Microstructural characterization and electrochemical properties of Nd0.5Sr0.5Co0.5Mn0.5O3-δ as a novel cathode for SOFCs

The microstructure and electrochemical properties of a novel cathode Nd0.5Sr0.5Co0.5Mn0.5O3-δ were discussed in the paper. After calcination at 1000 °C, a perfect single crystal was obtained. Some diffraction streaks along [200] and [112] patterns in HR-TEM appeared for the ordering of oxygen vacancies, or as the overlap of B-site cations (Co2+/Co3+ and Mn2+/Mn3+) with stacking fault-derived scattering of ordering. Area-specific resistance (ASR) was 0.023 Ω · cm2 when Nd0.5Sr0.5Co0.5Mn0.5O3-δ was deposited on the electrolyte as the electrode at 700 °C in air. The maximum power density and the maximum OCV were 592.80 mW·cm−2 at 650 °C and 0.89 V at 550 °C for a single cell, respectively. Hence, the material Nd0.5Sr0.5Co0.5Mn0.5O3-δ could be considered as an air-electrode in Intermediate-temperature solid oxide fuel cells (IT-SOFC).


Introduction
Recently, more attention has been directed towards IT-SOFCs (Intermediate Temperature Solid Oxide Fuel Cells) for the shorter start-up time, the lower requirements as electrode materials and the low maintenance [1,2]. However, lower operating temperatures raises some concerns, such as reducing kinetics of oxygen reduction reaction (ORR), which makes the catalytic activity of cathode materials poor [3,4]. Hence, the most critical technology to develop IT-SOFC's is searching for suitable electrode materials, especially those suitable for medium-temperature cathode materials [5,6]. High conductivities of electronic and ionic are necessary for the cathode materials to ensure enough catalytic activity for ORR [7].
Among cathode materials, perovskite oxides [8][9][10][11] and Ruddlesden-Popper (R-P) series [12] attract significant attention and have been widely studied as promising electrode materials for IT-SOFC. Bu et al [13] and Tsvetkov et al [14] have reported that Nd replaced La in the LnSrCoMnO 6-δ and Ln 1-x Sr x MnO 3 series of cathode materials (Ln represents the element from La to Gd) can obtain superior oxygen exchange activity. Kim et al [15][16][17] also proved that the addition of Nd at A-site could enhance the stability and improve the electrochemical performance of single cells. Electrode materials containing cobalt are considered to be the significantly promising since the element Co has three variable valences (Co 2+ , Co 3+ and Co 4+ ) [18]. Double perovskites containing Co, such as PrBaCo 2 O 5+δ and GdBaCo 2 O 5+δ have exhibited excellent electrocatalytic activities for ORR at low temperatures [19][20][21]. However, some properties limit their applications, such as mismatch of thermal expansion coefficients, poor of long-term stabilities [22,23]. Efforts have been made towards solving these problems. For example, Hong and Wang et al [24][25][26] proved that electrode materials mixed with Ce 0.8 Sm 0.2 O 1.9 (SDC) electrolyte in a certain ratio can decrease area specific resistance (ASR) and extend triple phase boundaries between fuel, electrode and electrolyte. Therefore, in this work, elements Nd and Sr were doped in A-site and Co and Mn were substituted with B-site to form Nd 0.5 Sr 0.5 Co 0.5 Mn 0.5 O 3-δ (NSCM). The electrolyte was SDC, and the cathode material was the mixture of SDC with NSCM. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Experimental sections
NSCM electrode material was prepared by the method of EDTA-citric acid-nitrate. Sr(NO 3 ) 2 , Nd(NO 3 ) 3 ·6H 2 O, Co(NO 3 ) 3 ·6H 2 O and C 4 H 14 MnO 8 as the raw materials were mixed in stoichiometric amounts (According to the molar ratio of the elements in the formula) in a beaker, added to deionized water, and stirred until all salts dissolved. In a separate mixture, EDTA and NH 3 ·H 2 O were mixed to obtain EDTA-NH 3 ·H 2 O. The molar ratio of citric acid as a chelating agent was added in the EDTA-NH 3 ·H 2 O, and then the complex was dropped into the nitrate mixture. Ammonia was added to balance pH value. Heating and stirring the nitrate mixture at 80°C until sol-gel was obtained. The sol-gel was placed in an oven and continually heated at 200°C until dry bubble film was obtained. Finally, the film was ground to get powder by ball mill, and then the powder was shifted into a muffle furnace to calcine at different temperatures for 12 h in air.
X-ray diffraction (XRD, Bruker D8) using Cu Kα radiation at RT was carried out to affirm phase formation of the cathode materials after calcination at different temperatures. Diffraction peaks were captured with 2θ range from 10 to 80 degrees when the step size is 0.02 degree.
The cathode material's powder morphology was observed by scanning electron microscope (SEM, S-4800IIFESEM, Japan). Mapping of every element and micromorphology of NSCM were collected on a High-Resolution Transmission Electron Microscopy (HR-TEM, Tecnai G2 F20, US). SDC was selected as the electrolyte for its wide application in IT-SOFC cathodes [27]. In order to decrease ASR and extend triple phase boundaries between electrode and electrolyte, SDC and NSCM were mixed with weight ratio 1: 5 (electrolyte: electrode material) [28][29][30]. The mixed powders were added in a mixed solution containing terpineol and ethocel at the heating conditions. Then the viscous solution was spread on SDC disks. Symmetrical cell NSCM-SDC|SDC|NSCM-SDC (Supported by electrolyte) was fabricated to collect ASR with an EG&G PAR model 273A. The EIS was recorded from 0.1 Hz to 100 kHz with an amplitude of 10 mV.

Results and discussion
XRD patterns at RT 2θ from 10°to 80°was seen for Nd 0.5 Sr 0.5 Cr 0.5 Mn 0.5 O 3-δ powder which were calcined at an interval of 50°C from 800°C to 1000°C were shown in figure 1(a). Some impurity peaks (at 2θ=27.264°, marked with * . According to the analysis of Jade 5, the impurity phase was SrMnO 3 corresponding to PDF# 84-1612) appeared when the samples were calcined before 950°C (including 950°C) as shown in figure 1(b). A perfect single crystal perovskite structure was prepared after calcination at 1000°C. As shown in figures 1(a) and (b), the NSCM sample after calcination in 1000°C, has a perfect orthorhombic perovskite structure with space group Imma(74). The diffraction patterns of NSCM samples had three main diffraction peaks at 2θ=33.08°, 47.53°, 59.10°, corresponding to Where D is the crystallite size, K is the Scherrer constant, λ is the x-ray radiation wavelength (Cu K α : λ=0.15406), θ is the Bragg angle, and β is the full width at half maxima. By fitting the Scherrer formula with equation (2), D can be obtained. It is obvious that β and θ for samples NSCM decreased with the raising of calcination temperatures while the particle sizes increased. Moreover, the lattice parameters of NSCM after calcination at 800, 850, 900, 950 and 1000°C samples at room temperature extracted from Rietveld refinement of XRD data are included in table 1 [29]. These data were in good accordance with the XRD patterns, and they showed the low reliability factors (R).
Energy-dispersive x-ray spectroscopy (EDS) in TEM was used to analyze atomic resolution chemical mapping in the NSCM material as shown in figures 2(a)-(e). Element mapping images (figures 2(b)-(e)) and EDX curves (figure 2(f)) confirmed that the elements of Nd, Sr, Co, Mn in the as-prepared composite were evenly distributed. The microscopic details of the sample were determined by HRTEM. As depicted in figure 2(g), the microstructure of NSCM was cuboid varying in size. The distance of diffraction fringe was 2.749 Å as shown in HRTEM images of NSCM. The result conformed to [200] planes. The indexed in orthorhombic lattice of the reflections in FFTs (fast Fourier transforms) can be judged from XRD patterns. The intensity streaks along [112] and [200] planes in (120) diffraction pattern could be found, which can be attributed to ordering of oxygen vacancies [31], or as the overlap of stacking derived fault scattering for B-site cations (Co(II) and Mn(III)) ordering [32,33]. Generally, in perovskite structure oxides, B-site ionic ordering is based on the differences between charge and size of the two cations [24]. Thus, it has a bearing on the presence of Co 2+ /Co 3+ and Mn 2+ /Mn 3+ couples in NSCM.
Symmetrical cell NSCM-SDC/SDC/NSCM-SDC was used to assess the effects of the cathode material in the electrocatalytic activity. The symmetrical cell was operating at 450°C −700°C in air, and EIS was used to determine ASRs.
In the Impedance spectrum, there are two parts in terms of the frequency occupying low and high: Ohmic resistance (R ohm ) and cathode polarization resistance (R p ). R ohm is the arc at high frequency which is considered from the connector resistances, electrolyte, the wire resistances, and other contactor's resistances. R p is the arcs at an intermediate-to-low frequency, which has a bearing on the electrode process [34][35][36]. Figure 3(a) gives the curve of the relationship between R p and temperature when the cell NSCM-SDC/SDC/NSCM-SDC was operating at different temperatures. With the temperature rising, semicircles formed by an arc intersecting a real axis associated with the cathode polarization process were obviously decreased; it was illustrated that R p decreased with increasing of operating temperatures. According to the change of polarization resistance with temperature, it can be seen that the polarization resistance decreases with the increase of temperature, indicating that the higher the temperature is, the greater the surface exchange coefficient and volume phase diffusion coefficient are. In other words, the higher the temperature is, the faster the oxygen ion migration will be. R p was 0.023 Ω·cm 2 at 700°C. The result is lower than the value (0.18 Ω·cm 2 ) at the same temperature reported from [37].
Commercial NiO was used as the anode material, and SDC with thickness about 19.6 μm was used as the electrolyte. NiO-SDC/SDC/NSCM-SDC (single cell with electrolyte for support) was fabricated to assess the electrochemical properties of cathode material NSCM. Figure 4 gave a cross section SEM image of electrodeelectrolyte-electrode.
I-V and I-P curves were obtained under the operating conditions (operating temperatures at 450°C to 650°C ; anode gas was 100% H 2 and cathode gas was O 2 ). The electrochemical performance results were provided in figure 5. The maximum power densities were 417.30 mWcm −2 , 565.45 mWcm −2 and 592.80 mWcm −2 at 550