Modification of a first-generation solid oxide fuel cell cathode with Co3O4 nanocubes having selectively exposed crystal planes
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Co3O4 nanocubes with exposed (001) planes were prepared and employed for use as first-generation Sr-doped LaMnO3 (LSM) cathodes in solid oxide fuel cells to improve the cell performance. Theoretical simulations suggest that the Co3O4 (001) plane has the smallest oxygen adsorption and oxygen dissociation energies compared with other planes, thus favouring cathode reactions in solid oxide fuel cells (SOFCs). Experimental studies consistently demonstrate that a cell using an LSM cathode made with Co3O4 nanocubes with selective (001) surfaces exhibits a peak power density of 500 mW cm−2 at 600 °C, while the power output for a cell using unselective (commercial) Co3O4 nanoparticles is only 179 mW cm−2 at the same temperature. The electrochemical study indicates that the use of Co3O4 nanoparticles with exposed (001) surfaces obviously accelerates the cathode reactions and thus decreases the polarisation resistance, which is the key to improving fuel cell performance. This study demonstrates the feasibility of using the crystal planes of metal oxides to improve the fuel cell performance and provides a new way to design SOFC cathodes.
KeywordsCo3O4 Cathode Density functional theory Nanocubes Solid oxide fuel cells
Solid oxide fuel cells (SOFCs), which can directly convert chemicals into electricity, are regarded as low-pollution devices that can be used to generate electricity . Cathodes are a key component of SOFCs and have received considerable attention, as they significantly govern SOFC performance [2, 3, 4]. Sr-doped LaMnO3 (LSM) is a first-generation cathode material used in SOFCs, and it is also one of the most traditional SOFC cathode materials that has been successfully employed in commercial SOFC systems . However, LSM is regarded as a good cathode material for applications in SOFCs at high temperatures (above 800 °C), but it is not an appropriate cathode material for use in SOFCs at intermediate temperatures (approximately, 600 °C) due to its pure electronic conductivity without any apparent ionic conductivity . Therefore, the design of nanostructured LSM cathodes  and the use of new materials with electron and oxygen ion mixed conductivity [8, 9] as cathodes in SOFCs at intermediate temperatures have been proposed.
Alternatively, the addition of traditional metal oxides to cathodes has been demonstrated to be beneficial for improving the cathode performance. Zhang et al.  have added a Co3O4 layer to the Sm0.5Sr0.5CoO3-δ cathode and found that the fuel cell performance could be doubled compared with the pure SSC cathode. Zhang et al.  coated a BaCe0.4Sm0.2Fe0.4O3 cathode with cobalt oxide, improving the fuel cell performance at intermediate temperatures. Li et al.  also found that the addition of Co3O4 to an LSM cathode can improve the cathode performance, although the performance is still mediocre at intermediate temperatures. It is generally accepted that Co3O4 is a good oxygen reduction catalyst for use in SOFCs operated at intermediate temperatures. However, note that these studies used ordinary Co3O4 particles and did not investigate the effect of selectively exposing different Co3O4 planes on the cathode reactions. Research in the catalysis field has demonstrated that the different Co3O4 planes have a significant impact on their activity for catalytic reactions, such as CO oxidation and CH4 combustion [13, 14, 15]. Therefore, it is reasonable to assume that the electrochemical performance of the cell could be improved by exposing a specific plane of Co3O4 instead of using ordinary Co3O4 particles, although such an attempt has not been performed in the SOFC community.
In this paper, a theoretical simulation was performed to investigate the adsorption and dissociation of oxygen on different Co3O4 planes, aiming to find a suitable plane for SOFC cathode reactions. Subsequently, Co3O4 nanoparticles with selectively exposed planes were synthesized and employed in the LSM cathode of an SOFC. The electrochemical performance of a cell made with an LSM cathode composed of Co3O4 nanocubes with specific exposed planes was studied and compared with that of a cell made with an LSM cathode composed of regular Co3O4 nanoparticles.
All calculations were performed using density functional theory (DFT)  and were implemented using VASP (Vienna ab initio simulation package) [17, 18]; the generalized gradient approximation (GGA) was performed using the Perdew–Burke–Ernzerhof approach to approximate the exchange–correlation term. Here, the core–valence interaction is described by the projector-augmented wave method . To correctly represent the electronic structure, the Hubbard correction was included [20, 21], using a Ueff (Co-3d) value of 3.32 eV . The plane wave cutoff energy was set to 520 eV, whereas the Brillouin zone was described by a 1 × 1 × 1 K-point gamma-centred mesh. To study the surface properties, a periodic slab consisting of a 2 × 2 repetition of the unit cell was built and had eight atomic layers and a 15 Å vacuum gap. The convergence criteria for the energy and force were set to 10−5 eV and 0.02 eV Å−1, respectively, during geometry optimization. The dissociation barriers on the (001), (110) and (111) facets were calculated by using climbing image nudged elastic band (CI-NEB) method , where the convergence criteria for energy and force were 10−5 eV and 0.02 eV Å−1, respectively.
The Co3O4 nanocubes were prepared by a hydrothermal process according to the literature [13, 24] with some modifications. Briefly, an NaOH solution (0.30 g NaOH + 10 mL distilled water) was first dispensed into a Co(NO3)2 solution (8.73 g Co(NO3)2·6H2O + 20 mL distilled water), and the resulting solution was placed in a Teflon liner and stirred for 30 min. Subsequently, the Teflon liner was put into a stainless steel autoclave and then treated at 180 °C for 3 h. After cooling it down to room temperature, the obtained product was filtered, washed several times with ethanol and deionized water and further dried at 60 °C. Finally, Co3O4 was obtained after firing at 500 °C for 3 h in air.
The cathode was used in an Sm0.2Ce0.8O2 (SDC)-based fuel cell system. The SDC powder was synthesized by a combustion method, and the preparation details can be found elsewhere . The SDC powder was calcined at 700 °C for 3 h to obtain a pure phase powder. The SDC powder was mixed with NiO and starch at a weight ratio of 2:3:1 (SDC powder:NiO:starch) and the resulting mixture was used as the anode powder. For preparing the anode powder, SDC, NiO and starch powders were mixed in an ethanol solution and mixed with ball milling. After milling for 24 h, the solution was put in an oven at 70 °C to evaporate ethanol. The dried powder was collected as the composite anode powder. The SDC-based half cells were prepared by co-pressing and co-sintering methods and sintered at 1450 °C for 6 h. Then, an La0.8Sr0.2MnO3-δ (LSM20)–SDC cathode slurry was deposited on the surface of the sintered SDC electrolyte and then co-fired at 1000 °C for 3 h to attach the cathode layer to the electrolyte, forming a complete cell with the structure of NiO–SDC(anode)/SDC(electrolyte)/LSM–SDC(cathode). The LSM20 powder was also synthesized by a combustion method, as mentioned above. To study the effect of exposing different Co3O4 surfaces on the fuel cell performance, the synthesized plane selective Co3O4 nanoparticles were dispersed in distilled water by using ultrasonic dispersion. Then, the dispersed Co3O4 particles were infiltrated into the porous LSM20–SDC composite cathode layer until the Co3O4:LSM20–SDC weight ratio was 1:5, forming the LSM20–SDC–Co3O4 (plane selective) cathode layer. For comparison, a similar procedure was performed to infiltrate the same amount of commercial Co3O4 nanoparticles into the LSM20–SDC cathode layer. The commercial Co3O4 nanoparticles were purchased from Aladdin (Product no. C131625), and the particle size was approximately 100 nm. The material was directly used without further treatment. After the deposition of the Co3O4 particles, both cells were fired at 800 °C. The fuel cell tests were performed using wet hydrogen as the fuel and static air as the oxidant. The electrochemical performance, including I–V and electrochemical impedance spectroscopy (EIS), was measured by using an electrochemical workstation (CHI 760E CH Instruments). The morphologies of the cells were observed using a field-emission scanning electron microscope (FESEM, JSM-7800F).
Results and discussion
Oxygen adsorption and dissociation energies for Co3O4 with (001), (110) and (111) surfaces
In this study, a new concept for using metal oxide nanocubes with specific exposed crystal planes in SOFC cathodes was provided. The theoretical simulations showed that the Co3O4 (001) plane had better oxygen adsorption and oxygen dissociation, which improved the cathode reactions in the SOFCs. To validate the simulations, Co3O4 nanocubes with (001) planes exposed at the surface were synthesized and then used in the cathode of SOFCs. The cell performance was much improved with the use of the Co3O4 nanocubes with (001) surfaces compared with that of the cell using conventional Co3O4 nanoparticles. The EIS studies indicated that the utilization of Co3O4 nanocubes with exposed (001) surfaces accelerated the cathode reactions. This led to a smaller polarization resistance and thus a higher fuel cell performance, although the structure and compositions of the cells were similar. This study demonstrated the feasibility of tailoring and utilizing the exposed crystal planes of metal oxides to enhance the performance of SOFC cathodes and may open new doors for the design of cathodes, which has not yet been revealed in the scientific community.
This work was supported by the National Natural Science Foundation of China (Grant no. 51602238), the Natural Science Foundation of Shandong Province (Grant no. ZR2018JL017), the Startup Funding for Talents at Qingdao University (Grant no. 41117010097), the Thousand Talents Plan, the World-Class Discipline Program of Shandong Province and the Taishan Scholar’s Advantageous and the Distinctive Discipline Program of Shandong Province, the China Postdoctoral Science Foundation (2017M622139) and the Qingdao Postdoctoral Application Research. The Australian Research Council (ARC) is also acknowledged for partially supporting this study under the ARC Laureate Fellowship Program (Project FL170100101).
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