Hierarchically nanoporous La1.7Ca0.3CuO4−δ and La1.7Ca0.3NixCu1−xO4−δ (0.25 ≤ x ≤ 0.75) as potential cathode materials for IT-SOFCs

School of Chemistry, University of St Andr st-andrews.ac.uk; Fax: +44 (0)1334463808; † Electronic supplementary information from XRD patterns, SEM and TEM image curves and the textural properties obt curves, impedance spectra of single cell different operating temperatures, XR adsorption/desorption curves for La1.7C citric acid method. See DOI: 10.1039/c5ta Cite this: J. Mater. Chem. A, 2015, 3, 13468


Introduction
Solid oxide fuel cells (SOFCs) have been considered as one of the most advanced power generation technologies for environmentally friendly power generators due to their high energy conversion efficiency, fuel adaptability and so on. 1,2 However, their high operating temperatures (800-1000 C) have brought in strict requirements (e.g., avoiding chemical reactions and thermal expansion mismatch between electrodes and electrolyte) on the electrode and interconnect materials, which limit the commercialization and wide usage of SOFCs. 1,3 Consequently, extensive efforts have been devoted to the development of IT-SOFCs between intermediate-temperature range (500-750 C) to improve long-term stability, provide more cell material choice and decrease costs. [4][5][6][7][8] However, the decrease of operating temperature usually leads to the reduction of electrical performance because of the polarization loss of the cathode. [9][10][11][12] Commercial cathode materials including Sr or Mn, like LSCF (La 1Àx Sr x Co y Fe 1Ày O 3 ) and LSM (La 1Àx Sr x MnO 3 ), present cation segregation at the surface over the long term that would reduce the cathode reactivity and stability in oxygen reduction reactions (ORR). [13][14][15][16][17][18] Therefore, it is essential to develop effective cathode materials for IT-SOFCs with high electrochemical activity.
Some perovskite-type oxides have mixed ionic-electronic conducting (MIEC) and have been widely investigated as the cathode materials for IT-SOFCs due to their good performance for ORR. 19,20 Recently, MIECs based on layered perovskites with K 2 NiF 4 -type structure have received extensive interest as cathode materials for IT-SOFCs, owing to their relatively high oxygen ion diffusivity, rapid surface exchange property, and compatible thermal expansion coefficients with solid electrolytes. 20,21 Most of the studies have been focused on the oxygen over-stoichiometry layered perovskites, especially La 2 NiO 4 based materials, which have been reported to have good ionic conductivity because of the interstitial oxygen. [22][23][24][25] However, the low electronic conductivity of La 2 NiO 4 limits its further practical use as SOFC cathodes. 26 In addition, as another archetypal material with K 2 NiF 4 structure, layered La 2 CuO 4 has previously been investigated for its electrical conduction properties, such as superconductivity at low temperature, and metallic/semi-conducting properties at room temperature. 27,28 According to many previous reports, introducing alkaline-earths (e.g., Sr, Ca, Ba) at the La-site provides effective routes to further improve their electrical conductivity through formation of electron-holes, oxygen vacancies or interstitial oxygen. [28][29][30][31] Partially substituting Cu with other transition metals (e.g., Ni) could also improve their ionic conductivity which would enhance the performance of these substituted La 2 CuO 4 oxides as cathode materials of SOFC, 32,33 however, the Sr-doped La 2 CuO 4 exhibits the issues of high-temperature synthesis, as well as low stability with electrolytes. 34 Although some encouraging results have been reported, good evidence regarding SOFC cathode with high performance has not yet been available for this system. 33 For the purpose of this study, we have chosen these methods: doping Ca on the La-site and Ni substituting on the Cu-site, to enhance both the oxygen diffusion and their electrochemical performance as a cathode of IT-SOFCs.
Besides the composition, the microstructure of cathode also has inuence on the electrochemical properties and performance of SOFCs. [35][36][37][38] Nanoscaled structure can accelerate oxygen reduction kinetics by providing enlarged number of active sites for surface oxygen exchange, which has great effect on the ORR of mixed ionic and electronic conductors. 36,37,39,40 Thus, in this study, we examined the applicability of the hierarchically nanostructured La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd (0 # x # 0.75) cathode materials which were prepared by a facile citrate-modied evaporation-induced self-assembly (EISA) method or conventional citric acid method with the purpose of enhancing electrochemical performance under IT-SOFCs operating conditions, as well as investigating the effect of nanoparticle size on the performance. We systemically investigated the ratio of nickel to copper on the B-site of La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd (0 # x # 0.75) and nanostructure morphologies with an effort to optimize their electrochemical properties. We report dramatically high power densities for the cell using nanostructured La 2 CuO 4 series cathodes which are optimized their electrochemical properties with YSZ electrolyte. product was ground to a ne powder in a mortar and pestle and calcined in air at 300 C for 3 h with the heating rate of 1 C min À1 , followed by 750 C for 2 h with 5 C min À1 . The resulting products were used for coating cathode materials onto YSZ electrolyte. Part of the resulting product obtained from 750 C was further calcined in air at 900 C for 2 h to check their crystal structure and morphology change.

Powder preparation
Fuel cell fabrication and testing 52 wt% NiO, 28 wt% YSZ powder (Praxair: SSA 56.4 m 2 g À1 ) and 20 wt% graphite ake pore-former were mixed by roll mixing with the addition of 2-propanol solvent. Porous NiO-YSZ pallets were prepared by conventional pressing and ring at 1150 C for 3 h. Then, a 55 wt% NiO-45 wt% YSZ slurry was dipcoated onto these sintered NiO-YSZ pallets to obtain a functional layer aer ring at 1150 C for 3 h. Followed, YSZ electrolyte was prepared by dip-coating YSZ ink onto the NiO-YSZ functional layer side of these sintered NiO-YSZ pallets. Finally, the pallets were heated at 1500 C for 6 h to obtain NiO-YSZ supported YSZ electrolyte. La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd cathodes were prepared by hand painting nanostructured La 1.7 Ca 0.3 Ni x -Cu 1Àx O 4Àd powders onto electrolyte with 5 mm in diameter. The cathodes were then red at 900 C for 2 h in air. And, silver wire (0.25 mm, 99.99%, Advent Research Material Ltd) and silver paste (9912-G, ESL EUROPE) were employed for current collection.
For electrochemical performance and impedance measurements of the unit cell, Ni-YSZ|YSZ|La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd , were performed in humidied H 2 and air or O 2 used as the fuels and oxidant, respectively, using an IM6 Electrochemical Workstation (ZAHNER, Germany) with frequency ranged from 0.1 Hz to 100 kHz with amplitude of 20 mV. Impedance plots were tted using the Zview soware.

Characterisation
Powder X-ray diffraction (XRD) patterns were recorded at room temperature on a PANalytical Empyrean Reection Diffractometer using Cu Ka radiation (l ¼ 1.541Å). The roomtemperature crystal structure and the unit cell parameters were analyzed using the GSAS soware. The morphologies of all samples and cells were observed on a JEOL JSM-6700 Field Scanning Electron Microscopy (FESEM). Transmission electron microscope (TEM) was performed using a JEOL JEM-2011 electron microscope at 200 kV. Nitrogen adsorption/desorption measurements were carried out under liquid nitrogen temperature (77 K) with a Tristar 3020 Instrument (Micrometrics Instrument Corp., Norcross, GA). The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specic surface areas. By using the Barrett-Joyner-Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. The thermal expansion coefficient of the pellets in air was investigated using a Netzsch Model DIL 402 C instrument that was equipped with Proteus analysis soware. Chemical compatibility between La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd cathode materials and YSZ electrolyte was evaluated by XRD analysis of an intimate mixture of 50 wt% La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd and 50 wt% YSZ red at 900 C for 5 h.  (Fig. 1a). The XRD patterns aer nal renement using tetragonal I4/mmm model were shown in Fig. S1. † Their room-temperature lattice parameters a and c, and cell volume were summarized in Table 1. The a parameter increases slightly while the c parameter decreases signicantly with the increasing Ni content in Cu-site, consistent with a decrease in the Jahn-Teller distortion for solid state samples induced by the presence of Cu(II) in the structure, similar to the reported results. 32 And the decrease in cell volume with the increasing Ni is considered resulting from the smaller ionic radii of Ni 2+ (r Ni 2+ ¼ 0.69Å) than that of Cu 2+ (r Cu 2+ ¼ 0.73Å). Their grain sizes were estimated from peaks (103), (110), (200) Fig. 3 and S2. † Although the primary particle size is about 50-400 nm, these samples possess hierarchically nanoporous structures aer calcination at 750 C for 2 h, as shown in Fig. 3a-d. 41,42 This is maybe caused by the addition of P123 copolymer and citric acid during the synthesis process to favour the formation of hierarchically nanoporous layered perovskite oxides, 43,44 which would be expected to positively increase the electrochemical performance. Even on further sintering the samples (Fig. 3e-h) at 900 C for 2 h, their hierarchically nanoporous structures are still maintained although the particle sizes are a bit bigger than those of samples obtained at 750 C. Their TEM images displayed in Fig. S3 † also show that these particles are inter-connected with each other to form hierarchically nanoporous structures, in agreement with their SEM results.

Results and discussion
The textural properties of La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd samples aer calcination at 750 C and 900 C for 2 h were evaluated by nitrogen adsorption/desorption. There are hysteresis loops between P/P o of 0.9 and 1.0, as presented in Fig. S4, † corresponding to the secondary mesoporous/macroporous structures produced by inter-aggregated particles. 45,46 Their pore size distribution curves shown in Fig. S4 † also demonstrate the coexistence of mesopores and macropores. Their BET specic surface areas (SSA), pore volume and average pore size calculated from N 2 adsorption isothermal curves were summarized in Table S1. † The results indicate that the BET SSA, pore volume and average pore size aer calcination at 900 C for 2 h are smaller than those of samples obtained at 750 C for 2 h. However the BET SSA and pore volumes are still high for samples obtained at 900 C. Consequently, these samples obtained at 750 C and 900 C could possess hierarchically mesoporous/macroporous nanostructures with high BET SSA and pore volumes, which are attributed to the large amount of secondary pores among particles, in agreement with the SEM and TEM results. represents the impurity phase La 2 CaCu 2 O 6 . In order to maintain the nanostructures, the hierarchically nanoporous cathode materials coated onto the YSZ electrolyte were red at 900 C for 2 h rather than being red at higher temperature which would benet the adherence between cathode and electrolyte if reaction could be avoided. The SEM images of the single cells using La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd oxides shown in Fig. S5 † indicate that the hierarchically nanoporous structures of La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd cathode materials on the YSZ electrolyte were still remaining aer sintering at 900 C for 2 h. For all these four samples, the YSZ electrolyte is approximately 20 mm in thickness prepared by dip coating process and is adhered well to both cathode and anode layers without any delamination. The thickness of the La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd cathode materials is around 15-20 mm with a highly hierarchically nanoporous structure that favours the gas diffusion. These cathode materials all show small particle size even aer heattreatment at 900 C for 2 h. Furthermore, these particles connected with one another, forming the secondary mesopores/ macropores, which would increase the tri-phase boundaries to enhance the ORR on the cathode surfaces.
The power generation property of the cells using the La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd powder as cathode, humidied H 2 as fuel and air as oxidant, respectively, with respect to the temperature, for the Ni-YSZ support and YSZ electrolyte systems is shown in Fig. 4. The maximum power densities (MPD) of the cells with the La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd (x ¼ 0, 0.25, 0.50, and 0.75) cathode materials were summarized in Fig. 5, in which their MPD at 850 C were 1.5, 0.67, 0.74 and 0.89 W cm À2 , respectively. Compared to previous data on electrochemical performance of cuprate cathodes fabricated at higher temperature, 32 much improved performance is achieved by the hierarchically nanoporous La 1.7 Ca 0.3 CuO 4Àd cathode. This suggests that low-temperature ceramic processing to fabricate nanostructured cathode layer offer a suitable potential on the cuprate cathode of SOFCs to effectively overcome its lower chemical compatibility in the long term. For La 1.7 Ca 0.3 Ni x -Cu 1Àx O 4Àd (x ¼ 0.25, 0.50, and 0.75) samples, their power density slightly increased with Ni content over the whole temperature range, which can be attributed to their increased oxygen ion mobility with Ni content. The highest MPD achieved by the La 1.7 Ca 0.3 CuO 4Àd cathode at high temperature range can be attributed to its high electrical conductivity while doping Lasite with Ca would result in enhanced oxygen ion mobility by the formation of oxygen vacancies and/or interstitial oxygen. 28,31 As shown in In order to further study the cathodic polarization (R p ) during cell performance, impedance spectroscopy results were considered, taking the impedance intercept of high, mid and low frequency with the real axis of the Nyquist plot in order to determine the ohmic resistance (R s ) and non-ohmic resistance (R p total ¼ R p1 + R p2 + R p3 ). Fig. 6 shows the impedance spectra of the cell, Ni-YSZ|YSZ|La 1.7 Ca 0.3 CuO 4Àd , with simulated tting curve under open circuit conditions at 700 C and the whole impedance spectra for La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd as thermal variation were represented in Fig. S6 and S7. † As shown in Fig. 6, two or three semicircles were roughly observed on impedance tting plot and considering response frequency, semicircle at  high (R p1 ) and mid frequency (R p2 ) could be assigned to the surface activation, oxygen and charge transfer while lower frequency (R p3 ) should be associated with gas diffusion. Clearly the high frequency interception (R s ) is barely changed or slightly decreased by elevating temperature because ohmic resistance mainly arises from electrolyte. Fig. 7a and b show Arrhenius plots of the thermal variation area-specic resistance (ASR) for ohmic and non-ohmic, respectively. As shown in Fig. 7c, activation energy for R p1 and R p2 were dramatically decreased with the increase of Ni content on the Cu-site which can be due to the increased oxygen overstoichiometry, similar to that of La 2 Ni 1Àx Cu x O 4+d . 47 The slight improvement of electrochemical performance (e.g., MPD) with Ni-Cu couple might also reect the changing of anisotropic oxygen transport mechanism (e.g., orthorhombic distortion, Jahn-Teller distortion of Cu 2+ , increased oxygen vacancies) in tetragonal layered perovskite structure with respect to Ni contents. 47 For La 1.7 Ca 0.3 CuO 4Àd , it also exhibits structural distortion and oxygen content change with the increasing of temperature, which may contribute to its excellent performance at high temperatures. 28 On the other hand, the slight increase of activation energy for R p3 with increasing Ni is because R p3 would be related with microstructure factor and gas diffusion in electrode. Accordingly, nanoporous factor, as shown in Table  S1, † might be in reasonable agreement with R p3 polarization result. Considering the same anode and electrolyte for these cells, excepting La 1.7 Ca 0.3 CuO 4Àd , it can be concluded that Ni-Cu couple of octahedral site in layered perovskite improves surface reactivity for oxygen transfer. In addition, the thermal expansion coefficient (TEC) values of La 1.7 Ca 0.3 Ni x Cu 1Àx O 4Àd were reduced with the increasing Ni contents in Cu site, suggesting La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 O 4Àd cathode shows relatively adaptable thermal compatibility with the typical SOFC electrolyte materials, as shown in Table 2. Therefore, considering a better stability, lower operating temperature, higher MPD at low temperature and more suitable TEC, hierarchically nanoporous La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 O 4Àd would be promising as a potential cathode for IT-SOFCs.

Effect of cathode particle size and morphology
As discussed in aforementioned paragraphs, La 1.7 Ca 0.3 Ni 0.75 -Cu 0.25 O 4Àd shows the highest powder density during intermediate-temperature range (#750 C). To further investigate the effect of particle size and morphology on the electrochemical performance, we prepared another kind of La 1.7 Ca 0.3 Ni 0.75 -Cu 0.25 O 4Àd using conventional citric acid method for comparison. The XRD patterns and SEM images in    The single cells were prepared by using Ni-YSZ|YSZ anode supported electrolyte, in which the thickness of electrolyte is ca. 30 mm. The cells were tested using pure H 2 and O 2 as the fuel and oxidant, respectively. The electrochemical performance, OCV, and MPD of the cells with La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 O 4Àd cathodes are shown in Fig. 8. The results indicate that La 1.7 -Ca 0.3 Ni 0.75 Cu 0.25 O 4Àd cathode prepared by citrate-modied EISA method shows a bit higher MPD than that of La 1.7 Ca 0.3 Ni 0.75 -Cu 0.25 O 4Àd by conventional citric acid method during the whole temperature range, especially at low temperature (e.g., 750 C), suggesting its higher ORR activity at low temperature based on the consideration of the same Ni-YSZ anode and YSZ electrolyte for these two cells. However, with the increasing of testing temperature, the difference between the two cathodes decreased and their MPD are almost the same at 850 C, indicating high ORR activity in both cathode samples at 850 C. However, during the whole testing temperature range, the OCV values are relatively higher when using La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 -O 4Àd obtained by citrate-modied EISA method as cathode, as shown in Fig. 8c, which might be obviously caused by its bigger grain size, higher surface area, pore volume and surface activity for ORR.
The impedance spectra, IR loss and overpotential h are displayed in Fig. 9. The results in Fig. 9a and b show that the R p is a bit smaller when using La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 O 4Àd from citrate-modied EISA method as cathode materials while there were no obvious differences in non-ohmic resistance. The smaller R p can be attributed to the improved oxygen ion mobility due to its bigger grain size. However, the overpotential h results shown in Fig. 9c indicate that the total overpotential h in the single cell using La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 -O 4Àd from citrate-modied EISA method as cathode was smaller than that from conventional citric acid method. It might be attributed to the more three-phase boundary active site, and better oxygen ion mobility and adhesion between electrolyte and the La 1.7 Ca 0.3 Ni 0.75 Cu 0.25 O 4Àd cathode from citrate-modied EISA method, since it has a bit bigger grain size, higher surface area and pore volume. However, which is the predominating effect, grain size, surface area or pore volume, is still under investigation.