Cathode Supports of SOFCs with a Hierarchical Pore Structure

D. H. Dong, X. Shao, Z. T. Wang, G. M. Parkinson and C. -Z. Li

Fuels and Energy Technology Institute, Curtin University of Technology, Perth, WA 6845, Australia

Cathode-supported SOFCs are attractive for the potential utilization of hydrocarbons as fuels to generate electricity because Cu-based anodes can be used to deal with coke formation in the Ni-base anodes. However, Cu-based anodes cannot be used in anode-supported SOFCs because the high sintering temperatures (above 1350 °C) required for the formation of a dense electrolyte film may melt the Cu-based anodes. Compared with anode materials, cathode materials show higher sinterability, which leads to a low porosity of the supporting cathodes and in turn high gas diffusion resistance within the cathodes. To get around this problem, a novel supporting cathode with efficient gas diffusion channels will be developed in this study in order to achieve high performances in power generation.

Introduction Although most studies focus on anode-supported solid oxide fuel cells (SOFCs) because of their high performances, cathode-supported SOFCs still attract some research interests owing to having some advantages over anode-supported SOFCs. Cu-base anodes can be used in cathode-supported SOFCs to cope with the coking that occurs in Ni-based anodes (1). However, the low melting points of Cu2O and CuO (1226-1335 °C) prevent their application in anode-supported SOFCs because of high sintering temperatures (above 1350 °C) required for the formation of a dense electrolyte film on the anode support. Secondly, cathode supports have higher stability compared with Ni-base anode supports because the Ni/NiO redox causes the variation of support volume (2). The longest test of SOFCs (30,000 h) was demonstrated by Siemens Westinghouse Power using cathode-supported SOFCs (3). In addition, cathode materials are cheaper than anode materials so that production costs can be reduced for cathode-supported SOFCs (4). The low performances of cathode-supported SOFCs are mainly attributed to cathode polarization resistances, including activation polarization and concentration polarization (5), which are determined by the cathode pore structure. A high surface area of a porous cathode would increase the oxygen reduction rate and in turn decrease activation polarization. A highly porous cathode would facilitate oxygen diffusion within supports and therefore decrease concentration polarization (6). Pore formers such as graphite and starch have been used to improve cathode pore structure, but they are not effective in producing connected pore channels for efficient gas diffusion because of the particle shape of the pore formers and the high sinterability of cathode materials (7).

10.1149/05701.0555ecst ©The Electrochemical SocietyECS Transactions, 57 (1) 555-560 (2013)

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A phase-inversion process had been used to form polymer membranes and ceramic membranes with two types of pores: finger-like pores and sponge-like pores (8,9). Finger-like pores around several micrometers in diameter act as gas diffusion channels, and small and numerous sponge-like pores could increase the surface area for reactions. Moreover, the pores are formed by the convection of solvent and coagulant. Therefore, the pores have higher connectivity compared with the pores formed by pore former templates and are expected to result in highly-efficient gas diffusion. In this study, a hierarchical pore structure of a cathode support has been developed by a phase-inversion process. The effect of finger-like pores on gas diffusion was studied. The cathode-supported SOFCs with a hierarchically structured support were investigated as a power generation device using hydrogen.

Experimental Cathode Support Preparation

Cathode supports were prepared by a modified phase-inversion process, as described in our previous study (10). Briefly, yttria-stabilized zirconia (YSZ, Sigma-Aldrich Australia) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF, Fuel Cell Materials, USA) ceramic powders were mixed with polyethersulfone (PESF) and 1-Methyl-2-pyrrolidinone (NMP, 99%) in a ball-mill (MTI Corporation, USA) to form a homogeneous slurry. The resulting slurry was cast into an aluminium mould, and then water as a coagulant was poured on top of the slurry to induce phase inversion. The green body of the membrane was obtained after being taken from the mould. Finally, the membrane was heated in a box furnace (ModuTemp Pty. Ltd., Australia) to remove organic components and then sintered at 1100 and 1280°C for 3 h respectively. For comparison, the supports without finger-like pores were also prepared by using a coagulant of the mixture of water and ethanol at the same ratio. Cathode Support Characterization

The microstructure of the prepared cathode supports was observed by Scanning Electron Microscopy (SEM, Zeiss Neon 40EsB FIBSEM). Porosity was tested by the Archimedes method. Gas (argon) permeation flux through the cathode supports sintered at 1280 °C was tested as a function of pressure difference. Cathode-supported SOFC Preparation and Test

A (Sc2O3)0.10(ZrO2)0.90 (ScSZ) electrolyte film was coated on the cathode supports pre-sintered at 1100 °C by slurry dip-coating, and the two layers were co-sintered at 1280 °C for 3 h to form a dense electrolyte film supported on cathodes. A NiO/YSZ (Fuel Cell Materials, USA) anode was coated on the cathode-supported electrolyte film by spray coating (11). After sintering at 1250 °C for 3 h, a cathode-supported SOFC was obtained. The power generation of the SOFC was tested at 750 °C using humid hydrogen (3% H2O) as a fuel and static air as an oxidant.

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Results and Discussions

A phase-inversion process has been used to prepare hollow fibre anodes for SOFCs, and phase-inversion occurs at the both sides of hollow fibre wall, resulting in a complicated microstructure: a central layer sandwiched by two groups of finger-like pores (12). In this study, plate cathode supports were prepared, and the phase-inversion was only occured on one side of membrane, forming a simpler microstructure. As shown in Figure 1, the cathode support prepared by phase-inversion has a hierarchical pore structure: finger-like pore channels with diameters from 40 to 100 µm cross the cathode thickness, ending with a porous layer about 100 µm in thickness; the porous layer and the walls of finger-like pore channels have sponge-like pores with sizes of between 1 and 5 µm. Accordingly, gas can diffuse quickly through the finger-like pore channels and then distribute into the sponge-like pores. A porous cathode without finger-like pore channel was also prepared using a mixture of water and ethanol (50:50 in mass) as a coagulant. The coagulant properties greatly affect the finger-like pore formation, and the presence of ethanol in coagulant (water) inhibits the formation of finger-like pores (13).

F

Figure 1. SEM images of the cathode support with sponge-like pore channels (a) and sponge-like pores within channel walls (b) and the cathode support without finger-like pore channels (c). The properties of the developed cathode supports are compared with those of the supports without finger-like pore channels in Table 1 and Figure 2. The support with finger-like pores show much higher gas permeation fluxes compared with the support without finger-like pores, indicating that the presence of finger-like pores with porous supports greatly facilitate gas diffusion within cathode supports. A higher porosity is also achieved within the support due to the existence of finger-like pores, e.g. 61.2% for the support with finger-like pores and 42.4% for the support without finger-like pores. The larger sintering shrinkage of the support with finger-like pores (e.g. 20.0% vs 15.8%) provides flexibility to adjust support shrinkage for matching with electrolyte shrinkage during co-sintering of the support and electrolyte coating to form a dense electrolyte film (14). TABLE I. Properties of Cathode Supports

Sample Gas flux at 10 psi (ml/ cm2 min)

Porosity (%) Sintering shrinkage (%)

Support with finger-like pore channels 4114 61.2 20.0 Support without finger-like pore channel 520 42.4 15.8

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Figure 2. Gas flux through the cathode supports with and without finger-like pores at different pressure differences. The cathode support has been made into a SOFC, and the sandwich structure of SOFCs is shown in Figure 3. Numerous finger-like pore channels cross the cathode support and end before approaching electrolyte film. A dense ScSZ electrolyte film about 14 µm in thickness was successfully prepared on the cathode support, and an anode layer about 50 µm in thickness was coated on the electrolyte film.

Figure 3. SEM images of cathode-supported SOFC: a, cathode with finger-like pores; b, sandwich structure of three layers: anode, electrolyte and cathode support.

The preliminary results of fuel cell performance tests are very low, as shown in Figure 4. The maximum power density is only 50 mW/cm2 at 750 °C, which is attributed to large cell resistances: an ohmic resistance of 1.8 Ω.cm2 and a polarization resistance of 4 Ω.cm2. The low performances might be a consequence of the high co-sintering temperature of 1280 °C which produced poorly conducting phases such as La2Zr2O7 and SrZrO3 in the cathode, and reduced the active sites of the cathode. Thereby, future work will focus on reduce the co-sintering temperature or using ceria-based electrolyte materials to replace YSZ and ScSZ .

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Figure 4. Electrochemical performances of the cathode-supported SOFC at 750 °C: a, power output; b, cell resistances.

Conclusions

Cathode-supported SOFCs have been successfully prepared with cathode supports

that have a hierarchical pore structure, which enables the cathode support to have efficient gas diffusion. The cathode supports prepared by phase-inversion have two types of pore structures: finger-like pore channels and small sponge-like pores within the walls of pore channels. The demonstrated hierarchical pore structure of the cathode represents a new type of supporting electrode with the potential to achieve high fuel cell performances.

Acknowledgements

This work was financially supported by Australia Research Council Discovery Project (DP120103317), the government of Western Australia via the Centre for Research into Energy for Sustainable Transport (CREST) and Curtin University of Technology through the Curtin Research Fellowship scheme. We also thank for the Curtin Centre for Materials Research for help with the SEM images.

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