High-Temperature Current Collection Enabled by the in Situ Phase

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High-Temperature Current Collection Enabled by In Situ Phase Transformation of Cobalt-Nickel Foam for Solid Oxide Fuel Cells Insung Lee, Mi Young Park, Hyo-Jin Kim, Jong-Ho Lee, Jun-Young Park, Jongsup Hong, Kyeong-Il Kim, Manho Park, Jung-Yeul Yun, and Kyung Joong Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13116 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

High-Temperature Current Collection Enabled by In Situ Phase Transformation of Cobalt-Nickel Foam for Solid Oxide Fuel Cells

Insung Lee1, Mi Young Park2.3, Hyo-Jin Kim2,3, Jong-Ho Lee2, Jun-Young Park4, Jongsup Hong5, Kyeong-Il Kim6, Manho Park7, Jung-Yeul Yun8, Kyung Joong Yoon2*

1

PG-NCM PJT, Research Institute of Industrial Science & Technology, Incheon, Korea

2

High-Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul, Korea 3

4

Department of Materials Science and Engineering, Korea University, Seoul, Korea

Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, Korea 5

School of Mechanical Engineering, Yonsei University, Seoul, Korea 6

R&D Group, EG Corporation, Geumsan, Korea 7

8

Alantum, Seongnam-City, Korea

Powder Technology Department, Korea Institute of Materials Science, Changwon, Korea

1

ACS Paragon Plus Environment


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*Corresponding Author at: Korea Institute of Science and Technology High-Temperature Energy Materials Research Center Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea Tel: +82-2-958-5515 Fax: +82-2-958-5529 E-mail: [email protected]

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Abstract For the commercial development of solid oxide fuel cells (SOFCs), cathode current collection has been one of the most challenging issues because it is extremely difficult to form the continuous electric paths between two rigid components in a high-temperature oxidizing atmosphere. Herein, we present a Co-Ni foam as an innovative cathode current collector that fulfills all the strict thermochemical and thermomechanical requirements for use in SOFCs. The Co-Ni foam is originally in the form of a metal alloy, offering excellent mechanical properties and manufacturing tolerance during stack assembly and start-up processes. Then, it is converted to the conductive spinel oxide in situ during operation and provides nearly ideal structural and chemical characteristics as a current collector, gas distributor and load-bearing component. The functionality and durability of the Co-Ni foam are verified by unit cell test and 1 kW-class stack operation, demonstrating performance that is equivalent to that of precious metals, as well as an exceptional stability under dynamic conditions with severe temperature- and current-variations. This work highlights a cost effective technique to achieve highly reliable electric contacts over the large area using the in situ metal-to-ceramic phase transformation that could be applied to various high-temperature electrochemical devices.

Key words: current collection, foam, cobalt-nickel alloy, spinel oxide, solid oxide fuel cell 3



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1. Introduction Solid oxide fuel cells (SOFCs) offer one of the most efficient and environmentally friendly means of converting the chemical energy of fuel to electric power

1-3

. With the

significant improvement of the unit cell performance, the focus is currently shifting to stack technologies because of the significant performance loss that is frequently encountered during the scale-up of the cell size and integration of multiple cells 4-5. Based on the extensive analysis of cell and stack performance, the electrical contact between the cathode and interconnect has been identified as one of the most important factors determining such performance loss because it is extremely difficult to form the continuous conduction path between two rigid components over the large area in a high-temperature oxidizing atmosphere

6-12

. Recently, considerable research efforts have been devoted to addressing this

issue. The simple and effective approach to form the cathode-interconnect contacts has been to incorporate the meshes or layers made of precious metal such as Pt, Au and Ag

13-15

, but

advancing this approach beyond the lab-scale experiments to practical application is not viable because of high costs. The application of the ceramic contact layer between the cathode and interconnect has been proposed as a more effective measure from the economic point of view. Various perovskite oxides, mostly standard or slightly modified cathode materials, have been evaluated for use as the contact layer

6-7, 14, 16-21

, and the positive effect

of some of the compositions has been verified in the stack-level experiments 7, 22-26. However, the lack of structural compliance is the major weakness of the contact layer approach. The contacts between the rigid components are inevitably exposed to a certain degree of structural irregularity, and furthermore, the stacks experience elastic and plastic deformation during the initial heat-up and subsequent operation. Under such circumstances, the thin contact layer cannot accommodate substantial geometric variations, making the electrical contacts prone to 4


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structural damage. To overcome the structural limitation of the thin contact layer, stainless steel mesh

27-28

and ceramic foams

29-31

have been proposed as an alternative. Stainless steel

mesh with a ceramic protective coating could provide a continuous electrical contact without impeding the air flow

27-28

. However, it is technically challenging to completely cover the

metal mesh with a dense ceramic coating layer, and the presence of the uncovered area or open porosity in the coating leads to the rapid growth of oxide scale and Cr evaporation. On the other hand, perovskite ceramic foams such as (La,Sr)(Mn,Co)O3

29

and (La,Sr)(Co,Fe)O3

(LSCF) 30-31 offer several advantages such as decent electrical conductivity, large contact area, high gas permeability and load bearing capability. However, their practical use in real stacks has not been successful because ceramic foams with sufficient porosity are extremely weak and brittle, causing mechanical damage during handling. To summarize, all existing techniques for cathode current collection have their inherent weaknesses, and technical innovation is urgently required to transfer the high unit cell performance to the practical stacks without significant performance loss. Herein, we report a Co-Ni foam as a novel cathode current collector that could overcome all the aforementioned limitations of the conventional techniques. The unique concept developed in this study is to use a metallic Co-Ni alloy foam in the initial assembly and to convert it into a conductive spinel oxide during operation. The metallic Co-Ni foam provides excellent mechanical properties to eliminate the risk of structural damage in assembly and start-up processes. The Co-Ni spinel oxide formed by in situ phase transformation at the initial stage of operation is electrically conductive, thermally stable and chemically compatible with other cell/stack components. The Co-Ni foam is cost-effective compared to the use of precious metals and offers higher dimensional tolerance than ceramic contact layers. Unlike stainless steel meshes, there is no risk of the resistive oxide scale 5



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growth and Cr evaporation, and it is significantly easier to handle compared to the ceramic foams. Therefore, the Co-Ni foam could potentially be the flawless current collector that fulfills all of the strict requirements for use in SOFCs. In this study, the basic properties of the Co-Ni foam such as phase, composition and structure were examined, and its functionality and stability were evaluated by area specific resistance (ASR) measurements under static and thermal cycling conditions. The performance of the Co-Ni foam was compared with that of the Pt mesh via unit cell test, and finally, it was applied to a 1 kW-class stack and evaluated under dynamic conditions with severe variations in temperature and electric current load. The impact of this innovative concept is discussed in detail, and a strategy for further improvement is proposed.

2. Experimental The Co-Ni alloy foam was custom-fabricated by Alantum (Korea). For fabrication of Co-Ni foam, polyurethane foam was used as a sacrificial skeleton. Thin Ni film was deposited on polyurethane foam by physical vapor deposition (PVD), and Co-Ni alloy was applied on the Ni layer by electroplating. Then, the polyurethane skeleton was removed by heat treatment at 900oC in H2 atmosphere, and pure Co-Ni alloy was obtained. Its photograph and optical microscope images are shown in Figure S1 (a) and (b), respectively, in Supporting Information. Phase analysis was performed using X-ray diffraction (XRD) after heattreatment at 400, 500, 600 and 700°C for 2-100 hours in air. The heating and cooling rates of 5oC min-1 were used for all the heat treatments. The microstructure of the foam was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The compositional analysis was performed using energy-dispersive Xray spectroscopy (EDS). 6


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For ASR measurements, the Co-Ni foam with the area of 1cm*1cm was placed between two stainless steel interconnect (SS460FC developed by POSCO, Korea) blocks, and pressure was applied using a spring load. Two Pt wires were welded to each interconnect block, and the ASR was measured by a 4-probe method. The schematic of the ASR measurement setup is displayed in Figure S2 in Supporting Information. The ASR data were collected at 700°C for over 10,000 hours in a static condition, and the ASR measurements were also performed through 30 thermal cycles. The thermal cycles were conducted between 700°C and room temperature at ramping and cooling rates of 5°C/min, and the interval between each cycle was 12 hours. The Co-Ni foam was evaluated as a cathode current collector in unit cell tests. The anode-supported planar cells were supplied by Research Institute of Industrial Science & Technology (Korea). The cell size was 5 cm × 5 cm, and the active cathode area was 4 cm × 4 cm. The cells consisted of a Ni–yttria-stabilized zirconia (YSZ) anode support, a Ni-YSZ anode functional layer, a YSZ electrolyte, a gadolinia-doped ceria (GDC) diffusion barrier layer, a La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-GDC cathode functional layer and an LSCF cathode top layer. The SEM image of the cross-section of the cell is displayed in Figure S3 in Supporting Information. The cells were placed between two fixtures made of Crofer 22 APU and sealed using a glass–ceramic sealant. As a cathode current collector, the Co-Ni foam (4 cm*4 cm) was used for one cell, and Pt mesh (4 cm*4 cm) was used for the other. Ni foam was used for current collection on the anode side. The schematic of the unit cell test setup is shown in Figure S4. The assembled test setup was placed in the furnace and heat treated at 850°C for 5 h with an applied load of 30 kg to form a tight seal. Then, 500 sccm of humidified hydrogen (3% H2O) and 1000 sccm of air was supplied to the anode and cathode, respectively. After the reduction of NiO to metallic Ni at the anode, electrochemical 7



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characterization was performed at 700°C using a Scribner 890C Fuel Cell Test System and a Solarton 1260/1287 potentiostat-frequency response analyzer. The Co-Ni foam was also evaluated in a 1 kW-class stack developed by EG Corporation (Korea) and Research Institute of Industrial Science & Technology (Korea). The 1 kW-class stack was constructed using 30 anode-supported cells with the cell size of 12 cm*12 cm, glass-ceramic sealants, stainless steel interconnects (SS460FC), Co-Ni foam cathode current collector and Ni foam anode current collector. After conditioning at 700°C, the current-voltage (I-V) curve was obtained up to the total current of 50 A, followed by durability tests in dynamic conditions. In each dynamic testing period, 15 load cycles (120 200 mA cm-2 @ 2 mA cm-2 min-1), 5 load trips (0 - 200 mA cm-2, abrupt on and off) and 5 thermal cycles (25 - 700°C @ 1°C min-1) were performed sequentially. The temperature- and current-profiles in one dynamic testing period are illustrated in Figure S5 in supporting information. A total of 5 dynamic testing periods were repeated while monitoring the change of the stack voltage. The total stack operation time reached ~3,000 hours. The details of the stack development will be reported in our forthcoming paper.

3. Results and Discussion Since the early stage of SOFC development, numerous advantages of the ceramic foams have been acknowledged as a stack component that can serve as a cathode current collector and gas distributor 29-31. However, their practical application has not been successful because of the brittle nature and poor mechanical strength that makes handling extremely difficult and inevitably causes breakage during the stack assembly. To overcome these inherent limitations of the ceramic foams, we developed a metallic Co-Ni foam that transforms into a conductive spinel oxide in situ during high-temperature operation. 8


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Specifically, the Co-Ni foam is originally in a metallic state and offers excellent strength and flexibility during stack integration and initial heating. It then transforms into a conductive spinel oxide at the operating temperature and functions as a current collector. The conductivity of Co-Ni spinel oxide with the same composition was measured to be 21 S cm-1 at 700oC (Figure S6 in Supporting Information), which is considered to be sufficient for functioning as a cathode current collector only if the desired structure is formed

19, 32

.

Therefore, for proper functioning, the intended phases must be formed at the individual stages. The thermal evolution of the Co-Ni phase was investigated using XRD with the results shown in Figure 1 (a). In this experiment, the XRD patterns of CO-Ni foam were collected after the heat treatment at 400-700°C for 2-100 hours. As-fabricated Co-Ni foam is found to be the mixture of face centered cubic (FCC) Co-Ni alloy and hexagonal close packed (HCP) Co. The HCP structure is stable for the pure Co metal, and the addition of a certain amount of Ni to Co could stabilize the FCC structure for the alloy 33. No significant change in the XRD pattern is observed up to 400°C, and the thermal oxidation that forms the Co-Ni spinel oxide and CoO is initiated at 500°C. The thermal oxidation progressively proceeds upon further heating, and after heat treatment at 700°C for 2 hours, the metallic phases completely disappear, resulting in the mixture of Co-Ni spinel oxide and CoO. Prolonged heat treatment at 700°C for 100 hours leads to the formation of single phase Co-Ni spinel oxide, and no indication of remaining CoO is found. This sample was crushed to fine powder and examined with XRD. All the peaks were indexed to be Co-Ni spinel, and no indication of the presence of Co-Ni alloy was detected, indicating that the entire sample was completely oxidized. The thermal evolution of the Co-Ni phase observed in Figure 1 (a) could be explained by the oxidation kinetics of Co and Ni. Cobalt and nickel form a continuous solid solution, and their oxides are mutually soluble oxides are cation-deficient p-type semiconductors

35-37

34

. Both cobalt and nickel

, and the oxidation predominantly

9



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proceeds through the outward diffusion of cations via lattice defects in the oxide scale to the surface where they react with adsorbed oxygen

36, 38

. In this process, the rate controlling step

is the diffusion of the cations through the oxide film, and the diffusion coefficient of Co is significantly higher than that of Ni 39. Therefore, at the initial stage of oxidation, the surface is populated with an excess amount of cobalt, leading to the formation of CoO and Co-Ni spinel oxide, and the subsequent incorporation of nickel gradually transforms CoO into the Co-Ni spinel. The XRD results in Figure 1 (a) suggest that single-phase Co-Ni spinel oxide foam can be obtained at the initial stage of operation within 100 hours, which is beneficial for performance because the electrical conductivity of CoO is significantly lower than that of the Co-Ni spinel 40-42 and the presence of CoO in the current collector would increase the ohmic resistance of the stack. Figure 1 (b) shows the elemental mapping results of Co and Ni at the cross-section of Co-Ni alloy foam, and both Co and Ni are homogeneously distributed. The composition was found to be 89 at% Co and 11 at% Ni by quantitative analysis. The elemental analysis was performed by TEM-EDS after thermal treatment at 700oC for 100 hours, and the uniform distribution of Co and Ni was confirmed in Figure 2 (c). The composition was measured to be 88.1 at% Co and 11.9 at% Ni by quantitative analysis, which indicates that the original composition is maintained through the thermal oxidation.

10


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Figure 1. (a) XRD patterns of the as-fabricated and thermally treated (400-700°C for 2-100 hours) Co-Ni foam. (b) SEM image and elemental distribution of Co and Ni in Co-Ni alloy 11



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foam. (c) TEM image and elemental distribution of Co, Ni and O after heat treatment at 700oC for 100 hours.

From a structural point of view, the foam-type current collector must have adequate amounts of solid phase and pores. Generally, the foams are treated as porous cellular solids and classified into open and closed cells; both the solid phase and pores are three dimensionally connected for the open cells, while the pores are isolated within the solid phase for the closed cells

29

. The open cell structure with sufficient porosity is required for the

current collector because the reactant gases must be supplied through the foams. On the other hand, the proper portion of the solid phase is required to provide sufficient conduction paths and mechanical strength. Figure 2 (a) shows the SEM images of the Co-Ni foam before and after the thermal treatment at 700°C. The open-cell structure with the three-dimensionally connected solid phase and sufficient porosity is observed, and no significant dimensional change is observed upon the phase transformation from metallic alloy to spinel oxide. The main structural element is the pentagonal ellipsoid, and the struts show a triangular shape with concave areas, which is the typical feature of the polyurethane foam used as a skeleton in the foaming process. The cell size is approximately 200-400 µm, and no processing defects are observed. Figure 2 (b) shows the change of the surface morphology upon the oxidation of the Co-Ni foam. The metallic foam exhibits a dense and smooth surface, and coarse grains with the grain size of ~6 µm are observed. Thermal oxidation leads to the formation of small pores over the surface. The formation of pores upon oxidation is driven by the Kirkendall effect, which is assigned to the differences in the diffusion rates between the cations and anions in the oxide scale, and could be the direct evidence of the outward diffusion of cations during oxidation, as discussed earlier 43-44. 12


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Figure 2. SEM images showing (a) the structures and (b) surface morphologies of the Co-Ni foam in metallic alloy and spinel oxide states.

The electrical performance and stability of the Co-Ni foam were evaluated by ASR measurements. Figure 3 (a) presents the ASR data collected for 100 hours while heating the sample from room temperature to 700°C at the ramping rate of 5°C/min and then maintaining the sample at that temperature. At the initial stage, the ASR rapidly increases upon heating 13



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because the metallic Co-Ni alloy is thermally oxidized. The ASR exhibits the maximum value at 500~600°C where metallic phases completely disappear and a significant amount of the poorly conductive CoO exists, as shown in the XRD analysis presented in Figure 1 (a). Upon further heating and prolonged exposure at 700°C, the two-phase mixture of Co-Ni spinel and CoO gradually transforms to single-phase Co-Ni spinel, resulting first in reduction and then stabilization of the ASR. Therefore, the electrical characteristics of the Co-Ni foam at the initial stage of operation are clearly correlated with the XRD results presented in Figure 1 (a). After stabilization, the ASR shows a nearly constant value at ~10 mOhm cm2 for 100 hours, which is comparable to the reported values for the state-of-the-art interconnects and precious metal current collectors

45-47

. Figure 3 (b) exhibits the ASR results in a prolonged

measurement for over 10,000 hours. The measurement was performed for 10,150 hours, which corresponds to 1 year and 2 months, and the ASR was stable with no indication of the degradation or failure of the Co-Ni foam for the entire time period. The slight change in the ASR with time observed in Figures 3 (a) and (b) could be attributed to the oxide scale growth of the metallic interconnects placed at both sides of the Co-Ni foam in the ASR measurements rather than the degradation of the Co-Ni foam itself because a similar trend was observed in our previous measurements using a Pt mesh instead of the Co-Ni foam

48

.

Therefore, the results in Figures 3 (a) and (b) demonstrate the chemical and mechanical stability of the Co-Ni foam in static operation. In real situations, the stability of the current collector with temperature variation is extremely important because the temperature fluctuations resulting from thermal and load cycles lead to the dimensional changes of the stack components and accordingly, brittle fracture of the ceramic foam could result. The stability of the Co-Ni foam was evaluated through the thermal cycles as shown in Figure 3 (c). Thermal cycles were performed between 700°C and room temperature at the heating and cooling rates of 5°C min-1. The increase of ASR was minimal through 30 thermal cycles as 14


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shown in Figure 3 (c), indicating that Co-Ni foam is structurally stable upon thermal stresses applied by the expansion and shrinkage of the metallic interconnect. Figure 3 (d) shows the cross-section SEM image of the Co-Ni foam sandwiched between two interconnects, taken after the ASR measurements. The well-connected current path between the two interconnects is revealed, and the porosity appears to be sufficient for facile gas transport. The contact between the Co-Ni foam and interconnects remain intact, and most importantly, no indication of mechanical damage caused by thermal cycles is found, verifying that the Co-Ni foam possesses adequate mechanical strength to endure the thermal stresses induced by the temperature changes. The SEM image of the Co-Ni foam in the lateral direction is displayed in Figure 3 (e). The contact area between the cathode and Co-Ni foam is estimated to be ~18% by image analysis. Based on high magnification SEM image in Figure 3 (f), the contribution of microstructural features to the overall contact area is considered to be negligible. The contact area is one of the most important factors determining the functionality of the cathode current collector

13, 49

. For woven meshes, the electrical contacts are mostly formed at the

crossover points of the wires, and the majority of the remaining part is considered as “dead area

13

.” The contact area of the woven meshes depends on the mesh geometry, softening

characteristics of the metal wires, operating temperature and applied pressure, and is estimated to be 4~6% under normal conditions

13, 49

. For the ceramic contact layers, on the

other hand, the contact interface is strongly influenced by the surface morphology because of the lack of the dimensional flexibility, and a subtle control of the surface roughness is required to obtain the intended contact area

50

. By contrast, the Co-Ni foam provides high

contact area without any delicate parameter control, offering clear advantages over the conventional current collecting techniques.

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Figure 3. (a-b) ASR of Co-Ni foam in static conditions measured for (a) 100 and (b) 10,150 hours. (c) ASR of Co-Ni foam in thermal cycling conditions. (d-e) SEM images of Co-Ni foam after ASR measurements in (d) vertical and (e) lateral directions. (f) SEM image of CoNi foam in higher magnification, showing the microscale geometries.

To evaluate its feasibility in practical applications, the Co-Ni foam was applied as a cathode current collector in unit cell testing. Figure 4 (a) compares the I-V curves and corresponding power densities of the two 5 cm* 5 cm cells tested using the Co-Ni foam and a Pt mesh as cathode current collectors. The cells are in the standard anode-supported configuration, consisting of a Ni-YSZ anode support (~800 µm), a Ni-YSZ anode functional layer (~15 µm), a YSZ electrolyte (~5 µm), a GDC diffusion barrier layer (~1 µm), an LSCFGDC cathode functional layer (~15 µm) and an LSCF cathode top layer (~30 µm)

51

. The

SEM image of the cross-section of the cell is displayed in Figure S3 in Supporting Information. The electrochemical measurements were performed at 700°C with humidified H2 (3% H2O) as the fuel and air as the oxidant. The open circuit voltages of the two cells, one with the Pt mesh and the other with the Co-Ni foam, are close to the theoretical values,

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indicating that the cells and seals are leak-tight. The I-V curves of the two cells are very similar, with the exception of small deviation at the high current range where the cell with a Pt mesh exhibited slightly higher performance. The maximum power densities of the cells with Co-Ni foam and Pt mesh are 1.03 and 1.06 W cm-2, respectively, and such a small difference is considered to be within the error range. Moreover, the power outputs of the two cells are nearly identical under the practical operating conditions at the current density of 200-500 mA cm-2, demonstrating that the Co-Ni foam could perform equivalently as a Pt mesh in real stacks. Figure 4 (b) shows the impedance spectra of the two cells in the form of Nyquist plots. The Nyquist plot displays the real and imaginary parts of the impedance on the x- and y-axes, respectively. The high-frequency intercept on the x-axis represents the ohmic resistance of the cell, which contains the cathode contact resistance, and the low-frequency intercept corresponds to the sum of the ohmic and polarization resistances. The impedance data measured at 750oC under open circuit voltage (OCV) and applied bias (1 and 2A) conditions (Figure S7 in Supporting Information) shows that polarization resistance associated with surface reaction and gas phase diffusion decreases with applied bias while ohmic resistance remains constant 52-53. As seen in Figure 4 (b), the ohmic resistances of the two cells are nearly identical, again validating that the functionality of the Co-Ni foam as a cathode current collector is equivalent to that of the Pt mesh. The ohmic resistance of the cell is composed of the contributions of the electrolyte, cathode, anode, cathode contact and anode contact. The ohmic resistance of the electrolyte is calculated to be ~0.038 Ohm cm2 based on the ionic conductivity of YSZ (0.013 S cm-1 at 700oC

54

) and the thickness of the

electrolyte layer (5 µm in Figure S3). The ohmic resistance of the cells with Pt and Co-Ni current collectors are 0.1147 and 0.1178 Ohm cm2, respectively, in the impedance spectra in Figure 4 (b), and the contributions of the electrodes and contacts are estimated to be 0.0770.080 Ohm cm2 for the two cells, which corresponds to ~67% of the total ohmic resistance. It 18


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is difficult to calculate the ohmic resistance values of the porous composite electrodes because the complicated geometric factors of the constituent phases, such as volume fraction, phase connectivity, particle contact area, tortuosity, etc. should be reflected, but the contributions of the cathode and anode are considered to be less significant than that of the electrolyte because the electrical conductivities of LSCF and Ni are higher than that of YSZ by several orders of magnitude. It was reported that the contact resistance of the anode is less than 5% of the cathode contact resistance because Ni in anode is extremely conductive

55

.

Based on this information, the cathode contact and electrolyte represent the major portions of the ohmic resistance, and the Co-Ni current collector significantly contributes to the overall cell performance in Figure 4 (a) and (b). The polarization resistance of the cells with Co-Ni foam and Pt mesh are 0.7996 and 0.7885 Ohm cm2, respectively. The difference between the two cells is only 0.0111 Ohm cm2, which is less than 2%. Considering the typical cell-to-cell variations of SOFCs, we believe this is within the error range. The comparison between the Co-Ni foam and Pt mesh in Figures 4 (a) and (b) suggests that the lower electrical conductivity of the Co-Ni spinel than that of Pt is compensated by the superior structural features of the foam compared to mesh, such as the large contact area and well-connected current paths, resulting in the similar overall cell performance. Among the various current collection techniques, it is well-known that the precious metals, such as Pt, Au and Ag, exhibits the best current collecting performance because of the high electronic conductivity and absence of oxide scale growth

14, 32, 56

. The results in Figure 4 (a) and (b), which

demonstrates the comparable performance of Co-Ni foam and Pt mesh, implies that Co-Ni foam must outperform most of the conventional methods based on conductive ceramics and stainless steels. The cell with the Co-Ni foam was tested at the constant current of 300 mA cm-2 for 350 hours, as shown in Figure 4 (c). The cell voltage decreased by 11 mV from 0.954 to 0.942 V in 350 hours, which is consistent with the intrinsic degradation rate of the 19



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same cell measured with the Pt mesh in our previous study

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51

. Figure 4 (d) compares the

impedance spectra collected before and after the long-term test. The polarization resistance slightly increased from 0.7996 to 0.8175 Ohm cm2, which could be ascribed to the cell degradation resulting from Ni coarsening, GDC agglomeration, chemical interaction between cathode and electrolyte, and Cr poisoning, based on our previous report

51

. However, the

increase of the ohmic resistance was negligible, indicating that Co-Ni foam is chemically and mechanically stable in SOFC operating conditions, and no additional performance degradation is caused by replacing Pt mesh with Co-Ni foam. Because the Co-Ni foam provides the equivalent level of performance and stability to that of the Pt mesh, it could significantly lower the manufacturing costs without causing any detrimental effects on the SOFC performance.

20


a

Voltage (V)

1.2

700oC / Wet H2 (3% H2O) 500 sccm / Air 1000 sccm

1.0

0.8

0.8

0.6

0.6

0.4

0.4 Pt Mesh Co-Ni Foam

0.0 0.0

1.0

1.5

2.0

0.4 700oC / Wet H2 (3% H2O) 500 sccm / Air 1000 sccm

0.3 0.2 0.1 0.0 -0.1

Pt Mesh Co-Ni Foam

-0.2 0.0

0.2 0.0

0.5

Current Density (A cm-2)

b

1.2

1.0

0.2

- ZIm (Ohm cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.4

0.6

0.8

ZRe (Ohm cm2)

21


1.0

Power Density (W cm-2)

Page 21 of 32


c 1.05 Voltage (V)

1.00 0.95 0.90

Temperature: 700oC Current: 4.8 A -2 Current Density: 0.3 A cm Fuel: Wet H2 (3% H2O) 500 sccm

0.85

Oxidant: Air 1000 sccm

0.80 0

50

100

150

200

250

300

350

Time (Hour)

d

0.4 700oC / Wet H2 (3% H2O) 500 sccm / Air 1000 sccm

0.3 -ZIm (Ohm cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2 0.1 0.0 -0.1

Initial Final

-0.2 0.0

0.2

0.4

0.6

0.8

1.0

ZRe (Ohm cm2)

Figure 4. (a) I-V and corresponding power densities and (b) impedance spectra of the anodesupported cells tested with Co-Ni foam and Pt mesh as a cathode current collector. (c) Longterm stability of the anode-supported cell measured with Co-Ni foam as a current collector. (d) Impedance spectra of the anode-supported cells tested with Co-Ni foam before and after long-term testing.

Finally, the Co-Ni foam was applied to the 1 kW-class stack, and its reliability was evaluated under severe dynamic operating conditions. Figure 5 (a) shows the image of the stack incorporating the Co-Ni foam as the cathode current collector. The stack is composed of 22


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30 anode-supported cells, metallic interconnects, glass-ceramic sealants, a Co-Ni foam cathode current collector and a Ni foam anode current collector. The cell size was 144 cm2 with an active electrode area of 100 cm2. At 700°C, the open circuit voltage was 38.3 V with dry H2 used as the fuel and air as the oxidant, indicating that the stack is leak-tight, and the output power reached 1 kW at the total current of 40 A, corresponding to the current density of 0.4 A cm-2, as shown in Figure S8 in Supporting Information. The durability test results in dynamic operating conditions are displayed in Figure 5 (b). In each dynamic testing period, 15 load cycles (120 - 200 mA cm-2 @ 2 mA cm-2 min-1), 5 load trips (0 - 200 mA cm-2, abrupt on and off) and 5 thermal cycles (25 - 700°C @ 1°C min-1) were performed sequentially. After conditioning of the stack for 350 hours, 5 testing periods were repeated, and the total operating time reached ~3000 hours. In Figure 5 (b), black, red and blue lines represent current, voltage and power, respectively. Throughout the cycle tests, the stack performance was extremely stable, and the degradation rate was minimal at 1.75%/3000 hours. Considering the strong impact of the cathode contact on the overall stack performance, the results in Figure 5 (b) demonstrate that the Co-Ni foam could remain intact through the harsh variations of electric load and temperature. Figure 5 (c) shows the SEM image of the Co-Ni foam used for stack testing. There was no indication of structural change or mechanical damage after 3000 hours of operation, confirming the tolerance of the Co-Ni foam for the dynamic variation of electric load and temperature that could be encountered in real operating conditions. Based on its performance, reliability and cost-effectiveness, the Co-Ni foam is considered to be an excellent choice for the formation of electrical contacts at hightemperature oxidizing atmosphere, and moreover, it presents a large range of possibilities for further improvement of its functionality via modification of its composition and structure.

23



1st Period 2nd Period 3rd Period 4th Period 5th Period

40

1000 800

30

600 20 400 10

Power (W)

b Current (A) / Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

0 0

500

1000

1500

2000

2500

0 3000

Time (Hr)

Figure 5. (a) Image of the 1 kW-class stack composed of 30 cells with the cell size of 144 cm2. (b) Stack voltage and power as a function of time measured during the cycling test for 3000 hours. (c) SEM image of Co-Ni foam after the stack test.

4. Conclusions Cathode current collection has been recognized as a critical factor that determines the successful transfer of the unit cell performance to the practical stacks in SOFC development. Various approaches developed to date have each revealed their inherent limitations in the formation of the electric connection between two rigid components in a high-temperature oxidizing atmosphere. The Co-Ni foam presented in this study could be an excellent 24


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candidate that overcomes the drawbacks of the existing techniques in various aspects. It is significantly cheaper than the precious metal meshes/foams, and there is no concern of the growth of the insulating scale or Cr evaporation that inevitably occur in stainless steel meshes/foams. It offers superior structural compliance compared to ceramic contact layers, and unlike ceramic foams, there is no risk of breakage during stack assembly. By the singlecomponent evaluation, unit cell test and stack operation, the excellent performance and durability of the Co-Ni foam was verified, even under extremely severe dynamic conditions. Moreover, there is a wide range of possibilities for further improvement by compositional modification because various conductive spinel oxides could be applied in this approach. The foam-type current collector that transforms from metallic alloy to conductive oxide via in situ phase transformation could be a superb choice for many aspects of SOFC stack development, and furthermore, this innovative concept could be applied in a wide range of applications where high-temperature electric contacts are required.

Supporting information Image of Co-Ni foam, Schematic of the ASR measurement setup, SEM image of the cross-section of the cell, Schematic of the unit cell test setup, Stack testing schedule, Electrical conductivity of Co-Ni spinel oxide, Impedance spectra before and after long-term test, Stack performance

Acknowledgements This research was financially supported by the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) 25



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granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153010031940) and the institutional research program of the Korea Institute of Science and Technology (KIST).

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References 1.

Minh, N. Q., Ceramic Fuel-Cells. Journal of the American Ceramic Society 1993, 76 (3),

563-588. 2.

Singhal, S. C., Advances in solid oxide fuel cell technology. Solid State Ionics 2000, 135

(1–4), 305-313. 3.

Wachsman, E. D.; Lee, K. T., Lowering the temperature of solid oxide fuel cells. Science

2011, 334 (6058), 935-939. 4.

Wen, T. L.; Wang, D.; Tu, H. Y.; Chen, M.; Lu, Z.; Zhang, Z.; Nie, H.; Huang, W., Research on

planar SOFC stack. Solid State Ionics 2002, 152–153, 399-404. 5.

Jung, H. Y.; Choi, S. H.; Kim, H.; Son, J. W.; Kim, J.; Lee, H. W.; Lee, J. H., Fabrication and

performance evaluation of 3-cell SOFC stack based on planar 10 cm × 10 cm anode-supported cells. Journal of Power Sources 2006, 159 (1), 478-483. 6.

Blum, L., An Analysis of Contact Problems in Solid Oxide Fuel Cell Stacks Arising from

Differences in Thermal Expansion Coefficients. Electrochimica Acta 2017, 223, 100-108. 7.

Guan, W. B.; Zhai, H. J.; Jin, L.; Li, T. S.; Wang, W. G., Effect of Contact between Electrode

and Interconnect on Performance of SOFC Stacks. Fuel Cells 2011, 11 (3), 445-450. 8.

Rolle, A.; Thoréton, V.; Rozier, P.; Capoen, E.; Mentré, O.; Boukamp, B.; Daviero-Minaud, S.,

Evidence of the Current Collector Effect: Study of the SOFC Cathode Material Ca3Co4O9+δ. Fuel

Cells 2012, 12 (2), 288-301. 9.

Zhu, W. Z.; Deevi, S. C., Opportunity of metallic interconnects for solid oxide fuel cells: a

status on contact resistance. Materials Research Bulletin 2003, 38 (6), 957-972. 10.

Liu, S.; Song, C.; Lin, Z., The effects of the interconnect rib contact resistance on the

performance of planar solid oxide fuel cell stack and the rib design optimization. Journal of Power

Sources 2008, 183 (1), 214-225. 11.

Lynch, M. E.; Liu, M., Investigation of sheet resistance in thin-film mixed-conducting solid

oxide fuel cell cathode test cells. Journal of Power Sources 2010, 195 (16), 5155-5166. 12.

Gazzarri, J. I.; Kesler, O., Short-stack modeling of degradation in solid oxide fuel cells: Part

I. Contact degradation. Journal of Power Sources 2008, 176 (1), 138-154. 13.

Jiang, S. P.; Love, J. G.; Apateanu, L., Effect of contact between electrode and current

collector on the performance of solid oxide fuel cells. Solid State Ionics 2003, 160 (1–2), 15-26. 14.

Sugita, S.; Yoshida, Y.; Orui, H.; Nozawa, K.; Arakawa, M.; Arai, H., Cathode contact

optimization and performance evaluation of intermediate temperature-operating solid oxide fuel cell stacks based on anode-supported planar cells with LaNi0.6Fe0.4O3 cathode. Journal of Power

Sources 2008, 185 (2), 932-936. 15.

Zhong, H.; Matsumoto, H.; Ishihara, T.; Toriyama, A., Ag current collector for honeycomb

solid oxide fuel cells using LaGaO3-based oxide electrolyte. Journal of Power Sources 2009, 186 (2), 238-243. 16.

Mori, M.; Sammes, N. M.; Suda, E.; Takeda, Y., Application of La0.6AE0.4MnO3 (AE=Ca and 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

Sr) to electric current collectors in high-temperature solid oxide fuel cells. Solid State Ionics 2003,

164 (1–2), 1-15. 17.

Tucker, M. C.; Cheng, L.; DeJonghe, L. C., Glass-containing composite cathode contact

materials for solid oxide fuel cells. Journal of Power Sources 2011, 196 (20), 8435-8443. 18.

Shong,

W.-J.;

Liu,

C.-K.;

Lu,

C.-W.;

Wu,

S.-H.;

Lee,

R.-Y.,

Characteristics

of

La0.6Sr0.4Co0.2Fe0.8O3–Cu2O mixture as a contact material in SOFC stacks. International Journal

of Hydrogen Energy 2017, 42 (2), 1170-1180. 19.

Tucker, M. C.; Cheng, L.; DeJonghe, L. C., Selection of cathode contact materials for solid

oxide fuel cells. Journal of Power Sources 2011, 196 (20), 8313-8322. 20.

Zhang, W.; Wang, F.; Wang, K.; Pu, J.; Chi, B.; Jian, L., Chemical compatibility and electrical

contact between Ni–Cr–Mo alloy and LaCo0.6Ni0.4O3−δ in intermediate temperature solid oxide fuel cells. International Journal of Hydrogen Energy 2012, 37 (22), 17253-17257. 21.

Wang, F.; Yan, D.; Zhang, W.; Chi, B.; Pu, J.; Jian, L., LaCo0.6Ni0.4O3−δ as cathode contact

material for intermediate temperature solid oxide fuel cells. International Journal of Hydrogen

Energy 2013, 38 (1), 646-651. 22.

Niewolak, L.; Blum, L.; Peters, R.; Grüner, D.; Quadakkers, W. J., Behavior of Metallic

Components During 4,000 h Operation of an SOFC Stack with Carbon Containing Fuel Gas. Fuel

Cells 2016, 16 (5), 600-610. 23.

Fang, Q.; Blum, L.; Peters, R.; Peksen, M.; Batfalsky, P.; Stolten, D., SOFC stack performance

under high fuel utilization. International Journal of Hydrogen Energy 2015, 40 (2), 1128-1136. 24.

Fang, Q.; Blum, L.; Batfalsky, P.; Menzler, N. H.; Packbier, U.; Stolten, D., Durability test and

degradation behavior of a 2.5 kW SOFC stack with internal reforming of LNG. International Journal

of Hydrogen Energy 2013, 38 (36), 16344-16353. 25.

Blum, L.; de Haart, L. G. J.; Malzbender, J.; Margaritis, N.; Menzler, N. H., Anode-Supported

Solid Oxide Fuel Cell Achieves 70 000 Hours of Continuous Operation. Energy Technology 2016, 4 (8), 939-942. 26.

Chou, Y.-S.; Stevenson, J. W.; Choi, J.-P., Long-term evaluation of solid oxide fuel cell

candidate materials in a 3-cell generic short stack fixture, part I: Test fixture, sealing, and electrochemical performance. Journal of Power Sources 2014, 255, 1-8. 27.

Morán-Ruiz, A.; Vidal, K.; Larrañaga, A.; Laguna-Bercero, M. A.; Porras-Vázquez, J. M.; Slater,

P. R.; Arriortua, M. I., LaNi0.6Co0 4O3−δ dip-coated on Fe–Cr mesh as a composite cathode contact material on intermediate solid oxide fuel cells. Journal of Power Sources 2014, 269, 509519. 28.

Morán-Ruiz, A.; Vidal, K.; Larrañaga, A.; Porras-Vázquez, J. M.; Slater, P. R.; Arriortua, M. I.,

Laser machining of LaNi0.6M0.4O3−δ (M: Co, Fe) dip-coated on a Fe–22Cr mesh material to obtain a new contact coating for SOFC: Interaction between Crofer22APU interconnect and La0.6Sr0.4FeO3 cathode. International Journal of Hydrogen Energy 2015, 40 (26), 8407-8418. 29.

Will, J.; Gauckler, L. J., Ceramic foams as current collectors in solid oxide fuel cells (SOFC):

Electrical conductivity and mechanical behaviour. Electrochemical Society Inc: Pennington, 1997; 28


Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vol. 97, p 757-764. 30.

Ma, Y. G.; Kim, E. Y.; Yoon, S. H.; Whang, C. M., Preparation of electrically conducting

ceramic foams. In Eco-Materials Processing & Design Vii, Kim, H. S.; Li, Y. B.; Lee, S. W., Eds. Trans Tech Publications Ltd: Zurich-Uetikon, 2006, pp 1050-1053. 31.

Gupta, R. K.; Kim, E. Y.; Noh, H. S.; Whang, C. M., Mechanical, electrical and micro-

structural properties of La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3) perovskite-based ceramic foams. J. Phys. D-

Appl. Phys. 2008, 41 (3), 5. 32.

Wang, G.; Wu, W.; Guan, W.; Jin, L.; Guo Wang, W., Effect of conductivity and adhesive

properties of cathode current-collecting layer on cell performance inside stack for planar solid oxide fuel cells. Ceramics International 2014, 40 (7, Part B), 11023-11030. 33.

D. Li, Y.; Q. Li, L.; W. Liao, H.; R. Wang, H., Preparation of pure nickel, cobalt, nickel-cobalt

and nickel-copper alloys by hydrothermal reduction. Journal of Materials Chemistry 1999, 9 (10), 2675-2677. 34.

Frederick, S. F.; Cornet, I., The Effect of Cobalt on the High Temperature Oxidation of

Nickel. Journal of The Electrochemical Society 1955, 102 (6), 285-291. Mrowec, S.; Walec, T.; Werber, T., Oxidation of cobalt at high temperatures. Corrosion

35.

Science 1966, 6 (6), 287-297. 36.

Koel, G. J.; Gellings, P. J., The contribution of different types of point defects to diffusion in

CoO and NiO during oxidation of the metals. Oxidation of Metals 1972, 5 (3), 185-203. 37.

Hüfner, S., Electronic structure of NiO and related 3d-transition-metal compounds.

Advances in Physics 1994, 43 (2), 183-356. Carter, R. E.; Richardson, F. D.; Wagner, C., Oxidation of Cobalt Metal. JOM 1955, 7 (2),

38. 336-343. 39.

Takasu, Y.; Kanagawa, H.; Matsuda, Y., Preferential oxidation characteristics in the oxidation

of cobalt-nickel alloys in nitric oxide and in oxygen. The Journal of Physical Chemistry 1977, 81 (14), 1407-1410. 40.

Lange, F.; Martin, M., The electrical conductivity of CoO: Experimental results and a new

conductivity model. Berichte der Bunsengesellschaft für physikalische Chemie 1997, 101 (2), 176184. 41.

Petric, A.; Ling, H., Electrical Conductivity and Thermal Expansion of Spinels at Elevated

Temperatures. Journal of the American Ceramic Society 2007, 90 (5), 1515-1520. 42.

Brabers, V. A. M.; Broemme, A. D. D., Low-spin-high-spin transition in the Co3O4 spinel.

Journal of Magnetism and Magnetic Materials 1992, 104, 405-406. 43.

Ha, D.-H.; Moreau, L. M.; Honrao, S.; Hennig, R. G.; Robinson, R. D., The Oxidation of

Cobalt Nanoparticles into Kirkendall-Hollowed CoO and Co3O4: The Diffusion Mechanisms and Atomic Structural Transformations. The Journal of Physical Chemistry C 2013, 117 (27), 1430314312. 44.

Wang, H.; Chen, C.; Zhang, Y.; Peng, L.; Ma, S.; Yang, T.; Guo, H.; Zhang, Z.; Su, D. S.; Zhang,

J., In situ oxidation of carbon-encapsulated cobalt nanocapsules creates highly active cobalt oxide 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

catalysts for hydrocarbon combustion. Nature Communications 2015, 6, 7181. 45.

Mehran, M. T.; Khan, M. S.; Lee, J.-W.; Song, R.-H.; Lee, S.-B.; Lee, J.-W.; Lim, T.-H.; Park, S.-

J., Nano-CeO2 and -LaCrO3 dispersed ferritic stainless steels as potential interconnect materials for solid oxide fuel cells. Journal of Alloys and Compounds 2017, 709, 453-463. 46.

Hosseini, S. N.; Karimzadeh, F.; Enayati, M. H.; Sammes, N. M., Oxidation and electrical

behavior of CuFe2O4 spinel coated Crofer 22 APU stainless steel for SOFC interconnect application. Solid State Ionics 2016, 289, 95-105. 47.

Magrasó, A.; Falk-Windisch, H.; Froitzheim, J.; Svensson, J.-E.; Haugsrud, R., Reduced long

term electrical resistance in Ce/Co-coated ferritic stainless steel for solid oxide fuel cell metallic interconnects. International Journal of Hydrogen Energy 2015, 40 (27), 8579-8585. 48.

Lee, S.-I.; Hong, J.; Kim, H.; Son, J.-W.; Lee, J.-H.; Kim, B.-K.; Lee, H.-W.; Yoon, K. J., Highly

Dense Mn-Co Spinel Coating for Protection of Metallic Interconnect of Solid Oxide Fuel Cells.

Journal of The Electrochemical Society 2014, 161 (14), F1389-F1394. 49.

Jiang, S. P., Issues on development of (La,Sr)MnO3 cathode for solid oxide fuel cells.

Journal of Power Sources 2003, 124 (2), 390-402. 50.

Wang, G.; Guan, W.; Miao, F.; Wang, W. G., Factors of cathode current-collecting layer

affecting cell performance inside solid oxide fuel cell stacks. International Journal of Hydrogen

Energy 2014, 39 (31), 17836-17844. 51.

Kim, S. J.; Choi, M.-B.; Park, M.; Kim, H.; Son, J.-W.; Lee, J.-H.; Kim, B.-K.; Lee, H.-W.; Kim, S.-

G.; Yoon, K. J., Acceleration tests: Degradation of anode-supported planar solid oxide fuel cells at elevated operating temperatures. Journal of Power Sources 2017, 360, 284-293. 52.

Schouler, E. J. L.; Kleitz, M., Electrocatalysis and Inductive Effects at the Gas, Pt/Stabilized

Zirconia Interface. J. Electrochem. Soc. 1987, 134 (5), 1045-1050. 53.

Lu, Z.; Hardy, J.; Templeton, J.; Stevenson, J., New insights in the polarization resistance of

anode-supported solid oxide fuel cells with La0.6Sr0.4Co0.2Fe0.8O3 cathodes. Journal of Power

Sources 2011, 196 (1), 39-45. 54.

Strickler, D. W.; Carlson, W. G., Electrical Conductivity in the ZrO2-Rich Region of Several

M2O3—ZrO2 Systems. Journal of the American Ceramic Society 1965, 48 (6), 286-289. 55.

Jin, L.; Guan, W.; Ma, X.; Zhai, H.; Wang, W. G., Quantitative contribution of resistance

sources of components to stack performance for planar solid oxide fuel cells. Journal of Power

Sources 2014, 253, 305-314. 56.

Wang, C.; Xin, X.; Xu, Y.; Ye, X.; Yu, L.; Wang, S.; Wen, T., Performance of a novel

La(Sr)MnO3-Pd composite current collector for solid oxide fuel cell cathode. Journal of Power

Sources 2011, 196 (8), 3841-3845.

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List of Figures Figuer 1. (a) XRD patterns of the as-fabricated and thermally treated (400-700°C for 2-100 hours) Co-Ni foam. (b) SEM image and elemental distribution of Co and Ni in Co-Ni foam. Figure 2. SEM images showing (a) the structures and (b) surface morphologies of the Co-Ni foam in metallic alloy and spinel oxide states. Figure 3. (a-b) ASR of Co-Ni foam in static conditions measured for (a) 100 and (b) 10,150 hours. (c) ASR of Co-Ni foam in thermal cycling conditions. (d-e) SEM images of Co-Ni foam after ASR measurements in (d) vertical and (e) lateral directions. Figure 4. (a) I-V and corresponding power densities and (b) impedance spectra of the anodesupported cells tested with Co-Ni foam and Pt mesh as a cathode current collector. (c) Longterm stability of the anode-supported cell measured with Co-Ni foam as a current collector. Figure 5. (a) Image of the 1 kW-class stack composed of 30 cells with the cell size of 144 cm2. (b) Stack voltage and power as a function of time measured during the cycling test for 3000 hours. (c) SEM image of Co-Ni foam after the stack test.

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High-Temperature Current Collection Enabled by the in Situ Phase

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