caused by its softness and suscepti- before glass sintering. This exposed the porous silver to the bility to damage. Also for SOFC application, silver alloys high temperature of 850 °C for 8 h. The reverse fabrica- should be considered instead of pure silver. Where higher tion, with sintering glass before coating the cathode with strength at elevated temperature is required, silver-palladium silver, reduced silver degradation. However, it increases the alloys are more suitable; however, that would increase the complexity of the cell mass production. The Ag current col- cost. Also the application of Ag(Al) alloys can enhance the lection film can be applied after the glass sintering for a thermal stability of silver [31]. The addition of aluminium single cell, although this process is impractical for the stack reduces silver agglomeration at elevated temperature and manufacture. prevents diffusion of silver into the matrix of support mate- It can be concluded that cells with silver require low- rial. Wires with high Ni, Fe or Cr content are less suitable temperature sealing materials. Tested cells were operated because of formation of a nonconductive oxide layer and at the moderate temperature of 650–700 °C to avoid silver reduced conductivity between the cathode and the intercon- exposure to high temperature. The cathode current collection nect. The addition of other elements and alloy formation on Ag wires were attached to the cathode after the glass sinter- the silver surface may be the solution for application of silver ing process. The cathode Ag coating was brush-painted after current collectors in SOFC. Silver wires coated with gold are the glass sintering. The porous structure of silver coating slightly more expensive than pure silver. allowed gas distribution and improved current collection. Observed agglomeration of silver (Fig. 6) reduced all these Cathode conductive layer benefits. Part of the cell power degradation is caused by silver agglomeration. Silver agglomeration and formation of The risk of vaporisation and sublimation of silver exposed holes and hillocks followed by formation of islands (Fig. 6a) to air increases at higher reaction and operating tempera- increased electrical resistivity. The formation of solid sil- tures. The vaporisation rate depends on temperature and ver increased after the long-term (850 h) testing. Faceting atmosphere, and is higher in air than in H /H O atmosphere. of agglomerated silver was visible (Fig. 6b). From the cell 2 2 Meulenberg et al. [2] extrapolated that up to 2% of silver can surface images, it is evident (Fig. 6) that the silver layer has evaporate after 40,000 h at 690 °C in air. The evaporation either evaporated or diffused into the LSCF coating layer will increase with the increase in the silver surface area. from the original deposition layer. Change in the surface The cathode silver layer used in presented experiments was morphology affected the electrical conductivity of the silver porous. Considering high porosity of the silver layer and layer. After several hours of operation and formation of iso- its thickness 10–30 μm, a significant part of the silver can lated silver ‘islands’ on the cathode surface, the role of silver evaporate after the cell’s lifetime. The glass sealant, which as an electron distribution to the whole cathode area was is applied to maintain gas tightness, also poses a problem as significantly reduced. The silver after agglomeration did not glass sealants usually require sintering at 850 °C for several provide a continuous electronic pathway through the cath- hours—leading to evaporation of silver into and through ode. This also affected the function of silver as an electron the glass matrix. In our previous report [29], the cathode conductor from the cathode to interconnect and increased was coated with silver ink to receive good current collection ASR of the connection between the cathode and attached Ag Fig. 6 SEM of the cathode surface covered by Ag: a after 100 h at 650 °C; b after 850 h at 650 °C 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 9 of 13 16 lead wire. Creating a long oxygen diffusion path through the However, from our experience, application of lower viscos- bulk silver limits also the quantity of oxygen transported to ity silver ink directly on the LSCF cathode can sometimes the electrolyte. Lower silver porosity reduced oxygen reduc- result in a cathode flaking. tion zone. Agglomeration of silver reduces the contribution Several things can cause cathode delamination. The of silver to the catalytic activity of the cathode. Lower silver peeling of the cathode was more frequently observed when porosity reduces the length of the three-phase boundary. On slightly thicker silver layers were utilised. The thickness of the other hand, during silver agglomeration and formation the silver layer is difficult to control during hand brush paint- of a dense silver phase, a significant part of the cathode was ing. Therefore, it can be speculated that increased shrinkage exposed allowing free gas diffusion. Camaratta and Wachs- of the outer layers of the silver film forced the cathode to curl man [1] observed a similar reduction in silver porosity for away. Shrinkage of silver created stress in the cathode film Ag-YSB and Ag-ESB cathode systems. They suggested that that can lead to cracking, curing and flaking. Cracks in the agglomeration of silver and reduction of porosity diminished silver coating were often initiated along the Ag lead wire cathodic reaction zone and reduced gas transport. Coating connection. However, the formation of cracks in the cath- the cathode with silver may deteriorate oxygen reduction ode structure near the edge of silver grains formed by silver reaction. Sasaki et al. [32] observed that dense silver reduced agglomeration was observed (Fig. 7b). That could increase gas transport to the significant part of the cathode. the risk of silver penetration through the electrolyte and cre- It can be concluded that initial morphological structure of ated the risk of cathode delamination. Other factors, which the porous silver current collector was destroyed as a reason may lead to delamination, are in the SDC layer fabrication of silver sintering and evaporation. The silver microstruc- during the cell preparation. The SDC layer prevents reac- ture was unstable at intermediate cell operating temperature. tion between cathode and electrolyte compounds. At the Therefore, for a long-term application, silver is not suitable interconnect/SCD interface, formation of the separation as a conductive material. Mixing silver with the ceramic area was occasionally observed. These cracks were paral- phase or infiltration of silver with ceramic precursors could lel to the electrolyte surface. However, usually the parallel prevent silver densification. The ceramic shell can restrain crack does not disturb the cathode stress field, as the stress densification of silver porous structure [9 ]. Another method also acts parallel to the electrolyte surface. However, if the is to incorporate silver into a ceramic matrix of the cath- parallel crack was followed by buckles formation, this would ode [19]. The decrease in temperature operation of SOFC generate a large tensile stress and result in cathode spalla- can increase the stability of silver. Zhu et al. [3] obtained tion. These parallel cracks increased the risk of interrupting high performance of the cathode with silver nanoparticles the electrical connection between interconnect and cathode. at 500 °C. The stability of Ag nanoparticles was increased It can be speculated that the calcination of SCD was con- by the strong metal-support (Ag–SNC) bonding interaction. ducted at very low temperature. The difference in thermal Mixing silver with SSZ can reduce silver agglomeration on expansion coefficient may create internal stress and cause the cathode. Morphology of this cermet is stable at 500 °C slow separation of the cathode layer from the electrolyte dur- [32]. Application of silver ink with thinner can improve the ing the cell operation. The cell delamination could increase wetting and impregnation of silver into the cathode matrix. the cell internal resistance. Usually, delamination is caused Fig. 7 SEM scans: a cells with cathode exfoliation, b the surface of the Ag-coated cathode, visible crack formation 1 3 16 Page 10 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 by mismatching in thermal expansion coefficient of cell may temporarily increase the cell performance because of parts. Changes to LSCF surface morphology caused by the increase in the three-phase boundary, but the silver deposits chemical and thermal reactions of LSCF may lead to oxide may short-circuit the cell. Deposition of silver increased the segregation. Decomposition and transformation of cath- risk of silver penetration into pinholes in the electrolyte and ode materials can also be a result of cathode delamination. consequently short-circuit formation [36]. The silver migra- Cobalt instability and decomposition of perovskite were tion was noticed only for the part of the cell near the cell reported in the literature [33]. If the cathode was partially inlet. This was the most active part of the cell with highest delaminated, the cathodic polarisation would be enlarged current density caused by high fuel partial pressure. This because of concentration in current conduction. The current confirmed that silver migration was responsible for short- collection efficiency was reduced. An electrode delamination circuit formation. No silver migration was detected for parts results in the interruption of the ionic path, and detachment of the cell beyond 2 cm from the cell inlet (Fig.  8a). No of the interconnect interrupts the electronic path. silver diffusion into or reaction with the YSZ electrolyte was Migration of silver and agglomeration at the electrolyte observed. Camaratta and Wachsman [1], after testing ESB interface could also result in cathode peeling [34]. Silver and YSB electrolytes with Ag in the cathode, also concluded tends to agglomerate at elevated temperature, which can lead that no silver diffused into the electrolyte. to cathode delamination. However, no silver was detected The mechanism responsible for migration of silver and in the SDC layer for samples where cathode delamination condensation across the SDC layer is unclear. It is unlikely or cracks in the SDC layer occurred. In addition, no silver that thermal effect alone is responsible for Ag migration was detected on the electrolyte for areas where the cathode to SDC. The temperature distribution along the cell was exfoliation occurred. Therefore, silver penetration into SDC relatively uniform and silver deposited in the SDC layer was not responsible for the cathode delamination. was observed only at the cell inlet (the most active part of For some cells, rapid degradation of performance was the cell). Silver migration could be a result of electromi- observed. Post-mortem inspection of these cells con- gration [6] or it can be associated with the formation of Ag firmed short-circuit formation. The short-circuit effect was (g) species and vapour transport to the cathode–electrolyte observed after few hours of cell operation. Short-circuits face followed by reduction to metal phase [11]. Ag (g) has were generated always at the cell area near the cell inlet. higher vapour pressure than A g O (g) at the SOFC opera- The exact location of short-circuit was difficult to deter - tion temperature. Therefore, it is thought Ag (g) formation mine. Looking at the SEM images of the cell cross-section and penetration in SDC are possible for this mechanism. (Fig. 8a), the Ag layer porous structure is visible. Results of However, electromigration and evaporation/deposition EDX elemental mapping of the cross-section of the cell after of silver are too slow to be alone directly responsible for testing at 700 °C are presented in Fig. 8. For some samples, the observed rate of silver migration during the electrode it was observed that silver accumulated in the SDC layer polarisation [37]. Mosialek et  al. [16] observed silver (Fig. 8b). Our results conr fi med De Silva et al. [ 35] observa- migration on the YSZ electrolyte in the potential range tions that silver may migrate from the cathode to the surface − 0.2–0.5 V; no migration was observed without polari- of the electrolyte. The contact of silver with an electrolyte sation. In addition, no migration was observed without Fig. 8 SEM micrographs of the cell after 48 h operation at 700 °C: a the cross-section of the cell 2 cm from the CPOX catalyst; b the cross- section of the part of the cell near the CPOX 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 11 of 13 16 the electrolyte. They concluded that for the formation of [11]. However, the performance of the Au conductive layer silver dendrites, it is necessary to have a potential differ - may decrease with time of operation. ence between the cathode (with Ag) and the oxygen ion conductor. Therefore, silver migration is not correlated directly with current density. Similar results were reported Conclusion for the GDC electrolyte [37]. The mechanism responsible for silver mass transport is difficult to distinguish. Silver The application of silver as a cathode conductive material, migration is possibly related to electron transfer and oxy- interconnect wires, and sealing for anode lead connection gen flux. The flow of electrons can promote silver electro - for a µSOFC was studied. The addition of silver as a cathode migration. This can explain the agglomeration of silver at conductive layer reduced the cell overpotential and increased the edge of electrolyte since electrolyte can conduct only the cell performance. However, the results showed that silver ions. The short 1–2 h OCV break in the cell power genera- was also responsible for the cell degradation. Using silver tion affected the performance of tested cells. Rapid cell in SOFC stacks reduces system durability. The silver ther- degradation was often seen directly after OCV. Since the mal expansion did not match with the thermal expansion OCV state is not conductive for thermal or electromigra- of the other cell materials, and therefore, caused damage to tion of silver [35], the cathode polarisation must promote the glass sealant. Silver was also found unstable in the dual silver migration. Moving from one state (including OCV) atmosphere. The results demonstrated that the microstruc- of operation to another could affect cell performance. The ture of the silver anode lead connection wire changed after process of cell passivation/activation resulted in the dif- it had been exposed to the dual atmosphere, the solid silver ference in temperature distribution along and across the wires and the seal became porous. The formation of striation cell and could result in some microcrack formation in structures and porosity affected the mechanical strength of the electrolyte. Also, the risk of coke deposition on the the Ag interconnect wires. anode during OCV was significant because no steam was The cathode polarisation process after OCV promotes formed from fuel electrooxidation and only 60–70% of silver migration what can lead to the cell short-circuit. fuel was converted on the CPOX catalyst [28]. Formed It can be concluded, that silver is not suitable as inter- coke and temperature gradient may also result in microc- connect at the intermediate temperature in the long-term racks formation in the electrolyte structure. However, SEM SOFC application if there is a risk of exposure to the dual scans did not confirm electrolyte cracks. Simner et  al. atmosphere. Silver migration was also responsible for short- [11] observed that fresh cell was not affected by holding circuit formation. it under OCV. This confirms our result that short-circuit occurs directly after a short OCV break in the cell opera- Acknowledgements The results are part of the outcome of the SAFARI project funded under Europe’s Fuel Cell and Hydrogen Joint Undertak- tion if the cell was in operation for several hours. Another ing (FCH JU), Grant Agreement No.325323. The Consortium grate- possible explanation is that during OCV period (no current fully acknowledges the support of the FCH JU. flow through electrodes), silver deposited in the SDC layer was saturated with oxygen. This oxygen was reduced after Open Access This article is distributed under the terms of the Crea- the cell returned to the operation mode with the flow of tive Commons Attribution 4.0 International License (http://creat iveco mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- electrons. According to Simner et al. [11], during OCV, it tion, and reproduction in any medium, provided you give appropriate is possible that the formation of oxygen-containing species credit to the original author(s) and the source, provide a link to the can block the oxygen charge transport. Creative Commons license, and indicate if changes were made. Silver metal diffusion through the electrolyte and ion migration can be a nanoscale phenomenon [38], which is impossible to be detected by SEM during post-mortem References analyses. This can explain the detection of the short-circuit (at room temperature) without any visible cracks in the 1. Camaratta, M., Wachsman, E.: Silver–bismuth oxide cathodes for electrolyte, despite many SEM/EDX cross-section scans. IT-SOFCs; part I—microstructural instability. Solid State Ion- However, this will require rigorous further investigation ics. 178(19–20), 1242–1247 (2007). https ://doi.org/10.1016/j. ssi.2007.06.009 to validate the theory proposed. 2. 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Kumar, A., Ciucci, F., Morozovska, A.N., Kalinin, S.V., Jesse, S.: Publisher’s Note Springer Nature remains neutral with regard to Measuring oxygen reduction/evolution reactions on the nanoscale. jurisdictional claims in published maps and institutional affiliations. Nat. Chem. 3(9), 707–713 (2011). doi: http://www .natur e.com/ nchem /journ al/v3/n9/abs/nchem .1112.html#suppl ement ary-infor matio n 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Application of silver in microtubular solid oxide fuel cells

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Springer Journals
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Copyright © 2018 by The Author(s)
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Materials Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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Abstract

In this paper, the behaviour of silver as cathode conductive material, interconnect wire, and sealing for anode lead connection for microtubular solid oxide fuel cells (µSOFC) is reported. The changes in silver morphology are examined by scanning electron microscopy on cells that had been operated under reformed methane. It is found that using silver in an solid oxide fuel cell (SOFC) stack can improve the cell performance. However, it is also concluded that silver may be responsible for cell degradation. This report brings together and explains all the known problems with application of silver for SOFCs. The results show that silver is unstable in interconnect and in cathode environments. It is found that the process of cell passiva- tion/activation promotes silver migration. The difference in thermal expansion of silver and sealant results in damage to the glass. It is concluded that when silver is exposed to a dual atmosphere condition, high levels of porosity formation is seen in the dense silver interconnect. The relevance of application of silver in SOFC stacks is discussed. Keywords Silver · SOFC · Microtubular SOFC · SOFC stacks Introduction cobalt ferrite (LSCF) decreases at lower operating tempera- ture. High ohmic losses in the cathode result in reduced cell Several configurations of solid oxide fuel cells (SOFC) are performance. This can be overcome by the addition of a commercially available; however, the selection of suitable metal-based current collection layer that enhances the elec- materials and development techniques is still the subject of tronic conductivity. The high-conductive layer ensures adhe- current research. Within an SOFC cell, the cathode is a sig- sion and connectivity between the cathode and the intercon- nificant contributor to the cell overpotential caused by the nect. This consequently reduces the contact resistance of the slow oxygen reduction reaction [1], which occurs at the elec- cathode/interconnect interface. trolyte–cathode–air boundary phase (triple-phase boundary). In planar cells, a silver mesh can be applied to reduce the When the temperature of SOFC is low, the oxygen dissocia- contact resistance between the interconnect and the cathode tive adsorption slows further because of the decrease in cata- [2]. The presence of silver will also act as a catalyst for lytic activity of the cathode oxygen reduction. Key require- oxygen reduction [3]. Silver has been proven as a cathode ments for cathode materials are that they have to be highly material showing good results at intermediate temperatures conductive for electrons and oxygen ions, and they should in SOFC [1, 4–6]. In these tests, the limiting feature was the maintain thermal stability in SOFC operating temperature. length of the oxygen diffusion paths. Moreover, silver does The conductivity of modern cathode materials such as lan- not form a nonconductive oxide layer when exposed to air thanum strontium manganite (LSM) or lanthanum strontium at elevated temperatures. The high resistance to oxidation at elevated temperatures is thus beneficial for SOFC applica - tion. Silver also has one of the highest electrical conductivi- −1 * Artur J. Majewski ties of metals (6.30 × 107 S m at 20 °C), thus the addition a.j.majewski@bham.ac.uk of silver improves electron percolation. Silver is often used Aman Dhir as a conductive material for the cathode in microtubular aman.dhir@wlv.ac.uk solid oxide fuel cells (µSOFC) [7–10]. This has been imple- mented for planar [11] or honeycomb [12] cell designs, too. School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK Silver in a µSOFC can also function as a sealant [13, 14]. Infiltration of silver into the porous cathode is an alternative Present Address: School of Engineering, University of Wolverhampton, Wolverhampton WV1 1LY, UK Vol.:(0123456789) 1 3 16 Page 2 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 method to improve current collection [1, 9]. Infiltration of as an additional cathode conductive layer and for the seal- silver or an alternative highly conductive material such as ing of an anode/lead connection. The aim of this work is to lanthanum strontium cobalt (LSC) can significantly improve examine the degradation mechanisms of a µSOFC cell. current density of the standard LSM cathode [6, 10, 15]. Adding silver into the cathode system has shown to reduce the overpotential related to oxygen reduction [16]. Porous Experimental silver can also function as a cathode for low-temperature SOFC [17] Inert properties and stability in the oxidising The current interruption method was applied for internal atmosphere can make silver a good candidate as a cathode resistance measurements. Electrochemical impedance interconnect material for intermediate-temperature SOFC. spectra were performed using Solartron cell test system However, silver tends to agglomerate during annealing in 1400A/1470E (Potentiostat/Galvanostat). The cell response air or in ambient oxygen [18]. Porous silver tends to become was measured over a frequency range of 1 MHz to 0.1 Hz dense at elevated temperatures. For the mixed Ag/cathode with AC voltage amplitude 10 mV at the open circuit voltage systems of yttrium or erbium stabilized bismuth oxide, 1 h (OCV) condition. Electrochemical performance measure- of sintering was shown to be enough to result in porosity ments were made using the Solartron in galvanostatic mode. reduction, separation of compounds, and increase in Ag Long-term cell tests were conducted under potentiostatic phase [1]. Silver is also known to be prone to migration mode at a set voltage of 0.7 V. Every 24 h the test was inter- at elevated temperature. Compson et al. [19] suggested to rupted by I–V test and impedance scan. use silver as an interconnect for SOFC only at temperatures Analysis of cell morphology: post-cell test the cells were below 650 °C. Above this temperature, loss of silver caused fractured, cell surface and cell cross-section were character- by sublimation, evaporation, and diffusion transport may ised using a Hitachi TM3030Plus scanning electron micro- affect cell performance. scope (SEM). Using silver as a current collector can reduce cell per- The cell temperature was maintained at the temperature formance since the low-temperature melting point of silver of 650 or 700 °C in a tubular furnace (Vecstar HZ/split-tube) requires changes in cell preparation technique [12]. Moreo- and monitored by thermocouples flanking the test chamber. ver, silver can evaporate even at low (300–350 °C) tempera- tures [20]. Evaporation of silver increases with temperature Cell fabrication and is similar in the air and in the reducing environment [21]. Silver does not have high gas penetrability. However, The cell tests were conducted using commercial tubular, silver has high oxygen solubility. The solubility allows appli- anode-supported µSOFCs, of size 15.2 cm in length and cation of silver as an anode for direct carbon SOFC [22]. 6.6 mm in external diameter, of composition Ni-YSZ/YSZ/ Silver tends to migrate when submitted to an electric field SDC/LSCF (yttria-stabilized zirconia (YSZ), samaria-doped at high temperature with of oxygen [23–25]. Silver ions Ag ceria (SDC)). Silver was applied to improve cell’s perfor- can move under the influence of the electrical field [23]. mance. Silver wires (99.9% pure silver, Scientific Wire The presence of Cr can increase the Ag migration into the Company) 2 × 0.71 mm were used to collect current, with cathode. Formation of compounds such as AgCrO with additional six silver wires 2 × 0.20 mm to fasten the current higher evaporation rate than pure Ag increases Ag migra- collector to the cathode. Silver paste (ESL 9907, Electrosci- tion [15]. Migration of silver and formation of conductive ence UK) was used to connect the current collecting wire to filaments can cause short-circuit failures [23]. Singh et al. the exposed anode. The paste was dried at 105 °C for 1 h. [26] observed that exposure to the dual oxidation–reduction After attaching the silver wire, an additional layer of silver environment can damage the silver microstructure. They paste was applied to cover the wire and to improve sealing. observed the formation of pores and cracks on the fuel side This was sintered at 750 °C for 2 h. In addition, the silver of the solid silver barrier. They suggested that dissociation was covered by a commercial glass sealant, which was sin- and dissolution of H and O into silver and formation of tered at 850 °C for 8 h. The cathode was coated with silver steam cause development of pores in solid silver barriers. ink (producer, SPI 5001) to reduce the lateral resistivity and Using silver in SOFC cells and stacks has many benefits to improve the electrical connection between the cathode and for that reason many researchers are trying to improve and the interconnect. The cathode area was 16.6 cm con- SOFC performance using silver compounds. However, the sisting of two separate parts (Fig. 1a–b). The silver ink was incorporation of silver will affect the cell durability. The aim applied by ink brushing the cathode, then drying at 105 °C of this work is to discuss the plausibility of Ag as an SOFC for 1 h. material and to highlight the problems that may occur for The cell exhaust was cemented (using high-temperature application of silver in SOFC. In this work, silver is utilised cement) to a manifold, and the gas feed was connected in the µSOFC systems, as an interconnect, as a cathode or 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 3 of 13 16 Fig. 1 a A tested cell in the furnace; b the anode current connection (sealed by glass); c the surface of glass sealant; d glass sealant at the anode wire connection, visible cracks using a silicone tube so the cell was free to move in the Results and discussion axial direction. Cells were operated at 650 and 700  °C using either hydrogen or methane/air mixture. Inside the Benefits from using silver cell/tube, at the inlet, a partial oxidation catalyst (CPOX) (0.1 g) was inserted in the shape of a honeycomb structure. Usually, tubular cells have large cell voltage and ohmic −1 Hydrogen was introduced into the cell at 145 ml min , losses since electrons have to be transported through the and after 1 h of OCV, a polarisation curve was recorded cathode along the cell. The addition of silver can reduce this (galvanostatic mode) with 0.5 V as a lower safe limit. Then problem. Thus, the surface of the cathode was covered with the voltage was set to 0.7 V (potentiostatic mode) and a silver ink (SPI 5001) for these test cells. The primary role short 1 h constant voltage test was performed. Afterwards, of the silver layer was to deliver and distribute the flux of the fuel was changed to a mixture of CH :air (dry air) with electrons over the whole cathode area. This also helped to molar ratio CH :air of 1:2.4 and a long-term durability test reduce the area-specific resistance (ASR) of the connection −1 was conducted at 0.7 V (CH 48 ml min ). These condi- between the cathode and the interconnect (LSCF-Ag wire). tions were chosen to simulate the realistic conditions of a An additional benefit of adding silver was to support oxygen µSOFC auxiliary power unit (APU) [27]. The fuel gas con- reduction reaction. The silver ink formed a porous layer of nection was outside the hot zone, which eliminated prob- 10–30 μm. The high porosity of the silver layer allowed free lems with sealing. All tests were conducted under ambient gas diffusion. air-condition on the cathode side. Details about the cell For cells tested without Ag coating on the cathode sur- test system were described in previous papers [28, 29]. face, cathode lead wires (0.71 mmAg) were coated with LSCF to improve contact interconnect/cathode. For cells with the Ag cathode coating layer, the wires were coated with Ag ink. 1 3 16 Page 4 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 The additional coating by silver improved power den- overpotential. By improving the cathode, the cell perfor- sity. The power density for the cell increased from 0.05 mance was enhanced mostly by decrease in ohmic resist- −2 to 0.3 W cm at 0.6 V, after the cathode was coated with ance and the overall cell polarisation. silver ink. This was also reflected in the impedance char - To confirm and compare how the various cathode coat- acteristic where covering by silver reduced cell impedance ings affect the cell performance, a selection of cells was (Fig. 2a). OCV was not affected. The power loss for the coated with Pt ink, Ag ink and LSCF ink (to improve con- cell with the LSCF cathode can be attributed to the inter- nection between already sintered LSCF cathode and Ag nal resistance of the cathode/interconnect contact area and wires). These cells were tested at 700 °C. Increasing the to the deteriorated electronic current distribution. Results cell-operating temperature from 650 to 700 °C reduced the presented in Fig.  2 show that the application of a good cell overpotential, for the cell covered with Ag (Fig. 2a–b). electronic conductor such as Ag or Pt not only improves The possible reason for this was the increased conductivity the contact between the cathode and the interconnector, of the cathode and the electrolyte and higher catalytical but also improves electron distribution. Consequently, this activity of the cathode. Slightly larger impedance for Ag leads to a perceived and more homogeneous polarisation was obtained for the cell covered by Pt ink (Fig. 2b). In of the cathode. Without the Ag layer, the cathode was not contrast, for the LSCF coating, the resistance of the cell used to its full performance capability, the cause of this— increased at all frequencies. The cell ohmic resistance for in-plane electronic resistance. Without Ag or Pt, not only Ag (0.03 Ω) was slightly lower than for Pt (0.04 Ω) and is the high-frequency intercept higher, but also the elec- much lower than for LSCF (0.28 Ω). In addition, the size trode impedance is larger. The silver coating significantly of both impedance arcs was slightly smaller for Ag than reduced the internal resistant loss. The ohmic resistance for Pt and much smaller than for LSCF. The cell perfor- decreased from 0.23 to 0.03 Ω. The cell polarisation resist- mance depends on the used current collector. The low- ance still consisted of at least two arcs. Interestingly, the frequency semicircle for Pt and Ag was slightly larger than characteristic frequency of arcs changed only slightly from the high-frequency. The characteristic frequency for high 20 to 50 Hz for the high-frequency arc and stayed around and low-frequency arcs was independent of the type of 0.2–0.3 Hz for the low-frequency arc. The contact resist- cathode coating. ance between the cathode and interconnect was reduced The EIS spectra confirmed the positive aspect of intro- after silver was added. The anode side of the cell and the duction silver to the cathode structure. The data and electrolyte remained unchanged. Altering the composition results presented clearly show that SOFC with the Ag- at one electrode does not affect the impedance of the sec- coated cathode, operating in the temperature region of ond electrode at OCV. A significant change to the cathode 650–700 °C, showed improved performance compared to impedance may overlap the anode impedance. The LSCF similar cells without silver in the cathode system. cathode system had a significant contribution to the cell −1 Fig. 2 Electrochemical impedance spectroscopy: a cells with LSCF and LSCF + Ag cathode. 650 °C, H 145 ml min ; b cells with the cathode −1 coated with Ag, Pt and LSCF 700 °C, H 145 ml min 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 5 of 13 16 interconnect is followed by diffusion and reaction of dis - Anode Ag sealing interconnect solved species and water vapour formation. The hydrogen and oxygen concentration in silver increases up to a critical The whole area of the exposed anode was covered by silver paste to improve the electrical connection and to part-seal pressure and bubbles are generated. The increase in the size of the pores in the silver layer can finally result in fuel leak - the anode. The silver was densified by sintering. However, as the thermal expansion coefficient of silver is higher than age. According to Jackson et al. [30], 24 h of exposure to the dual atmosphere is enough to form pores across a 1-mm that of the ceramic cell, it is expected that the connection between the cell and the interconnect further improves thick silver membrane. The high temperature of fuel com- bustion and possible anode oxidation at the middle of the through the silver expansion during heating. Silver is duc- tile and should easily deform under thermal stress avoiding anode can result in crack formation and damage to the cell. The fact that porosity always began to form from the fuel delamination and keep the cell hermeticity. However, using only silver paste as a sealant resulted in poor sealing. Fuel side of the silver layer suggests faster solubility of oxygen than hydrogen. Also the ratio of O:H 1:2 in water promotes leakage was seen, indicated by high temperatures detected by the thermocouples around the anode current connec- nucleation closer to the hydrogen source. The diffusiv- ity and highest concentration in silver under dual atmos- tion. The cells with only silver as a sealant could operate −4 2 −1 for around 24–50 h. To investigate the reason for the weak phere at 800 °C is, respectively [30]: H 2.9 × 10  cm s ; −7 −3 −5 2 −1 6.1 × 10   mol  cm, O 1.1 × 10   cm  s ; sealing, the cell was tested at 700 °C with H (3% H O) 2 2 2 −5 −3 for 8 h under OCV. The H leak test conducted after silver 1.1 × 10  mol cm . The rapid degradation of silver mechanical integrity and sintering indicated gas tightness of the silver/anode joint at room temperature. hermeticity after exposure to dual atmosphere affects the cell performance. Formation of pores in the silver also has During this test, the temperature at the anode current con- nection increased approximately 100 °C above the operating an influence on the stability of the silver seal. Such degrada- tion of the structure leads to fuel leakage. For this system of temperature and the connection became red-hot even at a furnace temperature of 700 °C. After this test, the surface tubular cells with the anode connection at the middle of the cell, the silver seal would be exposed to the dual atmosphere. of silver exposed to the fuel side indicated degradation, and the dense silver developed into a porous structure. The layer Therefore, if silver is applied as a sealant/current collector for a µSOFC, it has to be isolated to avoid exposure to the of silver delaminated from the anode surface creating a gap (Fig. 3a) indicated the increase in cell impedance. Further to dual atmosphere. The additional sealing of the silver by glass was needed to extend the cell operation. This was achieved this, SEM scans showed that the degradation of solid silver started from the fuel side of the silver by developing poros- by coating the silver anode connection by a 10 µm layer of glass sealant. This extended the life of the µSOFC up to ity (Fig. 3b). Singh et al. [26] suggested that pores and voids in silver more than 800 h. However, due to the difference in thermal expansion are developed because of water vapour formation. Adsorp- tion and dissolution of H and O gaseous molecules in silver coefficient of silver and glass, there is a risk of damage to Fig. 3 SEM scans: a the cross-section of the anode current connection (Ag-YSZ-Ni/YSZ, 8 h at 700 °C); a visible gap formed between the anode and the silver sealant; b the cross-section of silver sealant near the air side 1 3 16 Page 6 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 the glass coating. The components that have much lower heat generated by fuel combustion could be enough to melt thermal expansion coefficient than silver are YSZ 10.5, Ni/ the glass and produce pinholes. For future work, the glass YSZ 12.5, SDC 12.8 and cathode 14.6, compared to silver coating technique has to be improved. −6 −1 18.9 × 10  K . The glass sealant’s thermal expansion coef- In this study, the long-term stability of the Ag-based ficient was selected to match the expansion of the cell com- anode current collection was examined. For the cells tested −6 −1 ponents and was around 12 × 10  K . With this mismatch for more than 850 h, not only the degradation of the silver in thermal expansion, significant internal stress is created. sealing layer (Ag paste) was observed, but additionally, After the glass sintering (at 850 °C for 8 h), there was no vis- a significant disintegration of the silver current collect- ible damage to the glass surface. However, the long-term cell ing wire fastened to the anode. The cell with the CPOX operation at elevated temperature or thermo-cycling resulted catalyst was fed with the mixture of CH and air in the in the crack formation in the glass structure (Fig. 1d). These molar ratio 1:2.4. Approximately, 70% of the fuel was studies suggest that the high silver coefficient of thermal converted on the catalyst before it reached the anode [28]. −2 expansion can create problems for application of silver as an The power of the cell was around 0.14 W cm at 0.7 V. anode interconnect (and sealant) for the µSOFCs even with Low operation temperature (650 °C) decreased the rate of a glass coating. For this specific application, the rapid start oxygen reduction and conductivity of the YSZ electrolyte. and cooling required by APU could favour crack formation Post-mortem analysis showed part of the wire close to the within the glass sealant. anode surface was damaged. The silver anode current col- The glass surface showed visible pinhole formation, lecting wire and all the silver interconnect had become approximately 1 μm in diameter. Several effects, including highly porous. The solid silver appeared in a sponge-like the application/deposition technique, can cause this effect. structure (Fig. 4a–b). In this case, it has been attributed to the irregular sur- Pores formed across all surface areas of Ag wire. The face of silver to which it was applied. Due to the complex silver wire at the anode connection became brittle. Sur- shape of wire/anode connection, it was difficult to obtain a prisingly, the cell performance was stable for more than smooth silver surface. Pinholes usually indicate localised 850 h of operation under such gas leakage conditions. For- microbubbles or dewetting that degassed during sintering. mation of silver crystals damaged the glass sealing and Contaminants like dust particles can cause a formation of increased the exposure of the wire to air. These results localised microbubbles. This could come from insulation did not confirm Compson’s conclusion [19] that for silver material. The glass coating thickness is in a micron range, interconnects 650 °C is a safe operating temperature. After and it is possible for the debris to be larger in diameter 850 h of operation at 650 °C, all the interconnect wire than the coating thickness. The electrostatic attraction is and sealing paste was significantly deformed and dam- enough to hold small particulates on the surface. Some aged. It can be concluded that silver paste should not be particulates could also have a high moisture level. In addi- used as a sealant for the anode connection for long-term tion, if there are existing cracks on the glass surface, the cell operation. −2 Fig. 4 SEM scans: a the cross-section of the anode current collection wire; b the cross-section of silver wire; after 850 h at 650 °C ~ 0.2 A cm with CH reformate as a fuel 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 7 of 13 16 (Fig. 5c). This part of the wire was only 10 mm from the Ag wires anode connection (presented in Fig. 4), and after damage to the glass the coating was exposed to the dual atmosphere. Samples of Ag wire (taken 1 cm away from the cell) from the anode and the cathode side were analysed to check the The damage to the silver wire microstructure was observed even 10 mm from the point of wire exposure to the dual stability of silver as an interconnect wire. These samples of wires after long-term cell testing were compared with air–fuel atmosphere after long operational time. After sev- eral hours of operation, silver wires became brittle. This unused wires to detect any visible microstructural changes that had occurred during cell operation. An erosion of the affected the mechanical strength and integrity. Even low physical stress could break the wire. This is important if silver wire surface is visible on the SEM scans (Fig. 5a–d). The initially smooth surface of tested wires developed facet- a stack with silver wires is considered for an APU unit in vehicles where the system must resist vibrational stresses ing and striation structures after being exposed to air at 650–700 °C. Faceting is related to surface energy minimisa- and strains. A cell with such brittle wires would be prone to damage caused by vibration. The risk of breaking the tion. According to Lu and Zhu [21], striation and faceting of the silver surface occur only in an oxygen atmosphere. Facet- wire connection is significant. All these problems with silver will lead to degradation of the cell structural stability and ing made of large structures developed on the surface of Ag wire at the cathode side after 850 h at 650 °C (Fig. 5a–b). in long-term operation to the degradation of the cell/stack performance. Zhong et al. [12] using Ag mesh for cathode At higher operating temperature (450 h at 700 °C) the structures formed were more intensive; all surfaces of the current collector did not observe this type of degradation. However, they used Pt paste to improve the strength of the wire had changed; however, the structures were smaller (Fig. 5d). Wires at the anode side behaved differently. Some connection, and the test time was short. Silver commercially available has the purity of 99.95%. Silver is rarely used by faceting was visible, but significant wire area became porous Fig. 5 SEM scans of the surface of Ag interconnect wire (1 cm from the cell) after test under constant current for 850 h at 650 °C: a, b cathode lead; c anode lead; d the surface of Ag cathode wire after 450 h at 700 °C 1 3 16 Page 8 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 industry in pure form caused by its softness and suscepti- before glass sintering. This exposed the porous silver to the bility to damage. Also for SOFC application, silver alloys high temperature of 850 °C for 8 h. The reverse fabrica- should be considered instead of pure silver. Where higher tion, with sintering glass before coating the cathode with strength at elevated temperature is required, silver-palladium silver, reduced silver degradation. However, it increases the alloys are more suitable; however, that would increase the complexity of the cell mass production. The Ag current col- cost. Also the application of Ag(Al) alloys can enhance the lection film can be applied after the glass sintering for a thermal stability of silver [31]. The addition of aluminium single cell, although this process is impractical for the stack reduces silver agglomeration at elevated temperature and manufacture. prevents diffusion of silver into the matrix of support mate- It can be concluded that cells with silver require low- rial. Wires with high Ni, Fe or Cr content are less suitable temperature sealing materials. Tested cells were operated because of formation of a nonconductive oxide layer and at the moderate temperature of 650–700 °C to avoid silver reduced conductivity between the cathode and the intercon- exposure to high temperature. The cathode current collection nect. The addition of other elements and alloy formation on Ag wires were attached to the cathode after the glass sinter- the silver surface may be the solution for application of silver ing process. The cathode Ag coating was brush-painted after current collectors in SOFC. Silver wires coated with gold are the glass sintering. The porous structure of silver coating slightly more expensive than pure silver. allowed gas distribution and improved current collection. Observed agglomeration of silver (Fig. 6) reduced all these Cathode conductive layer benefits. Part of the cell power degradation is caused by silver agglomeration. Silver agglomeration and formation of The risk of vaporisation and sublimation of silver exposed holes and hillocks followed by formation of islands (Fig. 6a) to air increases at higher reaction and operating tempera- increased electrical resistivity. The formation of solid sil- tures. The vaporisation rate depends on temperature and ver increased after the long-term (850 h) testing. Faceting atmosphere, and is higher in air than in H /H O atmosphere. of agglomerated silver was visible (Fig. 6b). From the cell 2 2 Meulenberg et al. [2] extrapolated that up to 2% of silver can surface images, it is evident (Fig. 6) that the silver layer has evaporate after 40,000 h at 690 °C in air. The evaporation either evaporated or diffused into the LSCF coating layer will increase with the increase in the silver surface area. from the original deposition layer. Change in the surface The cathode silver layer used in presented experiments was morphology affected the electrical conductivity of the silver porous. Considering high porosity of the silver layer and layer. After several hours of operation and formation of iso- its thickness 10–30 μm, a significant part of the silver can lated silver ‘islands’ on the cathode surface, the role of silver evaporate after the cell’s lifetime. The glass sealant, which as an electron distribution to the whole cathode area was is applied to maintain gas tightness, also poses a problem as significantly reduced. The silver after agglomeration did not glass sealants usually require sintering at 850 °C for several provide a continuous electronic pathway through the cath- hours—leading to evaporation of silver into and through ode. This also affected the function of silver as an electron the glass matrix. In our previous report [29], the cathode conductor from the cathode to interconnect and increased was coated with silver ink to receive good current collection ASR of the connection between the cathode and attached Ag Fig. 6 SEM of the cathode surface covered by Ag: a after 100 h at 650 °C; b after 850 h at 650 °C 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 9 of 13 16 lead wire. Creating a long oxygen diffusion path through the However, from our experience, application of lower viscos- bulk silver limits also the quantity of oxygen transported to ity silver ink directly on the LSCF cathode can sometimes the electrolyte. Lower silver porosity reduced oxygen reduc- result in a cathode flaking. tion zone. Agglomeration of silver reduces the contribution Several things can cause cathode delamination. The of silver to the catalytic activity of the cathode. Lower silver peeling of the cathode was more frequently observed when porosity reduces the length of the three-phase boundary. On slightly thicker silver layers were utilised. The thickness of the other hand, during silver agglomeration and formation the silver layer is difficult to control during hand brush paint- of a dense silver phase, a significant part of the cathode was ing. Therefore, it can be speculated that increased shrinkage exposed allowing free gas diffusion. Camaratta and Wachs- of the outer layers of the silver film forced the cathode to curl man [1] observed a similar reduction in silver porosity for away. Shrinkage of silver created stress in the cathode film Ag-YSB and Ag-ESB cathode systems. They suggested that that can lead to cracking, curing and flaking. Cracks in the agglomeration of silver and reduction of porosity diminished silver coating were often initiated along the Ag lead wire cathodic reaction zone and reduced gas transport. Coating connection. However, the formation of cracks in the cath- the cathode with silver may deteriorate oxygen reduction ode structure near the edge of silver grains formed by silver reaction. Sasaki et al. [32] observed that dense silver reduced agglomeration was observed (Fig. 7b). That could increase gas transport to the significant part of the cathode. the risk of silver penetration through the electrolyte and cre- It can be concluded that initial morphological structure of ated the risk of cathode delamination. Other factors, which the porous silver current collector was destroyed as a reason may lead to delamination, are in the SDC layer fabrication of silver sintering and evaporation. The silver microstruc- during the cell preparation. The SDC layer prevents reac- ture was unstable at intermediate cell operating temperature. tion between cathode and electrolyte compounds. At the Therefore, for a long-term application, silver is not suitable interconnect/SCD interface, formation of the separation as a conductive material. Mixing silver with the ceramic area was occasionally observed. These cracks were paral- phase or infiltration of silver with ceramic precursors could lel to the electrolyte surface. However, usually the parallel prevent silver densification. The ceramic shell can restrain crack does not disturb the cathode stress field, as the stress densification of silver porous structure [9 ]. Another method also acts parallel to the electrolyte surface. However, if the is to incorporate silver into a ceramic matrix of the cath- parallel crack was followed by buckles formation, this would ode [19]. The decrease in temperature operation of SOFC generate a large tensile stress and result in cathode spalla- can increase the stability of silver. Zhu et al. [3] obtained tion. These parallel cracks increased the risk of interrupting high performance of the cathode with silver nanoparticles the electrical connection between interconnect and cathode. at 500 °C. The stability of Ag nanoparticles was increased It can be speculated that the calcination of SCD was con- by the strong metal-support (Ag–SNC) bonding interaction. ducted at very low temperature. The difference in thermal Mixing silver with SSZ can reduce silver agglomeration on expansion coefficient may create internal stress and cause the cathode. Morphology of this cermet is stable at 500 °C slow separation of the cathode layer from the electrolyte dur- [32]. Application of silver ink with thinner can improve the ing the cell operation. The cell delamination could increase wetting and impregnation of silver into the cathode matrix. the cell internal resistance. Usually, delamination is caused Fig. 7 SEM scans: a cells with cathode exfoliation, b the surface of the Ag-coated cathode, visible crack formation 1 3 16 Page 10 of 13 Materials for Renewable and Sustainable Energy (2018) 7:16 by mismatching in thermal expansion coefficient of cell may temporarily increase the cell performance because of parts. Changes to LSCF surface morphology caused by the increase in the three-phase boundary, but the silver deposits chemical and thermal reactions of LSCF may lead to oxide may short-circuit the cell. Deposition of silver increased the segregation. Decomposition and transformation of cath- risk of silver penetration into pinholes in the electrolyte and ode materials can also be a result of cathode delamination. consequently short-circuit formation [36]. The silver migra- Cobalt instability and decomposition of perovskite were tion was noticed only for the part of the cell near the cell reported in the literature [33]. If the cathode was partially inlet. This was the most active part of the cell with highest delaminated, the cathodic polarisation would be enlarged current density caused by high fuel partial pressure. This because of concentration in current conduction. The current confirmed that silver migration was responsible for short- collection efficiency was reduced. An electrode delamination circuit formation. No silver migration was detected for parts results in the interruption of the ionic path, and detachment of the cell beyond 2 cm from the cell inlet (Fig.  8a). No of the interconnect interrupts the electronic path. silver diffusion into or reaction with the YSZ electrolyte was Migration of silver and agglomeration at the electrolyte observed. Camaratta and Wachsman [1], after testing ESB interface could also result in cathode peeling [34]. Silver and YSB electrolytes with Ag in the cathode, also concluded tends to agglomerate at elevated temperature, which can lead that no silver diffused into the electrolyte. to cathode delamination. However, no silver was detected The mechanism responsible for migration of silver and in the SDC layer for samples where cathode delamination condensation across the SDC layer is unclear. It is unlikely or cracks in the SDC layer occurred. In addition, no silver that thermal effect alone is responsible for Ag migration was detected on the electrolyte for areas where the cathode to SDC. The temperature distribution along the cell was exfoliation occurred. Therefore, silver penetration into SDC relatively uniform and silver deposited in the SDC layer was not responsible for the cathode delamination. was observed only at the cell inlet (the most active part of For some cells, rapid degradation of performance was the cell). Silver migration could be a result of electromi- observed. Post-mortem inspection of these cells con- gration [6] or it can be associated with the formation of Ag firmed short-circuit formation. The short-circuit effect was (g) species and vapour transport to the cathode–electrolyte observed after few hours of cell operation. Short-circuits face followed by reduction to metal phase [11]. Ag (g) has were generated always at the cell area near the cell inlet. higher vapour pressure than A g O (g) at the SOFC opera- The exact location of short-circuit was difficult to deter - tion temperature. Therefore, it is thought Ag (g) formation mine. Looking at the SEM images of the cell cross-section and penetration in SDC are possible for this mechanism. (Fig. 8a), the Ag layer porous structure is visible. Results of However, electromigration and evaporation/deposition EDX elemental mapping of the cross-section of the cell after of silver are too slow to be alone directly responsible for testing at 700 °C are presented in Fig. 8. For some samples, the observed rate of silver migration during the electrode it was observed that silver accumulated in the SDC layer polarisation [37]. Mosialek et  al. [16] observed silver (Fig. 8b). Our results conr fi med De Silva et al. [ 35] observa- migration on the YSZ electrolyte in the potential range tions that silver may migrate from the cathode to the surface − 0.2–0.5 V; no migration was observed without polari- of the electrolyte. The contact of silver with an electrolyte sation. In addition, no migration was observed without Fig. 8 SEM micrographs of the cell after 48 h operation at 700 °C: a the cross-section of the cell 2 cm from the CPOX catalyst; b the cross- section of the part of the cell near the CPOX 1 3 Materials for Renewable and Sustainable Energy (2018) 7:16 Page 11 of 13 16 the electrolyte. They concluded that for the formation of [11]. However, the performance of the Au conductive layer silver dendrites, it is necessary to have a potential differ - may decrease with time of operation. ence between the cathode (with Ag) and the oxygen ion conductor. Therefore, silver migration is not correlated directly with current density. Similar results were reported Conclusion for the GDC electrolyte [37]. The mechanism responsible for silver mass transport is difficult to distinguish. Silver The application of silver as a cathode conductive material, migration is possibly related to electron transfer and oxy- interconnect wires, and sealing for anode lead connection gen flux. The flow of electrons can promote silver electro - for a µSOFC was studied. The addition of silver as a cathode migration. This can explain the agglomeration of silver at conductive layer reduced the cell overpotential and increased the edge of electrolyte since electrolyte can conduct only the cell performance. However, the results showed that silver ions. The short 1–2 h OCV break in the cell power genera- was also responsible for the cell degradation. Using silver tion affected the performance of tested cells. Rapid cell in SOFC stacks reduces system durability. The silver ther- degradation was often seen directly after OCV. Since the mal expansion did not match with the thermal expansion OCV state is not conductive for thermal or electromigra- of the other cell materials, and therefore, caused damage to tion of silver [35], the cathode polarisation must promote the glass sealant. Silver was also found unstable in the dual silver migration. Moving from one state (including OCV) atmosphere. The results demonstrated that the microstruc- of operation to another could affect cell performance. The ture of the silver anode lead connection wire changed after process of cell passivation/activation resulted in the dif- it had been exposed to the dual atmosphere, the solid silver ference in temperature distribution along and across the wires and the seal became porous. The formation of striation cell and could result in some microcrack formation in structures and porosity affected the mechanical strength of the electrolyte. Also, the risk of coke deposition on the the Ag interconnect wires. anode during OCV was significant because no steam was The cathode polarisation process after OCV promotes formed from fuel electrooxidation and only 60–70% of silver migration what can lead to the cell short-circuit. fuel was converted on the CPOX catalyst [28]. Formed It can be concluded, that silver is not suitable as inter- coke and temperature gradient may also result in microc- connect at the intermediate temperature in the long-term racks formation in the electrolyte structure. However, SEM SOFC application if there is a risk of exposure to the dual scans did not confirm electrolyte cracks. Simner et  al. atmosphere. Silver migration was also responsible for short- [11] observed that fresh cell was not affected by holding circuit formation. it under OCV. This confirms our result that short-circuit occurs directly after a short OCV break in the cell opera- Acknowledgements The results are part of the outcome of the SAFARI project funded under Europe’s Fuel Cell and Hydrogen Joint Undertak- tion if the cell was in operation for several hours. Another ing (FCH JU), Grant Agreement No.325323. The Consortium grate- possible explanation is that during OCV period (no current fully acknowledges the support of the FCH JU. flow through electrodes), silver deposited in the SDC layer was saturated with oxygen. This oxygen was reduced after Open Access This article is distributed under the terms of the Crea- the cell returned to the operation mode with the flow of tive Commons Attribution 4.0 International License (http://creat iveco mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- electrons. According to Simner et al. [11], during OCV, it tion, and reproduction in any medium, provided you give appropriate is possible that the formation of oxygen-containing species credit to the original author(s) and the source, provide a link to the can block the oxygen charge transport. Creative Commons license, and indicate if changes were made. Silver metal diffusion through the electrolyte and ion migration can be a nanoscale phenomenon [38], which is impossible to be detected by SEM during post-mortem References analyses. This can explain the detection of the short-circuit (at room temperature) without any visible cracks in the 1. Camaratta, M., Wachsman, E.: Silver–bismuth oxide cathodes for electrolyte, despite many SEM/EDX cross-section scans. IT-SOFCs; part I—microstructural instability. Solid State Ion- However, this will require rigorous further investigation ics. 178(19–20), 1242–1247 (2007). https ://doi.org/10.1016/j. ssi.2007.06.009 to validate the theory proposed. 2. 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Kumar, A., Ciucci, F., Morozovska, A.N., Kalinin, S.V., Jesse, S.: Publisher’s Note Springer Nature remains neutral with regard to Measuring oxygen reduction/evolution reactions on the nanoscale. jurisdictional claims in published maps and institutional affiliations. Nat. Chem. 3(9), 707–713 (2011). doi: http://www .natur e.com/ nchem /journ al/v3/n9/abs/nchem .1112.html#suppl ement ary-infor matio n 1 3

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