The Influence of the Substituting Element ( M = Ca , Sr , Ba ) in La 1 . 7 M 0 . 3 NiO 4 + δ on the Electrochemical Performance of the Composite Electrodes

The actual work focuses on the development of electrochemically active and stable electrodes for a high temperature proton-conducting electrolyte with a perspective of application in intermediate temperature electrochemical devices. The comparative study of the electrochemical performance of the La1.7M0.3NiO4+δ – based (M = Ca, Sr, Ba) composite cathodes with proton-conducting BaCe0.89Gd0.1Cu0.01O3 or oxygen-ionconducting Ce0.8Sm0.2O1.9 ceramic components in contact with the proton-conducting electrolyte BaCe0.89Gd0.1Cu0.01O3 was performed by an impedance spectroscopy in wet air during 1500 h. The composites were used as functional layers in bi-layered electrodes with current collector layers made of 98 wt.% LаNi0.6Fe0.4O3 + 2 wt.% CuO or 99.4 wt.% La0.6Sr0.4MnO3 + 0.6 wt.% CuO.


Introduction
High temperature proton-conducting oxide materials are of great fundamental and practical interest because of the phenomenon of proton conductivity which appears along with oxygen-ionic conductivity when placed in a humidified atmosphere [1,2]. Usage of such co-ionic electrolyte materials in solid oxide fuel cells (SOFCs) will give increased efficiency as a result of the higher open circuit voltage value and, correspondingly, the higher power output compared with SOFCs based on unipolar oxygen-ion-conducting electrolytes [3,4]. Among high temperature proton conductors, BaCeO 3 doped with acceptor dopants (such as Gd, Sm, Y) demonstrates the highest co-ionic conductivity [5,6]. BaCeO 3 -based materials, despite some difficulties with their practical application because of weak chemical stability in the presence of salt-forming gas components (such as CO 2 , H 2 S, SO 2 ), could be the ideal electrolytes to be used in the pure hydrogen-fed SOFCs and other electrochemical devices such as electrolyzers, hydrogen and combustible gas sensors etc [1,7].
The new electrolyte materials require well matched and electrochemically active electrode materials. The first studies concerning SOFCs based on BaCe(Zr)O 3 reported the use of a platinum cathode [7]. However, Pt is not preferred for practical applications due to its high cost. Moreover, the Pt electrodes were found to show significant polarization losses caused by the limited number of reaction sites at the cathode/electrolyte interface. From this viewpoint, cathode materials with mixed oxygen-ionic and electronic conductivity widely used in SOFCs on the base of unipolar oxygen-ion conductors (such as stabilized ZrO 2 , doped CeO 2 , doped LaGaO 3 ) so far as their application permits to broaden significantly a zone of electrochemical reaction [8], can be also applied in SOFCs with co-ionic conductors. However the development of an electrode material with proton conductivity would still be preferable since such cathodes allow the simultaneous transport of ionic (proton, oxygen-ion) and electronic defects under typical fuel cell operating conditions, thus offering to extend the electrochemically active area and correspondingly, decrease the polarization losses [9]. Additionally, electrode materials must be thermodynamically stable under working conditions: 400-900 °C, 10 −5 < pO 2 /atm ≤ 0.21, in the presence of H 2 O and CO 2 . Thermal affinity between electrolyte and cathode materials should be considered in order to attain both long-term stability and cycling.
Over the past 10 years the layered nickelate La 2 NiO 4+δ (LNO) with the coefficient of thermal expansion (CTE) α equal to 12.9-13.5 × 10 −6 K −1 , high value of the oxygen surface heteroexchange constant k (2.5 × 10 −6 cm s −1 at 800 °C) and oxygen diffusion coefficient D (1.7 × 10 −7 cm 2 s −1 at 800 °C) [10] has been the subject of intensive studies including its application in an SOFC with proton conducting electrolytes [11][12][13][14]. As far as La 2 NiO 4+δ is a p-type conductor in the temperature range 400-900 °C, appropriate acceptor doping in A-site could improve its electronic conductivity by generating extra electron holes for charge compensation. Experimental results reveal that doping with a certain amount of alkaline-earth metals (Ca, Sr. Ba) leads to an increase in total conductivity of substituted La 2 NiO 4+δ [15][16][17]. It has been found that LnNiO 4+δ (Ln = Pr, Nd) substituted by Ca and Sr show better cathodic properties than the undoped materials [18][19][20]. But there are few studies dealing with the electrochemical properties of substituted La 2 NiO 4+δ . In our previous work on the development of the cathodes for Ce 0.8 Sm 0.2 O 1.9 electrolyte we showed that despite having increased conductivity, Sr and Ca-substituted La 2 NiO 4+δ possessed lower polarization conductivity in comparison with the undoped one [17,21]. We found that one possible way to increase the electrochemical performance was to use the composite materials with an ionic conductor in their composition, preferably of the same chemical composition as the electrolyte support, as a functional layer. In the case of a proton conducting electrolyte support a proton conducting ceramic in the composite electrode material would be preferable in terms of broadening the zone of electrochemical reaction and having a better match in CTE with the electrolyte.
The actual work focuses on the comparative study of the electrochemical performance of the La 1.7 M 0.3 NiO 4+δ -based bi-layered electrodes (M = Ca, Sr, Ba) in contact with the proton-conducting electrolyte BaCe 0.89 Gd 0.1 Cu 0.01 O 3 . The electrolyte material was selected because of its superior densification at low sintering temperatures and high proton conductivity [2,6,22] [17,23]. The performance of the electrodes and their long-term stability during 1500 h were investigated by impedance spectroscopy in wet air (5% H 2 O). The electrolytes BaCe 0.89 Gd 0.1 Cu 0.01 O 3 (BCG-Cu) and Ce 0.8 Sm 0.2 O 1.9 (SDC) were synthesized using ceramic technology. After mixing, the initial components were calcined for 2 h at 1100 and 1050 °C, respectively, and ball-milled. The intermediate products were dry-pressed into disks at 150 MPa. The disks were then sintered at 1400 °C (BCGCu) and 1550 °C (SDC) for 5 h. Density of BCGCu and SDC electrolyte disks, determined from their geometrical dimensions and weights, was equal to 93-95% of the crystallographic values. XRD analysis of the powdered samples was performed by DMAX-2500, Rigaku Co. Ltd. diffractometer using Ni-filtered Cu Kα radiation in the range of 15 ≤ 2θ ≤ 85º. The thermal expansion of the materials was carried out using a Tesatronic TT-80 dilatometer between the room temperature and 900 °C with a heating/cooling rate of 3 °C min -1 in air. The average values of the coefficient of thermal expansion CTE were calculated on the linear section of dilatometric data.

Experimental
To prepare the composite electrode materials the electrolyte disks were ground up to no less than 2.5 m 2 /g and mixed with the electrode powders in 1:1 weight proportion with the addition of alcohol and a polyvinylbutyral organic binder. The electrodes were sited symmetrically on the BCGCu electrolyte substrates with the electrode surface area S equal to 0.25-0.36 cm 2 in the form of bi-layered coatings by brush-painting. The functional layers LCNO-BCGCu, LCNO-SDC, LSNO-BCG-Cu, LSNO-SDC, LBNO-BCGCu and LBNO-SDC were sintered at 1200 °C. After sintering, the thickness of the functional layers was about 20-25 µm. The collector layer was sited onto the pre-sintered functional layers. The sintering temperatures of the collector layers were 1050 and 1160 °C for LNF + 2 wt.% CuO and LSM + 0.6 wt.% CuO, respectively. The thickness of the collector layers was 50-90 µm to ensure a lower in-plane resistance of the electrodes, which is in a high importance for the electrodes in terms of practical application. The electrode morphology was characterized by scanning electron microscope JSM-5900LV (JEOL). Examples of bi-layer electrodes are presented in Fig. 1.
The electrochemical study was carried out using FRA-1260 with EI-1287 (Solartron Instruments Inc.). Up to six test samples were placed into a measuring cell. Each of them was pressed between two platinum current collecting nets with a thickness of 0.01 cm and a mesh size of 0.1 × 0.1 cm, which were connected to the measuring equipment by means of Pt wires via a two-electrode four-cable mode that permits the exclusion of the impedance of current-supplying cables from the overall impedance of the system. Measurements were performed in the temperature range 550-850 °C in wet air (5% H 2 O) in the frequency range of 0.01 Hz to 100 kHz at the amplitude of applied sinusoidal signal of 20 mV. Each measurement was finished by measuring a full dc resistance R dc of the cell.
The polarization resistance (R η ) of the electrochemical cell with symmetrically arranged electrodes was calculated as follows: serial resistance determined by extrapolation of the high-frequency region of an impedance spectrum to the intersection with the x-axis and related to the electrolyte resistance, contact electrolyte-electrode resistance and a part of lateral resistance of the electrode.
The long-term study on the samples with bi-layered electrodes was performed at 700 °C during 1500 h. The test samples with single-layered electrodes without collector were subjected to the same conditions and after holding for 1000 h were retrieved for XRD testing. At 500 and 1000 h of testing the samples were cooled down to room temperature and heated again.

Results and Discussion
According to XRD (Table 1), the individual electrode materials were single-phase with a tetragonal (LCNO, LSNO, LBNO), hexagonal (LNF) and rhombohedrally distorted perovskite (LSM) structure. The electrolyte materials (XRD analyses were performed after high-temperature sintering at 1400 and 1550 °C for BCGCu and SDC, respectively) were characterized by a perovskite structure for proton conducting BCGCu electrolyte and by a fluorite structure in the case of oxygen ion conducting SDC.
LCNO, LSNO and LBNO in combination with both BCGCu and SDC ceramic component demonstrated good adhesion to the BCGCu electrolyte after sintering at 1200 °C. XRD analysis of the sintered functional layers revealed the appearance of some secondary phases (  (1) where S is an electrode surface area, R dc is a dc resistance of the electrochemical cell, and R hf is a   interaction is strongly dependent on the ionic radius of the rare earth element (REE) and it decreases with radius increasing (0.99, 1.12 and 1.34 Å for Ca, Sr and Ba). It is probably connected with a decrease in the solubility limit of the elements in a fluorite structure, which, as is known from the literature, is equal to 20, 8 and less than 2 mol.% for Ca, Sr and Ba, respectively [24]. Nevertheless, the interaction of substituted nickelates with SDC is remarkably lower than that in the case of undoped La 2 NiO 4+δ [21]. The interaction in sintered composites with BCGCu was greater, except of Ba. The amount of the BaO extracted when evaluated from XRD data reached 4-6% in case of LCNO and 7-8% in case of LSNO, and in case of LBNO only trace amount of secondary phases was registered.  Table 2 in the recent work of J. Hou et al. [14]). All the developed electrodes in this work exhibited superior characteristics. Therefore, the experimental results reveal that fabrication of the bi-layered electrodes with a composite functional layer made of substituted LNO in a mixture with an ionic or co-ionic ceramic component and with an LNF collector layer is a feasible way to improve electrode characteristics.
The specific serial resistance of the cells with the composite electrodes studied is the highest for LSNO-BCGCu (Table 3, Fig. 3). It is most probably connected with the strong interaction between the components in the composite material. The close agreement in the values of the activation energies of the reciprocal serial resistance of the electrodes (0.42-0.46 eV) and the total conductivity of the BCGCu electrolyte (0.47 eV) suggests that the serial resistance is determined, mainly, by the resistance of the electrolyte support.  Table 3 Polarization (R η ) and serial resistance (R hf ) of the composite electrodes measured after sintering (0 h) and after long-term testing (1500 h); energies of activation of polarization, serial conductivity and total conductivity in air for BCGCu electrolyte  The long-term tests showed that serial resistance of all the electrodes investigated decreased during the first 500 h with subsequent total growth no more than 11% of the initial value (Table 3, Fig. 4 a). Polarization resistance of both composite electrodes with Ca-substituted nickelate increased dramatically during testing and in fact, the greatest changes took place during the first 500 h (Fig. 4 b,  c). The LBNO-SDC composite electrode showed moderate changes while the polarization resistance of LBNO-BCGCu, LSNO-SDC and LSNO-BCG-Cu decreased with time. Figure 5 shows the spectra of the composite electrodes before and after long-term testing. All the measured spectra were asymmetric in shape, implying more than one electrode process. Usually for the description of spectra of LNO-based electrodes the equivalent circuits containing two [14,25] or three distributed elements [26], composed of a constant phase element (Q) in parallel with a resistance (R) are used. The equivalent capacitance C and relaxation frequency f of an electrode process corresponding to a specific (RQ) can be applied as characteristic parameters to identify various electrode processes. The high-frequency electrode processes with the effective capacitances of approximately 10 −6 -10 −5 F cm −2 have been associated with the transport of oxygen ions across the interface between the electrode and electrolyte. In the middle frequency range the processes with the effective capacitances of 10 −4 -10 −3 is assigned to the diffusion of oxygen ions into the electrode volume accompanied by a charge transfer, while in the low-frequency range the processes with the effective capacitances of 10 −3 -10 −1 is ascribed to the adsorption and dissociation of the molecular oxygen on the electrode surface [26].  According to an estimation of the contribution of the high-and low-frequency processes in the total polarization resistance by the analysis of the spectra before and after 1500 h of testing the following features were found: the capacitance and resistance of the high-frequency process for LCNO-BCGCu did not change with time, while the capacitance of the low-frequency reduced by the order of magnitude and the resistance grew from 4.89 to 8.6 Ω (Fig. 5  a). For LCNO-SDC the capacitance and resistance of the high-frequency process did not show significant changes as well, where as the capacitance of the low-frequency process increased by the order of magnitude and its resistance grew from 2.46 to 6.66 Ω (Fig. 5 d). Such serious changes in spectra seemed to appear due to the strong interaction of the components of the composite electrodes based on LCNO, as was mentioned above ( Table 2).
LSNO-BCGCu and LSNO-SDC (Fig. 5 b, e) showed polarization characteristics stable in time. The resistance of high-frequency processes slightly decreased with time with simultaneous small increasing the resistance of the electrode processes in the middle and low frequency ranges. Similarly to the electrodes based on Ca-substituted nickelate these changes are related to the chemical interaction between the electrode components but for Sr-substituted nickelate the interaction is lower. The main feature of both electrodes based on Ba-substituted nickelates (Fig. 5 c, f) and LSNO-SDC (Fig. 5 e) is a significant increase in the serial resistance R hf . As far as the shape of the high-frequency region of spectra did not change during long-term testing, an increase in the serial resistance may be related to incompatibility in CTE for some composite electrodes and BCGCu electrolyte. To verify this assumption the temperature dependences of linear expansion of the electrolyte and electrode materials under investigation were measured and the CTE values of the composite materials were calculated as an arithmetic mean value of the individual components. The CTE value of the BCGCu electrolyte changes from 10.5 × 10 −6 K −1 in the temperature range 100-575 °C to 8.6 × 10 −6 K −1 in the temperature range 575-900 °C, respectively, the СTE value of the SDC electrolyte in the temperature range 100-900 °C is 12.5 × 10 −6 K −1 . The individual CTE values of the LCNO, LSNO and LBNO electrode materials are 13.9, 14.2 and 15.2 × 10 −6 K −1 , respectively. The CTE values of the composites with BCGCu and SDC are 11.3-11.9, 11.4-12.4, 11.9-12.9 × 10 −6 K −1 and 13.2, 13.4, 13.9 × 10 −6 K −1 , respectively. Despite having adequate adhesion during sintering a difference of more than 30% in CTE leads to partial delamination of the electrode, increasing the serial resistance.
It should be noted that in contrast to the LB-NO-SDC electrode, which showed an increase both in serial and total resistance (Fig. 5 f), the total resistance of the LBNO-BCGCu electrode remained almost constant with time. Besides the polarization resistance of the LBNO-BCGCu electrode decreased by approximately a factor of two and reached 0.30 Ω · cm 2 after the long-term testing.

Conclusions
This work aimed to study the influence of the substituting element in the bi-layered La 1.